RIFT: Routing in Fat Trees
draft-ietf-rift-rift-10

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RIFT Working Group                                    A. Przygienda, Ed.
Internet-Draft                                                   Juniper
Intended status: Standards Track                               A. Sharma
Expires: August 1, 2020                                          Comcast
                                                              P. Thubert
                                                                   Cisco
                                                          Bruno. Rijsman
                                                              Individual
                                                       Dmitry. Afanasiev
                                                                  Yandex
                                                        January 29, 2020

                       RIFT: Routing in Fat Trees
                        draft-ietf-rift-rift-10

Abstract

   This document defines a specialized, dynamic routing protocol for
   Clos and fat-tree network topologies optimized towards minimization
   of configuration and operational complexity.  The protocol

   o  deals with no configuration, fully automated construction of fat-
      tree topologies based on detection of links,

   o  minimizes the amount of routing state held at each level,

   o  automatically prunes and load balances topology flooding exchanges
      over a sufficient subset of links,

   o  supports automatic disaggregation of prefixes on link and node
      failures to prevent black-holing and suboptimal routing,

   o  allows traffic steering and re-routing policies,

   o  allows loop-free non-ECMP forwarding,

   o  automatically re-balances traffic towards the spines based on
      bandwidth available and finally

   o  provides mechanisms to synchronize a limited key-value data-store
      that can be used after protocol convergence to e.g.  bootstrap
      higher levels of functionality on nodes.

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Status of This Memo

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   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Authors . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   8
   3.  Reference Frame . . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Topology  . . . . . . . . . . . . . . . . . . . . . . . .  13
   4.  RIFT: Routing in Fat Trees  . . . . . . . . . . . . . . . . .  15
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  16
       4.1.1.  Properties  . . . . . . . . . . . . . . . . . . . . .  16
       4.1.2.  Generalized Topology View . . . . . . . . . . . . . .  17
         4.1.2.1.  Terminology . . . . . . . . . . . . . . . . . . .  17
         4.1.2.2.  Clos as Crossed Crossbars . . . . . . . . . . . .  18
       4.1.3.  Fallen Leaf Problem . . . . . . . . . . . . . . . . .  28
       4.1.4.  Discovering Fallen Leaves . . . . . . . . . . . . . .  30

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       4.1.5.  Addressing the Fallen Leaves Problem  . . . . . . . .  31
     4.2.  Specification . . . . . . . . . . . . . . . . . . . . . .  32
       4.2.1.  Transport . . . . . . . . . . . . . . . . . . . . . .  33
       4.2.2.  Link (Neighbor) Discovery (LIE Exchange)  . . . . . .  33
         4.2.2.1.  LIE FSM . . . . . . . . . . . . . . . . . . . . .  36
       4.2.3.  Topology Exchange (TIE Exchange)  . . . . . . . . . .  46
         4.2.3.1.  Topology Information Elements . . . . . . . . . .  46
         4.2.3.2.  South- and Northbound Representation  . . . . . .  46
         4.2.3.3.  Flooding  . . . . . . . . . . . . . . . . . . . .  49
         4.2.3.4.  TIE Flooding Scopes . . . . . . . . . . . . . . .  56
         4.2.3.5.  'Flood Only Node TIEs' Bit  . . . . . . . . . . .  59
         4.2.3.6.  Initial and Periodic Database Synchronization . .  60
         4.2.3.7.  Purging and Roll-Overs  . . . . . . . . . . . . .  60
         4.2.3.8.  Southbound Default Route Origination  . . . . . .  61
         4.2.3.9.  Northbound TIE Flooding Reduction . . . . . . . .  61
         4.2.3.10. Special Considerations  . . . . . . . . . . . . .  66
       4.2.4.  Reachability Computation  . . . . . . . . . . . . . .  67
         4.2.4.1.  Northbound SPF  . . . . . . . . . . . . . . . . .  67
         4.2.4.2.  Southbound SPF  . . . . . . . . . . . . . . . . .  68
         4.2.4.3.  East-West Forwarding Within a non-ToF Level . . .  68
         4.2.4.4.  East-West Links Within ToF Level  . . . . . . . .  68
       4.2.5.  Automatic Disaggregation on Link & Node Failures  . .  69
         4.2.5.1.  Positive, Non-transitive Disaggregation . . . . .  69
         4.2.5.2.  Negative, Transitive Disaggregation for Fallen
                   Leaves  . . . . . . . . . . . . . . . . . . . . .  72
       4.2.6.  Attaching Prefixes  . . . . . . . . . . . . . . . . .  74
       4.2.7.  Optional Zero Touch Provisioning (ZTP)  . . . . . . .  83
         4.2.7.1.  Terminology . . . . . . . . . . . . . . . . . . .  84
         4.2.7.2.  Automatic SystemID Selection  . . . . . . . . . .  85
         4.2.7.3.  Generic Fabric Example  . . . . . . . . . . . . .  86
         4.2.7.4.  Level Determination Procedure . . . . . . . . . .  87
         4.2.7.5.  ZTP FSM . . . . . . . . . . . . . . . . . . . . .  88
         4.2.7.6.  Resulting Topologies  . . . . . . . . . . . . . .  94
       4.2.8.  Stability Considerations  . . . . . . . . . . . . . .  96
     4.3.  Further Mechanisms  . . . . . . . . . . . . . . . . . . .  97
       4.3.1.  Overload Bit  . . . . . . . . . . . . . . . . . . . .  97
       4.3.2.  Optimized Route Computation on Leaves . . . . . . . .  97
       4.3.3.  Mobility  . . . . . . . . . . . . . . . . . . . . . .  97
         4.3.3.1.  Clock Comparison  . . . . . . . . . . . . . . . .  99
         4.3.3.2.  Interaction between Time Stamps and Sequence
                   Counters  . . . . . . . . . . . . . . . . . . . .  99
         4.3.3.3.  Anycast vs. Unicast . . . . . . . . . . . . . . . 100
         4.3.3.4.  Overlays and Signaling  . . . . . . . . . . . . . 100
       4.3.4.  Key/Value Store . . . . . . . . . . . . . . . . . . . 100
         4.3.4.1.  Southbound  . . . . . . . . . . . . . . . . . . . 100
         4.3.4.2.  Northbound  . . . . . . . . . . . . . . . . . . . 101
       4.3.5.  Interactions with BFD . . . . . . . . . . . . . . . . 101
       4.3.6.  Fabric Bandwidth Balancing  . . . . . . . . . . . . . 102

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         4.3.6.1.  Northbound Direction  . . . . . . . . . . . . . . 102
         4.3.6.2.  Southbound Direction  . . . . . . . . . . . . . . 104
       4.3.7.  Label Binding . . . . . . . . . . . . . . . . . . . . 105
       4.3.8.  Leaf to Leaf Procedures . . . . . . . . . . . . . . . 105
       4.3.9.  Address Family and Multi Topology Considerations  . . 105
       4.3.10. Reachability of Internal Nodes in the Fabric  . . . . 106
       4.3.11. One-Hop Healing of Levels with East-West Links  . . . 106
     4.4.  Security  . . . . . . . . . . . . . . . . . . . . . . . . 106
       4.4.1.  Security Model  . . . . . . . . . . . . . . . . . . . 106
       4.4.2.  Security Mechanisms . . . . . . . . . . . . . . . . . 108
       4.4.3.  Security Envelope . . . . . . . . . . . . . . . . . . 109
       4.4.4.  Weak Nonces . . . . . . . . . . . . . . . . . . . . . 112
       4.4.5.  Lifetime  . . . . . . . . . . . . . . . . . . . . . . 113
       4.4.6.  Key Management  . . . . . . . . . . . . . . . . . . . 113
       4.4.7.  Security Association Changes  . . . . . . . . . . . . 113
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . 114
     5.1.  Normal Operation  . . . . . . . . . . . . . . . . . . . . 114
     5.2.  Leaf Link Failure . . . . . . . . . . . . . . . . . . . . 115
     5.3.  Partitioned Fabric  . . . . . . . . . . . . . . . . . . . 116
     5.4.  Northbound Partitioned Router and Optional East-West
           Links . . . . . . . . . . . . . . . . . . . . . . . . . . 118
   6.  Implementation and Operation: Further Details . . . . . . . . 118
     6.1.  Considerations for Leaf-Only Implementation . . . . . . . 118
     6.2.  Considerations for Spine Implementation . . . . . . . . . 119
     6.3.  Adaptations to Other Proposed Data Center Topologies  . . 119
     6.4.  Originating Non-Default Route Southbound  . . . . . . . . 120
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . 120
     7.1.  General . . . . . . . . . . . . . . . . . . . . . . . . . 120
     7.2.  ZTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
     7.3.  Lifetime  . . . . . . . . . . . . . . . . . . . . . . . . 121
     7.4.  Packet Number . . . . . . . . . . . . . . . . . . . . . . 121
     7.5.  Outer Fingerprint Attacks . . . . . . . . . . . . . . . . 121
     7.6.  TIE Origin Fingerprint DoS Attacks  . . . . . . . . . . . 121
     7.7.  Host Implementations  . . . . . . . . . . . . . . . . . . 122
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . 122
     8.1.  Requested Multicast and Port Numbers  . . . . . . . . . . 122
     8.2.  Requested Registries with Suggested Values  . . . . . . . 122
       8.2.1.  Registry RIFT/common/AddressFamilyType  . . . . . . . 123
         8.2.1.1.  Requested Entries . . . . . . . . . . . . . . . . 123
       8.2.2.  Registry RIFT/common/HierarchyIndications . . . . . . 123
         8.2.2.1.  Requested Entries . . . . . . . . . . . . . . . . 123
       8.2.3.  Registry RIFT/common/IEEE802_1ASTimeStampType . . . . 123
         8.2.3.1.  Requested Entries . . . . . . . . . . . . . . . . 123
       8.2.4.  Registry RIFT/common/IPAddressType  . . . . . . . . . 124
         8.2.4.1.  Requested Entries . . . . . . . . . . . . . . . . 124
       8.2.5.  Registry RIFT/common/IPPrefixType . . . . . . . . . . 124
         8.2.5.1.  Requested Entries . . . . . . . . . . . . . . . . 124
       8.2.6.  Registry RIFT/common/IPv4PrefixType . . . . . . . . . 124

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         8.2.6.1.  Requested Entries . . . . . . . . . . . . . . . . 124
       8.2.7.  Registry RIFT/common/IPv6PrefixType . . . . . . . . . 124
         8.2.7.1.  Requested Entries . . . . . . . . . . . . . . . . 124
       8.2.8.  Registry RIFT/common/PrefixSequenceType . . . . . . . 125
         8.2.8.1.  Requested Entries . . . . . . . . . . . . . . . . 125
       8.2.9.  Registry RIFT/common/RouteType  . . . . . . . . . . . 125
         8.2.9.1.  Requested Entries . . . . . . . . . . . . . . . . 125
       8.2.10. Registry RIFT/common/TIETypeType  . . . . . . . . . . 125
         8.2.10.1.  Requested Entries  . . . . . . . . . . . . . . . 126
       8.2.11. Registry RIFT/common/TieDirectionType . . . . . . . . 126
         8.2.11.1.  Requested Entries  . . . . . . . . . . . . . . . 126
       8.2.12. Registry RIFT/encoding/Community  . . . . . . . . . . 126
         8.2.12.1.  Requested Entries  . . . . . . . . . . . . . . . 126
       8.2.13. Registry RIFT/encoding/KeyValueTIEElement . . . . . . 126
         8.2.13.1.  Requested Entries  . . . . . . . . . . . . . . . 127
       8.2.14. Registry RIFT/encoding/LIEPacket  . . . . . . . . . . 127
         8.2.14.1.  Requested Entries  . . . . . . . . . . . . . . . 127
       8.2.15. Registry RIFT/encoding/LinkCapabilities . . . . . . . 128
         8.2.15.1.  Requested Entries  . . . . . . . . . . . . . . . 128
       8.2.16. Registry RIFT/encoding/LinkIDPair . . . . . . . . . . 128
         8.2.16.1.  Requested Entries  . . . . . . . . . . . . . . . 128
       8.2.17. Registry RIFT/encoding/Neighbor . . . . . . . . . . . 129
         8.2.17.1.  Requested Entries  . . . . . . . . . . . . . . . 129
       8.2.18. Registry RIFT/encoding/NodeCapabilities . . . . . . . 129
         8.2.18.1.  Requested Entries  . . . . . . . . . . . . . . . 129
       8.2.19. Registry RIFT/encoding/NodeFlags  . . . . . . . . . . 130
         8.2.19.1.  Requested Entries  . . . . . . . . . . . . . . . 130
       8.2.20. Registry RIFT/encoding/NodeNeighborsTIEElement  . . . 130
         8.2.20.1.  Requested Entries  . . . . . . . . . . . . . . . 130
       8.2.21. Registry RIFT/encoding/NodeTIEElement . . . . . . . . 130
         8.2.21.1.  Requested Entries  . . . . . . . . . . . . . . . 131
       8.2.22. Registry RIFT/encoding/PacketContent  . . . . . . . . 131
         8.2.22.1.  Requested Entries  . . . . . . . . . . . . . . . 131
       8.2.23. Registry RIFT/encoding/PacketHeader . . . . . . . . . 131
         8.2.23.1.  Requested Entries  . . . . . . . . . . . . . . . 131
       8.2.24. Registry RIFT/encoding/PrefixAttributes . . . . . . . 132
         8.2.24.1.  Requested Entries  . . . . . . . . . . . . . . . 132
       8.2.25. Registry RIFT/encoding/PrefixTIEElement . . . . . . . 132
         8.2.25.1.  Requested Entries  . . . . . . . . . . . . . . . 133
       8.2.26. Registry RIFT/encoding/ProtocolPacket . . . . . . . . 133
         8.2.26.1.  Requested Entries  . . . . . . . . . . . . . . . 133
       8.2.27. Registry RIFT/encoding/TIDEPacket . . . . . . . . . . 133
         8.2.27.1.  Requested Entries  . . . . . . . . . . . . . . . 133
       8.2.28. Registry RIFT/encoding/TIEElement . . . . . . . . . . 133
         8.2.28.1.  Requested Entries  . . . . . . . . . . . . . . . 134
       8.2.29. Registry RIFT/encoding/TIEHeader  . . . . . . . . . . 134
         8.2.29.1.  Requested Entries  . . . . . . . . . . . . . . . 135
       8.2.30. Registry RIFT/encoding/TIEHeaderWithLifeTime  . . . . 135

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         8.2.30.1.  Requested Entries  . . . . . . . . . . . . . . . 135
       8.2.31. Registry RIFT/encoding/TIEID  . . . . . . . . . . . . 135
         8.2.31.1.  Requested Entries  . . . . . . . . . . . . . . . 136
       8.2.32. Registry RIFT/encoding/TIEPacket  . . . . . . . . . . 136
         8.2.32.1.  Requested Entries  . . . . . . . . . . . . . . . 136
       8.2.33. Registry RIFT/encoding/TIREPacket . . . . . . . . . . 136
         8.2.33.1.  Requested Entries  . . . . . . . . . . . . . . . 136
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 136
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . . 137
     10.1.  Normative References . . . . . . . . . . . . . . . . . . 137
     10.2.  Informative References . . . . . . . . . . . . . . . . . 139
   Appendix A.  Sequence Number Binary Arithmetic  . . . . . . . . . 141
   Appendix B.  Information Elements Schema  . . . . . . . . . . . . 142
     B.1.  common.thrift . . . . . . . . . . . . . . . . . . . . . . 143
     B.2.  encoding.thrift . . . . . . . . . . . . . . . . . . . . . 149
   Appendix C.  Constants  . . . . . . . . . . . . . . . . . . . . . 158
     C.1.  Configurable Protocol Constants . . . . . . . . . . . . . 158
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 160

1.  Authors

   This work is a product of a list of individuals which are all to be
   considered major contributors independent of the fact whether their
   name made it to the limited boilerplate author's list or not.

         Tony Przygienda, Ed. | Alankar Sharma | Pascal Thubert
         Juniper Networks     | Comcast        | Cisco

         Bruno Rijsman        | Ilya Vershkov  | Dmitry Afanasiev
         Individual           | Mellanox       | Yandex

         Don Fedyk            | Alia Atlas     | John Drake
         Individual           | Individual     | Juniper

                           Table 1: RIFT Authors

2.  Introduction

   Clos [CLOS] and Fat-Tree [FATTREE] topologies have gained prominence
   in today's networking, primarily as result of the paradigm shift
   towards a centralized data-center based architecture that is poised
   to deliver a majority of computation and storage services in the
   future.  Today's current routing protocols were geared towards a
   network with an irregular topology and low degree of connectivity
   originally but given they were the only available options,
   consequently several attempts to apply those protocols to Clos have
   been made.  Most successfully BGP [RFC4271] [RFC7938] has been
   extended to this purpose, not as much due to its inherent suitability

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   but rather because the perceived capability to easily modify BGP and
   the immanent difficulties with link-state [DIJKSTRA] based protocols
   to optimize topology exchange and converge quickly in large scale
   densely meshed topologies.  The incumbent protocols precondition
   normally extensive configuration or provisioning during bring up and
   re-dimensioning.  This tends to be viable only for a set of
   organizations with according networking operation skills and budgets.
   For many IP fabric builders a desirable protocol would be one that
   auto-configures itself and deals with failures and mis-configurations
   with a minimum of human intervention only.  Such a solution would
   allow local IP fabric bandwidth to be consumed in a 'standard
   component' fashion, i.e. provision it much faster and operate it at
   much lower costs than today, much like compute or storage is consumed
   already.

   In looking at the problem through the lens of data center
   requirements, RIFT addresses challenges in IP fabric routing not
   through an incremental modification of either a link-state
   (distributed computation) or distance-vector (diffused computation)
   but rather a mixture of both, colloquially best described as "link-
   state towards the spine" and "distance vector towards the leaves".
   In other words, "bottom" levels are flooding their link-state
   information in the "northern" direction while each node generates
   under normal conditions a "default route" and floods it in the
   "southern" direction.  This type of protocol allows naturally for
   highly desirable aggregation.  Alas, such aggregation could blackhole
   traffic in cases of misconfiguration or while failures are being
   resolved or even cause partial network partitioning and this has to
   be addressed by some adequate mechanism.  The approach RIFT takes is
   described in Section 4.2.5 and is basically based on automatic,
   sufficient disaggregation of prefixes in case of link and node
   failures.

   For the visually oriented reader, Figure 1 presents a first level
   simplified view of the resulting information and routes on a RIFT
   fabric.  The top of the fabric is holding in its link-state database
   the nodes below it and the routes to them.  In the second row of the
   database table we indicate that partial information of other nodes in
   the same level is available as well.  The details of how this is
   achieved will be postponed for the moment.  When we look at the
   "bottom" of the fabric, the leaves, we see that the topology is
   basically empty and they only hold a load balanced default route to
   the next level under normal conditions.

   The balance of this document details a dedicated IP fabric routing
   protocol, fills in the specification details and ultimately includes
   resulting security considerations.

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              .                                  [A,B,C,D]
              .                                  [E]
              .             +-----+      +-----+
              .             |  E  |      |  F  | A/32 @ [C,D]
              .             +-+-+-+      +-+-+-+ B/32 @ [C,D]
              .               | |          | |   C/32 @ C
              .               | |    +-----+ |   D/32 @ D
              .               | |    |       |
              .               | +------+     |
              .               |      | |     |
              .       [A,B] +-+---+  | | +---+-+ [A,B]
              .       [D]   |  C  +--+ +-+  D  | [C]
              .             +-+-+-+      +-+-+-+
              .  0/0  @ [E,F] | |          | |   0/0  @ [E,F]
              .  A/32 @ A     | |    +-----+ |   A/32 @ A
              .  B/32 @ B     | |    |       |   B/32 @ B
              .               | +------+     |
              .               |      | |     |
              .             +-+---+  | | +---+-+
              .             |  A  +--+ +-+  B  |
              . 0/0 @ [C,D] +-----+      +-----+ 0/0 @ [C,D]

                  Figure 1: RIFT information distribution

2.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 8174 [RFC8174].

3.  Reference Frame

3.1.  Terminology

   This section presents the terminology used in this document.  It is
   assumed that the reader is thoroughly familiar with the terms and
   concepts used in OSPF [RFC2328] and IS-IS [ISO10589-Second-Edition],
   [ISO10589] as well as the according graph theoretical concepts of
   shortest path first (SPF) [DIJKSTRA] computation and DAGs.

   Crossbar:  Physical arrangement of ports in a switching matrix
      without implying any further scheduling or buffering disciplines.

   Clos/Fat Tree:  This document uses the terms Clos and Fat Tree
      interchangeably whereas it always refers to a folded spine-and-
      leaf topology with possibly multiple Points of Delivery (PoDs) and
      one or multiple Top of Fabric (ToF) planes.  Several modifications

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      such as leaf-2-leaf shortcuts and multiple level shortcuts are
      possible and described further in the document.

   Directed Acyclic Graph (DAG):  A finite directed graph with no
      directed cycles (loops).  If links in Clos are considered as
      either being all directed towards the top or vice versa, each of
      such two graphs is a DAG.

   Folded Spine-and-Leaf:  In case Clos fabric input and output stages
      are analogous, the fabric can be "folded" to build a "superspine"
      or top which we will call Top of Fabric (ToF) in this document.

   Level:  Clos and Fat Tree networks are topologically partially
      ordered graphs and 'level' denotes the set of nodes at the same
      height in such a network, where the bottom level (leaf) is the
      level with lowest value.  A node has links to nodes one level down
      and/or one level up.  Under some circumstances, a node may have
      links to nodes at the same level.  As footnote: Clos terminology
      uses often the concept of "stage" but due to the folded nature of
      the Fat Tree we do not use it to prevent misunderstandings.

   Superspine vs. Aggregation and Spine vs. Edge/Leaf:
      Traditional level names in 5-stages folded Clos for Level 2, 1 and
      0 respectively.  We normalize this language to talk about top-of-
      fabric (ToF), top-of-pod (ToP) and leaves.

   Zero Touch Provisioning (ZTP):  Optional RIFT mechanism which allows
      to derive node levels automatically based on minimum configuration
      (only ToF property has to be provisioned on according nodes).

   Point of Delivery (PoD):  A self-contained vertical slice or subset
      of a Clos or Fat Tree network containing normally only level 0 and
      level 1 nodes.  A node in a PoD communicates with nodes in other
      PoDs via the Top-of-Fabric.  We number PoDs to distinguish them
      and use PoD #0 to denote "undefined" PoD.

   Top of PoD (ToP):  The set of nodes that provide intra-PoD
      communication and have northbound adjacencies outside of the PoD,
      i.e. are at the "top" of the PoD.

   Top of Fabric (ToF):  The set of nodes that provide inter-PoD
      communication and have no northbound adjacencies, i.e. are at the
      "very top" of the fabric.  ToF nodes do not belong to any PoD and
      are assigned "undefined" PoD value to indicate the equivalent of
      "any" PoD.

   Spine:  Any nodes north of leaves and south of top-of-fabric nodes.
      Multiple layers of spines in a PoD are possible.

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   Leaf:  A node without southbound adjacencies.  Its level is 0 (except
      cases where it is deriving its level via ZTP and is running
      without LEAF_ONLY which will be explained in Section 4.2.7).

   Top-of-fabric Plane or Partition:  In large fabrics top-of-fabric
      switches may not have enough ports to aggregate all switches south
      of them and with that, the ToF is 'split' into multiple
      independent planes.  Introduction and Section 4.1.2 explains the
      concept in more detail.  A plane is subset of ToF nodes that see
      each other through south reflection or E-W links.

   Radix:  A radix of a switch is basically number of switching ports it
      provides.  It's sometimes called fanout as well.

   North Radix:  Ports cabled northbound to higher level nodes.

   South Radix:  Ports cabled southbound to lower level nodes.

   South/Southbound and North/Northbound (Direction):
      When describing protocol elements and procedures, we will be using
      in different situations the directionality of the compass.  I.e.,
      'south' or 'southbound' mean moving towards the bottom of the Clos
      or Fat Tree network and 'north' and 'northbound' mean moving
      towards the top of the Clos or Fat Tree network.

   Northbound Link:  A link to a node one level up or in other words,
      one level further north.

   Southbound Link:  A link to a node one level down or in other words,
      one level further south.

   East-West Link:  A link between two nodes at the same level.  East-
      West links are normally not part of Clos or "fat-tree" topologies.

   Leaf shortcuts (L2L):  East-West links at leaf level will need to be
      differentiated from East-West links at other levels.

   Routing on the host (RotH):  Modern data center architecture variant
      where servers/leaves are multi-homed and consecutively participate
      in routing.

   Northbound representation:  Subset of topology information flooded
      towards higher levels of the fabric.

   Southbound representation:  Subset of topology information sent
      towards a lower level.

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   South Reflection:  Often abbreviated just as "reflection" it defines
      a mechanism where South Node TIEs are "reflected" from the level
      south back up north to allow nodes in the same level without E-W
      links to "see" each other's node TIEs.

   TIE:  This is an acronym for a "Topology Information Element".  TIEs
      are exchanged between RIFT nodes to describe parts of a network
      such as links and address prefixes, in a fashion similar to ISIS
      LSPs or OSPF LSAs.  A TIE has always a direction and a type.  We
      will talk about North TIEs (sometimes abbreviated as N-TIEs) when
      talking about TIEs in the northbound representation and South-TIEs
      (sometimes abbreviated as S-TIEs) for the southbound equivalent.
      TIEs have different types such as node and prefix TIEs.

   Node TIE:  This stands as acronym for a "Node Topology Information
      Element" that contains all adjacencies the node discovered and
      information about node itself.  Node TIE should NOT be confused
      with a N-TIE since "node" defines the type of TIE rather than its
      direction.

   Prefix TIE:  This is an acronym for a "Prefix Topology Information
      Element" and it contains all prefixes directly attached to this
      node in case of a North TIE and in case of South TIE the necessary
      default routes the node advertises southbound.

   Key Value TIE:  A South TIE that is carrying a set of key value pairs
      [DYNAMO].  It can be used to distribute information in the
      southbound direction within the protocol.

   TIDE:  Topology Information Description Element, equivalent to CSNP
      in ISIS.

   TIRE:  Topology Information Request Element, equivalent to PSNP in
      ISIS.  It can both confirm received and request missing TIEs.

   De-aggregation/Disaggregation:  Process in which a node decides to
      advertise more specific prefixes Southwards, either positively to
      attract the corresponding traffic, or negatively to repel it.
      Disaggregation is performed to prevent black-holing and suboptimal
      routing to the more specific prefixes.

   LIE:  This is an acronym for a "Link Information Element", largely
      equivalent to HELLOs in IGPs and exchanged over all the links
      between systems running RIFT to form three way adjacencies.

   Flood Repeater (FR):  A node can designate one or more northbound
      neighbor nodes to be flood repeaters.  The flood repeaters are
      responsible for flooding northbound TIEs further north.  They are

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      similar to MPR in OSLR.  The document sometimes calls them flood
      leaders as well.

   Bandwidth Adjusted Distance (BAD):  Each RIFT node can calculate the
      amount of northbound bandwidth available towards a node compared
      to other nodes at the same level and can modify the route distance
      accordingly to allow for the lower level to adjust their load
      balancing towards spines.

   Overloaded:  Applies to a node advertising `overload` attribute as
      set.  The semantics closely follow the meaning of the same
      attribute in [ISO10589-Second-Edition].

   Interface:  A layer 3 entity over which RIFT control packets are
      exchanged.

   Three-Way Adjacency:  RIFT tries to form a unique adjacency over an
      interface and exchange local configuration and necessary ZTP
      information.  An adjacency is only advertised in node TIEs and
      used for computations after it achieved three-way state, i.e. both
      routers reflected each other in LIEs including relevant security
      information.  LIEs before three-way state is reached may carry ZTP
      related information already.

   Bi-directional Adjacency:  Bidirectional adjacency is an adjacency
      where nodes of both sides of the adjacency advertised it in the
      node TIEs with the correct levels and system IDs.  Bi-
      directionality is used to check in different algorithms whether
      the link should be included.

   Neighbor:  Once a three-way adjacency has been formed a neighborship
      relationship contains the neighbor's properties.  Multiple
      adjacencies can be formed to a remote node via parallel interfaces
      but such adjacencies are NOT sharing a neighbor structure.  Saying
      "neighbor" is thus equivalent to saying "a three-way adjacency".

   Cost:  The term signifies the weighted distance between two
      neighbors.

   Distance:  Sum of costs (bound by infinite distance) between two
      nodes.

   Shortest-Path First (SPF):  A well-known graph algorithm attributed
      to Dijkstra that establishes a tree of shortest paths from a
      source to destinations on the graph.  We use SPF acronym due to
      its familiarity as general term for the node reachability
      calculations RIFT can employ to ultimately calculate routes of
      which Dijkstra algorithm is one.

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   North SPF (N-SPF):  A reachability calculation that is progressing
      northbound, as example SPF that is using South Node TIEs only.
      Normally it progresses a single hop only and installs default
      routes.

   South SPF (S-SPF):  A reachability calculation that is progressing
      southbound, as example SPF that is using North Node TIEs only.

   Security Envelope  RIFT packets are flooded within an authenticated
      security envelope that allows to protect the integrity of
      information a node accepts.

3.2.  Topology

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    .                +--------+          +--------+          ^ N
    .                |ToF   21|          |ToF   22|          |
    .Level 2         ++-+--+-++          ++-+--+-++        <-*-> E/W
    .                 | |  | |            | |  | |           |
    .             P111/2|  |P121          | |  | |         S v
    .                 ^ ^  ^ ^            | |  | |
    .                 | |  | |            | |  | |
    .  +--------------+ |  +-----------+  | |  | +---------------+
    .  |                |    |         |  | |  |                 |
    . South +-----------------------------+ |  |                 ^
    .  |    |           |    |         |    |  |              All TIEs
    .  0/0  0/0        0/0   +-----------------------------+     |
    .  v    v           v              |    |  |           |     |
    .  |    |           +-+    +<-0/0----------+           |     |
    .  |    |             |    |       |    |              |     |
    .+-+----++ optional +-+----++     ++----+-+           ++-----++
    .|       | E/W link |       |     |       |           |       |
    .|Spin111+----------+Spin112|     |Spin121|           |Spin122|
    .+-+---+-+          ++----+-+     +-+---+-+           ++---+--+
    .  |   |             |   South      |   |              |   |
    .  |   +---0/0--->-----+ 0/0        |   +----------------+ |
    . 0/0                | |  |         |                  | | |
    .  |   +---<-0/0-----+ |  v         |   +--------------+ | |
    .  v   |               |  |         |   |                | |
    .+-+---+-+          +--+--+-+     +-+---+-+          +---+-+-+
    .|       |  (L2L)   |       |     |       |  Level 0 |       |
    .|Leaf111~~~~~~~~~~~~Leaf112|     |Leaf121|          |Leaf122|
    .+-+-----+          +-+---+-+     +--+--+-+          +-+-----+
    .  +                  +    \        /   +              +
    .  Prefix111   Prefix112    \      /   Prefix121    Prefix122
    .                          multi-homed
    .                            Prefix
    .+---------- Pod 1 ---------+     +---------- Pod 2 ---------+

              Figure 2: A three level spine-and-leaf topology

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                    .+--------+  +--------+  +--------+  +--------+
                    .|ToF   A1|  |ToF   B1|  |ToF   B2|  |ToF   A2|
                    .++-+-----+  ++-+-----+  ++-+-----+  ++-+-----+
                    . | |         | |         | |         | |
                    . | |         | |         | +---------------+
                    . | |         | |         |           | |   |
                    . | |         | +-------------------------+ |
                    . | |         |           |           | | | |
                    . | +-----------------------+         | | | |
                    . |           |           | |         | | | |
                    . |           | +---------+ | +---------+ | |
                    . |           | |           | |       |   | |
                    . | +---------------------------------+   | |
                    . | |         | |           | |           | |
                    .++-+-----+  ++-+-----+  +--+-+---+  +----+-+-+
                    .|Spine111|  |Spine112|  |Spine121|  |Spine122|
                    .+-+---+--+  ++----+--+  +-+---+--+  ++---+---+
                    .  |   |      |    |       |   |      |   |
                    .  |   +--------+  |       |   +--------+ |
                    .  |          | |  |       |          | | |
                    .  |   -------+ |  |       |   +------+ | |
                    .  |   |        |  |       |   |        | |
                    .+-+---+-+   +--+--+-+   +-+---+-+  +---+-+-+
                    .|Leaf111|   |Leaf112|   |Leaf121|  |Leaf122|
                    .+-------+   +-------+   +-------+  +-------+

                  Figure 3: Topology with multiple planes

   We will use topology in Figure 2 (called commonly a fat tree/network
   in modern IP fabric considerations [VAHDAT08] as homonym to the
   original definition of the term [FATTREE]) in all further
   considerations.  This figure depicts a generic "single plane fat-
   tree" and the concepts explained using three levels apply by
   induction to further levels and higher degrees of connectivity.
   Further, this document will deal also with designs that provide only
   sparser connectivity and "partitioned spines" as shown in Figure 3
   and explained further in Section 4.1.2.

4.  RIFT: Routing in Fat Trees

   We present here a detailed outline of a protocol optimized for
   Routing in Fat Trees (RIFT) that in most abstract terms has many
   properties of a modified link-state protocol
   [RFC2328][ISO10589-Second-Edition] when distributing information
   northbound and distance vector [RFC4271] protocol when distributing
   information southbound.  While this is an unusual combination, it
   does quite naturally exhibit the desirable properties we seek.

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4.1.  Overview

4.1.1.  Properties

   The most singular property of RIFT is that it floods flat link-state
   information northbound only so that each level obtains the full
   topology of levels south of it.  Link-State information is, with some
   exceptions, never flooded East-West or back South again.  Exceptions
   like south reflection is explained in detail in Section 4.2.5.1 and
   east-west flooding at ToF level in multi-plane fabrics is outlined in
   Section 4.1.2.  In southbound direction, the protocol operates like a
   "fully summarizing, unidirectional" path vector protocol or rather a
   distance vector with implicit split horizon.  Routing information,
   normally just the default route, propagates one hop south and is 're-
   advertised' by nodes at next lower level.  However, RIFT uses
   flooding in the southern direction as well to avoid the overhead of
   building an update per adjacency.  We omit describing the East-West
   direction for the moment.

   Those information flow constraints create not only an anisotropic
   protocol (i.e. the information is not distributed "evenly" or
   "clumped" but summarized along the N-S gradient) but also a "smooth"
   information propagation where nodes do not receive the same
   information from multiple directions at the same time.  Normally,
   accepting the same reachability on any link, without understanding
   its topological significance, forces tie-breaking on some kind of
   distance metric.  And such tie-breaking leads ultimately in hop-by-
   hop forwarding to shortest paths only.  In constrast to that, RIFT,
   under normal conditions, does not need to tie-break same reachability
   information from multiple directions.  Its computation principles
   (south forwarding direction is always preferred) leads to valley-free
   forwarding behavior.  And since valley free routing is loop-free, it
   can use all feasible paths which is another highly desirable property
   if available bandwidth should be utilized to the maximum extent
   possible.

   To account for the "northern" and the "southern" information split
   the link state database is partitioned accordingly into "north
   representation" and "south representation" TIEs.  In simplest terms
   the North TIEs contain a link state topology description of lower
   levels and and South TIEs carry simply default routes towards the
   level above.  This oversimplified view will be refined gradually in
   following sections while introducing protocol procedures and state
   machines at the same time.

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4.1.2.  Generalized Topology View

   This section will shed some light on the topologies RIFT addresses,
   including multi plane fabrics and their implications.  Readers that
   are only interested in single plane designs, i.e. all top-of-fabric
   nodes being topologically equal and initially connected to all the
   switches at the level below them, can skip the rest of Section 4.1.2
   and resulting Section 4.2.5.2 as well.

   It is quite difficult to visualize multi plane design, which are
   effectively multi-dimensional switching matrices.  To cope with that,
   we will introduce a methodology allowing us to depict the
   connectivity in two-dimensional pictures.  Further, we will leverage
   the fact that we are dealing basically with stacked crossbar fabrics
   where ports align "on top of each other" in a regular fashion.

   A word of caution to the reader; at this point it should be observed
   that the language used to describe Clos variations, especially in
   multi-plane designs, varies widely between sources.  This description
   follows the terminology introduced in Section 3.1.  It is unavoidable
   to have it present to be able to follow the rest of this section
   correctly.

4.1.2.1.  Terminology

   This section describes the terminology and acronyms used in the rest
   of the text.

   P: Denotes the number of PoDs in a topology.

   S: Denotes the number of ToF nodes in a topology.

   K: Denotes the number of ports in radix of a switch pointing north or
      south.  Further, K_LEAF denotes number of ports pointing south,
      i.e. towards leaves, and K_TOP for number of ports pointing north
      towards a higher spine level.  To simplify the visual aids,
      notations and further considerations, K will be mostly set to
      Radix/2.

   ToF Plane:  Set of ToFs that are aware of each other by means of
      south reflection.  We number planes by capital letters, e.g.
      plane A.

   N: Denote the number of independent ToF planes in a topology.

   R: Denotes a redundancy factor, i.e. number of connections a spine
      has towards a ToF plane.  In single plane design K_TOP is equal to
      R.

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   Fallen Leaf:  A fallen leaf in a plane Z is a switch that lost all
      connectivity northbound to Z.

4.1.2.2.  Clos as Crossed Crossbars

   The typical topology for which RIFT is defined is built of P number
   of PoDs and connected together by S number of ToF nodes.  A PoD node
   has K number of ports (also called Radix).  We consider half of them
   (K=Radix/2) as connecting host devices from the south, and the other
   half connecting to interleaved PoD Top-Level switches to the north.
   Ratio K can be chosen differently without loss of generality when
   port speeds differ or the fabric is oversubscribed but K=R/2 allows
   for more readable representation whereby there are as many ports
   facing north as south on any intermediate node.  We represent a node
   hence in a schematic fashion with ports "sticking out" to its north
   and south rather than by the usual real-world front faceplate designs
   of the day.

   Figure 4 provides a view of a leaf node as seen from the north, i.e.
   showing ports that connect northbound.  For lack of a better symbol,
   we have chosen to use the "o" as ASCII visualisation of a single
   port.  In this example, K_LEAF has 6 ports.  Observe that the number
   of PoDs is not related to Radix unless the ToF Nodes are constrained
   to be the same as the PoD nodes in a particular deployment.

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       Top view
        +---+
        |   |
        | o |     e.g., Radix = 12, K_LEAF = 6
        |   |
        | o |
        |   |      -------------------------
        | o ------- Physical Port (Ethernet) ----+
        |   |      -------------------------     |
        | o |                                    |
        |   |                                    |
        | o |                                    |
        |   |                                    |
        | o |                                    |
        |   |                                    |
        +---+                                    |

          ||             ||      ||      ||      ||      ||      ||
        +----+       +------------------------------------------------+
        |    |       |                                                |
        +----+       +------------------------------------------------+
          ||             ||      ||      ||      ||      ||      ||
              Side views

                      Figure 4: A Leaf Node, K_LEAF=6

   The Radix of a PoD's topnode may be different than that of the leaf
   node.  Though, more often than not, a same type of node is used for
   both, effectively forming a square (K*K).  In general case, we could
   have switches with K_TOP southern ports on nodes at the top of the
   PoD which are not necessarily the same as K_LEAF.  For instance, in
   the representations below, we pick a 6 port K_LEAF and a 8 port
   K_TOP.  In order to form a crossbar, we need K_TOP Leaf Nodes as
   illustrated in Figure 5.

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             +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+
             |   |  |   |  |   |  |   |  |   |  |   |  |   |  |   |
             | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o |
             |   |  |   |  |   |  |   |  |   |  |   |  |   |  |   |
             | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o |
             |   |  |   |  |   |  |   |  |   |  |   |  |   |  |   |
             | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o |
             |   |  |   |  |   |  |   |  |   |  |   |  |   |  |   |
             | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o |
             |   |  |   |  |   |  |   |  |   |  |   |  |   |  |   |
             | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o |
             |   |  |   |  |   |  |   |  |   |  |   |  |   |  |   |
             | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o |
             |   |  |   |  |   |  |   |  |   |  |   |  |   |  |   |
             +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+

                 Figure 5: Southern View of a PoD, K_TOP=8

   As further visualized in Figure 6 the K_TOP Leaf Nodes are fully
   interconnected with the K_LEAF PoD-top nodes, providing connectivity
   that can be represented as a crossbar when "looked at" from the
   north.  The result is that, in the absence of a failure, a packet
   entering the PoD from the north on any port can be routed to any port
   in the south of the PoD and vice versa.  And that is precisely why it
   makes sense to talk about a "switching matrix".

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                                      E<-*->W

          +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+
          |   |  |   |  |   |  |   |  |   |  |   |  |   |  |   |
        +--------------------------------------------------------+
        |   o      o      o      o      o      o      o      o   |
        +--------------------------------------------------------+
        +--------------------------------------------------------+
        |   o      o      o      o      o      o      o      o   |
        +--------------------------------------------------------+
        +--------------------------------------------------------+
        |   o      o      o      o      o      o      o      o   |
        +--------------------------------------------------------+
        +--------------------------------------------------------+
        |   o      o      o      o      o      o      o      o   |
        +--------------------------------------------------------+
        +--------------------------------------------------------+
        |   o      o      o      o      o      o      o      o   |<-+
        +--------------------------------------------------------+  |
        +--------------------------------------------------------+  |
        |   o      o      o      o      o      o      o      o   |  |
        +--------------------------------------------------------+  |
          |   |  |   |  |   |  |   |  |   |  |   |  |   |  |   |    |
          +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+    |
                     ^                                              |
                     |                                              |
                     |     ----------        ---------------------  |
                     +----- Leaf Node        PoD top Node (Spine) --+
                           ----------        ---------------------

            Figure 6: Northern View of a PoD's Spines, K_TOP=8

   Side views of this PoD is illustrated in Figure 7 and Figure 8.

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                      Connecting to Spine

      ||      ||      ||      ||      ||      ||      ||      ||
  +----------------------------------------------------------------+   N
  |                    PoD top Node seen sideways                  |   ^
  +----------------------------------------------------------------+   |
      ||      ||      ||      ||      ||      ||      ||      ||       *
    +----+  +----+  +----+  +----+  +----+  +----+  +----+  +----+     |
    |    |  |    |  |    |  |    |  |    |  |    |  |    |  |    |     v
    +----+  +----+  +----+  +----+  +----+  +----+  +----+  +----+     S
      ||      ||      ||      ||      ||      ||      ||      ||

                           Connecting to Client nodes

              Figure 7: Side View of a PoD, K_TOP=8, K_LEAF=6

                   Connecting to Spine

          ||      ||      ||      ||      ||      ||
        +----+  +----+  +----+  +----+  +----+  +----+              N
        |    |  |    |  |    |  |    |  |    |  |   PoD top Nodes   ^
        +----+  +----+  +----+  +----+  +----+  +----+              |
          ||      ||      ||      ||      ||      ||                *
      +------------------------------------------------+            |
      |              Leaf seen sideways                |            v
      +------------------------------------------------+            S
          ||      ||      ||      ||      ||      ||

                   Connecting to Client nodes

    Figure 8: Other side View of a PoD, K_TOP=8, K_LEAF=6, 90o turn in
                                 E-W Plane

   As next step, let us observe that a resulting PoD can be abstracted
   as a bigger node with a number K of K_POD= K_TOP * K_LEAF, and the
   design can recurse.

   It will be critical at this point that, before progressing further,
   the concept and the picture of "crossed crossbars" is clear.  Else,
   the following considerations might be difficult to comprehend.

   To continue, the PoDs are interconnected with each other through a
   Top-of-Fabric (ToF) node at the very top or the north edge of the
   fabric.  The resulting ToF is NOT partitioned if, and only if (IIF),
   every PoD top level node (spine) is connected to every ToF Node.

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   This topology is also referred to as a single plane configuration and
   is quite popular due to its simplicity.  In order to reach a 1:1
   connectivity ratio between the ToF and the leaves, it results that
   there are K_TOP ToF nodes, because each port of a ToP node connects
   to a different ToF node, and K_LEAF ToP nodes for the same reason.
   Consequently, it will take (P * K_LEAF) ports on a ToF node to
   connect to each of the K_LEAF ToP nodes of the P PoDs, as shown in
   Figure 9.

        [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] <-----+
         |   |   |   |   |   |   |   |        |
      [=================================]     |     -----------
         |   |   |   |   |   |   |   |        +----- Top-of-Fabric
        [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ]       +----- Node      -------+
                                              |     -----------       |
                                              |                       v
        +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ <-----+                      +-+
        | | | | | | | | | | | | | | | |                              | |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ]                            | |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ] -------------------------  | |
      [ |o| |o| |o| |o| |o| |o| |o| |o<--- Physical Port (Ethernet)  | |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ] -------------------------  | |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ]                            | |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ]                            | |
        | | | | | | | | | | | | | | | |                              | |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ]                            | |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ]      --------------        | |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ] <---  PoD top level        | |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ]       node (Spine)  ---+   | |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ]      --------------    |   | |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ]                        |   | |
        | | | | | | | | | | | | | | | |  -+           +-   +-+   v   | |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ] |           |  --| |--[ ]--| |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ] |   -----   |  --| |--[ ]--| |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ] +--- PoD ---+  --| |--[ ]--| |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ] |   -----   |  --| |--[ ]--| |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ] |           |  --| |--[ ]--| |
      [ |o| |o| |o| |o| |o| |o| |o| |o| ] |           |  --| |--[ ]--| |
        | | | | | | | | | | | | | | | |  -+           +-   +-+       | |
        +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+                              +-+

      Figure 9: Fabric Spines and TOFs in Single Plane Design, 3 PoDs

   The top view can be collapsed into a third dimension where the hidden
   depth index is representing the PoD number.  We can then show one PoD
   as a class of PoDs and hence save one dimension in our

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   representation.  The Spine Node expands in the depth and the vertical
   dimensions, whereas the PoD top level Nodes are constrained, in
   horizontal dimension.  A port in the 2-D representation represents
   effectively the class of all the ports at the same position in all
   the PoDs that are projected in its position along the depth axis.
   This is shown in Figure 10.

            / / / / / / / / / / / / / / / /
           / / / / / / / / / / / / / / / /
          / / / / / / / / / / / / / / / /
         / / / / / / / / / / / / / / / /   ]
        +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+   ]]
        | | | | | | | | | | | | | | | |  ]   ---------------------------
      [ |o| |o| |o| |o| |o| |o| |o| |o| ] <-- PoD top level node (Spine)
      [ |o| |o| |o| |o| |o| |o| |o| |o| ]    ---------------------------
      [ |o| |o| |o| |o| |o| |o| |o| |o| ]]]]
      [ |o| |o| |o| |o| |o| |o| |o| |o| ]]]     ^^
      [ |o| |o| |o| |o| |o| |o| |o| |o| ]]     //  PoDs
      [ |o| |o| |o| |o| |o| |o| |o| |o| ]     // (in depth)
        | |/| |/| |/| |/| |/| |/| |/| |/     //
        +-+ +-+ +-+/+-+/+-+ +-+ +-+ +-+     //
                 ^
                 |     ----------------
                 +----- Top-of-Fabric Node
                       ----------------

   Figure 10: Collapsed Northern View of a Fabric for Any Number of PoDs

   As simple as single plane deployment is it introduces a limit due to
   the bound on the available radix of the ToF nodes that has to be at
   least P * K_LEAF.  Nevertheless, we will see that a distinct
   advantage of a connected or non-partitioned Top-of-Fabric is that all
   failures can be resolved by simple, non-transitive, positive
   disaggregation (i.e. nodes advertising more specific prefixes with
   the default to the level below them that is however not propagated
   further down the fabric) as described in Section 4.2.5.1 . In other
   words; non-partitioned ToF nodes can always reach nodes below or
   withdraw the routes from PoDs they cannot reach unambiguously.  And
   with this, positive disaggregation can heal all failures and still
   allow all the ToF nodes to see each other via south reflection.
   Disaggregation will be explained in further detail in Section 4.2.5.

   In order to scale beyond the "single plane limit", the Top-of-Fabric
   can be partitioned by a N number of identically wired planes where N
   is an integer divider of K_LEAF.  The 1:1 ratio and the desired
   symmetry are still served, this time with (K_TOP * N) ToF nodes, each
   of (P * K_LEAF / N) ports.  N=1 represents a non-partitioned Spine
   and N=K_LEAF is a maximally partitioned Spine.  Further, if R is any

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   integer divisor of K_LEAF, then N=K_LEAF/R is a feasible number of
   planes and R a redundancy factor.  If proves convenient for
   deployments to use a radix for the leaf nodes that is a power of 2 so
   they can pick a number of planes that is a lower power of 2.  The
   example in Figure 11 splits the Spine in 2 planes with a redundancy
   factor R=3, meaning that there are 3 non-intersecting paths between
   any leaf node and any ToF node.  A ToF node must have, in this case,
   at least 3*P ports, and be directly connected to 3 of the 6 PoD-ToP
   nodes (spines) in each PoD.

        +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+
      +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+
      | | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o | |
      +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+
      +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+
      | | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o | |
      +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+
      +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+
      | | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o | |
      +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+
        +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+

      Plane 1
     ----------- . ------------ . ------------ . ------------ . --------
      Plane 2

        +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+
      +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+
      | | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o | |
      +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+
      +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+
      | | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o | |
      +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+
      +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+
      | | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o | |
      +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+
        +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+
                 ^
                 |
                 |      ----------------
                 +----- Top-of-Fabric node
                        "across" depth
                        ----------------

    Figure 11: Northern View of a Multi-Plane ToF Level, K_LEAF=6, N=2

   At the extreme end of the spectrum it is even possible to fully
   partition the spine with N = K_LEAF and R=1, while maintaining

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   connectivity between each leaf node and each Top-of-Fabric node.  In
   that case the ToF node connects to a single Port per PoD, so it
   appears as a single port in the projected view represented in
   Figure 12.  The number of ports required on the Spine Node is more or
   equal to P, the number of PoDs.

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       Plane 1
         +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+  -+
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+ |
       | | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o | | |
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+ |
         +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+   |
      ----------- . ------------------- . ------------ . -------- |
         +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+   |
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+ |
       | | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o | | |
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+ |
         +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+   |
      ----------- . ------------ . ---- . ------------ . -------- |
         +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+   |
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+ |
       | | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o | | |
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+ |
         +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+   |
      ----------- . ------------ . ------------------- . -------- +<-+
         +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+   |  |
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+ |  |
       | | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o | | |  |
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+ |  |
         +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+   |  |
      ----------- . ------------ . ------------ . ---- . -------- |  |
         +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+   |  |
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+ |  |
       | | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o | | |  |
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+ |  |
         +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+   |  |
      ----------- . ------------ . ------------ . --------------- |  |
         +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+   |  |
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+ |  |
       | | o |  | o |  | o |  | o |  | o |  | o |  | o |  | o | | |  |
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |--|   |-+ |  |
         +---+  +---+  +---+  +---+  +---+  +---+  +---+  +---+  -+  |
       Plane 6    ^                                                  |
                  |                                                  |
                  |     ----------------           -------------     |
                  +-----  ToF       Node           Class of PoDs  ---+
                        ----------------           -------------

    Figure 12: Northern View of a Maximally Partitioned ToF Level, R=1

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4.1.3.  Fallen Leaf Problem

   As mentioned earlier, RIFT exhibits an anisotropic behaviour tailored
   for fabrics with a North / South orientation and a high level of
   interleaving paths.  A non-partitioned fabric makes a total loss of
   connectivity between a Top-of-Fabric node at the north and a leaf
   node at the south a very rare but yet possible occasion that is fully
   healed by positive disaggregation as described in Section 4.2.5.1.
   In large fabrics or fabrics built from switches with low radix, the
   ToF ends often being partitioned in planes which makes the occurrence
   of having a given leaf being only reachable from a subset of the ToF
   nodes more likely to happen.  This makes some further considerations
   necessary.

   We define a "Fallen Leaf" as a leaf that can be reached by only a
   subset, but not all, of Top-of-Fabric nodes due to missing
   connectivity.  If R is the redundancy factor, then it takes at least
   R breakages to reach a "Fallen Leaf" situation.

   In a maximally partitioned fabric, the redundancy factor is R= 1, so
   any breakage in the fabric may cause one or more fallen leaves.
   However, not all cases require disaggregation.  The following cases
   do not require particular action in such scenario:

      If a southern link on a leaf node goes down, then connectivity to
      any node attached to the leaf is lost.  There is no need to
      disaggregate since the connectivity is lost from all spine nodes
      to the leaf nodes in the same fashion.

      If a southern link on a leaf node goes down, then connectivity
      through that leaf is lost for all nodes.  There is no need to
      disaggregate since the connectivity to this leaf is lost for all
      spine nodes in a same fashion.

      If a ToF Node goes down, then northern traffic towards it is
      routed via alternate ToF nodes in the same plane and there is no
      need to disaggregate routes.

   In a general manner, the mechanism of non-transitive positive
   disaggregation is sufficient when the disaggregating ToF nodes
   collectively connect to all the ToP nodes in the broken plane.  This
   happens in the following case:

      If the breakage is the last northern link from a ToP node to a ToF
      node going down, then the fallen leaf problem affects only The ToF
      node, and the connectivity to all the nodes in the PoD is lost
      from that ToF node.  This can be observed by other ToF nodes

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      within the plane where the ToP node is located and positively
      disaggregated within that plane.

   On the other hand, there is a need to disaggregate the routes to
   Fallen Leaves in a transitive fashion, all the way to the other
   leaves in the following cases:

   o  If the breakage is the last northern link from a leaf node within
      a plane (there is only one such link in a maximally partitioned
      fabric) that goes down, then connectivity to all unicast prefixes
      attached to the leaf node is lost within the plane where the link
      is located.  Southern Reflection by a leaf node, e.g., between ToP
      nodes, if the PoD has only 2 levels, happens in between planes,
      allowing the ToP nodes to detect the problem within the PoD where
      it occurs and positively disaggregate.  The breakage can be
      observed by the ToF nodes in the same plane through the North
      flooding of TIEs from the ToP nodes.  The ToF nodes however need
      to be aware of all the affected prefixes for the negative,
      possibly transitive disaggregation to be fully effective (i.e.  a
      node advertising in control plane that it cannot reach a certain
      more specific prefix than default whereas such disaggregation must
      in extreme condition propagate further down southbound).  The
      problem can also be observed by the ToF nodes in the other planes
      through the flooding of North TIEs from the affected leaf nodes,
      together with non-node North TIEs which indicate the affected
      prefixes.  To be effective in that case, the positive
      disaggregation must reach down to the nodes that make the plane
      selection, which are typically the ingress leaf nodes.  The
      information is not useful for routing in the intermediate levels.

   o  If the breakage is a ToP node in a maximally partitioned fabric -
      in which case it is the only ToP node serving the plane in that
      PoD - goes down, then the connectivity to all the nodes in the PoD
      is lost within the plane where the ToP node is located.
      Consequently, all leaves of the PoD fall in this plane.  Since the
      Southern Reflection between the ToF nodes happens only within a
      plane, ToF nodes in other planes cannot discover fallen leaves in
      a different plane.  They also cannot determine beyond their local
      plane whether a leaf node that was initially reachable has become
      unreachable.  As the breakage can be observed by the ToF nodes in
      the plane where the breakage happened, the ToF nodes in the plane
      need to be aware of all the affected prefixes for the negative
      disaggregation to be fully effective.  The problem can also be
      observed by the ToF nodes in the other planes through the flooding
      of North TIEs from the affected leaf nodes, if there are only 3
      levels and the ToP nodes are directly connected to the leaf nodes,
      and then again it can only be effective it is propagated
      transitively to the leaf, and useless above that level.

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   For the sake of easy comprehension let us roll the abstractions back
   into a simple example and observe that in Figure 3 the loss of link
   Spine 122 to Leaf 122 will make Leaf 122 a fallen leaf for Top-of-
   Fabric plane B.  Worse, if the cabling was never present in first
   place, plane B will not even be able to know that such a fallen leaf
   exists.  Hence partitioning without further treatment results in two
   grave problems:

   o  Leaf 111 trying to route to Leaf 122 MUST choose Spine 111 in
      plane A as its next hop since plane B will inevitably blackhole
      the packet when forwarding using default routes or do excessive
      bow tying.  This information must be in its routing table.

   o  Any kind of "flooding" or distance vector trying to deal with the
      problem by distributing host routes will be able to converge only
      using paths through leaves.  The flooding of information on Leaf
      122 would have to go up to Top-of-Fabric A and then "loopback"
      over other leaves to ToF B leading in extreme cases to traffic for
      Leaf 122 when presented to plane B taking an "inverted fabric"
      path where leaves start to serve as TOFs, at least for the
      duration of a protocol's convergence.

4.1.4.  Discovering Fallen Leaves

   As illustrated later, and without further proof, the way to deal with
   fallen leaves in multi-plane designs, when aggregation is used, is
   that RIFT requires all the ToF nodes to share the same north topology
   database.  This happens naturally in single plane design by the means
   of northbound flooding and south reflection but needs additional
   considerations in multi-plane fabrics.  To satisfy this RIFT, in
   multi-plane designs, relies at the ToF level on ring interconnection
   of switches in multiple planes.  Other solutions are possible but
   they either need more cabling or end up having much longer flooding
   paths and/or single points of failure.

   In detail, by reserving two ports on each Top-of-Fabric node it is
   possible to connect them together by interplane bi-directional rings
   as illustrated in Figure 13.  The rings will be used to exchange full
   north topology information between planes.  All ToFs having same
   north topology allows by the means of transitive, negative
   disaggregation described in Section 4.2.5.2 to efficiently fix any
   possible fallen leaf scenario.  Somewhat as a side-effect, the
   exchange of information fulfills the ask to present full view of the
   fabric topology at the Top-of-Fabric level, without the need to
   collate it from multiple points by additional complexity of
   technologies like [RFC7752].

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           +---+  +---+  +---+  +---+  +---+  +---+  +--------+
           |   |  |   |  |   |  |   |  |   |  |   |  |        |
           |      |      |      |      |      |      |        |
         +-o-+  +-o-+  +-o-+  +-o-+  +-o-+  +-o-+  +-o-+      |
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |-+    |
       | | o |  | o |  | o |  | o |  | o |  | o |  | o | |    | Plane A
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |-+    |
         +-o-+  +-o-+  +-o-+  +-o-+  +-o-+  +-o-+  +-o-+      |
          |      |      |      |      |      |      |         |
         +-o-+  +-o-+  +-o-+  +-o-+  +-o-+  +-o-+  +-o-+      |
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |-+    |
       | | o |  | o |  | o |  | o |  | o |  | o |  | o | |    | Plane B
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |-+    |
         +-o-+  +-o-+  +-o-+  +-o-+  +-o-+  +-o-+  +-o-+      |
           |      |      |      |      |      |      |        |
                               ...                            |
           |      |      |      |      |      |      |        |
         +-o-+  +-o-+  +-o-+  +-o-+  +-o-+  +-o-+  +-o-+      |
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |-+    |
       | | o |  | o |  | o |  | o |  | o |  | o |  | o | |    | Plane X
       +-|   |--|   |--|   |--|   |--|   |--|   |--|   |-+    |
         +-o-+  +-o-+  +-o-+  +-o-+  +-o-+  +-o-+  +-o-+      |
           |      |      |      |      |      |      |        |
           |   |  |   |  |   |  |   |  |   |  |   |  |        |
           +---+  +---+  +---+  +---+  +---+  +---+  +--------+
    Rings    1      2      3      4      5      6      7

     Figure 13: Connecting Top-of-Fabric Nodes Across Planes by Rings

4.1.5.  Addressing the Fallen Leaves Problem

   One consequence of the "Fallen Leaf" problem is that some prefixes
   attached to the fallen leaf become unreachable from some of the ToF
   nodes.  RIFT proposes two methods to address this issue, the positive
   and the negative disaggregation.  Both methods flood South TIEs to
   advertise the impacted prefix(es).

   When used for the operation of disaggregation, a positive South TIE,
   as usual, indicates reachability to a prefix of given length and all
   addresses subsumed by it.  In contrast, a negative route
   advertisement indicates that the origin cannot route to the
   advertised prefix.

   The positive disaggregation is originated by a router that can still
   reach the advertised prefix, and the operation is not transitive.  In
   other words, the receiver does not generate its own flooding south as
   a consequence of receiving positive disaggregation advertisements

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   from a higher level node.  The effect of a positive disaggregation is
   that the traffic to the impacted prefix will follow the longest match
   and will be limited to the northbound routers that advertised the
   more specific route.

   In contrast, the negative disaggregation can be transitive, and is
   propagated south when all the possible routes have been advertised as
   negative exceptions.  A negative route advertisement is only
   actionable when the negative prefix is aggregated by a positive route
   advertisement for a shorter prefix.  In such case, the negative
   advertisement "punches out a hole" in the positive route in the
   routing table, making the positive prefix reachable through the
   originator with the special consideration of the negative prefix
   removing certain next hop neighbors.

   When the ToF is not partitioned, the collective southern flooding of
   the positive disaggregation by the ToF nodes that can still reach the
   impacted prefix is in general enough to cover all the switches at the
   next level south, typically the ToP nodes.  If all those switches are
   aware of the disaggregation, they collectively create a ceiling that
   intercepts all the traffic north and forwards it to the ToF nodes
   that advertised the more specific route.  In that case, the positive
   disaggregation alone is sufficient to solve the fallen leaf problem.

   On the other hand, when the fabric is partitioned in planes, the
   positive disaggregation from ToF nodes in different planes do not
   reach the ToP switches in the affected plane and cannot solve the
   fallen leaves problem.  In other words, a breakage in a plane can
   only be solved in that plane.  Also, the selection of the plane for a
   packet typically occurs at the leaf level and the disaggregation must
   be transitive and reach all the leaves.  In that case, the negative
   disaggregation is necessary.  The details on the RIFT approach to
   deal with fallen leaves in an optimal way are specified in
   Section 4.2.5.2.

4.2.  Specification

   This section specifies the protocol in a normative fashion by either
   prescriptive procedures or behavior defined by Finite State Machines
   (FSM).

   Some FSM figures are provided as [DOT] description due to limitations
   of ASCII art.

   "On Entry" actions on FSM state are performed every time and right
   before the according state is entered, i.e. after any transitions
   from previous state.

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   "On Exit" actions are performed every time and immediately when a
   state is exited, i.e. before any transitions towards target state are
   performed.

   Any attempt to transition from a state towards another on reception
   of an event where no action is specified MUST be considered an
   unrecoverable error.

   The FSMs and procedures are normative in the sense that an
   implementation MUST implement them either literally or an
   implementation MUST exhibit externally observable behavior that is
   identical to the execution of the specified FSMs.

   Where a FSM representation is inconvenient, i.e. the amount of
   procedures and kept state exceeds the amount of transitions, we defer
   to a more procedural description on data structures.

4.2.1.  Transport

   All packet formats are defined in Thrift [thrift] models in
   Appendix B.

   The serialized model is carried in an envelope within a UDP frame
   that provides security and allows validation/modification of several
   important fields without de-serialization for performance and
   security reasons.

4.2.2.  Link (Neighbor) Discovery (LIE Exchange)

   RIFT LIE exchange auto-discovers neighbors, negotiates ZTP parameters
   and discovers miscablings.  It uses a three-way handshake mechanism
   which is a cleaned up version of [RFC5303].  Observe that for easier
   comprehension the terminology of one/two and three-way states does
   NOT align with OSPF or ISIS FSMs albeit they use roughly same
   mechanisms.  The formation progresses under normal conditions from
   one-way to two-way and then three-way state at which point it is
   ready to exchange TIEs per Section 4.2.3.

   LIE exchange happens over well-known administratively locally scoped
   and configured or otherwise well-known IPv4 multicast address
   [RFC2365] and/or link-local multicast scope [RFC4291] for IPv6
   [RFC8200] using a configured or otherwise a well-known destination
   UDP port defined in Appendix C.1.  LIEs SHOULD be sent with an IPv4
   Time to Live (TTL) / IPv6 Hop Limit (HL) of 1 to prevent RIFT
   information reaching beyond a single L3 next-hop in the topology.
   LIEs SHOULD be sent with network control precedence.

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   Originating port of the LIE has no further significance other than
   identifying the origination point.  LIEs are exchanged over all links
   running RIFT.

   An implementation MAY listen and send LIEs on IPv4 and/or IPv6
   multicast addresses.  A node MUST NOT originate LIEs on an address
   family if it does not process received LIEs on that family.  LIEs on
   same link are considered part of the same negotiation independent on
   the address family they arrive on.  Observe further that the LIE
   source address may not identify the peer uniquely in unnumbered or
   link-local address cases so the response transmission MUST occur over
   the same interface the LIEs have been received on.  A node MAY use
   any of the adjacency's source addresses it saw in LIEs on the
   specific interface during adjacency formation to send TIEs.  That
   implies that an implementation MUST be ready to accept TIEs on all
   addresses it used as source of LIE frames.

   A three-way adjacency over any address family implies support for
   IPv4 forwarding if the `v4_forwarding_capable` flag is set to true
   and a node can use [RFC5549] type of forwarding in such a situation.
   It is expected that the whole fabric supports the same type of
   forwarding of address families on all the links.  Operation of a
   fabric where only some of the links are supporting forwarding on an
   address family and others do not is outside the scope of this
   specification.

   The protocol does NOT support selective disabling of address
   families, disabling v4 forwarding capability or any local address
   changes in three-way state, i.e. if a link has entered three-way IPv4
   and/or IPv6 with a neighbor on an adjacency and it wants to stop
   supporting one of the families or change any of its local addresses
   or stop v4 forwarding, it has to tear down and rebuild the adjacency.
   It also has to remove any information it stored about the adjacency
   such as LIE source addresses seen.

   Unless ZTP as described in Section 4.2.7 is used, each node is
   provisioned with the level at which it is operating.  It MAY be also
   provisioned with its PoD.  If any of those values is undefined, then
   accordingly a default level and/or an "undefined" PoD are assumed.
   This means that leaves do not need to be configured at all if initial
   configuration values are all left at "undefined" value.  Nodes above
   ToP MUST remain at "any" PoD value which has the same value as
   "undefined" PoD.  This information is propagated in the LIEs
   exchanged.

   Further definitions of leaf flags are found in Section 4.2.7 given
   they have implications in terms of level and adjacency forming here.

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   A node tries to form a three-way adjacency if and only if

   1.  the node is in the same PoD or either the node or the neighbor
       advertises "undefined/any" PoD membership (PoD# = 0) AND

   2.  the neighboring node is running the same MAJOR schema version AND

   3.  the neighbor is not member of some PoD while the node has a
       northbound adjacency already joining another PoD AND

   4.  the neighboring node uses a valid System ID AND

   5.  the neighboring node uses a different System ID than the node
       itself

   6.  the advertised MTUs match on both sides AND

   7.  both nodes advertise defined level values AND

   8.  [

          i) the node is at level 0 and has no three way adjacencies
          already to nodes at Highest Adjacency Three-Way level (HAT as
          defined later in Section 4.2.7.1) with level different than
          the adjacent node OR

          ii) the node is not at level 0 and the neighboring node is at
          level 0 OR

          iii) both nodes are at level 0 AND both indicate support for
          Section 4.3.8 OR

          iv) neither node is at level 0 and the neighboring node is at
          most one level away

       ].

   The rules checking PoD numbering MAY be optionally disregarded by a
   node if PoD detection is undesirable or has to be ignored.  This will
   not affect the correctness of the protocol except preventing
   detection of certain miscabling cases.

   A node configured with "undefined" PoD membership MUST, after
   building first northbound three way adjacencies to a node being in a
   defined PoD, advertise that PoD as part of its LIEs.  In case that
   adjacency is lost, from all available northbound three way
   adjacencies the node with the highest System ID and defined PoD is
   chosen.  That way the northmost defined PoD value (normally the ToP

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   nodes) can diffuse southbound towards the leaves "forcing" the PoD
   value on any node with "undefined" PoD.

   LIEs arriving with IPv4 Time to Live (TTL) / IPv6 Hop Limit (HL)
   larger than 1 MUST be ignored.

   A node SHOULD NOT send out LIEs without defined level in the header
   but in certain scenarios it may be beneficial for trouble-shooting
   purposes.

4.2.2.1.  LIE FSM

   This section specifies the precise, normative LIE FSM and can be
   omitted unless the reader is pursuing an implementation of the
   protocol.

   Initial state is `OneWay`.

   Event `MultipleNeighbors` occurs normally when more than two nodes
   see each other on the same link or a remote node is quickly
   reconfigured or rebooted without regressing to `OneWay` first.  Each
   occurrence of the event SHOULD generate a clear, according
   notification to help operational deployments.

   The machine sends LIEs on several transitions to accelerate adjacency
   bring-up without waiting for the timer tic.

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               Enter
                 |
                 V
           +-----------+
           | OneWay    |<----+
           |           |     | HALChanged [StoreHAL]
           | Entry:    |     | HALSChanged [StoreHALS]
           | [CleanUp] |     | HATChanged [StoreHAT]
           |           |     | HoldTimerExpired [-]
           |           |     | InstanceNameMismatch [-]
           |           |     | LevelChanged [UpdateLevel, PUSH SendLie]
           |           |     | LieReceived [ProcessLIE]
           |           |     | MTUMismatch [-]
           |           |     | NeighborAddressAdded [-]
           |           |     | NeighborChangedAddress [-]
           |           |     | NeighborChangedLevel [-]
           |           |     | NeighborChangedMinorFields [-]
           |           |     | NeighborDroppedReflection [-]
           |           |     | PODMismatch [-]
           |           |     | SendLIE [SendLIE]
           |           |     | TimerTick [PUSH SendLIE]
           |           |     | UnacceptableHeader
           |           |     | UpdateZTPOffer [SendOfferToZTPFSM]
           |           |-----+
           |           |
           |           |<--------------------- (ThreeWay)
           |           |--------------------->
           |           | ValidReflection [-]
           |           |
           |           |---------------------> (Multiple
           |           | MultipleNeighbors      Neighbors
           +-----------+  [StartMulNeighTimer]  Wait)
               ^    |
               |    |
               |    | NewNeighbor [PUSH SendLIE]
               |    V
              (TwoWay)

                                  LIE FSM

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              (OneWay)
               |    ^
               |    | HoldTimeExpired [-]
               |    | InstanceNameMismatch [-]
               |    | LevelChanged [StoreLevel]
               |    | MTUMismatch [-]
               |    | NeighborChangedAddress [-]
               |    | NeighborChangedLevel [-]
               |    | PODMismatch [-]
               |    | UnacceptableHeader [-]
               V    |
           +-----------+
           | TwoWay    |<----+
           |           |     | HALChanged [StoreHAL]
           |           |     | HALSChanged [StoreHALS]
           |           |     | HATChanged [StoreHAT]
           |           |     | LevelChanged [StoreLevel]
           |           |     | LIERcvd [ProcessLIE]
           |           |     | SendLIE [SendLIE]
           |           |     | TimerTick [PUSH SendLIE,
           |           |     |            IF HoldTimer expired
           |           |     |            PUSH HoldTimerExpired]
           |           |     | UpdateZTPOffer [SendOfferToZTPFSM]
           |           |-----+
           |           |
           |           |<----------------------
           |           |----------------------> (Multiple
           |           | NewNeighbor             Neighbors
           |           |   [StartMulNeighTimer]  Wait)
           |           | MultipleNeighbors
           +-----------+  [StartMulNeighTimer]
               ^    |
               |    | ValidReflection [-]
               |    V
             (ThreeWay)

                            LIE FSM (continued)

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                     (TwoWay)    (OneWay)
                      ^    |        ^
                      |    |        | HoldTimerExpired [-]
                      |    |        | InstanceNameMismatch [-]
                      |    |        | LevelChanged [UpdateLevel]
                      |    |        | MTUMismatch [-]
                      |    |        | NeighborChangedAddress [-]
                      |    |        | NeighborChangedLevel [-]
  NeighborDropped-    |    |        | PODMismatch [-]
       Reflection [-] |    |        | UnacceptableHeader [-]
                      |    V        |
                  +-----------+     |
                  | ThreeWay  |-----+
                  |           |
                  |           |<----+
                  |           |     | HALChanged [StoreHAL]
                  |           |     | HALSChanged [StoreHALS]
                  |           |     | HATChanged [StoreHAT]
                  |           |     | LieReceived [ProcessLIE]
                  |           |     | SendLIE [SendLIE]
                  |           |     | TimerTick [PUSH SendLie,
                  |           |     |            IF HoldTimer expired
                  |           |     |            PUSH HoldTimerExpired]
                  |           |     | UpdateZTPOffer [SendOfferToZTPFSM]
                  |           |     | ValidReflection [-]
                  |           |-----+
                  |           |----------------------> (Multiple
                  |           | MultipleNeighbors       Neighbors
                  +-----------+   [StartMulNeighTimer]  Wait)

                            LIE FSM (continued)

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          (TwoWay) (ThreeWay)
               |     |
               V     V
           +------------+
           | Multiple   |<----+
           | Neighbors  |     | HALChanged [StoreHAL]
           | Wait       |     | HALSChanged [StoreHALS]
           |            |     | HATChanged [StoreHAT]
           |            |     | MultipleNeighbors
           |            |     |   [StartMultipleNeighborsTimer]
           |            |     | TimerTick [IF MulNeighTimer expired
           |            |     |            PUSH MultipleNeighborsDone]
           |            |     | UpdateZTPOffer [SendOfferToZTP]
           |            |-----+
           |            |
           |            |<---------------------------
           |            |---------------------------> (OneWay)
           |            | LevelChanged [StoreLevel]
           +------------+ MultipleNeighborsDone [-]

                            LIE FSM (continued)

   Events

   o  TimerTick: one second timer tic

   o  LevelChanged: node's level has been changed by ZTP or
      configuration

   o  HALChanged: best HAL computed by ZTP has changed

   o  HATChanged: HAT computed by ZTP has changed

   o  HALSChanged: set of HAL offering systems computed by ZTP has
      changed

   o  LieRcvd: received LIE

   o  NewNeighbor: new neighbor parsed

   o  ValidReflection: received own reflection from neighbor

   o  NeighborDroppedReflection: lost previous own reflection from
      neighbor

   o  NeighborChangedLevel: neighbor changed advertised level

   o  NeighborChangedAddress: neighbor changed IP address

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   o  UnacceptableHeader: unacceptable header seen

   o  MTUMismatch: MTU mismatched

   o  InstanceNameMismatch: Instance mismatched

   o  PODMismatch: Unacceptable PoD seen

   o  HoldtimeExpired: adjacency hold down expired

   o  MultipleNeighbors: more than one neighbor seen on interface

   o  MultipleNeighborsDone: cooldown for multiple neighbors expired

   o  SendLie: send a LIE out

   o  UpdateZTPOffer: update this node's ZTP offer

   Actions

      on MultipleNeighbors in OneWay finishes in MultipleNeighborsWait:
      start multiple neighbors timer as 4 * DEFAULT_LIE_HOLDTIME

      on NeighborDroppedReflection in ThreeWay finishes in TwoWay: no
      action

      on NeighborDroppedReflection in OneWay finishes in OneWay: no
      action

      on PODMismatch in TwoWay finishes in OneWay: no action

      on NewNeighbor in TwoWay finishes in MultipleNeighborsWait: PUSH
      SendLie event

      on LieRcvd in OneWay finishes in OneWay: PROCESS_LIE

      on UnacceptableHeader in ThreeWay finishes in OneWay: no action

      on UpdateZTPOffer in TwoWay finishes in TwoWay: send offer to ZTP
      FSM

      on NeighborChangedAddress in ThreeWay finishes in OneWay: no
      action

      on HALChanged in MultipleNeighborsWait finishes in
      MultipleNeighborsWait: store new HAL

      on NeighborChangedAddress in TwoWay finishes in OneWay: no action

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      on MultipleNeighbors in TwoWay finishes in MultipleNeighborsWait:
      start multiple neighbors timer as 4 * DEFAULT_LIE_HOLDTIME

      on LevelChanged in ThreeWay finishes in OneWay: update level with
      event value

      on LieRcvd in ThreeWay finishes in ThreeWay: PROCESS_LIE

      on ValidReflection in OneWay finishes in ThreeWay: no action

      on NeighborChangedLevel in TwoWay finishes in OneWay: no action

      on MultipleNeighbors in ThreeWay finishes in
      MultipleNeighborsWait: start multiple neighbors timer as 4 *
      DEFAULT_LIE_HOLDTIME

      on InstanceNameMismatch in OneWay finishes in OneWay: no action

      on NewNeighbor in OneWay finishes in TwoWay: PUSH SendLie event

      on UpdateZTPOffer in OneWay finishes in OneWay: send offer to ZTP
      FSM

      on UpdateZTPOffer in ThreeWay finishes in ThreeWay: send offer to
      ZTP FSM

      on MTUMismatch in ThreeWay finishes in OneWay: no action

      on TimerTick in OneWay finishes in OneWay: PUSH SendLie event

      on SendLie in TwoWay finishes in TwoWay: SEND_LIE

      on ValidReflection in ThreeWay finishes in ThreeWay: no action

      on InstanceNameMismatch in TwoWay finishes in OneWay: no action

      on HoldtimeExpired in OneWay finishes in OneWay: no action

      on TimerTick in ThreeWay finishes in ThreeWay: PUSH SendLie event,
      if holdtime expired PUSH HoldtimeExpired event

      on HALChanged in TwoWay finishes in TwoWay: store new HAL

      on HoldtimeExpired in ThreeWay finishes in OneWay: no action

      on HALSChanged in TwoWay finishes in TwoWay: store HALS

      on HALSChanged in ThreeWay finishes in ThreeWay: store HALS

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      on ValidReflection in TwoWay finishes in ThreeWay: no action

      on MultipleNeighborsDone in MultipleNeighborsWait finishes in
      OneWay: no action

      on NeighborAddressAdded in OneWay finishes in OneWay: no action

      on TimerTick in MultipleNeighborsWait finishes in
      MultipleNeighborsWait: decrement MultipleNeighbors timer, if
      expired PUSH MultipleNeighborsDone

      on MTUMismatch in OneWay finishes in OneWay: no action

      on MultipleNeighbors in MultipleNeighborsWait finishes in
      MultipleNeighborsWait: start multiple neighbors timer as 4 *
      DEFAULT_LIE_HOLDTIME

      on LieRcvd in TwoWay finishes in TwoWay: PROCESS_LIE

      on HATChanged in MultipleNeighborsWait finishes in
      MultipleNeighborsWait: store HAT

      on HoldtimeExpired in TwoWay finishes in OneWay: no action

      on NeighborChangedLevel in ThreeWay finishes in OneWay: no action

      on LevelChanged in OneWay finishes in OneWay: update level with
      event value, PUSH SendLie event

      on SendLie in OneWay finishes in OneWay: SEND_LIE

      on HATChanged in OneWay finishes in OneWay: store HAT

      on LevelChanged in TwoWay finishes in TwoWay: update level with
      event value

      on HATChanged in TwoWay finishes in TwoWay: store HAT

      on PODMismatch in ThreeWay finishes in OneWay: no action

      on LevelChanged in MultipleNeighborsWait finishes in OneWay:
      update level with event value

      on UnacceptableHeader in TwoWay finishes in OneWay: no action

      on NeighborChangedLevel in OneWay finishes in OneWay: no action

      on InstanceNameMismatch in ThreeWay finishes in OneWay: no action

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      on HATChanged in ThreeWay finishes in ThreeWay: store HAT

      on HALChanged in OneWay finishes in OneWay: store new HAL

      on UnacceptableHeader in OneWay finishes in OneWay: no action

      on HALChanged in ThreeWay finishes in ThreeWay: store new HAL

      on UpdateZTPOffer in MultipleNeighborsWait finishes in
      MultipleNeighborsWait: send offer to ZTP FSM

      on NeighborChangedMinorFields in OneWay finishes in OneWay: no
      action

      on NeighborChangedAddress in OneWay finishes in OneWay: no action

      on MTUMismatch in TwoWay finishes in OneWay: no action

      on PODMismatch in OneWay finishes in OneWay: no action

      on SendLie in ThreeWay finishes in ThreeWay: SEND_LIE

      on TimerTick in TwoWay finishes in TwoWay: PUSH SendLie event, if
      holdtime expired PUSH HoldtimeExpired event

      on HALSChanged in OneWay finishes in OneWay: store HALS

      on HALSChanged in MultipleNeighborsWait finishes in
      MultipleNeighborsWait: store HALS

      on Entry into OneWay: CLEANUP

   Following words are used for well known procedures:

   1.  PUSH Event: pushes an event to be executed by the FSM upon exit
       of this action

   2.  CLEANUP: neighbor MUST be reset to unknown

   3.  SEND_LIE: create a new LIE packet

       1.  reflecting the neighbor if known and valid and

       2.  setting the necessary `not_a_ztp_offer` variable if level was
           derived from last known neighbor on this interface and

       3.  setting `you_are_not_flood_repeater` to computed value

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   4.  PROCESS_LIE:

       1.  if lie has wrong major version OR our own system ID or
           invalid system ID then CLEANUP else

       2.  if lie has non matching MTUs then CLEANUP, PUSH
           UpdateZTPOffer, PUSH MTUMismatch else

       3.  if PoD rules do not allow adjacency forming then CLEANUP,
           PUSH PODMismatch, PUSH MTUMismatch else

       4.  if lie has undefined level OR my level is undefined OR this
           node is leaf and remote level lower than HAT OR (lie's level
           is not leaf AND its difference is more than one from my
           level) then CLEANUP, PUSH UpdateZTPOffer, PUSH
           UnacceptableHeader else

       5.  PUSH UpdateZTPOffer, construct temporary new neighbor
           structure with values from lie, if no current neighbor exists
           then set neighbor to new neighbor, PUSH NewNeighbor event,
           CHECK_THREE_WAY else

           1.  if current neighbor system ID differs from lie's system
               ID then PUSH MultipleNeighbors else

           2.  if current neighbor stored level differs from lie's level
               then PUSH NeighborChangedLevel else

           3.  if current neighbor stored IPv4/v6 address differs from
               lie's address then PUSH NeighborChangedAddress else

           4.  if any of neighbor's flood address port, name, local
               linkid changed then PUSH NeighborChangedMinorFields and

           5.  CHECK_THREE_WAY

   5.  CHECK_THREE_WAY: if current state is one-way do nothing else

       1.  if lie packet does not contain neighbor then if current state
           is three-way then PUSH NeighborDroppedReflection else

       2.  if packet reflects this system's ID and local port and state
           is three-way then PUSH event ValidReflection else PUSH event
           MultipleNeighbors

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4.2.3.  Topology Exchange (TIE Exchange)

4.2.3.1.  Topology Information Elements

   Topology and reachability information in RIFT is conveyed by the
   means of TIEs which have good amount of commonalities with LSAs in
   OSPF.

   The TIE exchange mechanism uses the port indicated by each node in
   the LIE exchange and the interface on which the adjacency has been
   formed as destination.  It SHOULD use TTL of 1 as well and set inter-
   network control precedence on according packets.

   TIEs contain sequence numbers, lifetimes and a type.  Each type has
   ample identifying number space and information is spread across
   possibly many TIEs of a certain type by the means of a hash function
   that a node or deployment can individually determine.  One extreme
   design choice is a prefix per TIE which leads to more BGP-like
   behavior where small increments are only advertised on route changes
   vs. deploying with dense prefix packing into few TIEs leading to more
   traditional IGP trade-off with fewer TIEs.  An implementation may
   even rehash prefix to TIE mapping at any time at the cost of
   significant amount of re-advertisements of TIEs.

   More information about the TIE structure can be found in the schema
   in Appendix B.

4.2.3.2.  South- and Northbound Representation

   A central concept of RIFT is that each node represents itself
   differently depending on the direction in which it is advertising
   information.  More precisely, a spine node represents two different
   databases over its adjacencies depending whether it advertises TIEs
   to the north or to the south/sideways.  We call those differing TIE
   databases either south- or northbound (South TIEs and North TIEs)
   depending on the direction of distribution.

   The North TIEs hold all of the node's adjacencies and local prefixes
   while the South TIEs hold only all of the node's adjacencies, the
   default prefix with necessary disaggregated prefixes and local
   prefixes.  We will explain this in detail further in Section 4.2.5.

   The TIE types are mostly symmetric in both directions and Table 2
   provides a quick reference to main TIE types including direction and
   their function.

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   +--------------------+----------------------------------------------+
   | TIE-Type           | Content                                      |
   +--------------------+----------------------------------------------+
   | Node North TIE     | node properties and adjacencies              |
   +--------------------+----------------------------------------------+
   | Node South TIE     | same content as node North TIE               |
   +--------------------+----------------------------------------------+
   | Prefix North TIE   | contains nodes' directly reachable prefixes  |
   +--------------------+----------------------------------------------+
   | Prefix South TIE   | contains originated defaults and directly    |
   |                    | reachable prefixes                           |
   +--------------------+----------------------------------------------+
   | Positive           | contains disaggregated prefixes              |
   | Disaggregation     |                                              |
   | South TIE          |                                              |
   +--------------------+----------------------------------------------+
   | Negative           | contains special, negatively disaggregated   |
   | Disaggregation     | prefixes to support multi-plane designs      |
   | South TIE          |                                              |
   +--------------------+----------------------------------------------+
   | External Prefix    | contains external prefixes                   |
   | North TIE          |                                              |
   +--------------------+----------------------------------------------+
   | Key-Value North    | contains nodes northbound KVs                |
   | TIE                |                                              |
   +--------------------+----------------------------------------------+
   | Key-Value South    | contains nodes southbound KVs                |
   | TIE                |                                              |
   +--------------------+----------------------------------------------+

                            Table 2: TIE Types

   As an example illustrating a databases holding both representations,
   consider the topology in Figure 2 with the optional link between
   spine 111 and spine 112 (so that the flooding on an East-West link
   can be shown).  This example assumes unnumbered interfaces.  First,
   here are the TIEs generated by some nodes.  For simplicity, the key
   value elements which may be included in their South TIEs or North
   TIEs are not shown.

          ToF 21 South TIEs:
          Node South TIE:
            NodeElement(level=2, neighbors((Spine 111, level 1, cost 1),
            (Spine 112, level 1, cost 1), (Spine 121, level 1, cost 1),
            (Spine 122, level 1, cost 1)))
          Prefix South TIE:
            SouthPrefixesElement(prefixes(0/0, cost 1), (::/0, cost 1))

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          Spine 111 South TIEs:
          Node South TIE:
            NodeElement(level=1, neighbors((ToF 21, level 2, cost 1,
                        links(...)),
            (ToF 22, level 2, cost 1, links(...)),
            (Spine 112, level 1, cost 1, links(...)),
            (Leaf111, level 0, cost 1, links(...)),
            (Leaf112, level 0, cost 1, links(...))))
          Prefix South TIE:
            SouthPrefixesElement(prefixes(0/0, cost 1), (::/0, cost 1))

          Spine 111 North TIEs:
          Node North TIE:
            NodeElement(level=1,
            neighbors((ToF 21, level 2, cost 1, links(...)),
            (ToF 22, level 2, cost 1, links(...)),
            (Spine 112, level 1, cost 1, links(...)),
            (Leaf111, level 0, cost 1, links(...)),
            (Leaf112, level 0, cost 1, links(...))))
          Prefix North TIE:
            NorthPrefixesElement(prefixes(Spine 111.loopback)

          Spine 121 South TIEs:
          Node South TIE:
            NodeElement(level=1, neighbors((ToF 21,level 2,cost 1),
            (ToF 22, level 2, cost 1), (Leaf121, level 0, cost 1),
            (Leaf122, level 0, cost 1)))
          Prefix South TIE:
            SouthPrefixesElement(prefixes(0/0, cost 1), (::/0, cost 1))

          Spine 121 North TIEs:
          Node North TIE:
            NodeElement(level=1,
            neighbors((ToF 21, level 2, cost 1, links(...)),
            (ToF 22, level 2, cost 1, links(...)),
            (Leaf121, level 0, cost 1, links(...)),
            (Leaf122, level 0, cost 1, links(...))))
          Prefix North TIE:
            NorthPrefixesElement(prefixes(Spine 121.loopback)

          Leaf112 North TIEs:
          Node North TIE:
            NodeElement(level=0,
            neighbors((Spine 111, level 1, cost 1, links(...)),
            (Spine 112, level 1, cost 1, links(...))))
          Prefix North TIE:
            NorthPrefixesElement(prefixes(Leaf112.loopback, Prefix112,
            Prefix_MH))

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       Figure 14: example TIES generated in a 2 level spine-and-leaf
                                 topology

   It may be here not necessarily obvious why the node South TIEs
   contain all the adjacencies of the according node.  This will be
   necessary for algorithms given in Section 4.2.3.9 and Section 4.3.6.

4.2.3.3.  Flooding

   The mechanism used to distribute TIEs is the well-known (albeit
   modified in several respects to take advantage of fat tree topology)
   flooding mechanism used by today's link-state protocols.  Although
   flooding is initially more demanding to implement it avoids many
   problems with update style used in diffused computation such as
   distance vector protocols.  Since flooding tends to present an
   unscalable burden in large, densely meshed topologies (fat trees
   being unfortunately such a topology) we provide as solution a close
   to optimal global flood reduction and load balancing optimization in
   Section 4.2.3.9.

   As described before, TIEs themselves are transported over UDP with
   the ports indicated in the LIE exchanges and using the destination
   address on which the LIE adjacency has been formed.  For unnumbered
   IPv4 interfaces same considerations apply as in equivalent OSPF case.

4.2.3.3.1.  Normative Flooding Procedures

   On reception of a TIE with an undefined level value in the packet
   header the node SHOULD issue a warning and indiscriminately discard
   the packet.

   This section specifies the precise, normative flooding mechanism and
   can be omitted unless the reader is pursuing an implementation of the
   protocol.

   Flooding Procedures are described in terms of a flooding state of an
   adjacency and resulting operations on it driven by packet arrivals.
   The FSM itself has basically just a single state and is not well
   suited to represent the behavior.  An implementation MUST behave on
   the wire in the same way as the provided normative procedures of this
   paragraph.

   RIFT does not specify any kind of flood rate limiting since such
   specifications always assume particular points in available
   technology speeds and feeds and those points are shifting at faster
   and faster rate (speed of light holding for the moment).  The encoded
   packets provide hints to react accordingly to losses or overruns.

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   Flooding of all according topology exchange elements SHOULD be
   performed at highest feasible rate whereas the rate of transmission
   MUST be throttled by reacting to adequate features of the system such
   as e.g. queue lengths or congestion indications in the protocol
   packets.

   A node SHOULD NOT send out any topology information elements if the
   adjacancy is not in a "three-way" state.  No further tightening of
   this rule is possible due to possible link buffering and re-ordering
   of LIEs and TIEs/TIDEs/TIREs.

   A node MUST drop any received TIEs/TIDEs/TIREs unless it is in three-
   way state.

   TIDEs and TIREs MUST NOT be re-flooded the way TIEs of other nodes
   are are MUST be always generated by the node itself and cross only to
   the neighboring node.

4.2.3.3.1.1.  FloodState Structure per Adjacency

   The structure contains conceptually the following elements.  The word
   collection or queue indicates a set of elements that can be iterated:

   TIES_TX:  Collection containing all the TIEs to transmit on the
      adjacency.

   TIES_ACK:  Collection containing all the TIEs that have to be
      acknowledged on the adjacency.

   TIES_REQ:  Collection containing all the TIE headers that have to be
      requested on the adjacency.

   TIES_RTX:  Collection containing all TIEs that need retransmission
      with the according time to retransmit.

   Following words are used for well known procedures operating on this
   structure:

   TIE  Describes either a full RIFT TIE or accordingly just the
      `TIEHeader` or `TIEID`. The according meaning is unambiguously
      contained in the context of the algorithm.

   is_flood_reduced(TIE):  returns whether a TIE can be flood reduced or
      not.

   is_tide_entry_filtered(TIE):  returns whether a header should be
      propagated in TIDE according to flooding scopes.

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   is_request_filtered(TIE):  returns whether a TIE request should be
      propagated to neighbor or not according to flooding scopes.

   is_flood_filtered(TIE):  returns whether a TIE requested be flooded
      to neighbor or not according to flooding scopes.

   try_to_transmit_tie(TIE):

      A.  if not is_flood_filtered(TIE) then

          1.  remove TIE from TIES_RTX if present

          2.  if TIE" with same key on TIES_ACK then

              a.  if TIE" same or newer than TIE do nothing else

              b.  remove TIE" from TIES_ACK and add TIE to TIES_TX

          3.  else insert TIE into TIES_TX

   ack_tie(TIE):  remove TIE from all collections and then insert TIE
      into TIES_ACK.

   tie_been_acked(TIE):  remove TIE from all collections.

   remove_from_all_queues(TIE):  same as `tie_been_acked`.

   request_tie(TIE):  if not is_request_filtered(TIE) then
      remove_from_all_queues(TIE) and add to TIES_REQ.

   move_to_rtx_list(TIE):  remove TIE from TIES_TX and then add to
      TIES_RTX using TIE retransmission interval.

   clear_requests(TIEs):  remove all TIEs from TIES_REQ.

   bump_own_tie(TIE):  for self-originated TIE originate an empty or re-
      generate with version number higher then the one in TIE.

   The collection SHOULD be served with following priorities if the
   system cannot process all the collections in real time:

      Elements on TIES_ACK should be processed with highest priority

      TIES_TX

      TIES_REQ and TIES_RTX

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4.2.3.3.1.2.  TIDEs

   `TIEID` and `TIEHeader` space forms a strict total order (modulo
   incomparable sequence numbers in the very unlikely event that can
   occur if a TIE is "stuck" in a part of a network while the originator
   reboots and reissues TIEs many times to the point its sequence# rolls
   over and forms incomparable distance to the "stuck" copy) which
   implies that a comparison relation is possible between two elements.
   With that it is implicitly possible to compare TIEs, TIEHeaders and
   TIEIDs to each other whereas the shortest viable key is always
   implied.

   When generating and sending TIDEs an implementation SHOULD ensure
   that enough bandwidth is left to send elements of Floodstate
   structure.

4.2.3.3.1.2.1.  TIDE Generation

   As given by timer constant, periodically generate TIDEs by:

      NEXT_TIDE_ID: ID of next TIE to be sent in TIDE.

      TIDE_START: Begin of TIDE packet range.

   a.  NEXT_TIDE_ID = MIN_TIEID

   b.  while NEXT_TIDE_ID not equal to MAX_TIEID do

       1.  TIDE_START = NEXT_TIDE_ID

       2.  HEADERS = At most TIRDEs_PER_PKT headers in TIEDB starting at
           NEXT_TIDE_ID or higher that SHOULD be filtered by
           is_tide_entry_filtered and MUST either have a lifetime left >
           0 or have no content

       3.  if HEADERS is empty then START = MIN_TIEID else START = first
           element in HEADERS

       4.  if HEADERS' size less than TIRDEs_PER_PKT then END =
           MAX_TIEID else END = last element in HEADERS

       5.  send sorted HEADERS as TIDE setting START and END as its
           range

       6.  NEXT_TIDE_ID = END

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   The constant `TIRDEs_PER_PKT` SHOULD be generated and used by the
   implementation to limit the amount of TIE headers per TIDE so the
   sent TIDE PDU does not exceed interface MTU.

   TIDE PDUs SHOULD be spaced on sending to prevent packet drops.

4.2.3.3.1.2.2.  TIDE Processing

   On reception of TIDEs the following processing is performed:

      TXKEYS: Collection of TIE Headers to be send after processing of
      the packet

      REQKEYS: Collection of TIEIDs to be requested after processing of
      the packet

      CLEARKEYS: Collection of TIEIDs to be removed from flood state
      queues

      LASTPROCESSED: Last processed TIEID in TIDE

      DBTIE: TIE in the LSDB if found

   a.  LASTPROCESSED = TIDE.start_range

   b.  for every HEADER in TIDE do

       1.  DBTIE = find HEADER in current LSDB

       2.  if HEADER < LASTPROCESSED then report error and reset
           adjacency and return

       3.  put all TIEs in LSDB where (TIE.HEADER > LASTPROCESSED and
           TIE.HEADER < HEADER) into TXKEYS

       4.  LASTPROCESSED = HEADER

       5.  if DBTIE not found then

           I)     if originator is this node then bump_own_tie

           II)    else put HEADER into REQKEYS

       6.  if DBTIE.HEADER < HEADER then

           I)    if originator is this node then bump_own_tie else

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                 i.     if this is a North TIE header from a northbound
                        neighbor then override DBTIE in LSDB with HEADER

                 ii.    else put HEADER into REQKEYS

       7.  if DBTIE.HEADER > HEADER then put DBTIE.HEADER into TXKEYS

       8.  if DBTIE.HEADER = HEADER then

           I)     if DBTIE has content already then put DBTIE.HEADER
                  into CLEARKEYS

           II)    else put HEADER into REQKEYS

   c.  put all TIEs in LSDB where (TIE.HEADER > LASTPROCESSED and
       TIE.HEADER <= TIDE.end_range) into TXKEYS

   d.  for all TIEs in TXKEYS try_to_transmit_tie(TIE)

   e.  for all TIEs in REQKEYS request_tie(TIE)

   f.  for all TIEs in CLEARKEYS remove_from_all_queues(TIE)

4.2.3.3.1.3.  TIREs

4.2.3.3.1.3.1.  TIRE Generation

   There is not much to say here.  Elements from both TIES_REQ and
   TIES_ACK MUST be collected and sent out as fast as feasible as TIREs.
   When sending TIREs with elements from TIES_REQ the `lifetime` field
   MUST be set to 0 to force reflooding from the neighbor even if the
   TIEs seem to be same.

4.2.3.3.1.3.2.  TIRE Processing

   On reception of TIREs the following processing is performed:

      TXKEYS: Collection of TIE Headers to be send after processing of
      the packet

      REQKEYS: Collection of TIEIDs to be requested after processing of
      the packet

      ACKKEYS: Collection of TIEIDs that have been acked

      DBTIE: TIE in the LSDB if found

   a.  for every HEADER in TIRE do

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       1.  DBTIE = find HEADER in current LSDB

       2.  if DBTIE not found then do nothing

       3.  if DBTIE.HEADER < HEADER then put HEADER into REQKEYS

       4.  if DBTIE.HEADER > HEADER then put DBTIE.HEADER into TXKEYS

       5.  if DBTIE.HEADER = HEADER then put DBTIE.HEADER into ACKKEYS

   b.  for all TIEs in TXKEYS try_to_transmit_tie(TIE)

   c.  for all TIEs in REQKEYS request_tie(TIE)

   d.  for all TIEs in ACKKEYS tie_been_acked(TIE)

4.2.3.3.1.4.  TIEs Processing on Flood State Adjacency

   On reception of TIEs the following processing is performed:

      ACKTIE: TIE to acknowledge

      TXTIE: TIE to transmit

      DBTIE: TIE in the LSDB if found

   a.  DBTIE = find TIE in current LSDB

   b.  if DBTIE not found then

       1.  if originator is this node then bump_own_tie with a short
           remaining lifetime

       2.  else insert TIE into LSDB and ACKTIE = TIE

       else

       1.  if DBTIE.HEADER = TIE.HEADER then

           i.     if DBTIE has content already then ACKTIE = TIE

           ii.    else process like the "DBTIE.HEADER < TIE.HEADER" case

       2.  if DBTIE.HEADER < TIE.HEADER then

           i.     if originator is this node then bump_own_tie

           ii.    else insert TIE into LSDB and ACKTIE = TIE

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       3.  if DBTIE.HEADER > TIE.HEADER then

           i.     if DBTIE has content already then TXTIE = DBTIE

           ii.    else ACKTIE = DBTIE

   c.  if TXTIE is set then try_to_transmit_tie(TXTIE)

   d.  if ACKTIE is set then ack_tie(TIE)

4.2.3.3.1.5.  TIEs Processing When LSDB Received Newer Version on Other
              Adjacencies

   The Link State Database can be considered to be a switchboard that
   does not need any flooding procedures but can be given new versions
   of TIEs by a peer.  Consecutively, a peer receives from the LSDB
   newer versions of TIEs received by other peers and processes them
   (without any filtering) just like receiving TIEs from its remote
   peer.  This publisher model can be implemented in many ways.

4.2.3.3.1.6.  Sending TIEs

   On a periodic basis all TIEs with lifetime left > 0 MUST be sent out
   on the adjacency, removed from TIES_TX list and requeued onto
   TIES_RTX list.

4.2.3.4.  TIE Flooding Scopes

   In a somewhat analogous fashion to link-local, area and domain
   flooding scopes, RIFT defines several complex "flooding scopes"
   depending on the direction and type of TIE propagated.

   Every North TIE is flooded northbound, providing a node at a given
   level with the complete topology of the Clos or Fat Tree network that
   is reachable southwards of it, including all specific prefixes.  This
   means that a packet received from a node at the same or lower level
   whose destination is covered by one of those specific prefixes will
   be routed directly towards the node advertising that prefix rather
   than sending the packet to a node at a higher level.

   A node's Node South TIEs, consisting of all node's adjacencies and
   prefix South TIEs limited to those related to default IP prefix and
   disaggregated prefixes, are flooded southbound in order to allow the
   nodes one level down to see connectivity of the higher level as well
   as reachability to the rest of the fabric.  In order to allow an E-W
   disconnected node in a given level to receive the South TIEs of other
   nodes at its level, every *NODE* South TIE is "reflected" northbound
   to level from which it was received.  It should be noted that East-

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   West links are included in South TIE flooding (except at ToF level);
   those TIEs need to be flooded to satisfy algorithms in Section 4.2.4.
   In that way nodes at same level can learn about each other without a
   lower level, e.g. in case of leaf level.  The precise, normative
   flooding scopes are given in Table 3.  Those rules govern as well
   what SHOULD be included in TIDEs on the adjacency.  Again, East-West
   flooding scopes are identical to South flooding scopes except in case
   of ToF East-West links (rings) which are basically performing
   northbound flooding.

   Node South TIE "south reflection" allows to support positive
   disaggregation on failures describes in Section 4.2.5 and flooding
   reduction in Section 4.2.3.9.

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  +-----------+---------------------+----------------+-----------------+
  | Type /    | South               | North          | East-West       |
  | Direction |                     |                |                 |
  +-----------+---------------------+----------------+-----------------+
  | node      | flood if level of   | flood if level | flood only if   |
  | South TIE | originator is equal | of originator  | this node       |
  |           | to this node        | is higher than | is not ToF      |
  |           |                     | this node      |                 |
  +-----------+---------------------+----------------+-----------------+
  | non-node  | flood self-         | flood only if  | flood only if   |
  | South TIE | originated only     | neighbor is    | self-originated |
  |           |                     | originator of  | and this node   |
  |           |                     | TIE            | is not ToF      |
  +-----------+---------------------+----------------+-----------------+
  | all North | never flood         | flood always   | flood only if   |
  | TIEs      |                     |                | this node is    |
  |           |                     |                | ToF             |
  +-----------+---------------------+----------------+-----------------+
  | TIDE      | include at least    | include at     | if this node is |
  |           | all non-self        | least all node | ToF then        |
  |           | originated North    | South TIEs and | include all     |
  |           | TIE headers and     | all South TIEs | North TIEs,     |
  |           | self-originated     | originated by  | otherwise only  |
  |           | South TIE headers   | peer and       | self-originated |
  |           | and                 | all North TIEs | TIEs            |
  |           | node South TIEs of  |                |                 |
  |           | nodes at same       |                |                 |
  |           | level               |                |                 |
  +-----------+---------------------+----------------+-----------------+
  | TIRE as   | request all North   | request all    | if this node is |
  | Request   | TIEs and all peer's | South TIEs     | ToF then apply  |
  |           | self-originated     |                | North scope     |
  |           | TIEs and            |                | rules,          |
  |           | all node South TIEs |                | otherwise South |
  |           |                     |                | scope rules     |
  +-----------+---------------------+----------------+-----------------+
  | TIRE as   | Ack all received    | Ack all        | Ack all         |
  | Ack       | TIEs                | received TIEs  | received TIEs   |
  +-----------+---------------------+----------------+-----------------+

                    Table 3: Normative Flooding Scopes

   If the TIDE includes additional TIE headers beside the ones
   specified, the receiving neighbor must apply according filter to the
   received TIDE strictly and MUST NOT request the extra TIE headers
   that were not allowed by the flooding scope rules in its direction.

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   As an example to illustrate these rules, consider using the topology
   in Figure 2, with the optional link between spine 111 and spine 112,
   and the associated TIEs given in Figure 14.  The flooding from
   particular nodes of the TIEs is given in Table 4.

   +-----------+----------+--------------------------------------------+
   | Router    | Neighbor | TIEs                                       |
   | floods to |          |                                            |
   +-----------+----------+--------------------------------------------+
   | Leaf111   | Spine    | Leaf111 North TIEs, Spine 111 node South   |
   |           | 112      | TIE                                        |
   | Leaf111   | Spine    | Leaf111 North TIEs, Spine 112 node South   |
   |           | 111      | TIE                                        |
   |           |          |                                            |
   | Spine 111 | Leaf111  | Spine 111 South TIEs                       |
   | Spine 111 | Leaf112  | Spine 111 South TIEs                       |
   | Spine 111 | Spine    | Spine 111 South TIEs                       |
   |           | 112      |                                            |
   | Spine 111 | ToF 21   | Spine 111 North TIEs,         Leaf111      |
   |           |          | North TIEs, Leaf112 North TIEs, ToF 22     |
   |           |          | node South TIE                             |
   | Spine 111 | ToF 22   | Spine 111 North TIEs,         Leaf111      |
   |           |          | North TIEs, Leaf112 North TIEs, ToF 21     |
   |           |          | node South TIE                             |
   |           |          |                                            |
   | ...       | ...      | ...                                        |
   | ToF 21    | Spine    | ToF 21 South TIEs                          |
   |           | 111      |                                            |
   | ToF 21    | Spine    | ToF 21 South TIEs                          |
   |           | 112      |                                            |
   | ToF 21    | Spine    | ToF 21 South TIEs                          |
   |           | 121      |                                            |
   | ToF 21    | Spine    | ToF 21 South TIEs                          |
   |           | 122      |                                            |
   | ...       | ...      | ...                                        |
   +-----------+----------+--------------------------------------------+

             Table 4: Flooding some TIEs from example topology

4.2.3.5.  'Flood Only Node TIEs' Bit

   RIFT includes an optional ECN mechanism to prevent "flooding inrush"
   on restart or bring-up with many southbound neighbors.  A node MAY
   set on its LIEs the according bit to indicate to the neighbor that it
   should temporarily flood node TIEs only to it.  It SHOULD only set it
   in the southbound direction.  The receiving node SHOULD accommodate
   the request to lessen the flooding load on the affected node if south
   of the sender and SHOULD ignore the bit if northbound.

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   Obviously this mechanism is most useful in southbound direction.  The
   distribution of node TIEs guarantees correct behavior of algorithms
   like disaggregation or default route origination.  Furthermore
   though, the use of this bit presents an inherent trade-off between
   processing load and convergence speed since suppressing flooding of
   northbound prefixes from neighbors will lead to blackholes.

4.2.3.6.  Initial and Periodic Database Synchronization

   The initial exchange of RIFT is modeled after ISIS with TIDE being
   equivalent to CSNP and TIRE playing the role of PSNP.  The content of
   TIDEs and TIREs is governed by Table 3.

4.2.3.7.  Purging and Roll-Overs

   When a node exits the network, if "unpurged", residual stale TIEs may
   exist in the network until their lifetimes expire (which in case of
   RIFT is by default a rather long period to prevent ongoing re-
   origination of TIEs in very large topologies).  RIFT does however not
   have a "purging mechanism" in the traditional sense based on sending
   specialized "purge" packets.  In other routing protocols such
   mechanism has proven to be complex and fragile based on many years of
   experience.  RIFT simply issues a new, empty version of the TIE with
   a short lifetime and relies on each node to age out and delete such
   TIE copy independently.  Abundant amounts of memory are available
   today even on low-end platforms and hence keeping those relatively
   short-lived extra copies for a while is acceptable.  The information
   will age out and in the meantime all computations will deliver
   correct results if a node leaves the network due to the new
   information distributed by its adjacent nodes breaking bi-directional
   connectivity checks in different computations.

   Once a RIFT node issues a TIE with an ID, it SHOULD preserve the ID
   as long as feasible (also when the protocol restarts), even if the
   TIE looses all content.  The re-advertisement of empty TIE fulfills
   the purpose of purging any information advertised in previous
   versions.  The originator is free to not re-originate the according
   empty TIE again or originate an empty TIE with relatively short
   lifetime to prevent large number of long-lived empty stubs polluting
   the network.  Each node MUST timeout and clean up the according empty
   TIEs independently.

   Upon restart a node MUST, as any link-state implementation, be
   prepared to receive TIEs with its own system ID and supersede them
   with equivalent, newly generated, empty TIEs with a higher sequence
   number.  As above, the lifetime can be relatively short since it only
   needs to exceed the necessary propagation and processing delay by all
   the nodes that are within the TIE's flooding scope.

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   TIE sequence numbers are rolled over using the method described in
   Appendix A.  First sequence number of any spontaneously originated
   TIE (i.e. not originated to override a detected older copy in the
   network) MUST be a reasonably unpredictable random number in the
   interval [0, 2^10-1] which will prevent otherwise identical TIE
   headers to remain "stuck" in the network with content different from
   TIE originated after reboot.

4.2.3.8.  Southbound Default Route Origination

   Under certain conditions nodes issue a default route in their South
   Prefix TIEs with costs as computed in Section 4.3.6.1.

   A node X that

   1.  is NOT overloaded AND

   2.  has southbound or East-West adjacencies

   originates in its south prefix TIE such a default route IIF

   1.  all other nodes at X's' level are overloaded OR

   2.  all other nodes at X's' level have NO northbound adjacencies OR

   3.  X has computed reachability to a default route during N-SPF.

   The term "all other nodes at X's' level" describes obviously just the
   nodes at the same level in the PoD with a viable lower level
   (otherwise the node South TIEs cannot be reflected and the nodes in
   e.g.  PoD 1 and PoD 2 are "invisible" to each other).

   A node originating a southbound default route MUST install a default
   discard route if it did not compute a default route during N-SPF.

4.2.3.9.  Northbound TIE Flooding Reduction

   Section 1.4 of the Optimized Link State Routing Protocol [RFC3626]
   (OLSR) introduces the concept of a "multipoint relay" (MPR) that
   minimize the overhead of flooding messages in the network by reducing
   redundant retransmissions in the same region.

   A similar technique is applied to RIFT to control northbound
   flooding.  Important observations first:

   1.  a node MUST flood self-originated North TIEs to all the reachable
       nodes at the level above which we call the node's "parents";

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   2.  it is typically not necessary that all parents reflood the North
       TIEs to achieve a complete flooding of all the reachable nodes
       two levels above which we choose to call the node's
       "grandparents";

   3.  to control the volume of its flooding two hops North and yet keep
       it robust enough, it is advantageous for a node to select a
       subset of its parents as "Flood Repeaters" (FRs), which combined
       together deliver two or more copies of its flooding to all of its
       parents, i.e. the originating node's grandparents;

   4.  nodes at the same level do NOT have to agree on a specific
       algorithm to select the FRs, but overall load balancing should be
       achieved so that different nodes at the same level should tend to
       select different parents as FRs;

   5.  there are usually many solutions to the problem of finding a set
       of FRs for a given node; the problem of finding the minimal set
       is (similar to) a NP-Complete problem and a globally optimal set
       may not be the minimal one if load-balancing with other nodes is
       an important consideration;

   6.  it is expected that there will be often sets of equivalent nodes
       at a level L, defined as having a common set of parents at L+1.
       Applying this observation at both L and L+1, an algorithm may
       attempt to split the larger problem in a sum of smaller separate
       problems;

   7.  it is another expectation that there will be from time to time a
       broken link between a parent and a grandparent, and in that case
       the parent is probably a poor FR due to its lower reliability.
       An algorithm may attempt to eliminate parents with broken
       northbound adjacencies first in order to reduce the number of
       FRs.  Albeit it could be argued that relying on higher fanout FRs
       will slow flooding due to higher replication load reliability of
       FR's links seems to be a more pressing concern.

   In a fully connected Clos Network, this means that a node selects one
   arbitrary parent as FR and then a second one for redundancy.  The
   computation can be kept relatively simple and completely distributed
   without any need for synchronization amongst nodes.  In a "PoD"
   structure, where the Level L+2 is partitioned in silos of equivalent
   grandparents that are only reachable from respective parents, this
   means treating each silo as a fully connected Clos Network and solve
   the problem within the silo.

   In terms of signaling, a node has enough information to select its
   set of FRs; this information is derived from the node's parents' Node

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   South TIEs, which indicate the parent's reachable northbound
   adjacencies to its own parents, i.e. the node's grandparents.  A node
   may send a LIE to a northbound neighbor with the optional boolean
   field `you_are_flood_repeater` set to false, to indicate that the
   northbound neighbor is not a flood repeater for the node that sent
   the LIE.  In that case the northbound neighbor SHOULD NOT reflood
   northbound TIEs received from the node that sent the LIE.  If the
   `you_are_flood_repeater` is absent or if `you_are_flood_repeater` is
   set to true, then the northbound neighbor is a flood repeater for the
   node that sent the LIE and MUST reflood northbound TIEs received from
   that node.

   This specification proposes a simple default algorithm that SHOULD be
   implemented and used by default on every RIFT node.

   o  let |NA(Node) be the set of Northbound adjacencies of node Node
      and CN(Node) be the cardinality of |NA(Node);

   o  let |SA(Node) be the set of Southbound adjacencies of node Node
      and CS(Node) be the cardinality of |SA(Node);

   o  let |P(Node) be the set of node Node's parents;

   o  let |G(Node) be the set of node Node's grandparents.  Observe
      that |G(Node) = |P(|P(Node));

   o  let N be the child node at level L computing a set of FR;

   o  let P be a node at level L+1 and a parent node of N, i.e. bi-
      directionally reachable over adjacency A(N, P);

   o  let G be a grandparent node of N, reachable transitively via a
      parent P over adjacencies ADJ(N, P) and ADJ(P, G).  Observe that N
      does not have enough information to check bidirectional
      reachability of A(P, G);

   o  let R be a redundancy constant integer; a value of 2 or higher for
      R is RECOMMENDED;

   o  let S be a similarity constant integer; a value in range 0 .. 2
      for S is RECOMMENDED, the value of 1 SHOULD be used.  Two
      cardinalities are considered as equivalent if their absolute
      difference is less than or equal to S, i.e.

   o  |a-b|<=S.

   o  let RND be a 64-bit random number generated by the system once on
      startup.

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   The algorithm consists of the following steps:

   1.  Derive a 64-bits number by XOR'ing 'N's system ID with RND.

   2.  Derive a 16-bits pseudo-random unsigned integer PR(N) from the
       resulting 64-bits number by splitting it in 16-bits-long words
       W1, W2, W3, W4 (where W1 are the least significant 16 bits of the
       64-bits number, and W4 are the most significant 16 bits) and then
       XOR'ing the circularly shifted resulting words together:

       A.  (W1<<1) xor (W2<<2) xor (W3<<3) xor (W4<<4);

           where << is the circular shift operator.

   3.  Sort the parents by decreasing number of northbound adjacencies
       (using decreasing system id of the parent as tie-breaker):
       sort |P(N) by decreasing CN(P), for all P in |P(N), as ordered
       array |A(N)

   4.  Partition |A(N) in subarrays |A_k(N) of parents with equivalent
       cardinality of northbound adjacencies (in other words with
       equivalent number of grandparents they can reach):

       A.  set k=0; // k is the ID of the subarrray

       B.  set i=0;

       C.  while i < CN(N) do

           i)     set j=i;

           ii)    while i < CN(N) and CN(|A(N)[j]) - CN(|A(N)[i]) <= S

                  a.  place |A(N)[i] in |A_k(N) // abstract action,
                      maybe noop

                  b.  set i=i+1;

           iii)   /* At this point j is the index in |A(N) of the first
                  member of |A_k(N) and (i-j) is C_k(N) defined as the
                  cardinality of |A_k(N) */

                  set k=k+1;

       /* At this point k is the total number of subarrays, initialized
       for the shuffling operation below */

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   5.  shuffle individually each subarrays |A_k(N) of cardinality C_k(N)
       within |A(N) using the Durstenfeld variation of Fisher-Yates
       algorithm that depends on N's System ID:

       A.  while k > 0 do

           i)    for i from C_k(N)-1 to 1 decrementing by 1 do

                 a.  set j to PR(N) modulo i;

                 b.  exchange |A_k[j] and |A_k[i];

           ii)   set k=k-1;

   6.  For each grandparent G, initialize a counter c(G) with the number
       of its south-bound adjacencies to elected flood repeaters (which
       is initially zero):

       A.  for each G in |G(N) set c(G) = 0;

   7.  Finally keep as FRs only parents that are needed to maintain the
       number of adjacencies between the FRs and any grandparent G equal
       or above the redundancy constant R:

       A.  for each P in reshuffled |A(N);

           i)   if there exists an adjacency ADJ(P, G) in |NA(P) such
                that c(G) < R then

                a.  place P in FR set;

                b.  for all adjacencies ADJ(P, G') in |NA(P) increment
                    c(G')

       B.  If any c(G) is still < R, it was not possible to elect a set
           of FRs that covers all grandparents with redundancy R

   Additional rules for flooding reduction:

   1.  The algorithm MUST be re-evaluated by a node on every change of
       local adjacencies or reception of a parent South TIE with changed
       adjacencies.  A node MAY apply a hysteresis to prevent excessive
       amount of computation during periods of network instability just
       like in case of reachability computation.

   2.  Upon a change of the flood repeater set, a node SHOULD send out
       LIEs that grant flood repeater status to newly promoted nodes
       before it sends LIEs that revoke the status to the nodes that

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       have been newly demoted.  This is done to prevent transient
       behavior where the full coverage of grandparents is not
       guaranteed.  Such a condition is sometimes unavoidable in case of
       lost LIEs but it will correct itself though at possible transient
       hit in flooding propagation speeds.

   3.  A node MUST always flood its self-originated TIEs.

   4.  A node receiving a TIE originated by a node for which it is not a
       flood repeater SHOULD NOT reflood such TIEs to its neighbors
       except for rules in Paragraph 6.

   5.  The indication of flood reduction capability MUST be carried in
       the node TIEs and MAY be used to optimize the algorithm to
       account for nodes that will flood regardless.

   6.  A node generates TIDEs as usual but when receiving TIREs or TIDEs
       resulting in requests for a TIE of which the newest received copy
       came on an adjacency where the node was not flood repeater it
       SHOULD ignore such requests on first and only first request.
       Normally, the nodes that received the TIEs as flooding repeaters
       should satisfy the requesting node and with that no further TIREs
       for such TIEs will be generated.  Otherwise, the next set of
       TIDEs and TIREs MUST lead to flooding independent of the flood
       repeater status.  This solves a very difficult incast problem on
       nodes restarting with a very wide fanout, especially northbound.
       To retrieve the full database they often end up processing many
       in-rushing copies whereas this approach load-balances the
       incoming database between adjacent nodes and flood repeaters
       should guarantee that two copies are sent by different nodes to
       ensure against any losses.

4.2.3.10.  Special Considerations

   First, due to the distributed, asynchronous nature of ZTP, it can
   create temporary convergence anomalies where nodes at higher levels
   of the fabric temporarily see themselves lower than they belong.
   Since flooding can begin before ZTP is "finished" and in fact must do
   so given there is no global termination criteria, information may end
   up in wrong layers.  A special clause when changing level takes care
   of that.

   More difficult is a condition where a node (e.g. a leaf) floods a TIE
   north towards its grandparent, then its parent reboots, in fact
   partitioning the grandparent from leaf directly and then the leaf
   itself reboots.  That can leave the grandparent holding the "primary
   copy" of the leaf's TIE.  Normally this condition is resolved easily
   by the leaf re-originating its TIE with a higher sequence number than

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   it sees in northbound TIEs, here however, when the parent comes back
   it won't be able to obtain leaf's North TIE from the grandparent
   easily and with that the leaf may not issue the TIE with a higher
   sequence number that can reach the granparent for a long time.
   Flooding procedures are extended to deal with the problem by the
   means of special clauses that override the database of a lower level
   with headers of newer TIEs seen in TIDEs coming from the north.

4.2.4.  Reachability Computation

   A node has three possible sources of relevant information for
   reachability computation.  A node knows the full topology south of it
   from the received North Node TIEs or alternately north of it from the
   South Node TIEs.  A node has the set of prefixes with their
   associated distances and bandwidths from corresponding prefix TIEs.

   To compute prefix reachability, a node runs conceptually a northbound
   and a southbound SPF.  We call that N-SPF and S-SPF denoting the
   direction in which the computation front is progressing.

   Since neither computation can "loop", it is possible to compute non-
   equal-cost or even k-shortest paths [EPPSTEIN] and "saturate" the
   fabric to the extent desired but we use simple, familiar SPF
   algorithms and concepts here as example due to their prevalence in
   today's routing.

4.2.4.1.  Northbound SPF

   N-SPF MUST use exclusively northbound and East-West adjacencies in
   the computing node's node North TIEs (since if the node is a leaf it
   may not have generated a node South TIE) when starting SPF.  Observe
   that N-SPF is really just a one hop variety since Node South TIEs are
   not re-flooded southbound beyond a single level (or East-West) and
   with that the computation cannot progress beyond adjacent nodes.

   Once progressing, we are using the next higher level's node South
   TIEs to find according adjacencies to verify backlink connectivity.
   Just as in case of IS-IS or OSPF, two unidirectional links MUST be
   associated together to confirm bidirectional connectivity.
   Particular care MUST be paid that the Node TIEs do not only contain
   the correct system IDs but matching levels as well.

   Default route found when crossing an E-W link SHOULD be used IIF

   1.  the node itself does NOT have any northbound adjacencies AND

   2.  the adjacent node has one or more northbound adjacencies

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   This rule forms a "one-hop default route split-horizon" and prevents
   looping over default routes while allowing for "one-hop protection"
   of nodes that lost all northbound adjacencies except at Top-of-Fabric
   where the links are used exclusively to flood topology information in
   multi-plane designs.

   Other south prefixes found when crossing E-W link MAY be used IIF

   1.  no north neighbors are advertising same or supersuming non-
       default prefix AND

   2.  the node does not originate a non-default supersuming prefix
       itself.

   i.e. the E-W link can be used as a gateway of last resort for a
   specific prefix only.  Using south prefixes across E-W link can be
   beneficial e.g.  on automatic de-aggregation in pathological fabric
   partitioning scenarios.

   A detailed example can be found in Section 5.4.

4.2.4.2.  Southbound SPF

   S-SPF MUST use exclusively the southbound adjacencies in the node
   South TIEs, i.e. progresses towards nodes at lower levels.  Observe
   that E-W adjacencies are NEVER used in the computation.  This
   enforces the requirement that a packet traversing in a southbound
   direction must never change its direction.

   S-SPF MUST use northbound adjacencies in node North TIEs to verify
   backlink connectivity by checking for presence of the link beside
   correct SystemID and level.

4.2.4.3.  East-West Forwarding Within a non-ToF Level

   Using south prefixes over horizontal links MAY occur if the N-SPF
   includes East-West adjacencies in computation.  It can protect
   against pathological fabric partitioning cases that leave only paths
   to destinations that would necessitate multiple changes of forwarding
   direction between north and south.

4.2.4.4.  East-West Links Within ToF Level

   E-W ToF links behave in terms of flooding scopes defined in
   Section 4.2.3.4 like northbound links and MUST be used exclusively
   for control plane information flooding.  Even though a ToF node could
   be tempted to use those links during southbound SPF and carry traffic
   over them this MUST NOT be attempted since it may lead in, e.g.

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   anycast cases to routing loops.  An implementation MAY try to resolve
   the looping problem by following on the ring strictly tie-broken
   shortest-paths only but the details are outside this specification.
   And even then, the problem of proper capacity provisioning of such
   links when they become traffic-bearing in case of failures is vexing.

4.2.5.  Automatic Disaggregation on Link & Node Failures

4.2.5.1.  Positive, Non-transitive Disaggregation

   Under normal circumstances, node's South TIEs contain just the
   adjacencies and a default route.  However, if a node detects that its
   default IP prefix covers one or more prefixes that are reachable
   through it but not through one or more other nodes at the same level,
   then it MUST explicitly advertise those prefixes in an South TIE.
   Otherwise, some percentage of the northbound traffic for those
   prefixes would be sent to nodes without according reachability,
   causing it to be black-holed.  Even when not black-holing, the
   resulting forwarding could 'backhaul' packets through the higher
   level spines, clearly an undesirable condition affecting the blocking
   probabilities of the fabric.

   We refer to the process of advertising additional prefixes southbound
   as 'positive de-aggregation' or 'positive dis-aggregation'.  Such
   dis-aggregation is non-transitive, i.e. its' effects are always
   contained to a single level of the fabric only.  Naturally, multiple
   node or link failures can lead to several independent instances of
   positive dis-aggregation necessary to prevent looping or bow-tying
   the fabric.

   A node determines the set of prefixes needing de-aggregation using
   the following steps:

   1.  A DAG computation in the southern direction is performed first,
       i.e. the North TIEs are used to find all of prefixes it can reach
       and the set of next-hops in the lower level for each of them.
       Such a computation can be easily performed on a fat tree by e.g.
       setting all link costs in the southern direction to 1 and all
       northern directions to infinity.  We term set of those
       prefixes |R, and for each prefix, r, in |R, we define its set of
       next-hops to be |H(r).

   2.  The node uses reflected South TIEs to find all nodes at the same
       level in the same PoD and the set of southbound adjacencies for
       each.  The set of nodes at the same level is termed |N and for
       each node, n, in |N, we define its set of southbound adjacencies
       to be |A(n).

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   3.  For a given r, if the intersection of |H(r) and |A(n), for any n,
       is null then that prefix r must be explicitly advertised by the
       node in an South TIE.

   3.

   4.  Identical set of de-aggregated prefixes is flooded on each of the
       node's southbound adjacencies.  In accordance with the normal
       flooding rules for an South TIE, a node at the lower level that
       receives this South TIE SHOULD NOT propagate it south-bound or
       reflect the disaggregated prefixes back over its adjacencies to
       nodes at the level from which it was received.

   To summarize the above in simplest terms: if a node detects that its
   default route encompasses prefixes for which one of the other nodes
   in its level has no possible next-hops in the level below, it has to
   disaggregate it to prevent black-holing or suboptimal routing through
   such nodes.  Hence a node X needs to determine if it can reach a
   different set of south neighbors than other nodes at the same level,
   which are connected to it via at least one common south neighbor.  If
   it can, then prefix disaggregation may be required.  If it can't,
   then no prefix disaggregation is needed.  An example of
   disaggregation is provided in Section 5.3.

   A possible algorithm is described last:

   1.  Create partial_neighbors = (empty), a set of neighbors with
       partial connectivity to the node X's level from X's perspective.
       Each entry in the set is a south neighbor of X and a list of
       nodes of X.level that can't reach that neighbor.

   2.  A node X determines its set of southbound neighbors
       X.south_neighbors.

   3.  For each South TIE originated from a node Y that X has which is
       at X.level, if Y.south_neighbors is not the same as
       X.south_neighbors but the nodes share at least one southern
       neighbor, for each neighbor N in X.south_neighbors but not in
       Y.south_neighbors, add (N, (Y)) to partial_neighbors if N isn't
       there or add Y to the list for N.

   4.  If partial_neighbors is empty, then node X does not disaggregate
       any prefixes.  If node X is advertising disaggregated prefixes in
       its South TIE, X SHOULD remove them and re-advertise its
       according South TIEs.

   A node X computes reachability to all nodes below it based upon the
   received North TIEs first.  This results in a set of routes, each

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   categorized by (prefix, path_distance, next-hop-set).  Alternately,
   for clarity in the following procedure, these can be organized by
   next-hop-set as ( (next-hops), {(prefix, path_distance)}).  If
   partial_neighbors isn't empty, then the following procedure describes
   how to identify prefixes to disaggregate.

               disaggregated_prefixes = { empty }
               nodes_same_level = { empty }
               for each South TIE
                 if (South TIE.level == X.level and
                     X shares at least one S-neighbor with X)
                   add South TIE.originator to nodes_same_level
                   end if
                 end for

               for each next-hop-set NHS
                 isolated_nodes = nodes_same_level
                 for each NH in NHS
                   if NH in partial_neighbors
                     isolated_nodes =
                       intersection(isolated_nodes,
                                    partial_neighbors[NH].nodes)
                     end if
                   end for

                 if isolated_nodes is not empty
                   for each prefix using NHS
                     add (prefix, distance) to disaggregated_prefixes
                     end for
                   end if
                 end for

               copy disaggregated_prefixes to X's South TIE
               if X's South TIE is different
                 schedule South TIE for flooding
                 end if

             Figure 15: Computation of Disaggregated Prefixes

   Each disaggregated prefix is sent with the according path_distance.
   This allows a node to send the same South TIE to each south neighbor.
   The south neighbor which is connected to that prefix will thus have a
   shorter path.

   Finally, to summarize the less obvious points partially omitted in
   the algorithms to keep them more tractable:

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   1.  all neighbor relationships MUST perform backlink checks.

   2.  overload bits as introduced in Section 4.3.1 have to be respected
       during the computation.

   3.  all the lower level nodes are flooded the same disaggregated
       prefixes since we don't want to build an South TIE per node and
       complicate things unnecessarily.  The lower level node that can
       compute a southbound route to the prefix will prefer it to the
       disaggregated route anyway based on route preference rules.

   4.  positively disaggregated prefixes do NOT have to propagate to
       lower levels.  With that the disturbance in terms of new flooding
       is contained to a single level experiencing failures.

   5.  disaggregated Prefix South TIEs are not "reflected" by the lower
       level, i.e.  nodes within same level do NOT need to be aware
       which node computed the need for disaggregation.

   6.  The fabric is still supporting maximum load balancing properties
       while not trying to send traffic northbound unless necessary.

   In case positive disaggregation is triggered and due to the very
   stable but un-synchronized nature of the algorithm the nodes may
   issue the necessary disaggregated prefixes at different points in
   time.  This can lead for a short time to an "incast" behavior where
   the first advertising router based on the nature of longest prefix
   match will attract all the traffic.  An implementation MAY hence
   choose different strategies to address this behavior if needed.

   To close this section it is worth to observe that in a single plane
   ToF this disaggregation prevents blackholing up to (K_LEAF * P) link
   failures in terms of Section 4.1.2 or in other terms, it takes at
   minimum that many link failures to partition the ToF into multiple
   planes.

4.2.5.2.  Negative, Transitive Disaggregation for Fallen Leaves

   As explained in Section 4.1.3 failures in multi-plane Top-of-Fabric
   or more than (K_LEAF * P) links failing in single plane design can
   generate fallen leaves.  Such scenario cannot be addressed by
   positive disaggregation only and needs a further mechanism.

4.2.5.2.1.  Cabling of Multiple Top-of-Fabric Planes

   Let us return in this section to designs with multiple planes as
   shown in Figure 3.  Figure 16 highlights how the ToF is cabled in
   case of two planes by the means of dual-rings to distribute all the

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   North TIEs within both planes.  For people familiar with traditional
   link-state routing protocols ToF level can be considered equivalent
   to area 0 in OSPF or level-2 in ISIS which need to be "connected" as
   well for the protocol to operate correctly.

        .     ++==========++          ++==========++
        .     II          II          II          II
        .+----++--+  +----++--+  +----++--+  +----++--+
        .|ToF   A1|  |ToF   B1|  |ToF   B2|  |ToF   A2|
        .++-+-++--+  ++-+-++--+  ++-+-++--+  ++-+-++--+
        . | | II      | | II      | | II      | | II
        . | | ++==========++      | | ++==========++
        . | |         | |         | |         | |
        .
        . ~~~ Highlighted ToF of the previous multi-plane figure ~~

                 Figure 16: Topologically connected planes

   As described in Section 4.1.3 failures in multi-plane fabrics can
   lead to blackholes which normal positive disaggregation cannot fix.
   The mechanism of negative, transitive disaggregation incorporated in
   RIFT provides the according solution.

4.2.5.2.2.  Transitive Advertisement of Negative Disaggregates

   A ToF node that discovers that it cannot reach a fallen leaf
   disaggregates all the prefixes of such leaves.  It uses for that
   purpose negative prefix South TIEs that are, as usual, flooded
   southwards with the scope defined in Section 4.2.3.4.

   Transitively, a node explicitly loses connectivity to a prefix when
   none of its children advertises it and when the prefix is negatively
   disaggregated by all of its parents.  When that happens, the node
   originates the negative prefix further down south.  Since the
   mechanism applies recursively south the negative prefix may propagate
   transitively all the way down to the leaf.  This is necessary since
   leaves connected to multiple planes by means of disjoint paths may
   have to choose the correct plane already at the very bottom of the
   fabric to make sure that they don't send traffic towards another leaf
   using a plane where it is "fallen" at which in point a blackhole is
   unavoidable.

   When the connectivity is restored, a node that disaggregated a prefix
   withdraws the negative disaggregation by the usual mechanism of re-
   advertising TIEs omitting the negative prefix.

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4.2.5.2.3.  Computation of Negative Disaggregates

   The document omitted so far the description of the computation
   necessary to generate the correct set of negative prefixes.  Negative
   prefixes can in fact be advertised due to two different triggers.  We
   describe them consecutively.

   The first origination reason is a computation that uses all the node
   North TIEs to build the set of all reachable nodes by reachability
   computation over the complete graph and including ToF links.  The
   computation uses the node itself as root.  This is compared with the
   result of the normal southbound SPF as described in Section 4.2.4.2.
   The difference are the fallen leaves and all their attached prefixes
   are advertised as negative prefixes southbound if the node does not
   see the prefix being reachable within southbound SPF.

   The second mechanism hinges on the understanding how the negative
   prefixes are used within the computation as described in Figure 17.
   When attaching the negative prefixes at certain point in time the
   negative prefix may find itself with all the viable nodes from the
   shorter match nexthop being pruned.  In other words, all its
   northbound neighbors provided a negative prefix advertisement.  This
   is the trigger to advertise this negative prefix transitively south
   and normally caused by the node being in a plane where the prefix
   belongs to a fabric leaf that has "fallen" in this plane.  Obviously,
   when one of the northbound switches withdraws its negative
   advertisement, the node has to withdraw its transitively provided
   negative prefix as well.

4.2.6.  Attaching Prefixes

   After SPF is run, it is necessary to attach the resulting
   reachability information in form of prefixes.  For S-SPF, prefixes
   from an North TIE are attached to the originating node with that
   node's next-hop set and a distance equal to the prefix's cost plus
   the node's minimized path distance.  The RIFT route database, a set
   of (prefix, prefix-type, attributes, path_distance, next-hop set),
   accumulates these results.

   In case of N-SPF prefixes from each South TIE need to also be added
   to the RIFT route database.  The N-SPF is really just a stub so the
   computing node needs simply to determine, for each prefix in an South
   TIE that originated from adjacent node, what next-hops to use to
   reach that node.  Since there may be parallel links, the next-hops to
   use can be a set; presence of the computing node in the associated
   Node South TIE is sufficient to verify that at least one link has
   bidirectional connectivity.  The set of minimum cost next-hops from
   the computing node X to the originating adjacent node is determined.

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   Each prefix has its cost adjusted before being added into the RIFT
   route database.  The cost of the prefix is set to the cost received
   plus the cost of the minimum distance next-hop to that neighbor while
   taking into account its attributes such as mobility per
   Section 4.3.3.  Then each prefix can be added into the RIFT route
   database with the next_hop_set; ties are broken based upon type first
   and then distance and further on `PrefixAttributes` and only the best
   combination is used for forwarding.  RIFT route preferences are
   normalized by the according Thrift [thrift] model type.

   An example implementation for node X follows:

     for each South TIE
        if South TIE.level > X.level
           next_hop_set = set of minimum cost links to the
               South TIE.originator
           next_hop_cost = minimum cost link to
               South TIE.originator
           end if
        for each prefix P in the South TIE
           P.cost = P.cost + next_hop_cost
           if P not in route_database:
             add (P, P.cost, P.type,
                  P.attributes, next_hop_set) to route_database
             end if
           if (P in route_database):
             if route_database[P].cost > P.cost or
                   route_database[P].type > P.type:
               update route_database[P] with (P, P.type, P.cost,
                                              P.attributes,
                                              next_hop_set)
             else if route_database[P].cost == P.cost and
                   route_database[P].type == P.type:
               update route_database[P] with (P, P.type,
                                              P.cost, P.attributes,
                  merge(next_hop_set, route_database[P].next_hop_set))
             else
               // Not preferred route so ignore
               end if
             end if
           end for
        end for

       Figure 17: Adding Routes from South TIE Positive and Negative
                                 Prefixes

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   After the positive prefixes are attached and tie-broken, negative
   prefixes are attached and used in case of northbound computation,
   ideally from the shortest length to the longest.  The nexthop
   adjacencies for a negative prefix are inherited from the longest
   positive prefix that aggregates it, and subsequently adjacencies to
   nodes that advertised negative for this prefix are removed.

   The rule of inheritance MUST be maintained when the nexthop list for
   a prefix is modified, as the modification may affect the entries for
   matching negative prefixes of immediate longer prefix length.  For
   instance, if a nexthop is added, then by inheritance it must be added
   to all the negative routes of immediate longer prefixes length unless
   it is pruned due to a negative advertisement for the same next hop.
   Similarly, if a nexthop is deleted for a given prefix, then it is
   deleted for all the immediately aggregated negative routes.  This
   will recurse in the case of nested negative prefix aggregations.

   The rule of inheritance must also be maintained when a new prefix of
   intermediate length is inserted, or when the immediately aggregating
   prefix is deleted from the routing table, making an even shorter
   aggregating prefix the one from which the negative routes now inherit
   their adjacencies.  As the aggregating prefix changes, all the
   negative routes must be recomputed, and then again the process may
   recurse in case of nested negative prefix aggregations.

   Although these operations can be computationally expensive, the
   overall load on devices in the network is low because these
   computations are not run very often, as positive route advertisements
   are always preferred over negative ones.  This prevents recursion in
   most cases because positive reachability information never inherits
   next hops.

   To make the negative disaggregation less abstract and provide an
   example let us consider a ToP node T1 with 4 ToF parents S1..S4 as
   represented in Figure 18:

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                    +----+    +----+    +----+    +----+          N
                    | S1 |    | S1 |    | S1 |    | S1 |          ^
                    +----+    +----+    +----+    +----+       W< + >E
                     |         |         |         |              v
                     |+--------+         |         |              S
                     ||+-----------------+         |
                     |||+----------------+---------+
                     ||||
                    +----+
                    | T1 |
                    +----+

                   Figure 18: A ToP node with 4 parents

   If all ToF nodes can reach all the prefixes in the network; with
   RIFT, they will normally advertise a default route south.  An
   abstract Routing Information Base (RIB), more commonly known as a
   routing table, stores all types of maintained routes including the
   negative ones and "tie-breaks" for the best one, whereas an abstract
   Forwarding table (FIB) retains only the ultimately computed
   "positive" routing instructions.  In T1, those tables would look as
   illustrated in Figure 19:

                                  +---------+
                                  | Default |
                                  +---------+
                                       |
                                       |     +--------+
                                       +---> | Via S1 |
                                       |     +--------+
                                       |
                                       |     +--------+
                                       +---> | Via S2 |
                                       |     +--------+
                                       |
                                       |     +--------+
                                       +---> | Via S3 |
                                       |     +---------+
                                       |
                                       |     +--------+
                                       +---> | Via S4 |
                                             +--------+

                          Figure 19: Abstract RIB

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   In case T1 receives a negative advertisement for prefix 2001:db8::/32
   from S1 a negative route is stored in the RIB (indicated by a ~
   sign), while the more specific routes to the complementing ToF nodes
   are installed in FIB.  RIB and FIB in T1 now look as illustrated in
   Figure 20 and Figure 21, respectively:

           +---------+                 +-----------------+
           | Default | <-------------- | ~2001:db8::/32  |
           +---------+                 +-----------------+
                |                               |
                |     +--------+                |     +--------+
                +---> | Via S1 |                +---> | Via S1 |
                |     +--------+                      +--------+
                |
                |     +--------+
                +---> | Via S2 |
                |     +--------+
                |
                |     +--------+
                +---> | Via S3 |
                |     +---------+
                |
                |     +--------+
                +---> | Via S4 |
                      +--------+

       Figure 20: Abstract RIB after negative 2001:db8::/32 from S1

   The negative 2001:db8::/32 prefix entry inherits from ::/0, so the
   positive more specific routes are the complements to S1 in the set of
   next-hops for the default route.  That entry is composed of S2, S3,
   and S4, or, in other words, it uses all entries the the default route
   with a "hole punched" for S1 into them.  These are the next hops that
   are still available to reach 2001:db8::/32, now that S1 advertised
   that it will not forward 2001:db8::/32 anymore.  Ultimately, those
   resulting next-hops are installed in FIB for the more specific route
   to 2001:db8::/32 as illustrated below:

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           +---------+                  +---------------+
           | Default |                  | 2001:db8::/32 |
           +---------+                  +---------------+
                |                               |
                |     +--------+                |
                +---> | Via S1 |                |
                |     +--------+                |
                |                               |
                |     +--------+                |     +--------+
                +---> | Via S2 |                +---> | Via S2 |
                |     +--------+                |     +--------+
                |                               |
                |     +--------+                |     +--------+
                +---> | Via S3 |                +---> | Via S3 |
                |     +--------+                |     +--------+
                |                               |
                |     +--------+                |     +--------+
                +---> | Via S4 |                +---> | Via S4 |
                      +--------+                      +--------+

       Figure 21: Abstract FIB after negative 2001:db8::/32 from S1

   To illustrate matters further let us consider T1 receiving a negative
   advertisement for prefix 2001:db8:1::/48 from S2, which is stored in
   RIB again.  After the update, the RIB in T1 is illustrated in
   Figure 22:

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 +---------+        +----------------+         +------------------+
 | Default | <----- | ~2001:db8::/32 | <------ | ~2001:db8:1::/48 |
 +---------+        +----------------+         +------------------+
      |                     |                           |
      |     +--------+      |     +--------+            |
      +---> | Via S1 |      +---> | Via S1 |            |
      |     +--------+            +--------+            |
      |                                                 |
      |     +--------+                                  |     +--------+
      +---> | Via S2 |                                  +---> | Via S2 |
      |     +--------+                                        +--------+
      |
      |     +--------+
      +---> | Via S3 |
      |     +---------+
      |
      |     +--------+
      +---> | Via S4 |
            +--------+

      Figure 22: Abstract RIB after negative 2001:db8:1::/48 from S2

   Negative 2001:db8:1::/48 inherits from 2001:db8::/32 now, so the
   positive more specific routes are the complements to S2 in the set of
   next hops for 2001:db8::/32, which are S3 and S4, or, in other words,
   all entries of the parent with the negative holes "punched in" again.
   After the update, the FIB in T1 shows as illustrated in Figure 23:

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 +---------+         +---------------+         +-----------------+
 | Default |         | 2001:db8::/32 |         | 2001:db8:1::/48 |
 +---------+         +---------------+         +-----------------+
      |                     |                           |
      |     +--------+      |                           |
      +---> | Via S1 |      |                           |
      |     +--------+      |                           |
      |                     |                           |
      |     +--------+      |     +--------+            |
      +---> | Via S2 |      +---> | Via S2 |            |
      |     +--------+      |     +--------+            |
      |                     |                           |
      |     +--------+      |     +--------+            |     +--------+
      +---> | Via S3 |      +---> | Via S3 |            +---> | Via S3 |
      |     +--------+      |     +--------+            |     +--------+
      |                     |                           |
      |     +--------+      |     +--------+            |     +--------+
      +---> | Via S4 |      +---> | Via S4 |            +---> | Via S4 |
            +--------+            +--------+                  +--------+

      Figure 23: Abstract FIB after negative 2001:db8:1::/48 from S2

   Further, let us say that S3 stops advertising its service as default
   gateway.  The entry is removed from RIB as usual.  In order to update
   the FIB, it is necessary to eliminate the FIB entry for the default
   route, as well as all the FIB entries that were created for negative
   routes pointing to the RIB entry being removed (::/0).  This is done
   recursively for 2001:db8::/32 and then for, 2001:db8:1::/48.  The
   related FIB entries via S3 are removed, as illustrated in Figure 24.

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 +---------+         +---------------+         +-----------------+
 | Default |         | 2001:db8::/32 |         | 2001:db8:1::/48 |
 +---------+         +---------------+         +-----------------+
      |                     |                           |
      |     +--------+      |                           |
      +---> | Via S1 |      |                           |
      |     +--------+      |                           |
      |                     |                           |
      |     +--------+      |     +--------+            |
      +---> | Via S2 |      +---> | Via S2 |            |
      |     +--------+      |     +--------+            |
      |                     |                           |
      |                     |                           |
      |                     |                           |
      |                     |                           |
      |                     |                           |
      |     +--------+      |     +--------+            |     +--------+
      +---> | Via S4 |      +---> | Via S4 |            +---> | Via S4 |
            +--------+            +--------+                  +--------+

                 Figure 24: Abstract FIB after loss of S3

   Say that at that time, S4 would also disaggregate prefix
   2001:db8:1::/48.  This would mean that the FIB entry for
   2001:db8:1::/48 becomes a discard route, and that would be the signal
   for T1 to disaggregate prefix 2001:db8:1::/48 negatively in a
   transitive fashion with its own children.

   Finally, let us look at the case where S3 becomes available again as
   a default gateway, and a negative advertisement is received from S4
   about prefix 2001:db8:2::/48 as opposed to 2001:db8:1::/48.  Again, a
   negative route is stored in the RIB, and the more specific route to
   the complementing ToF nodes are installed in FIB.  Since
   2001:db8:2::/48 inherits from 2001:db8::/32, the positive FIB routes
   are chosen by removing S4 from S2, S3, S4.  The abstract FIB in T1
   now shows as illustrated in Figure 25:

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                                                +-----------------+
                                                | 2001:db8:2::/48 |
                                                +-----------------+
                                                        |
  +---------+       +---------------+    +-----------------+
  | Default |       | 2001:db8::/32 |    | 2001:db8:1::/48 |
  +---------+       +---------------+    +-----------------+
       |                    |                    |      |
       |     +--------+     |                    |      |     +--------+
       +---> | Via S1 |     |                    |      +---> | Via S2 |
       |     +--------+     |                    |      |     +--------+
       |                    |                    |      |
       |     +--------+     |     +--------+     |      |     +--------+
       +---> | Via S2 |     +---> | Via S2 |     |      +---> | Via S3 |
       |     +--------+     |     +--------+     |            +--------+
       |                    |                    |
       |     +--------+     |     +--------+     |     +--------+
       +---> | Via S3 |     +---> | Via S3 |     +---> | Via S3 |
       |     +--------+     |     +--------+     |     +--------+
       |                    |                    |
       |     +--------+     |     +--------+     |     +--------+
       +---> | Via S4 |     +---> | Via S4 |     +---> | Via S4 |
             +--------+            +--------+          +--------+

      Figure 25: Abstract FIB after negative 2001:db8:2::/48 from S4

4.2.7.  Optional Zero Touch Provisioning (ZTP)

   Each RIFT node can operate in zero touch provisioning (ZTP) mode,
   i.e. it has no configuration (unless it is a Top-of-Fabric at the top
   of the topology or the must operate in the topology as leaf and/or
   support leaf-2-leaf procedures) and it will fully configure itself
   after being attached to the topology.  Configured nodes and nodes
   operating in ZTP can be mixed and will form a valid topology if
   achievable.

   The derivation of the level of each node happens based on offers
   received from its neighbors whereas each node (with possibly
   exceptions of configured leaves) tries to attach at the highest
   possible point in the fabric.  This guarantees that even if the
   diffusion front reaches a node from "below" faster than from "above",
   it will greedily abandon already negotiated level derived from nodes
   topologically below it and properly peers with nodes above.

   The fabric is very consciously numbered from the top to allow for
   PoDs of different heights and minimize number of provisioning

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   necessary, in this case just a TOP_OF_FABRIC flag on every node at
   the top of the fabric.

   This section describes the necessary concepts and procedures for ZTP
   operation.

4.2.7.1.  Terminology

   The interdependencies between the different flags and the configured
   level can be somewhat vexing at first and it may take multiple reads
   of the glossary to comprehend them.

   Automatic Level Derivation:  Procedures which allow nodes without
      level configured to derive it automatically.  Only applied if
      CONFIGURED_LEVEL is undefined.

   UNDEFINED_LEVEL:  A "null" value that indicates that the level has
      not been determined and has not been configured.  Schemas normally
      indicate that by a missing optional value without an available
      defined default.

   LEAF_ONLY:  An optional configuration flag that can be configured on
      a node to make sure it never leaves the "bottom of the hierarchy".
      TOP_OF_FABRIC flag and CONFIGURED_LEVEL cannot be defined at the
      same time as this flag.  It implies CONFIGURED_LEVEL value of 0.

   TOP_OF_FABRIC flag:  Configuration flag that MUST be provided to all
      Top-of-Fabric nodes.  LEAF_FLAG and CONFIGURED_LEVEL cannot be
      defined at the same time as this flag.  It implies a
      CONFIGURED_LEVEL value.  In fact, it is basically a shortcut for
      configuring same level at all Top-of-Fabric nodes which is
      unavoidable since an initial 'seed' is needed for other ZTP nodes
      to derive their level in the topology.  The flag plays an
      important role in fabrics with multiple planes to enable
      successful negative disaggregation (Section 4.2.5.2).

   CONFIGURED_LEVEL:  A level value provided manually.  When this is
      defined (i.e. it is not an UNDEFINED_LEVEL) the node is not
      participating in ZTP.  TOP_OF_FABRIC flag is ignored when this
      value is defined.  LEAF_ONLY can be set only if this value is
      undefined or set to 0.

   DERIVED_LEVEL:  Level value computed via automatic level derivation
      when CONFIGURED_LEVEL is equal to UNDEFINED_LEVEL.

   LEAF_2_LEAF:  An optional flag that can be configured on a node to
      make sure it supports procedures defined in Section 4.3.8.  In a
      strict sense it is a capability that implies LEAF_ONLY and the

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      according restrictions.  TOP_OF_FABRIC flag is ignored when set at
      the same time as this flag.

   LEVEL_VALUE:  In ZTP case the original definition of "level" in
      Section 3.1 is both extended and relaxed.  First, level is defined
      now as LEVEL_VALUE and is the first defined value of
      CONFIGURED_LEVEL followed by DERIVED_LEVEL.  Second, it is
      possible for nodes to be more than one level apart to form
      adjacencies if any of the nodes is at least LEAF_ONLY.

   Valid Offered Level (VOL):  A neighbor's level received on a valid
      LIE (i.e. passing all checks for adjacency formation while
      disregarding all clauses involving level values) persisting for
      the duration of the holdtime interval on the LIE.  Observe that
      offers from nodes offering level value of 0 do not constitute VOLs
      (since no valid DERIVED_LEVEL can be obtained from those and
      consequently `not_a_ztp_offer` MUST be ignored).  Offers from LIEs
      with `not_a_ztp_offer` being true are not VOLs either.  If a node
      maintains parallel adjacencies to the neighbor, VOL on each
      adjacency is considered as equivalent, i.e. the newest VOL from
      any such adjacency updates the VOL received from the same node.

   Highest Available Level (HAL):  Highest defined level value seen from
      all VOLs received.

   Highest Available Level Systems (HALS):  Set of nodes offering HAL
      VOLs.

   Highest Adjacency Three Way (HAT):  Highest neighbor level of all the
      formed three way adjacencies for the node.

4.2.7.2.  Automatic SystemID Selection

   RIFT nodes require a 64 bit SystemID which SHOULD be derived as
   EUI-64 MA-L derive according to [EUI64].  The organizationally
   governed portion of this ID (24 bits) can be used to generate
   multiple IDs if required to indicate more than one RIFT instance."

   As matter of operational concern, the router MUST ensure that such
   identifier is not changing very frequently (or at least not without
   sending all its TIEs with fairly short lifetimes) since otherwise the
   network may be left with large amounts of stale TIEs in other nodes
   (though this is not necessarily a serious problem if the procedures
   described in Section 7 are implemented).

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4.2.7.3.  Generic Fabric Example

   ZTP forces us to think about miscabled or unusually cabled fabric and
   how such a topology can be forced into a "lattice" structure which a
   fabric represents (with further restrictions).  Let us consider a
   necessary and sufficient physical cabling in Figure 26.  We assume
   all nodes being in the same PoD.

          .        +---+
          .        | A |                      s   = TOP_OF_FABRIC
          .        | s |                      l   = LEAF_ONLY
          .        ++-++                      l2l = LEAF_2_LEAF
          .         | |
          .      +--+ +--+
          .      |       |
          .   +--++     ++--+
          .   | E |     | F |
          .   |   +-+   |   +-----------+
          .   ++--+ |   ++-++           |
          .    |    |    | |            |
          .    | +-------+ |            |
          .    | |  |      |            |
          .    | |  +----+ |            |
          .    | |       | |            |
          .   ++-++     ++-++           |
          .   | I +-----+ J |           |
          .   |   |     |   +-+         |
          .   ++-++     +--++ |         |
          .    | |         |  |         |
          .    +---------+ |  +------+  |
          .      |       | |         |  |
          .      +-----------------+ |  |
          .              | |       | |  |
          .             ++-++     ++-++ |
          .             | X +-----+ Y +-+
          .             |l2l|     | l |
          .             +---+     +---+

               Figure 26: Generic ZTP Cabling Considerations

   First, we must anchor the "top" of the cabling and that's what the
   TOP_OF_FABRIC flag at node A is for.  Then things look smooth until
   we have to decide whether node Y is at the same level as I, J (and as
   consequence, X is south of it) or at the same level as X.  This is
   unresolvable here until we "nail down the bottom" of the topology.
   To achieve that we choose to use in this example the leaf flags in X
   and Y.  In case where Y would not have a leaf flag it will try to

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   elect highest level offered and end up being in same level as I and
   J.

4.2.7.4.  Level Determination Procedure

   A node starting up with UNDEFINED_VALUE (i.e. without a
   CONFIGURED_LEVEL or any leaf or TOP_OF_FABRIC flag) MUST follow those
   additional procedures:

   1.  It advertises its LEVEL_VALUE on all LIEs (observe that this can
       be UNDEFINED_LEVEL which in terms of the schema is simply an
       omitted optional value).

   2.  It computes HAL as numerically highest available level in all
       VOLs.

   3.  It chooses then MAX(HAL-1,0) as its DERIVED_LEVEL.  The node then
       starts to advertise this derived level.

   4.  A node that lost all adjacencies with HAL value MUST hold down
       computation of new DERIVED_LEVEL for a short period of time
       unless it has no VOLs from southbound adjacencies.  After the
       holddown expired, it MUST discard all received offers, recompute
       DERIVED_LEVEL and announce it to all neighbors.

   5.  A node MUST reset any adjacency that has changed the level it is
       offering and is in three-way state.

   6.  A node that changed its defined level value MUST readvertise its
       own TIEs (since the new `PacketHeader` will contain a different
       level than before).  Sequence number of each TIE MUST be
       increased.

   7.  After a level has been derived the node MUST set the
       `not_a_ztp_offer` on LIEs towards all systems offering a VOL for
       HAL.

   8.  A node that changed its level SHOULD flush from its link state
       database TIEs of all other nodes, otherwise stale information may
       persist on "direction reversal", i.e.  nodes that seemed south
       are now north or east-west.  This will not prevent the correct
       operation of the protocol but could be slightly confusing
       operationally.

   A node starting with LEVEL_VALUE being 0 (i.e. it assumes a leaf
   function by being configured with the appropriate flags or has a
   CONFIGURED_LEVEL of 0) MUST follow those additional procedures:

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   1.  It computes HAT per procedures above but does NOT use it to
       compute DERIVED_LEVEL.  HAT is used to limit adjacency formation
       per Section 4.2.2.

   It MAY also follow modified procedures:

   1.  It may pick a different strategy to choose VOL, e.g.  use the VOL
       value with highest number of VOLs.  Such strategies are only
       possible since the node always remains "at the bottom of the
       fabric" while another layer could "invert" the fabric by picking
       its preferred VOL in a different fashion than always trying to
       achieve the highest viable level.

4.2.7.5.  ZTP FSM

   This section specifies the precise, normative ZTP FSM and can be
   omitted unless the reader is pursuing an implementation of the
   protocol.

   Initial state is ComputeBestOffer.

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  Enter
    |
    v
+------------------+
| ComputeBestOffer |
|                  |<----+
| Entry:           |     | BetterHAL [LEVEL_COMPUTE]
| [LEVEL_COMPUTE]  |     | BetterHAT [LEVEL_COMPUTE]
|                  |     | ChangeLocalConfiguredLevel [StoreConfigLevel,
|                  |     |                             LEVEL_COMPUTE]
|                  |     | ChangeLocalHierarchyIndications
|                  |     |   [StoreLeafFlags,
|                  |     |    LEVEL_COMPUTE]
|                  |     | LostHAT [LEVEL_COMPUTE]
|                  |     | NeighborOffer [IF NoLevelOffered
|                  |     |                  THEN REMOVE_OFFER
|                  |     |                  ELSE IF OfferedLevel > Leaf
|                  |     |                    THEN UPDATE_OFFER
|                  |     |                    ELSE REMOVE_OFFER
|                  |     | ShortTic [RemoveExpiredOffers]
|                  |-----+
|                  |
|                  |<---------------------
|                  |---------------------> (UpdatingClients)
|                  | ComputationDone [-]
+------------------+
    ^   |
    |   | LostHAL [IF AnySouthBoundAdjacenciesPresent
    |   |            THEN UpdateHoldDownTimerToNormalValue
    |   |            ELSE FireHoldDownTimerImmediately]
    |   V
(HoldingDown)

                                ZTP FSM FSM

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 (ComputeBestOffer)
     |   ^
     |   | ChangeLocalConfiguredLevel [StoreConfiguredLevel]
     |   | ChangeLocalHierarchyIndications [StoreLeafFlags]
     |   | HoldDownExpired [PURGE_OFFERS]
     V   |
 +------------------+
 | HoldingDown      |
 |                  |<----+
 |                  |     | BetterHAL [-]
 |                  |     | BetterHAT [-]
 |                  |     | ComputationDone [-]
 |                  |     | LostHAL [-]
 |                  |     | LostHat [-]
 |                  |     | NeighborOffer [IF NoLevelOffered
 |                  |     |                  THEN REMOVE_OFFER
 |                  |     |                  ELSE IF OfferedLevel > Leaf
 |                  |     |                    THEN UPDATE_OFFER
 |                  |     |                    ELSE REMOVE_OFFER
 |                  |     | ShortTic [RemoveExpiredOffers,
 |                  |     |           IF HoldDownTimer expired
 |                  |     |             THEN PUSH HoldDownExpired]
 |                  |-----+
 +------------------+
     ^
     |
   (UpdatingClients)

                          ZTP FSM FSM (continued)

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 (ComputeBestOffer)
     |   ^
     |   | BetterHAL [-]
     |   | BetterHAT [-]
     |   | LostHAT [-]
     |   | ChangeLocalHierarchyIndications [StoreLeafFlags]
     |   | ChangeLocalConfiguredLevel [StoreConfigLevel]
     V   |
 +------------------+
 | UpdatingClients  |
 |                  |<----+
 | Entry:           |     |
 | [UpdateAllLIE-   |     | NeighborOffer [IF NoLevelOffered
 |  FSMsWith-       |     |                  THEN REMOVE_OFFER
 |  Computation-    |     |                  ELSE IF OfferedLevel > Leaf
 |  Results]        |     |                    THEN UPDATE_OFFER
 |                  |     |                    ELSE REMOVE_OFFER
 |                  |     | ShortTic [RemoveExpiredOffers]
 |                  |-----+
 +------------------+
     |
     | LostHAL [IF AnySouthBoundAdjacenciesPresent
     |            THEN UpdateHoldDownTimerToNormalValue
     |            ELSE FireHoldDownTimerImmediately]
     V
 (HoldingDown)

                          ZTP FSM FSM (continued)

   Events

   o  ChangeLocalHierarchyIndications: node locally configured with new
      leaf flags

   o  ChangeLocalConfiguredLevel: node locally configured with a defined
      level

   o  NeighborOffer: a new neighbor offer with optional level and
      neighbor state

   o  BetterHAL: better HAL computed internally

   o  BetterHAT: better HAT computed internally

   o  LostHAL: lost last HAL in computation

   o  LostHAT: lost HAT in computation

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   o  ComputationDone: computation performed

   o  HoldDownExpired: holddown expired

   o  ShortTic: one second timer tick, to be ignored if transition does
      not exist

   Actions

      on ShortTic in HoldingDown finishes in HoldingDown: remove expired
      offers and if holddown timer expired PUSH_EVENT HoldDownExpired

      on ShortTic in ComputeBestOffer finishes in ComputeBestOffer:
      remove expired offers

      on HoldDownExpired in HoldingDown finishes in ComputeBestOffer:
      PURGE_OFFERS

      on ChangeLocalConfiguredLevel in HoldingDown finishes in
      ComputeBestOffer: store configured level

      on ShortTic in UpdatingClients finishes in UpdatingClients: remove
      expired offers

      on BetterHAT in ComputeBestOffer finishes in ComputeBestOffer:
      LEVEL_COMPUTE

      on BetterHAL in HoldingDown finishes in HoldingDown: no action

      on ChangeLocalHierarchyIndications in HoldingDown finishes in
      ComputeBestOffer: store leaf flags

      on BetterHAT in UpdatingClients finishes in ComputeBestOffer: no
      action

      on BetterHAL in UpdatingClients finishes in ComputeBestOffer: no
      action

      on ChangeLocalHierarchyIndications in UpdatingClients finishes in
      ComputeBestOffer: store leaf flags

      on LostHAL in HoldingDown finishes in HoldingDown:

      on LostHAT in ComputeBestOffer finishes in ComputeBestOffer:
      LEVEL_COMPUTE

      on LostHAT in HoldingDown finishes in HoldingDown: no action

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      on BetterHAT in HoldingDown finishes in HoldingDown: no action

      on NeighborOffer in UpdatingClients finishes in UpdatingClients:

         if no level offered then REMOVE_OFFER

         else

            if offered level > leaf then UPDATE_OFFER

            else REMOVE_OFFER

      on LostHAL in ComputeBestOffer finishes in HoldingDown: if any
      southbound adjacencies present then update holddown timer to
      normal duration else fire holddown timer immediately

      on LostHAL in UpdatingClients finishes in HoldingDown: if any
      southbound adjacencies present then update holddown timer to
      normal duration else fire holddown timer immediately

      on ComputationDone in ComputeBestOffer finishes in
      UpdatingClients: no action

      on LostHAT in UpdatingClients finishes in ComputeBestOffer: no
      action

      on ComputationDone in HoldingDown finishes in HoldingDown:

      on ChangeLocalConfiguredLevel in ComputeBestOffer finishes in
      ComputeBestOffer: store configured level and LEVEL_COMPUTE

      on ChangeLocalConfiguredLevel in UpdatingClients finishes in
      ComputeBestOffer: store configured level

      on NeighborOffer in ComputeBestOffer finishes in ComputeBestOffer:

         if no level offered then REMOVE_OFFER

         else

            if offered level > leaf then UPDATE_OFFER

            else REMOVE_OFFER

      on NeighborOffer in HoldingDown finishes in HoldingDown:

         if no level offered then REMOVE_OFFER

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         else

            if offered level > leaf then UPDATE_OFFER

            else REMOVE_OFFER

      on ChangeLocalHierarchyIndications in ComputeBestOffer finishes in
      ComputeBestOffer: store leaf flags and LEVEL_COMPUTE

      on BetterHAL in ComputeBestOffer finishes in ComputeBestOffer:
      LEVEL_COMPUTE

      on Entry into UpdatingClients: update all LIE FSMs with
      computation results

      on Entry into ComputeBestOffer: LEVEL_COMPUTE

   Following words are used for well known procedures:

   1.  PUSH Event: pushes an event to be executed by the FSM upon exit
       of this action

   2.  COMPARE_OFFERS: checks whether based on current offers and held
       last results the events BetterHAL/LostHAL/BetterHAT/LostHAT are
       necessary and returns them

   3.  UPDATE_OFFER: store current offer with adjancency holdtime as
       lifetime and COMPARE_OFFERS, then PUSH according events

   4.  LEVEL_COMPUTE: compute best offered or configured level and HAL/
       HAT, if anything changed PUSH ComputationDone

   5.  REMOVE_OFFER: remove the according offer and COMPARE_OFFERS, PUSH
       according events

   6.  PURGE_OFFERS: REMOVE_OFFER for all held offers, COMPARE OFFERS,
       PUSH according events

4.2.7.6.  Resulting Topologies

   The procedures defined in Section 4.2.7.4 will lead to the RIFT
   topology and levels depicted in Figure 27.

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                      .        +---+
                      .        | As|
                      .        | 24|
                      .        ++-++
                      .         | |
                      .      +--+ +--+
                      .      |       |
                      .   +--++     ++--+
                      .   | E |     | F |
                      .   | 23+-+   | 23+-----------+
                      .   ++--+ |   ++-++           |
                      .    |    |    | |            |
                      .    | +-------+ |            |
                      .    | |  |      |            |
                      .    | |  +----+ |            |
                      .    | |       | |            |
                      .   ++-++     ++-++           |
                      .   | I +-----+ J |           |
                      .   | 22|     | 22|           |
                      .   ++--+     +--++           |
                      .    |           |            |
                      .    +---------+ |            |
                      .              | |            |
                      .             ++-++     +---+ |
                      .             | X |     | Y +-+
                      .             | 0 |     | 0 |
                      .             +---+     +---+

              Figure 27: Generic ZTP Topology Autoconfigured

   In case we imagine the LEAF_ONLY restriction on Y is removed the
   outcome would be very different however and result in Figure 28.
   This demonstrates basically that auto configuration makes miscabling
   detection hard and with that can lead to undesirable effects in cases
   where leaves are not "nailed" by the accordingly configured flags and
   arbitrarily cabled.

   A node MAY analyze the outstanding level offers on its interfaces and
   generate warnings when its internal ruleset flags a possible
   miscabling.  As an example, when a node's sees ZTP level offers that
   differ by more than one level from its chosen level (with proper
   accounting for leaf's being at level 0) this can indicate miscabling.

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                       .        +---+
                       .        | As|
                       .        | 24|
                       .        ++-++
                       .         | |
                       .      +--+ +--+
                       .      |       |
                       .   +--++     ++--+
                       .   | E |     | F |
                       .   | 23+-+   | 23+-------+
                       .   ++--+ |   ++-++       |
                       .    |    |    | |        |
                       .    | +-------+ |        |
                       .    | |  |      |        |
                       .    | |  +----+ |        |
                       .    | |       | |        |
                       .   ++-++     ++-++     +-+-+
                       .   | I +-----+ J +-----+ Y |
                       .   | 22|     | 22|     | 22|
                       .   ++-++     +--++     ++-++
                       .    | |         |       | |
                       .    | +-----------------+ |
                       .    |           |         |
                       .    +---------+ |         |
                       .              | |         |
                       .             ++-++        |
                       .             | X +--------+
                       .             | 0 |
                       .             +---+

              Figure 28: Generic ZTP Topology Autoconfigured

4.2.8.  Stability Considerations

   The autoconfiguration mechanism computes a global maximum of levels
   by diffusion.  The achieved equilibrium can be disturbed massively by
   all nodes with highest level either leaving or entering the domain
   (with some finer distinctions not explained further).  It is
   therefore recommended that each node is multi-homed towards nodes
   with respective HAL offerings.  Fortunately, this is the natural
   state of things for the topology variants considered in RIFT.

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4.3.  Further Mechanisms

4.3.1.  Overload Bit

   The overload Bit MUST be respected in all according reachability
   computations.  A node with overload bit set SHOULD NOT advertise any
   reachability prefixes southbound except locally hosted ones.  A node
   in overload SHOULD advertise all its locally hosted prefixes north
   and southbound.

   The leaf node SHOULD set the 'overload' bit on its node TIEs, since
   if the spine nodes were to forward traffic not meant for the local
   node, the leaf node does not have the topology information to prevent
   a routing/forwarding loop.

4.3.2.  Optimized Route Computation on Leaves

   Since the leaves do see only "one hop away" they do not need to run a
   "proper" SPF.  Instead, they can gather the available prefix
   candidates from their neighbors and build the routing table
   accordingly.

   A leaf will have no North TIEs except its own and optionally from its
   East-West neighbors.  A leaf will have South TIEs from its neighbors.

   Instead of creating a network graph from its North TIEs and
   neighbor's South TIEs and then running an SPF, a leaf node can simply
   compute the minimum cost and next_hop_set to each leaf neighbor by
   examining its local adjacencies, determining bi-directionality from
   the associated North TIE, and specifying the neighbor's next_hop_set
   set and cost from the minimum cost local adjacency to that neighbor.

   Then a leaf attaches prefixes as described in Section 4.2.6.

4.3.3.  Mobility

   It is a requirement for RIFT to maintain at the control plane a real
   time status of which prefix is attached to which port of which leaf,
   even in a context of mobility where the point of attachment may
   change several times in a subsecond period of time.

   There are two classical approaches to maintain such knowledge in an
   unambiguous fashion:

   time stamp:  With this method, the infrastructure records the precise
      time at which the movement is observed.  One key advantage of this
      technique is that it has no dependency on the mobile device.  One
      drawback is that the infrastructure must be precisely synchronized

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      to be able to compare time stamps as observed by the various
      points of attachment, e.g., using the variation of the Precision
      Time Protocol (PTP) IEEE Std. 1588 [IEEEstd1588], [IEEEstd8021AS]
      designed for bridged LANs IEEE Std. 802.1AS [IEEEstd8021AS].  Both
      the precision of the synchronization protocol and the resolution
      of the time stamp must beat the highest possible roaming time on
      the fabric.  Another drawback is that the presence of the mobile
      device may be observed only asynchronously, e.g., after it starts
      using an IP protocol such as ARP [RFC0826], IPv6 Neighbor
      Discovery [RFC4861][RFC4862], or DHCP [RFC2131][RFC8415].

   sequence counter:  With this method, a mobile node notifies its point
      of attachment on arrival with a sequence counter that is
      incremented upon each movement.  On the positive side, this method
      does not have a dependency on a precise sense of time, since the
      sequence of movements is kept in order by the device.  The
      disadvantage of this approach is the lack of support for protocols
      that may be used by the mobile node to register its presence to
      the leaf node with the capability to provide a sequence counter.
      Well-known issues with wrapping sequence counters must be
      addressed properly, and many forms of sequence counters that vary
      in both wrapping rules and comparison rules.  A particular
      knowledge of the source of the sequence counter is required to
      operate it, and the comparison between sequence counters from
      heterogeneous sources can be hard to impossible.

   RIFT supports a hybrid approach contained in an optional
   `PrefixSequenceType` prefix attribute that we call a `monotonic
   clock` consisting of a timestamp and optional sequence number.  In
   case of presence of the attribute:

   o  The leaf node MAY advertise a time stamp of the latest sighting of
      a prefix, e.g., by snooping IP protocols or the node using the
      time at which it advertised the prefix.  RIFT transports the time
      stamp within the desired prefix North TIEs as 802.1AS timestamp.

   o  RIFT may interoperate with the "update to 6LoWPAN Neighbor
      Discovery" [RFC8505], which provides a method for registering a
      prefix with a sequence counter called a Transaction ID (TID).
      RIFT transports in such case the TID in its native form.

   o  RIFT also defines an abstract negative clock (ASNC) that compares
      as less than any other clock.  By default, the lack of a
      `PrefixSequenceType` in a Prefix North TIE is interpreted as ASNC.
      We call this also an `undefined` clock.

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   o  Any prefix present on the fabric in multiple nodes that has the
      `same` clock is considered as anycast.  ASNC is always considered
      smaller than any defined clock.

   o  RIFT implementation assumes by default that all nodes are being
      synchronized to 200 milliseconds precision which is easily
      achievable even in very large fabrics using [RFC5905].  An
      implementation MAY provide a way to reconfigure a domain to a
      different value.  We call this variable MAXIMUM_CLOCK_DELTA.

4.3.3.1.  Clock Comparison

   All monotonic clock values are comparable to each other using the
   following rules:

   1.  ASNC is older than any other value except ASNC AND

   2.  Clock with timestamp differing by more than MAXIMUM_CLOCK_DELTA
       are comparable by using the timestamps only AND

   3.  Clocks with timestamps differing by less than MAXIMUM_CLOCK_DELTA
       are comparable by using their TIDs only AND

   4.  An undefined TID is always older than any other TID AND

   5.  TIDs are compared using rules of [RFC8505].

4.3.3.2.  Interaction between Time Stamps and Sequence Counters

   For slow movements that occur less frequently than e.g. once per
   second, the time stamp that the RIFT infrastructure captures is
   enough to determine the freshest discovery.  If the point of
   attachment changes faster than the maximum drift of the time stamping
   mechanism (i.e.  MAXIMUM_CLOCK_DELTA), then a sequence counter is
   required to add resolution to the freshness evaluation, and it must
   be sized so that the counters stay comparable within the resolution
   of the time sampling mechanism.

   The sequence counter in [RFC8505] is encoded as one octet and wraps
   around using Appendix A.

   Within the resolution of MAXIMUM_CLOCK_DELTA the sequence counters
   captured during 2 sequential values of the time stamp SHOULD be
   comparable.  This means with default values that a node may move up
   to 127 times during a 200 milliseconds period and the clocks remain
   still comparable thus allowing the infrastructure to assert the
   freshest advertisement with no ambiguity.

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4.3.3.3.  Anycast vs. Unicast

   A unicast prefix can be attached to at most one leaf, whereas an
   anycast prefix may be reachable via more than one leaf.

   If a monotonic clock attribute is provided on the prefix, then the
   prefix with the `newest` clock value is strictly preferred.  An
   anycast prefix does not carry a clock or all clock attributes MUST be
   the same under the rules of Section 4.3.3.1.

   Observe that it is important that in mobility events the leaf is re-
   flooding as quickly as possible the absence of the prefix that moved
   away.

   Observe further that without support for [RFC8505] movements on the
   fabric within intervals smaller than 100msec will be seen as anycast.

4.3.3.4.  Overlays and Signaling

   RIFT is agnostic whether any overlay technology like [MIP, LISP,
   VxLAN, NVO3] and the associated signaling is deployed over it.  But
   it is expected that leaf nodes, and possibly Top-of-Fabric nodes can
   perform the correct encapsulation.

   In the context of mobility, overlays provide a classical solution to
   avoid injecting mobile prefixes in the fabric and improve the
   scalability of the solution.  It makes sense on a data center that
   already uses overlays to consider their applicability to the mobility
   solution; as an example, a mobility protocol such as LISP may inform
   the ingress leaf of the location of the egress leaf in real time.

   Another possibility is to consider that mobility as an underlay
   service and support it in RIFT to an extent.  The load on the fabric
   augments with the amount of mobility obviously since a move forces
   flooding and computation on all nodes in the scope of the move so
   tunneling from leaf to the Top-of-Fabric may be desired.

4.3.4.  Key/Value Store

4.3.4.1.  Southbound

   The protocol supports a southbound distribution of key-value pairs
   that can be used to e.g. distribute configuration information during
   topology bring-up.  The KV South TIEs can arrive from multiple nodes
   and hence need tie-breaking per key.  We use the following rules

   1.  Only KV TIEs originated by nodes to which the receiver has a bi-
       directional adjacency are considered.

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   2.  Within all such valid KV South TIEs containing the key, the value
       of the KV South TIE for which the according node South TIE is
       present, has the highest level and within the same level has
       highest originating system ID is preferred.  If keys in the most
       preferred TIEs are overlapping, the behavior is undefined.

   Observe that if a node goes down, the node south of it looses
   adjacencies to it and with that the KVs will be disregarded and on
   tie-break changes new KV re-advertised to prevent stale information
   being used by nodes further south.  KV information in southbound
   direction is not result of independent computation of every node over
   same set of TIEs but a diffused computation.

4.3.4.2.  Northbound

   Certain use cases seem to necessitate distribution of essentially KV
   information that is generated in the leaves in the northbound
   direction.  Such information is flooded in KV North TIEs.  Since the
   originator of northbound KV is preserved during northbound flooding,
   overlapping keys could be used.  However, to omit further protocol
   complexity, only the value of the key in TIE tie-broken in same
   fashion as southbound KV TIEs is used.

4.3.5.  Interactions with BFD

   RIFT MAY incorporate BFD [RFC5881] to react quickly to link failures.
   In such case following procedures are introduced:

      After RIFT three-way hello adjacency convergence a BFD session MAY
      be formed automatically between the RIFT endpoints without further
      configuration using the exchanged discriminators.  The capability
      of the remote side to support BFD is carried on the LIEs.

      In case established BFD session goes Down after it was Up, RIFT
      adjacency SHOULD be re-initialized and subsequently started from
      Init after it sees a consecutive BFD Up.

      In case of parallel links between nodes each link MAY run its own
      independent BFD session or they may share a session.

      In case RIFT changes link identifiers or BFD capability indication
      both the LIE as well as the BFD sessions SHOULD be brought down
      and back up again.

      Multiple RIFT instances MAY choose to share a single BFD session
      (in such case it is undefined what discriminators are used albeit
      RIFT MAY advertise the same link ID for the same interface in
      multiple instances and with that "share" the discriminators).

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      BFD TTL follows [RFC5082].

4.3.6.  Fabric Bandwidth Balancing

   A well understood problem in fabrics is that in case of link losses
   it would be ideal to rebalance how much traffic is offered to
   switches in the next level based on the ingress and egress bandwidth
   they have.  Current attempts rely mostly on specialized traffic
   engineering via controller or leaves being aware of complete topology
   with according cost and complexity.

   RIFT can support a very light weight mechanism that can deal with the
   problem in an approximate way based on the fact that RIFT is loop-
   free.

4.3.6.1.  Northbound Direction

   Every RIFT node SHOULD compute the amount of northbound bandwidth
   available through neighbors at higher level and modify distance
   received on default route from this neighbor.  Those different
   distances SHOULD be used to support weighted ECMP forwarding towards
   higher level when using default route.  We call such a distance
   Bandwidth Adjusted Distance or BAD.  This is best illustrated by a
   simple example.

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                             .   100  x             100 100 MBits
                             .    |   x              |   |
                             .  +-+---+-+          +-+---+-+
                             .  |       |          |       |
                             .  |Spin111|          |Spin112|
                             .  +-+---+++          ++----+++
                             .    |x  ||           ||    ||
                             .    ||  |+---------------+ ||
                             .    ||  +---------------+| ||
                             .    ||               || || ||
                             .    ||               || || ||
                             .   -----All Links 10 MBit-------
                             .    ||               || || ||
                             .    ||               || || ||
                             .    ||  +------------+| || ||
                             .    ||  |+------------+ || ||
                             .    |x  ||              || ||
                             .  +-+---+++          +--++-+++
                             .  |       |          |       |
                             .  |Leaf111|          |Leaf112|
                             .  +-------+          +-------+

                      Figure 29: Balancing Bandwidth

   All links from leaves in Figure 29 are assumed to 10 MBit/s bandwidth
   while the uplinks one level further up are assumed to be 100 MBit/s.
   Further, in Figure 29 we assume that Leaf111 lost one of the parallel
   links to Spine 111 and with that wants to possibly push more traffic
   onto Spine 112.  Leaf 112 has equal bandwidth to Spine 111 and Spine
   112 but Spine 111 lost one of its uplinks.

   The local modification of the received default route distance from
   upper level is achieved by running a relatively simple algorithm
   where the bandwidth is weighted exponentially while the distance on
   the default route represents a multiplier for the bandwidth weight
   for easy operational adjustments.

   On a node L use Node TIEs to compute for each non-overloaded
   northbound neighbor N three values:

      L_N_u: as sum of the bandwidth available to N

      N_u: as sum of the uplink bandwidth available on N

      T_N_u: as sum of L_N_u * OVERSUBSCRIPTION_CONSTANT + N_u

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   For all T_N_u determine the according M_N_u as
   log_2(next_power_2(T_N_u)) and determine MAX_M_N_u as maximum value
   of all M_N_u.

   For each advertised default route from a node N modify the advertised
   distance D to BAD = D * (1 + MAX_M_N_u - M_N_u) and use BAD instead
   of distance D to weight balance default forwarding towards N.

   For the example above a simple table of values will help the
   understanding.  We assume the default route distance is advertised
   with D=1 everywhere and OVERSUBSCRIPTION_CONSTANT = 1.

               +---------+-----------+-------+-------+-----+
               | Node    | N         | T_N_u | M_N_u | BAD |
               +---------+-----------+-------+-------+-----+
               | Leaf111 | Spine 111 | 110   | 7     | 2   |
               +---------+-----------+-------+-------+-----+
               | Leaf111 | Spine 112 | 220   | 8     | 1   |
               +---------+-----------+-------+-------+-----+
               | Leaf112 | Spine 111 | 120   | 7     | 2   |
               +---------+-----------+-------+-------+-----+
               | Leaf112 | Spine 112 | 220   | 8     | 1   |
               +---------+-----------+-------+-------+-----+

                         Table 5: BAD Computation

   If a calculation produces a result exceeding the range of the type,
   e.g. bandwidth, the result is set to the highest possible value for
   that type.

   BAD is only computed for default routes.  A node MAY compute and use
   BAD for any disaggregated prefixes or other RIFT routes.  A node MAY
   use another algorithm than BAD to weight northbound traffic based on
   bandwidth given that the algorithm is distributed and un-synchronized
   and ultimately, its correct behavior does not depend on uniformity of
   balancing algorithms used in the fabric.  E.g. it is conceivable that
   leaves could use real time link loads gathered by analytics to change
   the amount of traffic assigned to each default route next hop.

   Observe further that a change in available bandwidth will only affect
   at maximum two levels down in the fabric, i.e. blast radius of
   bandwidth changes is contained no matter its height.

4.3.6.2.  Southbound Direction

   Due to its loop free properties a node MAY take during S-SPF into
   account the available bandwidth on the nodes in lower levels and
   modify the amount of traffic offered to next level's "southbound"

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   nodes based as what it sees is the total achievable maximum flow
   through those nodes.  It is worth observing that such computations
   may work better if standardized but does not have to be necessarily.
   As long the packet keeps on heading south it will take one of the
   available paths and arrive at the intended destination.

4.3.7.  Label Binding

   A node MAY advertise on its LIEs a locally significant, downstream
   assigned, interface specific label.  One use of such label is a hop-
   by-hop encapsulation allowing to easily distinguish forwarding planes
   served by a multiplicity of RIFT instances.

4.3.8.  Leaf to Leaf Procedures

   RIFT can optionally allow special leaf East-West adjacencies under
   additional set of rules.  The leaf supporting those procedures MUST:

      advertise the LEAF_2_LEAF flag in node capabilities AND

      set the overload bit on all leaf's node TIEs AND

      flood only node's own north and south TIEs over E-W leaf
      adjacencies AND

      always use E-W leaf adjacency in both north as well as south
      computation AND

      install a discard route for any advertised aggregate in leaf's
      TIEs AND

      never form southbound adjacencies.

   This will allow the E-W leaf nodes to exchange traffic strictly for
   the prefixes advertised in each other's north prefix TIEs (since the
   southbound computation will find the reverse direction in the other
   node's TIE and install its north prefixes).

4.3.9.  Address Family and Multi Topology Considerations

   Multi-Topology (MT)[RFC5120] and Multi-Instance (MI)[RFC8202] is used
   today in link-state routing protocols to support several domains on
   the same physical topology.  RIFT supports this capability by
   carrying transport ports in the LIE protocol exchanges.  Multiplexing
   of LIEs can be achieved by either choosing varying multicast
   addresses or ports on the same address.

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   BFD interactions in Section 4.3.5 are implementation dependent when
   multiple RIFT instances run on the same link.

4.3.10.  Reachability of Internal Nodes in the Fabric

   RIFT does not precondition that its nodes have reachable addresses
   albeit for operational purposes this is clearly desirable.  Under
   normal operating conditions this can be easily achieved by e.g.
   injecting the node's loopback address into North and South Prefix
   TIEs or other implementation specific mechanisms.

   Things get more interesting in case a node looses all its northbound
   adjacencies but is not at the top of the fabric.  That is outside the
   scope of this document and may be covered in a separate document.

4.3.11.  One-Hop Healing of Levels with East-West Links

   Based on the rules defined in Section 4.2.4, Section 4.2.3.8 and
   given presence of E-W links, RIFT can provide a one-hop protection of
   nodes that lost all their northbound links or in other complex link
   set failure scenarios except at Top-of-Fabric where the links are
   used exclusively to flood topology information in multi-plane
   designs.  Section 5.4 explains the resulting behavior based on one
   such example.

4.4.  Security

4.4.1.  Security Model

   An inherent property of any security and ZTP architecture is the
   resulting trade-off in regard to integrity verification of the
   information distributed through the fabric vs. necessary provisioning
   and auto-configuration.  At a minimum, in all approaches, the
   security of an established adjacency can be ensured.  The stricter
   the security model the more provisioning must take over the role of
   ZTP.

   The most security conscious operators will want to have full control
   over which port on which router/switch is connected to the respective
   port on the "other side", which we will call the "port-association
   model" (PAM) achievable e.g. by configuring on each port pair a
   designated shared key or pair of private/public keys.  In secure data
   center locations, operators may want to control which router/switch
   is connected to which other router/switch only or choose a "node-
   association model" (NAM) which allows, for example, simplified port
   sparing.  In an even more relaxed environment, an operator may only
   be concerned that the router/switches share credentials ensuring that
   they belong to this particular data center network hence allowing the

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   flexible sparing of whole routers/switches.  We will define that case
   as the "fabric-association model" (FAM), equivalent to using a shared
   secret for the whole fabric.  Such flexibility may make sense for
   leaf nodes such as servers where the addition and swapping of servers
   is more frequent than the rest of the data center network.
   Generally, leaves of the fabric tend to be less trusted than
   switches.  The different models could be mixed throughout the fabric
   if the benefits outweigh the cost of increased complexity in
   provisioning.

   In each of the above cases, some configuration mechanism is needed to
   allow the operator to specify which connections are allowed, and some
   mechanism is needed to:

   a.  specify the according level in the fabric,

   b.  discover and report missing connections,

   c.  discover and report unexpected connections, and prevent such
       adjacencies from forming.

   On the more relaxed configuration side of the spectrum, operators
   might only configure the level of each switch, but don't explicitly
   configure which connections are allowed.  In this case, RIFT will
   only allow adjacencies to come up between nodes are that in adjacent
   levels.  The operators with lowest security requirements may not use
   any configuration to specify which connections are allowed.  Such
   fabrics could rely fully on ZTP for each router/switch to discover
   its level and would only allow adjacencies between adjacent levels to
   come up.  Figure 30 illustrates the tradeoffs inherent in the
   different security models.

   Ultimately, some level of verification of the link quality may be
   required before an adjacency is allowed to be used for forwarding.
   For example, an implementation may require that a BFD session comes
   up before advertising the adjacency.

   For the above outlined cases, RIFT has two approaches to enforce that
   a local port is connected to the correct port on the correct remote
   router/switch.  One approach is to piggy-back on RIFT's
   authentication mechanism.  Assuming the provisioning model (e.g. the
   YANG model) is flexible enough, operators can choose to provision a
   unique authentication key for:

   a.  each pair of ports in "port-association model" or

   b.  each pair of switches in "node-association model" or

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   c.  each pair of levels or

   d.  the entire fabric in "fabric-association model".

   The other approach is to rely on the system-id, port-id and level
   fields in the LIE message to validate an adjacency against the
   configured expected cabling topology, and optionally introduce some
   new rules in the FSM to allow the adjacency to come up if the
   expectations are met.

                   ^                 /\                  |
                  /|\               /  \                 |
                   |               /    \                |
                   |              / PAM  \               |
               Increasing        /        \          Increasing
               Integrity        +----------+         Flexibility
                   &           /    NAM     \            &
              Increasing      +--------------+         Less
              Provisioning   /      FAM       \     Configuration
                   |        +------------------+         |
                   |       / Level Provisioning \        |
                   |      +----------------------+      \|/
                   |     /    Zero Configuration  \      v
                        +--------------------------+

                         Figure 30: Security Model

4.4.2.  Security Mechanisms

   RIFT Security goals are to ensure authentication, message integrity
   and prevention of replay attacks.  Low processing overhead and
   efficient messaging are also a goal.  Message confidentiality is a
   non-goal.

   The model in the previous section allows a range of security key
   types that are analogous to the various security association models.
   PAM and NAM allow security associations at the port or node level
   using symmetric or asymmetric keys that are pre-installed.  FAM
   argues for security associations to be applied only at a group level
   or to be refined once the topology has been established.  RIFT does
   not specify how security keys are installed or updated it specifies
   how the key can be used to achieve goals.

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   The protocol has provisions for "weak" nonces to prevent replay
   attacks and includes authentication mechanisms comparable to
   [RFC5709] and [RFC7987].

4.4.3.  Security Envelope

   RIFT MUST be carried in a mandatory secure envelope illustrated in
   Figure 31.  Any value in the packet following a security fingerprint
   MUST be used only after the according fingerprint has been validated.

   Local configuration MAY allow to skip the checking of the envelope's
   integrity.

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

      UDP Header:
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Source Port         |       RIFT destination port   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           UDP Length          |        UDP Checksum           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Outer Security Envelope Header:
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           RIFT MAGIC          |         Packet Number         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Reserved   |  RIFT Major   | Outer Key ID  | Fingerprint   |
      |               |    Version    |               |    Length     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~       Security Fingerprint covers all following content       ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Weak Nonce Local              | Weak Nonce Remote             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Remaining TIE Lifetime (all 1s in case of LIE)     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      TIE Origin Security Envelope Header:
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              TIE Origin Key ID                |  Fingerprint  |
      |                                               |    Length     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~       Security Fingerprint covers all following content       ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Serialized RIFT Model Object
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                Serialized RIFT Model Object                   ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 31: Security Envelope

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   RIFT MAGIC:  16 bits.  Constant value of 0xA1F7 that allows to
      classify RIFT packets independent of used UDP port.

   Packet Number:  16 bits.  An optional, per packet type monotonically
      growing number rolling over using sequence number arithmetic
      defined inAppendix A.  A node SHOULD correctly set the number on
      subsequent packets or otherwise MUST set the value to
      `undefined_packet_number` as provided in the schema.  This number
      can be used to detect losses and misordering in flooding for
      either operational purposes or in implementation to adjust
      flooding behavior to current link or buffer quality.  This number
      MUST NOT be used to discard or validate the correctness of
      packets.

   RIFT Major Version:  8 bits.  It allows to check whether protocol
      versions are compatible, i.e. the serialized object can be decoded
      at all.  An implementation MUST drop packets with unexpected value
      and MAY report a problem.  Must be same as in encoded model
      object, otherwise packet is dropped.

   Outer Key ID:  8 bits to allow key rollovers.  This implies key type
      and used algorithm.  Value 0 means that no valid fingerprint was
      computed.  This key ID scope is local to the nodes on both ends of
      the adjacency.

   TIE Origin Key ID:  24 bits.  This implies key type and used
      algorithm.  Value 0 means that no valid fingerprint was computed.
      This key ID scope is global to the RIFT instance since it implies
      the originator of the TIE so the contained object does not have to
      be de-serialized to obtain it.

   Length of Fingerprint:  8 bits.  Length in 32-bit multiples of the
      following fingerprint not including lifetime or weak nonces.  It
      allows to navigate the structure when an unknown key type is
      present.  To clarify a common corner case when this value is set
      to 0 it signifies an empty (0 bytes long) security fingerprint.

   Security Fingerprint:  32 bits * Length of Fingerprint.  This is a
      signature that is computed over all data following after it.  If
      the significant bits of fingerprint are fewer than the 32 bits
      padded length than the significant bits MUST be left aligned and
      remaining bits on the right padded with 0s.  When using PKI the
      Security fingerprint originating node uses its private key to
      create the signature.  The original packet can then be verified
      provided the public key is shared and current.

   Remaining TIE Lifetime:  32 bits.  In case of anything but TIEs this
      field MUST be set to all ones and Origin Security Envelope Header

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      MUST NOT be present in the packet.  For TIEs this field represents
      the remaining lifetime of the TIE and Origin Security Envelope
      Header MUST be present in the packet.  The value in the serialized
      model object MUST be ignored.

   Weak Nonce Local:   16 bits.  Local Weak Nonce of the adjacency as
      advertised in LIEs.

   Weak Nonce Remote:   16 bits.  Remote Weak Nonce of the adjacency as
      received in LIEs.

   TIE Origin Security Envelope Header:  It MUST be present if and only
      if the Remaining TIE Lifetime field is NOT all ones.  It carries
      through the originators key ID and according fingerprint of the
      object to protect TIE from modification during flooding.  This
      ensures origin validation and integrity (but does not provide
      validation of a chain of trust).

   Observe that due to the schema migration rules per Appendix B the
   contained model can be always decoded if the major version matches
   and the envelope integrity has been validated.  Consequently,
   description of the TIE is available to flood it properly including
   unknown TIE types.

4.4.4.  Weak Nonces

   The protocol uses two 16 bit nonces to salt generated signatures.  We
   use the term "nonce" a bit loosely since RIFT nonces are not being
   changed on every packet as common in cryptography.  For efficiency
   purposes they are changed at a frequency high enough to dwarf replay
   attacks attempts for all practical purposes.  Therefore, we call them
   "weak" nonces.

   Any implementation including RIFT security MUST generate and wrap
   around local nonces properly.  When a nonce increment leads to
   `undefined_nonce` value the value SHOULD be incremented again
   immediately.  All implementation MUST reflect the neighbor's nonces.
   An implementation SHOULD increment a chosen nonce on every LIE FSM
   transition that ends up in a different state from the previous and
   MUST increment its nonce at least every 5 minutes (such
   considerations allow for efficient implementations without opening a
   significant security risk).  When flooding TIEs, the implementation
   MUST use recent (i.e. within allowed difference) nonces reflected in
   the LIE exchange.  The schema specifies maximum allowable nonce value
   difference on a packet compared to reflected nonces in the LIEs.  Any
   packet received with nonces deviating more than the allowed delta
   MUST be discarded without further computation of signatures to
   prevent computation load attacks.

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   In case where a secure implementation does not receive signatures or
   receives undefined nonces from neighbor indicating that it does not
   support or verify signatures, it is a matter of local policy how such
   packets are treated.  Any secure implementation MAY choose to either
   refuse forming an adjacency with an implementation not advertising
   signatures or valid nonces or simply keep on signing local packets
   while accepting neighbor's packets without further security
   verification.

   As a necessary exception, an implementation MUST advertise
   `undefined_nonce` for remote nonce value when the FSM is not in two-
   way or three-way state and accept an `undefined_nonce` for its local
   nonce value on packets in any other state than three-way.

   As optional optimization, an implementation MAY send one LIE with
   previously negotiated neighbor's nonce to try to speed up a
   neighbor's transition from three-way to one-way and MUST revert to
   sending `undefined_nonce` after that.

4.4.5.  Lifetime

   Protecting lifetime on flooding may lead to excessive number of
   security fingerprint computation and hence an application generating
   such fingerprints on TIEs MAY round the value down to the next
   `rounddown_lifetime_interval` defined in the schema when sending TIEs
   albeit such optimization in presence of security hashes over
   advancing weak nonces may not be feasible.

4.4.6.  Key Management

   As outlined in the Security Model a private shared key or a public/
   private key pair is used to Authenticate the adjacency.  The actual
   method of key distribution and key synchronization is assumed to be
   out of band from RIFT's perspective.  Both nodes in the adjacency
   must share the same keys and configuration of key type and algorithm
   for a key ID.  Mismatched keys will obviously not inter-operate due
   to unverifiable security envelope.

   Key roll-over while the adjacency is active is allowed and the
   technique is well known and described in e.g.  [RFC6518].  Key
   distribution procedures are out of scope for RIFT.

4.4.7.  Security Association Changes

   There in no mechanism to convert a security envelope for the same key
   ID from one algorithm to another once the envelope is operational.
   The recommended procedure to change to a new algorithm is to take the
   adjacency down and make the changes and then bring the adjacency up.

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   Obviously, an implementation MAY choose to stop verifying security
   envelope for the duration of key change to keep the adjacency up but
   since this introduces a security vulnerability window, such roll-over
   is not recommended.

5.  Examples

5.1.  Normal Operation

   This section describes RIFT deployment in the example topology
   without any node or link failures.  We disregard flooding reduction
   for simplicity's sake.

   As first step, the following bi-directional adjacencies will be
   created (and any other links that do not fulfill LIE rules in
   Section 4.2.2 disregarded):

   1.  ToF 21 (PoD 0) to Spine 111, Spine 112, Spine 121, and Spine 122

   2.  ToF 22 (PoD 0) to Spine 111, Spine 112, Spine 121, and Spine 122

   3.  Spine 111 to Leaf 111, Leaf 112

   4.  Spine 112 to Leaf 111, Leaf 112

   5.  Spine 121 to Leaf 121, Leaf 122

   6.  Spine 122 to Leaf 121, Leaf 122

   Consequently, North TIEs would be originated by Spine 111 and Spine
   112 and each set would be sent to both ToF 21 and ToF 22.  North TIEs
   also would be originated by Leaf 111 (w/ Prefix 111) and Leaf 112 (w/
   Prefix 112 and the multi-homed prefix) and each set would be sent to
   Spine 111 and Spine 112.  Spine 111 and Spine 112 would then flood
   these North TIEs to ToF 21 and ToF 22.

   Similarly, North TIEs would be originated by Spine 121 and Spine 122
   and each set would be sent to both ToF 21 and ToF 22.  North TIEs
   also would be originated by Leaf 121 (w/ Prefix 121 and the multi-
   homed prefix) and Leaf 122 (w/ Prefix 122) and each set would be sent
   to Spine 121 and Spine 122.  Spine 121 and Spine 122 would then flood
   these North TIEs to ToF 21 and ToF 22.

   At this point both ToF 21 and ToF 22, as well as any controller to
   which they are connected, would have the complete network topology.
   At the same time, Spine 111/112/121/122 hold only the N-ties of level
   0 of their respective PoD. leaves hold only their own North TIEs.

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   South TIEs with adjacencies and a default IP prefix would then be
   originated by ToF 21 and ToF 22 and each would be flooded to Spine
   111, Spine 112, Spine 121, and Spine 122.  Spine 111, Spine 112,
   Spine 121, and Spine 122 would each send the South TIE from ToF 21 to
   ToF 22 and the South TIE from ToF 22 to ToF 21.  (South TIEs are
   reflected up to level from which they are received but they are NOT
   propagated southbound.)

   A South TIE with a default IP prefix would be originated by Node 111
   and Spine 112 and each would be sent to Leaf 111 and Leaf 112.

   Similarly, an South TIE with a default IP prefix would be originated
   by Node 121 and Spine 122 and each would be sent to Leaf 121 and Leaf
   122.  At this point IP connectivity with maximum possible ECMP has
   been established between the leaves while constraining the amount of
   information held by each node to the minimum necessary for normal
   operation and dealing with failures.

5.2.  Leaf Link Failure

                    .  |   |              |   |
                    .+-+---+-+          +-+---+-+
                    .|       |          |       |
                    .|Spin111|          |Spin112|
                    .+-+---+-+          ++----+-+
                    .  |   |             |    |
                    .  |   +---------------+  X
                    .  |                 | |  X Failure
                    .  |   +-------------+ |  X
                    .  |   |               |  |
                    .+-+---+-+          +--+--+-+
                    .|       |          |       |
                    .|Leaf111|          |Leaf112|
                    .+-------+          +-------+
                    .      +                  +
                    .     Prefix111     Prefix112

                    Figure 32: Single Leaf link failure

   In case of a failing leaf link between spine 112 and leaf 112 the
   link-state information will cause re-computation of the necessary SPF
   and the higher levels will stop forwarding towards prefix 112 through
   spine 112.  Only spines 111 and 112, as well as both spines will see
   control traffic.  Leaf 111 will receive a new South TIE from spine
   112 and reflect back to spine 111.

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   Spine 111 will de-aggregate prefix 111 and prefix 112 but we will not
   describe it further here since de-aggregation is emphasized in the
   next example.  It is worth observing however in this example that if
   leaf 111 would keep on forwarding traffic towards prefix 112 using
   the advertised south-bound default of spine 112 the traffic would end
   up on Top-of-Fabric 21 and ToF 22 and cross back into pod 1 using
   spine 111.  This is arguably not as bad as black-holing present in
   the next example but clearly undesirable.  Fortunately, de-
   aggregation prevents this type of behavior except for a transitory
   period of time.

5.3.  Partitioned Fabric

   .                +--------+          +--------+   South TIE of ToF 21
   .                |        |          |        |   received by
   .                |ToF   21|          |ToF   22|   south reflection of
   .                ++-+--+-++          ++-+--+-++   spines 112 and 111
   .                 | |  | |            | |  | |
   .                 | |  | |            | |  | 0/0
   .                 | |  | |            | |  | |
   .                 | |  | |            | |  | |
   .  +--------------+ |  +--- XXXXXX +  | |  | +---------------+
   .  |                |    |         |  | |  |                 |
   .  |    +-----------------------------+ |  |                 |
   .  0/0  |           |    |         |    |  |                 |
   .  |    0/0       0/0    +- XXXXXXXXXXXXXXXXXXXXXXXXX -+     |
   .  |  1.1/16        |              |    |  |           |     |
   .  |    |           +-+    +-0/0-----------+           |     |
   .  |    |             |   1.1./16  |    |              |     |
   .+-+----++          +-+-----+     ++-----0/0          ++----0/0
   .|       |          |       |     |    1.1/16         |   1.1/16
   .|Spin111|          |Spin112|     |Spin121|           |Spin122|
   .+-+---+-+          ++----+-+     +-+---+-+           ++---+--+
   .  |   |             |    |         |   |              |   |
   .  |   +---------------+  |         |   +----------------+ |
   .  |                 | |  |         |                  | | |
   .  |   +-------------+ |  |         |   +--------------+ | |
   .  |   |               |  |         |   |                | |
   .+-+---+-+          +--+--+-+     +-+---+-+          +---+-+-+
   .|       |          |       |     |       |          |       |
   .|Leaf111|          |Leaf112|     |Leaf121|          |Leaf122|
   .+-+-----+          ++------+     +-----+-+          +-+-----+
   .  +                 +                  +              +
   .  Prefix111    Prefix112             Prefix121     Prefix122
   .                                       1.1/16

                        Figure 33: Fabric partition

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   Figure 33 shows the arguably a more catastrophic but also a more
   interesting case.  ToF 21 is completely severed from access to Prefix
   121 (we use in the figure 1.1/16 as example) by double link failure.
   However unlikely, if left unresolved, forwarding from leaf 111 and
   leaf 112 to prefix 121 would suffer 50% black-holing based on pure
   default route advertisements by ToF 21 and ToF 22.

   The mechanism used to resolve this scenario is hinging on the
   distribution of southbound representation by Top-of-Fabric 21 that is
   reflected by spine 111 and spine 112 to ToF 22.  ToF 22, having
   computed reachability to all prefixes in the network, advertises with
   the default route the ones that are reachable only via lower level
   neighbors that ToF 21 does not show an adjacency to.  That results in
   spine 111 and spine 112 obtaining a longest-prefix match to prefix
   121 which leads through ToF 22 and prevents black-holing through ToF
   21 still advertising the 0/0 aggregate only.

   The prefix 121 advertised by Top-of-Fabric 22 does not have to be
   propagated further towards leaves since they do no benefit from this
   information.  Hence the amount of flooding is restricted to ToF 21
   reissuing its South TIEs and south reflection of those by spine 111
   and spine 112.  The resulting SPF in ToF 22 issues a new prefix South
   TIEs containing 1.1/16.  None of the leaves become aware of the
   changes and the failure is constrained strictly to the level that
   became partitioned.

   To finish with an example of the resulting sets computed using
   notation introduced in Section 4.2.5, Top-of-Fabric 22 constructs the
   following sets:

      |R = Prefix 111, Prefix 112, Prefix 121, Prefix 122

      |H (for r=Prefix 111) = Spine 111, Spine 112

      |H (for r=Prefix 112) = Spine 111, Spine 112

      |H (for r=Prefix 121) = Spine 121, Spine 122

      |H (for r=Prefix 122) = Spine 121, Spine 122

      |A (for ToF 21) = Spine 111, Spine 112

   With that and |H (for r=prefix 121) and |H (for r=prefix 122) being
   disjoint from |A (for Top-of-Fabric 21), ToF 22 will originate an
   South TIE with prefix 121 and prefix 122, that is flooded to spines
   112, 112, 121 and 122.

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5.4.  Northbound Partitioned Router and Optional East-West Links

         .   +                  +                  +
         .   X N1               | N2               | N3
         .   X                  |                  |
         .+--+----+          +--+----+          +--+-----+
         .|       |0/0>  <0/0|       |0/0>  <0/0|        |
         .|  A01  +----------+  A02  +----------+  A03   | Level 1
         .++-+-+--+          ++--+--++          +---+-+-++
         . | | |              |  |  |               | | |
         . | | +----------------------------------+ | | |
         . | |                |  |  |             | | | |
         . | +-------------+  |  |  |  +--------------+ |
         . |               |  |  |  |  |          | |   |
         . | +----------------+  |  +-----------------+ |
         . | |             |     |     |          | | | |
         . | | +------------------------------------+ | |
         . | | |           |     |     |          |   | |
         .++-+-+--+        | +---+---+ |        +-+---+-++
         .|       |        +-+       +-+        |        |
         .|  L01  |          |  L02  |          |  L03   | Level 0
         .+-------+          +-------+          +--------+

                    Figure 34: North Partitioned Router

   Figure 34 shows a part of a fabric where level 1 is horizontally
   connected and A01 lost its only northbound adjacency.  Based on N-SPF
   rules in Section 4.2.4.1 A01 will compute northbound reachability by
   using the link A01 to A02 (whereas A02 will NOT use this link during
   N-SPF).  Hence A01 will still advertise the default towards level 0
   and route unidirectionally using the horizontal link.

   As further consideration, the moment A02 looses link N2 the situation
   evolves again.  A01 will have no more northbound reachability while
   still seeing A03 advertising northbound adjacencies in its south node
   tie.  With that it will stop advertising a default route due to
   Section 4.2.3.8.

6.  Implementation and Operation: Further Details

6.1.  Considerations for Leaf-Only Implementation

   RIFT can and is intended to be stretched to the lowest level in the
   IP fabric to integrate ToRs or even servers.  Since those entities
   would run as leaves only, it is worth to observe that a leaf only
   version is significantly simpler to implement and requires much less
   resources:

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   1.  Under normal conditions, the leaf needs to support a multipath
       default route only.  In most catastrophic partitioning case it
       has to be capable of accommodating all the leaf routes in its own
       PoD to prevent black-holing.

   2.  Leaf nodes hold only their own North TIEs and South TIEs of Level
       1 nodes they are connected to; so overall few in numbers.

   3.  Leaf node does not have to support any type of de-aggregation
       computation or propagation.

   4.  Leaf nodes do not have to support overload bit normally.

   5.  Unless optional leaf-2-leaf procedures are desired default route
       origination and South TIE origination is unnecessary.

6.2.  Considerations for Spine Implementation

   In case of spines, i.e. nodes that will never act as Top of Fabric a
   full implementation is not required, specifically the node does not
   need to perform any computation of negative disaggregation except
   respecting northbound disaggregation advertised from the north.

6.3.  Adaptations to Other Proposed Data Center Topologies

                         .  +-----+        +-----+
                         .  |     |        |     |
                         .+-+ S0  |        | S1  |
                         .| ++---++        ++---++
                         .|  |   |          |   |
                         .|  | +------------+   |
                         .|  | | +------------+ |
                         .|  | |              | |
                         .| ++-+--+        +--+-++
                         .| |     |        |     |
                         .| | A0  |        | A1  |
                         .| +-+--++        ++---++
                         .|   |  |          |   |
                         .|   |  +------------+ |
                         .|   | +-----------+ | |
                         .|   | |             | |
                         .| +-+-+-+        +--+-++
                         .+-+     |        |     |
                         .  | L0  |        | L1  |
                         .  +-----+        +-----+

                         Figure 35: Level Shortcut

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   Strictly speaking, RIFT is not limited to Clos variations only.  The
   protocol preconditions only a sense of 'compass rose direction'
   achieved by configuration (or derivation) of levels and other
   topologies are possible within this framework.  So, conceptually, one
   could include leaf to leaf links and even shortcut between levels

   As an example, short cutting levels illustrated in Figure 35 will
   lead either to suboptimal routing when L0 sends traffic to L1 (since
   using S0's default route will lead to the traffic being sent back to
   A0 or A1) or the leaves need each other's routes installed to
   understand that only A0 and A1 should be used to talk to each other.

   Whether such modifications of topology constraints make sense is
   dependent on many technology variables and the exhausting treatment
   of the topic is definitely outside the scope of this document.

6.4.  Originating Non-Default Route Southbound

   Obviously, an implementation MAY choose to originate southbound
   instead of a strict default route (as described in Section 4.2.3.8) a
   shorter prefix P' but in such a scenario all addresses carried within
   the RIFT domain must be contained within P'.

7.  Security Considerations

7.1.  General

   One can consider attack vectors where a router may reboot many times
   while changing its system ID and pollute the network with many stale
   TIEs or TIEs are sent with very long lifetimes and not cleaned up
   when the routes vanishes.  Those attack vectors are not unique to
   RIFT.  Given large memory footprints available today those attacks
   should be relatively benign.  Otherwise a node SHOULD implement a
   strategy of discarding contents of all TIEs that were not present in
   the SPF tree over a certain, configurable period of time.  Since the
   protocol, like all modern link-state protocols, is self-stabilizing
   and will advertise the presence of such TIEs to its neighbors, they
   can be re-requested again if a computation finds that it sees an
   adjacency formed towards the system ID of the discarded TIEs.

7.2.  ZTP

   Section 4.2.7 presents many attack vectors in untrusted environments,
   starting with nodes that oscillate their level offers to the
   possibility of a node offering a three-way adjacency with the highest
   possible level value with a very long holdtime trying to put itself
   "on top of the lattice" and with that gaining access to the whole
   southbound topology.  Session authentication mechanisms are necessary

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   in environments where this is possible and RIFT provides the
   according security envelope to ensure this if desired.

7.3.  Lifetime

   Traditional IGP protocols are vulnerable to lifetime modification and
   replay attacks that can be somewhat mitigated by using techniques
   like [RFC7987].  RIFT removes this attack vector by protecting the
   lifetime behind a signature computed over it and additional nonce
   combination which makes even the replay attack window very small and
   for practical purposes irrelevant since lifetime cannot be
   artificially shortened by the attacker.

7.4.  Packet Number

   Optional packet number is carried in the security envelope without
   any encryption protection and is hence vulnerable to replay and
   modification attacks.  Contrary to nonces this number must change on
   every packet and would present a very high cryptographic load if
   signed.  The attack vector packet number present is relatively
   benign.  Changing the packet number by a man-in-the-middle attack
   will only affect operational validation tools and possibly some
   performance optimizations on flooding.  It is expected that an
   implementation detecting too many "fake losses" or "misorderings" due
   to the attack on the packet number would simply suppress its further
   processing.

7.5.  Outer Fingerprint Attacks

   A node can try to inject LIE packets observing a conversation on the
   wire by using the outer key ID albeit it cannot generate valid hashes
   in case it changes the integrity of the message so the only possible
   attack is DoS due to excessive LIE validation.

   A node can try to replay previous LIEs with changed state that it
   recorded but the attack is hard to replicate since the nonce
   combination must match the ongoing exchange and is then limited to a
   single flap only since both nodes will advance their nonces in case
   the adjacency state changed.  Even in the most unlikely case the
   attack length is limited due to both sides periodically increasing
   their nonces.

7.6.  TIE Origin Fingerprint DoS Attacks

   A compromised node can attempt to generate "fake TIEs" using other
   nodes' TIE origin key identifiers.  Albeit the ultimate validation of
   the origin fingerprint will fail in such scenarios and not progress
   further than immediately peering nodes, the resulting denial of

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   service attack seems unavoidable since the TIE origin key id is only
   protected by the, here assumed to be compromised, node.

7.7.  Host Implementations

   It can be reasonably expected that with the proliferation of RotH
   servers, rather than dedicated networking devices, will constitute
   significant amount of RIFT devices.  Given their normally far wider
   software envelope and access granted to them, such servers are also
   far more likely to be compromised and present an attack vector on the
   protocol.  Hijacking of prefixes to attract traffic is a trust
   problem and cannot be addressed within the protocol if the trust
   model is breached, i.e. the server presents valid credentials to form
   an adjacency and issue TIEs.  However, in a move devious way, the
   servers can present DoS (or even DDos) vectors of issuing too many
   LIE packets, flood large amount of North TIEs and similar anomalies.
   A prudent implementation hosting leaves should implement thresholds
   and raise warnings when leaf is advertising number of TIEs in excess
   of those.

8.  IANA Considerations

   This specification requests multicast address assignments and
   standard port numbers.  Additionally registries for the schema are
   requested and suggested values provided that reflect the numbers
   allocated in the given schema.

8.1.  Requested Multicast and Port Numbers

   This document requests allocation in the 'IPv4 Multicast Address
   Space' registry the suggested value of 224.0.0.120 as
   'ALL_V4_RIFT_ROUTERS' and in the 'IPv6 Multicast Address Space'
   registry the suggested value of FF02::A1F7 as 'ALL_V6_RIFT_ROUTERS'.

   This document requests allocation in the 'Service Name and Transport
   Protocol Port Number Registry' the allocation of a suggested value of
   914 on udp for 'RIFT_LIES_PORT' and suggested value of 915 for
   'RIFT_TIES_PORT'.

8.2.  Requested Registries with Suggested Values

   This section requests registries that help govern the schema via
   usual IANA registry procedures.  A top level 'RIFT' registry should
   hold the according registries requested in following sections with
   their pre-defined values.  IANA is requested to store the schema
   version introducing the allocated value as well as, optionally, its
   description when present.  This will allow to assign different values
   to an entry depending on schema version.  Alternately, IANA is

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   requested to consider a root RIFT/2 registry to store RIFT schema
   major version 2 values and may be requested in the future to create a
   RIFT/3 registry under that.  In any case, IANA is requested to store
   the schema version in the entries since that will allow to
   distinguish between minor versions in the same major schema version.
   All values not suggested as to be considered `Unassigned`. The range
   of every registry is a 16-bit integer.  Allocation of new values is
   always performed via `Expert Review` action.

8.2.1.  Registry RIFT/common/AddressFamilyType

   Address family type.

8.2.1.1.  Requested Entries

          Name                  Value Schema Version Description
          Illegal                   0            2.0
          AddressFamilyMinValue     1            2.0
          IPv4                      2            2.0
          IPv6                      3            2.0
          AddressFamilyMaxValue     4            2.0

8.2.2.  Registry RIFT/common/HierarchyIndications

   Flags indicating node configuration in case of ZTP.

8.2.2.1.  Requested Entries

   Name                                 Value Schema Version Description
   leaf_only                                0            2.0
   leaf_only_and_leaf_2_leaf_procedures     1            2.0
   top_of_fabric                            2            2.0

8.2.3.  Registry RIFT/common/IEEE802_1ASTimeStampType

   Timestamp per IEEE 802.1AS, all values MUST be interpreted in
   implementation as unsigned.

8.2.3.1.  Requested Entries

                 Name    Value Schema Version Description
                 AS_sec      1            2.0
                 AS_nsec     2            2.0

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8.2.4.  Registry RIFT/common/IPAddressType

   IP address type.

8.2.4.1.  Requested Entries

             Name        Value Schema Version Description
             ipv4address     1            2.0  Content is IPv4
             ipv6address     2            2.0  Content is IPv6

8.2.5.  Registry RIFT/common/IPPrefixType

   Prefix advertisement.

   @note: for interface addresses the protocol can propagate the address
   part beyond the subnet mask and on reachability computation that has
   to be normalized.  The non-significant bits can be used for
   operational purposes.

8.2.5.1.  Requested Entries

                Name       Value Schema Version Description
                ipv4prefix     1            2.0
                ipv6prefix     2            2.0

8.2.6.  Registry RIFT/common/IPv4PrefixType

   IPv4 prefix type.

8.2.6.1.  Requested Entries

                Name      Value Schema Version Description
                address       1            2.0
                prefixlen     2            2.0

8.2.7.  Registry RIFT/common/IPv6PrefixType

   IPv6 prefix type.

8.2.7.1.  Requested Entries

                Name      Value Schema Version Description
                address       1            2.0
                prefixlen     2            2.0

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8.2.8.  Registry RIFT/common/PrefixSequenceType

   Sequence of a prefix in case of move.

8.2.8.1.  Requested Entries

   Name          Value      Schema Description
                           Version
   timestamp         1         2.0
   transactionid     2         2.0  Transaction ID set by client in e.g.
                                   in 6LoWPAN.

8.2.9.  Registry RIFT/common/RouteType

   RIFT route types.

   @note: route types which MUST be ordered on their preference PGP
   prefixes are most preferred attracting traffic north (towards spine)
   and then south normal prefixes are attracting traffic south (towards
   leaves), i.e. prefix in NORTH PREFIX TIE is preferred over SOUTH
   PREFIX TIE.

   @note: The only purpose of those values is to introduce an ordering
   whereas an implementation can choose internally any other values as
   long the ordering is preserved

8.2.9.1.  Requested Entries

           Name                Value Schema Version Description
           Illegal                 0            2.0
           RouteTypeMinValue       1            2.0
           Discard                 2            2.0
           LocalPrefix             3            2.0
           SouthPGPPrefix          4            2.0
           NorthPGPPrefix          5            2.0
           NorthPrefix             6            2.0
           NorthExternalPrefix     7            2.0
           SouthPrefix             8            2.0
           SouthExternalPrefix     9            2.0
           NegativeSouthPrefix    10            2.0
           RouteTypeMaxValue      11            2.0

8.2.10.  Registry RIFT/common/TIETypeType

   Type of TIE.

   This enum indicates what TIE type the TIE is carrying.  In case the
   value is not known to the receiver, the TIE MUST be re-flooded.  This

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   allows for future extensions of the protocol within the same major
   schema with types opaque to some nodes UNLESS the flooding scope is
   not the same as prefix TIE, then a major version revision MUST be
   performed.

8.2.10.1.  Requested Entries

   Name                                        Value  Schema Description
                                                     Version
   Illegal                                         0     2.0
   TIETypeMinValue                                 1     2.0
   NodeTIEType                                     2     2.0
   PrefixTIEType                                   3     2.0
   PositiveDisaggregationPrefixTIEType             4     2.0
   NegativeDisaggregationPrefixTIEType             5     2.0
   PGPrefixTIEType                                 6     2.0
   KeyValueTIEType                                 7     2.0
   ExternalPrefixTIEType                           8     2.0
   PositiveExternalDisaggregationPrefixTIEType     9     2.0
   TIETypeMaxValue                                10     2.0

8.2.11.  Registry RIFT/common/TieDirectionType

   Direction of TIEs.

8.2.11.1.  Requested Entries

            Name              Value Schema Version Description
            Illegal               0            2.0
            South                 1            2.0
            North                 2            2.0
            DirectionMaxValue     3            2.0

8.2.12.  Registry RIFT/encoding/Community

   Prefix community.

8.2.12.1.  Requested Entries

              Name   Value Schema Version Description
              top        1            2.0  Higher order bits
              bottom     2            2.0  Lower order bits

8.2.13.  Registry RIFT/encoding/KeyValueTIEElement

   Generic key value pairs.

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8.2.13.1.  Requested Entries

                Name      Value Schema Version Description
                keyvalues     1            2.0

8.2.14.  Registry RIFT/encoding/LIEPacket

   RIFT LIE Packet.

   @note: this node's level is already included on the packet header

8.2.14.1.  Requested Entries

   Name                        Value  Schema Description
                                     Version
   name                            1     2.0  Node or adjacency name.
   local_id                        2     2.0  Local link ID.
   flood_port                      3     2.0  UDP port to which we can
                                             receive flooded TIEs.
   link_mtu_size                   4     2.0  Layer 3 MTU, used to
                                             discover to mismatch.
   link_bandwidth                  5     2.0  Local link bandwidth on
                                             the interface.
   neighbor                        6     2.0  Reflects the neighbor once
                                             received to provide
                                             3-way connectivity.
   pod                             7     2.0  Node's PoD.
   node_capabilities              10     2.0  Node capabilities shown in
                                             the LIE. The capabilities
                                             MUST match the capabilities
                                             shown in the Node TIEs,
                                             otherwise
                                             the behavior is
                                             unspecified. A node
                                             detecting the mismatch
                                             SHOULD generate according
                                             error.
   link_capabilities              11     2.0  Capabilities of this link.
   holdtime                       12     2.0  Required holdtime of the
                                             adjacency, i.e. how much
                                             time
                                             MUST expire without LIE for
                                             the adjacency to drop.
   label                          13     2.0  Unsolicited, downstream
                                             assigned locally
                                             significant label
                                             value for the adjacency.
   not_a_ztp_offer                21     2.0  Indicates that the level

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                                             on the LIE MUST NOT be used
                                             to derive a ZTP level by
                                             the receiving node.
   you_are_flood_repeater         22     2.0  Indicates to northbound
                                             neighbor that it should
                                             be reflooding this node's
                                             N-TIEs to achieve flood
                                             reduction and
                                             balancing for northbound
                                             flooding. To be ignored if
                                             received from a
                                             northbound adjacency.
   you_are_sending_too_quickly    23     2.0  Can be optionally set to
                                             indicate to neighbor that
                                             packet losses are seen on
                                             reception based on packet
                                             numbers or the rate is too
                                             high. The receiver SHOULD
                                             temporarily slow down
                                             flooding rates.
   instance_name                  24     2.0  Instance name in case
                                             multiple RIFT instances
                                             running on same interface.

8.2.15.  Registry RIFT/encoding/LinkCapabilities

   Link capabilities.

8.2.15.1.  Requested Entries

    Name                  Value     Schema Description
                                   Version
    bfd                       1        2.0  Indicates that the link is
                                           supporting BFD.
    v4_forwarding_capable     2        2.0

8.2.16.  Registry RIFT/encoding/LinkIDPair

   LinkID pair describes one of parallel links between two nodes.

8.2.16.1.  Requested Entries

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   Name                       Value  Schema Description
                                    Version
   local_id                       1     2.0  Node-wide unique value for
                                            the local link.
   remote_id                      2     2.0  Received remote link ID for
                                            this link.
   platform_interface_index      10     2.0  Describes the local
                                            interface index of the link.
   platform_interface_name       11     2.0  Describes the local
                                            interface name.
   trusted_outer_security_key    12     2.0  Indication whether the link
                                            is secured, i.e. protected
                                            by outer key, absence
                                            of this element means no
                                            indication, undefined outer
                                            key means not secured.
   bfd_up                        13     2.0  Indication whether the link
                                            is protected by established
                                            BFD session.

8.2.17.  Registry RIFT/encoding/Neighbor

   Neighbor structure.

8.2.17.1.  Requested Entries

      Name       Value Schema Version Description
      originator     1            2.0  System ID of the originator.
      remote_id      2            2.0  ID of remote side of the link.

8.2.18.  Registry RIFT/encoding/NodeCapabilities

   Capabilities the node supports.

   @note: The schema may add to this field future capabilities to
   indicate whether it will support interpretation of future schema
   extensions on the same major revision.  Such fields MUST be optional
   and have an implicit or explicit false default value.  If a future
   capability changes route selection or generates blackholes if some
   nodes are not supporting it then a major version increment is
   unavoidable.

8.2.18.1.  Requested Entries

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   Name                   Value  Schema Description
                                Version
   protocol_minor_version     1     2.0  Must advertise supported minor
                                        version dialect that way.
   flood_reduction            2     2.0  Can this node participate in
                                        flood reduction.
   hierarchy_indications      3     2.0  Does this node restrict itself
                                        to be top-of-fabric or
                                        leaf only (in ZTP) and does it
                                        support leaf-2-leaf procedures.

8.2.19.  Registry RIFT/encoding/NodeFlags

   Indication flags of the node.

8.2.19.1.  Requested Entries

   Name     Value    Schema Description
                    Version
   overload     1       2.0  Indicates that node is in overload, do not
                            transit traffic through it.

8.2.20.  Registry RIFT/encoding/NodeNeighborsTIEElement

   neighbor of a node

8.2.20.1.  Requested Entries

    Name      Value  Schema Description
                    Version
    level         1     2.0  level of neighbor
    cost          3     2.0
    link_ids      4     2.0  can carry description of multiple parallel
                            links in a TIE
    bandwidth     5     2.0  total bandwidth to neighbor, this will be
                            normally sum of the
                            bandwidths of all the parallel links.

8.2.21.  Registry RIFT/encoding/NodeTIEElement

   Description of a node.

   It may occur multiple times in different TIEs but if either

      capabilities values do not match or

      flags values do not match or

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      neighbors repeat with different values

   the behavior is undefined and a warning SHOULD be generated.
   Neighbors can be distributed across multiple TIEs however if the sets
   are disjoint.  Miscablings SHOULD be repeated in every node TIE,
   otherwise the behavior is undefined.

   @note: Observe that absence of fields implies defined defaults.

8.2.21.1.  Requested Entries

   Name            Value  Schema Description
                         Version
   level               1     2.0  Level of the node.
   neighbors           2     2.0  Node's neighbors. If neighbor systemID
                                 repeats in other node TIEs of
                                 same node the behavior is undefined.
   capabilities        3     2.0  Capabilities of the node.
   flags               4     2.0  Flags of the node.
   name                5     2.0  Optional node name for easier
                                 operations.
   pod                 6     2.0  PoD to which the node belongs.
   miscabled_links    10     2.0  If any local links are miscabled, the
                                 indication is flooded.

8.2.22.  Registry RIFT/encoding/PacketContent

   Content of a RIFT packet.

8.2.22.1.  Requested Entries

                   Name Value Schema Version Description
                   lie      1            2.0
                   tide     2            2.0
                   tire     3            2.0
                   tie      4            2.0

8.2.23.  Registry RIFT/encoding/PacketHeader

   Common RIFT packet header.

8.2.23.1.  Requested Entries

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   Name          Value  Schema Description
                       Version
   major_version     1     2.0  Major version of protocol.
   minor_version     2     2.0  Minor version of protocol.
   sender            3     2.0  Node sending the packet, in case of
                               LIE/TIRE/TIDE                     also
                               the originator of it.
   level             4     2.0  Level of the node sending the packet,
                               required on everything except
                               LIEs. Lack of presence on LIEs indicates
                               UNDEFINED_LEVEL and is used
                               in ZTP procedures.

8.2.24.  Registry RIFT/encoding/PrefixAttributes

   Attributes of a prefix.

8.2.24.1.  Requested Entries

   Name              Value  Schema Description
                           Version
   metric                2     2.0  Distance of the prefix.
   tags                  3     2.0  Generic unordered set of route tags,
                                   can be redistributed to other
                                   protocols or use
                                   within the context of real time
                                   analytics.
   monotonic_clock       4     2.0  Monotonic clock for mobile
                                   addresses.
   loopback              6     2.0  Indicates if the interface is a node
                                   loopback.
   directly_attached     7     2.0  Indicates that the prefix is
                                   directly attached, i.e. should be
                                   routed to even if
                                   the node is in overload. *
   from_link            10     2.0  In case of locally originated
                                   prefixes, i.e. interface addresses
                                   this can                     describe
                                   which link the address belongs to.

8.2.25.  Registry RIFT/encoding/PrefixTIEElement

   TIE carrying prefixes

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8.2.25.1.  Requested Entries

   Name     Value  Schema Description
                  Version
   prefixes     1     2.0  Prefixes with the associated attributes.
                          If the same prefix repeats in multiple TIEs of
                          same node                     behavior is
                          unspecified.

8.2.26.  Registry RIFT/encoding/ProtocolPacket

   RIFT packet structure.

8.2.26.1.  Requested Entries

                 Name    Value Schema Version Description
                 header      1            2.0
                 content     2            2.0

8.2.27.  Registry RIFT/encoding/TIDEPacket

   TIDE with sorted TIE headers, if headers are unsorted, behavior is
   undefined.

8.2.27.1.  Requested Entries

   Name        Value Schema Version Description
   start_range     1            2.0  First TIE header in the tide
                                    packet.
   end_range       2            2.0  Last TIE header in the tide packet.
   headers         3            2.0  _Sorted_ list of headers.

8.2.28.  Registry RIFT/encoding/TIEElement

   Single element in a TIE.

   Schema enum `common.TIETypeType` in TIEID indicates which elements
   MUST be present in the TIEElement.  In case of mismatch the
   unexpected elements MUST be ignored.  In case of lack of expected
   element the TIE an error MUST be reported and the TIE MUST be
   ignored.

   This type can be extended with new optional elements for new
   `common.TIETypeType` values without breaking the major but if it is
   necessary to understand whether all nodes support the new type a node
   capability must be added as well.

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8.2.28.1.  Requested Entries

   Name                            Valu Schem Description
                                      e a Ver
                                         sion
   node                               1   2.0  Used in case of enum comm
                                              on.TIETypeType.NodeTIEType
                                              .
   prefixes                           2   2.0  Used in case of enum comm
                                              on.TIETypeType.PrefixTIETy
                                              pe.
   positive_disaggregation_prefixe    3   2.0  Positive prefixes (always
   s                                          southbound).
                                              It MUST NOT be advertised
                                              within a North TIE and
                                              ignored otherwise
   negative_disaggregation_prefixe    5   2.0  Transitive, negative
   s                                          prefixes (always
                                              southbound) which
                                              MUST be aggregated and
                                              propagated
                                              according to the
                                              specification
                                              southwards towards lower
                                              levels to heal
                                              pathological upper level
                                              partitioning, otherwise
                                              blackholes may occur in
                                              multiplane fabrics.
                                              It MUST NOT be advertised
                                              within a North TIE.
   external_prefixes                  6   2.0  Externally reimported
                                              prefixes.
   positive_external_disaggregatio    7   2.0  Positive external
   n_prefixes                                 disaggregated prefixes
                                              (always southbound).
                                              It MUST NOT be advertised
                                              within a North TIE and
                                              ignored otherwise.
   keyvalues                          9   2.0  Key-Value store elements.

8.2.29.  Registry RIFT/encoding/TIEHeader

   Header of a TIE.

   @note: TIEID space is a total order achieved by comparing the
   elements in sequence defined and comparing each value as an unsigned
   integer of according length.

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   @note: After sequence number the lifetime received on the envelope
   must be used for comparison before further fields.

   @note: `origination_time` and `origination_lifetime` are disregarded
   for comparison purposes and carried purely for debugging/security
   purposes if present.

8.2.29.1.  Requested Entries

   Name                 Value  Schema Description
                              Version
   tieid                    2     2.0  ID of the tie.
   seq_nr                   3     2.0  Sequence number of the tie.
   origination_time        10     2.0  Absolute timestamp when the TIE
                                      was generated. This can be used on
                                      fabrics with
                                      synchronized clock to prevent
                                      lifetime modification attacks.
   origination_lifetime    12     2.0  Original lifetime when the TIE
                                      was generated. This can be used on
                                      fabrics with
                                      synchronized clock to prevent
                                      lifetime modification attacks.

8.2.30.  Registry RIFT/encoding/TIEHeaderWithLifeTime

   Header of a TIE as described in TIRE/TIDE.

8.2.30.1.  Requested Entries

     Name               Value  Schema Description
                              Version
     header                 1     2.0
     remaining_lifetime     2     2.0  Remaining lifetime that expires
                                      down to 0 just like in ISIS.
                                      TIEs with lifetimes differing by
                                      less than `lifetime_diff2ignore`
                                      MUST                     be
                                      considered EQUAL.

8.2.31.  Registry RIFT/encoding/TIEID

   ID of a TIE.

   @note: TIEID space is a total order achieved by comparing the
   elements in sequence defined and comparing each value as an unsigned
   integer of according length.

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8.2.31.1.  Requested Entries

     Name       Value Schema Version Description
     direction      1            2.0  direction of TIE
     originator     2            2.0  indicates originator of the TIE
     tietype        3            2.0  type of the tie
     tie_nr         4            2.0  number of the tie

8.2.32.  Registry RIFT/encoding/TIEPacket

   TIE packet

8.2.32.1.  Requested Entries

                 Name    Value Schema Version Description
                 header      1            2.0
                 element     2            2.0

8.2.33.  Registry RIFT/encoding/TIREPacket

   TIRE packet

8.2.33.1.  Requested Entries

                 Name    Value Schema Version Description
                 headers     1            2.0

9.  Acknowledgments

   A new routing protocol in its complexity is not a product of a parent
   but of a village as the author list shows already.  However, many
   more people provided input, fine-combed the specification based on
   their experience in design or implementation.  This section will make
   an inadequate attempt in recording their contribution.

   Many thanks to Naiming Shen for some of the early discussions around
   the topic of using IGPs for routing in topologies related to Clos.
   Russ White to be especially acknowledged for the key conversation on
   epistemology that allowed to tie current asynchronous distributed
   systems theory results to a modern protocol design presented here.
   Adrian Farrel, Joel Halpern, Jeffrey Zhang, Krzysztof Szarkowicz,
   Nagendra Kumar, Melchior Aelmans provided thoughtful comments that
   improved the readability of the document and found good amount of
   corners where the light failed to shine.  Kris Price was first to
   mention single router, single arm default considerations.  Jeff
   Tantsura helped out with some initial thoughts on BFD interactions
   while Jeff Haas corrected several misconceptions about BFD's finer
   points.  Artur Makutunowicz pointed out many possible improvements

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   and acted as sounding board in regard to modern protocol
   implementation techniques RIFT is exploring.  Barak Gafni formalized
   first time clearly the problem of partitioned spine and fallen leaves
   on a (clean) napkin in Singapore that led to the very important part
   of the specification centered around multiple Top-of-Fabric planes
   and negative disaggregation.  Igor Gashinsky and others shared many
   thoughts on problems encountered in design and operation of large-
   scale data center fabrics.  Xu Benchong found a delicate error in the
   flooding procedures while implementing.

10.  References

10.1.  Normative References

   [EUI64]    IEEE, "Guidelines for Use of Extended Unique Identifier
              (EUI), Organizationally Unique Identifier (OUI), and
              Company ID (CID)", IEEE EUI,
              <http://standards.ieee.org/develop/regauth/tut/eui.pdf>.

   [ISO10589]
              ISO "International Organization for Standardization",
              "Intermediate system to Intermediate system intra-domain
              routeing information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473), ISO/IEC
              10589:2002, Second Edition.", Nov 2002.

   [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              DOI 10.17487/RFC1982, August 1996,
              <https://www.rfc-editor.org/info/rfc1982>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC2365]  Meyer, D., "Administratively Scoped IP Multicast", BCP 23,
              RFC 2365, DOI 10.17487/RFC2365, July 1998,
              <https://www.rfc-editor.org/info/rfc2365>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

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   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007,
              <https://www.rfc-editor.org/info/rfc5082>.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,
              <https://www.rfc-editor.org/info/rfc5120>.

   [RFC5303]  Katz, D., Saluja, R., and D. Eastlake 3rd, "Three-Way
              Handshake for IS-IS Point-to-Point Adjacencies", RFC 5303,
              DOI 10.17487/RFC5303, October 2008,
              <https://www.rfc-editor.org/info/rfc5303>.

   [RFC5549]  Le Faucheur, F. and E. Rosen, "Advertising IPv4 Network
              Layer Reachability Information with an IPv6 Next Hop",
              RFC 5549, DOI 10.17487/RFC5549, May 2009,
              <https://www.rfc-editor.org/info/rfc5549>.

   [RFC5709]  Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
              Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
              Authentication", RFC 5709, DOI 10.17487/RFC5709, October
              2009, <https://www.rfc-editor.org/info/rfc5709>.

   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
              DOI 10.17487/RFC5881, June 2010,
              <https://www.rfc-editor.org/info/rfc5881>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <https://www.rfc-editor.org/info/rfc5905>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [RFC7987]  Ginsberg, L., Wells, P., Decraene, B., Przygienda, T., and
              H. Gredler, "IS-IS Minimum Remaining Lifetime", RFC 7987,
              DOI 10.17487/RFC7987, October 2016,
              <https://www.rfc-editor.org/info/rfc7987>.

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8202]  Ginsberg, L., Previdi, S., and W. Henderickx, "IS-IS
              Multi-Instance", RFC 8202, DOI 10.17487/RFC8202, June
              2017, <https://www.rfc-editor.org/info/rfc8202>.

   [RFC8505]  Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Neighbor
              Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
              <https://www.rfc-editor.org/info/rfc8505>.

   [thrift]   Apache Software Foundation, "Thrift Interface Description
              Language", <https://thrift.apache.org/docs/idl>.

10.2.  Informative References

   [CLOS]     Yuan, X., "On Nonblocking Folded-Clos Networks in Computer
              Communication Environments", IEEE International Parallel &
              Distributed Processing Symposium, 2011.

   [DIJKSTRA]
              Dijkstra, E., "A Note on Two Problems in Connexion with
              Graphs", Journal Numer. Math. , 1959.

   [DOT]      Ellson, J. and L. Koutsofios, "Graphviz: open source graph
              drawing tools", Springer-Verlag , 2001.

   [DYNAMO]   De Candia et al., G., "Dynamo: amazon's highly available
              key-value store", ACM SIGOPS symposium on Operating
              systems principles (SOSP '07), 2007.

   [EPPSTEIN]
              Eppstein, D., "Finding the k-Shortest Paths", 1997.

   [FATTREE]  Leiserson, C., "Fat-Trees: Universal Networks for
              Hardware-Efficient Supercomputing", 1985.

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   [IEEEstd1588]
              IEEE, "IEEE Standard for a Precision Clock Synchronization
              Protocol for Networked Measurement and Control Systems",
              IEEE Standard 1588,
              <https://ieeexplore.ieee.org/document/4579760/>.

   [IEEEstd8021AS]
              IEEE, "IEEE Standard for Local and Metropolitan Area
              Networks - Timing and Synchronization for Time-Sensitive
              Applications in Bridged Local Area Networks",
              IEEE Standard 802.1AS,
              <https://ieeexplore.ieee.org/document/5741898/>.

   [ISO10589-Second-Edition]
              International Organization for Standardization,
              "Intermediate system to Intermediate system intra-domain
              routeing information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473)", Nov 2002.

   [RFC0826]  Plummer, D., "An Ethernet Address Resolution Protocol: Or
              Converting Network Protocol Addresses to 48.bit Ethernet
              Address for Transmission on Ethernet Hardware", STD 37,
              RFC 826, DOI 10.17487/RFC0826, November 1982,
              <https://www.rfc-editor.org/info/rfc826>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [RFC3626]  Clausen, T., Ed. and P. Jacquet, Ed., "Optimized Link
              State Routing Protocol (OLSR)", RFC 3626,
              DOI 10.17487/RFC3626, October 2003,
              <https://www.rfc-editor.org/info/rfc3626>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

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   [RFC6518]  Lebovitz, G. and M. Bhatia, "Keying and Authentication for
              Routing Protocols (KARP) Design Guidelines", RFC 6518,
              DOI 10.17487/RFC6518, February 2012,
              <https://www.rfc-editor.org/info/rfc6518>.

   [RFC7938]  Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
              BGP for Routing in Large-Scale Data Centers", RFC 7938,
              DOI 10.17487/RFC7938, August 2016,
              <https://www.rfc-editor.org/info/rfc7938>.

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,
              <https://www.rfc-editor.org/info/rfc8415>.

   [VAHDAT08]
              Al-Fares, M., Loukissas, A., and A. Vahdat, "A Scalable,
              Commodity Data Center Network Architecture", SIGCOMM ,
              2008.

   [Wikipedia]
              Wikipedia,
              "https://en.wikipedia.org/wiki/Serial_number_arithmetic",
              2016.

Appendix A.  Sequence Number Binary Arithmetic

   The only reasonably reference to a cleaner than [RFC1982] sequence
   number solution is given in [Wikipedia].  It basically converts the
   problem into two complement's arithmetic.  Assuming a straight two
   complement's subtractions on the bit-width of the sequence number the
   according >: and =: relations are defined as:

      U_1, U_2 are 12-bits aligned unsigned version number

      D_f is  ( U_1 - U_2 ) interpreted as two complement signed 12-bits
      D_b is  ( U_2 - U_1 ) interpreted as two complement signed 12-bits

      U_1 >: U_2 IIF D_f > 0 AND D_b < 0
      U_1 =: U_2 IIF D_f = 0

   The >: relationship is anti-symmetric but not transitive.  Observe
   that this leaves >: of the numbers having maximum two complement
   distance, e.g. ( 0 and 0x800 ) undefined in our 12-bits case since
   D_f and D_b are both -0x7ff.

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   A simple example of the relationship in case of 3-bit arithmetic
   follows as table indicating D_f/D_b values and then the relationship
   of U_1 to U_2:

           U2 / U1   0    1    2    3    4    5    6    7
           0        +/+  +/-  +/-  +/-  -/-  -/+  -/+  -/+
           1        -/+  +/+  +/-  +/-  +/-  -/-  -/+  -/+
           2        -/+  -/+  +/+  +/-  +/-  +/-  -/-  -/+
           3        -/+  -/+  -/+  +/+  +/-  +/-  +/-  -/-
           4        -/-  -/+  -/+  -/+  +/+  +/-  +/-  +/-
           5        +/-  -/-  -/+  -/+  -/+  +/+  +/-  +/-
           6        +/-  +/-  -/-  -/+  -/+  -/+  +/+  +/-
           7        +/-  +/-  +/-  -/-  -/+  -/+  -/+  +/+

          U2 / U1   0    1    2    3    4    5    6    7
          0         =    >    >    >    ?    <    <    <
          1         <    =    >    >    >    ?    <    <
          2         <    <    =    >    >    >    ?    <
          3         <    <    <    =    >    >    >    ?
          4         ?    <    <    <    =    >    >    >
          5         >    ?    <    <    <    =    >    >
          6         >    >    ?    <    <    <    =    >
          7         >    >    >    ?    <    <    <    =

Appendix B.  Information Elements Schema

   This section introduces the schema for information elements.  The IDL
   is Thrift [thrift].

   On schema changes that

   1.   change field numbers or

   2.   add new *required* fields or

   3.   remove any fields or

   4.   change lists into sets, unions into structures or

   5.   change multiplicity of fields or

   6.   changes name of any field or type or

   7.   change data types of any field or

   8.   adds, changes or removes a default value of any *existing* field
        or

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   9.   removes or changes any defined constant or constant value or

   10.  changes any enumeration type except extending `common.TIEType`
        (use of enumeration types is generally discouraged)

   major version of the schema MUST increase.  All other changes MUST
   increase minor version within the same major.

   Observe however that introducing an optional field does not cause a
   major version increase even if the fields inside the structure are
   optional with defaults.

   All signed integer as forced by Thrift [thrift] support must be cast
   for internal purposes to equivalent unsigned values without
   discarding the signedness bit.  An implementation SHOULD try to avoid
   using the signedness bit when generating values.

   The schema is normative.

B.1.  common.thrift

/**
    Thrift file with common definitions for RIFT
*/

namespace py common
namespace rs models

/** @note MUST be interpreted in implementation as unsigned 64 bits.
 *        The implementation SHOULD NOT use the MSB.
 */
typedef i64      SystemIDType
typedef i32      IPv4Address
/** this has to be long enough to accomodate prefix */
typedef binary   IPv6Address
/** @note MUST be interpreted in implementation as unsigned */
typedef i16      UDPPortType
/** @note MUST be interpreted in implementation as unsigned */
typedef i32      TIENrType
/** @note MUST be interpreted in implementation as unsigned */
typedef i32      MTUSizeType
/** @note MUST be interpreted in implementation as unsigned
    rolling over number */
typedef i16      SeqNrType
/** @note MUST be interpreted in implementation as unsigned */
typedef i32      LifeTimeInSecType
/** @note MUST be interpreted in implementation as unsigned */

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typedef i8       LevelType
/** optional, recommended monotonically increasing number
    _per packet type per adjacency_
    that can be used to detect losses/misordering/restarts.
    @note MUST be interpreted in implementation as unsigned
          rolling over number */
typedef i16      PacketNumberType
/** @note MUST be interpreted in implementation as unsigned */
typedef i32      PodType
/** @note MUST be interpreted in implementation as unsigned.
          This is carried in the
          security envelope and MUST fit into 8 bits. */
typedef i8       VersionType
/** @note MUST be interpreted in implementation as unsigned */
typedef i16      MinorVersionType
/** @note MUST be interpreted in implementation as unsigned */
typedef i32      MetricType
/** @note MUST be interpreted in implementation as unsigned
          and unstructured */
typedef i64      RouteTagType
/** @note MUST be interpreted in implementation as unstructured
          label value */
typedef i32      LabelType
/** @note MUST be interpreted in implementation as unsigned */
typedef i32      BandwithInMegaBitsType
/** @note Key Value key ID type */
typedef string   KeyIDType
/** node local, unique identification for a link (interface/tunnel
  * etc. Basically anything RIFT runs on). This is kept
  * at 32 bits so it aligns with BFD [RFC5880] discriminator size.
  */
typedef i32    LinkIDType
typedef string KeyNameType
typedef i8     PrefixLenType
/** timestamp in seconds since the epoch */
typedef i64    TimestampInSecsType
/** security nonce.
    @note MUST be interpreted in implementation as rolling
          over unsigned value */
typedef i16    NonceType
/** LIE FSM holdtime type */
typedef i16    TimeIntervalInSecType
/** Transaction ID type for prefix mobility as specified by RFC6550,
    value MUST be interpreted in implementation as unsigned  */
typedef i8     PrefixTransactionIDType
/** Timestamp per IEEE 802.1AS, all values MUST be interpreted in
    implementation as unsigned.  */
struct IEEE802_1ASTimeStampType {

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    1: required     i64     AS_sec;
    2: optional     i32     AS_nsec;
}
/** generic counter type */
typedef i64 CounterType
/** Platform Interface Index type, i.e. index of interface on hardware,
    can be used e.g. with RFC5837 */
typedef i32 PlatformInterfaceIndex

/** Flags indicating node configuration in case of ZTP.
 */
enum HierarchyIndications {
    /** forces level to `leaf_level` and enables according procedures */
    leaf_only                            = 0,
    /** forces level to `leaf_level` and enables according procedures */
    leaf_only_and_leaf_2_leaf_procedures = 1,
    /** forces level to `top_of_fabric` and enables according
        procedures */
    top_of_fabric                        = 2,
}

const PacketNumberType  undefined_packet_number    = 0
/** This MUST be used when node is configured as top of fabric in ZTP.
    This is kept reasonably low to alow for fast ZTP convergence on
    failures. */
const LevelType   top_of_fabric_level              = 24
/** default bandwidth on a link */
const BandwithInMegaBitsType  default_bandwidth    = 100
/** fixed leaf level when ZTP is not used */
const LevelType   leaf_level                  = 0
const LevelType   default_level               = leaf_level
const PodType     default_pod                 = 0
const LinkIDType  undefined_linkid            = 0

/** default distance used */
const MetricType  default_distance         = 1
/** any distance larger than this will be considered infinity */
const MetricType  infinite_distance       = 0x7FFFFFFF
/** represents invalid distance */
const MetricType  invalid_distance        = 0
const bool overload_default               = false
const bool flood_reduction_default        = true
/** default LIE FSM holddown time */
const TimeIntervalInSecType   default_lie_holdtime  = 3
/** default ZTP FSM holddown time */
const TimeIntervalInSecType   default_ztp_holdtime  = 1
/** by default LIE levels are ZTP offers */
const bool default_not_a_ztp_offer        = false

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/** by default everyone is repeating flooding */
const bool default_you_are_flood_repeater = true
/** 0 is illegal for SystemID */
const SystemIDType IllegalSystemID        = 0
/** empty set of nodes */
const set<SystemIDType> empty_set_of_nodeids = {}
/** default lifetime of TIE is one week */
const LifeTimeInSecType default_lifetime      = 604800
/** default lifetime when TIEs are purged is 5 minutes */
const LifeTimeInSecType purge_lifetime        = 300
/** round down interval when TIEs are sent with security hashes
    to prevent excessive computation. **/
const LifeTimeInSecType rounddown_lifetime_interval = 60
/** any `TieHeader` that has a smaller lifetime difference
    than this constant is equal (if other fields equal). This
    constant MUST be larger than `purge_lifetime` to avoid
    retransmissions */
const LifeTimeInSecType lifetime_diff2ignore  = 400

/** default UDP port to run LIEs on */
const UDPPortType     default_lie_udp_port       =  914
/** default UDP port to receive TIEs on, that can be peer specific */
const UDPPortType     default_tie_udp_flood_port =  915

/** default MTU link size to use */
const MTUSizeType     default_mtu_size           = 1400
/** default link being BFD capable */
const bool            bfd_default                = true

/** undefined nonce, equivalent to missing nonce */
const NonceType       undefined_nonce            = 0;
/** outer security key id, MUST be interpreted as in implementation
    as unsigned */
typedef i8            OuterSecurityKeyID
/** security key id, MUST be interpreted as in implementation
    as unsigned */
typedef i32           TIESecurityKeyID
/** undefined key */
const TIESecurityKeyID undefined_securitykey_id   = 0;
/** Maximum delta (negative or positive) that a mirrored nonce can
    deviate from local value to be considered valid. If nonces are
    changed every minute on both sides this opens statistically
    a `maximum_valid_nonce_delta` minutes window of identical LIEs,
    TIE, TI(x)E replays.
    The interval cannot be too small since LIE FSM may change
    states fairly quickly during ZTP without sending LIEs*/
const i16             maximum_valid_nonce_delta  = 5;

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/** Direction of TIEs. */
enum TieDirectionType {
    Illegal           = 0,
    South             = 1,
    North             = 2,
    DirectionMaxValue = 3,
}

/** Address family type. */
enum AddressFamilyType {
   Illegal                = 0,
   AddressFamilyMinValue  = 1,
   IPv4     = 2,
   IPv6     = 3,
   AddressFamilyMaxValue  = 4,
}

/** IPv4 prefix type. */
struct IPv4PrefixType {
    1: required IPv4Address    address;
    2: required PrefixLenType  prefixlen;
}

/** IPv6 prefix type. */
struct IPv6PrefixType {
    1: required IPv6Address    address;
    2: required PrefixLenType  prefixlen;
}

/** IP address type. */
union IPAddressType {
    /** Content is IPv4 */
    1: optional IPv4Address   ipv4address;
    /** Content is IPv6 */
    2: optional IPv6Address   ipv6address;
}

/** Prefix advertisement.

    @note: for interface
        addresses the protocol can propagate the address part beyond
        the subnet mask and on reachability computation that has to
        be normalized. The non-significant bits can be used
        for operational purposes.
*/
union IPPrefixType {
    1: optional IPv4PrefixType   ipv4prefix;
    2: optional IPv6PrefixType   ipv6prefix;

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}

/** Sequence of a prefix in case of move.
 */
struct PrefixSequenceType {
    1: required IEEE802_1ASTimeStampType  timestamp;
    /** Transaction ID set by client in e.g. in 6LoWPAN. */
    2: optional PrefixTransactionIDType   transactionid;
}

/** Type of TIE.

    This enum indicates what TIE type the TIE is carrying.
    In case the value is not known to the receiver,
    the TIE MUST be re-flooded. This allows for
    future extensions of the protocol within the same major schema
    with types opaque to some nodes UNLESS the flooding scope is not
    the same as prefix TIE, then a major version revision MUST
    be performed.
*/
enum TIETypeType {
    Illegal                                     = 0,
    TIETypeMinValue                             = 1,
    /** first legal value */
    NodeTIEType                                 = 2,
    PrefixTIEType                               = 3,
    PositiveDisaggregationPrefixTIEType         = 4,
    NegativeDisaggregationPrefixTIEType         = 5,
    PGPrefixTIEType                             = 6,
    KeyValueTIEType                             = 7,
    ExternalPrefixTIEType                       = 8,
    PositiveExternalDisaggregationPrefixTIEType = 9,
    TIETypeMaxValue                             = 10,
}

/** RIFT route types.

    @note: route types which MUST be ordered on their preference
           PGP prefixes are most preferred attracting
           traffic north (towards spine) and then south
           normal prefixes are attracting traffic south
           (towards leaves), i.e. prefix in NORTH PREFIX TIE
           is preferred over SOUTH PREFIX TIE.

    @note: The only purpose of those values is to introduce an
           ordering whereas an implementation can choose internally
           any other values as long the ordering is preserved
 */

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enum RouteType {
    Illegal               =  0,
    RouteTypeMinValue     =  1,
    /** First legal value. */
    /** Discard routes are most preferred */
    Discard               =  2,

    /** Local prefixes are directly attached prefixes on the
     *  system such as e.g. interface routes.
     */
    LocalPrefix           =  3,
    /** Advertised in S-TIEs */
    SouthPGPPrefix        =  4,
    /** Advertised in N-TIEs */
    NorthPGPPrefix        =  5,
    /** Advertised in N-TIEs */
    NorthPrefix           =  6,
    /** Externally imported north */
    NorthExternalPrefix   =  7,
    /** Advertised in S-TIEs, either normal prefix or positive
        disaggregation */
    SouthPrefix           =  8,
    /** Externally imported south */
    SouthExternalPrefix   =  9,
    /** Negative, transitive prefixes are least preferred */
    NegativeSouthPrefix   = 10,
    RouteTypeMaxValue     = 11,
}

B.2.  encoding.thrift

 /**
     Thrift file for packet encodings for RIFT
 */

 include "common.thrift"

 /** Represents protocol encoding schema major version */
 const common.VersionType protocol_major_version = 2
 /** Represents protocol encoding schema minor version */
 const common.MinorVersionType protocol_minor_version =  0

 /** Common RIFT packet header. */
 struct PacketHeader {
     /** Major version of protocol. */

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     1: required common.VersionType major_version =
             protocol_major_version;
     /** Minor version of protocol. */
     2: required common.VersionType minor_version =
             protocol_minor_version;
     /** Node sending the packet, in case of LIE/TIRE/TIDE
         also the originator of it. */
     3: required common.SystemIDType  sender;
     /** Level of the node sending the packet, required on everything
         except LIEs. Lack of presence on LIEs indicates UNDEFINED_LEVEL
         and is used in ZTP procedures.
      */
     4: optional common.LevelType            level;
 }

 /** Prefix community. */
 struct Community {
     /** Higher order bits */
     1: required i32          top;
     /** Lower order bits */
     2: required i32          bottom;
 }

 /** Neighbor structure.  */
 struct Neighbor {
     /** System ID of the originator. */
     1: required common.SystemIDType        originator;
     /** ID of remote side of the link. */
     2: required common.LinkIDType          remote_id;
 }

 /** Capabilities the node supports.

     @note: The schema may add to this
     field future capabilities to indicate whether it will support
     interpretation of future schema extensions on the same major
     revision. Such fields MUST be optional and have an implicit or
     explicit false default value. If a future capability changes route
     selection or generates blackholes if some nodes are not supporting
     it then a major version increment is unavoidable.
 */
 struct NodeCapabilities {
     /** Must advertise supported minor version dialect that way. */
     1: required common.MinorVersionType        protocol_minor_version =
             protocol_minor_version;
     /** Can this node participate in flood reduction. */
     2: optional bool                           flood_reduction =
             common.flood_reduction_default;

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     /** Does this node restrict itself to be top-of-fabric or
         leaf only (in ZTP) and does it support leaf-2-leaf
         procedures. */
     3: optional common.HierarchyIndications    hierarchy_indications;
 }

 /** Link capabilities. */
 struct LinkCapabilities {
     /** Indicates that the link is supporting BFD. */
     1: optional bool                           bfd =
             common.bfd_default;
     /** Indicates whether the interface will support v4 forwarding.

         @note: This MUST be set to true when LIEs from a v4 address are
                sent and MAY be set to true in LIEs on v6 address. If v4
                and v6 LIEs indicate contradicting information the
                behavior is unspecified. */
     2: optional bool                           v4_forwarding_capable =
             true;
 }

 /** RIFT LIE Packet.

     @note: this node's level is already included on the packet header
 */
 struct LIEPacket {
     /** Node or adjacency name. */
     1: optional string                    name;
     /** Local link ID. */
     2: required common.LinkIDType         local_id;
     /** UDP port to which we can receive flooded TIEs. */
     3: required common.UDPPortType        flood_port =
             common.default_tie_udp_flood_port;
     /** Layer 3 MTU, used to discover to mismatch. */
     4: optional common.MTUSizeType        link_mtu_size =
             common.default_mtu_size;
     /** Local link bandwidth on the interface. */
     5: optional common.BandwithInMegaBitsType
             link_bandwidth = common.default_bandwidth;
     /** Reflects the neighbor once received to provide
         3-way connectivity. */
     6: optional Neighbor                  neighbor;
     /** Node's PoD. */
     7: optional common.PodType            pod =
             common.default_pod;
     /** Node capabilities shown in LIE. The capabilities
         MUST match the capabilities shown in the Node TIEs, otherwise
         the behavior is unspecified. A node detecting the mismatch

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         SHOULD generate according error. */
    10: required NodeCapabilities          node_capabilities;
    /** Capabilities of this link. */
    11: optional LinkCapabilities          link_capabilities;
    /** Required holdtime of the adjacency, i.e. how much time
        MUST expire without LIE for the adjacency to drop. */
    12: required common.TimeIntervalInSecType
             holdtime = common.default_lie_holdtime;
    /** Unsolicited, downstream assigned locally significant label
        value for the adjacency. */
    13: optional common.LabelType          label;
     /** Indicates that the level on the LIE MUST NOT be used
         to derive a ZTP level by the receiving node. */
    21: optional bool                      not_a_ztp_offer =
             common.default_not_a_ztp_offer;
    /** Indicates to northbound neighbor that it should
        be reflooding this node's N-TIEs to achieve flood reduction and
        balancing for northbound flooding. To be ignored if received
        from a northbound adjacency. */
    22: optional bool                      you_are_flood_repeater =
              common.default_you_are_flood_repeater;
    /** Can be optionally set to indicate to neighbor that packet losses
        are seen on reception based on packet numbers or the rate is
        too high. The receiver SHOULD temporarily slow down
        flooding rates.
     */
    23: optional bool                      you_are_sending_too_quickly =
              false;
    /** Instance name in case multiple RIFT instances running on same
        interface. */
    24: optional string                    instance_name;
 }

 /** LinkID pair describes one of parallel links between two nodes. */
 struct LinkIDPair {
     /** Node-wide unique value for the local link. */
     1: required common.LinkIDType      local_id;
     /** Received remote link ID for this link. */
     2: required common.LinkIDType      remote_id;

     /** Describes the local interface index of the link. */
    10: optional common.PlatformInterfaceIndex platform_interface_index;
    /** Describes the local interface name. */
    11: optional string                        platform_interface_name;
    /** Indication whether the link is secured, i.e. protected by
        outer key, absence of this element means no indication,
        undefined outer key means not secured. */
    12: optional common.OuterSecurityKeyID

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                 trusted_outer_security_key;
    /** Indication whether the link is protected by established
        BFD session. */
    13: optional bool                          bfd_up;
 }

 /** ID of a TIE.

     @note: TIEID space is a total order achieved by comparing
            the elements in sequence defined and comparing each
            value as an unsigned integer of according length.
 */
 struct TIEID {
     /** direction of TIE */
     1: required common.TieDirectionType    direction;
     /** indicates originator of the TIE */
     2: required common.SystemIDType        originator;
     /** type of the tie */
     3: required common.TIETypeType         tietype;
     /** number of the tie */
     4: required common.TIENrType           tie_nr;
 }

 /** Header of a TIE.

    @note: TIEID space is a total order achieved by comparing
           the elements in sequence defined and comparing each
           value as an unsigned integer of according length.

    @note: After sequence number the lifetime received on the envelope
           must be used for comparison before further fields.

    @note: `origination_time` and `origination_lifetime` are disregarded
           for comparison purposes and carried purely for
           debugging/security purposes if present.
 */
 struct TIEHeader {
     /** ID of the tie. */
     2: required TIEID                             tieid;
     /** Sequence number of the tie. */
     3: required common.SeqNrType                  seq_nr;

     /** Absolute timestamp when the TIE
         was generated. This can be used on fabrics with
         synchronized clock to prevent lifetime modification attacks. */
    10: optional common.IEEE802_1ASTimeStampType   origination_time;
    /** Original lifetime when the TIE
        was generated. This can be used on fabrics with

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        synchronized clock to prevent lifetime modification attacks. */
    12: optional common.LifeTimeInSecType          origination_lifetime;
 }

 /** Header of a TIE as described in TIRE/TIDE.
 */
 struct TIEHeaderWithLifeTime {
     1: required     TIEHeader                       header;
     /** Remaining lifetime that expires down to 0 just like in ISIS.
         TIEs with lifetimes differing by less than
         `lifetime_diff2ignore` MUST be considered EQUAL. */
     2: required     common.LifeTimeInSecType        remaining_lifetime;
 }

 /** TIDE with sorted TIE headers, if headers are unsorted, behavior
     is undefined. */
 struct TIDEPacket {
     /** First TIE header in the tide packet. */
     1: required TIEID                       start_range;
     /** Last TIE header in the tide packet. */
     2: required TIEID                       end_range;
     /** _Sorted_ list of headers. */
     3: required list<TIEHeaderWithLifeTime> headers;
 }

 /** TIRE packet */
 struct TIREPacket {
     1: required set<TIEHeaderWithLifeTime>  headers;
 }

 /** neighbor of a node */
 struct NodeNeighborsTIEElement {
     /** level of neighbor */
     1: required common.LevelType                level;
     /**  Cost to neighbor.

          @note: All parallel links to same node
          incur same cost, in case the neighbor has multiple
          parallel links at different cost, the largest distance
          (highest numerical value) MUST be advertised.

          @note: any neighbor with cost <= 0 MUST be ignored
                 in computations */
     3: optional common.MetricType               cost
                 = common.default_distance;
     /** can carry description of multiple parallel links in a TIE */
     4: optional set<LinkIDPair>                 link_ids;

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     /** total bandwith to neighbor, this will be normally sum of the
         bandwidths of all the parallel links. */
     5: optional common.BandwithInMegaBitsType
                 bandwidth = common.default_bandwidth;
 }

 /** Indication flags of the node. */
 struct NodeFlags {
     /** Indicates that node is in overload, do not transit traffic
         through it. */
     1: optional bool         overload = common.overload_default;
 }

 /** Description of a node.

     It may occur multiple times in different TIEs but if either
         <list>
          <t>capabilities values do not match or</t>
         <t>flags values do not match or</t>
         <t>neighbors repeat with different values</t>
         </list>

     the behavior is undefined and a warning SHOULD be generated.
     Neighbors can be distributed across multiple TIEs however if
     the sets are disjoint. Miscablings SHOULD be repeated in every
     node TIE, otherwise the behavior is undefined.

     @note: Observe that absence of fields implies defined defaults.
 */
 struct NodeTIEElement {
     /** Level of the node. */
     1: required common.LevelType            level;
     /** Node's neighbors. If neighbor systemID repeats in other
         node TIEs of same node the behavior is undefined. */
     2: required map<common.SystemIDType,
                 NodeNeighborsTIEElement>    neighbors;
     /** Capabilities of the node. */
     3: required NodeCapabilities            capabilities;
     /** Flags of the node. */
     4: optional NodeFlags                   flags;
     /** Optional node name for easier operations. */
     5: optional string                      name;
     /** PoD to which the node belongs. */
     6: optional common.PodType              pod;

     /** If any local links are miscabled, the indication is flooded. */
    10: optional set<common.LinkIDType>      miscabled_links;

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 }

 /** Attributes of a prefix. */
 struct PrefixAttributes {
     /** Distance of the prefix. */
     2: required common.MetricType            metric
             = common.default_distance;
     /** Generic unordered set of route tags, can be redistributed
         to other protocols or use within the context of real time
         analytics. */
     3: optional set<common.RouteTagType>     tags;
     /** Monotonic clock for mobile addresses. */
     4: optional common.PrefixSequenceType    monotonic_clock;
     /** Indicates if the interface is a node loopback. */
     6: optional bool                         loopback = false;
     /** Indicates that the prefix is directly attached, i.e. should be
         routed to even if the node is in overload. */
     7: optional bool                         directly_attached = true;

     /** In case of locally originated prefixes, i.e. interface
         addresses this can describe which link the address
         belongs to. */
    10: optional common.LinkIDType            from_link;
 }

 /** TIE carrying prefixes */
 struct PrefixTIEElement {
     /** Prefixes with the associated attributes.
         If the same prefix repeats in multiple TIEs of same node
         behavior is unspecified. */
     1: required map<common.IPPrefixType, PrefixAttributes> prefixes;
 }

 /** Generic key value pairs. */
 struct KeyValueTIEElement {
     /** @note: if the same key repeats in multiple TIEs of same node
         or with different values, behavior is unspecified */
     1: required map<common.KeyIDType,string>    keyvalues;
 }

 /** Single element in a TIE.

     Schema enum `common.TIETypeType`
     in TIEID indicates which elements MUST be present
     in the TIEElement. In case of mismatch the unexpected
     elements MUST be ignored. In case of lack of expected
     element the TIE an error MUST be reported and the TIE
     MUST be ignored.

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     This type can be extended with new optional elements
     for new `common.TIETypeType` values without breaking
     the major but if it is necessary to understand whether
     all nodes support the new type a node capability must
     be added as well.
  */
 union TIEElement {
     /** Used in case of enum common.TIETypeType.NodeTIEType. */
     1: optional NodeTIEElement     node;
     /** Used in case of enum common.TIETypeType.PrefixTIEType. */
     2: optional PrefixTIEElement          prefixes;
     /** Positive prefixes (always southbound).
         It MUST NOT be advertised within a North TIE and
         ignored otherwise.
     */
     3: optional PrefixTIEElement   positive_disaggregation_prefixes;
     /** Transitive, negative prefixes (always southbound) which
         MUST be aggregated and propagated
         according to the specification
         southwards towards lower levels to heal
         pathological upper level partitioning, otherwise
         blackholes may occur in multiplane fabrics.
         It MUST NOT be advertised within a North TIE.
     */
     5: optional PrefixTIEElement   negative_disaggregation_prefixes;
     /** Externally reimported prefixes. */
     6: optional PrefixTIEElement          external_prefixes;
     /** Positive external disaggregated prefixes (always southbound).
         It MUST NOT be advertised within a North TIE and
         ignored otherwise.
     */
     7: optional PrefixTIEElement
             positive_external_disaggregation_prefixes;
     /** Key-Value store elements. */
     9: optional KeyValueTIEElement keyvalues;
 }

 /** TIE packet */
 struct TIEPacket {
     1: required TIEHeader  header;
     2: required TIEElement element;
 }

 /** Content of a RIFT packet. */
 union PacketContent {
     1: optional LIEPacket     lie;
     2: optional TIDEPacket    tide;
     3: optional TIREPacket    tire;

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     4: optional TIEPacket     tie;
 }

 /** RIFT packet structure. */
 struct ProtocolPacket {
     1: required PacketHeader  header;
     2: required PacketContent content;
 }

Appendix C.  Constants

C.1.  Configurable Protocol Constants

   This section gathers constants that are provided in the schema files
   and in the document.

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   +----------------+--------------+-----------------------------------+
   |                | Type         | Value                             |
   +----------------+--------------+-----------------------------------+
   | LIE IPv4       | Default      | 224.0.0.120 or all-rift-routers   |
   | Multicast      | Value,       | to be assigned in IPv4            |
   | Address        | Configurable | Multicast Address Space Registry  |
   |                |              | in Local Network Control Block    |
   +----------------+--------------+-----------------------------------+
   | LIE IPv6       | Default      | FF02::A1F7 or all-rift-routers to |
   | Multicast      | Value,       | be assigned in IPv6 Multicast     |
   | Address        | Configurable | Address Assignments               |
   +----------------+--------------+-----------------------------------+
   | LIE            | Default      | 914                               |
   | Destination    | Value,       |                                   |
   | Port           | Configurable |                                   |
   +----------------+--------------+-----------------------------------+
   | Level value    | Constant     | 24                                |
   | for            |              |                                   |
   | TOP_OF_FABRIC  |              |                                   |
   | flag           |              |                                   |
   +----------------+--------------+-----------------------------------+
   | Default LIE    | Default      | 3 seconds                         |
   | Holdtime       | Value,       |                                   |
   |                | Configurable |                                   |
   +----------------+--------------+-----------------------------------+
   | TIE            | Default      | 1 second                          |
   | Retransmission | Value        |                                   |
   | Interval       |              |                                   |
   +----------------+--------------+-----------------------------------+
   | TIDE           | Default      | 5 seconds                         |
   | Generation     | Value,       |                                   |
   | Interval       | Configurable |                                   |
   +----------------+--------------+-----------------------------------+
   | MIN_TIEID      | Constant     | TIE Key with minimal values:      |
   | signifies      |              | TIEID(originator=0,               |
   | start of TIDEs |              | tietype=TIETypeMinValue,          |
   |                |              | tie_nr=0, direction=South)        |
   +----------------+--------------+-----------------------------------+
   | MAX_TIEID      | Constant     | TIE Key with maximal values:      |
   | signifies end  |              | TIEID(originator=MAX_UINT64,      |
   | of TIDEs       |              | tietype=TIETypeMaxValue,          |
   |                |              | tie_nr=MAX_UINT64,                |
   |                |              | direction=North)                  |
   +----------------+--------------+-----------------------------------+

                          Table 6: all_constants

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Authors' Addresses

   Tony Przygienda (editor)
   Juniper
   1137 Innovation Way

   Sunnyvale, CA

   USA

   Email: prz@juniper.net

   Alankar Sharma
   Comcast
   1800 Bishops Gate Blvd
   Mount Laurel, NJ  08054
   US

   Email: Alankar_Sharma@comcast.com

   Pascal Thubert
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200
   MOUGINS - Sophia Antipolis  06254
   FRANCE

   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com

   Bruno Rijsman
   Individual

   Email: brunorijsman@gmail.com

   Dmitry Afanasiev
   Yandex

   Email: fl0w@yandex-team.ru

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