Internet Core Protocols: The Definitive Guide

Membership and Leave Reports

As was described in IGMP Message Type earlier in this chapter, the acts of joining and leaving multicast groups are pretty straightforward: hosts issue multiple join requests when they first start, and later issue a single leave request when they end. A good example of this process can be seen with Microsoft’s Windows Internet Name Service (WINS) server, shipped with Windows NT Server 4.0. WINS is a NetBIOS-to-TCP/IP resolution service, capable of automatically locating other WINS servers using IP multicasts. Once these other servers are located, they can replicate name resolution information with each other. Figure 4.16 shows a WINS server being started and then being shut down.

Figure 4.16. A Microsoft WINS server registering and then leaving a multicast group

Notice that the WINS server issues two Membership Reports back-to-back. This behavior is specified in RFC 2236, which states that hosts should issue at least two announcements whenever they are started, allowing for the possibility that the first announcement was lost or corrupted somehow. In the example shown in Figure 4.16, the Windows NT server sends the multicast Membership Report to a WINS-specific multicast address of 224.0.1.24, which is also the proper behavior according to RFC 2236.

After a while, the WINS server is terminated, so the Windows NT host issues an IGMPv2 Leave Report, stating ...