1 Design Decisions In Open vSwitch
2 ================================
4 This document describes design decisions that went into implementing
5 Open vSwitch. While we believe these to be reasonable decisions, it is
6 impossible to predict how Open vSwitch will be used in all environments.
7 Understanding assumptions made by Open vSwitch is critical to a
8 successful deployment. The end of this document contains contact
9 information that can be used to let us know how we can make Open vSwitch
10 more generally useful.
15 Over time, Open vSwitch has added many knobs that control whether a
16 given controller receives OpenFlow asynchronous messages. This
17 section describes how all of these features interact.
19 First, a service controller never receives any asynchronous messages
20 unless it changes its miss_send_len from the service controller
21 default of zero in one of the following ways:
23 - Sending an OFPT_SET_CONFIG message with nonzero miss_send_len.
25 - Sending any NXT_SET_ASYNC_CONFIG message: as a side effect, this
26 message changes the miss_send_len to
27 OFP_DEFAULT_MISS_SEND_LEN (128) for service controllers.
29 Second, OFPT_FLOW_REMOVED and NXT_FLOW_REMOVED messages are generated
30 only if the flow that was removed had the OFPFF_SEND_FLOW_REM flag
33 Third, OFPT_PACKET_IN and NXT_PACKET_IN messages are sent only to
34 OpenFlow controller connections that have the correct connection ID
35 (see "struct nx_controller_id" and "struct nx_action_controller"):
37 - For packet-in messages generated by a NXAST_CONTROLLER action,
38 the controller ID specified in the action.
40 - For other packet-in messages, controller ID zero. (This is the
41 default ID when an OpenFlow controller does not configure one.)
43 Finally, Open vSwitch consults a per-connection table indexed by the
44 message type, reason code, and current role. The following table
45 shows how this table is initialized by default when an OpenFlow
46 connection is made. An entry labeled "yes" means that the message is
47 sent, an entry labeled "---" means that the message is suppressed.
50 message and reason code other slave
51 ---------------------------------------- ------- -----
52 OFPT_PACKET_IN / NXT_PACKET_IN
55 OFPR_INVALID_TTL --- ---
57 OFPT_FLOW_REMOVED / NXT_FLOW_REMOVED
58 OFPRR_IDLE_TIMEOUT yes ---
59 OFPRR_HARD_TIMEOUT yes ---
67 The NXT_SET_ASYNC_CONFIG message directly sets all of the values in
68 this table for the current connection. The
69 OFPC_INVALID_TTL_TO_CONTROLLER bit in the OFPT_SET_CONFIG message
70 controls the setting for OFPR_INVALID_TTL for the "master" role.
76 The OpenFlow 1.0 specification requires the output port of the OFPAT_ENQUEUE
77 action to "refer to a valid physical port (i.e. < OFPP_MAX) or OFPP_IN_PORT".
78 Although OFPP_LOCAL is not less than OFPP_MAX, it is an 'internal' port which
79 can have QoS applied to it in Linux. Since we allow the OFPAT_ENQUEUE to apply
80 to 'internal' ports whose port numbers are less than OFPP_MAX, we interpret
81 OFPP_LOCAL as a physical port and support OFPAT_ENQUEUE on it as well.
87 The OpenFlow specification for the behavior of OFPT_FLOW_MOD is
88 confusing. The following tables summarize the Open vSwitch
89 implementation of its behavior in the following categories:
91 - "match on priority": Whether the flow_mod acts only on flows
92 whose priority matches that included in the flow_mod message.
94 - "match on out_port": Whether the flow_mod acts only on flows
95 that output to the out_port included in the flow_mod message (if
96 out_port is not OFPP_NONE). OpenFlow 1.1 and later have a
97 similar feature (not listed separately here) for out_group.
99 - "match on flow_cookie": Whether the flow_mod acts only on flows
100 whose flow_cookie matches an optional controller-specified value
103 - "updates flow_cookie": Whether the flow_mod changes the
104 flow_cookie of the flow or flows that it matches to the
105 flow_cookie included in the flow_mod message.
107 - "updates OFPFF_ flags": Whether the flow_mod changes the
108 OFPFF_SEND_FLOW_REM flag of the flow or flows that it matches to
109 the setting included in the flags of the flow_mod message.
111 - "honors OFPFF_CHECK_OVERLAP": Whether the OFPFF_CHECK_OVERLAP
112 flag in the flow_mod is significant.
114 - "updates idle_timeout" and "updates hard_timeout": Whether the
115 idle_timeout and hard_timeout in the flow_mod, respectively,
116 have an effect on the flow or flows matched by the flow_mod.
118 - "updates idle timer": Whether the flow_mod resets the per-flow
119 timer that measures how long a flow has been idle.
121 - "updates hard timer": Whether the flow_mod resets the per-flow
122 timer that measures how long it has been since a flow was
125 - "zeros counters": Whether the flow_mod resets per-flow packet
126 and byte counters to zero.
128 - "may add a new flow": Whether the flow_mod may add a new flow to
129 the flow table. (Obviously this is always true for "add"
130 commands but in some OpenFlow versions "modify" and
131 "modify-strict" can also add new flows.)
133 - "sends flow_removed message": Whether the flow_mod generates a
134 flow_removed message for the flow or flows that it affects.
136 An entry labeled "yes" means that the flow mod type does have the
137 indicated behavior, "---" means that it does not, an empty cell means
138 that the property is not applicable, and other values are explained
145 ADD MODIFY STRICT DELETE STRICT
146 === ====== ====== ====== ======
147 match on priority yes --- yes --- yes
148 match on out_port --- --- --- yes yes
149 match on flow_cookie --- --- --- --- ---
150 match on table_id --- --- --- --- ---
151 controller chooses table_id --- --- ---
152 updates flow_cookie yes yes yes
153 updates OFPFF_SEND_FLOW_REM yes + +
154 honors OFPFF_CHECK_OVERLAP yes + +
155 updates idle_timeout yes + +
156 updates hard_timeout yes + +
157 resets idle timer yes + +
158 resets hard timer yes yes yes
159 zeros counters yes + +
160 may add a new flow yes yes yes
161 sends flow_removed message --- --- --- % %
163 (+) "modify" and "modify-strict" only take these actions when they
164 create a new flow, not when they update an existing flow.
166 (%) "delete" and "delete_strict" generates a flow_removed message if
167 the deleted flow or flows have the OFPFF_SEND_FLOW_REM flag set.
168 (Each controller can separately control whether it wants to
169 receive the generated messages.)
174 OpenFlow 1.1 makes these changes:
176 - The controller now must specify the table_id of the flow match
177 searched and into which a flow may be inserted. Behavior for a
178 table_id of 255 is undefined.
180 - A flow_mod, except an "add", can now match on the flow_cookie.
182 - When a flow_mod matches on the flow_cookie, "modify" and
183 "modify-strict" never insert a new flow.
186 ADD MODIFY STRICT DELETE STRICT
187 === ====== ====== ====== ======
188 match on priority yes --- yes --- yes
189 match on out_port --- --- --- yes yes
190 match on flow_cookie --- yes yes yes yes
191 match on table_id yes yes yes yes yes
192 controller chooses table_id yes yes yes
193 updates flow_cookie yes --- ---
194 updates OFPFF_SEND_FLOW_REM yes + +
195 honors OFPFF_CHECK_OVERLAP yes + +
196 updates idle_timeout yes + +
197 updates hard_timeout yes + +
198 resets idle timer yes + +
199 resets hard timer yes yes yes
200 zeros counters yes + +
201 may add a new flow yes # #
202 sends flow_removed message --- --- --- % %
204 (+) "modify" and "modify-strict" only take these actions when they
205 create a new flow, not when they update an existing flow.
207 (%) "delete" and "delete_strict" generates a flow_removed message if
208 the deleted flow or flows have the OFPFF_SEND_FLOW_REM flag set.
209 (Each controller can separately control whether it wants to
210 receive the generated messages.)
212 (#) "modify" and "modify-strict" only add a new flow if the flow_mod
213 does not match on any bits of the flow cookie
218 OpenFlow 1.2 makes these changes:
220 - Only "add" commands ever add flows, "modify" and "modify-strict"
223 - A new flag OFPFF_RESET_COUNTS now controls whether "modify" and
224 "modify-strict" reset counters, whereas previously they never
225 reset counters (except when they inserted a new flow).
228 ADD MODIFY STRICT DELETE STRICT
229 === ====== ====== ====== ======
230 match on priority yes --- yes --- yes
231 match on out_port --- --- --- yes yes
232 match on flow_cookie --- yes yes yes yes
233 match on table_id yes yes yes yes yes
234 controller chooses table_id yes yes yes
235 updates flow_cookie yes --- ---
236 updates OFPFF_SEND_FLOW_REM yes --- ---
237 honors OFPFF_CHECK_OVERLAP yes --- ---
238 updates idle_timeout yes --- ---
239 updates hard_timeout yes --- ---
240 resets idle timer yes --- ---
241 resets hard timer yes yes yes
242 zeros counters yes & &
243 may add a new flow yes --- ---
244 sends flow_removed message --- --- --- % %
246 (%) "delete" and "delete_strict" generates a flow_removed message if
247 the deleted flow or flows have the OFPFF_SEND_FLOW_REM flag set.
248 (Each controller can separately control whether it wants to
249 receive the generated messages.)
251 (&) "modify" and "modify-strict" reset counters if the
252 OFPFF_RESET_COUNTS flag is specified.
257 OpenFlow 1.3 makes these changes:
259 - Behavior for a table_id of 255 is now defined, for "delete" and
260 "delete-strict" commands, as meaning to delete from all tables.
261 A table_id of 255 is now explicitly invalid for other commands.
263 - New flags OFPFF_NO_PKT_COUNTS and OFPFF_NO_BYT_COUNTS for "add"
266 The table for 1.3 is the same as the one shown above for 1.2.
272 The 802.1Q VLAN header causes more trouble than any other 4 bytes in
273 networking. More specifically, three versions of OpenFlow and Open
274 vSwitch have among them four different ways to match the contents and
275 presence of the VLAN header. The following table describes how each
278 Match NXM OF1.0 OF1.1 OF1.2
279 ----- --------- ----------- ----------- ------------
280 [1] 0000/0000 ????/1,??/? ????/1,??/? 0000/0000,--
281 [2] 0000/ffff ffff/0,??/? ffff/0,??/? 0000/ffff,--
282 [3] 1xxx/1fff 0xxx/0,??/1 0xxx/0,??/1 1xxx/ffff,--
283 [4] z000/f000 ????/1,0y/0 fffe/0,0y/0 1000/1000,0y
284 [5] zxxx/ffff 0xxx/0,0y/0 0xxx/0,0y/0 1xxx/ffff,0y
285 [6] 0000/0fff <none> <none> <none>
286 [7] 0000/f000 <none> <none> <none>
287 [8] 0000/efff <none> <none> <none>
288 [9] 1001/1001 <none> <none> 1001/1001,--
289 [10] 3000/3000 <none> <none> <none>
291 Each column is interpreted as follows.
293 - Match: See the list below.
295 - NXM: xxxx/yyyy means NXM_OF_VLAN_TCI_W with value xxxx and mask
296 yyyy. A mask of 0000 is equivalent to omitting
297 NXM_OF_VLAN_TCI(_W), a mask of ffff is equivalent to
300 - OF1.0 and OF1.1: wwww/x,yy/z means dl_vlan wwww, OFPFW_DL_VLAN
301 x, dl_vlan_pcp yy, and OFPFW_DL_VLAN_PCP z. ? means that the
302 given nibble is ignored (and conventionally 0 for wwww or yy,
303 conventionally 1 for x or z). <none> means that the given match
306 - OF1.2: xxxx/yyyy,zz means OXM_OF_VLAN_VID_W with value xxxx and
307 mask yyyy, and OXM_OF_VLAN_PCP (which is not maskable) with
308 value zz. A mask of 0000 is equivalent to omitting
309 OXM_OF_VLAN_VID(_W), a mask of ffff is equivalent to
310 OXM_OF_VLAN_VID. -- means that OXM_OF_VLAN_PCP is omitted.
311 <none> means that the given match is not supported.
315 [1] Matches any packet, that is, one without an 802.1Q header or with
316 an 802.1Q header with any TCI value.
318 [2] Matches only packets without an 802.1Q header.
320 NXM: Any match with (vlan_tci == 0) and (vlan_tci_mask & 0x1000)
321 != 0 is equivalent to the one listed in the table.
323 OF1.0: The spec doesn't define behavior if dl_vlan is set to
324 0xffff and OFPFW_DL_VLAN_PCP is not set.
326 OF1.1: The spec says explicitly to ignore dl_vlan_pcp when
327 dl_vlan is set to 0xffff.
329 OF1.2: The spec doesn't say what should happen if (vlan_vid == 0)
330 and (vlan_vid_mask & 0x1000) != 0 but (vlan_vid_mask != 0x1000),
331 but it would be straightforward to also interpret as [2].
333 [3] Matches only packets that have an 802.1Q header with VID xxx (and
336 [4] Matches only packets that have an 802.1Q header with PCP y (and
339 NXM: z is ((y << 1) | 1).
341 OF1.0: The spec isn't very clear, but OVS implements it this way.
343 OF1.2: Presumably other masks such that (vlan_vid_mask & 0x1fff)
344 == 0x1000 would also work, but the spec doesn't define their
347 [5] Matches only packets that have an 802.1Q header with VID xxx and
350 NXM: z is ((y << 1) | 1).
352 OF1.2: Presumably other masks such that (vlan_vid_mask & 0x1fff)
353 == 0x1fff would also work.
355 [6] Matches packets with no 802.1Q header or with an 802.1Q header
356 with a VID of 0. Only possible with NXM.
358 [7] Matches packets with no 802.1Q header or with an 802.1Q header
359 with a PCP of 0. Only possible with NXM.
361 [8] Matches packets with no 802.1Q header or with an 802.1Q header
362 with both VID and PCP of 0. Only possible with NXM.
364 [9] Matches only packets that have an 802.1Q header with an
365 odd-numbered VID (and any PCP). Only possible with NXM and
366 OF1.2. (This is just an example; one can match on any desired
369 [10] Matches only packets that have an 802.1Q header with an
370 odd-numbered PCP (and any VID). Only possible with NXM. (This
371 is just an example; one can match on any desired VID bit
376 - OF1.2: The top three bits of OXM_OF_VLAN_VID are fixed to zero,
377 so bits 13, 14, and 15 in the masks listed in the table may be
378 set to arbitrary values, as long as the corresponding value bits
379 are also zero. The suggested ffff mask for [2], [3], and [5]
380 allows a shorter OXM representation (the mask is omitted) than
381 the minimal 1fff mask.
387 OpenFlow 1.0 and later versions have the concept of a "flow cookie",
388 which is a 64-bit integer value attached to each flow. The treatment
389 of the flow cookie has varied greatly across OpenFlow versions,
394 - OFPFC_ADD set the cookie in the flow that it added.
396 - OFPFC_MODIFY and OFPFC_MODIFY_STRICT updated the cookie for
397 the flow or flows that it modified.
399 - OFPST_FLOW messages included the flow cookie.
401 - OFPT_FLOW_REMOVED messages reported the cookie of the flow
404 OpenFlow 1.1 made the following changes:
406 - Flow mod operations OFPFC_MODIFY, OFPFC_MODIFY_STRICT,
407 OFPFC_DELETE, and OFPFC_DELETE_STRICT, plus flow stats
408 requests and aggregate stats requests, gained the ability to
409 match on flow cookies with an arbitrary mask.
411 - OFPFC_MODIFY and OFPFC_MODIFY_STRICT were changed to add a
412 new flow, in the case of no match, only if the flow table
413 modification operation did not match on the cookie field.
414 (In OpenFlow 1.0, modify operations always added a new flow
415 when there was no match.)
417 - OFPFC_MODIFY and OFPFC_MODIFY_STRICT no longer updated flow
420 OpenFlow 1.2 made the following changes:
422 - OFPC_MODIFY and OFPFC_MODIFY_STRICT were changed to never
423 add a new flow, regardless of whether the flow cookie was
426 Open vSwitch support for OpenFlow 1.0 implements the OpenFlow 1.0
427 behavior with the following extensions:
429 - An NXM extension field NXM_NX_COOKIE(_W) allows the NXM
430 versions of OFPFC_MODIFY, OFPFC_MODIFY_STRICT, OFPFC_DELETE,
431 and OFPFC_DELETE_STRICT flow_mods, plus flow stats requests
432 and aggregate stats requests, to match on flow cookies with
433 arbitrary masks. This is much like the equivalent OpenFlow
436 - Like OpenFlow 1.1, OFPC_MODIFY and OFPFC_MODIFY_STRICT add a
437 new flow if there is no match and the mask is zero (or not
440 - The "cookie" field in OFPT_FLOW_MOD and NXT_FLOW_MOD messages
441 is used as the cookie value for OFPFC_ADD commands, as
442 described in OpenFlow 1.0. For OFPFC_MODIFY and
443 OFPFC_MODIFY_STRICT commands, the "cookie" field is used as a
444 new cookie for flows that match unless it is UINT64_MAX, in
445 which case the flow's cookie is not updated.
447 - NXT_PACKET_IN (the Nicira extended version of
448 OFPT_PACKET_IN) reports the cookie of the rule that
449 generated the packet, or all-1-bits if no rule generated the
450 packet. (Older versions of OVS used all-0-bits instead of
453 The following table shows the handling of different protocols when
454 receiving OFPFC_MODIFY and OFPFC_MODIFY_STRICT messages. A mask of 0
455 indicates either an explicit mask of zero or an implicit one by not
456 specifying the NXM_NX_COOKIE(_W) field.
458 Match Update Add on miss Add on miss
459 cookie cookie mask!=0 mask==0
460 ====== ====== =========== ===========
461 OpenFlow 1.0 no yes <always add on miss>
462 OpenFlow 1.1 yes no no yes
463 OpenFlow 1.2 yes no no no
466 * Updates the flow's cookie unless the "cookie" field is UINT64_MAX.
469 Multiple Table Support
470 ======================
472 OpenFlow 1.0 has only rudimentary support for multiple flow tables.
473 Notably, OpenFlow 1.0 does not allow the controller to specify the
474 flow table to which a flow is to be added. Open vSwitch adds an
475 extension for this purpose, which is enabled on a per-OpenFlow
476 connection basis using the NXT_FLOW_MOD_TABLE_ID message. When the
477 extension is enabled, the upper 8 bits of the 'command' member in an
478 OFPT_FLOW_MOD or NXT_FLOW_MOD message designates the table to which a
481 The Open vSwitch software switch implementation offers 255 flow
482 tables. On packet ingress, only the first flow table (table 0) is
483 searched, and the contents of the remaining tables are not considered
484 in any way. Tables other than table 0 only come into play when an
485 NXAST_RESUBMIT_TABLE action specifies another table to search.
487 Tables 128 and above are reserved for use by the switch itself.
488 Controllers should use only tables 0 through 127.
494 Open vSwitch supports stateless handling of IPv6 packets. Flows can be
495 written to support matching TCP, UDP, and ICMPv6 headers within an IPv6
496 packet. Deeper matching of some Neighbor Discovery messages is also
499 IPv6 was not designed to interact well with middle-boxes. This,
500 combined with Open vSwitch's stateless nature, have affected the
501 processing of IPv6 traffic, which is detailed below.
506 The base IPv6 header is incredibly simple with the intention of only
507 containing information relevant for routing packets between two
508 endpoints. IPv6 relies heavily on the use of extension headers to
509 provide any other functionality. Unfortunately, the extension headers
510 were designed in such a way that it is impossible to move to the next
511 header (including the layer-4 payload) unless the current header is
514 Open vSwitch will process the following extension headers and continue
517 * Fragment (see the next section)
518 * AH (Authentication Header)
521 * Destination Options
523 When a header is encountered that is not in that list, it is considered
524 "terminal". A terminal header's IPv6 protocol value is stored in
525 "nw_proto" for matching purposes. If a terminal header is TCP, UDP, or
526 ICMPv6, the packet will be further processed in an attempt to extract
532 IPv6 requires that every link in the internet have an MTU of 1280 octets
533 or greater (RFC 2460). As such, a terminal header (as described above in
534 "Extension Headers") in the first fragment should generally be
535 reachable. In this case, the terminal header's IPv6 protocol type is
536 stored in the "nw_proto" field for matching purposes. If a terminal
537 header cannot be found in the first fragment (one with a fragment offset
538 of zero), the "nw_proto" field is set to 0. Subsequent fragments (those
539 with a non-zero fragment offset) have the "nw_proto" field set to the
540 IPv6 protocol type for fragments (44).
545 An IPv6 jumbogram (RFC 2675) is a packet containing a payload longer
546 than 65,535 octets. A jumbogram is only relevant in subnets with a link
547 MTU greater than 65,575 octets, and are not required to be supported on
548 nodes that do not connect to link with such large MTUs. Currently, Open
549 vSwitch doesn't process jumbograms.
558 An OpenFlow switch must establish and maintain a TCP network
559 connection to its controller. There are two basic ways to categorize
560 the network that this connection traverses: either it is completely
561 separate from the one that the switch is otherwise controlling, or its
562 path may overlap the network that the switch controls. We call the
563 former case "out-of-band control", the latter case "in-band control".
565 Out-of-band control has the following benefits:
567 - Simplicity: Out-of-band control slightly simplifies the switch
570 - Reliability: Excessive switch traffic volume cannot interfere
571 with control traffic.
573 - Integrity: Machines not on the control network cannot
574 impersonate a switch or a controller.
576 - Confidentiality: Machines not on the control network cannot
577 snoop on control traffic.
579 In-band control, on the other hand, has the following advantages:
581 - No dedicated port: There is no need to dedicate a physical
582 switch port to control, which is important on switches that have
583 few ports (e.g. wireless routers, low-end embedded platforms).
585 - No dedicated network: There is no need to build and maintain a
586 separate control network. This is important in many
587 environments because it reduces proliferation of switches and
590 Open vSwitch supports both out-of-band and in-band control. This
591 section describes the principles behind in-band control. See the
592 description of the Controller table in ovs-vswitchd.conf.db(5) to
593 configure OVS for in-band control.
598 The fundamental principle of in-band control is that an OpenFlow
599 switch must recognize and switch control traffic without involving the
600 OpenFlow controller. All the details of implementing in-band control
601 are special cases of this principle.
603 The rationale for this principle is simple. If the switch does not
604 handle in-band control traffic itself, then it will be caught in a
605 contradiction: it must contact the controller, but it cannot, because
606 only the controller can set up the flows that are needed to contact
609 The following points describe important special cases of this
612 - In-band control must be implemented regardless of whether the
615 It is tempting to implement the in-band control rules only when
616 the switch is not connected to the controller, using the
617 reasoning that the controller should have complete control once
618 it has established a connection with the switch.
620 This does not work in practice. Consider the case where the
621 switch is connected to the controller. Occasionally it can
622 happen that the controller forgets or otherwise needs to obtain
623 the MAC address of the switch. To do so, the controller sends a
624 broadcast ARP request. A switch that implements the in-band
625 control rules only when it is disconnected will then send an
626 OFPT_PACKET_IN message up to the controller. The controller will
627 be unable to respond, because it does not know the MAC address of
628 the switch. This is a deadlock situation that can only be
629 resolved by the switch noticing that its connection to the
630 controller has hung and reconnecting.
632 - In-band control must override flows set up by the controller.
634 It is reasonable to assume that flows set up by the OpenFlow
635 controller should take precedence over in-band control, on the
636 basis that the controller should be in charge of the switch.
638 Again, this does not work in practice. Reasonable controller
639 implementations may set up a "last resort" fallback rule that
640 wildcards every field and, e.g., sends it up to the controller or
641 discards it. If a controller does that, then it will isolate
642 itself from the switch.
644 - The switch must recognize all control traffic.
646 The fundamental principle of in-band control states, in part,
647 that a switch must recognize control traffic without involving
648 the OpenFlow controller. More specifically, the switch must
649 recognize *all* control traffic. "False negatives", that is,
650 packets that constitute control traffic but that the switch does
651 not recognize as control traffic, lead to control traffic storms.
653 Consider an OpenFlow switch that only recognizes control packets
654 sent to or from that switch. Now suppose that two switches of
655 this type, named A and B, are connected to ports on an Ethernet
656 hub (not a switch) and that an OpenFlow controller is connected
657 to a third hub port. In this setup, control traffic sent by
658 switch A will be seen by switch B, which will send it to the
659 controller as part of an OFPT_PACKET_IN message. Switch A will
660 then see the OFPT_PACKET_IN message's packet, re-encapsulate it
661 in another OFPT_PACKET_IN, and send it to the controller. Switch
662 B will then see that OFPT_PACKET_IN, and so on in an infinite
665 Incidentally, the consequences of "false positives", where
666 packets that are not control traffic are nevertheless recognized
667 as control traffic, are much less severe. The controller will
668 not be able to control their behavior, but the network will
669 remain in working order. False positives do constitute a
672 - The switch should use echo-requests to detect disconnection.
674 TCP will notice that a connection has hung, but this can take a
675 considerable amount of time. For example, with default settings
676 the Linux kernel TCP implementation will retransmit for between
677 13 and 30 minutes, depending on the connection's retransmission
678 timeout, according to kernel documentation. This is far too long
679 for a switch to be disconnected, so an OpenFlow switch should
680 implement its own connection timeout. OpenFlow OFPT_ECHO_REQUEST
681 messages are the best way to do this, since they test the
682 OpenFlow connection itself.
687 This section describes how Open vSwitch implements in-band control.
688 Correctly implementing in-band control has proven difficult due to its
689 many subtleties, and has thus gone through many iterations. Please
690 read through and understand the reasoning behind the chosen rules
691 before making modifications.
693 Open vSwitch implements in-band control as "hidden" flows, that is,
694 flows that are not visible through OpenFlow, and at a higher priority
695 than wildcarded flows can be set up through OpenFlow. This is done so
696 that the OpenFlow controller cannot interfere with them and possibly
697 break connectivity with its switches. It is possible to see all
698 flows, including in-band ones, with the ovs-appctl "bridge/dump-flows"
701 The Open vSwitch implementation of in-band control can hide traffic to
702 arbitrary "remotes", where each remote is one TCP port on one IP address.
703 Currently the remotes are automatically configured as the in-band OpenFlow
704 controllers plus the OVSDB managers, if any. (The latter is a requirement
705 because OVSDB managers are responsible for configuring OpenFlow controllers,
706 so if the manager cannot be reached then OpenFlow cannot be reconfigured.)
708 The following rules (with the OFPP_NORMAL action) are set up on any bridge
709 that has any remotes:
711 (a) DHCP requests sent from the local port.
712 (b) ARP replies to the local port's MAC address.
713 (c) ARP requests from the local port's MAC address.
715 In-band also sets up the following rules for each unique next-hop MAC
716 address for the remotes' IPs (the "next hop" is either the remote
717 itself, if it is on a local subnet, or the gateway to reach the remote):
719 (d) ARP replies to the next hop's MAC address.
720 (e) ARP requests from the next hop's MAC address.
722 In-band also sets up the following rules for each unique remote IP address:
724 (f) ARP replies containing the remote's IP address as a target.
725 (g) ARP requests containing the remote's IP address as a source.
727 In-band also sets up the following rules for each unique remote (IP,port)
730 (h) TCP traffic to the remote's IP and port.
731 (i) TCP traffic from the remote's IP and port.
733 The goal of these rules is to be as narrow as possible to allow a
734 switch to join a network and be able to communicate with the
735 remotes. As mentioned earlier, these rules have higher priority
736 than the controller's rules, so if they are too broad, they may
737 prevent the controller from implementing its policy. As such,
738 in-band actively monitors some aspects of flow and packet processing
739 so that the rules can be made more precise.
741 In-band control monitors attempts to add flows into the datapath that
742 could interfere with its duties. The datapath only allows exact
743 match entries, so in-band control is able to be very precise about
744 the flows it prevents. Flows that miss in the datapath are sent to
745 userspace to be processed, so preventing these flows from being
746 cached in the "fast path" does not affect correctness. The only type
747 of flow that is currently prevented is one that would prevent DHCP
748 replies from being seen by the local port. For example, a rule that
749 forwarded all DHCP traffic to the controller would not be allowed,
750 but one that forwarded to all ports (including the local port) would.
752 As mentioned earlier, packets that miss in the datapath are sent to
753 the userspace for processing. The userspace has its own flow table,
754 the "classifier", so in-band checks whether any special processing
755 is needed before the classifier is consulted. If a packet is a DHCP
756 response to a request from the local port, the packet is forwarded to
757 the local port, regardless of the flow table. Note that this requires
758 L7 processing of DHCP replies to determine whether the 'chaddr' field
759 matches the MAC address of the local port.
761 It is interesting to note that for an L3-based in-band control
762 mechanism, the majority of rules are devoted to ARP traffic. At first
763 glance, some of these rules appear redundant. However, each serves an
764 important role. First, in order to determine the MAC address of the
765 remote side (controller or gateway) for other ARP rules, we must allow
766 ARP traffic for our local port with rules (b) and (c). If we are
767 between a switch and its connection to the remote, we have to
768 allow the other switch's ARP traffic to through. This is done with
769 rules (d) and (e), since we do not know the addresses of the other
770 switches a priori, but do know the remote's or gateway's. Finally,
771 if the remote is running in a local guest VM that is not reached
772 through the local port, the switch that is connected to the VM must
773 allow ARP traffic based on the remote's IP address, since it will
774 not know the MAC address of the local port that is sending the traffic
775 or the MAC address of the remote in the guest VM.
777 With a few notable exceptions below, in-band should work in most
778 network setups. The following are considered "supported' in the
779 current implementation:
781 - Locally Connected. The switch and remote are on the same
782 subnet. This uses rules (a), (b), (c), (h), and (i).
784 - Reached through Gateway. The switch and remote are on
785 different subnets and must go through a gateway. This uses
786 rules (a), (b), (c), (h), and (i).
788 - Between Switch and Remote. This switch is between another
789 switch and the remote, and we want to allow the other
790 switch's traffic through. This uses rules (d), (e), (h), and
791 (i). It uses (b) and (c) indirectly in order to know the MAC
792 address for rules (d) and (e). Note that DHCP for the other
793 switch will not work unless an OpenFlow controller explicitly lets this
794 switch pass the traffic.
796 - Between Switch and Gateway. This switch is between another
797 switch and the gateway, and we want to allow the other switch's
798 traffic through. This uses the same rules and logic as the
799 "Between Switch and Remote" configuration described earlier.
801 - Remote on Local VM. The remote is a guest VM on the
802 system running in-band control. This uses rules (a), (b), (c),
805 - Remote on Local VM with Different Networks. The remote
806 is a guest VM on the system running in-band control, but the
807 local port is not used to connect to the remote. For
808 example, an IP address is configured on eth0 of the switch. The
809 remote's VM is connected through eth1 of the switch, but an
810 IP address has not been configured for that port on the switch.
811 As such, the switch will use eth0 to connect to the remote,
812 and eth1's rules about the local port will not work. In the
813 example, the switch attached to eth0 would use rules (a), (b),
814 (c), (h), and (i) on eth0. The switch attached to eth1 would use
815 rules (f), (g), (h), and (i).
817 The following are explicitly *not* supported by in-band control:
819 - Specify Remote by Name. Currently, the remote must be
820 identified by IP address. A naive approach would be to permit
821 all DNS traffic. Unfortunately, this would prevent the
822 controller from defining any policy over DNS. Since switches
823 that are located behind us need to connect to the remote,
824 in-band cannot simply add a rule that allows DNS traffic from
825 the local port. The "correct" way to support this is to parse
826 DNS requests to allow all traffic related to a request for the
827 remote's name through. Due to the potential security
828 problems and amount of processing, we decided to hold off for
831 - Differing Remotes for Switches. All switches must know
832 the L3 addresses for all the remotes that other switches
833 may use, since rules need to be set up to allow traffic related
834 to those remotes through. See rules (f), (g), (h), and (i).
836 - Differing Routes for Switches. In order for the switch to
837 allow other switches to connect to a remote through a
838 gateway, it allows the gateway's traffic through with rules (d)
839 and (e). If the routes to the remote differ for the two
840 switches, we will not know the MAC address of the alternate
847 It seems likely that many controllers, at least at startup, use the
848 OpenFlow "flow statistics" request to obtain existing flows, then
849 compare the flows' actions against the actions that they expect to
850 find. Before version 1.8.0, Open vSwitch always returned exact,
851 byte-for-byte copies of the actions that had been added to the flow
852 table. The current version of Open vSwitch does not always do this in
853 some exceptional cases. This section lists the exceptions that
854 controller authors must keep in mind if they compare actual actions
855 against desired actions in a bytewise fashion:
857 - Open vSwitch zeros padding bytes in action structures,
858 regardless of their values when the flows were added.
860 - Open vSwitch "normalizes" the instructions in OpenFlow 1.1
861 (and later) in the following way:
863 * OVS sorts the instructions into the following order:
864 Apply-Actions, Clear-Actions, Write-Actions,
865 Write-Metadata, Goto-Table.
867 * OVS drops Apply-Actions instructions that have empty
870 * OVS drops Write-Actions instructions that have empty
873 Please report other discrepancies, if you notice any, so that we can
874 fix or document them.
880 Suggestions to improve Open vSwitch are welcome at discuss@openvswitch.org.
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