Ilpo Järvinen wrote:
On Tue, 4 Dec 2007, John Heffner wrote:
Ilpo Järvinen wrote:
...I'm still to figure out why tcp_cwnd_down uses snd_ssthresh/2
as lower bound even though the ssthresh was already halved, so snd_ssthresh
should suffice.
I remember this coming up at least once before, so it's probably worth a
comment in the code. Rate-halving attempts to actually reduce cwnd to half
the delivered window. Here, cwnd/4 (ssthresh/2) is a lower bound on how far
rate-halving can reduce cwnd. See the "Bounding Parameters" section of
<http://www.psc.edu/networking/papers/FACKnotes/current/>.
Thanks for the info! Sadly enough it makes NewReno recovery quite
inefficient when there are enough losses and high BDP link (in my case
384k/200ms, BDP sized buffer). There might be yet another bug in it as
well (it is still a bit unclear how tcp variables behaved during my
scenario and I'll investigate further) but reduction in the transfer
rate is going to last longer than a short moment (which is used as
motivation in those FACK notes). In fact, if I just use RFC2581 like
setting w/o rate-halving (and experience the initial "pause" in sending),
the ACK clock to send out new data works very nicely beating rate halving
fair and square. For SACK/FACK it works much nicer because recovery is
finished much earlier and slow start recovers cwnd quickly.
I believe this is exactly the reason why Matt (CC'd) and Jamshid
abandoned this line of work in the late 90's. In my opinion, it's
probably not such a bad idea to use cwnd/2 as the bound. In some
situations, the current rate-halving code will work better, but as you
point out, in others the cwnd is lowered too much.
...Mind if I ask another similar one, any idea why prior_ssthresh is
smaller (3/4 of it) than cwnd used to be (see tcp_current_ssthresh)?
Not sure on that one. I'm not aware of any publications this is based
on. Maybe Alexey knows?
-John
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
