Side_1_360

Two variations of distribution trees are source
trees and shared trees, see Figure 15.

A source tree means that each source has its tree.
When a number of sources share the distribution
tree, this is naturally referred to as a shared tree.
In the shared distribution tree, the traffic flows
from all sources must go to the shared tree root,
from which the information is distributed. Multi-
cast sessions are identified by a special multicast
address and all packets from the source to the
receivers carry this address.

In the following some multicast protocols are
described.

The Distance Vector Multicast Routing Protocol
(DVMRP) and Protocol-Independent Multicast

• Dense Mode (PIM-DM) use a source distribu-
  tion tree and basically work on the assumption
  that every sub-net in the network should receive
  the multicast traffic. Routers that do not have
  any receivers interested in a multicast session
  respond to a DVMRP or PIM-DM request by a
  prune message. This implies that these sub-nets
  are removed from the multicast distribution tree.
  Hosts that want to joint a multicast session or to
  leave it use Internet Group Management Proto-
  col (IGMP) messages.

The Protocol-Independent Multicast – Sparse
Mode (PIM-SM) uses a shared tree that assumes
that multicast traffic is not to be distributed
unless specifically requested by a node. A host
sends a join message when it wants to take part
in a multicast session.

Compared to ordinary multicast, when the num-
ber of receivers is fairly low, is may be more
efficient to explicitly list the receivers in the
packet. This is proposed for Small-Group Multi-
cast (SGM). Then every router that gets the
packet has to look at the header to decide

whether it should forward the packet on more

outgoing interfaces.

An overview of some multicast routing protocols

is given in Table 1 (from [ID_MPmc]).

Aggregationrefers to whether different destina-

tion addresses are aggregated into one entry in

the routing table. Flood and prunerefers to

cases when multicast protocols flood the net-

work with multicast data. Then, some branches

not used can be pruned if the nodes do not

longer want to receive data any more (pruning

starts from the point of the branch were no

receivers are active). This allows for dynamic

distribution trees. Tree types, being source or

Figure 15 Source tree and

shared tree

source A source B

receivers

i) Shared tree

source A source B

receivers

i) Source tree

DVMRP MOSPF CBT PIM-DM PIM-SM SSM SM

Aggregation no no no no no no no

Flood and yes no no yes no no option

prune

Tree type source source shared source both source shared

State no no no no yes no no

co-existence

Uni-/bi- NA NA bi NA uni uni bi

directional

Encapsulation no no yes no yes no yes

Loop free no no no no no no no

Table 1 Characteristics of

some routing protocols

(DVMRP = Distance Vector

Multicast Routing Protocol,

MOSPF = Multicast exten-

sions to OSPF, CBT = Core

Based Trees, PIM-DM = Pro-

tocol Independent Multicast

• Dense Mode, PIM-SM =
  Protocol Independent Multi-
  cast – Sparse Mode, SSM =
  Source Specific Multicast,
  SM = Simple Multicast)