IGMP/IP multicasting

IP multicast is documented in RFC 1112, which is included in STD 5 (the IP
standard). As such, IP multicast is a part of STD 5, and is therefore considered
to be an Internet Standard protocol. All hosts are required to implement multicast
into their IP stacks if they are to be compliant with Internet standards.
In addition to the IP multicast services, RFC 1112 also defined IGMPv1, a follow-
up to IGMPv0 (originally published in RFC 998). IGMPv2 was introduced in
RFC 2236 as an update to IGMPv1, and has gained a substantial number of implementations.
Some IGMP implementations only support IGMPv1 while others support
IGMPv2 and v1.*
Of the differences between IGMPv1 and v2, the most notable addition in IGMPv2
was the addition of the “Leave Report” message, which allows a host to inform the
network that it is no longer interested in receiving IP data grams for a specific multicast
group. This feature is required for networks that use data-link services that do
not support multicast directly (such as PPP). This chapter focuses on IGMPv2
primarily, and only mentions IGMPv1 when there is a significant difference.
RFC 2236 states that the Protocol ID for IGMP is 2. When a system receives an IP
data gram that is marked as containing Protocol 2, it should pass the contents of
the datagram to IGMP for further processing. However, note that IGMP is not a
transport protocol and is not used for the delivery of multicast data. Rather, IGMP
is a control protocol like ICMP, useful for informing devices of network events and
changes.

It is also important to note that although all hosts are required to implement IP
multicast, they are not required to implement IGMP. If they do not implement
IGMP, however, then they cannot tell local multicast routers that they wish to
receive multicast packets. Although a host can send any data it wants to a multicast
group IP address, hosts can only receive multicast data reliably if they implement
IGMP. Most of the multicast-aware systems in use today implement IGMP, as
well as the basic support for multicast services.
The default Time-to-Live value for IP data grams sent to a multicast address is “1
hop”, unless the sending application explicitly specifies a larger value. Also, it
should be noted that IGMP messages are only meant to be received and processed
by devices on the local network, and as such the Time-to-Live value for IP data grams
that contain IGMP messages should always be “1 hop”. Although multicast
data can be sent across router lines, IGMP messages should never be forwarded.