only if the flow that was removed had the OFPFF_SEND_FLOW_REM flag
set.
+Third, OFPT_PACKET_IN and NXT_PACKET_IN messages are sent only to
+OpenFlow controller connections that have the correct connection ID
+(see "struct nx_controller_id" and "struct nx_action_controller"):
+
+ - For packet-in messages generated by a NXAST_CONTROLLER action,
+ the controller ID specified in the action.
+
+ - For other packet-in messages, controller ID zero. (This is the
+ default ID when an OpenFlow controller does not configure one.)
+
Finally, Open vSwitch consults a per-connection table indexed by the
message type, reason code, and current role. The following table
shows how this table is initialized by default when an OpenFlow
MODIFY DELETE
ADD MODIFY STRICT DELETE STRICT
=== ====== ====== ====== ======
-match on priority --- --- yes --- yes
-match on out_port --- --- --- yes yes
+match on priority --- --- yes --- yes
+match on out_port --- --- --- yes yes
updates flow_cookie yes yes yes
updates OFPFF_SEND_FLOW_REM yes + +
honors OFPFF_CHECK_OVERLAP yes + +
In-Band Control
===============
-In-band control allows a single network to be used for OpenFlow traffic and
-other data traffic. See ovs-vswitchd.conf.db(5) for a description of
-configuring in-band control.
-
-This comment is an attempt to describe how in-band control works at a
-wire- and implementation-level. Correctly implementing in-band
-control has proven difficult due to its many subtleties, and has thus
-gone through many iterations. Please read through and understand the
-reasoning behind the chosen rules before making modifications.
-
-In Open vSwitch, in-band control is implemented as "hidden" flows (in that
-they are not visible through OpenFlow) and at a higher priority than
-wildcarded flows can be set up by through OpenFlow. This is done so that
-the OpenFlow controller cannot interfere with them and possibly break
-connectivity with its switches. It is possible to see all flows, including
-in-band ones, with the ovs-appctl "bridge/dump-flows" command.
+Motivation
+----------
+
+An OpenFlow switch must establish and maintain a TCP network
+connection to its controller. There are two basic ways to categorize
+the network that this connection traverses: either it is completely
+separate from the one that the switch is otherwise controlling, or its
+path may overlap the network that the switch controls. We call the
+former case "out-of-band control", the latter case "in-band control".
+
+Out-of-band control has the following benefits:
+
+ - Simplicity: Out-of-band control slightly simplifies the switch
+ implementation.
+
+ - Reliability: Excessive switch traffic volume cannot interfere
+ with control traffic.
+
+ - Integrity: Machines not on the control network cannot
+ impersonate a switch or a controller.
+
+ - Confidentiality: Machines not on the control network cannot
+ snoop on control traffic.
+
+In-band control, on the other hand, has the following advantages:
+
+ - No dedicated port: There is no need to dedicate a physical
+ switch port to control, which is important on switches that have
+ few ports (e.g. wireless routers, low-end embedded platforms).
+
+ - No dedicated network: There is no need to build and maintain a
+ separate control network. This is important in many
+ environments because it reduces proliferation of switches and
+ wiring.
+
+Open vSwitch supports both out-of-band and in-band control. This
+section describes the principles behind in-band control. See the
+description of the Controller table in ovs-vswitchd.conf.db(5) to
+configure OVS for in-band control.
+
+Principles
+----------
+
+The fundamental principle of in-band control is that an OpenFlow
+switch must recognize and switch control traffic without involving the
+OpenFlow controller. All the details of implementing in-band control
+are special cases of this principle.
+
+The rationale for this principle is simple. If the switch does not
+handle in-band control traffic itself, then it will be caught in a
+contradiction: it must contact the controller, but it cannot, because
+only the controller can set up the flows that are needed to contact
+the controller.
+
+The following points describe important special cases of this
+principle.
+
+ - In-band control must be implemented regardless of whether the
+ switch is connected.
+
+ It is tempting to implement the in-band control rules only when
+ the switch is not connected to the controller, using the
+ reasoning that the controller should have complete control once
+ it has established a connection with the switch.
+
+ This does not work in practice. Consider the case where the
+ switch is connected to the controller. Occasionally it can
+ happen that the controller forgets or otherwise needs to obtain
+ the MAC address of the switch. To do so, the controller sends a
+ broadcast ARP request. A switch that implements the in-band
+ control rules only when it is disconnected will then send an
+ OFPT_PACKET_IN message up to the controller. The controller will
+ be unable to respond, because it does not know the MAC address of
+ the switch. This is a deadlock situation that can only be
+ resolved by the switch noticing that its connection to the
+ controller has hung and reconnecting.
+
+ - In-band control must override flows set up by the controller.
+
+ It is reasonable to assume that flows set up by the OpenFlow
+ controller should take precedence over in-band control, on the
+ basis that the controller should be in charge of the switch.
+
+ Again, this does not work in practice. Reasonable controller
+ implementations may set up a "last resort" fallback rule that
+ wildcards every field and, e.g., sends it up to the controller or
+ discards it. If a controller does that, then it will isolate
+ itself from the switch.
+
+ - The switch must recognize all control traffic.
+
+ The fundamental principle of in-band control states, in part,
+ that a switch must recognize control traffic without involving
+ the OpenFlow controller. More specifically, the switch must
+ recognize *all* control traffic. "False negatives", that is,
+ packets that constitute control traffic but that the switch does
+ not recognize as control traffic, lead to control traffic storms.
+
+ Consider an OpenFlow switch that only recognizes control packets
+ sent to or from that switch. Now suppose that two switches of
+ this type, named A and B, are connected to ports on an Ethernet
+ hub (not a switch) and that an OpenFlow controller is connected
+ to a third hub port. In this setup, control traffic sent by
+ switch A will be seen by switch B, which will send it to the
+ controller as part of an OFPT_PACKET_IN message. Switch A will
+ then see the OFPT_PACKET_IN message's packet, re-encapsulate it
+ in another OFPT_PACKET_IN, and send it to the controller. Switch
+ B will then see that OFPT_PACKET_IN, and so on in an infinite
+ loop.
+
+ Incidentally, the consequences of "false positives", where
+ packets that are not control traffic are nevertheless recognized
+ as control traffic, are much less severe. The controller will
+ not be able to control their behavior, but the network will
+ remain in working order. False positives do constitute a
+ security problem.
+
+ - The switch should use echo-requests to detect disconnection.
+
+ TCP will notice that a connection has hung, but this can take a
+ considerable amount of time. For example, with default settings
+ the Linux kernel TCP implementation will retransmit for between
+ 13 and 30 minutes, depending on the connection's retransmission
+ timeout, according to kernel documentation. This is far too long
+ for a switch to be disconnected, so an OpenFlow switch should
+ implement its own connection timeout. OpenFlow OFPT_ECHO_REQUEST
+ messages are the best way to do this, since they test the
+ OpenFlow connection itself.
+
+Implementation
+--------------
+
+This section describes how Open vSwitch implements in-band control.
+Correctly implementing in-band control has proven difficult due to its
+many subtleties, and has thus gone through many iterations. Please
+read through and understand the reasoning behind the chosen rules
+before making modifications.
+
+Open vSwitch implements in-band control as "hidden" flows, that is,
+flows that are not visible through OpenFlow, and at a higher priority
+than wildcarded flows can be set up through OpenFlow. This is done so
+that the OpenFlow controller cannot interfere with them and possibly
+break connectivity with its switches. It is possible to see all
+flows, including in-band ones, with the ovs-appctl "bridge/dump-flows"
+command.
The Open vSwitch implementation of in-band control can hide traffic to
arbitrary "remotes", where each remote is one TCP port on one IP address.