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Merge branch 'doc-mptcp-new-general-doc-and-fixes'

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Matthieu Baerts says:

====================
doc: mptcp: new general doc and fixes

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>
====================

v2: https://lore.kernel.org/r/20240528-upstream-net-20240520-mptcp-doc-v2-0-47f2d5bc2ef3@kernel.org
v1: https://lore.kernel.org/r/20240520-upstream-net-20240520-mptcp-doc-v1-0-e3ad294382cb@kernel.org

Link: https://lore.kernel.org/r/20240530-upstream-net-20240520-mptcp-doc-v3-0-e94cdd9f2673@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Jakub Kicinski 2024-06-01 15:18:03 -07:00
commit d1f9e6513e
4 changed files with 197 additions and 36 deletions
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1
Documentation/networking/index.rst
View File

@ -72,6 +72,7 @@ Contents:
mac80211-injection
mctp
mpls-sysctl
mptcp
mptcp-sysctl
multiqueue
multi-pf-netdev

74
Documentation/networking/mptcp-sysctl.rst
View File

@ -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.
@ -85,11 +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"

156
Documentation/networking/mptcp.rst Normal file
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@ -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.

2
MAINTAINERS
View File

@ -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