21.5 Source-Specific Multicast
Multicasting on a WAN has been difficult to
deploy for several reasons. The biggest problem is that the
MRP, described in Section
21.4, needs to get the data from all the senders, which may be
located anywhere in the network, to all the receivers, which may
similarly be located anywhere. Another large problem is multicast
address allocation: There are not enough IPv4 multicast addresses
to statically assign them to everyone who wants one, as is done
with unicast addresses. To send wide-area multicast and not
conflict with other multicast senders, you need a unique address,
but there is not yet a global multicast address allocation
mechanism.
Source-specific
multicast, or SSM [Holbrook and Cheriton 1999], provides a
pragmatic solution to these problems. It combines the group address
with a system's source address, which solves the problems as
follows:
-
The receivers supply the sender's source address
to the routers as part of joining the group. This removes the
rendezvous problem from the network, as the network now knows
exactly where the sender is. However, it retains the scaling
properties of not requiring the sender to know who all the
receivers are. This simplifies multicast routing protocols
immensely.
-
It redefines the identifier from simply being a
multicast group address to being a combination of a unicast source
and multicast destination (which SSM now calls a channel. This means that the source may pick
any multicast address since it becomes the (source, destination)
combination that must be unique, and the source already makes it
unique. An SSM session is the combination of source, destination,
and port.
SSM also provides a certain amount of
anti-spoofing, that is, it is harder for source 2 to transmit on
source 1's channel since source 1's channel includes source 1's
source address. Spoofing is still possible, of course, but is much
harder.
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