How can I define the layered approach to protocols?

A layered approach to protocols can be seen in models like OSI and TCP/IP. The OSI model defines networking into a seven-layer process. Within the OSI model, the data is passed down from layer-to-layer. It begins at the application layer and ends at the physical layer. While the OSI model was great in theory what has been implemented in its place is TCP/IP. The DoD TCP/IP is a four layer model what both have in common is the ability to layer security. As an example, at OSI Layer 2 tunneling and VPN protocols can be used to protect data. Security controls like L2TP, WEP, and WPA are available. OSI Layer 3 is the home of IPSec. Even at the application layer security can be added by means of protocols like SSH, DNSSEC, PGP, or Secure RPC.