[Iccrg] An aggression metric?

[Lisong Xu]

Hi Michael,

This is an interesting idea. But it may be hard to get a single number 
N, since this number may be different for different packet loss rates 
and base RTTs.

A similar method is to compare the response function (where N is also a 
function), which captures more information of the "aggression" of a 
congestion control mechanism.

Lisong

On 7/14/2011 2:15 AM, Michael Welzl wrote:
> Hi!
>
> Here's an idea. Our group's charter says: "The key goal of the Internet
> Congestion Control Research Group (ICCRG) therefore is to move towards
> consensus on which technologies are viable long-term solutions for the
> Internet congestion control architecture, and what an appropriate
> cost/benefit tradeoff is."
>
> For a "viable long-term solution", I think that the "aggression" of a
> congestion control mechanism is important, but most evaluations focus on
> its efficiency in terms of bandwidth utilization, fairness among flows
> of their own kind, etc. By aggression, I mean:
> - what happens when it fights against a standard TCP?
> - what happens when it fights against its competitors, e.g. (insert your
> favorite mechanism here)?
> It's not uncommon to have a diagram that shows one of these things in
> papers too, but what would really be good, I think, would be to have a
> unified way of looking at it - an aggression metric. Something that lets
> me conclude that, e.g., CUBIC is 7-aggressive, HTCP is also 7-aggressive
> (of course these two are surely equal! he he :) ), FAST is 3-aggressive,
> Westwood is 12-aggressive, whatever! Something like that.
>
> Standard TCP could be the base unit (the number 1) here. We recently
> finished a paper in which we present an extension of the TCP
> steady-state throughput equation for multiple flows - i.e. from the
> packet size, loss event rate, RTT, and now also the number of flows (N)
> and the actual packet loss ratio, one can calculate the rate at which N
> flows would send. This is the paper:
>
> Dragana Damjanovic, Michael Welzl: "An Extension of the TCP Steady-State
> Throughput Equation for Parallel Flows and its Application in MulTFRC",
> accepted for publication in IEEE/ACM Transactions on Networking, 2011.
> http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5756471&tag=1
>
> The equation is also included in our MulTFRC draft:
> http://tools.ietf.org/html/draft-irtf-iccrg-multfrc-01
> (btw, we're just about to finish an update of this document, stay tuned)
>
> It wouldn't be hard to turn this equation around such that, from the
> packet size, loss event rate, RTT, packet loss ratio and sending rate,
> one could calculate how many standard TCP's (N) must have produced (or
> would be represented by) this rate. This would also work with floats,
> e.g. a calculation could yield N=3.52 or something like that. Thus, one
> could carry out a "benchmark test" simulation of a high-speed mechanism
> where it's confronted with different loss and RTT conditions, and from
> its resulting rate, one could then say that it's between X and
> Y-aggressive, i.e. representing between X and Y TCPs.
>
> Would that be a useful "aggression" metric?
>
> e.g. an alternative could be to produce a simpler equation which is not
> so much based on all the specifics of TCP (slow start etc), maybe just
> use N * MSS/RTT * (1/sqrt(p)), see where a modern TCP with slow start
> stands in relation to that, and where high-speed mechanisms stand in
> relation to that. Or should use an even simpler equation?
>
> Do we even need or want such a metric?
>
> Cheers,
> Michael
>
>
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-- 
Lisong Xu, Associate Professor
Computer Science & Engineering
University of Nebraska-Lincoln
http://cse.unl.edu/~xu