[e2e] TCP out of order
fred at cisco.com
Wed Jan 23 17:28:08 PST 2002
At 11:51 PM 1/22/2002, Michael Welzl wrote:
> > Note that you can get very consistent packet reordering even if the
> > speed-of-light delay difference is zero and the circuit bandwidths
> > are identical, if the packets being generated are sometimes different
> > sizes. That is, if you send a big packet followed by a small packet, the
> > small packet will always catch up to and pass the big packet if they
> > traverse enough store-and-forward hops.
>Forgive my ignorance - but: why?
in addition to issues mentioned, chaos theory applies. If two packets
arrive simultaneously at a router, cross a backplane, and go out the same
interface, either one might go "next", and the second will wait for the first.
Further, the queue depth on any given interface is not zero. Normally,
networks are well overprovisioned, so p/(1-p) comes up with a small number,
but it is a random number averaging a certain value, it is not a certain value.
At 01:16 AM 1/23/2002, Michael Welzl wrote:
>Just out of curiosity: doesn't this pose a problem for the packet pair
yes and no. PP generates an estimate of the link bandwidth available, and
one would expect that it would normally be an estimate of the *available*
bandwidth on a bottleneck link, since in a packet network any packet
presumably has O(p/(1-p)) packets in queue on arrival of each of the
packets in the pair. What it means is that there is some error in the
estimate that wouldn't be there if everything were indeed deterministic.
But please, what does the word "estimate" mean? I think we already know
that it is an initial approximation to a value that will vary over time.
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