510 Not Extended
[RFC2774][2] is an experimental RFC, that introduces a mechanism to allow developers to extend the HTTP protocol with vendor-specific features in a safe namespaced way.
Around this time many new protocols and systems started to get built on top of HTTP, and the authors might have felt there was a need to do this in a namespaced way.
This is not unlike XML namespaces, where it’s possible for one standards organization to define a new format (for example ATOM), and other corporations wanting to add new properties, avoiding the possibility of collision if a second vendor also introduced a property with the same name.
As far as I know this extension to HTTP can be considered dead.
The 510 Not Extended statuscode could be used when a server requires that
a client uses an ‘extended HTTP request’, but didn’t. When a server emits
this response, it should also tell the client how to extend the request,
but the specification doesn’t specify a format for this.
Because this error is a ‘client error’, it should probably have had a 4xx code, not a 5xx one.
I would not recommend for anyone to use this.
Should I use this?
No. RFC2774 is a very obscure extension to HTTP, and as far as I know has never really been picked up. If you feel you have a need for this, I believe there is likely to be a different way to extend HTTP. Perhaps via content negotiation, or just different kinds of messaging from client to server.
References
- RFC2774, Section 6 - 510 Not Extended