1.. SPDX-License-Identifier: GPL-2.0 2 3============================================= 4Open vSwitch datapath developer documentation 5============================================= 6 7The Open vSwitch kernel module allows flexible userspace control over 8flow-level packet processing on selected network devices. It can be 9used to implement a plain Ethernet switch, network device bonding, 10VLAN processing, network access control, flow-based network control, 11and so on. 12 13The kernel module implements multiple "datapaths" (analogous to 14bridges), each of which can have multiple "vports" (analogous to ports 15within a bridge). Each datapath also has associated with it a "flow 16table" that userspace populates with "flows" that map from keys based 17on packet headers and metadata to sets of actions. The most common 18action forwards the packet to another vport; other actions are also 19implemented. 20 21When a packet arrives on a vport, the kernel module processes it by 22extracting its flow key and looking it up in the flow table. If there 23is a matching flow, it executes the associated actions. If there is 24no match, it queues the packet to userspace for processing (as part of 25its processing, userspace will likely set up a flow to handle further 26packets of the same type entirely in-kernel). 27 28 29Flow key compatibility 30---------------------- 31 32Network protocols evolve over time. New protocols become important 33and existing protocols lose their prominence. For the Open vSwitch 34kernel module to remain relevant, it must be possible for newer 35versions to parse additional protocols as part of the flow key. It 36might even be desirable, someday, to drop support for parsing 37protocols that have become obsolete. Therefore, the Netlink interface 38to Open vSwitch is designed to allow carefully written userspace 39applications to work with any version of the flow key, past or future. 40 41To support this forward and backward compatibility, whenever the 42kernel module passes a packet to userspace, it also passes along the 43flow key that it parsed from the packet. Userspace then extracts its 44own notion of a flow key from the packet and compares it against the 45kernel-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 67How flow keys evolve over time is important to making this work, so 68the following sections go into detail. 69 70 71Flow key format 72--------------- 73 74A flow key is passed over a Netlink socket as a sequence of Netlink 75attributes. Some attributes represent packet metadata, defined as any 76information about a packet that cannot be extracted from the packet 77itself, e.g. the vport on which the packet was received. Most 78attributes, however, are extracted from headers within the packet, 79e.g. source and destination addresses from Ethernet, IP, or TCP 80headers. 81 82The <linux/openvswitch.h> header file defines the exact format of the 83flow key attributes. For informal explanatory purposes here, we write 84them as comma-separated strings, with parentheses indicating arguments 85and nesting. For example, the following could represent a flow key 86corresponding 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 92Often we ellipsize arguments not important to the discussion, e.g.:: 93 94 in_port(1), eth(...), eth_type(0x0800), ipv4(...), tcp(...) 95 96 97Wildcarded flow key format 98-------------------------- 99 100A wildcarded flow is described with two sequences of Netlink attributes 101passed over the Netlink socket. A flow key, exactly as described above, and an 102optional corresponding flow mask. 103 104A wildcarded flow can represent a group of exact match flows. Each '1' bit 105in the mask specifies a exact match with the corresponding bit in the flow key. 106A '0' bit specifies a don't care bit, which will match either a '1' or '0' bit 107of a incoming packet. Using wildcarded flow can improve the flow set up rate 108by reduce the number of new flows need to be processed by the user space program. 109 110Support for the mask Netlink attribute is optional for both the kernel and user 111space program. The kernel can ignore the mask attribute, installing an exact 112match flow, or reduce the number of don't care bits in the kernel to less than 113what was specified by the user space program. In this case, variations in bits 114that the kernel does not implement will simply result in additional flow setups. 115The kernel module will also work with user space programs that neither support 116nor supply flow mask attributes. 117 118Since the kernel may ignore or modify wildcard bits, it can be difficult for 119the userspace program to know exactly what matches are installed. There are 120two possible approaches: reactively install flows as they miss the kernel 121flow table (and therefore not attempt to determine wildcard changes at all) 122or use the kernel's response messages to determine the installed wildcards. 123 124When interacting with userspace, the kernel should maintain the match portion 125of the key exactly as originally installed. This will provides a handle to 126identify the flow for all future operations. However, when reporting the 127mask of an installed flow, the mask should include any restrictions imposed 128by the kernel. 129 130The behavior when using overlapping wildcarded flows is undefined. It is the 131responsibility of the user space program to ensure that any incoming packet 132can match at most one flow, wildcarded or not. The current implementation 133performs best-effort detection of overlapping wildcarded flows and may reject 134some but not all of them. However, this behavior may change in future versions. 135 136 137Unique flow identifiers 138----------------------- 139 140An alternative to using the original match portion of a key as the handle for 141flow identification is a unique flow identifier, or "UFID". UFIDs are optional 142for both the kernel and user space program. 143 144User space programs that support UFID are expected to provide it during flow 145setup in addition to the flow, then refer to the flow using the UFID for all 146future operations. The kernel is not required to index flows by the original 147flow key if a UFID is specified. 148 149 150Basic rule for evolving flow keys 151--------------------------------- 152 153Some care is needed to really maintain forward and backward 154compatibility for applications that follow the rules listed under 155"Flow key compatibility" above. 156 157The 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 165This rule does have less-obvious consequences so it is worth working 166through a few examples. Suppose, for example, that the kernel module 167did not already implement VLAN parsing. Instead, it just interpreted 168the 802.1Q TPID (0x8100) as the Ethertype then stopped parsing the 169packet. The flow key for any packet with an 802.1Q header would look 170essentially like this, ignoring metadata:: 171 172 eth(...), eth_type(0x8100) 173 174Naively, to add VLAN support, it makes sense to add a new "vlan" flow 175key attribute to contain the VLAN tag, then continue to decode the 176encapsulated headers beyond the VLAN tag using the existing field 177definitions. With this change, a TCP packet in VLAN 10 would have a 178flow key much like this:: 179 180 eth(...), vlan(vid=10, pcp=0), eth_type(0x0800), ip(proto=6, ...), tcp(...) 181 182But this change would negatively affect a userspace application that 183has not been updated to understand the new "vlan" flow key attribute. 184The application could, following the flow compatibility rules above, 185ignore the "vlan" attribute that it does not understand and therefore 186assume 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 188802.1Q header) and it could cause the application's behavior to change 189across kernel versions even though it follows the compatibility rules. 190 191The solution is to use a set of nested attributes. This is, for 192example, why 802.1Q support uses nested attributes. A TCP packet in 193VLAN 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 198Notice how the "eth_type", "ip", and "tcp" flow key attributes are 199nested inside the "encap" attribute. Thus, an application that does 200not understand the "vlan" key will not see either of those attributes 201and therefore will not misinterpret them. (Also, the outer eth_type 202is still 0x8100, not changed to 0x0800.) 203 204Handling malformed packets 205-------------------------- 206 207Don't drop packets in the kernel for malformed protocol headers, bad 208checksums, etc. This would prevent userspace from implementing a 209simple Ethernet switch that forwards every packet. 210 211Instead, in such a case, include an attribute with "empty" content. 212It doesn't matter if the empty content could be valid protocol values, 213as long as those values are rarely seen in practice, because userspace 214can always forward all packets with those values to userspace and 215handle them individually. 216 217For example, consider a packet that contains an IP header that 218indicates protocol 6 for TCP, but which is truncated just after the IP 219header, so that the TCP header is missing. The flow key for this 220packet would include a tcp attribute with all-zero src and dst, like 221this:: 222 223 eth(...), eth_type(0x0800), ip(proto=6, ...), tcp(src=0, dst=0) 224 225As another example, consider a packet with an Ethernet type of 0x8100, 226indicating that a VLAN TCI should follow, but which is truncated just 227after the Ethernet type. The flow key for this packet would include 228an all-zero-bits vlan and an empty encap attribute, like this:: 229 230 eth(...), eth_type(0x8100), vlan(0), encap() 231 232Unlike a TCP packet with source and destination ports 0, an 233all-zero-bits VLAN TCI is not that rare, so the CFI bit (aka 234VLAN_TAG_PRESENT inside the kernel) is ordinarily set in a vlan 235attribute expressly to allow this situation to be distinguished. 236Thus, the flow key in this second example unambiguously indicates a 237missing or malformed VLAN TCI. 238 239Other rules 240----------- 241 242The 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. 252