path: root/DESIGN

                     Design Decisions In Open vSwitch
                     ================================

This document describes design decisions that went into implementing
Open vSwitch.  While we believe these to be reasonable decisions, it is
impossible to predict how Open vSwitch will be used in all environments.
Understanding assumptions made by Open vSwitch is critical to a
successful deployment.  The end of this document contains contact
information that can be used to let us know how we can make Open vSwitch
more generally useful.

Asynchronous Messages
=====================

Over time, Open vSwitch has added many knobs that control whether a
given controller receives OpenFlow asynchronous messages.  This
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.

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
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
connection is made.  An entry labeled "yes" means that the message is
sent, an entry labeled "---" means that the message is suppressed.

                                             master/
  message and reason code                     other     slave
  ----------------------------------------   -------    -----
  OFPT_PACKET_IN / NXT_PACKET_IN
    OFPR_NO_MATCH                              yes       ---
    OFPR_ACTION                                yes       ---
    OFPR_INVALID_TTL                           ---       ---

  OFPT_FLOW_REMOVED / NXT_FLOW_REMOVED
    OFPRR_IDLE_TIMEOUT                         yes       ---
    OFPRR_HARD_TIMEOUT                         yes       ---
    OFPRR_DELETE                               yes       ---

  OFPT_PORT_STATUS
    OFPPR_ADD                                  yes       yes
    OFPPR_DELETE                               yes       yes
    OFPPR_MODIFY                               yes       yes

The NXT_SET_ASYNC_CONFIG message directly sets all of the values in
this table for the current connection.  The
OFPC_INVALID_TTL_TO_CONTROLLER bit in the OFPT_SET_CONFIG message
controls the setting for OFPR_INVALID_TTL for the "master" role.


OFPAT_ENQUEUE
=============

The OpenFlow 1.0 specification requires the output port of the OFPAT_ENQUEUE
action to "refer to a valid physical port (i.e. < OFPP_MAX) or OFPP_IN_PORT".
Although OFPP_LOCAL is not less than OFPP_MAX, it is an 'internal' port which
can have QoS applied to it in Linux.  Since we allow the OFPAT_ENQUEUE to apply
to 'internal' ports whose port numbers are less than OFPP_MAX, we interpret
OFPP_LOCAL as a physical port and support OFPAT_ENQUEUE on it as well.


OFPT_FLOW_MOD
=============

The OpenFlow 1.0 specification for the behavior of OFPT_FLOW_MOD is
confusing.  The following table summarizes the Open vSwitch
implementation of its behavior in the following categories:

    - "match on priority": Whether the flow_mod acts only on flows
      whose priority matches that included in the flow_mod message.

    - "match on out_port": Whether the flow_mod acts only on flows
      that output to the out_port included in the flow_mod message (if
      out_port is not OFPP_NONE).

    - "updates flow_cookie": Whether the flow_mod changes the
      flow_cookie of the flow or flows that it matches to the
      flow_cookie included in the flow_mod message.

    - "updates OFPFF_ flags": Whether the flow_mod changes the
      OFPFF_SEND_FLOW_REM flag of the flow or flows that it matches to
      the setting included in the flags of the flow_mod message.

    - "honors OFPFF_CHECK_OVERLAP": Whether the OFPFF_CHECK_OVERLAP
      flag in the flow_mod is significant.

    - "updates idle_timeout" and "updates hard_timeout": Whether the
      idle_timeout and hard_timeout in the flow_mod, respectively,
      have an effect on the flow or flows matched by the flow_mod.

    - "updates idle timer": Whether the flow_mod resets the per-flow
      timer that measures how long a flow has been idle.

    - "updates hard timer": Whether the flow_mod resets the per-flow
      timer that measures how long it has been since a flow was
      modified.

    - "zeros counters": Whether the flow_mod resets per-flow packet
      and byte counters to zero.

    - "sends flow_removed message": Whether the flow_mod generates a
      flow_removed message for the flow or flows that it affects.

An entry labeled "yes" means that the flow mod type does have the
indicated behavior, "---" means that it does not, an empty cell means
that the property is not applicable, and other values are explained
below the table.

                                          MODIFY          DELETE
                             ADD  MODIFY  STRICT  DELETE  STRICT
                             ===  ======  ======  ======  ======
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     +       +
updates idle_timeout         yes     +       +
updates hard_timeout         yes     +       +
resets idle timer            yes     +       +
resets hard timer            yes    yes     yes
zeros counters               yes     +       +
sends flow_removed message   ---    ---     ---      %       %

(+) "modify" and "modify-strict" only take these actions when they
    create a new flow, not when they update an existing flow.

(%) "delete" and "delete_strict" generates a flow_removed message if
    the deleted flow or flows have the OFPFF_SEND_FLOW_REM flag set.
    (Each controller can separately control whether it wants to
    receive the generated messages.)


Multiple Table Support
======================

OpenFlow 1.0 has only rudimentary support for multiple flow tables.
Notably, OpenFlow 1.0 does not allow the controller to specify the
flow table to which a flow is to be added.  Open vSwitch adds an
extension for this purpose, which is enabled on a per-OpenFlow
connection basis using the NXT_FLOW_MOD_TABLE_ID message.  When the
extension is enabled, the upper 8 bits of the 'command' member in an
OFPT_FLOW_MOD or NXT_FLOW_MOD message designates the table to which a
flow is to be added.

The Open vSwitch software switch implementation offers 255 flow
tables.  On packet ingress, only the first flow table (table 0) is
searched, and the contents of the remaining tables are not considered
in any way.  Tables other than table 0 only come into play when an
NXAST_RESUBMIT_TABLE action specifies another table to search.

Tables 128 and above are reserved for use by the switch itself.
Controllers should use only tables 0 through 127.


IPv6
====

Open vSwitch supports stateless handling of IPv6 packets.  Flows can be
written to support matching TCP, UDP, and ICMPv6 headers within an IPv6
packet.  Deeper matching of some Neighbor Discovery messages is also
supported.

IPv6 was not designed to interact well with middle-boxes.  This,
combined with Open vSwitch's stateless nature, have affected the
processing of IPv6 traffic, which is detailed below.

Extension Headers
-----------------

The base IPv6 header is incredibly simple with the intention of only
containing information relevant for routing packets between two
endpoints.  IPv6 relies heavily on the use of extension headers to
provide any other functionality.  Unfortunately, the extension headers
were designed in such a way that it is impossible to move to the next
header (including the layer-4 payload) unless the current header is
understood.

Open vSwitch will process the following extension headers and continue
to the next header:

    * Fragment (see the next section)
    * AH (Authentication Header)
    * Hop-by-Hop Options
    * Routing
    * Destination Options

When a header is encountered that is not in that list, it is considered
"terminal".  A terminal header's IPv6 protocol value is stored in
"nw_proto" for matching purposes.  If a terminal header is TCP, UDP, or
ICMPv6, the packet will be further processed in an attempt to extract
layer-4 information.

Fragments
---------

IPv6 requires that every link in the internet have an MTU of 1280 octets
or greater (RFC 2460).  As such, a terminal header (as described above in
"Extension Headers") in the first fragment should generally be
reachable.  In this case, the terminal header's IPv6 protocol type is
stored in the "nw_proto" field for matching purposes.  If a terminal
header cannot be found in the first fragment (one with a fragment offset
of zero), the "nw_proto" field is set to 0.  Subsequent fragments (those
with a non-zero fragment offset) have the "nw_proto" field set to the
IPv6 protocol type for fragments (44).

Jumbograms
----------

An IPv6 jumbogram (RFC 2675) is a packet containing a payload longer
than 65,535 octets.  A jumbogram is only relevant in subnets with a link
MTU greater than 65,575 octets, and are not required to be supported on
nodes that do not connect to link with such large MTUs.  Currently, Open
vSwitch doesn't process jumbograms.


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.


Suggestions
===========

Suggestions to improve Open vSwitch are welcome at discuss@openvswitch.org.