226 IM Used
The HTTP226 IM Usedsuccessful response status code indicates that the server is returning adelta in response to aGET request.It is used in the context ofHTTP delta encodings.
IM stands forinstance manipulation, which refers to the algorithm generating adelta.In delta encoding, a client sends aGET request with two headers:A-IM:, which indicates a preference for a differencing algorithm, andIf-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 the226 status code, anIM: header that describes the differencing algorithm used, and may include aDelta-Base: header with theETag 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 oncompression 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 UsedExamples
Receiving a208 with thevcdiff delta algorithm
In the followingGET request, a client requests a resource and has a cached version with the ETagabcd123.TheA-IM: header indicates a preference forvcdiff anddiffe delta algorithms:
GET /resource.txt HTTP/1.1Host: example.comA-IM: vcdiff, diffeIf-None-Match: "abcd123"Assuming the server supports delta encodings, it responds with the diff since the version with the ETagabcd123.TheIM header indicates that thevcdiff algorithm is used, and theDelta-Base: header indicates that the diff is based on a resource with ETagabcd123.
HTTP/1.1 226 IM UsedETag: "5678a23"IM: vcdiffContent-Type: text/plainContent-Length: 123Delta-Base: abcd123...Specifications
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| Unknown specification # section-10.4.1 |