rfc827.txt

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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


































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     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.



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     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



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     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

     comes  into  being,  it  will  be inappropriate to regard any one

     autonomous  system  as  a  "core"  system.   For  the   sake   of

     concreteness, however, and because the initial implementations of

     the Exterior Gateway Protocol are expected to focus  on  the  the

     case  of  connecting  "stub  gateways"  to  the DARPA gateways on

     ARPANET and SATNET, we will often use the term "core" gateways in

     our examples and discussion.


          The purpose of the Exterior Gateway  Protocol  (EGP)  is  to

     enable  one  or  more  autonomous systems to be used as transport

     media for traffic originating in some other autonomous system and

     destined  for yet another, while allowing the end-user to see the

     composite of all the autonomous systems  as  a  single  internet,

     with  a  flat, uniform address space.  The route which a datagram

     takes through the internet, and the number of autonomous  systems

     which  it  traverses,  are  to  be  transparent  to  the end-user

     (unless, of course, the end-user makes  use  of  the  IP  "source

     route" option).


          In  describing  the  Exterior  Gateway  Protocol,  we   have

     deliberately  left  a great deal of latitude to the designers and

     implementers of particular autonomous systems, particularly  with

     regard to timer values.  We have done this because we expect that



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



     different  gateway   implementations   and   different   internet

     environments  may  just have different requirements and goals, so

     that no single strict implementation specification could apply to

     all.   However,  this does NOT mean that ANY implementation which

     conforms to the specification will work well, or that  the  areas

     in  which  we  have left latitude are not crucial to performance.

     The fact that some time-out value, for example, is not  specified

     here does not mean that everything will work no matter what value

     is assigned.


          Autonomous systems will be  assigned  16-bit  identification

     numbers  (in  much  the same ways as network and protocol numbers

     are now assigned), and every EGP message header contains one word

     for  this  number.   Zero  will not be assigned to any autonomous

     system; rather, the  presence  of  a  zero  in  this  field  will

     indicate that no number is present.


          We need to introduce the concept  of  one  gateway  being  a

     NEIGHBOR  of  another.   In the simplest and most common case, we

     call two gateways "neighbors" if there is a network to which each

     has  an interface.  However, we will need a somewhat more general

     notion of "neighbor" to allow the following two cases:


          a) Two gateways may be regarded as  neighbors  if  they  are

             directly  connected  not by a network (in the usual sense




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



             of the term), but by a simple wire, or HDLC line, or some

             similar means of "direct connection".


          b) Two gateways may be regarded as  neighbors  if  they  are

             connected  by an "internet" which is transparent to them.

             That is, we would  like  to  be  able  to  say  that  two

             gateways  are  neighbors even if they are connected by an

             internet, as long as the gateways utilize no knowledge of

             the  internal  structure  of  that  internet in their own

             packet-forwarding algorithms.


     In order to handle all these cases, let us say that two  gateways

     are NEIGHBORS if they are connected by some communications medium

     whose internal structure is transparent to them.   (See  IEN  184

     for a more general discussion of this notion of neighbor.)


          If two neighbors are part of the same autonomous system,  we

     call  them  INTERIOR  NEIGHBORS; if two neighbors are not part of

     the same autonomous system, we call them EXTERIOR NEIGHBORS.   In

     order  for  one  system  to  use  another  as a transport medium,

     gateways which are exterior neighbors of each other must be  able

     to find out which networks can be reached through the other.  The

     Exterior Gateway Protocol enables this information to  be  passed

     between  exterior  neighbors.  Since it is a polling protocol, it

     also enables each gateway to control the rate at which  it  sends




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



     and  receives  network  reachability  information,  allowing each

     system to control its own overhead.  It also enables each  system

     to  have  an independent routing algorithm whose operation cannot

     be disrupted by failures of other systems.


          It must be clearly understood that any autonomous system  in

     which  routing  needs  to be performed among gateways within that

     system must implement its  own  routing  algorithm.   (A  routing

     algorithm  is  not  generally  necessary  for a simple autonomous

     system which consists of a single stub  gateway.)   The  Exterior

     Gateway Protocol is NOT a routing algorithm.  It enables exterior

     neighbors to exchange information which is likely to be needed by

     any  routing algorithm, but it does NOT specify what the gateways

     are to do with this information.  The "routing updates"  of  some

     autonomous  system's interior routing algorithm may or may not be

     similar in  format  to  the  messages  of  the  exterior  gateway

     protocol.  The gateways in the DARPA "core" system will initially

     use the GGP protocol (the old Gateway-Gateway protocol) as  their

     routing  algorithm, but this will be subject to change.  Gateways

     in other autonomous systems may use their  own  Interior  Gateway

     Protocols  (IGPs),  which may or may not be similar to the IGP of

     any other autonomous system.  They may, of course, use  GGP,  but

     will  not  be permitted to exchange GGP messages with gateways in

     other autonomous systems.




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



          It must also be clearly understood that the Exterior Gateway

     Protocol  is  NOT  intended to provide information which could be

     used as input  to  a  completely  general  area  or  hierarchical

     routing  algorithm.   It  is  intended  for  a  set of autonomous

     systems which are connected in a tree, with no cycles.   It  does

     not  enable  the  passing  of  sufficient  information to prevent

     routing loops if cycles in the topology do exist.


          The Exterior Gateway Protocol has three parts: (a)  Neighbor

     Acquisition Protocol, (b) Neighbor Reachability Protocol, and (c)

     Network  Reachability  determination.   Note  that  all  messages

     defined  by EGP are intended to travel only a single "hop".  That

     is, they originate at one gateway and are sent to  a  neighboring

     gateway   without  the  mediation  of  any  intervening  gateway.

     Therefore, the time-to-live field should be set to a  very  small

     value.   Gateways  which  encounter EGP messages in their message

     streams which are not addressed to them may discard them.


















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



     2  NEIGHBOR ACQUISITION