- The most common form of network communication is between
two host in the network and the network routing infra-structure
has been set up primarily to support this form of communication.
Examples of such "traditional" communication:
- File transfer
- telephone conversation (three or multiway calling is rare)
- Web browsing
- We can distinguish three forms of communication:
- Unicast communication: when messages from a sender is received
by one receiver in the network
Unicast communication has been supported efficiently by using routing tables.
- Broadcast communication: when messages from a sender is received
by all receivers in the network
- Multicast communication: when messages from a sender is received by zero or more receivers in the network.
- Unicast communication: when messages from a sender is received
by one receiver in the network
- What types of applications are well-served by multicast ?
- Entertainment - e.g., video distribution
- Tele-conference involving multiple parties
- Stock quotes ! (Lots of people will be interested in obtaining the quotes)
- In summary: any application where there are multiple recipients for the same information
- Important to distiguish between multicast communication
and multicast routing:
- multicast communication: when a message from a sender is received
by zero or more receivers in the network.
You can realise multicast communication by having the sender transmit a copy of the (same) message to each receiver:
- multicast routing: forward a message to a set of receivers
in the network:
- multicast communication: when a message from a sender is received
by zero or more receivers in the network.
- Multicast or group communication existed long before multicast
routing...
Applications simply send multiple messages - one message to each
receiver.
Obviously, multicast routing can support multicast communication much more efficiently than "naive multicast communication" (= sending multiple unicast messages).
- The IP Multicast communication model:
- An IP multicast group consists of a number of IP hosts
that are interested in receiving the same set of messages
(e.g., participants in a video conference)
- Multicast receiver (i.e., a multicast group member) must
explicitly join a multicast group to receive
multicast messages for that group.
- Receiver can dynamically join and leave multicast group.
- Any host - even those that did not join the multicast group -
can send multicast messages to any multicast group.
- The sender has NO knowledge on the current set of members
in a multicast group....
As a result:
IP multicast must adopt unreliable delivery (UDP) (because how can you achieve reliability if you don't even know who the receivers are ???)
- An IP multicast group consists of a number of IP hosts
that are interested in receiving the same set of messages
(e.g., participants in a video conference)
- IP Multicasting was implemented by Steve Deering
in his Ph.D. thesis work at Stanford in 1993.
- Identifying a multicast group
- Before Deering's work, there was no multicast available on
the Internet
- Deering needed group IDs to identify a multicast group, so
that he can send messages to the members of the multicast
group:
- What can he use for the destination address in an IP packet
that will allow the message to be delivered to the current members
of the multicast group ?
- None at that time
- Deering had to
- create a new set of addresses for himself and
- augment the IP module in the kernel to route IP messages with these special addresses
- Before Deering's work, there was no multicast available on
the Internet
- IP Multicast Addressing:
- Before IP multicast, there were 3 type of IP addresses:
- Class A: 0xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
- Class B: 10xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
- Class C: 110xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
- Reserved: 111xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
- IP multicast address has been allocated in half of the reserved address:
- Class D: 1110xxxxxxxxxxxxxxxxxxxxxxxxxxxx (multicast group address)
- Reserved D: 1111xxxxxxxxxxxxxxxxxxxxxxxxxxxx
See: click here
- In dotted decimal notation:
- Multicast address ranges from: 224.0.0.0 - 239.255.255.255
- Before IP multicast, there were 3 type of IP addresses:
- IP Multicast Programming:
- Deering defined and implemented the following API with UDP
Sockets for multicast.
- Steps to join a multicast group (in order to receive multicast messages
sent to this group of receivers):
- Create an ordinary UDP socket
s = socket(AF_INET, SOCK_DGRAM, 0);
- Bind the UDP socket to some (unused) port#
struct sockaddr_in in_addr; /* Internet Address socket structure */ in_addr.sin_family = AF_INET; /* Protocol domain */ in_addr.sin_addr.s_addr = INADDR_ANY; /* Use wildcard IP address */ in_addr.sin_port = htons(portNumber); /* Use this UDP port */ bind(s, (struct sockaddr *)&in_addr, sizeof(in_addr) );
You need to use a specific port# - you cannot allow the system to assign you with one, because all multicast program must use the same port# to communicate with each other...
- Set up an IP multicast address structure:
struct ip_mreq mreq; /* IP Multicast request */ mreq.imr_multiaddr.s_addr = inet_addr("224.0.0.1"); /* Multicast group address */ mreq.imr_interface.s_addr = htonl(INADDR_ANY);The address inet_addr("224.0.0.1") is the multicast group address
- Join a multicast group using a set socket option system call:
setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char *) &mreq, sizeof(mreq) );
The variable s is a UDP socket that you have binded to some UDP port on the local host (see UDP programming)
- Create an ordinary UDP socket
- NOTE: the socket that is set to receive multicast message cannot be use in a sendto call in some implementations. So create another socket to send multicast messages !
- Deering defined and implemented the following API with UDP
Sockets for multicast.
- Send IP Multicast messages:
- Simply use sendto with a UDP socket.
- Specify a multicast address in the socket address structure
and use the same port number specified by
the multicast receivers:
sendto(s, line, strlen(line)+1, 0 /* flags */, (struct sockaddr *)&dest_addr, sizeof(dest_addr)); ^^^^^^^^^ With: dest_addr.sin_addr.s_addr = inet_addr("224.0.0.1"); dest_addr.sin_port = Same port used by receivers... - WARNING: Before you call "sendto",
you should set the TTL (time to live) of the socket
that you use to send multicast message with this set socket option
call:
char ttl = ...; /* Set a value between 1 and 255 */ /* Multicast with TTL = 1 will only reach your own network */ /* Multicast with TTL = 255 will reach the whole Internet */ setsockopt(s, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl);
- Simply use sendto with a UDP socket.
- A multicast receiver: click here .
Usage:
mcast-r MCAST_addr Port# - A multicast sender: no need for special multicast sender.
Just use udp4-s2.c that allows you to specify an address
as dotted decimal notation:
click here .
Usage:
udp4-s2 MCAST_addr Port# - (Use lennon.cc.gatech.edu in multicast demo from GaTech)