The 101 (Switching Protocols) status code indicates that the server understands and is willing to comply with the client’s request, via the Upgrade header field, for a change in the application protocol being used on this connection. The server MUST generate an Upgrade header field in the response that indicates which protocol(s) will be switched to immediately after the empty line that terminates the 101 response.
This status code is generally used with the Upgrade header field to transition from HTTP/1.1 to some other protocol on the same connection:
A server that sends a 101 (Switching Protocols) response MUST send an Upgrade header field to indicate the new protocol(s) to which the connection is being switched; if multiple protocol layers are being switched, the sender MUST list the protocols in layer-ascending order.
For more information, see Section 6.7 of the spec.
Not cachable by default.