The OSI Reference Model and TCP/IP



===================================================================================================
| No. | ISO Layer Name | TCP/IP Layers                  | Responsibility                          |
===================================================================================================
|     |                |                                |                                         |
|     |                |                                | Applications running on a PC or other   |
|  7  | Application    |                                | device.  Applications like telnet, FTP, |
|     |                |                                | and HTTP operate at this layer.         |
|     |                |                                |                                         |
|----------------------|       There is no clear        |-----------------------------------------|
|     |                |       distinction              |                                         |
|     |                |       between these            | Provides an abstraction for data        |
|  6  | Presentation   |       layers in the            | representation differences between      |
|     |                |       TCP/IP protocol.         | applications.                           |
|     |                |                                |                                         |
|----------------------|                                |-----------------------------------------|
|     |                |                                |                                         |
|     |                |                                | Offers an optional bidirectional or     |
|  5  | Session        |                                | full-duplex service if not provided     |
|     |                |                                | by the transport layer.                 |
|     |                |                                |                                         |
|----------------------|--------------------------------|-----------------------------------------|
|     |                |                                |                                         |
|  4  | Transport      | TCP, UDP, and ICMP(1)          | Provides end-to-end management.  For    |
|     |                |                                | example, TCP provides flow control,     |
|     |                |                                | acknowledgement, sequencing, and error  |
|     |                |                                | correction; TCP also provides           |
|     |                |                                | connection-oriented "reliable"          |
|     |                |                                | communications.  UDP provides           |
|     |                | (1): ICMP could very well be   | connectionless communication, a very    |
|     |                |      positioned at layer 3     | thin layer on top of IP.  However, it   |
|     |                |      but it is positioned at   | provides "unreliable" communication,    |
|     |                |      layer 4 because ICMP      | that is, the data may get to its        |
|     |                |      must be first placed      | destination, or it may not.             |
|     |                |      within an IP packet       |                                         |
|     |                |      of which IP is a layer 3  | TCP also provides a concept of          |
|     |                |      protocol.                 | of session management at this layer.    |
|     |                |                                |                                         |
|----------------------|--------------------------------|-----------------------------------------|
|     |                |                                |                                         |
|     |                |                                | Is responsible for the routing of       |
|  3  | Network        |  IP, ARP, and RARP.            | packets (which encapsulates frames)     |
|     |                |                                | across the network.                     |
|     |                |                                |                                         |
|----------------------|--------------------------------|-----------------------------------------|
|     |                |                                |                                         |
|  2  | Data Link      | The data link                  | Regulates access to the network.        |
|     |                |                                | Provides point-to-point frame           |
|     |                | (Ethernet, ATM+IP, FDDI,       | formation and management for various    |
|     |                | & Token Ring all span          | protocols.  All frames contain          |
|     |                | the data link and physical     | source addresses and a destination      |
|     |                | layers)                        | addresses.                              |
|     |                |                                |                                         |
|----------------------|--------------------------------|-----------------------------------------|
|     |                |                                |                                         |
|     |                |                                | Controls the physical attachment        |
|  1  | Physical       | Physical, as mentioned         | to the network, including wiring        |
|     |                | above.                         | and signaling.                          |
|     |                |                                |                                         |
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Sources: The OSI Reference Model and Building Switched Networks by Darryl P. Black (1999)