[Iccrg] Flow Rate Fairness: Dismantling a Religion

Fri Oct 27 04:14:08 BST 2006

My comment on Bob's paper. It is a bit long (about 2 pages)

Overall, I believe this “cost-base fairness’ approach is refreshing 
because it enlarges the scope of congestion/fairness and potentially 
opens up a better class of solutions for both network operators and 
network users.
Solutions from this approach may allow a network operator to operate its 
network effectively by separating its internal concerns from its 
external concerns. Internally, it may manage its network in any way it 
sees fit (flow rate, implicit, explicit, etc.). Externally, it has more 
freedom in providing new classes of solutions to satisfy its customers 
from the network edge (cost-based, fairness, security, mobility, etc)
Solutions from this approach may deliver network users (customers) 
proper means to deploy better strategies (in using the network) to 
deliver the required performance for their applications.
I believe that this approach deserves further exploration.
However, to make effective the argument for cost-base fairness, the 
paper should define its scope more clearly. In particular, it should 
point out that
1) Congestion and Fairness are to distinct issues.
2) One can provide a perfect solution to congestion problem without 
being fair
3) Solving “fairness” problem might have noting to do with network 
congestion.
4) The users constitute only one part of the network congestion problem. 
Congestion occurs when the network operator do no provision network 
resources adequately, or manage their resources properly. It may occur 
when users exploit the network aggressively. It may occur as a result of 
some random events such as unpredictable traffic patterns.
5) When congestion is not caused by users, should they be penalized?

Most of the arguments for “cost-based fairness” appear in the 
introduction of the paper hence, I mainly comment on this section. 
Further comments will be provided if necessary. Bob’s words are in quotes.

Introduction

“Relative flow rates should be the outcome of another fairness 
mechanism, not the mechanism itself”
You can say this about the outcome of everything if you stand above the 
problem and see it from a larger perspective. However, doing so is not 
always appropriate as from this enlarged problem scope, undesirable 
complexities may entail.
In this case (cost-based), it is a good position to take as we need 
better solutions for congestion control and fairness.

“The metric required to arbitrate cost fairness is simply volume of 
congestion, that is congestion times the bit rate of each user causing 
it, taken over time.”
Very reasonable metric provided that information can be collected 
accurately and timely, otherwise it can be useless.

“this isn’t just some dry academic fairness debate”
Great. Many of us academic are not dry and quite “fair”.

“because flow rate fairness isn’t even capable of reasoning about 
questions like, “How many flows is it fair to start between two 
endpoints? or over different routes?” or, …..”

Flow rate fairness does not have to answer to these questions as they 
are outside of its scope.
Let me explain this. From the network operator view point, it will 
optimize its operational cost and maximize thee return for the use of 
its resources. The operation can be divided in two components: internal 
and external. Internally, it can use whatever congestion control 
mechanisms and fairness measure to maximize its performance. Externally, 
this is the job of edge devices that interface with the customers. The 
operator may monitor its customers, performs policing their traffic 
according to their SLA, or it may even play along with the customers, 
taking their welfare into account if it chooses to do so. For example, 
the network operator may take fairness among customers (or economic 
entities) into account if it thinks that would help in keeping its 
customers and raising its revenue and minimizing the cost of internal 
operation.
Here, your “cost-based” fairness may play an interesting role. It is a 
game between network operator (edge devices) and its customers. Network 
operator tries to satisfy its customers based on its current resources 
AND prevent customers from exploit its resources unfairly. The customers 
try everything to get at least what they pay for, even to get more than 
their “fair share” (regardless of how fairshare is defined). They may 
want to spit their flow, mark their packet favorably, etc.
Cost-based fairshare may be effective here, providing one can define the 
“cost” and its “atomic unit cost” on which “fairness” can be defined. 
Furthermore, there must be some rules by which customers and their 
network operator both agree upon; otherwise “cost-based fairness” will 
encounter the same problem that “flow-rate fairness” encountered.

“In 1997, the basis of the dominant consensus was completely undermined, 
but it didn’t even notice”
Even now we cannot describe congestion adequately due to the limitations 
of mathematics as well of our understanding of the phenomenon. A paper 
by Jean Bolot [1] deals nicely with one-bottleneck congestion and I 
stress “one bottle-neck”. More than that, who knows!
Even now we cannot describe essential characteristics of Internet 
traffic mathematically, again partly due to the limitations of 
mathematics, partly due to the lack of measurement data!
Kelly brilliantly introduced some nice ideas from another domain to 
networks with the hope that this price-based control could apply 
effectively in the new domain. Currently, we do not even have a proper 
model of how customers behave or react to priced-based admission control 
in a large scale. Unless you can do so, network operator will not take 
it up.

“novel p2p applications have started to thoroughly exploit this vacuum 
with no guilt or shame, by just running more flows for longer”
I would not blame P2P applications. Everyone wants to get as much as 
they can out of the network legitimately. Is it true that network 
operator has been squeezing as much as they can out of their customers 
legitimately? It is here that we may provide solutions that help both sides?

“Fair allocation of rates between flows isn’t based on any respected 
definition of fairness from philosophy or the social sciences.”
Who says that our society is fair and the social sciences that go with 
it are fair? They are not. Look at the global economic situation. It is 
only fair to those who can push their wares but not others. 
Occasionally, it breaks down just like any other systems with even more 
serious consequences.

I have done some work on congestion, DiffServ, Edge-to-Edge, and 
End-to-End QoS. We may continue the discussion in this forum if it is 
“fair” to other respected researchers.

[1] Bolot, J., and Shankar, A., “Dynamic Behaviour of Rate-based Flow 
Control Mechanisms,” ACM Computer Communication Review, 20(4): 35-49, 1990.

Cheers,

Doan

Bob Briscoe wrote:

> ICCRG folks,
>
> Here's the memo on deconstructing the Internet community's dominant 
> fairness ideology that I promised. It should appear in internet-drafts 
> shortly, but you can pull the tech report off my Web site now if you 
> prefer.
>
> To some of you it will not be anything new, but I have found that very 
> many people have got into a trap in their thinking, so I'm trying to 
> jump people out of what is actually a self-referential dogma. Concepts 
> of fairness in TCP, WFQ and so on are all afflicted with this dogma 
> which is actually way off beam relative to any reasonable views of 
> fairness in real life. It's deliberately blunt and simplified.
>
> Flow Rate Fairness: Dismantling a Religion
> <http://www.cs.ucl.ac.uk/staff/B.Briscoe/projects/2020comms/refb/fair_tr.pdf> 
>
>
> Abstract: We were moved to write this memo because the applied 
> research and standards communities in networking are using completely 
> unrealistic and impractical fairness criteria. The issue is not 
> whether they should use this or that allocation scheme; they don't 
> even allocate the right thing and they don't allocate it between the 
> right entities. We explain as bluntly as we can that sharing out flow 
> rates (as TCP and many other popular fairness mechanisms do) has no 
> intellectual heritage from any concept of fairness in philosophy or 
> social science, or indeed real life. Comparing and controlling flow 
> rates alone will never achieve fairness and should never again be 
> claimed as a fairness mechanism for production networks. Instead, a 
> realistic fairness mechanism must share out the `cost' of each users 
> actions on others.
>
> Here's the same text formatted as an I-D, but it becomes twice as long:
> <http://www.cs.ucl.ac.uk/staff/B.Briscoe/pubs.html#rateFairDis>
>
> I may not be able to get into discussions over the next week or so, 
> given other deadlines...
>
>
> Bob
>
>
>
> ____________________________________________________________________________ 
>
> Notice: This contribution is the personal view of the author and does 
> not necessarily reflect the technical nor commercial direction of BT plc.
> ____________________________________________________________________________ 
>
> Bob Briscoe, Networks Research Centre, BT Research
> B54/77 Adastral Park,Martlesham Heath,Ipswich,IP5 3RE,UK. +44 1473 645196
>
>
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-- 
Doan B. Hoang
University of Technology, Sydney	
URL: http://research.it.uts.edu.au/arn