rfc827.txt

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



          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.





                                  - 17 -

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



          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.




                                  - 18 -

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



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



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



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

     will be declared unreachable.)















































                                  - 21 -

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



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



     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.





                                  - 23 -

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



          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.











































                                  - 24 -

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



     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