[xcp] Questions about delta_throughput and reaction to packet loss

[Ted Faber]

On Mon, Nov 21, 2005 at 08:53:15PM -0000, sampad  mishra wrote:
> Don't you ppl think tht it is too much of an ask from a router to do
> the calculations like 'Calculations Upon Packet Arrival' ,
> 'Calculations Upon Packet Departure' and 'Calculations Upon Control
> Interval Timeout' ?

Nope.

It's much harder to get router designers to put the calculations in.
And there are some good reasons for that; if they put every idea that
someone throws out in an academic paper into a router, well, you're
starting to talk about an awful lot of code to write and maintain.  and
congestion control isn't something you can go alone.

We do think that the cost/benefit tradeoff for getting XCP into routers
is a good one - particularly because the cost of the actual calculations
are so low.  It's a few multiplies and adds per packet and a few words
of memory per router.  The per-CI calcucations - which are more involved
- can be done in a slower processor, and are not terribly time-critical.
They're trivial compared to running a routing algorithm.

We have a student working on an IXP implementation that we have every
reason to believe will saturate a Gb/s link using the fairly clumsy IXP
architecture.  We think that putting the few operations directly on a
chip will be very cheap in terms of processing time.

I do believe strongly that these per-packet computational costs are
pretty minor, but right now the onus is on XCP proponents (like me) to
prove that there really is a benefit and no bugaboos lurking in the
corner.  I haven't seen anything too awful, but there are lots of
corners unturned at this point.  And I would not say that it's a great
idea to shift all hosts over to XCP tomorrow, even if such a thing were
possible.

-- 
Ted Faber
http://www.isi.edu/~faber           PGP: http://www.isi.edu/~faber/pubkeys.asc
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