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HTTP/2

Binary framing + multiplexing — dozens of concurrent requests over a single TCP connection.

HTTP2MultiplexingHPACK
核心 · Key Idea

In one line: HTTP/2 replaces 1.1's text protocol with binary frames and multiplexes many requests on one TCP connection, slashing connection-setup cost and head-of-line blocking at the application layer.

What it is#

HTTP/1.1 pain points:

  • One in-flight request per connection — serial.
  • Browsers worked around this by opening 6 TCP connections per origin (still not enough).

HTTP/2's redesign:

FeatureMeaning
Binary framingRequests / responses become frames; many streams interleave on the same connection.
MultiplexingTens of streams concurrently share one connection without blocking.
HPACK header compressionStatic + dynamic tables compress repeated headers down to a few bytes.
Server pushServer can pre-push assets (now largely deprecated in browsers).
PrioritiesStreams have weights; browsers tell the server which to send first.

Analogy#

打个比方 · Analogy

HTTP/1.1 = single-lane road — only one car at a time. HTTP/2 = multi-lane highway — dozens of cars on the same road (TCP) running side-by-side.

Key concepts#

StreamStream
A request/response pair; each has a unique ID.
FrameFrame
Smallest unit inside a stream: HEADERS / DATA / SETTINGS / PING / GOAWAY etc.
HPACKHeader Compression
Static table (~80 common headers) + a shared dynamic table compress repeated headers to a few bytes.
Stream PriorityStream Priority
Tree-structured weights — browser tells server what to send first.
Head-of-line blockingHoL Blocking
HTTP/2 fixes application-layer HoL; TCP-layer HoL remains — only HTTP/3 fully fixes it.

How it works#

Open a page and dozens of assets arrive in parallel.

Practical notes#

  • Requires HTTPS. Modern browsers only do HTTP/2 over TLS (h2 over TLS). Cleartext h2c is server-to-server only.
  • Verify it's H2: curl -I --http2 https://... or DevTools' Network panel "Protocol" column.
  • Enable on the server: nginx listen 443 ssl http2;; Caddy is on by default.
  • Stop doing 1.1 micro-optimizations — domain sharding, sprite sheets, inlined assets are counter-productive under H2.
  • TCP HoL still bites. Any packet loss stalls every stream on that TCP connection — only HTTP/3 truly solves this.

Easy confusions#

HTTP/2
Runs over **TCP + TLS**.
Application-layer multiplexing, **TCP-layer HoL remains**.
HTTP/3
Runs over **QUIC (UDP)**.
Streams are truly independent — **no HoL**.

Further reading#