A protocol stack is organized in such a way that the highest level of communication resides in the top layer. Each layer in the stack is built on the services of the immediate lower layer.

The TCP/IP protocol stack has four layers, as follows:

• Application layer: This layer manages the high-level protocols, including representation, coding, and dialogue control issues. It handles everything related to applications, and the data is packed appropriately for the next layer. It is a user process that cooperates with other processes on the same host or a different one. Examples of protocols at this layer are TELNET, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).
• Transport layer: This layer handles quality of service, reliability, flow control, and error correction. One of its protocols is the TCP, which provides reliable network communications that are oriented to the connection, unlike UDP, which is not connection oriented. It also provides data transfer. Example protocols include TCP (connection oriented) and UDP (non-connection oriented).
• Network layer: The purpose of the internet layer is to send packets from the source of any network and make them reach their destination, regardless of the route they take to get there.
• Network access layer: This is also called a host-to-host network layer. It includes the LAN and WAN protocols, and the details in the physical and data link layers of the OSI model. Also known as the link layer or data link layer, the network interface layer is the interface to the current network hardware.

The following diagram represents the TCP/IP protocol stack:

The IP is the most important protocol of the network layer. It is a non-connection oriented protocol that does not assume reliability of the lower layers. IP does not provide reliability, flow control, or error recovery. These functions must be provided by the upper level, in the transport layer with TCP as the transport protocol, or in the application layer if UDP is being used as the transport protocol. The message unit in an IP network is called an IP datagram. This is the basic unit of information that is transmitted from one side of the TCP/IP network to the other.

The application layer is where all of the user interaction with the computer and services occurs. As an example of this, any browser can work, even without the TCP/IP stack installed. Usually, we use browsers such as Google Chrome, Mozilla, Firefox, Internet Explorer, and Opera for communicating with this layer.

When initiating a query for a remote document, the HTTP protocol is used. Each time we request a communication of this type, the browser interacts with the application layer, which, in turn, serves as an interface between the user's applications and the protocol stack, which will provide communication with the help of the lower layers.

The responsibilities of the application layer are to identify and establish the communication availability of the target destination, as well as to determine the resources for that communication to exist. Some of the protocols of the application layer are as follows:

• FTP
• HTTP
• Post Office Protocol version 3 (POP3)
• Internet Message Access Protocol (IMAP)
• SMTP
• Simple Network Management Protocol (SNMP)
• TELNET—TCP/IP Terminal Emulation Protocol