Switching Protocols

# Quick Definition

The 101 Switching Protocols status code indicates that the server understands and is willing to comply with the client's request to change the application protocol being used on this connection. The server will switch to the protocol defined in the response's Upgrade header immediately after the empty line that terminates the 101 response.

# When Does a 101 Response Occur?

A 101 response is returned when the client sends a request with an Upgrade header asking the server to switch to a different protocol, and the server agrees. The most common scenario is a WebSocket handshake, where the client requests an upgrade from HTTP/1.1 to the WebSocket protocol.

The client initiates the switch by including Connection: Upgrade and Upgrade: websocket headers (along with a security key). If the server supports WebSocket and accepts the request, it responds with 101 and the connection transitions from HTTP to a full-duplex WebSocket channel.

# Common Use Cases

• WebSocket connections for real-time chat applications and collaboration tools
• Live data feeds such as stock tickers, sports scores, or IoT sensor streams
• Multiplayer gaming servers that need persistent, low-latency bidirectional communication
• Server-Sent Events fallback when upgrading from HTTP polling to a persistent connection
• Protocol upgrades in reverse proxy configurations (Nginx, HAProxy forwarding WebSocket traffic)

# Best Practices

1. Always validate the Sec-WebSocket-Key -- The server must compute the correct Sec-WebSocket-Accept value by concatenating the key with the magic GUID and returning the Base64-encoded SHA-1 hash.
2. Configure reverse proxies correctly -- Nginx requires proxy_set_header Upgrade $http_upgrade and proxy_set_header Connection "upgrade" to forward WebSocket upgrades.
3. Implement heartbeat/ping frames -- Use WebSocket ping/pong frames to detect dead connections and prevent intermediaries from closing idle connections.
4. Handle upgrade failures gracefully -- If the server cannot upgrade, fall back to HTTP long-polling or Server-Sent Events for real-time functionality.
5. Use wss:// in production -- Always use secure WebSocket connections (WSS) over TLS to prevent intermediary interference and ensure data privacy.

# HTTP Example

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

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

# Related Status Codes

# Frequently Asked Questions