=pod

=head1 NAME

SSL_set_num_tickets,
SSL_get_num_tickets,
SSL_CTX_set_num_tickets,
SSL_CTX_get_num_tickets,
SSL_new_session_ticket
- control the number of TLSv1.3 session tickets that are issued

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 int SSL_set_num_tickets(SSL *s, size_t num_tickets);
 size_t SSL_get_num_tickets(SSL *s);
 int SSL_CTX_set_num_tickets(SSL_CTX *ctx, size_t num_tickets);
 size_t SSL_CTX_get_num_tickets(SSL_CTX *ctx);
 int SSL_new_session_ticket(SSL *s);

=head1 DESCRIPTION

SSL_CTX_set_num_tickets() and SSL_set_num_tickets() can be called for a server
application and set the number of TLSv1.3 session tickets that will be sent to
the client after a full handshake. Set the desired value (which could be 0) in
the B<num_tickets> argument. Typically these functions should be called before
the start of the handshake.

The default number of tickets is 2; the default number of tickets sent following
a resumption handshake is 1 but this cannot be changed using these functions.
The number of tickets following a resumption handshake can be reduced to 0 using
custom session ticket callbacks (see L<SSL_CTX_set_session_ticket_cb(3)>).

Tickets are also issued on receipt of a post-handshake certificate from the
client following a request by the server using
L<SSL_verify_client_post_handshake(3)>. These new tickets will be associated
with the updated client identity (i.e. including their certificate and
verification status). The number of tickets issued will normally be the same as
was used for the initial handshake. If the initial handshake was a full
handshake then SSL_set_num_tickets() can be called again prior to calling
SSL_verify_client_post_handshake() to update the number of tickets that will be
sent.

To issue tickets after other events (such as application-layer changes),
SSL_new_session_ticket() is used by a server application to request that a new
ticket be sent when it is safe to do so.  New tickets are only allowed to be
sent in this manner after the initial handshake has completed, and only for
TLS 1.3 connections.  By default, the ticket generation and transmission are
delayed until the server is starting a new write operation, so that it is
bundled with other application data being written and properly aligned to a
record boundary.  If the connection was at a record boundary when
SSL_new_session_ticket() was called, the ticket can be sent immediately
(without waiting for the next application write) by calling
SSL_do_handshake().  SSL_new_session_ticket() can be called more than once to
request additional tickets be sent; all such requests are queued and written
together when it is safe to do so and triggered by SSL_write() or
SSL_do_handshake().  Note that a successful return from
SSL_new_session_ticket() indicates only that the request to send a ticket was
processed, not that the ticket itself was sent.  To be notified when the
ticket itself is sent, a new-session callback can be registered with
L<SSL_CTX_sess_set_new_cb(3)> that will be invoked as the ticket or tickets
are generated.

SSL_CTX_get_num_tickets() and SSL_get_num_tickets() return the number of
tickets set by a previous call to SSL_CTX_set_num_tickets() or
SSL_set_num_tickets(), or 2 if no such call has been made.

=head1 RETURN VALUES

SSL_CTX_set_num_tickets(), SSL_set_num_tickets(), and
SSL_new_session_ticket() return 1 on success or 0 on failure.

SSL_CTX_get_num_tickets() and SSL_get_num_tickets() return the number of tickets
that have been previously set.

=head1 SEE ALSO

L<ssl(7)>

=head1 HISTORY

SSL_new_session_ticket() was added in OpenSSL 3.0.0.
SSL_set_num_tickets(), SSL_get_num_tickets(), SSL_CTX_set_num_tickets(), and
SSL_CTX_get_num_tickets() were added in OpenSSL 1.1.1.

=head1 COPYRIGHT

Copyright 2018-2021 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
