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 Wildcarded flow key format
95 --------------------------
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.
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.
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.
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.
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
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.
134 Basic rule for evolving flow keys
135 ---------------------------------
137 Some care is needed to really maintain forward and backward
138 compatibility for applications that follow the rules listed under
139 "Flow key compatibility" above.
141 The basic rule is obvious:
143 ------------------------------------------------------------------
144 New network protocol support must only supplement existing flow
145 key attributes. It must not change the meaning of already defined
147 ------------------------------------------------------------------
149 This rule does have less-obvious consequences so it is worth working
150 through a few examples. Suppose, for example, that the kernel module
151 did not already implement VLAN parsing. Instead, it just interpreted
152 the 802.1Q TPID (0x8100) as the Ethertype then stopped parsing the
153 packet. The flow key for any packet with an 802.1Q header would look
154 essentially like this, ignoring metadata:
156 eth(...), eth_type(0x8100)
158 Naively, to add VLAN support, it makes sense to add a new "vlan" flow
159 key attribute to contain the VLAN tag, then continue to decode the
160 encapsulated headers beyond the VLAN tag using the existing field
161 definitions. With this change, a TCP packet in VLAN 10 would have a
162 flow key much like this:
164 eth(...), vlan(vid=10, pcp=0), eth_type(0x0800), ip(proto=6, ...), tcp(...)
166 But this change would negatively affect a userspace application that
167 has not been updated to understand the new "vlan" flow key attribute.
168 The application could, following the flow compatibility rules above,
169 ignore the "vlan" attribute that it does not understand and therefore
170 assume that the flow contained IP packets. This is a bad assumption
171 (the flow only contains IP packets if one parses and skips over the
172 802.1Q header) and it could cause the application's behavior to change
173 across kernel versions even though it follows the compatibility rules.
175 The solution is to use a set of nested attributes. This is, for
176 example, why 802.1Q support uses nested attributes. A TCP packet in
177 VLAN 10 is actually expressed as:
179 eth(...), eth_type(0x8100), vlan(vid=10, pcp=0), encap(eth_type(0x0800),
180 ip(proto=6, ...), tcp(...)))
182 Notice how the "eth_type", "ip", and "tcp" flow key attributes are
183 nested inside the "encap" attribute. Thus, an application that does
184 not understand the "vlan" key will not see either of those attributes
185 and therefore will not misinterpret them. (Also, the outer eth_type
186 is still 0x8100, not changed to 0x0800.)
188 Handling malformed packets
189 --------------------------
191 Don't drop packets in the kernel for malformed protocol headers, bad
192 checksums, etc. This would prevent userspace from implementing a
193 simple Ethernet switch that forwards every packet.
195 Instead, in such a case, include an attribute with "empty" content.
196 It doesn't matter if the empty content could be valid protocol values,
197 as long as those values are rarely seen in practice, because userspace
198 can always forward all packets with those values to userspace and
199 handle them individually.
201 For example, consider a packet that contains an IP header that
202 indicates protocol 6 for TCP, but which is truncated just after the IP
203 header, so that the TCP header is missing. The flow key for this
204 packet would include a tcp attribute with all-zero src and dst, like
207 eth(...), eth_type(0x0800), ip(proto=6, ...), tcp(src=0, dst=0)
209 As another example, consider a packet with an Ethernet type of 0x8100,
210 indicating that a VLAN TCI should follow, but which is truncated just
211 after the Ethernet type. The flow key for this packet would include
212 an all-zero-bits vlan and an empty encap attribute, like this:
214 eth(...), eth_type(0x8100), vlan(0), encap()
216 Unlike a TCP packet with source and destination ports 0, an
217 all-zero-bits VLAN TCI is not that rare, so the CFI bit (aka
218 VLAN_TAG_PRESENT inside the kernel) is ordinarily set in a vlan
219 attribute expressly to allow this situation to be distinguished.
220 Thus, the flow key in this second example unambiguously indicates a
221 missing or malformed VLAN TCI.
226 The other rules for flow keys are much less subtle:
228 - Duplicate attributes are not allowed at a given nesting level.
230 - Ordering of attributes is not significant.
232 - When the kernel sends a given flow key to userspace, it always
233 composes it the same way. This allows userspace to hash and
234 compare entire flow keys that it may not be able to fully