Network Working Group J. Heinanen
Request for Comments: 1735 Telecom Finland
Category: EXPerimental R. Govindan
ISI
December 1994

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.

IESG Note:

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expect that it will be superceded by other work being developed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.

Abstract

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 RFCis a
prodUCt of the Routing over Large Clouds Working Group of the IETF.

1. Introduction

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.

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.

After receiving a NARP request, the NAS checks if it "serves" D. If
so, the NAS resolves D's NBMA address, using mechanisms beyond the
scope of this document (examples of such mechanisms include ARP [1,
2] and pre-configured tables). The NAS then either forwards the NARP
request to D or generates a positive NARP reply on its behalf. The
reply contains D's IP and NBMA address and is sent back to S. NARP
replies usually traverse the same sequence of NASs as the NARP
request (in reverse order, of course).

If the NAS does not serve D, it extracts from its forwarding table
the next hop towards D. If the next hop is a peer NAS, it forwards
the NARP request to the next hop. If the next hop is a peer router
outside the served NBMA or if no such next hop entry is found, the
NAS generates a negative NARP reply.

A NAS receiving a NARP reply may cache the NBMA address information
contained therein. If a subsequent NARP request for the same target
address does not desire an authorative reply, a caching NAS can then
respond with the cached non-authoritative NBMA address or with cached
negative information. A well behaving terminal should always first
accept a non-authoritative reply. Only if communication attempt
based on the non-authoritative information fails, the terminal can
choose to issue another request this time aSKINg for an authoritative
reply.

NARP requests and replies never cross the borders of an NBMA network.
Thus, IP traffic out off and into an NBMA network always traverses an
IP router at its border. Network layer filtering can then be
implemented at these border routers.

3. Configuration

Terminals

To participate in NARP, a terminal connected to an NBMA network
should to be configured with the IP address(es) of its NAS(s). If
the terminal is attached to several NBMA networks, it should also
be configured to receive reachability information from its NAS(s)
so that it can determine, which IP destinations are reachable
through which NBMA networks.

NBMA ARP Servers

A NAS is configured with a set of IP address prefixes that
correspond to the IP addresses of the terminals it is serving.
Moreover, the NAS must be configured to exchange reachability
information with its peer NASs (if any). In addition, the NAS may
be configured to exchange reachability information with routers

outside the served NBMA. And finally, if a served terminal is
attached to several NBMA networks, the NAS may need to be
configured to send reachability information to such a terminal.

4. Packet Formats

NARP requests and replies are carried in IP packets as protocol type
54. This section describes the packet formats of NARP requests and
replies:

NARP Request

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version Hop Count Checksum
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Code Unused
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Destination IP address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Source IP address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NBMA length NBMA address
+-+-+-+-+-+-+-+-+
(variable length)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version
The NARP version number. Currently this value is 1.

Hop Count
The Hop count indicates the maximum number of NASs that a request
or reply is allowed to traverse before being discarded.

Checksum
The standard IP checksum over the entire NARP packet (starting with
the fixed header).

Type
The NARP packet type. The NARP Request has a Type code 1.

Code
A response to an NARP request may contain cached information. If an
authoritative answer is desired, then code 2 (NARP Request for
Authoritative Information) should be used. Otherwise, a code value
of 1 (NARP Request) should be used.

Source and Destination IP Addresses
Respectively, these are the IP addresses of the NARP requestor and
the target terminal for which the NBMA address is desired.

NBMA Length and NBMA Address
The NBMA length field is the length of the NBMA address of the
source terminal in bits. The NBMA address itself is zero-filled to
the nearest 32-bit boundary.

NARP Reply

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version Hop Count Checksum
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Code Unused
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Destination IP address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Source IP address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
NBMA length NBMA address
+-+-+-+-+-+-+-+-+
(variable length)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version
The NARP version number. Currently this value is 1.

Hop Count
The Hop count indicates the maximum number of NASs that a request
or reply is allowed to traverse before being discarded.

Checksum
The standard IP checksum over the entire NARP packet (starting with
the fixed header).

Type
The NARP packet type. The NARP Reply has a Type code 2.

Code
NARP replies may be positive or negative. A Positive, Non-
authoritative Reply carries a code of 1, while a Positive,
Authoritative Reply carries a code of 2. A Negative, Non-
authoritative Reply carries a code of 3 and a Negative,
Authoritative reply carries a code of 4.

The general rule is that a NAS should not reply to an NARP request
for authoritative information with cached information, but may do
so for an NARP request. A NAS implementation is allowed to relax
this rule and return non-authoritative information even in case
authorative was desired if the NAS becomes heavily loaded and the
cached information is very recently updated.

Source and Destination IP Address
Respectively, these are the IP addresses of the NARP requestor and
the target terminal for which the NBMA address is desired.

NBMA Length and NBMA Address
The NBMA length field is the length of the NBMA address of the
destination terminal in bits. The NBMA address itself is zero-
filled to the nearest 32-bit boundary. Negative replies do not
carry the NBMA length or the NBMA address field.

A NAS may cache NBMA replies.

5. Protocol Operation

The external behavior of a NAS may be described in terms of two
procedures (processRequest and processReply) operating on two tables
(forwardingTable and cacheTable). In an actual implementation, the
code and data structures may be realized differently.

Each NAS has a forwardingTable consisting of entries with the fields:

<networkLayerAddrPrefix, type, outIf, outIfAddr>

The networkLayerAddrPrefix field identifies a set of IP addresses
known to the NAS. It consists of two subfields <ipAddr, mask>.

The type field indicates the type of the networkLayerAddrPrefix. The
possible values are:

- locallyServed: The NAS is itself serving the
networkLayerAddrPrefix. The outIf field denotes the NBMA interface
via which the served terminals can be reached and the outIfAddr
field has no meaning. Such a forwardingTable entry has been
created by manual configuration.

- nasLearned: The NAS has learned about the networkLayerAddrPrefix
from another NAS. The outIf and outIfAddr fields, respectively,
denote the NBMA interface and IP address of this next hop NAS.
Such a forwardingTable entry is a result of network layer address
prefix information exchange with one of the NAS' peer NASs.

- externallyLearned: The NAS has learned about the
networkLayerAddrPrefix from a peer router outside the served NBMA.
The outIf and outIfAddr fields, respectively, denote the NBMA
interface and IP address of this next hop NAS. Such a
forwardingTable entry is a result of network layer address prefix
information exchange with one of the NAS' peer routers.

The protocol used to exchange networkLayerAddrPrefix information
among the NASs can be any regular IP intra-domain or inter-domain
routing protocol.

In addition to the forwardingTable, each NAS has an NARP cacheTable
consisting of entries with the fields:

<networkLayerAddr, nbmaAddr, timeStamp>

The entries in the cacheTable are learned from NARP replies
traversing the NAS. In case of a negative cache entry the nbmaAddr
is empty. The timeStamp field records the time when the cacheTable
entry has been created or updated. It is used to determine if an
entry is a very recent one and to age old entries after a certain
hold period.

The following pseudocode defines how NBMA NARP requests and replies
are processed by an NAS.

procedure processRequest(request);
let bestMatch == matchForwardingTable(request.dIPa) do
if bestMatch then
if bestMatch.type == locallyServed then
let nbmaAddr == arp(request.dIPa) do
if nbmaAddr then
genPosAuthReply(request.sIPa, request.dIPa, nbmaAddr)
else
genNegAuthReply(request.sIPa, request.dIPa)
end
end
elseif bestMatch.type == nasLearned then
if not requestForAuthInfo?(request) or
realBusyRightNow?() then
let cacheMatch == matchCacheTable(request.dIPa) do
if cacheMatch and
(not requestForAuthInfo?(request) or
realRecentCacheEntry?(cacheMatch)) then
if cacheMatch.nbmaAddr == EMPTY then
genNegNonAuthReply(request.sIPa, request.dIPa)
else
genPosNonAuthReply(request.sIPa, request.dIPa,

cacheMatch.nbmaAddr)
end
else /* no cache match */
forwardRequest(request, bestMatch.OutIf,
bestMatch.OutIfAddr)
end
end
else /* request for authoritative information */
forwardRequest(request, bestMatch.OutIf,
bestMatch.OutIfAddr)
end
else /* bestMatch.type == externallyLearned */
genNegAuthReply(request.sIPa, request.dIPa)
end
else /* no match in forwardingTable */
genNegAuthReply(request.sIPa, request.dIPa)
end
end
end

procedure processReply(reply);
addCacheTableEntry(reply.dIPa, reply.nbmaAddr, currentTime);
if reply.sIPa == selfIpAddr then
/* reply is to the NAS itself */
else
let bestMatch == matchForwardingTable(reply.sIPa) do
if bestMatch then
forwardReply(reply, bestMatch.outIf, bestMatch.outIfAddr)
end
end
end
end

The semantics of the procedures used in the pseudocode are explained
below.

matchForwardingTable(ipAddress) returns the forwardingTable entry
whose networkLayerAddrPrefix field is the longest match for ipAddress
or FALSE if no match is found.

arp(ipAddress) resolves the NBMA address corresponding to ipAddress.
It returns FALSE if the resolution fails.

genPosAuthReply(sourceIpAddr, destIpAddr, destNbmaAddr) and
genPosNonAuthReply(sourceIpAddr, destIpAddr, destNbmaAddr) generate a
positive, authoritative and non-authoritative reply with
sourceIpAddr, destIpAddr, and destNbmaAddr in Source IP address,
Destination IP address, and NBMA Address fields, respectively.

genNegAuthReply(sourceIpAddr, destIpAddr) and
genNegNonAuthReply(sourceIpAddr, destIpAddr) respectively generate a
negative, authoritative and non-authoritative reply with sourceIpAddr
and destIpAddr in Source IP address and Destination IP address
fields, respectively.

requestForAuthInfo?(request) tests if request is a Request for
authoritative information.

realBusyRightNow?() returns TRUE if the NAS is heavily loaded and
FALSE otherwise.

realRecentCacheEntry?(cacheTableEntry) returns TRUE if the
cacheTableEntry is very recently updated and FALSE otherwise.

matchCacheTable(ipAddr) returns a cacheTable entry whose
networkLayerAddr field is equal to ipAddr or FALSE if no match is
found.

forwardRequest(request, interface, ipAddr) decrements the Hop count
field of request, recomputes the NARP Checksum field, and forwards
request to ipAddr of interface provided that the value of the Hop
count field remains positive.

addCacheTableEntry(ipAddr, nbmaAddr, time) adds a new entry to the
cacheTable or overwrites an existing entry whose networkLayerAddr
field is equal to ipAddr.

forwardReply(reply, interface, ipAddr) decrements the Hop count field
of request, recomputes the NARP Checksum field, and forwards reply to
ipAddr of interface provided that the value of the Hop count field
remains positive.

Like NASs, each NBMA terminal has a forwardingTable and a cacheTable.
The forwardingTable is either manually configured or filled via
reachability information exchange with the terminal's NASs or peer
routers.

When the terminal wishes to find out the NBMA address of a particular
destination terminal, it first checks if a matching entry is found in
the forwardingTable. If not, the destination is unreachable and the
terminal gives up. If a forwardingTable entry is found, and if the
next hop belongs to one of the terminal's NASs, the terminal next
consults its cacheTable to oBTain the NBMA address. If no cache
match is found, the terminal generates a NARP request to the next hop
NAS. If the reply to the NARP request is positive, the terminal
learns the NBMA address and updates its cacheTable with the new
information.

6. Discussion

The NARP semantics resembles closely the ATMARP semantics described
in [2]. The only actual differences are:

- NARP requests and replies include a hop count to prevent them from
looping forever in case of misconfigured NAS routing.

- NARP request and replies distinguish between authoritative and
non-authoritative information.

In order to keep the NBMA terminals as simple as possible, it would
be desirable to extend the the ATMARP protocol a little further so
that it could be also used as the terminal-NAS protocol. This could
be easily accomplished just by adding three new operation codes to
ATMARP to cover the different kinds of queries and responses. NARP
would then become the NAS-NAS protocol. Finally, if the NASs are
co-located with the "classical" ATM ARP servers, the terminals would
not need to make any distinction between between local and foreign IP
subnetworks.

The NASs can also act as "connectionless servers" for the terminal by
advertizing to it all destinations no matter if they are inside or
outside the served NBMA. Then, the terminal could choose either to
try to resolve the NBMA address of the destination or just to send
the IP packets to the NAS. The latter option may be desirable if
communication with the destination is short-lived and/or doesn't
require much network resources.

NARP supports portability of NBMA terminals. A terminal can be moved
anywhere within the NBMA network and still keep its original IP
address as long as its NAS(s) remain the same. Requests for
authoritative information will always return the correct NBMA
address.

References

[1] Plummer, D., "An Ethernet Address Resolution Protocol - or -
Converting Network Protocol Addresses to 48.bit Ethernet Address
for Transmission on Ethernet Hardware", STD 37, RFC826, MIT,
November 1982.

[2] Laubach, M., "Classical IP and ARP over ATM", RFC1577, Hewlett-
Packard Laboratories, January 1994.

[3] Piscitello, D., and J. Lawrence, "Transmission of IP Datagrams
over the SMDS Service, RFC1209, Bell Communications Research,
March 1991.

Acknowledgements

We would like to thank John Burnett of Adaptive, Dennis Ferguson of
ANS, Joel Halpern of Network Systems, and Paul Francis of Bellcore
for their valuable insight and comments to earlier versions of this
draft.

Security Considerations

Security issues are not discussed in this memo.

Authors' Addresses

Juha Heinanen
Telecom Finland
PO Box 228
SF-33101 Tampere
Finland

Phone: +358 49 500 958
EMail: Juha.Heinanen@datanet.tele.fi

Ramesh Govindan
USC/Information Sciences Institute
4676 Admiralty Way
Marina del Rey, CA 90292

Phone: +1 310-822-1511
EMail: govindan@isi.edu