Multicasting

· Receiving Multicast Messages

· Sending Multicast Message

· Multicast Control

· Multicast Examples

· Displaying Destination Address

· Binding Options

· Receiving Multicast Message

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

ü Create a UDP socket sd

sd = socket (PF_INET,SOCK_DGRAM, 0);

ü Bind it to a UDPPort, e.g., 10123.

#define SA struct sockaddr;

struct sockaddr_in LocalAddress;

LocalAddress.sin_family = AF_INET;
LocalAddress.sin_port = htons( UDPPort);
LocalAddresst.sin_addr.s_addr  =  htonl (INADDR_ANY);

bind (sd,  (SA *) &LocalAddress, sizeof(LocalAddress));

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

Thus all processes on the same host must execute the following statement before bind.

reusePort (sd);

ü Join a multicast IP address MulticatIPAddress ,  e.g., 224.0.0.1

joinGroup (sd, MulticastIPAddress);

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

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

Sendinging Multicast Message

For a process to send multicast messages it needs to perform the following. You may use the same UDP socket sd for both sending as well as receiving multicast messages or you can use  a 2^nd udp socket for sending, it does not have to join any multicast group.

struct sockaddr_in MulticatGroupAddress;

MulticatGroupAddress.sin_family = AF_INET;
MulticatGroupAddress.sin_port = UDPport;
MulticatGroupAddress.sin_addr.s_addr = inet_addr (MulticatIPAddress);

sendto (sd, sendBuf, Len,0,  (SA *) &MulticastGroupAddress, sizeof(MulticastGroupAddress)) ;

Control Utility:

Join Group:

To join a udp socket sd to multicast IP address in the range: 224.0.0.0 through 239.255.255.255 use:

joinGroup (sd, “224.0.0.1”);

Time-to-live:

To control how far a messages can go, e.g., 2 means 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; // 0 to turn it off

setLoopback (sd,  &loop);

Reuse-port:

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

reusePort (sd);

Display Distination Address:

To display destination address of a received packet:

displayDaddr (sd);

Multicast Examples

ü Group Chat

Usage: mcastChat <mcastIPAddress> <port>

E.g: % mcastChat 224.0.0.22 10022

The program creates one socket used for both sending and receiving.

It binds the socket to INADDR_ANY (see binding options below)

ü Textbook Example (chapter 21)

Usage: sendrecv <maddr> <port>

E.g: % sendrecv 224.0.0.33 10123

The program creates two sockets, one for sending and the other for receiving.

Note that the main program uses the wrapper function:

http://www.cs.odu.edu/~cs779/stevens2nd/lib/udp_client.c

To bind the receive socket to the mcast address (see binding options below)

Displaying Destination Address

msghdr structure:

êExample: DisplayDistAddr.c

struct msghdr mhdr;
struct iovec iov;
char data[8192];
char cdata[sizeof(struct sockaddr_in) + sizeof(struct cmsghdr)];
struct cmsghdr *cmsg;
struct sockaddr_in from;
socklen_t fromlen = sizeof(struct sockaddr_in);
struct in_addr ip_addr;
struct cmsghdr *cmptr;
char str[32];

int
main(int argc, char **argv)
{

int echo_s;
struct sockaddr_in laddr;
int ret;
int loop;

if (argc < 2) {
printf("usage: udpServerDaddr <group IP> <local_port>\n");
exit(0);
}
echo_s = socket(AF_INET, SOCK_DGRAM, 0);
reusePort(echo_s);

bzero(&laddr, sizeof(laddr));
laddr.sin_family = AF_INET;
laddr.sin_port = htons(atoi(argv[2]));
if (bind(echo_s, (struct sockaddr *) & laddr, sizeof(laddr)) < 0) {
perror("bind error");
exit(1);
}
displayDaddr(echo_s);

joinGroup(echo_s, argv[1]);

loop = 0;
setLoopback(echo_s, loop);


while (1) {

/* using recvmsg & sendmsg */
printf("\n.... Using recvMSG & sendMSG ....\n");
init_mhdr();

if ((ret = recvmsg(echo_s, &mhdr, MSG_WAITALL)) < 0) {
perror("recvmsg()");
}
printf("received:");
fflush(stdout);
write(1, data, ret);
display_addr();

iov.iov_len = ret;
if (sendmsg(echo_s, &mhdr, MSG_DONTWAIT) < 0) {
perror("sendmsg()");
exit(-1);
}
/* using recvfrom & sendto */
printf("\n.... Using recvFROM & sendTO ....\n");
if ((ret = recvfrom(echo_s, data, sizeof(data), 0,
(SA *) & from, &fromlen)) < 0) {
perror("recvfrom");
}
printf("received:");
fflush(stdout);
write(1, data, ret);
printf("udp message sent from: %s:%d\n",
inet_ntop(AF_INET, &from.sin_addr, str, sizeof(str)),
ntohs(from.sin_port));

if (sendto(echo_s, data, ret, 0, (SA *) & from, fromlen) < 0) {
printf("error in sendto\n");
exit(-1);
}
}
}
void
init_mhdr()
{
bzero(cdata, sizeof(cdata));
bzero(&mhdr, sizeof(mhdr));
mhdr.msg_name = &from;
mhdr.msg_namelen = fromlen;
mhdr.msg_iov = &iov;
mhdr.msg_iovlen = 1;
mhdr.msg_control = cdata;
mhdr.msg_controllen = sizeof(cdata);
iov.iov_base = data;
iov.iov_len = sizeof(data);
cmsg = (struct cmsghdr *) cdata;
}

void
display_addr()
{
for (cmptr = CMSG_FIRSTHDR(&mhdr); cmptr != NULL;
cmptr = CMSG_NXTHDR(&mhdr, cmptr)) {
if (cmptr->cmsg_level == IPPROTO_IP &&
cmptr->cmsg_type == IP_RECVDSTADDR) {
memcpy(&ip_addr, CMSG_DATA(cmptr), sizeof(struct in_addr));
printf("udp message sent to: %s\n",
inet_ntop(AF_INET, &ip_addr, str, sizeof(str)));
printf("udp message sent from: %s:%d\n",
inet_ntop(AF_INET, &from.sin_addr, str,
sizeof(str)), ntohs(from.sin_port));
}
}
}

To test:

· Server:

% cd ~/mcastChat

% DisplayDistAddr 224.0.0.1 10123

· Client:

% cd   ~/mcastChat
% mcastChat 224.0.0.1 10123

Notice

If both programs runs at the same host and DisplayDistAddr starts first,

then a message typed by mcastChat will be echoed back to the same port.

Only one process gets it (the one that starts first, DisplayDistAddr) and that will

cause infinite message circulation.

Binding Options:

BindChoice.c

main (int argc, char *argv[])

{

struct sockaddr_in LocalHost;

int UDPsocket;

if (argc != 4)

{

printf ("Usage: BindChoice <multicast ip> <port> <-l|-u|-m|-a>\n");

printf ("e.g.: BindChoice 224.0.0.1 10123 -a\n");

exit (1);

}

bzero(&LocalHost, sizeof(LocalHost));

if (strcmp (argv[3], "-a") == 0)

{

printf ("Using INADDR_ANY\n");

LocalHost.sin_addr.s_addr = htonl (INADDR_ANY);

}

else if (strcmp (argv[3], "-m") == 0)

{

printf ("Using MCAST ADDR: %s\n", argv[1]);

LocalHost.sin_addr.s_addr = inet_addr (argv[1]);

}

else if (strcmp (argv[3], "-l") == 0)

{

printf ("Using LOCAL ADDR: %s\n", "127.0.0.1");

LocalHost.sin_addr.s_addr = inet_addr ("127.0.0.1");

}

else if (strcmp (argv[3], "-u") == 0)

{

getIPaddress ();

printf ("Using UNICAST ADDR: %s\n", UnicastIPaddress);

LocalHost.sin_addr.s_addr = inet_addr (UnicastIPaddress);

}

MulticastIPAddress = argv[1];

UDPport = htons (atoi (argv[2]));

LocalHost.sin_family = AF_INET;

LocalHost.sin_port = UDPport;

if ((UDPsocket = socket (AF_INET, SOCK_DGRAM, 0)) < 0)

{

printf ("can't create UDP socket: \n");

exit (-1);

}

reusePort (UDPsocket);

if (bind (UDPsocket, (SA *) & LocalHost, sizeof (LocalHost)) < 0)

{

printf ("error in bind\n");

exit (-1);

}

TimeToLive = 2;

setTTLvalue (UDPsocket, &TimeToLive);

loop = 1; /* enable loopback */

setLoopback (UDPsocket, loop);

joinGroup (UDPsocket, argv[1]);

if (fork () == 0)

sendMessage (0, UDPsocket);

getMessage (UDPsocket, 1);

}

……

Option :

-a : Binding to INADDR_ANY receives both multicast and unicast messages.

-m : Binding to MCAST ADDR receives only multicast messages.

-l : Binding to LOCAL ADDR receives only unicast messages from local host

-u : Binding to UNICAST ADDR receives only unicast messages from other hosts.

Example Usage:

· Server: (run at antraes,128.82.4.98)

% cd ~/mcastChat

% BindChoice 224.0.0.1 10123 -a

· Clients: (run at antraes,128.82.4.98)

% cd   ~/mcastChat
% mcastChat 224.0.0.1 10123

% cd   ~/SocketProgramming/c
% UDPClient1 127.0.0.1 10123

% UDPClient1 128.82.4.98 10123

& Try other options and different hosts