HTTP & Web Protocols

Common methods: GET reads, POST creates, PUT/PATCH update, DELETE removes, HEAD fetches headers. A GET form puts fields in the URL query string (cached, logged); POST sends them in the body, preferred for sensitive data.

The first digit groups the semantics: 1xx informational, 2xx success (200), 3xx redirection (301/302), 4xx client error (bad or forbidden request), 5xx server error. A client reads that leading digit to react.

HTTP is a text-based, stateless application-layer protocol over TCP/IP for client-server request/response. Both messages share a start line, key:value headers, a blank line, then an optional body.

HTTPS is HTTP carried over a TLS (formerly SSL) encrypted connection, so traffic is encrypted, integrity-protected, and the server is authenticated by a certificate. Plain HTTP sends everything in cleartext anyone can read.

Return a 3xx status with a Location header naming the target URL — 301 permanent (browsers and SEO cache it) or 302 temporary. The browser then issues a fresh request to that URL, falling back to an HTML link.

An architectural style for web APIs: resources addressed by URLs, manipulated with HTTP methods (GET/POST/PUT/DELETE), typically exchanging JSON, leveraging status codes and caching. Key constraints: statelessness and a uniform interface.

curl resolves the host to an IP via DNS, opens a TCP connection (three-way handshake), negotiates TLS for https, then sends an HTTP GET request line plus headers. The response travels back down the network stack (IP, link, physical) through any proxy to the server, which builds the body and replies.

DNS (Domain Name System) maps a name like avito.ru to an IP. A resolver walks the hierarchy — root, then TLD (.ru), then the authoritative server — usually over UDP/53, falling back to TCP for large replies. Answers are cached at every level with a TTL, so repeat lookups skip the walk.

Via response headers. Cache-Control (and legacy Expires) set lifetime, visibility and revalidation; Last-Modified plus If-Modified-Since cache by date; ETag caches by a content hash. The client revalidates and reuses its copy.

TCP is a connection-oriented transport protocol: a three-way handshake opens the connection, then data is split into numbered segments that the receiver acknowledges (ACK). Lost segments are retransmitted, sequence numbers restore order, and a sliding window pipelines many segments while congestion control throttles the sender to avoid overrunning the network or receiver.

The Common Gateway Interface — a convention where the server runs an external program per request, passing request data via environment variables and reading the HTTP response from its stdout. Spawning a process per request is slow.

The server attaches an ETag validator to a resource; the client echoes it back via If-None-Match on its next request. If the resource still matches, the server replies 304 with an empty body and the client reuses its cached copy; if not, 200 with fresh content.

UDP is a connectionless transport: no handshake, no ACK, no retransmission — cheap, low-latency, but unreliable, so it suits voice, video, metrics, and games where a dropped datagram is tolerable. A socket is the OS abstraction for a transport endpoint (a file-like handle): stream sockets ride TCP, datagram sockets ride UDP, plus local Unix sockets.

REST is an architectural style — any format (JSON/XML/text), HTTP-only, resource-oriented, cacheable. SOAP is a protocol — XML-only, transport-agnostic (HTTP/SMTP), operation-oriented, with built-in WS-Security and ACID transactions.