The second approach to multicast routing is based
on the assumption that the multicast group members are sparsely
distributed throughout the network and bandwidth is not necessarily
widely available.
It is important to note that sparse mode does
not imply that the group has few members, just that they are
widely dispersed. In this case, flooding would unnecessarily waste
network bandwidth and could cause serious performance problems.
Therefore, sparse-mode multicast routing protocols must rely on more
selective techniques to set up and maintain multicast trees.
Sparse-Mode protocols begin with an empty distribution tree and add
branches only as the result of explicit requests to join the
distribution.
Sparse-mode routing protocols include the
following:
- Core-Based Trees (CBT)
- Protocol Independent Multicast Sparse Mode
(PIM SM)
Because SM protocols assume that relatively
few routers in each individual network will be involved in each multicast, these
protocols are more appropriate in WAN environments.
Sparse-mode PIM is optimized for environments in which many
multipoint data streams exist. Each data stream goes to a
relatively small number of users. For these types of groups,
reverse path forwarding techniques waste bandwidth.
Core-Based Trees
Core-based trees (CBT) are described in RFC
2201. The CBT protocol constructs a single tree that is shared by
all members of the group. Multicast traffic for the entire group is
sent and received over the same tree, regardless of the source. This
use of a shared tree can provide significant savings in the
amount of multicast state information that is stored in individual
routers.
A CBT shared tree has a core router that is
used to construct the tree. (see the Figure) Routers join the tree
by sending a join message to the core. When the core receives a join
request, it returns an acknowledgment over the reverse path, thus
forming a branch of the tree. Join messages need not travel all the
way to the core before being acknowledged. If a join message
encounters a router on the tree before the message reaches the core,
that router terminates the join message and acknowledges it. The
router that sent the join is then connected to the shared tree.
The Core-Based Tree (CBT) multicast routing
protocol is truly a work-in-progress and has been for several
years. CBTv2 (defined
in RFC 2189) which is not backward compatible with CBTv1
superseded the original version, CBTv1. However, because CBTv1 was implemented in so few production networks, backward compatibility is not an
issue. CBTv2 also has not seen significant deployment to date, which
is probably good, because a new draft specification for CBTv3 is
already out that supersedes and is not backward compatible with
CBTv2!
Protocol Independent Multicast Sparse Mode
Protocol Independent Multicast Sparse Mode is
optimized for environments where there are many multipoint data
streams. Sparse multicast is most useful when:
- There are few receivers in a group.
- The type of traffic is intermittent.
In sparse mode, each data stream goes to a
relatively small number of segments in the campus network. Instead
of flooding the network to determine the status of multicast
members, sparse-mode PIM defines a rendezvous point. When a sender
wants to send data, the sender first sends to the rendezvous point.
When a receiver wants to receive data, the receiver registers with
the rendezvous point. Once the data stream begins to flow from
sender to rendezvous point to receiver, the routers in the path will
optimize the path automatically to remove any unnecessary hops.
Sparse-mode PIM assumes that no hosts want the multicast traffic
unless they specifically ask for it. PIM is able to simultaneously
support dense mode for some multicast groups and sparse mode for
others.