101

Switching Protocols

1xx Informational

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Server perspective

Use 101 when the client requested a protocol upgrade and the server is switching this connection to that protocol.

When to use

  • •Return 101 for successful WebSocket upgrades
  • •Use it only when the protocol switch is explicitly requested and supported

How to respond

  • •Include Upgrade and Connection: Upgrade in the successful response
  • •Start speaking the new protocol immediately after the 101 response

Headers to consider

  • •Upgrade
  • •Connection

Response body

  • •Do not include a response body in a 101 response

Server-side pitfalls

  • •Do not use 101 unless the client actually requested an upgrade
  • •Do not treat 101 as a redirect or as a generic signal to retry elsewhere

Examples

Server upgrades to WebSocket

Request:GET /socket with Upgrade: websocket
Response:101 Switching Protocols with Connection: Upgrade

The server confirms the switch and the connection leaves ordinary HTTP messaging behind.

References

Related 1xx Informational Codes

100

Continue

The server is saying, 'I checked the start of your request. It looks okay so far, so go ahead and send the big part now.'

102

Processing

The server is saying, 'I’m still working on this. Don’t assume the request was lost just because it’s taking a while.'

WebDAV
103

Early Hints

The server is giving the browser a head start: 'You’ll probably need these files, so start getting them while I finish the real response.'