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Unformatted text preview: d packet. With TCP, the ACKs are cumulative-they acknowledge that the
receiver has correctly received all bytes up through the acknowledged sequence number
minus one. In this example three of the ACKs acknowledge 2048 bytes of data and two
acknowledge 1024 bytes of data. (This ignores the ACKs in the connection establishment
What we are watching with tcpdump are the dynamics of TCP in action. The ordering of
the packets that we see on the wire depends on many factors, most of which we have no
control over: the sending TCP implementation, the receiving TCP implementation, the
reading of data by the receiving process (which depends on the process scheduling by the
operating system), and the dynamics of the network (i.e., Ethernet collisions and backoffs).
There is no single correct way for two TCPs to exchange a given amount of data.
To show how things can change. Figure 20.2 shows another time line for the same exchange
of data between the same two hosts, captured a few minutes after the one in Figure 20.1. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_bulk.htm (3 of 24) [12/09/2001 14.47.22] Chapter 20. TCP Bulk Data Flow Figure 20.2 Another transfer of 8192 bytes from svr4 to bsdi.
A few things have changed. This time the receiver does not send an ACK of 3073; instead it
waits and sends the ACK of 4097. The receiver sends only four ACKs (segments 7, 10, 12,
and 15): three of these are for 2048 bytes and one for 1024 bytes. The ACK of the final 1024
bytes of data appears in segment 17, along with the ACK of the FIN. (Compare segment 17
in this figure with segments 16 and 18 in Figure 20.1.)
Fast Sender, Slow Receiver
Figure 20.3 shows another time line, this time from a fast sender (a Spare) to a slow receiver
(an 80386 with a slow Ethernet card). The dynamics are different again. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_bulk.htm (4 of 24) [12/09/2001 14.47....
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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