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What are MTU, TTL, and RWIN?

MTU (Maximum Transmission Unit), TTL (Time To Live), and RWIN (Receive Window) are terms that come up often when folks have good modem connections, but not so good throughput. They are TCP/IP settings that are stored in the Windows Registry, and can be tweaked for improving downstream performance (upstream has no tweaks). Sometimes tweaking these settings will make the connection seem much faster, sometimes it won't change anything, and sometimes it hoses the registry and/or TCP/IP and requires a re-install. Backup the registry before making any changes to it ever. Furthermore, these are things you can tell a customer about and let them research on their own, but DO NOT attempt to hack a customer's registry by hand. Furthermore, there are shareware/freeware programs that will do this for you and minimize the chance of error (EasyMTU is my personal favorite).

Nearly every source of information I've read about these fixes conflicts with every other report I've read about what does what. The basics are that, when data is divided into packets by a modem or DSLAM or whatever (not TCP/IP packets), they are sent in bursts and, if a TCP/IP packet is divided in the middle, it must be resent. With Windows 95, the MTU is set to 1500, but dialup modem servers send data in 576 bit chunks. Hence, two packets will be received, totalling 1152 bytes, and a third packet of 576 won't fit in the 348-bit space left. So setting the MTU to 576 on a dialup machine will improve performance. Note, though, that making this change and then signing up for DSL or Cable access will result in not-so-highspeed performance, and you'll need to make another tweak.

The RWIN setting may or may not change performance. In my testing, I have noticed no real change with any setting I've tried. Some say it needs to be a multiple of this or that, others say it needs to be as big as possible, and other say if it's too big, performance will degrade. I suggest you do the research and decide for yourself.

The TTL (Time To Live) is not actually a time setting, but a "number of hops" setting. The internet isn't big enough yet (and may never be, with new technologies and bigger hubs around the world) to need a TTL bigger than 64, unless you spend a lot of time hacking and bouncing signals off of servers in Czechoslovakia.


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