[Tmrg] Metrics for releasing bandwidth

[Lachlan Andrew]

Greetings TMRGers,

In a transient when new flows start (particularly non-rate-controlled
"UDP" sources), how can we measure how quickly a TCP algorithm backs
off?

One suggestion was measuring how long it takes until it halves its
window.  Issues with that metric are:
- if the UDP flow is less than 50% of the bandwidth, the flow can
respond ideally and instantly without ever halving its window.
- it ignores how quickly the TCP flow increases its window again after
the reduction
- it focuses on the experience of the TCP flow, not the impact on the UDP flow
- different TCP flows may take different amounts of time to experience
their first loss, especially if the new load arrives gradually as in a
flash crowd.

As an alternative, I propose measuring the number of packets dropped
by the UDP flow(s) from when it starts to 10(?) seconds after it
reaches its peak rate.  This
- is meaningful for any rate of UDP cross traffic
- captures the entire duration of the transient, not just the start
- applies equally well if there are many TCP flows, or the UDP rate
increases gradually.

Unfortunately, this has a "magic number" of 10s.  Can anyone see other
flaws, or better metrics?

Cheers,
Lachlan

-- 
Lachlan Andrew  Dept of Computer Science, Caltech
1200 E California Blvd, Mail Code 256-80, Pasadena CA 91125, USA
Ph: +1 (626) 395-8820    Fax: +1 (626) 568-3603
http://netlab.caltech.edu/~lachlan