Be Prepared for IP Multicasting Applications

s businesses use more intranet, groupware, multimedia, and client/server technologies, they need more IP multicasting applications. Video conferencing, audio conferencing, online training, news distribution, software distribution, and database replication are all good candidates for IP multicasting, one-to-many communication from a source to a group of selected destinations. Multicasting applications can minimize the demand for network bandwidth while delivering information from a source to multiple destinations via one stream.

Are you ready for IP multicasting? Before you open the door to IP multicasting applications, you need to learn what IP multicasting is and to find out whether your network infrastructure and IP multicasting applications support each other. This article will explain IP multicasting and then describe a multicasting session conducted with Microsoft NetShow Live.

Why Multicast?
Traditionally, network traffic is either unicast or broadcast. Unicasting is one-to-one communication from a source to a destination on the network. If n hosts request the same information from the same source, n copies of the information move from the source host to n destination hosts. Broadcasting is one-to-everyone communication from a source to the rest of hosts on the network. The broadcast information reaches every host regardless of whether the host needs the information. Broadcasting wastes network bandwidth by flooding the network when only a small number of users need the information.

Unicasting and broadcasting do not work for certain applications. For example, your CEO wants to conduct an audio conference with 50 senior managers through the network. If the multimedia application for the conference uses unicasting, the CEO's computer repeatedly sends out 50 audio streams to 50 managers' computers. Unicasting wastes bandwidth because it sends 50 duplicate copies over the network, and it causes a significant delay before the last manager hears the CEO.

Broadcasting cannot solve the problem: Broadcasting sends only one audio stream over the network, but it sends the information to every computer on the network. Therefore, the audio stream floods every corner of the network and can bring the network down.

Multicasting comes to the rescue. The multicast host sends out only one copy of the information, and only the destination hosts that need the information receive it. In the audio conferencing example, the CEO's computer sends only one audio stream to the network, and only the 50 senior managers receive the stream. The information uses only required network bandwidth and arrives at every manager's computer without a noticeable delay.

IP Multicasting and Its Addresses
IP multicasting transmits information from a source host to a group of destination hosts in a TCP/IP network. (The sidebar "A Quick Review of ISO/OSI, IEEE 802.2, and TCP/IP " describes computer communications protocols.) Multicasting is an integral part of the IP protocol on the network layer of the International Standards Organization/Open Systems Interconnection (ISO/OSI) Reference Model. This technology is not new. Steve Deering at Stanford University proposed the Internet Group Management Protocol (IGMP) in the Internet Engineering Task Force Request for Comments (IETF RFC) 1112 in 1989. IGMP defines most specifications of IP multicasting. You can download a free copy of RFC 1112 from http://www.cis.ohio-state.edu/htbin/rfc/rfc1112.html.

In the addressing scheme in the IP protocol, class A, B, and C addresses identify a host in a TCP/IP network. A class D address represents an IP multicast group in which many hosts participate in the same IP multicasting application. The first four bits of the 32-bit class D addresses are 1110, and the remaining 24 bits can range from all 0s to all 1s. Therefore, IP multicast addresses can range from 224.0.0.0 to 239.255.255. 255. The protocol reserves address 224.0.0.0 for operating systems, and the protocol cannot assign that address to applications. For example, the protocol permanently assigns 224.0.0.1 to the all-hosts group, which includes all computers and routers participating in IP multicasting in a local network. The sidebar "How IP Multicasting Works ," page 106, describes the steps in developing multicasting software.

Microsoft fully supports IP multicasting in its TCP/IP in Windows NT 3.5, 3.51, and 4.0 and Windows 95. IGMP defines three levels of support for IP multicasting. Level 0 has no support, Level 1 supports sending but not receiving multicast IP data, and Level 2 fully supports sending and receiving multicast IP data. The Microsoft TCP/IP supports Level 2.

The only component that uses IP multicasting in NT is Windows Internet Naming Service (WINS). When a WINS server starts up, it sends an IP multicasting packet to address 224.0.1.24 looking for other WINS servers that participate in WINS database replication. WINS sends out this multicast packet periodically by default.

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