Multicast Routing In Fat Trees

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 3 January 2025.¶

Copyright Notice

Copyright (c) 2024 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 (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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

Table of Contents

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¶

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 [I-D.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.¶

   .                +--------+          +--------+          ^ 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

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

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

    /** 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;
    }

3. Security Considerations

To be provided.¶

4. Acknowledgements

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

Authors' Addresses