I have lwIP setup with MSS = 1460, and TCP_WND = 2*MSS. As I understand it, this means that the transmitter will send lwIP up to 2*MSS of data before expecting an ACK from lwIP. In one example, the transmitter sends lwIP two consecutive segments in rapid succession, each carrying the full MSS. lwIP quickly ACKs the first segment only. The transmitter retransmits the second segment, then lwIP debug output complains of recieving a duplicate seqno. Could this be caused by a configuration problem, or is it likely something else? I feel like I'm finally starting to get a handle on understanding my lwIP issues, but don't understand why lwIP was so quick to ACK the first segment but not wait for the second, so one ACK could be sent. The trace was in another message I posted with the subject "Duplicate sequence numbers".
Thanks for everyone's help so far.
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