Subject: RE: initial window size

From: Vernon Schryver <[email-masked]>
Date: Thu, 29 Mar 2001 14:47:56 -0700 (MST)

> From: Martin McSweeney <[email-masked]>

> Hmmm... "a bit much", isn't 512K an extremely large window given any
> circumstances?

No, it is not so very large if we are not talking about the initial window
size and if you care about going fast.

512 KBytes is only 4 Mbits and a 45 Mbit/sec pipe for only 100 ms. 
A 5,000 mile wire has an RTT of 100 ms. from the speed of light alone.
Assuming zero delay from routers or other bottlenecks, you cannot reach
full speed with a window of only 512K on a 5,000 mile T3.

Even in a 100 Mbit/sec local area network, a window of 512K is not very
large, since 40 ms is not a lot to cover the delays added by a few routers
or bridges and the scheduling and other delays in the hosts at the ends
of a TCP connection.

You almost certainly don't want a tiny 8K window on a 100 Mbit/sec or
faster network, since 8KBytes occupies a 100 Mbit/sec wire for only
640 microseconds.

There are networks that are fast that 512K might be reasonable even
for an initial congestion window.  For example, what is the right
initial congestion window for a 10G Ethernet?

Vernon Schryver    [email-masked]

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From: Joe Touch <[email-masked]>
Date: Thu, 29 Mar 2001 14:10:48 -0800

> initial congestion window for a 10G Ethernet?

That might depend on whether the connection is local or not. Local
connections (same subnet as an attached interface) ignore the congestion
window in some TCP implementations.

Joe

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From: "Oleg Vishnepolsky" <[email-masked]>
Date: Thu, 29 Mar 2001 17:58:10 -0500

Since TCP stack does not have direct knowledge of the speed, the MTU gives a good starting point.
A while back the reasonable ratio was for the window to be about 6-8 times the MTU size for the
local connections (slow start algorithm for non-local).
Is it still the case these days ? 

Oleg Vishnepolsky
-----Original Message-----
From: [email-masked]
[mailto:[email-masked]]On Behalf Of Vernon Schryver
Sent: Thursday, March 29, 2001 4:48 PM
To: [email-masked]
Subject: RE: initial window size

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From: "David St. Germain" <[email-masked]>
Date: Thu, 29 Mar 2001 18:26:05 -0500

Although I agree with you that this is how one would tweak the window size,
the OS still has to default to something.  With Windows NT 4.0, I know the
default upon installation is 512 (I just recently saw that statistic), I'm
not sure what the other OS's default to upon installation, but I believe the
original RFC recommendation was to start with a default of 512 and use the
slow start algorithm to ramp up.

Dave.