Exterior Gateway Protocol (EGP)
RFC 827

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RFC 827

                      EXTERIOR GATEWAY PROTOCOL (EGP)

                               Eric C. Rosen

                       Bolt Beranek and Newman Inc.

                               October 1982

It is proposed to establish a standard for Gateway to Gateway procedures
that allow the Gateways to be mutually suspicious.  This document is a
DRAFT for that standard.  Your comments are strongly encouraged.



     RFC 827                              Bolt Beranek and Newman Inc.
                                                         Eric C. Rosen

                             Table of Contents

     1   INTRODUCTION.......................................... 1
     2   NEIGHBOR ACQUISITION.................................. 8
     3   NEIGHBOR REACHABILITY PROTOCOL....................... 11
     4   NETWORK REACHABILITY (NR) MESSAGE.................... 15
     5   POLLING FOR NR MESSAGES.............................. 22
     6   SENDING NR MESSAGES.................................. 25
     7   INDIRECT NEIGHBORS................................... 27
     8   HOW TO BE A STUB GATEWAY............................. 28
     9   LIMITATIONS.......................................... 32

                                   - i -



     RFC 827                              Bolt Beranek and Newman Inc.
                                                         Eric C. Rosen

     1  INTRODUCTION

          The DARPA Catenet is expected to be a continuously expanding

     system,  with  more  and  more  hosts  on  more and more networks

     participating in it.  Of course, this will require more and  more

     gateways.   In  the  past,  such  expansion  has taken place in a

     relatively unstructured manner.  New gateways,  often  containing

     radically different software than the existing gateways, would be

     added and would immediately begin  participating  in  the  common

     routing algorithm via the GGP protocol.  However, as the internet

     grows larger and larger, this simple method of expansion  becomes

     less and less feasible.  There are a number of reasons for this:

          - the overhead of the routing algorithm becomes  excessively

            large;

          - the  proliferation   of   radically   different   gateways

            participating  in  a single common routing algorithm makes

            maintenance and fault isolation nearly  impossible,  since

            it  becomes  impossible  to  regard  the  internet  as  an

            integrated communications system;

          - the  gateway  software  and  algorithms,  especially   the

            routing  algorithm, become too rigid and inflexible, since

            any proposed change must be made  in  too  many  different

            places and by too many different people.

                                   - 1 -



     RFC 827                              Bolt Beranek and Newman Inc.
                                                         Eric C. Rosen

          In the future, the internet is expected to evolve into a set

     of  separate  domains  or  "autonomous  systems",  each  of which

     consists of a set of one or more relatively homogeneous gateways.

     The  protocols,  and  in  particular  the routing algorithm which

     these gateways use among themselves, will be  a  private  matter,

     and  need never be implemented in gateways outside the particular

     domain or system.

          In the simplest case, an autonomous system might consist  of

     just a single gateway connecting, for example, a local network to

     the ARPANET.  Such a gateway might be called  a  "stub  gateway",

     since  its  only purpose is to interface the local network to the

     rest of the internet, and it is  not  intended  to  be  used  for

     handling  any traffic which neither originated in nor is destined

     for that particular local network.  In the near-term  future,  we

     will  begin  to  think  of  the  internet  as a set of autonomous

     systems, one of which consists of the DARPA gateways  on  ARPANET

     and  SATNET,  and  the others of which are stub gateways to local

     networks.   The former system, which we  shall  call  the  "core"

     system,  will be used as a transport or "long-haul" system by the

     latter systems.

          Ultimately, however, the internet may consist of a number of

     co-equal  autonomous  systems,  any  of  which  may be used (with

     certain  restrictions  which  will  be  discussed  later)  as   a

                                   - 2 -



     RFC 827                              Bolt Beranek and Newman Inc.
                                                         Eric C. Rosen

     transport  medium  for  traffic  originating  in  any  system and

     destined for any system.  When this  more  complex  configuration