View source for Source-specific multicast

{{Short description|Delivering multicast packets based on source and destination addressing}}
'''Source-specific multicast''' ('''SSM''') is a method of delivering [[Engineering:Multicast|multicast]] packets in which the only packets that are delivered to a receiver are those originating from a specific source address requested by the receiver. By so limiting the source, SSM reduces demands on the network and improves security.

SSM requires that the receiver specify the source address and explicitly excludes the use of the (*,G) join for all multicast groups in RFC 3376, which is possible only in IPv4's IGMPv3 and IPv6's [[Multicast Listener Discovery|MLDv2]].

==Any-source multicast (as counterexample)==
Source-specific multicast  is best understood in contrast to [[Any-source multicast|any-source multicast]] (ASM). In the ASM service model a receiver expresses interest in traffic ''to'' a [[Multicast address|multicast address]]. The multicast network must
# discover all multicast sources sending to that address, and
# route data from all sources to all interested receivers.
This behavior is particularly well suited to [[Groupware|groupware]] applications where 
# all participants in the group want to be aware of all other participants, and 
# the list of participants is not known in advance.
The source discovery burden on the network can become significant when the number of sources is large.

==Operation==
In the SSM service model, in addition to the receiver expressing interest in traffic ''to'' a multicast address, the receiver expresses interest in receiving traffic ''from'' only one specific source sending to that multicast address.  This relieves the network of discovering many multicast sources and reduces the amount of multicast routing information that the network must maintain.

SSM requires support in last-hop routers and in the receiver's [[Operating system|operating system]]. SSM support is not required in other network components, including routers and even the sending host.  Interest in multicast traffic from a specific source is conveyed from hosts to routers using IGMPv3 as specified in RFC 4607.

SSM destination addresses must be in the ranges <code>232.0.0.0/8</code> for [[IPv4]]. For [[IPv6]] current allowed SSM destination addresses are specified by <code>ff3x::/96</code>, where the hexadecimal digit <code>x</code> represents the scope.<ref>{{cite journal |author=S. Deering, B. Haberman, T. Jinmei, E. Nordmark, B. Zill |date=March 2005 |title=RFC 4007: IPv6 Scoped Address Architecture |doi=10.17487/RFC4007 |url=http://tools.ietf.org/html/rfc4007 |journal=IETF }}</ref> Note however that the allocation may be extended in the future so receivers and network equipment should treat any <code>ff3x::/32</code> address as SSM.<ref>{{cite journal |author=Hugh Holbrook, Brad Cain |date=August 2006 |title=RFC 4607: Source-Specific Multicast for IP |doi=10.17487/RFC4607 |url=http://tools.ietf.org/html/rfc4607 |journal=IETF |accessdate= May 25, 2013 |doi-access=free }}</ref>

== References ==
{{reflist}}

== Further reading ==
* {{cite IETF |author=S. Bhattacharyya |editor-first1=S. |editor-last1=Bhattacharyya |date=July 2003 |title=An Overview of Source-Specific Multicast (SSM) |rfc=3569}}

== External links ==
* [https://web.archive.org/web/20070618133657/https://jssm.dev.java.net/ JAVA Source-specific multicast support library]

[[Category:Internet broadcasting]]
[[Category:Internet Protocol]]
[[Category:Network protocols]]

{{Sourceattribution|Source-specific multicast}}