Lost message detection (RFC0534)
Original Publication Date: 1973-Jul-01
Included in the Prior Art Database: 2019-Feb-12
Internet Society Requests For Comment (RFCs)
Network Working Group David Walden Request for Comments: 534 BBN-NET NIC: 17453 17 July 1973 References: 512, 516, 533
Lost Message Detection
As an aside to RFC 533, note that if sending Hosts do uniquely identify messages on a given link using the extra four bits and receiving Hosts do look at these bits, a lost message detection system such as those suggested in RFCs 512 and 516 drops right out of using of the unique message-id. These extra four bits can be treated as Hathaway’s SCB of RFC 512 providing a 16 element sequence number on a per connection basis. A 16 element sequence is sufficient as the IMPs never allow more than four outstanding messages at one time between a given pair of Hosts. As Hathaway also suggests, the 0 element in the sequence can be used to indicate to the receiving Host that sequence numbers are not being used.
To summarize, there appear to be three modes of using the message-id number under Host/Host protocol:
1. The sender can always set the extra four bits to 0 and only transmit one message over a given link at a time -- this is slow but it allows orderly retransmission of messages without any help from the receiver.
2. The receiver can give no help to the sender. In this case it doesn’t matter whether the sender uses the extra four bits to uniquely identify the messages or not -- the sender has no method of orderly retransmission, although the sender can accurately identify which message was lost if the sender has uniquely identified the messages.
3. The sender can have multiple messages outstanding (i.e., RFNMs not received) on a given link and the receiver can h...