ls-qpack

QPACK compression library for use with HTTP/3

Introduction

QPACK is the compression mechanism used by HTTP/3 to compress HTTP headers. It is in the process of being standardazed by the QUIC Working Group. The QPACK Internet-Draft is has been stable for some time and we don't expect functional changes to it before the final RFC is released.

Functionality

ls-qpack is a full-featured, tested, and fast QPACK library. The QPACK encoder produces excellent compression results based on an innovative mnemonic technique (to be described in a future article). It boasts the fastest Huffman encoder and decoder.

The library is production quality. It is used in OpenLiteSpeed, LiteSpeed Web Server, and LiteSpeed Web ADC.

The library is robust:

1. The encoder does not assume anything about usual HTTP headers such as Server or User-Agent. Instead, it uses its mnemonic compression technique to achieve good compression results for any input.
2. The decoder uses modulo arithmetic to track dynamic table insertions. This is in contrast to all other QPACK implementations, which use an integer counter, meaning that at some point, the decoder will break.
3. The decoder processes input in streaming fashion. The caller does not have to buffer the contents of HTTP/3 HEADERS frame. Instead, the decoder can be supplied input byte-by-byte.

Other Languages

The ls-qpack library is implemented in vanilla C99. It makes it a good candidate for wrapping into a library for a higher-level language. As of this writing, we know of the following wrappers:

• Go: ls-qpack-go
• Python: pylsqpack
• TypeScript: quicker

Versioning

Before the QPACK RFC is released, the three parts of the version are:

• MAJOR: set to zero;
• MINOR: set to the number of QPACK Internet-Draft the lirbary supports; and
• PATCH: set to the patch number

Once the RFC is released, MARJO will be set to 1 and the version will follow the usual MAJOR.MINOR.PATCH pattern.

API

The API is documented in the header file, lsqpack.h. One example how it is used in real code can be seen in lsquic, a QUIC and HTTP/3 library developed by LiteSpeed Technologies.