The ngtcp2 programmers' guide for early adopters
================================================

This document is written for early adopters of ngtcp2 library.  It
describes a brief introduction of programming ngtcp2.

Prerequisites
-------------

Reading :rfc:`9000` and :rfc:`9001` helps you a lot to write QUIC
application.  They describes how TLS is integrated into QUIC and why
the existing TLS stack cannot be used with QUIC.

QUIC requires the special interface from TLS stack, which is probably
not available from most of the existing TLS stacks.  As far as I know,
the TLS stacks maintained by the active participants of QUIC working
group only get this interface at the time of this writing.  In order
to build QUIC application you have to choose one of them.  Here is the
list of TLS stacks which are supposed to provide such interface and
for which we provide crypto helper libraries:

* `OpenSSL with QUIC support
  <https://github.com/quictls/openssl/tree/OpenSSL_1_1_1q+quic>`_
* GnuTLS >= 3.7.2
* BoringSSL
* Picotls
* wolfSSL

Creating ngtcp2_conn object
---------------------------

:type:`ngtcp2_conn` is the primary object to present a single QUIC
connection.  Use `ngtcp2_conn_client_new()` for client application,
and `ngtcp2_conn_server_new()` for server.

They require :type:`ngtcp2_callbacks`, :type:`ngtcp2_settings`, and
:type:`ngtcp2_transport_params` objects.

The :type:`ngtcp2_callbacks` contains the callback functions which
:type:`ngtcp2_conn` calls when a specific event happens, say,
receiving stream data or stream is closed, etc.  Some of the callback
functions are optional.  For client application, the following
callback functions must be set:

* :member:`client_initial <ngtcp2_callbacks.client_initial>`:
  `ngtcp2_crypto_client_initial_cb()` can be passed directly.
* :member:`recv_crypto_data <ngtcp2_callbacks.recv_crypto_data>`:
  `ngtcp2_crypto_recv_crypto_data_cb()` can be passed directly.
* :member:`encrypt <ngtcp2_callbacks.encrypt>`:
  `ngtcp2_crypto_encrypt_cb()` can be passed directly.
* :member:`decrypt <ngtcp2_callbacks.decrypt>`:
  `ngtcp2_crypto_decrypt_cb()` can be passed directly.
* :member:`hp_mask <ngtcp2_callbacks.hp_mask>`:
  `ngtcp2_crypto_hp_mask_cb()` can be passed directly.
* :member:`recv_retry <ngtcp2_callbacks.recv_retry>`:
  `ngtcp2_crypto_recv_retry_cb()` can be passed directly.
* :member:`rand <ngtcp2_callbacks.rand>`
* :member:`get_new_connection_id
  <ngtcp2_callbacks.get_new_connection_id>`
* :member:`update_key <ngtcp2_callbacks.update_key>`:
  `ngtcp2_crypto_update_key_cb()` can be passed directly.
* :member:`delete_crypto_aead_ctx
  <ngtcp2_callbacks.delete_crypto_aead_ctx>`:
  `ngtcp2_crypto_delete_crypto_aead_ctx_cb()` can be passed directly.
* :member:`delete_crypto_cipher_ctx
  <ngtcp2_callbacks.delete_crypto_cipher_ctx>`:
  `ngtcp2_crypto_delete_crypto_cipher_ctx_cb()` can be passed
  directly.
* :member:`get_path_challenge_data
  <ngtcp2_callbacks.get_path_challenge_data>`:
  `ngtcp2_crypto_get_path_challenge_data_cb()` can be passed directly.
* :member:`version_negotiation
  <ngtcp2_callbacks.version_negotiation>`:
  `ngtcp2_crypto_version_negotiation_cb()` can be passed directly.

For server application, the following callback functions must be set:

* :member:`recv_client_initial
  <ngtcp2_callbacks.recv_client_initial>`:
  `ngtcp2_crypto_recv_client_initial_cb()` can be passed directly.
* :member:`recv_crypto_data <ngtcp2_callbacks.recv_crypto_data>`:
  `ngtcp2_crypto_recv_crypto_data_cb()` can be passed directly.
* :member:`encrypt <ngtcp2_callbacks.encrypt>`:
  `ngtcp2_crypto_encrypt_cb()` can be passed directly.
* :member:`decrypt <ngtcp2_callbacks.decrypt>`:
  `ngtcp2_crypto_decrypt_cb()` can be passed directly.
* :member:`hp_mask <ngtcp2_callbacks.hp_mask>`:
  `ngtcp2_crypto_hp_mask_cb()` can be passed directly.
* :member:`rand <ngtcp2_callbacks.rand>`
* :member:`get_new_connection_id
  <ngtcp2_callbacks.get_new_connection_id>`
* :member:`update_key <ngtcp2_callbacks.update_key>`:
  `ngtcp2_crypto_update_key_cb()` can be passed directly.
* :member:`delete_crypto_aead_ctx
  <ngtcp2_callbacks.delete_crypto_aead_ctx>`:
  `ngtcp2_crypto_delete_crypto_aead_ctx_cb()` can be passed directly.
* :member:`delete_crypto_cipher_ctx
  <ngtcp2_callbacks.delete_crypto_cipher_ctx>`:
  `ngtcp2_crypto_delete_crypto_cipher_ctx_cb()` can be passed
  directly.
* :member:`get_path_challenge_data
  <ngtcp2_callbacks.get_path_challenge_data>`:
  `ngtcp2_crypto_get_path_challenge_data_cb()` can be passed directly.
* :member:`version_negotiation
  <ngtcp2_callbacks.version_negotiation>`:
  `ngtcp2_crypto_version_negotiation_cb()` can be passed directly.

``ngtcp2_crypto_*`` functions are a part of :doc:`ngtcp2 crypto API
<crypto_apiref>` which provides easy integration with the supported
TLS backend.  It vastly simplifies TLS integration and is strongly
recommended.

:type:`ngtcp2_settings` contains the settings for QUIC connection.
All fields must be set.  Application should call
`ngtcp2_settings_default()` to set the default values.  It would be
very useful to enable debug logging by setting logging function to
:member:`ngtcp2_settings.log_printf` field.  ngtcp2 library relies on
the timestamp fed from application.  The initial timestamp must be
passed to :member:`ngtcp2_settings.initial_ts` field in nanosecond
resolution.  ngtcp2 cares about the difference from that initial
value.  It could be any timestamp which increases monotonically, and
actual value does not matter.

:type:`ngtcp2_transport_params` contains QUIC transport parameters
which is sent to a remote endpoint during handshake.  All fields must
be set.  Application should call `ngtcp2_transport_params_default()`
to set the default values.

Client application has to supply Connection IDs to
`ngtcp2_conn_client_new()`.  The *dcid* parameter is the destination
connection ID (DCID), and which should be random byte string and at
least 8 bytes long.  The *scid* is the source connection ID (SCID)
which identifies the client itself.  The *version* parameter is the
QUIC version to use.  It should be :macro:`NGTCP2_PROTO_VER_V1`.

Similarly, server application has to supply these parameters to
`ngtcp2_conn_server_new()`.  But the *dcid* must be the same value
which is received from client (which is client SCID).  The *scid* is
chosen by server.  Don't use DCID in client packet as server SCID.
The *version* parameter is the QUIC version to use.  It should be
:macro:`NGTCP2_PROTO_VER_V1`.

A path is very important to QUIC connection.  It is the pair of
endpoints, local and remote.  The path passed to
`ngtcp2_conn_client_new()` and `ngtcp2_conn_server_new()` is a network
path that handshake is performed.  The path must not change during
handshake.  After handshake is confirmed, client can migrate to new
path.  An application must provide actual path to the API function to
tell the library where a packet comes from.  The "write" API function
takes path parameter and fills it to which the packet should be sent.

TLS integration
---------------

Use of :doc:`ngtcp2 crypto API <crypto_apiref>` is strongly
recommended because it vastly simplifies the TLS integration.

The most of the TLS work is done by the callback functions passed to
:type:`ngtcp2_callbacks` object.  There are some operations left to
application in order to make TLS integration work.  We have a set of
helper functions to make it easier for applications to configure TLS
stack object to work with QUIC and ngtcp2.  They are specific to each
supported TLS stack:

- OpenSSL

  * `ngtcp2_crypto_openssl_configure_client_context`
  * `ngtcp2_crypto_openssl_configure_server_context`

- BoringSSL

  * `ngtcp2_crypto_boringssl_configure_client_context`
  * `ngtcp2_crypto_boringssl_configure_server_context`

- GnuTLS

  * `ngtcp2_crypto_gnutls_configure_client_session`
  * `ngtcp2_crypto_gnutls_configure_server_session`

- Picotls

  * `ngtcp2_crypto_picotls_configure_client_context`
  * `ngtcp2_crypto_picotls_configure_server_context`
  * `ngtcp2_crypto_picotls_configure_client_session`
  * `ngtcp2_crypto_picotls_configure_server_session`

- wolfSSL

  * `ngtcp2_crypto_wolfssl_configure_client_context`
  * `ngtcp2_crypto_wolfssl_configure_server_context`

They make the minimal QUIC specific changes to TLS stack object.  See
the ngtcp2 crypto API header files for each supported TLS stack.  In
order to make these functions work, we require that a pointer to
:type:`ngtcp2_crypto_conn_ref` must be set as a user data in TLS stack
object, and its :member:`ngtcp2_crypto_conn_ref.get_conn` must point
to a function which returns :type:`ngtcp2_conn` of the underlying QUIC
connection.

If you do not use the above helper functions, you need to generate and
install keys to :type:`ngtcp2_conn`, and pass handshake messages to
:type:`ngtcp2_conn` as well.  When TLS stack generates new secrets,
they have to be installed to :type:`ngtcp2_conn` by calling
`ngtcp2_crypto_derive_and_install_rx_key()` and
`ngtcp2_crypto_derive_and_install_tx_key()`.  When TLS stack generates
new crypto data to send, they must be passed to :type:`ngtcp2_conn` by
calling `ngtcp2_conn_submit_crypto_data()`.

When QUIC handshake is completed,
:member:`ngtcp2_callbacks.handshake_completed` callback function is
called.  The local and remote endpoint independently declare handshake
completion.  The endpoint has to confirm that the other endpoint also
finished handshake.  When the handshake is confirmed, client side
:type:`ngtcp2_conn` will call
:member:`ngtcp2_callbacks.handshake_confirmed` callback function.
Server confirms handshake when it declares handshake completion,
therefore, separate handshake confirmation callback is not called.

Read and write packets
----------------------

`ngtcp2_conn_read_pkt()` processes the incoming QUIC packets.  In
order to write QUIC packets, call `ngtcp2_conn_writev_stream()` or
`ngtcp2_conn_write_pkt()`.  The *destlen* parameter must be at least
the value returned from `ngtcp2_conn_get_max_udp_payload_size()`.

In order to send stream data, the application has to first open a
stream.  Use `ngtcp2_conn_open_bidi_stream()` to open bidirectional
stream.  For unidirectional stream, call
`ngtcp2_conn_open_uni_stream()`.  Call `ngtcp2_conn_writev_stream()`
to send stream data.

If BBR congestion control algorithm is used, the additional API
functions are required when sending QUIC packets.  BBR needs pacing
packets.  `ngtcp2_conn_get_send_quantum()` returns the number of bytes
that can be sent without packet spacing.  After one or more calls of
`ngtcp2_conn_writev_stream()` (it can be called multiple times to fill
the buffer sized up to `ngtcp2_conn_get_send_quantum()` bytes), call
`ngtcp2_conn_update_pkt_tx_time()` to set the timer when the next
packet should be sent.  The timer is integrated into
`ngtcp2_conn_get_expiry()`.

Packet handling on server side
------------------------------

Any incoming UDP datagram should be first processed by
`ngtcp2_pkt_decode_version_cid()`.  It can handle Connection ID more
than 20 bytes which is the maximum length defined in QUIC v1.  If the
function returns :macro:`NGTCP2_ERR_VERSION_NEGOTIATION`, server
should send Version Negotiation packet.  Use
`ngtcp2_pkt_write_version_negotiation()` for this purpose.  If
`ngtcp2_pkt_decode_version_cid()` succeeds, then check whether the UDP
datagram belongs to any existing connection by looking up connection
tables by Destination Connection ID.  If it belongs to an existing
connection, pass the UDP datagram to `ngtcp2_conn_read_pkt()`.  If it
does not belong to any existing connection, it should be passed to
`ngtcp2_accept()`.  If it returns :macro:`NGTCP2_ERR_RETRY`, the
server should send Retry packet (use `ngtcp2_crypto_write_retry()` to
create Retry packet).  If it returns an other negative error code,
just drop the packet to the floor and take no action, or send
Stateless Reset packet (use `ngtcp2_pkt_write_stateless_reset()` to
create Stateless Reset packet).  Otherwise, the UDP datagram is
acceptable as a new connection.  Create :type:`ngtcp2_conn` object and
pass the UDP datagram to `ngtcp2_conn_read_pkt()`.

Dealing with early data
-----------------------

Client application has to load resumed TLS session.  It also has to
set the remembered transport parameters using
`ngtcp2_conn_set_early_remote_transport_params()` function.

Other than that, there is no difference between early data and 1RTT
data in terms of API usage.

If early data is rejected by a server, client must call
`ngtcp2_conn_early_data_rejected`.  All connection states altered
during early data transmission are undone.  The library does not
retransmit early data to server as 1RTT data.  If an application
wishes to resend data, it has to reopen streams and writes data again.
See `ngtcp2_conn_early_data_rejected`.

Stream data ownership
--------------------------------

Stream data passed to :type:`ngtcp2_conn` must be held by application
until :member:`ngtcp2_callbacks.acked_stream_data_offset` callbacks is
invoked, telling that the those data are acknowledged by the remote
endpoint and no longer used by the library.

Timers
------

The library does not ask an operating system for any timestamp.
Instead, an application has to supply timestamp to the library.  The
type of timestamp in ngtcp2 library is :type:`ngtcp2_tstamp` which is
nanosecond resolution.  The library only cares the difference of
timestamp, so it does not have to be a system clock.  A monotonic
clock should work better.  It should be same clock passed to
:member:`ngtcp2_settings.initial_ts`.  The duration in ngtcp2 library
is :type:`ngtcp2_duration` which is also nanosecond resolution.

`ngtcp2_conn_get_expiry()` tells an application when timer fires.
When timer fires, call `ngtcp2_conn_handle_expiry()` and
`ngtcp2_conn_write_pkt()` (or `ngtcp2_conn_writev_stream()`).

After calling these functions, new expiry will be set.  The
application should call `ngtcp2_conn_get_expiry()` to restart timer.
If `ngtcp2_conn_get_expiry()` returned :macro:`NGTCP2_ERR_IDLE_CLOSE`,
it means that an idle timer has expired for this particular
connection.  In this case, drop the connection without calling
`ngtcp2_conn_write_connection_close()`.

Connection migration
--------------------

In QUIC, client application can migrate to a new local address.
`ngtcp2_conn_initiate_immediate_migration()` migrates to a new local
address without checking reachability.  On the other hand,
`ngtcp2_conn_initiate_migration()` migrates to a new local address
after a new path is validated (thus reachability is established).

Closing connection abruptly
---------------------------

In order to close QUIC connection abruptly, call
`ngtcp2_conn_write_connection_close()` and get a terminal packet.
After the call, the connection enters the closing state.

The closing and draining state
------------------------------

After the successful call of `ngtcp2_conn_write_connection_close()`,
the connection enters the closing state.  When
`ngtcp2_conn_read_pkt()` returns :macro:`NGTCP2_ERR_DRAINING`, the
connection has entered the draining state.  In these states,
`ngtcp2_conn_writev_stream()` and `ngtcp2_conn_read_pkt()` return an
error (either :macro:`NGTCP2_ERR_CLOSING` or
:macro:`NGTCP2_ERR_DRAINING` depending on the state).
`ngtcp2_conn_write_connection_close()` returns 0 in these states.  If
an application needs to send a packet containing CONNECTION_CLOSE
frame in the closing state, resend the packet produced by the first
call of `ngtcp2_conn_write_connection_close()`.  Therefore, after a
connection has entered one of these states, the application can
discard :type:`ngtcp2_conn` object.  The closing and draining state
should persist at least 3 times the current PTO.

Error handling in general
-------------------------

In general, when error is returned from the ngtcp2 library function,
call `ngtcp2_conn_write_connection_close()` to get terminal packet.
If the successful call of the function creates non-empty packet, the
QUIC connection enters the closing state.

If :macro:`NGTCP2_ERR_DROP_CONN` is returned from
`ngtcp2_conn_read_pkt`, a connection should be dropped without calling
`ngtcp2_conn_write_connection_close()`.  Similarly, if
:macro:`NGTCP2_ERR_IDLE_CLOSE` is returned from
`ngtcp2_conn_handle_expiry`, a connection should be dropped without
calling `ngtcp2_conn_write_connection_close()`.  If
:macro:`NGTCP2_ERR_DRAINING` is returned from `ngtcp2_conn_read_pkt`,
a connection has entered the draining state, and no further packet
transmission is allowed.

The following error codes must be considered as transitional, and
application should keep connection alive:

* :macro:`NGTCP2_ERR_STREAM_DATA_BLOCKED`
* :macro:`NGTCP2_ERR_STREAM_SHUT_WR`
* :macro:`NGTCP2_ERR_STREAM_NOT_FOUND`
* :macro:`NGTCP2_ERR_STREAM_ID_BLOCKED`

Version negotiation
-------------------

Version negotiation is configured with the following
:type:`ngtcp2_settings` fields:

* :member:`ngtcp2_settings.preferred_versions` and
  :member:`ngtcp2_settings.preferred_versionslen`
* :member:`ngtcp2_settings.other_versions` and
  :member:`ngtcp2_settings.other_versionslen`
* :member:`ngtcp2_settings.original_version`

*client_chosen_version* passed to `ngtcp2_conn_client_new` also
influence the version negotiation process.

By default, client sends *client_chosen_version* passed to
`ngtcp2_conn_client_new` in other_versions field of
version_information QUIC transport parameter.  That means there is no
chance for server to select the other compatible version.  Meanwhile,
ngtcp2 supports QUIC v2 draft version
(:macro:`NGTCP2_PROTO_VER_V2_DRAFT`).  Including both
:macro:`NGTCP2_PROTO_VER_V1` and :macro:`NGTCP2_PROTO_VER_V2_DRAFT` in
:member:`ngtcp2_settings.other_versions` field allows server to choose
:macro:`NGTCP2_PROTO_VER_V2_DRAFT` which is compatible to
:macro:`NGTCP2_PROTO_VER_V1`.

By default, server sends :macro:`NGTCP2_PROTO_VER_V1` in
other_versions field of version_information QUIC transport parameter.
Because there is no particular preferred versions specified, server
will accept any supported version.  In order to set the version
preference, specify :member:`ngtcp2_settings.preferred_versions`
field.  If it is specified, server sends them in other_versions field
of version_information QUIC transport parameter unless
:member:`ngtcp2_settings.other_versionslen` is not zero.  Specifying
:member:`ngtcp2_settings.other_versions` overrides the above mentioned
default behavior.  Even if there is no overlap between
:member:`ngtcp2_settings.preferred_versions` and other_versions field
plus *client_chosen_version* from client, as long as
*client_chosen_version* is supported by server, server accepts
*client_chosen_version*.

If client receives Version Negotiation packet from server,
`ngtcp2_conn_read_pkt` returns
:macro:`NGTCP2_ERR_RECV_VERSION_NEGOTIATION`.
:member:`ngtcp2_callbacks.recv_version_negotiation` is also invoked if
set.  It will provide the versions contained in the packet.  Client
then either gives up the connection attempt, or selects the version
from Version Negotiation packet, and starts new connection attempt
with that version.  In the latter case, the initial version that used
in the first connection attempt must be set to
:member:`ngtcp2_settings.original_version`.  The client version
preference that is used when selecting a version from Version
Negotiation packet must be set to
:member:`ngtcp2_settings.preferred_versions`.
:member:`ngtcp2_settings.other_versions` must include the selected
version.  The selected version becomes *client_chosen_version* in the
second connection attempt, and must be passed to
`ngtcp2_conn_client_new`.

Server never know whether client reacted upon Version Negotiation
packet or not, and there is no particular setup for server to make
this incompatible version negotiation work.
