doc: describe the default behaviour (#313)

* doc: describe the default behaviour

Because for someone new, it might not be clear what it does.

The idea is also to modify the welcome page on mptcpd.mptcp.dev with the
same description.

Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>

* doc: how to change the behaviour

While describing what's being done by default, best to mention how to
adapt that.

Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>

---------

Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>

README.md

@@ -16,6 +16,27 @@ information (e.g. available remote addresses), available network
 interfaces, request new MPTCP subflows, handle requests for subflows,
 etc.
 
+## Behavior
+By default, this daemon will load the `addr_adv` plugin, which will
+add MPTCP endpoints with the `subflow` flag ("client" mode) for the
+default in-kernel path-manager. Note that this is something
+[NetworkManager 1.40 or newer](https://networkmanager.dev/blog/networkmanager-1-40/#mptcp-support)
+does by default. Having several daemons configuring the MPTCP
+endpoints at the same time should be avoided. This daemon is usually
+recommended when NetworkManager 1.40 or newer is not available, or
+when advanced per-connection path management is needed, using the
+userspace path-manager and a custom made
+[plugin](https://github.com/multipath-tcp/mptcpd/wiki/Plugins) using
+the [C API](https://mptcpd.mptcp.dev/doc/html/).
+
+To change this behavior, with NetworkManager, look for the
+`connection.mptcp-flags` option in the
+[settings](https://networkmanager.dev/docs/api/latest/nm-settings-nmcli.html#nm-settings-nmcli.property.connection.mptcp-flags),
+while for `mptcpd`, look at the `/etc/mptcpd/mptcpd.conf` config
+file, or disable the service if it is not needed. Make sure not to
+have both NetworkManager and `mptcpd` conflicting to configure the
+MPTCP endpoints.
+
 ## Installing `mptcpd`
 `mptcpd` is packaged in most major distributions:
 

mptcpd.dox

@@ -16,8 +16,28 @@
  * interfaces, request new MPTCP subflows, handle requests for
  * subflows, etc.
  *
+ * By default, this daemon will load the `addr_adv` plugin, which will
+ * add MPTCP endpoints with the `subflow` flag ("client" mode) for the
+ * default in-kernel path-manager. Note that this is something
+ * [NetworkManager 1.40 or newer](https://networkmanager.dev/blog/networkmanager-1-40/#mptcp-support)
+ * does by default. Having several daemons configuring the MPTCP
+ * endpoints at the same time should be avoided. This daemon is usually
+ * recommended when NetworkManager 1.40 or newer is not available, or
+ * when advanced per-connection path management is needed, using the
+ * userspace path-manager and a custom made
+ * [plugin](https://github.com/multipath-tcp/mptcpd/wiki/Plugins) using
+ * the C API that is described here.
+ *
+ * To change this behavior, with NetworkManager, look for the
+ * `connection.mptcp-flags` option in the
+ * [settings](https://networkmanager.dev/docs/api/latest/nm-settings-nmcli.html#nm-settings-nmcli.property.connection.mptcp-flags),
+ * while for `mptcpd`, look at the `/etc/mptcpd/mptcpd.conf` config
+ * file, or disable the service if it is not needed. Make sure not to
+ * have both NetworkManager and `mptcpd` conflicting to configure the
+ * MPTCP endpoints.
+ *
  * @see Additional higher level documentation is available in the
- *      [`mptcpd` Wiki](https://github.com/multipath-tcp/mptcpd/wiki).
+ *      [`mptcpd` Wiki](https://github.com/multipath-tcp/mptcpd/wiki/Plugins).
  */
 
 /*