9. Extensions

The WebSocket Protocol provides an extension mechanism that allows adding additional functionality.

9.1 Negotiating Extensions

Extensions are negotiated through the Sec-WebSocket-Extensions header during the handshake.

Client Requests Extension

GET /chat HTTP/1.1
Host: server.example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13
Sec-WebSocket-Extensions: permessage-deflate; client_max_window_bits

Server Response

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

Extension Format

extension = extension-token *( ";" extension-param )
extension-param = token [ "=" (token | quoted-string) ]

9.2 Known Extensions

permessage-deflate (RFC 7692)

The most commonly used extension, providing message compression functionality.

Advantages:

• Reduces bandwidth usage
• Suitable for text data

Disadvantages:

• Increases CPU overhead
• May increase latency

Usage Example (Node.js):

const WebSocket = require('ws');

const wss = new WebSocket.Server({
  port: 8080,
  perMessageDeflate: {
    zlibDeflateOptions: {
      chunkSize: 1024,
      memLevel: 7,
      level: 3
    },
    zlibInflateOptions: {
      chunkSize: 10 * 1024
    },
    clientNoContextTakeover: true,
    serverNoContextTakeover: true,
    serverMaxWindowBits: 10,
    concurrencyLimit: 10,
    threshold: 1024 // Only compress messages larger than 1KB
  }
});

Other Extensions

• permessage-bzip2: Bzip2 compression
• multiplexing: Multiplexing (draft)
• Custom extensions

Extension Development

When developing custom extensions:

1. Define extension name
2. Define parameters
3. Specify how to modify frame structure
4. Implement handshake negotiation
5. Register extension

Reference Links

• Previous Chapter: 8. Error Handling
• Next Chapter: 10. Security Considerations
• RFC 7692: WebSocket Compression Extensions

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