RFC 2226 (rfc2226) - Page 2 of 14
IP Broadcast over ATM Networks
Alternative Format: Original Text Document
RFC 2226 IP Broadcast over ATM Networks October 1997 Broadcasting typically allows a single transmit operation to cause a packet to be received by all IP hosts that are members of a particular 'subnet'. To address the need for multicast support (represented by transmission to IP addresses in the Class D space), RFC 2022 ("Support for Multicast over UNI 3.0/3.1 based ATM Networks") [2] was created. This memo creates an analog of the RFC 1577 ARP Server - a new entity known as the MARS (Multicast Address Resolution Server). The MARS operates as a centralized registry and distribution mechanism for mappings between IP multicast addresses and groups of ATM unicast addresses. Host behavior is also defined for establishing and managing point to multipoint VCs, based on the information returned by the MARS, when hosts wish to transmit packets to a multicast group. This memo aims to show how RFC 2022 may be used to emulate IP broadcast within Logical IP Subnets. While the broadcast technique does not align itself well with the underlying point-to-point nature of ATM, clearly, some applications will still wish to use IP broadcasts. Client-server applications where the client searches for a server by sending out a broadcast is one scenario. Routing protocols, most notably RIP, are other examples. 2. Review of Unicast and Multicast. Both the unicast and multicast cases take advantage of the point-to- point and point-to-multipoint capabilities defined in the ATM Forum UNI 3.1 document [4]. A unicast IP address has a single ATM level destination. Unicast transmissions occur over point to point Virtual Channels (VCs) between the source and destination. The ARP Server holds mappings between IP destination addresses and their associated ATM destination address. Hosts issue an ARP_REQUEST to the ARP Server when they wish to ascertain a particular mapping. The ARP Server replies with either an ARP_REPLY containing the ATM address of the destination, or an ARP_NAK when the ARP Server is unable to resolve the address. If the request is successful the host establishes a VC to the destination interface. This VC is then used to forward the first (and subsequent) packets to that particular IP destination. RFC 1577 describes in further detail how hosts are administratively grouped in to Logical IP Subnets (LISs), and how the ARP Server establishes the initial mappings for members of the LIS it serves. The basic host behavior for multicasting is similar - the sender must establish and manage a point to multipoint VC whose leaf nodes are the group's actual members. Under UNI 3.1 these VCs can only be established and altered by the source (root) interface. Smith & Armitage Standards Track
