Skip to main content

Listener overview

This topic provides an overview of listeners. After you create a Server Load Balancer ( SLB) instance, you must configure one or more listeners for the SLB instance. 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 TCP and UDP listeners for Layer 4 load balancing, and HTTP and HTTPS listeners for Layer 7 load balancing. The following table describes the features and use scenarios of these listeners.

ProtocolDescriptionScenario
TCP● A connection-oriented protocol. A reliable connection must be established before data can be sent and received.
● Session persistence is based on source IP addresses.
● Source IP addresses are visible at the network layer.
● Data is transmitted at a fast rate.		● 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 logins.
● Web applications that do not have custom requirements.
UDP● 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.
HTTP● An application-layer protocol that is used to package data.
● Cookie-based session persistence.
● Use the X-Forward-For header to obtain the real IP addresses of clients.		Applicable to scenarios that require data content identification, such as web applications and small mobile games.
HTTPS● Encrypted data transmission that prevents unauthorized access.
● Centralized certificate management service. You can upload certificates to CLB. The decryption operations are completed directly on CLB.		Applicable to scenarios that require encrypted transmission.