The application layer is the topmost layer, closest to the user, where programs exchange data; in TCP/IP it rolls the OSI application, presentation, and session layers into one.

Application Layer:

The layer closest to the end user that is used to exchange data between programs running on the source and destination hosts.

OSI vs TCP/IP Model Mapping:

Key point: The upper three layers of the OSI model (Application, Presentation, Session) define the functions of the TCP/IP Application layer.

Common Application Layer Protocols: HTTP, FTP, TFTP, IMAP, DNS, SMTP, POP, DHCP

Go deeper:

• TCP/IP and the OSI Model Explained! — walks through how the layered models line up.

• Application layer (Wikipedia) — how the layer is defined in both TCP/IP and OSI.

• OSI model (Wikipedia) — the seven layers and how TCP/IP folds the top three into one.

Presentation formats, compresses, and encrypts/decrypts data so both ends understand it; session creates, maintains, and restarts the dialogs between the two applications.

These two OSI layers are folded into the single TCP/IP Application layer, but it helps to understand them separately. The presentation layer is about making the data itself usable: it converts it into a common format both ends agree on (a JPG opens the same on any machine), shrinks it to save bandwidth, and scrambles it for privacy in transit. The session layer is about the conversation around that data — it sets up the dialog between the two applications, keeps it alive, and can restart it if the link drops, so an app does not have to start over after a brief interruption.

* Presentation (L6) makes data usable — format, compress, encrypt; Session (L5) manages the dialog — create, maintain, restart. *

Presentation Layer (OSI Layer 6) - Three Functions:

1. Formatting/Presenting data - Converts data at the source into a compatible format for the destination device
2. Compressing data - Reduces data size in a way that can be decompressed by the destination
3. Encrypting data - Encrypts for transmission and decrypts upon receipt

Common formats: MKV, MPG, MOV, GIF, JPG, PNG

Session Layer (OSI Layer 5) - Functions:

1. Creates and maintains dialogs between source and destination applications
2. Handles exchange of information to:
   • Initiate dialogs
   • Keep sessions active
   • Restart sessions that are disrupted or idle for a long period

Go deeper:

• Presentation layer (Wikipedia) — formatting, compression and encryption duties of L6.

• Session layer (Wikipedia) — opening, managing and resynchronizing dialogs at L5.