The bursty sources can be characterized as sending short packet bursts separated by silent periods on the order of several minutes. One example is when a tool like sdr is used to periodically advertise a session. A problem occurs when trying to establish a multicast tree for this kind of source. The problem begins when one or a few packets are sent to the RP. The RP will hear the packet and flood an SA message, and RPs in other domains will send join messages back to the source. However, because no multicast forwarding state existed when the packet was originally sent, and because it takes time to forward SA messages and have other RPs establish forwarding state, the original burst will not reach new receivers. Once state is established, all subsequent packets should reach these receivers. The problem occurs when the period of silence between packet bursts exceeds the forwarding state timeout value (typically 3 min). Because no packets are sent, the forwarding state is discarded. When another session announcement is sent, the same process of establishing state but losing the initial burst is repeated. In this way, no packets from bursty sources ever reach group members. The solution, specified in the MSDP protocol, is to have SA messages carry the first n data packets. This is not a particularly elegant solution, but it does solve the problem. The lack of elegance is making the protocol harder to standardize. Because data packets are delivered via SA messages, which are delivered over TCP connections, some in the multicast community wonder if this will have undesirable side effects or break assumptions of higher-layer protocols. As a result, recent discussions in the MSDP working group have generated proposals which allow data to be carried in either GRE or UDP packets.
