Networking - The TCP/IP Five-Layer Model - The Application Layer

The Application layer in the TCP/IP model corresponds to the top layers of the OSI model. It is where user programs and services interact with the network. All of the lower TCP/IP layers exist to make Application layer services possible.

In particular, the Application layer is responsible for:

• Presenting standard network services to user programs,
• Implementing protocols that define how those services behave, and
• Handling application-specific concerns such as user authentication, formatting, and resource naming.

Examples of protocols commonly associated with the TCP/IP Application layer include:

• HTTP and HTTPS for web access,
• SMTP, POP, and IMAP for electronic mail,
• FTP and SFTP for file transfer, and
• DNS for translating human readable names into IP addresses.

The Application layer is responsible for several processes, each of which is responsible for a series of methods:

1. Service Definition and Interfaces
   • Defining the commands, replies, and data formats used by networked applications, and
   • Providing program interfaces (APIs) that hide the details of lower layers.
2. Resource Location and Naming
   • Using names and identifiers (such as URLs, host names, and mail addresses) to refer to resources, and
   • Resolving those names into network level addresses and parameters.
3. Application Level Reliability and Policy
   • Providing acknowledgments, retries, or sequencing when the underlying Transport layer does not,
   • Enforcing application specific limits and security checks, and
   • Coordinating interactions among multiple users or processes that share the same resources.

We have now reached the top of the TCP/IP five-layer model. To wrap things up, let us move on to the Conclusion.

Nah, I want to skip around:

• Introduction
• Physical (Layer 1)
• Data Link (Layer 2)
• Network (Layer 3)
• Transport (Layer 4)
• Application (Layer 5)
• Conclusion

Copyright 1999, Marc Elliot Hall, DBA Sensation! Services