From: Janet Davis (jlnd_at_cs.washington.edu)
Date: Wed Feb 25 2004 - 16:29:32 PST
Here's the promised information on three unrelated subjects.
===
Last week, I posed a problem: What could cause ACK clocking to break?
Suppose that the acknowledgements, nicely spaced out according to the
bottleneck bandwidth, arrive at a full queue. Further suppose that they
arrive during a lull in the traffic, so that they end up back-to-back in
the queue, even though they arrived at different times. When the router
forwards them, they will be forwarded back-to-back -- faster than the
bottleneck bandwidth, thus breaking the ACK clock.
A total lull in traffic isn't necessary; the link just needs to be at less
than full capacity when the packets arrive at a non-empty queue to mess up
the ACK spacing.
This has been observed to happen on the Internet, and it's called ACK
compression. You can read about it here:
http://citeseer.nj.nec.com/mogul92observing.html
==
We talked a bit about why TCP should provide proportional fairness;
however, TCP is often not fair in the real world. There are several
reasons for this. In particular:
1. ACK compression and the slow start/AIMD algorithms mean that packets
from the same flow tend to be clustered together. As a result, a FIFO
queue overflowing typically causes many packets to be lost from the same
flow, rather than being evenly distributed across all the flows. This is
one of the reasons we like RED, which makes an effort to spread packet
drops out over time.
This could why reloading a web page makes it load faster -- if you are
unlucky enough to have many packets dropped from your connection, then you
have to time out and start over with slow start. If you start a new
connection, you may be more lucky.
2. TCP connections with a longer RTT take longer to increase their window
size in AIMD (since the rule is increase by one packet per RTT), and are
thus at a disadvantage. If there are many connections with very short
RTTs on a bottleneck link, they may crowd out connections with longer
RTTs.
===
This paper describes the process of deciding whether to peer in more
detail.
http://citeseer.nj.nec.com/norton00internet.html
-- Janet Davis jlnd_at_cs.washington.edu http://www.cs.washington.edu/homes/jlnd/ _______________________________________________ Cse461 mailing list Cse461_at_cs.washington.edu http://mailman.cs.washington.edu/mailman/listinfo/cse461
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