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The packet format of IGMP and PIM is useful in understanding the operation of PIM. Understanding the packet format also helps you in troubleshooting PIM problems, in case sniffer traces need to be looked at. This section covers the important packet format of IMGP and PIM. IGMP Packet FormatIGMP messages are always sent with a TTL of 1 and are IP-encapsulated with a protocol number of 2. Figure 12-11 shows the IGMP version 2 packet format. The IGMP version 1 packet format is a little different than the format of version 2. The IGMP version 1 packet splits the Type field in version 2 into two parts , to include both the version number and the type. Figure 12-11. IGMP Packet Format
The Type field indicates different types of IGMP packets:
The types listed are the most important ones. You can find other Type field information in RFC 2236. The Maximum Response Time field is used only in membership query messages. It spec-ifies the maximum time in units of 1/10 of a second that a host might wait to respond to a query message. The Checksum field is the checksum of the IGMP message to verify packet integrity. The Group Address field contains the multicast group that the receiver is interested in receiving. When the general IGMP query is sent, this group address field is set to 0. PIM Packet/Message FormatsPIM version 1 packets are encapsulated in IGMP Type 14 packets. PIM version 2 uses its own protocol number of 103 and is not encapsulated into IGMP. PIM version 2 packets are sent to the multicast address 224.0.0.13. Figure 12-12 shows the format for PIM hello messages. Figure 12-12. PIM Hello Packet Format
The PIM hello message belongs to the PIM Type 0 packet. Option Type is always set to 1. The PIM hello packets are used to establish the PIM neighbor relationship. Figure 12-13 shows the PIM register message for PIM sparse mode. Figure 12-13. PIM Register Packet
The PIM register message belongs to PIM Type 1 packets. The DR encapsulates the multi-cast packet into the register packet and sends it to the RP. From the packet format, the B bit is set to 1 if the router is a PIM multicast border router for the source. The B bit is set to 0 if the sending router is a DR for a directly connected source. The N bit is the null register, and it is used to prevent unnecessary bursts of traffic being sent to the RP between receiving REGISTER STOP messages. Figure 12-14 shows the REGISTER STOP message format. Figure 12-14. PIM REGISTER STOP Packet Format
The encoded group address contains the groups of multicast addresses being encapsulated into the REGISTER message. The Encoded unicast-source address contains the unicast address of the source of the multicast stream. From the previous discussion, the REGISTER STOP message is sent to the router that sends the REGISTER message, to indicate that a native multicast stream has been received and that the DR can stop sending the REGISTER packets. Figure 12-15 illustrates the PIM JOIN/PRUNE message that is used in both dense and sparse modes. Figure 12-15. PIM JOIN/PRUNE Packet Format
In the PIM JOIN/PRUNE packet, the encoded unicast upstream neighbor address is the IP address of the RPF neighbor that performs the join or prune. The number of groups contains the number of multicast group sets in the message. Each set consists of an encoded multi-cast group address, followed by a list of encoded source addresses to join or prune. Figure 12-16 shows the format for the PIM assert message. The assert mechanism allows the router to select an active router to forward the multicast packets to avoid packet duplication on the LAN. Figure 12-16. PIM Assert Packet Format
The encoded group address is the multicast group address. The encoded unicast source address is the IP address of the source of the multicast stream. The metric is the routing metric to the source of the multicast stream associated with the routing protocol used. The metric could be hop count if the routing protocol used is RIP. The R bit is the RPT bit and is set to 1 if the multicast packet that triggered the assert packet is routed down the shared tree. The R bit is set to 0 if the multicast packet is routed down the shortest- path tree. |
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