How crawlers avoid re-fetching unchanged pages: conditional requests with ETag and Last-Modified, 304 handling, content hashing for change detection, and recrawl scheduling driven by per-page change rate.
Traces how the ALPN protocol list in a TLS ClientHello, and its predecessor NPN, fingerprints a client, and why the offered protocols and their order must agree with the HTTP layer that follows or the whole session looks forged.
Traces how HTTP/1.1 header order and field-name casing fingerprint a client, why every browser and library emits a fixed sequence, and how HTTP/2's mandatory lowercasing erased half the signal while keeping the rest.
Traces how the three Accept request headers, their exact default values, q-value syntax, and ordering form a per-browser signature, and how a missing or mismatched triad marks a request as a non-browser client.
A primary-source reference for HTTP caching: how Cache-Control directives, Expires, ETag and Last-Modified revalidation, Vary, and the stale-* extensions actually behave in private and shared caches under RFC 9111.
Traces how proxies append to the X-Forwarded-For and Forwarded chains, why the client-facing end is trivially spoofable, how trusted-proxy and rightmost-IP resolution actually works, and the security bugs that follow from getting it wrong.
Traces the low-bandwidth slow attacks: Slowloris, slow POST (RUDY), and slow read, how each pins a worker thread on thread-per-connection servers, why event-driven servers shrug them off, and what actually times them out.
Traces how HTTP/1.1's two ways of measuring a request body let a front-end and back-end disagree on where one request ends, how CL.TE and TE.CL desync turns that into socket poisoning, and what actually fixes it.
Traces HTTP from Berners-Lee's one-line 1991 protocol through RFC 1945, the RFC 2068/2616/7230 era of HTTP/1.1, Google's SPDY, HTTP/2 (RFC 7540/9113), and HTTP/3 over QUIC (RFC 9114).
Traces the user-agent string from RFC 1945 through the Mozilla token, the Mosaic-Netscape-IE spoofing spiral, and Chrome's 2020-2023 freeze and reduction into User-Agent Client Hints.