LSQUIC Documentation

This is the documentation for LSQUIC 2.26.2, last updated Dec 23, 2020.

LiteSpeed QUIC (LSQUIC) Library is an open-source implementation of QUIC and HTTP/3 functionality for servers and clients. LSQUIC is:

  • fast;
  • flexible; and
  • production-ready.

Most of the code in this distribution has been used in our own products – LiteSpeed Web Server, LiteSpeed Web ADC, and OpenLiteSpeed – since 2017.

Currently supported QUIC versions are Q043, Q046, Q050, ID-27, ID-28, ID-29, and ID-32. Support for newer versions will be added soon after they are released.

LSQUIC is licensed under the MIT License; see LICENSE in the source distribution for details.


LSQUIC supports nearly all QUIC and HTTP/3 features, including

  • ECN
  • Spin bits (allowing network observer to calculate a connection’s RTT)
  • Path migration
  • NAT rebinding
  • Push promises
  • TLS Key updates
  • Extensions:
  • Extensible HTTP Priorities
  • Datagrams
  • Loss bits extension (allowing network observer to locate source of packet loss)
  • Timestamps extension (allowing for one-way delay calculation, improving performance of some congestion controllers)
  • Delayed ACKs (this reduces number of ACK frames sent and processed, improving throughput)
  • QUIC grease bit to reduce ossification opportunities


The LSQUIC library does not use sockets to receive and send packets; that is handled by the user-supplied callbacks. The library also does not mandate the use of any particular event loop. Instead, it has functions to help the user schedule events. (Thus, using an event loop is not even strictly necessary.) The various callbacks and settings are supplied to the engine constructor. LSQUIC keeps QUIC connections in several data structures in order to process them efficiently. Connections that need processing are kept in two priority queues: one holds connections that are ready to be processed (or “ticked”) and the other orders connections by their next timer value. As a result, no connection is processed needlessly.

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