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

[Fred Baker <fred@cisco.com>]

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.