OSSL-GUIDE-QUIC-INTRODUCTION(7SSL)

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💖 OSSL-GUIDE-QUIC-INTRODUCTION 💖

okay bestie, so this is the OpenSSL Guide: An introduction to QUIC in OpenSSL ✨ it's part of the OpenSSL 7SSL manual fam, and it's basically the ✨vibe check✨ page before you dive into the real QUIC stuff.

🌸 INTRODUCTION 🌸

So this page is gonna give you the lowdown on some basic QUIC concepts + background, and how it all works inside OpenSSL. It assumes you already have a basic understanding of UDP/IP and sockets (iykyk), and it also assumes you're familiar with some OpenSSL and TLS fundamentals — go read ossl-guide-libraries-introduction(7) and ossl-guide-tls-introduction(7) first if you haven't, no cap 🫶

🦋 WHAT IS QUIC? 🦋

QUIC is a general purpose protocol that lets applications securely communicate over a network. It's defined in RFC9000 (peep <https://datatracker.ietf.org/doc/rfc9000/>). QUIC integrates parts of the TLS protocol for connection establishment, but it independently protects packets, which is lowkey iconic of it. It gives you similar security guarantees to TLS like confidentiality, integrity and authentication (see ossl-guide-tls-introduction(7)).

QUIC really said "let me deliver" and gave us a bunch of advantages:

  • Multiple streams — It supports multiple streams of communication (see "QUIC STREAMS" below), so application protocols built on QUIC can create arbitrarily many bytestreams for communication between a client and server. This lets an application protocol dodge the problem where one packet of data is held up waiting on another packet to be delivered (aka "head-of-line blocking", the villain 💀). It also means an application can open additional logical streams without needing a round-trip exchange of packets between client and server — unlike opening an additional TLS/TCP connection, which is giving extra work energy.
  • HTTP/3 — Since QUIC is the basis of HTTP/3, supporting QUIC also lets applications use HTTP/3 through a suitable third-party library. slay.
  • Fast connection initiation — Future versions of OpenSSL will offer support for 0-RTT connection initiation, letting a connection be initiated to a server and application data be transmitted with no waiting time. It's similar to TLS 1.3's 0-RTT functionality but also avoids the round trip needed to open a TCP socket, so like… it's basically TLS 1.3 0-RTT and TCP Fast Open combined. obsessed.
  • Connection migration — Future versions of OpenSSL will offer support for connection migration, letting connections seamlessly survive IP address changes. main character energy fr fr.
  • Datagram based use cases — Future versions of OpenSSL will offer support for the QUIC datagram extension, allowing support for both TLS and DTLS-style use cases on a single connection. 🔥
  • Implemented as application library — Because most QUIC implementations (including OpenSSL's) are implemented as an application library rather than by an operating system, an application can gain the benefit of QUIC without waiting for an OS update to be deployed. Future evolutions and enhancements to the QUIC protocol can be delivered as quickly as an application can be updated — no dependency on an OS update cadence, periodt.
  • Multiplexing over a single UDP socket — Because QUIC is UDP-based, you can multiplex a QUIC connection on the same UDP socket as some other UDP-based protocols, like RTP. 💅

⏰ QUIC TIME BASED EVENTS ⏰

okay so here's a KEY difference between the TLS implementation and the QUIC implementation in OpenSSL: how time is handled. The QUIC protocol requires various actions to be performed on a regular basis regardless of whether application data is being transmitted or received. it's a needy protocol and we love it for that.

OpenSSL introduces a new function SSL_handle_events(3) that will automatically process any outstanding time based events that must be handled. Alternatively, calling any I/O function such as SSL_read_ex(3) or SSL_write_ex(3) will also process these events. There's also SSL_get_event_timeout(3) which tells an application the amount of time that remains until SSL_handle_events(3) (or any I/O function) must be called. 🎀

Fortunately, a blocking application that does not leave the QUIC connection idle, and is regularly calling I/O functions, does not typically need to worry about this. But — and this is the important part — if you are developing a nonblocking application, or one that may leave the QUIC connection idle for a period of time, then you will need to arrange to call these functions. don't skip it bestie, it's giving required. 😭

OpenSSL provides an optional "thread assisted mode" that will automatically create a background thread and will regularly call SSL_handle_events(3) in a thread safe manner. This gives an application a simple way to satisfy the QUIC requirements for time based events without having to implement special logic to accomplish it. we love a shortcut that's also correct. 💖

🧚 QUIC AND TLS 🧚

QUIC reuses parts of the TLS protocol in its implementation. Specifically, the TLS handshake also exists in QUIC. The TLS handshake messages get wrapped up in QUIC protocol messages in order to send them to the peer. Once the TLS handshake is complete, all application data is sent entirely using QUIC protocol messages without using TLS — although some TLS handshake messages may still be sent in some circumstances. sneaky but valid.

This relationship between QUIC and TLS means that many of the API functions in OpenSSL that apply to TLS connections also apply to QUIC connections, and applications can use them in exactly the same way. Some functions do not apply to QUIC at all, and others have altered semantics. You should refer to the documentation pages for each function for info on how it applies to QUIC. Typically, if QUIC is not mentioned in the manual pages, then the functions apply to both TLS and QUIC. so if you're unsure… read the docs. i know. i know. 🙈

🌊 QUIC STREAMS 🌊

QUIC introduces the concept of "streams". A stream provides a reliable mechanism for sending and receiving application data between the endpoints. The bytes transmitted are guaranteed to be received in the same order they were sent, with no loss of data and no reordering of the bytes. iconic behaviour honestly. A TLS application effectively has one bi-directional stream available to it per TLS connection. A QUIC application can have multiple uni-directional or bi-directional streams available to it for each connection. go off.

In OpenSSL, an SSL object is used to represent both connections and streams. A QUIC application creates an initial SSL object to represent the connection (known as the connection SSL object). Once the connection is complete, additional SSL objects can be created to represent streams (known as stream SSL objects). Unless configured otherwise, a "default" stream is also associated with the connection SSL object so you can still write data and read data to/from it. Some OpenSSL API functions can only be used with connection SSL objects, and some can only be used with stream SSL objects. Check the documentation for each function to confirm what type of SSL object can be used in any particular context. A connection SSL object that has a default stream attached to it can be used in contexts that require a connection SSL object or in contexts that require a stream SSL object. it's versatile like that 💅

🔌 SOCKETS AND BLOCKING 🔌

TLS assumes "stream" type semantics for its underlying transport layer protocol (usually achieved by using TCP). But QUIC assumes "datagram" type semantics by using UDP. An OpenSSL application using QUIC is responsible for creating a BIO to represent the underlying transport layer. This BIO must support datagrams and is typically BIO_s_datagram(3), but other BIO choices are available. See bio(7) for an introduction to OpenSSL's BIO concept. 🫶

A significant difference between OpenSSL TLS applications and OpenSSL QUIC applications is the way that blocking is implemented. In TLS, if your application expects blocking behaviour, then you configure the underlying socket for blocking. Conversely, if your application wants nonblocking behaviour, then the underlying socket is configured to be nonblocking.

With an OpenSSL QUIC application, the underlying socket must always be configured to be nonblocking. Howevever the SSL object will, by default, still operate in blocking mode. So from an application's perspective, calls to functions such as SSL_read_ex(3), SSL_write_ex(3) and other I/O functions will still block. OpenSSL itself provides that blocking capability for QUIC instead of the socket. If nonblocking behaviour is desired, then the application must call SSL_set_blocking_mode(3).

⚠️ no cap on this one: the socket is always nonblocking for QUIC. OpenSSL is the one doing the blocking, not your socket. don't fight it bestie.

📚 FURTHER READING 📚

See ossl-guide-quic-client-block(7) to see an example of applying these concepts in order to write a simple blocking QUIC client. 🌈

See ossl-guide-quic-server-block(7) to see an example of applying these concepts in order to write a simple blocking QUIC server. ⭐

👀 SEE ALSO 👀

Copyright 2023-2025 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.

okay that's the whole page bestie, you ate it up 💅✨ this little guide lives in the OpenSSL 4.0.3 docs and was dated 2026-09-29 — go forth and build something iconic with QUIC 💖🦋🎀

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