RIFT Working Group                                    T. Przygienda, Ed.
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                               A. Sharma
Expires: October 27, 2018                                        Comcast
                                                                A. Atlas
                                                                J. Drake
                                                        Juniper Networks
                                                            Apr 25, 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 September 2, 2018.

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Copyright Notice

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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  . . . . . . . . . . . . . . . . . . . . . . . .  12
     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)  . . . . . . . . . .  14  Topology Information Elements . . . . . . . . . .  14  South- and Northbound Representation  . . . . . .  15  Flooding  . . . . . . . . . . . . . . . . . . . .  18  TIE Flooding Scopes . . . . . . . . . . . . . . .  18  Initial and Periodic Database Synchronization . .  20  Purging . . . . . . . . . . . . . . . . . . . . .  20  Southbound Default Route Origination  . . . . . .  21  Optional Automatic Flooding Reduction and
                   Partitioning  . . . . . . . . . . . . . . . . . .  21
       4.2.4.  Policy-Guided Prefixes  . . . . . . . . . . . . . . .  23  Ingress Filtering . . . . . . . . . . . . . . . .  24  Applying Policy . . . . . . . . . . . . . . . . .  24  Store Policy-Guided Prefix for Route Computation
                   and Regeneration  . . . . . . . . . . . . . . . .  25  Re-origination  . . . . . . . . . . . . . . . . .  26  Overlap with Disaggregated Prefixes . . . . . . .  26
       4.2.5.  Reachability Computation  . . . . . . . . . . . . . .  26  Northbound SPF  . . . . . . . . . . . . . . . . .  27  Southbound SPF  . . . . . . . . . . . . . . . . .  27

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  East-West Forwarding Within a Level . . . . . . .  28
       4.2.6.  Attaching Prefixes  . . . . . . . . . . . . . . . . .  28
       4.2.7.  Attaching Policy-Guided Prefixes  . . . . . . . . . .  29
       4.2.8.  Automatic Disaggregation on Link & Node Failures  . .  30
       4.2.9.  Optional Autoconfiguration  . . . . . . . . . . . . .  33  Terminology . . . . . . . . . . . . . . . . . . .  34  Automatic SystemID Selection  . . . . . . . . . .  35  Generic Fabric Example  . . . . . . . . . . . . .  35  Level Determination Procedure . . . . . . . . . .  36  Resulting Topologies  . . . . . . . . . . . . . .  37
       4.2.10. Stability Considerations  . . . . . . . . . . . . . .  39
     4.3.  Further Mechanisms  . . . . . . . . . . . . . . . . . . .  40
       4.3.1.  Overload Bit  . . . . . . . . . . . . . . . . . . . .  40
       4.3.2.  Optimized Route Computation on Leafs  . . . . . . . .  40
       4.3.3.  Key/Value Store . . . . . . . . . . . . . . . . . . .  40  Southbound  . . . . . . . . . . . . . . . . . . .  40  Northbound  . . . . . . . . . . . . . . . . . . .  41
       4.3.4.  Interactions with BFD . . . . . . . . . . . . . . . .  41
       4.3.5.  Fabric Bandwidth Balancing  . . . . . . . . . . . . .  41  Northbound Direction  . . . . . . . . . . . . . .  42  Southbound Direction  . . . . . . . . . . . . . .  43
       4.3.6.  Segment Routing Support with RIFT . . . . . . . . . .  43  Global Segment Identifiers Assignment . . . . . .  43  Distribution of Topology Information  . . . . . .  44
       4.3.7.  Leaf to Leaf Procedures . . . . . . . . . . . . . . .  44
       4.3.8.  Other End-to-End Services . . . . . . . . . . . . . .  45
       4.3.9.  Address Family and Multi Topology Considerations  . .  45
       4.3.10. Reachability of Internal Nodes in the Fabric  . . . .  45
       4.3.11. One-Hop Healing of Levels with East-West Links  . . .  45
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  46
     5.1.  Normal Operation  . . . . . . . . . . . . . . . . . . . .  46
     5.2.  Leaf Link Failure . . . . . . . . . . . . . . . . . . . .  47
     5.3.  Partitioned Fabric  . . . . . . . . . . . . . . . . . . .  48
     5.4.  Northbound Partitioned Router and Optional East-West
           Links . . . . . . . . . . . . . . . . . . . . . . . . . .  50
   6.  Implementation and Operation: Further Details . . . . . . . .  51
     6.1.  Considerations for Leaf-Only Implementation . . . . . . .  51
     6.2.  Adaptations to Other Proposed Data Center Topologies  . .  51
     6.3.  Originating Non-Default Route Southbound  . . . . . . . .  52
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  52
   8.  Information Elements Schema . . . . . . . . . . . . . . . . .  53
     8.1.  common.thrift . . . . . . . . . . . . . . . . . . . . . .  53
     8.2.  encoding.thrift . . . . . . . . . . . . . . . . . . . . .  57
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  63
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  63
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  63
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  63
     11.2.  Informative References . . . . . . . . . . . . . . . . .  65

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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  66

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.

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

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              .                                  [A,B,C,D]
              .                                  [E]
              .             +-----+      +-----+
              .             |  E  |      |  F  | A/32 @ A
              .             +-+-+-+      +-+-+-+ B/32 @ B
              .               | |          | |   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 [RFC1142], [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
      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.

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   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 the PoD value 0 to indicate this.

   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.

   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.

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   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.

   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.

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

   Overloaded:  Applies to a node advertising `overload` attribute as
      set.  The semantics closely follow the meaning of the same
      attribute in [RFC1142].

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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   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][RFC1142] 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.

4.1.  Overview

   The singular property of RIFT is that it floods northbound "flat"
   link-state information so that each level understands the full
   topology of levels south of it.  In contrast, in the southbound
   direction the protocol operates like a path vector protocol or rather
   a distance vector with implicit split horizon since the topology
   constraints make a diffused computation front propagating in all
   directions unnecessary.

   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

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   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 for
   IPv6 [RFC4291] 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.

   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).  Nodes in the spine are configured with an
   "undefined" PoD.  This information is propagated in the LIEs

   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" 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

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   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 neighboring node is at level 0 OR

          iii) both nodes are at level 0 AND both indicate support for
          Section 4.3.7 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.

   A node configured with "undefined" PoD membership MUST, after
   building first northbound adjacency making it participant in a PoD,
   advertise that PoD as part of its LIEs.

   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

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   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.  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.  Albeit
   initially more demanding to implement it avoids many problems with
   diffused computation update style used by path vector.  As described
   before, TIEs themselves are transported over UDP with the ports
   indicates 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 with 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 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

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   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 nad 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.  Optional Automatic Flooding Reduction and Partitioning

   Several nodes can, but strictly only under conditions defined below,
   run a hashing function based on TIE originator value and partition
   flooding between them.

   Steps for flooding reduction and partitioning:

   1.  select all nodes in the same level for which

       A.  node S-TIEs have been received AND

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       B.  which have precisely the same non-empty sets of respectively
           north and south neighbor adjacencies AND

       C.  have at least one shared southern neighbor including backlink
           verification and

       D.  support flooding reduction (overload bits are ignored)

       and then

   2.  run on the chosen set a hash algorithm using nodes flood
       priorities and IDs to select flooding leader and backup per TIE
       originator ID, i.e.  each node floods immediately through to all
       its necessary neighbors TIEs that it received with an originator
       ID that makes it the flooding leader or backup for this
       originator.  The preference (higher is better) is computed as
       XOR(TIE-ORIGINATOR-ID<<1,~OWN-SYSTEM-ID)), whereas << is a non-
       circular shift and ~ is bit-wise NOT.

   3.  In the very unlikely case of hash collisions on either of the two
       nodes with highest values (i.e.  either does NOT produce unique
       hashes as compared to all other hash values), the node running
       the election does not attempt to reduce flooding.

   Additional rules for flooding reduction and partitioning:

   1.  A node always floods its own TIEs

   2.  A node generates TIDEs as usual but when receiving TIREs with
       requests for TIEs for a node for which it is not a flooding
       leader or backup it ignores such TIDEs on first request only.
       Normally, the flooding leader should satisfy the requestor and
       with that no further TIREs for such TIEs will be generated.
       Otherwise, the next set of TIDEs and TIREs will lead to flooding
       independent of the flooding leader status.

   3.  A node receiving a TIE originated by a node for which it is not a
       flooding leader floods such TIEs only when receiving an out-of-
       date TIDE for them, except for the first one.

   The mechanism can be implemented optionally in each node.  The
   capability is carried in the node S-TIE (and for symmetry purposes in
   node N-TIE as well but it serves no purpose there currently).

   Obviously flooding reduction does NOT apply to self originated TIEs.
   Observe further that all policy-guided information consists of self-
   originated TIEs.

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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.

   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.

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   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
   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.

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   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).

   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.

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   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.  Overlap with Disaggregated Prefixes

   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.

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Internet-Draft                    RIFT                          Apr 2018  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

   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.

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Internet-Draft                    RIFT                          Apr 2018  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.

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 it MUST operate 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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Internet-Draft                    RIFT                          Apr 2018  Terminology

   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 4.3.7.
      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.  As simple
   examples either system MAC and two random bytes can be used or an
   IPv4/IPv6 router ID interface address recycled as System ID.  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.  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

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       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.

4.3.4.  Interactions with BFD

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

      After RIFT 3-way hello adjacency convergence a BFD session MAY be
      formed automatically between the RIFT endpoints without further

      In case RIFT looses 3-way hello adjacency, the BFD session should
      be brought down until 3-way adjacency is formed again.

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

      In case of parallel links between nodes each link may run its own
      independent BFD session.

      In case RIFT changes link identifiers both the hello as well as
      the BFD sessions will be brought down and back up again.

4.3.5.  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

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   engineering via controller or leafs being aware of complete topology
   with according cost and complexity.

   RIFT presents 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

   In a first step, a node can compare the amount of northbound bandwith
   available to neighbors at the same level and modify metric on its
   advertised default route (or even other routes) to present a
   different distance leading to e.g. e.g. weighted ECMP forwarding on
   leafs.  We call such a distance Bandwidth Adjusted Distance or BAD.
   This is best illustrated by a simple example.

                      .    |   x              |   |
                      .    |   x              |   |
                      .  +-+---+-+          +-+---+-+
                      .  |       |          |       |
                      .  |Node111|          |Node112|
                      .  +-+---+++          ++----+++
                      .    |x  ||           ||    ||
                      .    ||  |+---------------+ ||
                      .    ||  +---------------+| ||
                      .    ||               || || ||
                      .    ||  +------------+| || ||
                      .    ||  |+------------+ || ||
                      .    |x  ||              || ||
                      .  +-+---+++          +--++-+++
                      .  |       |          |       |
                      .  |Leaf111|          |Leaf112|
                      .  +-------+          +-------+

                      Figure 10: Balancing Bandwidth

   All links in Figure 10 are assumed to have the same bandwith for
   simplicity.  Node 111 sees in the node S-TIE of 112 that Node 112 has
   twice the amount of bandwidth going northbound and therefore Node 111
   will advertise its default route cost (BAD) as twice the default
   which without further failures would lead to Leaf 111 and Leaf 112
   distributing 2/3 of the traffic to Node 111 and 1/3 to Node 112.

   Further, in Figure 10 we assume that Leaf111 lost one of the parallel
   links to Node 111 and with that wants to push more traffic onto Node
   112.  This leads to local modification of the received BADs and each

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   node can choose the ratio here independently based on understanding
   of e.g. traffic distribution between E-W and N-S or queue occupancy.
   If we assume that 50% of the leaf's traffic is for Leaf112 and 50%
   exits northbound we would modify the BADs accordingly to the
   bandwidth available towards each of them and end in Leaf 111 with a
   weight of 4 to Node 111 and weight of 1 to Node 112 which gives us
   roughly 4/5 of the traffic going to Node 112.

   Future version of this document will provide the precise algorithm to
   compute BADs from all other nodes at the same level using the same
   algorithm as Section while ignoring overloaded nodes.

   Observe that since BAD is only computed for default routes any
   disaggregated prefixes or PGP are not affected.

   Observe further that a change in available bandwidth will only affect
   one level 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.
   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.

4.3.6.  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

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   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.  RIFT can be extended beside the
   mechanism in Section to provide an algorithm for globally
   optimized flooding minimalization should demand for such a use case

4.3.7.  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).

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4.3.8.  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.

4.3.9.  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 4.3.4 are implementation dependent when
   multiple RIFT instances run on the same link.

4.3.10.  Reachability of Internal Nodes in the Fabric

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

4.3.11.  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.

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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

   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

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   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

                    .  |   |              |   |
                    .+-+---+-+          +-+---+-+
                    .|       |          |       |
                    .|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

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   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

    .                +--------+          +--------+   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

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   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
   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.

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5.4.  Northbound Partitioned Router and Optional East-West Links

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

                    Figure 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 4.3.10 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.

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6.  Implementation and Operation: Further Details

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.  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

namespace * common

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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 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

/** 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,

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/** default bandwidth on a link */
const BandwithInMegaBitsType  default_bandwidth    = 10
/** 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       = 0x70000000
/** 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
/** 0 is illegal for SystemID */
const SystemIDType IllegalSystemID        = 0
/** empty set of nodes */
const set<SystemIDType> empty_set_of_nodeids = {}

/** normalized bandwidth metric maximum, i.e. node with lowest northbound bandwidth
 *  at its level uses this metric to advertise its default route */
const MetricType normalized_bw_metric_max = 0x1fff
/** normalized bandwidth metric minimum, i.e. node with highest northbound bandwidth
 *  at its level uses this metric to advertise its default route */
const MetricType normalized_bw_metric_min = 0x00ff

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

/** 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

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/** 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;

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

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/** @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

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 = 8
/** represents protocol encoding schema minor version */
const i32 protocol_minor_version = 0

/** common RIFT packet header */
struct PacketHeader {

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    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
        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 =

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    /** 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 =

/** 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;

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/** 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;

/** Neighbor of a node */
struct NodeNeighborsTIEElement {
    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;

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/** 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;
    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 {
    /** Observe that in default metric case the node is supposed to advertise
      * metric calculated from comparison of bandwidths at all nodes at its
      * level. **/
    2: required common.MetricType       metric = common.default_distance;

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/** 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.
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;

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/** 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.

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

              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.

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   [RFC1142]  Oran, D., Ed., "OSI IS-IS Intra-domain Routing Protocol",
              RFC 1142, DOI 10.17487/RFC1142, February 1990,

   [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,

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

   [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>.

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   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
              DOI 10.17487/RFC5881, June 2010,

   [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,

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.

              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.

              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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   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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