Introduction to Network Protocols

A network protocol is a standard written down on a piece of paper (or, more precisely, with a text editor in a computer). The standards that are used for the Internet are called Requests For Comment (RFC). RFCs are numbered from 1 onwards. There are more than 4,500 RFCs today. Many of them have become out of date, so only a handful of the first thousand RFCs are still used today. 


The International Standardization Office (ISO) has standardized a system of network protocols called as ISO OSI. Another organization that issues communication standards is the International Telecommunication Union (ITU) located in Geneva. The ITU was formerly known as the CCITT and, being founded in 1865, is one of the oldest worldwide organizations for comparison, the Red Cross was founded in 1863). 

Some standards are also issued by the Institute of Electrical and Electronics Engineers (IEEE). RFC, standards released by RIPE (Réseaux IP Européens), and PKCS (Public Key Cryptography Standard) are freely available on the Internet and are easy to get hold of. Other organizations (ISO, ITU, and so on) do not provide their standards free of charge—you have to pay for them. If that presents a problem, then you have to spend some time doing some library research.

1.Communication between the two foreigners and between the two interpreters is only virtual.

2. In fact, the only real communication happens between the foreigner and his or her interpreter. 

3. Even more layers are used in computer networks. 

4. The number of layers depends on which system of network protocols you choose to use. The system of network protocols is sometimes referred to as the network model. 

5. You most commonly work with a system that uses the Internet, which is also referred to as the TCP/IP family. 


6. In addition to TCP/IP, we will also come across the ISO OSI model that was standardized by the ISO.

1. ISO OSI

Communication between two computers is shown in the following figure:


Seven-layer architecture of ISO OSI

(I) Physical Layer 

The physical layer is responsible for activating the physical circuit between the Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE), communicating through it, and then deactivating it. 

Additionally, the physical layer is also responsible for the communication between DCEs. A computer or router can represent the DTE. The DCE, on the other hand, is usually represented by a modem or a multiplexer.

(II) Data Link Layer 

As for serial links, the link layer provides data exchange between neighboring computers as well as data exchange between computers within a local network. For the link layer, the basic unit of data transfer is the data link packet frame data frame is composed of a header, payload, and trailer.

A serial port or an Ethernet card can serve as a link interface. A link interface has a link address that is unique within a particular Local Area Network (LAN).

(III) Network Layer 

The network layer ensures the data transfer between two remote computers within a particular Wide Area Network (WAN). The basic unit of transfer is a datagram that is wrapped (encapsulated) in a frame. The datagram is also composed of a header and data field. Trailers are not very common in network protocols.

(IV) Transport Layer 

A network layer facilitates the connection between two remote computers. As far as the transport layer is concerned, it acts as if there were no modems, repeaters, bridges, or routers along the way. 

The transport layer relies completely on the services of lower layers. It also expects that the connection between two computers has been established, and it can therefore fully dedicate its efforts to the cooperation between two distant computers. 

Generally, the transport layer is responsible for communication between two applications running on different computers.

(V) Session Layer 

The session layer facilitates exchange of data between two applications. In other words, it serves as a checkpoint and is involved in synchronizing transactions, correctly closing files, and so on. Sharing a network disk is a good example of a session. 

The disk can be shared for a certain period of time, but the disk is not used for the entire time. When we need to work with a file on the network disk, a connection is established on the transport layer from the time when the file is opened to when it is closed. 

The session, however, exists on the session layer for the entire time the disk is being shared.

(VI) Presentation Layer 

The presentation layer is responsible for representing and securing data. The representation can differ on different computers. For example, it deals with the problem of whether the highest bit is in the byte on the right or on the left. By securing, we mean encrypting, ensuring data integrity, digital signing, and so forth. 

(VII) Application Layer 

The application layer defines the format in which the data should be received from or handed over to the applications. 

For example, the OSI Virtual Terminal protocol describes how data should be formatted as well as the dialogue used between the two ends of the connection.

2. TCP/IP 

With a few exceptions, the TCP/IP family does not deal with the physical or link layers.

1. In practice, Internet protocols often use protocols that adhere to the ISO OSI standards for the physical and link layers. 

2. What is the correlation between the ISO OSI protocols and TCP/IP? Each group of protocols has its definition of its own layers as well as the protocols used on these layers. 

4. Generally speaking, ISO OSI protocols and TCP/IP are incompatible. 

5. In practice, ISO OSI-compliant communication appliances need to be used for transferring IP datagrams, or on the other hand, services based on ISO OSI need to be provided via the Internet.

3. Internet Protocol 

Internet Protocol (IP) basically corresponds to the network layer. IP is used for transmitting IP datagrams between remote computers.

Each IP datagram header contains the destination address, which is the complete routing information used for delivering the IP datagram to its destination.

 Therefore, the network can only transmit each datagram individually. IP datagrams of one session can be transmitted through different paths and can thus be received by the destination in a different order than they were sent.

4. TCP and UDP 

TCP and UDP correspond to the transportation layer. 

1. TCP transports data using TCP segments that are addressed to individual applications. 

2. UDP transports data using UDP datagrams. 

3. TCP and UDP arrange a connection between applications that run on remote computers. 

4. TCP and UDP can also facilitate communication between processes running on the same computer, but this is not very interesting for our purposes.