tailieunhanh - Internetworking with TCP/IP- P27

Internetworking with TCP/IP- P27: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | 228 Reliable Stream Transport Service TCP Chap. 13 We can summarize the ideas presented so far To accommodate the varying delays encountered in an internet environment TCP uses an adaptive retransmission algorithm that monitors delays on each connection and adjusts its timeout parameter accordingly. Accurate Measurement Of Round Trip Samples In theory measuring a round trip sample is trivial it consists of subtracting the time at which the segment is sent from the time at which the acknowledgement arrives. However complications arise because TCP uses a cumulative acknowledgement scheme in which an acknowledgement refers to data received and not to the instance of a specific datagram that carried the data. Consider a retransmission. TCP forms a segment places it in a datagram and sends it the timer expires and TCP sends the segment again in a second datagram. Because both datagrams carry exactly the same data the sender has no way of knowing whether an acknowledgement corresponds to the original or retransmitted datagram. This phenomenon has been called acknowledgement ambiguity and TCP acknowledgements are said to be ambiguous. Should TCP assume acknowledgements belong with the earliest . original transmission or the latest . the most recent retransmission Surprisingly neither assumption works. Associating the acknowledgement with the original transmission can make the estimated round trip time grow without bound in cases where an internet loses datagrams . If an acknowledgement arrives after one or more retransmissions TCP will measure the round trip sample from the original transmission and compute a new RTT using the excessively long sample. Thus RTT will grow slightly. The next time TCP sends a segment the larger RTT will result in slightly longer timeouts so if an acknowledgement arrives after one or more retransmissions the next sample round trip time will be even larger and so on. Associating the acknowledgement with the most recent .