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mandatory field that must be calculated and stored by the sender, and then verified by the
receiver. The TCP checksum is calculated similar to the UDP checksum, using a pseudoheader as described in Section 11.3.
The urgent pointer is valid only if the URG flag is set. This pointer is a positive offset that
must be added to the sequence number field of the segment to yield the sequence number of
the last byte of urgent data. TCP's urgent mode is a way for the sender to transmit
emergency data to the other end. We'll look at this feature in Section 20.8.
The most common option field is the maximum segment size option, called the MSS. Each
end of a connection normally specifies this option on the first segment exchanged (the one
with the SYN flag set to establish the connection). It specifies the maximum sized segment
that the sender wants to receive. We describe the MSS option in more detail in Section 18.4,
and some of the other TCP options in Chapter 24.
In Figure 17.2 we note that the data portion of the TCP segment is optional. We'll see in
Chapter 18 that when a connection is established, and when a connection is terminated,
segments are exchanged that contain only the TCP header with possible options. A header
without any data is also used to acknowledge received data, if there is no data to be
transmitted in that direction. There are also some cases dealing with timeouts when a
segment can be sent without any data. 17.4 Summary file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/tcp_tran.htm (5 of 6) [12/09/2001 14.47.10] Chapter 17. TCP: Transmission Control Protocol TCP provides a reliable, connection-oriented, byte stream, transport layer service. We
looked briefly at all the fields in the TCP header and will examine them in detail in the
TCP packetizes the user data into segments, sets a timeout any time it sends data,
acknowledges data received by the other end, reorders out-of-order data, discards duplicate
data, provides end-to-end flow control, and calcula...
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