[PATCH net-next v3 0/3] doc: mptcp: new general doc and fixes

[PATCH net-next v3 0/3] doc: mptcp: new general doc and fixes

[Matthieu Baerts (NGI0)]

A general documentation about MPTCP was missing since its introduction
in v5.6. The last patch adds a new 'mptcp' page in the 'networking'
documentation.

The first patch is a fix for a missing sysctl entry introduced in v6.10
rc0, and the second one reorder the sysctl entries.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Changes in v3:
- Patch 3/3: Fix a few run-on sentences (Randy)
- Link to v2: https://lore.kernel.org/r/20240528-upstream-net-20240520-mptcp-doc-v2-0-47f2d5bc2ef3@kernel.org

Changes in v2:
- Patch 3/3: fixed mptcp.dev link syntax.
- Rebased on top of net-next (Paolo).
- Link to v1: https://lore.kernel.org/r/20240520-upstream-net-20240520-mptcp-doc-v1-0-e3ad294382cb@kernel.org

---
Matthieu Baerts (NGI0) (3):
      doc: mptcp: add missing 'available_schedulers' entry
      doc: mptcp: alphabetical order
      doc: new 'mptcp' page in 'networking'

 Documentation/networking/index.rst        |   1 +
 Documentation/networking/mptcp-sysctl.rst |  74 +++++++-------
 Documentation/networking/mptcp.rst        | 156 ++++++++++++++++++++++++++++++
 MAINTAINERS                               |   2 +-
 4 files changed, 197 insertions(+), 36 deletions(-)
---
base-commit: c53a46b16ce2605181688ea6af5f6c8d7fb3c9c1
change-id: 20240520-upstream-net-20240520-mptcp-doc-e57ac322e4ac

Best regards,
-- 
Matthieu Baerts (NGI0) <matttbe@kernel.org>

[PATCH net-next v3 1/3] doc: mptcp: add missing 'available_schedulers' entry

[Matthieu Baerts (NGI0)]

This sysctl knob has been added recently, but the documentation has not
been updated.

This knob is used to show the available schedulers choices that are
registered, similar to 'net.ipv4.tcp_available_congestion_control'.

Fixes: 73c900aa3660 ("mptcp: add net.mptcp.available_schedulers")
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 Documentation/networking/mptcp-sysctl.rst | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Documentation/networking/mptcp-sysctl.rst b/Documentation/networking/mptcp-sysctl.rst
index 69975ce25a02..102a45e7bfa8 100644
--- a/Documentation/networking/mptcp-sysctl.rst
+++ b/Documentation/networking/mptcp-sysctl.rst
@@ -93,3 +93,7 @@ scheduler - STRING
 	sysctl.
 
 	Default: "default"
+
+available_schedulers - STRING
+	Shows the available schedulers choices that are registered. More packet
+	schedulers may be available, but not loaded.

-- 
2.43.0

[PATCH net-next v3 2/3] doc: mptcp: alphabetical order

[Matthieu Baerts (NGI0)]

Similar to what is done in other 'sysctl' pages: it looks clearer from a
readability perspective.

This might cause troubles in the short/mid-term with the backports, but
by not putting new entries at the end, this can help to reduce conflicts
in case of backports in the long term. We don't change the information
here too often, so it looks OK to do that.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 Documentation/networking/mptcp-sysctl.rst | 78 +++++++++++++++----------------
 1 file changed, 39 insertions(+), 39 deletions(-)

diff --git a/Documentation/networking/mptcp-sysctl.rst b/Documentation/networking/mptcp-sysctl.rst
index 102a45e7bfa8..fd514bba8c43 100644
--- a/Documentation/networking/mptcp-sysctl.rst
+++ b/Documentation/networking/mptcp-sysctl.rst
@@ -7,14 +7,6 @@ MPTCP Sysfs variables
 /proc/sys/net/mptcp/* Variables
 ===============================
 
-enabled - BOOLEAN
-	Control whether MPTCP sockets can be created.
-
-	MPTCP sockets can be created if the value is 1. This is a
-	per-namespace sysctl.
-
-	Default: 1 (enabled)
-
 add_addr_timeout - INTEGER (seconds)
 	Set the timeout after which an ADD_ADDR control message will be
 	resent to an MPTCP peer that has not acknowledged a previous
@@ -25,25 +17,6 @@ add_addr_timeout - INTEGER (seconds)
 
 	Default: 120
 
-close_timeout - INTEGER (seconds)
-	Set the make-after-break timeout: in absence of any close or
-	shutdown syscall, MPTCP sockets will maintain the status
-	unchanged for such time, after the last subflow removal, before
-	moving to TCP_CLOSE.
-
-	The default value matches TCP_TIMEWAIT_LEN. This is a per-namespace
-	sysctl.
-
-	Default: 60
-
-checksum_enabled - BOOLEAN
-	Control whether DSS checksum can be enabled.
-
-	DSS checksum can be enabled if the value is nonzero. This is a
-	per-namespace sysctl.
-
-	Default: 0
-
 allow_join_initial_addr_port - BOOLEAN
 	Allow peers to send join requests to the IP address and port number used
 	by the initial subflow if the value is 1. This controls a flag that is
@@ -57,6 +30,37 @@ allow_join_initial_addr_port - BOOLEAN
 
 	Default: 1
 
+available_schedulers - STRING
+	Shows the available schedulers choices that are registered. More packet
+	schedulers may be available, but not loaded.
+
+checksum_enabled - BOOLEAN
+	Control whether DSS checksum can be enabled.
+
+	DSS checksum can be enabled if the value is nonzero. This is a
+	per-namespace sysctl.
+
+	Default: 0
+
+close_timeout - INTEGER (seconds)
+	Set the make-after-break timeout: in absence of any close or
+	shutdown syscall, MPTCP sockets will maintain the status
+	unchanged for such time, after the last subflow removal, before
+	moving to TCP_CLOSE.
+
+	The default value matches TCP_TIMEWAIT_LEN. This is a per-namespace
+	sysctl.
+
+	Default: 60
+
+enabled - BOOLEAN
+	Control whether MPTCP sockets can be created.
+
+	MPTCP sockets can be created if the value is 1. This is a
+	per-namespace sysctl.
+
+	Default: 1 (enabled)
+
 pm_type - INTEGER
 	Set the default path manager type to use for each new MPTCP
 	socket. In-kernel path management will control subflow
@@ -74,6 +78,14 @@ pm_type - INTEGER
 
 	Default: 0
 
+scheduler - STRING
+	Select the scheduler of your choice.
+
+	Support for selection of different schedulers. This is a per-namespace
+	sysctl.
+
+	Default: "default"
+
 stale_loss_cnt - INTEGER
 	The number of MPTCP-level retransmission intervals with no traffic and
 	pending outstanding data on a given subflow required to declare it stale.
@@ -85,15 +97,3 @@ stale_loss_cnt - INTEGER
 	This is a per-namespace sysctl.
 
 	Default: 4
-
-scheduler - STRING
-	Select the scheduler of your choice.
-
-	Support for selection of different schedulers. This is a per-namespace
-	sysctl.
-
-	Default: "default"
-
-available_schedulers - STRING
-	Shows the available schedulers choices that are registered. More packet
-	schedulers may be available, but not loaded.

-- 
2.43.0

[PATCH net-next v3 3/3] doc: new 'mptcp' page in 'networking'

[Matthieu Baerts (NGI0)]

A general documentation about MPTCP was missing since its introduction
in v5.6.

Most of what is there comes from our recently updated mptcp.dev website,
with additional links to resources from the kernel documentation.

This is a first version, mainly targeting app developers and users.

Link: https://www.mptcp.dev
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Notes:
  - v2:
    - Fix mptcp.dev link syntax.
  - v3:
    - Fix a few run-on sentences (Randy)
Cc: Randy Dunlap <rdunlap@infradead.org>
---
 Documentation/networking/index.rst |   1 +
 Documentation/networking/mptcp.rst | 156 +++++++++++++++++++++++++++++++++++++
 MAINTAINERS                        |   2 +-
 3 files changed, 158 insertions(+), 1 deletion(-)

diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst
index 7664c0bfe461..a6443851a142 100644
--- a/Documentation/networking/index.rst
+++ b/Documentation/networking/index.rst
@@ -72,6 +72,7 @@ Contents:
    mac80211-injection
    mctp
    mpls-sysctl
+   mptcp
    mptcp-sysctl
    multiqueue
    multi-pf-netdev
diff --git a/Documentation/networking/mptcp.rst b/Documentation/networking/mptcp.rst
new file mode 100644
index 000000000000..17f2bab61164
--- /dev/null
+++ b/Documentation/networking/mptcp.rst
@@ -0,0 +1,156 @@
+.. 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.
diff --git a/MAINTAINERS b/MAINTAINERS
index 27367ad339ea..1a65444adb21 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -15753,7 +15753,7 @@ B:	https://github.com/multipath-tcp/mptcp_net-next/issues
 T:	git https://github.com/multipath-tcp/mptcp_net-next.git export-net
 T:	git https://github.com/multipath-tcp/mptcp_net-next.git export
 F:	Documentation/netlink/specs/mptcp_pm.yaml
-F:	Documentation/networking/mptcp-sysctl.rst
+F:	Documentation/networking/mptcp*.rst
 F:	include/net/mptcp.h
 F:	include/trace/events/mptcp.h
 F:	include/uapi/linux/mptcp*.h

-- 
2.43.0

Re: [PATCH net-next v3 0/3] doc: mptcp: new general doc and fixes

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net-next.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Thu, 30 May 2024 16:07:29 +0200 you wrote:
> A general documentation about MPTCP was missing since its introduction
> in v5.6. The last patch adds a new 'mptcp' page in the 'networking'
> documentation.
> 
> The first patch is a fix for a missing sysctl entry introduced in v6.10
> rc0, and the second one reorder the sysctl entries.
> 
> [...]

Here is the summary with links:
  - [net-next,v3,1/3] doc: mptcp: add missing 'available_schedulers' entry
    https://git.kernel.org/netdev/net-next/c/ccf45c92d746
  - [net-next,v3,2/3] doc: mptcp: alphabetical order
    https://git.kernel.org/netdev/net-next/c/a32c6966b23d
  - [net-next,v3,3/3] doc: new 'mptcp' page in 'networking'
    https://git.kernel.org/netdev/net-next/c/c049275f24de

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
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