As with Lecture 1, not a lot that’s substantive here.
- 1.5 Protocol layers and their service models
- 1.6 Networks under attack
Section 1.5 of the book walks through the layers of the Internet. All of the practical issues I’ve ever come across in my work are in the outermost layers: application, transport, and network. The link layer on down is not really relevant.
I will still watch the video lectures on these layers, and take notes. However, given the practical focus of my 2024 sabbatical, I will not spend time on their readings unless a specific need or interest arises.
Five-layer vs seven-layer models
The model used in the book has five layers: application, transport, network, link, physical. The model I’ve always learned has seven layers: between application (7) and transport (4), it adds presentation (6) and session (5). This is called the Open Systems Interconnect (OSI) model. These layers are absent from the Internet itself, though they are implemented in applications. The presentation layer would have included encryption and compression; the session layer would have provided integrity checking. We depend on the application layer, and the application itself, to provide these capabilities on the modern Internet.