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Unformatted text preview: ltiple networks are involved and the multicast datagrams must pass through
In this chapter we'll look at the Internet Group Management Protocol (IGMP), which is used by hosts and
routers that support multicasting. It lets all the systems on a physical network know which hosts currently
belong to which multicast groups. This information is required by the multicast routers, so they know which
multicast datagrams to forward onto which interfaces. IGMP is defined in RFC 1112 [Deering 1989].
Like ICMP, IGMP is considered part of the IP layer. Also like ICMP, IGMP messages are transmitted in IP
datagrams. Unlike other protocols that we've seen, IGMP has a fixed-size message, with no optional data.
Figure 13.1 shows the encapsulation of an IGMP message within an IP datagram. Figure 13.1 Encapsulation of an IGMP message within an IP datagram.
IGMP messages are specified in the IP datagram with a protocol value of 2. 13.2 IGMP Message
Figure 13.2 shows the format of the 8-byte IGMP message. Figure 13.2 Format of fields in IGMP message.
The IGMP version is 1. An IGMP type of 1 is a query sent by a multicast router, and 2 is a response sent by a
host. The checksum is calculated in the same manner as the ICMP checksum.
The group address is a class D IP address. In a query the group address is set to 0, and in a report it contains
the group address being reported. We'll say more about it in the next section when we see how IGMP
operates. 13.3 IGMP Protocol
file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/igmp_int.htm (1 of 7) [12/09/2001 14.47.02] Chapter 13. IGMP: Internet Group Management Protocol Joining a Multicast Group
Fundamental to multicasting is the concept of a process joining a multicast group on a given interface on a
host. (We use the term process to mean a program being executed by the operating system.) Membership in a
multicast group on a given interface is dynamic-it changes over time as processes join and leave the group.
We imply her...
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- Spring '12