Appropriate HTTP status code for request specifying invalid Content-Encoding header?

Question

What status code should be returned if a client sends an HTTP request and specifies a Content-Encoding header which cannot be decoded by the server?

Example

A client POSTs JSON data to a REST resource and encodes the entity body using the gzip coding. However, the server can only decode DEFLATE codings because it failed the gzip class in server school.

What HTTP response code should be returned? I would say 415 Unsupported Media Type but it's not the entity's Content-Type that is the problem -- it's the encoding of the otherwise supported entity body.

Which is more appropriate: 415? 400? Perhaps a custom response code?

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Addendum: I have, of course, thoroughly checked rfc2616. If the answer is there I may need some new corrective eyewear, but I don't believe that it is.

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Update:

This has nothing to do with sending a response that might be unacceptable to a client. The problem is that the client is sending the server what may or may not be a valid media type in an encoding the server cannot understand (as per the Content-Encoding header the client packaged with the request message).

It's an edge-case and wouldn't be encountered when dealing with browser user-agents, but it could crop up in REST APIs accepting entity bodies to create/modify resources.

Solution 1

As i'm reading it, 415 Unsupported Media Type sounds like the most appropriate.

From RFC 2616:

10.4.16 415 Unsupported Media Type

The server is refusing to service the request because the entity of the request is in a format not supported by the requested resource for the requested method.

Yeah, the text part says "media type" rather than "encoding", but the actual description doesn't include any mention of that distinction.

The new hotness, RFC 7231, is even explicit about it:

6.5.13. 415 Unsupported Media Type

The 415 (Unsupported Media Type) status code indicates that the
origin server is refusing to service the request because the payload
is in a format not supported by this method on the target resource.
The format problem might be due to the request's indicated
Content-Type or Content-Encoding, or as a result of inspecting the
data directly.

Solution 2

They should make that the final question on Who Wants To Be a Millionaire!

Well the browser made a request that the server cannot service because the information the client provided is in a format that cannot be handled by the server. However, this isn't the server's fault for not supporting the data the client provided, it's the client's fault for not listening to the server's Acccept-* headers and providing data in an inappropriate encoding. That would make it a Client Error (400 series error code).

• My first instinct is 400 Bad Request is the appropriate response in this case.
• 405 Method Not Allowed isn't right because it refers to the HTTP verb being one that isn't allowed.
• 406 Not Acceptable looks like it might have promise, but it refers to the server being unable to provide data to the client that satisfies the Accept-* request headers that it sent. This doesn't seem like it would fit your case.
• 412 Precondition Failed is rather vaguely defined. It might be appropriate, but I wouldn't bet on it.
• 415 Unsupported Media Type isn't right because it's not the data type that's being rejected, it's the encoding format.

After that we get into the realm of non-standard response codes.

• 422 Unprocessable Entity describes a response that should be returned if the request was well-formed but if it was semantically incorrect in some way. This seems like a good fit, but it's a WebDAV extension to HTTP and not standard.

Given the above, I'd personally opt for 400 Bad Request. If any other HTTP experts have a better candidate though, I'd listen to them instead. ;)

UPDATE: I'd previously been referencing the HTTP statuses from their page on Wikipedia. Whilst the information there seems to be accurate, it's also less than thorough. Looking at the specs from W3C gives a lot more information on HTTP 406, and it's leading me to think that 406 might be the right code after all.

10.4.7 406 Not Acceptable

The resource identified by the request is only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request.

Unless it was a HEAD request, the response SHOULD include an entity containing a list of available entity characteristics and location(s) from which the user or user agent can choose the one most appropriate. The entity format is specified by the media type given in the Content-Type header field. Depending upon the format and the capabilities of the user agent, selection of the most appropriate choice MAY be performed automatically. However, this specification does not define any standard for such automatic selection.

  Note: HTTP/1.1 servers are allowed to return responses which are
  not acceptable according to the accept headers sent in the
  request. In some cases, this may even be preferable to sending a
  406 response. User agents are encouraged to inspect the headers of
  an incoming response to determine if it is acceptable.

If the response could be unacceptable, a user agent SHOULD temporarily stop receipt of more data and query the user for a decision on further actions.

While it does mention the Content-Type header explicitly, the wording mentions "entity characteristics", which you could read as covering stuff like GZIP versus DEFLATE compression.

One thing worth noting is that the spec says that it may be appropriate to just send the data as is, along with the headers to tell the client what format it's in and what encoding it uses, and just leave it for the client to sort out. So if the client sends a header indicating it accepts GZIP compression, but the server can only generate a response with DEFLATE, then sending that along with headers saying it's DEFLATE should be okay (depending on the context).

• Client: Give me a GZIPPED page.
• Server: Sorry, no can do. I can DEFLATE pack it for you. Here's the DEFLATE packed page. Is that okay for you?
• Client: Welllll... I didn't really want DEFLATE, but I can decode it okay so I'll take it.

(or)

• Client: I think I'll have to clear that with my user. Hold on.