One common problem that rears its head again and again on forums is the
inability for users to view certain sites, in particular Microsoft
sites, hotmail and ebay. 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
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
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.
pingable can cure many of the problems associated of 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 my 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.
If your ISP is AOL then it is recommended you use the MTU of 1400, for all
other ISPs try 1430 or 1458.
The MTU settings I recommend for a 2Mb connection
are MTU 1430 RWin 55600
Speedguide make a tool which makes changing your MTU settings very easy - SG TCP Optimizer
~ 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.
|| How to tweak
and suggested settings || Tweaking History || Understanding
MTU and RWIN ||