Add FIP port forwarding to Networking Guide

This patch adds and introduction and configuration instructions to the
Networking Guide for floating IPs port forwarding

Change-Id: I821b242f4ba58d92b8e9491db65232ec0a85f73b
Partially-Implements: blueprint port-forwarding
Partial-Bug: #1491317

doc/source/admin/config-fip-port-forwardings.rst

@@ -0,0 +1,55 @@
+.. _config-fip-port-forwardings:
+
+===========================
+Floating IP port forwarding
+===========================
+
+Floating IP port forwarding enables users to forward traffic from a
+TCP/UDP/other protocol port of a floating IP to a TCP/UDP/other protocol port
+associated to one of the fixed IPs of a Neutron port. This is accomplished by
+associating ``port_forwarding`` sub-resource to a floating IP.
+
+CRUD operations for port forwarding are implemented by a Neutron API extension
+and a service plug-in. Please refer to the Neutron API Reference documentation
+for details on the CRUD operations.
+
+Configuring floating IP port forwarding
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To configure floating IP port forwarding, take the following steps:
+
+* Add the ``port_forwarding`` service to the ``service_plugins`` setting in
+  ``/etc/neutron/neutron.conf``. For example:
+
+  .. code-block:: console
+
+     service_plugins = router,segments,port_forwarding
+
+* Set the ``extensions`` option in the ``[agent]`` section of
+  ``/etc/neutron/l3_agent.ini`` to include ``port_forwarding``. This has to be
+  done in each network and compute node where the L3 agent is running. For
+  example:
+
+  .. code-block:: console
+
+     extensions = port_forwarding
+
+.. note::
+
+   The ``router`` service plug-in manages floating IPs and routers. As a
+   consequence, it has to be configured along with the ``port_forwarding``
+   service plug-in.
+
+.. note::
+
+   After updating the options in the configuration files, the neutron-server
+   and every neutron-l3-agent need to be restarted for the new values to take
+   effect.
+
+After configuring floating IP port forwarding, the
+``floating-ip-port-forwarding`` extension alias will be included in the output
+of the following command:
+
+.. code-block:: console
+
+   $ openstack extension list --network

doc/source/admin/config.rst

@@ -18,6 +18,7 @@ Configuration
    config-dns-int-ext-serv
    config-dns-res
    config-dvr-ha-snat
+   config-fip-port-forwardings
    config-ipam
    config-ipv6
    config-lbaas