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Using ARP to implement transparent subnet gateways (RFC1027)

IP.com Disclosure Number: IPCOM000001831D
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2000-Sep-12
Document File: 7 page(s) / 20K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

S. Carl-Mitchell: AUTHOR [+2]

Abstract

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.

This text was extracted from a ASCII Text document.
This is the abbreviated version, containing approximately 15% of the total text.

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.

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 ga...