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+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
+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
+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=, dst=, 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
+ 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.