linux-kernel - [PATCH AUTOSEL for 4.4 071/162] sctp: adjust ssthresh when transport is idle

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Date:   Mon, 9 Apr 2018 00:28:34 +0000
From:   Sasha Levin <Alexander.Levin@...rosoft.com>
To:     "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Sasha Levin <Alexander.Levin@...rosoft.com>
Subject: [PATCH AUTOSEL for 4.4 071/162] sctp: adjust ssthresh when transport
 is idle

From: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>

[ Upstream commit a02d036c027e9070ca27c156f59eb445d5405480 ]

RFC 4960 Errata 3.27 identifies that ssthresh should be adjusted to cwnd
because otherwise it could cause the transport to lock into congestion
avoidance phase specially if ssthresh was previously reduced by some
packet drop, leading to poor performance.

The Errata says to adjust ssthresh to cwnd only once, though the same
goal is achieved by updating it every time we update cwnd too. The
caveat is that we could take longer to get back up to speed but that
should be compensated by the fact that we don't adjust on RTO basis (as
RFC says) but based on Heartbeats, which are usually way longer.

See-also: https://tools.ietf.org/html/draft-ietf-tsvwg-rfc4960-errata-01#section-3.27
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
---
 net/sctp/transport.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/sctp/transport.c b/net/sctp/transport.c
index aab9e3f29755..d0889e560615 100644
--- a/net/sctp/transport.c
+++ b/net/sctp/transport.c
@@ -546,6 +546,8 @@ void sctp_transport_lower_cwnd(struct sctp_transport *transport,
 		 */
 		transport->cwnd = max(transport->cwnd/2,
 					 4*asoc->pathmtu);
+		/* RFC 4960 Errata 3.27.2: also adjust sshthresh */
+		transport->ssthresh = transport->cwnd;
 		break;
 	}

-- 
2.15.1