TCP vs UDP: What is the Difference?

TCP and UDP are the backbone of internet communication, handling the transfer of data. While their names might be well-known, choosing the right protocol can still be a challenge.

This guide will take you through a deep dive into these two protocols. We will unveil the key differences between TCP and UDP, and share the scenarios where each is most effective, helping you grasp the essence of these fundamental protocols.

What is TCP?

TCP, short for Transmission Control Protocol, is a connection-oriented, reliable, byte-stream communication protocol used in the transport layer. Playing a crucial role in the internet protocol suite (also known as the TCP/IP protocol stack), TCP's main purpose is to ensure data is transferred reliably and in order between two applications across the network, no matter where on Earth these applications are located.

TCP is ideal for applications that require high accuracy in data, such as web browsing, file transfers, emailing, and database management. Because of its reliability, TCP is the preferred protocol for most critical internet applications. However, this reliability comes at the cost of some speed, which is one of the main differences between TCP and UDP.

How TCP Works

Before starting data transfer, the client device sends a synchronization signal (SYN) with a sequence number to the server, indicating the starting point of the data transfer.
Upon receiving the client's SYN, the server acknowledges it and sends back a synchronization acknowledgment signal (SYN-ACK), a process often referred to as "synchronization acknowledgment."
After that, the client sends an acknowledgment signal (ACK) back to the server to confirm the server's synchronization response. With this, a direct connection is established, and the two parties begin transferring data.

During the transfer, the connection is maintained, and for each data packet sent, the receiving end needs to send back an acknowledgment signal. If the sender doesn't receive this acknowledgment, it will resend the data packet.

If errors occur during the transfer, the faulty data packets are discarded, and the sender needs to resend new packets.

When the network is congested or other issues arise, data transfer can be delayed, but the connection doesn't break. TCP avoids causing network congestion by adjusting the rate of data transmission.

It's due to these detailed control mechanisms that TCP can ensure successful data transfer and maintain reliability.

What is UDP?

UDP, short for User Datagram Protocol, is a connectionless communication protocol used for packet data transfer. Unlike TCP, UDP does not guarantee the reliable arrival of packets, meaning it does not provide services such as ordered delivery, error checking, or retransmission mechanisms. This characteristic makes UDP have lower latency during communication, making it suitable for real-time applications where reliability is less critical, such as video conferencing, live streaming, and online gaming.

How UDP Works

UDP operates by immediately sending data to the receiving end upon request until the transfer is complete or terminated. Sometimes referred to as the "fire-and-forget" protocol, UDP sends data to the recipient without ensuring delivery confirmation or checking if packets arrive as intended, in an uncertain order.

Connectionless: UDP does not require establishing a connection before transferring data. Data can be sent directly to the recipient, reducing the overhead of connection establishment and maintenance.
No Guarantee of Delivery: UDP does not ensure the order, integrity, or reliability of packets. If the network conditions are poor, packets may be lost or arrive in the wrong order, and UDP will not correct these issues.
Simple and Fast: UDP has a small header length of only 8 bytes, compared to TCP's minimum of 20 bytes, which allows for quicker processing of packets.
No Congestion Control: UDP does not perform congestion control. Therefore, it may continue to send packets even when network conditions are poor, potentially leading to network congestion.

Key Differences Between TCP and UDP

Here are the main differences between TCP and UDP:

1. Connection Type

TCP and UDP are two distinct network communication protocols, with the fundamental difference being that TCP requires a connection to transfer data reliably, while UDP can send data directly without ensuring reliable transmission.

TCP, a connection-oriented protocol, relies on a server that is in a waiting state, constantly listening for clients trying to establish a connection. Clients must establish a connection with the server before transmitting or receiving data. This connection is made through a three-way handshake process: the client initiates a synchronization request, the server responds with an acknowledgment, and then the client sends a synchronization acknowledgment signal to establish the connection.

UDP, on the other hand, is a connectionless protocol. During data transmission, the sender sends information directly into the network without checking if the receiver is ready or capable of receiving the message. The information is sent directly, regardless of whether the destination is prepared to receive it. Although this connectionless protocol may lead to the loss of some packets, such losses may not be noticeable to the receiver in certain scenarios, like in video calls.

2. Data Sequence

TCP follows a strict sequence during data transfer. It uses port numbers to identify data for correct delivery to applications. It also synchronizes with the remote host using sequence numbers, and each data segment sent and received is sequenced, allowing the system to track the exact order of data sent to ensure data is transmitted in the predetermined order.

In contrast, UDP does not implement sequential transmission. It sends packets independently and without a predetermined order. These packets are reassembled after reaching the receiving application. Notably, packets are reassembled based on the order of receipt, not the original sending order, meaning the protocol itself cannot recognize which packet should arrive first. If they arrive out of order, the application may incorrectly receive packets. Moreover, UDP will simply discard packets when it cannot handle them.

3. Data Retransmission

TCP uses an acknowledgment mechanism to ensure successful packet transmission. After receiving a packet, the receiver sends an ACK back to the sender to confirm receipt. If the sender does not receive the ACK within a timeout period, it assumes the packet is lost and retransmits it.

UDP does not provide a data retransmission function. As a connectionless protocol, it does not wait for confirmation after sending a packet and does not track whether the packet reaches its destination. As such, UDP is faster in processing data transfers, but it also means that it does not guarantee the reliable arrival of data.

4. Error Detection

TCP employs a triple mechanism to detect errors, ensuring the integrity of data during transmission. These include:

Pausing the connection after timeout: Each connection has a specific timeout limit. If no acknowledgment message is received by the server or client within this period, the connection will break, and data transmission can only continue after re-establishing the connection.
Checksum in the header: A 16-bit value known as a checksum is included in the packet's header. TCP adds a checksum for each data segment and verifies the integrity of data during transmission.
Acknowledgment (ACK) messages: When a connection is established or data is sent, the server issues acknowledgment messages (ACK). The client, upon receiving an acknowledgment, increases its value by one and sends it back.

These three measures ensure the correctness of TCP's data stream, preventing data loss or corruption. On the other hand, UDP only performs basic error checking, primarily through the checksum.

5. Reliability

If a connection is interrupted, TCP immediately pauses data transmission to prevent packet loss.

TCP also ensures packets are sent in the correct order through sequencing mechanisms, ensuring that images, web pages, data files, and other information arrive intact.

TCP guarantees accurate delivery of data, confirming each received packet, and only sending the next packet after receiving an acknowledgment (ACK).

TCP uses flow control and congestion control mechanisms to ensure that data is not lost, damaged, duplicated, or out of order during transmission.

UDP, in contrast, is designed to send packets continuously to the receiver without waiting for acknowledgments. Under poor network conditions, both TCP and UDP may experience packet loss, but TCP can detect lost packets and retransmit them, whereas UDP cannot identify which packets are lost and is unable to resend them.

6. Speed

UDP's transmission speed and efficiency are higher than TCP's. Since UDP does not need to establish a connection before transmitting data, it reduces the time to start a server and wait for it to be ready to receive data. UDP also does not sort or acknowledge packets, further shortening processing time.

In terms of bandwidth usage, UDP is more efficient. Unlike TCP, which involves many error checks and acknowledgment mechanisms during transmission, UDP simplifies these processes, allowing data to be transferred quickly from one end to the other. This makes UDP especially suitable for use in environments with limited network conditions, on mobile devices, or where resources are tight.

TCP, due to its need to ensure reliable data transmission, is relatively slower and consumes more resources. In the event of a data transmission error, TCP will re-establish the connection. UDP, however, will discard problematic packets and continue transmitting subsequent packets, greatly enhancing transmission efficiency.

7. Flow Control

Flow control is a mechanism that enables the server to assess the processing capacity of the receiving end to determine the amount and speed of data it can handle.

TCP adjusts the data transmission speed through flow control information to match the processing speed of the receiver, ensuring the data transfer does not overwhelm the recipient. However, this also leads to slower data transfers and lower efficiency as it requires waiting for the acknowledgment from the receiver.

UDP does not implement flow control. It sends data at the optimal speed based on the sending server's conditions, which can sometimes overwhelm the receiver with a continuous stream of data. If the UDP transmission speed is too fast for the receiver to handle, UDP will discard packets that cannot be received.

8. Congestion Control

Additive Increase/Multiplicative Decrease (AIMD) is one of the key algorithms used in TCP. It prevents network overload by linearly increasing and exponentially decreasing the size of the congestion window, a complex yet effective strategy. During network congestion, TCP pauses data sending until the network clears, ensuring complete packet transfer.

UDP does not have built-in congestion control features. In times of heavy network traffic, UDP may drop queued packets and continue sending subsequent data.

9. Broadcast

TCP provides a reliable end-to-end connection. It establishes a stable connection between both ends of communication and tracks packets and bytes in transit with accurate record-keeping systems. Each synchronization and acknowledgment message increases by one from the previous message's number for easy tracking and positioning. Packet headers also contain sequence information to ensure data flows in the correct order. Therefore, TCP is very suitable for one-to-one data transfers but not as well suited for broadcasting data to multiple endpoints simultaneously.

In multicasting or broadcasting scenarios, the server sends data to multiple recipients at once. It does not wait for acknowledgment from the receivers and proceeds with data transmission, which is exactly how UDP operates. UDP's basic architecture makes it particularly suited for broadcasting packets to a group of endpoints. In such cases, the data transfer target is not a specific network host but a group of hosts.

10. Header Size

In various communication protocols, data exchange is accomplished through a sequence of bytes composed of multiple fields, each containing specific information related to the protocol used. The byte sequence is mainly divided into header and data payload, with the payload carrying the actual message content, while the header contains a series of information for identifying and maintaining the operation of the protocol. TCP and UDP use different header structures during data transmission.

The TCP header is more complex, with variable lengths ranging from 20 to 60 bytes, allowing TCP to perform more intricate communication tasks and ensure data transfer reliability. In contrast, the UDP header is fixed at only 8 bytes, which helps improve transmission speed and efficiency, although it is not as functionally flexible as TCP.

Regarding specific fields in the header, the TCP header includes several dedicated fields such as sequence numbers, acknowledgment numbers, checksums, control flags, sliding window sizes, source port numbers, and destination port numbers, which are used to maintain complex transmission control functions. The UDP header, however, contains only a few basic fields, including source port numbers, destination port numbers, and checksums, making its structure simpler.

Let's summarize the differences between TCP and UDP in a chart format:

Feature	TCP	UDP
Connection Type	Requires establishing a connection before data transfer	No connection needed to start and end data transfer
Data Sequence	Packets in TCP have sequence numbers in the header to maintain transfer order	No specific order
Data Retransmission	Data can be retransmitted if packets fail to arrive	No data retransmission, lost data cannot be recovered
Error Detection	Uses timeouts, checksums, and acknowledgments to prevent and correct errors	Only uses checksums to avoid errors, and cannot correct them
Reliability	Ensures successful data transfer, highly reliable	Does not guarantee
Speed	Slower but complete data transfer	Fast speed, but there is a risk of incomplete data transfer
Flow Control	Regulates data sending speed through flow control mechanisms to ensure the receiver can effectively handle data	No flow control function, sends data at the original rate from the data source without adjusting transmission speed
Congestion Control	Avoids network congestion by adjusting data transmission rates	Cannot control network congestion and will drop packets
Broadcast	Does not support	Supports
Header Length	Uses variable length headers, up to 60 bytes	Fixed-length header of only eight bytes

Which is better, TCP or UDP?

Whether TCP or UDP is better depends on the nature of your network activities and the requirements for data transmission. For example, UDP is preferred for online gaming because it provides fast data transfer to reduce lag during gameplay. However, for transferring important files, TCP is a better choice as it ensures the accurate and complete transmission of file data.

Pros of TCP

If you prioritize the reliability of transmission and the integrity of the data, then TCP is your best bet. It might not be the fastest, but it will get the job done correctly.

Establishes and maintains a stable connection between the sender and receiver.
Operates independently of the operating system.
Adaptable to various routing protocols.
Features error detection to ensure data arrives at its destination intact.
Provides data acknowledgment and can retransmit data if necessary.
Transfers data in a specific order.
Adjusts the data transmission speed based on the receiver's processing capabilities.

Cons of TCP

However, for some data transmission that requires higher speed, TCP might not be the best option.

It uses more bandwidth and is slower than UDP.
It can be particularly slow at the start of the file transfer.
If some data is lost, TCP might hinder the loading of the remaining data; for example, images on a webpage might not load until all the data is transferred.
Reduces transmission rates during network congestion, resulting in slower speeds.
Not as well-suited for local area networks (LAN) and personal area networks (PAN).
Cannot perform multicasting or broadcasting.

Pros of UDP

UDP is capable of high-speed data transfers and does not slow down to retransmit in case of data loss, making it well-suited for streaming continuous data or broadcasting.

Can send or receive data without establishing a connection.
Supports multicast and broadcast transmission.
Capable of transferring data even if it's incomplete.
Smaller packet size can reduce latency.
Operable under a broader range of network conditions.
More efficient communication.
Able to transmit real-time data.

Cons of UDP

Despite UDP's ability to provide quick data transfers to meet our speed needs, it lacks the reliability of TCP.

It is connectionless, with less reliable data transfer.
Lacks mechanisms to ensure successful data transfer.
Difficult to determine if data has arrived completely.
Does not have error control mechanisms; it discards packets upon error detection.
Routers tend to drop UDP packets over TCP packets when there is a conflict.
Can cause network congestion when multiple users receive UDP data simultaneously, with no way to alleviate the situation.
Cannot sort data, leading to potentially disordered or scrambled arrival of packets.

In conclusion, both TCP and UDP have their strengths and are crucial for network communication. Therefore, it's not comprehensive to judge solely based on a comparison of TCP versus UDP. Choosing between TCP and UDP may be more appropriate based on the specific needs of data transmission. Here are some examples of scenarios where each protocol is typically used:

Applications of TCP

Sending emails or text messages
Transferring files
Browsing web pages
Remote server or network administration

Applications of UDP

Watching live streams
Online gaming
Video calls/video conferencing
Domain Name System (DNS) queries (translating domain names to IP addresses)

Conclusion

Whether you use TCP or UDP for your connections, ensuring the privacy and security of your network is very important. You can encrypt your network communication by using a VPN or proxy server to ensure that your online activities remain private.

Additionally, to further enhance privacy protection, you can use tools like BrowserScan fingerprint detection to check whether your real digital fingerprint is protected to prevent information leakage.