Layer 6 is the presentation layer and is responsible for proper data formatting, along with optional compression and encryption. The presentation layer ensures that the data is in the proper format, either before presenting the data to the application, or before sending it to the network. For example, if you download a file from the internet with the .gz extension, then the presentation layer will search for an application to associate it with, so the OS can open the file correctly. If the application is not installed, then you will see a message, as shown in the following screenshot:

If you do not have the application installed, then you can go in and manually select the application with which you want to open the file, or obtain and install the correct application.

The presentation layer also provides optional services to compress and decompress data. Compression removes redundancy and makes data smaller. This function is optional, as not all data is compressed.

This layer also handles encryption, which is scrambling data by using a key so that it is in an unreadable form that does not make sense to anyone unless they have the key. Because encryption is also an optional function, this may not be required. 

In the presentation layer, we do see a few protocols, which we'll investigate in the following section, along with the PDU.