Protocols

A protocol is a rule or a complete set of rules defining how computers communicate. In other words, a protocol is an agreement that must be obeyed by sender and receiver in order to communicate. To be able to communicate, people speak same language. Similarly, to establish communication between computers there must be a set of rules that both computers understand.

Following are the three different types of protocols:

Application Protocol
Transport Protocol
Network Protocol

Application protocols provide interaction within applications and facilitate data exchange (e.g., SMTP). Transport protocols ensure that data is able to move across computers by adding error-handling information to the data. Network protocols handle addressing, error checking and retransmission requests.

Some most common high-level network protocols are: File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP), Telnet and Hypertext Transfer Protocol (HTTP). Most of these protocols sit on a lower level protocol like Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP).

Reference Models

A network model is a pattern that provides guidelines for moving data within a network.

Two most important reference models are:

Open Systems Interconnection (OSI) reference model
TCP/IP reference model

OSI Reference Model

The OSI model is a seven-layer model developed by the International Standards Organization (ISO).

The seven layers are:

Application Layer.
Presentation Layer.
Session Layer.
Transport Layer.
Network Layer.
Data link Layer.
Physical Layer.

Computers running on the Internet communicate to each other using either the TCP or the UDP.