Using ARP to implement transparent subnet gateways
RFC 1027
|
Document |
Type |
|
RFC - Unknown
(October 1987; No errata)
|
|
Authors |
|
|
|
Last updated |
|
2020-07-29
|
|
Stream |
|
Legacy stream
|
|
Formats |
|
plain text
html
pdf
htmlized (tools)
htmlized
bibtex
|
Stream |
Legacy state
|
|
(None)
|
|
Consensus Boilerplate |
|
Unknown
|
|
RFC Editor Note |
|
(None)
|
IESG |
IESG state |
|
RFC 1027 (Unknown)
|
|
Telechat date |
|
|
|
Responsible AD |
|
(None)
|
|
Send notices to |
|
(None)
|
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
Show full document text