netdev - TCP ssthresh in Linux/RFC

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Message-ID: <AANLkTinZsZGHFodjh0O4WqQ1HhL9w2N9H=9yUDNjo_Fi@mail.gmail.com>
Date:	Thu, 22 Jul 2010 10:56:39 -0700
From:	Yuchung Cheng <ycheng@...gle.com>
To:	netdev <netdev@...r.kernel.org>
Subject: TCP ssthresh in Linux/RFC

The Linux set ssthresh upon entering recovery stage (tcp_fastretrans_alert()) to
1) max(snd_cwnd/2, 2) in tcp_cong.c
2) max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U) in tcp_cubic.c

but RFC 5681 says  ssthresh = max (FlightSize / 2, 2*SMSS). It also
specifically mentions
" Implementation Note: An easy mistake to make is to simply use cwnd,
   rather than FlightSize, which in some implementations may
   incidentally increase well beyond rwnd."

I can think of a few pros and cons of both design, but would like to
hear Linux developers opinions first.

Yuchung
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