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Versions: 00 01

RIFT                                                            Z. Zhang
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                              P. Thubert
Expires: January 14, 2021                                          Cisco
                                                           July 13, 2020


                     Multicast Routing In Fat Trees
                     draft-zzhang-rift-multicast-01

Abstract

   This document specifies multicast procedures with RIFT.  Multicast in
   RIFT is similar to Bidirectional Protocol Independent Multicast (PIM-
   Bidir), with the Rendezvous Point Link (RP-Link) simulated by a
   spanning tree of some Top of Fabric (TOF) nodes and sub-TOF nodes.

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."

   This Internet-Draft will expire on January 14, 2021.

Copyright Notice

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





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Specifications  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Multicast Capability  . . . . . . . . . . . . . . . . . .   4
     2.2.  Optional Per-neighbor Flooding Scope  . . . . . . . . . .   5
     2.3.  Multicast TIE . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Building Spanning Tree among TOFs and sub-TOFs  . . . . .   6
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Because of the simple north-south regular topology in Fat Tree
   networks, the PIM-Bidir [RFC5015] solution is extended for multicast
   in RIFT (referred to as MRIFT in this document).  The following is a
   summary of the changes and adaptations compared to PIM-Bidir.

   With PIM-Bidir, PIM joins are sent towards a Rendezvous Point Address
   (RPA), which could be an address not belonging to any router.  The
   RPA does belong to a RP Link (RPL), which could be attached to a
   single router or multiple routers (e.g.  RPL is a LAN).  With MRIFT,
   there is no concept of RPA any more (joins are simply sent
   northbound).  The joins are terminated on some sub-TOF nodes and the
   RPL is simulated by a spanning tree among some TOF and sub-TOF nodes.

   Instead of (*,G) trees in PIM-Bidir, MRIFT uses (*,G-Prefix) trees,
   where the G-Prefix could be *, G, or anything in between (e.g.,
   225.1.1.0/24).  For light flows, they could just follow the (*,*)
   tree.  For heavy flows, individual (*,G) trees could be built.  For
   medium flows, some (*,G-prefix) trees could be shared.  All the First
   Hop Routers (FHRs, connecting to sources) and the Last Hop Routers
   (LHRs, connecting to receivers) of a particular (*,G) flow must agree
   on whether a (*,*) or (*,G) or (*,G-prefix) tree is used for the flow



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   so that they all join the same tree.  This is done via out of band
   control outside the scope of this document.

   Because of the rich connections in Fat Trees, a router has to choose
   one of its many north neighbors to send join to.  This is done
   through hashing.  The hashing algorithm should lead to several but
   not too many routers choosing the same north neighbor, so that fewer
   routers are involved in multicast traffic forwarding, yet none of
   those routers are overburdened by replicating to too many downstream
   neighbors.

   Instead of PIM messages, RIFT's own TIEs are used, similar to the
   concept in [draft-zzhang-pim-pds].  This introduces the concept of
   neighbor-scoped flooding - a multicast TIE is sent only to a chosen
   upstream north neighbor that consumes it and then regenerates a new
   TIE for the next upstream.

   When a join reaches a sub-TOF node, the normal join process stops.
   This forms a sub-tree rooted at this sub-TOF node.  Multiple sub-
   trees of the same tree may be joined by a single TOF node, or they
   may have to be connected by a spanning tree serving as the RPL.  For
   example, in the following topology, in normal situations the two sub-
   tree roots for the two pods, say Spine111 and Spine121, may be joined
   by TOF21, but if the TOF21-Spine121 link is down, then TOF22 may be
   used, and if the TOF22-Spine111 link is also down, then Spine111 and
   Spine121 will have to be joined via
   Spine111-TOF21-Spine112-TOF22-Spine121.
























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


2.  Specifications

2.1.  Multicast Capability

   A new optional field is added to the NodeCapabilities to indicate
   that the node is enabled for multicast:

    struct NodeCapabilities {
        ...
        4: optional bool           multicast_enabled;
    }





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2.2.  Optional Per-neighbor Flooding Scope

   This document introduces an optional per-neighbor flooding scope for
   TIEs:

    struct TIEHeader {
       ...
       13: optional common.SystemIDType    flooding_scope_neighbor;
    }

   When a node originates a TIE with a per-neighbor flooding scope, it
   is sent to the specified neighbor only.  When a node receives a TIE
   with per-neighbor flooding scope, it is accepted only if the node is
   the specified neighbor, and it is not reflooded any further.

2.3.  Multicast TIE

   Currently the multicast TIEs are only N-TIEs with per-neighbor
   flooding scope except on TOFs and sub-TOFs.  If a multicast TIE is
   received from a node south of sub-TOFs without the per-neighbor
   flooding scope specified, it MUST be discarded.






























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 /** TIE for multicast */
 struct IPMulticastTIEElement {
     /** Multicast TIEs are for (*, group-prefix) joins.
         The '*' is not encoded in the TIE. */
     1: required common.IPPrefixType            group_prefix;

     /** fields used by TOFs and sub-TOFs to build spanning tree RPL */
     2: optional common.SystemIDType            chosen_or_highest_parent;
     3: optional list<common.SystemIDType>  sub_tof_children;
 }

/** Type of TIE.
    ...
*/
enum TIETypeType {
    ...
    TIETypeIPMulticast                                                  = 11,
    TIETypeMaxValue                             = 12,
}

 /** Single element in a TIE.
     ...
  */
 union TIEElement {
     ...
     /** IP multicast elements. */
     10: optional IPMulticastTIEElement ip_multicast;
 }

2.4.  Building Spanning Tree among TOFs and sub-TOFs

   Note: this is still subject to further discussion/change.  It may be
   replaced by another scheme upon further discussions.

   If a sub-TOF node is the root of a sub-tree for a (*, G-prefix) tree,
   it hashes to a TOF neighbor as its parent for the tree, and
   originates a corresponding multicast N-TIE without the per-neighbor
   flooding scope - flooded to all its north TOF neighbors.  The
   chosen_or_highest_parent field is set to the chosen TOF neighbor.

   A receiving TOF node originates a corresponding S-TIE without the
   per-neighbor flooding scope.  The chosen_or_highest_parent field is
   set to the highest chosen_or_highest_parent of all received N-TIEs
   and S-TIEs for the tree, identifying the root of all sub-trees from
   that TOF node's point of view.  The sub_tof_children list all of sub-
   TOF nodes that have chosen the root as parent.





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   If a sub-TOF node that is the root of a sub-tree receives from TOF
   neighbors some S-TIE for the same tree but with different
   chosen_or_highest_parent values, it chooses, from all its TOF
   neighbors that are recorded as a chosen_or_highest_parent, the one
   with the highest system-id and (re)parent to that neighbor if that
   neighbor is not already its parent.

   After the above steps, if a TOF node remains as the chosen parent of
   some sub-TOF nodes but its system-id does not match the highest
   chosen_or_highest_parent of all N-TIEs and S-TIEs (i.e. the root),
   the TOF node needs to join towards the root through some intermediate
   sub-TOF and TOF nodes.  If it has a sub-TOF neighbor listed in the
   sub_tof_children of the root, it originates an S-TIE with the per-
   neighbor flooding scope set to the sub-TOF neighbor, i.e. the sub-TOF
   neighbor now becomes the parent of the TOF node (that is a parent of
   some other sub-TOF nodes).

   In case the TOF node does not have a neighbor listed in the
   sub_tof_children of the S-TIE for the root, further study is needed.
   It could be that the topology is so partitioned that a spanning tree
   could not be built.

3.  Security Considerations

   To be provided.

4.  Acknowledgements

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

5.  References

5.1.  Normative References

   [I-D.ietf-rift-rift]
              Przygienda, T., Sharma, A., Thubert, P., Rijsman, B., and
              D. Afanasiev, "RIFT: Routing in Fat Trees", draft-ietf-
              rift-rift-12 (work in progress), May 2020.

   [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>.







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5.2.  Informative References

   [I-D.zzhang-pim-pds]
              Zhang, J. and K. Patel, "Protocol Dependent Multicast
              Signaling", draft-zzhang-pim-pds-00 (work in progress),
              October 2015.

   [RFC5015]  Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
              "Bidirectional Protocol Independent Multicast (BIDIR-
              PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,
              <https://www.rfc-editor.org/info/rfc5015>.

Authors' Addresses

   Zhaohui Zhang
   Juniper Networks

   EMail: zzhang@juniper.net


   Pascal Thubert
   Cisco Systems, Inc

   EMail: pthubert@cisco.com



























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