Re: [Bloat] The wireless problem in a nutshell

[Justin McCann <jneilm@gmail.com>]

On Mon, Feb 7, 2011 at 6:31 AM, Dave Täht <d@taht.net> wrote:
>...
> How long can the RTT get before TCP throughput drops?
>
> Good question.
>
> That depends on your tolerance for latency and packet loss. You can
> increase your RTT, and reduce packet drops, by using buffering at the
> expense of latency. ...

There's a rule of thumb for the upper bound in two SIGCOMM-award
papers; I generally use the simple one from the Mathis paper.

bandwidth = (MSS / RTT) * (C / sqrt(loss))

   MSS = maximum segment size
   RTT = round trip time
   C = a constant between 0.87 and 1.31, depending on the ACK strategy
and type of loss
   sqrt(loss) = square root of the probability of losing a packet

So, for a given connection, your expected throughput scales linearly
with the size of your packets, and inversely with the RTT and
sqrt(loss).

I have an unverified/untested Python implementation of the equations
at http://www.cs.umd.edu/~jmccann/tcp_tput.py

I wonder how jumbo frames play into bufferbloat. When queues are in
terms of number of packets (not bytes), jumbo frames make the problem
even worse.

    Justin

* The Macroscopic Behavior of the TCP Congestion Avoidance Algorithm,
by Matthew Mathis, Jeffrey Semke, Jamshid Mahdavi, and Teunis Ott, CCR
27(3), 1997. (pdf:
http://ccr.sigcomm.org/archive/1997/jul97/ccr-9707-mathis.pdf)
* Modeling TCP Throughput:  A Simple Model and Its Empirical
Validation, by Jitendra Padhye, Victor Firoiu, Don Towsley, and Jim
Kurose, Proc. of ACM SIGCOMM 1998. (pdf:
http://conferences.sigcomm.org/sigcomm/1998/tp/paper25.pdf)