doc: net: Add more info for Arm FVP user mode networking

Added more detailed information about Arm FVP user mode networking and
how to use it.

Signed-off-by: Huifeng Zhang <Huifeng.Zhang@arm.com>

doc/connectivity/networking/armfvp_user_networking_setup.rst

@@ -0,0 +1,51 @@
+.. _networking_with_armfvp:
+
+Networking with Arm FVP User Mode
+#################################
+
+.. contents::
+    :local:
+    :depth: 2
+
+This page is intended to serve as a starting point for anyone interested in
+using Arm FVP user mode networking with Zephyr.
+
+Introduction
+*************
+
+User mode networking emulates a built-in IP router and DHCP server, and routes
+TCP and UDP traffic between the guest and host. It uses the user mode socket
+layer of the host to communicate with other hosts. This allows the use of
+a significant number of IP network services without requiring administrative
+privileges, or the installation of a separate driver on the host on which
+the model is running.
+
+By default, Arm FVP uses the ``172.20.51.0/24`` network and runs a gateway at
+``172.20.51.254``. This gateway also functions as a DHCP server for the GOS,
+allowing it to be automatically assigned with an IP address ``172.20.51.1``.
+
+More details about Arm FVP user mode networking can be obtained from here:
+https://developer.arm.com/documentation/100964/latest/Introduction-to-Fast-Models/User-mode-networking
+
+Using Arm FVP User Mode Networking with Zephyr
+***********************************************
+
+Arm FVP user mode networking can be enabled in any applications and it doesn't
+need any configurations on the host system. This feature has been enabled in
+DHCPv4 client sample.
+See :ref:`Sample DHCPv4 client application <dhcpv4-client-sample>`
+
+Limitations
+*************
+
+* You can use TCP and UDP over IP, but not ICMP (ping).
+* User mode networking does not support forwarding UDP ports on the host to
+  the model.
+* You can only use DHCP within the private network.
+* You can only make inward connections by mapping TCP ports on the host to
+  the model. This is common to all implementations that provide host
+  connectivity using NAT.
+* Operations that require privileged source ports, for example NFS in its
+  default configuration, do not work.
+* If setup fails, or the parameter syntax is incorrect, there is no error
+  reporting.

doc/connectivity/networking/networking_with_host.rst

@@ -14,6 +14,7 @@ Networking with the host system
    qemu_user_setup.rst
    networking_with_multiple_instances.rst
    qemu_802154_setup.rst
+   armfvp_user_networking_setup.rst
 
 While developing networking software, it is usually necessary to connect and
 exchange data with the host system like a Linux desktop computer.
@@ -42,6 +43,16 @@ possible:
     limitations including performance which makes it less valuable for practical
     purposes. See :ref:`networking_with_user_qemu` for details.
 
+* Arm FVP (User Mode Networking).
+
+  * User mode networking emulates a built-in IP router and DHCP server, and
+    routes TCP and UDP traffic between the guest and host. It uses the user mode
+    socket layer of the host to communicate with other hosts. This allows
+    the use of a significant number of IP network services without requiring
+    administrative privileges, or the installation of a separate driver on
+    the host on which the model is running. See :ref:`networking_with_armfvp`
+    for details.
+
 * native_posix board.
 
   * The Zephyr instance can be executed as a user space process in the host