[lwip-devel] [bug #24493] Dropped incoming packets cause a burst of duplicate ACKs

[Kieran Mansley]

Follow-up Comment #3, bug #24493 (project lwip):

Perhaps I had relative sequence numbers turned on, and you didn't, which
would explain the difference in sequence numbers seen in the trace, sorry.

The reason it sends 14 identical packets (the duplicate ACKs) is that it's
just processed 14 out of order packets.  It correctly sends one duplicate ACK
for each out of order packet it sees.  These packets are out of order because
it's expecting to see a packet that was lost.

I.e.

1) Frames 13-25 are sent by the PC, and recorded in the capture

2) A short time later these are received by lwIP and are processed one by
one.

3) Each is judged to be out of order because it's not the next packet
expected in the stream: it's still looking for the first lost one.

4) lwIP sends a duplicate ACK for each packet it receives to tell the host
that it's still waiting for the lost packet (frames 32-45)

Because steps 1) and 4) don't overlap in time, you don't see a single packet
sent by the PC followed by a single ACK in response, but the many duplicate
ACKs are each in response to an earlier out of order packet sent by the PC. 


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