Discussion

Multicast senders and receivers are generally PCs or hosts connected to a multicastenabled
router, which is the DR for the sender or receiver. Receivers use a group
membership protocol to tell their DRs which multicast streams they want to receive
and to dynamically join and leave multicast groups. Routers run a multicast routing
protocol to direct the streams from the sources to the correct receiving networks.
Using this protocol, the routers build a delivery tree, called a distribution tree,
between the sender (or senders) and receivers of a multicast group. Multicast data
follows the path of the distribution tree. Data flows downstream on an outgoing
interface toward the receiver and upstream on an incoming interface toward the
source.
Hosts use the Internet Group Membership Protocol (IGMP) to inform routers about
which multicast groups they want to join, and routers use IGMP to verify that a host
is still interested in listening to a group. There are three versions of IGMP, all supported
by JUNOS software. Version 1 (RFC 1112) runs on Windows 95 computers,
and Version 2 (RFC 2236) runs on most Unix hosts, including Mac OS X, and on
Windows 98, Windows 2000, and Windows NT systems. IGMPv2 adds explicit
leave functionality so hosts can report to the router when they are no longer interested
in a group. (In IGMPv1, the host simply stops sending report messages, and
after some time, the router assumes the host is no longer interested in the group and
stops forwarding traffic for that group.) IGMPv3 (RFC 3376), supported by Windows
XP systems, adds source filtering so the host can include and exclude specific
sources when requesting multicast packets. Source filtering is required for SSM.
Perhaps the biggest difference between unicast and multicast is that unicast routing
is concerned about where a packet is going and multicast routing is concerned about
where a packet comes from. Unicast routing looks up a packet’s destination address
in the routing table to determine which interface leads toward the destination. The
result is that unicast routing forwards packets from their source to (or toward) a destination.
Multicast routing uses RPF to set up forwarding state from the receiver to
the source (or root) of the distribution tree. RPF checks the routing table to determine
the interface that is closest to the root of the tree, and this RPF interface
becomes the incoming interface for the multicast group.
Multicast uses two methods to build distribution trees. With shortest path tree (SPT),
the root of the tree is the multicast source. When a router learns that it has a directly
connected listener for a group, it tries to join the tree for that group, building an SPT
for that group. The router sends a Join message (specifically, an (S,G) Join message)
out the upstream router for that group to let the upstream router know it wants to
receive packets for the group. The upstream routers repeat this process until the Join
message either reaches the DR for the multicast source or reaches a router that
already has multicast forwarding state for the (S,G) pair. This process creates a
branch from the receiver to the source.
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