Re: [Bloat] Best practices for paced TCP on Linux?

[Dave Taht <dave.taht@gmail.com>]

On Sat, Apr 7, 2012 at 2:49 PM, Fred Baker <fred@cisco.com> wrote:
>
> On Apr 7, 2012, at 11:50 AM, Dave Taht wrote:
>
>> http://www.kernel.org/doc/man-pages/online/pages/man7/tcp.7.html
>>
>> 64-256k seems about right but the math is eluding me this morning.
>
> For a 5 MBPS data stream, Path MTU = 1460, 100 ms RTT, you're looking at
>
>                       rate in bps    rtt in microseconds
> cwnd_limit = ceiling ( ----------- * ------------------- )
>                          8*pmtu         1e6
>
>                        5e6          100 e 3
>           = ceiling ( ------ * ------------------- )
>                       8*1460          1e6
>
>           = ceiling ( 428 * 0.100 )
>
>           = 43
>
> you probably want to bump that by one or two to account for 43*40 bytes of IP and TCP headers.
>
> 43*1460 is 62780 bytes per RTT, which is frightfully close to 65K bytes per RTT, 524,280 bits per RTT, or 5,242,800 bits per second with the stated RTT. Hmmm.
>
> Speaking strictly for myself, I would throw in one caveat, which is that a variable bit rate codec that averages 5 MBPS sometimes sends faster, and there may be good reason to allow it to. I think I'd recalculate for 6 MBPS on average, and carefully insert the RTT I cared about into the calculation. Doing that also accounts for the Mathis formula, which is far more complex and requires a lot more assumptions, but will come up with a number below 6 MBPS for a .1% loss rate.

In my case I'm 196ms away and running this for the past hour or so

vlc -6 http://pannekake.samfundet.no:3015/

seems to show it never really getting out of slow start.

Regrettably my favorite graph crashes xplot (grr)... so I can't see
the canonical bloat pattern or not.

Could be a problem in my lab (but the udp stream is ok), am checking now...

Captures are up here:
http://huchra.bufferbloat.net/~d/captures/gathering/pannekake_3015.cap

-- 
Dave Täht
SKYPE: davetaht
US Tel: 1-239-829-5608
http://www.bufferbloat.net