Large bandwidth-delay product

The amount of data in transit on a link at nay point of time is the product of bandwidth and the RTT . Because of the high latency of the satellite link , the bandwidth -delay product is large . TCP/IP allows the remot ehost to send a certain amount of data in advance without acknowledgment . An achnowledgment is usually required fro all incoming data on a TCP/Ip connection . However , the remote host is always allowed to send a certain amount of data without acknowledgment , which is important to achieve a good transfer rate on large bandwidth delay product connections . This amount of data is called the TCP window size . Th window size is usually 64kb in modern TCP / IP implementations .

On satellite networks , the value of the bandwidth -delay product is important . To utilize the link fully , the window size of the connection should be equal to the bandwidth -delay product . If the largest window size allowed is 64kb , the maximum theoretical throughput achievable via satellite is ( window size ) / RTT , or 54KB/520ms . This gives a maximum data rate of 123 KB/s which is 984 kbps , regardless of the fact that the capactiy of the link may be much greater .

Each TVP segment header contains a field called advertised window , whcih specifies how many additional bytes of data the receiver is prepared to accept . The advertised window is the receiver's current avaiable buffer size .

The sender is not allowed to send more bytes than the advertised window . To maximize performance , the sender should set its send buffer size and the receiver should set its receive buffer size to no less than the bandwidth -delay product . This buffer size has a maximum value of 64KB in most modern TCP/IP implementations .

TO overcome the problem of TCP / IP stacks from operating systems that don't increase the window size beyond 64KB , a technique known as TCP acknowledgment spoofing can be used ( see Performance Enhancing Proxy ,below ) .