=pod

=head1 NAME

ossl-guide-quic-introduction
- OpenSSL Guide: An introduction to QUIC in OpenSSL

=head1 INTRODUCTION

This page will provide an introduction to some basic QUIC concepts and
background and how it is used within OpenSSL. It assumes that you have a basic
understanding of UDP/IP and sockets. It also assumes that you are familiar with
some OpenSSL and TLS fundamentals (see L<ossl-guide-libraries-introduction(7)>
and L<ossl-guide-tls-introduction(7)>).

=head1 WHAT IS QUIC?

QUIC is a general purpose protocol for enabling applications to securely
communicate over a network. It is defined in RFC9000 (see
L<https://datatracker.ietf.org/doc/rfc9000/>). QUIC integrates parts of the
TLS protocol for connection establishment but independently protects packets.
It provides similar security guarantees to TLS such as confidentiality,
integrity and authentication (see L<ossl-guide-tls-introduction(7)>).

QUIC delivers a number of advantages:

=over 4

=item Multiple streams

It supports multiple streams of communication (see L</QUIC STREAMS> below),
allowing application protocols built on QUIC to create arbitrarily many
bytestreams for communication between a client and server. This allows an
application protocol to avoid problems where one packet of data is held up
waiting on another packet being delivered (commonly referred to as
"head-of-line blocking"). It also enables an application to open additional
logical streams without requiring a round-trip exchange of packets between the
client and server as is required when opening an additional TLS/TCP
connection.

=item HTTP/3

Since QUIC is the basis of HTTP/3, support for QUIC also enables applications
to use HTTP/3 using a suitable third-party library.

=item Fast connection initiation

Future versions of OpenSSL will offer support for 0-RTT connection initiation,
allowing a connection to be initiated to a server and application data to be
transmitted without any waiting time. This is similar to TLS 1.3's 0-RTT
functionality but also avoids the round trip needed to open a TCP socket; thus,
it is similar to a combination of TLS 1.3 0-RTT and TCP Fast Open.

=item Connection migration

Future versions of OpenSSL will offer support for connection migration, allowing
connections to seamlessly survive IP address changes.

=item 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.

=item Implemented as application library

Because most QUIC implementations, including OpenSSL's implementation, are
implemented as an application library rather than by an operating system, an
application can gain the benefit of QUIC without needing to wait 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 without dependency
on an OS update cadence.

=item Multiplexing over a single UDP socket

Because QUIC is UDP-based, it is possible to multiplex a QUIC connection on the
same UDP socket as some other UDP-based protocols, such as RTP.

=back

=head1 QUIC TIME BASED EVENTS

A key difference between the TLS implementation and the QUIC implementation in
OpenSSL is 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.

OpenSSL introduces a new function L<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 L<SSL_read_ex(3)> or
L<SSL_write_ex(3)> will also process these events. There is also
L<SSL_get_event_timeout(3)> which tells an application the amount of time that
remains until L<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. However 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.

OpenSSL provides an optional "thread assisted mode" that will automatically
create a background thread and will regularly call L<SSL_handle_events(3)> in a
thread safe manner. This provides a simple way for an application to satisfy the
QUIC requirements for time based events without having to implement special
logic to accomplish it.

=head1 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 are 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.

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 information 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.

=head1 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
without any loss of data or reordering of the bytes. 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.

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

=head1 SOCKETS AND BLOCKING

TLS assumes "stream" type semantics for its underlying transport layer protocol
(usually achieved by using TCP). However 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 L<BIO_s_datagram(3)>, but other B<BIO> choices are available.
See L<bio(7)> for an introduction to OpenSSL's B<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 B<SSL> object will, by default, still operate
in blocking mode. So, from an application's perspective, calls to functions such
as L<SSL_read_ex(3)>, L<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
L<SSL_set_blocking_mode(3)>.

=head1 FURTHER READING

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

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

=head1 SEE ALSO

L<ossl-guide-introduction(7)>, L<ossl-guide-libraries-introduction(7)>,
L<ossl-guide-libssl-introduction(7)>, L<ossl-guide-tls-introduction(7)>,
L<ossl-guide-tls-client-block(7)>, L<ossl-guide-quic-client-block(7)>,
L<ossl-guide-quic-client-non-block(7)>, L<ossl-guide-quic-multi-stream(7)>,
L<ossl-guide-quic-server-block(7)>, L<ossl-guide-quic-server-non-block(7)>,
L<bio(7)>,

=head1 COPYRIGHT

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
L<https://www.openssl.org/source/license.html>.

=cut
