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:
21 ofproto/bond.c slave bond
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 (with an optional wildcard mask).
152 This allows fast hash lookups in software, but makes
153 inefficient use of TCAMs in hardware 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 is only required
179 to support exact-match flows, that is, flows without wildcards. When a
180 packet arrives on a network device, the datapath looks for it in this
181 table. If there is a match, then it performs the associated actions.
182 If there is no match, the datapath passes the packet up to ofproto-dpif,
183 which maintains the full OpenFlow flow table. If the packet matches in
184 this flow table, then ofproto-dpif executes its actions and inserts a
185 new entry into the dpif flow table. (Otherwise, ofproto-dpif passes the
186 packet up to ofproto to send the packet to the OpenFlow controller, if
189 When calculating the dpif flow, ofproto-dpif generates an exact-match
190 flow that describes the missed packet. It makes an effort to figure out
191 what fields can be wildcarded based on the switch's configuration and
192 OpenFlow flow table. The dpif is free to ignore the suggested wildcards
193 and only support the exact-match entry. However, if the dpif supports
194 wildcarding, then it can use the masks to match multiple flows with
195 fewer entries and potentially significantly reduce the number of flow
196 misses handled by ofproto-dpif.
198 The "dpif" library in turn delegates much of its functionality to a
199 "dpif provider". The following diagram shows how dpif providers fit
200 into the Open vSwitch architecture:
203 | +-------------------+
204 | | ovs-vswitchd |<-->ovsdb-server
205 | +-------------------+
206 | | ofproto |<-->OpenFlow controllers
207 | +--------+-+--------+ _
208 | | netdev | |ofproto-| |
209 userspace | +--------+ | dpif | |
210 | | netdev | +--------+ |
211 | |provider| | dpif | |
212 | +---||---+ +--------+ |
213 | || | dpif | | implementation of
214 | || |provider| | ofproto provider
217 _ +---||-----+---||---+ |
219 kernel | | +--------+ _|
221 |_ +--------||---------+
226 struct dpif_class, in lib/dpif-provider.h, defines the interfaces
227 required to implement a dpif provider for new hardware or software.
228 That structure contains many function pointers, each of which has a
229 comment that is meant to describe its behavior in detail. If the
230 requirements are unclear, please report this as a bug.
232 There are two existing dpif implementations that may serve as
233 useful examples during a port:
235 * lib/dpif-linux.c is a Linux-specific dpif implementation that
236 talks to an Open vSwitch-specific kernel module (whose sources
237 are in the "datapath" directory). The kernel module performs
238 all of the switching work, passing packets that do not match any
239 flow table entry up to userspace. This dpif implementation is
240 essentially a wrapper around calls into the kernel module.
242 * lib/dpif-netdev.c is a generic dpif implementation that performs
243 all switching internally. This is how the Open vSwitch
244 userspace switch is implemented.
250 Open vSwitch source code uses uint16_t, uint32_t, and uint64_t as
251 fixed-width types in host byte order, and ovs_be16, ovs_be32, and
252 ovs_be64 as fixed-width types in network byte order. Each of the
253 latter is equivalent to the one of the former, but the difference in
254 name makes the intended use obvious.
256 The default "fail-mode" for Open vSwitch bridges is "standalone",
257 meaning that, when the OpenFlow controllers cannot be contacted, Open
258 vSwitch acts as a regular MAC-learning switch. This works well in
259 virtualization environments where there is normally just one uplink
260 (either a single physical interface or a bond). In a more general
261 environment, it can create loops. So, if you are porting to a
262 general-purpose switch platform, you should consider changing the
263 default "fail-mode" to "secure", which does not behave this way. See
264 documentation for the "fail-mode" column in the Bridge table in
265 ovs-vswitchd.conf.db(5) for more information.
267 lib/entropy.c assumes that it can obtain high-quality random number
268 seeds at startup by reading from /dev/urandom. You will need to
269 modify it if this is not true on your platform.
271 vswitchd/system-stats.c only knows how to obtain some statistics on
272 Linux. Optionally you may implement them for your platform as well.
275 Why OVS Does Not Support Hybrid Providers
276 -----------------------------------------
278 The "Porting Strategies" section above describes the "ofproto
279 provider" and "dpif provider" porting strategies. Only an ofproto
280 provider can take advantage of hardware TCAM support, and only a dpif
281 provider can take advantage of the OVS built-in implementations of
282 various features. It is therefore tempting to suggest a hybrid
283 approach that shares the advantages of both strategies.
285 However, Open vSwitch does not support a hybrid approach. Doing so
286 may be possible, with a significant amount of extra development work,
287 but it does not yet seem worthwhile, for the reasons explained below.
289 First, user surprise is likely when a switch supports a feature only
290 with a high performance penalty. For example, one user questioned why
291 adding a particular OpenFlow action to a flow caused a 1,058x slowdown
292 on a hardware OpenFlow implementation [1]. The action required the
293 flow to be implemented in software.
295 Given that implementing a flow in software on the slow management CPU
296 of a hardware switch causes a major slowdown, software-implemented
297 flows would only make sense for very low-volume traffic. But many of
298 the features built into the OVS software switch implementation would
299 need to apply to every flow to be useful. There is no value, for
300 example, in applying bonding or 802.1Q VLAN support only to low-volume
303 Besides supporting features of OpenFlow actions, a hybrid approach
304 could also support forms of matching not supported by particular
305 switching hardware, by sending all packets that might match a rule to
306 software. But again this can cause an unacceptable slowdown by
307 forcing bulk traffic through software in the hardware switch's slow
308 management CPU. Consider, for example, a hardware switch that can
309 match on the IPv6 Ethernet type but not on fields in IPv6 headers. An
310 OpenFlow table that matched on the IPv6 Ethernet type would perform
311 well, but adding a rule that matched only UDPv6 would force every IPv6
312 packet to software, slowing down not just UDPv6 but all IPv6
315 [1] Aaron Rosen, "Modify packet fields extremely slow",
316 openflow-discuss mailing list, June 26, 2011, archived at
317 https://mailman.stanford.edu/pipermail/openflow-discuss/2011-June/002386.html.
323 Please direct porting questions to dev@openvswitch.org. We will try
324 to use questions to improve this porting guide.