White Paper: Internet Group Management Protocol

Streaming video over the broadband Internet and IP networks can be costly in terms of bandwidth and networking resources. IP multicasting, based on the IGMP protocol, is designed to make video streaming as efficient as possible. The following paper describes IP multicasting based on IGMP.

IGMP (Internet Group Management Protocol) is how IP Multicasts are enabled on an IP network. IGMP allows broadcasters or webcasters to send content to a large number of subscribers effectively without choking the network, because traffic is sent only to a Group Destination Address (GDA).

Clients use IGMP to register themselves as receivers of certain Multicast Groups. Through IGMP, clients can announce their willingness to join, accept, or leave, Multicast streaming from the multicast group. Clients may join and leave the group at will. Only receivers that are registered to a specific GDA are influenced by the multicast traffic, which generally has no impact on other hosts using the network.

Why not use IP Broadcast or Unicast?

Were webcasters or broadcasters to stream content as an IP broadcast, every host on the network, even those that don’t want or need the information, would still have to process it. IP broadcasts are handled on each host as a Level III event, in its own broadcast domain. Thus, every event involves the CPU of the receiving hosts, forcing these to process unnecessary information. Another problem associated with IP broadcasts is that not all routers support this type of transmission. As a result many clients are unable to receive content.

One way of solving the lack of router support for IP broadcasts is by transmitting a Unicast stream. When using Unicast, a copy of the same content is sent to everyone on the network. Although this method solves the problem of broadcasts that do not cross routers, it creates a serious bandwidth problem. Networks, unlike radio or RF domains, have a limited amount of available bandwidth. It is therefore important to ensure that the network is used efficiently and that transmissions take up only the bandwidth needed. Some applications, such as video-on-demand, which do not share networking resources with other services, use the Multiple Unicast to distribute content.

IP Multicasting over IGMP – The most Efficient Way to Stream Rich Media

IP multicasting is the most efficient way to stream rich media over networks. In IP multicasting, everyone who wants to get the content can receive it while the network itself manages the groups of receivers and clients. To implement IP multicastng, the network must be Multicast aware of when and where the stream is duplicated. When using the IGMP protocol, a transmitter sends a stream and additional messages to the router closest to it or to the router that is located on the same SUBNET. Upon receiving the information, the router creates a GDA (Group Destination Address) that is defined by a particular Class D IP Address. (Class D IP addresses range from 224.0.0.0 to 239.255.255.255). The router then checks to see if there are any clients for this group. If there are no clients, the router drops each packet that it receives from the transmitter without sending them anywhere.

Figure 1

If however, a client - even if located on a remote network – wants to receive the stream, several things happen;

• First the receiver sends a dedicated Multicast IP address (to which all Multicast enabled routers listen) to all routers on its subnet, stating that it wishes to join a multicast group.

• If the router on the subnet recognized the group, it begins to release packets to the requesting receiver. If the router, however, does not recognize the IGMP group, it sends out messages and begins to search for the group.

• By communicating with other routers, the router to which the original request was made, is able to find the multicast group. Routers communicate with each other through "routing" protocols such as MOSPF and DVMRP, that have been adapted for IGMP use.

• When the multicast group is found, the other router along the way acts as the "source" router, either releasing or duplicating the stream.

Figure 2: Conserving Bandwidth by Using IGMP

The above diagram shows how IGMP conserves bandwidth. The remote receiver in Network A is receiving a stream from the first router after the source router. This router, which is IGMP version 2 enabled, duplicates the stream at the point where the stream needs to be duplicated and not at the source router, thus conserving bandwidth.The IGMP version is use today is version 2.

The major difference between IGMP version 1 and version 2 is in how clients are dropped from multicast groups. IGMP version 1 specified that routers would continue to release the stream to the receiver for a number of minutes, even after the receiver had ceased listening to the stream. In IGMP version 1, receivers couldn’t tell routers when they wanted to stop receiving the stream. IGMP version 2 lets receivers send a message telling the router to stop releasing packets if there are no other receivers present. In this way version 2 enhances bandwidth conservation over version 1.

By Arthur Rabinowitz, Optibase Networking Support Engineer