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 OpenFlow 1.1 specification for OFPT_PACKET_IN is confusing. The
273 definition in OF1.1 openflow.h is[*]:
275 /* Packet received on port (datapath -> controller). */
276 struct ofp_packet_in {
277 struct ofp_header header;
278 uint32_t buffer_id; /* ID assigned by datapath. */
279 uint32_t in_port; /* Port on which frame was received. */
280 uint32_t in_phy_port; /* Physical Port on which frame was received. */
281 uint16_t total_len; /* Full length of frame. */
282 uint8_t reason; /* Reason packet is being sent (one of OFPR_*) */
283 uint8_t table_id; /* ID of the table that was looked up */
284 uint8_t data[0]; /* Ethernet frame, halfway through 32-bit word,
285 so the IP header is 32-bit aligned. The
286 amount of data is inferred from the length
287 field in the header. Because of padding,
288 offsetof(struct ofp_packet_in, data) ==
289 sizeof(struct ofp_packet_in) - 2. */
291 OFP_ASSERT(sizeof(struct ofp_packet_in) == 24);
293 The confusing part is the comment on the data[] member. This comment
294 is a leftover from OF1.0 openflow.h, in which the comment was correct:
295 sizeof(struct ofp_packet_in) is 20 in OF1.0 and offsetof(struct
296 ofp_packet_in, data) is 18. When OF1.1 was written, the structure
297 members were changed but the comment was carelessly not updated, and
298 the comment became wrong: sizeof(struct ofp_packet_in) and
299 offsetof(struct ofp_packet_in, data) are both 24 in OF1.1.
301 That leaves the question of how to implement ofp_packet_in in OF1.1.
302 The OpenFlow reference implementation for OF1.1 does not include any
303 padding, that is, the first byte of the encapsulated frame immediately
304 follows the 'table_id' member without a gap. Open vSwitch therefore
305 implements it the same way for compatibility.
307 For an earlier discussion, please see the thread archived at:
308 https://mailman.stanford.edu/pipermail/openflow-discuss/2011-August/002604.html
310 [*] The quoted definition is directly from OF1.1. Definitions used
311 inside OVS omit the 8-byte ofp_header members, so the sizes in
312 this discussion are 8 bytes larger than those declared in OVS
319 The 802.1Q VLAN header causes more trouble than any other 4 bytes in
320 networking. More specifically, three versions of OpenFlow and Open
321 vSwitch have among them four different ways to match the contents and
322 presence of the VLAN header. The following table describes how each
325 Match NXM OF1.0 OF1.1 OF1.2
326 ----- --------- ----------- ----------- ------------
327 [1] 0000/0000 ????/1,??/? ????/1,??/? 0000/0000,--
328 [2] 0000/ffff ffff/0,??/? ffff/0,??/? 0000/ffff,--
329 [3] 1xxx/1fff 0xxx/0,??/1 0xxx/0,??/1 1xxx/ffff,--
330 [4] z000/f000 ????/1,0y/0 fffe/0,0y/0 1000/1000,0y
331 [5] zxxx/ffff 0xxx/0,0y/0 0xxx/0,0y/0 1xxx/ffff,0y
332 [6] 0000/0fff <none> <none> <none>
333 [7] 0000/f000 <none> <none> <none>
334 [8] 0000/efff <none> <none> <none>
335 [9] 1001/1001 <none> <none> 1001/1001,--
336 [10] 3000/3000 <none> <none> <none>
338 Each column is interpreted as follows.
340 - Match: See the list below.
342 - NXM: xxxx/yyyy means NXM_OF_VLAN_TCI_W with value xxxx and mask
343 yyyy. A mask of 0000 is equivalent to omitting
344 NXM_OF_VLAN_TCI(_W), a mask of ffff is equivalent to
347 - OF1.0 and OF1.1: wwww/x,yy/z means dl_vlan wwww, OFPFW_DL_VLAN
348 x, dl_vlan_pcp yy, and OFPFW_DL_VLAN_PCP z. ? means that the
349 given nibble is ignored (and conventionally 0 for wwww or yy,
350 conventionally 1 for x or z). <none> means that the given match
353 - OF1.2: xxxx/yyyy,zz means OXM_OF_VLAN_VID_W with value xxxx and
354 mask yyyy, and OXM_OF_VLAN_PCP (which is not maskable) with
355 value zz. A mask of 0000 is equivalent to omitting
356 OXM_OF_VLAN_VID(_W), a mask of ffff is equivalent to
357 OXM_OF_VLAN_VID. -- means that OXM_OF_VLAN_PCP is omitted.
358 <none> means that the given match is not supported.
362 [1] Matches any packet, that is, one without an 802.1Q header or with
363 an 802.1Q header with any TCI value.
365 [2] Matches only packets without an 802.1Q header.
367 NXM: Any match with (vlan_tci == 0) and (vlan_tci_mask & 0x1000)
368 != 0 is equivalent to the one listed in the table.
370 OF1.0: The spec doesn't define behavior if dl_vlan is set to
371 0xffff and OFPFW_DL_VLAN_PCP is not set.
373 OF1.1: The spec says explicitly to ignore dl_vlan_pcp when
374 dl_vlan is set to 0xffff.
376 OF1.2: The spec doesn't say what should happen if (vlan_vid == 0)
377 and (vlan_vid_mask & 0x1000) != 0 but (vlan_vid_mask != 0x1000),
378 but it would be straightforward to also interpret as [2].
380 [3] Matches only packets that have an 802.1Q header with VID xxx (and
383 [4] Matches only packets that have an 802.1Q header with PCP y (and
386 NXM: z is ((y << 1) | 1).
388 OF1.0: The spec isn't very clear, but OVS implements it this way.
390 OF1.2: Presumably other masks such that (vlan_vid_mask & 0x1fff)
391 == 0x1000 would also work, but the spec doesn't define their
394 [5] Matches only packets that have an 802.1Q header with VID xxx and
397 NXM: z is ((y << 1) | 1).
399 OF1.2: Presumably other masks such that (vlan_vid_mask & 0x1fff)
400 == 0x1fff would also work.
402 [6] Matches packets with no 802.1Q header or with an 802.1Q header
403 with a VID of 0. Only possible with NXM.
405 [7] Matches packets with no 802.1Q header or with an 802.1Q header
406 with a PCP of 0. Only possible with NXM.
408 [8] Matches packets with no 802.1Q header or with an 802.1Q header
409 with both VID and PCP of 0. Only possible with NXM.
411 [9] Matches only packets that have an 802.1Q header with an
412 odd-numbered VID (and any PCP). Only possible with NXM and
413 OF1.2. (This is just an example; one can match on any desired
416 [10] Matches only packets that have an 802.1Q header with an
417 odd-numbered PCP (and any VID). Only possible with NXM. (This
418 is just an example; one can match on any desired VID bit
423 - OF1.2: The top three bits of OXM_OF_VLAN_VID are fixed to zero,
424 so bits 13, 14, and 15 in the masks listed in the table may be
425 set to arbitrary values, as long as the corresponding value bits
426 are also zero. The suggested ffff mask for [2], [3], and [5]
427 allows a shorter OXM representation (the mask is omitted) than
428 the minimal 1fff mask.
434 OpenFlow 1.0 and later versions have the concept of a "flow cookie",
435 which is a 64-bit integer value attached to each flow. The treatment
436 of the flow cookie has varied greatly across OpenFlow versions,
441 - OFPFC_ADD set the cookie in the flow that it added.
443 - OFPFC_MODIFY and OFPFC_MODIFY_STRICT updated the cookie for
444 the flow or flows that it modified.
446 - OFPST_FLOW messages included the flow cookie.
448 - OFPT_FLOW_REMOVED messages reported the cookie of the flow
451 OpenFlow 1.1 made the following changes:
453 - Flow mod operations OFPFC_MODIFY, OFPFC_MODIFY_STRICT,
454 OFPFC_DELETE, and OFPFC_DELETE_STRICT, plus flow stats
455 requests and aggregate stats requests, gained the ability to
456 match on flow cookies with an arbitrary mask.
458 - OFPFC_MODIFY and OFPFC_MODIFY_STRICT were changed to add a
459 new flow, in the case of no match, only if the flow table
460 modification operation did not match on the cookie field.
461 (In OpenFlow 1.0, modify operations always added a new flow
462 when there was no match.)
464 - OFPFC_MODIFY and OFPFC_MODIFY_STRICT no longer updated flow
467 OpenFlow 1.2 made the following changes:
469 - OFPC_MODIFY and OFPFC_MODIFY_STRICT were changed to never
470 add a new flow, regardless of whether the flow cookie was
473 Open vSwitch support for OpenFlow 1.0 implements the OpenFlow 1.0
474 behavior with the following extensions:
476 - An NXM extension field NXM_NX_COOKIE(_W) allows the NXM
477 versions of OFPFC_MODIFY, OFPFC_MODIFY_STRICT, OFPFC_DELETE,
478 and OFPFC_DELETE_STRICT flow_mods, plus flow stats requests
479 and aggregate stats requests, to match on flow cookies with
480 arbitrary masks. This is much like the equivalent OpenFlow
483 - Like OpenFlow 1.1, OFPC_MODIFY and OFPFC_MODIFY_STRICT add a
484 new flow if there is no match and the mask is zero (or not
487 - The "cookie" field in OFPT_FLOW_MOD and NXT_FLOW_MOD messages
488 is used as the cookie value for OFPFC_ADD commands, as
489 described in OpenFlow 1.0. For OFPFC_MODIFY and
490 OFPFC_MODIFY_STRICT commands, the "cookie" field is used as a
491 new cookie for flows that match unless it is UINT64_MAX, in
492 which case the flow's cookie is not updated.
494 - NXT_PACKET_IN (the Nicira extended version of
495 OFPT_PACKET_IN) reports the cookie of the rule that
496 generated the packet, or all-1-bits if no rule generated the
497 packet. (Older versions of OVS used all-0-bits instead of
500 The following table shows the handling of different protocols when
501 receiving OFPFC_MODIFY and OFPFC_MODIFY_STRICT messages. A mask of 0
502 indicates either an explicit mask of zero or an implicit one by not
503 specifying the NXM_NX_COOKIE(_W) field.
505 Match Update Add on miss Add on miss
506 cookie cookie mask!=0 mask==0
507 ====== ====== =========== ===========
508 OpenFlow 1.0 no yes <always add on miss>
509 OpenFlow 1.1 yes no no yes
510 OpenFlow 1.2 yes no no no
513 * Updates the flow's cookie unless the "cookie" field is UINT64_MAX.
516 Multiple Table Support
517 ======================
519 OpenFlow 1.0 has only rudimentary support for multiple flow tables.
520 Notably, OpenFlow 1.0 does not allow the controller to specify the
521 flow table to which a flow is to be added. Open vSwitch adds an
522 extension for this purpose, which is enabled on a per-OpenFlow
523 connection basis using the NXT_FLOW_MOD_TABLE_ID message. When the
524 extension is enabled, the upper 8 bits of the 'command' member in an
525 OFPT_FLOW_MOD or NXT_FLOW_MOD message designates the table to which a
528 The Open vSwitch software switch implementation offers 255 flow
529 tables. On packet ingress, only the first flow table (table 0) is
530 searched, and the contents of the remaining tables are not considered
531 in any way. Tables other than table 0 only come into play when an
532 NXAST_RESUBMIT_TABLE action specifies another table to search.
534 Tables 128 and above are reserved for use by the switch itself.
535 Controllers should use only tables 0 through 127.
541 Open vSwitch supports stateless handling of IPv6 packets. Flows can be
542 written to support matching TCP, UDP, and ICMPv6 headers within an IPv6
543 packet. Deeper matching of some Neighbor Discovery messages is also
546 IPv6 was not designed to interact well with middle-boxes. This,
547 combined with Open vSwitch's stateless nature, have affected the
548 processing of IPv6 traffic, which is detailed below.
553 The base IPv6 header is incredibly simple with the intention of only
554 containing information relevant for routing packets between two
555 endpoints. IPv6 relies heavily on the use of extension headers to
556 provide any other functionality. Unfortunately, the extension headers
557 were designed in such a way that it is impossible to move to the next
558 header (including the layer-4 payload) unless the current header is
561 Open vSwitch will process the following extension headers and continue
564 * Fragment (see the next section)
565 * AH (Authentication Header)
568 * Destination Options
570 When a header is encountered that is not in that list, it is considered
571 "terminal". A terminal header's IPv6 protocol value is stored in
572 "nw_proto" for matching purposes. If a terminal header is TCP, UDP, or
573 ICMPv6, the packet will be further processed in an attempt to extract
579 IPv6 requires that every link in the internet have an MTU of 1280 octets
580 or greater (RFC 2460). As such, a terminal header (as described above in
581 "Extension Headers") in the first fragment should generally be
582 reachable. In this case, the terminal header's IPv6 protocol type is
583 stored in the "nw_proto" field for matching purposes. If a terminal
584 header cannot be found in the first fragment (one with a fragment offset
585 of zero), the "nw_proto" field is set to 0. Subsequent fragments (those
586 with a non-zero fragment offset) have the "nw_proto" field set to the
587 IPv6 protocol type for fragments (44).
592 An IPv6 jumbogram (RFC 2675) is a packet containing a payload longer
593 than 65,535 octets. A jumbogram is only relevant in subnets with a link
594 MTU greater than 65,575 octets, and are not required to be supported on
595 nodes that do not connect to link with such large MTUs. Currently, Open
596 vSwitch doesn't process jumbograms.
605 An OpenFlow switch must establish and maintain a TCP network
606 connection to its controller. There are two basic ways to categorize
607 the network that this connection traverses: either it is completely
608 separate from the one that the switch is otherwise controlling, or its
609 path may overlap the network that the switch controls. We call the
610 former case "out-of-band control", the latter case "in-band control".
612 Out-of-band control has the following benefits:
614 - Simplicity: Out-of-band control slightly simplifies the switch
617 - Reliability: Excessive switch traffic volume cannot interfere
618 with control traffic.
620 - Integrity: Machines not on the control network cannot
621 impersonate a switch or a controller.
623 - Confidentiality: Machines not on the control network cannot
624 snoop on control traffic.
626 In-band control, on the other hand, has the following advantages:
628 - No dedicated port: There is no need to dedicate a physical
629 switch port to control, which is important on switches that have
630 few ports (e.g. wireless routers, low-end embedded platforms).
632 - No dedicated network: There is no need to build and maintain a
633 separate control network. This is important in many
634 environments because it reduces proliferation of switches and
637 Open vSwitch supports both out-of-band and in-band control. This
638 section describes the principles behind in-band control. See the
639 description of the Controller table in ovs-vswitchd.conf.db(5) to
640 configure OVS for in-band control.
645 The fundamental principle of in-band control is that an OpenFlow
646 switch must recognize and switch control traffic without involving the
647 OpenFlow controller. All the details of implementing in-band control
648 are special cases of this principle.
650 The rationale for this principle is simple. If the switch does not
651 handle in-band control traffic itself, then it will be caught in a
652 contradiction: it must contact the controller, but it cannot, because
653 only the controller can set up the flows that are needed to contact
656 The following points describe important special cases of this
659 - In-band control must be implemented regardless of whether the
662 It is tempting to implement the in-band control rules only when
663 the switch is not connected to the controller, using the
664 reasoning that the controller should have complete control once
665 it has established a connection with the switch.
667 This does not work in practice. Consider the case where the
668 switch is connected to the controller. Occasionally it can
669 happen that the controller forgets or otherwise needs to obtain
670 the MAC address of the switch. To do so, the controller sends a
671 broadcast ARP request. A switch that implements the in-band
672 control rules only when it is disconnected will then send an
673 OFPT_PACKET_IN message up to the controller. The controller will
674 be unable to respond, because it does not know the MAC address of
675 the switch. This is a deadlock situation that can only be
676 resolved by the switch noticing that its connection to the
677 controller has hung and reconnecting.
679 - In-band control must override flows set up by the controller.
681 It is reasonable to assume that flows set up by the OpenFlow
682 controller should take precedence over in-band control, on the
683 basis that the controller should be in charge of the switch.
685 Again, this does not work in practice. Reasonable controller
686 implementations may set up a "last resort" fallback rule that
687 wildcards every field and, e.g., sends it up to the controller or
688 discards it. If a controller does that, then it will isolate
689 itself from the switch.
691 - The switch must recognize all control traffic.
693 The fundamental principle of in-band control states, in part,
694 that a switch must recognize control traffic without involving
695 the OpenFlow controller. More specifically, the switch must
696 recognize *all* control traffic. "False negatives", that is,
697 packets that constitute control traffic but that the switch does
698 not recognize as control traffic, lead to control traffic storms.
700 Consider an OpenFlow switch that only recognizes control packets
701 sent to or from that switch. Now suppose that two switches of
702 this type, named A and B, are connected to ports on an Ethernet
703 hub (not a switch) and that an OpenFlow controller is connected
704 to a third hub port. In this setup, control traffic sent by
705 switch A will be seen by switch B, which will send it to the
706 controller as part of an OFPT_PACKET_IN message. Switch A will
707 then see the OFPT_PACKET_IN message's packet, re-encapsulate it
708 in another OFPT_PACKET_IN, and send it to the controller. Switch
709 B will then see that OFPT_PACKET_IN, and so on in an infinite
712 Incidentally, the consequences of "false positives", where
713 packets that are not control traffic are nevertheless recognized
714 as control traffic, are much less severe. The controller will
715 not be able to control their behavior, but the network will
716 remain in working order. False positives do constitute a
719 - The switch should use echo-requests to detect disconnection.
721 TCP will notice that a connection has hung, but this can take a
722 considerable amount of time. For example, with default settings
723 the Linux kernel TCP implementation will retransmit for between
724 13 and 30 minutes, depending on the connection's retransmission
725 timeout, according to kernel documentation. This is far too long
726 for a switch to be disconnected, so an OpenFlow switch should
727 implement its own connection timeout. OpenFlow OFPT_ECHO_REQUEST
728 messages are the best way to do this, since they test the
729 OpenFlow connection itself.
734 This section describes how Open vSwitch implements in-band control.
735 Correctly implementing in-band control has proven difficult due to its
736 many subtleties, and has thus gone through many iterations. Please
737 read through and understand the reasoning behind the chosen rules
738 before making modifications.
740 Open vSwitch implements in-band control as "hidden" flows, that is,
741 flows that are not visible through OpenFlow, and at a higher priority
742 than wildcarded flows can be set up through OpenFlow. This is done so
743 that the OpenFlow controller cannot interfere with them and possibly
744 break connectivity with its switches. It is possible to see all
745 flows, including in-band ones, with the ovs-appctl "bridge/dump-flows"
748 The Open vSwitch implementation of in-band control can hide traffic to
749 arbitrary "remotes", where each remote is one TCP port on one IP address.
750 Currently the remotes are automatically configured as the in-band OpenFlow
751 controllers plus the OVSDB managers, if any. (The latter is a requirement
752 because OVSDB managers are responsible for configuring OpenFlow controllers,
753 so if the manager cannot be reached then OpenFlow cannot be reconfigured.)
755 The following rules (with the OFPP_NORMAL action) are set up on any bridge
756 that has any remotes:
758 (a) DHCP requests sent from the local port.
759 (b) ARP replies to the local port's MAC address.
760 (c) ARP requests from the local port's MAC address.
762 In-band also sets up the following rules for each unique next-hop MAC
763 address for the remotes' IPs (the "next hop" is either the remote
764 itself, if it is on a local subnet, or the gateway to reach the remote):
766 (d) ARP replies to the next hop's MAC address.
767 (e) ARP requests from the next hop's MAC address.
769 In-band also sets up the following rules for each unique remote IP address:
771 (f) ARP replies containing the remote's IP address as a target.
772 (g) ARP requests containing the remote's IP address as a source.
774 In-band also sets up the following rules for each unique remote (IP,port)
777 (h) TCP traffic to the remote's IP and port.
778 (i) TCP traffic from the remote's IP and port.
780 The goal of these rules is to be as narrow as possible to allow a
781 switch to join a network and be able to communicate with the
782 remotes. As mentioned earlier, these rules have higher priority
783 than the controller's rules, so if they are too broad, they may
784 prevent the controller from implementing its policy. As such,
785 in-band actively monitors some aspects of flow and packet processing
786 so that the rules can be made more precise.
788 In-band control monitors attempts to add flows into the datapath that
789 could interfere with its duties. The datapath only allows exact
790 match entries, so in-band control is able to be very precise about
791 the flows it prevents. Flows that miss in the datapath are sent to
792 userspace to be processed, so preventing these flows from being
793 cached in the "fast path" does not affect correctness. The only type
794 of flow that is currently prevented is one that would prevent DHCP
795 replies from being seen by the local port. For example, a rule that
796 forwarded all DHCP traffic to the controller would not be allowed,
797 but one that forwarded to all ports (including the local port) would.
799 As mentioned earlier, packets that miss in the datapath are sent to
800 the userspace for processing. The userspace has its own flow table,
801 the "classifier", so in-band checks whether any special processing
802 is needed before the classifier is consulted. If a packet is a DHCP
803 response to a request from the local port, the packet is forwarded to
804 the local port, regardless of the flow table. Note that this requires
805 L7 processing of DHCP replies to determine whether the 'chaddr' field
806 matches the MAC address of the local port.
808 It is interesting to note that for an L3-based in-band control
809 mechanism, the majority of rules are devoted to ARP traffic. At first
810 glance, some of these rules appear redundant. However, each serves an
811 important role. First, in order to determine the MAC address of the
812 remote side (controller or gateway) for other ARP rules, we must allow
813 ARP traffic for our local port with rules (b) and (c). If we are
814 between a switch and its connection to the remote, we have to
815 allow the other switch's ARP traffic to through. This is done with
816 rules (d) and (e), since we do not know the addresses of the other
817 switches a priori, but do know the remote's or gateway's. Finally,
818 if the remote is running in a local guest VM that is not reached
819 through the local port, the switch that is connected to the VM must
820 allow ARP traffic based on the remote's IP address, since it will
821 not know the MAC address of the local port that is sending the traffic
822 or the MAC address of the remote in the guest VM.
824 With a few notable exceptions below, in-band should work in most
825 network setups. The following are considered "supported' in the
826 current implementation:
828 - Locally Connected. The switch and remote are on the same
829 subnet. This uses rules (a), (b), (c), (h), and (i).
831 - Reached through Gateway. The switch and remote are on
832 different subnets and must go through a gateway. This uses
833 rules (a), (b), (c), (h), and (i).
835 - Between Switch and Remote. This switch is between another
836 switch and the remote, and we want to allow the other
837 switch's traffic through. This uses rules (d), (e), (h), and
838 (i). It uses (b) and (c) indirectly in order to know the MAC
839 address for rules (d) and (e). Note that DHCP for the other
840 switch will not work unless an OpenFlow controller explicitly lets this
841 switch pass the traffic.
843 - Between Switch and Gateway. This switch is between another
844 switch and the gateway, and we want to allow the other switch's
845 traffic through. This uses the same rules and logic as the
846 "Between Switch and Remote" configuration described earlier.
848 - Remote on Local VM. The remote is a guest VM on the
849 system running in-band control. This uses rules (a), (b), (c),
852 - Remote on Local VM with Different Networks. The remote
853 is a guest VM on the system running in-band control, but the
854 local port is not used to connect to the remote. For
855 example, an IP address is configured on eth0 of the switch. The
856 remote's VM is connected through eth1 of the switch, but an
857 IP address has not been configured for that port on the switch.
858 As such, the switch will use eth0 to connect to the remote,
859 and eth1's rules about the local port will not work. In the
860 example, the switch attached to eth0 would use rules (a), (b),
861 (c), (h), and (i) on eth0. The switch attached to eth1 would use
862 rules (f), (g), (h), and (i).
864 The following are explicitly *not* supported by in-band control:
866 - Specify Remote by Name. Currently, the remote must be
867 identified by IP address. A naive approach would be to permit
868 all DNS traffic. Unfortunately, this would prevent the
869 controller from defining any policy over DNS. Since switches
870 that are located behind us need to connect to the remote,
871 in-band cannot simply add a rule that allows DNS traffic from
872 the local port. The "correct" way to support this is to parse
873 DNS requests to allow all traffic related to a request for the
874 remote's name through. Due to the potential security
875 problems and amount of processing, we decided to hold off for
878 - Differing Remotes for Switches. All switches must know
879 the L3 addresses for all the remotes that other switches
880 may use, since rules need to be set up to allow traffic related
881 to those remotes through. See rules (f), (g), (h), and (i).
883 - Differing Routes for Switches. In order for the switch to
884 allow other switches to connect to a remote through a
885 gateway, it allows the gateway's traffic through with rules (d)
886 and (e). If the routes to the remote differ for the two
887 switches, we will not know the MAC address of the alternate
894 It seems likely that many controllers, at least at startup, use the
895 OpenFlow "flow statistics" request to obtain existing flows, then
896 compare the flows' actions against the actions that they expect to
897 find. Before version 1.8.0, Open vSwitch always returned exact,
898 byte-for-byte copies of the actions that had been added to the flow
899 table. The current version of Open vSwitch does not always do this in
900 some exceptional cases. This section lists the exceptions that
901 controller authors must keep in mind if they compare actual actions
902 against desired actions in a bytewise fashion:
904 - Open vSwitch zeros padding bytes in action structures,
905 regardless of their values when the flows were added.
907 - Open vSwitch "normalizes" the instructions in OpenFlow 1.1
908 (and later) in the following way:
910 * OVS sorts the instructions into the following order:
911 Apply-Actions, Clear-Actions, Write-Actions,
912 Write-Metadata, Goto-Table.
914 * OVS drops Apply-Actions instructions that have empty
917 * OVS drops Write-Actions instructions that have empty
920 Please report other discrepancies, if you notice any, so that we can
921 fix or document them.
927 Suggestions to improve Open vSwitch are welcome at discuss@openvswitch.org.