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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.




                                   - 6 -

 
     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