section describes how all of these features interact.
First, a service controller never receives any asynchronous messages
-unless it explicitly configures a miss_send_len greater than zero with
-an OFPT_SET_CONFIG message.
+unless it changes its miss_send_len from the service controller
+default of zero in one of the following ways:
+
+ - Sending an OFPT_SET_CONFIG message with nonzero miss_send_len.
+
+ - Sending any NXT_SET_ASYNC_CONFIG message: as a side effect, this
+ message changes the miss_send_len to
+ OFP_DEFAULT_MISS_SEND_LEN (128) for service controllers.
Second, OFPT_FLOW_REMOVED and NXT_FLOW_REMOVED messages are generated
only if the flow that was removed had the OFPFF_SEND_FLOW_REM flag
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 + +
receive the generated messages.)
+Flow Cookies
+============
+
+OpenFlow 1.0 and later versions have the concept of a "flow cookie",
+which is a 64-bit integer value attached to each flow. The treatment
+of the flow cookie has varied greatly across OpenFlow versions,
+however.
+
+In OpenFlow 1.0:
+
+ - OFPFC_ADD set the cookie in the flow that it added.
+
+ - OFPFC_MODIFY and OFPFC_MODIFY_STRICT updated the cookie for
+ the flow or flows that it modified.
+
+ - OFPST_FLOW messages included the flow cookie.
+
+ - OFPT_FLOW_REMOVED messages reported the cookie of the flow
+ that was removed.
+
+OpenFlow 1.1 made the following changes:
+
+ - Flow mod operations OFPFC_MODIFY, OFPFC_MODIFY_STRICT,
+ OFPFC_DELETE, and OFPFC_DELETE_STRICT, plus flow stats
+ requests and aggregate stats requests, gained the ability to
+ match on flow cookies with an arbitrary mask.
+
+ - OFPFC_MODIFY and OFPFC_MODIFY_STRICT were changed to add a
+ new flow, in the case of no match, only if the flow table
+ modification operation did not match on the cookie field.
+ (In OpenFlow 1.0, modify operations always added a new flow
+ when there was no match.)
+
+ - OFPFC_MODIFY and OFPFC_MODIFY_STRICT no longer updated flow
+ cookies.
+
+OpenFlow 1.2 made the following changes:
+
+ - OFPC_MODIFY and OFPFC_MODIFY_STRICT were changed to never
+ add a new flow, regardless of whether the flow cookie was
+ used for matching.
+
+Open vSwitch support for OpenFlow 1.0 implements the OpenFlow 1.0
+behavior with the following extensions:
+
+ - An NXM extension field NXM_NX_COOKIE(_W) allows the NXM
+ versions of OFPFC_MODIFY, OFPFC_MODIFY_STRICT, OFPFC_DELETE,
+ and OFPFC_DELETE_STRICT flow_mods, plus flow stats requests
+ and aggregate stats requests, to match on flow cookies with
+ arbitrary masks. This is much like the equivalent OpenFlow
+ 1.1 feature.
+
+ - Like OpenFlow 1.1, OFPC_MODIFY and OFPFC_MODIFY_STRICT add a
+ new flow if there is no match and the mask is zero (or not
+ given).
+
+ - The "cookie" field in OFPT_FLOW_MOD and NXT_FLOW_MOD messages
+ is used as the cookie value for OFPFC_ADD commands, as
+ described in OpenFlow 1.0. For OFPFC_MODIFY and
+ OFPFC_MODIFY_STRICT commands, the "cookie" field is used as a
+ new cookie for flows that match unless it is UINT64_MAX, in
+ which case the flow's cookie is not updated.
+
+ - NXT_PACKET_IN (the Nicira extended version of
+ OFPT_PACKET_IN) reports the cookie of the rule that
+ generated the packet, or all-1-bits if no rule generated the
+ packet. (Older versions of OVS used all-0-bits instead of
+ all-1-bits.)
+
+The following table shows the handling of different protocols when
+receiving OFPFC_MODIFY and OFPFC_MODIFY_STRICT messages. A mask of 0
+indicates either an explicit mask of zero or an implicit one by not
+specifying the NXM_NX_COOKIE(_W) field.
+
+ Match Update Add on miss Add on miss
+ cookie cookie mask!=0 mask==0
+ ====== ====== =========== ===========
+OpenFlow 1.0 no yes <always add on miss>
+OpenFlow 1.1 yes no no yes
+OpenFlow 1.2 yes no no no
+NXM yes yes* no yes
+
+* Updates the flow's cookie unless the "cookie" field is UINT64_MAX.
+
+
Multiple Table Support
======================
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.
+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.
-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-band control, on the other hand, has the following advantages:
-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.
+ - 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.
gateway.
+Action Reproduction
+===================
+
+It seems likely that many controllers, at least at startup, use the
+OpenFlow "flow statistics" request to obtain existing flows, then
+compare the flows' actions against the actions that they expect to
+find. Before version 1.8.0, Open vSwitch always returned exact,
+byte-for-byte copies of the actions that had been added to the flow
+table. The current version of Open vSwitch does not always do this in
+some exceptional cases. This section lists the exceptions that
+controller authors must keep in mind if they compare actual actions
+against desired actions in a bytewise fashion:
+
+ - Open vSwitch zeros padding bytes in action structures,
+ regardless of their values when the flows were added.
+
+Please report other discrepancies, if you notice any, so that we can
+fix or document them.
+
+
Suggestions
===========
git://git.onelab.eu / sliver-openvswitch.git / blobdiff