Definitions
Return to Introduction  Previous page  Next page
Maximum Transmission Unit (MTU)

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.

RWIN (TCP Receive Window)

RWIN determines how much data the receiving computer is prepared to receive. An RWIN value that is set too high will result in greater data loss if the packet is lost or damaged in transit. An RWIN value that is set too low will produce very poor throughput. Typically, an RWIN value should be set that is 3 or 4 times the size of the Maximum Segment Size (MSS).

Maximum Segment Size (MSS)

The MSS is the largest segment of TCP data that the Winsock is prepared to receive on a particular connection. When the TCP connection is initially established, both sides agree to use the minimum of each other's advertised MSS value.If the MSS is too low, the data/header ratio will be low. And if the MSS is too high, this will lead to large IP datagrams and the packets will tend to fragment in transit where other networks may have small MTU's. Hence, performance can increase at times by reducing the MSS value.Since packet headers are normally 40 bytes in length, your MSS value should always be at least 40 less than the value of the MTU. As a rule of thumb, first try setting the MSS value to be the greatest power of 2 that is at least 40 less than the MTU value.

TTL (Time To Live)

TTL is a field in the IP header which indicates how long a packet should be allowed to survive before it is discarded. TTL essentially determines the maximum number of hops permitted. Windows 95 default is 32, but with the Internet growing larger all the time, it is worth changing it to the larger setting of 128. Windows 98 does default to 128 now.




Page URL http://www.pointstone.com/products/InternetAccelerator/index.html?definitions.htm