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

for supporting SSM, with pointers to more detailed documents where
appropriate. Issues of interoperability with the multicast service
model defined by RFC1112 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, RFC2119 [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 RFC1112 [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
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". Note
that while SFM is a different model than ASM from a receiver
standpoint, there is no distinction between the two for a sender.

For the purpose of this document, we treat the scoped multicast model
of [12] to be a variant of ASM since it does not explicitly restrict
the number of sources, but only requires that they be located within
the scope zone of the group.

3. The IGMP/PIM-SM/MSDP/MBGP Protocol Suite for ASM

As of this writing, all multicast-capable networks support the ASM
service model. One of the most common multicast protocol suites for
supporting ASM consists of IGMP version 2 [2], PIM-SM [8,9], MSDP
[13] and MBGP [26]. IGMPv2 is the most commonly used protocol for
hosts to specify membership in a multicast group, and nearly all
multicast routers support (at least) IGMPv2. In case of IPv6, MLDv1
[21] is the commonly used protocol.

Although a number of protocols such as PIM-DM [10], CBT [24,11],
DVMRP [6], etc. exist for building multicast tree among all receivers
and sources in the same administrative domain, PIM-SM [8,9] is the
most widely used protocol. PIM-SM builds a spanning multicast tree
rooted at a core rendezvous point or RP for all group members within
a single administrative domain. A 'first-hop' router adjacent to a
multicast source sends the source's traffic to the RP for its domain.
The RP forwards the data down the shared spanning tree to all
interested receivers within the domain. PIM-SM also allows receivers
to switch to a source-based shortest path tree.

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.

Inter-domain multicast service (i.e., where sources and receivers are
located in different domains) requires additional protocols - MSDP
[13] and MBGP [26] are the most commonly used ones. An RP uses the
MSDP protocol to announce multicast sources to 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.

MBGP defines extensions to the BGP protocol 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.

However, the last-hop routers of interested receivers may eventually
switch to a shortest-path tree rooted at the source that is
transmitting the data.

4. Problems with Current Architecture

There are several deployment problems associated with current
multicast architecture:

A) 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 much
less serious for IPv6 than for IPv4 since the size of the
multicast address space is much larger. A static address
allocation scheme, GLOP [17] 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. RFC3138
expands on RFC2770 to allow routing registries to assign
multicast addresses from the GLOP space corresponding to the
RFC1930 private AS space [27]. This space is referred to as
the EGLOP (Extended GLOP) address space. Proposed longer-term
solutions such as the Multicast Address Allocation Architecture
[14] are generally perceived as being too complex (with respect
to the dynamic nature of multicast address allocation) for
widespread deployment.

B) Lack of Access control:

In the ASM service model, a receiver cannot 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. Moreover, even when a source is allocated a
multicast group address to transmit on, it has no way of
enforcing that no other source will use the same address. This
is true even in the case of IPv6, where address collisions are
less likely due to the much larger size of the address space.

C) 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 are not necessary.

5. 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:

A) Address Allocation: 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.

B) Access Control: 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 only by the source S. In
contrast, any host can transmit to an ASM host group. At the
same time, when a sender picks a channel (S,G) to transmit on,
it is automatically ensured that no other sender will be
transmitting on the same channel (except in the case of
malicious acts such as address spoofing). This makes it much
harder to "spam" an SSM channel than an ASM multicast group.

C) Handling of well-known sources: SSM requires only
source-based forwarding trees; this eliminates the need for a
shared tree infrastructure. 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. Note that there is no difference in how
MBGP is used for ASM and SSM.

6. 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::/96 for IPv6
--------------------------------------------------------------

v
+--------------+ 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)
--------------------------------------------------------------
v
+-----------------+ Querier Router
IGMPv3/MLDv2 QUERIER
--------------------------------------------------------------
PIM-SSM PIM-SSM ROUTING
+------------+ Designated Router

(S,G) Join only
v
+-----------+ Backbone Router
PIM-SSM
+-----------+

(S,G) Join only
V

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

We now discuss the framework elements in detail:

6.1. 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 [9] which recommend that routers should
not send SSM traffic 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
range. However, SSM service, as defined in [5], is available only in
this address range for IPv4.

In case of IPv6, [23] has defined an extension to the addressing
architecture to allow for unicast prefix-based multicast addresses.
See RFC3306 for details.

6.2. 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.
Channel discovery is the responsibility of applications. This
information can be made available in a number of ways, including via
web pages, sessions announcement applications, etc. This is similar
to what is used for ASM applications where a multicast session needs
to be announced so that potential subscribers can know of the
multicast group address, encoding schemes used, etc. In fact, the
only additional piece of information that needs to be announced is
the source address for the channel being advertised. However, the
exact mechanisms for doing this is outside the scope of this
framework document.

6.3. SSM-Aware Applications

There are two main issues in making multicast applications
"SSM-aware":

- An application that wants to receive an SSM session must first
discover the channel address in use.

- A receiving application must be able to specify both a source
address and a destination address to the network layer protocol
module on the end-host.

Specific API requirements are identified in [16]. [16] describes
a recommended application programming interface for a host
operating system to support the SFM service model. Although it is
intended for SFM, a subset of this interface is sufficient for
supporting SSM.

6.4. IGMPv3/MLDv2 Host Reporting and Querier

In order to use SSM service, an end-host must be able to specify a
channel address, consisting of a source's unicast address and an SSM
destination address. IGMP version 2 [3] and MLD version 1 [19]
allows an end-host to specify only a destination multicast address.
The ability to specify an SSM channel address c is provided by IGMP
version 3 [3] and MLD version 2 [20]. These protocols support
"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. In fact, IGMPv3 provides a
superset of the capabilities required to realize the SSM service
model.

A detailed discussion of the use of IGMPv3 in the SSM destination
address range is provided in [4].

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. MLD version 1 is derived
from IGMPv2 and does not provide the source filtering capability
required for the SSM service model. MLD version 2 is derived from,
and provides the same support for source-filtering as, IGMPv3. Thus
IGMPv3 (or MLDv2 for IPv6) provides a host with the ability to
request the network for an SSM channel subscription.

6.5. PIM-SSM Routing

[9] provides guidelines for how a PIM-SM implementation should handle
source-specific host reports as required by SSM. Earlier versions of
the PIM protocol specifications did not describe how to do this.

The router requirements for operation in the SSM range are detailed
in [5]. These rules are primarily concerned with preventing
ASM-style behaviour in the SSM address range. In order to comply
with [5] several changes to the PIM-SM protocol are required, as
described in [9]. The most important changes in PIM-SM required for
compliance with [5] are:

- 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.

Note that only a small subset of the full PIM-SM protocol
functionality is needed to support the SSM service model. This
subset is explicitly documented in [9].

7. Interoperability with Existing Multicast Service Models

Interoperability with ASM is one of the most important issues in
moving to SSM deployment, since both models are expected to be used
at least in the foreseeable future. SSM is the ONLY service model
for the SSM address range - the correct protocol behaviour for this
range is specified in [5]. The ASM service model will be offered for
the non-SSM address 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 interoperability issue concerns the MSDP protocol, which is
used between PIM-SM rendezvous points (RPs) to discover multicast
sources across multiple domains. MSDP is not needed for SSM, but is
needed if ASM is supported. [9] specifies operational
recommendations to help ensure that MSDP does not interfere with the
ability of a network to support the SSM service model. Specifically,
[9] states that RPs must not accept, originate or forward MSDP SA
messages for the SSM address range.

8. 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.

9. Acknowledgments

We would like to thank Gene Bowen, Ed Kress, Bryan Lyles, Timothy
Roscoe, Hugh Holbrook, Isidor Kouvelas, Tony Speakman and Nidhi
Bhaskar for participating in lengthy discussions and design work on
SSM, and providing feedback on this document. Thanks are also due to
Mujahid Khan, Ted Seely, Tom Pusateri, Bill Fenner, Kevin Almeroth,
Brian Levine, Brad Cain, Hugh LaMaster and Pekka Savola for their
valuable insights and continuing support.

10. References

10.1. Informative References

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

[2] Fenner, W., "Internet Group Management Protocol, Version 2", RFC
2236, November 1997.

[3] Cain, B., Deering, S., Kouvelas, I. and A. Thyagarajan,
"Internet Group Management Protocol, Version 3.", RFC3376,
October 2002.

[4] Holbrook, H. and B. Cain, "Using IGMPv3 and MLDv2 for
Source-Specific Multicast", Work In Progress.

[5] Holbrook, H. and B. Cain, "Source-Specific Multicast for IP",
Work in Progress.

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

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

[8] Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering, S.,
Handley, M., Jacobson, V., Liu, C., Sharma, P. and L. Wei,
"Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol
Specification", RFC2362, June 1998.

[9] Fenner, B., Handley, M., Holbrook, H. and I. Kouvelas, "Protocol
Independent Multicast - Sparse Mode (PIM-SM): Protocol
Specification (Revised)", Work In Progress.

[10] Adams, A., Nicholas, J. and W. Siadek, "Protocol Independent
Multicast - Dense Mode (PIM-DM): Protocol Specification
(Revised)", Work In Progress.

[11] Ballardie, A., "Core-Based Trees (CBT) Multicast Routing
Architecture", RFC2201, September 1997.

[12] Meyer, D., "Adminstratively Scoped IP Multicast", BCP 23, RFC
2365, July 1998.

[13] Farinacci, D. et al., "Multicast Source Discovery Protocol",
Work In Progress.

[14] Thaler, D., Handley, M. and D. Estrin, "The Internet Multicast
Address Allocation Architecture", RFC2908, September 2000.

[15] Diot, C., Levine, B., Lyles, B., Kassem, H. and D. Balensiefen,
"Deployment Issues for the IP Multicast Service and
Architecture", In IEEE Networks Magazine's Special Issue on
Multicast, January, 2000.

[16] Thaler, D., Fenner B. and B. Quinn, "Socket Interface Extensions
for Multicast Source Filters", Work in Progress.

[17] Meyer, D. and P. Lothberg, "GLOP Addressing in 233/8", BCP 53,
RFC3180, September 2001.

[18] Levine, B. et al., "Consideration of Receiver Interest for IP
Multicast Delivery", In Proceedings of IEEE Infocom, March 2000.

[19] Deering, S., Fenner, W. and B. Haberman, "Multicast Listener
Discovery for IPv6", RFC2710, October 1999.

[20] Vida, R. et. al., "Multicast Listener Discovery Version 2(MLDv2)
for IPv6", Work In Progress.

[21] Haberman, B. and D. Thaler, "Unicast-Prefix-Based IPv6 Multicast
Addresses", RFC3306, August 1992.

[22] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC2401, November 1998.

[23] Haberman, B., "Allocation Guidelines for IPv6 Multicast
Addresses", RFC3307, August 2002.

[24] Ballardie, A., "Core-Based Trees (CBT Version 2) Multicast
Routing -- Protocol Specification", RFC2189, September 2001.

[25] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
1112, August 1989.

[26] Bates, T., Rekhter, Y., Chandra, R. and D. Katz, "Multiprotocol
Extensions for BGP-4", RFC2858, June 2000.

[27] Meyer, D., "Extended Assignments in 233/8", RFC3138, June 2001.

10.2. Normative References

[28] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.

11. Contributors

Christophe Diot
Intel
EMail: christophe.diot@intel.com

Leonard Giuliano
Juniper Networks
EMail: lenny@juniper.net

Greg Shepherd
Procket Networks
EMail: shep@procket.com

Robert Rockell
Sprint
EMail: rrockell@sprint.net

David Meyer
Sprint
EMail: dmm@1-4-5.net

John Meylor
Cisco Systems
EMail: jmeylor@cisco.com

Brian Haberman
CASPian Networks
EMail: bkhabs@nc.rr.com

12. Editor's Address

Supratik Bhattacharyya
Sprint

EMail: supratik@sprintlabs.com

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