Reference Models
The process of creating data packets is based on two connection models—the OSI and DARPA reference models. The Open Systems Interconnection (OSI) model is a standard reference model for how network data is transmitted between any two points in a computer network. TCP/IP in its most basic form supports the Defense Advanced Research Projects Agency (DARPA) model of internetworking and its network-defined layers. Much like the DARPA model, the OSI was designed to connect dissimilar computer network systems. The OSI reference model defines seven layers of functions that take place at each end of a network communication:
OSI Reference Model
|
Layer |
Description |
|---|---|
|
Application (7) |
This is the layer at which programs are identified; user authentication and privacy are implemented here. |
|
Presentation (6) |
This is a layer—usually part of an operating system—that converts incoming and outgoing data from one presentation format to another. |
|
Session (5) |
This layer sets up, coordinates, ends conversations, exchanges, and dialogs between the applications at each end of the dialog. |
|
Transport (4) |
This layer manages the end-to-end control and error checking. |
|
Network (3) |
This layer handles the routing and forwarding of the data. |
|
Data link (2) |
This layer provides error control and synchronization for the physical level. |
|
Physical (1) |
This layer transmits the bit stream through the network at the electrical and mechanical level. |
TCP/IP also has a much simpler protocol model called the DARPA model:
DARPA Model
|
Layer |
Description |
|---|---|
|
Process (4) |
This is the layer where higher-level processes such as |
|
Host to Host (3) |
This is where TCP lives. This is the mechanism that actually ports the data to the correct application. TCP ports are defined here. |
|
Internet (2) |
IP addresses are used to direct packets to the correct destination. Routing protocols live here along with Address Resolution Protocol (ARP) and Internet Control Message Protocol (ICMP) . |
|
Network Interface (1) |
This is the physical connection to the network: Ethernet, token ring, and so on. The packets are placed onto the network at this point. |
