MaxMTU

MTU Defined

MTU stands for "Maximum Transmission Unit". It defines the size of the data packets that Windows® uses as a negotiation basis. When a connection is initiated between two computers they must agree on an MTU. This is done by comparing MTUs and selecting the smaller of the two.

How it Works

When you go online your Internet provider compares its MaxMTU with yours and chooses the smaller of the two. In theory, you might think that setting MaxMTU to the highest possible value would be the best solution. Then your provider would always choose its maximum value, which would maximize throughput. In practice, however, it's not that simple.

Packet Fragmentation

The problem is that your provider may allow a MaxMTU that is larger than those permitted by some of the other servers that carry the data along the way. If this happens the routers must break your packets up, and this almost always causes a drastic reduction in throughput. Among other things, the software routines used to break up and reassemble the packets are usually not very efficient, because it's not something that the programmers are particularly interested in (they don't really want to have to break up and reassemble packets at all).

As a general rule of thumb, it's possible to say that the optimum MaxMTU figure increases with the available bandwidth. The trick is to choose a MaxMTU that is as large as possible without causing packet fragmentation.

The Effect of Windows "MTU Discovery"

Windows® has a built-in MTU Discovery feature that can prevent fragmentation automatically. It does this by sending out a packet with a "Not Fragmentable" tag at the beginning of every connection attempt (e.g. when you access a web page). Since the router can't fragment this packet it's forced to return an error message. Windows® then reduces the packet size and tries again, continuing until no error is returned.

For example, if your Internet provider uses an MTU of 576 and your MTU is set to the Windows® default of 1500. Windows® must adjust your setting downwards every time you go online. This process is automatic, but it still takes it time. Setting the correct value greatly reduces the amount of work Windows® has to do to negotiate a connection, increasing access time.