Based on kernel version 3.19. Page generated on 2015-02-13 21:22 EST.
1 Open vSwitch datapath developer documentation 2 ============================================= 3 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, 8 and so on. 9 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 16 implemented. 17 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). 24 25 26 Flow key compatibility 27 ---------------------- 28 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. 37 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: 43 44 - If userspace's notion of the flow key for the packet matches the 45 kernel's, then nothing special is necessary. 46 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. 53 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.) 63 64 How flow keys evolve over time is important to making this work, so 65 the following sections go into detail. 66 67 68 Flow key format 69 --------------- 70 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 77 headers. 78 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: 84 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) 88 89 Often we ellipsize arguments not important to the discussion, e.g.: 90 91 in_port(1), eth(...), eth_type(0x0800), ipv4(...), tcp(...) 92 93 94 Wildcarded flow key format 95 -------------------------- 96 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. 100 101 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. 106 107 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. 114 115 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. 120 121 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 125 by the kernel. 126 127 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. 132 133 134 Basic rule for evolving flow keys 135 --------------------------------- 136 137 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. 140 141 The basic rule is obvious: 142 143 ------------------------------------------------------------------ 144 New network protocol support must only supplement existing flow 145 key attributes. It must not change the meaning of already defined 146 flow key attributes. 147 ------------------------------------------------------------------ 148 149 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: 155 156 eth(...), eth_type(0x8100) 157 158 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: 163 164 eth(...), vlan(vid=10, pcp=0), eth_type(0x0800), ip(proto=6, ...), tcp(...) 165 166 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. 174 175 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: 178 179 eth(...), eth_type(0x8100), vlan(vid=10, pcp=0), encap(eth_type(0x0800), 180 ip(proto=6, ...), tcp(...))) 181 182 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.) 187 188 Handling malformed packets 189 -------------------------- 190 191 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. 194 195 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. 200 201 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 205 this: 206 207 eth(...), eth_type(0x0800), ip(proto=6, ...), tcp(src=0, dst=0) 208 209 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: 213 214 eth(...), eth_type(0x8100), vlan(0), encap() 215 216 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. 222 223 Other rules 224 ----------- 225 226 The other rules for flow keys are much less subtle: 227 228 - Duplicate attributes are not allowed at a given nesting level. 229 230 - Ordering of attributes is not significant. 231 232 - 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 235 interpret.