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@@ -564,6 +564,32 @@ A: Open vSwitch has two kinds of flows (see the previous question), so A: Open vSwitch maintains snooping tables for each VLAN. +### Q: Can OVS populate the kernel flow table in advance instead of in reaction to packets? + +A: No. There are several reasons: + + - Kernel flows are not as sophisticated as OpenFlow flows, which + means that some OpenFlow policies could require a large number of + kernel flows. The "conjunctive match" feature is an extreme + example: the number of kernel flows it requires is the product of + the number of flows in each dimension. + + - With multiple OpenFlow flow tables and simple sets of actions, the + number of kernel flows required can be as large as the product of + the number of flows in each dimension. With more sophisticated + actions, the number of kernel flows could be even larger. + + - Open vSwitch is designed so that any version of OVS userspace + interoperates with any version of the OVS kernel module. This + forward and backward compatibility requires that userspace observe + how the kernel module parses received packets. This is only + possible in a straightforward way when userspace adds kernel flows + in reaction to received packets. + + For more relevant information on the architecture of Open vSwitch, + please read "The Design and Implementation of Open vSwitch", + published in USENIX NSDI 2015. + Performance ----------- |