 change, and source list change. The current state report is used in response to the router's periodic queries, and the filter mode change and source list change (collectively known as state change records) are used when the host's state changes in response to an application request. Queries are backwards compatible, SO that an IGMPv3 router may send a single message and get responses back from IGMPv3, IGMPv2 and IGMPv1 hosts. IGMP client support is offered in the TCP/IP stacks of most hosts, including most flavors of Windows. IGMP must also be supported at the router level; CISCO, Bay Networks and others offer routers that can cope with it. Multicast-Aware Routing There are several standards available for routing IP Multicast traffic. These are described in three separate IETF documents. RFC 1075 defines the Distance Vector Multicast Routing Protocol (DVMRP). RFC 1584 defines the Multicast Open Shortest Path First (MOSPF) protocol-an IP Multicast extension to the Internet's OSPF. The Protocol-Independent Multicast (PIM) Sparse Mode was defined by the IETF in RFC 2117; PIM-Dense Mode (PIM-DM) is, at the time of this writing, still a draft. PIM is a multicast protocol that can be used in conjunction with all unicast IP routing protocols. DVMRP uses a technique known as Reverse Path Forwarding to flood a multicast packet out of all paths except the one that leads back to the packet's source. Doing SO permits a data stream to reach all LANs. If a router is attached to a set of LANs that do not wish to receive a particular multicast group, it can send a prune message back up the distribution tree. The prune message stops subsequent packets from flowing to LANs that do not support group members. DVMRP refloods on a periodic basis in order to absorb hosts who are just tuning in to the multicast. The more often that DVMRP floods, the shorter the time that a new recipient will have to wait to be added to a multicast. Flooding frequently, on the other hand, greatly increases network traffic. Early versions of DVMRP did no