TCP/IP Receive Window

[Paul McHale]

Hi,

I just had DSL installed and was told to adjust the DefaultReceiveWindow in
the windows registry.  This dramatically improved performance.  To be clear,
my understanding of the setting is this.  It defines the amount of data
which can be sent before an ACK must be received.

The logic is the overhead of ACKs becomes tremendous as the link speeds up.
I believe the default is 8KB.  That is to say only 8KB can be sent before an
ACK must be received.  With a slow link, this worked great.  Missing ACK -
retransmit only 8KB.  With faster links, one is constantly waiting for ACKs.

My question regards the setting for Linux.  What is the default size?  Where
is it defined?  Please keep in mind, latency is the critical factor here.
When an ACK takes 200ms to be received, it is very detrimental to wait for
ACKs.  Even fast lines often suffer this latency.  Of course, Bandwidth and
Latency are separate issues.  Bandwidth is reduced as a function of latency
due to handshaking nature of the protocol.  This also implies LANs would
suffer as much as the latency should be very low.

Side Issue:  This is what led to dramatic performance increases when sliding
window protocols such as zmodem emerged.  I wonder if TCP/IP uses any
sliding window technology.  Probably not since the parameters of the
exchange are not rigidly defined as in zmodem.  I don't believe it does use
sliding window but would love to hear if it does or will.




Regards,

Paul




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Paul McHale
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Re: TCP/IP Receive Window

[Nate Amsden]

Paul McHale wrote:

 I just had DSL installed and was told to adjust the DefaultReceiveWindow in
 the windows registry.  This dramatically improved performance.  To be clear,
 my understanding of the setting is this.  It defines the amount of data
 which can be sent before an ACK must be received.

for some reason win9x is not tweaked by default for networks, this kind
of adjustments is not required(or needed) in linux.

default linux 2.2 settings for defaultrecieve window is 65535

you can tweak it all you like but i suggest keeping it as it is unless
you are having problems, i have setup many linux boxes on dsl
connections and they all absolutly SCREAM.

look at /proc/sys/net/core

each of the files in there stores a setting, you can change it by:

echo some updated setting filename

changes will take effect right away(no reboot required!) you can also
look at the configuration in /proc/sys/net/ipv4

if you want more info on how to tune network subsystems on various OSs
check this page out:

http://www.psc.edu/networking/perf_tune.html

hth

nate

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