RFC 1735 (rfc1735) - Page 2 of 11
NBMA Address Resolution Protocol (NARP)
Alternative Format: Original Text Document
RFC 1735 NBMA ARP (NARP) December 1994 A conventional address resolution protocol, such as ARP [1, 2] for IP, may not be sufficient to resolve the NBMA address of the destination terminal, since it only applies to terminals belonging to the same IP subnetwork, whereas an NBMA network can consist of multiple logically independent IP subnets (LISs, [3]). Once the NBMA address of the destination terminal is resolved, the source may either start sending IP packets to the destination (in a connectionless NBMA network such as SMDS) or may first establish a connection to the destination with the desired bandwidth and QOS characteristics (in a connection oriented NBMA network such as ATM). An NBMA network can be non-broadcast either because it technically doesn't support broadcasting (e.g., an X.25 network) or because broadcasting is not feasible for one reason or another (e.g., an SMDS broadcast group or an extended Ethernet would be too large). 2. Protocol Overview In this section, we briefly describe how a source S uses NARP to determine the NBMA address of a destination D or to find out that such an address doesn't exist. S first checks if the destination terminal belongs to the same IP subnetwork as S itself. If so, S resolves the NBMA address of D using conventional means, such as ARP [1, 2] or preconfigured tables. If D resides in another subnetwork, S formulates a NARP request containing the source and destination IP addresses. S then forwards the request to an entity called the "NBMA ARP Server" (NAS). For administrative and policy reasons, a physical NBMA network may be partitioned into several disjoint logical NBMA networks. NASs cooperatively resolve the NBMA next hop within their logical NBMA network. In the following we'll always use the term "NBMA network" to mean a logical NBMA network. If S is connected to several NBMA networks, it should have at least one NAS in each of them. In order to know which NAS(s) to query for which destination addresses, a multi-homed S should also be configured to receive reachability information from its NASs. Each NAS "serves" a pre-configured set of terminals and peers with a pre-configured set of NASs, which all belong to the same NBMA network. A NAS may also peer with routers outside the served NBMA. A NAS exchanges reachability information with its peers (and possibly with the terminals it serves) using regular routing protocols. This exchange is used to construct a forwarding table in every NAS. The forwarding table determines the next hop NAS towards the NARP request's destination or a next hop router outside the NBMA. Heinanen & Govindan
