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RFC 827 - Exterior Gateway Protocol (EGP)


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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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          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.
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     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.
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     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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     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

          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  three-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

     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

     Neighbor Acquisition Reply message.  It must also send a Neighbor

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

     Acquisition Request message, unless it has done so already.

          Two gateways become direct neighbors when each  has  sent  a

     Neighbor  Acquisition  Message to, and received the corresponding

     Neighbor Acquisition Reply from, the other.

          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  Message  from  a  gateway  which  is

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

     a Neighbor Acquisition Message.

          If  a  Neighbor  Acquisition  Reply  is  received   from   a

     prospective neighbor, but a period of time passes during which no

     Neighbor Acquisition Message is received  from  that  prospective

     neighbor,  the  neighbor  acquisition  protocol  shall  be deemed

     incomplete.  A Neighbor Cease message (see below) should then  be

     sent.   If  one  gateway  still desires to acquire the other as a

     neighbor, the protocol must be repeated from the beginning.

          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

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

     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.

          NOTE THAT WE HAVE NOT SPECIFIED THE WAY IN WHICH ONE GATEWAY

     INITIALLY  DECIDES THAT IT WANTS TO BECOME A NEIGHBOR OF ANOTHER.

     While this is hardly a trivial problem, it is  not  part  of  the

     External Gateway Protocol.

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

     3  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.

          At  the  current  time,   the   reachability   determination

     algorithm  is  left to the designers of a particular gateway.  We

     have in mind algorithms like the following:

          A reachable  neighbor  shall  be  declared  unreachable  if,

     during the time in which we sent our last n "Hello"s, we received

     fewer than k "I Heard You"s in return.  An  unreachable  neighbor

     shall  be declared reachable if, during the time in which we sent

     our last m "Hello"s, we received at least j  "I  Heard  You"s  in

     return.

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

          However, 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  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.

          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 one gateway to

     make its reachability determination parasitic on the other:  only

     one  gateway  actually  needs  to  send "Hello" messages, and the

     other can declare it up or down based on the status field in  the

     "Hello".   That  is,  the  "passive" gateway (which sends only "I

     Heard  You"s)  declares  the  "active"  one  (which  sends   only

     "Hello"s)  to  be reachable when the "Hello"s from the active one

     indicate that it has declared the passive one  to  be  reachable.

     Of  course,  this can only work if there is prior agreement as to

     which neighbor is to be the active one.  (Ways of coming to  this

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     RFC 827                              Bolt Beranek and Newman Inc.
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     "prior agreement" are not part of the Exterior Gateway Protocol.)

          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

     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" and "I Heard You" messages from gateway G to gateway

     G'  also  carry  the identification number of the NR poll message

     (see below) which G has most recently received from G'.

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

          "Hello" and "I Heard You" messages from gateway G to gateway

     G'  also  carry  the  minimum interval in minutes with which G is

     willing to be polled by G' for NR messages (see below).

          "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 Gateway Going Down message to all direct  neighbors

     which  will  no longer be able to reach it.  It should retransmit

     that message (up to some number of times)  until  it  receives  a

     Gateway  Going  Down Acknowledgment.  This provides 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 827                              Bolt Beranek and Newman Inc.
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     4  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,

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

     transferring this information.

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          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 contains a

     byte  which  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.

     This information might (or might not) be useful in  the  interior

     routing algorithm of gateway G', or for diagnostic purposes.

          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.

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          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 shall be assumed that N is still reachable from  G.   HOWEVER,

     IF  N IS NOT MENTIONED IN TWO SUCCESSIVE NR MESSAGES FROM G, THAT

     SHALL 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 may often be the case that where G and  G'  are  exterior

     neighbors on network N, G knows of many more gateway neighbors on

     network N, and knows for which networks those other neighbors are

     the appropriate first hop.  Since G' may not know about all these

     other neighbors, it is convenient and often more efficient for it

     to be able to obtain this information from G.  Therefore, the EGP

     NR message also contains fields which  allow  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.  Gateways should be included in this list only

             if  G  is  actively  running  its  neighbor  reachability

             protocol with them.

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          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.

          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, and the distance from that

     gateway to each network.

          Preceding the list of data blocks is:

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

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

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

             network   N.   For  convenience,  four  bytes  have  been

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

             three bytes should be zero.

          b) 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.

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

             of G for which this message contains data blocks.

          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.

          Each block includes  a  one-byte  count  of  the  number  of

     networks for which that gateway is the appropriate first hop.  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

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     class C network.  No trailing bytes are used.

          It may sometimes be necessary to fragment  the  NR  message.

     The  NR  message  contains  a  byte indicating the number of this

     fragment (fragments will be  numbered  from  zero),  and  a  byte

     containing  the  number  of  the last fragment (NOT the number of

     fragments).  If fragmentation is not used, these bytes must  both

     be  zero.   EACH  FRAGMENT  MUST  BE  A  FULLY  SELF-CONTAINED NR

     MESSAGE.  That is, each fragment  will  begin  with  a  count  of

     interior  and  exterior  neighbors,  and  will have some integral

     number of gateway data blocks.  The number of data blocks in each

     fragment  must correspond to the neighbor counts at the beginning

     of that fragment.  However, only the first fragment should  begin

     with a data block describing the sending gateway.

          This  scheme  enables  each   fragment   to   be   processed

     independently, and requires no complex reassembly mechanisms.  It

     also enables processing of a message all of whose fragments  have

     not been received.  If, after some amount of time and some number

     of retransmissions  of  a  poll,  not  all  fragments  have  been

     received,  the  fragments which are present shall be processed as

     if they constituted the complete NR message.   (This  means  that

     networks  mentioned  only in the missing fragment will retain the

     "distance" values they had in the previous NR message  from  that

     gateway.   However,  if  no new value for a particular network is

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     received in the next NR message from that  gateway,  the  network

     will be declared unreachable.)

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     5  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 Hello and I Heard  You  messages  which  gateway  G

     sends  to  gateway  G' indicate the minimum interval which G will

     accept as the polling interval from G'.  That  is,  G'  will  not

     guarantee  to  respond to polls from G that arrive less than that

     interval apart.

          Polls must only  be  sent  to  direct  neighbors  which  are

     declared reachable by the neighbor reachability protocol.

          An NR Poll message contains an identification 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

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     polls.  It will be the responsibility of the polling  gateway  to

     choose an identification number which is sufficiently "unique" to

     allow detection of out-of-date NR messages  which  may  still  be

     floating   around   the  network.   Since  polls  are  relatively

     infrequent, this is  not  expected  to  be  much  of  a  problem.

     However,  to  aid in choosing an identification number, the Hello

     and I Heard You messages carry the identification number  of  the

     last  NR  poll received from the neighbor to which they are being

     sent.

          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.

          If only some fragments of an NR message are  received  after

     the  maximum  number  of  retransmissions, the fragments that are

     present shall be treated as constituting  the  whole  of  the  NR

     message.

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          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.

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     6  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

     identification    number   of   the   poll   message   in   their

     "identification 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.)

          To facilitate the sending of unsolicited NR messages, the NR

     poll  message  has  a  byte  indicating  the  polling interval in

     minutes.

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          Polls from  non-neighbors,  from  neighbors  which  are  not

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

     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".

          Duplicated   polls   (successive   polls   with   the   same

     identification  number) should be responded to with duplicates of

     the same NR message.  If that message  is  fragmented,  the  same

     fragments  shall  be  sent  each  time.   Note  that  there is no

     provision for handling multiple outstanding polls from  a  single

     neighbor.   NOTE  THAT  IF  THE  SAME  FRAGMENTS  ARE NOT SENT IN

     RESPONSE TO DUPLICATED POLLS, INCORRECT REASSEMBLY  WILL  BE  THE

     PROBABLE  RESULT.   If  fragmentation is not being used, however,

     then no harm should result from responding to  a  duplicate  poll

     with a different (presumably more recent) NR message.

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     7  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.

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     8  HOW TO BE A STUB GATEWAY

          The most common application of EGP will probably be its  use

     to  enable  a  stub  gateway to communicate with one of the DARPA

     core gateways,  so  as  to  enable  data  flow  between  networks

     accessible only via the stub and networks accessible only via the

     system of core gateways.  As discussed previously, a stub gateway

     can  be  considered  to  be a one-gateway internet system with no

     interior neighbors.  It is probably used  to  interface  a  local

     network  or  networks  to a long range transport network (such as

     ARPANET or SATNET) on which there is  a  core  gateway.  In  this

     case,  the stub will not want the core gateways to forward it any

     traffic other than traffic which is destined for the  network  or

     networks which can be reached only via the stub.  In general, the

     stub will not want to  perform  any  services  for  the  internet

     transport system which are not needed in order to be able to pass

     traffic to  and  from  the  networks  that  cannot  be  otherwise

     reached.

          The 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.  When a stub and a  core  gateway  become  direct

     neighbors,  the  core  gateway will begin sending Hello messages.

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     When the  stub  declares  the  core  gateways  which  are  direct

     neighbors  to  be reachable, it should poll those gateways for NR

     messages at a rate not to exceed once per minute (or as specified

     in the Hello messages from the core gateways).  The core gateways

     will also poll the stub for NR messages.

          The NR message sent by  the  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.

          The core gateways will send complete NR messages, containing

     information about all other gateways on the common networks, 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 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 core gateways will report distances less than 128 if the

     network  can  be  reached  without leaving the core system (i.e.,

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     without traversing any gateway other than a  core  gateway),  and

     greater than or equal to 128 otherwise.

          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.

          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, 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.)

          If the stub gateway does not respond in a timely  manner  to

     Hello  messages  from  the  core  gateway,  it  may  be  declared

     unreachable.  If it does not respond to NR  poll  messages  in  a

     timely manner, its networks may be declared unreachable.  In both

     these cases, the core gateways may discard traffic  destined  for

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     those  networks, returning ICMP "destination network unreachable"

     to the source hosts.

          The stub gateway is  expected  to  fully  execute  the  ICMP

     protocol,  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.

                                  - 31 -

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     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 of the set of autonomous systems is not  tree-structured

     (i.e.,  if it has cycles), 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.

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     RFC 827                              Bolt Beranek and Newman Inc.
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                       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 #     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !       Identification #        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Description:

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

     EGP Version #

         1

     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

         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.

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     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.

     Identification Number

         An identification number to aid in matching requests and
         replies.

                                  - 34 -

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                   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 #               !Min Poll Intvl !    Zero       !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !      Last Poll Id #           !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     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 #

         1

     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.

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     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

     Last Poll Id Number

             The  identification  number of the most recently received
             NR poll message from the neighbor to which  this  message
             is being sent.

     Minimum Polling Interval

             This  gateway  should  not be polled for NR messages more
             often than once in this number of minutes.

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     RFC 827                              Bolt Beranek and Newman Inc.
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                           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 #     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !             IP Source Network                 !  Interval     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !    Identification #           !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     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 #

         1

     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
         containing the gateway which is the source of this message.

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     Identification Number

          An  identification  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.

     Interval

          The polling interval in minutes.

                                  - 38 -

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

                         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 #     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !  Fragment #   !# of last frg. !       Identification #        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !                      IP Source Network                        !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ! # of Int Gwys ! # of Ext Gwys !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !  # of Nets    !                                 ; # of nets for
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Gateway 1
     ! Gateway 1 IP address (without network #)      ! ; 1, 2 or 3 bytes
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !   net 1,1     !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ! distance      !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !   net 1,2     !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! ; 1, 2 or 3 bytes
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ! distance      !
     +-+-+-+-+-+-+-+-+
                  .
                  .

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !   net 1,m     !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!  ; m nets reachable
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  ; via Gateway 1
                  .
                  .
     +-+-+-+-+-+-+-+-+
     !  # of nets    !       ;number of nets for Gateway n
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !             Gateway  n IP address (without network #)         !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !   net n,1     !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!  ; 1, 2 or 3 bytes
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ! distance      !
     +-+-+-+-+-+-+-+-+

                                  - 39 -

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

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

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !   net n,m     !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!  ; m nets reachable
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  ; via Gateway n
     ! distance      !
     +-+-+-+-+-+-+-+-+

                                  - 40 -

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

     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 #

         1

     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.

     Identification Number

         The  identification  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.

     Fragment Number

          Which  Fragment  this  is  in  the  NR  Message.   Zero,  if
          fragmentation is not used.

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                                                         Eric C. Rosen

     Number of Last Fragment

          Number  of  the  last  fragment in the NR Message.  Zero, if
          fragmentation is not used.

     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.

     # of Networks

          The  number  of  networks  for  which  the  gateway whose IP
          address immediately follows is the appropriate first hop.

     Gateway IP address

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

     Network address

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

     Distance

         1 byte of distance in # of hops.

                                  - 42 -

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

                              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 #     !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ! Error Type    !  Error Code   !    Id. # of Erroneous Msg.    !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     !       Sequence #              !
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     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 #

         1

     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 #

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

                                  - 43 -

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

     Sequence Number

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

     Error Type

          The Type of the EGP message that was in error.

     Error Code

          The Code of the EGP message that was in error.

     Identification number of erroneous message

          The Sequence number of the EGP message that was in error.

     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
          9  -  Not all fragments of NR message received

                                  - 44 -

 

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