The TCP/IP Guide - TCP Connection Establishment Process: The "Three-Way Handshake"

To establish a connection, each device must send a SYN and receive an ACK for it from the other device. Thus, conceptually, we need to have four control messages pass between the devices. However, it's inefficient to send a SYN and an ACK in separate messages when one could communicate both simultaneously. Thus, in the normal sequence of events in connection establishment, one of the SYNs and one of the ACKs is sent together by setting both of the relevant bits (a message sometimes called a SYN+ACK). This makes a total of three messages, and for this reason the connection procedure is called a three-way handshake.

Table 152 describes in detail how the three-way handshake works (including a summary of the preparation discussed in the previous topic). It is adapted from the table describing the TCP finite state machine, but shows what happens for both the server and the client over time. Each row shows the state the device begins in, what action it takes in that state and the state to which it transitions. The transmit and receive parts of each of the three steps of the handshake process are shown in the table, as well as in Figure 211.

**Table 152: TCP “Three-Way Handshake” Connection Establishment Procedure**
Client			Server
Start State	Action	Move To State	Start State	Action	Move To State
*CLOSED*	The client cannot do anything until the server has performed a passive OPEN and is ready to accept a connection. (Well, it can try, but nothing will be accomplished until the server is ready.)	—	*CLOSED*	The server performs a passive OPEN, creating a transmission control block (TCB) for the connection and readying itself for the receipt of a connection request (SYN) from a client.	*LISTEN*
*CLOSED*	Step #1 Transmit: The client performs an active OPEN, creating a transmission control block (TCB) for the connection and sending a SYN message to the server.	*SYN-SENT*	*LISTEN*	The server waits for contact from a client.	—
*SYN-SENT*	The client waits to receive an ACK to the SYN it has sent, as well as the server's SYN.	—	*LISTEN*	Step #1 Receive, Step #2 Transmit: The server receives the SYN from the client. It sends a single SYN+ACK message back to the client that contains an ACK for the client's SYN, and the server's own SYN.	*SYN-RECEIVED*
*SYN-SENT*	Step #2 Receive, Step #3 Transmit: The client receives from the server the SYN+ACK containing the ACK to the client's SYN, and the SYN from the server. It sends the server an ACK for the server's SYN. The client is now done with the connection establishment.	*ESTABLISHED*	*SYN-RECEIVED*	The server waits for an ACK to the SYN it sent previously.	—
*ESTABLISHED*	The client is waiting for the server to finish connection establishment so they can operate normally.		*SYN-RECEIVED*	Step #3 Receive: The server receives the ACK to its SYN and is now done with connection establishment.	*ESTABLISHED*
*ESTABLISHED*	The client is ready for normal data transfer operations.		*ESTABLISHED*	The server is ready for normal data transfer operations.

**Figure 211: TCP “Three-Way Handshake” Connection Establishment Procedure**
This diagram illustrates how a conventional connection is established between a client and server, showing the three messages sent during the process and how each device transitions from the *CLOSED* state through intermediate states until the session is *ESTABLISHED*.

TCP Connection Preparation: Transmission Control Blocks (TCBs) and Passive and Active Socket OPENs

TCP Connection Establishment Sequence Number Synchronization and Parameter Exchange

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