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Unformatted text preview: This is normally to avoid fragmentation (Section 11.5). We discuss the
maximum segment size in Section 18.4, and show the format of the various TCP options in
Figure 18.3 shows the time line for this sequence of packets. (We described some general
features of these time lines when we showed the first one in Figure 6.11.) This figure shows
which end is sending packets. We also expand some of the tcpdump output (e.g., printing
SYN instead of S). In this time line we have also removed the window size values, since they
add nothing to the discussion.
Connection Establishment Protocol
Now let's return to the details of the TCP protocol that are shown in Figure 18.3. To establish a
1. The requesting end (normally called the client) sends a SYN segment specifying the port
number of the server that the client wants to connect to, and the client's initial sequence
number (ISN, 1415531521 in this example). This is segment 1.
2. The server responds with its own SYN segment containing the server's initial sequence
number (segment 2). The server also acknowledges the client's SYN by ACKing the
client's ISN plus one. A SYN consumes one sequence number.
3. The client must acknowledge this SYN from the server by ACKing the server's ISN plus file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_conn.htm (3 of 37) [12/09/2001 14.47.16] Chapter 18. TCP Connection Establishment and Termination one (segment 3).
These three segments complete the connection establishment. This is often called the three-way
handshake. Figure 18.3 Time line of connection establishment and connection termination.
The side that sends the first SYN is said to perform an active open. The other side, which
receives this SYN and sends the next SYN, performs a passive open. (In Section 18.8 we
describe a simultaneous open where both sides can do an active open.)
When each end sends its SYN to establish the connection, it chooses an initial sequence number
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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