[Tmrg] Round table: Buffer sizes

[John Heffner]

Lachlan Andrew wrote:
> Greetings all,
> 
> Another question on the list is buffer sizes.
> 
> I'd obviously like to standardize on the buffer sizes that WAN-in-Lab
> supports "natively", namely 128 packets at 1Gbps and 16384 packet at
> 2.5Gbps, but they're fairly ad-hoc choices.  WAN-in-Lab can
> alternatively be cajoled into using buffer sizes of any power of 2
> from 128 to 8192 packets.
> 
> An obvious buffer size to set is some multiple of the BDP, but that is
> not well defined if flows have different RTTs.  Setting the buffer too
> large will mask the effects of RTT unfairness; for example, setting
> the buffer to be the size of the maximum BDP would mean that all RTTs
> are within a factor of 2, even if the actual path delays differ by a
> factor of 10.
> 
> Also, Cisco buffer sizes seem to be specified in numbers of packets
> not bytes, and I believe Dummynet has an option to do that too.  This
> makes a "BDP-sized " buffer hard to define with bidirectional traffic,
> since the number of packets depends on the fraction of ACKs vs
> full-sized packets.
> 
> Given all that, can someone suggest suitable buffer sizes for this
> core set of tests?


Given the relatively wide range of buffer sizes in hardware out there at 
any given speed, and the significant effect buffer size can have on 
congestion control, it seems like this should be an extra dimension 
rather than fixed per link type.

Maybe select a couple different sizes based on a set of drain times? 
Say, {1 ms, 10 ms, 100 ms, 1 sec}?  Convert this to bytes or packets by 
dividing by the link byte or MTU packet rate.

   -John