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Versions: 00 01 02 03

Networking Working Group                                   T. Przygienda
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                               A. Sharma
Expires: December 28, 2017                                       Comcast
                                                                J. Drake
                                                                A. Atlas
                                                        Juniper Networks
                                                           June 26, 2017


                       RIFT: Routing in Fat Trees
                        draft-przygienda-rift-02

Abstract

   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 and ultimately (6) 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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 28, 2017.







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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Reference Frame . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Topology  . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Requirement Considerations  . . . . . . . . . . . . . . . . .   8
   4.  RIFT: Routing in Fat Trees  . . . . . . . . . . . . . . . . .  10
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Specification . . . . . . . . . . . . . . . . . . . . . .  10
       4.2.1.  Transport . . . . . . . . . . . . . . . . . . . . . .  10
       4.2.2.  Link (Neighbor) Discovery (LIE Exchange)  . . . . . .  11
       4.2.3.  Topology Exchange (TIE Exchange)  . . . . . . . . . .  11
         4.2.3.1.  Topology Information Elements . . . . . . . . . .  11
         4.2.3.2.  South- and Northbound Representation  . . . . . .  12
         4.2.3.3.  Flooding  . . . . . . . . . . . . . . . . . . . .  14
         4.2.3.4.  TIE Flooding Scopes . . . . . . . . . . . . . . .  14
         4.2.3.5.  Initial and Periodic Database Synchronization . .  16
         4.2.3.6.  Purging . . . . . . . . . . . . . . . . . . . . .  16
         4.2.3.7.  Southbound Default Route Origination  . . . . . .  16
         4.2.3.8.  Optional Automatic Flooding Reduction and
                   Partitioning  . . . . . . . . . . . . . . . . . .  16
       4.2.4.  Policy-Guided Prefixes  . . . . . . . . . . . . . . .  17
         4.2.4.1.  Ingress Filtering . . . . . . . . . . . . . . . .  19
         4.2.4.2.  Applying Policy . . . . . . . . . . . . . . . . .  19
         4.2.4.3.  Store Policy-Guided Prefix for Route Computation
                   and Regeneration  . . . . . . . . . . . . . . . .  20
         4.2.4.4.  Re-origination  . . . . . . . . . . . . . . . . .  21
         4.2.4.5.  Overlap with Disaggregated Prefixes . . . . . . .  21
       4.2.5.  Reachability Computation  . . . . . . . . . . . . . .  21
         4.2.5.1.  Northbound SPF  . . . . . . . . . . . . . . . . .  21
         4.2.5.2.  Southbound SPF  . . . . . . . . . . . . . . . . .  22



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         4.2.5.3.  East-West Forwarding Within a Level . . . . . . .  22
       4.2.6.  Attaching Prefixes  . . . . . . . . . . . . . . . . .  22
       4.2.7.  Attaching Policy-Guided Prefixes  . . . . . . . . . .  23
       4.2.8.  Automatic Disaggregation on Link & Node Failures  . .  24
     4.3.  Further Mechanisms  . . . . . . . . . . . . . . . . . . .  27
       4.3.1.  Overload Bit  . . . . . . . . . . . . . . . . . . . .  27
       4.3.2.  Optimized Route Computation on Leafs  . . . . . . . .  27
       4.3.3.  Key/Value Store . . . . . . . . . . . . . . . . . . .  28
       4.3.4.  Interactions with BFD . . . . . . . . . . . . . . . .  28
       4.3.5.  Leaf to Leaf Procedures . . . . . . . . . . . . . . .  29
       4.3.6.  Other End-to-End Services . . . . . . . . . . . . . .  29
       4.3.7.  Address Family and Multi Topology Considerations  . .  29
       4.3.8.  Reachability of Internal Nodes in the Fabric  . . . .  30
       4.3.9.  One-Hop Healing of Levels with East-West Links  . . .  30
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  30
     5.1.  Normal Operation  . . . . . . . . . . . . . . . . . . . .  30
     5.2.  Leaf Link Failure . . . . . . . . . . . . . . . . . . . .  32
     5.3.  Partitioned Fabric  . . . . . . . . . . . . . . . . . . .  32
     5.4.  Northbound Partitioned Router and Optional East-West
           Links . . . . . . . . . . . . . . . . . . . . . . . . . .  34
   6.  Implementation and Operation: Further Details . . . . . . . .  35
     6.1.  Considerations for Leaf-Only Implementation . . . . . . .  36
     6.2.  Adaptations to Other Proposed Data Center Topologies  . .  36
     6.3.  Originating Non-Default Route Southbound  . . . . . . . .  37
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  37
   8.  Information Elements Schema . . . . . . . . . . . . . . . . .  37
     8.1.  common.thrift . . . . . . . . . . . . . . . . . . . . . .  38
     8.2.  encoding.thrift . . . . . . . . . . . . . . . . . . . . .  40
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  45
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  45
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  45
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  45
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  45
     12.2.  Informative References . . . . . . . . . . . . . . . . .  47
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  48

1.  Introduction

   Clos [CLOS] and Fat-Tree [FATTREE] have gained prominence in today's
   networking, primarily as a result of a 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.  The
   existing set of dynamic routing protocols was geared originally
   towards a network with an irregular topology and low degree of
   connectivity and consequently several attempts to adapt those 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



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   modify it "quicker" and the immanent difficulties with link-state
   [DIJKSTRA] based protocols to fulfill certain of the resulting
   requirements.

   In looking at the problem through the very lens of its requirements
   an optimal approach does not seem 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'.  The balance of this document
   details the resulting protocol.

1.1.  Requirements Language

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

   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.

   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.




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   Leaf:  A node at level 0.

   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 for 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.

   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
      itself.

   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.




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   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.

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 1: 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.







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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 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.

   REQ2:    Very high degree of ECMP (and ideally non equal cost) must
            be supported.  Maximum ECMP is currently understood as the
            most efficient routing approach to maximize the throughput
            of switching fabrics [MAKSIC2013].

   REQ3:    Traffic engineering should be allowed by modification of
            prefixes and/or their next-hops.

   REQ4:    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
            prevented.

   REQ5:    The solution should allow for access to link states of the
            whole topology to enable efficient support for modern
            control architectures like SPRING [RFC7855] or PCE
            [RFC4655].

   REQ6:    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.

   REQ7:    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
            feasible.

   REQ8:    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



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            failures.  The de-aggregation should support maximum
            possible ECMP/N-ECMP remaining after failure.

   REQ9:    A node without any configuration beside default values
            should come up as leaf 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.

   REQ10:   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.

   REQ11:   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.

   REQ12:   Parallel links between same set of nodes must be
            distinguishable for SPF, failure and traffic engineering
            purposes.

   REQ13:   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.

   REQ14:   Optionally, the protocol should allow to provision data
            centers where the individual switches carry no configuration
            information and are all determining the topology 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 mode.

   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.





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   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
              routers.

   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 novel 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.

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.







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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 link in the topology.  LIEs are
   exchanged over all links running RIFT.

   Each node is provisioned with the level at which it is operating and
   its PoD.  A default level and PoD of zero are assumed, meaning that
   leafs do not need to be configured with a level (or even PoD).  Nodes
   in the spine are configured with a PoD of zero.  This information is
   propagated in the LIEs exchanged.  Adjacencies are formed if and only
   if

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

   2.  the neighboring node is at most one level away AND

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

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

   A node configured with "any" 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.

   LIE exchange uses three-way handshake mechanism [RFC5303].  Precise
   final 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)

4.2.3.1.  Topology Information Elements

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




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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
   side of the scale 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.

4.2.3.2.  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 south or to
   the north/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 neighbors 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.

   As an example illustrating a databases holding both representations,
   consider the topology in Figure 1 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.











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           Spine21 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)))
           SouthPrefixesElement(prefixes(0/0, cost 1), (::/0, cost 1))

           Node111 S-TIE:
           NodeElement(layer=1, neighbors((Spine21,layer 2,cost 1),
           (Spine22, layer 2, cost 1), (Node112, layer 1, cost 1),
           (Leaf111, layer 0, cost 1), (Leaf112, layer 0, cost 1)))
           SouthPrefixesElement(prefixes(0/0, cost 1), (::/0, cost 1))

           Node111 N-TIE:
           NodeLinkElement(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(...))))
           NorthPrefixesElement(prefixes(Node111.loopback)

           Node121 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)))
           SouthPrefixesElement(prefixes(0/0, cost 1), (::/0, cost 1))

           Node121 N-TIE: NodeLinkElement(layer=1,
           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(...))))
           NorthPrefixesElement(prefixes(Node121.loopback)

           Leaf112 N-TIE:
           NodeLinkElement(layer=0,
           neighbors((Node111, layer 1, cost 1, links(...)),
           (Node112, layer 1, cost 1, links(...))))
           NorthPrefixesElement(prefixes(Leaf112.loopback, Prefix112,
           Prefix_MH))


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








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4.2.3.3.  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.

   Precise final state machines and procedures will be provided in later
   versions of this specification.

4.2.3.4.  TIE Flooding Scopes

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

   Every 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 more southern level, e.g. in case of leaf level.  The
   precise flooding scopes are given in Table 1.  Those rules govern in
   a symmetric fashion what SHOULD be included in TIDEs towards
   neighbors.

   Node S-TIE "reflection" allows to support disaggregation on failures
   describes in Section 4.2.8 and flooding reduction in Section 4.2.3.8.





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   Packet Type   South                      North              East-West
   vs. Peer
   Direction
   ------------- -------------------------- ------------------ ---------
   node S-TIE    flood own only             flood always       same as
                                                               South
   other S-TIE   flood own only             flood only if TIE  same as
                                            originator is      South
                                            equal peer
   all N-TIEs    never flood                flood always       same as
                                                               South
   TIDE          include TIEs in flooding   include TIEs in    same as
                 scope                      flooding scope     South
   TIRE          include all N-TIEs and all include only if    same as
                 peer's self-originated     TIE originator is  South
                 TIEs and all node S-TIEs   equal peer

                         Table 1: Flooding Scopes

   As an example to illustrate these rules, consider using the topology
   in Figure 1, with the optional link between node 111 and node 112,
   and the associated TIEs given in Figure 2.  The flooding from
   particular nodes of the TIEs is given in Table 2.

   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, Node112 N-TIEs, Leaf111 N-TIEs,
                       Leaf112 N-TIEs, Spine22 node S-TIE
   Node111    Spine22  Node111 N-TIEs, Node112 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 2: Flooding some TIEs from example topology





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4.2.3.5.  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 1.

4.2.3.6.  Purging

   RIFT does not purge information that has been distributed by the
   protocol.  Purging mechanisms in other routing protocols have proven
   through many years of experience to be complex and fragile.  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.
   Each node will timeout and clean up the according empty TIEs
   independently.

4.2.3.7.  Southbound Default Route Origination

   Under certain conditions nodes issue a default route in their South
   Prefix TIEs.

   A node X that is NOT overloaded AND has southbound or east-west
   adjacencies originates in its south prefix TIE such a default route
   IIF all other nodes at X's' level are overloaded OR have NO
   northbound adjacencies OR X has computed reachability to a default
   route during N-SPF.

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

4.2.3.8.  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:




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   1.  select all nodes in the same level for which node S-TIEs have
       been received and which have precisely the same non-empty sets of
       respectively north and south neighbor adjacencies and 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.

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



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   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.

   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



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   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.

4.2.4.1.  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
      received.

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

4.2.4.2.  Applying Policy

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

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

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



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   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.

4.2.4.3.  Store Policy-Guided Prefix for Route Computation and
          Regeneration

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

   There are three different policies that can be used:

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

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

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

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

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

   Two databases of PGPs - from N-TIEs and from S-TIEs are stored.  When
   a PGP is inserted into the appropriate database, the usual tie-
   breaking on cost is performed.  Observe that the node retains all PGP



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   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.

4.2.4.4.  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.

4.2.4.5.  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.

4.2.5.1.  Northbound SPF

   N-SPF uses northbound adjacencies in north node TIEs when progressing
   Dijkstra.  N-SPF uses E-W adjacencies during N-SPF ONLY if the node
   itself does NOT have any northbound adjacencies and the adjacent node
   has one or more northbound adjacencies.  This forms a "one-hop split-
   horizon" and prevents looping over default routes while allowing for
   "one-hop protection" of nodes that lost all northbound adjacencies.
   A detailed example can be found in Section 4.3.8.




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   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, unidirectional links are associated together to confirm
   bidirectional connectivity.

4.2.5.2.  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 the direction.

   S-SPF uses northbound adjacencies in north node TIEs to verify
   backlink connectivity.

4.2.5.3.  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
   connected).

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




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   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:


  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))
          else
            // Not preferred route so ignore
            end if
          end if
        end for
     end for


                Figure 3: 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 4.2.4.3;
   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))
          else
            // Not preferred route so ignore
            end if
          end if
      end for


            Figure 4: 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
       X.south_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,
                                                partial_neighbors[NH].nodes)
                  end if
                end for

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

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


              Figure 5: 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.

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.




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   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

   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 TIEs (which are always S-TIEs) can arrive
   from multiple nodes and need tie-breaking per key uses 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
       S-TIE with the highest level and within the same level highest
       originator ID is preferred.

   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 is not result of
   independent computation of every node but a diffused computation.

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
      configuration.

      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.



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      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.  Leaf to Leaf Procedures

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

      Only nodes supporting Leaf to Leaf Procedures CAN advertise LIEs
      on E-W links at level 0 and MUST in such a case advertise the
      according flag in node capabilities as "true".

      The overload bit MUST be set on all leaf's node TIEs.

      Only node's own north and south TIEs are flooded over E-W leaf
      adjacency.

      E-W leaf adjacency is always used in both north as well as south
      computation.

      Any advertised aggregate in leaf's south TIE MUST install a
      discard route.

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

4.3.6.  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.

4.3.7.  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.




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   BFD interactions in Section 4.3.4 are implementation dependent when
   multiple RIFT instances run on the same link.

4.3.8.  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.

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

   Based on the rules defined in Section 4.2.5, Section 4.2.3.7 and
   given presence of E-W links, RIFT can provide a one-hop protection of
   nodes that lost all their northbound links.  Section 5.4 explains the
   resulting behavior based on an example.

5.  Examples

5.1.  Normal Operation

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

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

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

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

   3.  Node 111 to Leaf 111, Leaf 112

   4.  Node 112 to Leaf 111, Leaf 112

   5.  Node 121 to Leaf 121, Leaf 122



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   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
   N-TIEs.

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

   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.








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5.2.  Leaf Link Failure

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


                    Figure 6: Single Leaf link failure

   In case of a failing leaf link between node 112 and leaf 112 the
   link-state information will cause re-computation of the necessary SPF
   and the higher levels will stop forwarding towards prefix 112 through
   node 112.  Only nodes 111 and 112, as well as both spines will see
   control traffic.  Leaf 111 will receive a new S-TIE from node 112 and
   reflect back to node 111.  Node 111 will de-aggregate prefix 111 and
   prefix 112 but we will not describe it further here since de-
   aggregation is emphasized in the next example.  It is worth observing
   however in this example that if leaf 111 would keep on forwarding
   traffic towards prefix 112 using the advertised south-bound default
   of node 112 the traffic would end up on spine 21 and spine 22 and
   cross back into pod 1 using node 111.  This is arguably not as bad as
   black-holing present in the next example but clearly undesirable.
   Fortunately, de-aggregation prevents this type of behavior except for
   a transitory period of time.

5.3.  Partitioned Fabric












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   .                +--------+          +--------+   S-TIE of Spine21
   .                |        |          |        |   received by
   .                |Spine 21|          |Spine 22|   reflection of
   .                ++-+--+-++          ++-+--+-++   Nodes 112 and 111
   .                 | |  | |            | |  | |
   .                 | |  | |            | |  | 0/0
   .                 | |  | |            | |  | |
   .                 | |  | |            | |  | |
   .  +--------------+ |  +--- XXXXXX +  | |  | +---------------+
   .  |                |    |         |  | |  |                 |
   .  |    +-----------------------------+ |  |                 |
   .  0/0  |           |    |         |    |  |                 |
   .  |    0/0       0/0    +- XXXXXXXXXXXXXXXXXXXXXXXXX -+     |
   .  |  1.1/16        |              |    |  |           |     |
   .  |    |           +-+    +-0/0-----------+           |     |
   .  |    |             |   1.1./16  |    |              |     |
   .+-+----++          +-+-----+     ++-----0/0          ++----0/0
   .|       |          |       |     |    1.1/16         |   1.1/16
   .|Node111|          |Node112|     |Node121|           |Node122|
   .+-+---+-+          ++----+-+     +-+---+-+           ++---+--+
   .  |   |             |    |         |   |              |   |
   .  |   +---------------+  |         |   +----------------+ |
   .  |                 | |  |         |                  | | |
   .  |   +-------------+ |  |         |   +--------------+ | |
   .  |   |               |  |         |   |                | |
   .+-+---+-+          +--+--+-+     +-+---+-+          +---+-+-+
   .|       |          |       |     |       |          |       |
   .|Leaf111|          |Leaf112|     |Leaf121|          |Leaf122|
   .+-+-----+          ++------+     +-----+-+          +-+-----+
   .  +                 +                  +              +
   .  Prefix111    Prefix112             Prefix121     Prefix122
   .                                       1.1/16


                        Figure 7: Fabric partition

   Figure 7 shows the arguably most catastrophic but also the most
   interesting case.  Spine 21 is completely severed from access to
   Prefix 121 (we use in the figure 1.1/16 as example) by double link
   failure.  However unlikely, if left unresolved, forwarding from leaf
   111 and leaf 112 to prefix 121 would suffer 50% black-holing based on
   pure default route advertisements by spine 21 and spine 22.

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



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

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

   To finish with an example of the resulting sets computed using
   notation introduced in Section 4.2.8, spine 22 constructs the
   following sets:

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

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

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

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

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

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

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

5.4.  Northbound Partitioned Router and Optional East-West Links
















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


                    Figure 8: North Partitioned Router

   Figure 8 shows a part of a fabric where level 1 is horizontally
   connected and A01 lost its only northbound adjacency.  Based on N-SPF
   rules in Section 4.2.5.1 A01 will compute northbound reachability by
   using the link A01 to A02 (whereas A02 will NOT use this link during
   N-SPF).  Hence A01 will still advertise the default towards level 0
   and route unidirectionally using the horizontal link.  Moreover,
   based on Section 4.3.8 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 4.2.3.7.  Moreover, A02 may now inject its loopback address
   as south PGP.

6.  Implementation and Operation: Further Details








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6.1.  Considerations for Leaf-Only Implementation

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

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

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

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

   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 9: 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 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 9 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 4.2.3.7) 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.

8.  Information Elements Schema

   This section introduces the schema for information elements.

   On schema changes that




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   1.  change field numbers or

   2.  add new required fields or

   3.  change lists into sets, unions into structures or

   4.  change multiplicity of fields or

   5.  change datatypes of any field or

   6.  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.

   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 for common definitions in RIFT
   */

   namespace * models

   typedef i64    SystemID
   typedef i32    IPv4Address
   /** this has to be of length long enough to accomodate prefix */
   typedef binary IPv6Address
   typedef i16    UDPPortType
   typedef i32    TIENrType
   typedef i16    MTUSizeType
   typedef i32    SeqNrType
   /** lifetime in seconds */
   typedef i32    LifeTimeType
   typedef i16    LevelType
   typedef i16    PodType
   typedef i16    VersionType
   typedef i32    MetricType
   typedef string KeyIDType



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   /** node local, unique identification for a link, this is kept
     * at 32 bits so it aligns with BFD [RFC5880] discriminator size
     */
   typedef i32    LinkIDType
   typedef string KeyNameType
   /** indicates whether the direction is northbound/east-west
     * or southbound */
   typedef bool   TieDirectionType
   typedef byte   PrefixLenType
   /** timestamp in seconds since the epoch */
   typedef i64    TimestampInSecsType
   /** security nonce */
   typedef i64    NonceType

   const LevelType   default_level           = 0
   const PodType     default_pod             = 0
   const LinkIDType  undefined_linkid        = 0
   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 bool leaf_2_leaf_procedures_default = false;

   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 IPPrefixType {
       1: optional IPv4PrefixType   ipv4prefix;



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       2: optional IPv6PrefixType   ipv6prefix;
   }

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

   /** @note: route types which MUST be ordered on preference
    *  PGP prefixes are most preferred attracting
    *  traffic north (towards spine)
    *  normal prefixes are attracting traffic south (towards leafs),
    *  i.e. prefix in NORTH PREFIX TIE is preferred
    */
   enum RouteType {
       Illegal           = 0,
       RouteTypeMinValue = 1,
       /** First legal value. Local prefixes are
        *  locally hosted routes on the system.
        */
       LocalPrefix       = 2,
       SouthPGPPrefix    = 3,
       NorthPGPPrefix    = 4,
       NorthPrefix       = 5,
       SouthPrefix       = 6,
       /** advertised in N-TIEs */
       RouteTypeMaxValue = 7
   }

8.2.  encoding.thrift

/**
    Thrift file for packet encodings for RIFT
*/

include "common.thrift"

namespace * models

/** represents protocol major version */
const i32 current_major_version = 2
/** represents protocol minor version */



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const i32 current_minor_version = 1

/** coomon RIFT packet header */
struct PacketHeader {
    1: required common.VersionType major_version = current_major_version;
    2: required common.VersionType minor_version = current_minor_version;
    /** this is the node sending the packet, in case of LIE/TIRE/TIDE
        also the originator of it */
    3: required common.SystemID  sender;
    /** level of the node sending the packet */
    4: required common.LevelType level;
}

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

union Content {
    1: optional LIE          hello;
    2: optional TIDEPacket   tide;
    3: optional TIREPacket   tire;
    4: optional TIEPacket    tie;
}

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

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

/** RIFT LIE packet

    @note this node's level is already included on the packet header */
struct LIE {
    1: optional string                 name;
    /** UDP port to which we can flood TIEs, same address
        as the hello TX this hello has been received on */
    /** local link ID */



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    2: required common.LinkIDType      local_id;
    3: required common.UDPPortType     flood_port;
    /** this will reflect the neighbor once received */
    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;
}

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

/** 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 whether N or S-TIE, True > False */
    1: required common.TieDirectionType    northbound;
    2: required common.SystemID            originator;
    3: required common.TIETypeType         tietype;
    4: required common.TIENrType           tie_nr;
}

/** Header of a TIE */
struct TIEHeader {
    2: required TIEID                      tieid;
    3: required common.SeqNrType           seq_nr;
    4: required common.LifeTimeType        lifetime;



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}

/** 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;
    3: optional common.MetricType      cost = common.default_distance;
    /** can carry description of multiple parallel links in a TIE
     *  all parallel links to same node incur same cost
     **/
    4: optional set<LinkIDPair>            link_ids;
}

/** Capabilities the node supports */
struct NodeCapabilities {
    /** can this node participate in flood reduction,
        only relevant at level > 0 */
    1: required bool         flood_reduction;
    /** does this node support leaf-2-leaf procedures,
        only relevant at level 0 */
    2: optional bool         leaf_2_leaf_procedures;
}

/** Flags the node sets */
struct NodeFlags {
    /** node is in overload, do not transit traffic through it */
    1: required bool         overload;
}

/** 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
        * neighbors repeat with different values



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    the behavior is undefined and a warning
    SHOULD be generated.

    @note: observe that absence of fields implies defined defaults
*/
struct NodeTIEElement {
    1: required common.LevelType                             level;
    2: optional NodeCapabilities                             capabilities =
            {
              "flood_reduction": common.flood_reduction_default,
              "leaf_2_leaf_procedures": common.leaf_2_leaf_procedures_default
            };
    3: optional NodeFlags                                    flags =
            {
              "overload": common.overload_default
            };
    /** if neighbor systemID repeats in other node TIEs of same node
        the behavior is undefined */
    4: required map<common.SystemID,NodeNeighborsTIEElement> neighbors;
}

/** multiple prefixes */
struct PrefixTIEElement {
    /** prefixes with the associated cost.
        if the same prefix repeats in multiple TIEs of same node
        or with different metrics, behavior is unspecified */
    1: required map<common.IPPrefixType,common.MetricType> 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;
    3: optional KeyValueTIEElement        keyvalues;
    /** @todo: policy guided prefixes */



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}

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.  Security Considerations

   Security mechanisms will be addressed in upcoming versions of this
   specification.

11.  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.
   Adrian Farrel 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.

12.  References

12.1.  Normative References

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

   [RFC1142]  Oran, D., Ed., "OSI IS-IS Intra-domain Routing Protocol",
              RFC 1142, DOI 10.17487/RFC1142, February 1990,
              <http://www.rfc-editor.org/info/rfc1142>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.




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

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

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

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <http://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,
              <http://www.rfc-editor.org/info/rfc4655>.

   [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,
              <http://www.rfc-editor.org/info/rfc5120>.

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

   [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, <http://www.rfc-editor.org/info/rfc5709>.

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

   [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,
              <http://www.rfc-editor.org/info/rfc6234>.




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   [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,
              <http://www.rfc-editor.org/info/rfc6822>.

   [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, <http://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,
              <http://www.rfc-editor.org/info/rfc7938>.

   [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,
              <http://www.rfc-editor.org/info/rfc7987>.

12.2.  Informative References

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

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

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

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

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

   [MAKSIC2013]
              Maksic et al., N., "Improving Utilization of Data Center
              Networks", IEEE Communications Magazine, Nov 2013.

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




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

Authors' Addresses

   Tony Przygienda
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA  94089
   US

   Email: prz@juniper.net


   Alankar Sharma
   Comcast
   1800 Bishops Gate Blvd
   Mount Laurel, NJ  08054
   US

   Email: Alankar_Sharma@comcast.com


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

   Email: jdrake@juniper.net


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

   Email: akatlas@juniper.net










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