1 .. SPDX-License-Identifier: GPL-2.0 2 3 ============================================= 4 Open vSwitch datapath developer documentation 5 ============================================= 6 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. 12 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. 20 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). 27 28 29 Flow key compatibility 30 ---------------------- 31 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. 40 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: 46 47 - If userspace's notion of the flow key for the packet matches the 48 kernel's, then nothing special is necessary. 49 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. 56 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.) 66 67 How flow keys evolve over time is important to making this work, so 68 the following sections go into detail. 69 70 71 Flow key format 72 --------------- 73 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. 81 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:: 87 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) 91 92 Often we ellipsize arguments not important to the discussion, e.g.:: 93 94 in_port(1), eth(...), eth_type(0x0800), ipv4(...), tcp(...) 95 96 97 Wildcarded flow key format 98 -------------------------- 99 100 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. 103 104 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. 109 110 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. 117 118 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. 123 124 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. 129 130 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. 135 136 137 Unique flow identifiers 138 ----------------------- 139 140 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. 143 144 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. 148 149 150 Basic rule for evolving flow keys 151 --------------------------------- 152 153 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. 156 157 The basic rule is obvious:: 158 159 ================================================================== 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 ================================================================== 164 165 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:: 171 172 eth(...), eth_type(0x8100) 173 174 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:: 179 180 eth(...), vlan(vid=10, pcp=0), eth_type(0x0800), ip(proto=6, ...), tcp(...) 181 182 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. 190 191 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:: 194 195 eth(...), eth_type(0x8100), vlan(vid=10, pcp=0), encap(eth_type(0x0800), 196 ip(proto=6, ...), tcp(...))) 197 198 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.) 203 204 Handling malformed packets 205 -------------------------- 206 207 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. 210 211 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. 216 217 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:: 222 223 eth(...), eth_type(0x0800), ip(proto=6, ...), tcp(src=0, dst=0) 224 225 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:: 229 230 eth(...), eth_type(0x8100), vlan(0), encap() 231 232 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. 238 239 Other rules 240 ----------- 241 242 The other rules for flow keys are much less subtle: 243 244 - Duplicate attributes are not allowed at a given nesting level. 245 246 - Ordering of attributes is not significant. 247 248 - 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.
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