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                                  RFC 888


                     "STUB" EXTERIOR GATEWAY PROTOCOL


                            Linda J. Seamonson

                               Eric C. Rosen


                            BBN Communications


                               January 1984










This note describes the Exterior Gateway Protocol used to connect Stub
Gateways to an Autonomous System of core Gateways.  This document specifies
the working protocol, and defines an ARPA official protocol.  All
implementers of Gateways should carefully review this document.















     RFC 888                                              JANUARY 1984



                             Table of Contents





     1   INTRODUCTION.......................................... 1

     2   DEFINITIONS AND OVERVIEW.............................. 4

     3   NEIGHBOR ACQUISITION.................................. 7

     4   NEIGHBOR REACHABILITY PROTOCOL....................... 10

     5   NETWORK REACHABILITY (NR) MESSAGE.................... 15

     6   POLLING FOR NR MESSAGES.............................. 22

     7   SENDING NR MESSAGES.................................. 24

     8   INDIRECT NEIGHBORS................................... 26

     9   LIMITATIONS.......................................... 27

     A   APPENDIX A - EGP MESSAGE FORMATS..................... 28
     A.1   NEIGHBOR ACQUISITION MESSAGE....................... 28
     A.2   NEIGHBOR HELLO/I HEARD YOU MESSAGE................. 30
     A.3   NR POLL MESSAGE.................................... 32
     A.4   NETWORK REACHABILITY MESSAGE....................... 34
     A.5   EGP ERROR MESSAGE.................................. 37

















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     RFC 888                                              JANUARY 1984



     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



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     RFC 888                                              JANUARY 1984



            any proposed change  must be made in  too  many  different

            places   and   by   too   many   different        people.




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

     of  separate  sections 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

     sections 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"




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     RFC 888                                              JANUARY 1984



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

     latter systems.


          Ultimately, the internet may consist of a number of co-equal

     autonomous  systems,  any  of  which  may  be used as a transport

     medium for traffic originating in any system and destined for any

     system.  This more general case is still the subject of research.

     This paper describes only how stub gateways connect to  the  core

     system using the Exterior Gateway Protocol (EGP).































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     RFC 888                                              JANUARY 1984



     2  DEFINITIONS AND OVERVIEW


          For the purposes of this paper, a "stub gateway" is  defined

     as follows:


          - it is not a core gateway

          - it shares a network with at least one core gateway (has an

            interface on the same network as some core gateway)

          - it has interfaces to one or more networks  which  have  no

            core gateways

          - all other nets which are reachable from  the  core  system

            via  the stub have no other path to the core system except

            via the stub



          The stub gateway is expected to fully execute  the  Internet

     Control Message Protocol (ICMP), as well as the EGP protocol.  In

     particular, it must respond to ICMP echo requests, and must  send

     ICMP  destination  dead  messages  as  appropriate.   It  is also

     required to send ICMP Redirect messages as appropriate.



          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 a  field




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     RFC 888                                              JANUARY 1984



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

     system; the use  of  zero  as  an  autonomous  system  number  is

     reserved for future use.


          We call two gateways "neighbors" if there is  a  network  to

     which  each  has  an interface.  If two neighbors are part of the

     same autonomous system, we  call  them  INTERIOR  NEIGHBORS;  for

     example,  any  two core gateways on the same network are interior

     neighbors of each other.  If two neighbors are not  part  of  the

     same  autonomous  system,  we  call  them EXTERIOR NEIGHBORS; for

     example, a stub gateway and any core gateway that share a network

     are exterior neighbors of each other.  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  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.





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     RFC 888                                              JANUARY 1984



          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.


          Each EGP message contains a sequence  number.   The  gateway

     should maintain one sequence number per neighbor.

























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     RFC 888                                              JANUARY 1984



     3  NEIGHBOR ACQUISITION


          Before it is possible to obtain routing information from  an

     exterior  gateway,  it  is necessary to acquire that gateway as a

     direct neighbor.  (The distinction between  direct  and  indirect

     neighbors  will  be  made  in a later section.)  In order for two

     gateways to become direct neighbors, they must be  neighbors,  in

     the  sense  defined  above,  and  they  must execute the NEIGHBOR

     ACQUISITION  PROTOCOL,  which  is  simply  a   standard   two-way

     handshake.


          A gateway that wishes to initiate neighbor acquisition  with

     another  sends  it  a Neighbor Acquisition Request.  This message

     should be repeatedly transmitted (at a reasonable  rate,  perhaps

     once  every  30 seconds or so) until a Neighbor Acquisition Reply

     or a Neighbor Acquisition Refusal is received.  The Request  will

     contain  an  identification number which is copied into the reply

     so that request and reply can be matched up.


          A gateway receiving  a  Neighbor  Acquisition  Request  must

     determine  whether  it  wishes to become a direct neighbor of the

     source of the Request.  If not, it may, at  its  option,  respond

     with   a   Neighbor   Acquisition   Refusal  message,  optionally

     specifying the reason for refusal.  Otherwise, it should  send  a



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     RFC 888                                              JANUARY 1984



     Neighbor Acquisition Reply message.


          The gateway  that  sent  the  Request  should  consider  the

     Neighbor Acquisition complete when it has received the neighbor's

     Reply.  The gateway that  sent  the  Reply  should  consider  the

     acquisition complete when it has sent the Reply.


          Unmatched Replies or Refusals should be  discarded  after  a

     reasonable  period  of time.  However, information about any such

     unmatched messages may be useful for diagnostic purposes.


          A Neighbor Acquisition  Request  from  a  gateway  which  is

     already a direct neighbor should be responded to with a Reply.


          A Neighbor Acquisition Request or Reply from  gateway  G  to

     gateway  G'  carries the minimum interval in seconds with which G

     is willing to answer Neighbor Reachability Hello Messages from G'

     and the minimum interval in seconds with which G is willing to be

     polled for NR messages (see below).


          If  a  gateway  wishes  to  cease  being  a  neighbor  of  a

     particular  exterior  gateway, it sends a Neighbor Cease message.

     A gateway  receiving  a  Neighbor  Cease  message  should  always

     respond with a Neighbor Cease Acknowledgment.  It should cease to

     treat the sender of the message as a neighbor in any way.   Since



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     RFC 888                                              JANUARY 1984



     there  is  a  significant  amount  of protocol run between direct

     neighbors (see below), if some gateway no longer needs  to  be  a

     direct  neighbor  of  some other, it is "polite" to indicate this

     fact with a Neighbor Cease Message.  The Neighbor  Cease  Message

     should  be  retransmitted  (up  to some number of times) until an

     acknowledgment for it is received.


          Once  a  Neighbor  Cease  message  has  been  received,  the

     Neighbor   Reachability  Protocol  (below)  should  cease  to  be

     executed.


          A stub should have tables configured in with  the  addresses

     of  a  small  number  of  the  core gateways (no more than two or

     three) with which it has  a  common  network.   It  will  be  the

     responsibility  of the stub to initiate neighbor acquisition with

     these gateways.  If the direct neighbors of  a  stub  should  all

     fail,  it  will  be  the responsibility of the stub to acquire at

     least one new direct neighbor.  It can do so by choosing  one  of

     the  core  gateways which it has had as an indirect neighbor (see

     below), and executing the neighbor acquisition protocol with  it.

     (It  is  possible  that  no  more than one core gateway will ever

     agree to become a direct neighbor with any given stub gateway  at

     any one time.)




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     RFC 888                                              JANUARY 1984



     4  NEIGHBOR REACHABILITY PROTOCOL


          It is important for a gateway to keep real-time  information

     as  to the reachability of its neighbors.  If a gateway concludes

     that a particular neighbor cannot be  reached,  it  should  cease

     forwarding  traffic to that gateway.  To make that determination,

     a NEIGHBOR REACHABILITY protocol is  needed.   The  EGP  protocol

     provides two messages types for this purpose -- a "Hello" message

     and an "I Heard You" message.


          When a "Hello" message is received from a  direct  neighbor,

     an "I Heard You" must be returned to that neighbor "immediately".

     The delay between receiving a "Hello" and returning an  "I  Heard

     You" should never be more than a few seconds.


          Core  gateways  will  use  the   following   algorithm   for

     determining reachablility of an exterior neighbor:


          A reachable  neighbor  shall  be  declared  unreachable  if,

     during  the  time  in  which  the  core  gateway  sent its last n

     "Hello"s, it received fewer than k "I Heard You"s in return.   An

     unreachable  neighbor  shall be declared reachable if, during the

     time in which the core gateway  sent  its  last  m  "Hello"s,  it

     received at least j "I Heard You"s in return.




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     RFC 888                                              JANUARY 1984



          Stub  gateways  may  also  send  "Hello"s  to  their  direct

     neighbors  and  receive  "I Heard You"s in return.  The algorithm

     for determining reachability may  be  similar  to  the  algorithm

     described  above.  However, it is not necessary for stubs to send

     "Hello"s.  The "Hello" and "I Heard You" messages have  a  status

     field  which  the  sending  gateway  uses  to indicate whether it

     thinks  the  receiving  gateway  is  reachable  or   not.    This

     information  can  be  useful  for  diagnostic  purposes.  It also

     allows a stub gateway  to  make  its  reachability  determination

     parasitic  on  its  core neighbor: only the core gateway actually

     needs to send "Hello" messages, and the stub can declare it up or

     down based on the status field in the "Hello".  That is, the stub

     gateway (which sends only  "I  Heard  You"s)  declares  the  core

     gateway  (which  sends  only  "Hello"s)  to be reachable when the

     "Hello"s from the core indicate that it has declared the stub  to

     be reachable.


          The frequency with which the  "Hello"s  are  sent,  and  the

     values of the parameters k, n, j, and m cannot be specified here.

     For best results, this will depend on the characteristics of  the

     neighbor  and  of the network which the neighbors have in common.

     THIS IMPLIES THAT THE PROPER PARAMETERS MAY NEED TO BE DETERMINED

     JOINTLY  BY THE DESIGNERS AND IMPLEMENTERS OF THE TWO NEIGHBORING



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     RFC 888                                              JANUARY 1984



     GATEWAYS;  choosing  algorithms  and  parameters  in   isolation,

     without  considering  the characteristics of the neighbor and the

     connecting network, would not be expected to  result  in  optimum

     reachability determinations.


          However, the Neighbor Acquisition Request and Reply messages

     provide  neighbors with a way to inform each other of the minimum

     frequency at which they  are  willing  to  answer  Hellos.   When

     gateway  G sends a Neighbor Acquisition Request to gateway G', it

     states that it does not  wish  to  answer  Hellos  from  G'  more

     frequently  than  once  every  X  seconds.   G'  in  its Neighbor

     Acquisition Reply states that it does not wish to  answer  Hellos

     from  G  more  frequently  than  once  every  Y seconds.  The two

     frequencies do not have to be the same, but  each  neighbor  must

     conform  to  the  interval requested by the other.  A gateway may

     send Hellos less frequently than requested, but not more.


          A  direct  neighbor  gateway   should   also   be   declared

     unreachable  if  the  network  connecting it supplies lower level

     protocol information from which this can be deduced.   Thus,  for

     example,  if  a gateway receives an 1822 Destination Dead message

     from the ARPANET which indicates that a direct neighbor is  dead,

     it should declare that neighbor unreachable.  The neighbor should




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     RFC 888                                              JANUARY 1984



     not be declared reachable again until  the  requisite  number  of

     Hello/I-Heard-You packets have been exchanged.


          A direct neighbor which  has  become  unreachable  does  not

     thereby  cease  to  be  a  direct  neighbor.  The neighbor can be

     declared reachable again without  any  need  to  go  through  the

     neighbor  acquisition  protocol  again.  However, if the neighbor

     remains unreachable for an extremely long period of time, such as

     an  hour,  the  gateway  should  cease to treat it as a neighbor,

     i.e., should cease sending Hello messages to  it.   The  neighbor

     acquisition  protocol  would  then  need to be repeated before it

     could become a direct neighbor again.


          "Hello" messages from sources other  than  direct  neighbors

     should  simply  be ignored.  However, logging the presence of any

     such messages might provide useful diagnostic information.


          A gateway which is going down, or  whose  interface  to  the

     network which connects it to a particular neighbor is going down,

     should send a Neighbor Cease  message  to  all  direct  neighbors

     which  will  no  longer  be  able to reach it.  The Cease message

     should use the info field to specify the reason as "going  down".

     It  should  retransmit  that message (up to some number of times)

     until it receives a Neighbor Cease Acknowledgment.  This provides



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     RFC 888                                              JANUARY 1984



     the  neighbors  with an advance warning of an outage, and enables

     them to prepare for it in a way which will minimize disruption to

     existing traffic.










































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     RFC 888                                              JANUARY 1984



     5  NETWORK REACHABILITY (NR) MESSAGE


          Terminology: Let gateway G have an interface to  network  N.

     We  say  that G is AN APPROPRIATE FIRST HOP to network M relative

     to network N (where M and N are distinct networks) if and only if

     the following condition holds:


          Traffic which is destined for network M, and  which  arrives

          at gateway G over its network N interface, will be forwarded

          to M by G over a path  which  does  not  include  any  other

          gateway with an interface to network N.


          In short, G is  an  appropriate  first  hop  for  network  M

     relative  to network N just in case there is no better gateway on

     network N through which to route traffic which  is  destined  for

     network  M.   For  optimal routing, traffic in network N which is

     destined for network M ought always to be forwarded to a  gateway

     which is an appropriate first hop.


          In  order  for  exterior  neighbors  G  and  G'  (which  are

     neighbors  over network N) to be able to use each other as packet

     switches for forwarding traffic to remote networks, each needs to

     know  the  list of networks for which the other is an appropriate

     first hop.  The Exterior  Gateway  Protocol  defines  a  message,




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     RFC 888                                              JANUARY 1984



     called  the  Network  Reachability  Message  (or NR message), for

     transferring this information.


          Let G be a gateway on network N.  Then the NR message  which

     G sends about network N must contain the following information:


          A list of all the networks for which  G  is  an  appropriate

          first hop relative to network N.


     If G' can obtain this information from exterior neighbor G,  then

     it  knows  that no traffic destined for networks which are NOT in

     that list should be forwarded to G.  (It cannot simply  conclude,

     however,  that all traffic for any networks in that list ought to

     be forwarded via G, since G' may also have other neighbors  which

     are also appropriate first hops to network N.  For example, G and

     G'' might each be neighbors of G',  but  might  be  "equidistant"

     from  some  network  M.   Then each could be an appropriate first

     hop.)


          For each network in the list, the NR message also  specifies

     the "distance" (according to some metric whose definition is left

     to the designers of the autonomous system of which gateway G is a

     member)  from  G  to  that  network.   Core  gateways will report

     distances less than 128 for networks that can be reached  without




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     RFC 888                                              JANUARY 1984



     leaving  the  core  system,  and  greater  than  or  equal to 128

     otherwise.  A stub gateway should report distances less than  128

     for all networks listed in its NR messages.


          The maximum value of distance (255.) shall be taken to  mean

     that  the network is UNREACHABLE.  ALL OTHER VALUES WILL BE TAKEN

     TO MEAN THAT THE NETWORK IS REACHABLE.


          If an NR message from some gateway G fails to  mention  some

     network  N which was mentioned in the previous NR message from G,

     it is possible that N has become unreachable from G.  If  several

     successive  NR  messages  from  G omit mention of N, it should be

     taken to mean that  N  is  no  longer  reachable  from  G.   This

     procedure  is  necessary  to  ensure  that  networks which can no

     longer be  reached,  but  which  are  never  explicitly  declared

     unreachable, are timed out and removed from the list of reachable

     networks.


          It will often be the case that where a core gateway G and  a

     stub  gateway  G'  are  direct neighbors on network N, G knows of

     many more gateway neighbors on network N,  and  knows  for  which

     networks  those  gateway neighbors are the appropriate first hop.

     Since the stub G' may not know about all these  other  neighbors,

     it  is  convenient  and often more efficient for it to be able to



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     RFC 888                                              JANUARY 1984



     obtain this information from G.  Therefore, the  EGP  NR  message

     also  contains  fields  which allow the core gateway G to specify

     the following information:


          a) A list of all neighbors (both interior and exterior) of G

             (on  network  N)  which  G  has reliably determined to be

             reachable.  G may also include indirect neighbors in this

             list (see below.)


          b) For each of those neighbors, the  list  of  networks  for

             which that neighbor is an appropriate first hop (relative

             to network N).


          c) For each such <neighbor, network>  pair,  the  "distance"

             from that neighbor to that network.


          Thus the NR message provides a means of allowing  a  gateway

     to  "discover" new neighbors by seeing whether a neighbor that it

     already knows  of  has  any  additional  neighbors  on  the  same

     network.  This information also makes possible the implementation

     of the INDIRECT NEIGHBOR strategy defined below.


          A  more  precise  description  of  the  NR  message  is  the

     following.





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     RFC 888                                              JANUARY 1984



          The data portion of the  message  will  consist  largely  of

     blocks  of data.  Each block will be headed by a gateway address,

     which will be the address  either  of  the  gateway  sending  the

     message  or  of  one  of  that gateway's neighbors.  Each gateway

     address will be followed by a list of the networks for which that

     gateway  is  an  appropriate first hop.  All networks at the same

     distance from the gateway will be grouped together in this  list,

     preceded  by  the  distance  itself and the number of networks at

     that distance.  The whole list is preceded  by  a  count  of  the

     distance-groups in the list.


          Preceding the list of data blocks is:

          a) The count (one byte) of the number of interior  neighbors

             of  G  for  which  this message contains data blocks.  By

             convention, this count will include the data block for  G

             itself, which should be the first one to appear.


          b) The count (one byte) of the number of exterior  neighbors

                of  G  for  which  this  message contains data blocks.




          c) The address of the network which this message  is  about.

             If  G  and  G' are neighbors on network N, then in the NR

             message going from G  to  G',  this  is  the  address  of



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     RFC 888                                              JANUARY 1984



             network   N.   For  convenience,  four  bytes  have  been

             allocated for this address -- the trailing one,  two,  or

             three bytes should be zero.


          Then follow the data blocks themselves, first the block  for

     G itself, then the blocks for all the interior neighbors of G (if

     any), then the blocks for  the  exterior  neighbors.   Since  all

     gateways  mentioned  are  on  the same network, whose address has

     already been given, the gateway  addresses  are  given  with  the

     network  address part (one, two, or three bytes) omitted, to save

     space.


          In the list of networks, each network address is either one,

     two,  or three bytes, depending on whether it is a class A, class

     B, or class C network.  No trailing bytes are used.


          The NR message  sent  by  a  stub  should  be  the  simplest

     allowable.   That  is,  it  should have only a single data block,

     headed by its own address (on the network it has in  common  with

     the neighboring core gateway), listing just the networks to which

     it is an appropriate first hop.  These will be just the  networks

     that can be reached no other way, in general.







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     RFC 888                                              JANUARY 1984



          The core gateways will send complete NR messages, containing

     information  about all other gateways on the common network, both

     core gateways (which shall be listed as interior  neighbors)  and

     other  gateways (which shall be listed as exterior neighbors, and

     may include the stub itself).  This information will  enable  the

     stub  to  become  an  indirect  neighbor (see below) of all these

     other gateways.  That is, the stub shall forward traffic directly

     to  these  other  gateways  as  appropriate, but shall not become

     direct neighbors with them.


          The  stub  should  NEVER  forward  to   any   (directly   or

     indirectly)  neighboring  core gateway any traffic for which that

     gateway is not an appropriate first hop, as indicated  in  an  NR

     message.   Of  course, this does not apply to datagrams which are

     using the source route option; any such datagrams  should  always

     be  forwarded as indicated in the source route option field, even

     if that  requires  forwarding  to  a  gateway  which  is  not  an

     appropriate first hop.













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     RFC 888                                              JANUARY 1984



     6  POLLING FOR NR MESSAGES


          No gateway is required to send  NR  messages  to  any  other

     gateway,  except  as  a  response  to  an  NR  Poll from a direct

     neighbor.  However, a gateway is required to  respond  to  an  NR

     Poll  from  a  direct neighbor within several seconds (subject to

     the qualification two paragraphs  hence),  even  if  the  gateway

     believes that neighbor to be down.


          The EGP NR Poll message is defined  for  this  purpose.   No

     gateway  may  poll another for an NR message more often than once

     per minute.  A gateway receiving more than one  poll  per  minute

     may  simply  ignore  the  excess  polls,  or  may return an error

     message.


          The minimum interval which gateway  G  will  accept  as  the

     polling  interval  from gateway G' and the minimum interval which

     G' will accept as the polling interval from G  are  specified  at

     the  time  that  G  and  G'  become  direct  neighbors.  Both the

     Neighbor Acquisition Request and the Neighbor  Acquisition  Reply

     allow  the  sender  to  specify,  in seconds, its desired minimum

     polling interval.  If G specifies to G' that its minimum  polling

     interval  is  X,  G'  should not poll G more frequently than once

     every X seconds.  G will not guarantee to  answer  more  frequent



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     RFC 888                                              JANUARY 1984



     polls.


          Polls must only  be  sent  to  direct  neighbors  which  are

     declared reachable by the neighbor reachability protocol.


          An NR Poll message contains a sequence number chosen by  the

     polling  gateway.   The polled gateway will return this number in

     the NR message it sends in response to the poll,  to  enable  the

     polling gateway to match up received NR messages with polls.


          In general, a poll should be retransmitted  some  number  of

     times  (with a reasonable interval between retransmissions) until

     an NR message is received.  IF NO NR MESSAGE  IS  RECEIVED  AFTER

     THE MAXIMUM NUMBER OF RETRANSMISSIONS, THE POLLING GATEWAY SHOULD

     ASSUME THAT THE POLLED GATEWAY IS NOT AN  APPROPRIATE  FIRST  HOP

     FOR  ANY  NETWORK  WHATSOEVER.   The  optimum  parameters for the

     polling/retransmission  algorithm  will  be  dependent   on   the

     characteristics   of   the  two  neighbors  and  of  the  network

     connecting them.


          Received NR messages whose  identification  numbers  do  not

     match  the  identification  number of the most recently sent poll

     shall be ignored.  There is no provision for multiple outstanding

     polls to the same neighbor.




                                  - 23 -






     RFC 888                                              JANUARY 1984



     7  SENDING NR MESSAGES


          In general, NR messages are to be sent only in response to a

     poll.   However,  between  two  successive polls from an exterior

     neighbor, a gateway may send one  and  only  one  unsolicited  NR

     message  to  that  neighbor.   This  gives  it limited ability to

     quickly announce  network  reachability  changes  that  may  have

     occurred in the interval since the last poll.  Excess unsolicited

     NR messages may be ignored, or an error message may be returned.


          An NR message should be sent within  several  seconds  after

     receipt  of  a poll.  Failure to respond in a timely manner to an

     NR poll may result in the polling  gateway's  deciding  that  the

     polled gateway is not an appropriate first hop to any network.


          NR messages sent in response to  polls  carry  the  sequence

     number  of  the  poll  message in their "sequence number" fields.

     Unsolicited NR messages carry the identification  number  of  the

     last  poll  received,  and have the "unsolicited" bit set.  (Note

     that this allows for only a single  unsolicited  NR  message  per

     polling period.)


          Polls from  non-neighbors,  from  neighbors  which  are  not

     declared  reachable, or with bad IP source network fields, should




                                  - 24 -






     RFC 888                                              JANUARY 1984



     be responded to with an EGP error message  with  the  appropriate

     "reason"  field.   If  G  sends  an  NR poll to G' with IP source

     network N, and G' is not a neighbor of  G  on  its  interface  to

     network  N  (or G' does not have an interface to network N), then

     the source network field is considered "bad".


          A gateway is normally not required to send more than one  NR

     message  within the minimum interval specified at the time of the

     neighbor acquisition.  An exception to  this  must  be  made  for

     duplicate polls (successive polls with the same sequence number),

     which occur when an NR message is lost  in  transit.   A  gateway

     should  send an NR message containing its most recent information

     in response to a duplicate poll.























                                  - 25 -






     RFC 888                                              JANUARY 1984



     8  INDIRECT NEIGHBORS


          Becoming a "direct neighbor" of an exterior gateway requires

     three  steps:  (a)  neighbor  acquisition, (b) running a neighbor

     reachability protocol, and (c) polling the neighbor  periodically

     for NR messages.  Suppose, however, that gateway G receives an NR

     message from G', in which G'  indicates  the  presence  of  other

     neighbors  G1, ..., Gn, each of which is an appropriate first hop

     for some set of networks to which G' itself is not an appropriate

     first hop.  Then G should be allowed to forward traffic for those

     networks directly to the appropriate one of G1, ..., Gn,  without

     having to send it to G' first.  In this case, G may be considered

     an INDIRECT NEIGHBOR of G1, ..., Gn, since it is  a  neighbor  of

     these  other  gateways for the purpose of forwarding traffic, but

     does not perform neighbor acquisition, neighbor reachability,  or

     exchange   of  NR  messages  with  them.   Neighbor  and  network

     reachability information is obtained indirectly via G', hence the

     designation  "indirect  neighbor".   We say that G is an indirect

     neighbor of G1, ..., Gn VIA G'.


          If G is an indirect neighbor of  G'  via  G'',  and  then  G

     receives  an  NR  message  from  G'' which does not mention G', G

     should treat G' as having become unreachable.




                                  - 26 -






     RFC 888                                              JANUARY 1984



     9  LIMITATIONS


          It must be clearly  understood  that  the  Exterior  Gateway

     Protocol   does  not  in  itself  constitute  a  network  routing

     algorithm.  In addition, it does not provide all the  information

     needed  to  implement  a  general area routing algorithm.  If the

     topology does not obey the  rules  given  for  stubs  above,  the

     Exterior  Gateway  Protocol  does  not provide enough topological

     information to prevent loops.


          If any gateway sends an NR message with  false  information,

     claiming  to be an appropriate first hop to a network which it in

     fact cannot even reach, traffic  destined  to  that  network  may

     never be delivered.  Implementers must bear this in mind.






















                                  - 27 -






     RFC 888                                              JANUARY 1984



     A  APPENDIX A - EGP MESSAGE FORMATS

          The Exterior Gateway Protocol runs under Internet Protocol as
     protocol number 8 (decimal).




     A.1  NEIGHBOR ACQUISITION MESSAGE

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ! EGP Version # !     Type      !     Code      !    Info       !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !        Checksum               !       Autonomous System #     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !        Sequence #             !       NR Hello interval       !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !        NR poll interval       !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Description:

          The Neighbor Acquisition messages are used by interior and
          exterior gateways to become neighbors of each other.

     EGP Version #

         2

     Type

         3

     Code

          Code = 0      Neighbor Acquisition Request
          Code = 1      Neighbor Acquisition Reply
          Code = 2      Neighbor Acquisition Refusal (see Info field)
          Code = 3      Neighbor Cease Message (see Info field)
          Code = 4      Neighbor Cease Acknowledgment

     Checksum



                                  - 28 -






     RFC 888                                              JANUARY 1984



         The  EGP checksum is the 16-bit one's complement of the one's
         complement sum of the  EGP  message  starting  with  the  EGP
         version  number  field.   For  computing  the  checksum,  the
         checksum field should be zero.

     Autonomous System #

         This   16-bit   number   identifies   the  autonomous  system
         containing the gateway which is the source of this message.

     Info

         For Refusal message, gives reason for refusal:

             0  Unspecified
             1  Out of table space
             2  Administrative prohibition

         For Cease message, gives reason for ceasing to be neighbor:

             0 Unspecified
             1 Going down
             2 No longer needed

         Otherwise, this field MUST be zero.

     Sequence Number

         A sequence number to aid in matching requests and
         replies.

     NR Hello Interval

         Minimum Hello polling interval(seconds).

     NR Poll Interval

         Minumum NR polling interval(seconds).









                                  - 29 -






     RFC 888                                              JANUARY 1984



     A.2  NEIGHBOR HELLO/I HEARD YOU MESSAGE

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ! EGP Version # !    Type       !     Code      !    Status     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !    Checksum                   !    Autonomous System #        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !      Sequence #               !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Description:

         Exterior  neighbors  use  EGP  Neighbor Hello and I Heard You
         Messages to determine neighbor connectivity.  When a  gateway
         receives  an  EGP  Neighbor  Hello message from a neighbor it
         should respond with an EGP I Heard You message.

     EGP Version #

         2

     Type

         5

     Code

          Code = 0 for Hello
          Code = 1 for I Heard you

     Checksum

         The  EGP checksum is the 16-bit one's complement of the one's
         complement sum of the  EGP  message  starting  with  the  EGP
         version  number  field.   For  computing  the  checksum,  the
         checksum field should be zero.

     Autonomous System #

         This   16-bit   number   identifies   the  autonomous  system
         containing the gateway which is the source of this message.




                                  - 30 -






     RFC 888                                              JANUARY 1984



     Sequence Number

         A sequence number to aid in matching requests and replies.

     Status

             0  No status given
             1  You appear reachable to me
             2  You appear unreachable to me due to neighbor
                reachability protocol
             3  You appear unreachable to me due to network
                reachability information (such as 1822 "destination
                dead" messages from ARPANET)
             4  You appear unreachable to me due to problems
                with my network interface
































                                  - 31 -






     RFC 888                                              JANUARY 1984



     A.3  NR POLL MESSAGE

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ! EGP Version # !    Type       !     Code      !    Unused     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !         Checksum              !       Autonomous System #     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !         Sequence #            !       Unused                  !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !             IP Source Network                                 !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


     Description:

          A  gateway  that  wants  to  receive  an  NR message from an
          Exterior Gateway will send an NR Poll message.  Each gateway
          mentioned in the NR message will have an  interface  on  the
          network that is in the IP source network field.

     EGP Version #

         2

     Type

         2

     Code

         0

     Checksum

          The EGP checksum is the 16-bit one's complement of the one's
          complement  sum  of  the  EGP  message starting with the EGP
          version number  field.   For  computing  the  checksum,  the
          checksum field should be zero.

     Autonomous System #

         This   16-bit   number   identifies   the  autonomous  system



                                  - 32 -






     RFC 888                                              JANUARY 1984



         containing the gateway which is the source of this message.

     Sequence Number

          A sequence  number  to  aid in matching requests and
          replies.

     IP Source Network

          Each  gateway  mentioned  in  the  NR  message  will have an
          interface on the network that is in the  IP  source  network
          field.   The  IP  source  network  is  coded  as one byte of
          network number followed by two bytes of  zero  for  class  A
          networks,  two  bytes of network number followed by one byte
          of zero for class B networks, and  three  bytes  of  network
          number for class C networks.































                                  - 33 -






     RFC 888                                              JANUARY 1984



     A.4  NETWORK REACHABILITY MESSAGE

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ! EGP Version # !     Type      !   Code        !U! Zeroes      !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !    Checksum                   !       Autonomous System #     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !    Sequence #                 ! # of Int Gwys ! # of Ext Gwys !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !                      IP Source Network                        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ! Gateway 1 IP address (without network #)      ! ; 1, 2 or 3 bytes
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !  # Distances  !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !  Distance 1   !   # Nets      !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !   net 1,1,1   !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !   net 1,1,2   !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !  Distance 2   !   # Nets      !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !   net 1,2,1   !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !   net 1,2,2   !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !             Gateway  n IP address (without network #)         !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !  # Distances  !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !  Distance 1   !  # Nets       !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !   net n,1,1   !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!  ; 1, 2 or 3 bytes
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !   net n,1,2   !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!  ; 1, 2 or 3 bytes
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !  Distance 2   !  # Nets       !



                                  - 34 -






     RFC 888                                              JANUARY 1984



     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !   net n,2,1   !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!  ; 1, 2 or 3 bytes
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !   net n,2,2   !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!  ; 1, 2 or 3 bytes
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           ...



     Description:

          The  Network  Reachability  message (NR) is used to discover
     which networks may be reached through Exterior Gateways.  The  NR
     message is sent in response to an NR Poll message.

     EGP Version #

         2

     Type

         1

     Code

         0

     Checksum

         The  EGP checksum is the 16-bit one's complement of the one's
         complement sum of the  EGP  message  starting  with  the  EGP
         version  number  field.   For  computing  the  checksum,  the
         checksum field should be zero.

     Autonomous System #

         This   16-bit   number   identifies   the  autonomous  system
         containing the gateway which is the source of this message.

     U (Unsolicited) bit

         This bit is set if the NR message is being sent unsolicited.





                                  - 35 -






     RFC 888                                              JANUARY 1984



     Sequence Number

         The  sequence  number  of  the  last  NR  poll  message
         received from the neighbor to whom this NR message  is  being
         sent.   This  number  is  used  to  aid in matching polls and
         replies.

     IP Source Network

          Each  gateway  mentioned  in  the  NR  message  will have an
          interface on the network that is in the  IP  source  network
          field.

     # of Interior Gateways

          The  number  of interior gateways that are mentioned in this
          message.

     # of Exterior Gateways

          The  number  of exterior gateways that are mentioned in this
          message.

     Gateway IP address

          1, 2 or 3 bytes of Gateway IP address (without network #).

     # of Distances

          The number of distances in the gateway block.

     Distance

          The distance.

     # of Nets

          The number of nets at this distance.

     Network address

          1, 2,  or 3 bytes of network address of network which can be
          reached via the preceding gateway.




                                  - 36 -






     RFC 888                                              JANUARY 1984



     A.5  EGP ERROR MESSAGE

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ! EGP Version # !    Type       !     Code      !    Unused     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !    Checksum                   !       Autonomous System #     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !       Sequence #              !          Reason               !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !                                                               !
     !                     Error Message Header                      !
     !            (first three 32-bit words of EGP header)           !
     !                                                               !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Description:

         An  EGP  Error  Message is sent in response to an EGP Message
         that has a bad checksum or has an incorrect value in  one  of
         its fields.

     EGP Version #

         2

     Type

         8

     Code

         0

     Checksum

          The EGP checksum is the 16-bit one's complement of the one's
          complement  sum  of  the  EGP  message starting with the EGP
          version number  field.   For  computing  the  checksum,  the
          checksum field should be zero.

     Autonomous System #




                                  - 37 -






     RFC 888                                              JANUARY 1984



         This   16-bit   number   identifies   the  autonomous  system
         containing the gateway which is the source of this message.

     Sequence Number

          A  sequence number assigned by the gateway sending the error
          message.

     Reason

          The reason that the EGP message was in error.  The following
          reasons are defined:

          0  -  unspecified
          1  -  Bad EGP checksum
          2  -  Bad IP Source address in NR Poll or Response
          3  -  Undefined EGP Type or Code
          4  -  Received poll from non-neighbor
          5  -  Received excess unsolicted NR message
          6  -  Received excess poll
          7  -  Erroneous counts in received NR message
          8  -  No response received to NR poll

























                                  - 38 -


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