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          Design Decisions In Open vSwitch

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

          Asynchronous Messages

          Over time, Open vSwitch has added many knobs that control whether a given controller receives OpenFlow asynchronous messages. This section describes how all of these features interact.

          First, a service controller never receives any asynchronous messages unless it changes its misssendlen from the service controller default of zero in one of the following ways:

          Second, OFPTFLOWREMOVED and NXTFLOWREMOVED messages are generated only if the flow that was removed had the OFPFFSENDFLOW_REM flag set.

          Third, OFPTPACKETIN and NXTPACKETIN messages are sent only to OpenFlow controller connections that have the correct connection ID (see "struct nxcontrollerid" and "struct nxactioncontroller"):

          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 ---------------------------------------- ------- ----- OFPTPACKETIN / NXTPACKETIN OFPRNOMATCH yes --- OFPRACTION yes --- OFPRINVALIDTTL --- --- OFPRACTIONSET (OF1.4+) yes --- OFPRGROUP (OF1.4+) yes ---

          OFPTFLOWREMOVED / NXTFLOWREMOVED OFPRRIDLETIMEOUT yes --- OFPRRHARDTIMEOUT yes --- OFPRRDELETE yes --- OFPRRGROUPDELETE (OF1.4+) yes --- OFPRRMETERDELETE (OF1.4+) yes --- OFPRREVICTION (OF1.4+) yes ---

          OFPTPORTSTATUS OFPPRADD yes yes OFPPRDELETE yes yes OFPPR_MODIFY yes yes

          OFPTROLEREQUEST / OFPTROLEREPLY (OF1.4+) OFPCRRMASTERREQUEST --- --- OFPCRRCONFIG --- --- OFPCRREXPERIMENTER --- ---

          OFPTTABLESTATUS (OF1.4+) OFPTRVACANCYDOWN --- --- OFPTRVACANCYUP --- ---

          OFPTREQUESTFORWARD (OF1.4+) OFPRFRGROUPMOD --- --- OFPRFRMETER_MOD --- --- ```

          The NXTSETASYNCCONFIG message directly sets all of the values in this table for the current connection. The OFPCINVALIDTTLTOCONTROLLER bit in the OFPTSETCONFIG message controls the setting for OFPRINVALID_TTL for the "master" role.

          OFPAT_ENQUEUE

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

          OFPTFLOWMOD

          The OpenFlow specification for the behavior of OFPTFLOWMOD is confusing. The following tables summarize the Open vSwitch implementation of its behavior in the following categories:

          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 outport --- --- --- yes yes match on flowcookie --- --- --- --- --- match on tableid --- --- --- --- --- controller chooses tableid --- --- --- updates flowcookie yes yes yes updates OFPFFSENDFLOWREM yes + + honors OFPFFCHECKOVERLAP yes + + updates idletimeout yes + + updates hardtimeout 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 "deletestrict" generates a flowremoved message if the deleted flow or flows have the OFPFFSENDFLOW_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:

          ``` MODIFY DELETE ADD MODIFY STRICT DELETE STRICT === ====== ====== ====== ====== match on priority yes --- yes --- yes match on outport --- --- --- yes yes match on flowcookie --- yes yes yes yes match on tableid yes yes yes yes yes controller chooses tableid yes yes yes updates flowcookie yes --- --- updates OFPFFSENDFLOWREM yes + + honors OFPFFCHECKOVERLAP yes + + updates idletimeout yes + + updates hardtimeout 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 "deletestrict" generates a flowremoved message if the deleted flow or flows have the OFPFFSENDFLOW_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:

          ``` MODIFY DELETE ADD MODIFY STRICT DELETE STRICT === ====== ====== ====== ====== match on priority yes --- yes --- yes match on outport --- --- --- yes yes match on flowcookie --- yes yes yes yes match on tableid yes yes yes yes yes controller chooses tableid yes yes yes updates flowcookie yes --- --- updates OFPFFSENDFLOWREM yes --- --- honors OFPFFCHECKOVERLAP yes --- --- updates idletimeout yes --- --- updates hardtimeout 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 "deletestrict" generates a flowremoved message if the deleted flow or flows have the OFPFFSENDFLOW_REM flag set. (Each controller can separately control whether it wants to receive the generated messages.)

          (&) "modify" and "modify-strict" reset counters if the OFPFFRESETCOUNTS flag is specified. ```

          OpenFlow 1.3

          OpenFlow 1.3 makes these changes:

          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:

          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 flowmod. For example, if a controller removes all flows with a single OpenFlow "flowmod", 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 flowmods. Bundles are supported for flowmod and portmod messages only. For flowmods, 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', 'modifystrict', 'delete', or 'deletestrict' 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 flowmod, and in case of an error the earlier successful flowmods are not rolled back.

          OFPTPACKETIN

          The OpenFlow 1.1 specification for OFPTPACKETIN 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 ofppacketin) is 20 in OF1.0 and offsetof(struct ofppacketin, 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 ofppacketin) and offsetof(struct ofppacketin, data) are both 24 in OF1.1.

          That leaves the question of how to implement ofppacketin 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.

          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:

          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:

          OpenFlow 1.1 made the following changes:

          OpenFlow 1.2 made the following changes:

          Open vSwitch support for OpenFlow 1.0 implements the OpenFlow 1.0 behavior with the following extensions:

          The following table shows the handling of different protocols when receiving OFPFCMODIFY and OFPFCMODIFYSTRICT messages. A mask of 0 indicates either an explicit mask of zero or an implicit one by not specifying the NXMNXCOOKIE(W) field.

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

          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 NXTFLOWMODTABLEID message. When the extension is enabled, the upper 8 bits of the 'command' member in an OFPTFLOWMOD or NXTFLOWMOD 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 NXASTRESUBMITTABLE 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 OFPTCMISS* 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 OFPTTABLEMOD message to set OFPTCMISS* for a flow table and added the 'config' field to the OFPSTTABLE 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 OFPTCMISS* without adding any more OFPTC* constants. This meant that OFPTTABLEMOD 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 OFPMPTABLEFEATURES that included a field 'config' documented as reporting the OFPTC* values set with OFPTTABLEMOD; of course this served no real purpose because no OFPTC* values are defined. OF1.3 did remove the OFPTC* field from OFPMPTABLE (previously named OFPSTTABLE).

          OpenFlow 1.4 defined two new OFPTC* constants, OFPTCEVICTION and OFPTCVACANCYEVENTS, using bits that did not overlap with OFPTCMISS* even though those bits had not been defined since OF1.2. OFPTTABLEMOD still controlled these settings. The field for OFPTC* values in OFPMPTABLEFEATURES was renamed from 'config' to 'capabilities' and documented as reporting the flags that are supported in a OFPTTABLEMOD message. The OFPMPTABLEDESC 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 OFPTC* flags TABLEMOD stats? TABLEFEATURES TABLEDESC


          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/VACANCYEVENTS 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:

          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 "nwproto" field for matching purposes. If a terminal header cannot be found in the first fragment (one with a fragment offset of zero), the "nwproto" 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:

          In-band control, on the other hand, has the following advantages:

          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.

          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:

          The following are explicitly not supported by in-band control:

          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:

          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.

          现金李逵劈鱼