The Delta-Base HTTP response header is included by the server in a delta-encoded response to specify the entity tag (ETag) of the base instance.

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When a response with an IM header is sent to a client, the Delta-Base HTTP response header must be included if there was more than one ETag specified in the If-None-Match HTTP request header.


Any HTTP response that includes an IM header and a delta-encoding might also include the Delta-Base response header.


In the following example, the client requests a resource that it has received in a previous response. The cached version has ETag 12340001 and to avoid resending the same resource, the client indicates this in the If-None-Match header. Using the A-IM request header, the client specifies that it is willing to accept a delta that is encoded using diffe encoding.

In response, the server returns the delta. The Delta-Base response header indicates the ETag for the version that the delta must be applied. The client then applies it to the cached version and applies the new ETag to the freshly-cached update.


GET /news.html HTTP/1.1
If-None-Match: 123400001
A-IM: diffe


HTTP/1.1 226 IM Used
IM: diffe
ETag: abcd00001
Delta-Base: 12340001


The Delta-Base header is sent by a server to direct the client to the cached resource for which the delta contained in the response must be applied. It is used for resources that were subject to IM.

Last updated: August 2, 2023