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Unformatted text preview: • The window scale option for supporting large TCP window sizes (larger than 64KB). • Selective acknowledgment ( SACK ) to enable faster recovery from transmission errors. • Timestamps for calculating appropriate RTT and retransmission timeout values for the link in use. Long round-trip time (RTT) Satellite links have an average RTT of around 520ms to the f rst hop. TCP uses the slow-start mechanism at the start of a connection to f nd the appro- priate TCP/IP parameters for that connection. Time spent in the slow-start stage is proportional to the RTT, and for a satellite link it means that TCP stays in slow-start mode for a longer time than would otherwise be the case. This drastically decreases the throughput of short-duration TCP connections. This is can be seen in the way that a small website might take surprisingly long to load, but when a large f le is transferred acceptable data rates are achieved after a while. Furthermore, when packets are lost, TCP enters the congestion-control phase, and owing to the higher RTT, remains in this phase for a longer time, thus re- ducing the throughput of both short- and long-duration TCP connections. Large bandwidth-delay product The amount of data in transit on a link at any 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 remote host to send a cer- tain amount of data in advance without acknowledgment. An acknowledgment is usually required for all incoming data on a TCP/IP connection. However, the re- mote host is always allowed to send a certain amount of data without acknowl- edgment, 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 . The 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 maxi- mum theoretical throughput achievable via satellite is (window size) / RTT, or 64KB / 520 ms. This gives a maximum data rate of 123 KB/s, which is 984 kbps, regardless of the fact that the capacity of the link may be much greater. Each TCP segment header contains a f eld called advertised window , which speci f es how many additional bytes of data the receiver is prepared to accept. The advertised window is the receiver's current available buffer size. Chapter 3: Network Design 91 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....
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