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New Member

bandwidth, delay and tcp window size

We have recently installed a DS3. RTT is 40ms. The wan routers are 7500 running 12.1.17 image.

Will I ever be able to used the entire 45mbps bandwidth ?

IOS 12.1.17 has max tcp windows size of 65535

rfc1323 Window Scale Option is only supported on IOS 12.2.

TCP Window = Bandwidth (bytes/sec) * RTT (seconds)


Equals to 13.107 Mbps.

Should I upgrade the routers to 12.2 which supports rfc 1323 Windows Scale option ?

I have tried increasing tcp window size in my NT machines at both ends. The max transfer I get is 282Kbytes/Sec. I used ftp.


Re: bandwidth, delay and tcp window size

This is a very interesting issue. Strange that there are no other replies so far. I studied the case a bit and thus the following remarks:

When do the routers need to know about TCP window size? Their main task is IP routing, which requires packets only to be examined at layer 3. The window size is something that is negotiated between the end nodes that have a TCP connection established. Exeptions to this occur when the router does something like protocol translation, tunneling through TCP or when a router on one end is also the termination point for the TCP connection. So I would not think that an upgrade will help you much.

Another remark is that the actual speed is much lower than what might be expected from the calculations. 13 Mbps should equal an FTP-rate of about 1Mbyte/s. So your achieved results are 4 to 5 times lower than they should.

When you get this straight, you will just need three simultaneous connections to fill the link almost completely. That should be pretty sufficient as this type of link is never installed to serve only one end node.

Goodluck with it, and please let me know if you solved the issue somehow.

New Member

Re: bandwidth, delay and tcp window size


Can I know how FTP rate is calculated above.


Praful Jaded

New Member

Re: bandwidth, delay and tcp window size

sorry, I was on vacation and just back.

I am using an NT 4.0 workstation and ftp bundled with that. If you have tried ftp from a command prompt, it displays the transfer rate after each transfer is complete. I hope, I can trust Microsoft.



Cisco Employee

Re: bandwidth, delay and tcp window size

Prahlad, What lgijssel telling is(which i agree with), it is the endstations that negotiate the tcp window. Router is not a end station. It is a transit device that route/forward traffic.

pc1, pc2------R1---DS3link---R2-----Pc3, pc4

The Ds3 utilization depends on the tcp window size of the PCs because they are the ones which are the end points of a tcp session.(send/recv data).

PLs let us know if that answers your question.



New Member

Re: bandwidth, delay and tcp window size

Leo and Gopal,

I agree with both of you. but I have tried increasing the windows size to 65535 and still don't see the improvement.

I am trying to arrange Win2k (it supports rfc1323) or Unix machines at both ends to do further testing. Will let you know. Thanks - Prahlad

New Member

Re: bandwidth, delay and tcp window size

I had similar issues. Of course I agree with the previous concensus, that the TCP WINDOW size in the router only needs to be adjusted for TCP connections that are made to the router (rsh, rlogin, rcp, telnet, tacacs, dlsw, etc).

I increased TCP Window Size to 256950, enabled Window Scaling, RFC 1323, disabled TimeStamps, enabled PathMTU discovery, and blackhole detection. This was on two Windows 2000 Servers, seperated by ATM WAN, using 20 Mbps SCR.

Previously the hosts were only able to transfer at about 1.6 Mbps, increasing the settings from default to the above, I was able to reach a transfer rate of 8mbps.

I tested with multiple FTP servers, and multiple FTP Clients, and found that other FTP Clients do not utilize the RFC1323 options. Microsofts FTP client does for certain.

I used a quick little utility called DRTCP from Supposedly they say that cisco recommends them, but I have never seen mention of DRTCP anywhere else.. At any rate it worked well and quick, and there is a definition of the settings there as well.

I am actually looking for advanced information on the RFC1323 options and how they affect DLSW if anyone has any information on that, it would be greatly appreciated.

Dan Price

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