Resending old frame when a damaged frame has been received in one bit sliding window protocol

Question

I'm reading Tanenbaum's and Wheterall's book on Computer Networks. I'm trying out some examples of situations that can occur in the one bit sliding window protocol for a datalink layer.

The code for the protocol can be found here on page 214.

The situation I was investigating was the following: Let's say client A successfully sends a frame to client B. Client B then sends his frame containing data along with the acknowledgement (ACK) that he received client A's frame correctly (piggybacking). Now somewhere along the way this frame gets damaged. Therefore, in the code, a chksum_err event occurs. Then the exact same frame client A sent before is sent again. Why? Shouldn't it be the case that client A now sends a frame with the sequence number inverted, new data but the acknowledgement is not inverted? This way, assuming nothing special happens along the way, client B correctly receives a new frame (instead of a duplicate) and client B knows that the frame he sent was damaged so he needs to send it again.

Isn't this the beauty of the protocol? Is the code wrong or what am I missing?

Answer 1

Shouldn't it be the case that client A now sends a frame with the sequence number inverted, new data but the acknowledgement is not inverted?

This could indeed be done when the error occured in the data. i.e. you're sure that it wasn't the acknowledgement or sequence number that was damaged. In that case, sender A could indeed send a new frame as described and avoid sending unnecessary duplicates.

However, although an error can be detected, it can not be detected where the error occured (in the data, acknowledgement or sequence number). If the error occured in the aknowledgement number then the client may unrightfully think that his previously sent frame was received correctly while it actually wasn't received correctly and so, if he sents a new frame with new data as proposed in the question, a client will miss a frame!