netdev - The ssthresh reduce during 3WHS

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Message-ID: <CAH4NVoT4ofCNKAmcnO1AgEbs+ywbXom=E28k2VU4dB+yK79S_A@mail.gmail.com>
Date:	Wed, 12 Mar 2014 21:46:04 +0800
From:	Yong Zhang <fell198904@...il.com>
To:	netdev@...r.kernel.org
Cc:	Yong Zhang <fell198904@...il.com>
Subject: The ssthresh reduce during 3WHS

hi，

I am trying to to apply patch 9ad7c049f0f79c418e293b1b68cf10d68f54fcdb
to my old version kernel, but I am not understanding the following
changes.

The comments int the patch say ssthresh may reduce unnecessarily
druing 3WHS,  but i can not find the reduce operation of ssthresh
during 3WHS.

Can you please explain when dose this may hanppen?


diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index bef9f04..ea0d218 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -880,6 +880,11 @@ static void tcp_init_metrics(struct sock *sk)

        tp->snd_ssthresh = dst_metric(dst, RTAX_SSTHRESH);
                 if (tp->snd_ssthresh > tp->snd_cwnd_clamp)
                         tp->snd_ssthresh = tp->snd_cwnd_clamp;
+       } else {
+                /* ssthresh may have been reduced unnecessarily during.
+                * 3WHS. Restore it back to its initial default.
+                */
+                tp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
         }


Thanks,
Fel
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