Dr TCP MTU query

CurlyWhirly · 25 Apr 2006 at 16:25

I have seen references to Dr TCP http://www.aopr43.dsl.pipex.com/ppages/tweaks.shtml

In the above article it is mentioned that BT themselves recommend an MTU of 1458.

Firstly is there any way that I know what my MTU is set to as I can see no mention of it in my routers stats :confused:

If so is there a way to alter the router setting I wonder?

Also I have run Dr TCP and changed the MTU to 1458 but I assume this only changes the XP setting and not the routers setting :confused:

Should I have left the MTU where it was before (whatever that was) and if so I can get my original back by simply deleting the existing MTU value and selecting 'save' and it then goes back to default settings.

Does altering the MTU make any difference or is it a waste of time :confused:

Burbleflop · 25 Apr 2006 at 16:27

Yep, playing with Dr TCP will only change the MTU on the PC, you'll need to do the router manually.

The default would be 1500. You may, or may not, notice a difference if you change the MTU. For what it is worth, my speeds increased marginally when I set the MTU at home to 1458.

Skilldibop · 25 Apr 2006 at 16:31

XP shouldn't store MTU settings. This should all be on the router. Default for most models is 1500 unless they have an "auto" setting. Deleting the value wont necessarily fix it, your only sure way is to reset the whole router to defaults, or revert to a saved config.

Gavstar · 25 Apr 2006 at 16:33

Skilldibop said:
XP shouldn't store MTU settings. This should all be on the router.

Is that correct? Isn't the way I understand it, I thought that the MTU should be changed for both the PC's LAN connection AND the router.

There seems little point in doing one without the other.

CurlyWhirly · 25 Apr 2006 at 16:33

Burbleflop said:
Yep, playing with Dr TCP will only change the MTU on the PC, you'll need to do the router manually.

Mmm. I would like to have a go and change it but I don't think my router supports it :confused:

It is a Thomson SpeedTouch 546 Ethernet ADSL Router.
This is why I wondered about a firmware upgrade in another thread but I already have the latest version.

I don't expect there's a way around this problem so I will just have to live with it

Skilldibop · 25 Apr 2006 at 16:52

Gavstar said:
Is that correct? Isn't the way I understand it, I thought that the MTU should be changed for both the PC's LAN connection AND the router.

There seems little point in doing one without the other.

The router cuts packets up to send over a link with a lower MTU if the packet size on the inbound link is larger than the outbound MTU.

All matching MTUs doe is relieve a little CPU load on the router. Which in terms of speed makes no difference over a connection upto 8Mb. If you were routing 100mbit ethernet on both sides it would be important because the throughput is much higher and the workload so much more. For home ADSL routers the're is little point matching MTUs.
The only reason you change the MTU on the ADSL side is so that the routing kit at the other end doesn't have to split your packets up because it's doing a lot more work as it's dealing with many ADSL connections.

So yeah, provided your router isn't being massacred on the CPU front it's a non issue.

Mmm. I would like to have a go and change it but I don't think my router supports it

in that case it's probably fixed on auto, and will adjust to match the kit at the other end anyway.