Pragmatic General Multicast Overview

Multicast applications often require real-time operation. These applications cannot take advantage of Transmission Control Protocol (TCP) reliability features such as sequencing, retransmission, and flow control through windowing between sender and receiver. The User Datagram Protocol (UDP), the major transport layer alternative to TCP, leaves much to be desired in its reliability for multicast traffic. Pragmatic General Multicast (PGM) is a special protocol layer for multicast traffic that can be used between the IP layer and the multicast application to add reliability to multicast traffic. PGM allows a receiver to detect missing information in all cases and to request replacement information if the receiver application requires it. PGM is IP protocol number 113.

Although PGM is mainly concerned with the operation of multicast source and receiver, PGM-enabled routers (called PGM network elements) play a router assistance role in the initial delivery and potential replacement of multicast traffic. PGM routers are not mandatory in PGM, but they can provide the following benefits when placed anywhere between the source and receivers:

• Reduce the load on the multicast source by aggregating duplicate messages to the source. PGM routers are required to perform this function.
• Limit the flooding of repair data (replacement information) to only those downstream receivers that requested the repair data. PGM routers are required to perform this function.
• Act as designated local repairers (DLRs) by caching the repair data and resending it to receivers that request it later. DLR functions are a PGM option, and PGM routers are not required to perform this role.

PGM adds reliability to multicast traffic streams. It is not a complete multicast protocol like the Distance Vector Routing Multicast Protocol (DVMRP) or Protocol Independent Multicast (PIM). Adding PGM to a router does not enable the router to perform multicast functions. Instead, a PGM router with multicast capabilities and a preconfigured multicast protocol such as PIM can offer more reliable multicast services to PGM sources and receivers. PGM is not an alternative to multicast routing protocols, but an enhancement of the multicast capabilities already present and configured on the router.

For information about supported standards for PGM, see Junos OS Multicast Standards.