SecWebSocketExtensions

SecWebSocketExtensions refers to the Sec-WebSocket-Extensions HTTP header used during the WebSocket handshake to negotiate optional protocol extensions between client and server. Defined by the WebSocket protocol (RFC 6455), this header enables features such as compression or alternate framing, with permessage-deflate being the most common extension described in RFC 7692.

During the handshake, the client sends Sec-WebSocket-Extensions in the upgrade request, listing one or more extension names and optional parameters in a comma-separated form. For example, a client might propose permessage-deflate with parameters like client_max_window_bits. The server may accept one or more of the proposed extensions and returns the chosen ones in its Sec-WebSocket-Extensions response header, including any accepted parameters. If no extension is negotiated, the header may be omitted or empty in the response, and the connection proceeds without extensions.

Commonly negotiated extensions include permessage-deflate, which provides compressed WebSocket frames to reduce bandwidth, and may involve parameters such as server_no_context_takeover or client_no_context_takeover. Extensions are optional and negotiable; both sides must support the selected extensions, and the exact behavior is defined by the specific extension’s specification.

Security and compatibility considerations include ensuring that extensions are only enabled when both parties support and validate them, and understanding that negotiated extensions alter how frames are processed and decompressed. Implementations should handle negotiation failures gracefully and fall back to a standard, unextended WebSocket if needed.

See also: RFC 6455, RFC 7692, WebSocket extensions.

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