Upgrade

Description

The Upgrade request header is used by a client to request an upgrade to a different communication protocol on an existing connection. It is commonly used to transition from HTTP/1.1 to WebSockets or HTTP/2 when supported by the server. The server responds with the 101 Switching Protocols status if it agrees to the upgrade request.

This header is often seen in WebSocket communications where an HTTP connection is upgraded to a WebSocket connection.

Syntax

The Upgrade header follows this syntax:

Upgrade: <protocol>[, <protocol>]*

Where:

• <protocol> is the name of the protocol the client wishes to upgrade to.
• Multiple protocols can be specified, separated by commas, indicating preference order.

Examples:

Upgrade: websocket
Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9

Examples

HTTP to WebSocket Upgrade

A client requests an upgrade to a WebSocket connection.

Client Request:

GET /chat HTTP/1.1
Host: example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13

Server Response:

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=

HTTP/1.1 to HTTP/2 Upgrade

A client requests to upgrade an HTTP/1.1 connection to HTTP/2.

Client Request:

GET /index.html HTTP/1.1
Host: example.com
Upgrade: h2c
Connection: Upgrade

Server Response:

HTTP/1.1 101 Switching Protocols
Upgrade: h2c
Connection: Upgrade

If the server does not support the requested protocol, it will simply ignore the Upgrade header and continue using the original protocol.

Summary

The Upgrade request header allows clients to request a change in communication protocols, such as upgrading an HTTP connection to WebSockets or HTTP/2. If the server supports the requested protocol, it responds with a 101 Switching Protocols status and confirms the upgrade. This header plays a crucial role in modern web applications, particularly for real-time communication and improved efficiency with newer protocols.