RFC 7692 in one paragraph

permessage-deflate (RFC 7692) is the WebSocket compression extension: it deflate-compresses message payloads before framing. It's negotiated during the handshake — the client offers it in Sec-WebSocket-Extensions: permessage-deflate, the server accepts (optionally with parameters), and from then on either side may send compressed messages.

Three facts anchor everything else:

1. It's per-MESSAGE, not per-frame. A fragmented message is compressed as one stream, then split into frames. Only the FIRST frame of a compressed message sets the marker bit.
2. The marker is RSV1. That reserved bit in the frame header — which conformance rules say must be 0 — becomes legal only after this extension is negotiated. RSV1=1 on a message means "inflate the payload".
3. The stream is raw DEFLATE. No zlib header, no gzip wrapper, no checksum (RFC 7692 §7.2). If your library emits 78 9c ..., you're producing zlib, not raw deflate — every interoperable peer will reject it.

Compression is optional per message: the sender can leave RSV1=0 and send any message uncompressed even after negotiation — worth remembering for payloads that won't shrink.