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Dismantling a Religion: Is TCP wrong?

Posted by David Mayer on December 21, 2006


Fairness in communication networks is a topic of great interest. It has been treated in hundreds of papers and led to design of some of the most widely used protocols (e.g., TCP). But Bob Briscoe in his Internet Draft from October 2006 [1] argues that the fairness criteria used by the networking world today is fundamentally wrong.

Specifically, Briscoe

  • Rejects flow rate fairness (widely accepted and used) as completely inappropriate, wrong & ignorant. This also applies to the TCP algorithm and fair queueing algorithms.
  • Suggests that all proposals using flow rate fairness be discarded or someone should write a defence of flow rate fairness.
  • Argues that weighted proportional fairness is a correct fairness criteria. It also notices that the paper introducing weighted proportional fairness rejected flow rate fairness, but the community did not notice.
  • Proposes that weighted proportional fairness is implemented and deployed.
  • Accuses the research community of creating a hegemony in this case and ignoring voices from outside.

To give a short introduction, let us say that fairness is a measure characterising division of resources among users. Once a fairness criteria is defined, a policy can be evaluated with respect to that criteria and policies can be designed which achieve fairness in terms of a given fairness criteria.

The most widely used fairness is based on bit rates of individual flows. It is called flow rate fairness and it attempts at making flows fair to each other in terms of their bit rates. An example of a flow rate fairness is max-min fairness. It is defined as an allocation of rates where no rate can be increased without decreasing some other, already lower, rate.

Flow Rate vs. Weighted Proportional Fairness

Most importantly, Briscoe says that flow rate fairness allocates a wrong measure among wrong entities, namely rate among flows, and that the right way to do it is to allocate cost among economic entities as it is done in Weighted Proportional Fairness proposal by Kelly [2].

Kelly’s paper [2] “struck an intellectual death blow” to the idea of flow rate fairness, but the dominant belief system did not notice, since cost fairness proposal stated its case in terms of the dominant belief system.

Dismantling the Flow Rate Fairness religion involves analysing 2 basic questions: what should be allocated (rates vs. cost) and among what (flows vs. economic entities) should that something be allocated. Following is the compilation of Briscoe’s arguments against Flow Rate Fairness and pro Cost Fairness.

1. What should be allocated? (Fairness of what?)
Regarding this question, Briscoe rejects fairness between flows and justifies cost fairness by arguing that:

  • Arguments against fairness between flows:
    • Flow is not a good measure of either benefit or cost.
    • Equalising rates does not equalise benefits or cost.
    • “Fair allocation of rates isn’t based on any respected definition of fairness from philosophy or the social sciences.”
  • Arguments for cost fairness:
    • Kelly showed that, using cost, each user will want their congestion algorithm to adjust its rate to maximise their net utility—benefit minus cost. Users will allocate themselves rates in proportion to the share of the cost that they cause – Weighted Proportional Fairness.
    • Cost fairness maximises total utility across the network.
    • “Cost in any part of the world has an exchange value with cost in any other part, because, wherever there’s an Internet attachment, ther’s a connection with the global economy.”

Further it is argued that it is not realistic to define fairness within the system (the Internet) without reference to fairness outside the system (society and market). But cost fairness naturally relates to monetary cost. And monetary cost allows to proceed with the concept “you get what you pay for” — a fairness concept we live in.

2. What entities should it be allocated among? (Fairness among what?)
Briscoe argues that fairness should be enforced among economic entities and not flows, as

  • “There is no evidence that fairness between flows relates in any way to fairness between any real-world entities that one would expect to treat fairly, such as people or organisations.”
  • Flow rate fairness is trivial to cheat by creating arbitrary number of smaller flows.
  • “Fairness can only be meaningful between entities with real-world identities—humans, organisations, institutions, businesses.”
  • Allocating resources among economic entities reflects the commercial reality and prevents multiple flows cheating.

Summary

I think Briscoe’s draft brings up a few extremely important points.

The accusation of research community creating a hegemony ignoring voices from outside is a serious one and I would encourage readers to express their views about this.

The lesson for beginning researchers is that one should question everything, even the seemingly fundamental truths in our field. If these are truths indeed, one gains a deeper understanding, if not, one gains a huge potential to publish.

I find a bit vague the statement that cost fairness would achieve outcomes “hopelessly unlike” the outcomes that result from flow rate fairness [1, p.3]. Although the intellectual superiority of Cost Fairness matters a great deal, maybe the practical difference would be negligible due to some large-scale effects, and thus not worth deployment. It would be nice to see a simulation illustrating the difference, but that does not have to be Briscoe’s job, of course. It is the networking research community who should react.

Unfortunately, the page dedicated to FAQ about this draft is empty as of this writing. A suitable place for a timely respond could be the online Computer Communication Review or this very journal (PaperPicker).

It is noteworthy that the draft was written with some help of such networking luminaries like Scott Shenker or Jon Crowcroft.

Bibliography

[1] B. Briscoe. Flow Rate Fairness: Dismantling a Religion. (Internet-Draft. Expires: April 18, 2007), October 2006. http://www.ietf.org/internet-drafts/draft-briscoe-tsvarea-fair-00.pdf.

[2] F. Kelly. Charging and Rate Control for Elastic Traffic. European Transactions on Telecommunications, 8(1):33-37, 1997. http://www.statslab.cam.ac.uk/~frank/elastic.html.

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