[lwip-devel] [bug #20287] tcp_output_nagle sends too early

[Simon Goldschmidt]

Follow-up Comment #25, bug #20287 (project lwip):

If in tcp_enqueue, the number of enqueued pbufs has reached the maximum
limit, TCP has to wait for data to be acknowledged to enqueue more. If this
limit is reached at a point where nagle would wait and not send (e.g. the only
unsent segment is half full; at least one segment in unacked), lwIP is stuck
until the remote side acknowledges unacked data. This typically leads to an
ACK being sent by a remote TCP timer because of the delayed ACK strategy.

In consequence, when TCP_SND_QUEUELEN is set low and an application enqueues
many small packets (e.g. 12 bytes), this can happen very fast and leads to
very poor throughput (2 packets every ~200 ms).

As the current implementation always sends what is enqueued, this was not a
problem until now.

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