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HN · HTTP/2Curated

The secret message in every HTTP/2 connection

Summary

John Graham-Cumming explains the start of an HTTP/2 connection and the deliberately conspicuous client preface string. The preamble contains the string PRI. HTTP/2 then uses binary frames.

Ideas

  • HTTP/2 starts with a fixed plain-text preamble.
  • The preamble detects servers and proxies without HTTP/2 support.
  • Binary frames follow once the protocol has been successfully detected.
  • SETTINGS frames negotiate connection parameters for both directions.

Insights

  • Protocols need unambiguous transitions between incompatible states.
  • Readable test strings make diagnosis easier and prevent misinterpretation.
  • A connection preface can serve detection and protection at the same time.

Facts

  • It is 24 bytes long.

Recommendations

  • Examine handshakes with packet captures and protocol decoders.
  • Explicitly test intermediate components for their HTTP/2 behaviour.

References

Read the original article

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