Data is being sent from a source PC to a destination server. Which three statements correctly describe the function of TCP or UDP in this situation? (Choose three.)

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Data is being sent from a source PC to a destination server. Which three statements correctly describe the function of TCP or UDP in this situation? (Choose three.)

  • TCP is the preferred protocol when a function requires lower network overhead.
  • The source port field identifies the running application or service that will handle data returning to the PC.
  • The TCP source port number identifies the sending host on the network.
  • UDP segments are encapsulated within IP packets for transport across the network.
  • The UDP destination port number identifies the application or service on the server which will handle the data.
  • The TCP process running on the PC randomly selects the destination port when establishing a session with the server.

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✔️ Understanding the Role of TCP and UDP in Data Transmission

When data is being transmitted from a source PC to a destination server, the Transport Layer (Layer 4 of the OSI model) comes into action to manage the delivery of that data. This layer uses two key protocols — TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) — depending on the type of application or service. These protocols serve critical functions, including process-to-process delivery, error checking, and segmentation of data.

Out of the given options, the three correct statements that describe the function of TCP or UDP are:

✅ 1. The source port field identifies the running application or service that will handle data returning to the PC.

The source port is a crucial part of the TCP or UDP header. When a client device like a PC initiates a connection to a server, it includes a source port number in the segment it sends. This source port is randomly chosen from a range of ephemeral (temporary) ports and is used to map the response back to the correct application or process on the client device. When the server replies, it uses this port as the destination port in its response. This way, the operating system on the PC knows which application should receive the incoming data.

  • Example: If a web browser on a PC connects to a web server on port 80, it may choose source port 49532. The reply from the server will use destination port 49532, and the OS on the PC will know to forward the response to the browser that initiated the request.

✅ 2. UDP segments are encapsulated within IP packets for transport across the network.

Like TCP, UDP operates at Layer 4, while IP operates at Layer 3. For any communication to traverse a network, data must be encapsulated into packets. In the case of UDP:

  • The UDP segment is encapsulated into an IP packet.

  • The IP packet includes source and destination IP addresses.

  • The UDP segment includes source and destination port numbers, along with data and a checksum.

This encapsulation allows network devices to route the packet using IP, while still maintaining the port-level targeting needed for the end applications.

  • UDP is commonly used in services where speed is more critical than reliability, such as video streaming, VoIP, or DNS queries.

✅ 3. The UDP destination port number identifies the application or service on the server which will handle the data.

The destination port number plays a vital role in directing incoming data to the correct application or service on the destination server. In UDP communication, since it is connectionless and does not involve a handshake, the destination port number is essential to identify which service should receive the data.

  • Example: If a client sends a DNS request to a server, it will target UDP port 53, because DNS servers are configured to listen on that port for incoming DNS queries.

Unlike TCP, which establishes a connection and can handle flow control, UDP delivers the data with minimal overhead. Therefore, it relies heavily on port numbers to function efficiently and correctly.

❌ Incorrect Statements and Why They Are Wrong

Let’s now examine the incorrect choices and explain why they don’t accurately describe TCP or UDP behavior:

❌ TCP is the preferred protocol when a function requires lower network overhead.

This is incorrect because TCP is actually the protocol with higher overhead compared to UDP. TCP includes features like:

  • Three-way handshake (SYN, SYN-ACK, ACK)

  • Sequence numbers

  • Acknowledgments

  • Flow control

  • Congestion control

  • Error checking and correction

These features ensure reliable and ordered delivery, but they also increase the overhead. UDP is the preferred protocol when low overhead is needed and where some data loss is acceptable, such as in voice or video communication.

❌ The TCP source port number identifies the sending host on the network.

This is also incorrect. The source port number is used to identify the specific application or process on the sending device, not the host itself.

The identification of the sending host on the network is done via the IP address, which is part of the Layer 3 (IP layer) header, not the TCP or UDP port numbers.

Think of the IP address as the “street address” of a building and the port number as the “apartment number” — together, they specify exactly where the data should go.

❌ The TCP process running on the PC randomly selects the destination port when establishing a session with the server.

This is false because the destination port is not randomly selected by the TCP process on the client. Instead, it is usually predetermined and corresponds to the standard port associated with the service being requested.

  • For example:

    • HTTP uses TCP port 80

    • HTTPS uses TCP port 443

    • FTP uses TCP port 21

    • SSH uses TCP port 22

These destination ports are well-known port numbers, and the server listens for requests on them. The client must send data to the correct destination port for the server to accept and process the request.

📚 Summary Table

Statement Correct? Explanation
TCP is the preferred protocol when a function requires lower network overhead. ❌ UDP is used when low overhead is needed.
The source port field identifies the running application or service that will handle data returning to the PC. ✅ Source port maps return traffic to the correct process.
The TCP source port number identifies the sending host on the network. ❌ The host is identified by IP address, not by port number.
UDP segments are encapsulated within IP packets for transport across the network. ✅ All Layer 4 segments (TCP/UDP) are encapsulated in IP packets.
The UDP destination port number identifies the application or service on the server which will handle the data. ✅ This is how the server knows which process to forward the data to.
The TCP process running on the PC randomly selects the destination port when establishing a session with the server. ❌ The destination port is predefined (e.g., port 80 for HTTP).

🧠 Conclusion

In networking, TCP and UDP serve different purposes, and understanding how they use ports and IP addresses is essential for diagnosing, designing, or troubleshooting network communications. TCP ensures reliability and order, making it ideal for file transfers, web browsing, and email. UDP offers speed with minimal overhead, ideal for streaming and real-time applications. Knowing how port numbers, encapsulation, and addressing work at the transport and network layers enables network professionals to build more efficient and reliable systems.