Settling TCP Retransmission Ambiguity With Setting Algorithm

The most important and complex conception in TCP is the way to deal with time-out and retransmission. As other reliable protocols, TCP expects that a receiver can send a ACK, after receiving new bytes from data stream. Once sending a segment, TCP sets a timer and waits for the ACK. If the timer expires before data in the segment is acknowledged, TCP assumes that the segment has lost or being damaged, and a retransmission is sent.To measure a RTT sample is easy in theory. Only using the value of time when ACK is received subtracts the value of time when sent the segment. But, there is a situation that after TCP data segments are retransmitted, when a ACK segment arrives, it can not be identified whether the ACK segment is for the first segment or for a segment later, i.e. the problem of TCP retransmission ambiguity.For the problem of TCP retransmission ambiguity, there have been 2 ways to deal with it mainly. One is Karn's Algorithm, and the other is the TCP timestamps option. This paper will give a new resolution:4 reserved bits in TCP header will be used to resolve the problem, and above 2 algorithms as well as Setting Algorithm will be discussed.Although, traditionally, TCP is regarded as a part of the TCP/IP Internet Protocol Suite, but it is a generally independent protocol, and can be used in other transmission systems. Forexample, because TCP hardly involves the network operation of low layer, except running on the complex Internet, TCP can run on some simple network such as Ethernet.In fact, the application of TCP is rather widely. Nowadays, anytime, a great quantity of communication is about TCP. So, the spurious retransmissions of TCP segments will waste a lot of resource of network. If use what put forward Setting algorithm in the thesis, for a TCP connection utilizing large windows, comparing Karn's Algorithm, the spurious retransmissions which are due to the situation that sample rate is awfully less than data frequency can be avoided. So, it has positive significance in practice. And, what Setting algorithm uses bits is far less than the TCP timestamps option.It is described in RFC 2988 that "the only case when TCP can safely take time sample from retransmitted segments is when the TCP timestamp option is employed, since the timestamps option removes the ambiguity regarding which instance of the date segment triggered the acknowledgement." The appearing of Setting Algorithm does not make the case only.