Multicast Architecture 549
This section provides only general descriptions about applications and functions of
the Layer 2 and Layer 3 multicast protocols in a network. For details of these
protocols, refer to the respective chapters.
Layer 3 multicast protocols
Layer 3 multicast protocols include multicast group management protocols and
multicast routing protocols. Figure 168 describes where these multicast protocols
are in a network.
Figure 168 Positions of Layer 3 multicast protocols
1 Multicast management protocols
Typically, the internet group management protocol (IGMP) or multicast listener
discovery protocol (MLD) is used between hosts and Layer 3 multicast devices
directly connected with the hosts. These protocols define the mechanism of
establishing and maintaining group memberships between hosts and Layer 3
multicast devices.
2 Multicast routing protocols
A multicast routing protocol runs on Layer 3 multicast devices to establish and
maintain multicast routes and forward multicast packets correctly and efficiently.
Multicast routes constitute a loop-free data transmission path from a data source
to multiple receivers, namely, a multicast distribution tree.
In the ASM model, multicast routes come in intra-domain routes and inter-domain
routes.
■ An intra-domain multicast routing protocol is used to discover multicast
sources and build multicast distribution trees within an AS so as to deliver
multicast data to receivers. Among a variety of mature intra-domain multicast
routing protocols, protocol independent multicast (PIM) is a popular one. Based
on the forwarding mechanism, PIM comes in two modes - dense mode (often
referred to as PIM-DM) and sparse mode (often referred to as PIM-SM).
■ An inter-domain multicast routing protocol is used for delivery of multicast
information between two ASs. So far, mature solutions include multicast
source discovery protocol (MSDP).
AS 1 AS 2
Source
Receiver
Receiver
Receiver
PIM/IPv6 PIM
PIM/IPv6 PIM
MSDP
IGMP/MLD
IGMP/MLD
IGMP/MLD
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