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
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