OPENSSL-QUIC(7ossl)

📦
version 15.1 · 2 versions
, in freebsd 15.1

💖 NAME 💖

openssl-quic — OpenSSL QUIC. Yes bestie, this is the openssl-quic(7ossl) page, and it's all about the QUIC transport protocol in OpenSSL.

✨ DESCRIPTION ✨

Okay so OpenSSL 3.2 and later features support for the QUIC transport protocol. You can use OpenSSL's QUIC capabilities for both client and server applications. This man page describes how to let applications use the QUIC protocol using the libssl API.

The QUIC protocol maps to 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 which use libssl API to bring QUIC protocol support in. QUIC clients can use OSSL_QUIC_client_method(3) or OSSL_QUIC_client_thread_method(3) with SSL_CTX_new(3). See below for more details about the difference between the two. For servers, there is only one option: SSL method OSSL_QUIC_server_method(3) with SSL_CTX_new(3).

The remainder of this man page discusses, in order:

  • Default stream mode versus multi-stream mode for clients;
  • 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.

🌈 CLIENT MODES OF OPERATION 🌈

When a client creates a QUIC connection, 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.

Default stream mode is primarily for compatibility with existing applications. For new applications utilizing QUIC, it's recommended to disable this mode and instead adopt the multi-stream API. See the RECOMMENDATIONS FOR NEW APPLICATIONS section for more details.

🎀 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.

✨ CHANGES TO EXISTING APIS ✨

Okay so here's the tea, bestie: most of the SSL APIs — the ones you already know and love, like SSL_read(3) and SSL_write(3) — work exactly the same way they do for TLS connections. Same semantics, no glow-up needed. But there are some exceptions, and you need to know about them, so let's go 💅

The changes to the semantics of existing APIs are as follows:

  • Since QUIC rides on UDP, SSL_set_bio(3), SSL_set0_rbio(3) and SSL_set0_wbio(3) still function like before, but now they must receive a BIO with datagram semantics. Broadly speaking, you've got four options for what to use as a network BIO:
    • BIO_s_datagram(3) — recommended for most applications, it replaces BIO_s_socket(3) and gives you a UDP socket.
    • BIO_s_dgram_pair(3) — gives you BIO pair-like functionality but with datagram semantics, and it's the recommendation for existing applications that use a BIO pair or memory BIO to manage libssl's communication with the network.
    • BIO_s_dgram_mem(3) — a simple memory BIO-like interface, again with datagram semantics. Heads up though: unlike BIO_s_dgram_pair(3), it's unidirectional.
    • An application can also choose to implement a custom BIO. If you do, the new BIO_sendmmsg(3) and BIO_recvmmsg(3) APIs must be supported. No pressure 🫶
  • SSL_set_fd(3), SSL_set_rfd(3) and SSL_set_wfd(3) traditionally instantiate a BIO_s_socket(3). For QUIC, these functions instantiate a BIO_s_datagram(3) instead. That's equivalent to instantiating a BIO_s_datagram(3) and then using SSL_set0_rbio(3) and SSL_set0_wbio(3).
  • Traditionally, whether the application-level I/O APIs (like 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. That is no longer the case. Applications must now explicitly configure the desired application-level blocking mode using SSL_set_blocking_mode(3). See SSL_set_blocking_mode(3) for the details.
  • Network-level I/O must always be performed in a nonblocking manner. Like, always. 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 you hand 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. So if your app uses this, congrats, you don't need to change the BIO you use at all 🎀
  • BIO_new_buffer_ssl_connect(3) cannot be used with QUIC, periodt. Applications must switch to using 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. That might not be it for short-lived processes which should exit immediately once they're done with their QUIC connection. A rapid shutdown mode is available for exactly those 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. Instead, they 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's 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 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. Straight up. For info on how to configure ALPN, see SSL_set_alpn_protos(3).
  • Whether QUIC operates in client or server mode is determined by the SSL_METHOD used, not by calls to SSL_set_connect_state(3) or SSL_set_accept_state(3). It's not necessary to call either of SSL_set_connect_state(3) or SSL_set_accept_state(3) before connecting, but if you do call one, the function you call must be congruent with the SSL_METHOD being used.
  • 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, because OpenSSL QUIC currently always uses TLS 1.3. Yeah. Always 💀
  • The following libssl functionality is not available when used with QUIC:
    • Async functionality
    • SSL_MODE_AUTO_RETRY
    • 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:

🦋 CONSIDERATIONS FOR EXISTING APPLICATIONS 🦋

okay so if you already have an app and you're trying to adopt QUIC, bestie, run through this checklist to figure out what you gotta change:

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. slay, no extra work 💅

  • 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. no cap, it's giving mismatch 💀

  • 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 can 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 client 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.

Client 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 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. highkey the better vibe 🔥

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

Simple demos for QUIC usage under these various scenarios can be found at https://github.com/openssl/openssl/tree/master/doc/designs/ddd.

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.

✨ QUIC-SPECIFIC APIS ✨

Okay bestie, this next bit is literally the new-API dump, and it's highkey all about APIs that are directly or indirectly related to QUIC. For the full deets on how each API actually behaves, go read the man page it points to. 💅

First up, these SSL APIs are brand new but they're lowkey relevant to both QUIC and DTLS:

  • SSL_get_event_timeout(3) — tells you when the QUIC implementation should next be woken up via a call to SSL_handle_events(3) (or another I/O function like SSL_read(3) or SSL_write(3)), if ever. Also works with DTLS and supersedes DTLSv1_get_timeout(3) for new usage. ⏰
  • SSL_handle_events(3) — this is a non-specific I/O operation that makes a best effort attempt to perform any pending I/O or timeout processing. It can 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, like SSL_read(3) and SSL_write(3), implicitly perform event handling on the SSL object, so you only need to call this function if no other I/O function is going to be called. Also works with DTLS and supersedes DTLSv1_handle_timeout(3) for new usage. 🔥

Now for the SSL APIs that are specifically QUIC:

  • SSL_new_listener(3) — creates a listener SSL object. It's different from a normal SSL object because it's used to provide an abstraction for the acceptance of network connections in a protocol-agnostic manner. Currently, listener SSL objects are only supported for QUIC server usage or client-only usage. The listener interface may expand to support additional protocols in the future. 👑
  • SSL_new_listener_from(3) — creates a listener SSL object that is subordinate to a QUIC domain SSL object ssl. See SSL_new_domain(3) and openssl-quic-concurrency(7) for details on QUIC domain SSL objects.
  • SSL_is_listener(3) — returns 1 if and only if an SSL object is a listener SSL object.
  • SSL_get0_listener(3) — returns an SSL object pointer (potentially to the same object on which it is called) or NULL.
  • SSL_listen(3) — begins listening after a listener has been created. You ordinarily don't need to call this because it will be called automatically on the first call to SSL_accept_connection(3).
  • SSL_accept_connection(3) — accepts a new incoming connection for a listener SSL object. A new SSL object representing the accepted connection is created and returned on success. If no incoming connection is available and the listener SSL object is configured in nonblocking mode, NULL is returned. 🫶
  • SSL_get_accept_connection_queue_len(3) — returns an informational value listing the number of connections waiting to be popped from the queue via calls to SSL_accept_connection().
  • SSL_new_from_listener(3) — creates a client connection under a given listener SSL object. For QUIC, you can also use SSL_new_from_listener() in conjunction with a listener that does accept incoming connections (i.e., one that was not created using SSL_LISTENER_FLAG_NO_ACCEPT), leading to a UDP network endpoint that has both incoming and outgoing connections. 🌈
  • SSL_new_domain(3) — creates a new QUIC event domain, represented as an SSL object. This is known as a QUIC domain SSL object. The concept of a QUIC event domain is discussed in detail in openssl-quic-concurrency(7).
  • SSL_is_domain(3) — returns 1 if an SSL object is a QUIC domain SSL object.
  • SSL_get0_domain(3) — obtains a pointer to the QUIC domain SSL object in an SSL object hierarchy (if any).
  • SSL_set_blocking_mode(3), SSL_get_blocking_mode(3) — configure 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 that supports polling, SSL_get_rpoll_descriptor(3) outputs an OS resource that 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 should be used only 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.
  • SSL_set1_initial_peer_addr(3) — sets the initial peer address for an outgoing QUIC connection. You must use this 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.
  • SSL_shutdown_ex(3) — augments SSL_shutdown(3) by letting you specify an application error code. It also lets an application decide how quickly it wants a shutdown to be performed, potentially by trading off strict RFC compliance. 😭
  • SSL_stream_conclude(3) — lets an application 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.
  • SSL_stream_reset(3) — lets an application 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) — let an application 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) — let an application determine the application error code that 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.
  • SSL_get_conn_close_info(3) — lets an application determine the error code that was signalled when the local or remote endpoint terminated the QUIC connection. 💀
  • SSL_get0_connection(3) — gets the QUIC connection SSL object from a QUIC stream SSL object.
  • SSL_is_connection(3) — returns 1 if an SSL object is not a QUIC stream SSL object.
  • SSL_get_stream_type(3) — provides information on the kind of QUIC stream attached to the SSL object.
  • SSL_get_stream_id(3) — returns the QUIC stream ID that the QUIC protocol has associated with a QUIC stream.
  • SSL_new_stream(3) — creates a new QUIC stream SSL object representing a new, locally-initiated QUIC stream.
  • SSL_accept_stream(3) — 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.
  • SSL_get_accept_stream_queue_len(3) — provides information on the number of pending remotely-initiated streams.
  • SSL_set_incoming_stream_policy(3) — 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).
  • SSL_set_default_stream_mode(3) — used to configure or disable default stream mode; see the MODES OF OPERATION section for details.

Finally, these BIO APIs aren't specific to QUIC, but they've been added to facilitate QUIC-specific requirements and they're closely associated with its use:

🧵 THREAD ASSISTED MODE 🧵

Okay so bestie, if you're a client and you want the laziest possible life, this is your era. The optional thread assisted mode for clients works with OSSL_QUIC_client_thread_method(3). In this mode, a background thread gets created for you automatically — no cap, it just spawns. Then the OpenSSL QUIC implementation takes full responsibility for making sure timeout events get handled on a timely basis, even if your app doesn't call any SSL I/O function like SSL_read(3) or SSL_write(3) for a long time. Like, you can just go touch grass and it's fine 🌱.

All the necessary locking is handled automatically internally, but — and this is the important bit — the thread safety guarantees for the public SSL API are unchanged. So an application must still do its own locking if it wants to make concurrent use of the public SSL APIs. Internal locking ≠ your locking, bestie.

Because this method relies on threads, it's not available on platforms where threading support isn't available or isn't supported by OpenSSL. Sad but true 😭. However, it does provide the simplest mode of usage for an application. Trade-offs, we love them.

The implementation may or may not use a common thread or thread pool to service multiple SSL objects in the same SSL_CTX. Like, it's giving "we're not promising anything" energy, and honestly? Respect.

🔁 APPLICATION-DRIVEN EVENT LOOPS 🔁

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

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. Customisation is the moment 💅.

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

Once any of these events occurs, SSL_handle_events(3) should be called.

  • If the network read and write BIOs provided were not pollable (for example, in the case of BIO_s_dgram_pair(3)), the application is responsible for managing and synchronising network I/O. It should call SSL_handle_events(3) after it writes data to a BIO_s_dgram_pair(3) or otherwise takes action so that the QUIC implementation can read new datagrams via a call to BIO_recvmmsg(3) on the underlying network BIO. The QUIC implementation may output datagrams via a call to BIO_sendmmsg(3) and the application is responsible for ensuring these are transmitted.

The application must call SSL_get_event_timeout(3) after every call to SSL_handle_events(3) (or another I/O function on the SSL object), and ensure that a call to SSL_handle_events(3) is performed after the specified timeout (if any). That's not a suggestion, that's a requirement, bestie 🫶.

🪟 WINDOWS APPLICATION NOTES 🪟

QUIC protocol uses UDP sockets. The recvfrom() function on Windows may fail with "WSAECONNRESET" error, causing the OpenSSL QUIC stack to enter permanent error, which prevents further communication over QUIC protocol. Yeah. Permanent. 😭

Applications should disable SIO_UDP_CONNRESET and SIO_UDP_NETRESET error notification on UDP sockets they pass to the OpenSSL QUIC stack. More details can be found here: https://learn.microsoft.com/en-us/windows/win32/winsock/winsock-ioctls#sio_udp_connreset-opcode-setting-i-t3

OpenSSL attempts to always disable SIO_UDP_CONNRESET and SIO_UDP_NETRESET on UDP sockets it receives from the application, but no error is reported back if the respective WSAIoctl() calls fail. So a robust application should set those options itself so it can handle error notifications from WSAIoctl() properly. Do it yourself and you'll actually know if it worked 💀.

Don't sleep on this one — silent failure is the villain of this whole section 🦋.

👀 SEE ALSO 👀

The whole squad, assembled:

Copyright 2022-2026 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.

<aqua>And that's a wrap on the whole page, bestie 🎀✨ This edition was served to you as version 3.5.6, dated 2026-04-07, filed under OPENSSL-QUIC(7ossl) — go forth and QUIC responsibly 💜🦋</aqua>

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