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 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.