1 How to Port Open vSwitch to New Software or Hardware
2 ====================================================
4 Open vSwitch (OVS) is intended to be easily ported to new software and
5 hardware platforms. This document describes the types of changes that
6 are most likely to be necessary in porting OVS to Unix-like platforms.
7 (Porting OVS to other kinds of platforms is likely to be more
10 Open vSwitch Architectural Overview
11 -----------------------------------
13 The following diagram shows the conceptual architecture of Open
14 vSwitch from a porter's perspective.
16 | +-------------------+ |
17 | | ovs-vswitchd | |Generic
18 | +-------------------+ |code
19 userspace | | ofproto | _|
20 | +---------+---------+ _
21 | | netdev |dpif/wdp | |
22 |_ +---||----+----||---+ |Code that
24 | +---||-----+---||---+ |porting
28 |_ +-------||----------+
33 Some of the components are generic. Modulo bugs, these components
34 should not need to be modified as part of a port:
36 - Near the top of the diagram, "ofproto" is the library in Open vSwitch
37 that contains the core OpenFlow protocol implementation and switching
38 functionality. It is built from source files in the "ofproto"
41 - Above ofproto, "ovs-vswitchd", the main Open vSwitch userspace
42 program, is the primary client for ofproto. It is built
43 from source files in the "vswitchd" directory of the Open
46 ovs-vswitchd is the most sophisticated of ofproto's clients, but
47 ofproto can have other clients as well. Notably, ovs-openflowd,
48 in the utilities directory, is much simpler (though less
49 capable) than ovs-vswitchd, and it may be easier to get up and
50 running as part of a port.
52 The other components require attention during a port:
54 - "dpif" or "wdp" is what ofproto uses to directly monitor and
55 control a "datapath", which is the term used in OVS for a
56 collection of physical or virtual ports that are exposed over
57 OpenFlow as a single switch. A datapath implements a flow
60 - "netdev" is the interface to "network devices", e.g. eth0 on
61 Linux. ofproto expects that every port exposed by a datapath
62 has a corresponding netdev that it can open with netdev_open().
64 The following sections talk about these components in more detail.
69 The architectural diagram shows "dpif" and "wdp" as alternatives.
70 These alternatives correspond to the "master" and "wdp" branches,
71 respectively, of the Open vSwitch Git repository at
72 git://openvswitch.org/openvswitch. Both of these branches currently
73 represent reasonable porting targets for different purposes:
75 - The "master" branch is more mature and better tested. Open
76 vSwitch releases are made from this branch, and most OVS
77 development and testing occurs on this branch.
79 - The "wdp" branch has a software architecture that can take
80 advantage of hardware with support for wildcards (e.g. TCAMs or
81 similar). This branch has known important bugs, but is the basis
82 of a few ongoing hardware projects, so we expect the quality to
85 Since its architecture is better, in the medium to long term we will
86 fix the problems in the "wdp" branch and merge it into "master".
88 In porting OVS, the major difference between the two branches is the
89 form of the flow table in the datapath:
91 - On "master", the "dpif" datapath interface maintains a simple
92 flow table, one that does not support any kind of wildcards.
93 This flow table essentially acts as a cache. When a packet
94 arrives on an interface, the datapath looks for it in this
95 exact-match table. If there is a match, then it performs the
96 associated actions. If there is no match, the datapath passes
97 the packet up to "ofproto", which maintains a flow table that
98 supports wildcards. If the packet matches in this flow table,
99 then ofproto executes its actions and inserts a new exact-match
100 entry into the dpif flow table. (Otherwise, ofproto sends the
101 packet to the OpenFlow controller, if one is configured.)
103 Thus, on the "master" branch, the datapath has little
104 opportunity to take advantage of hardware support for wildcards,
105 since it is only ever presented with exact-match flow entries.
107 - On "wdp", the "wdp" datapath interface maintains a flow table
108 similar to that of OpenFlow, one that supports wildcards. Thus,
109 a wdp datapath can take advantage of hardware support for
110 wildcards, since it is free to implement the flow table any way
113 The following sections describe the two datapath interfaces in a
116 dpif: The "master" Branch Datapath
117 ----------------------------------
119 struct dpif_class, in lib/dpif-provider.h, defines the
120 interfaces required to implement a dpif for new hardware or
121 software. That structure contains many function pointers, each
122 of which has a comment that is meant to describe its behavior in
123 detail. If the requirements are unclear, please report this as
124 a bug and we will clarify.
126 There are two existing dpif implementations that may serve as
127 useful examples during a port:
129 * lib/dpif-linux.c is a Linux-specific dpif implementation that
130 talks to an Open vSwitch-specific kernel module (whose sources
131 are in the "datapath" directory). The kernel module performs
132 all of the switching work, passing packets that do not match any
133 flow table entry up to userspace. This dpif implementation is
134 essentially a wrapper around calls to "ioctl".
136 * lib/dpif-netdev.c is a generic dpif implementation that performs
137 all switching internally. It delegates most of its work to the
138 "netdev" library (described below). Using dpif-netdev, instead
139 of writing a new dpif, can be a simple way to get OVS up and
140 running on new platforms, but other solutions are likely to
141 yield higher performance.
143 "wdp": The "wdp" Branch Datapath
144 --------------------------------
146 struct wdp_class, in ofproto/wdp-provider.h, defines the interfaces
147 required to implement a wdp ("wildcarded datapath") for new hardware
148 or software. That structure contains many function pointers, each of
149 which has a comment that is meant to describe its behavior in detail.
150 If the requirements are unclear, please report this as a bug and we
153 The wdp interface is preliminary. Please let us know if it seems
154 unsuitable for your purpose. We will try to improve it.
156 There is currently only one wdp implementation:
158 * ofproto/wdp-xflow.c is an adaptation of "master" branch code
159 that breaks wildcarded flows up into exact-match flows in the
160 same way that ofproto always does on the "master" branch. It
161 delegates its work to exact-match datapath implementations whose
162 interfaces are identical to "master" branch datapaths, except
163 that names have been changed from "dpif" to "xfif" ("exact-match
164 flow interface") and similar.
166 "netdev": Interface to network devices
167 --------------------------------------
169 The netdev interface can be roughly divided into functionality for the
172 * Functions required to properly implement OpenFlow features. For
173 example, OpenFlow requires the ability to report the Ethernet
174 hardware address of a port. These functions must be implemented
175 for minimally correct operation.
177 * Functions required to implement optional Open vSwitch features.
178 For example, the Open vSwitch support for in-band control
179 requires netdev support for inspecting the TCP/IP stack's ARP
180 table. These functions must be implemented if the corresponding
181 OVS features are to work, but may be omitted initially.
183 * Functions that may be needed in some implementations but not
184 others. The dpif-netdev described above, for example, needs to
185 be able to send and receive packets on a netdev.
187 struct netdev_class, in lib/netdev-provider.h, defines the interfaces
188 required to implement a netdev. That structure contains many function
189 pointers, each of which has a comment that is meant to describe its
190 behavior in detail. If the requirements are unclear, please report
191 this as a bug and we will clarify.
193 The existing netdev implementations may serve as useful examples
196 * lib/netdev-linux.c implements netdev functionality for Linux
197 network devices, using Linux kernel calls. It may be a good
198 place to start for full-featured netdev implementations.
200 * lib/netdev-gre.c and lib/netdev-patch.c are minimal
201 implementations for "virtual ports" implemented by the Open
202 vSwitch datapath module for the Linux kernel. They may serve as
203 a model for minimal netdev implementations.
208 Please direct porting questions to dev@openvswitch.org. We will try
209 to use questions to improve this porting guide.