/ HTTP Headers


The A-IM HTTP header stands for Accept-Instance Manipulations, a less commonly used HTTP header that has crucial implications for HTTP-based data transfer protocols. It’s primarily utilized in HTTP extensions and specified in the HTTP/1.1 protocol (RFC 3229), used for HTTP-based delta encoding, and it provides methods for partial instance manipulation.

The Purpose of A-IM

This HTTP request header field allows the client to specify what forms of instance manipulation (such as deltas or patches) it supports. HTTP servers use the A-IM header to determine the client’s capabilities and preferences. These manipulations help to reduce bandwidth usage and improve HTTP efficiency by allowing clients to only download parts of a resource that have changed since the last time they accessed it.


The A-IM header field uses the following syntax:

A-IM: <im-type>

Here, <im-type> denotes the instance manipulation mechanisms that the client can handle. Instance manipulations are named by strings (tokens) and are case-insensitive. Multiple values can be listed, separated by commas.

Possible Values

Typically, there are two significant instance manipulations used with A-IM:

  • vcdiff: A general delta encoding algorithm.
  • gzip: A method of compressing data.

However, other values may be used, depending on the specific extensions a server or client may have.

How it Works: Request and Response

Here’s an example of how the A-IM header works:


A client might send a request with an A-IM header like this:

GET /resource HTTP/1.1
Host: example.com
A-IM: vcdiff

In this request, the client is informing the server that it can handle instance manipulations of type vcdiff.


If the server supports the specified instance manipulation, it will respond with a 226 IM Used status code and an IM header field. The IM header will contain the instance manipulations that have been applied to the payload body. The server may also include an ETag header field to provide a validator for the new instance.

Here’s an example response to the above request:

HTTP/1.1 226 IM Used
IM: vcdiff
ETag: "737060cd8c284d8af7ad3082f209582d"
Content-Type: application/vcdiff
Content-Length: 2731

[Payload body]

In this response, the server indicates that it has applied the vcdiff instance manipulation to the payload body. The ETag header provides a validator for the new instance of the resource.

It’s essential to understand that the server isn’t obligated to apply instance manipulations even if the client sends an A-IM header. If the server cannot or chooses not to apply the requested instance manipulations, it should ignore the A-IM header and respond normally.


The A-IM header is not universally supported across all browsers and HTTP servers. It is mainly used in HTTP extensions and often omitted in standard HTTP operations.


The A-IM HTTP header is a powerful tool for enhancing HTTP efficiency by enabling instance manipulations like delta encoding. Though it’s not universally supported, it provides valuable capabilities when working with HTTP extensions.

Always ensure to understand both the server and client’s capabilities and preferences when working with A-IM. It’s an excellent tool for optimizing network traffic, but like any tool, it must be used appropriately.

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