Layer 4 of the OSI model is the transport layer. The transport layer is in the middle of the OSI model, with three layers below and three layers above. In this article, I discuss fingerprinting and how it is related to the transport layer. Fingerprinting is the act of operating system (OS) To better understand how fingerprinting works, we first need to review some transport layer basics. identification. For example, is the client running MAC OS 10, BSD or Windows 2003 Server?