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
14 For historical reasons, different words are used for essentially the
15 same concept in different areas of the Open vSwitch source tree. Here
16 is a concordance, indexed by the area of the source tree:
23 lib/netdev.c netdev ---
24 database Interface Port
27 Open vSwitch Architectural Overview
28 -----------------------------------
30 The following diagram shows the very high-level architecture of Open
31 vSwitch from a porter's perspective.
34 | ovs-vswitchd |<-->ovsdb-server
36 | ofproto |<-->OpenFlow controllers
44 Some of the components are generic. Modulo bugs or inadequacies,
45 these components should not need to be modified as part of a port:
47 - "ovs-vswitchd" is the main Open vSwitch userspace program, in
48 vswitchd/. It reads the desired Open vSwitch configuration from
49 the ovsdb-server program over an IPC channel and passes this
50 configuration down to the "ofproto" library. It also passes
51 certain status and statistical information from ofproto back
54 - "ofproto" is the Open vSwitch library, in ofproto/, that
55 implements an OpenFlow switch. It talks to OpenFlow controllers
56 over the network and to switch hardware or software through an
57 "ofproto provider", explained further below.
59 - "netdev" is the Open vSwitch library, in lib/netdev.c, that
60 abstracts interacting with network devices, that is, Ethernet
61 interfaces. The netdev library is a thin layer over "netdev
62 provider" code, explained further below.
64 The other components may need attention during a port. You will
65 almost certainly have to implement a "netdev provider". Depending on
66 the type of port you are doing and the desired performance, you may
67 also have to implement an "ofproto provider" or a lower-level
68 component called a "dpif" provider.
70 The following sections talk about these components in more detail.
73 Writing a netdev Provider
74 -------------------------
76 A "netdev provider" implements an operating system and hardware
77 specific interface to "network devices", e.g. eth0 on Linux. Open
78 vSwitch must be able to open each port on a switch as a netdev, so you
79 will need to implement a "netdev provider" that works with your switch
80 hardware and software.
82 struct netdev_class, in lib/netdev-provider.h, defines the interfaces
83 required to implement a netdev. That structure contains many function
84 pointers, each of which has a comment that is meant to describe its
85 behavior in detail. If the requirements are unclear, please report
88 The netdev interface can be divided into a few rough categories:
90 * Functions required to properly implement OpenFlow features. For
91 example, OpenFlow requires the ability to report the Ethernet
92 hardware address of a port. These functions must be implemented
93 for minimally correct operation.
95 * Functions required to implement optional Open vSwitch features.
96 For example, the Open vSwitch support for in-band control
97 requires netdev support for inspecting the TCP/IP stack's ARP
98 table. These functions must be implemented if the corresponding
99 OVS features are to work, but may be omitted initially.
101 * Functions needed in some implementations but not in others. For
102 example, most kinds of ports (see below) do not need
103 functionality to receive packets from a network device.
105 The existing netdev implementations may serve as useful examples
108 * lib/netdev-linux.c implements netdev functionality for Linux
109 network devices, using Linux kernel calls. It may be a good
110 place to start for full-featured netdev implementations.
112 * lib/netdev-vport.c provides support for "virtual ports"
113 implemented by the Open vSwitch datapath module for the Linux
114 kernel. This may serve as a model for minimal netdev
117 * lib/netdev-dummy.c is a fake netdev implementation useful only
124 After a netdev provider has been implemented for a system's network
125 devices, you may choose among three basic porting strategies.
127 The lowest-effort strategy is to use the "userspace switch"
128 implementation built into Open vSwitch. This ought to work, without
129 writing any more code, as long as the netdev provider that you
130 implemented supports receiving packets. It yields poor performance,
131 however, because every packet passes through the ovs-vswitchd process.
132 See INSTALL.userspace for instructions on how to configure a userspace
135 If the userspace switch is not the right choice for your port, then
136 you will have to write more code. You may implement either an
137 "ofproto provider" or a "dpif provider". Which you should choose
138 depends on a few different factors:
140 * Only an ofproto provider can take full advantage of hardware
141 with built-in support for wildcards (e.g. an ACL table or a
144 * A dpif provider can take advantage of the Open vSwitch built-in
145 implementations of bonding, LACP, 802.1ag, 802.1Q VLANs, and
146 other features. An ofproto provider has to provide its own
147 implementations, if the hardware can support them at all.
149 * A dpif provider is usually easier to implement, but most
150 appropriate for software switching. It "explodes" wildcard
151 rules into exact-match entries. This allows fast hash lookups
152 in software, but makes inefficient use of TCAMs in hardware
153 that support wildcarding.
155 The following sections describe how to implement each kind of port.
161 An "ofproto provider" is what ofproto uses to directly monitor and
162 control an OpenFlow-capable switch. struct ofproto_class, in
163 ofproto/ofproto-provider.h, defines the interfaces to implement an
164 ofproto provider for new hardware or software. That structure contains
165 many function pointers, each of which has a comment that is meant to
166 describe its behavior in detail. If the requirements are unclear,
167 please report this as a bug.
169 The ofproto provider interface is preliminary. Please let us know if
170 it seems unsuitable for your purpose. We will try to improve it.
173 Writing a dpif Provider
174 -----------------------
176 Open vSwitch has a built-in ofproto provider named "ofproto-dpif",
177 which is built on top of a library for manipulating datapaths, called
178 "dpif". A "datapath" is a simple flow table, one that supports only
179 exact-match flows, that is, flows without wildcards. When a packet
180 arrives on a network device, the datapath looks for it in this
181 exact-match table. If there is a match, then it performs the
182 associated actions. If there is no match, the datapath passes the
183 packet up to ofproto-dpif, which maintains an OpenFlow flow table
184 (that supports wildcards). If the packet matches in this flow table,
185 then ofproto-dpif executes its actions and inserts a new exact-match
186 entry into the dpif flow table. (Otherwise, ofproto-dpif passes the
187 packet up to ofproto to send the packet to the OpenFlow controller, if
190 The "dpif" library in turn delegates much of its functionality to a
191 "dpif provider". The following diagram shows how dpif providers fit
192 into the Open vSwitch architecture:
195 | +-------------------+
196 | | ovs-vswitchd |<-->ovsdb-server
197 | +-------------------+
198 | | ofproto |<-->OpenFlow controllers
199 | +--------+-+--------+ _
200 | | netdev | |ofproto-| |
201 userspace | +--------+ | dpif | |
202 | | netdev | +--------+ |
203 | |provider| | dpif | |
204 | +---||---+ +--------+ |
205 | || | dpif | | implementation of
206 | || |provider| | ofproto provider
209 _ +---||-----+---||---+ |
211 kernel | | +--------+ _|
213 |_ +--------||---------+
218 struct dpif_class, in lib/dpif-provider.h, defines the interfaces
219 required to implement a dpif provider for new hardware or software.
220 That structure contains many function pointers, each of which has a
221 comment that is meant to describe its behavior in detail. If the
222 requirements are unclear, please report this as a bug.
224 There are two existing dpif implementations that may serve as
225 useful examples during a port:
227 * lib/dpif-linux.c is a Linux-specific dpif implementation that
228 talks to an Open vSwitch-specific kernel module (whose sources
229 are in the "datapath" directory). The kernel module performs
230 all of the switching work, passing packets that do not match any
231 flow table entry up to userspace. This dpif implementation is
232 essentially a wrapper around calls into the kernel module.
234 * lib/dpif-netdev.c is a generic dpif implementation that performs
235 all switching internally. This is how the Open vSwitch
236 userspace switch is implemented.
242 Open vSwitch source code uses uint16_t, uint32_t, and uint64_t as
243 fixed-width types in host byte order, and ovs_be16, ovs_be32, and
244 ovs_be64 as fixed-width types in network byte order. Each of the
245 latter is equivalent to the one of the former, but the difference in
246 name makes the intended use obvious.
248 The default "fail-mode" for Open vSwitch bridges is "standalone",
249 meaning that, when the OpenFlow controllers cannot be contacted, Open
250 vSwitch acts as a regular MAC-learning switch. This works well in
251 virtualization environments where there is normally just one uplink
252 (either a single physical interface or a bond). In a more general
253 environment, it can create loops. So, if you are porting to a
254 general-purpose switch platform, you should consider changing the
255 default "fail-mode" to "secure", which does not behave this way. See
256 documentation for the "fail-mode" column in the Bridge table in
257 ovs-vswitchd.conf.db(5) for more information.
259 lib/entropy.c assumes that it can obtain high-quality random number
260 seeds at startup by reading from /dev/urandom. You will need to
261 modify it if this is not true on your platform.
263 vswitchd/system-stats.c only knows how to obtain some statistics on
264 Linux. Optionally you may implement them for your platform as well.
267 Why OVS Does Not Support Hybrid Providers
268 -----------------------------------------
270 The "Porting Strategies" section above describes the "ofproto
271 provider" and "dpif provider" porting strategies. Only an ofproto
272 provider can take advantage of hardware TCAM support, and only a dpif
273 provider can take advantage of the OVS built-in implementations of
274 various features. It is therefore tempting to suggest a hybrid
275 approach that shares the advantages of both strategies.
277 However, Open vSwitch does not support a hybrid approach. Doing so
278 may be possible, with a significant amount of extra development work,
279 but it does not yet seem worthwhile, for the reasons explained below.
281 First, user surprise is likely when a switch supports a feature only
282 with a high performance penalty. For example, one user questioned why
283 adding a particular OpenFlow action to a flow caused a 1,058x slowdown
284 on a hardware OpenFlow implementation [1]. The action required the
285 flow to be implemented in software.
287 Given that implementing a flow in software on the slow management CPU
288 of a hardware switch causes a major slowdown, software-implemented
289 flows would only make sense for very low-volume traffic. But many of
290 the features built into the OVS software switch implementation would
291 need to apply to every flow to be useful. There is no value, for
292 example, in applying bonding or 802.1Q VLAN support only to low-volume
295 Besides supporting features of OpenFlow actions, a hybrid approach
296 could also support forms of matching not supported by particular
297 switching hardware, by sending all packets that might match a rule to
298 software. But again this can cause an unacceptable slowdown by
299 forcing bulk traffic through software in the hardware switch's slow
300 management CPU. Consider, for example, a hardware switch that can
301 match on the IPv6 Ethernet type but not on fields in IPv6 headers. An
302 OpenFlow table that matched on the IPv6 Ethernet type would perform
303 well, but adding a rule that matched only UDPv6 would force every IPv6
304 packet to software, slowing down not just UDPv6 but all IPv6
307 [1] Aaron Rosen, "Modify packet fields extremely slow",
308 openflow-discuss mailing list, June 26, 2011, archived at
309 https://mailman.stanford.edu/pipermail/openflow-discuss/2011-June/002386.html.
315 Please direct porting questions to dev@openvswitch.org. We will try
316 to use questions to improve this porting guide.