[Iccrg] Re: [tcpm] RFC5681: why halving FlightSize not cwnd?
Mark Allman
mallman at icir.org
Thu Sep 6 20:54:02 BST 2012
A few things ...
- I don't buy Ethan's argument that the burden on the network is 4
packets if you lose the 17th. It seems to me the burden is measured
from the front of the window not the back. So, in this case it was
a burden of 17 packets that caused the loss.
- The historical reason why we use FlightSize instead of cwnd is that
cwnd was not always reflective of the burden placed on the network.
For instance, some implementations just let the cwnd grow without
bound regardless of whether it was used or not. So, the cwnd might
be (say) 100 even though you have never sent more than 20 segments
per RTT when you go to set ssthresh.
- (The note about the rwnd is not elegant! All we mean is that the
cwnd is growing without being used and, hey, it could even grow
higher than the rwnd. But, really who cares about the rwnd ... the
point is that it grows unused and so is not reflective of anything.)
- I may well buy that the current scheme leaves us too conservative
due to cases like you sketch. Exactly how important this case is
I'll leave to you and Ethan to fight out! :-)
- So, without additional schemes we're left with being too aggressive
(using cwnd) or too conservative (using FlightSize). But, if we're
going to error that is probably the right direction.
- I probably would not have all that much heartburn making the
ssthresh 10 in the case you describe as long as there was some
knowledge that a cwnd of 20 was used recently. I.e., it isn't the
result of some large storage of permission to send that was built up
over time, but was in fact the result of the application's sending
pattern. I think one could design some rules around that notion
that would be OK.
allman
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