Delta-Base
The HTTP Delta-Base response header specifies the ETag of the base instance against which a delta-encoded response was computed.
Usage
Delta encoding allows a server to send only the differences between two versions of a resource instead of the full representation. When a client holds a cached copy and requests an update using the A-IM header, the server computes a delta between the cached version and the current version.
The Delta-Base header identifies the specific version used as the starting point for the delta calculation. The value is the ETag of the base instance. The client matches this ETag against stored cached representations to find the correct base, applies the delta, and produces the updated resource.
This header appears in 226 (IM Used) responses alongside an IM header describing the delta encoding algorithm. When the client's If-None-Match header listed multiple ETags, Delta-Base clarifies which cached version the server selected as the base.
Example
A client requests an update to a cached resource.
The request carries the cached ETag in
If-None-Match and specifies
diffe as an acceptable delta encoding through the
A-IM header.
GET /news.html HTTP/1.1
Host: www.example.re
If-None-Match: "12340001"
A-IM: diffe
The server responds with a 226 status, a new
ETag for the updated resource, and the
Delta-Base pointing to the cached version.
The client applies the delta to the copy tagged
"12340001" and stores the result under the new
ETag.
HTTP/1.1 226 IM Used
IM: diffe
ETag: "abcd00001"
Delta-Base: "12340001"
Takeaway
The Delta-Base header identifies the cached version a server used as the base for a delta-encoded response. Together with IM and A-IM, the header enables efficient partial updates to cached resources.