Tranmission errors

In order TCP/IP implementations , packet loss is always considered to have been caused by congestion ( as opposed to link errors ) . When this happens , TCP performs congestion avoidance , requiring three duplicate ACKs or slow start in the case of a timeout . Because of the long RTT values  , once this congestion-control phase is started , TCP/IP on satellite links will take a longert time to return to the previous throughput level . THerefore errors on a satellite link have a more seruious effect on the performance of TCP than over low latency links . To overcome this limitation , mechanisms such as Selective Acknowledgment (SACK) have been developed . SACK specifies exactly hose packets that have been received , allowing the sender to retransmit only those segments that are missing because of link errors .

The mircosoft Windows 2000 TCP/IP Implementation Details White Paper states

"Windows 2000 introduces upport for an important performance feature known as Selective Acknowledgment (SACK ) . SACK is especially important for connections using large TCP window sizes' .

SACK has been a standard feature in Linux and BSD kernels for quite some time . be sure that your Internet router and your ISP;s remote side both support SACK .