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Network Working Group D. Walden Request for Comments: 547 BBN-NET NIC: 17793 13 August 1973

           Change to the Very Distant Host Specification
 Attached is a new version of figure F-4 for BBN Report 1822,
 Specification for the Interconnection of a Host and an IMP.  Also
 attached is replacement text for the paragraph beginning at the
 bottom of page F-7 and continuing through page F-8.
 Please put this RFC with your copy of 1822 pending update of 1822.
                 SPECIAL PACKET BIT ___
    ___HELLO/I-HEARD-YOU BIT           |      ___ UNUSED __
   |                                   |     |             |
   |                                   |     |             |
   V                                   V     V             V
 |   |   |                       |   |   |///////|   |   |///|   |
 |   |   |                       |   |   |///////|   |   |///|   |
   ^   ^     PACKET WORD COUNT     ^               ^   ^       ^
   |   |         ( 6 BITS )        |               |   |       |
   |   |                           |               |   |    CHANNEL
   |   |                           |               |   |    NUMBER
   |   |                           |               |   |
   |  PACKET                  HOST/IMP BIT         |  CHANNEL ZERO
   |  ODD/EVEN BIT                                 |  ACKNOWLEDGE BIT
   |                                               |
  LAST PACKET BIT                                CHANNEL ONE
                                                 ACKNOWLEDGEMENT BIT
                        FIG. F-4   CONTROL WORD FORMAT

Walden [Page 1] RFC 547 Change to the Very Distant Host Specification13 August 1973

 The following algorithm is used to decide whether the circuit between
 an IMP and a very distant Host is dead or alive.  We first define
 what we call a special packet -- this is (logically) a one word
 packet consisting of only the control word and having the SPECIAL
 PACKET bit set to one.  All packets which are not special packets
 (i.e., which are regular data packets or null packets) have the
 SPECIAL PACKET bit set to zero.  In a special packet, none of the
 control word fields or bits have their usual meanings; consequently,
 a special packet cannot be used to acknowledge data packets or send
 data.  In a special packet, only one bit other than the SPECIAL
 PACKET bit has any meaning, the HELLO/I-HEARD-YOU bit.
 Every r seconds both IMP and Host (independently) send a HELLO
 packet, a special packet with the HELLO/I-HEARD-YOU bit set to zero.
 When either IMP or Hosts receives a HELLO packet, it must promptly
 (with highest priority) send the other an I-HEARD-YOU packet, a
 special packet with the HELLO/I-HEAR-YOU bit set to one.  In other
 words, the I-HEARD-YOU packet is an acknowledgement of the periodic
 HELLO packet, and a I-HEARD-YOU packet must only be sent as
 acknowledgement for a HELLO packet.  If either IMP or Host sends more
 than t HELLO packets without receiving an I-HEARD-YOU packet in
 acknowledgement, the IMP or Host declares the line dead.  Once either
 IMP or Host declares the line dead, it must send or accept no packets
 (either special or regular) for 2*t*r* seconds to allow the other
 party also to declare the line dead.  After waiting 2*t*r* seconds,
 an attempt is made to bring the line alive.  This is done by sending
 HELLO packets (but no regular packets) every r seconds while noting
 received I-HEARD-YOU packets until k HELLO packets in a row are
 acknowledged with I-HEARD-YOU packets.  While doing this, received
 HELLO packets must be acknowledged with I-HEARD-YOU packets.  Once
 acknowledgement for k HELLO packets have been received in a row
 (i.e., one acknowledgement every r seconds for k intervals[1]), the
 line is declared alive, and regular packets again may be sent,
 received, and acknowledged along with the periodic (every r seconds)
 HELLO packets.  If a regular data packet is received while a party is
 trying to bring the line up (due perhaps to slight timing differences
 between the parties at the ends of the line), the data packet must
 not be acknowledged.
 The odd/even bits, the used/unused bits, and the channel filling and
 emptying sequences must be initialized at start up[2] and
 reinitialized every time the line is declared dead.  If either the
 IMP or Host decides the line is dead, the same action is taken as the
 IMP or Host normally takes when the other's ready line is down.  The
 line being up causes the same action as is normally taken when the
 ready line is up.  The value of r is currently 1.25 seconds, the
 value of t is currently 4, and the value of k is currently also 4.

Walden [Page 2] RFC 547 Change to the Very Distant Host Specification13 August 1973

 It is likely that the values of r, t, and k will be adjusted in the
 future; very distant Host programmers are advised to make it easy to
 change these parameters.


 [1] In particular, the IMP implementation requires the receipt of an
 acknowledgement within r seconds of the transmission of a HELLO
 packet in order to consider that the HELLO packet was successfully
 [2] At start-up, the line must be assumed to be dead and the
 procedure of waiting 2*t*r* seconds before sending HELLO packets,
 etc. must be used to bring the line alive initially.
       [ This RFC was put into machine readable form for entry ]
        [ into the online RFC archives by Jeff McClellan 1/98 ]

Walden [Page 3]

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