# Copyright (c) 2017 - 2021 LiteSpeed Technologies Inc.  See LICENSE.
LSQUIC APIs
===========

LSQUIC exposes the following object types to the user:

    - Engine Settings   (struct lsquic_engine_settings)
    - Stream Interface  (struct lsquic_stream_if) 
    - Engine API        (struct lsquic_engine_api)
    - Engine
    - Connection
    - Stream

The first three -- engine settings, engine APIs, and stream interface --
are used to instantiate the engine.  After engine is instantiated, the
user code need only concern itself with engine, connections, and streams.


Engine Settings
---------------

Engine settings is the struct lsquic_engine_settings.  It contains various
QUIC settings and LSQUIC parameters.  The usual way to use it is to initialize
it to default values using lsquic_engine_init_settings(), modify any values
if necessary, and pass it as parameter to lsquic_engine_new().

QUIC settings are specified by the following members:

    lsquic_engine_settings      QUIC
    member                      parameter
    ----------------------      ---------
    es_cfcw                     CFCW
    es_sfcw                     SFCW
    es_max_streams_in           MIDS
    es_ua                       UAID
    es_versions                 VER
    es_idle_conn_to             ICSL
    es_silent_close             SCLS
    es_support_srej             COPT/SREJ
    es_support_nstp             COPT/NSTP
    es_support_tcid0            TCID

The following parameters affect run-time behavior:

    es_rw_once                  Important: affects event dispatch
    es_handshake_to
    es_support_push
    es_pace_packets

Other noteworthy settings:

    es_max_header_list_size
    es_progress_check

To be sure your settings are good (in other words, passing this struct won't
trip up the engine constructor), use lsquic_engine_check_settings().


Stream Interface
----------------

The stream interface, lsquic_stream_if, specifies callbacks LSQUIC engine
will call for connections and streams.

The following callbacks should be specified for connection:

    on_new_conn             This is called when connection is created.

    on_goaway_received      This function is called when we receive GOAWAY
                            frame from peer.  This callback is optional.

    on_conn_closed          Connection is closed: all streams have been
                            destroyed.

The streams have four callbacks:

    on_new_stream           Stream has been created.

    on_read                 Stream can be read from (see Events).

    on_write                Stream can be written to (see Events).

    on_close                Stream has been closed.

For both connections and streams, the "on new" callback return value can
be used to specify user-supplied data.  This data pointer is optional and
can be NULL.  It can also refer to the same data for the connection and
its streams.  "on close" callbacks should be used to free user-supplied
data.


Engine API
----------

The engine API, struct lsquic_engine_api, is a combination structure to
make calling lsquic_engine_new() manageable.  It holds references to
struct lsquic_engine_settings and struct lsquic_stream_if, as well as:

    - Interface for sending outgoing packets, ea_packets_out
    - Interface for allocating memory for outgoing packet buffers
      (optional).
    - Interface for share memory hash, ea_shi
    - Optional interface for reporting connections whose handshake
      did not complete (ea_bad_handshake)

ea_packets_out is a pointer to a function of type lsquic_packets_out_f.
The engine calls this function when it is appropriate to send out packets
for one or more connections, which it gives to the function in a batch.
This batch is an array of struct lsquic_out_spec.


Engine
------

The engine is instantiated using lsquic_engine_new().  The first parameter
is a list flags and the second parameter is the reference to the engine
api.  The engine settings are specified, they are copied; changing
the setting after the engine has been created will not affect engine's
behavior.  If the settings are not specified, the engine will use default
settings created by lsquic_engine_init_settings().

Once the engine is instantiated, there are four main ways to use it to
drive QUIC connections:

    1. Create a connection using lsquic_engine_connect().
    2. Feed it incoming packets using lsquic_engine_packet_in() function.
    3. Process connections using one of the connection queue functions
       (see Connection Queues).
    4. Accept outgoing packets for sending (and send them!) using
       ea_packets_out callback.


Connection Management
---------------------

A connection needs to be processed once in a while.  It needs to be
processed when one of the following is true:

    - There are incoming packets;
    - A stream is both readable by the user code and the user code wants
      to read from it;
    - A stream is both writeable by the user code and the user code wants
      to write to it;
    - User has written to stream outside of on_write() callbacks (that is
      allowed) and now there are packets ready to be sent;
    - A timer (pacer, retransmission, idle, etc) has expired;
    - A control frame needs to be sent out;
    - A stream needs to be serviced or created.

Each of these use cases is handled by a single function:

    lsquic_engine_process_conns()

The connections to which the conditions above apply are processed (or
"ticked") in the least recently ticked order.  After calling this function,
you can see when is the next time a connection needs to be processed using

    lsquic_engine_earliest_adv_tick()

Based on this value, next event can be scheduled (in the event loop of
your choice).

Connection
----------

A connection is created using lsquic_engine_connect().  When on_new_conn()
is called, the client code should call lsquic_conn_make_stream() one or
more times.  One new stream will be created for each one of those calls.

Several auxiliary functions are available:

    - lsquic_conn_id()
    - lsquic_conn_going_away()
    - lsquic_conn_get_peer_ctx()
    - lsquic_conn_get_stream_by_id()
    - lsquic_conn_get_ctx()


Stream
------

LSQUIC stream hides QUIC and HTTP/2 framing complexities from the user.
What it presents is a way to send HTTP headers and, optionally, body to
peer.  On read side, the user gets what looks like HTTP/1.1 stream.

Expected usage for client is to express the desire to write to stream
using lsquic_stream_wantwrite() call.  Once on_write() is called:

    1. Write headers using lsquic_stream_send_headers()
    2. Optionally write payload body using of of lsquic_stream_write(),
       lsquic_stream_writev(), or lsquic_stream_writef().

That done, shutdown write side using lsquic_stream_shutdown(), unregister
for write events and register for read events using lsquic_stream_wantread().

Read and parse HTTP/1.1 stream from on_read() callback until end-of-stream
or an error is encountered.

Then unregister the read event and shutdown the read side.  The stream will
be closed after that at some point and on_close() callback will be called,
at which point resources can be freed.  (Internally, the stream object is
not destroyed until either all the packets carrying its data are ACKed or
the connection is destroyed).

on_read() and on_write() callbacks are dispatched differently based on the
value of es_rw_once:

If es_rw_once is false, then the callbacks are dispatched in a loop until
the user unregisters the event or the stream becomes unreadable (or
unwriteable).

If es_rw_once is true, on_read() and on_write() are called once "per tick".
It is the up to the user to read and write enough data.


Events
------

Stream events are persistent: once call lsquic_stream_wantwrite() or
lsquic_stream_wantread(), the event stays active until turned off.

Note that when an error is encountered (such as a stream reset), the
stream becomes readable and writeable: this allows user code to collect
the error.


Versions
--------

QUIC version are listed in enum lsquic_version.  To specify a list of
versions, they are usually placed in a bitmask, e.g. es_versions.
