netdev - Re: Problem about Setting TCP Congestion Window to a Small Constant

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

Date:	Sat, 22 Nov 2014 12:34:23 -0500
From:	John Heffner <johnwheffner@...il.com>
To:	wc8348@...utexas.edu
Cc:	Netdev <netdev@...r.kernel.org>
Subject: Re: Problem about Setting TCP Congestion Window to a Small Constant

On Sat, Nov 22, 2014 at 11:29 AM,  <wc8348@...utexas.edu> wrote:
> Hi all,
>      My name is Wenzhi Cui, a graduate student working on TCP congestion
> control in Linux Kernel. I have a problem about congestion control in
> linux kernel.
>
>      Recently I am playing with TCP Congestion Control Protocol by setting
> the congestion window size to a constant, say 1, and measure the TCP
> flow throughput to see the relationship between cwnd size and
> transmission rate (on a stable environment).
>
>      The problem is, when I am setting the snd_cwnd to 1, the real
> transmission rate is around 25MBps. However, since I am testing TCP
> on our department network with 1Gbps (which is 125MBps) bandwidth and
> 300 micro seconds Round trip time (measured by analyzing tcpdump
> trace). So the theoretical sending rate should be
> CWND * MSS / RTT = 1 * 1460 Byte / 300 us = around 5 MBps
> which is far less than the observed 25 MBps bandwidth. I have taken a look
> at tcp_cong.c, tcp_input output.c, etc. but I still cannot find the
> problem.
>
>      Can somebody help me figure out what is missing, maybe what may
> affect the real TCP transmission other than CWND or what will happen
> when CWND is set to a very small constant?

Looking at a tcptrace "outstanding data graph" may be illuminating.

  -John
--
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