Cryptography and SSL/TLS Toolkit



openssl-quic - OpenSSL QUIC


OpenSSL 3.2 and later features support for the QUIC transport protocol. Currently, only client connectivity is supported. This man page describes the usage of QUIC client functionality for both existing and new applications.

QUIC functionality uses the standard SSL API. A QUIC connection is represented by an SSL object in the same way that a TLS connection is. Only minimal changes are needed to existing applications making use of the libssl APIs to make use of QUIC client functionality. To make use of QUIC, use the SSL method OSSL_QUIC_client_method(3) or OSSL_QUIC_client_thread_method(3) with SSL_CTX_new(3).

When a QUIC connection is created, by default, it operates in default stream mode, which is intended to provide compatibility with existing non-QUIC application usage patterns. In this mode, the connection has a single stream associated with it. Calls to SSL_read(3) and SSL_write(3) on the QUIC connection SSL object read and write from that stream. Whether the stream is client-initiated or server-initiated from a QUIC perspective depends on whether SSL_read(3) or SSL_write(3) is called first. See the MODES OF OPERATION section for more information.

The default stream mode is intended for compatibility with existing applications. New applications using QUIC are recommended to disable default stream mode and use the multi-stream API; see the MODES OF OPERATION section and the RECOMMENDATIONS FOR NEW APPLICATIONS section for more information.

The remainder of this man page discusses, in order:

  • Default stream mode versus multi-stream mode;

  • The changes to existing libssl APIs which are driven by QUIC-related implementation requirements, which existing applications should bear in mind;

  • Aspects which must be considered by existing applications when adopting QUIC, including potential changes which may be needed.

  • Recommended usage approaches for new applications.

  • New, QUIC-specific APIs.


Default Stream Mode

A QUIC client connection can be used in either default stream mode or multi-stream mode. By default, a newly created QUIC connection SSL object uses default stream mode.

In default stream mode, a stream is implicitly created and bound to the QUIC connection SSL object; SSL_read(3) and SSL_write(3) calls to the QUIC connection SSL object work by default and are mapped to that stream.

When default stream mode is used, any API function which can be called on a QUIC stream SSL object can also be called on a QUIC connection SSL object, in which case it affects the default stream bound to the connection.

The identity of a QUIC stream, including its stream ID, varies depending on whether a stream is client-initiated or server-initiated. In default stream mode, if a client application calls SSL_read(3) first before any call to SSL_write(3) on the connection, it is assumed that the application protocol is using a server-initiated stream, and the SSL_read(3) call will not complete (either blocking, or failing appropriately if nonblocking mode is configured) until the server initiates a stream. Conversely, if the client application calls SSL_write(3) before any call to SSL_read(3) on the connection, it is assumed that a client-initiated stream is to be used and such a stream is created automatically.

Default stream mode is intended to aid compatibility with legacy applications. New applications adopting QUIC should use multi-stream mode, described below, and avoid use of the default stream functionality.

It is possible to use additional streams in default stream mode using SSL_new_stream(3) and SSL_accept_stream(3); note that the default incoming stream policy will need to be changed using SSL_set_incoming_stream_policy(3) in order to use SSL_accept_stream(3) in this case. However, applications using additional streams are strongly recommended to use multi-stream mode instead.

Calling SSL_new_stream(3) or SSL_accept_stream(3) before a default stream has been associated with the QUIC connection SSL object will inhibit future creation of a default stream.

Multi-Stream Mode

The recommended usage mode for new applications adopting QUIC is multi-stream mode, in which no default stream is attached to the QUIC connection SSL object and attempts to call SSL_read(3) and SSL_write(3) on the QUIC connection SSL object fail. Instead, an application calls SSL_new_stream(3) or SSL_accept_stream(3) to create individual stream SSL objects for sending and receiving application data using SSL_read(3) and SSL_write(3).

To use multi-stream mode, call SSL_set_default_stream_mode(3) with an argument of SSL_DEFAULT_STREAM_MODE_NONE; this function must be called prior to initiating the connection. The default stream mode cannot be changed after initiating a connection.

When multi-stream mode is used, meaning that no default stream is associated with the connection, calls to API functions which are defined as operating on a QUIC stream fail if called on the QUIC connection SSL object. For example, calls such as SSL_write(3) or SSL_get_stream_id(3) will fail.


Most SSL APIs, such as SSL_read(3) and SSL_write(3), function as they do for TLS connections and do not have changed semantics, with some exceptions. The changes to the semantics of existing APIs are as follows:

  • Since QUIC uses UDP, SSL_set_bio(3), SSL_set0_rbio(3) and SSL_set0_wbio(3) function as before, but must now receive a BIO with datagram semantics. There are broadly four options for applications to use as a network BIO:

    • BIO_s_datagram(3), recommended for most applications, replaces BIO_s_socket(3) and provides a UDP socket.

    • BIO_s_dgram_pair(3) provides BIO pair-like functionality but with datagram semantics, and is recommended for existing applications which use a BIO pair or memory BIO to manage libssl's communication with the network.

    • BIO_s_dgram_mem(3) provides a simple memory BIO-like interface but with datagram semantics. Unlike BIO_s_dgram_pair(3), it is unidirectional.

    • An application may also choose to implement a custom BIO. The new BIO_sendmmsg(3) and BIO_recvmmsg(3) APIs must be supported.

  • SSL_set_fd(3), SSL_set_rfd(3) and SSL_set_wfd(3) traditionally instantiate a BIO_s_socket(3). For QUIC, these functions instead instantiate a BIO_s_datagram(3). This is equivalent to instantiating a BIO_s_datagram(3) and using SSL_set0_rbio(3) and SSL_set0_wbio(3).

  • Traditionally, whether the application-level I/O APIs (such as SSL_read(3) and SSL_write(3) operated in a blocking fashion was directly correlated with whether the underlying network socket was configured in a blocking fashion. This is no longer the case; applications must explicitly configure the desired application-level blocking mode using SSL_set_blocking_mode(3). See SSL_set_blocking_mode(3) for details.

  • Network-level I/O must always be performed in a nonblocking manner. The application can still enjoy blocking semantics for calls to application-level I/O functions such as SSL_read(3) and SSL_write(3), but the underlying network BIO provided to QUIC (such as a BIO_s_datagram(3)) must be configured in nonblocking mode. For application-level blocking functionality, see SSL_set_blocking_mode(3).

  • BIO_new_ssl_connect(3) has been changed to automatically use a BIO_s_datagram(3) when used with QUIC, therefore applications which use this do not need to change the BIO they use.

  • BIO_new_buffer_ssl_connect(3) cannot be used with QUIC and applications must change to use BIO_new_ssl_connect(3) instead.

  • SSL_shutdown(3) has significant changes in relation to how QUIC connections must be shut down. In particular, applications should be advised that the full RFC-conformant QUIC shutdown process may take an extended amount of time. This may not be suitable for short-lived processes which should exit immediately after their usage of a QUIC connection is completed. A rapid shutdown mode is available for such applications. For details, see SSL_shutdown(3).

  • SSL_want(3), SSL_want_read(3) and SSL_want_write(3) no longer reflect the I/O state of the network BIO passed to the QUIC SSL object, but instead reflect the flow control state of the QUIC stream associated with the SSL object.

    When used in nonblocking mode, SSL_ERROR_WANT_READ indicates that the receive part of a QUIC stream does not currently have any more data available to be read, and SSL_ERROR_WANT_WRITE indicates that the stream's internal buffer is full.

    To determine if the QUIC implementation currently wishes to be informed of incoming network datagrams, use the new function SSL_net_read_desired(3); likewise, to determine if the QUIC implementation currently wishes to be informed when it is possible to transmit network datagrams, use the new function SSL_net_write_desired(3). Only applications which wish to manage their own event loops need to use these functions; see APPLICATION-DRIVEN EVENT LOOPS for further discussion.

  • The use of ALPN is mandatory when using QUIC. Attempts to connect without configuring ALPN will fail. For information on how to configure ALPN, see SSL_set_alpn_protos(3).

  • Whether QUIC operates in a client or server mode is determined by the SSL_METHOD used, rather than by calls to SSL_set_connect_state(3) or SSL_set_accept_state(3). It is not necessary to call either of SSL_set_connect_state(3) or SSL_set_accept_state(3) before connecting, but if either of these are called, the function called must be congruent with the SSL_METHOD being used. Currently, only client mode is supported.

  • The SSL_set_min_proto_version(3) and SSL_set_max_proto_version(3) APIs are not used and the values passed to them are ignored, as OpenSSL QUIC currently always uses TLS 1.3.

  • The following libssl functionality is not available when used with QUIC.

    • Async functionality


    • Record Padding and Fragmentation (SSL_set_block_padding(3), etc.)

    • SSL_stateless(3) support

    • SRTP functionality

    • TLSv1.3 Early Data

    • TLS Next Protocol Negotiation cannot be used and is superseded by ALPN, which must be used instead. The use of ALPN is mandatory with QUIC.

    • Post-Handshake Client Authentication is not available as QUIC prohibits its use.

    • QUIC requires the use of TLSv1.3 or later, therefore functionality only relevant to older TLS versions is not available.

    • Some cipher suites which are generally available for TLSv1.3 are not available for QUIC, such as TLS_AES_128_CCM_8_SHA256. Your application may need to adjust the list of acceptable cipher suites it passes to libssl.

    • CCM mode is not currently supported.

    The following libssl functionality is also not available when used with QUIC, but calls to the relevant functions are treated as no-ops:


Existing applications seeking to adopt QUIC should apply the following list to determine what changes they will need to make:

  • An application wishing to use QUIC must use OSSL_QUIC_client_method(3) or OSSL_QUIC_client_thread_method(3) as its SSL method. For more information on the differences between these two methods, see THREAD ASSISTED MODE.

  • Determine how to provide QUIC with network access. Determine which of the below apply for your application:

    • Your application uses BIO_s_socket(3) to construct a BIO which is passed to the SSL object to provide it with network access.

      Changes needed: Change your application to use BIO_s_datagram(3) instead when using QUIC. The socket must be configured in nonblocking mode. You may or may not need to use SSL_set1_initial_peer_addr(3) to set the initial peer address; see the QUIC-SPECIFIC APIS section for details.

    • Your application uses BIO_new_ssl_connect(3) to construct a BIO which is passed to the SSL object to provide it with network access.

      Changes needed: No changes needed. Use of QUIC is detected automatically and a datagram socket is created instead of a normal TCP socket.

    • Your application uses any other I/O strategy in this list but combines it with a BIO_f_buffer(3), for example using BIO_push(3).

      Changes needed: Disable the usage of BIO_f_buffer(3) when using QUIC. Usage of such a buffer is incompatible with QUIC as QUIC requires datagram semantics in its interaction with the network.

    • Your application uses a BIO pair to cause the SSL object to read and write network traffic to a memory buffer. Your application manages the transmission and reception of buffered data itself in a way unknown to libssl.

      Changes needed: Switch from using a conventional BIO pair to using BIO_s_dgram_pair(3) instead, which has the necessary datagram semantics. You will need to modify your application to transmit and receive using a UDP socket and to use datagram semantics when interacting with the BIO_s_dgram_pair(3) instance.

    • Your application uses a custom BIO method to provide the SSL object with network access.

      Changes needed: The custom BIO must be re-architected to have datagram semantics. BIO_sendmmsg(3) and BIO_recvmmsg(3) must be implemented. These calls must operate in a nonblocking fashion. Optionally, implement the BIO_get_rpoll_descriptor(3) and BIO_get_wpoll_descriptor(3) methods if desired. Implementing these methods is required if blocking semantics at the SSL API level are desired.

  • An application must explicitly configure whether it wishes to use the SSL APIs in blocking mode or not. Traditionally, an SSL object has automatically operated in blocking or nonblocking mode based on whether the underlying network BIO operates in blocking or nonblocking mode. QUIC requires the use of a nonblocking network BIO, therefore the blocking mode at the application level must be explicitly configured by the application using the new SSL_set_blocking_mode(3) API. The default mode is blocking. If an application wishes to use the SSL object APIs at application level in a nonblocking manner, it must add a call to SSL_set_blocking_mode(3) to disable blocking mode.

  • If your application does not choose to use thread assisted mode, it must ensure that it calls an I/O function on the SSL object (for example, SSL_read(3) or SSL_write(3)), or the new function SSL_handle_events(3), regularly. If the SSL object is used in blocking mode, an ongoing blocking call to an I/O function satisfies this requirement. This is required to ensure that timer events required by QUIC are handled in a timely fashion.

    Most applications will service the SSL object by calling SSL_read(3) or SSL_write(3) regularly. If an application does not do this, it should ensure that SSL_handle_events(3) is called regularly.

    SSL_get_event_timeout(3) can be used to determine when SSL_handle_events(3) must next be called.

    If the SSL object is being used with an underlying network BIO which is pollable (such as BIO_s_datagram(3)), the application can use SSL_get_rpoll_descriptor(3), SSL_get_wpoll_descriptor(3) to obtain resources which can be used to determine when SSL_handle_events(3) should be called due to network I/O.

    Applications which use thread assisted mode do not need to be concerned with this requirement, as the QUIC implementation ensures timeout events are handled in a timely manner. See THREAD ASSISTED MODE for details.

  • Ensure that your usage of SSL_want(3), SSL_want_read(3) and SSL_want_write(3) reflects the API changes described in CHANGES TO EXISTING APIS. In particular, you should use these APIs to determine the ability of a QUIC stream to receive or provide application data, not to to determine if network I/O is required.

  • Evaluate your application's use of SSL_shutdown(3) in light of the changes discussed in CHANGES TO EXISTING APIS. Depending on whether your application wishes to prioritise RFC conformance or rapid shutdown, consider using the new SSL_shutdown_ex(3) API instead. See QUIC-SPECIFIC APIS for details.

The recommended usage in new applications varies depending on three independent design decisions:

  • Whether the application will use blocking or nonblocking I/O at the application level (configured using SSL_set_blocking_mode(3)).

    If the application does nonblocking I/O at the application level it can choose to manage its own polling and event loop; see APPLICATION-DRIVEN EVENT LOOPS.

  • Whether the application intends to give the QUIC implementation direct access to a network socket (e.g. via BIO_s_datagram(3)) or whether it intends to buffer transmitted and received datagrams via a BIO_s_dgram_pair(3) or custom BIO.

    The former is preferred where possible as it reduces latency to the network, which enables QUIC to achieve higher performance and more accurate connection round trip time (RTT) estimation.

  • Whether thread assisted mode will be used (see THREAD ASSISTED MODE).

Simple demos for QUIC usage under these various scenarios can be found at

Applications which wish to implement QUIC-specific protocols should be aware of the APIs listed under QUIC-SPECIFIC APIS which provide access to QUIC-specific functionality. For example, SSL_stream_conclude(3) can be used to indicate the end of the sending part of a stream, and SSL_shutdown_ex(3) can be used to provide a QUIC application error code when closing a connection.

Regardless of the design decisions chosen above, it is recommended that new applications avoid use of the default stream mode and use the multi-stream API by calling SSL_set_default_stream_mode(3); see the MODES OF OPERATION section for details.


This section details new APIs which are directly or indirectly related to QUIC. For details on the operation of each API, see the referenced man pages.

The following SSL APIs are new but relevant to both QUIC and DTLS:


Determines when the QUIC implementation should next be woken up via a call to SSL_handle_events(3) (or another I/O function such as SSL_read(3) or SSL_write(3)), if ever.

This can also be used with DTLS and supersedes DTLSv1_get_timeout(3) for new usage.


This is a non-specific I/O operation which makes a best effort attempt to perform any pending I/O or timeout processing. It can be used to advance the QUIC state machine by processing incoming network traffic, generating outgoing network traffic and handling any expired timeout events. Most other I/O functions on an SSL object, such as SSL_read(3) and SSL_write(3) implicitly perform event handling on the SSL object, so calling this function is only needed if no other I/O function is to be called.

This can also be used with DTLS and supersedes DTLSv1_handle_timeout(3) for new usage.

The following SSL APIs are specific to QUIC:

SSL_set_blocking_mode(3), SSL_get_blocking_mode(3)

Configures whether blocking semantics are used at the application level. This determines whether calls to functions such as SSL_read(3) and SSL_write(3) will block.

SSL_get_rpoll_descriptor(3), SSL_get_wpoll_descriptor(3)

These functions facilitate operation in nonblocking mode.

When an SSL object is being used with an underlying network read BIO which supports polling, SSL_get_rpoll_descriptor(3) outputs an OS resource which can be used to synchronise on network readability events which should result in a call to SSL_handle_events(3). SSL_get_wpoll_descriptor(3) works in an analogous fashion for the underlying network write BIO.

The poll descriptors provided by these functions need only be used when SSL_net_read_desired(3) and SSL_net_write_desired(3) return 1, respectively.

SSL_net_read_desired(3), SSL_net_write_desired(3)

These functions facilitate operation in nonblocking mode and are used in conjunction with SSL_get_rpoll_descriptor(3) and SSL_get_wpoll_descriptor(3) respectively. They determine whether the respective poll descriptor is currently relevant for the purposes of polling.


This function can be used to set the initial peer address for an outgoing QUIC connection. This function must be used in the general case when creating an outgoing QUIC connection; however, the correct initial peer address can be autodetected in some cases. See SSL_set1_initial_peer_addr(3) for details.


This augments SSL_shutdown(3) by allowing an application error code to be specified. It also allows a client to decide how quickly it wants a shutdown to be performed, potentially by trading off strict RFC compliance.


This allows an application to indicate the normal end of the sending part of a QUIC stream. This corresponds to the FIN flag in the QUIC RFC. The receiving part of a stream remains usable.


This allows an application to indicate the non-normal termination of the sending part of a stream. This corresponds to the RESET_STREAM frame in the QUIC RFC.

SSL_get_stream_write_state(3) and SSL_get_stream_read_state(3)

This allows an application to determine the current stream states for the sending and receiving parts of a stream respectively.

SSL_get_stream_write_error_code(3) and SSL_get_stream_read_error_code(3)

This allows an application to determine the application error code which was signalled by a peer which has performed a non-normal stream termination of the respective sending or receiving part of a stream, if any.


This allows an application to determine the error code which was signalled when the local or remote endpoint terminated the QUIC connection.


Gets the QUIC connection SSL object from a QUIC stream SSL object.


Returns 1 if a SSL object is not a QUIC stream SSL object.


Provides information on the kind of QUIC stream which is attached to the SSL object.


Returns the QUIC stream ID which the QUIC protocol has associated with a QUIC stream.


Creates a new QUIC stream SSL object representing a new, locally-initiated QUIC stream.


Potentially yields a new QUIC stream SSL object representing a new remotely-initiated QUIC stream, blocking until one is available if the connection is configured to do so.


Provides information on the number of pending remotely-initiated streams.


Configures how incoming, remotely-initiated streams are handled. The incoming stream policy can be used to automatically reject streams created by the peer, or allow them to be handled using SSL_accept_stream(3).


Used to configure or disable default stream mode; see the MODES OF OPERATION section for details.

The following BIO APIs are not specific to QUIC but have been added to facilitate QUIC-specific requirements and are closely associated with its use:


This is a new BIO method which is similar to a conventional BIO pair but provides datagram semantics.

BIO_get_rpoll_descriptor(3), BIO_get_wpoll_descriptor(3)

This is a new BIO API which allows a BIO to expose a poll descriptor. This API is used to implement the corresponding SSL APIs SSL_get_rpoll_descriptor(3) and SSL_get_wpoll_descriptor(3).

BIO_sendmmsg(3), BIO_recvmmsg(3)

This is a new BIO API which can be implemented by BIOs which implement datagram semantics. It is implemented by BIO_s_datagram(3) and BIO_s_dgram_pair(3). It is used by the QUIC implementation to send and receive UDP datagrams.

BIO_dgram_set_no_trunc(3), BIO_dgram_get_no_trunc(3)

By default, BIO_s_dgram_pair(3) has semantics comparable to those of Berkeley sockets being used with datagram semantics. This allows an alternative mode to be enabled in which datagrams will not be silently truncated if they are too large.

BIO_dgram_set_caps(3), BIO_dgram_get_caps(3)

These functions are used to allow the user of one end of a BIO_s_dgram_pair(3) to indicate its capabilities to the other end of a BIO_s_dgram_pair(3). In particular, this allows an application to inform the QUIC implementation of whether it is prepared to handle local and/or peer addresses in transmitted datagrams and to provide the applicable information in received datagrams.

BIO_dgram_get_local_addr_cap(3), BIO_dgram_set_local_addr_enable(3), BIO_dgram_get_local_addr_enable(3)

Local addressing support refers to the ability of a BIO with datagram semantics to allow a source address to be specified on transmission and to report the destination address on reception. These functions can be used to determine if a BIO can support local addressing and to enable local addressing support if it can.


This is used to determine if an error while calling BIO_sendmmsg(3) or BIO_recvmmsg(3) is ephemeral in nature, such as "would block" errors.


The optional thread assisted mode can be used with OSSL_QUIC_client_thread_method(3). In this mode, a background thread is created automatically. The OpenSSL QUIC implementation then takes responsibility for ensuring that timeout events are handled on a timely basis even if no SSL I/O function such as SSL_read(3) or SSL_write(3) is called by the application for a long time.

All necessary locking is handled automatically internally, but the thread safety guarantees for the public SSL API are unchanged. Therefore, an application must still do its own locking if it wishes to make concurrent use of the public SSL APIs.

Because this method relies on threads, it is not available on platforms where threading support is not available or not supported by OpenSSL. However, it does provide the simplest mode of usage for an application.

The implementation may or may not use a common thread or thread pool to service multiple SSL objects in the same SSL_CTX.


OpenSSL's QUIC implementation is designed to facilitate applications which wish to use the SSL APIs in a blocking fashion, but is also designed to facilitate applications which wish to use the SSL APIs in a nonblocking fashion and manage their own event loops and polling directly. This is useful when it is desirable to host OpenSSL's QUIC implementation on top of an application's existing nonblocking I/O infrastructure.

This is supported via the concept of poll descriptors; see BIO_get_rpoll_descriptor(3) for details. Broadly, a BIO_POLL_DESCRIPTOR is a structure which expresses some kind of OS resource which can be used to synchronise on I/O events. The QUIC implementation provides a BIO_POLL_DESCRIPTOR based on the poll descriptor provided by the underlying network BIO. This is typically an OS socket handle, though custom BIOs could choose to implement their own custom poll descriptor format.

Broadly, an application which wishes to manage its own event loop should interact with the SSL object as follows:


SSL_handle_events(3), SSL_get_event_timeout(3), SSL_net_read_desired(3), SSL_net_write_desired(3), SSL_get_rpoll_descriptor(3), SSL_get_wpoll_descriptor(3), SSL_set_blocking_mode(3), SSL_shutdown_ex(3), SSL_set1_initial_peer_addr(3), SSL_stream_conclude(3), SSL_stream_reset(3), SSL_get_stream_read_state(3), SSL_get_stream_read_error_code(3), SSL_get_conn_close_info(3), SSL_get0_connection(3), SSL_get_stream_type(3), SSL_get_stream_id(3), SSL_new_stream(3), SSL_accept_stream(3), SSL_set_incoming_stream_policy(3), SSL_set_default_stream_mode(3)

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