TCP/IP Models
Network
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The US Departement of Defense (DoD) created the TCP/IP reference model, because it wanted to design a network that could service any conditions, including a nuclear war. In a world connected bydifferent type of communication media such as copper wire, microwapes, optical fibers and satellite links, the DoD wanted transmission of packet every time and under any condition. This very difficult design problem brought about the creation of the TCP/IP model.
Unlike the proprietary networking technilogies mentioned earlier, TCP/IP was developed as an open standard. This meant that anyone was free to use TCP/IP. This helped speed up the development of TCP/IP as a standard.
The TCP/IP model has the following four layers : Aplications layer, Transport layer, internet layer, and network access layer.
Although some of the layers in the TCP/IP model have the same name as layer in the OSI model, the layers of the two models do not correspond exactly. Most notably, the application layer has different functions in each model.The designers of TCp/IP flet that the application layer should include the OSI session and presentation layer details. They created an application layer that handles issues of presentation, encoding, and dialog control.
The transport layer deals with the quality of service issues of reliability, flow control, and error correction. One of its protocols, the transmission control protocol (TCP), provides excellent and flexible way to create reliable, well-flowing, low-error network communications.
TCP is a connection-oriented protocol. It maintains a dialogue between source and destination while packaging application layer information into units called segment. Connection-oriented does notmean that a circuit exists between the communicatingcomputers. It does mean that eyer 4 segments travel back and foth between two host to acknowledge the connection exists logically for some period.
The purpose of the internet layer is to divide TCP segment into packets and send them from any network. The packets arrive at the destination network independent of the path they took to get there. The specific protocol that governs this layer is called the internet protocol (IP). Best path determination and packet switching occur at this layer.
The relationship between IP and TCP is an important one. IP can be thought to point the way for the packets, while TCP provides a reliable transport.
The name of the network access layer is very broad and somewhat confusing. It is also known as the host-to-network layer. This layer is concerned with all of the components, both physical and logical, that are required to make physiical link. It includes the networking technology details, including all the details in the OSI physical and data link layer.
Follow the illustrates some of the common protocols specified by the TCP/IP reference model layer. Some of the most commonly used application layer protocols include the follow :
File transfer Protocol (FTP)
Hypertext transfer protocol (HTTP)• Domain Name System (DNS)
Trivial File Transfer Protocol (TFTP)
The common transfort layer protocols include :
Transport Control Protocol (TCP)
User Datagram Protocol (UDP)
The primary protocol of the internet layer is :
Internet Protocol (IP)
The network access layer lefers to any particular technology used on a specific network.
A comparison of the OSI model and the TCP/IP model will point out some similarities and differences.
OSI Model TCP/IP Model
Smilarities include :
Both have layers
Both have application layers, though they include very different service
Both have comparable transport nd network layers
Both models need to be know by networking professionals.
Both assume packets are switched. This mean individual packets may take different paths to rech the same destination. This is contrasted with circuit-switched networks where all the packets take the same path.
Differences include :
TCP/IP combines the presentation and session layer issues into it s applications layer.
TCP/IP combines the OSI data link and physical layers into the network access layer.• TCP/IP appears simpler because it has fewer layers.
TCP/IP protocol are the standards around which th internet developed, so the TCP/IP model gains credibility just because of its protocols. In contrast, networks are not usually built on the OSI protocol, even though the Osi model is used as a guide.
Although TCP/IP protocols are the standards around which the internet has grown, this curriculum will used the OSI model for the following reasons :
It is geberic, protocol-independent standard.
It has more detils, wich make it more helpful for teaching and learning.
It has more detils, wich can be helpful when troubleshooting.
Networking profesionals differ in their opinions on which model to use. Due to the nature of the industry it is necessary to become familiar with both. Both the OSI and TCP/IP models will be referred to throughout the curiculum. The focus will be on yhe followoing :
TCP as an TCP layer 4 protocol
IP as an OSI layer 3 protocol
Ethernet as a layer 2 and layer 1technology
Remember that there is a difference between a model and an actual protocol the is used in networking. The OSI model will be used to describe TCP/IP protocols.
