[Iccrg] An aggression metric?

[Michael Welzl]

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