Hi Chris, In the slides from last week's lecture, we should be recalculating the checksum for the acknowledgement before sending it, though this wasn't in the C code: [CITS3002]⬈ ...though I suppose checking the checksum from the acknowledgement isn't included on the previous slide's code either so I understand if the code has just omitted this for simplicity. I'm assuming that we should recalculate & reassign after setting the acknowledgement? Let me know your thoughts, Henry