Bug #359

Fwd: [PATCH] tcp: fix tcp_rcv_rtt_update() use of an unscaled RTT sample

Added by David Taht on Apr 10, 2012. Updated on Apr 19, 2012.
Closed Normal Dave Täht

Description

———- Forwarded message ———-
From: David Miller davem@davemloft.net
Date: Tue, Apr 10, 2012 at 11:47 AM
Subject: Re: [PATCH] tcp: fix tcp_rcv_rtt_update() use of an unscaled RTT sample
To: ncardwell@google.com
Cc: netdev@vger.kernel.org, nanditad@google.com, ycheng@google.com,
edumazet@google.com, therbert@google.com

From: Neal Cardwell ncardwell@google.com
Date: Tue, 10 Apr 2012 13:59:20 -0400

> Fix a code path in tcp_rcv_rtt_update() that was comparing scaled and
> unscaled RTT samples.
>
> The intent in the code was to only use the ’m’ measurement if it was a
> new minimum.  However, since ’m’ had not yet been shifted left 3 bits
> but ‘new_sample’ had, this comparison would nearly always succeed,
> leading us to erroneously set our receive-side RTT estimate to the ’m’
> sample when that sample could be nearly 8x too high to use.
>
> The overall effect is to often cause the receive-side RTT estimate to
> be significantly too large (up to 40% too large for brief periods in
> my tests).
>
> Signed-off-by: Neal Cardwell ncardwell@google.com

Applied, thanks.

History

Updated by Dave Täht on Apr 19, 2012.
Tested in cerowrt 3.3.2-6. Will probably be fixed in 3.3.3

This is a static export of the original bufferbloat.net issue database. As such, no further commenting is possible; the information is solely here for archival purposes.
RSS feed

Recent News & Articles

Mar 21, 2019 Wiki page
Dave Taht's Take on TCP
Mar 17, 2019 Wiki page
Jake Holland's Stance on ECN
Sep 6, 2018 Wiki page
Pete Heist's Thoughts on ECN
Sep 5, 2018 Wiki page
Dave Taht's Stance on ECN
Sep 4, 2018 Wiki page
Jonathan Morton's Take on ECN

Find us elsewhere

Bufferbloat Mailing Lists
#bufferbloat on Twitter
Google+ group
Archived Bufferbloat pages from the Wayback Machine

Sponsors

Comcast Research Innovation Fund
Nlnet Foundation
Shuttleworth Foundation
GoFundMe

Bufferbloat Related Projects

Congestion Control Blog
Lede Project (OpenWrt)
Flent Network Test Suite
Sqm-Scripts
The Cake shaper
AQMs in BSD
IETF AQM WG

Network Performance Related Resources


Jim Gettys' Blog - The chairman of the Fjord
Toke's Blog - Karlstad University's work on bloat
Voip Users Conference - Weekly Videoconference mostly about voip
Candelatech - A wifi testing company that "gets it".