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1 .. SPDX-License-Identifier: GPL-2.0
3 =============================================
4 Open vSwitch datapath developer documentation
5 =============================================
7 The Open vSwitch kernel module allows flexible userspace control over
8 flow-level packet processing on selected network devices. It can be
9 used to implement a plain Ethernet switch, network device bonding,
10 VLAN processing, network access control, flow-based network control,
11 and so on.
13 The kernel module implements multiple "datapaths" (analogous to
14 bridges), each of which can have multiple "vports" (analogous to ports
15 within a bridge). Each datapath also has associated with it a "flow
16 table" that userspace populates with "flows" that map from keys based
17 on packet headers and metadata to sets of actions. The most common
18 action forwards the packet to another vport; other actions are also
19 implemented.
21 When a packet arrives on a vport, the kernel module processes it by
22 extracting its flow key and looking it up in the flow table. If there
23 is a matching flow, it executes the associated actions. If there is
24 no match, it queues the packet to userspace for processing (as part of
25 its processing, userspace will likely set up a flow to handle further
26 packets of the same type entirely in-kernel).
29 Flow key compatibility
30 ----------------------
32 Network protocols evolve over time. New protocols become important
33 and existing protocols lose their prominence. For the Open vSwitch
34 kernel module to remain relevant, it must be possible for newer
35 versions to parse additional protocols as part of the flow key. It
36 might even be desirable, someday, to drop support for parsing
37 protocols that have become obsolete. Therefore, the Netlink interface
38 to Open vSwitch is designed to allow carefully written userspace
39 applications to work with any version of the flow key, past or future.
41 To support this forward and backward compatibility, whenever the
42 kernel module passes a packet to userspace, it also passes along the
43 flow key that it parsed from the packet. Userspace then extracts its
44 own notion of a flow key from the packet and compares it against the
45 kernel-provided version:
47 - If userspace's notion of the flow key for the packet matches the
48 kernel's, then nothing special is necessary.
50 - If the kernel's flow key includes more fields than the userspace
51 version of the flow key, for example if the kernel decoded IPv6
52 headers but userspace stopped at the Ethernet type (because it
53 does not understand IPv6), then again nothing special is
54 necessary. Userspace can still set up a flow in the usual way,
55 as long as it uses the kernel-provided flow key to do it.
57 - If the userspace flow key includes more fields than the
58 kernel's, for example if userspace decoded an IPv6 header but
59 the kernel stopped at the Ethernet type, then userspace can
60 forward the packet manually, without setting up a flow in the
61 kernel. This case is bad for performance because every packet
62 that the kernel considers part of the flow must go to userspace,
63 but the forwarding behavior is correct. (If userspace can
64 determine that the values of the extra fields would not affect
65 forwarding behavior, then it could set up a flow anyway.)
67 How flow keys evolve over time is important to making this work, so
68 the following sections go into detail.
71 Flow key format
72 ---------------
74 A flow key is passed over a Netlink socket as a sequence of Netlink
75 attributes. Some attributes represent packet metadata, defined as any
76 information about a packet that cannot be extracted from the packet
77 itself, e.g. the vport on which the packet was received. Most
78 attributes, however, are extracted from headers within the packet,
79 e.g. source and destination addresses from Ethernet, IP, or TCP
80 headers.
82 The <linux/openvswitch.h> header file defines the exact format of the
83 flow key attributes. For informal explanatory purposes here, we write
84 them as comma-separated strings, with parentheses indicating arguments
85 and nesting. For example, the following could represent a flow key
86 corresponding to a TCP packet that arrived on vport 1::
88 in_port(1), eth(src=e0:91:f5:21:d0:b2, dst=00:02:e3:0f:80:a4),
89 eth_type(0x0800), ipv4(src=172.16.0.20, dst=172.18.0.52, proto=17, tos=0,
90 frag=no), tcp(src=49163, dst=80)
92 Often we ellipsize arguments not important to the discussion, e.g.::
94 in_port(1), eth(...), eth_type(0x0800), ipv4(...), tcp(...)
97 Wildcarded flow key format
98 --------------------------
100 A wildcarded flow is described with two sequences of Netlink attributes
101 passed over the Netlink socket. A flow key, exactly as described above, and an
102 optional corresponding flow mask.
104 A wildcarded flow can represent a group of exact match flows. Each '1' bit
105 in the mask specifies a exact match with the corresponding bit in the flow key.
106 A '0' bit specifies a don't care bit, which will match either a '1' or '0' bit
107 of a incoming packet. Using wildcarded flow can improve the flow set up rate
108 by reduce the number of new flows need to be processed by the user space program.
110 Support for the mask Netlink attribute is optional for both the kernel and user
111 space program. The kernel can ignore the mask attribute, installing an exact
112 match flow, or reduce the number of don't care bits in the kernel to less than
113 what was specified by the user space program. In this case, variations in bits
114 that the kernel does not implement will simply result in additional flow setups.
115 The kernel module will also work with user space programs that neither support
116 nor supply flow mask attributes.
118 Since the kernel may ignore or modify wildcard bits, it can be difficult for
119 the userspace program to know exactly what matches are installed. There are
120 two possible approaches: reactively install flows as they miss the kernel
121 flow table (and therefore not attempt to determine wildcard changes at all)
122 or use the kernel's response messages to determine the installed wildcards.
124 When interacting with userspace, the kernel should maintain the match portion
125 of the key exactly as originally installed. This will provides a handle to
126 identify the flow for all future operations. However, when reporting the
127 mask of an installed flow, the mask should include any restrictions imposed
128 by the kernel.
130 The behavior when using overlapping wildcarded flows is undefined. It is the
131 responsibility of the user space program to ensure that any incoming packet
132 can match at most one flow, wildcarded or not. The current implementation
133 performs best-effort detection of overlapping wildcarded flows and may reject
134 some but not all of them. However, this behavior may change in future versions.
137 Unique flow identifiers
138 -----------------------
140 An alternative to using the original match portion of a key as the handle for
141 flow identification is a unique flow identifier, or "UFID". UFIDs are optional
142 for both the kernel and user space program.
144 User space programs that support UFID are expected to provide it during flow
145 setup in addition to the flow, then refer to the flow using the UFID for all
146 future operations. The kernel is not required to index flows by the original
147 flow key if a UFID is specified.
150 Basic rule for evolving flow keys
151 ---------------------------------
153 Some care is needed to really maintain forward and backward
154 compatibility for applications that follow the rules listed under
155 "Flow key compatibility" above.
157 The basic rule is obvious::
159 ==================================================================
160 New network protocol support must only supplement existing flow
161 key attributes. It must not change the meaning of already defined
162 flow key attributes.
163 ==================================================================
165 This rule does have less-obvious consequences so it is worth working
166 through a few examples. Suppose, for example, that the kernel module
167 did not already implement VLAN parsing. Instead, it just interpreted
168 the 802.1Q TPID (0x8100) as the Ethertype then stopped parsing the
169 packet. The flow key for any packet with an 802.1Q header would look
170 essentially like this, ignoring metadata::
172 eth(...), eth_type(0x8100)
174 Naively, to add VLAN support, it makes sense to add a new "vlan" flow
175 key attribute to contain the VLAN tag, then continue to decode the
176 encapsulated headers beyond the VLAN tag using the existing field
177 definitions. With this change, a TCP packet in VLAN 10 would have a
178 flow key much like this::
180 eth(...), vlan(vid=10, pcp=0), eth_type(0x0800), ip(proto=6, ...), tcp(...)
182 But this change would negatively affect a userspace application that
183 has not been updated to understand the new "vlan" flow key attribute.
184 The application could, following the flow compatibility rules above,
185 ignore the "vlan" attribute that it does not understand and therefore
186 assume that the flow contained IP packets. This is a bad assumption
187 (the flow only contains IP packets if one parses and skips over the
188 802.1Q header) and it could cause the application's behavior to change
189 across kernel versions even though it follows the compatibility rules.
191 The solution is to use a set of nested attributes. This is, for
192 example, why 802.1Q support uses nested attributes. A TCP packet in
193 VLAN 10 is actually expressed as::
195 eth(...), eth_type(0x8100), vlan(vid=10, pcp=0), encap(eth_type(0x0800),
196 ip(proto=6, ...), tcp(...)))
198 Notice how the "eth_type", "ip", and "tcp" flow key attributes are
199 nested inside the "encap" attribute. Thus, an application that does
200 not understand the "vlan" key will not see either of those attributes
201 and therefore will not misinterpret them. (Also, the outer eth_type
202 is still 0x8100, not changed to 0x0800.)
204 Handling malformed packets
205 --------------------------
207 Don't drop packets in the kernel for malformed protocol headers, bad
208 checksums, etc. This would prevent userspace from implementing a
209 simple Ethernet switch that forwards every packet.
211 Instead, in such a case, include an attribute with "empty" content.
212 It doesn't matter if the empty content could be valid protocol values,
213 as long as those values are rarely seen in practice, because userspace
214 can always forward all packets with those values to userspace and
215 handle them individually.
217 For example, consider a packet that contains an IP header that
218 indicates protocol 6 for TCP, but which is truncated just after the IP
219 header, so that the TCP header is missing. The flow key for this
220 packet would include a tcp attribute with all-zero src and dst, like
221 this::
223 eth(...), eth_type(0x0800), ip(proto=6, ...), tcp(src=0, dst=0)
225 As another example, consider a packet with an Ethernet type of 0x8100,
226 indicating that a VLAN TCI should follow, but which is truncated just
227 after the Ethernet type. The flow key for this packet would include
228 an all-zero-bits vlan and an empty encap attribute, like this::
230 eth(...), eth_type(0x8100), vlan(0), encap()
232 Unlike a TCP packet with source and destination ports 0, an
233 all-zero-bits VLAN TCI is not that rare, so the CFI bit (aka
234 VLAN_TAG_PRESENT inside the kernel) is ordinarily set in a vlan
235 attribute expressly to allow this situation to be distinguished.
236 Thus, the flow key in this second example unambiguously indicates a
237 missing or malformed VLAN TCI.
239 Other rules
240 -----------
242 The other rules for flow keys are much less subtle:
244 - Duplicate attributes are not allowed at a given nesting level.
246 - Ordering of attributes is not significant.
248 - When the kernel sends a given flow key to userspace, it always
249 composes it the same way. This allows userspace to hash and
250 compare entire flow keys that it may not be able to fully
251 interpret.