Network+/Standards/TCP/IP Model/Introduction

The Transmission Control Protocol/Internet protocol model 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 modularization allows the system to be maintained and updated without causing trouble to other parts of the system.

Introduction
The TCP/IP Model is a Department of Defense created system for network transmission. Similar 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 transmission into four layers which are:

4. The Application Layer 3. The Transport Layer 2. The Internet Layer 1. The Link Layer

You may see some layers called by different names in different places but the concept of operation remains the same.

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 many important protocols including HTTP for web-browsing, FTP for file transfer, IMAPv4 for instant messaging, and SMTP for e-mail delivery.

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

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 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 do not implement Logical Link Control at Network Interface Layer.

Internet 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. Currently, a transition is occurring between IP protocol version 4 (IPv4), and the more flexible and scalable version 6 (IPv6). It is not uncommon to see a combination of the two protocols in similar settings. The IP datagram is composed of 20 bytes for the header plus a variable data field that varies based on optimization.

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. Signaling 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.

Also included in the link layer of the TCP/IP model is the physical mediums (this is a separate layer in the OSI model). The physical mediums consist of twisted copper cables and also wireless transmission mediums that allow bits to travel from machine to machine. The physical layer is how the signal is physically transported from location A to location B across a network.

Differences from the OSI Model
The Application Layer (Layer 4) of the TCP/IP Model combines the Application (Layer 7), Presentation (Layer 6), and Session (Layer 5) layers of the OSI Model.

The Link Layer of the TCP/IP Model (Layer 1) combines the Data Link (Layer 2) and Physical (Layer 1) layers of the OSI Model.