Application-layer protocols: which listed items are TCP/IP protocols operating at the OSI Application layer? (IP, TCP, Telnet, FTP, TFTP)

Introduction / Context:
Protocols live at different layers with distinct responsibilities. Correctly classifying them aids design and troubleshooting. Application-layer protocols directly serve end-user applications, while lower layers provide transport or routing.

Given Data / Assumptions:

• List includes: 1) IP, 2) TCP, 3) Telnet, 4) FTP, 5) TFTP.
• OSI Application layer covers end-user services, not transport or network functions.

Concept / Approach:
Classify each item: IP (Layer 3, Internet), TCP (Layer 4, Transport), Telnet/FTP/TFTP (Application layer). Therefore, the set of application-layer protocols is Telnet, FTP, and TFTP.

Step-by-Step Solution:

Verification / Alternative check:
Check common port assignments: Telnet 23/TCP, FTP 21/TCP (and data ports), TFTP 69/UDP—all classic application services.

Why Other Options Are Wrong:
1 and 3 / 1, 3 and 5: Include IP, which is not application-layer.
All of the above: Incorrect because IP and TCP are not application-layer protocols.

Common Pitfalls:
Equating “uses a port” with “application layer.” TCP and UDP use ports but remain transport protocols; the services using them are application-layer.

Final Answer:
3, 4 and 5