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Multicast forwarding

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

Multicast forwarding

Multicast forwarding, the intelligent forwarding of multicast traffic, is provided by the TCP/IP protocol and the IGMP routing protocol component for interfaces running in IGMP router mode, on a server running Routing and Remote Access.

The TCP/IP protocol performs the following multicast forwarding functions:

  • Receives and processes all multicast traffic

    The Windows Server 2003 family implementation of the TCP/IP protocol receives and processes all multicast traffic on interfaces that are configured for IGMP router mode. All IP multicast traffic is either forwarded to a process running on the router or intelligently forwarded to network segments that contain either multicast group members or routers connected to downstream network segments that contain multicast group members.

  • Forwards multicast packets to appropriate interfaces

    Upon receipt of a multicast packet, Routing and Remote Access consults a multicast forwarding table to decide whether to forward the packet to any of its other attached interfaces.

Interfaces running in IGMP router mode perform the following multicast forwarding functions:

  • Places the network adapter in multicast-promiscuous mode

    IGMP router mode sets the network adapter to multicast-promiscuous mode. In multicast-promiscuous mode, all multicast packets received by the network adapter are passed to the networking layers for processing. Not all network adapters are capable of multicast-promiscuous mode.

  • Tracks multicast group membership

    The IGMP router mode interface listens for IGMP Membership Report messages on locally attached networks and compiles a list of multicast accounting information as a series of {receiver network, multicast group} pairs. The receiver network is the IP network ID of a network segment containing a listening node. The multicast group is the specific multicast address that is registered by a listening node. The multicast-capable router does not track the IP addresses of the hosts that are listening, only the IP network ID where at least one host is listening.

    To ensure that hosts are still listening on their registered multicast addresses, the IGMP router periodically queries each network. The response to the query is an IGMP Membership Report message. If there are multiple IGMP routers on a single network, one IGMP router is elected the querier and performs all periodic queries.

  • Updates to the TCP/IP multicast forwarding table

    Based on the current state of listening hosts on IGMP router mode interfaces, the TCP/IP multicast forwarding table is updated with the appropriate entries.