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PGM-Enabled Receivers

The PGM architecture requires one or more PGM-enabled receivers of the multicast content generated by a PGM source. PGM receivers accept all types of downstream PGM messages: ODATA, SPMs, NCFs, and RDATA.

Receivers process the ODATA packets as they arrive from the source, constantly checking the 32-bit sequence number in the ODATA PGM header for gaps in the sequence. If the receiver detects missing information, it generates a NAK for that sequence number. The NAK is unicast upstream to the PGM next hop, which is a router or the source, as determined by the last address in the received SPM.

A receiver knows that its NAK was received by the PGM next hop when it gets an NCF in response to its NAK. If several receivers on a subnet are missing the same ODATA packet, receivers getting an NCF for the packet before sending a NAK suppress the NAK. If a receiver does not get an NCF in response to a NAK, the receiving application can send a NAK again or continue, with the certainty that information is missing.

After the NCF, PGM receivers get an RDATA packet with the same sequence number indicated in the NAK and a copy of the missing ODATA. NCFs and RDATA can originate from the source or a router acting as a DLR for a subnet. The receiver now has complete information or knows for certain what is missing.

PGM receivers can request almost anything from the PGM source. However, because the source determines the window size, there is no guarantee that older information is available.

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