NBMA Address Resolution Protocol (NARP)
Network Working Group J. Heinanen
Request for Comments: 1735 Telecom Finland
Category: Experimental R. Govindan
NBMA Address Resolution Protocol (NARP)
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Note that the work contained in this memo does not describe an
Internet standard. This work represents an early stage in the
ongoing efforts to resolve direct communication over NBMA subnets.
It is a suitable experimental protocol for early deployment. It is
expect that it will be superceded by other work being developed
within the IETF.
This document describes the NBMA Address Resolution Protocol (NARP).
NARP can be used by a source terminal (host or router) connected to a
Non-Broadcast, Multi-Access link layer (NBMA) network to find out the
NBMA addresses of the a destination terminal provided that the
destination terminal is connected to the same NBMA network. Although
this document focuses on NARP in the context of IP, the technique is
applicable to other network layer protocols as well. This RFC is a
product of the Routing over Large Clouds Working Group of the IETF.
The NBMA Address Resolution Protocol (NARP) allows a source terminal
(a host or router), wishing to communicate over a Non-Broadcast,
Multi-Access link layer (NBMA) network, to find out the NBMA
addresses of a destination terminal if the destination terminal is
connected to the same NBMA network as the source.
Heinanen & Govindan [Page 1]
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, ).
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.
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