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.                          |
|     |                |                                |                                         |
---------------------------------------------------------------------------------------------------

Sources: The OSI Reference Model and Building Switched Networks by Darryl P. Black (1999)

The Difference Between TCP/IP and the OSI Reference Model

TCP/IP and OSI actually represent opposite approaches to the problem of robust network implementations. The node title is your first clue. The OSI Reference Model was conceptualized first, then implemented (poorly). The TCP/IP Reference Model was actually drawn up to describe the already existing network stack. Both play an important role in the education of future network engineers; OSI demonstrates principles while TCP/IP offers a great example of practical design.

A Reference Model is one conceptual step more general than a technical specification. It generalizes the principles behind the network layer architecture so that the reasoning behind implementation details is not lost. Because networking is such an ubuiquitous and difficult problem, a good reference model is handy to keep the big picture in mind when designing new network protocols.

The OSI Reference Model has proven to be invaluable in the realm of education because it was conceived on an abstract level, these are the things you need to think about when designing networks. It offers a very clean model with clear distinctions between the roles of each layer. Unfortunately in practice things are often not so clear cut. For example, error correction may need to be done at each layer to satisfy its correctness, but in practice we may want a higher layer to correct everything below it for efficiency's sake. Another problem with OSI is that some layers are overkill. For instance, the Session and Presentation layers both do very little, and what they do is likely to vary from application to application. Thinking about them conceptually is useful, but implementing usually isn't.

The TCP/IP Reference Model, on the other hand, isn't even a proper model. For one thing, only the middle two layers, Network and Transport are well-defined. The bottom layer mostly defines an interface to some heterogenous hardware below it. This is done so that TCP/IP can be implemented on top of any simple network, but it renders the model incomplete because it tells us nothing about how data transmission is accomplished at the frame or physical levels. The result is something that's handy to know, but not great for teaching principles.

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