tailieunhanh - Internetworking with TCP/IP- P39

Internetworking with TCP/IP- P39: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | Sec. Reverse Path Multicasting 339 router does not know about distant group members it does know about local members . members on each of its directly-attached networks . As a consequence routers attached to leaf networks can decide whether to forward over the leaf network if a leaf network contains no members for a given group the router connecting that network to the rest of the internet does not forward on the network. In addition to taking local action the leaf router informs the next router along the path back to the source. Once it learns that no group members lie beyond a given network interface the next router stops forwarding datagrams for the group across the network. When a router finds that no group members lie beyond it the router informs the next router along the path to the root. Using graph-theoretic terminology we say that when a router learns that a group has no members along a path and stops forwarding it has pruned . removed the path from the forwarding tree. In fact RPM is called a broadcast and prune strategy because a router broadcasts using RPF until it receives information that allows it to prune a path. Researchers also use another term for the RPM algorithm they say that the system is data-driven because a router does not send group membership information to any other routers until datagrams arrive for that group. In the data-driven model a router must also handle the case where a host decides to join a particular group after the router has pruned the path for that group. RPM handles joins bottom-up when a host informs a local router that it has joined a group the router consults its record of the group and obtains the address of the router to which it had previously sent a prune request. The router sends a new message that undoes the effect of the previous prune and causes datagrams to flow again. Such messages are known as graft requests and the algorithm is said to graft the previously pruned branch back onto the tree. .