Protocol-independent multicast (PIM) is a set of four specifications that define modes of Internet multicasting to allow one-to-many and many-to-many transmission of information.
The four modes are:
- sparse mode (SM)
- dense mode (DM)
- source-specific multicast (SSM)
The most common mode in PIM is the sparse mode. It is used for transmission of data to nodes in multiple Internet domains, where it is expected that only a small proportion of the potential nodes will actually subscribe. Dense mode, in contrast to sparse mode, is used when it is expected that a large proportion of the potential nodes will subscribe to the multicast. In source-specific multicast, paths (also called trees) originate (or are rooted) at a single, defined source, whereas bidirectional PIM is not source-specific.
The term "protocol independent" means that PIM can function by making use of routing information supplied by a variety of communications protocols. In information technology, a protocol is a defined set of rules that end points in a circuit or network employ to facilitate communication.