curl: (on linux) add MPTCP support

Multipath TCP (MPTCP), standardized in RFC8684 [1], is a TCP extension
that enables a TCP connection to use different paths.

Multipath TCP has been used for several use cases. On smartphones, MPTCP
enables seamless handovers between cellular and Wi-Fi networks while
preserving established connections. This use-case is what pushed Apple
to use MPTCP since 2013 in multiple applications [2]. On dual-stack
hosts, Multipath TCP enables the TCP connection to automatically use the
best performing path, either IPv4 or IPv6. If one path fails, MPTCP
automatically uses the other path.

To benefit from MPTCP, both the client and the server have to support
it. Multipath TCP is a backward-compatible TCP extension that is enabled
by default on recent Linux distributions (Debian, Ubuntu, Redhat, ...).
Multipath TCP is included in the Linux kernel since version 5.6 [3]. To
use it on Linux, an application must explicitly enable it when creating
the socket. No need to change anything else in the application.

This attached patch adds an --mptcp option which allows the creation of
an MPTCP socket instead of TCP on Linux. If Multipath TCP is not
supported on the system, an error will be reported. It is important to
note that if the end server doesn't support MPTCP, the connection will
continue after a seamless fallback to TCP.

Link: https://www.rfc-editor.org/rfc/rfc8684.html [1]
Link: https://www.tessares.net/apples-mptcp-story-so-far/ [2]
Link: https://www.mptcp.dev [3]
Co-developed-by: Dorian Craps (@CrapsDorian) <doriancraps@gmail.com>
Co-developed-by: Olivier Bonaventure (@obonaventure) <Olivier.Bonaventure@uclouvain.be>
Co-developed-by: Matthieu Baerts (@matttbe) <matttbe@kernel.org>
Signed-off-by: Dorian Craps <dorian.craps@student.vinci.be>

Closes #13278

docs/cmdline-opts/Makefile.inc

@@ -162,6 +162,7 @@ DPAGES = \
   max-redirs.md \
   max-time.md \
   metalink.md \
+  mptcp.md \
   negotiate.md \
   netrc-file.md \
   netrc-optional.md \

docs/cmdline-opts/mptcp.md

@@ -0,0 +1,41 @@
+---
+c: Copyright (C) Dorian Craps, <dorian.craps@student.vinci.be>
+SPDX-License-Identifier: curl
+Long: mptcp
+Added: 8.9.0
+Help: Enable Multipath TCP
+Category: connection
+Multi: boolean
+See-also:
+  - tcp-fastopen
+Example:
+  - --mptcp $URL
+---
+
+# `--mptcp`
+
+Enables the use of Multipath TCP (MPTCP) for connections. MPTCP is an extension
+to the standard TCP that allows multiple TCP streams over different network
+paths between the same source and destination. This can enhance bandwidth and
+improve reliability by using multiple paths simultaneously.
+
+MPTCP is beneficial in networks where multiple paths exist between clients and
+servers, such as mobile networks where a device may switch between WiFi and
+cellular data or in wired networks with multiple Internet Service Providers.
+
+## Usage
+
+To use MPTCP for your connections, add the `--mptcp` option when using `curl'.
+
+## Requirements
+
+This feature is currently only supported on Linux starting from kernel 5.6. Only
+TCP connections are modified, hence this option does not effect HTTP/3 (QUIC)
+connections.
+
+The server you are connecting to must also support MPTCP. If not, the connection
+seamlessly falls back to TCP.
+
+## Availability
+
+The `--mptcp` option is available starting from `curl` version 8.9.0.

docs/options-in-versions

@@ -127,6 +127,7 @@
 --max-redirs                         7.5
 --max-time (-m)                      4.0
 --metalink                           7.27.0
+--mptcp                              8.9.0
 --negotiate                          7.10.6
 --netrc (-n)                         4.6
 --netrc-file                         7.21.5