[lwip-devel] [bug #48839] Possible bug with snd_wnd semantics

[Ambroz Bizjak]

Follow-up Comment #3, bug #48839 (project lwip):

Hi Simon,
I think retransmissions can trigger a lastack update without a window update.

- Remote sends two data segments with consecutive data; the first has a lower
sequence number than the second.
- Lwip receives the second segment only as the first one is lost. It updates
the window based on the second segment.
- The remote later retransmits the first segment. It will have the lower
sequence number of the first segment but it will have the newest ack value
(rcv_nxt of remote), greater than the one of both the first and second segment
originally sent, due to the remote having received some data.
- Lwip receives the retransmitted first segment. A window update is inhibited
(details [1]) since the ack number acknowledges new data, lastack is updated.
The window is effectively now too large.

The other kind of inconsistency seems less likely to me.

[1] The retransmitted segment has SEQ lesser than snd_wl1 and ACK different
from the snd_wl2, which is sufficient to pass /* Update window. */



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