TCP IP Illustrated

8 ms figure 244 shows what happens if we send sixteen

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Unformatted text preview: make their routing decision, and start sending the output packet. In this figure we're assuming the ideal case where it takes no time for these operations to occur at the router (the horizontal dashed lines). Nevertheless, it takes four units of time to send all 8192 bytes from Rl to R4. The time for each hop is (4096 + 40 bytes) x 8 bits/bytes / 1'544'000 bits/sec = 21.4 ms per hop (4096 + 40 bytes) x 8 bits/bytes = 21.4 ms per hop 1,544,000 bits/sec file:///D|/Documents%20and%20Settings/bigini/Docum...i/homenet2run/tcpip/tcp-ip-illustrated/tcp_fut.htm (5 of 20) [12/09/2001 14.47.33] Chapter 24. TCP Futures and Performance (We account for the 40 bytes of IP and TCP header.) The total time to send the data is the number of packets plus the number of hops, minus one, which we can see visually in this example is four units of time, or 85.6 ms. Each link is idle for two units of time, or 42.8 ms. Figure 24.4 shows what happens if we send sixteen 512-byte packets. Figure 24.4 Sending sixteen 512-byte packets through four routers. It takes more units of time, but the units are shorter since a smaller packet is being sent. (512 + 40 bytes) x 8 bits/byte / 1,544,000 bits/sec = 2.9 ms per hop The total time is now (18 x 2.9) = 52.2 ms. Each link is again idle for two units of time, which is now 5.8 ms. In this example we have ignored the time required for the ACKs to be returned, the connection establishment and termination times, and the possible sharing of the links with other traffic. Nevertheless, measurements in [Bellovin 1993] indicate that bigger is not always better. More research is required in this area on various networks. 24.3 Long Fat Pipes In Section 20.7 we showed the capacity of a connection as capacity (bits) = bandwidth (bits/sec) x round-trip time (sec) and called this the bandwidth-delay product. This is also called the size of the pipe between the end points. Existing limits in TCP are being encountered as this product increases to larger and larger values. Figure 24.5 shows some values for various types of...
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