MSDP Packets
MSDP packets are encapsulated in TCP datagrams, in the format of Type Length Value (TLV), as shown in Figure 1.
- Type: indicates the packet type. Table 1 lists types of MSDP packet.
- Length: indicates the packet length.
- Value: indicates the packet content, which varies depending on the packet type.
Value |
MSDP Packet Type |
Function |
Major Information Contained |
|---|---|---|---|
1 |
Source-Active (SA) |
Carries multiple (S, G) entries and is transmitted among several RPs. |
|
Encapsulates PIM-SM multicast data. |
|
||
2 |
Source-Active Request (SA-Req) |
Requests (S, G) list of a specified group G to reduce the delay in joining the group. |
Address of the group G |
3 |
Source-Active Response (SA-Resp) |
Responds to SA request messages. |
|
4 |
Keepalive |
Maintains MSDP peer connections. Keepalive packets are sent only when no other protocol packets are exchanged between MSDP peers. |
- |
5 |
Reserved |
This packet type is reserved and used as Notification messages at present. |
- |
6 |
Traceroute in Progress |
Traces and detects the RPF path along which SA messages are transmitted. |
|
7 |
Traceroute Reply |
As described in Table 1, SA messages carry (S, G) information and encapsulate multicast packets. MSDP peers share (S, G) information by exchanging SA messages. If an SA message contains only (S, G) information, remote users may not receive multicast data because the (S, G) entry has already timed out when reaching the remote domain. If multicast data packets are encapsulated in an SA message, remote users can still receive multicast data when the (S, G) entry times out.
When a user joins a group, the user must wait for the SA message sent from the MSDP peer in the next period because SA messages are sent periodically. To reduce the delay for the user to join the source SPT, MSDP supports SA request and response messages of Type 2 and Type 3, which enables active source information to be updated more quickly.