What is the purpose of the ETag HTTP header?

• A. ETag provides a version identifier for a cached resource to validate freshness.
• B. ETag specifies the content type.
• C. ETag controls server-side caching duration.
• D. ETag stores the last modification timestamp.

Answer: (A)

ETag is a version identifier for a resource's specific representation. The server assigns a unique tag whenever the resource changes. A client can store the cached copy and later send that ETag back in an If-None-Match header. If the resource hasn’t changed, the server responds with 304 Not Modified, letting the client use its cached copy without transferring the full data. If it has changed, the server returns the new representation along with a new ETag, so the client updates its cache.

This mechanism helps validate freshness and save bandwidth by avoiding sending unchanged content. It’s different from the content type (that’s indicated by Content-Type), the caching duration controlled by directives like Cache-Control, and the last modification time tracked by Last-Modified. ETag provides a robust, versioned fingerprint of the resource that supports efficient conditional requests.