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author | Ben Pfaff <blp@nicira.com> | 2015-01-09 08:28:42 -0800 |
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committer | Ben Pfaff <blp@nicira.com> | 2015-01-11 10:00:59 -0800 |
commit | ae99ee4554db1ede9aadef5ee8d3e7c8e7303feb (patch) | |
tree | 7800ad06e57257511d357038efde1b09b65d28ed | |
parent | d06c1ff8ea022f616fbb47b756a29964be037a97 (diff) |
FAQ.md: Describe OpenFlow packet buffering.
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>
-rw-r--r-- | FAQ.md | 41 |
1 files changed, 41 insertions, 0 deletions
@@ -1622,6 +1622,47 @@ A: Reconfiguring your bridge can change your bridge's datapath-id because ovs-vsctl set bridge br0 other-config:datapath-id=0123456789abcdef +### Q: My controller is getting errors about "buffers". What's going on? + +A: When a switch sends a packet to an OpenFlow controller using a + "packet-in" message, it can also keep a copy of that packet in a + "buffer", identified by a 32-bit integer "buffer_id". There are + two advantages to buffering. First, when the controller wants to + tell the switch to do something with the buffered packet (with a + "packet-out" OpenFlow request), it does not need to send another + copy of the packet back across the OpenFlow connection, which + reduces the bandwidth cost of the connection and improves latency. + This enables the second advantage: the switch can optionally send + only the first part of the packet to the controller (assuming that + the switch only needs to look at the first few bytes of the + packet), further reducing bandwidth and improving latency. + + However, buffering introduces some issues of its own. First, any + switch has limited resources, so if the controller does not use a + buffered packet, the switch has to decide how long to keep it + buffered. When many packets are sent to a controller and buffered, + Open vSwitch can discard buffered packets that the controller has + not used after as little as 5 seconds. This means that + controllers, if they make use of packet buffering, should use the + buffered packets promptly. (This includes sending a "packet-out" + with no actions if the controller does not want to do anything with + a buffered packet, to clear the packet buffer and effectively + "drop" its packet.) + + Second, packet buffers are one-time-use, meaning that a controller + cannot use a single packet buffer in two or more "packet-out" + commands. Open vSwitch will respond with an error to the second + and subsequent "packet-out"s in such a case. + + Finally, a common error early in controller development is to try + to use buffer_id 0 in a "packet-out" message as if 0 represented + "no buffered packet". This is incorrect usage: the buffer_id with + this meaning is actually 0xffffffff. + + ovs-vswitchd(8) describes some details of Open vSwitch packet + buffering that the OpenFlow specification requires implementations + to document. + Development ----------- |