An Overview of Source-Specific Multicast (SSM)
RFC 3569

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Network Working Group                              S. Bhattacharyya, Ed.
Request for Comments: 3569                                        Sprint
Category: Informational                                        July 2003

             An Overview of Source-Specific Multicast (SSM)

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   The purpose of this document is to provide an overview of
   Source-Specific Multicast (SSM) and issues related to its deployment.
   It discusses how the SSM service model addresses the challenges faced
   in inter-domain multicast deployment, changes needed to routing
   protocols and applications to deploy SSM and interoperability issues
   with current multicast service models.

1.  Introduction

   This document provides an overview of the Source-Specific Multicast
   (SSM) service and its deployment using the PIM-SM and IGMP/MLD
   protocols.  The network layer service provided by SSM is a "channel",
   identified by an SSM destination IP address (G) and a source IP
   address S.  An IPv4 address range has been reserved by IANA for use
   by the SSM service.  An SSM destination address range already exists
   for IPv6.  A source S transmits IP datagrams to an SSM destination
   address G.  A receiver can receive these datagrams by subscribing to
   the channel (Source, Group) or (S,G).  Channel subscription is
   supported by version 3 of the IGMP protocol for IPv4 and version2 of
   the MLD protocol for IPv6.  The interdomain tree for forwarding IP
   multicast datagrams is rooted at the source S, and is constructed
   using the PIM Sparse Mode [9] protocol.

   This document is not intended to be a standard for Source-Specific
   Multicast (SSM).  Instead, its goal is to serve as an introduction to
   SSM and its benefits for anyone interested in deploying SSM services.
   It provides an overview of SSM and how it solves a number of problems
   faced in the deployment of inter-domain multicast.  It outlines
   changes to protocols and applications both at end-hosts and routers

Bhattacharyya                Informational                      [Page 1]
RFC 3569                   An Overview of SSM                  July 2003

   for supporting SSM, with pointers to more detailed documents where
   appropriate.  Issues of interoperability with the multicast service
   model defined by RFC 1112 are also discussed.

   This memo is a product of the Source-Specific Multicast (SSM) Working
   Group of the Internet Engineering Task Force.

   The keywords "MUST"", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as defined in BCP 14, RFC 2119 [28].

2.  Terminology

   This section defines some terms that are used in the rest of this
   document:

      Any-Source Multicast (ASM): This is the IP multicast service model
      defined in RFC 1112 [25].  An IP datagram is transmitted to a
      "host group", a set of zero or more end-hosts (or routers)
      identified by a single IP destination address (224.0.0.0 through
      239.255.255.255 for IPv4).  End-hosts may join and leave the group
      any time, and there is no restriction on their location or number.
      Moreover, this model supports multicast groups with arbitrarily
      many senders - any end-host (or router) may transmit to a host
      group, even if it is not a member of that group.

      Source-Specific Multicast (SSM): This is the multicast service
      model defined in [5].  An IP datagram is transmitted by a source S
      to an SSM destination address G, and receivers can receive this
      datagram by subscribing to channel (S,G).  SSM provides host
      applications with a "channel" abstraction, in which each channel
      has exactly one source and any number of receivers.  SSM is
      derived from earlier work in EXPRESS [1].  The address range 232/8
      has been assigned by IANA for SSM service in IPv4.  For IPv6, the
      range FF3x::/96 is defined for SSM services [21].

      Source-Filtered Multicast (SFM): This is a variant of the ASM
      service model, and uses the same address range as ASM
      (224.0.0.0-239.255.255.255).  It extends the ASM service model as
      follows.  Each "upper layer protocol module" can now request data
      sent to a host group G by only a specific set of sources, or can
      request data sent to host group G from all BUT a specific set of
      sources.  Support for source filtering is provided by version 3 of
      the Internet Group Management Protocol (or IGMPv3) [3] for IPv4,
      and version 2 of the Multicast Listener Discovery (or MLDv2) [22]
      protocol for IPv6.  We shall henceforth refer to these two