net: doc: Add Ethernet over USB basic documentation

Add basic documentation about Ethernet over USB and describe
prj_netusb.conf configuration file.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>

doc/subsystems/networking/networking.rst

@@ -19,3 +19,4 @@ operation of the stacks and how they were implemented.
    net-app-api.rst
    buffers.rst
    qemu_setup.rst
+   usbnet_setup.rst

doc/subsystems/networking/usbnet_setup.rst

@@ -0,0 +1,101 @@
+.. _usb_device_networking_setup:
+
+USB Device Networking
+#####################
+
+This page describes how to set up networking between a Linux host
+and a Zephyr application running on USB supported devices.
+
+The board is connected to Linux host using USB cable
+and provides an Ethernet interface to the host.
+The :ref:`echo-server-sample` application from the Zephyr source
+distribution is run on supported board.  The board is connected to a
+Linux host using a USB cable providing an Ethernet interface to the host.
+
+Basic Setup
+***********
+
+To communicate with the Zephyr application over a newly created Ethernet
+interface, we need to assign IP addresses and set up a routing table for
+the Linux host.
+After plugging a USB cable from the board to the Linux host, the
+``cdc_ether`` driver registers a new Ethernet device with a provided MAC
+address.
+
+You can check that network device is created and MAC address assigned by
+running dmesg from the Linux host.
+
+.. code-block:: console
+
+   cdc_ether 1-2.7:1.0 eth0: register 'cdc_ether' at usb-0000:00:01.2-2.7, CDC Ethernet Device, 00:00:5e:00:53:01
+
+We need to set it up and assign IP addresses as explained in the following
+section.
+
+Choosing IP addresses
+=====================
+
+To establish network connection to the board we need to choose IP address
+for the interface on the Linux host.
+
+It make sense to choose addresses in the same subnet we have in Zephyr
+application. IP addresses usually set in the project configuration files
+and may be checked also from the shell with following commands. Connect
+a serial console program (such as puTTY) to the board, and enter this
+command to the Zephyr shell:
+
+.. code-block:: console
+
+   shell> net iface
+
+   Interface 0xa800e580 (Ethernet)
+   ===============================
+   Link addr : 00:00:5E:00:53:00
+   MTU       : 1500
+   IPv6 unicast addresses (max 2):
+           fe80::200:5eff:fe00:5300 autoconf preferred infinite
+           2001:db8::1 manual preferred infinite
+   ...
+   IPv4 unicast addresses (max 1):
+           192.0.2.1 manual preferred infinite
+
+This command shows that one IPv4 address and two IPv6 addresses have
+been assigned to the board. We can use either IPv4 or IPv6 for network
+connection depending on the board network configuration.
+
+Next step is to assign IP addresses to the new Linux host interface, in
+the following steps ``enx00005e005301`` is the name of the interface on my
+Linux system.
+
+Setting IPv4 address and routing
+================================
+
+.. code-block:: console
+
+   # ip address add dev enx00005e005301 192.0.2.2
+   # ip link set enx00005e005301 up
+   # ip route add 192.0.2.0/24 dev enx00005e005301
+
+Setting IPv6 address and routing
+================================
+
+.. code-block:: console
+
+   # ip address add dev enx00005e005301 2001:db8::2
+   # ip link set enx00005e005301 up
+   # ip -6 route add 2001:db8::/64 dev enx00005e005301
+
+Testing connection
+******************
+
+From the host we can test the connection by pinging Zephyr IP address of
+the board with:
+
+.. code-block:: console
+
+   $ ping 192.0.2.1
+   PING 192.0.2.1 (192.0.2.1) 56(84) bytes of data.
+   64 bytes from 192.0.2.1: icmp_seq=1 ttl=64 time=2.30 ms
+   64 bytes from 192.0.2.1: icmp_seq=2 ttl=64 time=1.43 ms
+   64 bytes from 192.0.2.1: icmp_seq=3 ttl=64 time=2.45 ms
+   ...

samples/net/echo_server/README.rst

@@ -60,6 +60,10 @@ echo-server directory:
 - :file:`prj_sam_e70_xplained.conf`
   Use this for Atmel SMART SAM E70 Xplained board with ethernet.
 
+- :file:`prj_netusb.conf`
+  Use this for Ethernet over USB setup with supported boards. The setup is
+  described in :ref:`usb_device_networking_setup`.
+
 Build echo-server sample application like this:
 
 .. zephyr-app-commands::