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)

3 files changed, 75 insertions, 2 deletions

diff --git a/doc/man3/SSL_CTX_sess_set_get_cb.pod b/doc/man3/SSL_CTX_sess_set_get_cb.pod
index ebea4c54cd..d0caa8a155 100644
--- a/doc/man3/SSL_CTX_sess_set_get_cb.pod
+++ b/doc/man3/SSL_CTX_sess_set_get_cb.pod
@@ -57,7 +57,17 @@ and session caching is enabled (see
 L<SSL_CTX_set_session_cache_mode(3)>).
 The new_session_cb() is passed the B<ssl> connection and the ssl session
 B<sess>. If the callback returns B<0>, the session will be immediately
-removed again.
+removed again. Note that in TLSv1.3 sessions are established after the main
+handshake has completed. The server decides when to send the client the session
+information and this may occur some time after the end of the handshake (or not
+at all). This means that applications should expect the new_session_cb()
+function to be invoked during the handshake (for <= TLSv1.2) or after the
+handshake (for TLSv1.3). It is also possible in TLSv1.3 for multiple sessions to
+be established with a single connection. In these case the new_session_cb()
+function will be invoked multiple times.
+
+In TLSv1.3 it is recommended that each SSL_SESSION object is only used for
+resumption once.
 
 The remove_session_cb() is called, whenever the SSL engine removes a session
 from the internal cache. This happens when the session is removed because
diff --git a/doc/man3/SSL_SESSION_is_resumable.pod b/doc/man3/SSL_SESSION_is_resumable.pod
new file mode 100644
index 0000000000..d3fae9dd12
--- /dev/null
+++ b/doc/man3/SSL_SESSION_is_resumable.pod
@@ -0,0 +1,40 @@
+=pod
+
+=head1 NAME
+
+SSL_SESSION_is_resumable
+- determine whether an SSL_SESSION object can be used for resumption
+
+=head1 SYNOPSIS
+
+ #include <openssl/ssl.h>
+
+ const SSL_CIPHER *SSL_SESSION_is_resumable(const SSL_SESSSION *s);
+
+=head1 DESCRIPTION
+
+SSL_SESSION_is_resumable() determines whether an SSL_SESSION object can be used
+to resume a session or not. Returns 1 if it can or 0 if not. Note that
+attempting to resume with a non-resumable session will result in OpenSSL
+performing a full handshake.
+
+=head1 SEE ALSO
+
+L<ssl(7)>,
+L<SSL_get_session(3)>,
+L<SSL_CTX_sess_set_new_cb(3)>
+
+=head1 HISTORY
+
+SSL_SESSION_is_resumable() was first added to OpenSSL 1.1.1
+
+=head1 COPYRIGHT
+
+Copyright 2017 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the OpenSSL license (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
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