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MTU (Maximum Transmission Unit)
This is the default packet size that Windows uses to negotiate
with. When a connection is opened between two computers, they must agree on an MTU. This is done by comparing MTUs
and selecting the smaller of the two.
If the MTU is set too large for routers that are between the computers, these routers then fragment this information
into a packet size that the router can handle. This fragmentation can double the amount of time it takes to send
a single packet. Windows has a built-in MTU Discovery that will adjust for this by sending out a packet that is
marked as "Not Fragmentable". Then the router sends back an error to the computer saying that the packet
was too large, and Windows then lowers the MTU until there are no more errors.
If your ISP uses an MTU of 576, then every time you start a connection, Windows must adjust down to this value.
Even though Windows automatically adjusts the packet size, it still takes it time to negotiate an acceptable MTU.
By setting this value manually, you greatly reduce the amount of work that Windows must do to negotiate.
NOTE: Negotiation always adjusts downwards the value you have set, never upwards. Windows 95 Default is 1500. Windows
98 Default is 576.
Windows 98/Me Information
Windows 98/Me has a built in option to change the MaxMTU for a Dial-Up Adapter. SpeedConnect will also change these
settings. Windows 98 has a new "Automatic" setting that enables programmatic control and is the default
and recommended setting. When you use Automatic mode, all Point-to-Point Protocol (PPP) connections at speeds below
128kbps are made with an MTU of 576. At speeds above 128kbps, all connections are made with an MTU of 1500. Regardless
of the MTU setting, the MTU will be further reduced to account for the network overhead if Virtual Private Networking
(VPN) is used.
With Windows 98/Me, you may still find that choosing an optimal combination of MaxMTU, RWIN and TTL permits faster
data transfer rates than simply using the default options. 512, 552, 556, 1006, 1024, 1064 and 1152 are all values
that other people have reported better luck with. 1024 is a value that some people using ISDN have reported success
with. Some NT users have reported better results with default MaxMTU and a specified RWIN like 4x or 6x. Unfortunately,
there is no shortcut for experimentation if you truly wish to optimize your connection. Try various combinations
of settings and watch your download speeds using any good throughput monitor and see what works best for your particular
combination of ISP, modem, phone lines, and system configuration.
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