Lectures notes for 2010 - KTH

Core-Based Trees (CBT)
A fixed point in the network chosen to be the center of the multicast group, i.e., “core”. Nodes desiring to be
recipients send “join” commands toward this core. These commands will be processed by all intermediate
routers, which will mark the interface on which they received the command as belonging to the group’s tree.
The routers need to keep one piece of state information per group, listing all the interface that belong to the
tree. If the router that receives a join command is already a member of the tree, it will mark only one more
interface as belong to the group. If this is the first join command that the router receives, it will forward the
command one step further toward the core.
Advantages
• CBT limits the expansion of multicast transmissions to precisely the set of all recipients (so it is
demand-driven). This is in contrast with RPF where the first packet is sent to the whole network.
• The amount of state is less; it depends only on the number of the groups, not the number of pairs
of sources and groups ⇒ Group-shared multicast trees (*, G)
• Routing is based on a spanning tree, thus CBT does not depend on multicast or unicast routing
tables
Disadvantages
• The path between some sources and some receivers may be suboptimal.
• Senders sends multicast datagrams to the core router encapsulated in unicast datagrams
Maguire Core-Based Trees (CBT) 8: 44 of 78
maguire@kth.se 2010.03.21 Internetworking/Internetteknik