path: root/DESIGN.rst

..
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================================
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
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 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.

.. table:: ``OFPT_PACKET_IN`` / ``NXT_PACKET_IN``

  =========================================== ======= =====
                                              master/
           message and reason code            other   slave
  =========================================== ======= =====
  ``OFPR_NO_MATCH``                             yes    ---
  ``OFPR_ACTION``                               yes    ---
  ``OFPR_INVALID_TTL``                          ---    ---
  ``OFPR_ACTION_SET`` (OF1.4+)                  yes    ---
  ``OFPR_GROUP`` (OF1.4+)                       yes    ---
  =========================================== ======= =====

.. table:: ``OFPT_FLOW_REMOVED`` / ``NXT_FLOW_REMOVED``

  =========================================== ======= =====
                                              master/
           message and reason code            other   slave
  =========================================== ======= =====
  ``OFPRR_IDLE_TIMEOUT``                        yes    ---
  ``OFPRR_HARD_TIMEOUT``                        yes    ---
  ``OFPRR_DELETE``                              yes    ---
  ``OFPRR_GROUP_DELETE`` (OF1.4+)               yes    ---
  ``OFPRR_METER_DELETE`` (OF1.4+)               yes    ---
  ``OFPRR_EVICTION`` (OF1.4+)                   yes    ---
  =========================================== ======= =====

.. table:: ``OFPT_PORT_STATUS``

  =========================================== ======= =====
                                              master/
           message and reason code            other   slave
  =========================================== ======= =====
  ``OFPPR_ADD``                                 yes    yes
  ``OFPPR_DELETE``                              yes    yes
  ``OFPPR_MODIFY``                              yes    yes
  =========================================== ======= =====

.. table:: ``OFPT_ROLE_REQUEST`` / ``OFPT_ROLE_REPLY`` (OF1.4+)

  =========================================== ======= =====
                                              master/
           message and reason code            other   slave
  =========================================== ======= =====
  ``OFPCRR_MASTER_REQUEST``                     ---    ---
  ``OFPCRR_CONFIG``                             ---    ---
  ``OFPCRR_EXPERIMENTER``                       ---    ---
  =========================================== ======= =====

.. table:: ``OFPT_TABLE_STATUS`` (OF1.4+)

  =========================================== ======= =====
                                              master/
           message and reason code            other   slave
  =========================================== ======= =====
  ``OFPTR_VACANCY_DOWN``                        ---    ---
  ``OFPTR_VACANCY_UP``                          ---    ---
  =========================================== ======= =====


.. table:: ``OFPT_REQUESTFORWARD`` (OF1.4+)

  =========================================== ======= =====
                                              master/
           message and reason code            other   slave
  =========================================== ======= =====
  ``OFPRFR_GROUP_MOD``                          ---    ---
  ``OFPRFR_METER_MOD``                          ---    ---
  =========================================== ======= =====

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 specification for the behavior of ``OFPT_FLOW_MOD`` is confusing.
The following tables summarize 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``).
  OpenFlow 1.1 and later have a similar feature (not listed separately here)
  for ``out_group``.

"match on flow_cookie":
  Whether the ``flow_mod`` acts only on flows whose ``flow_cookie`` matches an
  optional controller-specified value and mask.

"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.

"may add a new flow":
  Whether the ``flow_mod`` may add a new flow to the flow table.  (Obviously
  this is always true for "add" commands but in some OpenFlow versions "modify"
  and "modify-strict" can also add new flows.)

"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.

OpenFlow 1.0
~~~~~~~~~~~~

================================ === ====== ====== ====== ======
                                            MODIFY        DELETE
RULE                             ADD MODIFY STRICT DELETE STRICT
================================ === ====== ====== ====== ======
match on ``priority``            yes  ---    yes    ---    yes
match on ``out_port``            ---  ---    ---    yes    yes
match on ``flow_cookie``         ---  ---    ---    ---    ---
match on ``table_id``            ---  ---    ---    ---    ---
controller chooses ``table_id``  ---  ---    ---
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   +      +
may add a new flow               yes  yes    yes
sends ``flow_removed`` message   ---  ---    ---     %      %
================================ === ====== ====== ====== ======

where:

``+``
  "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.)

OpenFlow 1.1
~~~~~~~~~~~~

OpenFlow 1.1 makes these changes:

- The controller now must specify the ``table_id`` of the flow match searched
  and into which a flow may be inserted.  Behavior for a ``table_id`` of 255 is
  undefined.

- A ``flow_mod``, except an "add", can now match on the ``flow_cookie``.

- When a ``flow_mod`` matches on the ``flow_cookie``, "modify" and
  "modify-strict" never insert a new flow.

================================ === ====== ====== ====== ======
                                            MODIFY        DELETE
RULE                             ADD MODIFY STRICT DELETE STRICT
================================ === ====== ====== ====== ======
match on ``priority``            yes  ---    yes    ---    yes
match on ``out_port``            ---  ---    ---    yes    yes
match on ``flow_cookie``         ---  yes    yes    yes    yes
match on ``table_id``            yes  yes    yes    yes    yes
controller chooses ``table_id``  yes  yes    yes
updates ``flow_cookie``          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   +      +
may add a new flow               yes   #      #
sends ``flow_removed`` message   ---  ---    ---     %      %
================================ === ====== ====== ====== ======

where:

``+``
  "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.)

``#``
  "modify" and "modify-strict" only add a new flow if the flow_mod does not
  match on any bits of the flow cookie

OpenFlow 1.2
~~~~~~~~~~~~

OpenFlow 1.2 makes these changes:

- Only "add" commands ever add flows, "modify" and "modify-strict" never do.

- A new flag ``OFPFF_RESET_COUNTS`` now controls whether "modify" and
  "modify-strict" reset counters, whereas previously they never reset counters
  (except when they inserted a new flow).

================================ === ====== ====== ====== ======
                                            MODIFY        DELETE
RULE                             ADD MODIFY STRICT DELETE STRICT
================================ === ====== ====== ====== ======
match on ``priority``            yes  ---    yes    ---    yes
match on ``out_port``            ---  ---    ---    yes    yes
match on ``flow_cookie``         ---  yes    yes    yes    yes
match on ``table_id``            yes  yes    yes    yes    yes
controller chooses ``table_id``  yes  yes    yes
updates ``flow_cookie``          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   &      &
may add a new flow               yes  ---    ---
sends ``flow_removed`` message   ---  ---    ---     %      %
================================ === ====== ====== ====== ======

``%``
  "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.)

``&``
  "modify" and "modify-strict" reset counters if the ``OFPFF_RESET_COUNTS``
  flag is specified.

OpenFlow 1.3
~~~~~~~~~~~~

OpenFlow 1.3 makes these changes:

- Behavior for a table_id of 255 is now defined, for "delete" and
  "delete-strict" commands, as meaning to delete from all tables.  A table_id
  of 255 is now explicitly invalid for other commands.

- New flags ``OFPFF_NO_PKT_COUNTS`` and ``OFPFF_NO_BYT_COUNTS`` for "add"
  operations.

The table for 1.3 is the same as the one shown above for 1.2.

OpenFlow 1.4
~~~~~~~~~~~~

OpenFlow 1.4 makes these changes:

- Adds the "importance" field to ``flow_mods``, but it does not explicitly
  specify which kinds of ``flow_mods`` set the importance.  For consistency,
  Open vSwitch uses the same rule for importance as for ``idle_timeout`` and
  ``hard_timeout``, that is, only an "ADD" flow_mod sets the importance.  (This
  issue has been filed with the ONF as EXT-496.)

.. TODO(stephenfin) Link to EXT-496

- Eviction Mechanism to automatically delete entries of lower importance to
  make space for newer entries.

OpenFlow 1.4 Bundles
--------------------

Open vSwitch makes all flow table modifications atomically, i.e., any datapath
packet only sees flow table configurations either before or after any change
made by any ``flow_mod``.  For example, if a controller removes all flows with
a single OpenFlow ``flow_mod``, no packet sees an intermediate version of the
OpenFlow pipeline where only some of the flows have been deleted.

It should be noted that Open vSwitch caches datapath flows, and that the cached
flows are *NOT* flushed immediately when a flow table changes.  Instead, the
datapath flows are revalidated against the new flow table as soon as possible,
and usually within one second of the modification.  This design amortizes the
cost of datapath cache flushing across multiple flow table changes, and has a
significant performance effect during simultaneous heavy flow table churn and
high traffic load.  This means that different cached datapath flows may have
been computed based on a different flow table configurations, but each of the
datapath flows is guaranteed to have been computed over a coherent view of the
flow tables, as described above.

With OpenFlow 1.4 bundles this atomicity can be extended across an arbitrary
set of ``flow_mod``.  Bundles are supported for ``flow_mod`` and port_mod
messages only.  For ``flow_mod``, both ``atomic`` and ``ordered`` bundle flags
are trivially supported, as all bundled messages are executed in the order they
were added and all flow table modifications are now atomic to the datapath.
Port mods may not appear in atomic bundles, as port status modifications are
not atomic.

To support bundles, ovs-ofctl has a ``--bundle`` option that makes the
flow mod commands (``add-flow``, ``add-flows``, ``mod-flows``, ``del-flows``,
and ``replace-flows``) use an OpenFlow 1.4 bundle to operate the
modifications as a single atomic transaction.  If any of the flow mods
in a transaction fail, none of them are executed.  All flow mods in a
bundle appear to datapath lookups simultaneously.

Furthermore, ovs-ofctl ``add-flow`` and ``add-flows`` commands now accept
arbitrary flow mods as an input by allowing the flow specification to
start with an explicit ``add``, ``modify``, ``modify_strict``, ``delete``, or
``delete_strict`` keyword.  A missing keyword is treated as ``add``, so
this is fully backwards compatible.  With the new ``--bundle`` option
all the flow mods are executed as a single atomic transaction using an
OpenFlow 1.4 bundle.  Without the ``--bundle`` option the flow mods are
executed in order up to the first failing ``flow_mod``, and in case of an
error the earlier successful ``flow_mod`` calls are not rolled back.

``OFPT_PACKET_IN``
------------------

The OpenFlow 1.1 specification for ``OFPT_PACKET_IN`` is confusing.  The
definition in OF1.1 ``openflow.h`` is[*]:

::

    /* Packet received on port (datapath -> controller). */
    struct ofp_packet_in {
        struct ofp_header header;
        uint32_t buffer_id;     /* ID assigned by datapath. */
        uint32_t in_port;       /* Port on which frame was received. */
        uint32_t in_phy_port;   /* Physical Port on which frame was received. */
        uint16_t total_len;     /* Full length of frame. */
        uint8_t reason;         /* Reason packet is being sent (one of OFPR_*) */
        uint8_t table_id;       /* ID of the table that was looked up */
        uint8_t data[0];        /* Ethernet frame, halfway through 32-bit word,
                                   so the IP header is 32-bit aligned.  The
                                   amount of data is inferred from the length
                                   field in the header.  Because of padding,
                                   offsetof(struct ofp_packet_in, data) ==
                                   sizeof(struct ofp_packet_in) - 2. */
    };
    OFP_ASSERT(sizeof(struct ofp_packet_in) == 24);

The confusing part is the comment on the ``data[]`` member.  This comment is a
leftover from OF1.0 ``openflow.h``, in which the comment was correct:
``sizeof(struct ofp_packet_in)`` is 20 in OF1.0 and ``ffsetof(struct
ofp_packet_in, data)`` is 18.  When OF1.1 was written, the structure members
were changed but the comment was carelessly not updated, and the comment became
wrong: ``sizeof(struct ofp_packet_in)`` and offsetof(struct ofp_packet_in,
data) are both 24 in OF1.1.

That leaves the question of how to implement ``ofp_packet_in`` in OF1.1.  The
OpenFlow reference implementation for OF1.1 does not include any padding, that
is, the first byte of the encapsulated frame immediately follows the
``table_id`` member without a gap.  Open vSwitch therefore implements it the
same way for compatibility.

For an earlier discussion, please see the thread archived at:
https://mailman.stanford.edu/pipermail/openflow-discuss/2011-August/002604.html

[*] The quoted definition is directly from OF1.1.  Definitions used inside OVS
omit the 8-byte ``ofp_header`` members, so the sizes in this discussion are
8 bytes larger than those declared in OVS header files.

VLAN Matching
-------------

The 802.1Q VLAN header causes more trouble than any other 4 bytes in
networking.  More specifically, three versions of OpenFlow and Open vSwitch
have among them four different ways to match the contents and presence of the
VLAN header.  The following table describes how each version works.

======== ============= =============== =============== ================
 Match        NXM          OF1.0            OF1.1           OF1.2
======== ============= =============== =============== ================
 ``[1]`` ``0000/0000`` ``????/1,??/?`` ``????/1,??/?`` ``0000/0000,--``
 ``[2]`` ``0000/ffff`` ``ffff/0,??/?`` ``ffff/0,??/?`` ``0000/ffff,--``
 ``[3]`` ``1xxx/1fff`` ``0xxx/0,??/1`` ``0xxx/0,??/1`` ``1xxx/ffff,--``
 ``[4]`` ``z000/f000`` ``????/1,0y/0`` ``fffe/0,0y/0`` ``1000/1000,0y``
 ``[5]`` ``zxxx/ffff`` ``0xxx/0,0y/0`` ``0xxx/0,0y/0`` ``1xxx/ffff,0y``
 ``[6]`` ``0000/0fff`` ``<none>``      ``<none>``      ``<none>``
 ``[7]`` ``0000/f000`` ``<none>``      ``<none>``      ``<none>``
 ``[8]`` ``0000/efff`` ``<none>``      ``<none>``      ``<none>``
 ``[9]`` ``1001/1001`` ``<none>``      ``<none>``      ``1001/1001,--``
``[10]`` ``3000/3000`` ``<none>``      ``<none>``      ``<none>``
``[11]`` ``1000/1000`` ``<none>``      ``fffe/0,??/1`` ``1000/1000,--``
======== ============= =============== =============== ================

where:

Match:
  See the list below.

NXM:
  ``xxxx/yyyy`` means ``NXM_OF_VLAN_TCI_W`` with value ``xxxx`` and mask
  ``yyyy``.  A mask of ``0000`` is equivalent to omitting
  ``NXM_OF_VLAN_TCI(_W)``, a mask of ``ffff`` is equivalent to
  ``NXM_OF_VLAN_TCI``.

OF1.0, OF1.1:
  ``wwww/x,yy/z`` means ``dl_vlan`` ``wwww``, ``OFPFW_DL_VLAN`` ``x``,
  ``dl_vlan_pcp`` ``yy``, and ``OFPFW_DL_VLAN_PCP`` ``z``.  If
  ``OFPFW_DL_VLAN`` or ``OFPFW_DL_VLAN_PCP`` is 1, the corresponding field
  value is wildcarded, otherwise it is matched.  ``?`` means that the given
  bits are ignored (their conventional values are ``0000/x,00/0`` in OF1.0,
  ``0000/x,00/1`` in OF1.1; ``x`` is never ignored).  ``<none>`` means that the
  given match is not supported.

OF1.2:
  ``xxxx/yyyy,zz`` means ``OXM_OF_VLAN_VID_W`` with value ``xxxx`` and mask
  ``yyyy``, and ``OXM_OF_VLAN_PCP`` (which is not maskable) with value ``zz``.
  A mask of ``0000`` is equivalent to omitting ``OXM_OF_VLAN_VID(_W)``, a mask
  of ``ffff`` is equivalent to ``OXM_OF_VLAN_VID``.  ``--`` means that
  ``OXM_OF_VLAN_PCP`` is omitted.  ``<none>`` means that the given match is not
  supported.

The matches are:

``[1]``:
  Matches any packet, that is, one without an 802.1Q header or with an 802.1Q
  header with any TCI value.

``[2]``
  Matches only packets without an 802.1Q header.

  NXM:
    Any match with ``vlan_tci == 0`` and ``(vlan_tci_mask & 0x1000) != 0`` is
    equivalent to the one listed in the table.

  OF1.0:
    The spec doesn't define behavior if ``dl_vlan`` is set to ``0xffff`` and
    ``OFPFW_DL_VLAN_PCP`` is not set.

  OF1.1:
    The spec says explicitly to ignore ``dl_vlan_pcp`` when ``dl_vlan`` is set
    to ``0xffff``.

  OF1.2:
    The spec doesn't say what should happen if ``vlan_vid == 0`` and
    ``(vlan_vid_mask & 0x1000) != 0`` but ``vlan_vid_mask != 0x1000``, but it
    would be straightforward to also interpret as ``[2]``.

``[3]``
  Matches only packets that have an 802.1Q header with VID ``xxx`` (and any
  PCP).

``[4]``
  Matches only packets that have an 802.1Q header with PCP ``y`` (and any VID).

  NXM:
    ``z`` is ``(y << 1) | 1``.

  OF1.0:
    The spec isn't very clear, but OVS implements it this way.

  OF1.2:
    Presumably other masks such that ``(vlan_vid_mask & 0x1fff) == 0x1000``
    would also work, but the spec doesn't define their behavior.

``[5]``
  Matches only packets that have an 802.1Q header with VID ``xxx`` and PCP
  ``y``.

   NXM:
     ``z`` is ``((y << 1) | 1)``.

   OF1.2:
     Presumably other masks such that ``(vlan_vid_mask & 0x1fff) == 0x1fff``
     would also work.

``[6]``
  Matches packets with no 802.1Q header or with an 802.1Q header with a VID of
  0.  Only possible with NXM.

``[7]``
  Matches packets with no 802.1Q header or with an 802.1Q header with a PCP of
  0.  Only possible with NXM.

``[8]``
  Matches packets with no 802.1Q header or with an 802.1Q header with both VID
  and PCP of 0.  Only possible with NXM.

``[9]``
  Matches only packets that have an 802.1Q header with an odd-numbered VID (and
  any PCP).  Only possible with NXM and OF1.2.  (This is just an example; one
  can match on any desired VID bit pattern.)

``[10]``
  Matches only packets that have an 802.1Q header with an odd-numbered PCP (and
  any VID).  Only possible with NXM.  (This is just an example; one can match
  on any desired VID bit pattern.)

``[11]``
  Matches any packet with an 802.1Q header, regardless of VID or PCP.

Additional notes:

OF1.2:
  The top three bits of ``OXM_OF_VLAN_VID`` are fixed to zero, so bits 13, 14,
  and 15 in the masks listed in the table may be set to arbitrary values, as
  long as the corresponding value bits are also zero.  The suggested ``ffff``
  mask for [2], [3], and [5] allows a shorter OXM representation (the mask is
  omitted) than the minimal ``1fff`` mask.

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_mod`` calls, 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    (add on miss)  (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
----------------------

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.

``OFPTC_*`` Table Configuration
-------------------------------

This section covers the history of the ``OFPTC_*`` table configuration bits
across OpenFlow versions.

OpenFlow 1.0 flow tables had fixed configurations.

OpenFlow 1.1 enabled controllers to configure behavior upon flow table miss and
added the ``OFPTC_MISS_*`` constants for that purpose.  ``OFPTC_*`` did not
control anything else but it was nevertheless conceptualized as a set of
bit-fields instead of an enum.  OF1.1 added the ``OFPT_TABLE_MOD`` message to
set ``OFPTC_MISS_*`` for a flow table and added the ``config`` field to the
``OFPST_TABLE`` reply to report the current setting.

OpenFlow 1.2 did not change anything in this regard.

OpenFlow 1.3 switched to another means to changing flow table miss behavior and
deprecated ``OFPTC_MISS_*`` without adding any more ``OFPTC_*`` constants.
This meant that ``OFPT_TABLE_MOD`` now had no purpose at all, but OF1.3 kept it
around "for backward compatibility with older and newer versions of the
specification."  At the same time, OF1.3 introduced a new message
OFPMP_TABLE_FEATURES that included a field ``config`` documented as reporting
the ``OFPTC_*`` values set with ``OFPT_TABLE_MOD``; of course this served no
real purpose because no ``OFPTC_*`` values are defined.  OF1.3 did remove the
``OFPTC_*`` field from ``OFPMP_TABLE`` (previously named ``OFPST_TABLE``).

OpenFlow 1.4 defined two new ``OFPTC_*`` constants, ``OFPTC_EVICTION`` and
``OFPTC_VACANCY_EVENTS``, using bits that did not overlap with ``OFPTC_MISS_*``
even though those bits had not been defined since OF1.2.  ``OFPT_TABLE_MOD``
still controlled these settings.  The field for ``OFPTC_*`` values in
``OFPMP_TABLE_FEATURES`` was renamed from ``config`` to ``capabilities`` and
documented as reporting the flags that are supported in a ``OFPT_TABLE_MOD``
message.  The ``OFPMP_TABLE_DESC`` message newly added in OF1.4 reported the
``OFPTC_*`` setting.

OpenFlow 1.5 did not change anything in this regard.

.. list-table:: Revisions
   :header-rows: 1

   * - OpenFlow
     - ``OFPTC_*`` flags
     - ``TABLE_MOD``
     - Statistics
     - ``TABLE_FEATURES``
     - ``TABLE_DESC``
   * - OF1.0
     - none
     - no (\*)(+)
     - no (\*)
     - nothing (\*)(+)
     - no (\*)(+)
   * - OF1.1/1.2
     - ``MISS_*``
     - yes
     - yes
     - nothing (+)
     - no (+)
   * - OF1.3
     - none
     - yes (\*)
     - no (\*)
     - config (\*)
     - no (\*)(+)
   * - OF1.4/1.5
     - ``EVICTION``/``VACANCY_EVENTS``
     - yes
     - no
     - capabilities
     - yes

where:

OpenFlow:
  The OpenFlow version(s).

``OFPTC_*`` flags:
  The ``OFPTC_*`` flags defined in those versions.

``TABLE_MOD``:
  Whether ``OFPT_TABLE_MOD`` can modify ``OFPTC_*`` flags.

Statistics:
  Whether ``OFPST_TABLE/OFPMP_TABLE`` reports the ``OFPTC_*`` flags.

``TABLE_FEATURES``:
  What ``OFPMP_TABLE_FEATURES`` reports (if it exists): either the current
  configuration or the switch's capabilities.

``TABLE_DESC``:
  Whether ``OFPMP_TABLE_DESC`` reports the current configuration.

(\*): Nothing to report/change anyway.

(+): No such message.

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.

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.

- Open vSwitch "normalizes" the instructions in OpenFlow 1.1 (and later) in the
  following way:

  * OVS sorts the instructions into the following order: Apply-Actions,
    Clear-Actions, Write-Actions, Write-Metadata, Goto-Table.

  * OVS drops Apply-Actions instructions that have empty action lists.

  * OVS drops Write-Actions instructions that have empty action sets.

Please report other discrepancies, if you notice any, so that we can fix or
document them.

Suggestions
-----------

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