Application, Presentation, and Session Layer
Application, Presentation, and Session Layer

Application, Presentation, and Session

Application, Presentation, and Session


This topic explain how the functions of the application layer, presentation layer, and session layer work together to provide network services to end user applications. Start learning CCNA 200-301 for free right now!!

Note: Welcome: This topic is part of Chapter 15 of the Cisco CCNA 1 course, for a better follow up of the course you can go to the CCNA 1 section to guide you through an order.

Application Layer

In the OSI and the TCP/IP models, the application layer is the closest layer to the end user. As shown in the figure, it is the layer that provides the interface between the applications used to communicate, and the underlying network over which messages are transmitted. Application layer protocols are used to exchange data between programs running on the source and destination hosts.

Application Layer CCNA
Application Layer CCNA

Based on the TCP/IP model, the upper three layers of the OSI model (application, presentation, and session) define functions of the TCP/IP application layer.

There are many application layer protocols, and new protocols are always being developed. Some of the most widely known application layer protocols include Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Trivial File Transfer Protocol (TFTP), Internet Message Access Protocol (IMAP), and Domain Name System (DNS) protocol.

Presentation and Session Layer

Presentation Layer

The presentation layer has three primary functions:

  • Formatting, or presenting, data at the source device into a compatible format for receipt by the destination device.
  • Compressing data in a way that can be decompressed by the destination device.
  • Encrypting data for transmission and decrypting data upon receipt.

As shown in the figure, the presentation layer formats data for the application layer, and it sets standards for file formats. Some well-known standards for video include Matroska Video (MKV), Motion Picture Experts Group (MPG), and QuickTime Video (MOV). Some well-known graphic image formats are Graphics Interchange Format (GIF), Joint Photographic Experts Group (JPG), and Portable Network Graphics (PNG) format.

Presentation Layer
Presentation Layer

Session Layer

As the name implies, functions at the session layer create and maintain dialogs between source and destination applications. The session layer handles the exchange of information to initiate dialogs, keep them active, and to restart sessions that are disrupted or idle for a long period of time.

TCP/IP Application Layer Protocols

The TCP/IP application protocols specify the format and control information necessary for many common internet communication functions. Application layer protocols are used by both the source and destination devices during a communication session. For the communications to be successful, the application layer protocols that are implemented on the source and destination host must be compatible.

Click each application protocol type to learn more about each protocol.

DNS - Domain Name System (or Service)

  • TCP, UDP client 53
  • Translates domain names, such as cisco.com, into IP addresses.

BOOTP - Bootstrap Protocol

  • UDP client 68, server 67
  • Enables a diskless workstation to discover its own IP address, the IP address of a BOOTP server on the network, and a file to be loaded into memory to boot the machine
  • BOOTP is being superseded by DHCP

DHCP - Dynamic Host Configuration Protocol

  • UDP client 68, server 67
  • Dynamically assigns IP addresses to be re-used when no longer needed

SMTP - Simple Mail Transfer Protocol

  • TCP 25
  • Enables clients to send email to a mail server
  • Enables servers to send email to other servers

POP3 - Post Office Protocol

  • TCP 110
  • Enables clients to retrieve email from a mail server
  • Downloads the email to the local mail application of the client

IMAP - Internet Message Access Protocol

  • TCP 143
  • Enables clients to access email stored on a mail server
  • Maintains email on the server

FTP - File Transfer Protocol

  • TCP 20 to 21
  • Sets rules that enable a user on one host to access and transfer files to and from another host over a network
  • FTP is a reliable, connection-oriented, and acknowledged file delivery protocol

TFTP - Trivial File Transfer Protocol

  • UDP client 69
  • A simple, connectionless file transfer protocol with best-effort, unacknowledged file delivery
  • It uses less overhead than FTP

HTTP - Hypertext Transfer Protocol

  • TCP 80, 8080
  • A set of rules for exchanging text, graphic images, sound, video, and other multimedia files on the World Wide Web


  • TCP, UDP 443
  • The browser uses encryption to secure HTTP communications
  • Authenticates the website to which you are connecting your browser

Glossary: If you have doubts about any special term, you can consult this computer network dictionary.

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