395 lines
18 KiB
Plaintext
395 lines
18 KiB
Plaintext
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.\" ========================================================================
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.\"
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.IX Title "OPENSSL-QUIC-CONCURRENCY 7ossl"
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.TH OPENSSL-QUIC-CONCURRENCY 7ossl "2025-04-23" "3.5.1-dev" "OpenSSL"
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.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.\" way too many mistakes in technical documents.
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.if n .ad l
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.nh
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.SH "NAME"
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openssl\-quic\-concurrency \- OpenSSL QUIC Concurrency Model
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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A \s-1QUIC\s0 domain is a group of \s-1QUIC\s0 resources such as listeners (see
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\&\fISSL_new_listener\fR\|(3)) and connections which share common event processing
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resources, such as internal pollers, timers and locks. All usage of OpenSSL \s-1QUIC\s0
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happens inside a \s-1QUIC\s0 domain.
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.PP
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These resources can be accessed and used concurrently depending on the
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circumstances. This man page discusses the available concurrency models and how
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they can be used.
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.SH "EXPLICIT AND IMPLICIT QUIC DOMAINS"
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.IX Header "EXPLICIT AND IMPLICIT QUIC DOMAINS"
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A \s-1QUIC\s0 domain is instantiated either explicitly (\fISSL_new_domain\fR\|(3)) or
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implicitly by calling \fISSL_new\fR\|(3) or \fISSL_new_listener\fR\|(3):
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.IP "\(bu" 4
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An explicit \s-1QUIC\s0 domain is created by and visible to the application as a \s-1QUIC\s0
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domain \s-1SSL\s0 object and has other \s-1QUIC SSL\s0 objects created underneath it, such as
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listeners or connections.
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.IP "\(bu" 4
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An implicit \s-1QUIC\s0 domain is one which is created internally due to the direct
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creation of a \s-1QUIC\s0 connection or listener \s-1SSL\s0 object; the application does not
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explicitly create a \s-1QUIC\s0 domain \s-1SSL\s0 object and never directly references the
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domain.
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.PP
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Explicit creation of a \s-1QUIC\s0 domain provides the greatest level of control for an
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application. Applications can use an implicit \s-1QUIC\s0 domain for ease of use and to
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avoid needing to create a separate \s-1QUIC\s0 domain \s-1SSL\s0 object.
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.PP
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Regardless of whether a \s-1QUIC\s0 domain is explicitly created, the internal
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processing model is the same and the application must choose an appropriate
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concurrency model as discussed below.
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.SH "CONCURRENCY MODELS"
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.IX Header "CONCURRENCY MODELS"
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The OpenSSL \s-1QUIC\s0 implementation supports multiple concurrency models to support
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a wide variety of usage scenarios.
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.PP
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The available concurrency models are as follows:
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.IP "\(bu" 4
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The \fBSingle-Threaded Concurrency Model (\s-1SCM\s0)\fR, which supports only
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application-synchronised single-threaded usage.
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.IP "\(bu" 4
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The \fBContentive Concurrency Model (\s-1CCM\s0)\fR, which supports multi-threaded usage.
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.IP "\(bu" 4
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The \fBThread-Assisted Concurrency Model (\s-1TACM\s0)\fR, which also supports
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multi-threaded usage and provides assistance to an application for handling \s-1QUIC\s0
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timer events.
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.PP
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The merits of these models are as follows:
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.IP "\(bu" 4
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The \fBSingle-Threaded Concurrency Model (\s-1SCM\s0)\fR performs no locking or
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synchronisation. It is entirely up to the application to synchronise access to
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the \s-1QUIC\s0 domain and its subsidiary \s-1SSL\s0 objects.
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.Sp
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This concurrency model is also useful for an application which wants to use the
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OpenSSL \s-1QUIC\s0 implementation as a pure state machine.
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.IP "\(bu" 4
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The \fBContentive Concurrency Model (\s-1CCM\s0)\fR performs automatic locking when making
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\&\s-1API\s0 calls to \s-1SSL\s0 objects in a \s-1QUIC\s0 domain. This provides automatic
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synchronisation for multi-threaded usage of \s-1QUIC\s0 objects. For example, different
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\&\s-1QUIC\s0 stream \s-1SSL\s0 objects in the same \s-1QUIC\s0 connection can be safely accessed from
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different threads.
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.Sp
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This concurrency model adds the overhead of locking over the Single-Threaded
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Concurrency Model in order to support multi-threaded usage, but provides limited
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performance in highly contended multi-threaded usage due to its simple approach.
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However, it may still prove a good solution for a broad class of applications
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which spend the majority of their time in application logic and not in \s-1QUIC I/O\s0
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processing.
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.Sp
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An advantage of this model relative to the more sophisticated concurrency models
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below is that it does not create any \s-1OS\s0 threads.
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.IP "\(bu" 4
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The \fBThread-Assisted Concurrency Model (\s-1TACM\s0)\fR is identical to the Contentive
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Concurrency Model except that a thread is spun up in the background to ensure
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that \s-1QUIC\s0 timer events are handled in a timely fashion. This ensures that \s-1QUIC\s0
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timeout events are handled even if an application does not periodically call
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into the \s-1QUIC\s0 domain to ensure that any outstanding QUIC-related timer or
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network I/O events are handled. The assist thread contends for the same
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resources like any other thread. However, handshake layer events (\s-1TLS\s0) are never
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processed by the assist thread.
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.PP
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The default concurrency model is \s-1CCM\s0 or \s-1TACM,\s0 depending on the \fB\s-1SSL_METHOD\s0\fR
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used with a \fB\s-1SSL_CTX\s0\fR. Using \fIOSSL_QUIC_client_method\fR\|(3) results in a default
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concurrency model of \s-1CCM,\s0 whereas using \fIOSSL_QUIC_client_thread_method\fR\|(3)
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results in a default concurrency model of \s-1TACM.\s0
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.PP
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Additional concurrency models may be offered in future releases of OpenSSL.
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.SH "BLOCKING I/O CAPABILITIES"
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.IX Header "BLOCKING I/O CAPABILITIES"
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All of the supported concurrency models are capable of supporting blocking I/O
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calls, where application-level I/O calls (for example, to \fISSL_read_ex\fR\|(3) or
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\&\fISSL_write_ex\fR\|(3) on a \s-1QUIC\s0 stream \s-1SSL\s0 object) block until the request can be
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serviced. This includes the use of \fISSL_poll\fR\|(3) in a blocking fashion.
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.PP
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Supporting blocking \s-1API\s0 calls reliably with multi-threaded usage requires the
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creation of additional \s-1OS\s0 resources such as internal file descriptors to allow
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threads to be woken when necessary. This creation of internal \s-1OS\s0 resources is
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optional and may need to be explicitly requested by an application depending on
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the chosen concurrency model. If this functionality is disabled, depending on
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the chosen concurrency model, blocking \s-1API\s0 calls may not be available and calls
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to \fISSL_set_blocking_mode\fR\|(3) attempting to enable blocking mode may fail,
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notwithstanding the following section.
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.SS "Legacy Blocking Support Compatibility"
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.IX Subsection "Legacy Blocking Support Compatibility"
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OpenSSL 3.2 and 3.3 contained a buggy implementation of blocking \s-1QUIC I/O\s0 calls
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which is only reliable under single-threaded usage. This functionality is always
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available in the Single-Threaded Concurrency Model (\s-1SCM\s0), where it works
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reliably.
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.PP
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For compatibility reasons, this functionality is also available under the
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default concurrency model if the application does not explicitly specify a
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concurrency model or disable it. This is known as Legacy Blocking Compatibility
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Mode, and its usage is not recommended for multi-threaded applications.
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.SH "RECOMMENDED USAGE"
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.IX Header "RECOMMENDED USAGE"
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New applications are advised to choose a concurrency model as follows:
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.IP "\(bu" 4
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A purely single-threaded application, or an application which wishes to use
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OpenSSL \s-1QUIC\s0 as a state machine and manage synchronisation itself, should
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explicitly select the \s-1SCM\s0 concurrency model.
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.IP "\(bu" 4
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An application which wants to engage in multi-threaded usage of different \s-1QUIC\s0
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connections or streams in the same \s-1QUIC\s0 domain should a) select the \s-1CCM\s0 or \s-1TACM\s0
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concurrency model and b) explicitly opt in or out of blocking I/O support
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(depending on whether the application wishes to make blocking I/O calls),
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disabling Legacy Blocking Compatibility Mode.
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.Sp
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An application should select the \s-1CCM\s0 concurrency model if the application can
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guarantee that a \s-1QUIC\s0 domain will be serviced regularly (for example, because
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the application can guarantee that the timeout returned by
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\&\fISSL_get_event_timeout\fR\|(3) will be handled). If an application is unable to do
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this, it should select the \s-1TACM\s0 concurrency model.
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.IP "\(bu" 4
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Applications should explicitly configure a concurrency model during
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initialisation.
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.SH "CONFIGURING A CONCURRENCY MODEL"
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.IX Header "CONFIGURING A CONCURRENCY MODEL"
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If using an explicit \s-1QUIC\s0 domain, a concurrency model is chosen when calling
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\&\fISSL_new_domain\fR\|(3) by specifying zero or more of the following flags:
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.IP "\fB\s-1SSL_DOMAIN_FLAG_SINGLE_THREAD\s0\fR" 4
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.IX Item "SSL_DOMAIN_FLAG_SINGLE_THREAD"
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Specifying this flag configures the Single-Threaded Concurrency Model (\s-1SCM\s0).
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.IP "\fB\s-1SSL_DOMAIN_FLAG_MULTI_THREAD\s0\fR" 4
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.IX Item "SSL_DOMAIN_FLAG_MULTI_THREAD"
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Speciyfing this flag configures the Contentive Concurrency Model (\s-1CCM\s0) (unless
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\&\fB\s-1SSL_DOMAIN_FLAG_THREAD_ASSISTED\s0\fR is also specified).
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.IP "\fB\s-1SSL_DOMAIN_FLAG_THREAD_ASSISTED\s0\fR" 4
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.IX Item "SSL_DOMAIN_FLAG_THREAD_ASSISTED"
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Specifying this flag configures the Thread-Assisted Concurrency Model (\s-1TACM\s0).
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It implies \fB\s-1SSL_DOMAIN_FLAG_MULTI_THREAD\s0\fR.
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.IP "\fB\s-1SSL_DOMAIN_FLAG_BLOCKING\s0\fR" 4
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.IX Item "SSL_DOMAIN_FLAG_BLOCKING"
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Enable reliable support for blocking I/O calls, allocating whatever \s-1OS\s0 resources
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are necessary to realise this. If this flag is specified,
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\&\fB\s-1SSL_DOMAIN_FLAG_LEGACY_BLOCKING\s0\fR is ignored.
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.Sp
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Details on the allocated \s-1OS\s0 resources can be found under \*(L"\s-1CONSUMPTION OF OS
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RESOURCES\*(R"\s0 below.
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.IP "\fB\s-1SSL_DOMAIN_FLAG_LEGACY_BLOCKING\s0\fR" 4
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.IX Item "SSL_DOMAIN_FLAG_LEGACY_BLOCKING"
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Enables legacy blocking compatibility mode. See \*(L"Legacy Blocking Support
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Compatibility\*(R".
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.PP
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Mutually exclusive flag combinations result in an error (for example, combining
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\&\fB\s-1SSL_DOMAIN_FLAG_SINGLE_THREAD\s0\fR and \fB\s-1SSL_DOMAIN_FLAG_MULTI_THREADED\s0\fR).
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.PP
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The concurrency model for a domain cannot be changed after the domain is
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created.
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.SS "Default Behaviour"
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.IX Subsection "Default Behaviour"
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If none of \fB\s-1SSL_DOMAIN_FLAG_SINGLE_THREAD\s0\fR, \fB\s-1SSL_DOMAIN_FLAG_MULTI_THREAD\s0\fR or
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\&\fB\s-1SSL_DOMAIN_FLAG_THREAD_ASSISTED\s0\fR are provided to \fISSL_new_domain\fR\|(3) or
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another constructor function which can accept the above flags, the default
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concurrency model set on the \fB\s-1SSL_CTX\s0\fR is used. This default can be set and get
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using \fISSL_CTX_set_domain_flags\fR\|(3) and \fISSL_CTX_get_domain_flags\fR\|(3). Any
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additional flags provided (for example, \fB\s-1SSL_DOMAIN_FLAG_BLOCCKING\s0\fR) are added
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to the set of inherited flags.
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.PP
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The default concurrency model set on a newly created \fB\s-1SSL_CTX\s0\fR is determined as
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follows:
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.IP "\(bu" 4
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If an \fB\s-1SSL_METHOD\s0\fR of \fIOSSL_QUIC_client_thread_method\fR\|(3) is used, the
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Thread-Assisted Concurrency Model (\s-1TACM\s0) is used with the
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\&\fB\s-1SSL_DOMAIN_FLAG_BLOCKING\s0\fR flag. This provides reliable blocking functionality.
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.IP "\(bu" 4
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Otherwise, if OpenSSL was built without threading support, the Single-Threaded
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Concurrency Model (\s-1SCM\s0) is used, with the \fB\s-1SSL_DOMAIN_FLAG_LEGACY_BLOCKING\s0\fR
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flag.
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.IP "\(bu" 4
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Otherwise, if an \fB\s-1SSL_METHOD\s0\fR of \fIOSSL_QUIC_client_method\fR\|(3) is used, the
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Contentive Concurrency Model (\s-1CCM\s0) is used with the
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\&\fB\s-1SSL_DOMAIN_FLAG_LEGACY_BLOCKING\s0\fR flag.
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.IP "\(bu" 4
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Otherwise, the Contentive Concurrency Model (\s-1CCM\s0) is used.
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.PP
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The default concurrency model may vary between releases of OpenSSL. An
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application may specify one or more of the domain flags above to ensure
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consistent usage of a specific concurrency model between releases.
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.SS "Configuration of Concurrency Models with Implicit \s-1QUIC\s0 Domains"
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.IX Subsection "Configuration of Concurrency Models with Implicit QUIC Domains"
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If an explicit \s-1QUIC\s0 domain is not explicitly created using \fISSL_new_domain\fR\|(3),
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an implicit \s-1QUIC\s0 domain is created when calling \fISSL_new_listener\fR\|(3) or
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\&\fISSL_new\fR\|(3). Such a domain will use the default domain flags configured on the
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\&\fB\s-1SSL_CTX\s0\fR as described above.
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.SH "CONSUMPTION OF OS RESOURCES"
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.IX Header "CONSUMPTION OF OS RESOURCES"
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If full blocking I/O support is selected using \fB\s-1SSL_DOMAIN_FLAG_BLOCKING\s0\fR, at
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least one socket, socket-like \s-1OS\s0 handle or file descriptor must be allocated to
|
|
allow one thread to wake other threads which may be blocking in calls to \s-1OS\s0
|
|
socket polling interfaces such as \fIselect\fR\|(2) or \fIpoll\fR\|(2). This is allocated
|
|
automatically internally by OpenSSL.
|
|
.PP
|
|
If the Thread-Assisted Concurrency Model (\s-1TACM\s0) is selected, a background thread
|
|
is spawned. This also implies \fB\s-1SSL_DOMAIN_FLAG_BLOCKING\s0\fR and the above.
|
|
.PP
|
|
The internal consumption by OpenSSL of mutexes, condition variables, spin locks
|
|
or other similar thread synchronisation primitives is unspecified under all
|
|
concurrency models.
|
|
.PP
|
|
The internal consumption by OpenSSL of threads is unspecified under the
|
|
Thread-Assisted Concurrency Model.
|
|
.PP
|
|
The internal consumption by OpenSSL of sockets, socket-like \s-1OS\s0 handles or file
|
|
descriptors, or other resources as needed to support inter-thread notification,
|
|
is unspecified under the Thread-Assisted Concurrency Model or when using
|
|
\&\fB\s-1SSL_DOMAIN_FLAG_BLOCKING\s0\fR.
|
|
.SH "BEHAVIOUR OF SSL OBJECTS"
|
|
.IX Header "BEHAVIOUR OF SSL OBJECTS"
|
|
A \s-1QUIC SSL\s0 object has blocking mode enabled by default where \fBall\fR of the
|
|
following criteria are met:
|
|
.IP "\(bu" 4
|
|
\&\fB\s-1SSL_DOMAIN_FLAG_BLOCKING\s0\fR or \fB\s-1SSL_DOMAIN_FLAG_LEGACY_BLOCKING\s0\fR is enabled;
|
|
and
|
|
.IP "\(bu" 4
|
|
The \s-1QUIC\s0 connection is being used with network read and write BIOs which expose
|
|
supported poll descriptors. See \fIopenssl\-quic\fR\|(7) for details.
|
|
.PP
|
|
In all other cases, a \s-1QUIC SSL\s0 object has blocking mode disabled by default. The
|
|
blocking mode can be changed explicitly using \fISSL_set_blocking_mode\fR\|(3).
|
|
.SH "SEE ALSO"
|
|
.IX Header "SEE ALSO"
|
|
\&\fIopenssl\-quic\fR\|(7), \fISSL_handle_events\fR\|(3), \fISSL_get_event_timeout\fR\|(3),
|
|
\&\fIOSSL_QUIC_client_thread_method\fR\|(3), \fISSL_CTX_set_domain_flags\fR\|(3),
|
|
\&\fISSL_new_domain\fR\|(3)
|
|
.SH "COPYRIGHT"
|
|
.IX Header "COPYRIGHT"
|
|
Copyright 2024\-2025 The OpenSSL Project Authors. All Rights Reserved.
|
|
.PP
|
|
Licensed under the Apache License 2.0 (the \*(L"License\*(R"). You may not use
|
|
this file except in compliance with the License. You can obtain a copy
|
|
in the file \s-1LICENSE\s0 in the source distribution or at
|
|
<https://www.openssl.org/source/license.html>.
|