netdev - [RFC] cubic: backoff after slow start

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <20070807143739.7badea0a@oldman>
Date:	Tue, 7 Aug 2007 14:37:39 -0400
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Injong Rhee <rhee@....ncsu.edu>, Sangtae Ha <sha2@...u.edu>
Cc:	netdev@...r.kernel.org
Subject: [RFC] cubic: backoff after slow start

CUBIC takes several unnecessary iterations to converge out of slow start. This
is most noticable over a link where the bottleneck queue size is much larger than BDP,
and the sender has to "fill the pipe" in slow start before the first loss. Typical
consumer broadband links seem to have large (up to 2secs) of queue that needs
to get filled before the first loss.

A possible fix is to use a beta of .5 (same as original TCP) when leaving
slow start.  Originally, the Linux version didn't do slow start so it probably
never was observed.

--- a/net/ipv4/tcp_cubic.c	2007-08-02 12:16:22.000000000 +0100
+++ b/net/ipv4/tcp_cubic.c	2007-08-03 15:57:12.000000000 +0100
@@ -289,7 +289,11 @@ static u32 bictcp_recalc_ssthresh(struct

 	ca->loss_cwnd = tp->snd_cwnd;

-	return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
+	/* Initial backoff when leaving slow start */
+	if (tp->snd_ssthresh == 0x7fffffff)
+		return max(tp->snd_cwnd >> 1U, 2U);
+	else
+		return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
 }

 static u32 bictcp_undo_cwnd(struct sock *sk)
-
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html