Differences Between TCP and UDP Explained

TCP and UDP are the two most important protocols at the transport layer, with significant differences in design philosophy and application scenarios.

Core Differences

1. Connectivity

• TCP: Connection-oriented, requires three-way handshake to establish connection, four-way wave to terminate connection
• UDP: Connectionless, sends data directly without establishing connection

2. Reliability

• TCP: Reliable transmission, provides acknowledgment, retransmission, checksum and other mechanisms
• UDP: Unreliable transmission, does not guarantee data arrival, possible packet loss and out-of-order

3. Ordering

• TCP: Guarantees data arrives in order, implemented through sequence numbers and acknowledgments
• UDP: Does not guarantee order, packets may arrive out of order

4. Flow Control

• TCP: Has sliding window mechanism to prevent sender from sending too fast causing receiver buffer overflow
• UDP: No flow control, sender does not care about receiver's processing capability

5. Congestion Control

• TCP: Has congestion control mechanism (slow start, congestion avoidance, fast retransmit, fast recovery)
• UDP: No congestion control, may cause network congestion

6. Transmission Efficiency

• TCP: Large header overhead (20-60 bytes), relatively lower transmission efficiency
• UDP: Small header overhead (8 bytes), high transmission efficiency

7. Transmission Mode

• TCP: Byte-stream oriented, treats data as a continuous byte stream
• UDP: Message-oriented, preserves message boundaries, one message is one data unit

8. Connection Count

• TCP: One-to-one connection, point-to-point communication
• UDP: Supports one-to-one, one-to-many, many-to-many, many-to-one communication

Application Scenarios

TCP Applicable Scenarios

• File Transfer: FTP, HTTP, HTTPS, etc., requiring reliable transmission
• Email Transfer: SMTP, POP3, IMAP, etc.
• Remote Login: SSH, Telnet, etc.
• Applications requiring reliability: Database connections, financial transactions, etc.

UDP Applicable Scenarios

• Real-time Audio/Video: Video conferencing, live streaming, VoIP, etc., sensitive to latency
• Online Gaming: FPS, MOBA, etc., requiring low latency
• DNS Query: Domain name resolution, small data volume, requiring fast response
• Broadcast and Multicast: IGMP, DHCP, etc.
• Real-time Monitoring: Sensor data reporting, etc.

Performance Comparison

Feature	TCP	UDP
Connection Establishment	Required (3-way handshake)	Not required
Reliability	High	Low
Transmission Efficiency	Relatively lower	Higher
Latency	Relatively higher	Lower
Resource Consumption	Higher	Lower
Data Order	Guaranteed	Not guaranteed

Related Questions

• Why use UDP instead of TCP for video calls?
• Can TCP and UDP be used simultaneously?
• How to implement reliable transmission over UDP?