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Unformatted text preview: ip to vangogh.
We can immediately see the three retransmissions around times 10, 14, and 21 in Figure
21.6. At each of these three points we can also see that only one segment is retransmitted,
because only one dot dips below the upward slope.
Let's examine the first of these dips in detail (around the 10-second mark). From the
tcpdump output we can put together Figure 21.7. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (12 of 29) [12/09/2001 14.47.27] Chapter 21. TCP Timeout and Retransmission Figure 21.7 Packet exchange for retransmission around the 10-second mark.
We have removed all the window advertisements from this figure, except for segment 72,
which we discuss below, slip always advertised a window of 4096, and vangogh
advertised a window of 8192. The segments are numbered in this figure as a continuation of
Figure 21.2, where the first data segment across the connection was numbered 1. As in file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_time.htm (13 of 29) [12/09/2001 14.47.27] Chapter 21. TCP Timeout and Retransmission Figure 21.2, the segments are numbered according to their send or receive order on the host
slip, where tcpdump was being run. We have also removed a few segments that have no
relevance to the discussion (44, 47, and 49, all ACKs from vangogh).
It appears that segment 45 got lost or arrived damaged-we can't tell from this output. What
we see on the host slip is the acknowledgment for everything up through but not including
byte 6657 (segment 58), followed by eight more ACKs of this same sequence number. It is
the reception of segment 62, the third of the duplicate ACKs, that forces the retransmission
of the data starting at sequence number 6657 (segment 63). Indeed, Berkeley-derived
implementations count the number of duplicate ACKs received, and when the third one is
received, assume that a segment has been lost and retransmit only one segment, sta...
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