Open vSwitch datapath developer documentation
  =============================================
  
  The Open vSwitch kernel module allows flexible userspace control over
  flow-level packet processing on selected network devices.  It can be
  used to implement a plain Ethernet switch, network device bonding,
  VLAN processing, network access control, flow-based network control,
  and so on.
  
  The kernel module implements multiple "datapaths" (analogous to
  bridges), each of which can have multiple "vports" (analogous to ports
  within a bridge).  Each datapath also has associated with it a "flow
  table" that userspace populates with "flows" that map from keys based
  on packet headers and metadata to sets of actions.  The most common
  action forwards the packet to another vport; other actions are also
  implemented.
  
  When a packet arrives on a vport, the kernel module processes it by
  extracting its flow key and looking it up in the flow table.  If there
  is a matching flow, it executes the associated actions.  If there is
  no match, it queues the packet to userspace for processing (as part of
  its processing, userspace will likely set up a flow to handle further
  packets of the same type entirely in-kernel).
  
  
  Flow key compatibility
  ----------------------
  
  Network protocols evolve over time.  New protocols become important
  and existing protocols lose their prominence.  For the Open vSwitch
  kernel module to remain relevant, it must be possible for newer
  versions to parse additional protocols as part of the flow key.  It
  might even be desirable, someday, to drop support for parsing
  protocols that have become obsolete.  Therefore, the Netlink interface
  to Open vSwitch is designed to allow carefully written userspace
  applications to work with any version of the flow key, past or future.
  
  To support this forward and backward compatibility, whenever the
  kernel module passes a packet to userspace, it also passes along the
  flow key that it parsed from the packet.  Userspace then extracts its
  own notion of a flow key from the packet and compares it against the
  kernel-provided version:
  
      - If userspace's notion of the flow key for the packet matches the
        kernel's, then nothing special is necessary.
  
      - If the kernel's flow key includes more fields than the userspace
        version of the flow key, for example if the kernel decoded IPv6
        headers but userspace stopped at the Ethernet type (because it
        does not understand IPv6), then again nothing special is
        necessary.  Userspace can still set up a flow in the usual way,
        as long as it uses the kernel-provided flow key to do it.
  
      - If the userspace flow key includes more fields than the
        kernel's, for example if userspace decoded an IPv6 header but
        the kernel stopped at the Ethernet type, then userspace can
        forward the packet manually, without setting up a flow in the
        kernel.  This case is bad for performance because every packet
        that the kernel considers part of the flow must go to userspace,
        but the forwarding behavior is correct.  (If userspace can
        determine that the values of the extra fields would not affect
        forwarding behavior, then it could set up a flow anyway.)
  
  How flow keys evolve over time is important to making this work, so
  the following sections go into detail.
  
  
  Flow key format
  ---------------
  
  A flow key is passed over a Netlink socket as a sequence of Netlink
  attributes.  Some attributes represent packet metadata, defined as any
  information about a packet that cannot be extracted from the packet
  itself, e.g. the vport on which the packet was received.  Most
  attributes, however, are extracted from headers within the packet,
  e.g. source and destination addresses from Ethernet, IP, or TCP
  headers.
  
  The <linux/openvswitch.h> header file defines the exact format of the
  flow key attributes.  For informal explanatory purposes here, we write
  them as comma-separated strings, with parentheses indicating arguments
  and nesting.  For example, the following could represent a flow key
  corresponding to a TCP packet that arrived on vport 1:
  
      in_port(1), eth(src=e0:91:f5:21:d0:b2, dst=00:02:e3:0f:80:a4),
      eth_type(0x0800), ipv4(src=172.16.0.20, dst=172.18.0.52, proto=17, tos=0,
      frag=no), tcp(src=49163, dst=80)
  
  Often we ellipsize arguments not important to the discussion, e.g.:
  
      in_port(1), eth(...), eth_type(0x0800), ipv4(...), tcp(...)
  
  
  Basic rule for evolving flow keys
  ---------------------------------
  
  Some care is needed to really maintain forward and backward
  compatibility for applications that follow the rules listed under
  "Flow key compatibility" above.
  
  The basic rule is obvious:
  
      ------------------------------------------------------------------
      New network protocol support must only supplement existing flow
      key attributes.  It must not change the meaning of already defined
      flow key attributes.
      ------------------------------------------------------------------
  
  This rule does have less-obvious consequences so it is worth working
  through a few examples.  Suppose, for example, that the kernel module
  did not already implement VLAN parsing.  Instead, it just interpreted
  the 802.1Q TPID (0x8100) as the Ethertype then stopped parsing the
  packet.  The flow key for any packet with an 802.1Q header would look
  essentially like this, ignoring metadata:
  
      eth(...), eth_type(0x8100)
  
  Naively, to add VLAN support, it makes sense to add a new "vlan" flow
  key attribute to contain the VLAN tag, then continue to decode the
  encapsulated headers beyond the VLAN tag using the existing field
  definitions.  With this change, an TCP packet in VLAN 10 would have a
  flow key much like this:
  
      eth(...), vlan(vid=10, pcp=0), eth_type(0x0800), ip(proto=6, ...), tcp(...)
  
  But this change would negatively affect a userspace application that
  has not been updated to understand the new "vlan" flow key attribute.
  The application could, following the flow compatibility rules above,
  ignore the "vlan" attribute that it does not understand and therefore
  assume that the flow contained IP packets.  This is a bad assumption
  (the flow only contains IP packets if one parses and skips over the
  802.1Q header) and it could cause the application's behavior to change
  across kernel versions even though it follows the compatibility rules.
  
  The solution is to use a set of nested attributes.  This is, for
  example, why 802.1Q support uses nested attributes.  A TCP packet in
  VLAN 10 is actually expressed as:
  
      eth(...), eth_type(0x8100), vlan(vid=10, pcp=0), encap(eth_type(0x0800),
      ip(proto=6, ...), tcp(...)))
  
  Notice how the "eth_type", "ip", and "tcp" flow key attributes are
  nested inside the "encap" attribute.  Thus, an application that does
  not understand the "vlan" key will not see either of those attributes
  and therefore will not misinterpret them.  (Also, the outer eth_type
  is still 0x8100, not changed to 0x0800.)
  
  Handling malformed packets
  --------------------------
  
  Don't drop packets in the kernel for malformed protocol headers, bad
  checksums, etc.  This would prevent userspace from implementing a
  simple Ethernet switch that forwards every packet.
  
  Instead, in such a case, include an attribute with "empty" content.
  It doesn't matter if the empty content could be valid protocol values,
  as long as those values are rarely seen in practice, because userspace
  can always forward all packets with those values to userspace and
  handle them individually.
  
  For example, consider a packet that contains an IP header that
  indicates protocol 6 for TCP, but which is truncated just after the IP
  header, so that the TCP header is missing.  The flow key for this
  packet would include a tcp attribute with all-zero src and dst, like
  this:
  
      eth(...), eth_type(0x0800), ip(proto=6, ...), tcp(src=0, dst=0)
  
  As another example, consider a packet with an Ethernet type of 0x8100,
  indicating that a VLAN TCI should follow, but which is truncated just
  after the Ethernet type.  The flow key for this packet would include
  an all-zero-bits vlan and an empty encap attribute, like this:
  
      eth(...), eth_type(0x8100), vlan(0), encap()
  
  Unlike a TCP packet with source and destination ports 0, an
  all-zero-bits VLAN TCI is not that rare, so the CFI bit (aka
  VLAN_TAG_PRESENT inside the kernel) is ordinarily set in a vlan
  attribute expressly to allow this situation to be distinguished.
  Thus, the flow key in this second example unambiguously indicates a
  missing or malformed VLAN TCI.
  
  Other rules
  -----------
  
  The other rules for flow keys are much less subtle:
  
      - Duplicate attributes are not allowed at a given nesting level.
  
      - Ordering of attributes is not significant.
  
      - When the kernel sends a given flow key to userspace, it always
        composes it the same way.  This allows userspace to hash and
        compare entire flow keys that it may not be able to fully
        interpret.