CyberSecurity - The OSI Model

THE OSI Model

The Open Systems Interconnect (OSI) model is used to describe networking protocols. Because it is rarely implemented in actual networks, it is considered a reference to standardize the development of actual networks. OSI was the first nonproprietary open definition for networking.

The OSI model defines groups of functionality required for network computers into layers, with each layer implementing a standard protocol for its functionality. There are seven layers in the OSI model

1. Application

2. Presentation

3. Session

4. Transport

5. Network

6. Data Link

7. Physical

 Each OSI layer performs a specific function for the network:

physical layer—Manages signals among network systems

Data link layer—Divides data into frames that can be transmitted by the physical layer network layer—Translates network addresses and routes data from sender to receiver Transport layer—Ensures that data are transferred reliably in the correct sequence Session layer—Coordinates and manages user connections

presentation layer—Formats, encrypts and compresses data

Application layer—Mediates between software applications and other layers of network services

TCP/IP

The protocol suite used as the de facto standard for the Internet is known as the Transmission Control Protocol/Internet Protocol (TCP/IP). The TCP/IP suite includes both network-oriented protocols and application support protocols. Exhibit 3.4 shows some of the standards associated with the TCP/IP suite and where these

fit within the OSI model. It is interesting to note that the TCP/IP set of protocols was developed before the OSI

framework; therefore, there is no direct match between the TCP/IP standards and the layers of the framework.

Encapsulation

Encapsulation is the process of adding addressing information to data as it is transmitted down the OSI stack. Each layer relies on the services provided by the layer below. Each layer of the OSI model only communicates with its destination peer. It does so using datagrams or Protocol Data Units (PDUs). 

Upper layer data are passed down to the transport layer as segments and are“wrapped” with a header for identification.

These segments are passed down to the network layer as packets again with a header. Data are broken down to frames at the data link layer and also have control information appended. At the physical layer, data take the form of bits

(1s and 0s) for delivery to destination network.

Once at the destination—each layer on the receiving end strips off the appropriate addressing information and passes it up the OSI stack until the message is delivered. This process is called decapsulation.

Communication services at layer 4 are categorized as either connection-oriented or connectionless. TCP provides reliable, sequenced delivery with error-checking. Connections are established using a three-way handshake, and thus are connection-oriented, User Datagram Protocol (UDP) is a connectionless protocol used where speed is more important than error-checking and guaranteed delivery. UDP does use checksums