This topic provides an overview of listeners. After you create a Server Load Balancer (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 Layer 4 (TCP and UDP) and Layer 7 (HTTP and HTTPS) listeners. The following table lists the features and use cases of these listeners.

Protocol Feature Use case
  • A connection-oriented protocol. A logical connection must be established before data can be sent and received.
  • Source IP address-based session persistence.
  • Source IP addresses readable at the network layer.
  • Fast data transmission
  • Applicable to scenarios that require high reliability and data accuracy but can tolerate low speeds, such as file transmission, sending or receiving emails, and remote logons.
  • Web applications that do not have special requirements.

For more information, see Add a TCP listener.

  • A connectionless protocol. UDP transmits data packets directly instead of making a three-way handshake with the other party before UDP sends data. UDP does not provide error recovery or data re-transmission.
  • Fast data transmission but relatively low reliability.

Applicable to scenarios where real-time transmission is more important than reliability, such as video chats and real-time financial market pushes.

For more information, see Add a UDP listener.

  • An application-layer protocol that is used to package data.
  • Cookie-based session persistence.
  • Use X-Forward-For to obtain source IP addresses.

Applicable to scenarios that require data content to be identified, such as web applications and small mobile games.

For more information, see Add an HTTP listener.

  • Encrypted data transmission that prevents unauthorized access.
  • Centralized certificate management service. You can upload certificates to SLB. The decryption operations are directly completed on SLB.

Applicable to scenarios that require encrypted transmission.

For more information, see Add an HTTPS listener.