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+Network Working Group                                          D. Walden
+Request for Comments: 547                                        BBN-NET
+NIC: 17793                                                13 August 1973
+
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+             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.
+
+   DCW/ph
+
+
+                   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
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+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.
+
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+
+
+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.
+
+Endnotes
+
+   [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
+   acknowledged.
+
+   [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 ]
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+Walden                                                          [Page 3]
+