0x430. Peeling Back the Lower Layers

When you use a web browser, all seven OSI layers are taken care of for you, allowing you to focus on browsing and not protocols. At the upper layers of OSI, many protocols can be plaintext since all the other details of the connection are already taken care of by the lower layers. Sockets exist on the session layer (5), providing an interface to send data from one host to another. TCP on the transport layer (4) provides reliability and transport control, while IP on the network layer (3) provides addressing and packet-level communication. Ethernet on the data-link layer (2) provides addressing between Ethernet ports, suitable for basic LAN (Local Area Network) communications. At the bottom, the physical layer (1) is simply the wire and the protocol used to send bits from one device to another. A single HTTP message will be wrapped in multiple layers as it is passed through different aspects of communication.

This process can be thought of as an intricate interoffice bureaucracy, reminiscent of the movie Brazil. At each layer, there is a highly specialized receptionist who only understands the language and protocol of that layer. As data packets are transmitted, each receptionist performs the necessary duties of her particular layer, puts the packet in an interoffice envelope, writes the header on the outside, and passes it on to the receptionist at the next layer below. That receptionist, in turn, performs the necessary duties of his layer, puts the entire envelope in another envelope, writes the header on the outside, and passes it on. Network traffic is a chattering bureaucracy of servers, clients, and peer-to-peer connections. At the higher layers, the traffic could be financial data, email, or basically anything. Regardless of what the packets contain, the protocols used at the lower layers to move the data from point A to point B are usually the same. Once you understand the office bureaucracy of these common lower layer protocols, you can peek inside envelopes in transit, and even falsify documents to manipulate the system.