3com 8807 User Guide
414
C
HAPTER
37: IP M
ULTICAST
O
VERVIEW
Figure 100 Application positions of multicast-related protocols
Multicast group management protocol
Multicast groups use Internet group management protocol (IGMP) as the
management protocols. IGMP runs between the host and multicast router and
defines the membership establishment and maintenance mechanism between
them.
management protocols. IGMP runs between the host and multicast router and
defines the membership establishment and maintenance mechanism between
them.
Multicast routing protocols
A multicast routing protocol runs between multicast routers to create and
maintain multicast routes for correct and efficient forwarding of multicast packet.
The multicast routing creates a loop-free data transmission path from one source
to multiple receivers. The task of multicast routing protocols is to build up the
distribution tree architecture. A multicast router can use multiple methods to build
up a path for data transmission, that is, a distribution tree.
maintain multicast routes for correct and efficient forwarding of multicast packet.
The multicast routing creates a loop-free data transmission path from one source
to multiple receivers. The task of multicast routing protocols is to build up the
distribution tree architecture. A multicast router can use multiple methods to build
up a path for data transmission, that is, a distribution tree.
As in unicast routing, the multicast routing can also be intra-domain or
inter-domain. Intra-domain multicast routing is rather mature and protocol
independent multicast (PIM) is the most wildly used intra-domain protocol, which
can work in collaboration with unicast routing protocols. The inter-domain routing
first needs to solve how to transfer routing information between ASs. Since the
ASs may belong to different telecom carriers, the inter-domain routing information
must contain carriers’ policies, in addition to distance information. Currently,
inter-domain routing protocols include multicast source discovery protocol (MSDP)
and MBGP multicast extension.
inter-domain. Intra-domain multicast routing is rather mature and protocol
independent multicast (PIM) is the most wildly used intra-domain protocol, which
can work in collaboration with unicast routing protocols. The inter-domain routing
first needs to solve how to transfer routing information between ASs. Since the
ASs may belong to different telecom carriers, the inter-domain routing information
must contain carriers’ policies, in addition to distance information. Currently,
inter-domain routing protocols include multicast source discovery protocol (MSDP)
and MBGP multicast extension.
RPF Mechanism for IP
Multicast Packets
Multicast Packets
To ensure that multicast packets reach a router along the shortest path, the
multicast router must check the receiving interface of multicast packets depending
on the unicast routing table or a unicast routing table independently provided for
multicast. This check mechanism is the basis for most multicast routing protocols
to perform multicast forwarding, and is known as Reverse Path Forwarding (RPF)
check. A multicast router uses the source address of a received multicast packet to
query the unicast routing table or the independent multicast routing table to
determine that the receiving interface is on the shortest path from the receiving
station to the source. If a source tree is used, the source address is the address of
the source host sending the multicast packet. If a shared tree is used, the source
multicast router must check the receiving interface of multicast packets depending
on the unicast routing table or a unicast routing table independently provided for
multicast. This check mechanism is the basis for most multicast routing protocols
to perform multicast forwarding, and is known as Reverse Path Forwarding (RPF)
check. A multicast router uses the source address of a received multicast packet to
query the unicast routing table or the independent multicast routing table to
determine that the receiving interface is on the shortest path from the receiving
station to the source. If a source tree is used, the source address is the address of
the source host sending the multicast packet. If a shared tree is used, the source
Server
Multicast
User D
User E
User C
IGMP
IGMP
User A
User B
IGMP
MBGP/MSDP
PIM
PIM
AS1
AS2