A protocol is an agreed set of rules for exchanging information between systems. Web collection relies on several protocols: DNS resolves names, TCP or QUIC carries connections, TLS protects many connections, and HTTP defines request and response semantics.
Diagnose one layer at a time
When a fetch fails, identify the stage. A DNS lookup error happens before a connection; a TLS certificate failure occurs during secure connection setup; an HTTP 429 means the server responded and is rate-limiting the client. Treating all of these as “the site is down” makes troubleshooting harder.
Tools may negotiate different protocol versions or use different proxies. Capture the relevant error, status, and negotiated protocol when available. Avoid changing several layers at once, because that can hide the cause of a problem.
Understanding the protocol stack also helps choose the right client: a plain HTTP library is enough for many pages, while browser automation is needed only when browser execution or interaction is part of the task.
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