9.5 TCP Communication Process

Question 1

TCP Connection Establishment

Answer 1

Each application process running on a server is configured to use a port number. The port number is either automatically assigned or configured manually by a system administrator.

An individual server cannot have two services assigned to the same port number within the same transport layer services.

Question 2

Three way handshacke

Answer 2

The three-way handshake validates that the destination host is available to communicate.

In TCP connections, the host client establishes the connection with the server using the three-way handshake process.

Step 1. SYN
The initiating client requests a client-to-server communication session with the server.

Step 2. ACK and SYN
The server acknowledges the client-to-server communication session and requests a server-to-client communication session.

Step 3. ACK
The initiating client acknowledges the server-to-client communication session.

Question 3

Session Termination

Answer 3

To close a connection, the Finish (FIN) control flag must be set in the segment header. To end each one-way TCP session, a two-way handshake, consisting of a FIN segment and an Acknowledgment (ACK) segment, is used. Therefore, to terminate a single conversation supported by TCP, four exchanges are needed to end both sessions. Either the client or the server can initiate the termination.

Step 1. FIN
When the client has no more data to send in the stream, it sends a segment with the FIN flag set.

Step 2. ACK
The server sends an ACK to acknowledge the receipt of the FIN to terminate the session from client to server.

Step 3. FIN
The server sends a FIN to the client to terminate the server-to-client session.

Step 4. ACK
The client responds with an ACK to acknowledge the FIN from the server.

When all segments have been acknowledged, the session is closed.

Question 4

TCP Three-way Handshake Analysis

Answer 4

Hosts maintain state, track each data segment within a session, and exchange information about what data is received using the information in the TCP header. TCP is a full-duplex protocol, where each connection represents two one-way communication sessions. To establish the connection, the hosts perform a three-way handshake. As shown in the figure, control bits in the TCP header indicate the progress and status of the connection.

These are the functions of the three-way handshake:

.It establishes that the destination device is present on the network.

.It verifies that the destination device has an active service and is accepting requests on the destination port number that the initiating client intends to use.

.It informs the destination device that the source client intends to establish a communication session on that port number.

After the communication is completed the sessions are closed, and the connection is terminated. The connection and session mechanisms enable TCP reliability function.

Question 5

Control Bits Field

Answer 5

The six bits in the Control Bits field of the TCP segment header are also known as flags. A flag is a bit that is set to either on or off.

The six control bits flags are as follows:

URG - Urgent pointer field significant
ACK - Acknowledgment flag used in connection establishment and session termination
PSH - Push function
RST - Reset the connection when an error or timeout occurs
SYN - Synchronize sequence numbers used in connection establishment
FIN - No more data from sender and used in session termination