Interview Practice
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Notes

Networking essentials

1 min read

The path of a web request

  1. DNS: the resolver finds the IP address for the host name, using caches with TTLs.
  2. TCP handshake: SYN, SYN-ACK, ACK establishes a connection (one round trip).
  3. TLS handshake: TLS 1.3 negotiates keys in one round trip, or zero with resumption.
  4. HTTP request and response over the encrypted connection.
  5. Connection reuse (keep-alive, HTTP/2 multiplexing) amortizes the setup cost over many requests.

TCP versus UDP

TCP provides reliable, ordered byte streams with flow control and congestion control. UDP sends independent datagrams with no delivery guarantee; applications that need low latency and can tolerate loss (games, voice) or that implement their own reliability (QUIC) use it. HTTP/3 runs over QUIC, which is built on UDP and avoids TCP's head-of-line blocking across streams.

HTTP

  • Methods: GET and HEAD are safe; GET, PUT and DELETE are idempotent; POST is neither.
  • Status classes: 2xx success, 3xx redirection, 4xx client error, 5xx server error. Know 200, 201, 204, 301, 304, 400, 401, 403, 404, 409, 429, 500, 502, 503 and 504.
  • Caching: Cache-Control, ETag with If-None-Match, and CDNs at the edge.
  • HTTP/2 multiplexes many streams over one TCP connection; HTTP/3 does the same over QUIC.

Load balancing layers

Layer 4 balancers route by IP and port and are fast; layer 7 balancers understand HTTP and can route by path, header or cookie, terminate TLS and retry.

Questions to answer aloud

  • What happens when you type a URL into a browser and press Enter?
  • Why does a TCP connection take longer to reach full speed on a long-distance link?
  • When would you choose WebSockets over server-sent events, and over long polling?
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