SRIFT: Segment Routing in Fat Trees
draft-ietf-rift-sr-01

RIFT                                                            Z. Zhang
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                             J. Tantsura
Expires: 3 January 2025                                           Nvidia
                                                                 J. Head
                                                        Juniper Networks
                                                             2 July 2024

                  SRIFT: Segment Routing in Fat Trees
                         draft-ietf-rift-sr-01

Abstract

   This document specifies signaling procedures for Segment Routing in
   RIFT.  Each node's loopback address, Segment Routing Global Block
   (SRGB) and Node Segment Identifier (Node-SID), which are typically
   assigned by a configuration management system and distibuted by
   routing protocols, are distributed southbound from the Top Of Fabric
   (TOF) nodes via RIFT's Key-Value distribution mechanism, so that each
   node can compute how to reach a segment represented by the active SID
   in a packet.  An SR controller signals SR policies to ingress nodes
   so that they can send packets with a desired segment list to steer
   traffic.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on 3 January 2025.

Copyright Notice

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  SR in RIFT (SRIFT)  . . . . . . . . . . . . . . . . . . . . .   4
   3.  SRIFT-Node Key-Type . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  SRIFT Node Key-Type . . . . . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   7.  Informative References  . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Before we discuss the SR procedures for RIFT, let us first review how
   SR works with OSPF [RFC8665] and IS-IS [RFC8667].

   Each node is provisioned with a loopback address as well as SRGB and
   Node-SID values.  The loopback address and Node-SID are centrally
   coordinated and are unique per-node within the SR network.  These
   values are then communicated to each node out-of-band and stored as
   configuration information.  Communication could be done via primitive
   pen and paper or via modern signaling (Netconf/YANG) from a
   configuration management system.

   SRGB information represents the label range of the "global" labels
   that can be allocated on a particular node for SR.  SRGB could have
   more than one contiguous range of labeks allocated to it.  It is
   comprised of the first available label value and the total number of
   available labels per range.  While in modern networks it is common
   for each node to have identical SRGB values so that a Node-SID will
   correspond to the same label on each node, this is not required as to

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   allow for flexible label allocation.  In either scenario, SRGB is
   part of each node's configuration.  In today's networks, it is likely
   pushed to nodes by a configuration management system.

   Each node then signals its SRGB and Node-SID to the other nodes.  A
   Node-SID is an index value assigned to a node (say node X), and
   another node (say node Y) uses the Node-SID to derive (from Y's SRGB)
   the label to use when sending traffic to node X.

   Consider the following example illustrating Node A's computed IP
   route and label values.

                       B
                     *   *
                   *       *
                 *           *
               A               D
                 *           *
                   *       *
                     *   *
                       C

   Node Name   Loopback   Node SID   SRGB Label Base   SRGB Label Range
   ---------   --------   --------   ---------------   ----------------

   A           10.1.1.1   1          100               50
   B           10.1.1.2   2          100               50
   C           10.1.1.3   3          200               50
   D           10.1.1.4   4          100               50

      Destination    Next Hop
      -----------    --------
      10.1.1.1       local
      10.1.1.2       if_ab
      10.1.1.3       if_ac
      10.1.1.4       if_ab, if_ac

      Label           Next Hop
      -----           --------
      100 (La_a)      pop and look up next header
      101 (Lb_a)      swap to 101 (Lb_b), via if_ab
      102 (Lc_a)      swap to 202 (Lc_c), via if_ac
      103 (Ld_a)      swap to 103 (Ld_b), via if_ab
                      swap to 203 (Ld_c), via if_ac

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   The specific notation Lb_a refers to the label derived for node B,
   using B's Node-SID as index into A's SRGB.  Similarly, Ld_c refers to
   the label derived for Node D, using D's Node-SID as index into C's
   SRGB.

   Node A computes the route to Node D's loopback address.  The next
   hops are Node B (via if_ab) and Node C (via if_ac).  Node A uses Node
   D's Node-SID (which was advertised along with the loopback address)
   to index into its local SRGB to obtain a label value of 103 (Ld_a).
   Furthermore, Node A also uses Node D's Node-SID to derive label
   values for Node B and Node C, 103 (Ld_b) and 203 (Ld_c) respectively,
   using D's Node-SIDs as index into B and C' SRGBs respectively.
   Notice that Node C's SRGB is different from the other nodes.  Node A
   can now program its label forwarding state with (Ld_a --> (via if_ab
   swap to Ld_b, via if_ac swap to Ld_c)).

   Similarly, Node B computes the route to Node D's loopback address,
   but this time finds that the next hop is Node D itself (via if_bd).
   Node B will also use Node D's Node-SID (again, advertised with the
   loopback address) to index into its local SRGB and obtain a label
   value of 103 (Ld_b) and index into Node D's SRGB and obtain a label
   value of 103 (Ld_d).  The label forwarding state can be programmed
   with (Ld_b --> via if_bd swap to Ld_d).  Finally, Node D programs its
   label forwarding state with (Ld_d -> pop and lookup next header).

2.  SR in RIFT (SRIFT)

   In referring to the previous section, it is clear that each RIFT node
   participating in a SR domain requires the following information:

   *  SRGB values of all adjacent nodes

   *  Node-SID values of all nodes participating in the routing domain

   *  Loopback addresses or System IDs of all other nodes

   In OSPF and IS-IS, each node's SR information is simply flooded.
   With RIFT, Node TIEs could be used to flood SR information, but each
   node would have to learn its own SR information first.  With RIFT's
   Key-Value mechanism, KV-TIEs can be used for TOF nodes to flood all
   nodes' SR information that it learns from an SR controller, therefore
   accommodating both provisioning and signalling of SR.  The non-TOF
   nodes do not need any SR related provisioning, which goes very well
   with RIFT's ZTP concept.

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   ToF nodes in an SR domain MUST populate KV South TIEs with the
   minimum required SR information for each node.  Specifically SRGB
   Label Base, SRGB Label Range, Node-SID, RIFT System ID, and Loopback
   Address.  While the Loopback Address must be included, it MAY be set
   to an empty value in cases if loopbacks are not configured for nodes.

   Traffic forwarding in an SR network is typically done in two ways.

   The first option is to use Prefix-SIDs and allow traffic to follow
   the shortest paths for the prefixes.  Prefix-SIDs for node prefixes,
   i.e. Node-SIDs (for loopback addresses), can be used both for
   encapsulating service traffic to service nodes (e.g.  VPN PEs) and
   for SR-TE traffic steering purposes (see below), but the benefits of
   other Prefix-SIDs are not clear, so currently only Node-SIDs are
   supported with RIFT.

   The second option is to use SR-TE and follow a specific segment list
   in the packet header.  Each node in the path steers the packet to the
   currently active segment in the list, following the natural path for
   that segment (see above).  Since a node only has the full topology
   south of it, and a leaf node does not have any south topology, the
   traffic steering information (i.e. the segment list) must be
   programmed by controllers into ingress nodes via SR policies.

   Support for Adjacency SIDs will be considered in future revisions.

   Consider the following 4-level topology:

                         ToF1                      ToF2

                Spine1_11    Spine1_21  |  Spine1_21    Spine1_22
                                        |
                Spine2_11    Spine2_21  |  Spine2_21    Spine2_22
                                        |
                  Leaf11       Leaf12   |   Leaf21       Leaf22

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   Suppose the TE controller instructs Leaf11 to send a packet to
   Spine2_11 with label stack (Label_TOF2, Label_Spine2_21,
   Label_Leaf21).  Spine2_11 recognizes that Label_TOF2 maps to node
   TOF2 and it should not simply follow the default route (because the
   default route could lead to an unintended path via TOF1).  In other
   words, each node needs to have a specific route to every node (that
   may appear in the segment list).  That means for RIFT the southbound
   distance vector routing needs to additionally advertise routes for
   the nodes in the north, and they must be propagated all the way down.
   Each node originates a route for its own loopback address and
   advertises it southbound, with a special marking that allows a south
   node to re-advertise it further south.

   If loopback addresses are not used, similar "routes" for System IDs
   must be used.  It is RECOMMENDED to use loopback addresses to reuse
   existing mechanisms.

3.  SRIFT-Node Key-Type

   This section requests an entry from the RIFT Key-Types Registry for
   RIFT networks that use SR along with suggested values in accordance
   with [I-D.ietf-rift-kv-tie-structure-and-processing].

   +============+=======+==================================+
   | Name       | Value | Description                      |
   +============+=======+==================================+
   | SRIFT-Node | TBD   | Key-Type describing a SRIFT node |
   +------------+-------+----------------------------------+

                   Table 1: Requested Entries

3.1.  SRIFT Node Key-Type

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      TBD      |      Key Identifier for a SRIFT Node          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     (System ID,                                               |
      |      Loopback Address,                                        |
      |      SRGB Label Base,                                         |
      |      SRGB Label Range,                                        |
      |      Node-SID)                                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

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      System ID:
         A node's 64-bit RIFT System ID.

      Loopback Address:
         A node's loopback address.  This MAY be set to 0 if loopback
         addresses are not used.

      SRGB Label Base:
         The first valid label within the corresponding node's SRGB.

      SRGB Label Range:
         The total number of valid labels in the corresponding node's
         SRGB.

      Node-SID:
         The corresponding node's Node-SID value.

   The Key Identifier for a RIFT node is assigned by an orchestrator,
   which distributes the KVs to all ToF nodes.  In the simplest
   scenario, the Key Identifier could be the SID index into the SRGB.
   The ToF nodes all have the same KVs learned from the orchestrator and
   distribute them southbound.  A node may learn the same KV from
   multiple north nodes and the tie-breaking will lead to the same
   information to be selected and distributed further south.

4.  Security Considerations

   This document does not introduce any new security concerns with RIFT
   or any other referenced protocols.  RIFT KV TIEs are already
   extensively secured via RIFT's specification.

5.  Acknowledgements

   The authors thank Bruno Rijsman and Antoni Przygenda for their review
   and suggestions.

6.  Normative References

   [I-D.ietf-rift-kv-tie-structure-and-processing]
              Head, J. and T. Przygienda, "RIFT Key/Value TIE Structure
              and Processing", Work in Progress, Internet-Draft, draft-
              ietf-rift-kv-tie-structure-and-processing-00, 4 March
              2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
              rift-kv-tie-structure-and-processing-00>.

   [I-D.ietf-rift-rift]
              Przygienda, T., Head, J., Sharma, A., Thubert, P.,
              Rijsman, B., and D. Afanasiev, "RIFT: Routing in Fat

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              Trees", Work in Progress, Internet-Draft, draft-ietf-rift-
              rift-24, 23 May 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-rift-
              rift-24>.

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

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

7.  Informative References

   [RFC8665]  Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
              H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", RFC 8665,
              DOI 10.17487/RFC8665, December 2019,
              <https://www.rfc-editor.org/info/rfc8665>.

   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

Authors' Addresses

   Zhaohui Zhang
   Juniper Networks
   Email: zzhang@juniper.net

   Jeff Tantsura
   Nvidia
   Email: jefftant.ietf@gmail.com

   Jordan Head
   Juniper Networks
   Email: jhead@juniper.net

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