Protocol Independent Multicast
Protocol Independent Multicast (PIM) is a method in network technology that enables dynamic routing of multicast packets on the Internet. In contrast to traditional methods such as DVMRP or MOSPF , PIM uses the routing information of the unicast routing protocol active in the router and works regardless of its specific form (e.g. OSPF , IS-IS , RIP ). It can therefore also be used with a heterogeneous network infrastructure and supports both scenarios with a low subscriber density or widely dispersed subscribers (i.e. members of a multicast group) and scenarios with a high (i.e. highly concentrated) subscriber density. PIM therefore provides the two functional modes sparse mode (low subscriber density, scattered subscribers) and dense mode (high subscriber density) as well as a mode (BIDIR-PIM) that is optimized for bidirectional data flow.
Dense mode
In the so-called dense mode , the forwarding of a multicast packet generates a broadcast to all neighboring PIM routers. In order to avoid circulating packets, reverse path forwarding is used so that a multicast packet is only flooded if its input network interface is on the shortest path - from the point of view of the unicast routing protocol - towards the source (IP sender address). The routers then log off from the multicast distribution tree, in whose subnetworks there are no participants for this multicast group (so-called pruning , i.e. cutting back the multicast distribution tree). If group members become active, routers can also actively reintegrate into the distribution tree (so-called grafting), otherwise the flooding is repeated periodically, i.e. H. as soon as the pruning status of the previous router has expired. The dense mode is therefore only suitable for networks with a high subscriber density, since in this case the packets are replicated to every outgoing network interface anyway due to the dense group structure , i.e. H. must be flooded. The periodic flooding of PIM-DM therefore does not result in any significant additional effort.
Sparse mode
The sparse mode is used for networks with a low subscriber density or a wide range of subscribers across different subnetworks . This includes the agreement of a rendezvous point router that accepts multicast publications. At this rendezvous point, other routers can inquire whether multicasts have been received for relevant interest groups. If this is the case, the rendezvous point mediates a connection between the sender of the multicast and the router in whose subnet an interested participant is located. This mode creates an enormous load on the network links downstream of the rendezvous point. By using Switchover-SPT ( Shortest Path Tree or Minimum Spanning Tree ), a last-hop router can establish a direct connection to the multicast sender after the first packet received and thus relieve the network, since the network section now has the lowest costs is used.