Overview

At least one listener must be added to a load balancer. A listener receives requests from clients and routes requests to backend servers using the protocol, port, and load balancing algorithm you select.

Supported Protocols

ELB provides load balancing at both Layer 4 and Layer 7. Select a protocol that meets your needs in a specific scenario.

**Table 1** Protocols supported by ELB
Protocol		Description	Application Scenario
Layer 4	TCP	Source IP address-based sticky sessions Fast data transfer	Scenarios that require high reliability and data accuracy, such as file transfer, email sending and receiving, and remote login Web applications with a number of concurrent connections or requiring high performance
Layer 4	UDP	Low reliability Fast data transfer	Scenarios that focus more on timeliness than on reliability, such as video chats, games, and real-time financial quotations
Layer 7	HTTP	Cookie-based sticky sessions X-Forward-For request header	Applications where data content needs to be identified, such as web applications and mobile games
Layer 7	HTTPS	An extension of HTTP for encrypted data transmission to prevent unauthorized access Encryption and decryption performed on load balancers to reduce the workload of backend servers.	Applications that require encrypted transmission