In the TCP or IP protocol suite, which layer 4 transport protocol is used for a Telnet connection?

Introduction / Context:
Telnet is one of the earliest application layer protocols used for remote terminal access over a network. It allows a user to log in to a remote host and execute commands as though sitting at a local terminal. In the TCP or IP stack, application protocols such as Telnet rely on transport protocols at layer four to provide data delivery. This question asks which transport protocol Telnet uses, testing your knowledge of how application and transport layers interact.

Given Data / Assumptions:

• Telnet is an application layer protocol in the TCP or IP suite.
• Transport layer protocols available include TCP and UDP.
• Telnet requires a reliable, ordered stream of data between client and server.
• The question asks which layer four protocol supports Telnet connections.

Concept / Approach:
Telnet provides interactive remote terminal sessions, which are sensitive to errors and require that characters arrive in the correct order without loss or duplication. The TCP protocol at the transport layer provides reliable, connection oriented byte streams with flow control and congestion control, making it suitable for such interactive applications. UDP is connectionless and does not guarantee delivery or ordering, so it is not typically used for Telnet. Therefore, Telnet uses TCP as its transport protocol, usually on port 23.

Step-by-Step Solution:
Step 1: Recall that Telnet provides a virtual terminal connection where keystrokes and responses must be delivered reliably and in order.Step 2: Identify that TCP is the transport protocol in the TCP or IP suite that offers reliable, ordered, connection oriented delivery.Step 3: Recognize that UDP is connectionless and does not provide guaranteed delivery or sequencing, which is not ideal for Telnet.Step 4: Note that IP is the network layer protocol and TCP or IP refers to the entire protocol suite, not specifically to a transport protocol.Step 5: Conclude that Telnet uses TCP at layer four to carry its application data.

Verification / Alternative check:
Standard port assignments list Telnet as using TCP port 23. Packet captures of Telnet sessions show TCP segments carrying Telnet data between source and destination. Protocol stacks in operating systems enable Telnet over TCP, while Telnet over UDP is not a common or standard configuration. These practical observations confirm that TCP is the correct transport protocol for Telnet.

Why Other Options Are Wrong:
Option A is incorrect because UDP does not provide the reliable, ordered delivery that Telnet requires and is not the standard transport for Telnet. Option B is wrong because IP is a network layer protocol responsible for addressing and routing, not a transport protocol. Option D, TCP or IP, refers to the overall protocol suite and not to a specific layer four protocol; it is imprecise for answering which transport protocol Telnet actually uses.

Common Pitfalls:
Students sometimes select TCP or IP in multiple choice questions because it sounds familiar, but that choice mixes layers and does not identify the transport protocol specifically. Another pitfall is to assume that any simple application might use UDP for lower overhead, but Telnet predates such optimizations and depends on the reliability that TCP provides. Remembering the association between Telnet and TCP port 23 can help keep this straight.

Final Answer:
The transport protocol used by Telnet at layer four is TCP.