Comparing OSI and Internet Models

There is a rough correspondence between the two models, apart from the missing and merged layers. And there are big differences.

The OSI model was developed before an implementation, whereas the Internet model was developed after TCP/IP was implemented and is more a description of what happened. OSI makes a clear distinction between the model and implementation, while the Internet is more fuzzy.

OSI is very general, whereas Internet is very specific. OSI is more flexible in that is it not tied to a specific protocol and is better able to adapt to changes in technology. On the other hand, the OSI model had many problems when it came to an implementation where it was found that the layers provided did not correspond well to reality. Extra sublayers were developed and the simplicity of the OSI model was lost.

As it turns out, TCP/IP has been widely successful. Many reasons for this have been given, but the major ones seem to be that the committee defining OSI took so long that TCP/IP was already widely established by the time the standard was published. Also, the standard was so complex that only poor implementations of OSI were made, while the simpler layering of TCP/IP was fairly easy to make run well. Seven is not a magic number and other proposals had more layers (splitting up several layers into smaller, easier ones), or fewer (in particular the Internet model). It appears that seven was chosen as IBM already had a seven layer protocol (Systems Network Architecture, SNA).

It is important to realize that layering is there for structuring only. Layers must be followed for interoperability, but they need not be followed for implementation. The TCP/IP model is not all-singing, all-dancing either, it does have problems. The specification is confused with the implementation; it is only really good for describing TCP/IP and no other protocol stack; the physical and data link layers are merged, making it hard to talk about (say) copper wire vs. fibre installations.

The OSI model is widely used; the OSI protocols are virtually never used. The Internet model is rarely used; the Internet protocols are extremely widespread. Split the link layer of the Internet model into a physical and data link layer:

1. Physical
2. Data link
3. Network
4. Transport
5. Application

This appears to be a good compromise in that it matches well with reality (i.e., TCP/IP) and is somewhat more flexible than the Internet model.