This topic provides an overview of listeners. After you create an SLB instance, you must configure one or more listeners for it. A listener checks for connection requests and then distributes the requests to backend servers based on the forwarding rules that are defined by a specified scheduling algorithm.

SLB provides listeners for Layer-4 (TCP and UDP) and Layer-7 (HTTP and HTTPS) load balancing. The following table lists the features and use cases of these listeners.

Protocol Feature Use case
TCP
  • A connection-oriented protocol that requires a logical connection to be established before any data can be transmitted
  • Source IP address-based session persistence
  • Source IP addresses readable at the network layer
  • Fast data transmission
  • Services that prioritize high reliability and data accuracy over high-speed data transmission, such as file transmission, email sending or receiving, and remote logon
  • Basic web applications

For more information, see Add a TCP listener.

UDP
  • A connectionless protocol that transmits data packets without implementing the three-way handshake and does not provide error recovery or data retransmission
  • Fast data transmission with relatively low reliability

Services that focus on real-time content delivery other than reliability, such as video chats and real-time quotes

For more information, see Add a UDP listener.

HTTP
  • An application-layer protocol used to package data
  • Cookie-based session persistence
  • Support for using the X-Forwarded-For (XFF) header to identify original client IP addresses

Applications that need to identify data content, such as web applications and small-sized mobile games

For more information, see Add an HTTP listener.

HTTPS
  • Encrypted data transmission that prevents unauthorized access
  • Centralized certificate management that allows certificate uploading and decryption on SLB

Applications that require encrypted transmission

For more information, see Add an HTTPS listener.

Note The WebSocket Secure (WSS)/WebSocket (WS) protocol is supported in all regions. For more information, see WS/WSS support FAQ.