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The transmission control protocol/Internet protocol refers to a set of rules governing communication between devices across IP networks like the Ethernet. It is implemented as a series of layers. Why layers? Layers allow us to deal with complex systems by separating the problem into its individual components, then dealing with it in a modular manner. This modularisation allows the system to be maintained and updated without causing trouble to other parts of the system.

Contents 1 Introduction 2 Application Layer 3 Transport Layer 4 Network Layer 5 Link Layer 6 Physical Layer 7 Differences from the OSI Model

[edit] Introduction

The TCP/IP Model is a Department of Defense created system for network transmission. Similarly to the OSI Model, the TCP/IP Model uses layers to describe the process of data transmission across a network. Instead of using a seven layers, the TCP/IP Model breaks down the transmissions into four layers which are (in descending order):

1. The Application Layer
2. The Transport Layer
3. The Network Layer
4. The Link Layer
5. The Physical Layer

[edit] Application Layer

The Application Layer, as the name implies, is where the applications or high level protocols reside. Protocols such as FTP, SMTP, DNS, SNMP, and HTTP all function at this layer. Some programs or applications that are built upon these protocols include the humble internet browser, email, and bittorent.

The application layer has protocols like HTTP; which supports web-browsing, FTP; which supports file transfer, IMAPv4; which supports instants messaging, SMTP; which supports e-mail delivery...the list is quite large, but as you can clearly see they are very important.

The application layer hosts high level protocols such as HTTP and FTP, which enable us to surf the web and transfer files

[edit] Transport Layer

The Transport Layer handles session management between host computers. TCP, UDP, IGMP and ICMP all function at this layer. Raw data from the application layer is broken down and encapsulated into segments, which are then passed into the lower layer.

The Transport Layer attempts to enable the successful transfer of of information by maintaining an end to end perspective, ensuring data that exits one end reaches the other.

The Logical Link Control was handled to this layer from a sub layer of Data Link Layer at OSI Model, TCP/IP Model not implement Logical Link Control at Network Interface Layer.

[edit] Network Layer

The Internet Layer packages information from the higher layers, determines the path the information is to take across the network, and then encapsulates the segments from the Transport layer into "envelopes" called packets. Path selection happens here, therefore IP and IPSec both operate at this level. Routers and some switches operate at this level.

The internet layer has quite a number of protocols like ICMP (Internet control message protocol), IPv4, IPv6 (v4 and v6 refer to version numbers) The internet layer defines the set of rules used to transmit datagrams. In order for both the transmitter and the receiver to understand each other a structure needs to be defined for the datagram. The most widely IP protocol currently used in version 4 (IPv4) but there is currently a push to use a more flexible and scalable version (IPv6) that still needs to be adapted by many vendors. The IP datagram is composed of a 20 byte for header plus a variable data field that varies based on optimization.

[edit] Link Layer

The Link Layer breaks down the packets from the Internet Layer into frames and then eventually into bits for transmission across the physical network medium. Signalling and network medium standards such as Ethernet, Token Ring, FDDI, X .25, Frame Relay, RS-232, and v.35 are defined in this layer. Network Interface Controller (NIC) Cards, network hubs, repeaters, bridges, and switches operate at this level.

[edit] Physical Layer

The physical layer usually comprises of twisted copper cables and wireless transmission mediums that allow bits to travel from machine to machine.

[edit] Differences from the OSI Model

The Application Layer of the TCP/IP Model combines the Application (Layer 7), Presentation (Layer 6), and Session (Layer 5) layers of the OSI Model. The Network Interface Layer of the TCP/IP Model combines the Data Link and Physical layers of the OSI Model.