The Linux Programming Interface

1266 Chapter 61

61.6 A Closer Look at TCP

Knowing some of the details of the operation of TCP helps us to debug applications

that use TCP sockets, and, in some cases, to make such applications more efficient. In

the following sections, we look at:

z the format of TCP segments;

z the TCP acknowledgement scheme;

z the TCP state machine;

z TCP connection establishment and termination; and

z the TCP TIME_WAIT state.

61.6.1 Format of a TCP Segment

Figure 61-2 shows the format of the TCP segments that are exchanged between the

endpoints of a TCP connection. The meanings of these fields are as follows:

z Source port number: This is the port number of the sending TCP.

z Destination port number: This is the port number of the destination TCP.

z Sequence number: This is the sequence number for this segment. This is the offset

of the first byte of data in this segment within the stream of data being transmit-

ted in this direction over the connection, as described in Section 58.6.3.

Figure 61-2: Format of a TCP segment

Header

Header

length

(4 bits)

Reser-

ved

(4 bits)

Source port number Destination port number

Window size

TCP checksum Urgent pointer

Options (if present)

(0 – 40 bytes)

Sequence number

Acknowledgement number

20 bytes

(^)
Control
(8 bits)
Data (if present)
(0+ bytes)
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