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TCP Connection Establishment Process: The "Three-Way Handshake"
(Page 2 of 4)
Control Messages Used for Connection Establishment: SYN and ACK
TCP uses control messages to manage
the process of contact and communication. There aren't, however, any
special TCP control message types; all TCP messages use the
same segment format. A set of control
flags in the TCP header indicates whether a segment is being used for
control purposes or just to carry data. As I introduced in the
discussion of the TCP finite state machine,
two control message types are used in connection setup, which are specified
by setting the following two flags:
- SYN: This bit indicates that the
segment is being used to initialize a connection. SYN stands
for synchronize, in reference to the sequence number synchronization
I mentioned above.
- ACK: This bit indicates that the
device sending the segment is conveying an acknowledgment for
a message it has received (such as a SYN).
There are also other control bits
(FIN, RST, PSH and URG), which aren't important
to connection establishment, so we will set them aside for now. We'll
discuss them in other topics. In common TCP parlance, a message with
a control bit set is often named for that bit. For example, if the SYN
control bit is set the segment is often called a SYN message.
Similarly, one with the ACK bit set is an ACK message
or even just an ACK.
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Version 3.0 - Version Date: September 20, 2005
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