RFC 534

Lost Message Detection ✨

Bestie, this one is by David Walden from BBN-NET (NIC 17453), dated 17 July 1973 🦋 It's a Network Working Group doc, and it's basically a sidequest to RFC 533. The header lists 512, 516, 533 as references, and it doesn't say anything about obsoleting or updating another RFC, so that's the full tea 🫶

Okay so, the aside is that if sending Hosts uniquely identify messages on a given link using the extra four bits, and receiving Hosts actually look at those bits, a lost message detection system like the ones suggested in RFCs 512 and 516 just drops right out of using the unique message-id 💖 Those four bits can work as Hathaway's SCB from RFC 512, giving you a 16 element sequence number on a per connection basis 🧚 Sixteen is enough because the IMPs never allow more than four outstanding messages at one time between a given pair of Hosts 💅 And as Hathaway also suggests, the 0 element can tell the receiving Host that sequence numbers aren't being used at all 🌸

So there seem to be three modes for using the message-id number under the Host/Host protocol. It's giving options 👑

1. The sender always sets the extra four bits to 0 and only sends one message over a given link at a time. It's slow, ngl, but it allows orderly retransmission of messages with no help from the receiver 🙈
2. The receiver gives no help to the sender. In this case it doesn't matter whether the sender uses the extra four bits to uniquely identify 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 (meaning RFNMs not received) on a given link, and the receiver can help the sender. If the sender uses the extra four bits to uniquely identify messages in a way that can be synchronized with the receiver (for example, sequential id numbers), the receiver can reliably detect lost messages main character energy 🔥

Now, it probably seems like not enough to some people, but if the sender and receiver use synchronized unique message-id numbers, very reliable retransmission schemes are readily available 😭 For instance, the sender can retransmit the appropriate messages in response to incomplete transmissions, and the receiver can use the unique message-ids to sort the retransmitted messages into the proper order with the other received messages 🎀 Or, alternatively, the receiver can discard all messages received out of order, and the sender can back up and retransmit a message for which an incomplete transmission was received, plus all subsequent messages. Iconic, periodt ✨

[ This RFC was put into machine readable form for entry ] [ into the online RFC archives by Alex McKenzie with 10/99 ] 🍓