The layer under everything: TCP vs UDP, what TLS actually does, and the realtime menu — WebSockets, SSE, and push vs polling
Connection vs connectionless. Reliability, ordering, the 3-way handshake, flow & congestion control — and the head-of-line blocking that TCP bakes in.
Confidentiality, integrity, authentication. The handshake, TLS 1.3's 1-RTT / 0-RTT, ALPN, and the chain of trust.
The HTTP Upgrade handshake, full-duplex framing, message vs frame, and ping/pong keepalive.
Server-Sent Events over a long-lived HTTP response — auto-reconnect, event IDs — and when it beats a socket.
Short poll → long poll → SSE → WebSockets. The latency, overhead, and complexity tradeoffs, with a decision flow.