4 Layers of the TCP/IP Model

The Internet Protocol Suite, TCP/IP, is a suite of protocols used for communication over the internet.  The TCP/IP model was created after the OSI 7 layer model for two major reasons.  First, the foundation of the Internet was built using the TCP/IP suite and through the spread of the World Wide Web and Internet, TCP/IP has been preferred.  Second, a project researched by the Department of Defense (DOD) consisted of creating the TCP/IP protocols.  The DOD's goal was to bring international standards which could not be met by the OSI model. Since the DOD was the largest software consumer and they preferred the TCP/IP suite, most vendors used this model rather then the OSI.  Below is a side by side comparison of the TCP/IP and OSI models.

 

TCP/IP Model	VS.	OSI Model
Application	Layer 7	Application
	Layer 6	Presentation
	Layer 5	Session
Transport	Layer 4	Transport
Internet	Layer 3	Network
Network Access	Layer 2	Data Link
	Layer 1	Physical

 

The TCP/IP model, similar to the OSI model, is comprised of layers.  The OSI has seven layers and the TCP/IP model has four or five layers depending on different preferences.  Some people use the Application, Transport, Internet and Network Access layers.  Others split the Network Access layer into the Physical and Data Link components.

LAYER 4 - APPLICATION

This layer is comparable to the application, presentation, and session layers of the OSI model all combined into one.  It provides a way for applications to have access to networked services.  This layer also contains the high level protocols.  The main issue with this layer is the ability to use both TCP¹ and UDP² protocols.  For example TFTP³ uses UDP because usually on a LAN⁴ the physical links are short enough to ensure quick and reliable packet delivery without many errors.  SMTP⁵ instead uses TCP because of the error checking capabilities.  Since we consider our email important information we would like to ensure a safe delivery.

LAYER 3 - TRANSPORT

This layer acts as the delivery service used by the application layer.  Again the two protocols used are TCP and UDP.  The choice is made based on the application's transmission reliability requirements.  The transport layer also handles all error detection and recovery.  It uses checksums, acknowledgements, and timeouts to control transmissions and end to end verification.  Unlike the OSI model, TCP/IP treats reliability as an end-to-end problem.

LAYER 2 - INTERNET

The routing and delivery of data is the responsibility of this layer and is the key component of this architecture.  It allows communication across networks of the same and different types and carries out translations to deal with dissimilar data addressing schemes.  It inject packets into any network and deliver them to the destination independently to one another.  Because the path through the network is not predetermined, the packets may be received out of order.  The upper layers are responsible for the reordering of the data.  This layer can be compared to the network layer of the OSI model.  IP and ARP⁶are the major protocols used at this layer.

LAYER 1 - NETWORK ACCESS

This a combination of the Data Link and Physical layers of the OSI model which consists of the actual hardware.  This includes wires, network interface cards, etc.  Other related details within this layer are connectors, signal strength, and wavelength along with various others.  It will use the required LAN operating algorithms, such as Carrier Sense Multiple Access with Collision Detect (CMSA/CD)⁷ or IBM Token Passing⁸ etc. and is responsible for placing the data within a frame.  The frame format is dependent on the system being used, for example Ethernet LAN, Frame relay⁹, etc. The frame is the package that holds the data, in the same way as an envelope holds a letter.  The frame holds the hardware address of the host and checking algorithms for data integrity.   This layer has actually not been specified in details because it depends on which technology is being used such as Ethernet.  So freedom is given to this layer as far as implementation is concerned.

TERMS 1.  TCP (Transmission Control Protocol)  - enables two to establish a connection and exchange streams of data. 
2.  UDP (User Datagram Protocol)  - offering a direct way to send and receive datagrams over an IP network with very few error recovery services.
3.  TFTP (Trivial File Transfer Protocol)  - Simplified version of the FTP protocol which has no security features.
4.  LAN (Local Area Network)  - A computer network that spans a small area.
5.  SMTP (Simple Mail Transfer Protocol)  - Protocol used to send email messages between servers.
6.  ARP (Address Resolution Protocol)  - used to convert an IP address to a physical address.
7.  CMSA/CD (Carrier Sense Multiple Access with Collision Detect)  - protocol for carrier transmission access for Ethernet networks.
8.  IBM Token Passing  - uses a token to grant a device the chance to transmit over the network.
9.  Frame Relay  - a packet switching protocol for connecting devices on a Wide Area Network.