[p2p-hackers] Hard question....

Bob Harris bob.harris.spamcontrol at gmail.com
Mon Apr 3 06:30:11 UTC 2006

Two reminders: (1) you gotta keep in mind where the bottlenecks are, and
(2) network usage is bursty.

So (1): TCP flows will achieve "min-max fair share" of the bandwidth, i.e.
they will saturate a link to the maximum capacity of the bottleneck between
the source and the sink. Suppose you have:

                              ------------ D ------ F
A ---- B ---- C -----<
                              ------------ E

Suppose DF is the bottleneck, and AB has 6 Mb bandwidth. Flow A-F might
consume somewhere between 0.5 DF to 1 DF on AB. The AE will have
plenty of bandwidth left over.

On (2): Suppose there is an AF flow, with the bottleneck link at AB at 6Mb. The
will not be consuming bandwidth constantly - there will be bursts of
AIM may not have anything to send most of the time. When it does, it will
likely slow-start to bottleneck capacity pretty quickly. Another flow, say
should get 5.9 Mb by bursting to 6 Mb when the link is free, and throttling
3Mb when there is competition. So achieving 5.9 depends on the "over time"
behavior of the protocol as opposed to how it shares the bandwidth "over

So that's two different scenarios where there would be unused capacity on
the link.
I think I summarized David's scenario accurately.


On 4/3/06, Matthew Kaufman <matthew at matthew.at> wrote:
> David Barrett:
> > Um... most connections aren't saturated 24x7.  Like, I have a
> > 6Mbps connection and sometimes I'm just using AIM.  In this
> > situation, I'd like to measure that 5.9Mbps is free.
> 5.9Mbps is free to where?
> I'll bet that 5.9 Mbps isn't even free to the first IP hop you see, much
> of
> the time.
> What really matters is how much bandwidth is available between you *and
> the
> source or sink you are trying to communicate with*
> Matthew Kaufman
> matthew at matthew.at
> http://www.amicima.com
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