 layer produces information that is exchanged with a peer application, with a mutually understood presentation format, and capability to establish a communication. The internet's transport layer, using a protocol such as TCP, establishes a connection between the two applications and ensures received packets are error free. The network layer functions are covered by the IP which provides addressing as well as control mechanisms to deliver data packets produced by the transport later protocol. The network access (link) layer establishes logical and physical links OSI Model TCP/IP Application Presentation Application Session Transport Transport (Host to Host) Network Internet Data Link Network Access Physical Physical Figure 8.6 A comparison of the OSI and TCP/IP stacks Packetised Data Communications Application layer UDP/TCP header Transport layer IP data header Internet layer Network Frame data header Link layer Figure 8.7 IP suite stack headers between two nodes in the network. An example of such protocols is the Ethernet which includes the physical means of data transmission such as digital modulation, and includes the physical layer functions as part of the network access layer. Each layer of the IP stack adds control information that enables the receiver to retrieve the embedded information. Each layer effectively creates an envelope that carries information necessary for extracting the data transmitted between two end applications. A typical internet stack and header construction is shown in Figure 8.7. A brief description of different layers and typical protocols are given below. Application Layer Protocols Countless applications use the IP to communicate with applications running in other devices. A Skype application may communicate with a Skype server in a remote telephone exchange to facilitate an international telephone conversation, a browser application may communicate with a Google server to retrieve search results and SO on. Applications may encrypt the data they communicate (an email application), a