Change to the Very Distant Host specification (RFC0547)
Original Publication Date: 1973-Aug-01
Included in the Prior Art Database: 2019-Feb-12
Internet Society Requests For Comment (RFCs)
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...