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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
time (ms) Bandwidthdelay
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
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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.
- Spring '12