The Osi Framework - D-Link NetDefendOS User Manual

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
NetDefendOS features such as ARP, Services and ALGs.
Layer Functions
The different layers perform the following functions:
Layer 7 - Application Layer
Layer 6 - Presentation Layer
Layer 5 - Session Layer
Layer 4 - Transport Layer
Layer 3 - Network Layer
Layer 2 - Data-Link Layer
Layer 1 - Physical Layer
Layer number
Layer 7
Layer 6
Layer 5
Layer 4
Layer 3
Layer 2
Layer 1
Figure D.1. The 7 Layers of the OSI Model
Defines the user interface that supports applications
directly. Protocols: HTTP, FTP, TFTP. DNS, SMTP, Telnet,
SNMP and similar. The ALGs operate at this level.
Translates the various applications to uniform network
formats that the rest of the layers can understand.
Establishes, maintains and terminates sessions across the
network. Protocols: NetBIOS, RPC and similar.
Controls data flow and provides error-handling. Protocols:
TCP, UDP and similar.
Performs addressing and routing. Protocols: IP, OSPF, ICMP,
IGMP and similar.
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.
Defines the physical hardware connection.
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Layer purpose
Application
Presentation
Session
Transport
Network
Data-Link
Physical