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.
git://git.onelab.eu / sliver-openvswitch.git / commitdiff