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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/REA

+--------------+----+----+---+--+

DLCI (lower) FECNBECNDE 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

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

 
 
 
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