RFC2390 - Inverse Address Resolution Protocol

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　　Network Working Group T. Bradley
Request for Comments: 2390 Avici Systems, Inc.
Obsoletes: 1293 C. Brown
Category: Standards Track Consultant
A. Malis
Ascend Communications, Inc.
September 1998

Inverse Address Resolution Protocol

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

2. Abstract

This memo describes additions to ARP that will allow a station to
request a protocol address corresponding to a given hardware address.
Specifically, this applies to Frame Relay stations that may have a
Data Link Connection Identifier (DLCI), the Frame Relay equivalent of
a hardware address, associated with an established Permanent Virtual
Circuit (PVC), but do not know the protocol address of the station on
the other side of this connection. It will also apply to other
networks with similar circumstances.

This memo replaces RFC1293. The changes from RFC1293 are minor
changes to formalize the language, the additions of a packet diagram
and an example in section 7.2, and a new security section.

3. Conventions

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [5].

4. Introduction

This document will rely heavily on Frame Relay as an example of how
the Inverse Address Resolution Protocol (InARP) can be useful. It is
not, however, intended that InARP be used exclusively with Frame
Relay. InARP may be used in any network that provides destination
hardware addresses without indicating corresponding protocol
addresses.

5. Motivation

The motivation for the development of Inverse ARP is a result of the
desire to make dynamic address resolution within Frame Relay both
possible and efficient. Permanent virtual circuits (PVCs) and
eventually switched virtual circuits (SVCs) are identified by a Data
Link Connection Identifier (DLCI). These DLCIs define a single
virtual connection through the wide area network (WAN) and may be
thought of as the Frame Relay equivalent to a hardware address.
Periodically, through the exchange of signaling messages, a network
may announce a new virtual circuit with its corresponding DLCI.
Unfortunately, protocol addressing is not included in the
announcement. The station receiving such an indication will learn of
the new connection, but will not be able to address the other side.
Without a new configuration or a mechanism for discovering the
protocol address of the other side, this new virtual circuit is
unusable.

Other resolution methods were considered to solve the problems, but
were rejected. Reverse ARP [4], for example, seemed like a good
candidate, but the response to a request is the protocol address of
the requesting station, not the station receiving the request. IP
specific mechanisms were limiting since they would not allow
resolution of other protocols other than IP. For this reason, the ARP
protocol was expanded.

Inverse Address Resolution Protocol (InARP) will allow a Frame Relay
station to discover the protocol address of a station associated with
the virtual circuit. It is more efficient than sending ARP messages
on every VC for every address the system wants to resolve and it is
more flexible than relying on static configuration.

6. Packet Format

Inverse ARP is an extension of the existing ARP. Therefore, it has
the same format as standard ARP.

ar$hrd 16 bits Hardware type
ar$pro 16 bits Protocol type
ar$hln 8 bits Byte length of each hardware address (n)
ar$pln 8 bits Byte length of each protocol address (m)
ar$op 16 bits Operation code
ar$sha nbytes source hardware address
ar$spa mbytes source protocol address
ar$tha nbytes target hardware address
ar$tpa mbytes target protocol address

Possible values for hardware and protocol types are the same as those
for ARP and may be found in the current Assigned Numbers RFC[2].

Length of the hardware and protocol address are dependent on the
environment in which InARP is running. For example, if IP is running
over Frame Relay, the hardware address length is either 2, 3, or 4,
and the protocol address length is 4.

The operation code indicates the type of message, request or
response.

InARP request = 8
InARP response = 9

These values were chosen so as not to conflict with other ARP
extensions.

7. Protocol Operation

Basic InARP operates essentially the same as ARP with the exception
that InARP does not broadcast requests. This is because the hardware
address of the destination station is already known.

When an interface supporting InARP becomes active, it should initiate
the InARP protocol and format InARP requests for each active PVC for
which InARP is active. To do this, a requesting station simply
formats a request by inserting its source hardware, source protocol
addresses and the known target hardware address. It then zero fills
the target protocol address field. Finally, it will encapsulate the
packet for the specific network and send it directly to the target
station.

Upon receiving an InARP request, a station may put the requester's
protocol address/hardware address mapping into its ARP cache as it
would any ARP request. Unlike other ARP requests, however, the
receiving station may assume that any InARP request it receives is
destined for it. For every InARP request, the receiving station
should format a proper response using the source addresses from the
request as the target addresses of the response. If the station is
unable or unwilling to reply, it ignores the request.

When the requesting station receives the InARP response, it may
complete the ARP table entry and use the provided address
information. Note: as with ARP, information learned via InARP may be
aged or invalidated under certain circumstances.

7.1. Operation with Multi-Addressed Hosts

In the context of this discussion, a multi-addressed host will refer
to a host that has multiple protocol addresses assigned to a single
interface. If such a station receives an InARP request, it must
choose one address with which to respond. To make such a selection,
the receiving station must first look at the protocol address of the
requesting station, and then respond with the protocol address
corresponding to the network of the requester. For example, if the
requesting station is probing for an IP address, the responding
multi-addressed station should respond with an IP address which
corresponds to the same subnet as the requesting station. If the
station does not have an address that is appropriate for the request
it should not respond. In the IP example, if the receiving station
does not have an IP address assigned to the interface that is a part
of the requested subnet, the receiving station would not respond.

A multi-addressed host should send an InARP request for each of the
addresses defined for the given interface. It should be noted,
however, that the receiving side may answer some or none of the
requests depending on its configuration.

7.2. Protocol Operation Within Frame Relay

One case where Inverse ARP can be used is on a frame relay interface
which supports signaling of DLCIs via a data link management
interface. An InARP equipped station connected to such an interface
will format an InARP request and address it to the new virtual
circuit. If the other side supports InARP, it may return a response
indicating the protocol address requested.

In a frame relay environment, InARP packets are encapsulated using
the NLPID/SNAP format defined in [3] which indicates the ARP
protocol. Specifically, the packet encapsulation will be as follows:

+----------+----------+
| Q.922 address |
+----------+----------+
|ctrl 0x03 | pad 00 |
+----------+----------+
|nlpid 0x80| oui 0x00 |
+----------+ +
| oui (cont) 0x00 00 |
+----------+----------+
| pid 0x08 06 |
+----------+----------+
| . |
| . |

The format for an InARP request itself is defined by the following:

ar$hrd - 0x000F the value assigned to Frame Relay
ar$pro - protocol type for which you are searching
(i.e. IP = 0x0800)
ar$hln - 2,3, or 4 byte addressing length
ar$pln - byte length of protocol address for which you
are searching (for IP = 4)
ar$op - 8; InARP request
ar$sha - Q.922 [6] address of requesting station
ar$spa - protocol address of requesting station
ar$tha - Q.922 address of newly announced virtual circuit
ar$tpa - 0; This is what is being requested

The InARP response will be completed similarly.

ar$hrd - 0x000F the value assigned to Frame Relay
ar$pro - protocol type for which you are searching
(i.e. IP = 0x0800)
ar$hln - 2,3, or 4 byte addressing length
ar$pln - byte length of protocol address for which you
are searching (for IP = 4)
ar$op - 9; InARP response
ar$sha - Q.922 address of responding station
ar$spa - protocol address requested
ar$tha - Q.922 address of requesting station
ar$tpa - protocol address of requesting station

Note that the Q.922 addresses specified have the C/R, FECN, BECN, and
DE bits set to zero.

Procedures for using InARP over a Frame Relay network are as follows:

Because DLCIs within most Frame Relay networks have only local
significance, an end station will not have a specific DLCI assigned
to itself. Therefore, such a station does not have an address to put
into the InARP request or response. Fortunately, the Frame Relay
network does provide a method for obtaining the correct DLCIs. The
solution proposed for the locally addressed Frame Relay network below
will work equally well for a network where DLCIs have global
significance.

The DLCI carried within the Frame Relay header is modified as it
traverses the network. When the packet arrives at its destination,
the DLCI has been set to the value that, from the standpoint of the
receiving station, corresponds to the sending station. For example,
in figure 1 below, if station A were to send a message to station B,
it would place DLCI 50 in the Frame Relay header. When station B
received this message, however, the DLCI would have been modified by
the network and would appear to B as DLCI 70.

~~~~~~~~~~~~~~~
( )
+-----+ ( ) +-----+
| |-50------(--------------------)---------70-| |
| A | ( ) | B |
| |-60-----(---------+ ) | |
+-----+ ( | ) +-----+
( | )
( | ) <---Frame Relay
~~~~~~~~~~~~~~~~ network
80
|
+-----+
| |
| C |
| |
+-----+

Figure 1

Lines between stations represent data link connections (DLCs).
The numbers indicate the local DLCI associated with each
connection.

DLCI to Q.922 Address Table for Figure 1

DLCI (decimal) Q.922 address (hex)
50 0x0C21
60 0x0CC1
70 0x1061
80 0x1401

For authoritative description of the correlation between DLCI and
Q.922 [6] addresses, the reader should consult that specification.
A summary of the correlation is included here for convenience. The
translation between DLCI and Q.922 address is based on a two byte
address length using the Q.922 encoding format. The format is:

8 7 6 5 4 3 2 1
+------------------------+---+--+
| DLCI (high order) |C/R|EA|
+--------------+----+----+---+--+
| DLCI (lower) |FECN|BECN|DE |EA|
+--------------+----+----+---+--+

For InARP, the FECN, BECN, C/R and DE bits are assumed to be 0.

When an InARP message reaches a destination, all hardware addresses
will be invalid. The address found in the frame header will,
however, be correct. Though it does violate the purity of layering,
Frame Relay may use the address in the header as the sender hardware
address. It should also be noted that the target hardware address,
in both the InARP request and response, will also be invalid. This
should not cause problems since InARP does not rely on these fields
and in fact, an implementation may zero fill or ignore the target
hardware address field entirely.

Using figure 1 as an example, station A may use Inverse ARP to
discover the protocol address of the station associated with its DLCI
50. The Inverse ARP request would be as follows:

InARP Request from A (DLCI 50)
ar$op 8 (InARP request)
ar$sha unknown
ar$spa pA
ar$tha 0x0C21 (DLCI 50)
ar$tpa unknown

When Station B receives this packet, it will modify the source
hardware address with the Q.922 address from the Frame Relay header.
This way, the InARP request from A will become:

ar$op 8 (InARP request)
ar$sha 0x1061 (DLCI 70)
ar$spa pA
ar$tha 0x0C21 (DLCI 50)
ar$tpa unknown.

Station B will format an Inverse ARP response and send it to station
A:

ar$op 9 (InARP response)
ar$sha unknown
ar$spa pB
ar$tha 0x1061 (DLCI 70)
ar$tpa pA

The source hardware address is unknown and when the response is
received, station A will extract the address from the Frame Relay
header and place it in the source hardware address field. Therefore,
the response will become:

ar$op 9 (InARP response)
ar$sha 0x0C21 (DLCI 50)
ar$spa pB
ar$tha 0x1061 (DLCI 70)
ar$tpa pA

This means that the Frame Relay interface must only intervene in the
processing of incoming packets.

Also, see [3] for a description of similar procedures for using ARP
[1] and RARP [4] with Frame Relay.

8. Security Considerations

This document specifies a functional enhancement to the ARP family of
protocols, and is subject to the same security constraints that
affect ARP and similar address resolution protocols. Because
authentication is not a part of ARP, there are known security issues
relating to its use (e.g., host impersonation). No additional
security mechanisms have been added to the ARP family of protocols by
this document.

9. 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, November
1982.

[2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC1700,
October 1994. See also: http://www.iana.org/numbers.html

[3] Bradley, T., Brown, C., and A. Malis, "Multiprotocol Interconnect
over Frame Relay", RFC1490, July 1993.

[4] Finlayson, R., Mann, R., Mogul, J., and M. Theimer, "A Reverse
Address Resolution Protocol", STD 38, RFC903, June 1984.

[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.

[6] Information technology - Telecommunications and Information
Exchange between systems - Protocol Identification in the Network
Layer, ISO/IEC TR 9577: 1992.

10. Authors' Addresses

Terry Bradley
Avici Systems, Inc.
12 Elizabeth Drive
Chelmsford, MA 01824

Phone: (978) 250-3344
EMail: tbradley@avici.com

Caralyn Brown
Consultant

EMail: cbrown@juno.com

Andrew Malis
Ascend Communications, Inc.
1 Robbins Road
Westford, MA 01886

Phone: (978) 952-7414
EMail: malis@ascend.com

11. Full Copyright Statement

Copyright (C) The Internet Society (1998). All Rights Reserved.

This document and translations of it may be copied and furnished to
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