Differences Between TCP and UDP

Description
TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are two core protocols at the transport layer, responsible for data transmission between applications. Their fundamental difference lies in whether they provide reliable transmission service. Understanding their differences helps in selecting the appropriate protocol for real-world scenarios.

Detailed Explanation of Key Points

1. Connection Method

   • TCP is a connection-oriented protocol: Before data transmission, a stable connection channel must be established via a "three-way handshake," and the connection is terminated via a "four-way handshake" after transmission. For example, dialing and establishing a connection is required before making a phone call.
   • UDP is a connectionless protocol: It sends data packets directly without needing to establish a connection beforehand. It is like mailing a letter; after sending, there is no guarantee the recipient will receive it.

2. Reliability

   • TCP ensures reliability: It guarantees data integrity and order through the following mechanisms:
     • Acknowledgement (ACK): The receiver returns an acknowledgement signal for each data packet received.
     • Retransmission on Timeout: The sender retransmits data if an ACK is not received.
     • Data Sequencing: Each data packet is numbered, allowing the receiver to reassemble them in order.
     • Flow Control: The sending rate is adjusted via a sliding window mechanism to prevent receiver buffer overflow.
   • UDP does not guarantee reliability: After sending, there is no confirmation of arrival; packet loss or out-of-order delivery may occur.

3. Transmission Efficiency

   • TCP has lower transmission efficiency: Due to mechanisms like connection establishment/termination, acknowledgements, and retransmissions, the overhead is significant, resulting in higher latency.
   • UDP has high transmission efficiency: Without complex control mechanisms, its packet header is smaller (only 8 bytes), leading to lower transmission latency.

4. Data Boundaries

   • TCP is a byte-stream protocol: Data is treated as an unstructured continuous byte stream, requiring the application layer to define boundaries (e.g., using specific delimiters).
   • UDP preserves message boundaries: Each data packet is sent independently, and the receiver reads one complete packet at a time.

5. Application Scenarios

   • TCP is suitable for scenarios requiring high reliability: Such as web browsing (HTTP), file transfer (FTP), and email (SMTP).
   • UDP is suitable for scenarios prioritizing real-time performance or efficiency: Such as video streaming, voice calls (where packet loss is more acceptable than delay), and DNS queries.

Comparison Summary Table

Key Selection Principles

• Choose TCP when data integrity and order are required (e.g., important file transfers).
• Choose UDP when low latency is prioritized or partial packet loss is tolerable (e.g., real-time gaming or live streaming).