Subject: [tmrg] Fwd: An aggression metric?

From: Michael Welzl <[email-masked]>
Date: Thu, 14 Jul 2011 11:17:16 +0200

Hi TMRGers,

I sent this message to ICCRG, but I think it should be of interest to  
TMRG'ers too, so here I'm forwarding it. Please carry out discussion  
on the ICCRG list, though.

Cheers,
Michael

Begin forwarded message:

> From: Michael Welzl <[email-masked]>
> Date: July 14, 2011 11:15:19 AM GMT+02:00
> To: "[email-masked] list" <[email-masked]>
> Subject: An aggression metric?
>
> 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
>