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+.. SPDX-License-Identifier: GPL-2.0
+
+=====================
+Multipath TCP (MPTCP)
+=====================
+
+Introduction
+============
+
+Multipath TCP or MPTCP is an extension to the standard TCP and is described in
+`RFC 8684 (MPTCPv1) <https://www.rfc-editor.org/rfc/rfc8684.html>`_. It allows a
+device to make use of multiple interfaces at once to send and receive TCP
+packets over a single MPTCP connection. MPTCP can aggregate the bandwidth of
+multiple interfaces or prefer the one with the lowest latency. It also allows a
+fail-over if one path is down, and the traffic is seamlessly reinjected on other
+paths.
+
+For more details about Multipath TCP in the Linux kernel, please see the
+official website: `mptcp.dev <https://www.mptcp.dev>`_.
+
+
+Use cases
+=========
+
+Thanks to MPTCP, being able to use multiple paths in parallel or simultaneously
+brings new use-cases, compared to TCP:
+
+- Seamless handovers: switching from one path to another while preserving
+  established connections, e.g. to be used in mobility use-cases, like on
+  smartphones.
+- Best network selection: using the "best" available path depending on some
+  conditions, e.g. latency, losses, cost, bandwidth, etc.
+- Network aggregation: using multiple paths at the same time to have a higher
+  throughput, e.g. to combine fixed and mobile networks to send files faster.
+
+
+Concepts
+========
+
+Technically, when a new socket is created with the ``IPPROTO_MPTCP`` protocol
+(Linux-specific), a *subflow* (or *path*) is created. This *subflow* consists of
+a regular TCP connection that is used to transmit data through one interface.
+Additional *subflows* can be negotiated later between the hosts. For the remote
+host to be able to detect the use of MPTCP, a new field is added to the TCP
+*option* field of the underlying TCP *subflow*. This field contains, amongst
+other things, a ``MP_CAPABLE`` option that tells the other host to use MPTCP if
+it is supported. If the remote host or any middlebox in between does not support
+it, the returned ``SYN+ACK`` packet will not contain MPTCP options in the TCP
+*option* field. In that case, the connection will be "downgraded" to plain TCP,
+and it will continue with a single path.
+
+This behavior is made possible by two internal components: the path manager, and
+the packet scheduler.
+
+Path Manager
+------------
+
+The Path Manager is in charge of *subflows*, from creation to deletion, and also
+address announcements. Typically, it is the client side that initiates subflows,
+and the server side that announces additional addresses via the ``ADD_ADDR`` and
+``REMOVE_ADDR`` options.
+
+Path managers are controlled by the ``net.mptcp.pm_type`` sysctl knob -- see
+mptcp-sysctl.rst. There are two types: the in-kernel one (type ``0``) where the
+same rules are applied for all the connections (see: ``ip mptcp``) ; and the
+userspace one (type ``1``), controlled by a userspace daemon (i.e. `mptcpd
+<https://mptcpd.mptcp.dev/>`_) where different rules can be applied for each
+connection. The path managers can be controlled via a Netlink API; see
+netlink_spec/mptcp_pm.rst.
+
+To be able to use multiple IP addresses on a host to create multiple *subflows*
+(paths), the default in-kernel MPTCP path-manager needs to know which IP
+addresses can be used. This can be configured with ``ip mptcp endpoint`` for
+example.
+
+Packet Scheduler
+----------------
+
+The Packet Scheduler is in charge of selecting which available *subflow(s)* to
+use to send the next data packet. It can decide to maximize the use of the
+available bandwidth, only to pick the path with the lower latency, or any other
+policy depending on the configuration.
+
+Packet schedulers are controlled by the ``net.mptcp.scheduler`` sysctl knob --
+see mptcp-sysctl.rst.
+
+
+Sockets API
+===========
+
+Creating MPTCP sockets
+----------------------
+
+On Linux, MPTCP can be used by selecting MPTCP instead of TCP when creating the
+``socket``:
+
+.. code-block:: C
+
+    int sd = socket(AF_INET(6), SOCK_STREAM, IPPROTO_MPTCP);
+
+Note that ``IPPROTO_MPTCP`` is defined as ``262``.
+
+If MPTCP is not supported, ``errno`` will be set to:
+
+- ``EINVAL``: (*Invalid argument*): MPTCP is not available, on kernels < 5.6.
+- ``EPROTONOSUPPORT`` (*Protocol not supported*): MPTCP has not been compiled,
+  on kernels >= v5.6.
+- ``ENOPROTOOPT`` (*Protocol not available*): MPTCP has been disabled using
+  ``net.mptcp.enabled`` sysctl knob; see mptcp-sysctl.rst.
+
+MPTCP is then opt-in: applications need to explicitly request it. Note that
+applications can be forced to use MPTCP with different techniques, e.g.
+``LD_PRELOAD`` (see ``mptcpize``), eBPF (see ``mptcpify``), SystemTAP,
+``GODEBUG`` (``GODEBUG=multipathtcp=1``), etc.
+
+Switching to ``IPPROTO_MPTCP`` instead of ``IPPROTO_TCP`` should be as
+transparent as possible for the userspace applications.
+
+Socket options
+--------------
+
+MPTCP supports most socket options handled by TCP. It is possible some less
+common options are not supported, but contributions are welcome.
+
+Generally, the same value is propagated to all subflows, including the ones
+created after the calls to ``setsockopt()``. eBPF can be used to set different
+values per subflow.
+
+There are some MPTCP specific socket options at the ``SOL_MPTCP`` (284) level to
+retrieve info. They fill the ``optval`` buffer of the ``getsockopt()`` system
+call:
+
+- ``MPTCP_INFO``: Uses ``struct mptcp_info``.
+- ``MPTCP_TCPINFO``: Uses ``struct mptcp_subflow_data``, followed by an array of
+  ``struct tcp_info``.
+- ``MPTCP_SUBFLOW_ADDRS``: Uses ``struct mptcp_subflow_data``, followed by an
+  array of ``mptcp_subflow_addrs``.
+- ``MPTCP_FULL_INFO``: Uses ``struct mptcp_full_info``, with one pointer to an
+  array of ``struct mptcp_subflow_info`` (including the
+  ``struct mptcp_subflow_addrs``), and one pointer to an array of
+  ``struct tcp_info``, followed by the content of ``struct mptcp_info``.
+
+Note that at the TCP level, ``TCP_IS_MPTCP`` socket option can be used to know
+if MPTCP is currently being used: the value will be set to 1 if it is.
+
+
+Design choices
+==============
+
+A new socket type has been added for MPTCP for the userspace-facing socket. The
+kernel is in charge of creating subflow sockets: they are TCP sockets where the
+behavior is modified using TCP-ULP.
+
+MPTCP listen sockets will create "plain" *accepted* TCP sockets if the
+connection request from the client didn't ask for MPTCP, making the performance
+impact minimal when MPTCP is enabled by default.