Re: Increase tcp initial window

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

>> Imagine this in the presence of routers which implement RED - it
>> would not (and should not) win.  Absent RED, tragedy of the commons
>> ensues (and, possibly, "congestion collapse" - google that and
>> Nagle's RFC on gateways with infinite storage for enlightening
>> historical perspective).

> Why would that preclude making the initial congestion window a sysctl
> tunable?

It doesn't, per se.  What it does argue against is making it tunable by
end-user admins over a nontrivial range.

> There are numerous white papers (including the original Google one
> which started off the IETF effort) which show that having a larger
> initial window is a win,

I'd have to read the paper to tell, but I suspect it either is "theory
indicates this ought to be a win" (with underspecified or unrealistic
assumptions) or "this is a win when dropped as an isolated instance
into [then-]today's Internet".

I think Erik is right.  A large initial window on an end-user host, in
the presence of routers doing RED, just pushes it into congestion
instantly as packets get REDded (hopefully congestion backoff, because
if not it then leads to congestion collapse); absent RED, well, as Erik
put it, tragedy of the commons ensues, with endpoints effectively
fighting over who gets to monopolize choke-point bandwidth.  "You're
going to send ten packets?  _I_'ll send _twenty_!".  Like most
tragedies of the commons, until mechanisms to defend the commons
against the greedy are put in place, this ends up rewarding the greedy
and penalizing the well-behaved good neighbours.  (In this case, the
defense mechanism is something that, like RED, preferentially penalizes
those sending packet floods.)

The only cases where it will win, as far as I can see, are (a) the case
where either there is sufficient capacity that there is no contention
and (b) the case where everyone else is well-behaved, backing off in
the presence of initial packet storms from hosts starting off with
unreasonably large windows for the paths in question.  It's essentially
the opposite of slow start, and loses for the reasons slow start wins.

Of course, rewarding the greedy and penalizing the well-behaved good
neighbours _is_ the direction the net is going these days, so perhaps
this would be appropriate.

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