[9fans] tcp!

[erik quanstrom <quanstro@quanstro.net>]

since it came up, i put my working copy of tcp along with some testing
scripts in /n/sources/contrib/quanstro/tcp.

there are a number of fixes rolled into this, but the main fixes are
- add support for new reno,
- properly handle zero-window probes (on both ends),
- don't confuse the cwind with the receiver's advertized window.  this
particular condition can lead to livelock.
- don't confuse the window scale with the amount of local buffering
we'd like to do.
- and, don't queue tcp infinitely, which can crash kernels.  :-)

i don't have the numbers for the old tcp handy, but i think you'll
be surprised at how much difference there can be.  i saw differences
of 20x when the sender was limited in how fast it could read by the
read rate from user space.

i've included "testscript."  for the two machines i have handy, i get
the following results with new and old tcp.

machine		stack	kernel	0ms delay	1ms delay
ideal		-	386	unlimited	8.19mb/s

xeon x5550	old	386	138mb/s		0.49mb/s  (!)
intel atom	old	386	37.2mb/s		0.10mb/s

amd x4 964	new	386	145mb/s		8.03mb/s
intel e31220	new	amd64	303mb/s		8.15mb/s
intel atom	new	386	67mb/s		8.03mb/s
	# note: i can get up to 80mb/s using forsyth's qmalloc.


- erik