Ideally, should router and computer MTU's be matched?

Have recently upgraded my Internet connection to VDSL. Because it's now PTM, the router (integrated router-modem) MTU setting has changed to 1492. However, in the Network settings of the computer(s) on the LAN, the MTU value is still 1500. The computer's value can be user-changed quite easily if required, and I notice that even a custom value can be set. So, should the router's and the computer's values be matched for optimum throughput from WAN side to browser? In other words, should I adjust the MTU value in the computer to 1492? The router-to-computer(s) connections are wired Ethernet, BTW.

Yes. With Vista onwards the OS should auto work out the correct MTU to use. What OS are you on?

For fibre 1492 is the correct value; 1500 is usually on ADSL.

If, as my Win 10, the 1500 is just a default I should leave it alone.

OSX Mavericks (soon to be upgraded by me to Sierra). In its Network settings you can set the MTU to whatever value you wish, within a specified range. And so, based on what you've said, I've changed it from 1500 to 1492. For such a minor change, I was not anticipating any great difference in 'zippiness to screen' but presumably if there's a better match, the efficiency of getting packets across the Ethernet connection to/from the router will be slightly improved (probably less buffering required). It seems to be running okay with this change.

On my home network I do also have a WinXP machine but I very rarely use it for any Internet access, as Microsoft stopped support for WinXP quite some time ago (except for certain commercial requirements) and the browser I have on it (IE8) isn't up to the required standard any longer for safe Internet usage.

Many years ago I recall looking into the general workings of MTU but have long since forgotten most of it. I imagine it's also possible to change the MTU value on XP but I don't remember where, exactly, that could be done. At present, I don't see any obvious place - say, the TCP/IP settings. I've a vague feeling you have to go into the Windows Registry to change it. Does that ring any bells with you?

Yes XP Mtu is set in the registry. There are a number of programs that will set it for you and reboot. Or follow this link to a Microsoft step by step guide.

On XP, the MTU is to be found in the Registry at:

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\{03CB512B-EEA2-456C-AC4E-5D2D53A2BBF4}]

"MTU"=dword:000005dc

"DhcpIPAddress"="192.168.1.105"

where the Id within {} is the 1 that has your recognisable values like "DhcpIPAddress" above.

Or you can get SpeedGuide.net :: TCP Optimizer to do the dirty work, as well as adjust RWIN to be suitable.

Edited by XRaySpeX (Mon 13-Feb-17 00:52:30)

Fibre can use mini jumbo frames to still deliver 1500 MTU.

In reply to a post by meditator:

OSX Mavericks (soon to be upgraded by me to Sierra). In its Network settings you can set the MTU to whatever value you wish, within a specified range. And so, based on what you've said, I've changed it from 1500 to 1492. For such a minor change, I was not anticipating any great difference in 'zippiness to screen' but presumably if there's a better match, the efficiency of getting packets across the Ethernet connection to/from the router will be slightly improved (probably less buffering required). It seems to be running okay with this change.

It shouldn't really make any noticeable difference for tcp, there won't be any buffering saved.

On uploading save of a one off, couple of ms, when you first send a 1500 packet and the router says "too big".

For downloads the router will mss clamp tcp anyway so with or without changing mtu on PC the incoming would be 1492.

I wouldn't change anything on a modern OS that does window scaling - especially rwin type tweaks that may have helped some ancient windows 15 years ago.

I can't remember what Win XP defaults were. If tcp window scaling was off, then for FTTC speeds finding a way to enable it would gain a lot for single thread.

As has already been said - you can do 1500 on BTW pppoe, kit permitting. It's not a ptm limitation just that using pppoe historically meant using 1492 before jumbo/mini jumbo frames were possible on ethernet.