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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
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 |
Protocol rule being enforced
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
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