1 Open vSwitch datapath developer documentation
2 =============================================
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,
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
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).
26 Flow key compatibility
27 ----------------------
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.
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:
44 - If userspace's notion of the flow key for the packet matches the
45 kernel's, then nothing special is necessary.
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.
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.)
64 How flow keys evolve over time is important to making this work, so
65 the following sections go into detail.
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
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:
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)
89 Often we ellipsize arguments not important to the discussion, e.g.:
91 in_port(1), eth(...), eth_type(0x0800), ipv4(...), tcp(...)
94 Basic rule for evolving flow keys
95 ---------------------------------
97 Some care is needed to really maintain forward and backward
98 compatibility for applications that follow the rules listed under
99 "Flow key compatibility" above.
101 The basic rule is obvious:
103 ------------------------------------------------------------------
104 New network protocol support must only supplement existing flow
105 key attributes. It must not change the meaning of already defined
107 ------------------------------------------------------------------
109 This rule does have less-obvious consequences so it is worth working
110 through a few examples. Suppose, for example, that the kernel module
111 did not already implement VLAN parsing. Instead, it just interpreted
112 the 802.1Q TPID (0x8100) as the Ethertype then stopped parsing the
113 packet. The flow key for any packet with an 802.1Q header would look
114 essentially like this, ignoring metadata:
116 eth(...), eth_type(0x8100)
118 Naively, to add VLAN support, it makes sense to add a new "vlan" flow
119 key attribute to contain the VLAN tag, then continue to decode the
120 encapsulated headers beyond the VLAN tag using the existing field
121 definitions. With this change, a TCP packet in VLAN 10 would have a
122 flow key much like this:
124 eth(...), vlan(vid=10, pcp=0), eth_type(0x0800), ip(proto=6, ...), tcp(...)
126 But this change would negatively affect a userspace application that
127 has not been updated to understand the new "vlan" flow key attribute.
128 The application could, following the flow compatibility rules above,
129 ignore the "vlan" attribute that it does not understand and therefore
130 assume that the flow contained IP packets. This is a bad assumption
131 (the flow only contains IP packets if one parses and skips over the
132 802.1Q header) and it could cause the application's behavior to change
133 across kernel versions even though it follows the compatibility rules.
135 The solution is to use a set of nested attributes. This is, for
136 example, why 802.1Q support uses nested attributes. A TCP packet in
137 VLAN 10 is actually expressed as:
139 eth(...), eth_type(0x8100), vlan(vid=10, pcp=0), encap(eth_type(0x0800),
140 ip(proto=6, ...), tcp(...)))
142 Notice how the "eth_type", "ip", and "tcp" flow key attributes are
143 nested inside the "encap" attribute. Thus, an application that does
144 not understand the "vlan" key will not see either of those attributes
145 and therefore will not misinterpret them. (Also, the outer eth_type
146 is still 0x8100, not changed to 0x0800.)
148 Handling malformed packets
149 --------------------------
151 Don't drop packets in the kernel for malformed protocol headers, bad
152 checksums, etc. This would prevent userspace from implementing a
153 simple Ethernet switch that forwards every packet.
155 Instead, in such a case, include an attribute with "empty" content.
156 It doesn't matter if the empty content could be valid protocol values,
157 as long as those values are rarely seen in practice, because userspace
158 can always forward all packets with those values to userspace and
159 handle them individually.
161 For example, consider a packet that contains an IP header that
162 indicates protocol 6 for TCP, but which is truncated just after the IP
163 header, so that the TCP header is missing. The flow key for this
164 packet would include a tcp attribute with all-zero src and dst, like
167 eth(...), eth_type(0x0800), ip(proto=6, ...), tcp(src=0, dst=0)
169 As another example, consider a packet with an Ethernet type of 0x8100,
170 indicating that a VLAN TCI should follow, but which is truncated just
171 after the Ethernet type. The flow key for this packet would include
172 an all-zero-bits vlan and an empty encap attribute, like this:
174 eth(...), eth_type(0x8100), vlan(0), encap()
176 Unlike a TCP packet with source and destination ports 0, an
177 all-zero-bits VLAN TCI is not that rare, so the CFI bit (aka
178 VLAN_TAG_PRESENT inside the kernel) is ordinarily set in a vlan
179 attribute expressly to allow this situation to be distinguished.
180 Thus, the flow key in this second example unambiguously indicates a
181 missing or malformed VLAN TCI.
186 The other rules for flow keys are much less subtle:
188 - Duplicate attributes are not allowed at a given nesting level.
190 - Ordering of attributes is not significant.
192 - When the kernel sends a given flow key to userspace, it always
193 composes it the same way. This allows userspace to hash and
194 compare entire flow keys that it may not be able to fully