Update README-QUIC.md

We move some of the "why QUIC" content into the guide and just provide a
summary in README-QUIC.md.

We also clarify how to use s_client with QUIC.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22505)

(cherry picked from commit 355fd1f45b707d2b066d6dff555dd53928e55627)

1 files changed, 56 insertions, 3 deletions

diff --git a/doc/man7/ossl-guide-quic-introduction.pod b/doc/man7/ossl-guide-quic-introduction.pod
index bb79e5b438..3141dfd209 100644
--- a/doc/man7/ossl-guide-quic-introduction.pod
+++ b/doc/man7/ossl-guide-quic-introduction.pod
@@ -20,9 +20,62 @@ 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)>). It
-additionally provides multiplexing capabilities through the use of "streams"
-(see L</QUIC STREAMS> below).
+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