A Tutorial on IP Multicast

Introduction

This tutorial assumes basic familiarity with the socket
programming abstraction found in many variants of the UNIX

Operating System. This tutorial will illustrate how to use sockets to
join an IP multicast group and send and receive data from multicast
groups.

Multicast Groups and Addresses

Every IP multicast group has a group address. IP multicast
provides only open groups: That is, it is not necessary to be
a member of a group in order to send datagrams to the group.

Multicast address are like IP addresses used for single hosts, and is
written in the same way: A.B.C.D. Multicast addresses will never
clash with host addresses because a portion of the IP address space is
specifically reserved for multicast. This reserved range consists of
addresses from 224.0.0.0 to 239.255.255.255. However, the multicast
addresses from 224.0.0.0 to 224.0.0.255 are reserved for multicast
routing information; Application programs should use multicast
addresses outside this range.

Time-To-Live (TTL) for Multicast Packets

The IP multicast routing protocol uses the Time To Live (TTL)
field of IP datagrams to decide how “far” from a sending host a given
multicast packet should be forwarded. The default TTL for multicast
datagrams is 1, which will result in multicast packets going only to
other hosts on the local network. A setsockopt(2) call may be
used to change the TTL:

	unsigned char ttl;

	setsockopt(sock,IPPROTO_IP,IP_MULTICAST_TTL,&ttl,sizeof(ttl));

As the values of the TTL field increase, routers will expand the
number of hops they will forward a multicast packet. To provide
meaningful scope control, multicast routers enforce the following
“thresholds” on forwarding based on the TTL field:

0

restricted to the same host

1

restricted to the same subnet

32

restricted to the same site

64

restricted to the same region

128

restricted to the same continent

255

unrestricted

Sending Multicast Datagrams

Sending a multicast datagram is easy: The sender simply specifies a
multicast address as the destination of an ordinary sendto(2)
system call.

Receiving Multicast Datagrams

To receive multicast packets, an application must first request that
the host join a particular multicast group. This is done using
another call to setsockopt(2):

	struct ip_mreq mreq;

	setsockopt(sock,IPPROTO_IP,IP_ADD_MEMBERSHIP,&mreq,sizeof(mreq));

The definition of struct ip_mreq is as follows:

	struct ip_mreq {
	    struct in_addr imr_multiaddr; /* multicast group to join */
	    struct in_addr imr_interface; /* interface to join on */
	}

For now, imr_interface can be safely set to INADDR_ANY
to join on the default multicast network interface.

In addition to the host part of an IP multicast address, there is also
a port number, as in TCP or UDP sockets. This port number
information is used by the kernel to decide which port on the local
machine to route packets to. However, unlike in TCP or UDP, there can
be many sockets which receive IP multicast packets off a single local
port. Binding the socket to a local port is done with bind(2),
and is done in the same manner as binding to a UDP address.

After the bind(2) has been performed and we have used

setsockopt(2) to join a multicast group, we are ready to
receive. An ordinary recvfrom(2) call may be used to read
datagrams off of the receive socket.

An Example of Multicast Programming: “Hello, World!”

Here is an example which illustrates all
the facilities described here for using multicast. It consists of two
programs: sender and
listener. The listener program
simply echoes everything it receives to its standard out, and the
sender multicasts “hello, world!” to everyone else in the multicast group.