In transport layer protocols, what is the key difference between TCP (Transmission Control Protocol) and UDP (User Datagram Protocol)?

Introduction / Context:
Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are the two most common transport layer protocols used in IP networks. Interviewers often ask candidates to compare TCP and UDP, focusing on reliability, connection management, and typical application use cases. Understanding their differences helps explain why some applications prefer TCP while others rely on UDP for speed and simplicity.

Given Data / Assumptions:

• Both TCP and UDP operate on top of IP at the transport layer (Layer 4).
• TCP is used by applications like web browsing, email, and file transfer.
• UDP is used by applications like DNS queries, streaming, voice over IP, and online gaming.
• We are comparing reliability, ordering, connection behaviour, and overhead.

Concept / Approach:
TCP is connection-oriented. Before data is exchanged, TCP establishes a session using a three-way handshake. It provides reliable delivery by using sequence numbers, acknowledgements, retransmissions, and flow control mechanisms such as sliding windows. TCP also implements congestion control algorithms to avoid overloading the network. UDP, by contrast, is connectionless: it simply sends datagrams without establishing a session or tracking state. UDP does not guarantee delivery, ordering, or protection against duplication; it is a best-effort protocol with minimal header overhead, which makes it suitable for time sensitive applications that can tolerate some loss.

Step-by-Step Solution:
Step 1: Describe TCP as connection-oriented, requiring a handshake to establish a session between client and server. Step 2: Explain that TCP provides reliability through sequence numbers, acknowledgements, and retransmissions of lost segments. Step 3: Highlight that TCP ensures in-order delivery and uses flow and congestion control to manage data rates. Step 4: Describe UDP as connectionless, sending independent datagrams without establishing or maintaining a session. Step 5: Note that UDP offers no built-in reliability, ordering, or congestion control, but it has lower overhead and latency.

Verification / Alternative check:
Inspecting packet captures shows that TCP segments contain sequence and acknowledgement numbers and flags such as SYN, ACK, and FIN, which are used to manage connections. UDP datagrams have a much simpler header with only ports, length, and checksum. Observing application behaviour also supports this: if you drop some UDP packets in a voice call, the conversation continues with minor glitches, whereas dropping TCP segments may stall or slow down file transfers until retransmissions succeed.

Why Other Options Are Wrong:
Option B incorrectly states that TCP is broadcast, which it is not; both TCP and UDP typically use unicast, though UDP can support multicast. Option C misplaces TCP and UDP in the OSI model, as both work at the transport layer, not at the network or physical layers. Option D invents a distinction between wired and wireless networks that does not exist; both protocols operate over any medium that supports IP.

Common Pitfalls:
A common error is assuming that UDP is always better for performance; while it has less overhead, applications must handle reliability themselves if needed. Another pitfall is misunderstanding that TCP's reliability comes at the cost of latency and throughput under certain conditions, which may be unacceptable for real-time media. Choosing between TCP and UDP requires understanding the specific application's tolerance for latency, loss, and complexity.

Final Answer:
The key difference is that TCP is a connection-oriented, reliable protocol with ordered delivery and control mechanisms, while UDP is a connectionless, best-effort protocol with no built-in reliability but lower overhead.