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Appendix D: The OSI Framework

Overview

The Open Systems Interconnection (OSI) model defines a framework for inter-computer communications. It categorizes different protocols for a great variety of network applications into seven smaller, more manageable layers. The model describes how data from an application in one computer can be transferred through a network medium to an application on another computer.

Control of data traffic is passed from one layer to the next, starting at the application layer in one computer, proceeding to the bottom layer, traversing over the medium to another computer and then delivering up to the top of the hierarchy. Each layer handles a certain set of protocols, so that the tasks for achieving an application can be distributed to different layers and be implemented independently. The model is relevant to understanding the operation of many cOS Core features such as ARP, Services and ALGs.

Layer number Layer purpose
Layer 7 Application
Layer 6 Presentation
Layer 5 Session
Layer 4 Transport
Layer 3 Network
Layer 2 Data-Link
Layer 1 Physical

Figure D.1. The 7 Layers of the OSI Model

Layer Functions

The different layers perform the following functions:

Layer 7 - Application Layer
Defines the user interface that supports applications directly. Protocols: HTTP, FTP, TFTP. DNS, SMTP, Telnet, SNMP and similar. The ALGs operate at this level.
Layer 6 - Presentation Layer
Translates the various applications to uniform network formats that the rest of the layers can understand.
Layer 5 - Session Layer
Establishes, maintains and terminates sessions across the network. Protocols: NetBIOS, RPC and similar.
Layer 4 - Transport Layer
Controls data flow and provides error-handling. Protocols: TCP, UDP and similar.
Layer 3 - Network Layer
Performs addressing and routing. Protocols: IP, OSPF, ICMP, IGMP and similar.
Layer 2 - Data-Link Layer
Creates frames of data for transmission over the physical layer and includes error checking/correction. Protocols: Ethernet, PPP and similar. ARP operates at this level.
Layer 1 - Physical Layer
Defines the physical hardware connection.
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