Byte side: Can TCP and UDP use the same port number?

Hello, everyone, I am amazing.

Today I saw an interview question from ByteDance on Zhihu. I thought it was quite interesting, so I shared it with you.

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Before answering this question, we can first understand what TCP and UDP are.

1. TCP protocol

TCP (Transmission Control Protocol) is a connection-oriented, reliable, byte-stream-based transport layer communication protocol. It is one of the most commonly used protocols in the Internet protocol suite. Its main features include:

1. Connection-oriented: Before data transmission, TCP needs to establish a connection between the sender and the receiver. This process is often called a "three-way handshake."
2. Reliability: TCP guarantees the order and integrity of data packets. If any data is lost or corrupted, it will request retransmission.
3. Flow control and congestion control: TCP can control the rate at which data is transmitted to avoid overloading the network.
4. Bidirectional communication: Once a connection is established, data can be transferred in both directions.

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2. UDP protocol

UDP (User Datagram Protocol) is a simple connectionless transport layer protocol. Compared with TCP, UDP has different characteristics:

1. Connectionless: UDP does not need to establish a connection before transmitting data and can send data directly.
2. No reliability guarantees: UDP does not guarantee the order, integrity, or non-duplicate of data packets.
3. Lightweight: UDP header overhead is small and processing is fast, suitable for applications with high real-time requirements, such as video streaming and online games.
4. No flow control and congestion control: It does not adjust the sending rate, which may cause data loss when the network is congested.

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3. Port number

Now that we understand TCP and UDP, let's take a look at what port numbers are.

In computer networks, a port is an abstraction of a communication endpoint used to distinguish different processes or network services. A port number is a 16-bit number with a valid range from 0 to 65535.

Some people may ask, why do computers have the concept of port numbers?

We know that in a network environment, a computer often runs multiple network applications at the same time.

For example, a person may be using a web browser, an email client, and an online chat software at the same time. In order to be able to distinguish and manage network traffic from different applications, there needs to be a mechanism to identify these different communication flows, that is, to distinguish multiple tasks through port numbers.

A port number is a numerical label that works with an IP address to form a network address that identifies a specific process or network service on a computer network. In short, if an IP address can be thought of as the "address of a house," then a port number is like the "specific room number in a house."

4. Can TCP and UDP use the same port number?

The answer is yes.

For TCP and UDP, although they share the same port number space as transport layer protocols, their ports are managed independently. This means that TCP and UDP can use the same port number without conflicting with each other. For example, TCP port 80 is usually used for HTTP service, while UDP port 80 can be used by another service without interfering with each other.

The reason is that both TCP and UDP datagram formats contain port information, but since TCP and UDP are two completely different protocols, network devices and operating systems will correctly process and route data based on the protocol type (TCP or UDP) and port number. In fact, in the operating system, TCP and UDP ports are maintained and managed separately, so they can use the same port number independently.

There is an obvious use case where some applications might use the same port for both TCP and UDP. For example, DNS often uses both TCP and UDP on port 53. UDP is used for common queries, while TCP is used for larger or more complex queries such as zone transfers.

Therefore, TCP and UDP can use the same port number because they are based on different protocols, which are distinguished and processed by the operating system and network devices according to the protocol type and port number. This design makes network communication more flexible and allows different network applications and services to effectively share network resources.