What Is IPv4 Multicasting?

Updated: March 28, 2003

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

IP Multicasting Overview

The concept of group membership is central to IP multicasting. IP multicast datagrams are sent to a group, and only members of the group receive the datagrams. A group is identified by a single IP multicast address, which is an IP address in the Class D range of to (designated as in classless interdomain routing (CIDR) notation). These Class D addresses are known as group addresses. A source host sends multicast datagrams to a group address. Destination hosts inform a local router that they need to join the group.

In an IP multicast-enabled intranet, any host can send IP multicast datagrams to any group address, and any host can receive IP multicast datagrams from any group address regardless of its location. To facilitate this capability, the hosts and routers on the intranet must support IP multicasting. Hosts use the Internet Group Management Protocol (IGMP) for establishing group membership. Routers use multicast routing protocols for forwarding multicast data.

The following figure illustrates a multicast-enabled intranet.

Multicast-Enabled Network

Multicast-Enabled Network

In this illustration, the hosts and routers are multicast-enabled so that the following can occur:

  • The sending host sends multicast datagrams to a designated group address.

  • The routers forward the multicast datagrams to any network segments that include group members. Routers can forward multicast traffic across a network, between networks, and across the Internet.

  • The receiving hosts inform a local router to join the group, and then they receive all subsequent datagrams sent to the group address.

  • If a receiving host leaves the group and detects that it might be the last group member on the subnet, it can contact the local router to leave the group, informing the router to stop forwarding the multicast datagrams to that subnet.

Benefits of IP Multicasting

Multicasting provides an efficient way to support high-bandwidth, one-to-many applications on a network:

  • Multicasting can dramatically reduce network traffic by sending a single copy of the data.

  • Hosts can be configured for multicasting without hardware upgrades.

  • Because newer routers already support multicast forwarding and multicast routing protocols, enabling multicasting on a network is practical and cost-effective.

Multicasting is useful for many types of one-to-many applications, such as the following:

  • Multimedia, such as video conferencing and collaborative computing.

  • Automatic discovery of resources in an internetwork (in Windows Server 2003 for example, TCP/IP router discovery uses multicasting by default, and WINS uses multicasting during automatic discovery of replication partners).

  • Datacasting such as file distribution or database synchronization.

  • Mobile computer support such as remote address book updating.

  • Distribution of organizational publications.

IP Multicasting with Routing and Remote Access

Windows Server 2003 does not provide multicast routing protocols, such as Distance Vector Multicast Routing Protocol (DVMRP), Multicast Extensions to Open Shortest Path First (MOSPF), and Protocol Independent Multicast (PIM), although Routing and Remote Access does support multicast routing protocols developed by independent software vendors (ISVs).

As an alternative, you can use the Routing and Remote Access service to forward multicast traffic. In this case, the Routing and Remote Access service uses IGMP as an IP routing protocol component. Router interfaces are configured in one of two operating modes: IGMP router mode or IGMP proxy mode. The purpose of IGMP router mode is to forward multicast traffic in a single-router intranet. The purpose of IGMP proxy mode is to connect a single-router intranet to a multicast-capable intranet or the Internet.

Although Routing and Remote Access uses IGMP in a limited way to enable multicast forwarding on an intranet, it is not the equivalent of a true multicast routing protocol. The Routing and Remote Access IGMP routing protocol component supports multicast forwarding for several specific internetwork topologies. For more information about the supported topologies, see “How IPv4 Multicasting Works.”

Related Information

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