Acked-by: Jarno Rajahalme <jrajaha...@nicira.com> On Jan 9, 2015, at 8:28 AM, Ben Pfaff <b...@nicira.com> wrote:
> Signed-off-by: Ben Pfaff <b...@nicira.com> > --- > FAQ.md | 41 +++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 41 insertions(+) > > diff --git a/FAQ.md b/FAQ.md > index 458e07a..9e95d07 100644 > --- a/FAQ.md > +++ b/FAQ.md > @@ -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 > ----------- > -- > 2.1.3 > > _______________________________________________ > dev mailing list > dev@openvswitch.org > http://openvswitch.org/mailman/listinfo/dev _______________________________________________ dev mailing list dev@openvswitch.org http://openvswitch.org/mailman/listinfo/dev
