[Tmrg] Round table: Buffer sizes

[John Heffner]

Lachlan Andrew wrote:
> Greetings again,
> 
> On 02/10/2007, Lachlan Andrew <lachlan.andrew at gmail.com> wrote:
>> On 01/10/2007, Sally Floyd <sallyfloyd at mac.com> wrote:
>>> some links that have been measured, a realistic mix means
>>> 90% of data packets with 1500 bytes, with a mix for the
>>> remaining data packets of 500 bytes, 4000 bytes, 200 bytes,
>>> and the like.
>> Since the TCP algorithms themselves determine the percentages of
>> traffic, we should specify the traffic in terms of the number of
>> flows   with each MTU, rather than the amount of traffic.  How about
>> specifying 90% of flows use 1500-byte, and 10% of flows use 536-byte?
> 
> On second thoughts,  TSO and iperf's blocking themselves produce a
> significant number of packets below the MTU.  If we have 100% of flows
> with an MTU of 1500 and use TSO, we may automatically get 90% 1500
> byte packets, and 10% smaller ones.
> 
> If we rely on this artifact, we should control for it (specify iperf
> parameters?), and specify how to get comparable results in
> simulations.
> 
> Having one flow with 10% of its packets small is very different from
> having 10% of flows with all of their packets small.  Sally, I assume
> the study you referred to was pre-TSO, but many of the smaller packets
> could still have been runts from connections with larger MTUs.
> 
> Thoughts?

TSO traffic will often give you only MSS-sized segments until the window 
gets large enough that you start sending full 64k packets down to the 
driver.  (If I remember correctly how it works now -- there have been so 
many changes..)

One thing to try might be using setsockopt(TCP_MAXSEG) on some flows. 
That's probably easier than changing interface MTUs.

   -John