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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Transport Layer Protocols            9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)                 9  TCP/IP Transmission Control Protocol (TCP)                      9  TCP Basic Operation: Connection Establishment, Management and Termination

TCP Connection Preparation: Transmission Control Blocks (TCBs) and Passive and Active Socket OPENs 1 2 3 4 TCP Connection Establishment Sequence Number Synchronization and Parameter Exchange

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”.

TCP Connection Preparation: Transmission Control Blocks (TCBs) and Passive and Active Socket OPENs 1 2 3 4 TCP Connection Establishment Sequence Number Synchronization and Parameter Exchange