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The Maximum Transmission Unit (MTU) and Related Registry Setting Explained and Tweaked  to Increase Your Surfing Speeds


The Maximum Transmission Unit (MTU) of a network is the largest packet size that can be transferred in a frame, including headers and trailers. It follows that if a too-large packet is sent, you get reduced performance as packets are fragmented and reassembled. In addition, when you start, smaller packets will be saved to fill the MTU size before being transmitted, slowing the build-up speed. The Internet standard is 576 bytes, but Windows ' 95 sets 1500 by default - '98 adjusts it automatically.

Here are some industry standard MTU sizes:

  • 16 Mbit/sec Token Ring 17914

  • 4 Mbit/sec Token Ring 4464

  • FDDI 4352

  • Ethernet 1500

  • 802.3/802.2 1492

  • X.25 576

MTU (think of it as an envelope) is usually set in conjunction with:

MSS, the Maximum Segment Size, or letter inside the envelope, which must be smaller than MTU by at least 40 bytes, or the size of the headers and trailers (e.g. 536).

RWIN, the TCP Receive WINdow, which is a global setting that determines how much data gets through at the receiving end (e.g. the size of the letterbox). It actually specifies the number of bytes a sender can transmit without receiving acknowledgements. If it is too large, more data is lost when a packet is lost or damaged. On the other hand, transmission will be very slow if it is too small. Its normal setting is 4x, but 6x (3216) MSS I have found to be better - you will have to experiment with this one. Whatever figure you use, it must be an even multiple of the MSS, or you will get fragmentation.

The settings are inside the Registry, and are not there by default; you have to put them specifically into the relevant section. 

"Insert" a MaxMTU key into:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Class\NetTrans\0000

Assuming 0000 is your dialup connection (check all the others as well). Give
it a value of 576. Also under:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\VXD\MSTCP

"Alter" the DefaultRcvWindow and DefaultTTL keys to what you want (see below). Try 2144 (536 x4) and 64 respectively (the defaults are 8192 and 32).

In NT 4.0, you need to run regedt32.exe and navigate to this key:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameters

"Add" the value entry MTU of datatype REG_DWORD to the value of MTU you
require (see rules above). Try 576.

"Add" the value entry TcpWindowSize of datatype REG_DWORD to the value of RWIN you require (see rules above). Try 2144 (4x) or 3216 (6x).

"Add" the value entry DefaultTTL of datatype REG_DWORD to the value of TTL you require in seconds (see rules above). TTL stands for Time To Live, or a flag in a frame that indicates to a router how long the packet has been on the network, or, in other words, the maximum time an IP packet may exist on the network without reaching its destination, so as to limit the number of routers it passes through. 64 seconds is suggested.

There are a number of other settings, too specialized to mention here, but information about them can be found in the Microsoft Knowledge Base. The above, however, should be enough to get you started and increase your surfing speeds.

As far as I know, a Mac is self-adjusting in this respect, as is NT5, sorry, Windows 2000.

This is an article from  Phil Croucher, author of  Communications and Networks.  Phil has a way of explaining in "plain" English. The information is well presented and is well above A+ standard.


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Updated 07/06/04

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