Network Working Group T. Bradley
INTERNET-DRAFT StarBurst Communications Corp.
Obsoletes: 1293 C. Brown
<draft-ietf-ion-inarp-update-00.txt> Cadia Networks, Inc.
A. Malis
Ascom Nexion
Expires May 1997
Inverse Address Resolution Protocol
1. Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
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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
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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
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.
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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 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.
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
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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 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
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 reply
indicating the protocol address requested.
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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$sha - Q.922 address of requesting station
ar$spa - protocol address of requesting station
ar$tha - Q.922 addressed of newly announced virtual circuit
ar$tpa - 0; This is what 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
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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 [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 1700,
October 1994.
[3] Brown, C., Malis, A., "Multiprotocol Interconnect over Frame
Relay", RFC 1490, July 1993.
[4] Finlayson, Mann, Mogul, Theimer, "A Reverse Address Resolution
Protocol", RFC 903, June 1984.
9. Security Considerations
Security issues are not addressed in this memo.
10. Authors' Addresses
Terry Bradley
StarBurst Communications Corporation
150 Baker Avenue,
Concord, MA 01742
Phone: (508) 287-5560
Email: tbradley@starburstcom.com
Caralyn Brown
Cadia Networks, Inc.
1 Corporate Drive
Andover, MA 01810
Phone: (508) 689-2400 x133
Email: cbrown@cadia.com
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Andrew Malis
Ascom Nexion
289 Great Rd.
Acton, MA 01720
Phone: (508) 266-4522
Email: malis@nexen.com
Bradley, Brown, Malis [Page 7]
