[Tmrg] Tmrg-interest Digest, Vol 11, Issue 4

[Lars Eggert]

On 2007-8-22, at 10:26, ext Douglas Leith wrote:
> Re dummynet, my experience these days is that it can run up to 1Gb
> using modern hardware.  In my experience more of an issue is that end
> hosts can still have difficulty at high bandwidth-delay products due
> to sack processing overhead etc. and it is this that has placed the
> upper limit on test speeds rather than anything else.  I'm not sure
> where things break these days but it would be easy enough to check.

It may be interesting to compare the results in a dummynet setup with  
one that uses real hardwarde (Lachlan's setup, for example.)

What I saw a few years back was that dummynet bunched together  
packets in bursts, due to the way device interrupts were handled by  
BSD at the time. Essentially, device driver processing interrupts IP- 
layer processing, and so dummynet only got cycles intermittently. The  
result was that although you'd get a simulated path that had the  
desired bandwidth/delay properties over longer timescales (>> CPU  
quantum), if you looked at the packet-level trace, there were some  
oddities there. For my stuff (queueing), that caused some issues, but  
I'm not sure if it'd matter much for TCP evaluation.

Lars
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