[e2e] why fair sharing? ( Are we doing sliding window in the Internet?)
gorinsky at arl.wustl.edu
Fri Jan 12 14:54:49 PST 2007
> How hard it is to turn the Fair Queueing knob to "on" on the gateways?
To put my 2 kopecks in... First, since an application can masquerade as
multiple flows, fairness enforcement with FQ is not effective. To lend
itself to meaningful enforcement, fairness should be defined not in terms
of flows or even hosts/processes generating them. Instead, fairness
should be linked to humans behind the communications but this requires
a very different network architecture.
Second, packet-by-packet FQ and end-to-end TCP strive to approximate
instantaneous PS (Processor Sharing) which is not a good fit for any
natural application. Multimedia streams need a minimal rate, not a fair
share. Elastic applications are not well served by PS either because
average message delay is much larger than under SRPT (Shortest Remaining
Processing Time), in agreement with Internet experiences where deviations
from short-term fair sharing improves overall efficiency.
While minimizing the average message delay, SRPT also might starve large
messages. However, one can have it both ways: the rich can get richer
without making the poor poorer. ViFi (Virtual Finish Time First), which
schedules messages preemptively in the order of their finish times under
PS, is close to SRPT (and much better than PS) with respect to the average
message delay and guarantees that no message is delivered later than
under PS. You can read more on ViFi in:
S. Gorinsky and N. S. V. Rao, "Dedicated Channels as an Optimal
Network Support for Effective Transfer of Massive Data", Proceedings of
High-Speed Networking (HSN 2006), April 2006.
The paper and respective simulation suite are available at:
In the context of web servers, ViFi was independently proposed under
a name of FSP (Fair Sojourn Protocol):
E. J. Friedman and S. G. Henderson, "Fairness and Efficiency in
Web Server Protocols", Proceedings of ACM SIGMETRICS 2003, June 2003,
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