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.

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

  OFPT_FLOW_REMOVED / NXT_FLOW_REMOVED
    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       ---

  OFPT_PORT_STATUS
    OFPPR_ADD                                  yes       yes
    OFPPR_DELETE                               yes       yes
    OFPPR_MODIFY                               yes       yes

  OFPT_ROLE_REQUEST / OFPT_ROLE_REPLY (OF1.4+)
    OFPCRR_MASTER_REQUEST                      ---       ---
    OFPCRR_CONFIG                              ---       ---
    OFPCRR_EXPERIMENTER                        ---       ---

  OFPT_TABLE_STATUS (OF1.4+)
    OFPTR_VACANCY_DOWN                         ---       ---
    OFPTR_VACANCY_UP                           ---       ---

  OFPT_REQUESTFORWARD (OF1.4+)
    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
                             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   ---    ---     ---      %       %

(+) "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
                             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   ---    ---     ---      %       %

(+) "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
                             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.)

  - 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_mods.  Bundles are supported for flow_mod and
port_mod messages only.  For flow_mods, 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_mods 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 offsetof(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>

Each column is interpreted as follows.

  - 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 and 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.)

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_mods, plus flow stats requests
    and aggregate stats requests, to match on flow cookies with
    arbitrary masks.  This is much like the equivalent OpenFlow
    1.1 feature.

  - Like OpenFlow 1.1, OFPC_MODIFY and OFPFC_MODIFY_STRICT add a
    new flow if there is no match and the mask is zero (or not
    given).

  - The "cookie" field in OFPT_FLOW_MOD and NXT_FLOW_MOD messages
    is used as the cookie value for OFPFC_ADD commands, as
    described in OpenFlow 1.0.  For OFPFC_MODIFY and
    OFPFC_MODIFY_STRICT commands, the "cookie" field is used as a
    new cookie for flows that match unless it is UINT64_MAX, in
    which case the flow's cookie is not updated.

  - NXT_PACKET_IN (the Nicira extended version of
    OFPT_PACKET_IN) reports the cookie of the rule that
    generated the packet, or all-1-bits if no rule generated the
    packet.  (Older versions of OVS used all-0-bits instead of
    all-1-bits.)

The following table shows the handling of different protocols when
receiving OFPFC_MODIFY and OFPFC_MODIFY_STRICT messages.  A mask of 0
indicates either an explicit mask of zero or an implicit one by not
specifying the NXM_NX_COOKIE(_W) field.

```
                Match   Update   Add on miss   Add on miss
                cookie  cookie     mask!=0       mask==0
                ======  ======   ===========   ===========
OpenFlow 1.0      no     yes        <always add on miss>
OpenFlow 1.1     yes      no          no           yes
OpenFlow 1.2     yes      no          no            no
NXM              yes     yes*         no           yes

* Updates the flow's cookie unless the "cookie" field is UINT64_MAX.
```

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

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.

The following table summarizes.  The columns say:

  - OpenFlow version(s).

  - The OFPTC_* flags defined in those versions.

  - Whether OFPT_TABLE_MOD can modify OFPTC_* flags.

  - Whether OFPST_TABLE/OFPMP_TABLE reports the OFPTC_* flags.

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

  - Whether OFPMP_TABLE_DESC reports the current configuration.

OpenFlow   OFPTC_* flags            TABLE_MOD stats? 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

   [*] 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.
