MTU Auto Discovery : Specifies whether Microsoft TCP/IP will attempt to discover the largest packet size (the MTU) over the path to a remote computer. By discovering the path MTU and limiting TCP segments to this size, TCP can eliminate fragmentation along the path at routers connecting networks with different MTUs. If you enable, it will instruct your connection to try to find the max MTU over the path to the remote host. Default is On. The disadvantage of Path MTU Discovery is that it may cause a delay before TCP is able to start sending data.

MTU : All data sent over the internet goes in data packets. The size of these packets is called MTU (Maximum Transmission Unit). If these data packet sizes are too large or too small, they will have to be broken apart. Therefore this will slow down your connection speed. (e.g.: Windows 95 Default: 1500) You must use Advanced Users tab, if you know what you do correctly. For example, if you have a DSL connection running PPPoE, you must set MTU value between 1400 and 1492.

RcvWindow : (TcpRcvWindow) Specifies the default receive window advertised by TCP. So Rcvwindow value determines how much information the receiving system can receive.

Instead of fixed MTU values, you can customize MTU and RcvWindow values. Just set MTU value to "Custom MTU..." and then type the values you want in the blanks. You cannot customize MSS value by using BeFaster.

MSS : Maximum Segment Size (MSS) defines how much actual data can put in the packet, and it is therefore a marginally smaller value than MTU to allow for packet addressing and other information. So MSS value determines the maximum segment size of TCP information that Winsock can receive on a given connection. You cannot customize MSS value, BeFaster does not allow this:
MSS Value = MTU value - 40