Question about tweaking my MTU

[madriss]

Hi,

This is a question rather than a problem. I read a thing about finding your maximum MTU the other day. At the time the router was set to 1500 and I think my computer was set to 1492 on DRTCP. I tried pinging a website with various numbers of bytes of data, apparently if it says "Packet needs to be fragmented but DF set." then it is too high. I got as low as 1372 (not 1472) before it started sending the packets without that error, so I've now set my MTU to that, but everywhere I read people have at least 1400. Wondering why mine might be so low and if that's normal/correct? To be perfectly honest I don't notice what difference the MTU makes, never have really, but may as well have things optimised if I can and I'm curious :)

Thanks for your help!

Edit: Just read something about adding 28 for the packet header, so perhaps my max is 1400? That would seem more normal, though odd as I believe ADSL24 recommends 1478 or something.

[deezel]

I run mine at 1500 on a netgear 834 gt it's best to have your MTU the same on your pc as well as your router , this is a good site for testing RWIN and MTU ..........

http://www.dslreports.com/tweaks

My results

4. Notes and Recommendationshttp://i.dslr.net/bb/vbull_coll/icon7.gif RWIN is in range
http://i.dslr.net/bb/vbull_coll/icon7.gif Looking good

Observations:
http://i.dslr.net/bb/vbull_coll/icon7.gif Just a few re-transmissions seen (FAQ #705 (http://www.dslreports.com/faq/705)) <<< entas off peak time

Observations:
http://i.dslr.net/bb/vbull_coll/icon2.gif Become pingable
if you need basic packet loss tests done
http://i.dslr.net/bb/vbull_coll/icon2.gif Check tweak FAQ (http://www.dslreports.com/faq/tweaks)

[madriss]

Handy page, thanks for the link! :)

[Beta_Tester]

Well, according to various bits of research I have found on the web, an MTU of 1478 performs the best, as the IP packets fit neatly into the ATM cells as your traffic is routed over BT's ATM network.

Anything up to 1500 should work with no problems however. Your router can be set to a higher MTU, as your PC is a device that creates the packets, not the router.

[wfnuk]

For some peculiar reason, Enta connections don't seem to 'like' an MTU setting of 1500 with certain routers. For example, when I moved to ADSL24 from F2S, where I had no problems with an MTU setting of 1500 on a D-Link router, I found that I could no longer browse any Microsoft sites until I lowered the MTU setting.

ADSL24 used to recommend an MTU setting of 1500 on their Activation Details e-mail details but I believe that this recommendation was dropped after I reported my findings to James back in 2007.

MTU settings were also discussed on this thread here (http://adsl24.co.uk/forum/showthread.php?t=6689).

[James]

It's definitely each to their own. We've seem some lines do better on an MTU of 1400, some on 1478 and some on 1500 and a few in between! Best way to find out what's good for you is to try them all :)

[madriss]

Thanks for the info and comments :) I think 1400 is my optimal so will stick with that for a while!