Internet DRAFT - draft-heinanen-inarp-uni
Internet Engineering Task Force Juha Heinanen
INTERNET DRAFT Telia Finland
Expires September 2001 March, 2001
Inverse ARP over Unidirectional Virtual Circuits
Status of this Memo
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This memo describes operation of an Inverse Address Resolution
Protocol (InARP) over unidirectional virtual circuits such as MPLS
Inverse Address Resolution Protocol (InARP)  is commonly used by
stations (usually routers) connected via Frame Relay or ATM virtual
circuits to automatically learn the protocol addresses of their
peers. InARP is needed when a station only knows that a virtual
circuit to another station exists, but doesn't have any knowledge of
protocol layer identity of the other station. This can happen either
if the virtual circuit is network provisioned or if some other
address than the protocol address of the other station is used in the
virtual circuit setup.
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When a Frame Relay or ATM local station has discovered the hardware
address (Frame Relay DLCI or ATM VPI/VCI) of a remote station, it
sends an InARP Request to query the protocol address of the remote
station. The remote station learns the protocol address of the local
station from the source protocol address field of the InARP request
and the corresponding hardware address from the frame header of the
InARP request. The remote station then sends an InARP response
containing its own protocol address to the learned hardware address.
The above procedure does not work if the stations are connected via
unidirectional virtual circuits, such as network provisioned MPLS
LSPs. In order to be able migrate from network provisioned Frame
Relay or ATM virtual circuits to network provisioned MPLS LSPs, a new
version of InARP is needed. This memo describes the operation of
InARP in situations where one or more unidirectional virtual circuits
are used to implement bidirectional connectivity between two
2. Protocol Operation
Once the local station (A) learns the hardware address (label) of an
outgoing unidirectional virtual circuit, it constructs an InARP
request to find out the protocol address of the remote station (B) to
which this virtual circuit leads to. The InARP request contains the
protocol address (pA) and hardware address (hA) of the local station
in the source protocol and hardware address fields, respectively:
ar$op 8 (InARP request)
When the remote station (B) receives the request, it constructs a
response by including its own protocol address (pB) in the source
protocol address field and by copying the source protocol and
hardware addresses from the request to the target protocol and
hardware address fields, respectively:
ar$op 9 (InARP response)
Because of unidirectionality of the virtual circuits, the remote
station can't use the either the source hardware address in the
request or the hardware address in the frame header to send the
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response back to the local station. Instead, the remote station
first checks if it itself has already learned about a virtual
circuit, which has the same target protocol address as the source
protocol address in the request. If so, the remote station sends the
response to such a virtual circuit. If not, the remote station sends
the response to every virtual circuit whose target protocol address
is still unknown to it.
When the local station receives an InARP response, it first checks if
the target address pair of the response matches an existing outgoing
virtual circuit. If so, it creates a new protocol address/hardware
address mapping for the virtual circuit based on the protocol address
and hardware address fields of the response. I not, it silently
discards the response.
Once the local station unlearns the hardware address (label) of an
outgoing virtual circuit, it deletes the protocol address/hardware
address mapping that was associated with it. Even if the local
station doesn't unlearn a hardware address, it may be desirable to
age the address/hardware address mapping after a time period. The
implementation of aging (if any) is outside the scope of this memo.
3. Scalability Considerations
The above operation could potentially result in generation of a large
number of simultaneous InARP responses. The worst case occurs when a
full mesh of virtual circuits connecting N stations is created
simultaneously and each local station sends simultaneously N-1 InARP
requests to each of which every remote station (having not yet
learned any addresses) replies with N-1 InARP responses.
Although it is not likely in practice that all virtual circuits are
created simultaneously, InARP implementations can also help to
alleviate the problem. The local stations could wait a random time
interval after virtual circuit discovery before sending out their
InARP requests. That would creating an effect similar to as if the
stations and their virtual circuits had been added one at a time.
4. 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
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I would like to thank Joel Halpern of Longitude Systems for his
constructive comments on earlier versions of this memo.
 Bradley, T., Brown, C., and Malis, A., Inverse Address Resolution
Protocol. RFC 2390, September 1998.
Telia Finland, Inc.
33200 Tampere, Finland
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