# Overview

You need to add at least one listener after you have created a load balancer. This 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 TCP or UDP for load balancing at Layer 4 and HTTP or HTTPS at Layer 7.

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, and remote login
- Web applications that receive a large number of concurrent requests and require high performance

Layer 4

UDP

- Low reliability
- Fast data transfer

Scenarios that require quick response, such as video chat, gaming, and real-time financial quotations

Layer 4

SSL

- TCP-based security encryption
- High reliability
- Supported by only classic load balancers

Web applications that require encrypted transmission

Layer 7

HTTP

- Cookie-based sticky sessions
- X-Forward-For request header

Web applications where data content needs to be identified, such as mobile games

Layer 7

HTTPS

- An extension of HTTP for encrypted data transmission to prevent unauthorized access
- Encryption and decryption performed on load balancers
- Multiple versions of encryption protocols and cipher suites

Web applications that require encrypted transmission

Parent topic: Listener