RFC 9204 - QPACK: Field Compression for HTTP/3

• Status: Proposed Standard
• Published: June 2022
• Stream: IETF
• Errata: No Errata

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Abstract

This specification defines QPACK: a compression format for efficiently representing HTTP fields that is to be used in HTTP/3. This is a variation of HPACK compression that seeks to reduce head-of-line blocking.

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Contents

• 1. Introduction
  • 1.1 Conventions and Definitions
  • 1.2 Notational Conventions
• 2. Compression Process Overview
  • 2.1 Encoder
  • 2.2 Decoder
• 3. Reference Tables
  • 3.1 Static Table
  • 3.2 Dynamic Table
• 4. Wire Format
  • 4.1 Primitives
  • 4.2 Encoder and Decoder Streams
  • 4.3 Encoder Instructions
  • 4.4 Decoder Instructions
  • 4.5 Field Line Representations
• 5. Configuration
• 6. Error Handling
• 7. Security Considerations
  • 7.1 Probing Dynamic Table State
  • 7.2 Static Huffman Encoding
  • 7.3 Memory Consumption
  • 7.4 Implementation Limits
• 8. IANA Considerations
• 9. References

Appendices

• Appendix A. Static Table
• Appendix B. Encoding and Decoding Examples
• Appendix C. Sample Single-Pass Encoding Algorithm

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Related Resources

• Official RFC: RFC 9204
• Official Page: RFC 9204 DataTracker
• Errata: RFC Editor Errata

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Quick Reference

What is QPACK?

QPACK is HTTP/3's header compression format, based on HPACK but optimized for QUIC's out-of-order delivery characteristics.

Core Improvements

Major changes compared to HPACK:

• ✅ Reduced Head-of-Line Blocking - Allows out-of-order decoding
• ✅ Independent Streams - Encoder/decoder use separate unidirectional streams
• ✅ Flexible Trade-offs - Balance between compression ratio and latency
• ✅ Dynamic Table Management - More fine-grained control mechanisms

QPACK vs HPACK

Feature	HPACK (HTTP/2)	QPACK (HTTP/3)
Transport Protocol	TCP	QUIC (UDP)
Head-of-Line Blocking	Severe	Significantly Reduced
Dynamic Table	Single	Encoder/Decoder Separated
Compression Ratio	High	Close to HPACK
Out-of-Order Processing	Not Supported	Supported
Reference Modes	Absolute Index Only	Multiple Index Types

Use Cases

• 🌐 HTTP/3 - Next generation HTTP protocol
• 🚀 QUIC Protocol - Low latency transport
• 📱 Mobile Applications - Unstable network environments
• ⚡ Real-time Communication - WebRTC over HTTP/3