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

From: Ambroz Bizjak
Subject: [lwip-devel] [bug #50476] active open, ssthresh is really low with auto-tuning receiver
Date: Tue, 7 Mar 2017 07:17:11 -0500 (EST)
User-agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36

Follow-up Comment #1, bug #50476 (project lwip):

I see that lwIP sets ssthresh to the advertised window (snd_wnd) when it
receives the SYN-ACK. This must be causing lwIP to prematurely enter and
remain in congestion avoidance.

/** Initial slow start threshold value: we use the full window */
#define LWIP_TCP_INITIAL_SSTHRESH(pcb)  ((pcb)->snd_wnd)

RFC 5681 says:

"The initial value of ssthresh SHOULD be set arbitrarily high (e.g.,to the
size of the largest possible advertised window), but ssthresh MUST be reduced
in response to congestion. Setting ssthresh as high as possible allows the
network conditions, rather than some arbitrary host limit, to dictate the
sending rate."

Note that it says "largest *possible* advertised window", which is not the
same as window advertised at SYN-ACK reception. Maybe the fix is:



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