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Many computer users may have heard of the Open Systems Interconnection Model (OSI Model). This is a seven-layer model used to describe and standardize networking and other forms of telecommunications. However, there are layers above the OSI Model and between the layers. Although, they are not standard or official entities/layers.
The OSI Model consists of seven layers as listed below
- Physical
- Data Link
- Network
- Transport
- Session
- Presentation
- Application
Bruce Schneier and RSA invented the concept of layers above the OSI layer.
- Layer 8 - The user
- Layer 9 - The organization
- Layer 10 - Government or legal compliance
Some alternative models view layer 8 as the financial layer and layer 9 as the political layer.
The "extended OSI model" is important. For instance, with the USA government making new laws that control the Internet, the behavior and usage of the lower levels are influenced. Also, think about some of the cryptographic algorithms that are used to secure connections (such as TLS and SSL). These cryptographic algorithms are standardized by large organizations or the government. For example, all of the versions of TLS are standardized - TLS v1.0 is RFC 2246 (https://tools.ietf.org/html/rfc2246), TLS v1.1 is RFC 4346 (https://tools.ietf.org/html/rfc4346), and TLS v1.2 is RFC 5246 (https://tools.ietf.org/html/rfc5246). Remember, TLS runs on OSI layer 6. In addition, TCP v4 is standard RFC 793 (http://tools.ietf.org/html/rfc793) and runs on layer 4. Clearly, the government or political layer (Layer 10) is an important layer to incorporate into the OSI model due to layer-10's strong influence on the lower levels.
Not only are there extended layers above the OSI model, some entities exist between the standard layers. For example, Network Driver Interface Specification (NDIS) is an API interface between Layer 2 (data link) and Layer 3 (network). The alternative to NDIS, Open Data-Link Interface (ODI), also functions between the two layers. Some software and code may exist between many layers. For instance, Unix's Berkeley Sockets and the System V Transport Layer Interface provide layer 4 with interfaces to layer 5, 6, and 7. Therefore, some code can exist between multiple layers, but not be a part of the layer.
