TCP IP Illustrated

TCP IP Illustrated

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Unformatted text preview: networks. file:///D|/Documents%20and%20Settings/bigini/Docum...i/homenet2run/tcpip/tcp-ip-illustrated/tcp_fut.htm (6 of 20) [12/09/2001 14.47.33] Chapter 24. TCP Futures and Performance Network Ethernet LAN T1 telephone line, transcontinental T1 telephone line, satellite T3 telephone line, transcontinental gigabit, transcontinental Bandwidth (bits/sec) Roundtrip time (ms) Bandwidthdelay product (bytes) 10,000,000 1,544,000 1,544,000 45,000,000 1,000,000,000 3 60 500 60 60 3,750 11,580 95,500 337,500 7,500,000 Figure 24.5 Bandwidth-delay product for various networks. We show the bandwidth-delay product in bytes, because that's how we typically measure the buffer sizes and window sizes required on each end. Networks with large bandwidth-delay products are called long fat networks (LFNs, pronounced "elefan(t)s"), and a TCP connection operating on an LFN is called a long fat pipe. Going back to Figure 20.11 and Figure 20.12, the pipe can be stretched in the horizontal direction (a longer RTT), or stretched in the vertical direction (a higher bandwidth), or both. Numerous problems are encountered with long fat pipes. 1. The TCP window size is a 16-bit field in the TCP header, limiting the window to 65535 bytes. As we can see from the final column in Figure 24.5, existing networks already require a larger window than this, for maximum throughput. The window scale option described in Section 24.4 solves this problem. 2. Packet loss in an LFN can reduce throughput drastically. If only a single segment is lost, the fast retransmit and fast recovery algorithm that we described in Section 21.7 is required to keep the pipe from draining. But even with this algorithm, the loss of more than one packet within a window typically causes the pipeline to drain. (If the pipe drains, slow start gets things going again, but that takes multiple round-trip times to get the pipe filled again.) Selective acknowledgments (SACKs) were proposed in RFC 1072 [Jacobson and Braden 1988] to handle multiple dropped packets within a window. But this feature was omitted from RFC 1323, because the authors felt several technical prob...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.

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