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Diffstat (limited to 'FAQ.md')
| -rw-r--r-- | FAQ.md | 64 |
1 files changed, 15 insertions, 49 deletions
@@ -1970,55 +1970,21 @@ 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. - - Note that the packet buffering support is deprecated in OVS 2.6 - release, and will be removed in OVS 2.7. After the change OVS - always sends the 'buffer_id' as 0xffffffff in "packet-in" messages - and will send an error response if any other value of this field is - included in "packet-out" and "flow mod" sent by a controller. - Controllers are already expected to work properly in cases where - the switch can not buffer packets, so this change should not affect - existing users. +### Q: My controller complains that OVS is not buffering packets. + What's going on? + +A: "Packet buffering" is an optional OpenFlow feature, and controllers + should detect how many "buffers" an OpenFlow switch implements. It + was recently noticed that OVS implementation of the buffering + feature was not compliant to OpenFlow specifications. Rather than + fix it and risk controller incompatibility, the buffering feature + is removed as of OVS 2.7. Controllers are already expected to work + properly in cases where the switch can not buffer packets, but + sends full packets in "packet-in" messages instead, so this change + should not affect existing users. After the change OVS always + sends the 'buffer_id' as 0xffffffff in "packet-in" messages and + will send an error response if any other value of this field is + included in a "packet-out" or a "flow mod" sent by a controller. ### Q: How does OVS divide flows among buckets in an OpenFlow "select" group? |