What's a sensible MTU?

What's a sensible value to set for the MTU of a router, running at around 3M bps downstream rate, under Windows XP? With a new router, I've been noticing that, sometimes, one or two websites I visit daily become unobtainable; that is, they don't resolve and the browser (IE7) simply gives up, stating that it can't get the website. Other websites are fine, though. I suspect this is an MTU problem of some sort. However, changing the router's MTU value from its default of 1458 has had no effect.

I've performed a ping fragmentation test in Windows (pinging a normally reachable popular website) and found that the Windows MTU was sometimes 1430, sometimes 1432. I gather that Windows XP sets its own MTU automatically and dynamically.

So knowing that the router's MTU should preferably equal the Windows MTU, or be slightly greater, I tweaked the router's MTU up slightly, to 1460 (1432 + 28). This seems to have had no effect, though.

I'm assuming, of course, that the router's firmware is bug-free in this respect.

Do I actually need to increase the router's MTU a lot further beyond 1460?

1500 works for me.
Sometimes it depends on your ISP (but I remember you don't wish to say who they are) so it might be worth asking them.

Your problems could also be DNS related.

-------------------------------------------
Freeola[EntaNet] Faraday IPSC node
Hmmm... Understanding ADSL | Map Coordinates | and more...
SNR: Netgear DG834>2Wire 2700>Thomson ST585v6 My Router Experience

Yes, I've seen it written (vaguely so) that sometimes it can be due to the ISP. However, in this case, the non resolving of particular websites started only after getting the new router.

What would have to be wrong or ill-configured in either Windows or the router to make it a DNS-related problem?

the MTU should be set to 1500 as this is the ethernet and windows default.

If you want to run a smaller MTU set it on the PC to avoid potential fragmentation issues in the router. Lack of access to secure sites (inc Ebay etc) is often blocked if packet fragmentation results from the router MTU being less than that of windows.

Phil

MaxDSL - goes as fast as it can and doesn't read the line checker first.

MaxDSL diagnostics
Are your kids pirates ? Limewire, Bearshare, Kazaa, BitTorrent, eMule are all tools of the trade.

The occasionally-unobtainable websites I'm encountering aren't secure sites, and the router's MTU has never been less than Windows XP's MTU, so I think we can rule that one out. But I wasn't aware that MTU was also controllable in Ethernet. Clearly, optimised packeting needs to take place at the Ethernet level also, but is an Ethernet MTU actually user-configurable? If so, how? And would the Ethernet MTU value depend on whether the connection was wired or wireless?

It was my impression that nowadays, with Windows XP and 2K, the operating system automatically sets its own optimised MTU. Thus, although there may be a Windows default value of 1500 when the OS is first installed, the value that Windows uses adapts to the type of Internet connection being used. In fact, this appears to be supported by the fact that my ping test gave a Windows value (without the packet header overhead) of about 1430.

Perhaps the best approach should be for me to just gradually bump up the existing router MTU toward 1500, from 1460, and see if and at what point the non-resolving of those few particular websites disappears? It could take me a while, though, as on some days the problem is there, on other days it isn't.

Edited by meditator (Tue 17-Nov-09 13:59:14)

Seems more like a DNS problem, which could occur with any ISP, rather than an MTU issue. Why not try OpenDNS and see if it goes away?

1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg BB

the best approach is to just set the router's MTU to 1500. I've done this to other Netgears and the "page won't display" issue goes away.

If you use ping tests to find the MTU at which it breaks you are merely finding the lowest MTU in the system and not measuring the Windows MTU. If path MTU discovery is enabled and working then XP is supposed to reduce its MTU.

if you haven't changed the setting then Windows is at 1500 MTU. DrTCP is a tool for changing this. Wireless uses a bigger packet "over the air" to cover the overheads.

Phil

MaxDSL - goes as fast as it can and doesn't read the line checker first.

MaxDSL diagnostics
Are your kids pirates ? Limewire, Bearshare, Kazaa, BitTorrent, eMule are all tools of the trade.

In reply to a post by meditator:

...Perhaps the best approach should be for me to just gradually bump up the existing router MTU toward 1500...

OR just try 1500 as I and others have mentioned???

Previously you mentioned you talk to your mystery ISP "I interact with their professional engineers" - it would be interesting to hear what they say...

-------------------------------------------
Freeola[EntaNet] Faraday IPSC node
Hmmm... Understanding ADSL | Map Coordinates | and more...
SNR: Netgear DG834>2Wire 2700>Thomson ST585v6 My Router Experience

In reply to a post by yarwell:

the MTU should be set to 1500 as this is the ethernet and windows default.

If you want to run a smaller MTU set it on the PC to avoid potential fragmentation issues in the router. Lack of access to secure sites (inc Ebay etc) is often blocked if packet fragmentation results from the router MTU being less than that of windows.

Mmmm........... I remember tweaking the MTU on my computer using the advice of a well-known American site, and when I finished, I was unable to send out any emails !

"If it ain't broke, don't fix it"

Home ADSL: Sky Max LLU
Business ADSL:
Andrews & Arnold
Orange 3G Business Everywhere Mobile BB

Yes, zebedee, I had much the same experience back in the days when I was using Windows 2000. Using Dr TCP caused me more problems than it solved. Nowadays, though, I gather that Windows XP automatically adjusts its own MTU to an optimum value, so the use of an app such as Dr TCP (there are several others around, I think) should not be necessary.

The issue I'm trying to resolve here, though, is not the Windows value for the MTU but the router's value. With what little I've learned about MTU, the router's value needs to be at least the same value as that of Windows's. If that condition's not satisfied, then either the packets will tend to get fragmented or packets will not be used efficiently. In one extreme, packets can be completely dropped and the website become unobtainable, which is precisely what I'm occasionally observing.