1. INTRODUCTION

The Exterior Gateway Protocol (EGP) [Rosen82; Seamonson&Rosen84; Mills84a]
has been specified to allow autonomous development of different gateway systems
while  still  maintaining  global distribution of internet routing information.
EGP provides a means for  different  autonomous  gateway  systems  to  exchange
information about the networks that are reachable via them.

This  report  mainly  describes  an  implementation  of EGP that runs as a user
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process under the Berkeley Unix  4.2 operating system run on a VAX    computer.
Some  related issues concerning local autonomous system configurations are also
discussed.

The EGP implementation is experimental and is not a part of Unix 4.2 BSD. It is
anticipated that Berkeley will incorporate a version of EGP in the future.

The program is written in C. The EGP  part  is  based  on  the  C-Gateway  code
written  by  Liza  Martin at MIT and the route management part is based on Unix
4.2 BSD route management daemon, "routed".

The EGP functions are consistent with the specification of [Mills84a]  except
where noted.

A  knowledge  of  EGP  as  described  in  [Seamonson&Rosen84; Mills84a] is
assumed.

This chapter discusses the motivation for the project, Chapter 2 describes  the
gateway  design,  Chapter 3 is on testing, Chapter 4 suggests some enhancements
and Chapter 5 discusses topology issues.

Further information about running the EGP program and describing  the  software
is being published in an ISI Research Report ISI/RR-84-145 [Kirton84].

Requests  for  documentation  and  copies  of the EGP program should be sent to
Joyce Reynolds (JKReynolds@USC-ISIF.ARPA). Software support is not provided.

1.1. Motivation for Development

With the introduction of EGP, the internet gateways  will  be  divided  into  a
"core"  autonomous  system  (AS)  of  gateways  maintained by Bolt, Beranek and
Newman  (BBN)  and  many  "stub"  AS's  that  are   maintained   by   different
organizations  and  have at least one network in common with a core AS gateway.
The core AS will act as a  hub  for  passing  on  routing  information  between

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   Unix is a trade mark of AT&T
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    VAX is a trade mark of Digital Equipment Corporation


different  stub AS's so that it will only be necessary for stub AS's to conduct
EGP with a core gateway. Further detail is given in [Rosen82].

At the time of this  project  there  were  28  "non-routing"  gateways  in  the
internet.  Non-routing  gateways  did  not  exchange  routing  information  but
required static entries in the core gateway routing tables.   Since  August  1,
1984  these  static  entries  have  been  eliminated and previously non-routing
gateways are required to communicate this  information  to  the  core  gateways
dynamically via EGP [Postel84].

At the USC Information Sciences Institute (ISI) there was a non-routing gateway
to  the  University  of  California  at  Irvine  network  (UCI-ICS).  With  the
elimination of  non-routing  gateways  from  the  core  gateway  tables  it  is
necessary to inform the core ISI gateway of the route to UCI-ICS using EGP.

Also,  we  would  like a backup gateway between ISI-NET and the ARPANET in case
the core ISI gateway is down. Such, a gateway  would  need  to  convey  routing
information  via EGP. Details of the ISI network configuration are discussed in
Section 5.2.

Of the 28 non-routing gateways 23 were implemented by Unix  systems,  including
ISI's.  Also, ISI's proposed backup gateway was a Unix system. Thus there was a
local and general need for an EGP implementation to run under Unix. The current
version  of  Unix  that  included  Department  of  Defense  (DoD) protocols was
Berkeley Unix 4.2 so this was selected.

1.2. Overview of EGP

This report assumes a knowledge of EGP, however a brief overview is given  here
for completeness. For further details refer to [Rosen82] for the background to
EGP,  [Seamonson&Rosen84] for an informal description, and [Mills84a] for a
more formal specification and implementation details.

EGP is generally conducted between gateways in  different  AS's  that  share  a
common network, that is, neighbor gateways.

EGP  consists  of three procedures, neighbor acquisition, neighbor reachability
and network reachability.

Neighbor acquisition is a two way handshake in which gateways agree to  conduct
EGP  by exchanging Request and Confirm messages which include the minimum Hello
and Poll  intervals.    Acquisition  is  terminated  by  exchanging  Cease  and
Cease-ack messages.

Neighbor  reachability  is  a  periodic exchange of Hello commands and I-H-U (I
heard you) responses to ensure that each gateway is up. Currently a  30  second
minimum interval is used across ARPANET. Only one gateway need send commands as
the   other   can  use  them  to  determine  reachability.  A  gateway  sending
reachability commands is said to be in the active mode, while  a  gateway  that
just responds is in the passive mode.

Network  reachability  is  determined by periodically sending Poll commands and
receiving Update responses which indicate the networks  reachable  via  one  or
more  gateways  on  the  shared network. Currently 2 minute minimum interval is
used across ARPANET.