Re: 2 cents re: MTU

Bernard Matthews (
Thu, 25 Jun 1998 14:57:12 -0400

Hi Brian,

Are the rules changing because of the overall speed of the
connections/technologies involved, or because of the asymetry
of these higher speed technologies between the user and the ISP?

Just curious, and possibly off topic.

----- bernard

At 11:49 AM 6/25/98 -0600, Brian Godette wrote:
>Hmmm, geee, I hope you do know that RWIN is what controls how much data you
>can receive on a TCP stream before sending an ACK. Reducing this can
>potentialy slow down a TCP transfer drasticly. Setting it larger increases
>the response time for lost/corrupted packets. RWIN wants to be large enough
>so that there's always slightly more payload in transit than it takes for
>an ACK to reach the other end (which is where unloaded ping times become of
>some interest). It takes around 420ms to receive a 1500 byte packet (1460
>byte payload), as soon as the RTT of an unloaded ping goes over 840ms RWIN
>needs to become 3x MTU. So yeah 4x works, but only for *analog* modem
>connections, soon as you go to x2/k56flex/v.90 the rules change, the rules
>change even more for ISDN/FR/xDSL/cable modem/etc.