Planning IP Multicast-Enabled Routers

To implement IP multicasting on a multiple-router intranet, you must install routers enabled for multicast routing and configured with one or more multicast routing protocols.

Windows Server 2003 does not provide any multicast routing protocols. To provide multicast forwarding within a single-router intranet or when connecting a single-router intranet to the Internet, you can configure the Internet Group Management Protocol (IGMP) routing protocol component of the Routing and Remote Access service with interfaces set to IGMP router mode and IGMP proxy mode. The IGMP routing protocol component exchanges and updates information in the IP multicast forwarding table about host membership in specific groups.

The IGMP routing protocol is not a multicast routing protocol. To support efficient multicast forwarding and routing on a multiple-router intranet, you must also install IP multicast-enabled routers that use one or more multicast routing protocols. Multicast routers use multicast routing protocols to communicate multicast group information with each other.

Although Windows Server 2003 does not include any multicast routing protocols, the Routing and Remote Access service is an extensible platform that can support multicast routing protocols. Multicast routing protocols include Protocol-Independent Multicast (PIM) in both Sparse Mode (PIM-SM) and Dense Mode (PIM-DM), Multicast Extensions to OSPF (MOSPF), and the Distance Vector Multicast Routing Protocol (DVMRP). Your choice of multicast routing protocol will depend on the size and type of network and the distribution of multicast group members.

Protocol-Independent Multicast (PIM). The PIM protocol routes to multicast groups whose members span wide-area and interdomain internetworks. PIM functions independently of any unicast routing protocol. A multicast group that uses PIM can declare itself sparse or dense, using either Sparse Mode or Dense Mode:

Protocol-Independent Multicast Sparse Mode (PIM-SM), the most widely used multicast routing protocol, is designed for multicast groups whose members are distributed sparsely across a large region. PIM-SM can operate in a LAN environment but is most efficient in a WAN environment. Using a dense-mode protocol for a multicast group whose members are distributed thinly can cause unnecessary transmission and router storage of data packets or membership report information. This overhead might be acceptable where multicast group members are populated densely, but it is inefficient for a sparse mode multicast group. In sparse mode, routers must explicitly join and leave multicast groups, which eliminates unnecessary traffic and storage.

Protocol-Independent Multicast Dense Mode (PIM-DM) is a dense-mode multicast routing protocol designed for multicast groups whose members are distributed thickly over an area where bandwidth is plentiful. PIM-DM is interoperable with the sparse mode, PIM-SM. PIM-DM does not scale well.

Multicast Extensions to OSPF (MOSPF). The MOSPF protocol, an extension of OSPF, is also a dense-mode multicast routing protocol. MOSPF employs a unicast routing protocol that requires that each router in a network be aware of all available links. MOSPF is intended for use on a single organization's network, and does not scale well. MOSPF requires OSPF as its accompanying unicast routing protocol. It can sometimes put a heavy load on router CPU bandwidth.

Distance Vector Multicast Routing Protocol (DVMRP). The original IPv4 multicast routing protocol, DVMRP runs over multicast-capable LANs such as Ethernet. DVMRP can also tunnel IP multicast packets as unicast packets through routers with no multicast capability. DVMRP is a dense-mode multicast routing protocol that does not scale well.

Configuring IGMP

To support IPv4 multicast applications on a single-router intranet or when connecting a single-router intranet to the Internet, you can use the Routing and Remote Access service on one or more computers running Windows Server 2003, add the IGMP routing protocol component on each server, and configure the server's outbound interface for IGMP router mode and its inbound interface for IGMP proxy mode. If your multicast applications cross the Internet, the outbound interface is the intranet interface and the inbound interface is the Internet interface.

IGMP router mode on the outbound interface. In Windows Server 2003, an outbound interface running in IGMP router mode listens for IGMP Membership Report messages and tracks group membership. Enable IGMP router mode on the interfaces to listening multicast hosts. The TCP/IP protocol and the IGMP routing protocol component for interfaces running in IGMP router mode forward multicast traffic.

IGMP proxy mode on the inbound interface. IGMP proxy mode is designed to pass IGMP Membership Report messages within a single-router intranet or from a single-router intranet to the MBone. (As explained earlier, in a multiple-router intranet, you must install routers that use one or more multicast routing protocols.) With IGMP proxy mode enabled on the inbound interface, hosts can receive multicast traffic from multicast sources and can send multicast traffic to other hosts.

Within a single-router intranet, or when connecting a single-router intranet to the Internet, you do not need routers running multicast routing protocols. However, within a multiple-router intranet that uses multicast routers running multicast routing protocols, you can still use the Routing and Remote Access service as a multicast forwarding router on the periphery of your intranet.

RFC 1112, "Host Extensions for IP Multicasting," defines address and host extensions for IP hosts that support multicasting, and defines IGMP Version 1. RFC 2236, "Internet Group Management Protocol (IGMP), Version 2," defines IGMP Version 2. Windows Server 2003 supports IGMP Version 3, described in the Internet Draft "Internet Group Management Protocol, Version 3." Under IGMP Version 3, hosts can specify interest in receiving multicast traffic from specified sources or from all but a specific set of sources.