[ovs-discuss] in-band openflow control messages

Ben Pfaff blp at nicira.com
Thu Nov 20 04:39:55 UTC 2014

On Wed, Nov 19, 2014 at 07:29:29PM -0600, swair shah wrote:
> On Wed, Nov 19, 2014 at 6:45 PM, Ben Pfaff <blp at nicira.com> wrote:
> > On Wed, Nov 19, 2014 at 06:46:34PM -0600, swair shah wrote:
> > > Is it possible for openvswitch to interpret a data plane message as a
> > > controller message and change forwarding rules?
> >
> > What's an example?
> I don't know of one. I don't think its implemented in ovs. If I want my
> switch to expect modify_state message even from ports other than controller
> port, what parts of the code base should I look into?

I mean, what's an example of what you want OVS to do?  I don't
understand what you're looking for.

> also what does the in-band module do?

It's heavily documented in DESIGN.md:

In-Band Control


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

  - 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

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.


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

 - 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

   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.


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"

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.
Currently the remotes are automatically configured as the in-band OpenFlow
controllers plus the OVSDB managers, if any.  (The latter is a requirement
because OVSDB managers are responsible for configuring OpenFlow controllers,
so if the manager cannot be reached then OpenFlow cannot be reconfigured.)

The following rules (with the OFPP_NORMAL action) are set up on any bridge
that has any remotes:

   (a) DHCP requests sent from the local port.
   (b) ARP replies to the local port's MAC address.
   (c) ARP requests from the local port's MAC address.

In-band also sets up the following rules for each unique next-hop MAC
address for the remotes' IPs (the "next hop" is either the remote
itself, if it is on a local subnet, or the gateway to reach the remote):

   (d) ARP replies to the next hop's MAC address.
   (e) ARP requests from the next hop's MAC address.

In-band also sets up the following rules for each unique remote IP address:

   (f) ARP replies containing the remote's IP address as a target.
   (g) ARP requests containing the remote's IP address as a source.

In-band also sets up the following rules for each unique remote (IP,port)

   (h) TCP traffic to the remote's IP and port.
   (i) TCP traffic from the remote's IP and port.

The goal of these rules is to be as narrow as possible to allow a
switch to join a network and be able to communicate with the
remotes.  As mentioned earlier, these rules have higher priority
than the controller's rules, so if they are too broad, they may
prevent the controller from implementing its policy.  As such,
in-band actively monitors some aspects of flow and packet processing
so that the rules can be made more precise.

In-band control monitors attempts to add flows into the datapath that
could interfere with its duties.  The datapath only allows exact
match entries, so in-band control is able to be very precise about
the flows it prevents.  Flows that miss in the datapath are sent to
userspace to be processed, so preventing these flows from being
cached in the "fast path" does not affect correctness.  The only type
of flow that is currently prevented is one that would prevent DHCP
replies from being seen by the local port.  For example, a rule that
forwarded all DHCP traffic to the controller would not be allowed,
but one that forwarded to all ports (including the local port) would.

As mentioned earlier, packets that miss in the datapath are sent to
the userspace for processing.  The userspace has its own flow table,
the "classifier", so in-band checks whether any special processing
is needed before the classifier is consulted.  If a packet is a DHCP
response to a request from the local port, the packet is forwarded to
the local port, regardless of the flow table.  Note that this requires
L7 processing of DHCP replies to determine whether the 'chaddr' field
matches the MAC address of the local port.

It is interesting to note that for an L3-based in-band control
mechanism, the majority of rules are devoted to ARP traffic.  At first
glance, some of these rules appear redundant.  However, each serves an
important role.  First, in order to determine the MAC address of the
remote side (controller or gateway) for other ARP rules, we must allow
ARP traffic for our local port with rules (b) and (c).  If we are
between a switch and its connection to the remote, we have to
allow the other switch's ARP traffic to through.  This is done with
rules (d) and (e), since we do not know the addresses of the other
switches a priori, but do know the remote's or gateway's.  Finally,
if the remote is running in a local guest VM that is not reached
through the local port, the switch that is connected to the VM must
allow ARP traffic based on the remote's IP address, since it will
not know the MAC address of the local port that is sending the traffic
or the MAC address of the remote in the guest VM.

With a few notable exceptions below, in-band should work in most
network setups.  The following are considered "supported' in the
current implementation:

 - Locally Connected.  The switch and remote are on the same
   subnet.  This uses rules (a), (b), (c), (h), and (i).

 - Reached through Gateway.  The switch and remote are on
   different subnets and must go through a gateway.  This uses
   rules (a), (b), (c), (h), and (i).

 - Between Switch and Remote.  This switch is between another
   switch and the remote, and we want to allow the other
   switch's traffic through.  This uses rules (d), (e), (h), and
   (i).  It uses (b) and (c) indirectly in order to know the MAC
   address for rules (d) and (e).  Note that DHCP for the other
   switch will not work unless an OpenFlow controller explicitly lets this
   switch pass the traffic.

 - Between Switch and Gateway.  This switch is between another
   switch and the gateway, and we want to allow the other switch's
   traffic through.  This uses the same rules and logic as the
   "Between Switch and Remote" configuration described earlier.

 - Remote on Local VM.  The remote is a guest VM on the
   system running in-band control.  This uses rules (a), (b), (c),
   (h), and (i).

 - Remote on Local VM with Different Networks.  The remote
   is a guest VM on the system running in-band control, but the
   local port is not used to connect to the remote.  For
   example, an IP address is configured on eth0 of the switch.  The
   remote's VM is connected through eth1 of the switch, but an
   IP address has not been configured for that port on the switch.
   As such, the switch will use eth0 to connect to the remote,
   and eth1's rules about the local port will not work.  In the
   example, the switch attached to eth0 would use rules (a), (b),
   (c), (h), and (i) on eth0.  The switch attached to eth1 would use
   rules (f), (g), (h), and (i).

The following are explicitly *not* supported by in-band control:

 - Specify Remote by Name.  Currently, the remote must be
   identified by IP address.  A naive approach would be to permit
   all DNS traffic.  Unfortunately, this would prevent the
   controller from defining any policy over DNS.  Since switches
   that are located behind us need to connect to the remote,
   in-band cannot simply add a rule that allows DNS traffic from
   the local port.  The "correct" way to support this is to parse
   DNS requests to allow all traffic related to a request for the
   remote's name through.  Due to the potential security
   problems and amount of processing, we decided to hold off for
   the time-being.

 - Differing Remotes for Switches.  All switches must know
   the L3 addresses for all the remotes that other switches
   may use, since rules need to be set up to allow traffic related
   to those remotes through.  See rules (f), (g), (h), and (i).

 - Differing Routes for Switches.  In order for the switch to
   allow other switches to connect to a remote through a
   gateway, it allows the gateway's traffic through with rules (d)
   and (e).  If the routes to the remote differ for the two
   switches, we will not know the MAC address of the alternate

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