RIFT Working Group                                    T. Przygienda, Ed.
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                               A. Sharma
Expires: October 28, 2018                                        Comcast
                                                              P. Thubert
                                                                A. Atlas
                                                                J. Drake
                                                        Juniper Networks
                                                            Apr 26, 2018

                       RIFT: Routing in Fat Trees


   This document outlines a specialized, dynamic routing protocol for
   Clos and fat-tree network topologies.  The protocol (1) deals with
   automatic construction of fat-tree topologies based on detection of
   links, (2) minimizes the amount of routing state held at each level,
   (3) automatically prunes the topology distribution exchanges to a
   sufficient subset of links, (4) supports automatic disaggregation of
   prefixes on link and node failures to prevent black-holing and
   suboptimal routing, (5) allows traffic steering and re-routing
   policies, (6) allows non-ECMP forwarding, (7) automatically re-
   balances traffic towards the spines based on bandwidth available and
   ultimately (8) 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.

Status of This Memo

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   This Internet-Draft will expire on October 28, 2018.

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Copyright Notice

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   2.  Reference Frame . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Topology  . . . . . . . . . . . . . . . . . . . . . . . .   8
   3.  Requirement Considerations  . . . . . . . . . . . . . . . . .  10
   4.  RIFT: Routing in Fat Trees  . . . . . . . . . . . . . . . . .  12
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.2.  Specification . . . . . . . . . . . . . . . . . . . . . .  13
       4.2.1.  Transport . . . . . . . . . . . . . . . . . . . . . .  13
       4.2.2.  Link (Neighbor) Discovery (LIE Exchange)  . . . . . .  13
       4.2.3.  Topology Exchange (TIE Exchange)  . . . . . . . . . .  15  Topology Information Elements . . . . . . . . . .  15  South- and Northbound Representation  . . . . . .  16  Flooding  . . . . . . . . . . . . . . . . . . . .  19  TIE Flooding Scopes . . . . . . . . . . . . . . .  19  Initial and Periodic Database Synchronization . .  21  Purging . . . . . . . . . . . . . . . . . . . . .  21  Southbound Default Route Origination  . . . . . .  22  Northbound TIE Flooding Reduction . . . . . . . .  22
       4.2.4.  Policy-Guided Prefixes  . . . . . . . . . . . . . . .  26  Ingress Filtering . . . . . . . . . . . . . . . .  27  Applying Policy . . . . . . . . . . . . . . . . .  28  Store Policy-Guided Prefix for Route Computation
                   and Regeneration  . . . . . . . . . . . . . . . .  29  Re-origination  . . . . . . . . . . . . . . . . .  29  Overlap with Disaggregated Prefixes . . . . . . .  30
       4.2.5.  Reachability Computation  . . . . . . . . . . . . . .  30  Northbound SPF  . . . . . . . . . . . . . . . . .  30  Southbound SPF  . . . . . . . . . . . . . . . . .  31  East-West Forwarding Within a Level . . . . . . .  31

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       4.2.6.  Attaching Prefixes  . . . . . . . . . . . . . . . . .  32
       4.2.7.  Attaching Policy-Guided Prefixes  . . . . . . . . . .  33
       4.2.8.  Automatic Disaggregation on Link & Node Failures  . .  34
       4.2.9.  Optional Autoconfiguration  . . . . . . . . . . . . .  37  Terminology . . . . . . . . . . . . . . . . . . .  38  Automatic SystemID Selection  . . . . . . . . . .  39  Generic Fabric Example  . . . . . . . . . . . . .  39  Level Determination Procedure . . . . . . . . . .  40  Resulting Topologies  . . . . . . . . . . . . . .  41
       4.2.10. Stability Considerations  . . . . . . . . . . . . . .  43
     4.3.  Further Mechanisms  . . . . . . . . . . . . . . . . . . .  44
       4.3.1.  Overload Bit  . . . . . . . . . . . . . . . . . . . .  44
       4.3.2.  Optimized Route Computation on Leafs  . . . . . . . .  44
       4.3.3.  Mobility  . . . . . . . . . . . . . . . . . . . . . .  44  Clock Comparison  . . . . . . . . . . . . . . . .  46  Interaction between Time Stamps and Sequence
                   Counters  . . . . . . . . . . . . . . . . . . . .  46  Anycast vs. Unicast . . . . . . . . . . . . . . .  47  Overlays and Signaling  . . . . . . . . . . . . .  47
       4.3.4.  Key/Value Store . . . . . . . . . . . . . . . . . . .  48  Southbound  . . . . . . . . . . . . . . . . . . .  48  Northbound  . . . . . . . . . . . . . . . . . . .  48
       4.3.5.  Interactions with BFD . . . . . . . . . . . . . . . .  48
       4.3.6.  Fabric Bandwidth Balancing  . . . . . . . . . . . . .  49  Northbound Direction  . . . . . . . . . . . . . .  49  Southbound Direction  . . . . . . . . . . . . . .  51
       4.3.7.  Label Binding . . . . . . . . . . . . . . . . . . . .  52
       4.3.8.  Segment Routing Support with RIFT . . . . . . . . . .  52  Global Segment Identifiers Assignment . . . . . .  52  Distribution of Topology Information  . . . . . .  52
       4.3.9.  Leaf to Leaf Procedures . . . . . . . . . . . . . . .  53
       4.3.10. Other End-to-End Services . . . . . . . . . . . . . .  53
       4.3.11. Address Family and Multi Topology Considerations  . .  53
       4.3.12. Reachability of Internal Nodes in the Fabric  . . . .  54
       4.3.13. One-Hop Healing of Levels with East-West Links  . . .  54
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  54
     5.1.  Normal Operation  . . . . . . . . . . . . . . . . . . . .  54
     5.2.  Leaf Link Failure . . . . . . . . . . . . . . . . . . . .  55
     5.3.  Partitioned Fabric  . . . . . . . . . . . . . . . . . . .  56
     5.4.  Northbound Partitioned Router and Optional East-West
           Links . . . . . . . . . . . . . . . . . . . . . . . . . .  58
   6.  Implementation and Operation: Further Details . . . . . . . .  59
     6.1.  Considerations for Leaf-Only Implementation . . . . . . .  60
     6.2.  Adaptations to Other Proposed Data Center Topologies  . .  60
     6.3.  Originating Non-Default Route Southbound  . . . . . . . .  61
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  61
   8.  Information Elements Schema . . . . . . . . . . . . . . . . .  62
     8.1.  common.thrift . . . . . . . . . . . . . . . . . . . . . .  62

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     8.2.  encoding.thrift . . . . . . . . . . . . . . . . . . . . .  67
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  72
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  73
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  73
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  73
     11.2.  Informative References . . . . . . . . . . . . . . . . .  75
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  77

1.  Introduction

   Clos [CLOS] and Fat-Tree [FATTREE] 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 routing protocols were geared towards a network with an
   irregular topology and low degree of connectivity originally but
   given they were the only available mechanisms, consequently several
   attempts to apply those to Clos have been made.  Most successfully
   BGP [RFC4271] [RFC7938] has been extended to this purpose, not as
   much due to its inherent suitability to solve the problem but rather
   because the perceived capability to modify it "quicker" and the
   immanent difficulties with link-state [DIJKSTRA] based protocols to
   perform in large scale densely meshed topologies.

   In looking at the problem through the lens of its requirements an
   optimal approach does not seem however to be a simple modification of
   either a link-state (distributed computation) or distance-vector
   (diffused computation) approach but rather a mixture of both,
   colloquially best described as "link-state towards the spine" and
   "distance vector towards the leafs".  In other words, "bottom" levels
   are flooding their link-state information in the "northern" direction
   while each switch generates under normal conditions a default route
   and floods it in the "southern" direction.  Obviously, such
   aggregation can blackhole in cases of misconfiguration or failures
   and this has to be addressed somehow.

   For the visually oriented reader, Figure 1 presents a first
   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 routes to them.  In the second row of the
   database we indicate that a partial information of other nodes in the
   same level is available as well; the details of how this is achieved
   should be postponed for the moment.  Whereas when we look at the
   "bottom" of the fabric we see that the topology of the leafs is
   basically empty and they only hold a load balanced default route to
   the next level.

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   The balance of this document details the resulting protocol and fills
   in the missing details.

              .                                  [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

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Reference Frame

2.1.  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 directed acyclic
   graphs (DAG).

   Level:  Clos and Fat Tree networks are trees and 'level' denotes the
      set of nodes at the same height in such a network, where the
      bottom level is level 0.  A node has links to nodes one level down

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      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.

   Spine/Aggregation/Edge Levels:  Traditional names for Level 2, 1 and
      0 respectively.  Level 0 is often called leaf as well.

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

   Spine:  The set of nodes that provide inter-PoD communication.  These
      nodes are also organized into levels (typically one, three, or
      five levels).  Spine nodes do not belong to any PoD and are
      assigned "undefined" PoD value to indicate the equivalent of "any"

   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 4.2.9).

   Connected Spine:  In case a spine level represents a connected graph
      (discounting links terminating at different levels), we call it a
      "connected spine", in case a spine level consists of multiple
      partitions, we call it a "disconnected" or "partitioned spine".
      In other terms, a spine without East-West links is disconnected
      and is the typical configuration forf Clos and Fat Tree networks.

   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.

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

   Southbound representation:  Information sent towards a lower level
      representing only limited amount of information.

   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.  It can be thought of as
      largely equivalent to ISIS LSPs or OSPF LSA.  We will talk about
      N-TIEs when talking about TIEs in the northbound representation
      and S-TIEs for the southbound equivalent.

   Node TIE:  This is an acronym for a "Node Topology Information
      Element", largely equivalent to OSPF Node LSA, i.e. it contains
      all neighbors the node discovered and information about node

   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 N-TIE and in case of S-TIE the necessary default
      and de-aggregated prefixes the node passes southbound.

   Policy-Guided Information:  Information that is passed in either
      southbound direction or north-bound direction by the means of
      diffusion and can be filtered via policies.  Policy-Guided
      Prefixes and KV Ties are examples of Policy-Guided Information.

   Key Value TIE:  A S-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.

   PGP:  Policy-Guided Prefixes allow to support traffic engineering
      that cannot be achieved by the means of SPF computation or normal
      node and prefix S-TIE origination.  S-PGPs are propagated in south
      direction only and N-PGPs follow northern direction strictly.

   De-aggregation/Disaggregation:  Process in which a node decides to
      advertise certain prefixes it received in N-TIEs to prevent black-
      holing and suboptimal routing upon link failures.

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   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 adjacencies.

   FL:  Flooding Leader for a specific system has a dedicated role to
      flood TIEs of that system.

   FR:  Flooding Repeater for a specific system has a dedicated role to
      flood TIEs of that system northbound.  Similar to MPR in OSLR.

   BAD:  This is an acronym for Bandwidth Adjusted Distance.  RIFT
      calculates the amount of northbound bandwidth available towards a
      node compared to other nodes at the same level and adjusts the
      default route distance accordingly to allow for the lower level to
      adjust their load balancing.

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

2.2.  Topology

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    .                +--------+          +--------+
    .                |        |          |        |          ^ N
    .                |Spine 21|          |Spine 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 |       |     |       |           |       |
    .|Node111+----------+Node112|     |Node121|           |Node122|
    .+-+---+-+          ++----+-+     +-+---+-+           ++---+--+
    .  |   |             |   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 two level spine-and-leaf topology

   We will use this topology (called commonly a fat tree/network in
   modern DC considerations [VAHDAT08] as homonym to the original
   definition of the term [FATTREE]) in all further considerations.  It
   depicts a generic "fat-tree" and the concepts explained in three
   levels here carry by induction for further levels and higher degrees
   of connectivity.  However, this document will deal with designs that
   provide only sparser connectivity as well.

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3.  Requirement Considerations

   [RFC7938] gives the original set of requirements augmented here based
   upon recent experience in the operation of fat-tree networks.

   REQ1:    The control protocol should discover the physical links
            automatically and be able to detect cabling that violates
            fat-tree topology constraints.  It must react accordingly to
            such mis-cabling attempts, at a minimum preventing
            adjacencies between nodes from being formed and traffic from
            being forwarded on those mis-cabled links.  E.g.  connecting
            a leaf to a spine at level 2 should be detected and ideally

   REQ2:    A node without any configuration beside default values
            should come up at the correct level in any PoD it is
            introduced into.  Optionally, it must be possible to
            configure nodes to restrict their participation to the
            PoD(s) targeted at any level.

   REQ3:    Optionally, the protocol should allow to provision data
            centers where the individual switches carry no configuration
            information and are all deriving their level from a "seed".
            Observe that this requirement may collide with the desire to
            detect cabling misconfiguration and with that only one of
            the requirements can be fully met in a chosen configuration

   REQ4:    The solution should allow for minimum size routing
            information base and forwarding tables at leaf level for
            speed, cost and simplicity reasons.  Holding excessive
            amount of information away from leaf nodes simplifies
            operation and lowers cost of the underlay.

   REQ5:    Very high degree of ECMP must be supported.  Maximum ECMP is
            currently understood as the most efficient routing approach
            to maximize the throughput of switching fabrics

   REQ6:    Non equal cost anycast must be supported to allow for easy
            and robust multi-homing of services without regressing to
            careful balancing of link costs.

   REQ7:    Traffic engineering should be allowed by modification of
            prefixes and/or their next-hops.

   REQ8:    The solution should allow for access to link states of the
            whole topology to enable efficient support for modern

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            control architectures like SPRING [RFC7855] or PCE

   REQ9:    The solution should easily accommodate opaque data to be
            carried throughout the topology to subsets of nodes.  This
            can be used for many purposes, one of them being a key-value
            store that allows bootstrapping of nodes based right at the
            time of topology discovery.

   REQ10:   Nodes should be taken out and introduced into production
            with minimum wait-times and minimum of "shaking" of the
            network, i.e.  radius of propagation (often called "blast
            radius") of changed information should be as small as

   REQ11:   The protocol should allow for maximum aggregation of carried
            routing information while at the same time automatically de-
            aggregating the prefixes to prevent black-holing in case of
            failures.  The de-aggregation should support maximum
            possible ECMP/N-ECMP remaining after failure.

   REQ12:   Reducing the scope of communication needed throughout the
            network on link and state failure, as well as reducing
            advertisements of repeating, idiomatic or policy-guided
            information in stable state is highly desirable since it
            leads to better stability and faster convergence behavior.

   REQ13:   Once a packet traverses a link in a "southbound" direction,
            it must not take any further "northbound" steps along its
            path to delivery to its destination under normal conditions.
            Taking a path through the spine in cases where a shorter
            path is available is highly undesirable.

   REQ14:   Parallel links between same set of nodes must be
            distinguishable for SPF, failure and traffic engineering

   REQ15:   The protocol must not rely on interfaces having discernible
            unique addresses, i.e. it must operate in presence of
            unnumbered links (even parallel ones) or links of a single
            node having same addresses.

   REQ16:   It would be desirable to achieve fast re-balancing of flows
            when links, especially towards the spines are lost or
            provisioned without regressing to per flow traffic
            engineering which introduces significant amount of
            complexity while possibly not being reactive enough to
            account for short-lived flows.

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   REQ17:   The control plane should be able to unambiguously determine
            the current point of attachment (which port on which leaf
            node) of a prefix, even in a context of fast mobility, e.g.,
            when the prefix is a host address on a wireless node that 1)
            may associate to any of multiple access points (APs) that
            are attached to different ports on a same leaf node or to
            different leaf nodes, and 2) may move and reassociate
            several times to a different AP within a sub-second period.

   Following list represents possible requirements and requirements
   under discussion:

   PEND1:   Supporting anything but point-to-point links is a non-
            requirement.  Questions remain: for connecting to the
            leaves, is there a case where multipoint is desirable?  One
            could still model it as point-to-point links; it seems there
            is no need for anything more than a NBMA-type construct.

   PEND2:   What is the maximum scale of number leaf prefixes we need to
            carry.  Is 500'000 enough ?

   Finally, following are the non-requirements:

   NONREQ1:   Broadcast media support is unnecessary.

   NONREQ2:   Purging is unnecessary given its fragility and complexity
              and today's large memory size on even modest switches and

   NONREQ3:   Special support for layer 3 multi-hop adjacencies is not
              part of the protocol specification.  Such support can be
              easily provided by using tunneling technologies the same
              way IGPs today are solving the problem.

4.  RIFT: Routing in Fat Trees

   Derived from the above requirements we present 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 "pointing north" and path-
   vector [RFC4271] protocol when "pointing south".  Albeit an unusual
   combination, it does quite naturally exhibit the desirable properties
   we seek.

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4.1.  Overview

   The singular property of RIFT is that it floods only northbound
   "flat" link-state information so that each level understands the full
   topology of levels south of it.  That information is never flooded
   East-West or back South again.  In the southbound direction the
   protocol operates like a "unidirectional" path vector protocol or
   rather a distance vector with implicit split horizon whereas the
   information only propagates one hop south and is 're-advertised' by
   nodes at next lower level.  However, we use flooding in the southern
   direction as well to avoid the necessity to build an update per
   neighbor.  We leave the East-West direction out for the moment.

   Those information flow constraints create a "smooth" information
   propagation where nodes do not receive the same information from
   multiple fronts which would force them to perform a diffused
   computation to tie-break the same reachability information arriving
   on arbitrary links and ultimately force hop-by-hop forwarding on
   shortest-paths only.

   To account for the "northern" and the "southern" information split
   the link state database is partitioned into "north representation"
   and "south representation" TIEs, whereas in simplest terms the N-TIEs
   contain a link state topology description of lower levels and and
   S-TIEs carry simply default routes.  This oversimplified view will be
   refined gradually in following sections while introducing protocol
   procedures aimed to fulfill the described requirements.

4.2.  Specification

4.2.1.  Transport

   All protocol elements are carried over UDP.  Once QUIC [QUIC]
   achieves the desired stability in deployments it may prove a valuable
   candidate for TIE transport.

   All packet formats are defined in Thrift models in Section 8.

   Future versions may include a [PROTOBUF] schema.

4.2.2.  Link (Neighbor) Discovery (LIE Exchange)

   LIE exchange happens over well-known administratively locally scoped
   IPv4 multicast address [RFC2365] or link-local multicast scope
   [RFC4291] for IPv6 [RFC8200] and SHOULD be sent with a TTL of 1 to
   prevent RIFT information reaching beyond a single L3 next-hop in the
   topology.  LIEs are exchanged over all links running RIFT.

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   Unless Section 4.2.9 is used, each node is provisioned with the level
   at which it is operating and its PoD (or otherwise a default level
   and "undefined" PoD are assumed; meaning that leafs do not need to be
   configured at all if initial configuration values are all left at 0).
   Nodes in the spine are configured with "any" PoD which has the same
   value "undefined" PoD hence we will talk about "undefined/any" PoD.
   This information is propagated in the LIEs exchanged.

   A node tries to form a three way adjacency if and only if
   (definitions of LEAF_ONLY are found in Section 4.2.9)

   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

   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 with level higher than the neighboring node

          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 4.3.9 OR

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


   Rule in Paragraph 3 MAY be optionally disregarded by a node if PoD
   detection is undesirable or has to be disregarded.

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   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
   spines in a PoD) can diffuse southbound towards the leafs "forcing"
   the PoD value on any node with "undefined" PoD.

   LIEs arriving with a TTL 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

   LIE exchange uses three way handshake mechanism [RFC5303].  Precise
   finite state machines will be provided in later versions of this
   specification.  LIE packets contain nonces and may contain an SHA-1
   [RFC6234] over nonces and some of the LIE data which prevents
   corruption and replay attacks.  TIE flooding reuses those nonces to
   prevent mismatches and can use those for security purposes in case it
   is using QUIC [QUIC].  Section 7 will address the precise security
   mechanisms in the future.

4.2.3.  Topology Exchange (TIE Exchange)  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

   TIE exchange mechanism uses 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.

   TIEs contain sequence numbers, lifetimes and a type.  Each type has a
   large 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
   point of the design space is a prefix per TIE which leads to BGP-like
   behavior vs. dense packing into few TIEs leading to more traditional
   IGP trade-off with fewer TIEs.  An implementation may even rehash at
   the cost of significant amount of re-advertisements of TIEs.

   More information about the TIE structure can be found in the schema
   in Section 8.

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Internet-Draft                    RIFT                          Apr 2018  South- and Northbound Representation

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

   The N-TIEs hold all of the node's adjacencies, local prefixes and
   northbound policy-guided prefixes while the S-TIEs hold only all of
   the node's adjacencies and the default prefix with necessary
   disaggregated prefixes and southbound policy-guided prefixes.  We
   will explain this in detail further in Section 4.2.8 and
   Section 4.2.4.

   The TIE types are symmetric in both directions and Table 1 provides a
   quick reference to the different TIE types including direction and
   their function.

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   | TIE-Type | Content                                                |
   | node     | node properties, adjacencies and information helping   |
   | N-TIE    | in complex disaggregation scenarios                    |
   | node     | same content as node N-TIE except the information to   |
   | S-TIE    | help disaggregation                                    |
   | Prefix   | contains nodes' directly reachable prefixes            |
   | N-TIE    |                                                        |
   | Prefix   | contains originated defaults and de-aggregated         |
   | S-TIE    | prefixes                                               |
   | PGP      | contains nodes north PGPs                              |
   | N-TIE    |                                                        |
   | PGP      | contains nodes south PGPs                              |
   | S-TIE    |                                                        |
   | KV       | contains nodes northbound KVs                          |
   | N-TIE    |                                                        |
   | KV       | contains nodes southbound KVs                          |
   | S-TIE    |                                                        |

                            Table 1: TIE Types

   As an example illustrating a databases holding both representations,
   consider the topology in Figure 2 with the optional link between node
   111 and node 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 and the PGP elements which may be included in their S-TIEs
   or N-TIEs are not shown.

        Spine21 S-TIEs:
        Node S-TIE:
          NodeElement(layer=2, neighbors((Node111, layer 1, cost 1),
          (Node112, layer 1, cost 1), (Node121, layer 1, cost 1),
          (Node122, layer 1, cost 1)))
        Prefix S-TIE:
          SouthPrefixesElement(prefixes(0/0, cost 1), (::/0, cost 1))

        Node111 S-TIEs:

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        Node S-TIE:
          NodeElement(layer=1, neighbors((Spine21, layer 2, cost 1, links(...)),
          (Spine22, layer 2, cost 1, links(...)),
          (Node112, layer 1, cost 1, links(...)),
          (Leaf111, layer 0, cost 1, links(...)),
          (Leaf112, layer 0, cost 1, links(...))))
        Prefix S-TIE:
          SouthPrefixesElement(prefixes(0/0, cost 1), (::/0, cost 1))

        Node111 N-TIEs:
        Node N-TIE:
          neighbors((Spine21, layer 2, cost 1, links(...)),
          (Spine22, layer 2, cost 1, links(...)),
          (Node112, layer 1, cost 1, links(...)),
          (Leaf111, layer 0, cost 1, links(...)),
          (Leaf112, layer 0, cost 1, links(...))))
        Prefix N-TIE:

        Node121 S-TIEs:
        Node S-TIE:
          NodeElement(layer=1, neighbors((Spine21,layer 2,cost 1),
          (Spine22, layer 2, cost 1), (Leaf121, layer 0, cost 1),
          (Leaf122, layer 0, cost 1)))
        Prefix S-TIE:
          SouthPrefixesElement(prefixes(0/0, cost 1), (::/0, cost 1))

        Node121 N-TIEs:
        Node N-TIE:
          neighbors((Spine21, layer 2, cost 1, links(...)),
          (Spine22, layer 2, cost 1, links(...)),
          (Leaf121, layer 0, cost 1, links(...)),
          (Leaf122, layer 0, cost 1, links(...))))
        Prefix N-TIE:

        Leaf112 N-TIEs:
        Node N-TIE:
          neighbors((Node111, layer 1, cost 1, links(...)),
          (Node112, layer 1, cost 1, links(...))))
        Prefix N-TIE:
          NorthPrefixesElement(prefixes(Leaf112.loopback, Prefix112,

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   Figure 3: example TIES generated in a 2 level spine-and-leaf topology  Flooding

   The mechanism used to distribute TIEs is the well-known (albeit
   modified in several respects to address fat tree requirements)
   flooding mechanism used by today's link-state protocols.  Although
   flloding is initially more demanding to implement it avoids many
   problems with update style used in diffused computation such as path
   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

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

   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.

   Precise finite state machines and procedures will be provided in
   later versions of this specification.  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 N-TIE is flooded northbound, providing a node at a given level
   with the complete topology of the Clos or Fat Tree network underneath
   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 may 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 S-TIEs, consisting of all node's adjacencies and prefix
   S-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 a E-W
   disconnected node in a given level to receive the S-TIEs of other
   nodes at its level, every *NODE* S-TIE is "reflected" northbound to

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   level from which it was received.  It should be noted that East-West
   links are included in South TIE flooding; those TIEs need to be
   flooded to satisfy algorithms in Section 4.2.5.  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 flooding scopes are given in
   Table 2.  Those rules govern as well what SHOULD be included in TIDEs
   towards neighbors.  East-West flooding scopes are identical to South
   flooding scopes.

   Node S-TIE "reflection" allows to support disaggregation on failures
   describes in Section 4.2.8 and flooding reduction in Section

   | Packet Type  | South                      | North                 |
   | vs. Peer     |                            |                       |
   | Direction    |                            |                       |
   | node S-TIE   | flood self-originated only | flood if TIE          |
   |              |                            | originator's level is |
   |              |                            | higher than own level |
   | non-node     | flood self-originated only | flood only if TIE     |
   | S-TIE        |                            | originator is equal   |
   |              |                            | peer                  |
   | all N-TIEs   | never flood                | flood always          |
   | TIDE         | include TIEs in flooding   | include TIEs in       |
   |              | scope                      | flooding scope        |
   | TIRE         | include all N-TIEs and all | include only if TIE   |
   |              | peer's self-originated     | originator is equal   |
   |              | TIEs and all node S-TIEs   | peer                  |

                         Table 2: Flooding Scopes

   As an example to illustrate these rules, consider using the topology
   in Figure 2, with the optional link between node 111 and node 112,
   and the associated TIEs given in Figure 3.  The flooding from
   particular nodes of the TIEs is given in Table 3.

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   | Router     | Neighbor | TIEs                                      |
   | floods to  |          |                                           |
   | Leaf111    | Node112  | Leaf111 N-TIEs, Node111 node S-TIE        |
   | Leaf111    | Node111  | Leaf111 N-TIEs, Node112 node S-TIE        |
   |            |          |                                           |
   | Node111    | Leaf111  | Node111 S-TIEs                            |
   | Node111    | Leaf112  | Node111 S-TIEs                            |
   | Node111    | Node112  | Node111 S-TIEs                            |
   | Node111    | Spine21  | Node111 N-TIEs, Leaf111 N-TIEs, Leaf112   |
   |            |          | N-TIEs, Spine22 node S-TIE                |
   | Node111    | Spine22  | Node111 N-TIEs, Leaf111 N-TIEs, Leaf112   |
   |            |          | N-TIEs, Spine21 node S-TIE                |
   |            |          |                                           |
   | ...        | ...      | ...                                       |
   | Spine21    | Node111  | Spine21 S-TIEs                            |
   | Spine21    | Node112  | Spine21 S-TIEs                            |
   | Spine21    | Node121  | Spine21 S-TIEs                            |
   | Spine21    | Node122  | Spine21 S-TIEs                            |
   | ...        | ...      | ...                                       |

             Table 3: Flooding some TIEs from example topology  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 2.  Purging

   RIFT does not purge information that has been distributed by the
   protocol.  Purging mechanisms in other routing protocols have proven
   to be complex and fragile over many years of experience.  Abundant
   amounts of memory are available today even on low-end platforms.  The
   information will age out and all computations will deliver correct
   results if a node leaves the network due to the new information
   distributed by its adjacent nodes.

   Once a RIFT node issues a TIE with an ID, it MUST 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.

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   Each node will timeout and clean up the according empty TIEs

   Upon restart a node MUST, as any link-state implementation, be
   prepared to receive TIEs with its own system ID and supercede 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.  Southbound Default Route Origination

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

   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 layer
   (otherwise the node S-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.  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:

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

   2.  it is typically not necessary that all parents reflood the N-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

   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.

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   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
   S-TIEs, which indicate the parent's reachable northbound adjacencies
   to its own parents, i.e. the node's grandparents.  An optional
   boolean information `you_are_not_flood_repeater` in a LIE packet to a
   parent is set to indicate that the parent is not an FR and that it
   SHOULD NOT reflood N-TIEs.

   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

   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.  |a-b|<=S.

   The algorithm consists of the following steps:

   1.  derive a 16-bits pseudo-random unsigned integer PR(N) from N's
       system ID by splitting it in 16-bits-long words W1, W2, ..., Wn
       and then XOR'ing the circularly shifted resulting words together,
       and casting the resulting representation:

       1.  (unsigned integer) (W1<<1 xor (W2<<2) xor ... xor (Wn<<n) );

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   2.  sort the parents by decreasing number of northbound adjacencies:
       sort |P(N) by decreasing CN(P), for all P in |P(N), as ordered
       array |A(N);

   3.  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):

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

       2.  set i=0;

       3.  while i < CN(N) do

           1.  set k=k+1;

           2.  set j=i;

           3.  while CN(|A(N)[j]) - CN(|A(N)[i]) <= S

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

               2.  set i=i+1;

           /* 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) */

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

   4.  shuffle individually each subarrays |A_k(N) of cardinality C_k(N)
       within |A(N) using a Fisher-Yates method that depends on N's
       System ID:

       1.  while k > 0 do

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

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

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

           2.  set k=k-1;

   5.  for each grandparent, initialize a counter with the number of its
       Southbound adjacencies :

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       1.  for each G in |G(N) set c(G) = CS(G);

   6.  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:

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

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

               1.  place P in FR set;

           2.  else

               1.  for all adjacencies ADJ(P, G) in |NA(P)

                   1.  decrement c(G);

   The algorithm MUST be re-evaluated by a node on every change of local
   adjacencies or reception of a parent S-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.

4.2.4.  Policy-Guided Prefixes

   In a fat tree, it can be sometimes desirable to guide traffic to
   particular destinations or keep specific flows to certain paths.  In
   RIFT, this is done by using policy-guided prefixes with their
   associated communities.  Each community is an abstract value whose
   meaning is determined by configuration.  It is assumed that the
   fabric is under a single administrative control so that the meaning
   and intent of the communities is understood by all the nodes in the
   fabric.  Any node can originate a policy-guided prefix.

   Since RIFT uses distance vector concepts in a southbound direction,
   it is straightforward to add a policy-guided prefix to an S-TIE.  For
   easier troubleshooting, the approach taken in RIFT is that a node's
   southbound policy-guided prefixes are sent in its S-TIE and the
   receiver does inbound filtering based on the associated communities
   (an egress policy is imaginable but would lead to different S-TIEs
   per neighbor possibly which is not considered in RIFT protocol
   procedures).  A southbound policy-guided prefix can only use links in
   the south direction.  If an PGP S-TIE is received on an East-West or
   northbound link, it must be discarded by ingress filtering.

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   Conceptually, a southbound policy-guided prefix guides traffic from
   the leaves up to at most the north-most layer.  It is also necessary
   to to have northbound policy-guided prefixes to guide traffic from
   the north-most layer down to the appropriate leaves.  Therefore, RIFT
   includes northbound policy-guided prefixes in its N PGP-TIE and the
   receiver does inbound filtering based on the associated communities.
   A northbound policy-guided prefix can only use links in the northern
   direction.  If an N PGP TIE is received on an East-West or southbound
   link, it must be discarded by ingress filtering.

   By separating southbound and northbound policy-guided prefixes and
   requiring that the cost associated with a PGP is strictly
   monotonically increasing at each hop, the path cannot loop.  Because
   the costs are strictly increasing, it is not possible to have a loop
   between a northbound PGP and a southbound PGP.  If East-West links
   were to be allowed, then looping could occur and issues such as
   counting to infinity would become an issue to be solved.  If complete
   generality of path - such as including East-West links and using both
   north and south links in arbitrary sequence - then a Path Vector
   protocol or a similar solution must be considered.

   If a node has received the same prefix, after ingress filtering, as a
   PGP in an S-TIE and in an N-TIE, then the node determines which
   policy-guided prefix to use based upon the advertised cost.

   A policy-guided prefix is always preferred to a regular prefix, even
   if the policy-guided prefix has a larger cost.  Section 8 provides
   normative indication of prefix preferences.

   The set of policy-guided prefixes received in a TIE is subject to
   ingress filtering and then re-originated to be sent out in the
   receiver's appropriate TIE.  Both the ingress filtering and the re-
   origination use the communities associated with the policy-guided
   prefixes to determine the correct behavior.  The cost on re-
   advertisement MUST increase in a strictly monotonic fashion.  Ingress Filtering

   When a node X receives a PGP S-TIE or a PGP N-TIE that is originated
   from a node Y which does not have an adjacency with X, all PGPs in
   such a TIE MUST be filtered.  Similarly, if node Y is at the same
   layer as node X, then X MUST filter out PGPs in such S- and N-TIEs to
   prevent loops.

   Next, policy can be applied to determine which policy-guided prefixes
   to accept.  Since ingress filtering is chosen rather than egress
   filtering and per-neighbor PGPs, policy that applies to links is done
   at the receiver.  Because the RIFT adjacency is between nodes and

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   there may be parallel links between the two nodes, the policy-guided
   prefix is considered to start with the next-hop set that has all
   links to the originating node Y.

   A policy-guided prefix has or is assigned the following attributes:

   cost:   This is initialized to the cost received

   community_list:   This is initialized to the list of the communities

   next_hop_set:   This is initialized to the set of links to the
      originating node Y.  Applying Policy

   The specific action to apply based upon a community is deployment
   specific.  Here are some examples of things that can be done with
   communities.  The length of a community is a 64 bits number and it
   can be written as a single field M or as a multi-field (S = M[0-31],
   T = M[32-63]) in these examples.  For simplicity, the policy-guided
   prefix is referred to as P, the processing node as X and the
   originator as Y.

   Prune Next-Hops: Community Required:   For each next-hop in
      P.next_hop_set, if the next-hop does not have the community, prune
      that next-hop from P.next_hop_set.

   Prune Next-Hops: Avoid Community:   For each next-hop in
      P.next_hop_set, if the next-hop has the community, prune that
      next-hop from P.next_hop_set.

   Drop if Community:   If node X has community M, discard P.

   Drop if not Community:   If node X does not have the community M,
      discard P.

   Prune to ifIndex T:   For each next-hop in P.next_hop_set, if the
      next-hop's ifIndex is not the value T specified in the community
      (S,T), then prune that next-hop from P.next_hop_set.

   Add Cost T:   For each appearance of community S in P.community_list,
      if the node X has community S, then add T to P.cost.

   Accumulate Min-BW T:   Let bw be the sum of the bandwidth for
      P.next_hop_set.  If that sum is less than T, then replace (S,T)
      with (S, bw).

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   Add Community T if Node matches S:   If the node X has community S,
      then add community T to P.community_list.  Store Policy-Guided Prefix for Route Computation and

   Once a policy-guided prefix has completed ingress filtering and
   policy, it is almost ready to store and use.  It is still necessary
   to adjust the cost of the prefix to account for the link from the
   computing node X to the originating neighbor node Y.

   There are three different policies that can be used:

   Minimum Equal-Cost:   Find the loWest cost C next-hops in
      P.next_hop_set and prune to those.  Add C to P.cost.

   Minimum Unequal-Cost:   Find the loWest cost C next-hop in
      P.next_hop_set.  Add C to P.cost.

   Maximum Unequal-Cost:   Find the highest cost C next-hop in
      P.next_hop_set.  Add C to P.cost.

   The default policy is Minimum Unequal-Cost but well-known communities
   can be defined to get the other behaviors.

   Regardless of the policy used, a node MUST store a PGP cost that is
   at least 1 greater than the PGP cost received.  This enforces the
   strictly monotonically increasing condition that avoids loops.

   Two databases of PGPs - from N-TIEs and from S-TIEs are stored.  When
   a PGP is inserted into the appropriate database, the usual tie-
   breaking on cost is performed.  Observe that the node retains all PGP
   TIEs due to normal flooding behavior and hence loss of the best
   prefix will lead to re-evaluation of TIEs present and re-
   advertisement of a new best PGP.  Re-origination

   A node must re-originate policy-guided prefixes and retransmit them.
   The node has its database of southbound policy-guided prefixes to
   send in its S-TIE and its database of northbound policy-guided
   prefixes to send in its N-TIE.

   Of course, a leaf does not need to re-originate southbound policy-
   guided prefixes.

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   PGPs may overlap with prefixes introduced by automatic de-
   aggregation.  The topic is under further discussion.  The break in
   connectivity that leads to infeasibility of a PGP is mirrored in
   adjacency tear-down and according removal of such PGPs.
   Nevertheless, the underlying link-state flooding will be likely
   reacting significantly faster than a hop-by-hop redistribution and
   with that the preference for PGPs may cause intermittent black-holes.

4.2.5.  Reachability Computation

   A node has three sources of relevant information.  A node knows the
   full topology south from the received N-TIEs.  A node has the set of
   prefixes with associated distances and bandwidths from received
   S-TIEs.  A node can also have a set of PGPs.

   To compute reachability, a node runs conceptually a northbound and a
   southbound SPF.  We call that N-SPF and S-SPF.

   Since neither computation can "loop" (with due considerations given
   to PGPs), it is possible to compute non-equal-cost or even k-shortest
   paths [EPPSTEIN] and "saturate" the fabric to the extent desired.  Northbound SPF

   N-SPF uses northbound and East-West adjacencies in North Node TIEs
   when progressing Dijkstra.  Observe that this is really just a one
   hop variety since South Node TIEs are not re-flooded southbound
   beyond a single level (or East-West) and with that the computation
   cannot progress beyond adjacent nodes.

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

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

   2.  the adjacent node has one or more northbound adjacencies

   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.

   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

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

   i.e. the E-W link can be used as the 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.

   For N-SPF we are using the South Node TIEs to find according
   adjacencies to verify backlink connectivity.  Just as in case of IS-
   IS or OSPF, two unidirectional links are associated together to
   confirm bidirectional connectivity.  Southbound SPF

   S-SPF uses only the southbound adjacencies in the south node 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 uses northbound adjacencies in north node TIEs to verify
   backlink connectivity.  East-West Forwarding Within a Level

   Ultimately, it should be observed that in presence of a "ring" of E-W
   links in a level neither SPF will provide a "ring protection" scheme
   since such a computation would have to deal necessarily with breaking
   of "loops" in generic Dijkstra sense; an application for which RIFT
   is not intended.  It is outside the scope of this document how an
   underlay can be used to provide a full-mesh connectivity between
   nodes in the same layer that would allow for N-SPF to provide
   protection for a single node loosing all its northbound adjacencies
   (as long as any of the other nodes in the level are northbound

   Using south prefixes over horizontal links is optional and 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.

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4.2.6.  Attaching Prefixes

   After the SPF is run, it is necessary to attach according prefixes.
   For S-SPF, prefixes from an N-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, type=spf, path_distance, next-hop
   set), accumulates these results.  Obviously, the prefix retains its
   type which is used to tie-break between the same prefix advertised
   with different types.

   In case of N-SPF prefixes from each S-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 S-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
   S-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.

   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 cost next-hop to that neighbor.  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.  RIFT route preferences are normalized by the according
   thrift model type.

   An exemplary implementation for node X follows:

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

                Figure 4: Adding Routes from S-TIE Prefixes

4.2.7.  Attaching Policy-Guided Prefixes

   Each policy-guided prefix P has its cost and next_hop_set already
   stored in the associated database, as specified in Section;
   the cost stored for the PGP is already updated to considering the
   cost of the link to the advertising neighbor.  By definition, a
   policy-guided prefix is preferred to a regular prefix.

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    for each policy-guided prefix P:
      if P not in route_database:
         add (P, type=PolicyGuided, P.cost, next_hop_set)
         end if
      if P in route_database :
          if (route_database[P].type is not PolicyGuided) or
             (route_database[P].cost > P.cost):
            update route_database[P] with (P, PolicyGuided, P.cost, next_hop_set)
          else if route_database[P].cost == P.cost
            update route_database[P] with (P, PolicyGuided, P.cost,
               merge(next_hop_set, route_database[P].next_hop_set))
            // Not preferred route so ignore
            end if
          end if
      end for

            Figure 5: Adding Routes from Policy-Guided Prefixes

4.2.8.  Automatic Disaggregation on Link & Node Failures

   Under normal circumstances, node's S-TIEs contain just the
   adjacencies, a default route and policy-guided prefixes.  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 S-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 as 'de-
   aggregation' or 'dis-aggregation'.

   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 N-TIEs are used to find all of prefixes it can reach and
       the set of next-hops in the lower level for each.  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

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       be |H(r).  Observe that policy-guided prefixes are NOT affected
       since their scope is controlled by configuration.

   2.  The node uses reflected S-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).

   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 S-TIE.

   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 S-TIE, a node at the lower level that
       receives this S-TIE will not propagate it south-bound.  Neither
       is it necessary for the receiving node to 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.  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 or East-West 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 layer from X's perspective.
       Each entry is a list of south neighbor of X and a list of nodes
       of X.layer that can't reach that neighbor.

   2.  A node X determines its set of southbound neighbors

   3.  For each S-TIE originated from a node Y that X has which is at
       X.layer, 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.

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

   A node X computes its SPF based upon the received N-TIEs.  This
   results in a set of routes, each 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_layer = { empty }
            for each S-TIE
              if (S-TIE.layer == X.layer and
                  X shares at least one S-neighbor with X)
                add S-TIE.originator to nodes_same_layer
                end if
              end for

            for each next-hop-set NHS
              isolated_nodes = nodes_same_layer
              for each NH in NHS
                if NH in partial_neighbors
                  isolated_nodes = intersection(isolated_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 S-TIE
            if X's S-TIE is different
              schedule S-TIE for flooding
              end if

              Figure 6: Computation to Disaggregate Prefixes

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   Each disaggregated prefix is sent with the accurate path_distance.
   This allows a node to send the same S-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:

   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 S-TIE per node and
       complicate things unnecessarily.  The PoD containing the prefix
       will prefer southbound anyway.

   4.  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 only.

   5.  disaggregated prefix S-TIEs are not "reflected" by the lower
       layer, 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.

   Ultimately, complex partitions of superspine on sparsely connected
   fabrics can lead to necessity of transitive disaggregation through
   multiple layers.  The topic will be described and standardized in
   later versions of this document.

4.2.9.  Optional Autoconfiguration

   Each RIFT node can optionally operate in zero touch provisioning
   (ZTP) mode, i.e. it has no configuration (unless it is a superspine
   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.  This section describes the necessary
   concepts and procedures.

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

   UNDEFINED_LEVEL:  An imaginary value that indicates that the level
      has not beeen 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".
      SUPERSPINE_FLAG and CONFIGURED_LEVEL cannot be defined at the same
      time as this flag.  It implies CONFIGURED_LEVEL value of 0.

   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.  SUPERSPINE_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

   LEAF_2_LEAF:  An optional flag that can be configured on a node to
      make sure it supports procedures defined in Section 4.3.9.
      SUPERSPINE_FLAG is ignored when set at the same time as this flag.
      LEAF_2_LEAF implies LEAF_ONLY and the according restrictions.

   LEVEL_VALUE:  In ZTP case the original definition of "level" in
      Section 2.1 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).  Offers
      from LIEs with `not_a_ztp_offer` being true are not VOLs either.

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

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

   SUPERSPINE_FLAG:  Configuration flag provided to all superspines.
      LEAF_FLAG and CONFIGURED_LEVEL cannot be defined at the same time
      as this flag.  It implies CONFIGURED_LEVEL value of 16.  In fact,
      it is basically a shortcut for configuring same level at all
      superspine nodes which is unavoidable since an initial 'seed' is
      needed for other ZTP nodes to derive their level in the topology.  Automatic SystemID Selection

   RIFT identifies each node via a SystemID which is a 64 bits wide
   integer.  It is relatively simple to derive a, for all practical
   purposes collision free, value for each node on startup.  For that
   purpose, a node MUST use as system ID EUI-64 MA-L format where the
   organizationally governed 24 bits can be used to generate system IDs
   for multiple RIFT instances running on the system.

   The router MUST ensure that such identifier is not changing very
   frequently (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 suggested in Section 7 are
   implemented).  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 7.  We assume all
   nodes being in the same PoD.

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

               Figure 7: Generic ZTP Cabling Considerations

   First, we need to anchor the "top" of the cabling and that's what the
   SUPERSPINE_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 or at the
   same level as Y and consequently, X is south of it.  This is
   unresolvable here until we "nail down the bottom" of the topology.
   To achieve that we use the the leaf flags.  We will see further then
   whether Y chooses to form adjacencies to F or I, J successively.  Level Determination Procedure

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

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   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 chooses on an ongoing basis from all VOLs the value of
       MAX(HAL-1,0) as its DERIVED_LEVEL.  The node then starts to
       advertise this derived level.

   3.  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.

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

   5.  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

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

   A node starting with LEVEL_VALUE being 0 (i.e. it assumes a leaf
   function or has a CONFIGURED_LEVEL of 0) MUST follow those additional

   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.

   Precise finite state machines will be provided in later versions of
   this specification.  Resulting Topologies

   The procedures defined in Section will lead to the RIFT
   topology and levels depicted in Figure 8.

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                      .        +---+
                      .        | As|
                      .        | 64|
                      .        ++-++
                      .         | |
                      .      +--+ +--+
                      .      |       |
                      .   +--++     ++--+
                      .   | E |     | F |
                      .   | 63+-+   | 63+-----------+
                      .   ++--+ |   ++-++           |
                      .    |    |    | |            |
                      .    | +-------+ |            |
                      .    | |  |      |            |
                      .    | |  +----+ |            |
                      .    | |       | |            |
                      .   ++-++     ++-++           |
                      .   | I +-----+ J |           |
                      .   | 62|     | 62|           |
                      .   ++--+     +--++           |
                      .    |           |            |
                      .    +---------+ |            |
                      .              | |            |
                      .             ++-++     +---+ |
                      .             | X |     | Y +-+
                      .             | 0 |     | 0 |
                      .             +---+     +---+

               Figure 8: 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 9.  This
   demonstrates basically that auto configuration prevents miscabling
   detection and with that can lead to undesirable effects when leafs
   are not "nailed" and arbitrarily cabled.

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                       .        +---+
                       .        | As|
                       .        | 64|
                       .        ++-++
                       .         | |
                       .      +--+ +--+
                       .      |       |
                       .   +--++     ++--+
                       .   | E |     | F |
                       .   | 63+-+   | 63+-------+
                       .   ++--+ |   ++-++       |
                       .    |    |    | |        |
                       .    | +-------+ |        |
                       .    | |  |      |        |
                       .    | |  +----+ |        |
                       .    | |       | |        |
                       .   ++-++     ++-++     +-+-+
                       .   | I +-----+ J +-----+ Y |
                       .   | 62|     | 62|     | 62|
                       .   ++-++     +--++     ++-++
                       .    | |         |       | |
                       .    | +-----------------+ |
                       .    |           |         |
                       .    +---------+ |         |
                       .              | |         |
                       .             ++-++        |
                       .             | X +--------+
                       .             | 0 |
                       .             +---+

               Figure 9: Generic ZTP Topology Autoconfigured

4.2.10.  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.  Fortuntately, 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

   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.

   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 Leafs

   Since the leafs do see only "one hop away" they do not need to run a
   full SPF but can simply gather prefix candidates from their neighbors
   and build the according routing table.

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

   Instead of creating a network graph from its N-TIEs and neighbor's
   S-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 interfaces, determining bi-directionality from the associated
   N-TIE, and specifying the neighbor's next_hop_set set and cost from
   the minimum cost local interfaces to that neighbor.

   Then a leaf attaches prefixes as in Section 4.2.6 as well as the
   policy-guided prefixes as in Section 4.2.7.

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 attachement 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 memorizes 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 to be able to compare time stamps as observed by the
      various points of attachment, e.g., using the variation of the

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      Precision Time Protocol (PTP) IEEE Std. 1588 [IEEEstd1588],
      [IEEEstd8021AS] designed for bridged LANs IEEE Std. 802.1AS
      [IEEEstd8021AS].  Both the precision of the synchronisation
      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

   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 MUST advertise a time stamp of the latest sighting
      of a prefix, e.g., by snooping IP protocols or the switch using
      the time at which it advertised the prefix.  RIFT transports the
      time stamp within the desired prefix N-TIEs as 802.1AS timestamp.

   o  RIFT may interoperate with the "update to 6LoWPAN Neighbor
      Discovery" [I-D.ietf-6lo-rfc6775-update], 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 (ANSC) that compares
      as less than any other clock.  By default, the lack of a
      `PrefixSequenceType` in a Prefix N-TIE is interpreted as ANSC.  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.  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 [I-D.ietf-6lo-rfc6775-update].  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 infrastruture captures is enough
   to determine the freshest discovery.  If the point of attachement
   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
   stampling mechanism.

   The sequence counter in [I-D.ietf-6lo-rfc6775-update] is encoded as
   one octet, wraps after 127 increments, and, by default, values are
   defined as comparable as long as they are less than SEQUENCE_WINDOW =
   16 apart.  An implementation MAY allow this to be configurable
   throughout the domain, and the number can be pushed up to 64 and
   still preserve the capability to discover an error situation where
   counters are not comparable.

   Within the resolution of MAXIMUM_CLOCK_DELTA the sequence counters
   captured during 2 sequential values of the time stamp must be

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   comparable.  This means with default values that a node may move up
   to 16 times during a 200 milliseconds period and the clocks remain
   still comparable thus allowing the infrastructure to assert the
   freshest advertisement with no ambiguity.  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 prefered.  An
   anycast prefix does not carry a clock or all clock attributes MUST be
   the same under the rules of Section

   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

   Observe further that without support for
   [I-D.ietf-6lo-rfc6775-update] movements on the fabric within
   intervals smaller than 100msec will be seen as anycast.  Overlays and Signaling

   RIFT is agnostic whichever the overlay technology [MIP, LISP, VxLAN,
   NVO3] and the associated signaling is deployed over it.  But it is
   expected that leaf nodes, and possibly superspine nodes can perform
   the according 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 superspines may be desired.  Future
   versions of this document may describe support for such tunneling in

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4.3.4.  Key/Value Store  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 S-TIEs can arrive from multiple nodes and
   hence need tie-breaking per key.  We use the following rules

   1.  Only KV TIEs originated by a node to which the receiver has an
       adjacency are considered.

   2.  Within all valid KV S-TIEs containing the key, the value of the
       KV S-TIE for which the according node S-TIE is present, has the
       highest level and within the same level has highest originator 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 but
   a diffused computation.  Northbound

   Certain use cases seem to necessitate distribution of essentialy KV
   information that is generated in the leafs in the northbound
   direction.  Such information is flooded in KV N-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.

      In case established BFD session goes Down after it was Up, RIFT
      adjacency should be re-initialized started from Init.

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      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 both the hello 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 CAN advertise the same link ID for the same interface in
      multiple instances and with that "share" the discriminators).

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 layer based on the ingress and egress bandwidth
   they have.  Current attempts rely mostly on specialized traffic
   engineering via controller or leafs 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 approximative way based on the fact that RIFT is loop-
   free.  Northbound Direction

   Every RIFT node SHOULD compute the amount of northbound bandwith
   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              |   |
                   .  +-+---+-+          +-+---+-+
                   .  |       |          |       |
                   .  |Node111|          |Node112|
                   .  +-+---+++          ++----+++
                   .    |x  ||           ||    ||
                   .    ||  |+---------------+ ||
                   .    ||  +---------------+| ||
                   .    ||               || || ||
                   .    ||               || || ||
                   .   -----All Links 10 MBit-------
                   .    ||               || || ||
                   .    ||               || || ||
                   .    ||  +------------+| || ||
                   .    ||  |+------------+ || ||
                   .    |x  ||              || ||
                   .  +-+---+++          +--++-+++
                   .  |       |          |       |
                   .  |Leaf111|          |Leaf112|
                   .  +-------+          +-------+

                      Figure 10: Balancing Bandwidth

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

   The local modification of the received default route distance from
   upper layer 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 adjustements.

   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 | Node111 | 110   | 7     | 2   |
                | Leaf111 | Node112 | 220   | 8     | 1   |
                | Leaf112 | Node111 | 120   | 7     | 2   |
                | Leaf112 | Node112 | 220   | 8     | 1   |

                         Table 4: BAD Computation

   All the multiplications and additions are saturating, i.e. when
   exceeding range of the bandwidth type are set to highest possible
   value of the type.

   Observe that since BAD is only computed for default routes any
   disaggregated prefixes so PGP or disaggregated routes are not
   affected, however, a node MAY choose to compute and use BAD for other

   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.  Southbound Direction

   Due to its loop free properties a node could take during S-SPF into
   account the available bandwidth on the nodes in lower layers and
   modify the amount of traffic offered to next level's "southbound"
   nodes based as what it sees is the total achievable maximum flow
   through those nodes.  It is worth observing that such computations
   will work better if standardized but does not have to be necessarily.

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   As long the packet keeps on heading south it will take one of the
   available paths and arrive at the intended destination.

   Future versions of this document will fill in more details.

4.3.7.  Label Binding

   A node MAY advertise on its TIEs a locally significant, downstream
   assigned label for the according interface.  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.  Segment Routing Support with RIFT

   Recently, alternative architecture to reuse labels as segment
   identifiers [I-D.ietf-spring-segment-routing] has gained traction and
   may present use cases in DC fabric that would justify its deployment.
   Such use cases will either precondition an assignment of a label per
   node (or other entities where the mechanisms are equivalent) or a
   global assignment and a knowledge of topology everywhere to compute
   segment stacks of interest.  We deal with the two issues separately.  Global Segment Identifiers Assignment

   Global segment identifiers are normally assumed to be provided by
   some kind of a centralized "controller" instance and distributed to
   other entities.  This can be performed in RIFT by attaching a
   controller to the superspine nodes at the top of the fabric where the
   whole topology is always visible, assign such identifiers and then
   distribute those via the KV mechanism towards all nodes so they can
   perform things like probing the fabric for failures using a stack of
   segments.  Distribution of Topology Information

   Some segment routing use cases seem to precondition full knowledge of
   fabric topology in all nodes which can be performed albeit at the
   loss of one of highly desirable properties of RIFT, namely minimal
   blast radius.  Basically, RIFT can function as a flat IGP by
   switching off its flooding scopes.  All nodes will end up with full
   topology view and albeit the N-SPF and S-SPF are still performed
   based on RIFT rules, any computation with segment identifiers that
   needs full topology can use it.

   Beside blast radius problem, excessive flooding may present
   significant load on implementations.

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4.3.9.  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).

4.3.10.  Other End-to-End Services

   Losing full, flat topology information at every node will have an
   impact on some of the end-to-end network services.  This is the price
   paid for minimal disturbance in case of failures and reduced flooding
   and memory requirements on nodes lower south in the level hierarchy.

4.3.11.  Address Family and Multi Topology Considerations

   Multi-Topology (MT)[RFC5120] and Multi-Instance (MI)[RFC6822] 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.

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

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4.3.12.  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 Prefix TIEs.

   Things get more interesting in case a node looses all its northbound
   adjacencies but is not at the top of the fabric.  In such a case a
   node that detects that some other members at its level are
   advertising northbound adjacencies MAY inject its loopback address
   into southbound PGP TIE and become reachable "from the south" that
   way.  Further, a solution may be implemented where based on e.g. a
   "well known" community such a southbound PGP is reflected at level 0
   and advertised as northbound PGP again to allow for "reachability
   from the north" at the cost of additional flooding.

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

   Based on the rules defined in Section 4.2.5, Section 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.  Section 5.4 explains the resulting behavior
   based on one such example.

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

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

   3.  Node 111 to Leaf 111, Leaf 112

   4.  Node 112 to Leaf 111, Leaf 112

   5.  Node 121 to Leaf 121, Leaf 122

   6.  Node 122 to Leaf 121, Leaf 122

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   Consequently, N-TIEs would be originated by Node 111 and Node 112 and
   each set would be sent to both Spine 21 and Spine 22.  N-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
   Node 111 and Node 112.  Node 111 and Node 112 would then flood these
   N-TIEs to Spine 21 and Spine 22.

   Similarly, N-TIEs would be originated by Node 121 and Node 122 and
   each set would be sent to both Spine 21 and Spine 22.  N-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
   Node 121 and Node 122.  Node 121 and Node 122 would then flood these
   N-TIEs to Spine 21 and Spine 22.

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

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

   An S Tie with a default IP prefix would be originated by Node 111 and
   Node 112 and each would be sent to Leaf 111 and Leaf 112.  Leaf 111
   and Leaf 112 would each send the S-TIE from Node 111 to Node 112 and
   the S-TIE from Node 112 to Node 111.

   Similarly, an S Tie with a default IP prefix would be originated by
   Node 121 and Node 122 and each would be sent to Leaf 121 and Leaf
   122.  Leaf 121 and Leaf 122 would each send the S-TIE from Node 121
   to Node 122 and the S-TIE from Node 122 to Node 121.  At this point
   IP connectivity with maximum possible ECMP has been established
   between the leafs 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

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                    .  |   |              |   |
                    .+-+---+-+          +-+---+-+
                    .|       |          |       |
                    .|Node111|          |Node112|
                    .+-+---+-+          ++----+-+
                    .  |   |             |    |
                    .  |   +---------------+  X
                    .  |                 | |  X Failure
                    .  |   +-------------+ |  X
                    .  |   |               |  |
                    .+-+---+-+          +--+--+-+
                    .|       |          |       |
                    .|Leaf111|          |Leaf112|
                    .+-------+          +-------+
                    .      +                  +
                    .     Prefix111     Prefix112

                    Figure 11: Single Leaf link failure

   In case of a failing leaf link between node 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
   node 112.  Only nodes 111 and 112, as well as both spines will see
   control traffic.  Leaf 111 will receive a new S-TIE from node 112 and
   reflect back to node 111.  Node 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 node 112 the traffic would end up on spine 21 and spine 22 and
   cross back into pod 1 using node 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

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    .                +--------+          +--------+   S-TIE of Spine21
    .                |        |          |        |   received by
    .                |Spine 21|          |Spine 22|   reflection of
    .                ++-+--+-++          ++-+--+-++   Nodes 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
    .|Node111|          |Node112|     |Node121|           |Node122|
    .+-+---+-+          ++----+-+     +-+---+-+           ++---+--+
    .  |   |             |    |         |   |              |   |
    .  |   +---------------+  |         |   +----------------+ |
    .  |                 | |  |         |                  | | |
    .  |   +-------------+ |  |         |   +--------------+ | |
    .  |   |               |  |         |   |                | |
    .+-+---+-+          +--+--+-+     +-+---+-+          +---+-+-+
    .|       |          |       |     |       |          |       |
    .|Leaf111|          |Leaf112|     |Leaf121|          |Leaf122|
    .+-+-----+          ++------+     +-----+-+          +-+-----+
    .  +                 +                  +              +
    .  Prefix111    Prefix112             Prefix121     Prefix122
    .                                       1.1/16

                        Figure 12: Fabric partition

   Figure 12 shows the arguably most catastrophic but also the most
   interesting case.  Spine 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 spine 21 and spine 22.

   The mechanism used to resolve this scenario is hinging on the
   distribution of southbound representation by spine 21 that is
   reflected by node 111 and node 112 to spine 22.  Spine 22, having
   computed reachability to all prefixes in the network, advertises with
   the default route the ones that are reachable only via lower level

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   neighbors that spine 21 does not show an adjacency to.  That results
   in node 111 and node 112 obtaining a longest-prefix match to prefix
   121 which leads through spine 22 and prevents black-holing through
   spine 21 still advertising the 0/0 aggregate only.

   The prefix 121 advertised by spine 22 does not have to be propagated
   further towards leafs since they do no benefit from this information.
   Hence the amount of flooding is restricted to spine 21 reissuing its
   S-TIEs and reflection of those by node 111 and node 112.  The
   resulting SPF in spine 22 issues a new prefix S-TIEs containing
   1.1/16.  None of the leafs 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 4.2.8, spine 22 constructs the
   following sets:

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

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

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

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

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

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

   With that and |H (for r=prefix 121) and |H (for r=prefix 122) being
   disjoint from |A (for spine 21), spine 22 will originate an S-TIE
   with prefix 121 and prefix 122, that is flooded to nodes 112, 112,
   121 and 122.

5.4.  Northbound Partitioned Router and Optional East-West Links

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

                    Figure 13: North Partitioned Router

   Figure 13 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 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.  Moreover,
   based on Section 4.3.12 it may advertise its loopback address as
   south PGP to remain reachable "from the south" for operational
   purposes.  This is necessary since A02 will NOT route towards A01
   using the E-W link (doing otherwise may form routing loops).

   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  Moreover, A02 may now inject its loopback address
   as south PGP.

6.  Implementation and Operation: Further Details

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6.1.  Considerations for Leaf-Only Implementation

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

   1.  Under normal conditions, the leaf needs to support a multipath
       default route only.  In worst 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 N-TIEs and S-TIEs of Level 1 nodes
       they are connected to; so overall few in numbers.

   3.  Leaf node does not have to support flooding reduction and de-

   4.  Unless optional leaf-2-leaf procedures are desired default route
       origination, S-TIE origination is unnecessary.

6.2.  Adaptations to Other Proposed Data Center Topologies

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

                         Figure 14: 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 layers but
   certain requirements in Section 3 will not be met anymore.  As an
   example, shortcutting levels illustrated in Figure 14 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 leafs 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.

6.3.  Originating Non-Default Route Southbound

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

7.  Security Considerations

   The protocol has provisions for nonces and can include authentication
   mechanisms in the future comparable to [RFC5709] and [RFC7987].

   One can consider additionally 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 can implement a strategy of e.g. discarding contents of all
   TIEs of nodes that were not present in the SPF tree over a certain
   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.

   Section 4.2.9 presents many attack vectors in untrusted environments,
   starting with nodes that oscillate their level offers to the
   possiblity 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

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   southbound topology.  Session authentication mechanisms are necessary
   in environments where this is possible.

8.  Information Elements Schema

   This section introduces the schema for information elements.

   On schema changes that

   1.  change field numbers or

   2.  add new required fields or

   3.  remove fields or

   4.  change lists into sets, unions into structures or

   5.  change multiplicity of fields or

   6.  changes name of any field

   7.  change datatypes of any field or

   8.  adds or removes a default value of any field or

   9.  changes default value of any field

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

   Thrift serializer/deserializer MUST not discard optional, unknown
   fields but preserve and serialize them again when re-flooding whereas
   missing optional fields MAY be replaced with according default values
   if present.

   All signed integer as forced by 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.

8.1.  common.thrift

    Thrift file with common definitions for RIFT

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/** @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 of length long enough to accomodate prefix */
typedef binary IPv6Address
/** @note MUST be interpreted in implementation as unsigned 16 bits */
typedef i16    UDPPortType
/** @note MUST be interpreted in implementation as unsigned 32 bits */
typedef i32    TIENrType
/** @note MUST be interpreted in implementation as unsigned 32 bits */
typedef i32    MTUSizeType
/** @note MUST be interpreted in implementation as unsigned 32 bits */
typedef i32    SeqNrType
/** @note MUST be interpreted in implementation as unsigned 32 bits */
typedef i32    LifeTimeInSecType
/** @note MUST be interpreted in implementation as unsigned 16 bits */
typedef i16    LevelType
/** @note MUST be interpreted in implementation as unsigned 32 bits */
typedef i32    PodType
/** @note MUST be interpreted in implementation as unsigned 16 bits */
typedef i16    VersionType
/** @note MUST be interpreted in implementation as unsigned 32 bits */
typedef i32    MetricType
/** @note MUST be interpreted in implementation as unstructured 64 bits */
typedef i64    RouteTagType
/** @note MUST be interpreted in implementation as unstructured 32 bits label value */
typedef i32    LabelType
/** @note MUST be interpreted in implementation as unsigned 32 bits */
typedef i32    BandwithInMegaBitsType
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 */
typedef i64    NonceType
/** adjacency holdtime */
typedef i16    HoldTimeInSecType
/** Transaction ID type for prefix mobility as specified by RFC6550,  value
    MUST be interpreted in implementation as unsigned  */
typedef i8     PrefixTransactionIDType

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/** timestamp per IEEE 802.1AS, values MUST be interpreted in implementation as unsigned  */
struct IEEE802_1ASTimeStampType {
    1: required     i64     AS_sec;
    2: optional     i32     AS_nsec;

/** Flags indicating nodes behavior in case of ZTP and support
    for special optimization procedures. It will force level to `leaf_level`
enum LeafIndications {
    leaf_only                            =0,
    leaf_only_and_leaf_2_leaf_procedures =1,

/** 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
/** This MUST be used when node is configured as superspine in ZTP.
    This is kept reasonably low to alow for fast ZTP convergence on
    failures. */
const LevelType   default_superspine_level = 24
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
/** any element with 0 distance will be ignored,
 *  missing metrics will be replaced with default_distance
const MetricType  invalid_distance        = 0
const bool overload_default               = false
const bool flood_reduction_default        = true
const HoldTimeInSecType default_holdtime  = 3
/** by default LIE levels are ZTP offers */
const bool default_not_a_ztp_offer        = false
/** by default e'one is repeating flooding */
const bool default_you_are_not_flood_repeater = false
/** 0 is illegal for SystemID */
const SystemIDType IllegalSystemID        = 0
/** empty set of nodes */
const set<SystemIDType> empty_set_of_nodeids = {}

/** default UDP port to run LIEs on */
const UDPPortType     default_lie_udp_port       =  6949
const UDPPortType     default_tie_udp_flood_port =  6950

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/** default MTU size to use */
const MTUSizeType     default_mtu_size           =  1400
/** default mcast is v4, we make it i64 to
 *  help languages struggling with highest bit */
const i64             default_lie_v4_mcast_group =  3758096790

/** indicates whether the direction is northbound/east-west
  * or southbound */
enum TieDirectionType {
    Illegal           = 0,
    South             = 1,
    North             = 2,
    DirectionMaxValue = 3,

enum AddressFamilyType {
   Illegal                = 0,
   AddressFamilyMinValue  = 1,
   IPv4     = 2,
   IPv6     = 3,
   AddressFamilyMaxValue  = 4,

struct IPv4PrefixType {
    1: required IPv4Address    address;
    2: required PrefixLenType  prefixlen;

struct IPv6PrefixType {
    1: required IPv6Address    address;
    2: required PrefixLenType  prefixlen;

union IPAddressType {
    1: optional IPv4Address   ipv4address;
    2: optional IPv6Address   ipv6address;

union IPPrefixType {
    1: optional IPv4PrefixType   ipv4prefix;
    2: optional IPv6PrefixType   ipv6prefix;

/** @note: Sequence of a prefix. Comparison function:
    if diff(timestamps) < 200 milliseconds better transactionid wins
    else better time wins
struct PrefixSequenceType {

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    1: required IEEE802_1ASTimeStampType  timestamp;
    2: optional PrefixTransactionIDType   transactionid;

enum TIETypeType {
    Illegal                 = 0,
    TIETypeMinValue         = 1,
    /** first legal value */
    NodeTIEType             = 2,
    PrefixTIEType           = 3,
    TransitivePrefixTIEType = 4,
    PGPrefixTIEType         = 5,
    KeyValueTIEType         = 6,
    TIETypeMaxValue         = 7,

/** @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 leafs),
 *  i.e. prefix in NORTH PREFIX TIE is preferred over SOUTH PREFIX TIE
enum RouteType {
    Illegal               = 0,
    RouteTypeMinValue     = 1,
    /** First legal value. */
    /** Discard routes are most prefered */
    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,
    /** advertised in S-TIEs */
    SouthPrefix           = 7,
    /** transitive southbound are least preferred */
    TransitiveSouthPrefix = 8,
    RouteTypeMaxValue     = 9

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8.2.  encoding.thrift

    Thrift file for packet encodings for RIFT

include "common.thrift"

/** represents protocol encoding schema major version */
const i32 protocol_major_version = 10
/** represents protocol encoding schema minor version */
const i32 protocol_minor_version = 0

/** common RIFT packet header */
struct PacketHeader {
    1: required common.VersionType major_version = protocol_major_version;
    2: required common.VersionType minor_version = protocol_minor_version;
    /** this is the 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;

/** Community serves as community for PGP purposes */
struct Community {
    1: required i32          top;
    2: required i32          bottom;

/** Neighbor structure  */
struct Neighbor {
    1: required common.SystemIDType        originator;
    2: required common.LinkIDType          remote_id;

/** Capabilities the node supports */
struct NodeCapabilities {
    /** can this node participate in flood reduction,
        only relevant at level > 0 */
    1: optional bool                      flood_reduction =
    /** does this node restrict itself to be leaf only (in ZTP) and

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        does it support leaf-2-leaf procedures */
    2: optional common.LeafIndications    leaf_indications;

/** RIFT LIE packet

    @note this node's level is already included on the packet header */
struct LIEPacket {
    /** optional 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 =
    /** layer 3 MTU */
    4: optional common.MTUSizeType        link_mtu_size =
    /** this will reflect the neighbor once received to provid
        3-way connectivity */
    5: optional Neighbor                  neighbor;
    6: optional common.PodType            pod = common.default_pod;
    /** optional nonce used for security computations */
    7: optional common.NonceType          nonce;
    /** optional node capabilities shown in the LIE. The capabilies
        MUST match the capabilities shown in the Node TIEs, otherwise
        the behavior is unspecified. A node detecting the mismatch
        SHOULD generate according error.
    8: optional NodeCapabilities          capabilities;
    /** required holdtime of the adjacency, i.e. how much time
        MUST expire without LIE for the adjacency to drop
    9: required common.HoldTimeInSecType  holdtime =
    /** indicates that the level on the LIE MUST NOT be used
        to derive a ZTP level by the receiving node. */
   10: optional bool                      not_a_ztp_offer =
   /** indicates to northbound neighbor that it should not
       be reflooding this node's N-TIEs to flood reduce and
       balance northbound flooding. To be ignored if received from a
       northbound adjacency. */
   11: optional bool                      you_are_not_flood_repeater=
   /** optional downstream assigned locally significant label
       value for the adjacency. */
   12: optional common.LabelType          label;

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/** 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;
    /** more properties of the link can go in here */

/** 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 {
    /** indicates direction of the TIE */
    1: required common.TieDirectionType    direction;
    /** indicates originator of the TIE */
    2: required common.SystemIDType        originator;
    3: required common.TIETypeType         tietype;
    4: required common.TIENrType           tie_nr;

/** Header of a TIE */
struct TIEHeader {
    2: required TIEID                      tieid;
    3: required common.SeqNrType           seq_nr;
    /** lifetime expires down to 0 just like in ISIS */
    4: required common.LifeTimeInSecType   lifetime;

/** A sorted TIDE packet, if unsorted, behavior is undefined */
struct TIDEPacket {
    /** all 00s marks starts */
    1: required TIEID           start_range;
    /** all FFs mark end */
    2: required TIEID           end_range;
    /** _sorted_ list of headers */
    3: required list<TIEHeader> headers;

/** A TIRE packet */
struct TIREPacket {
    1: required set<TIEHeader> headers;

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/** Neighbor of a node */
struct NodeNeighborsTIEElement {
    /** Level of neighbor */
    2: 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;

    /** total bandwith to neighbor, this will be normally sum of the
     *   bandwidths of all the parallel links.
    5: optional common.BandwithInMegaBitsType   bandwidth =

/** Flags the node sets */
struct NodeFlags {
    /** 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
        * capabilities values do not match or
        * flags values do not match or
        * neighbors repeat with different values or
        * visible in same level/having partition upper do not match
    the behavior is undefined and a warning SHOULD be generated.
    Neighbors can be distributed across multiple TIEs however if
    the sets are disjoint.

    @note: observe that absence of fields implies defined defaults
struct NodeTIEElement {
    1: required common.LevelType            level;
    /** if neighbor systemID repeats in other node TIEs of same node
        the behavior is undefined. Equivalent to |A_(n,s)(N) in spec. */
    2: required map<common.SystemIDType,
                NodeNeighborsTIEElement>    neighbors;
    3: optional NodeCapabilities            capabilities;

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    4: optional NodeFlags                   flags;
    /** optional node name for easier operations */
    5: optional string                      name;

    /** Nodes seen an the same level through reflection through nodes
        having backlink to both nodes. They are equivalent to |V(N) in
        future specifications. Ignored in Node S-TIEs if present.
    6: optional set<common.SystemIDType>    visible_in_same_level
            = common.empty_set_of_nodeids;
    /** Non-overloaded nodes in |V seen as attached to another north
      * level partition due to the fact that some nodes in its |V have
      * adjacencies to higher level nodes that this node doesn't see.
      * This may be used in the computation at higher levels to prevent
      * blackholing. Ignored in Node S-TIEs if present.
      * Equivalent to |PUL(N) in spec. */
    7: optional set<common.SystemIDType>    same_level_unknown_north_partitions
            = common.empty_set_of_nodeids;

struct PrefixAttributes {
    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;
    /** optional monotonic clock for mobile addresses */
    4: optional common.PrefixSequenceType    monotonic_clock;

/** multiple 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;

/** keys with their values */
struct KeyValueTIEElement {
    /** 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. enum common.TIETypeType
    in TIEID indicates which elements MUST be present
    in the TIEElement. In case of mismatch the unexpected
    elements MUST be ignored.

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union TIEElement {
    /** in case of enum common.TIETypeType.NodeTIEType */
    1: optional NodeTIEElement            node;
    /** in case of enum common.TIETypeType.PrefixTIEType */
    2: optional PrefixTIEElement          prefixes;
    /** transitive prefixes (always southbound) which SHOULD be propagated
     *   southwards towards lower levels to heal
     *   pathological upper level partitioning, otherwise
     *   blackholes may occur. MUST NOT be advertised within a North TIE.
    3: optional PrefixTIEElement          transitive_prefixes;
    4: optional KeyValueTIEElement        keyvalues;
    /** @todo: policy guided prefixes */

/** @todo: flood header separately in UDP to allow caching to TIEs
           while changing lifetime?
struct TIEPacket {
    1: required TIEHeader  header;
    2: required TIEElement element;

union PacketContent {
    1: optional LIEPacket     lie;
    2: optional TIDEPacket    tide;
    3: optional TIREPacket    tire;
    4: optional TIEPacket     tie;

/** protocol packet structure */
struct ProtocolPacket {
    1: required PacketHeader  header;
    2: required PacketContent content;

9.  IANA Considerations

   This specification will request at an opportune time multiple
   registry points to exchange protocol packets in a standardized way,
   amongst them multicast address assignments and standard port numbers.
   The schema itself defines many values and codepoints which can be
   considered registries themselves.

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10.  Acknowledgments

   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
   epistomology that allowed to tie current asynchronous distributed
   systems theory results to a modern protocol design presented here.
   Adrian Farrel, Joel Halpern and Jeffrey Zhang 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 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 on a (clean)
   napkin in Singapore.

11.  References

11.1.  Normative References

              Thubert, P., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for 6LoWPAN Neighbor
              Discovery", draft-ietf-6lo-rfc6775-update-19 (work in
              progress), April 2018.

              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.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,

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   [RFC2365]  Meyer, D., "Administratively Scoped IP Multicast", BCP 23,
              RFC 2365, DOI 10.17487/RFC2365, July 1998,

   [RFC3626]  Clausen, T., Ed. and P. Jacquet, Ed., "Optimized Link
              State Routing Protocol (OLSR)", RFC 3626,
              DOI 10.17487/RFC3626, October 2003,

   [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,

   [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>.

   [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,

   [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,

   [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,

   [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,

   [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,

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   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,

   [RFC6822]  Previdi, S., Ed., Ginsberg, L., Shand, M., Roy, A., and D.
              Ward, "IS-IS Multi-Instance", RFC 6822,
              DOI 10.17487/RFC6822, December 2012,

   [RFC7855]  Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
              Litkowski, S., Horneffer, M., and R. Shakir, "Source
              Packet Routing in Networking (SPRING) Problem Statement
              and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
              2016, <https://www.rfc-editor.org/info/rfc7855>.

   [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,

   [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,

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

11.2.  Informative References

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

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

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

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

              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-15 (work
              in progress), January 2018.

              IEEE, "IEEE Standard for a Precision Clock Synchronization
              Protocol for Networked Measurement and Control Systems",
              IEEE Standard 1588,

              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,

              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.

              Maksic et al., N., "Improving Utilization of Data Center
              Networks", IEEE Communications Magazine, Nov 2013.

              Google, Inc., "Protocol Buffers,

   [QUIC]     Iyengar et al., J., "QUIC: A UDP-Based Multiplexed and
              Secure Transport", 2016.

   [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,

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   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <https://www.rfc-editor.org/info/rfc3315>.

   [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,

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,

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

Authors' Addresses

   Tony Przygienda (editor)
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA  94089

   Email: prz@juniper.net

   Alankar Sharma
   1800 Bishops Gate Blvd
   Mount Laurel, NJ  08054

   Email: Alankar_Sharma@comcast.com

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   Pascal Thubert
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200
   MOUGINS - Sophia Antipolis  06254

   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com

   Alia Atlas
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886

   Email: akatlas@juniper.net

   John Drake
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA  94089

   Email: jdrake@juniper.net

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