PGM-Enabled Source

A PGM-enabled source of multicast content generates sequenced packets of ODATA that are multicast to receivers. Interleaved with the content packets are source path messages (SPMs), which tell PGM routers and receivers about their upstream next-hop PGM device—either another PGM router or the PGM source.

ODATA packets and SPMs are multicast from the source. A PGM router always appends its own IP address to the SPM before it is multicast on the downstream interfaces. The SPMs are sent by the source and upstream PGM routers with the router alert option set in the IP headers so that PGM routers do not have to examine every packet in the session for SPM packets.

The PGM source acknowledges a received NAK by multicasting an NCF downstream to the next PGM device on the path to the receiver. NCFs make sure that PGM routers and receivers do not bombard sources with NAKs. Downstream PGM routers suppress all subsequent NAKs that indicate the same missing information once one NCF is received from the upstream device.

The PGM source also responds to NAKs by multicasting RDATA packets with the same sequence number as the one indicated by the NAK. RDATA packets have the router alert option set in the IP header so that PGM routers can distinguish them from ODATA packets.

PGM sources organize their packets in sessions. PGM sources are not required to retain copies of information older than the current session, although they might. Long sessions are not necessarily kept on the source in their entirety.

PGM sources identify themselves through a global source ID (GSID). This globally unique source identifier is formed from the low-order 48 bits of the Message Digest 5 (MD5) signature of the Domain Name System (DNS) name of the source.