How does setting MTU at router change anything?

I ask because large HTTP transfers were timing out after a few minutes until I readjusted the MTU size on the router to 1472 from 1500, now they work fine.

Because you can't squeeze size 1500 packets through a size 1472 pipe. The computers at either end of the connection agree how large the packets are going to be, and by default this tends to be the standard ethernet maximum which is 1500. They don't know that your router was configured only to allow size 1472. An alternative solution would have been to set your computer MTU to 1472.

Err, I read it as he changed to 1472 from 1500 i.e. he reduced the MTU, not increased it.

Maybe if the PC was set to a value higher than that recommended by the ISP?

The value on my PC didn't change (that I know of, unless DHCP is able to readjust this).

If the router was set to a smaller size than the PC, it would determine the packet size sent to the net (and received from it). It's probably a good idea to change the MTU size in Windows as well.

When you connect to a server using TCP your PC sends a TCP syn packet with the maximum segment size (MSS) which is 40 bytes less than the PC's MTU, the server responds with its MSS in the reply.

Some routers adjust the MSS requested in the TCP Syn & Syn/Ack packets, it is called MSS clamping http://www.google.co.uk/search?hl=en&q=mss+clamping This tricks the PC and server into communicating using the MTU determined by the router.

Edited by AbandonShip (Mon 18-Jun-07 13:32:49)

In reply to:

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I read it as he changed to 1472 from 1500

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You're right of course - I misread the post. In that case I can only imagine that the MSS clamping explanation is correct, although many home routers don't support this. Reducing the PC MTU is still a better solution in general.

I'm using the 3Com OfficeConnect line of equipment and in the literature online it mentions MSS clamping, so I guess that's why. Thanks for explanation.

Just to confuse things further however, I am using 2 routers (see picture below).

http://i140.photobucket.com/albums/r20/norm360/network.gif

The first router had its MTU set at 1472, the second was 1500 before I readjusted it. Could this be the reason HTTP transfers were failing?

I also don't understand the process behind a download working to begin with then suddenly stopping minutes later because of an incorrect MTU setting. If it were wrong wouldn't it fail outright?

Edited by deleted (Thu 21-Jun-07 12:59:16)