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mvfst (Pronounced move fast) is a client and server implementation of IETF QUIC protocol in C++ by Facebook. QUIC is a UDP based reliable, multiplexed transport protocol that will become an internet standard. The goal of mvfst is to build a performant implementation of the QUIC transport protocol that applications could adapt for use cases on both the internet and the data-center. mvfst has been tested at scale on android, iOS apps, as well as servers and has several features to support large scale deployments.


Server features:

  • Multi-threaded UDP socket server with a thread local architecture to be able to scale to multi-core servers
  • Customizable Connection-Id routing. The default Connection-Id routing implementation integrates seamlessly with katran
  • APIs to enable zero-downtime restarts of servers, so that applications do not have to drop connections when restarting.
  • APIs to expose transport and server statistics for debuggability
  • Zero-Rtt connection establishment and customizable zero rtt path validation
  • Support for UDP Generic segmentation offloading (GSO) for faster UDP writes.

Client features:

  • Native happy eyeballs support between ipv4 and ipv6 so that applications do not need to implement it themselves
  • Pluggable congestion control and support for turning off congestion control to plug in application specific control algorithms

Source Layout

  • quic/api: Defines API that applications can use to interact with the QUIC transport layer.
  • quic/client: Client transport implementation
  • quic/codec: Read and write codec implementation for the protocol
  • quic/common: Implementation of common utility functions
  • quic/congestion_control: Implementation of different congestion control algorithms such as Cubic and Copa
  • quic/flowcontrol: Implementations of flow control functions
  • quic/handshake: Implementations cryptographic handshake layer
  • quic/happyeyeballs: Implementation of mechanism to race IPV4 and IPV6 connection and pick a winner
  • quic/logging: Implementation of logging framework
  • quic/loss: Implementations of different loss recovery algorithms
  • quic/samples: Example client and server
  • quic/server: Server transport implementation
  • quic/state: Defines and implements both connection and stream level state artifacts and state machines


mvfst largely depends on two libraries: folly and fizz.

Building mvfst

Ubuntu 16+

To begin, you should install the dependencies we need for build. This largely consists of dependencies from folly as well as fizz.

sudo apt-get install         \
    g++                      \
    cmake                    \
    libboost-all-dev         \
    libevent-dev             \
    libdouble-conversion-dev \
    libgoogle-glog-dev       \
    libgflags-dev            \
    libiberty-dev            \
    liblz4-dev               \
    liblzma-dev              \
    libsnappy-dev            \
    make                     \
    zlib1g-dev               \
    binutils-dev             \
    libjemalloc-dev          \
    libssl-dev               \
    pkg-config               \

Then, build and install folly and fizz

Alternatively, run the helper script in this subdirectory. It will install and link the required dependencies and also build folly and fizz. This may take several minutes the first time.


After building, the directory _build/ will contain the dependencies (under _build/deps) whereas _build/build will contain all the built libraries and binaries for mvfst.

You can also install mvfst as well as its dependencies folly and fizz to a custom directory using the build script, by supplying an INSTALL_PREFIX env var.

./ -i /usr/local

See ./ --help for more options

You might need to run the script as root to install to certain directories.

By default the build script enables the building of test target (i.e. runs with -DBUILD_TEST=ON option). Since some of tests in mvfst require some test artifacts of Fizz, it is necessary to supply the path of the Fizz src directory (via option DFIZZ_PROJECT) to correctly build all test targets in mvfst.

Run a sample client and server

Building the test targets of mvfst (or via should automatically build the sample client and server binaries into the default _build/build directory (or whichever target directory was specified). The server will automatically bind to ::1 by default if no host is used, but you can then spin a simple echo server by running:

./_build/build/quic/samples/echo -mode=server -host=<host> -port=<port>

and to run a client:

./_build/build/quic/samples/echo -mode=client -host=<host> -port=<port>

For more options, see

./_build/build/quic/samples/echo --help


This repo implements the QUIC transport. For an HTTP/3 implementation that uses Mvfst, please check out Proxygen.


We'd love to have your help in making mvfst better. If you're interested, please read our guide to guide to contributing

Please also join our slack to ask questions or start discussions.


mvfst is MIT licensed, as found in the LICENSE file.


The API should be considered in alpha. We can't predict all the use cases that people will have, so we're waiting some time before declaring a more stable API. We are open to have several different APIs for different constraints.

Reporting and Fixing Security Issues

Please do not open GitHub issues or pull requests - this makes the problem immediately visible to everyone, including malicious actors. Security issues in mvfst can be safely reported via Facebook's Whitehat Bug Bounty program:

Facebook's security team will triage your report and determine whether or not is it eligible for a bounty under our program.


An implementation of the QUIC transport protocol.




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