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Unformatted text preview: the left one unit, back to the sender. We have a round-trip time (RTT) of 8 units of time.
We have purposely drawn the ACK segment smaller than the data segment, since it's
normally just an IP header and a TCP header. We're showing only a unidirectional flow of
data here. Also, we assume that the ACK moves at the same speed as the data segment,
which isn't always true. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_bulk.htm (14 of 24) [12/09/2001 14.47.22] Chapter 20. TCP Bulk Data Flow In general the time to send a packet depends on two factors: a propagation delay (caused by the finite speed
of light, latencies in transmission equipment, etc.) and a transmission delay that depends on the speed of the
media (how many bits per second the media can transmit). For a given path between two nodes the
propagation delay is fixed while the transmission delay depends on the packet size. At lower speeds the
transmission delay dominates (e.g., Exercise 7.2 where we didn't even consider the propagation delay),
whereas at gigabit speeds the propagation delay dominates (e.g.. Figure 24.6). When the sender receives the ACK it can transmit two more segments (which we've
numbered 2 and 3), at times 8 and 9. Its congestion window is now two segments. These two
segments move right toward the receiver, where the ACKs are generated at times 12 and 13.
The spacing of the ACKs returned to the sender is identical to the spacing of the data
segments. This is called the self-clocking behavior of TCP. Since the receiver can only
generate ACKs when the data arrives, the spacing of the ACKs at the sender identifies the
arrival rate of the data at the receiver. (In actuality, however, queueing on the return path can
change the arrival rate of the ACKs.)
Figure 20.10 shows the next 16 time units. The arrival of the two ACKs increases the
congestion window from two to four segments, and these four segments are sent at times 1619. The first of the ACKs retur...
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