1Open vSwitch datapath developer documentation 2============================================= 3 4The Open vSwitch kernel module allows flexible userspace control over 5flow-level packet processing on selected network devices. It can be 6used to implement a plain Ethernet switch, network device bonding, 7VLAN processing, network access control, flow-based network control, 8and so on. 9 10The kernel module implements multiple "datapaths" (analogous to 11bridges), each of which can have multiple "vports" (analogous to ports 12within a bridge). Each datapath also has associated with it a "flow 13table" that userspace populates with "flows" that map from keys based 14on packet headers and metadata to sets of actions. The most common 15action forwards the packet to another vport; other actions are also 16implemented. 17 18When a packet arrives on a vport, the kernel module processes it by 19extracting its flow key and looking it up in the flow table. If there 20is a matching flow, it executes the associated actions. If there is 21no match, it queues the packet to userspace for processing (as part of 22its processing, userspace will likely set up a flow to handle further 23packets of the same type entirely in-kernel). 24 25 26Flow key compatibility 27---------------------- 28 29Network protocols evolve over time. New protocols become important 30and existing protocols lose their prominence. For the Open vSwitch 31kernel module to remain relevant, it must be possible for newer 32versions to parse additional protocols as part of the flow key. It 33might even be desirable, someday, to drop support for parsing 34protocols that have become obsolete. Therefore, the Netlink interface 35to Open vSwitch is designed to allow carefully written userspace 36applications to work with any version of the flow key, past or future. 37 38To support this forward and backward compatibility, whenever the 39kernel module passes a packet to userspace, it also passes along the 40flow key that it parsed from the packet. Userspace then extracts its 41own notion of a flow key from the packet and compares it against the 42kernel-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 64How flow keys evolve over time is important to making this work, so 65the following sections go into detail. 66 67 68Flow key format 69--------------- 70 71A flow key is passed over a Netlink socket as a sequence of Netlink 72attributes. Some attributes represent packet metadata, defined as any 73information about a packet that cannot be extracted from the packet 74itself, e.g. the vport on which the packet was received. Most 75attributes, however, are extracted from headers within the packet, 76e.g. source and destination addresses from Ethernet, IP, or TCP 77headers. 78 79The <linux/openvswitch.h> header file defines the exact format of the 80flow key attributes. For informal explanatory purposes here, we write 81them as comma-separated strings, with parentheses indicating arguments 82and nesting. For example, the following could represent a flow key 83corresponding 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 89Often we ellipsize arguments not important to the discussion, e.g.: 90 91 in_port(1), eth(...), eth_type(0x0800), ipv4(...), tcp(...) 92 93 94Basic rule for evolving flow keys 95--------------------------------- 96 97Some care is needed to really maintain forward and backward 98compatibility for applications that follow the rules listed under 99"Flow key compatibility" above. 100 101The basic rule is obvious: 102 103 ------------------------------------------------------------------ 104 New network protocol support must only supplement existing flow 105 key attributes. It must not change the meaning of already defined 106 flow key attributes. 107 ------------------------------------------------------------------ 108 109This rule does have less-obvious consequences so it is worth working 110through a few examples. Suppose, for example, that the kernel module 111did not already implement VLAN parsing. Instead, it just interpreted 112the 802.1Q TPID (0x8100) as the Ethertype then stopped parsing the 113packet. The flow key for any packet with an 802.1Q header would look 114essentially like this, ignoring metadata: 115 116 eth(...), eth_type(0x8100) 117 118Naively, to add VLAN support, it makes sense to add a new "vlan" flow 119key attribute to contain the VLAN tag, then continue to decode the 120encapsulated headers beyond the VLAN tag using the existing field 121definitions. With this change, an TCP packet in VLAN 10 would have a 122flow key much like this: 123 124 eth(...), vlan(vid=10, pcp=0), eth_type(0x0800), ip(proto=6, ...), tcp(...) 125 126But this change would negatively affect a userspace application that 127has not been updated to understand the new "vlan" flow key attribute. 128The application could, following the flow compatibility rules above, 129ignore the "vlan" attribute that it does not understand and therefore 130assume that the flow contained IP packets. This is a bad assumption 131(the flow only contains IP packets if one parses and skips over the 132802.1Q header) and it could cause the application's behavior to change 133across kernel versions even though it follows the compatibility rules. 134 135The solution is to use a set of nested attributes. This is, for 136example, why 802.1Q support uses nested attributes. A TCP packet in 137VLAN 10 is actually expressed as: 138 139 eth(...), eth_type(0x8100), vlan(vid=10, pcp=0), encap(eth_type(0x0800), 140 ip(proto=6, ...), tcp(...))) 141 142Notice how the "eth_type", "ip", and "tcp" flow key attributes are 143nested inside the "encap" attribute. Thus, an application that does 144not understand the "vlan" key will not see either of those attributes 145and therefore will not misinterpret them. (Also, the outer eth_type 146is still 0x8100, not changed to 0x0800.) 147 148Handling malformed packets 149-------------------------- 150 151Don't drop packets in the kernel for malformed protocol headers, bad 152checksums, etc. This would prevent userspace from implementing a 153simple Ethernet switch that forwards every packet. 154 155Instead, in such a case, include an attribute with "empty" content. 156It doesn't matter if the empty content could be valid protocol values, 157as long as those values are rarely seen in practice, because userspace 158can always forward all packets with those values to userspace and 159handle them individually. 160 161For example, consider a packet that contains an IP header that 162indicates protocol 6 for TCP, but which is truncated just after the IP 163header, so that the TCP header is missing. The flow key for this 164packet would include a tcp attribute with all-zero src and dst, like 165this: 166 167 eth(...), eth_type(0x0800), ip(proto=6, ...), tcp(src=0, dst=0) 168 169As another example, consider a packet with an Ethernet type of 0x8100, 170indicating that a VLAN TCI should follow, but which is truncated just 171after the Ethernet type. The flow key for this packet would include 172an all-zero-bits vlan and an empty encap attribute, like this: 173 174 eth(...), eth_type(0x8100), vlan(0), encap() 175 176Unlike a TCP packet with source and destination ports 0, an 177all-zero-bits VLAN TCI is not that rare, so the CFI bit (aka 178VLAN_TAG_PRESENT inside the kernel) is ordinarily set in a vlan 179attribute expressly to allow this situation to be distinguished. 180Thus, the flow key in this second example unambiguously indicates a 181missing or malformed VLAN TCI. 182 183Other rules 184----------- 185 186The other rules for flow keys are much less subtle: 187 188 - Duplicate attributes are not allowed at a given nesting level. 189 190 - Ordering of attributes is not significant. 191 192 - When the kernel sends a given flow key to userspace, it always 193 composes it the same way. This allows userspace to hash and 194 compare entire flow keys that it may not be able to fully 195 interpret. 196