doc/man3/SSL_read_early_data.pod - third_party/openssl - Git at Google

 =pod

 =head1 NAME

 SSL_set_max_early_data,
 SSL_CTX_set_max_early_data,
 SSL_get_max_early_data,
 SSL_CTX_get_max_early_data,
 SSL_set_recv_max_early_data,
 SSL_CTX_set_recv_max_early_data,
 SSL_get_recv_max_early_data,
 SSL_CTX_get_recv_max_early_data,
 SSL_SESSION_get_max_early_data,
 SSL_SESSION_set_max_early_data,
 SSL_write_early_data,
 SSL_read_early_data,
 SSL_get_early_data_status,
 SSL_allow_early_data_cb_fn,
 SSL_CTX_set_allow_early_data_cb,
 SSL_set_allow_early_data_cb
 - functions for sending and receiving early data

 =head1 SYNOPSIS

  #include <openssl/ssl.h>

  int SSL_CTX_set_max_early_data(SSL_CTX *ctx, uint32_t max_early_data);
  uint32_t SSL_CTX_get_max_early_data(const SSL_CTX *ctx);
  int SSL_set_max_early_data(SSL *s, uint32_t max_early_data);
  uint32_t SSL_get_max_early_data(const SSL *s);

  int SSL_CTX_set_recv_max_early_data(SSL_CTX *ctx, uint32_t recv_max_early_data);
  uint32_t SSL_CTX_get_recv_max_early_data(const SSL_CTX *ctx);
  int SSL_set_recv_max_early_data(SSL *s, uint32_t recv_max_early_data);
  uint32_t SSL_get_recv_max_early_data(const SSL *s);

  uint32_t SSL_SESSION_get_max_early_data(const SSL_SESSION *s);
  int SSL_SESSION_set_max_early_data(SSL_SESSION *s, uint32_t max_early_data);

  int SSL_write_early_data(SSL *s, const void *buf, size_t num, size_t *written);

  int SSL_read_early_data(SSL *s, void *buf, size_t num, size_t *readbytes);

  int SSL_get_early_data_status(const SSL *s);


  typedef int (*SSL_allow_early_data_cb_fn)(SSL *s, void *arg);

  void SSL_CTX_set_allow_early_data_cb(SSL_CTX *ctx,
                                       SSL_allow_early_data_cb_fn cb,
                                       void *arg);
  void SSL_set_allow_early_data_cb(SSL *s,
                                   SSL_allow_early_data_cb_fn cb,
                                   void *arg);

 =head1 DESCRIPTION

 These functions are used to send and receive early data where TLSv1.3 has been
 negotiated. Early data can be sent by the client immediately after its initial
 ClientHello without having to wait for the server to complete the handshake.
 Early data can be sent if a session has previously been established with the
 server or when establishing a new session using an out-of-band PSK, and only
 when the server is known to support it. Additionally these functions can be used
 to send data from the server to the client when the client has not yet completed
 the authentication stage of the handshake.

 Early data has weaker security properties than other data sent over an SSL/TLS
 connection. In particular the data does not have forward secrecy. There are also
 additional considerations around replay attacks (see L</REPLAY PROTECTION>
 below). For these reasons extreme care should be exercised when using early
 data. For specific details, consult the TLS 1.3 specification.

 When a server receives early data it may opt to immediately respond by sending
 application data back to the client. Data sent by the server at this stage is
 done before the full handshake has been completed. Specifically the client's
 authentication messages have not yet been received, i.e. the client is
 unauthenticated at this point and care should be taken when using this
 capability.

 A server or client can determine whether the full handshake has been completed
 or not by calling L<SSL_is_init_finished(3)>.

 On the client side, the function SSL_SESSION_get_max_early_data() can be used to
 determine if a session established with a server can be used to send early data.
 If the session cannot be used then this function will return 0. Otherwise it
 will return the maximum number of early data bytes that can be sent.

 The function SSL_SESSION_set_max_early_data() sets the maximum number of early
 data bytes that can be sent for a session. This would typically be used when
 creating a PSK session file (see L<SSL_CTX_set_psk_use_session_callback(3)>). If
 using a ticket based PSK then this is set automatically to the value provided by
 the server.

 A client uses the function SSL_write_early_data() to send early data. This
 function is similar to the L<SSL_write_ex(3)> function, but with the following
 differences. See L<SSL_write_ex(3)> for information on how to write bytes to
 the underlying connection, and how to handle any errors that may arise. This
 page describes the differences between SSL_write_early_data() and
 L<SSL_write_ex(3)>.

 When called by a client, SSL_write_early_data() must be the first IO function
 called on a new connection, i.e. it must occur before any calls to
 L<SSL_write_ex(3)>, L<SSL_read_ex(3)>, L<SSL_connect(3)>, L<SSL_do_handshake(3)>
 or other similar functions. It may be called multiple times to stream data to
 the server, but the total number of bytes written must not exceed the value
 returned from SSL_SESSION_get_max_early_data(). Once the initial
 SSL_write_early_data() call has completed successfully the client may interleave
 calls to L<SSL_read_ex(3)> and L<SSL_read(3)> with calls to
 SSL_write_early_data() as required.

 If SSL_write_early_data() fails you should call L<SSL_get_error(3)> to determine
 the correct course of action, as for L<SSL_write_ex(3)>.

 When the client no longer wishes to send any more early data then it should
 complete the handshake by calling a function such as L<SSL_connect(3)> or
 L<SSL_do_handshake(3)>. Alternatively you can call a standard write function
 such as L<SSL_write_ex(3)>, which will transparently complete the connection and
 write the requested data.

 A server may choose to ignore early data that has been sent to it. Once the
 connection has been completed you can determine whether the server accepted or
 rejected the early data by calling SSL_get_early_data_status(). This will return
 SSL_EARLY_DATA_ACCEPTED if the data was accepted, SSL_EARLY_DATA_REJECTED if it
 was rejected or SSL_EARLY_DATA_NOT_SENT if no early data was sent. This function
 may be called by either the client or the server.

 A server uses the SSL_read_early_data() function to receive early data on a
 connection for which early data has been enabled using
 SSL_CTX_set_max_early_data() or SSL_set_max_early_data(). As for
 SSL_write_early_data(), this must be the first IO function
 called on a connection, i.e. it must occur before any calls to
 L<SSL_write_ex(3)>, L<SSL_read_ex(3)>, L<SSL_accept(3)>, L<SSL_do_handshake(3)>,
 or other similar functions.

 SSL_read_early_data() is similar to L<SSL_read_ex(3)> with the following
 differences. Refer to L<SSL_read_ex(3)> for full details.

 SSL_read_early_data() may return 3 possible values:

 =over 4

 =item SSL_READ_EARLY_DATA_ERROR

 This indicates an IO or some other error occurred. This should be treated in the
 same way as a 0 return value from L<SSL_read_ex(3)>.

 =item SSL_READ_EARLY_DATA_SUCCESS

 This indicates that early data was successfully read. This should be treated in
 the same way as a 1 return value from L<SSL_read_ex(3)>. You should continue to
 call SSL_read_early_data() to read more data.

 =item SSL_READ_EARLY_DATA_FINISH

 This indicates that no more early data can be read. It may be returned on the
 first call to SSL_read_early_data() if the client has not sent any early data,
 or if the early data was rejected.

 =back

 Once the initial SSL_read_early_data() call has completed successfully (i.e. it
 has returned SSL_READ_EARLY_DATA_SUCCESS or SSL_READ_EARLY_DATA_FINISH) then the
 server may choose to write data immediately to the unauthenticated client using
 SSL_write_early_data(). If SSL_read_early_data() returned
 SSL_READ_EARLY_DATA_FINISH then in some situations (e.g. if the client only
 supports TLSv1.2) the handshake may have already been completed and calls
 to SSL_write_early_data() are not allowed. Call L<SSL_is_init_finished(3)> to
 determine whether the handshake has completed or not. If the handshake is still
 in progress then the server may interleave calls to SSL_write_early_data() with
 calls to SSL_read_early_data() as required.

 Servers must not call L<SSL_read_ex(3)>, L<SSL_read(3)>, L<SSL_write_ex(3)> or
 L<SSL_write(3)>  until SSL_read_early_data() has returned with
 SSL_READ_EARLY_DATA_FINISH. Once it has done so the connection to the client
 still needs to be completed. Complete the connection by calling a function such
 as L<SSL_accept(3)> or L<SSL_do_handshake(3)>. Alternatively you can call a
 standard read function such as L<SSL_read_ex(3)>, which will transparently
 complete the connection and read the requested data. Note that it is an error to
 attempt to complete the connection before SSL_read_early_data() has returned
 SSL_READ_EARLY_DATA_FINISH.

 Only servers may call SSL_read_early_data().

 Calls to SSL_read_early_data() may, in certain circumstances, complete the
 connection immediately without further need to call a function such as
 L<SSL_accept(3)>. This can happen if the client is using a protocol version less
 than TLSv1.3. Applications can test for this by calling
 L<SSL_is_init_finished(3)>. Alternatively, applications may choose to call
 L<SSL_accept(3)> anyway. Such a call will successfully return immediately with no
 further action taken.

 When a session is created between a server and a client the server will specify
 the maximum amount of any early data that it will accept on any future
 connection attempt. By default the server does not accept early data; a
 server may indicate support for early data by calling
 SSL_CTX_set_max_early_data() or
 SSL_set_max_early_data() to set it for the whole SSL_CTX or an individual SSL
 object respectively. The B<max_early_data> parameter specifies the maximum
 amount of early data in bytes that is permitted to be sent on a single
 connection. Similarly the SSL_CTX_get_max_early_data() and
 SSL_get_max_early_data() functions can be used to obtain the current maximum
 early data settings for the SSL_CTX and SSL objects respectively. Generally a
 server application will either use both of SSL_read_early_data() and
 SSL_CTX_set_max_early_data() (or SSL_set_max_early_data()), or neither of them,
 since there is no practical benefit from using only one of them. If the maximum
 early data setting for a server is nonzero then replay protection is
 automatically enabled (see L</REPLAY PROTECTION> below).

 If the server rejects the early data sent by a client then it will skip over
 the data that is sent. The maximum amount of received early data that is skipped
 is controlled by the recv_max_early_data setting. If a client sends more than
 this then the connection will abort. This value can be set by calling
 SSL_CTX_set_recv_max_early_data() or SSL_set_recv_max_early_data(). The current
 value for this setting can be obtained by calling
 SSL_CTX_get_recv_max_early_data() or SSL_get_recv_max_early_data(). The default
 value for this setting is 16,384 bytes.

 The recv_max_early_data value also has an impact on early data that is accepted.
 The amount of data that is accepted will always be the lower of the
 max_early_data for the session and the recv_max_early_data setting for the
 server. If a client sends more data than this then the connection will abort.

 The configured value for max_early_data on a server may change over time as
 required. However, clients may have tickets containing the previously configured
 max_early_data value. The recv_max_early_data should always be equal to or
 higher than any recently configured max_early_data value in order to avoid
 aborted connections. The recv_max_early_data should never be set to less than
 the current configured max_early_data value.

 Some server applications may wish to have more control over whether early data
 is accepted or not, for example to mitigate replay risks (see L</REPLAY PROTECTION>
 below) or to decline early_data when the server is heavily loaded. The functions
 SSL_CTX_set_allow_early_data_cb() and SSL_set_allow_early_data_cb() set a
 callback which is called at a point in the handshake immediately before a
 decision is made to accept or reject early data. The callback is provided with a
 pointer to the user data argument that was provided when the callback was first
 set. Returning 1 from the callback will allow early data and returning 0 will
 reject it. Note that the OpenSSL library may reject early data for other reasons
 in which case this callback will not get called. Notably, the built-in replay
 protection feature will still be used even if a callback is present unless it
 has been explicitly disabled using the SSL_OP_NO_ANTI_REPLAY option. See
 L</REPLAY PROTECTION> below.

 =head1 NOTES

 The whole purpose of early data is to enable a client to start sending data to
 the server before a full round trip of network traffic has occurred. Application
 developers should ensure they consider optimisation of the underlying TCP socket
 to obtain a performant solution. For example Nagle's algorithm is commonly used
 by operating systems in an attempt to avoid lots of small TCP packets. In many
 scenarios this is beneficial for performance, but it does not work well with the
 early data solution as implemented in OpenSSL. In Nagle's algorithm the OS will
 buffer outgoing TCP data if a TCP packet has already been sent which we have not
 yet received an ACK for from the peer. The buffered data will only be
 transmitted if enough data to fill an entire TCP packet is accumulated, or if
 the ACK is received from the peer. The initial ClientHello will be sent in the
 first TCP packet along with any data from the first call to
 SSL_write_early_data(). If the amount of data written will exceed the size of a
 single TCP packet, or if there are more calls to SSL_write_early_data() then
 that additional data will be sent in subsequent TCP packets which will be
 buffered by the OS and not sent until an ACK is received for the first packet
 containing the ClientHello. This means the early data is not actually
 sent until a complete round trip with the server has occurred which defeats the
 objective of early data.

 In many operating systems the TCP_NODELAY socket option is available to disable
 Nagle's algorithm. If an application opts to disable Nagle's algorithm
 consideration should be given to turning it back on again after the handshake is
 complete if appropriate.

 In rare circumstances, it may be possible for a client to have a session that
 reports a max early data value greater than 0, but where the server does not
 support this. For example, this can occur if a server has had its configuration
 changed to accept a lower max early data value such as by calling
 SSL_CTX_set_recv_max_early_data(). Another example is if a server used to
 support TLSv1.3 but was later downgraded to TLSv1.2. Sending early data to such
 a server will cause the connection to abort. Clients that encounter an aborted
 connection while sending early data may want to retry the connection without
 sending early data as this does not happen automatically. A client will have to
 establish a new transport layer connection to the server and attempt the SSL/TLS
 connection again but without sending early data. Note that it is inadvisable to
 retry with a lower maximum protocol version.

 =head1 REPLAY PROTECTION

 When early data is in use the TLS protocol provides no security guarantees that
 the same early data was not replayed across multiple connections. As a
 mitigation for this issue OpenSSL automatically enables replay protection if the
 server is configured with a nonzero max early data value. With replay
 protection enabled sessions are forced to be single use only. If a client
 attempts to reuse a session ticket more than once, then the second and
 subsequent attempts will fall back to a full handshake (and any early data that
 was submitted will be ignored). Note that single use tickets are enforced even
 if a client does not send any early data.

 The replay protection mechanism relies on the internal OpenSSL server session
 cache (see L<SSL_CTX_set_session_cache_mode(3)>). When replay protection is
 being used the server will operate as if the SSL_OP_NO_TICKET option had been
 selected (see L<SSL_CTX_set_options(3)>). Sessions will be added to the cache
 whenever a session ticket is issued. When a client attempts to resume the
 session, OpenSSL will check for its presence in the internal cache. If it exists
 then the resumption is allowed and the session is removed from the cache. If it
 does not exist then the resumption is not allowed and a full handshake will
 occur.

 Note that some applications may maintain an external cache of sessions (see
 L<SSL_CTX_sess_set_new_cb(3)> and similar functions). It is the application's
 responsibility to ensure that any sessions in the external cache are also
 populated in the internal cache and that once removed from the internal cache
 they are similarly removed from the external cache. Failing to do this could
 result in an application becoming vulnerable to replay attacks. Note that
 OpenSSL will lock the internal cache while a session is removed but that lock is
 not held when the remove session callback (see L<SSL_CTX_sess_set_remove_cb(3)>)
 is called. This could result in a small amount of time where the session has
 been removed from the internal cache but is still available in the external
 cache. Applications should be designed with this in mind in order to minimise
 the possibility of replay attacks.

 The OpenSSL replay protection does not apply to external Pre Shared Keys (PSKs)
 (e.g. see SSL_CTX_set_psk_find_session_callback(3)). Therefore, extreme caution
 should be applied when combining external PSKs with early data.

 Some applications may mitigate the replay risks in other ways. For those
 applications it is possible to turn off the built-in replay protection feature
 using the B<SSL_OP_NO_ANTI_REPLAY> option. See L<SSL_CTX_set_options(3)> for
 details. Applications can also set a callback to make decisions about accepting
 early data or not. See SSL_CTX_set_allow_early_data_cb() above for details.

 =head1 RETURN VALUES

 SSL_write_early_data() returns 1 for success or 0 for failure. In the event of a
 failure call L<SSL_get_error(3)> to determine the correct course of action.

 SSL_read_early_data() returns SSL_READ_EARLY_DATA_ERROR for failure,
 SSL_READ_EARLY_DATA_SUCCESS for success with more data to read and
 SSL_READ_EARLY_DATA_FINISH for success with no more to data be read. In the
 event of a failure call L<SSL_get_error(3)> to determine the correct course of
 action.

 SSL_get_max_early_data(), SSL_CTX_get_max_early_data() and
 SSL_SESSION_get_max_early_data() return the maximum number of early data bytes
 that may be sent.

 SSL_set_max_early_data(), SSL_CTX_set_max_early_data() and
 SSL_SESSION_set_max_early_data() return 1 for success or 0 for failure.

 SSL_get_early_data_status() returns SSL_EARLY_DATA_ACCEPTED if early data was
 accepted by the server, SSL_EARLY_DATA_REJECTED if early data was rejected by
 the server, or SSL_EARLY_DATA_NOT_SENT if no early data was sent.

 =head1 SEE ALSO

 L<SSL_get_error(3)>,
 L<SSL_write_ex(3)>,
 L<SSL_read_ex(3)>,
 L<SSL_connect(3)>,
 L<SSL_accept(3)>,
 L<SSL_do_handshake(3)>,
 L<SSL_CTX_set_psk_use_session_callback(3)>,
 L<ssl(7)>

 =head1 HISTORY

 All of the functions described above were added in OpenSSL 1.1.1.

 =head1 COPYRIGHT

 Copyright 2017-2020 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
	=pod

	=head1 NAME

	SSL_set_max_early_data,
	SSL_CTX_set_max_early_data,
	SSL_get_max_early_data,
	SSL_CTX_get_max_early_data,
	SSL_set_recv_max_early_data,
	SSL_CTX_set_recv_max_early_data,
	SSL_get_recv_max_early_data,
	SSL_CTX_get_recv_max_early_data,
	SSL_SESSION_get_max_early_data,
	SSL_SESSION_set_max_early_data,
	SSL_write_early_data,
	SSL_read_early_data,
	SSL_get_early_data_status,
	SSL_allow_early_data_cb_fn,
	SSL_CTX_set_allow_early_data_cb,
	SSL_set_allow_early_data_cb
	- functions for sending and receiving early data

	=head1 SYNOPSIS

	#include <openssl/ssl.h>

	int SSL_CTX_set_max_early_data(SSL_CTX *ctx, uint32_t max_early_data);
	uint32_t SSL_CTX_get_max_early_data(const SSL_CTX *ctx);
	int SSL_set_max_early_data(SSL *s, uint32_t max_early_data);
	uint32_t SSL_get_max_early_data(const SSL *s);

	int SSL_CTX_set_recv_max_early_data(SSL_CTX *ctx, uint32_t recv_max_early_data);
	uint32_t SSL_CTX_get_recv_max_early_data(const SSL_CTX *ctx);
	int SSL_set_recv_max_early_data(SSL *s, uint32_t recv_max_early_data);
	uint32_t SSL_get_recv_max_early_data(const SSL *s);

	uint32_t SSL_SESSION_get_max_early_data(const SSL_SESSION *s);
	int SSL_SESSION_set_max_early_data(SSL_SESSION *s, uint32_t max_early_data);

	int SSL_write_early_data(SSL s, const void buf, size_t num, size_t *written);

	int SSL_read_early_data(SSL s, void buf, size_t num, size_t *readbytes);

	int SSL_get_early_data_status(const SSL *s);


	typedef int (SSL_allow_early_data_cb_fn)(SSL s, void *arg);

	void SSL_CTX_set_allow_early_data_cb(SSL_CTX *ctx,
	SSL_allow_early_data_cb_fn cb,
	void *arg);
	void SSL_set_allow_early_data_cb(SSL *s,
	SSL_allow_early_data_cb_fn cb,
	void *arg);

	=head1 DESCRIPTION

	These functions are used to send and receive early data where TLSv1.3 has been
	negotiated. Early data can be sent by the client immediately after its initial
	ClientHello without having to wait for the server to complete the handshake.
	Early data can be sent if a session has previously been established with the
	server or when establishing a new session using an out-of-band PSK, and only
	when the server is known to support it. Additionally these functions can be used
	to send data from the server to the client when the client has not yet completed
	the authentication stage of the handshake.

	Early data has weaker security properties than other data sent over an SSL/TLS
	connection. In particular the data does not have forward secrecy. There are also
	additional considerations around replay attacks (see L</REPLAY PROTECTION>
	below). For these reasons extreme care should be exercised when using early
	data. For specific details, consult the TLS 1.3 specification.

	When a server receives early data it may opt to immediately respond by sending
	application data back to the client. Data sent by the server at this stage is
	done before the full handshake has been completed. Specifically the client's
	authentication messages have not yet been received, i.e. the client is
	unauthenticated at this point and care should be taken when using this
	capability.

	A server or client can determine whether the full handshake has been completed
	or not by calling L<SSL_is_init_finished(3)>.

	On the client side, the function SSL_SESSION_get_max_early_data() can be used to
	determine if a session established with a server can be used to send early data.
	If the session cannot be used then this function will return 0. Otherwise it
	will return the maximum number of early data bytes that can be sent.

	The function SSL_SESSION_set_max_early_data() sets the maximum number of early
	data bytes that can be sent for a session. This would typically be used when
	creating a PSK session file (see L<SSL_CTX_set_psk_use_session_callback(3)>). If
	using a ticket based PSK then this is set automatically to the value provided by
	the server.

	A client uses the function SSL_write_early_data() to send early data. This
	function is similar to the L<SSL_write_ex(3)> function, but with the following
	differences. See L<SSL_write_ex(3)> for information on how to write bytes to
	the underlying connection, and how to handle any errors that may arise. This
	page describes the differences between SSL_write_early_data() and
	L<SSL_write_ex(3)>.

	When called by a client, SSL_write_early_data() must be the first IO function
	called on a new connection, i.e. it must occur before any calls to
	L<SSL_write_ex(3)>, L<SSL_read_ex(3)>, L<SSL_connect(3)>, L<SSL_do_handshake(3)>
	or other similar functions. It may be called multiple times to stream data to
	the server, but the total number of bytes written must not exceed the value
	returned from SSL_SESSION_get_max_early_data(). Once the initial
	SSL_write_early_data() call has completed successfully the client may interleave
	calls to L<SSL_read_ex(3)> and L<SSL_read(3)> with calls to
	SSL_write_early_data() as required.

	If SSL_write_early_data() fails you should call L<SSL_get_error(3)> to determine
	the correct course of action, as for L<SSL_write_ex(3)>.

	When the client no longer wishes to send any more early data then it should
	complete the handshake by calling a function such as L<SSL_connect(3)> or
	L<SSL_do_handshake(3)>. Alternatively you can call a standard write function
	such as L<SSL_write_ex(3)>, which will transparently complete the connection and
	write the requested data.

	A server may choose to ignore early data that has been sent to it. Once the
	connection has been completed you can determine whether the server accepted or
	rejected the early data by calling SSL_get_early_data_status(). This will return
	SSL_EARLY_DATA_ACCEPTED if the data was accepted, SSL_EARLY_DATA_REJECTED if it
	was rejected or SSL_EARLY_DATA_NOT_SENT if no early data was sent. This function
	may be called by either the client or the server.

	A server uses the SSL_read_early_data() function to receive early data on a
	connection for which early data has been enabled using
	SSL_CTX_set_max_early_data() or SSL_set_max_early_data(). As for
	SSL_write_early_data(), this must be the first IO function
	called on a connection, i.e. it must occur before any calls to
	L<SSL_write_ex(3)>, L<SSL_read_ex(3)>, L<SSL_accept(3)>, L<SSL_do_handshake(3)>,
	or other similar functions.

	SSL_read_early_data() is similar to L<SSL_read_ex(3)> with the following
	differences. Refer to L<SSL_read_ex(3)> for full details.

	SSL_read_early_data() may return 3 possible values:

	=over 4

	=item SSL_READ_EARLY_DATA_ERROR

	This indicates an IO or some other error occurred. This should be treated in the
	same way as a 0 return value from L<SSL_read_ex(3)>.

	=item SSL_READ_EARLY_DATA_SUCCESS

	This indicates that early data was successfully read. This should be treated in
	the same way as a 1 return value from L<SSL_read_ex(3)>. You should continue to
	call SSL_read_early_data() to read more data.

	=item SSL_READ_EARLY_DATA_FINISH

	This indicates that no more early data can be read. It may be returned on the
	first call to SSL_read_early_data() if the client has not sent any early data,
	or if the early data was rejected.

	=back

	Once the initial SSL_read_early_data() call has completed successfully (i.e. it
	has returned SSL_READ_EARLY_DATA_SUCCESS or SSL_READ_EARLY_DATA_FINISH) then the
	server may choose to write data immediately to the unauthenticated client using
	SSL_write_early_data(). If SSL_read_early_data() returned
	SSL_READ_EARLY_DATA_FINISH then in some situations (e.g. if the client only
	supports TLSv1.2) the handshake may have already been completed and calls
	to SSL_write_early_data() are not allowed. Call L<SSL_is_init_finished(3)> to
	determine whether the handshake has completed or not. If the handshake is still
	in progress then the server may interleave calls to SSL_write_early_data() with
	calls to SSL_read_early_data() as required.

	Servers must not call L<SSL_read_ex(3)>, L<SSL_read(3)>, L<SSL_write_ex(3)> or
	L<SSL_write(3)> until SSL_read_early_data() has returned with
	SSL_READ_EARLY_DATA_FINISH. Once it has done so the connection to the client
	still needs to be completed. Complete the connection by calling a function such
	as L<SSL_accept(3)> or L<SSL_do_handshake(3)>. Alternatively you can call a
	standard read function such as L<SSL_read_ex(3)>, which will transparently
	complete the connection and read the requested data. Note that it is an error to
	attempt to complete the connection before SSL_read_early_data() has returned
	SSL_READ_EARLY_DATA_FINISH.

	Only servers may call SSL_read_early_data().

	Calls to SSL_read_early_data() may, in certain circumstances, complete the
	connection immediately without further need to call a function such as
	L<SSL_accept(3)>. This can happen if the client is using a protocol version less
	than TLSv1.3. Applications can test for this by calling
	L<SSL_is_init_finished(3)>. Alternatively, applications may choose to call
	L<SSL_accept(3)> anyway. Such a call will successfully return immediately with no
	further action taken.

	When a session is created between a server and a client the server will specify
	the maximum amount of any early data that it will accept on any future
	connection attempt. By default the server does not accept early data; a
	server may indicate support for early data by calling
	SSL_CTX_set_max_early_data() or
	SSL_set_max_early_data() to set it for the whole SSL_CTX or an individual SSL
	object respectively. The B<max_early_data> parameter specifies the maximum
	amount of early data in bytes that is permitted to be sent on a single
	connection. Similarly the SSL_CTX_get_max_early_data() and
	SSL_get_max_early_data() functions can be used to obtain the current maximum
	early data settings for the SSL_CTX and SSL objects respectively. Generally a
	server application will either use both of SSL_read_early_data() and
	SSL_CTX_set_max_early_data() (or SSL_set_max_early_data()), or neither of them,
	since there is no practical benefit from using only one of them. If the maximum
	early data setting for a server is nonzero then replay protection is
	automatically enabled (see L</REPLAY PROTECTION> below).

	If the server rejects the early data sent by a client then it will skip over
	the data that is sent. The maximum amount of received early data that is skipped
	is controlled by the recv_max_early_data setting. If a client sends more than
	this then the connection will abort. This value can be set by calling
	SSL_CTX_set_recv_max_early_data() or SSL_set_recv_max_early_data(). The current
	value for this setting can be obtained by calling
	SSL_CTX_get_recv_max_early_data() or SSL_get_recv_max_early_data(). The default
	value for this setting is 16,384 bytes.

	The recv_max_early_data value also has an impact on early data that is accepted.
	The amount of data that is accepted will always be the lower of the
	max_early_data for the session and the recv_max_early_data setting for the
	server. If a client sends more data than this then the connection will abort.

	The configured value for max_early_data on a server may change over time as
	required. However, clients may have tickets containing the previously configured
	max_early_data value. The recv_max_early_data should always be equal to or
	higher than any recently configured max_early_data value in order to avoid
	aborted connections. The recv_max_early_data should never be set to less than
	the current configured max_early_data value.

	Some server applications may wish to have more control over whether early data
	is accepted or not, for example to mitigate replay risks (see L</REPLAY PROTECTION>
	below) or to decline early_data when the server is heavily loaded. The functions
	SSL_CTX_set_allow_early_data_cb() and SSL_set_allow_early_data_cb() set a
	callback which is called at a point in the handshake immediately before a
	decision is made to accept or reject early data. The callback is provided with a
	pointer to the user data argument that was provided when the callback was first
	set. Returning 1 from the callback will allow early data and returning 0 will
	reject it. Note that the OpenSSL library may reject early data for other reasons
	in which case this callback will not get called. Notably, the built-in replay
	protection feature will still be used even if a callback is present unless it
	has been explicitly disabled using the SSL_OP_NO_ANTI_REPLAY option. See
	L</REPLAY PROTECTION> below.

	=head1 NOTES

	The whole purpose of early data is to enable a client to start sending data to
	the server before a full round trip of network traffic has occurred. Application
	developers should ensure they consider optimisation of the underlying TCP socket
	to obtain a performant solution. For example Nagle's algorithm is commonly used
	by operating systems in an attempt to avoid lots of small TCP packets. In many
	scenarios this is beneficial for performance, but it does not work well with the
	early data solution as implemented in OpenSSL. In Nagle's algorithm the OS will
	buffer outgoing TCP data if a TCP packet has already been sent which we have not
	yet received an ACK for from the peer. The buffered data will only be
	transmitted if enough data to fill an entire TCP packet is accumulated, or if
	the ACK is received from the peer. The initial ClientHello will be sent in the
	first TCP packet along with any data from the first call to
	SSL_write_early_data(). If the amount of data written will exceed the size of a
	single TCP packet, or if there are more calls to SSL_write_early_data() then
	that additional data will be sent in subsequent TCP packets which will be
	buffered by the OS and not sent until an ACK is received for the first packet
	containing the ClientHello. This means the early data is not actually
	sent until a complete round trip with the server has occurred which defeats the
	objective of early data.

	In many operating systems the TCP_NODELAY socket option is available to disable
	Nagle's algorithm. If an application opts to disable Nagle's algorithm
	consideration should be given to turning it back on again after the handshake is
	complete if appropriate.

	In rare circumstances, it may be possible for a client to have a session that
	reports a max early data value greater than 0, but where the server does not
	support this. For example, this can occur if a server has had its configuration
	changed to accept a lower max early data value such as by calling
	SSL_CTX_set_recv_max_early_data(). Another example is if a server used to
	support TLSv1.3 but was later downgraded to TLSv1.2. Sending early data to such
	a server will cause the connection to abort. Clients that encounter an aborted
	connection while sending early data may want to retry the connection without
	sending early data as this does not happen automatically. A client will have to
	establish a new transport layer connection to the server and attempt the SSL/TLS
	connection again but without sending early data. Note that it is inadvisable to
	retry with a lower maximum protocol version.

	=head1 REPLAY PROTECTION

	When early data is in use the TLS protocol provides no security guarantees that
	the same early data was not replayed across multiple connections. As a
	mitigation for this issue OpenSSL automatically enables replay protection if the
	server is configured with a nonzero max early data value. With replay
	protection enabled sessions are forced to be single use only. If a client
	attempts to reuse a session ticket more than once, then the second and
	subsequent attempts will fall back to a full handshake (and any early data that
	was submitted will be ignored). Note that single use tickets are enforced even
	if a client does not send any early data.

	The replay protection mechanism relies on the internal OpenSSL server session
	cache (see L<SSL_CTX_set_session_cache_mode(3)>). When replay protection is
	being used the server will operate as if the SSL_OP_NO_TICKET option had been
	selected (see L<SSL_CTX_set_options(3)>). Sessions will be added to the cache
	whenever a session ticket is issued. When a client attempts to resume the
	session, OpenSSL will check for its presence in the internal cache. If it exists
	then the resumption is allowed and the session is removed from the cache. If it
	does not exist then the resumption is not allowed and a full handshake will
	occur.

	Note that some applications may maintain an external cache of sessions (see
	L<SSL_CTX_sess_set_new_cb(3)> and similar functions). It is the application's
	responsibility to ensure that any sessions in the external cache are also
	populated in the internal cache and that once removed from the internal cache
	they are similarly removed from the external cache. Failing to do this could
	result in an application becoming vulnerable to replay attacks. Note that
	OpenSSL will lock the internal cache while a session is removed but that lock is
	not held when the remove session callback (see L<SSL_CTX_sess_set_remove_cb(3)>)
	is called. This could result in a small amount of time where the session has
	been removed from the internal cache but is still available in the external
	cache. Applications should be designed with this in mind in order to minimise
	the possibility of replay attacks.

	The OpenSSL replay protection does not apply to external Pre Shared Keys (PSKs)
	(e.g. see SSL_CTX_set_psk_find_session_callback(3)). Therefore, extreme caution
	should be applied when combining external PSKs with early data.

	Some applications may mitigate the replay risks in other ways. For those
	applications it is possible to turn off the built-in replay protection feature
	using the B<SSL_OP_NO_ANTI_REPLAY> option. See L<SSL_CTX_set_options(3)> for
	details. Applications can also set a callback to make decisions about accepting
	early data or not. See SSL_CTX_set_allow_early_data_cb() above for details.

	=head1 RETURN VALUES

	SSL_write_early_data() returns 1 for success or 0 for failure. In the event of a
	failure call L<SSL_get_error(3)> to determine the correct course of action.

	SSL_read_early_data() returns SSL_READ_EARLY_DATA_ERROR for failure,
	SSL_READ_EARLY_DATA_SUCCESS for success with more data to read and
	SSL_READ_EARLY_DATA_FINISH for success with no more to data be read. In the
	event of a failure call L<SSL_get_error(3)> to determine the correct course of
	action.

	SSL_get_max_early_data(), SSL_CTX_get_max_early_data() and
	SSL_SESSION_get_max_early_data() return the maximum number of early data bytes
	that may be sent.

	SSL_set_max_early_data(), SSL_CTX_set_max_early_data() and
	SSL_SESSION_set_max_early_data() return 1 for success or 0 for failure.

	SSL_get_early_data_status() returns SSL_EARLY_DATA_ACCEPTED if early data was
	accepted by the server, SSL_EARLY_DATA_REJECTED if early data was rejected by
	the server, or SSL_EARLY_DATA_NOT_SENT if no early data was sent.

	=head1 SEE ALSO

	L<SSL_get_error(3)>,
	L<SSL_write_ex(3)>,
	L<SSL_read_ex(3)>,
	L<SSL_connect(3)>,
	L<SSL_accept(3)>,
	L<SSL_do_handshake(3)>,
	L<SSL_CTX_set_psk_use_session_callback(3)>,
	L<ssl(7)>

	=head1 HISTORY

	All of the functions described above were added in OpenSSL 1.1.1.

	=head1 COPYRIGHT

	Copyright 2017-2020 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