Протоколы маршрутизации многоадресной рассылки PIM

Multicast routing protocol

Protocol Independent Multicast (PIM) is a family of multicast routing protocols for Internet Protocol (IP) networks that provide one to many and many to many distribution of data over a LAN, WAN or the Internet. It is termed protocol independent because PIM does not include its own topology discovery mechanism, but instead uses routing information supplied by other routing protocols. PIM is not dependent on a specific unicast routing protocol; it can make use of any unicast routing protocol in use on the network. PIM does not build its own routing tables. PIM uses the unicast routing table for reverse path forwarding. There are four variants of PIM:
PIM Sparse Mode (PIM SM) explicitly builds unidirectional shared trees rooted at a rendezvous point (RP) per group, and optionally creates shortest path trees per source. PIM SM generally scales fairly well for wide area usage. PIM Dense Mode (PIM DM) uses dense multicast routing. It implicitly builds shortest path trees by flooding multicast traffic domain wide, and then pruning back branches of the tree where no receivers are present. PIM DM is straightforward to implement but generally has poor scaling properties. The first multicast routing protocol, DVMRP used dense mode multicast routing. See RFC 3973. Bidirectional PIM (Bidir PIM) explicitly builds shared bi directional trees. It never builds a shortest path tree, so may have longer end to end delays than PIM SM, but scales well because it needs no source specific state.

There are two PIM versions. The versions are not directly compatible though may coexist on the same network. Network equipment may implement both versions. PIMv2 has the following improvements over PIMv1: A single RP is used per group. RP discovery is accomplished by a Bootstrap Router (BSR). Groups are either sparse or dense mode; Interfaces can be either. General improvements to protocol flexibility and efficiency.

When a data source first sends to a group, its Designated Router (DR) unicasts Register messages to the Rendezvous Point (RP) with the source's data packets encapsulated within. If the data rate is high, the RP can send source specific Join/Prune messages back towards the source and the source's data packets will follow the resulting forwarding state and travel un encapsulated to the RP. Whether they arrive encapsulated or natively, the RP forwards the source's de capsulated data packets down the RP centered distribution tree toward group members. If the data rate warrants it, routers with local receivers can join a source specific, shortest path, distribution tree, and prune this source's packets off the shared RP centered tree. For low data rate sources, neither the RP, nor last hop routers need join a source specific shortest path tree and data packets can be delivered via the shared RP tree. Once the other routers which need to receive those group packets have subscribed, the RP will unsubscribe to that multicast group, unless it also needs to forward packets to another router or node. Additionally, the routers will use reverse path forwarding to ensure that there are no loops for packet forwarding among routers that wish to receive multicast packets.

Dense mode multicast is one mode that multicast can use to construct a tree for sending packets to the multicast subscribers. It is an alternative to sparse mode. The basic assumption behind dense mode is that the multicast packet stream has receivers at most locations. Sparse mode assumes relatively fewer receivers. Dense mode is ideal for groups where many of the nodes will subscribe to receive the multicast packets, so that most of the routers must receive and forward these packets (groups of a high density). This difference shows up in the initial behavior and mechanisms of the two protocols. Dense Mode uses a fairly simple approach to handle IP multicast routing. The source initially broadcasts to every router directly connected to it. These neighboring routers further forward the data to their neighbors. When a router does not wish to receive this group's data (if no other neighboring PIM routers are present and no host is interested in the group), it sends a Prune message to indicate its lack of interest. Upon receiving a Prune message, the router will modify its state so that it will not forward those packets out that interface. If every interface on a router is pruned, the router will also be pruned. In older Cisco IOS releases, PIM DM would re flood all the multicast traffic every 3 minutes. This is fine for low volume multicast, but not higher bandwidth multicast packet streams. More recent Cisco IOS versions support a new feature called PIM Dense Mode State Refresh, since 12.1(5)T. This feature uses a PIM state refresh messages to refresh the Prune state on outgoing interfaces. Another benefit is that topology changes are recognized more quickly. By default, the PIM state refresh messages are sent every 60 seconds. Additionally, the routers will use reverse path forwarding to ensure that there are no loops for packet forwarding among routers that wish to receive multicast packets. When a data packet is received on a non RPF interface, a mechanism is required to prevent loops. If the non RPF interface is a LAN, an Assert message is sent. Non Forwarder routers then send a Prune on their RPF interface if they don't need the multicast stream. Only one such Prune is sent, at the time of the transition to having no interfaces in the Outgoing Interface List (OILIST). The LAN Prune receiver delays acting on it for 3 seconds, so that if another LAN router still needs the multicast stream, it can send a PIM Join message to counteract (cancel) the Prune. ("That router doesn't need it, but I still do!") Suppose a router has Pruned, and some time later a receiver requests the multicast stream with an IGMP message. The router then sends a Graft message. In effect, "hey, I need that multicast stream over here now".