226 IM Used
The HTTP 226 IM Used successful response status code indicates that the server is returning a delta in response to a GET request.
It is used in the context of HTTP delta encodings.
IM stands for instance manipulation, which refers to the algorithm generating a delta.
In delta encoding, a client sends a GET request with two headers: A-IM:, which indicates a preference for a differencing algorithm, and If-None-Match, which specifies the version of a resource it has.
The server responds with deltas relative to a given base document, rather than the document in full.
This response uses the 226 status code, an IM: header that describes the differencing algorithm used, and may include a Delta-Base: header with the ETag matching the base document associated to the delta.
Warning: Poor support for HTTP delta encodings means there are few implementations. Instead, most systems rely solely on compression methods to reduce bandwidth, although a combination of compression and delta encodings is possible.
Even if the client and server support delta encodings, proxies or caches may not, and the complexity of adding HTTP delta encodings to a system may outweigh the benefits.
Status
226 IM Used
Examples
Receiving a 208 with the vcdiff delta algorithm
In the following GET request, a client requests a resource and has a cached version with the ETag abcd123.
The A-IM: header indicates a preference for vcdiff and diffe delta algorithms:
GET /resource.txt HTTP/1.1
Host: example.com
A-IM: vcdiff, diffe
If-None-Match: "abcd123"
Assuming the server supports delta encodings, it responds with the diff since the version with the ETag abcd123.
The IM header indicates that the vcdiff algorithm is used, and the Delta-Base: header indicates that the diff is based on a resource with ETag abcd123.
HTTP/1.1 226 IM Used
ETag: "5678a23"
IM: vcdiff
Content-Type: text/plain
Content-Length: 123
Delta-Base: abcd123
...
Specifications
| Specification |
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| Unknown specification # section-10.4.1 |