net: tcp: Use correct seqnum for tcp_out and tcp_out_ext #90636
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Based on TCP Spec., the outgoing TCP packets shoud use SND.NXT as the seqnum. In Zephyr, the conn->seq works as the SND.UNA and the conn->seq + conn->unacked_len works as the SND.NXT. Currently, it uses SND.UNA in tcp_out() as the seqnum, which might get dropped as old packets and could not deliver the message to the peer. A few exceptions use SND.NXT - 1 as the seqnum are: keepalive, zero-window-probe, FIN/SYN retransmissions. And, for closing a connection, Zephyr won't send out FIN until all the data has been ACKed, so the conn->unacked_len is 0 and it is ok to use conn->seq as the SND.NXT. Signed-off-by: Shrek Wang <[email protected]>
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jukkar
approved these changes
May 27, 2025
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Based on TCP Spec., the outgoing TCP packets shoud use SND.NXT as the seqnum. In Zephyr, the conn->seq works as the SND.UNA and the conn->seq + conn->unacked_len works as the SND.NXT. Currently, it uses SND.UNA in tcp_out() as the seqnum, which might get dropped as old packets and could not deliver the message to the peer. A few exceptions use SND.NXT - 1 as the seqnum are: keepalive, zero-window-probe, FIN/SYN retransmissions.
And, for closing a connection, Zephyr won't send out FIN until all the data has been ACKed, so the conn->unacked_len is 0 and it is ok to use conn->seq as the SND.NXT.