226 IM Used

Note

Browsers don't support delta encoding with HTTP. This status code is sent back by custom servers used by specific clients.

Takeaways

1. The server has fulfilled a GET request for the resource, and the response is a representation of the result of one or more instance-manipulations applied to the current instance.
2. The actual current instance might not be available except by combining this response with other previous or future responses, as appropriate for the specific instance-manipulation(s). If so, the headers of the resulting instance are the result of combining the headers from the 226 response and the other instances.
3. The request must have included an A-IM header field listing at least one instance-manipulation. The response must include an ETag header field giving the entity tag of the current instance.
4. A response received with a status code of 226 may be stored by a cache and used in reply to a subsequent request, subject to the HTTP expiration mechanism and any Cache-Control headers.
5. A response received with a status code of 226 may be used by a cache, in conjunction with a cache entry for the base instance, to create a cache entry for the current instance.

• In the context of delta encodings, the HTTP 226 IM Used status code is set by the server to indicate that it is returning a delta to the GET request that it received.
• With delta encoding a server responds to GET requests with differences (called deltas) relative to a given base document (rather than the current document).
• The client uses the A-IM: HTTP header to indicate which differencing algorithm to use and the If-None-Match: header to hint the server about the last version it got.
• The server generates a delta, sending it back in an HTTP response with the 226 status code and containing the IM: (with the name of the algorithm used) and Delta-Base: (with the ETag matching the base document associated to the delta) HTTP headers.

Information

IM stands for instance manipulations the term used to describe an algorithm generating a delta.