TCP/IP Tutorial and Technical Overview - IBM Redbooks

messages from each of the dependent downstream routers on an interface, the
upstream router can remove this interface from the multicast delivery tree.
If the upstream router is able to prune all of its interfaces from the tree, it sends a
prune message to its upstream router. This continues until all unnecessary
branches have been removed from the delivery tree.
Maintaining prune information
In order to remove outdated prune information, each prune message contains a
prune lifetime timer. This indicates the length of time that the prune will remain in
effect. If the interface is still pruned when the timer expires, the interface is
reconnected to the multicast delivery tree. If this causes unwanted multicast
datagrams to be delivered to a downstream device, the prune mechanism is
reinitiated.
Grafting pruned networks
Because IP multicast supports dynamic group membership, hosts can join a
multicast group at any time. When this occurs, DVMRP routers use graft
messages to reattach the network to the multicast delivery tree. A graft message
is sent as a result of receiving a IGMP membership report for a group that has
previously been pruned. Separate graft messages are sent to the appropriate
upstream neighbor for each source network that has been pruned.
Receipt of a graft message is acknowledged with a graft ACK message. This
enables the sender to differentiate between a lost graft packet and an inactive
device. If an acknowledgment is not received within the graft timeout period, the
request is retransmitted. The purpose of the graft ACK message is to
acknowledge the receipt of a graft message. It does not imply any action has
been taken as a result of the request. Therefore, all graft request messages are
acknowledged even if they do not cause any action to be taken by the receiving
router.
Chapter 6. IP multicast 257