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Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Multicasting is a way of efficiently transmitting text, audio, and video on the Internet or an internal network to a select group of people, much like a conference call includes a select group of people. Instead of sending information in individual packets to each recipient, a single message is sent to a multicast group, which includes all the people that want to participate in the multicast session. While multicasting is possible on a variety of networks, this topic concentrates on Internet multicasting.

Multicasting is a one-to-many transmission. In contrast, the traditional method of sending messages on the Internet, called unicasting, is a one-to-one transmission. If multicasting is comparable to a conference call, then unicasting is like a private call between two people. Broadcasting is a one-to-all technique in which messages are sent to everybody. Internet routers block broadcasts from propagating everywhere.

Multicasting provides a way for one host to send packets to a selective group of hosts. The key word is "selective." Users choose to be part of a specific multicast. Multicast packets then travel to the user from the multicast source. An important point is that multicast packets only travel across routes where there is an end user that has requested to be part of the multicast. This keeps multicast packets from crossing parts of the network that do not have multicast participants. Still, on the Internet, a multicast group is potentially huge, with members located around the world.

The trick to multicasting is that users indicate to their local router that they want to be part of a particular multicast group. That router then indicates to the next router closest to the source of the multicast that it wants to receive the multicast. This process continues until a path is established between the multicast source and the person who wants to join the multicast. The result is that only routers that need multicast packets for end systems actually receive those packets. Nonparticipating routers do not receive the packets, making the process more efficient.

As shown in Figure M-9, you can imagine a tree of routers that branch from the multicast source and connect to end systems that want to receive the multicast. If a router has no hosts that want to receive the multicast, it excludes itself from the tree.

Most multicasts are multimedia related, although a multicast host may simply broadcast a message or an occasional news item to participants. This sounds similar to electronic mailing lists, but IP Multicast uses special addressing and special protocols to achieve high- performance and efficiency. See "Webcasting" for an example of how multicasting is used.

RTP (Real-time Transport Protocol) is a protocol that works in conjunction with multicasting to transport real-time audio, video, simulation data, and other information over multicast networks. While IP Multicast defines how to set up multicast groups, RTP defines how to transport real-time information to the members of the groups and monitor the quality of the information that is delivered. Refer to "RTP (Real-time Transport Protocol)" for more information.

IP Multicast Protocols

IP Multicast is an open IETF (Internet Engineering Task Force) standard for distributing data to multiple recipients. The multicast recipient group can change dynamically. A host may decide to join or leave a group at any time, and a host may be a member of more than one multicast group. In addition, any host can be a multicast source by simply sending packets addressed to a particular multicast group.

Routers in this scheme must be "multicast enabled." When a multicast source transmits a multicast datagram, the local router forwards the packet to other routers with attached networks that include members of the multicast group.

IP Multicast uses class D addressing, which is a special form of the IP address designed for multicasting. The class scheme for IP addresses is discussed under "IP (Internet Protocol)." The first four bits of a class D address identify it as a class D address. The remaining 28 bits identify a particular multicast group. A class D address can be compared to the channel number of a TV station. When you tune in to a particular class D address, you receive packets that are being multicast by other systems that multicast on the address.

Several protocols are related to IP multicast. These are outlined below:

  • IGMP (Internet Group Management Protocol)    Multicast receivers must indicate their desire to be included in a multicast session. This is done with IGMP, a protocol that runs between hosts and their immediately neighboring multicast routers. In addition, a multicast router can use IGMP to occasionally broadcast a query on a network to determine if any hosts still want to receive broadcasts from the multicast transmitters from which it is accepting packets.

  • DVMRP (Distance Vector Multicast Routing Protocol)    DVMRP is a distance vector routing protocol for multicast. DVMRP is used when a router receives a multicast packet and it wants to find out if other multicast routers it has connections to need to receive the packet (that is, they have attached hosts that are members of the group). DVMRP sends the packet to all attached routers and waits for a reply. Routers with no group members return a "prune" message, which essentially prevents further multicast messages for that group from reaching the router.

  • MOSPF (Multicast Open Shortest Path First)    As the name implies, MOSPF is an Open Shortest Path First routing protocol. MOSPF routers build maps of the network topology, including the location of islands and tunnels, and then determine the best path through the network to a particular multicast router. Note that MOSPF is designed for use within autonomous systems.

  • PIM (Protocol-Independent Multicast)    PIM is a new concept for multicast routing and is an alternative to DVMRP and MOSPF. It uses two modes: PIM-dense and PIM-sparse. Dense mode operates like DVMRP. It floods the network with traffic-a bad thing unless the group has enough participants to warrant such an action. PIM-sparse avoids flooding (and wasted bandwidth) in cases where a group is small. Instead, a rendezvous point is established and all members send packets to it.

  • MTP (Multicast Transport Protocol)    MTP defines a flow-controlled multicasting transport protocol that operates on top of any network protocol as long as the data link layer includes multicast. MTP ensures that all messages are delivered reliably, in order, and at the same time. MTP can retransmit packets to a specific branch of a multicast group, thus reducing excess traffic in branches that don't need the retransmission.

For more information, refer to the IETF Working Groups page listed on the related entries page. A number of working groups are defining various aspects of multicasting, and their Web sites provide links to drafts and related RFCs. See "Webcasting" for related information.

An industry consortium called the IPMI (IP Multicast Initiative) is dedicated to advancing IP Multicasting and making information about it available.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.