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226 IM Used

The response is the result of applying one or more instance manipulations, such as a delta, to the current representation.

Status

HTTP/1.1 226 IM Used

Details

  • Category: 2xx
  • Status: Experimental. Defined by an experimental or informational RFC.
  • Specification: RFC 3229 §10.4.1
  • Cacheable: Cacheable by default
  • Response body: Allowed
  • Retry: Safe to retry

Delta encoding in HTTP lets a server send only what changed since the copy the client already holds. The client advertises A-IM (accept instance manipulations) with its cached ETag, and the server replies 226 with the delta plus an IM header naming the manipulation applied.

The mechanism never saw broad deployment. Modern systems solve the same problem with compression, conditional requests returning 304, and application-level sync protocols. Expect to encounter 226 only in specialized feed and replication systems.

Headers

  • IM: Required. Names the instance manipulations applied, e.g. vcdiff or feed.
  • Delta-Base: The ETag of the base instance the delta was computed against.

Common causes

  • A client sent A-IM and the server supports delta encoding for that resource.
  • An RFC 3229+feed aware aggregator requested only new entries from a syndication feed.

Usage notes

For clients

  • Apply the delta to the exact instance named in Delta-Base. If you no longer hold it, re-request without A-IM.

For servers

  • Only send 226 when the client advertised support. Unsolicited deltas break ordinary clients.
  • Fall back to a normal 200 when no suitable base instance is available.

Examples

Fetching a feed with delta encoding

HTTP request

GET /feed.xml HTTP/1.1
Host: news.example.com
A-IM: feed
If-None-Match: "b8e1"

HTTP response

HTTP/1.1 226 IM Used
IM: feed
Delta-Base: "b8e1"
ETag: "c0a4"
Content-Type: application/atom+xml

Specifications

See also