Cisco Networking: OSI Model Layer 6 - Presentation
The presentation layer of the Open System Interconnection (OSI) model is responsible for how that data looks or is formatted. Consider an example in which spies exchange encoded messages. The manner of passing the messages back and forth is defined by the session layer, but how the messages are encoded (or the cipher the spies used to obscure the message) is the responsibility of the presentation layer.
Naturally, this has to be negotiated between the participants because it would be useless for one spy to encode a message that the other spy did not know how to decode. So with the presentation layer, all participants need to agree with the methods of encoding that are used at this layer.
The same is true in the computer world — all participants, such as the servers and clients, need to agree with how the data will be formatted in order to exchange it. This is why standards for items like the HTML and XML languages allow servers to present data to clients and the clients to display this data to the users.
Differences among browsers make the actual display of the data slightly different on each browser, partially due to how they honor or interpret the data formatting presented by the web page. This formatting variance is why so many people have multiple web browsers installed on their computers.
Encryption is one of the key translations that takes place at the presentation layer. On outbound traffic from the server, the presentation layer encrypts data that is sent, and on the other end of the connection, it decrypts the data that is sent to the application layer. The following figure illustrates the flow of the data between a network client and the server.
If the client computer is running an e-mail program and the server is the user’s e-mail server, then on either end of the connection (both the client and the server sides), they are likely using the Simple Mail Transfer Protocol (SMTP) application layer protocol, or rather the encrypted version, SMTPS. The data flow would be as follows:
Using SMTPS, the client side of the application passes the text to the presentation layer services and requests encryption.
A Transport Layer Security (TLS) component at the presentation layer receives the unencrypted message and proceeds to encrypt the message using standard TLS processes.
The encrypted message flows down through the remaining OSI layers, over the physical network to the server.
At the server, the message is sent up through all of the layers until it arrives at the presentation layer.
Now, the servers TLS processes will take over and decrypt the message so that it is clearly readable.
The clear text message is then delivered to the SMTP application layer protocol for processing.
In this case, the next step would be to deliver the message to the recipient’s mailbox.