HTTP headers / response header
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
An opaque version identifier for the response. Clients echo it back in If-None-Match (cache validation, → 304) or If-Match (optimistic concurrency, → 412).
Weak ETags (W/"...") survive gzip-level changes; strong ones may differ between servers behind a load balancer — that breaks revalidation subtly.
The fastest way to see what ETag is actually doing is to send the request and read both sides raw — ReqPad shows auto-generated and custom headers for every request, on all six protocols, with history. Related references: status codes · WWW-Authenticate · Content-Encoding · Transfer-Encoding · Last-Modified · Expires · Location
Build the request, send it, read raw headers and timing — from your iPhone. Free to start.