Documentation/networking/openvswitch.txt

   1 Open vSwitch datapath developer documentation
   2 =============================================
   3
   4 The Open vSwitch kernel module allows flexible userspace control over
   5 flow-level packet processing on selected network devices.  It can be
   6 used to implement a plain Ethernet switch, network device bonding,
   7 VLAN processing, network access control, flow-based network control,
   8 and so on.
   9
  10 The kernel module implements multiple "datapaths" (analogous to
  11 bridges), each of which can have multiple "vports" (analogous to ports
  12 within a bridge).  Each datapath also has associated with it a "flow
  13 table" that userspace populates with "flows" that map from keys based
  14 on packet headers and metadata to sets of actions.  The most common
  15 action forwards the packet to another vport; other actions are also
  16 implemented.
  17
  18 When a packet arrives on a vport, the kernel module processes it by
  19 extracting its flow key and looking it up in the flow table.  If there
  20 is a matching flow, it executes the associated actions.  If there is
  21 no match, it queues the packet to userspace for processing (as part of
  22 its processing, userspace will likely set up a flow to handle further
  23 packets of the same type entirely in-kernel).
  24
  25
  26 Flow key compatibility
  27 ----------------------
  28
  29 Network protocols evolve over time.  New protocols become important
  30 and existing protocols lose their prominence.  For the Open vSwitch
  31 kernel module to remain relevant, it must be possible for newer
  32 versions to parse additional protocols as part of the flow key.  It
  33 might even be desirable, someday, to drop support for parsing
  34 protocols that have become obsolete.  Therefore, the Netlink interface
  35 to Open vSwitch is designed to allow carefully written userspace
  36 applications to work with any version of the flow key, past or future.
  37
  38 To support this forward and backward compatibility, whenever the
  39 kernel module passes a packet to userspace, it also passes along the
  40 flow key that it parsed from the packet.  Userspace then extracts its
  41 own notion of a flow key from the packet and compares it against the
  42 kernel-provided version:
  43
  44     - If userspace's notion of the flow key for the packet matches the
  45       kernel's, then nothing special is necessary.
  46
  47     - If the kernel's flow key includes more fields than the userspace
  48       version of the flow key, for example if the kernel decoded IPv6
  49       headers but userspace stopped at the Ethernet type (because it
  50       does not understand IPv6), then again nothing special is
  51       necessary.  Userspace can still set up a flow in the usual way,
  52       as long as it uses the kernel-provided flow key to do it.
  53
  54     - If the userspace flow key includes more fields than the
  55       kernel's, for example if userspace decoded an IPv6 header but
  56       the kernel stopped at the Ethernet type, then userspace can
  57       forward the packet manually, without setting up a flow in the
  58       kernel.  This case is bad for performance because every packet
  59       that the kernel considers part of the flow must go to userspace,
  60       but the forwarding behavior is correct.  (If userspace can
  61       determine that the values of the extra fields would not affect
  62       forwarding behavior, then it could set up a flow anyway.)
  63
  64 How flow keys evolve over time is important to making this work, so
  65 the following sections go into detail.
  66
  67
  68 Flow key format
  69 ---------------
  70
  71 A flow key is passed over a Netlink socket as a sequence of Netlink
  72 attributes.  Some attributes represent packet metadata, defined as any
  73 information about a packet that cannot be extracted from the packet
  74 itself, e.g. the vport on which the packet was received.  Most
  75 attributes, however, are extracted from headers within the packet,
  76 e.g. source and destination addresses from Ethernet, IP, or TCP
  77 headers.
  78
  79 The <linux/openvswitch.h> header file defines the exact format of the
  80 flow key attributes.  For informal explanatory purposes here, we write
  81 them as comma-separated strings, with parentheses indicating arguments
  82 and nesting.  For example, the following could represent a flow key
  83 corresponding to a TCP packet that arrived on vport 1:
  84
  85     in_port(1), eth(src=e0:91:f5:21:d0:b2, dst=00:02:e3:0f:80:a4),
  86     eth_type(0x0800), ipv4(src=172.16.0.20, dst=172.18.0.52, proto=17, tos=0,
  87     frag=no), tcp(src=49163, dst=80)
  88
  89 Often we ellipsize arguments not important to the discussion, e.g.:
  90
  91     in_port(1), eth(...), eth_type(0x0800), ipv4(...), tcp(...)
  92
  93
  94 Wildcarded flow key format
  95 --------------------------
  96
  97 A wildcarded flow is described with two sequences of Netlink attributes
  98 passed over the Netlink socket. A flow key, exactly as described above, and an
  99 optional corresponding flow mask.
 100
 101 A wildcarded flow can represent a group of exact match flows. Each '1' bit
 102 in the mask specifies a exact match with the corresponding bit in the flow key.
 103 A '0' bit specifies a don't care bit, which will match either a '1' or '0' bit
 104 of a incoming packet. Using wildcarded flow can improve the flow set up rate
 105 by reduce the number of new flows need to be processed by the user space program.
 106
 107 Support for the mask Netlink attribute is optional for both the kernel and user
 108 space program. The kernel can ignore the mask attribute, installing an exact
 109 match flow, or reduce the number of don't care bits in the kernel to less than
 110 what was specified by the user space program. In this case, variations in bits
 111 that the kernel does not implement will simply result in additional flow setups.
 112 The kernel module will also work with user space programs that neither support
 113 nor supply flow mask attributes.
 114
 115 Since the kernel may ignore or modify wildcard bits, it can be difficult for
 116 the userspace program to know exactly what matches are installed. There are
 117 two possible approaches: reactively install flows as they miss the kernel
 118 flow table (and therefore not attempt to determine wildcard changes at all)
 119 or use the kernel's response messages to determine the installed wildcards.
 120
 121 When interacting with userspace, the kernel should maintain the match portion
 122 of the key exactly as originally installed. This will provides a handle to
 123 identify the flow for all future operations. However, when reporting the
 124 mask of an installed flow, the mask should include any restrictions imposed
 125 by the kernel.
 126
 127 The behavior when using overlapping wildcarded flows is undefined. It is the
 128 responsibility of the user space program to ensure that any incoming packet
 129 can match at most one flow, wildcarded or not. The current implementation
 130 performs best-effort detection of overlapping wildcarded flows and may reject
 131 some but not all of them. However, this behavior may change in future versions.
 132
 133
 134 Unique flow identifiers
 135 -----------------------
 136
 137 An alternative to using the original match portion of a key as the handle for
 138 flow identification is a unique flow identifier, or "UFID". UFIDs are optional
 139 for both the kernel and user space program.
 140
 141 User space programs that support UFID are expected to provide it during flow
 142 setup in addition to the flow, then refer to the flow using the UFID for all
 143 future operations. The kernel is not required to index flows by the original
 144 flow key if a UFID is specified.
 145
 146
 147 Basic rule for evolving flow keys
 148 ---------------------------------
 149
 150 Some care is needed to really maintain forward and backward
 151 compatibility for applications that follow the rules listed under
 152 "Flow key compatibility" above.
 153
 154 The basic rule is obvious:
 155
 156     ------------------------------------------------------------------
 157     New network protocol support must only supplement existing flow
 158     key attributes.  It must not change the meaning of already defined
 159     flow key attributes.
 160     ------------------------------------------------------------------
 161
 162 This rule does have less-obvious consequences so it is worth working
 163 through a few examples.  Suppose, for example, that the kernel module
 164 did not already implement VLAN parsing.  Instead, it just interpreted
 165 the 802.1Q TPID (0x8100) as the Ethertype then stopped parsing the
 166 packet.  The flow key for any packet with an 802.1Q header would look
 167 essentially like this, ignoring metadata:
 168
 169     eth(...), eth_type(0x8100)
 170
 171 Naively, to add VLAN support, it makes sense to add a new "vlan" flow
 172 key attribute to contain the VLAN tag, then continue to decode the
 173 encapsulated headers beyond the VLAN tag using the existing field
 174 definitions.  With this change, a TCP packet in VLAN 10 would have a
 175 flow key much like this:
 176
 177     eth(...), vlan(vid=10, pcp=0), eth_type(0x0800), ip(proto=6, ...), tcp(...)
 178
 179 But this change would negatively affect a userspace application that
 180 has not been updated to understand the new "vlan" flow key attribute.
 181 The application could, following the flow compatibility rules above,
 182 ignore the "vlan" attribute that it does not understand and therefore
 183 assume that the flow contained IP packets.  This is a bad assumption
 184 (the flow only contains IP packets if one parses and skips over the
 185 802.1Q header) and it could cause the application's behavior to change
 186 across kernel versions even though it follows the compatibility rules.
 187
 188 The solution is to use a set of nested attributes.  This is, for
 189 example, why 802.1Q support uses nested attributes.  A TCP packet in
 190 VLAN 10 is actually expressed as:
 191
 192     eth(...), eth_type(0x8100), vlan(vid=10, pcp=0), encap(eth_type(0x0800),
 193     ip(proto=6, ...), tcp(...)))
 194
 195 Notice how the "eth_type", "ip", and "tcp" flow key attributes are
 196 nested inside the "encap" attribute.  Thus, an application that does
 197 not understand the "vlan" key will not see either of those attributes
 198 and therefore will not misinterpret them.  (Also, the outer eth_type
 199 is still 0x8100, not changed to 0x0800.)
 200
 201 Handling malformed packets
 202 --------------------------
 203
 204 Don't drop packets in the kernel for malformed protocol headers, bad
 205 checksums, etc.  This would prevent userspace from implementing a
 206 simple Ethernet switch that forwards every packet.
 207
 208 Instead, in such a case, include an attribute with "empty" content.
 209 It doesn't matter if the empty content could be valid protocol values,
 210 as long as those values are rarely seen in practice, because userspace
 211 can always forward all packets with those values to userspace and
 212 handle them individually.
 213
 214 For example, consider a packet that contains an IP header that
 215 indicates protocol 6 for TCP, but which is truncated just after the IP
 216 header, so that the TCP header is missing.  The flow key for this
 217 packet would include a tcp attribute with all-zero src and dst, like
 218 this:
 219
 220     eth(...), eth_type(0x0800), ip(proto=6, ...), tcp(src=0, dst=0)
 221
 222 As another example, consider a packet with an Ethernet type of 0x8100,
 223 indicating that a VLAN TCI should follow, but which is truncated just
 224 after the Ethernet type.  The flow key for this packet would include
 225 an all-zero-bits vlan and an empty encap attribute, like this:
 226
 227     eth(...), eth_type(0x8100), vlan(0), encap()
 228
 229 Unlike a TCP packet with source and destination ports 0, an
 230 all-zero-bits VLAN TCI is not that rare, so the CFI bit (aka
 231 VLAN_TAG_PRESENT inside the kernel) is ordinarily set in a vlan
 232 attribute expressly to allow this situation to be distinguished.
 233 Thus, the flow key in this second example unambiguously indicates a
 234 missing or malformed VLAN TCI.
 235
 236 Other rules
 237 -----------
 238
 239 The other rules for flow keys are much less subtle:
 240
 241     - Duplicate attributes are not allowed at a given nesting level.
 242
 243     - Ordering of attributes is not significant.
 244
 245     - When the kernel sends a given flow key to userspace, it always
 246       composes it the same way.  This allows userspace to hash and
 247       compare entire flow keys that it may not be able to fully
 248       interpret.