[ovs-discuss] OVS in-band mode: working but need help understanding the reason

Dani Camps danicamps81 at gmail.com
Thu May 22 15:53:56 UTC 2014


Hi Ben,

I think the problem I am having is that the "ARP request to controller"
hidden rule is not matching. This is what happens:

1) In the switch (192.168.56.203) connected to the controller
(192.168.56.103) I see ARP Requests coming from the other switch:

# tcpdump -i s3-eth0
19:47:32.818889 ARP, Request who-has 192.168.56.103 tell 192.168.56.202,
length 28
19:47:33.816421 ARP, Request who-has 192.168.56.103 tell 192.168.56.202,
length 28
...

2) However when I dump the status of the hidden flows in the switch
connected to the controller I get this:

duration=485s, n_packets=851, n_bytes=924434,
priority=180007,tcp,nw_dst=192.168.56.103,tp_dst=6633,actions=NORMAL
duration=485s, n_packets=1, n_bytes=60,
priority=180006,arp,arp_spa=192.168.56.103,arp_op=1,actions=NORMAL
duration=485s, n_packets=0, n_bytes=0,
priority=180005,arp,arp_tpa=192.168.56.103,arp_op=2,actions=NORMAL
duration=485s, n_packets=860, n_bytes=63226,
priority=180008,tcp,nw_src=192.168.56.103,tp_src=6633,actions=NORMAL
duration=485s, n_packets=1, n_bytes=60,
priority=180001,arp,dl_dst=f6:b8:41:3c:d6:45,arp_op=2,actions=NORMAL
duration=478s, n_packets=0, n_bytes=0,
priority=180003,arp,dl_dst=08:00:27:a9:08:16,arp_op=2,actions=NORMAL
duration=485s, n_packets=0, n_bytes=0,
priority=180000,udp,in_port=LOCAL,dl_src=f6:b8:41:3c:d6:45,tp_src=68,tp_dst=67,actions=NORMAL
duration=485s, n_packets=1, n_bytes=42,
priority=180002,arp,dl_src=f6:b8:41:3c:d6:45,arp_op=1,actions=NORMAL
duration=478s, n_packets=0, n_bytes=0,
priority=180004,arp,dl_src=08:00:27:a9:08:16,arp_op=1,actions=NORMAL
table_id=254, duration=486s, n_packets=0, n_bytes=0,
priority=0,reg0=0x3,actions=drop
table_id=254, duration=486s, n_packets=320, n_bytes=30696,
priority=0,reg0=0x1,actions=controller(reason=no_match)
table_id=254, duration=486s, n_packets=0, n_bytes=0,
priority=0,reg0=0x2,actions=drop


Where specially the following rule is interesting:

duration=485s, n_packets=0, n_bytes=0,
priority=180005,arp,arp_tpa=192.168.56.103,arp_op=2,actions=NORMAL

I understand that this rule is saying that there are no matches on ARP
Request to the controller from other switches. But this is wrong because we
are seeing the ARP Request on the interface. As I said in my previous
email, eventually something changes and the rules starts to match.

Is this behavior correct?

Best Regards

Daniel
















On Thu, May 22, 2014 at 4:43 PM, Ben Pfaff <blp at nicira.com> wrote:

> The OFPP_NORMAL action does act as a normal L2 bridge.
>
> On Thu, May 22, 2014 at 12:29:12PM +0200, Dani Camps wrote:
> > Dear Ben,
> >
> > Thanks, I had already read it. What I do not understand from that
> > explanation though is how relaying happens when there is a switch in the
> > middle between another switch and the controller. For instance, the
> middle
> > switch gets an ARP Request towards the controller, and according to rule
> > (f) in the design document this packet is treated as NORMAL (OFPP_NORMAL
> > action). What I do not understand is why the normal processing of a node
> > should be to relay, or flood, an ARP Request that is not addressed to
> him?
> >
> > Flooding would be the normal behavior if you had a L2 switch, but in the
> > case of running OVS in a Linux box there is no bridging behavior as a
> > default behavior unless you create a bridge explicitly (with brctl). Or
> is
> > it the case that an ovs bridge defaults to a normal L2 bridge for packets
> > matching OFPP_NORMAL action?
> >
> > Best Regards
> >
> > Daniel
> >
> >
> > On Thu, May 22, 2014 at 12:47 AM, Ben Pfaff <blp at nicira.com> wrote:
> >
> > > On Wed, May 21, 2014 at 06:05:56PM +0200, Dani Camps wrote:
> > > > Could anyone explain how is in-band supposed to
> > > > work? Especially the part where an ARP Request to the controller
> from a
> > > > connected switch should be treated as a NORMAL packet but still be
> > > > forwarded to the controller?
> > >
> > > There's a lot of documentation in DESIGN in the OVS source tree:
> > >
> > > 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.
> > >
> > > 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.
> > > 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)
> > > pair:
> > >
> > >    (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
> > >      gateway.
> > >
>
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