Internet
Protocol (IP) multicast is a
routing technique that allows IP traffic to be sent from one source or
multiple sources and delivered to multiple destinations. Instead of
sending individual packets to each destination, a single packet is sent
to a multicast group, which is identified by a single IP destination
group address. IP multicast routing arose because unicast and broadcast
techniques do not handle the requirements of new applications
efficiently. Multicast addressing, for example, supports the
transmission of a single IP datagram to multiple hosts. This chapter
focuses on the leading multicast routing options. Figure 39-1
illustrates the general nature of a multicast environment
Figure 39-1: IP
multicast provides a means to deliver high-bandwidth traffic
to multiple destinations.
A
principle component of
IP multicast is the Internet Group-Membership Protocol (IGMP). IGMP
relies on Class D IP addresses for the creation of multicast groups and
is defined in RFC 1112. IGMP is used to dynamically register
individual hosts in a multicast group with a Class D address. Hosts
identify group memberships by sending IGMP messages, and traffic is sent
to all members of that multicast group. Under IGMP, routers listen to
IGMP messages and periodically send out queries to discover which groups
are active or inactive on particular LANs. Routers communicate with each
other by using one or more protocols to build multicast routes for each
group.
Several routing protocols
are used to discover multicast groups and to build routes for each
group. These include Protocol-Independent Multicast
(PIM), Distance-Vector Multicast Routing Protocol (DVMRP),
and Multicast Open Shortest Path First (MOSPF). The following table
summarizes the unicast requirements needed and flooding algorithms used
for each protocol. Table 39-1 summarizes the multicast routing option.
Table 39-1: Summary
of Multicast Routing Options
| Protocol |
Unicast Protocol
Requirements |
Flooding Algorithm |
|
PIM-dense mode
|
Any
|
Reverse path flooding (RPF)
|
|
PIM-sparse mode
|
Any
|
RPF
|
|
DVMRP
|
Internal, RIP-like routing
protocol
|
RPF
|
|
MOSPF
|
Open Shortest Path First (OSPF)
|
Shortest-path first (SPF)
|
Protocol-Independent
Multicast (PIM) is addressed in an Internet draft RFC (under discussion
by the IETF Multicast Routing Working Group). It includes two different
modes of behavior for dense and sparse traffic environments: dense
mode and sparse mode.
The PIM dense mode uses a process of
reverse path flooding
that is similar to the DVMRP. Differences exist, however, between dense
mode PIM and DVMRP. PIM, for example, does not require a particular
unicast protocol to determine which interface leads back to the source
of a data stream. DVMRP employs its own unicast protocol, while PIM uses
whatever unicast protocol the internetwork is using.
The
PIM sparse mode is
optimized for internetworks with many data streams but relatively few
LANs. It defines a rendezvous point that is then used as a registration
point to facilitate the proper routing of packets.
When a sender wants to transmit data, the
first-hop router (with respect to the source) node sends data to the
rendezvous point. When a receiver wants to receive data, the last-hop
router (with respect to the receiver) registers with the rendezvous
point. A data stream then can flow from the sender to the rendezvous
point and to the receiver. Routers in the path optimize the path and
automatically remove any unnecessary hops,
even at the rendezvous point.
The Distance-Vector
Multicast Routing Protocol (DVMRP) uses a reverse path-flooding
technique and is used as the basis for the Internet's multicast backbone
(MBONE). DVMRP is defined in RFC 1075 and has certain some
shortcomings. In particular, DVMRP is notorious for poor network
scaling, resulting from reflooding, particularly with versions that do
not implement pruning. DVMRP's flat unicast routing mechanism also
affects its capability to scale.
The reverse path-flooding
operation involves a router sending a copy of a packet out to all paths
(except the path back to the origin) upon the packet's receipt. Routers
then send a prune message back to the source to stop a data stream if
the router is attached to a LAN that does not want to receive a
particular multicast group.
Reflooding and DVMRP unicast
are used in DVMRP path-flooding operations. In reflooding,
DVMRP routers periodically reflood an attached network to reach new
hosts. The flooding mechanism uses an algorithm that takes into account
the frequency of flooding and the time required for a new multicast
group member to receive the data stream. DVMRP unicast is used to
determine which interface leads back to the source of a data stream. It
is unique to DVMRP but is similar to RIP in that it is based on hop
count. The DVMRP unicast environment permits the use of a different path
than the path used for multicast traffic.
The Multicast
Open Shortest Path First (MOSPF) is an extension of OSPF. In general,
MOSPF employs a unicast routing protocol that requires each router in a
network to be aware of all available links.
An
MOSPF router calculates routes from the source to all possible group
members for a particular multicast group. MOSPF routers include
multicast information in OSPF link states. MOSPF calculates the routes
for each source/multicast group pair when the router receives traffic
for that pair, and routes are cached until a topology change occurs.
MOSPF then recalculates the topology.
Several MOSPF implementation issues have
been identified and require consideration. First, MOSPF works only in
internetworks that use OSPF. In addition, MOSPF is best suited for
environments with relatively few active source/group pairs. MOSPF can
take up significant router CPU bandwidth in environments that have many
active source/group pairs or that are unstable.
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