[e2e] Are we doing sliding window in the Internet?
David P. Reed
dpreed at reed.com
Sun Jan 6 11:05:15 PST 2008
Thinking deeply about "absence of congestion collapse" and "fairness" -
it is clearly not possible for a protocol (or the network itself) to
absolutely guarantee either property. This is the core of the argument
we made in the paper called End-to-end arguments in system design".
For a simple example by analogy: it is not possible to design a bridge
that will not collapse when hundreds of thousands of people endeavor to
stand on it with the purpose of collapsing it. You MAY be able to do
such a design if you *assume* that the behavior of people is (for
example) random, without coordination, and "normal". Similarly,
telephone switches and sewage systems are not guaranteed to be either
"fair" or to avoid under all circumstances violating the desirable
As long as the implementation of the Internet is finite and has a finite
capacity, it would seem extremely likely that certain kinds of traffic
patterns that would be "desirable" would also make the Internet
vulnerable to congestion collapse and unfairness according to reasonable
definitions of fairness as viewed by end users [Nick McKeown is known as
an example of a radical who has written often that he thinks fairness is
defined at each router in isolation - but such "router-eyed" views seem
to me irrelevant, since fairness at a router can lead to very non-fair
It seems very likely that attempting to operate the Internet as a whole
at very high loads (the goal of protocol optimizers seems to be to
eliminate any slack time on any links that can be eliminated) create the
very risk of "congestion collapse" that people fear. Systems operating
at extreme points are often quite unstable and non-resilient.
So the safest thing the network as a whole can do to insure lack of
"congestion collapse" is to trade excess capacity for stability. This
can be done, not by legislating end-to-end protocols, but by investing
in new capacity well ahead of demand.
Lars Eggert wrote:
> It's important to remember the two reasons for congestion control from
> Sally's RFC2914: preventing congestion collapse and establishing some
> degree of fairness.
More information about the end2end-interest