TCP Connection Establishment Process: The "Three-Way Handshake"

Normal Connection Establishment: 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.

Key Concept: The normal process of establishing a connection between a TCP client and server involves three steps: the client sends a SYN message; the server sends a message that combines an ACK for the client’s SYN and contains the server’s SYN; and then the client sends an ACK for the server’s SYN. This is called the TCP 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+ACKmessage 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+ACKcontaining 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 SYNand 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.

 

From:http://www.tcpipguide.com/free/t_TCPConnectionEstablishmentProcessTheThreeWayHandsh-3.htm