Documentation updates for TLSv1.3 sessions

Add documentation for SSL_SESSION_is_resumable(). Also describe the interaction
of the various session functions and TLSv1.3 post-handshake sessions.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3008)

1 files changed, 24 insertions, 1 deletions

diff --git a/doc/man3/SSL_get_session.pod b/doc/man3/SSL_get_session.pod
index d753b271ee..33b365d337 100644
--- a/doc/man3/SSL_get_session.pod
+++ b/doc/man3/SSL_get_session.pod
@@ -26,7 +26,30 @@ count of the B<SSL_SESSION> is incremented by one.
 =head1 NOTES
 
 The ssl session contains all information required to re-establish the
-connection without a new handshake.
+connection without a full handshake for SSL versions <= TLSv1.2. In TLSv1.3 the
+same is true, but sessions are established after the main handshake has occurred.
+The server will send the session information to the client at a time of its
+choosing which may be some while after the initial connection is established (or
+not at all). Calling these functions on the client side in TLSv1.3 before the
+session has been established will still return an SSL_SESSION object but it
+cannot be used for resuming the session. See L<SSL_SESSION_is_resumable(3)> for
+information on how to determine whether an SSL_SESSION object can be used for
+resumption or not.
+
+Additionally, in TLSv1.3, a server can send multiple session messages for a
+single connection. In that case the above functions will only return information
+on the last session that was received.
+
+The preferred way for applications to obtain a resumable SSL_SESSION object is
+to use a new session callback as described in L<SSL_CTX_sess_set_new_cb(3)>.
+The new session callback is only invoked when a session is actually established,
+so this avoids the problem described above where an application obtains an
+SSL_SESSION object that cannot be used for resumption in TLSv1.3. It also
+enables applications to obtain information about all sessions sent by the
+server.
+
+In TLSv1.3 it is recommended that each SSL_SESSION object is only used for
+resumption once.
 
 SSL_get0_session() returns a pointer to the actual session. As the
 reference counter is not incremented, the pointer is only valid while