rfc827.txt

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



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



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



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



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



     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.




































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



     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.

























                                  - 32 -

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



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



     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 -

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



                   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 #