datapath: Use compat directory only for backported code.

[openvswitch] / PORTING

         How to Port Open vSwitch to New Software or Hardware
         ====================================================

Open vSwitch (OVS) is intended to be easily ported to new software and
hardware platforms.  This document describes the types of changes that
are most likely to be necessary in porting OVS to Unix-like platforms.
(Porting OVS to other kinds of platforms is likely to be more
difficult.)

Open vSwitch Architectural Overview
-----------------------------------

The following diagram shows the conceptual architecture of Open
vSwitch from a porter's perspective.
                _                         _
               |   +-------------------+   |
               |   |   ovs-vswitchd    |   |Generic
               |   +-------------------+   |code
     userspace |   |      ofproto      |  _|
               |   +---------+---------+  _
               |   | netdev  |dpif/wdp |   |
               |_  +---||----+----||---+   |Code that
                _      ||         ||       |may need
               |   +---||-----+---||---+   |porting
               |   |          |datapath|  _|
        kernel |   |          +--------+
               |   |                   |
               |_  +-------||----------+
                           ||
                        physical
                          NIC

Some of the components are generic.  Modulo bugs, these components
should not need to be modified as part of a port:

    - Near the top of the diagram, "ofproto" is the library in Open vSwitch
      that contains the core OpenFlow protocol implementation and switching
      functionality.  It is built from source files in the "ofproto"
      directory.

    - Above ofproto, "ovs-vswitchd", the main Open vSwitch userspace
      program, is the primary client for ofproto.  It is built
      from source files in the "vswitchd" directory of the Open
      vSwitch distribution.

      ovs-vswitchd is the most sophisticated of ofproto's clients, but
      ofproto can have other clients as well.  Notably, ovs-openflowd,
      in the utilities directory, is much simpler (though less
      capable) than ovs-vswitchd, and it may be easier to get up and
      running as part of a port.

The other components require attention during a port:

    - "dpif" or "wdp" is what ofproto uses to directly monitor and
      control a "datapath", which is the term used in OVS for a
      collection of physical or virtual ports that are exposed over
      OpenFlow as a single switch.  A datapath implements a flow
      table.

    - "netdev" is the interface to "network devices", e.g. eth0 on
      Linux.  ofproto expects that every port exposed by a datapath
      has a corresponding netdev that it can open with netdev_open().

The following sections talk about these components in more detail.

Which Branch?
-------------

The architectural diagram shows "dpif" and "wdp" as alternatives.
These alternatives correspond to the "master" and "wdp" branches,
respectively, of the Open vSwitch Git repository at
git://openvswitch.org/openvswitch.  Both of these branches currently
represent reasonable porting targets for different purposes:

    - The "master" branch is more mature and better tested.  Open
      vSwitch releases are made from this branch, and most OVS
      development and testing occurs on this branch.

    - The "wdp" branch has a software architecture that can take
      advantage of hardware with support for wildcards (e.g. TCAMs or
      similar).  This branch has known important bugs, but is the basis
      of a few ongoing hardware projects, so we expect the quality to
      improve rapidly.

Since its architecture is better, in the medium to long term we will
fix the problems in the "wdp" branch and merge it into "master".

In porting OVS, the major difference between the two branches is the
form of the flow table in the datapath:

    - On "master", the "dpif" datapath interface maintains a simple
      flow table, one that does not support any kind of wildcards.
      This flow table essentially acts as a cache.  When a packet
      arrives on an interface, the datapath looks for it in this
      exact-match table.  If there is a match, then it performs the
      associated actions.  If there is no match, the datapath passes
      the packet up to "ofproto", which maintains a flow table that
      supports wildcards.  If the packet matches in this flow table,
      then ofproto executes its actions and inserts a new exact-match
      entry into the dpif flow table.  (Otherwise, ofproto sends the
      packet to the OpenFlow controller, if one is configured.)

      Thus, on the "master" branch, the datapath has little
      opportunity to take advantage of hardware support for wildcards,
      since it is only ever presented with exact-match flow entries.

    - On "wdp", the "wdp" datapath interface maintains a flow table
      similar to that of OpenFlow, one that supports wildcards.  Thus,
      a wdp datapath can take advantage of hardware support for
      wildcards, since it is free to implement the flow table any way
      it likes.
      
The following sections describe the two datapath interfaces in a
little more detail.

dpif: The "master" Branch Datapath
----------------------------------

struct dpif_class, in lib/dpif-provider.h, defines the
interfaces required to implement a dpif for new hardware or
software.  That structure contains many function pointers, each
of which has a comment that is meant to describe its behavior in
detail.  If the requirements are unclear, please report this as
a bug and we will clarify.

There are two existing dpif implementations that may serve as
useful examples during a port:

    * lib/dpif-linux.c is a Linux-specific dpif implementation that
      talks to an Open vSwitch-specific kernel module (whose sources
      are in the "datapath" directory).  The kernel module performs
      all of the switching work, passing packets that do not match any
      flow table entry up to userspace.  This dpif implementation is
      essentially a wrapper around calls to "ioctl".

    * lib/dpif-netdev.c is a generic dpif implementation that performs
      all switching internally.  It delegates most of its work to the
      "netdev" library (described below).  Using dpif-netdev, instead
      of writing a new dpif, can be a simple way to get OVS up and
      running on new platforms, but other solutions are likely to
      yield higher performance.

"wdp": The "wdp" Branch Datapath
--------------------------------

struct wdp_class, in ofproto/wdp-provider.h, defines the interfaces
required to implement a wdp ("wildcarded datapath") for new hardware
or software.  That structure contains many function pointers, each of
which has a comment that is meant to describe its behavior in detail.
If the requirements are unclear, please report this as a bug and we
will clarify.

The wdp interface is preliminary.  Please let us know if it seems
unsuitable for your purpose.  We will try to improve it.

There is currently only one wdp implementation:

    * ofproto/wdp-xflow.c is an adaptation of "master" branch code
      that breaks wildcarded flows up into exact-match flows in the
      same way that ofproto always does on the "master" branch.  It
      delegates its work to exact-match datapath implementations whose
      interfaces are identical to "master" branch datapaths, except
      that names have been changed from "dpif" to "xfif" ("exact-match
      flow interface") and similar.

"netdev": Interface to network devices
--------------------------------------

The netdev interface can be roughly divided into functionality for the
following purposes:

    * Functions required to properly implement OpenFlow features.  For
      example, OpenFlow requires the ability to report the Ethernet
      hardware address of a port.  These functions must be implemented
      for minimally correct operation.

    * Functions required to implement optional Open vSwitch features.
      For example, the Open vSwitch support for in-band control
      requires netdev support for inspecting the TCP/IP stack's ARP
      table.  These functions must be implemented if the corresponding
      OVS features are to work, but may be omitted initially.

    * Functions that may be needed in some implementations but not
      others.  The dpif-netdev described above, for example, needs to
      be able to send and receive packets on a netdev.

struct netdev_class, in lib/netdev-provider.h, defines the interfaces
required to implement a netdev.  That structure contains many function
pointers, each of which has a comment that is meant to describe its
behavior in detail.  If the requirements are unclear, please report
this as a bug and we will clarify.

The existing netdev implementations may serve as useful examples
during a port:

    * lib/netdev-linux.c implements netdev functionality for Linux
      network devices, using Linux kernel calls.  It may be a good
      place to start for full-featured netdev implementations.

    * lib/netdev-vport.c provides support for "virtual ports" 
      implemented by the Open vSwitch datapath module for the Linux
      kernel.  This may serve as a model for minimal netdev
      implementations.

Miscellaneous Notes
-------------------

lib/entropy.c assumes that it can obtain high-quality random number
seeds at startup by reading from /dev/urandom.  You will need to
modify it if this is not true on your platform.

vswitchd/system-stats.c only knows how to obtain some statistics on
Linux.  Optionally you may implement them for your platform as well.

Questions
---------

Please direct porting questions to dev@openvswitch.org.  We will try
to use questions to improve this porting guide.