PGM-Enabled Routers

Multicast-capable routers can implement the PGM router assistance functions, although not all multicast routers must be PGM-enabled routers. Mandatory PGM router assistance functions include aggregating duplicate NAKs sent to the source to reduce the load on the multicast source, generating NCFs in response to NAKs, and flooding RDATA packets to only those downstream receivers that requested it with a NAK. Optionally, a PGM router can be a DLR, caching PGM information and cutting down on network traffic by resending RDATA packets locally.

There can be zero or more PGM-enabled network elements (routers) between the source and receiver. If there are no PGM routers between the source and receiver, then all PGM messages flow directly between the source and receiver, and no router assistance functions are possible. Both PGM and non-PGM routers can be freely mixed on a network because PGM is a transport layer protocol and is not involved with router multicast functions.

PGM routers also receive SPMs from the source or an upstream PGM router and forward them downstream, inserting the router’s own downstream IP interface address into the SPM so that receivers always know their upstream PGM next hop.

When a PGM router receives unicast NAKs from a downstream PGM router or receiver, the router unicasts one NAK for each missing sequence number to the next-hop PGM device upstream toward the source. The address of the PGM next-hop device is determined by received SPMs.

The PGM router multicasts NCFs in response to received NAKs on the downstream interfaces that received the NAKs. NCFs are not multicast on interfaces that have not received NAKs.

PGM routers must multicast all ODATA and RDATA packets they receive from upstream PGM devices. Normal multicast protocols are used to determine downstream interfaces.

If the PGM router is a DLR, it responds to received NAKs with an NCF and with its own RDATA packet. NAKs are not forwarded upstream from a DLR.

Figure 33 shows the overall PGM architecture and the role of PGM-enabled routers.

The figure shows only NAKs, NCFs, and RDATA flows. RDATA can come from either the source (left) or a DLR router (right). In both cases, unicast NAKs from a receiver are forwarded upstream by the routers, and multicast NCFs are generated downstream. Subnet NAK suppression is shown, as well as RDATA from the source or DLR sent only to the portions of the network requesting it.