[PA3] Out-of-Order Packet Handling and Sender Responsibilities

[JayunKu]

I remember from the PA3 lab session that the implementation of handling out-of-order packets is up to us, but discarding them might be simpler. If we can discard out-of-order packets, it would align with the Go-Back-N (GBN) protocol. However, the sender responsibilities in the instructions state 'retransmit only packets that have timed out.' If this means retransmitting only the timed-out packet (not all unacknowledged packets), how is this different from GBN? Did I misunderstand the lab session?

PA3 랩 세션에서 기억하기로는, out of order packet에 관한 구현은 저희에게 달려있지만, discard가 간단할 것이라고 들었던 것으로 기억합니다. out of order 패킷을 discard 해도 된다면 Go Back N 프로토콜처럼 구현하게 될텐데, instruction의 sender responsibilities 부분에 "retransmit only packets that have timed out"이라고 나와 있습니다. 이게 타임아웃난 패킷만 재전송해야 한다는 의미라면(unacked 패킷을 모두 전송하는 것이 아니라) GBN 프로토콜과 다른 것이 아닌지 궁금합니다. 제가 랩세션에서 잘못 이해한 것일까요?

[lgm9]

Actually, from the sender's side, it would be impossible to distinguish whether a packet was dropped or not.
For example, if the sender sends A B C D E, and only C is dropped by the receiver.
Then, the ACK until B would be sent back to the sender, so you should go for C D E F G.
From the lab session, the situation was from the receiver.
If you only get legit A B D E packets and a corrupted C packet, then you should send an ACK for B (of course, you didn't get C). Then the sender will send back from C D E F G. (This method is not implemented by you, of course. The grader will use the model solution for the sender)