Using ARP to implement transparent subnet gateways
RFC 1027

Document Type RFC - Unknown (October 1987; Errata)
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Network Working Group                                Smoot Carl-Mitchell
Request for Comments: 1027                     Texas Internet Consulting
                                                      John S. Quarterman
                                               Texas Internet Consulting
                                                            October 1987

           Using ARP to Implement Transparent Subnet Gateways

Status of this Memo

    This RFC describes the use of the Ethernet Address Resolution
    Protocol (ARP) by subnet gateways to permit hosts on the connected
    subnets to communicate without being aware of the existence of
    subnets, using the technique of "Proxy ARP" [6].  It is based on
    RFC-950 [1], RFC-922 [2], and RFC-826 [3] and is a restricted subset
    of the mechanism of RFC-925 [4].  Distribution of this memo is
    unlimited.

Acknowledgment

    The work described in this memo was performed while the authors were
    employed by the Computer Sciences Department of the University of
    Texas at Austin.

Introduction

    The purpose of this memo is to describe in detail the implementation
    of transparent subnet ARP gateways using the technique of Proxy ARP.
    The intent is to document this widely used technique.

1.  Motivation

    The Ethernet at the University of Texas at Austin is a large
    installation connecting over ten buildings.  It currently has more
    than one hundred hosts connected to it [5].  The size of the
    Ethernet and the amount of traffic it handles prohibit tying it
    together by use of repeaters.  The use of subnets provided an
    attractive alternative for separating the network into smaller
    distinct units.

    This is exactly the situation for which Internet subnets as
    described in RFC-950 are intended.  Unfortunately, many vendors had
    not yet implemented subnets, and it was not practical to modify the
    more than half a dozen different operating systems running on hosts
    on the local networks.

Carl-Mitchell & Quarterman                                      [Page 1]
RFC 1027          ARP and Transparent Subnet Gateways       October 1987

    Therefore a method for hiding the existence of subnets from hosts
    was highly desirable.  Since all the local area networks supported
    ARP, an ARP-based method (commonly known as "Proxy ARP" or the "ARP
    hack") was chosen.  In this memo, whenever the term "subnet" occurs
    the "RFC-950 subnet method" is assumed.

2.  Design

2.1  Basic method

    On a network that supports ARP, when host A (the source) broadcasts
    an ARP request for the network address corresponding to the IP
    address of host B (the target), host B will recognize the IP address
    as its own and will send a point-to-point ARP reply.  Host A keeps
    the IP-to-network-address mapping found in the reply in a local
    cache and uses it for later communication with host B.

    If hosts A and B are on different physical networks, host B will not
    receive the ARP broadcast request from host A and cannot respond to
    it.  However, if the physical network of host A is connected by a
    gateway to the physical network of host B, the gateway will see the
    ARP request from host A.  Assuming that subnet numbers are made to
    correspond to physical networks, the gateway can also tell that the
    request is for a host that is on a different physical network from
    the requesting host.  The gateway can then respond for host B,
    saying that the network address for host B is that of the gateway
    itself.  Host A will see this reply, cache it, and send future IP
    packets for host B to the gateway.  The gateway will forward such
    packets to host B by the usual IP routing mechanisms.  The gateway
    is acting as an agent for host B, which is why this technique is
    called "Proxy ARP"; we will refer to this as a transparent subnet
    gateway or ARP subnet gateway.

    When host B replies to traffic from host A, the same algorithm
    happens in reverse: the gateway connected to the network of host B
    answers the request for the network address of host A, and host B
    then sends IP packets for host A to gateway.  The physical networks
    of host A and B need not be connected to the same gateway. All that
    is necessary is that the networks be reachable from the gateway.

    With this approach, all ARP subnet handling is done in the ARP
    subnet gateways.  No changes to the normal ARP protocol or routing
    need to be made to the source and target hosts.  From the host point
    of view, there are no subnets, and their physical networks are
    simply one big IP network.  If a host has an implementation of
    subnets, its network masks must be set to cover only the IP network
    number, excluding the subnet bits, for the system to work properly.

Carl-Mitchell & Quarterman                                      [Page 2]
RFC 1027          ARP and Transparent Subnet Gateways       October 1987
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