Internet Core Protocols: The Definitive Guide

Time-to-Live considerations

Blindly forwarding multicast data around the Internet could cause significant utilization problems. For this reason, another important aspect of wide-area multicasting is the Time-to-Live value of the multicast IP datagram. As was discussed in Chapter 2, the IP datagram’s Time-to-Live header field dictates the maximum number of hops that a datagram can take before it must be discarded. Since all multicast datagrams are sent using IP, this field is also used with multicast datagrams, and is also used by multicast routers to keep traffic from propagating across too many multicast routers.

Multicast group addresses in the range of 224.0.0.0 through 224.0.0.255 are reserved for administrative use only, and as such are restricted to the local network. Any data sent to those addresses must have a Time-to-Live setting of 1 in order to keep those messages from being forwarded across multicast routers. RFC 2236 goes so far as to state that even if the Time-to-Live is not set to 1 on those datagrams, they still should not be forwarded.

For example, the all-hosts group address of 224.0.0.1 refers to all of the multicast-aware devices on the local network. Whenever a multicast router wants to locate the active multicast groups in use on the local network, it will send an IGMP query to the 224.0.0.1 group address. In addition, the multicast router must set the Time-to-Live value of that query to 1, preventing any other multicast routers on the local network from forwarding ...