[e2e] RES: Why Buffering?

[Lachlan Andrew]

2009/6/25 Detlef Bosau <detlef.bosau at web.de>:
> 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.

Absolutely.  If buffers elsewhere in the network are emptying or
filling, then the ack rate isn't exactly the rate at which packets
leave this buffer.  However, Jon seemed to be saying that the arrival
rate at the queue could somehow exceed the departure rate in the long
term.  That can only happen if (a) the window is increasing or (b) a
buffer somewhere else is emptying.

> (Although I'm still a bit reluctant towards the term "rate".)

Why?  *Long term* rates are meaningful, even if there is short-term
fluctuation in the spacing between adjacent pairs of packets.

>> 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 unavailable.

True.  Buffers get their name from providing protection against (short
timescale) fluctuation in rate.  However, on the timescale of TCP's
window halving (many RTTs), packets do travel at the same speed over
all parts of the path.

> 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 ;-)

Yes, enabling window scaling does allow TCP to behave as it was
intended on large BDP paths.  If the others weren't amused, they could
also configure their systems correctly.

The comment suggests that you thought that window scaling somehow
makes one user's window larger than TCP intended.  It doesn't.  It
simply allows large windows to be signalled correctly, without
imposing an artificially small limit.

Cheers,
Lachlan

-- 
Lachlan Andrew  Centre for Advanced Internet Architectures (CAIA)
Swinburne University of Technology, Melbourne, Australia
<http://caia.swin.edu.au/cv/landrew> <http://netlab.caltech.edu/lachlan>
Ph +61 3 9214 4837