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Versions: 00 01 02 03 04 05 rfc3569                                     
INTERNET-DRAFT                                    Supratik Bhattacharyya
Expires 18 February 2002                                 Christophe Diot
                                                              Sprint ATL
                                                        Leonard Giuliano
                                                        Juniper Networks
                                                             Rob Rockell
                                                      Sprint E|Solutions
                                                             John Meylor
                                                           Cisco Systems
                                                             David Meyer
                                                      Sprint E|Solutions
                                                           Greg Shepherd
                                                        Juniper Networks
                                                          Brian Haberman
                                                         Nortel Networks
                                                          18 August 2001

        An Overview of Source-Specific Multicast(SSM) Deployment

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   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."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   The key words "MUST"", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC 2119].

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INTERNET-DRAFT        An Overview of SSM Deployment          18 May 2000


   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.  The IPv4 address range 232/8 has been reserved by IANA fo
   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 (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. Although a number of protocols
   exists for constructing source-rooted forwarding trees, this document
   discusses one of the most widely implemented one - PIM Sparse Mode

   This document is intended as a starting point for deploying SSM
   services.  It provides an architectural overview of SSM and describes
   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 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

1. 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 [RFC1112]. An IP datagram is transmitted to a
  "host group", a set of zero or more end-hosts identified by a single
  IP destination address ( through for IPv4).
  This model supports one-to-many and and many-to-many multicast groups.
  End-hosts may join and leave the group any time, and there is no
  restriction on their location or number. Moreover, any end-host 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 [SSM-ARCH]. 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 is derived from EXPRESS [EXPRESS]
  and supports one-to-many multicast.The address range 232/8 has been

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INTERNET-DRAFT        An Overview of SSM Deployment          18 May 2000

  assigned by IANA [IANA-ALLOC] for SSM service in IPv4. For IPv6, the
  range FF3x::/12 is defined for SSM services [SSM-IPV6].

  Source-Filtered Multicast (SFM) : This is a variant of the multicast
  service model defined in RFC 1112. A source transmits IP datagrams to
  a host group address in the range of to
  However, 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.
  Such support for source filtering is provided by version 3 of the
  Internet Group Management Protocol (or IGMPv3) [IGMPv3] for IPv4, and
  version 2 of the Multicast Listener Discovery (or MLD) protocol for
  IPv6 [MLDv2]. We shall henceforth refer to these two protocols as
  "SFM-capable". Earlier versions of these protocols - IGMPv1/IGMPv2 and
  MLDv1 - do not provide support for source-filtering, and are referred
  to as "non-SFM-capable".

2. The IGMP/PIM-SM/MSDP/MBGP Architecture for ASM

   All multicast-capable networks of today support the ASM service
   model.  One of the most common multicast protocol architectures for
   supporting ASM in wide-area backbones consists of IGMP version 2
   protocols.  To become a member of a particular host group end-hosts
   report multicast group membership with querier routers handling
   multicast group membership function using the IGMP version 2 (IGMPv2)
   protocol [RFC2236] for IPv4 or the MLD version 1 (MLDv1) protocol
   [RFC2710] for IPv6.  Routers then exchange messages with each other
   according to a routing protocol to construct a distribution tree
   connecting all the end-hosts. A number of different protocols exist
   for building multicast forwarding trees, which differ mainly in the
   type of delivery tree constructed [IPMULTICAST,PIM-ARCH, PIM-SM, PIM-
   SM-NEW, PIM-DM]. For scalability reasons, sparse-mode protocols
   (e.g., PIM-SM) are preferred over dense-mode protocols (e.g., DVMRP,
   PIM-DM)  for deployment in large backbone networks (though many
   smaller networks deploy dense-mode protocols). PIM-SM,  most widely
   deployed sparse-mode protocol, builds a spanning multicast tree
   rooted at a core rendezvous point or RP for all group members within
   a single administrative domain. Multicast sources within this domain
   send their data to this RP which forwards the data down the shared
   tree to interested receivers within the domain. As of this writing,
   multicast end-hosts with SFM capabilities are not widely available.
   Hence a client can only specify interest in an entire host group and
   receives data sent from any source to this group. PIM-SM also allows
   receivers to switch to a source-based shortest path tree.

   An RP uses the MSDP [MSDP] protocol to announce multicast sources to

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   RPs in other domains. When an RP discovers a source in a different
   domain transmitting data to a multicast group for which there are
   interested receivers in its own domain, it joins the shortest-path
   source based tree rooted at that source. It then redistributes the
   data received to all interested receivers via the intra-domain shared
   tree rooted at itself.

   The MBGP protocol [MBGP] defines extensions to the BGP protocol [BGP]
   to support the advertisement of reachability information for
   multicast routes. This allows an autonomous system (AS) to support
   incongruent unicast and multicast routing topologies, and thus
   implement separate routing policies for each.

3. Problems with Current Architecture

   There are several deployment problems associated with current
   multicast architecture:

   A) Inefficient handling of well-known sources :

       In cases where the address of the source is well known in advance
      of the receiver joining the group, and when the shortest
      forwarding path is the preferred forwarding mode, then shared tree
      mechanisms and MSDP are not necessary.

      B) Lack of access control :

       In the ASM service model, a receiver can not specify which
      specific sources it would like to receive when it joins a given
      group. A receiver will be forwarded data sent to a host group by
      any source.

      C) Address Allocation :

      Address allocation is one of core deployment challenges posed by
      the ASM service model. The current multicast architecture does not
      provide a deployable solution to prevent address collisions among
      multiple applications. The problem is more serious for IPv4 than
      IPv6 since the total number of multicast addresses is smaller. A
      static address allocation scheme, GLOP [GLOP00] has been proposed
      as an interim solution for IPv4; however, GLOP addresses are
      allocated per registered AS, which is inadequate in cases where
      the number of sources exceeds the AS numbers available for
      mapping. Proposed longer-term solutions such as the Multicast
      Address Allocation Architecture [MAAA] are generally perceived as
      being too complex (with respect to the dynamic nature of multicast
      address allocation) for widespread deployment.

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4. Source Specific Multicast (SSM) : Benefits and Requirements

   As mentioned before, the Source Specific Multicast (SSM) service
   model defines a "channel" identified by an (S,G) pair, where S is a
   source address and G is an SSM destination address. Channel
   subscriptions are described using an SFM-capable group management
   protocol such as IGMPv3 or MLDv2. Only source-based forwarding trees
   are needed to implement this model.

   The SSM service model alleviates all of the deployment problems
   described earlier :

      4.1 SSM lends itself to an elegant solution to the access control
      problem. When a receiver subscribes to an (S,G) channel, it
      receives data sent by a only the source S. In contrast, any host
      can transmit to an ASM host group. Hence, it is more difficult to
      spam an SSM channel than an ASM host group.

      4.2 SSM defines channels on a per-source basis, i.e., the channel
      (S1,G) is distinct from the channel (S2,G), where S1 and S2 are
      source addresses, and G is an SSM destination address. This averts
      the problem of global allocation of SSM destination addresses, and
      makes each source independently responsible for resolving address
      collisions for the various channels that it creates.

      4.3 SSM requires only source-based forwarding trees; this
      eliminates the need for a shared tree infrastructure. In terms of
      the IGMP/PIM-SM/MSDP/MBGP protocol suite, this implies that
      neither the RP-based shared tree infrastructure of PIM-SM nor the
      MSDP protocol is required. Thus the complexity of the multicast
      routing infrastructure for SSM is low, making it viable for
      immediate deployment.

      4.4 It is widely held that point-to-multipoint applications such
      as Internet TV will dominate the Internet multicast application
      space in the near future. The SSM model is ideally suited for such

5. SSM Framework

Figure 1 illustrates the elements in an end-to-end implementation
framework for SSM :

    IANA assigned 232/8 for IPv4             ADDRESS ALLOCATION
         FF3x::/12 for IPv6

Bhattacharyya et. al.                                           [Page 5]

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       +--------------+ session directory/web page
       | source,group |                      SESSION DESCRIPTION
              ^ |
        Query | | (S,G)
              | v
     +-----------------+ host
     |   SSM-aware app |                     CHANNEL DISCOVERY
     |   SSM-aware app |                   SSM-AWARE APPLICATION
     |   IGMPv3/MLDv2  |              IGMPv3/MLDv2 HOST REPORTING
               |(source specific host report)
     +-----------------+  Querier Router
     |   IGMPv3/MLDv2  |                         QUERIER
       |   PIM-SSM  |                        PIM-SSM ROUTING
       +------------+     Designated Router
               | (S,G) Join only
         +-----------+  Backbone Router
         |  PIM-SSM  |
               | (S,G) Join only

     Figure 1  : SSM Framework: elements in end-to-end model

   We now discuss the framework elements in detail :

   5.1 Address Allocation

   For IPv4, the address range of 232/8 has been assigned by IANA for
   SSM. To ensure global SSM functionality in 232/8, including in
   networks where routers run non-SFM-capable protocols, operational
   policies are being proposed [SSM-BCP] which prevent data sent to
   232/8 from being delivered to parts of the network that do not have
   channel subscribers.

   Note that IGMPv3/MLDv2 does not limit (S,G) joins to only the 232/8

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   range. However, SSM service, as defined in [SSM-ARCH], is available
   only in this address range for IPv4.

   In case of IPv6, [HABE1] has defined an extension to the addressing
   architecture to allow for unicast prefix-based multicast addresses.
   In this case, bytes 0-3 (starting from the least significant byte) of
   the IP address is used to specify a multicast group id, bytes 4-11 is
   be used to specify a unicast address prefix (of up to 64 bits) that
   owns this multicast group id, and byte 12 is used to specify the
   length of the prefix. A source-specific multicast address can be
   specified by setting both the prefix length field and the prefix
   field to zero.

   5.2 Session Description and Channel Discovery

      An SSM receiver application must know both the SSM destination
      address G and the source address S before subscribing to a
      channel. Thus the function of channel discovery becomes the
      responsibility of applications.  This information can be made
      available in a number of ways, including via web pages, sessions
      announcement applications, etc.  The exact mechanisms for doing
      this is outside the scope of this framework document.

   5.3. SSM-Aware Applications

      -- For applications sourcing content via SSM channels, the session
      must be advertised including a source address as well as an SSM

      -- Applications expecting to subscribe to an SSM channel must be
      capable of specifying a source address in addition to an SSM
      destination address. In other words, the application must be "SSM-

      Specific API requirements are identified in [THAL00].

   5.4. IGMPv3/MLDv2 Host Reporting and Querier

      IGMP version 2 [IGMPv2] allows end-hosts to report their interest
      in a multicast group by specifying a class-D IP address for IPv4.
      However in order to implement the SSM service model, an end-host
      must specify a source's unicast address as well as an SSM
      destination address. This capability is provided by IGMP version 3
      [IGMPv3]. IGMPv3 supports "source filtering", i.e., the ability of
      an end-system to express interest in receiving data packets sent
      only by SPECIFIC sources, or from ALL BUT some specific sources.
      Thus, IGMPv3 provides a superset of the capabilities required to

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      realize the SSM service model.

      There are a number of backward compatibility issues between IGMP
      versions 2 and 3 which have to be addressed. A detailed discussion
      of the use of IGMPv3 in the SSM destination address range is
      provided in [SSM-IGMPv3].

      The Multicast Listener Discovery (MLD) protocol used by an IPv6
      router to discover the presence of multicast listeners on its
      directly attached links, and to discover the multicast addresses
      that are of interest to those neighboring nodes.  Version 1 of MLD
      [DEER99] is  derived from IGMPv2 and allows a multicast listener
      to specify the multicast group(s) that it is interested in.
      Version 2 of MLD [VIDA01] is derived from, and provides the same
      support for source-filtering as, IGMPv3.

5.5. PIM-SSM Routing

   PIM-SM [PIM-SM-NEW] itself supports two types of trees, a shared tree
   rooted at a core (RP), and a source-based shortest path tree. Thus
   PIM-SM already supports source-based trees. The original
    PIM-SM [PIM-SM] did not allow a router to choose between a shared
   tree and a source-based tree. In fact, a receiver always joined a PIM
   shared tree to start with, and may later be switched to a per-source
   tree by its adjacent edge router. However, the more recent PIM-SM
   specification [PIM-SM-NEW] has support for source-specific join.

   Supporting SSM with PIM-SM involves several changes to PIM-SM as
   described in [PIM-SM-NEW]. The resulting PIM functionality is
   described as PIM-SSM. The specific architectural issues associated
   with PIM-SSM and IGMPv3/MLDv2 are detailed in [SSM-ARCH]. The most
   important changes to PIM-SM with respect to SSM are as follows:

      -- When a DR receives an (S,G) join request with the address G in
      the SSM address range, it must initiate a (S,G) join and NEVER a
      (*,G) join.

      --Backbone routers (i.e. routers that do not have directly
      attached hosts) must not propagate (*,G) joins for group addresses
      in the SSM address range.

      --Rendezvous Points (RPs) must not accept PIM Register messages or
      (*,G) Join messages in the SSM address range.

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INTERNET-DRAFT        An Overview of SSM Deployment          18 May 2000

6. Interoperability with Existing Multicast Service Models

   Interoperability with ASM is one of the most important issues in
   moving to SSM deployment. ASM and SSM will always coexist; hence
   there will be two service models for Internet multicast. SSM is the
   ONLY service model for the SSM address range - the correct protocol
   behaviour for this range is specified in [SSM-ARCH]. The ASM service
   model will be offered for the non-SSM adddress range, where receivers
   can issue (*,G) join requests to receive multicast data. A receiver
   is also allowed to issue an (S,G) join request in the non-SSM address
   range; however, in that case there is no guarantee that it will
   receive service according to the SSM model.

   Another backward compatibility issue concerns the MSDP protocol,
   which is used between PIM-SM rendezvous points (RPs) to discover
   multicast sources across multiple domains. SSM obviates the needs for
   MSDP, but MSDP is still required to support ASM for non-SSM class-D
   IPv4 addresses. In order to ensure that SSM is the sole forwarding
   model in 232/8, RPs must not accept, originate or forward MSDP SA
   messages for the SSM address range [SSM-BCP].

7. Security Considerations

   SSM does not introduce new security considerations for IP multicast.
   It can help in preventing denial-of-service attacks resulting from
   unwanted sources transmitting data to a multicast channel (S, G).
   However no guarantee is provided.

8. Acknowledgments

   We would like to thank Gene Bowen, Ed Kress, Bryan Lyles, Sue Moon
   and Timothy Roscoe at Sprintlabs, Hugh Holbrook, Isidor Kouvelas,
   Tony Speakman and Nidhi Bhaskar at Cisco Systems for participating in
   lengthy discussions and design work on SSM, and providing feedback on
   this document. Thanks are also due to Mujahid Khan and Ted Seely at
   SprintLink, Tom Pusateri at Juniper Networks, Bill Fenner at AT&T
   Research, Kevin Almeroth at the University of California Santa
   Barbara, Brian Levine at the University of  Massachusetts Amherst,
   Brad Cain at Cereva Networks and Hugh LaMaster at NASA for their
   valuable insights and continuing support.

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INTERNET-DRAFT        An Overview of SSM Deployment          18 May 2000

9. References:

   [EXPRESS] H. Holbrook and D.R. Cheriton.  IP Multicast Channels :
   EXPRESS Support for Large-scale Single-Source Applications. In
   Proceedings of SIGCOMM 1999.

   [IANA-ALLOCATION] Internet Assigned Numbers Authority.

   [RFC2236] W. Fenner. Internet Group Management Protocol, Version 2.
   Request For Comments 2236.

   [IGMPv3] B. Cain and S. Deering, I. Kouvelas and A. Thyagarajan.
   Internet Group Management Protocol, Version 3. Work in Progress.

   [SSM-IGMPv3] H. Holbrook and B. Cain.  IGMPv3 for SSM. Work in

   [SSM-ARCH] H. Holbrook and B. Cain.  Source-Specific Multicast for
   IP. Work in Progress.

   [IPMULTICAST] S. Deering and D. Cheriton.  Multicast Routing in
   Datagram Networks and Extended LANs. ACM Transactions on Computer
   Systems, 8(2):85-110, May 1990.

   [PIM-ARCH] S. Deering et al.  PIM Architecture for Wide-Area
   Multicast Routing. IEEE/ACM Transaction on Networking, pages 153-162,
   April 1996.

   [PIM-SM] D. Estrin et al.  Protocol Independent Multicast - Sparse
   Mode (PIM-SM) : Protocol Specification. Request for Comments, 2362.

   [PIM-SM-NEW] B. Fenner, M. Handley, H. Holbrook, I. Kouvelas.
   Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol
   Specification (Revised)", Work In Progress, 2000.  <draft-ietf-pim-

   [PIM-DM] S. Deering et al.  Protocol Independent Multicast Version 2
   Dense Mode Specification.  Work in Progress.

   [MSDP] Farinacci et al.  Multicast Source Discovery Protocol. Work in

   [MAAA] M. Handley, D. Thaler and D. Estrin.  The Internet Multicast
   Address Allocation Architecture.  Work in Progress (draft-ietf-
   malloc-arch-**.txt) June 2000.

   [MCAST-DEPLOY] C. Diot, B. Levine, B. Lyles, H. Kassem and D.

Bhattacharyya et. al.                                          [Page 10]

INTERNET-DRAFT        An Overview of SSM Deployment          18 May 2000

   Balensiefen.  Deployment Issues for the IP Multicast Service and
   Architecture.  In IEEE Networks Magazine's Special Issue on
   Multicast, January, 2000.

   [SSM-RULES] H. Sandick and B. Cain.  PIM-SM Rules for Support of
   Single-Source Multicast. Work in Progress.

   [MSF-API] Dave Thaler, Bill Fenner and Bob Quinn.  Socket Interface
   Extensions for Multicast Source Filters. Work in Progress.

   [RFC2770] GLOP Addressing in 233/8. Request For Comments 2770.

   [RCVR-INTEREST] B. Levine et al.  Consideration of Receiver Interest
   for IP Multicast Delivery.  In Proceedings of IEEE Infocom, March

   [SSM-BCP]   G. Shepherd et al.  Source-Specific Protocol Independent
   Multicast in 232/8.  Work in Progress.

   [RFC2710] S. Deering, W. Fenner and B. Haberman.  Multicast Listener
   Discovery for IPv6. Request for Comments 2710.

   [MLDv2] R. Vida, et. al.
            Multicast Listener Discovery Version 2 (MLDv2) for IPv6.
            Work in progress.

   [SSM-IPv6] B. Haberman and D. Thaler.
            Unicast-Prefix-Based IPv6 Multicast Addresses. Work in

   [IPSEC] S. Kent, R. Atkinson.
            Security Architecture for the Internet Protocol. Request for
            Comments 2401.

   [IPv6-ALLOC] B. Haberman.
            Dynamic Allocation Guidelines for IPv6 Multicast Addresses.
            Work in Progress.

12. Authors' Address:

   Supratik Bhattacharyya
   Christophe Diot
   Sprint Advanced Technology Labs
   One Adrian Court
   Burlingame CA 94010 USA

Bhattacharyya et. al.                                          [Page 11]

INTERNET-DRAFT        An Overview of SSM Deployment          18 May 2000

   Leonard Giuliano
   Greg Shepherd
   Juniper Networks, Inc.
   1194 North Mathilda Avenue
   Sunnyvale, CA 94089 USA

   Robert Rockell
   David Mayer
   Sprint E|Solutions
   Reston Virginia USA

   John Meylor
   Cisco Systems
   San Jose CA USA

   Brian Haberman
   Nortel Networks

Bhattacharyya et. al.                                          [Page 12]