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[lwip-devel] [bug #50476] active open, ssthresh is really low with auto-

From: Joel Cunningham
Subject: [lwip-devel] [bug #50476] active open, ssthresh is really low with auto-tuning receiver
Date: Thu, 9 Mar 2017 15:41:16 -0500 (EST)
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_3) AppleWebKit/602.4.8 (KHTML, like Gecko) Version/10.0.3 Safari/602.4.8

Follow-up Comment #11, bug #50476 (project lwip):

I debugged the unit test and here's what's going on:

Before the change to set ssthresh in tcp_allloc(), ssthresh is 0 in
test_tcp_fast_rexmit_wraparound because we don't perform either an
active/passive open, instead just manually setting the pcb state via

Then when we call test_tcp_input(), cwnd enters congestion avoidance because
it is larger than ssthresh.  Congestion avoidance doesn't grow the window
large enough for a second segment to be sent in the tcp_output() call at the
end of tcp_input()

With my change, we stay in slow start (due to large ssthresh) and cwnd grows
large enough for multiple segments to be sent in tcp_output()

This could be fixed by moving ssthresh assignment back to the active/passive
open spots, but I'm not sure.  Maybe the unit tests should be manually setting
ssthresh to meet expectations like it is doing for cwnd


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