NAME

       SSL_CTX_set_max_send_fragment, SSL_set_max_send_fragment, SSL_CTX_set_split_send_fragment,
       SSL_set_split_send_fragment, SSL_CTX_set_max_pipelines, SSL_set_max_pipelines,
       SSL_CTX_set_default_read_buffer_len, SSL_set_default_read_buffer_len,
       SSL_CTX_set_tlsext_max_fragment_length, SSL_set_tlsext_max_fragment_length,
       SSL_SESSION_get_max_fragment_length - Control fragment size settings and pipelining
       operations

SYNOPSIS

        #include <openssl/ssl.h>

        long SSL_CTX_set_max_send_fragment(SSL_CTX *ctx, long);
        long SSL_set_max_send_fragment(SSL *ssl, long m);

        long SSL_CTX_set_max_pipelines(SSL_CTX *ctx, long m);
        long SSL_set_max_pipelines(SSL_CTX *ssl, long m);

        long SSL_CTX_set_split_send_fragment(SSL_CTX *ctx, long m);
        long SSL_set_split_send_fragment(SSL *ssl, long m);

        void SSL_CTX_set_default_read_buffer_len(SSL_CTX *ctx, size_t len);
        void SSL_set_default_read_buffer_len(SSL *s, size_t len);

        int SSL_CTX_set_tlsext_max_fragment_length(SSL_CTX *ctx, uint8_t mode);
        int SSL_set_tlsext_max_fragment_length(SSL *ssl, uint8_t mode);
        uint8_t SSL_SESSION_get_max_fragment_length(const SSL_SESSION *session);

DESCRIPTION

       Some engines are able to process multiple simultaneous crypto operations. This capability
       could be utilised to parallelise the processing of a single connection. For example a
       single write can be split into multiple records and each one encrypted independently and
       in parallel. Note: this will only work in TLS1.1+. There is no support in SSLv3, TLSv1.0
       or DTLS (any version). This capability is known as "pipelining" within OpenSSL.

       In order to benefit from the pipelining capability. You need to have an engine that
       provides ciphers that support this. The OpenSSL "dasync" engine provides AES128-SHA based
       ciphers that have this capability. However, these are for development and test purposes
       only.

       SSL_CTX_set_max_send_fragment() and SSL_set_max_send_fragment() set the max_send_fragment
       parameter for SSL_CTX and SSL objects respectively. This value restricts the amount of
       plaintext bytes that will be sent in any one SSL/TLS record. By default its value is
       SSL3_RT_MAX_PLAIN_LENGTH (16384). These functions will only accept a value in the range
       512 - SSL3_RT_MAX_PLAIN_LENGTH.

       SSL_CTX_set_max_pipelines() and SSL_set_max_pipelines() set the maximum number of
       pipelines that will be used at any one time. This value applies to both "read" pipelining
       and "write" pipelining. By default only one pipeline will be used (i.e. normal non-
       parallel operation). The number of pipelines set must be in the range 1 -
       SSL_MAX_PIPELINES (32). Setting this to a value > 1 will also automatically turn on
       "read_ahead" (see SSL_CTX_set_read_ahead(3)). This is explained further below. OpenSSL
       will only ever use more than one pipeline if a cipher suite is negotiated that uses a
       pipeline capable cipher provided by an engine.

       Pipelining operates slightly differently for reading encrypted data compared to writing
       encrypted data. SSL_CTX_set_split_send_fragment() and SSL_set_split_send_fragment() define
       how data is split up into pipelines when writing encrypted data. The number of pipelines
       used will be determined by the amount of data provided to the SSL_write_ex() or
       SSL_write() call divided by split_send_fragment.

       For example if split_send_fragment is set to 2000 and max_pipelines is 4 then:

       SSL_write/SSL_write_ex called with 0-2000 bytes == 1 pipeline used

       SSL_write/SSL_write_ex called with 2001-4000 bytes == 2 pipelines used

       SSL_write/SSL_write_ex called with 4001-6000 bytes == 3 pipelines used

       SSL_write/SSL_write_ex called with 6001+ bytes == 4 pipelines used

       split_send_fragment must always be less than or equal to max_send_fragment. By default it
       is set to be equal to max_send_fragment.  This will mean that the same number of records
       will always be created as would have been created in the non-parallel case, although the
       data will be apportioned differently. In the parallel case data will be spread equally
       between the pipelines.

       Read pipelining is controlled in a slightly different way than with write pipelining.
       While reading we are constrained by the number of records that the peer (and the network)
       can provide to us in one go. The more records we can get in one go the more opportunity we
       have to parallelise the processing. As noted above when setting max_pipelines to a value
       greater than one, read_ahead is automatically set. The read_ahead parameter causes OpenSSL
       to attempt to read as much data into the read buffer as the network can provide and will
       fit into the buffer. Without this set data is read into the read buffer one record at a
       time. The more data that can be read, the more opportunity there is for parallelising the
       processing at the cost of increased memory overhead per connection. Setting read_ahead can
       impact the behaviour of the SSL_pending() function (see SSL_pending(3)). In addition the
       default size of the internal read buffer is multiplied by the number of pipelines
       available to ensure that we can read multiple records in one go. This can therefore have a
       significant impact on memory usage.

       The SSL_CTX_set_default_read_buffer_len() and SSL_set_default_read_buffer_len() functions
       control the size of the read buffer that will be used. The len parameter sets the size of
       the buffer. The value will only be used if it is greater than the default that would have
       been used anyway. The normal default value depends on a number of factors but it will be
       at least SSL3_RT_MAX_PLAIN_LENGTH + SSL3_RT_MAX_ENCRYPTED_OVERHEAD (16704) bytes.

       SSL_CTX_set_tlsext_max_fragment_length() sets the default maximum fragment length
       negotiation mode via value mode to ctx.  This setting affects only SSL instances created
       after this function is called.  It affects the client-side as only its side may initiate
       this extension use.

       SSL_set_tlsext_max_fragment_length() sets the maximum fragment length negotiation mode via
       value mode to ssl.  This setting will be used during a handshake when extensions are
       exchanged between client and server.  So it only affects SSL sessions created after this
       function is called.  It affects the client-side as only its side may initiate this
       extension use.

       SSL_SESSION_get_max_fragment_length() gets the maximum fragment length negotiated in
       session.

RETURN VALUES

       All non-void functions return 1 on success and 0 on failure.

NOTES

       The Maximum Fragment Length extension support is optional on the server side.  If the
       server does not support this extension then SSL_SESSION_get_max_fragment_length() will
       return: TLSEXT_max_fragment_length_DISABLED.

       The following modes are available:

       TLSEXT_max_fragment_length_DISABLED
           Disables Maximum Fragment Length Negotiation (default).

       TLSEXT_max_fragment_length_512
           Sets Maximum Fragment Length to 512 bytes.

       TLSEXT_max_fragment_length_1024
           Sets Maximum Fragment Length to 1024.

       TLSEXT_max_fragment_length_2048
           Sets Maximum Fragment Length to 2048.

       TLSEXT_max_fragment_length_4096
           Sets Maximum Fragment Length to 4096.

       With the exception of SSL_CTX_set_default_read_buffer_len()
       SSL_set_default_read_buffer_len(), SSL_CTX_set_tlsext_max_fragment_length(),
       SSL_set_tlsext_max_fragment_length() and SSL_SESSION_get_max_fragment_length() all these
       functions are implemented using macros.

SEE ALSO

       ssl(7), SSL_CTX_set_read_ahead(3), SSL_pending(3)

HISTORY

       The SSL_CTX_set_max_pipelines(), SSL_set_max_pipelines(),
       SSL_CTX_set_split_send_fragment(), SSL_set_split_send_fragment(),
       SSL_CTX_set_default_read_buffer_len() and  SSL_set_default_read_buffer_len() functions
       were added in OpenSSL 1.1.0.

       The SSL_CTX_set_tlsext_max_fragment_length(), SSL_set_tlsext_max_fragment_length() and
       SSL_SESSION_get_max_fragment_length() functions were added in OpenSSL 1.1.1.

COPYRIGHT

       Copyright 2016-2023 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 <https://www.openssl.org/source/license.html>.