Delta-Base

Delta encoding is a technique used to reduce the amount of data transmitted between a client and a server by sending only the differences (or deltas) between the current version of a resource and a previously cached version. This is particularly useful in situations where resources are frequently updated with small changes, as it can significantly reduce the amount of bandwidth and processing power required to transmit and receive updates.

To enable delta encoding in HTTP, the client and server must both support the necessary mechanisms for generating and applying deltas. This is typically achieved through the use of specific HTTP headers, such as Delta-Base, IM, and A-IM.

The Role of the Delta-Base Header

The Delta-Base header is used by the server to indicate the base version of a resource from which the delta is generated. This header is included in the HTTP response when the server sends a delta-encoded representation of a resource to the client. The client can then use the value of the Delta-Base header to determine which cached version of the resource should be used as the base for applying the delta.

The Delta-Base header is not used in HTTP requests, as the client typically indicates its willingness to receive delta-encoded responses by including the A-IM header in the request.

Example of Delta-Base in an HTTP Response

Let’s consider a scenario where a client has previously requested and cached a version of a resource, and now wants to request an updated version of that resource. The client sends an HTTP GET request with the A-IM header to indicate that it is capable of handling delta-encoded responses:

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

The server, upon receiving the request, determines that it can generate a delta-encoded representation of the updated resource based on a previously cached version. The server then includes the Delta-Base header in the HTTP response to indicate the base version of the resource:

HTTP/1.1 226 IM Used
Delta-Base: "1234567890abcdef"
Content-Type: application/vcdiff
Content-Length: 1024

...delta-encoded representation of the updated resource...

In this example, the server has generated a delta-encoded representation of the updated resource using the base version identified by the entity tag (ETag) "1234567890abcdef". The client can then use this ETag to locate the correct cached version of the resource and apply the delta to obtain the updated version.

Summary

The Delta-Base HTTP header plays a crucial role in enabling delta encoding in HTTP by indicating the base version of a resource from which a delta is generated. This header is included in HTTP responses when the server sends a delta-encoded representation of a resource, allowing the client to determine the appropriate cached version to use as the base for applying the delta.