IM Used

Overview

226 IM Used means the server returned the result of one or more instance-manipulations (delta encoding) rather than the full resource. The technique sends only the difference (delta) from the version the client already holds, reducing bandwidth.

It is defined by HTTP delta encoding in RFC 3229: the client advertises supported manipulations with the A-IM header and its baseline version with If-None-Match.

When it happens

• Use it for resources that change often but by small amounts (RSS/Atom feeds), where the client wants only the newly added entries as a delta instead of a full re-download.
• It is returned when a client requests a delta method (for example A-IM: feed) and the server supports it.

Request / Response example

GET /feed.xml HTTP/1.1
Host: example.com
A-IM: feed
If-None-Match: "v41"

HTTP/1.1 226 IM Used
IM: feed
ETag: "v42"
Content-Type: application/xml

(only the entries added since v41)

In code

GET /feed.xml HTTP/1.1
A-IM: feed
If-None-Match: "v41"

Notes

• Browser support and real-world use are extremely rare, so you almost never see this code in practice.
• The response's IM header states which instance-manipulations were applied; the client merges them onto its baseline to reconstruct the current representation.

Related status codes

Related headers

Specification