Multicast Extensions to OSPF
RFC 1584

Document Type RFC - Historic (March 1994; No errata)
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Network Working Group                                             J. Moy
Request for Comments: 1584                                 Proteon, Inc.
Category: Standards Track                                     March 1994

                      Multicast Extensions to OSPF

Status of this Memo

    This document specifies an Internet standards track protocol for the
    Internet community, and requests discussion and suggestions for
    improvements.  Please refer to the current edition of the "Internet
    Official Protocol Standards" (STD 1) for the standardization state
    and status of this protocol.  Distribution of this memo is
    unlimited.

Abstract

    This memo documents enhancements to the OSPF protocol enabling the
    routing of IP multicast datagrams. In this proposal, an IP multicast
    packet is routed based both on the packet's source and its multicast
    destination (commonly referred to as source/destination routing). As
    it is routed, the multicast packet follows a shortest path to each
    multicast destination. During packet forwarding, any commonality of
    paths is exploited; when multiple hosts belong to a single multicast
    group, a multicast packet will be replicated only when the paths to
    the separate hosts diverge.

    OSPF, a link-state routing protocol, provides a database describing
    the Autonomous System's topology. A new OSPF link state
    advertisement is added describing the location of multicast
    destinations. A multicast packet's path is then calculated by
    building a pruned shortest-path tree rooted at the packet's IP
    source. These trees are built on demand, and the results of the
    calculation are cached for use by subsequent packets.

    The multicast extensions are built on top of OSPF Version 2. The
    extensions have been implemented so that a multicast routing
    capability can be introduced piecemeal into an OSPF Version 2
    routing domain. Some of the OSPF Version 2 routers may run the
    multicast extensions, while others may continue to be restricted to
    the forwarding of regular IP traffic (unicasts).

    Please send comments to mospf@gated.cornell.edu.

Moy                                                             [Page 1]
RFC 1584              Multicast Extensions to OSPF            March 1994

Table of Contents

    1       Introduction ........................................... 4
    1.1     Terminology ............................................ 5
    1.2     Acknowledgments ........................................ 6
    2       Multicast routing in MOSPF ............................. 6
    2.1     Routing characteristics ................................ 6
    2.2     Sample path of a multicast datagram .................... 8
    2.3     MOSPF forwarding mechanism ............................ 10
    2.3.1   IGMP interface: the local group database .............. 10
    2.3.2   A datagram's shortest-path tree ....................... 14
    2.3.3   Support for Non-broadcast networks .................... 16
    2.3.4   Details concerning forwarding cache entries ........... 16
    3       Inter-area multicasting ............................... 18
    3.1     Extent of group-membership-LSAs ....................... 19
    3.2     Building inter-area datagram shortest-path trees ...... 22
    4       Inter-AS multicasting ................................. 27
    4.1     Building inter-AS datagram shortest-path trees ........ 28
    4.2     Stub area behavior .................................... 30
    4.3     Inter-AS multicasting in a core Autonomous System ..... 31
    5       Modelling internal group membership ................... 31
    6       Additional capabilities ............................... 33
    6.1     Mixing with non-multicast routers ..................... 34
    6.2     TOS-based multicast ................................... 35
    6.3     Assigning multiple IP networks to a physical network .. 36
    6.4     Networks on Autonomous System boundaries .............. 37
    6.5     Recommended system configuration ...................... 38
    7       Basic implementation requirements ..................... 40
    8       Protocol data structures .............................. 40
    8.1     Additions to the OSPF area structure .................. 41
    8.2     Additions to the OSPF interface structure ............. 42
    8.3     Additions to the OSPF neighbor structure .............. 43
    8.4     The local group database .............................. 43
    8.5     The forwarding cache .................................. 44
    9       Interaction with the IGMP protocol .................... 45
    9.1     Sending IGMP Host Membership Queries .................. 46
    9.2     Receiving IGMP Host Membership Reports ................ 46
    9.3     Aging local group database entries .................... 47
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