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+FPM
+===
+
+FPM stands for Forwarding Plane Manager and it's a module for use with Zebra.
+
+The encapsulation header for the messages exchanged with the FPM is
+defined by the file :file:`fpm/fpm.h` in the frr tree. The routes
+themselves are encoded in Netlink or protobuf format, with Netlink
+being the default.
+
+Netlink is standard format for encoding messages to talk with kernel space
+in Linux and it is also the name of the socket type used by it.
+The FPM netlink usage differs from Linux's in:
+
+- Linux netlink sockets use datagrams in a multicast fashion, FPM uses
+  as a stream and it is unicast.
+- FPM netlink messages might have more or less information than a normal
+  Linux netlink socket message (example: RTM_NEWROUTE might add an extra
+  route attribute to signalize VxLAN encapsulation).
+
+Protobuf is one of a number of new serialization formats wherein the
+message schema is expressed in a purpose-built language. Code for
+encoding/decoding to/from the wire format is generated from the
+schema. Protobuf messages can be extended easily while maintaining
+backward-compatibility with older code. Protobuf has the following
+advantages over Netlink:
+
+- Code for serialization/deserialization is generated automatically. This
+  reduces the likelihood of bugs, allows third-party programs to be integrated
+  quickly, and makes it easy to add fields.
+- The message format is not tied to an OS (Linux), and can be evolved
+  independently.
+
+.. note::
+
+   Currently there are two FPM modules in ``zebra``:
+
+   * ``fpm``
+   * ``dplane_fpm_nl``
+
+fpm
+^^^
+
+The first FPM implementation that was built using hooks in ``zebra`` route
+handling functions. It uses its own netlink/protobuf encoding functions to
+translate ``zebra`` route data structures into formatted binary data.
+
+
+dplane_fpm_nl
+^^^^^^^^^^^^^
+
+The newer FPM implementation that was built using ``zebra``'s data plane
+framework as a plugin. It only supports netlink and it shares ``zebra``'s
+netlink functions to translate route event snapshots into formatted binary
+data.
+
+
+Protocol Specification
+----------------------
+
+FPM (in any mode) uses a TCP connection to talk with external applications.
+It operates as TCP client and uses the CLI configured address/port to connect
+to the FPM server (defaults to port ``2620``).
+
+FPM frames all data with a header to help the external reader figure how
+many bytes it has to read in order to read the full message (this helps
+simulates datagrams like in the original netlink Linux kernel usage).
+
+Frame header:
+
+::
+
+    0                   1                   2                   3
+    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+   +---------------+---------------+-------------------------------+
+   | Version       | Message type  | Message length                |
+   +---------------+---------------+-------------------------------+
+   | Data...                                                       |
+   +---------------------------------------------------------------+
+
+
+Version
+^^^^^^^
+
+Currently there is only one version, so it should be always ``1``.
+
+
+Message Type
+^^^^^^^^^^^^
+
+Defines what underlining protocol we are using: netlink (``1``) or protobuf (``2``).
+
+
+Message Length
+^^^^^^^^^^^^^^
+
+Amount of data in this frame in network byte order.
+
+
+Data
+^^^^
+
+The netlink or protobuf message payload.