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SSL_CTX_set_split_send_fragment(3) OpenSSL SSL_CTX_set_split_send_fragment(3)
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- Control fragment size settings and pipelining operations
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(SSL_SESSION *session);
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 every use more than one pipeline if a cipher
suite is negotiated that uses a pipeline capable cipher provided by an
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
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)).
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
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
SSL_SESSION_get_max_fragment_length() gets the maximum fragment length
negotiated in session.
All non-void functions return 1 on success and 0 on failure.
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:
The following modes are available:
Disables Maximum Fragment Length Negotiation (default).
Sets Maximum Fragment Length to 512 bytes.
Sets Maximum Fragment Length to 1024.
Sets Maximum Fragment Length to 2048.
Sets Maximum Fragment Length to 4096.
With the exception of SSL_CTX_set_default_read_buffer_len()
SSL_SESSION_get_max_fragment_length() all these functions are
implemented using macros.
The SSL_CTX_set_max_pipelines(), SSL_set_max_pipelines(),
SSL_set_default_read_buffer_len() functions were added in OpenSSL
SSL_SESSION_get_max_fragment_length() functions were added in OpenSSL
Copyright 2016-2020 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