clientmaxwindowbits

clientmaxwindowbits, more commonly written as client_max_window_bits, is a parameter used in the WebSocket permessage-deflate extension to negotiate the DEFLATE compression window size. It is part of the negotiation described in RFC 7692 and is conveyed by the client in the Sec-WebSocket-Extensions header when initiating a WebSocket connection.

The parameter indicates the desired base-2 logarithm of the LZ77 sliding window size that the client would like the server to use for compression. The resulting window size is 2^n bytes, with n typically in the range of 8 to 15. A value may be provided explicitly by the client (for example, requesting a particular window size) or omitted, in which case the server may choose a default. The server may accept the request or override it with its own choice, which it signals back in the response via the server_max_window_bits parameter if applicable.

The chosen window size affects performance and resource usage. Larger window sizes can improve compression efficiency, especially for long or repetitive data streams, but they require more memory and processing power. Smaller window sizes reduce memory usage at the potential cost of a weaker compression ratio. The actual window size used becomes part of the negotiated permessage-deflate state and governs subsequent DEFLATE operations on compressed WebSocket messages.

Security and compatibility considerations apply. Compression can introduce side-channel risks in some contexts, and not all servers or intermediaries support permessage-deflate or honor all negotiations. Therefore, both client and server should implement robust handling and provide clear fallbacks when negotiation cannot be completed.