Internet Core Protocols: The Definitive Guide

Troubleshooting Multicasts and IGMP

Since IGMP is a very small and lightweight protocol, there aren’t many things that can go wrong with it. The only real errors that are likely to occur are problems with programs that implement the specification poorly. For example, the party.exe application that is provided by Microsoft does not use the appropriate system calls for generating multicast traffic. As such, it does not cause a Membership Report to be sent when the program is started, and does not cause a Leave Report to be issued when it is stopped.

Another implementation problem can be seen with the SNTP client provided with RedHat Linux 5.2. Although this same client code executes properly on Solaris and Digital Unix systems, RedHat Linux 5.2 had some bugs in the multicasting code that caused some malformed IGMP messages to be sent. Figure 4.18 shows this in detail.

Figure 4.18. A malformed IGMP Leave Report

The first mistake that is made is that the SNTP client does not issue multiple Membership Reports when it is started. Instead, only one Membership Report is issued, although RFC 2236 specifically states that applications should issue at least two separate Membership Reports whenever they join a multicast group.

The second mistake—and one that is much more problematic—is that the Leave Report issued by this implementation is completely malformed. Although the client does send the ...