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Author Topic: MTU Advice Please  (Read 11771 times)

joebloggs

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MTU Advice Please
« on: July 07, 2008, 08:41:37 PM »

I firt came across this site about a year ago when i had a problem. My ISP (orange) made some changes at the Exchange and suddenly I couldn't get internet pages. I am using a Belkin N1 modem model no F5D8631-4. I eventually found outthe problem was related to MTU settings. When I followed the advice n your site I got connected, using DR TCP for Windows XP and registry hacks for my Windows Vista PC.
However, I now find that every device that I try to connect to my Network needs the MTU dropping below the default 1500 before I can see web pages. Other people's laptops behave exactly the same. Thwy can connect to other networks but can't view web pages on mine unless we drop the MTU.

My son also has a Playstation 3 and we had exactly the same problem, and again it was necessary to drop the MTU and then I could connect fine.

My son now wants to connect a Sony PSP. I am having the usual issue - I can get google website up, so know I am connected correctly, but practically all other sites will not come up. However, I have searched the Internet and can find no mention of how to change the MTU settings on the PSP.

So my questions are:
1. Why is it necessary to change the MTU settings of every device that I connect to my router. Should the router manage the MTU packet size of the devices. Does this mean my router is not configured correctly, or that it is faulty.
2. Is it an issue with my ISP, but Orange are notorius at tech support.
3. If it is normal behaviour, do you know of any way of changing the MTU settings on a Sony PSP.

Thanks in advance

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roseway

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Re: MTU Advice Please
« Reply #1 on: July 07, 2008, 10:54:50 PM »

Hi and welcome

I'm not familiar with that particular router, but most routers allow you to set the MTU to different values. This would save your having to set the MTU of every connected computer separately, but it's not ideal. If a computer has an MTU of 1500 and it's connected to a router with the MTU set to (say) 1430, then nearly every outgoing packet will be fragmented. This won't stop the connection working, but it will slow it down to a degree. So the best technical option is either to have the router and computers all set to the same MTU value, or to leave the router MTU at 1500 and only set the MTU on the computers.

I guess that the best option in your case would be to set the MTU value in the router to 1430 (or whatever value gives the best result) and set the computers to the same value where they have the capability. If that particular router doesn't have the capability to change its MTU value then you're stuck with setting the value in every connected device, and if this can't be done in the PSP then I can't see a solution other than changing to a different router.
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kitz

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Re: MTU Advice Please
« Reply #2 on: July 07, 2008, 11:07:29 PM »

Check and see if your router has any settings that allow you to change the MTU.
The MTU set in your router should never be below that of the devices on the local network (PCs) or it can cause the "black holing effect".


The other thing to check for - see if your router is blocking ICMP pings.
Go to somewhere like grc.com and run a security (stealth) check.
Although grc.com has some good advice the one thing that I any many others disagree with is that on grc.com you can fail his stealth check simply because your router hasnt blocked pings.

When you block pings, you turn off ICMP, which is a valid internet protocol and needed for path MTU discovery.. therefore a side effect of making your router unpingable is that it stops PMTU discovery working and the "internet breaks".

>> 1. Why is it necessary to change the MTU settings of every device that I connect to my router.

The correct packet size needs to be end to end point - your PC is the end point receiving the data, the server sending the data is the other end point. ie its the 2 devices that are communicating with each other.

>> Should the router manage the MTU packet size of the devices.

No - packets just flow through the router.  On the internet packets will pass through many, many routers and switches, its the routers job to forward them to the correct destination. 
A router firewall may block bad packets and protect your network, but it doesnt have anything to do with "wrapping" and "unwrapping" the data packets which are done at the end points.

>> Is it an issue with my ISP, but Orange are notorius at tech support.

Unfortunately this is an issue that does seem to affect a lot of orange users, so much so that orange problems asked permission to replicate my MTU pages for orange users :/
Its therefore likely that its one/some of Oranges' network routers have a low MTU setting which is the "blackhole".


>> do you know of any way of changing the MTU settings on a Sony PSP.

I dont.. I dont have a PSP, so not the best person to ask Im afraid. :/

What is happening is somewhere along the line data isnt getting back to your PC, the place where the packets are being dropped because they are too large is known as the blackhole.
So your PC is saying to a remote server send me info.
The server is going ok then - here you go and its sending packets at the default size of 1500.
Some-where on the path back to you theres a router which has an MTU set lower than 1500 therefore the packets are being dropped because theyre too big to pass through.

When you set the MTU on the local machine your PC says
Send me data but send the packets no bigger than xsize.
The remote server then wraps up the packets to send to you making sure that they are the size requested and not default.

Above version is simplified or I'd be here all night typing stuff out :D

..  and whilst I was typing all the above out - erics already beaten me to it and posted.. but Im still going to post anyhow
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joebloggs

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Re: MTU Advice Please
« Reply #3 on: July 08, 2008, 12:44:03 AM »

Thanks for the prompt and detailed response. My issue seems to me to be that the router will only allow a maximum setting of 1492 for the MTU, but the PS3 and I assume the PSP are using a defualt value of 1500. This also applies to the computers.
By lowering the MTU values of the connected devices to 1430 or thereabouts I can get a perfectly good connection, but have now come across a device where I can't lower this setting.
Looks like a new router might be my best bet?
Still can't really understand why everyone with these routers doesn't suffer the same issues.
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kitz

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Re: MTU Advice Please
« Reply #4 on: July 08, 2008, 06:53:36 AM »

>>  the router will only allow a maximum setting of 1492

Thats a bit of a pain..  1492 is default for PPPoE (-8 bytes for the header) ATM is 1500 :/

However this may be a bit of a moot point really.. as I suspect that a router with the lower MTU is somewhere within Orange's Network, thats causing the fragmentation.
"My ISP (orange) made some changes at the Exchange" < --- This indicates youve been LLU'd and therefore rather than using BTw's kit at the exchange, theyve installed their own.

Did you check ICMP in the router?  As mentioned above this stops PMTU Discovery working.
Hopefully (as long as Orange havent disabled PMTU on their routers too) this may just do the trick and get the 2 end points talking to be able to negiotiate the correct packet size.

The setting is likely to be somewhere in your router firewall settings.  Look for something like ICMP ping and make sure it isnt blocked or disabled.

>> Still can't really understand why everyone with these routers doesn't suffer the same issues.

Theyre possibly still on BTw's kit..  ICMP may not be blocked and PMTU is working..  or it could simply be that only some of Orange's kit at certain exchanges are configured to a lower MTU.

Some of us adsl "oldies" may remember the time many years ago when adsl was in its infancy, and BT kit was configured with too low of an MTU.  BT in the end ended up having to reconfigure their dslams at the exchange.  IIRC They now use an MTU of around 1600 on their kit  to allow for ATM and other overheads.. which ensures that 1500 packets dont get fragmented.
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joebloggs

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Re: MTU Advice Please
« Reply #5 on: July 10, 2008, 01:08:47 PM »

I did try the ping blocking. This was ticked originally but unticking it made no difference.

I also have an Orange Livebox that I keep in it's box. Yesterday I set this up just for testing purposes and using this the PSP connected with no problems.
This sort of confirmed by suspicions as I am aware of other Orange users in my area who haven't had the MTU issue.
This appears to me to definitely point at it being a problem with the Belkin modem. According to the Orange web site, the default value of the MTU for the Livebox is 1492 and they advise you to set 3rd party routers to that setting.
The default value of the Belkin N1 is 1432. You can change it up to 1492 using the Admin screens and I have done this but with no joy. My best guess is that this setting is not taking for some reason even though it appears to be in the settings.
Any suggestions on how I should proceed from here? 
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kitz

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Re: MTU Advice Please
« Reply #6 on: July 11, 2008, 02:12:06 PM »

It could well point to being the belkin then :(
One of my old routers wasnt very good with the MTU and Im sure they got the MSS and MTU mixed up in the firmware as I had to set it to 1500 or else some sites wouldnt display.

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