David Miller wrote:
From: Rick Jones <[EMAIL PROTECTED]>
Date: Wed, 29 Aug 2007 15:29:03 -0700
David Miller wrote:
None of the research folks want to commit to saying a lower value is
OK, even though it's quite clear that on a local 10 gigabit link a
minimum value of even 200 is absolutely and positively absurd.
So what do these cellphone network people want to do, increate the
minimum RTO or increase it? Exactly how does it help them?
They want to increase it. The folks who triggered this want to make it
3 seconds to avoid spurrious RTOs. Their experience the "other
platform" they widh to replace suggests that 3 seconds is a good value
for their network.
If the issue is wireless loss, algorithms like FRTO might help them,
because FRTO tries to make a distinction between capacity losses
(which should adjust cwnd) and radio losses (which are not capacity
based and therefore should not affect cwnd).
I was looking at that. FRTO seems only to affect the cwnd calculations,
and not the RTO calculation, so it seems to "deal with" spurrious RTOs
rather than preclude them. There is a strong desire here to not have
spurrious RTO's in the first place. Each spurrious retransmission will
increase a user's charges.
All of this seems to suggest that the RTO calculation is wrong.
I think there's definitely room for improving the RTO calculation.
However, this may not be the end-all fix...
It seems that packets in this network can be delayed several orders of
magnitude longer than the usual round trip as measured by TCP.
What exactly causes such a huge delay? What is the TCP measured RTO
in these circumstances where spurious RTOs happen and a 3 second
minimum RTO makes things better?
I haven't done a lot of work on wireless myself, but my understanding is
that one of the biggest problems is the behavior link-layer
retransmission schemes. They can suddenly increase the delay of packets
by a significant amount when you get a burst of radio interference.
It's hard for TCP to gracefully handle this kind of jump without some
minimum RTO, especially since wlan RTTs can often be quite small.
-John
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