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E_PROTOCOL_EXTENSIONS_NOT_SUPPORTED
E_PROTOCOL_EXTENSIONS_NOT_SUPPORTED (0x83760003) means the Windows protocol component encountered requested or selected WebSocket extension is unsupported. Extension negotiation names a feature the Windows WebSocket implementation or application did not offer or cannot activate. Identify the failing layer before changing HTTP, TLS, or application payload settings.
Compare both offers
| Source | Record |
|---|---|
| Client request | Every Sec-WebSocket-Extensions token and parameter. |
| Server response | Only extensions actually selected by the peer. |
| API arrays | The zero-based extension indices supplied to handshake completion. |
Security and interoperability
Do not accept an unknown extension by merely ignoring its parameters: extensions can change reserved bits and frame semantics. The negotiated result must be a subset of the offer and understood by both sides.
Test plan
- Repeat with no extensions
- Offer one supported extension with canonical parameters
- Reject a server-selected extension that was never offered
Resolution
Remove the extension, implement it end to end, or route the connection through a stack that supports the required negotiation. Keep the unextended path as a control.
Extension selection must be symmetric
The server may select only an extension offered by the client, and both sides must agree on its parameters. Keep the exact comma-separated offer and response rather than normalized feature names; parameter order, duplicate parameters, and unsupported values can make an otherwise familiar extension unusable. Test the handshake once with the extension omitted: success in that control confirms the base WebSocket path and isolates negotiation from DNS, TLS, authentication, and HTTP upgrade handling.
Before changing configuration
- Confirm this is the first failure rather than a cleanup result
- Preserve the exact input or object state selected at the failure
- Identify limits, negotiation choices, or lifecycle transitions immediately before it
- Design a one-variable experiment instead of enabling broad retry or compatibility behavior
Acceptance evidence
Close the result investigation only when the original supported operation succeeds repeatedly, an invalid control still fails, and resource ownership remains correct; success on different data or another endpoint is not equivalent evidence.
Opening handshake evidence
| Boundary | Required evidence |
|---|---|
| Client request | Method, target, Host, Upgrade, Connection, key, version, protocols, and extensions |
| Server response | Status, Upgrade, Connection, accept value, selected protocol, and extensions |
| Windows API inputs | Arrays, counts, and selected indices passed before the return |
| Post-handshake actions | Action types, buffers, context tokens, and completion calls |
Deterministic protocol boundary
Capture the first peer bytes or API transition yielding this result; backoff can limit a reconnect storm but cannot repair negotiation or state, and direct versus proxied tests may differ because an intermediary can terminate and recreate the WebSocket.
Regression sequence
- Complete a baseline RFC 6455 handshake without optional extensions
- Exchange text and binary messages through the documented action loop
- Perform a clean close in both directions
- Introduce only the version, protocol, extension, frame, or call-order defect represented here
Wire and API ownership
| Boundary | Responsibility |
|---|---|
| HTTP upgrade | Owns request and response header syntax before the session opens |
| WebSocket framing | Owns opcode, masking, reserved bits, length encoding, fragmentation, and control-frame rules |
| Negotiated feature | Owns extension or subprotocol selection and its parameters |
| Windows action loop | Owns local handle state, buffers, action completion, and close sequencing |
Technical references
- Microsoft Open Specifications: HRESULT values
- Microsoft WebSocket handshake completion
- Microsoft WebSocket action queue API
- RFC 6455: The WebSocket Protocol
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