=pod

=begin comment

NB: Changes to the source code samples in this file should also be reflected in
demos/guide/quic-multi-stream.c

=end comment

=head1 NAME

ossl-guide-quic-multi-stream
- OpenSSL Guide: Writing a simple multi-stream QUIC client

=head1 INTRODUCTION

This page will introduce some important concepts required to write a simple
QUIC multi-stream application. It assumes a basic understanding of QUIC and how
it is used in OpenSSL. See L<ossl-guide-quic-introduction(7)> and
L<ossl-guide-quic-client-block(7)>.

=head1 QUIC STREAMS

In a QUIC multi-stream application we separate out the concepts of a QUIC
"connection" and a QUIC "stream". A connection object represents the overarching
details of the connection between a client and a server including all its
negotiated and configured parameters. We use the B<SSL> object for that in an
OpenSSL application (known as the connection B<SSL> object). It is created by an
application calling L<SSL_new(3)>.

Separately a connection can have zero or more streams associated with it
(although a connection with zero streams is probably not very useful, so
normally you would have at least one). A stream is used to send and receive
data between the two peers. Each stream is also represented by an B<SSL>
object. A stream is logically independent of all the other streams associated
with the same connection. Data sent on a stream is guaranteed to be delivered
in the order that it was sent within that stream. The same is not true across
streams, e.g. if an application sends data on stream 1 first and then sends some
more data on stream 2 second, then the remote peer may receive the data sent on
stream 2 before it receives the data sent on stream 1.

Once the connection B<SSL> object has completed its handshake (i.e.
L<SSL_connect(3)> has returned 1), stream B<SSL> objects are created by the
application calling L<SSL_new_stream(3)> or L<SSL_accept_stream(3)> (see
L</CREATING NEW STREAMS> below).

The same threading rules apply to B<SSL> objects as for most OpenSSL objects
(see L<ossl-guide-libraries-introduction(7)>). In particular most OpenSSL
functions are thread safe, but the B<SSL> object is not. This means that you can
use an B<SSL> object representing one stream at the same time as another thread
is using a different B<SSL> object for a different stream on the same
connection. But you cannot use the same B<SSL> object on two different threads
at the same time (without additional application level locking).

=head1 THE DEFAULT STREAM

A connection B<SSL> object may also (optionally) be associated with a stream.
This stream is known as the default stream. The default stream is automatically
created and associated with the B<SSL> object when the application calls
L<SSL_read_ex(3)>, L<SSL_read(3)>, L<SSL_write_ex(3)> or L<SSL_write(3)> and
passes the connection B<SSL> object as a parameter.

If a client application calls L<SSL_write_ex(3)> or L<SSL_write(3)> first then
(by default) the default stream will be a client-initiated bi-directional
stream. If a client application calls L<SSL_read_ex(3)> or L<SSL_read(3)>
first then the first stream initiated by the server will be used as the default
stream (whether it is bi-directional or uni-directional).

This behaviour can be controlled via the default stream mode. See
L<SSL_set_default_stream_mode(3)> for further details.

It is recommended that new multi-stream applications should not use a default
stream at all and instead should use a separate stream B<SSL> object for each
stream that is used. This requires calling L<SSL_set_default_stream_mode(3)>
and setting the mode to B<SSL_DEFAULT_STREAM_MODE_NONE>.

=head1 CREATING NEW STREAMS

An endpoint can create a new stream by calling L<SSL_new_stream(3)>. This
creates a locally initiated stream. In order to do so you must pass the QUIC
connection B<SSL> object as a parameter. You can also specify whether you want a
bi-directional or a uni-directional stream.

The function returns a new QUIC stream B<SSL> object for sending and receiving
data on that stream.

The peer may also initiate streams. An application can use the function
L<SSL_get_accept_stream_queue_len(3)> to determine the number of streams that
the peer has initiated that are waiting for the application to handle. An
application can call L<SSL_accept_stream(3)> to create a new B<SSL> object for
a remotely initiated stream. If the peer has not initiated any then this call
will block until one is available if the connection object is in blocking mode
(see L<SSL_set_blocking_mode(3)>).

When using a default stream OpenSSL will prevent new streams from being
accepted. To override this behaviour you must call
L<SSL_set_incoming_stream_policy(3)> to set the policy to
B<SSL_INCOMING_STREAM_POLICY_ACCEPT>. See the man page for further details. This
is not relevant if the default stream has been disabled as described in
L</THE DEFAULT STREAM> above.

Any stream may be bi-directional or uni-directional. If it is uni-directional
then the initiator can write to it but not read from it, and vice-versa for the
peer. You can determine what type of stream an B<SSL> object represents by
calling L<SSL_get_stream_type(3)>. See the man page for further details.

=head1 USING A STREAM TO SEND AND RECEIVE DATA

Once you have a stream B<SSL> object (which includes the connection B<SSL>
object if a default stream is in use) then you can send and receive data over it
using the L<SSL_write_ex(3)>, L<SSL_write(3)>, L<SSL_read_ex(3)> or
L<SSL_read(3)> functions. See the man pages for further details.

In the event of one of these functions not returning a success code then
you should call L<SSL_get_error(3)> to find out further details about the error.
In blocking mode this will either be a fatal error (e.g. B<SSL_ERROR_SYSCALL>
or B<SSL_ERROR_SSL>), or it will be B<SSL_ERROR_ZERO_RETURN> which can occur
when attempting to read data from a stream and the peer has indicated that the
stream is concluded (i.e. "FIN" has been signalled on the stream). This means
that the peer will send no more data on that stream. Note that the
interpretation of B<SSL_ERROR_ZERO_RETURN> is slightly different for a QUIC
application compared to a TLS application. In TLS it occurs when the connection
has been shutdown by the peer. In QUIC this only tells you that the current
stream has been concluded by the peer. It tells you nothing about the underlying
connection. If the peer has concluded the stream then no more data will be
received on it, however an application can still send data to the peer until
the send side of the stream has also been concluded. This can happen by the
application calling L<SSL_stream_conclude(3)>. It is an error to attempt to
send more data on a stream after L<SSL_stream_conclude(3)> has been called.

It is also possible to abandon a stream abnormally by calling
L<SSL_stream_reset(3)>.

Once a stream object is no longer needed it should be freed via a call to
L<SSL_free(3)>. An application should not call L<SSL_shutdown(3)> on it since
this is only meaningful for connection level B<SSL> objects. Freeing the stream
will automatically signal STOP_SENDING to the peer.

=head1 STREAMS AND CONNECTIONS

Given a stream object it is possible to get the B<SSL> object corresponding to
the connection via a call to L<SSL_get0_connection(3)>. Multi-threaded
restrictions apply so care should be taken when using the returned connection
object. Specifically, if you are handling each of your stream objects in a
different thread and call L<SSL_get0_connection(3)> from within that thread then
you must be careful to not to call any function that uses the connection object
at the same time as one of the other threads is also using that connection
object (with the exception of L<SSL_accept_stream(3)> and
L<SSL_get_accept_stream_queue_len(3)> which are thread-safe).

A stream object does not inherit all its settings and values from its parent
B<SSL> connection object. Therefore certain function calls that are relevant to
the connection as a whole will not work on a stream. For example the function
L<SSL_get_certificate(3)> can be used to obtain a handle on the peer certificate
when called with a connection B<SSL> object. When called with a stream B<SSL>
object it will return NULL.

=head1 SIMPLE MULTI-STREAM QUIC CLIENT EXAMPLE

This section will present various source code samples demonstrating how to write
a simple multi-stream QUIC client application which connects to a server, send
some HTTP/1.0 requests to it, and read back the responses. Note that HTTP/1.0
over QUIC is non-standard and will not be supported by real world servers. This
is for demonstration purposes only.

We will build on the example code for the simple blocking QUIC client that is
covered on the L<ossl-guide-quic-client-block(7)> page and we assume that you
are familiar with it. We will only describe the differences between the simple
blocking QUIC client and the multi-stream QUIC client. Although the example code
uses blocking B<SSL> objects, you can equally use nonblocking B<SSL> objects.
See L<ossl-guide-quic-client-non-block(7)> for more information about writing a
nonblocking QUIC client.

The complete source code for this example multi-stream QUIC client is available
in the C<demos/guide> directory of the OpenSSL source distribution in the file
C<quic-multi-stream.c>. It is also available online at
L<https://github.com/openssl/openssl/blob/master/demos/guide/quic-multi-stream.c>.

=head2 Disabling the default stream

As discussed above in L</THE DEFAULT STREAM> we will follow the recommendation
to disable the default stream for our multi-stream client. To do this we call
the L<SSL_set_default_stream_mode(3)> function and pass in our connection B<SSL>
object and the value B<SSL_DEFAULT_STREAM_MODE_NONE>.

    /*
     * We will use multiple streams so we will disable the default stream mode.
     * This is not a requirement for using multiple streams but is recommended.
     */
    if (!SSL_set_default_stream_mode(ssl, SSL_DEFAULT_STREAM_MODE_NONE)) {
        printf("Failed to disable the default stream mode\n");
        goto end;
    }

=head2 Creating the request streams

For the purposes of this example we will create two different streams to send
two different HTTP requests to the server. For the purposes of demonstration the
first of these will be a bi-directional stream and the second one will be a
uni-directional one:

    /*
     * We create two new client initiated streams. The first will be
     * bi-directional, and the second will be uni-directional.
     */
    stream1 = SSL_new_stream(ssl, 0);
    stream2 = SSL_new_stream(ssl, SSL_STREAM_FLAG_UNI);
    if (stream1 == NULL || stream2 == NULL) {
        printf("Failed to create streams\n");
        goto end;
    }

=head2 Writing data to the streams

Once the streams are successfully created we can start writing data to them. In
this example we will be sending a different HTTP request on each stream. To
avoid repeating too much code we write a simple helper function to send an HTTP
request to a stream:

    int write_a_request(SSL *stream, const char *request_start,
                        const char *hostname)
    {
        const char *request_end = "\r\n\r\n";
        size_t written;

        if (!SSL_write_ex(stream, request_start, strlen(request_start), &written))
            return 0;
        if (!SSL_write_ex(stream, hostname, strlen(hostname), &written))
            return 0;
        if (!SSL_write_ex(stream, request_end, strlen(request_end), &written))
            return 0;

        return 1;
    }

We assume the strings B<request1_start> and B<request2_start> hold the
appropriate HTTP requests. We can then call our helper function above to send
the requests on the two streams. For the sake of simplicity this example does
this sequentially, writing to B<stream1> first and, when this is successful,
writing to B<stream2> second. Remember that our client is blocking so these
calls will only return once they have been successfully completed. A real
application would not need to do these writes sequentially or in any particular
order. For example we could start two threads (one for each stream) and write
the requests to each stream simultaneously.

    /* Write an HTTP GET request on each of our streams to the peer */
    if (!write_a_request(stream1, request1_start, hostname)) {
        printf("Failed to write HTTP request on stream 1\n");
        goto end;
    }

    if (!write_a_request(stream2, request2_start, hostname)) {
        printf("Failed to write HTTP request on stream 2\n");
        goto end;
    }

=head2 Reading data from a stream

In this example B<stream1> is a bi-directional stream so, once we have sent the
request on it, we can attempt to read the response from the server back. Here
we just repeatedly call L<SSL_read_ex(3)> until that function fails (indicating
either that there has been a problem, or that the peer has signalled the stream
as concluded).

    printf("Stream 1 data:\n");
    /*
     * Get up to sizeof(buf) bytes of the response from stream 1 (which is a
     * bidirectional stream). We keep reading until the server closes the
     * connection.
     */
    while (SSL_read_ex(stream1, buf, sizeof(buf), &readbytes)) {
        /*
        * OpenSSL does not guarantee that the returned data is a string or
        * that it is NUL terminated so we use fwrite() to write the exact
        * number of bytes that we read. The data could be non-printable or
        * have NUL characters in the middle of it. For this simple example
        * we're going to print it to stdout anyway.
        */
        fwrite(buf, 1, readbytes, stdout);
    }
    /* In case the response didn't finish with a newline we add one now */
    printf("\n");

In a blocking application like this one calls to L<SSL_read_ex(3)> will either
succeed immediately returning data that is already available, or they will block
waiting for more data to become available and return it when it is, or they will
fail with a 0 response code.

Once we exit the while loop above we know that the last call to
L<SSL_read_ex(3)> gave a 0 response code so we call the L<SSL_get_error(3)>
function to find out more details. Since this is a blocking application this
will either return B<SSL_ERROR_SYSCALL> or B<SSL_ERROR_SSL> indicating a
fundamental problem, or it will return B<SSL_ERROR_ZERO_RETURN> indicating that
the stream is concluded and there will be no more data available to read from
it. Care must be taken to distinguish between an error at the stream level (i.e.
a stream reset) and an error at the connection level (i.e. a connection closed).
The L<SSL_get_stream_read_state(3)> function can be used to distinguish between
these different cases.

    /*
     * Check whether we finished the while loop above normally or as the
     * result of an error. The 0 argument to SSL_get_error() is the return
     * code we received from the SSL_read_ex() call. It must be 0 in order
     * to get here. Normal completion is indicated by SSL_ERROR_ZERO_RETURN. In
     * QUIC terms this means that the peer has sent FIN on the stream to
     * indicate that no further data will be sent.
     */
    switch (SSL_get_error(stream1, 0)) {
    case SSL_ERROR_ZERO_RETURN:
        /* Normal completion of the stream */
        break;

    case SSL_ERROR_SSL:
        /*
         * Some stream fatal error occurred. This could be because of a stream
         * reset - or some failure occurred on the underlying connection.
         */
        switch (SSL_get_stream_read_state(stream1)) {
        case SSL_STREAM_STATE_RESET_REMOTE:
            printf("Stream reset occurred\n");
            /* The stream has been reset but the connection is still healthy. */
            break;

        case SSL_STREAM_STATE_CONN_CLOSED:
            printf("Connection closed\n");
            /* Connection is already closed. Skip SSL_shutdown() */
            goto end;

        default:
            printf("Unknown stream failure\n");
            break;
        }
        break;

    default:
        /* Some other unexpected error occurred */
        printf ("Failed reading remaining data\n");
        break;
    }

=head2 Accepting an incoming stream

Our B<stream2> object that we created above was a uni-directional stream so it
cannot be used to receive data from the server. In this hypothetical example
we assume that the server initiates a new stream to send us back the data that
we requested. To do that we call L<SSL_accept_stream(3)>. Since this is a
blocking application this will wait indefinitely until the new stream has
arrived and is available for us to accept. In the event of an error it will
return B<NULL>.

    /*
     * In our hypothetical HTTP/1.0 over QUIC protocol that we are using we
     * assume that the server will respond with a server initiated stream
     * containing the data requested in our uni-directional stream. This doesn't
     * really make sense to do in a real protocol, but its just for
     * demonstration purposes.
     *
     * We're using blocking mode so this will block until a stream becomes
     * available. We could override this behaviour if we wanted to by setting
     * the SSL_ACCEPT_STREAM_NO_BLOCK flag in the second argument below.
     */
    stream3 = SSL_accept_stream(ssl, 0);
    if (stream3 == NULL) {
        printf("Failed to accept a new stream\n");
        goto end;
    }

We can now read data from the stream in the same way that we did for B<stream1>
above. We won't repeat that here.

=head2 Cleaning up the streams

Once we have finished using our streams we can simply free them by calling
L<SSL_free(3)>. Optionally we could call L<SSL_stream_conclude(3)> on them if
we want to indicate to the peer that we won't be sending them any more data, but
we don't do that in this example because we assume that the HTTP application
protocol supplies sufficient information for the peer to know when we have
finished sending request data.

We should not call L<SSL_shutdown(3)> or L<SSL_shutdown_ex(3)> on the stream
objects since those calls should not be used for streams.

    SSL_free(stream1);
    SSL_free(stream2);
    SSL_free(stream3);

=head1 SEE ALSO

L<ossl-guide-introduction(7)>, L<ossl-guide-libraries-introduction(7)>,
L<ossl-guide-libssl-introduction(7)> L<ossl-guide-quic-introduction(7)>,
L<ossl-guide-quic-client-block(7)>

=head1 COPYRIGHT

Copyright 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
L<https://www.openssl.org/source/license.html>.

=cut
