Network Working Group T. Bradley
Request for Comments: 1293 C. Brown
Wellfleet Communications, Inc.
January 1992
Inverse Address Resolution Protocol
1. Status of this Memo
This RFC specifies an IAB standards track protocol for the Internet
community, and requests discussion and suggestions for improvements.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.
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.
3. Conventions
The following language conventions are used in the items of
specification in this document:
o Must, Will, Shall or Mandatory -- the item is an absolute
requirement of the specification.
o Should or Recommended -- the item should generally be
followed for all but exceptional circumstances.
o May or Optional -- the item is truly optional and may be
followed or ignored according to the needs of the
implementor.
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
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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 are the
Frame Relay equivalent to a hardware address. Periodically, through
the exchange of signalling 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 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 as we
wanted. IP specific mechanisms were limiting since we wished to
allow protocol address resolution of many protocols. For this
reason, we expanded the ARP protocol.
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 efficiently than simulating a
broadcast with multiple copies of the same message 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
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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 between 2 and 4, and
the protocol address length is 4.
The operation code indicates the type of message, request or reply.
InARP request = 8
InARP reply = 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. A requesting
station simply formats a request by inserting its source hardware and
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 may
format a proper reply using the source addresses from the request as
the target addresses of the reply. If the station is unable or
unwilling to reply, it ignores the request.
When the requesting station receives the InARP reply, 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
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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 may choose to 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 when a new virtual circuit
is signalled. The Frame Relay station may format an InARP request
addressed to the new virtual circuit. If the other side supports
InARP, it may return a reply indicating the protocol address
requested.
The format for an InARP request is a follows:
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$sha - Q.922 address of requesting station
ar$spa - protocol address of requesting station
ar$tha - Q.922 addressed of newly announced virtual circuit
ar$tpa - 0; This is what we're looking for
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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 identical
to those for using ARP and RARP discussed in section 10 of the
Multiprotocol Interconnect over Frame Relay Networks document [3].
8. References
[1] Plummer, David C., "An Ethernet Address Resolution Protocol",
RFC-826, November 1982.
[2] Reynolds, J. and Postel, J., "Assigned Numbers", RFC-1060, ISI,
March 1990.
[3] Bradley, T., Brown, C., Malis, A., "Multiprotocol Interconnect
over Frame Relay Networks", RFC-1294, January 1992.
[4] Finlayson, Mann, Mogul, Theimer, "A Reverse Address Resolution
Protocol", RFC-903, Stanford University, June 1984.
9. Security Considerations
Security issues are not addressed in this memo.
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RFC 1293 Inverse ARP January 1992
10. Authors' Addresses
Terry Bradley
Wellfleet Communications, Inc.
15 Crosby Drive
Bedford, MA 01730
Phone: (617) 275-2400
Email: tbradley@wellfleet.com
Caralyn Brown
Wellfleet Communications, Inc.
15 Crosby Drive
Bedford, MA 01730
Phone: (617) 275-2400
Email: cbrown@wellfleet.com
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