In a building, each floor depends on the strength of the floors below it. Ensuring that the fifth floor is reinforced provides very little comfort if there is a structural problem on the third floor. In order to ensure that the building keeps standing, you need to reinforce every floor starting from the bottom.
And so it is with protocol stacks. Providing security at the transport layer (e.g., TLS) has questionable value if the packet exchange is compromised at the network or data link layer. Yet, we rarely worry about protecting these layers.
This new technical brief from Northforge describes some of the common attacks that can occur at the data link layer and how MAC-layer Security or MACsec (IEEE std 802.1AE™) can be used to provide hop-by-hop or end-to-end authentication and encryption to protect the lowest floors of your protocol building.