[ovs-dev] [PATCH] FAQ: Describe why OVS can't prepopulate the kernel flow table.
Ben Pfaff
blp at nicira.com
Tue Oct 13 16:10:35 UTC 2015
Signed-off-by: Ben Pfaff <blp at nicira.com>
---
FAQ.md | 26 ++++++++++++++++++++++++++
1 file changed, 26 insertions(+)
diff --git a/FAQ.md b/FAQ.md
index 512bd2c..4c259bf 100644
--- a/FAQ.md
+++ b/FAQ.md
@@ -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
-----------
--
2.1.3
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