Cryptography and SSL/TLS Toolkit



SSL_set_max_early_data, SSL_CTX_set_max_early_data, SSL_get_max_early_data, SSL_CTX_get_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 - functions for sending and receiving early data


 #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);
 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);


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 only be sent if a session has previously been established with the server, and 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 and there are no guarantees that the same early data was not replayed across multiple connections. For this reason 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 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 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 SSL_write_ex(3) function, but with the following differences. See 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 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 SSL_write_ex(3), SSL_read_ex(3), SSL_connect(3), 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 SSL_read_ex(3) and SSL_read(3) with calls to SSL_write_early_data() as required.

If SSL_write_early_data() fails you should call SSL_get_error(3) to determine the correct course of action, as for 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 SSL_connect(3) or SSL_do_handshake(3). Alternatively you can call a standard write function such as 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. 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 SSL_write_ex(3), SSL_read_ex(3), SSL_accept(3), SSL_do_handshake(3), or other similar functions.

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

SSL_read_early_data() may return 3 possible values:


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


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


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.

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 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 SSL_read_ex(3), SSL_read(3), SSL_write_ex(3) or 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 SSL_accept(3) or SSL_do_handshake(3). Alternatively you can call a standard read function such as 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 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 SSL_is_init_finished(3). Alternatively, applications may choose to call 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 this is approximately 16k. A server may override this default value 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. 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.

In the event that the current maximum early data setting for the server is different to that originally specified in a session that a client is resuming with then the lower of the two values will apply.


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 as the first TCP packet, causing the early application data from calls to SSL_write_early_data() to be buffered by the OS and not sent until an ACK is received for the ClientHello packet. 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.


SSL_write_early_data() returns 1 for success or 0 for failure. In the event of a failure call 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 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.


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


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

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