[e2e] Skype and congestion collapse.

[Jon Crowcroft]

In missive <003701c521af$89a6f1a0$0200a8c0 at fun>, "Michael Welzl" typed:

 >>But: what about my ftp download?
 >>It's supposed to be a multi service network, after all...
 
not yet it isnt:) - its a best effort service network (read the fine print in your
ISP contract:)
 
 >>> btw, TCP friendliness _is_ selfish - its in your own interest to do as you
 >>would be done by.
 >>that seems strange to me. can you elaborate?
 
read the selfish gene (latest edition) for example, or the
hostdter work on the iterated prisoners dilemma - cooperative behaviour
can (and does) evolve _out of_ purely selfish strategies - its basd on the 
notion investigated in social situations by axelrod where you _evolve_ your 
strategies over a number of encounters - this gets over any explicit cooperation, but 
allows the system of individuals to exchange experience effectively but implicitly 

so think of it like this:

Anyone can think of "cheating on" tcp friendliness.
The fact that _anyone_ can think this, means that more than 1 person will.
If they stop for more than a moment, they'll realiase that a TCP unfriendly
"meme" will propagate very rapidly. It only takes a small percentage of
cheaters to cause the Internet to revert to the congestion collapse pre 1988.
Then all that so called selfish advantage in fact turns quite explicitly to disadvantage 
(as with the oft over-cited commons tragedy). 

i.e. 1 cheater leads many to cheat which means no-one whether good or bad
gets even a fair share, let alone more than a fair share.
So the vast majority of people selfishly, ironically, paradoxically, cooperate.

Of course, there are always a very few stealthy cheaters. The system can tolerate those.

There's some nice simulations of this in several other areas where the idea has become
important, such as peer-to-peer systems, and ad hoc mobile nets and mesh wireless nets, 
where we depend on other nodes  -
(hm, one for lloyd wood here: I have always depended on the kindness of strangers:)
(in manet, this is battery as well as radio capacity, and in p2p its cache storage
as well as uplink v. downlink capacity).

In MANETs and P2P, the "presence" or lifetime of a node, 
coupled with a lot of mechanism for anonimty has led to
the possibility that stealthy cheating pays off because 
the cheater leaves the system before the system acquires to
many bad behaving users or detects them - As a result of this,
researchers have devised lots of schemes to
enforce cooperation (at least statistically) - viz
bittorrent has tokens you need to download, which
you acquire for upload, and systems like CORE and CONFIDANT
(from eurecom and epfl) for MANET...

but in the internet, while noone knows if you are a dog,
they do know where you live and what you did last summer:)

 cheers

   jon