[e2e] RES: Why Buffering?

Detlef Bosau detlef.bosau at web.de
Thu Jun 25 11:54:09 PDT 2009

Lachlan Andrew wrote:
> ACK clocking means that the arrival rate equals the packet rate, over

Hm. First of all, ACK clocking means that packets are sent to the 
network when other packets leave the network.

So, the arrival rate is determined by the ack rate. (Although I'm still 
a bit reluctant towards the term "rate".)

> a long time.  The arrival rate is halved if the buffer is negligible,
> but not if the buffer is large.
One problem is that packets don't travel all parts of a path with the 
same speed. TCP traffic may be bursty, perhaps links are temporarily 
>>  >>Surely not a different one for any possible flow.
>> removing the RT dependence in TCP congestion window is a goal of quite
>> a few researchers...
> The real motivation for large buffers is minimising complaints from
> customers.  If a handful of important customers buy cards and use them
> to send a few TCP (Reno) flows over a long-RTT path, then the cards
> that those customers buy have to have "big" buffers to get high
> throughput.  That causes Cisco to design for 100ms of buffering.  (Not
> "one RTT", which is ambiguous, as Detlef pointed out.)

I once was told that a guy could drastically improve his throughput by 
enabling window scaling.....

On a path from the US to Germany.

I'm not quite sure whether the other users of the path were all amused 
about the one guy who enabled window scaling ;-)

But I'm convinced that the RAM manufacturers were enthusiastic to see 
their chips fully used for the first time....;-)
>>  >>So, after a long list of scenarios where Alexandre is right: Is there a
>>  >>scenario, where he is wrong? ;-)
>> probably.....
> Yes, the one I listed earlier in this thread:  A small number of
> long-lived Reno-like flows with large BDPs are bottlenecked at this
> router.  Most routers don't carry this load, but when it was cheap to
> put in enough memory, it was a common enough case to influence router
> design, for better or worse (but not 'til death do us part).

Say's theorem ;-)  Every offer gets his market ;-)

Detlef Bosau		Galileistraße 30	70565 Stuttgart
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