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I don't understand the meaning of "258EAFA5-E914-47DA-95CA-C5AB0DC85B11" in RFC 6455. Why does the server need this magic string? And why does the WebSocket protocol need this mechanism?
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The WebSocket protocol is an independent TCP-based protocol. Its only relationship to HTTP is that its handshake is interpreted by HTTP servers as an Upgrade request. By default the WebSocket protocol uses port 80 for regular WebSocket connections and port 443 for WebSocket connections tunneled over TLS [RFC2818].
WebSocket uses a unified TCP connection and needs one party to terminate the connection. Until it happens, the connection remains active. HTTP needs to build a distinct connection for separate requests. Once the request is completed, the connection breaks automatically.
A ping or pong is just a regular frame, but it's a control frame. Pings have an opcode of 0x9 , and pongs have an opcode of 0xA . When you get a ping, send back a pong with the exact same Payload Data as the ping (for pings and pongs, the max payload length is 125).
The |Sec-WebSocket-Key| header field is used in the WebSocket opening handshake. It is sent from the client to the server to provide part of the information used by the server to prove that it received a valid WebSocket opening handshake.
The RFC explains it. It is a GUID, selected because it is "unlikely to be used by network endpoints that do not understand the WebSocket Protocol". See RFC 6455.
If you're interested in the specifics of the format for GUIDs like that, see RFC 4122.
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From the Quora answer:
There is no reason for choosing the magic string. The particular magic string GUID was chosen to add some level of integrity to the WebSockets protocol, because the string is globally unique.
The RFC (RFC 6455 - The WebSocket Protocol) only says:
...concatenate this with the Globally Unique Identifier (GUID, [RFC4122]) "258EAFA5-E914-47DA-95CA-C5AB0DC85B11" in string form, which is unlikely to be used by network endpoints that do not understand the WebSocket Protocol.
Hope that answers your question.
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