PORTING

   1          How to Port Open vSwitch to New Software or Hardware
   2          ====================================================
   3
   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
   8 difficult.)
   9
  10
  11 Vocabulary
  12 ----------
  13
  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:
  17
  18         datapath/       vport           ---
  19         vswitchd/       iface           port
  20         ofproto/        port            bundle
  21         lib/bond.c      slave           bond
  22         lib/lacp.c      slave           lacp
  23         lib/netdev.c    netdev          ---
  24         database        Interface       Port
  25
  26
  27 Open vSwitch Architectural Overview
  28 -----------------------------------
  29
  30 The following diagram shows the very high-level architecture of Open
  31 vSwitch from a porter's perspective.
  32
  33                    +-------------------+
  34                    |    ovs-vswitchd   |<-->ovsdb-server
  35                    +-------------------+
  36                    |      ofproto      |<-->OpenFlow controllers
  37                    +--------+-+--------+
  38                    | netdev | | ofproto|
  39                    +--------+ |provider|
  40                    | netdev | +--------+
  41                    |provider|
  42                    +--------+
  43
  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:
  46
  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
  52       into the database.
  53
  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.
  58
  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.
  63
  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.
  69
  70 The following sections talk about these components in more detail.
  71
  72
  73 Writing a netdev Provider
  74 -------------------------
  75
  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.
  81
  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
  86 this as a bug.
  87
  88 The netdev interface can be divided into a few rough categories:
  89
  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.
  94
  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.
 100
 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.
 104
 105 The existing netdev implementations may serve as useful examples
 106 during a port:
 107
 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.
 111
 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
 115       implementations.
 116
 117     * lib/netdev-dummy.c is a fake netdev implementation useful only
 118       for testing.
 119
 120
 121 Porting Strategies
 122 ------------------
 123
 124 After a netdev provider has been implemented for a system's network
 125 devices, you may choose among three basic porting strategies.
 126
 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
 133 switch.
 134
 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:
 139
 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
 142       TCAM).
 143
 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.
 148
 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.
 154
 155 The following sections describe how to implement each kind of port.
 156
 157
 158 ofproto Providers
 159 -----------------
 160
 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.
 168
 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.
 171
 172
 173 Writing a dpif Provider
 174 -----------------------
 175
 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
 188 one is configured.)
 189
 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:
 193
 194                 _
 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
 207                |_      ||     +---||---+   |
 208                        ||         ||       |
 209                 _  +---||-----+---||---+   |
 210                |   |          |datapath|   |
 211         kernel |   |          +--------+  _|
 212                |   |                   |
 213                |_  +--------||---------+
 214                             ||
 215                          physical
 216                            NIC
 217
 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.
 223
 224 There are two existing dpif implementations that may serve as
 225 useful examples during a port:
 226
 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.
 233
 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.
 237
 238
 239 Miscellaneous Notes
 240 -------------------
 241
 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.
 247
 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.
 258
 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.
 262
 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.
 265
 266
 267 Why OVS Does Not Support Hybrid Providers
 268 -----------------------------------------
 269
 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.
 276
 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.
 280
 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.
 286
 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
 293 traffic.
 294
 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
 305 processing.
 306
 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.
 310
 311
 312 Questions
 313 ---------
 314
 315 Please direct porting questions to dev@openvswitch.org.  We will try
 316 to use questions to improve this porting guide.