Multicasting Overview

Receiving multicast message

For a process to recieve multicast messages it needs to perform the following steps:

Ø Craeate a UDP socket sd

sd = socket(PF_INET,SOCK_DGRAM, 0);

Ø Bind it to a UDPport, e.g., 1234.

struct sockaddr_in groupHost;

groupHost.sin_family = AF_INET;
groupHost.sin_port = htons(UDPport);
groupHost.sin_addr.s_addr  =  htonl (INADDR_ANY);

bind(sd,  (struct sockaddr *) &groupHost, sizeof(groupHost));

All processes must bind to the same port in order to receive the multicast messages.

Ø Join a multicast group address GroupIPaddress ,  e.g., 224.111.112.113

joinGroup (sd, GroupIPaddress);

Ø Use recv or recvfrom to read the messages, e.g.,

nbytes = recv (sd, recvBuf, BufLen,0);

Sendinging multicast message

For a process to send multicast messages it needs to perform the following:

You may use the UDPsocket sd for sending multicast messages or

You can use  any other udp socket (it does not have to join any multicast group):

struct sockaddr_in dest;

dest.sin_family = AF_INET;
dest.sin_port = UDPport;
dest.sin_addr.s_addr = inet_addr(GroupIPaddress);

sendto (sd, sendBuf, BufLen,0,  (struct sockaddr *) &dest, sizeof(dest)) ;

Other issues:

Time-to-live:

To control how far the messages can go,
e.g., 2 mean at most 2 routers away.

u_char TimeToLive;
TimeToLive = 2;

setTTLvalue (sd, &TimeToLive);

Loop-back:

To allow the process to get a copy of its own transmission we use:

u_char loop;
loop = 1;

setLoopback (sd,  &loop);

Reuse-port:

To allow multiple multicast processes to run on the same host:

reusePort (sd);

Multicast Examples in C

· Textbook Example: (chapter 21):

main.c & send.c &  recv.c

Usage: sendrecv <maddr> <port>

E.g: % sendrecv 224.2.2.2 1234

· Additional  Example:

· group chat

Usage: mchat <group>

E.g: % mchat 22

· IPV6 version of group chat

Usage: mchat <group>

E.g: % mchat 22

Multicasting Examples in Java