RFC 1735 (rfc1735) - Page 2 of 11

NBMA Address Resolution Protocol (NARP)

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