Which of the following application protocols uses both TCP and UDP as transport protocols, depending on the type of message being sent?

Introduction / Context:
Most well-known application protocols use either TCP or UDP as their transport layer protocol, but not both. However, some protocols can use both TCP and UDP depending on the specific operation. Cisco exams often test this nuance, particularly for DNS.

Given Data / Assumptions:

• We are comparing common protocols: FTP, SMTP, Telnet, and DNS.
• We want to know which protocol can operate over both TCP and UDP.
• We are focusing on standard, widely implemented behavior.

Concept / Approach:
DNS primarily uses UDP on port 53 for simple, fast queries and responses because these messages are small and do not require the overhead of TCP. However, DNS also uses TCP on port 53 for tasks such as zone transfers and for responses that are too large to fit in a single UDP packet. FTP, SMTP, and Telnet always use TCP, because they require reliable, connection-oriented sessions for data transfer and interactive communication.

Step-by-Step Solution:
Step 1: Recall that FTP (ports 20 and 21), SMTP (port 25), and Telnet (port 23) are all designed around TCP session semantics and do not use UDP in normal operation.Step 2: Recall that DNS uses UDP port 53 for standard name resolution queries and replies because it is lightweight and efficient.Step 3: Recognize that DNS also uses TCP port 53 when performing zone transfers between servers or when a UDP response would be truncated due to size limitations.Step 4: Therefore, DNS is the protocol in this list that uses both TCP and UDP.

Verification / Alternative check:
Checking standard RFCs and network documentation confirms that DNS supports both UDP and TCP on port 53, whereas FTP, SMTP, and Telnet are defined strictly over TCP. Packet captures from real networks also show both UDP and TCP flows for DNS traffic.

Why Other Options Are Wrong:
Option A (FTP) uses TCP for command and data channels and does not rely on UDP.Option B (SMTP) uses TCP for reliable mail transfer between servers.Option C (Telnet) uses TCP for interactive terminal sessions and does not use UDP.

Common Pitfalls:
Some learners mistakenly think that because FTP and SMTP move large amounts of data, they might use UDP for performance. In fact, reliability requirements dictate TCP usage. DNS is the notable protocol here that can use both transports, depending on the transaction type and message size.

Final Answer:
The protocol that uses both TCP and UDP is DNS (Domain Name System).