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MTU Problems


One common problem that rears its head again and again on the forums is the inability for users to view certain sites, in particular Microsoft sites, Hotmail, Ebay and of course Orange's own web site. Quite often this is the result of an MTU problem.


What is happening?

When your PC requests data from these sites, they use something called Path MTU Discovery to check what size packets to send back to your PC. If the site is unable to check what size packet to send it uses the default MTU of 1500 (for windows PCs). However there are there are certain gateways/routers on the internet which use a smaller MTU, therefore the packets are too large to be transmitted and are rejected or dropped.


It is the job of Path MTU Discovery to find and report the smallest MTU setting between your PC and the remote site, so that the remote server sends out the correct sized packets.


What can I do to correct this?

You need to lower your MTU settings and/or make sure that Path MTU Discovery is working properly.


Why is Path MTU Discovery not working?

Path MTU Discovery uses the ICMP protocol to check the maximum packet size. One of the other uses of ICMP is something called the ICMP PING. Many security sites recommend that you make your router/PC unpingable, so when you turn of pings on your router or firewall quite often you are blocking the ICMP protocol completely and therefore Path MTU Discovery stops working.


Despite what some security sites say, there is no harm in making yourself pingable. So what if someone can ping you and check if you are online - it really makes no difference particulary these days when more and more users are using routers which are online 24/7.


Making yourself pingable can cure many of the problems associated with being unable to view certain websites due to MTU problems.


What else do I need to do?

Tweak your PC


It is also recommended that you tweak your own MTU settings to make sure that your PC is set at the correct MTU. Details on how to do this can be found on the page tweaking your MTU settings. When tweaking your MTU you also need to change your RWIN setting. You can find a list of recommended RWIN settings in the table on the same page.


Tweak your router?

Many routers have a setting in which you can alter its MTU size. You can change this, but you must make sure that the MTU setting on your router is the same as on your PC. It does not matter too much if the MTU on your router is higher than that on your PC, but the setting in your router must not be less than the MTU on your PC.


When determining the packet size to send the end to end points are used, therefore if your router has a lower MTU than the PC, your router will be the point at which the data packets become fragmented. If the MTU on your router is higher, the packets will traverse though the router unhindered.




Original source: MTU Problems - Reproduced with permission from who kindly allowed to use this information



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