Updates to the early data documentation

SSL_CTX_get_max_early_data,

SSL_SESSION_get_max_early_data,

SSL_write_early,

SSL_write_early_finish,

SSL_read_early,

SSL_get_early_data_status

- functions for sending and receiving early data

 uint32_t SSL_SESSION_get_max_early_data(const SSL_SESSION *s);

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

 int SSL_write_early_finish(SSL *s);

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

connections. For this reason extreme care should be exercised when using early

data.

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.

A server or client can determine whether the full handshake has been completed

or not by calling L<SSL_is_init_finished(3)>.

[[TODO(TLS1.3): The server uses SSL_write_ex()/SSL_write() to send data to an

unauthenticated client. Should we create a separate function for this to avoid

accidents??]]

On the client side the function SSL_SESSION_get_max_early_data() can be used to

determine whether 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

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().

SSL_SESSION_get_max_early_data(). Once the initial SSL_write_early() 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() as required.

Once finished writing early data you must then call SSL_write_early_finish().

This sends a message to the server signalling the end of early data.

If SSL_write_early() fails you should call L<SSL_get_error(3)> to determine the

correct course of action, as for L<SSL_write_ex(3)>.

If either SSL_write_early() or SSL_write_early_finish() fail 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.

Following an SSL_write_early_finish() call the connection to the server still

needs to be completed. Complete the connection by calling a function such as

L<SSL_connect(3)> or L<SSL_do_handshake(3)>. Alternatively you can call a

"normal" read/write function such as L<SSL_read_ex(3)> or L<SSL_write_ex(3)>,

which will transparently complete the connection and read/write the requested

data.

Only clients may call SSL_write_early() or SSL_write_early_finish().

Only clients may call SSL_write_early().

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

=back

Once SSL_read_early() returns SSL_READ_EARLY_FINISH 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

"normal" read/write function such as L<SSL_read_ex(3)> or L<SSL_write_ex(3)>,

which will transparently complete the connection and read/write the requested

data. Note that it is an error to attempt to complete the connection before

SSL_read_early() has returned SSL_READ_EARLY_FINISH.

Once the initial SSL_write_early() call has completed successfully the client

may interleave calls to L<SSL_write_ex(3)> and L<SSL_write(3)> with calls to

SSL_read_early() as required. As noted above data sent via L<SSL_write_ex(3)> or

L<SSL_write(3)> in this way is sent to an unauthenticated client.

Servers must not call L<SSL_read_ex(3)> or L<SSL_read(3)> until SSL_read_early()

has returned with SSL_READ_EARLY_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() has

returned SSL_READ_EARLY_FINISH.

Only servers may call SSL_read_early().

Calls to SSL_read_early() may, in certain circumstances, complete the connection

immediately without further need to call a function such as L<SSL_accept(3)>.

Applications can test for this by calling L<SSL_is_init_finished(3)>.

Alternatively, applications may choose to call L<SSL_accept(3)> anway. 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 this is approximately 16k. A server may override

=head1 RETURN VALUES

SSL_write_early() and SSL_write_early_finish() return 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_write_early() 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() returns SSL_READ_EARLY_ERROR for failure,

SSL_READ_EARLY_SUCCESS for success with more data to read and