ovn-northd(8)                     OVN Manual                     ovn-northd(8)



NAME
       ovn-northd  and ovn-northd-ddlog - Open Virtual Network central control
       daemon

SYNOPSIS
       ovn-northd [options]

DESCRIPTION
       ovn-northd is a centralized  daemon  responsible  for  translating  the
       high-level  OVN  configuration into logical configuration consumable by
       daemons such as ovn-controller. It translates the logical network  con‐
       figuration  in  terms  of conventional network concepts, taken from the
       OVN Northbound Database (see ovn-nb(5)), into logical datapath flows in
       the OVN Southbound Database (see ovn-sb(5)) below it.

       ovn-northd is implemented in C. ovn-northd-ddlog is a compatible imple‐
       mentation written in DDlog, a language for  incremental  database  pro‐
       cessing.  This documentation applies to both implementations, with dif‐
       ferences indicated where relevant.

OPTIONS
       --ovnnb-db=database
              The OVSDB database containing the OVN  Northbound  Database.  If
              the  OVN_NB_DB environment variable is set, its value is used as
              the default. Otherwise, the default is unix:/ovnnb_db.sock.

       --ovnsb-db=database
              The OVSDB database containing the OVN  Southbound  Database.  If
              the  OVN_SB_DB environment variable is set, its value is used as
              the default. Otherwise, the default is unix:/ovnsb_db.sock.

       --ddlog-record=file
              This option is for ovn-north-ddlog only. It causes the daemon to
              record  the  initial database state and later changes to file in
              the text-based DDlog command format. The ovn_northd_cli  program
              can  later  replay  these  changes  for debugging purposes. This
              option has a performance impact. See debugging-ddlog.rst in  the
              OVN documentation for more details.

       --dry-run
              Causes   ovn-northd  to  start  paused.  In  the  paused  state,
              ovn-northd does not apply any changes to the databases, although
              it  continues  to  monitor  them.  For more information, see the
              pause command, under Runtime Management Commands below.

              For  ovn-northd-ddlog,  one   could   use   this   option   with
              --ddlog-record  to  generate  a  replay log without restarting a
              process or disturbing a running system.

       database in the above options must be an OVSDB active or  passive  con‐
       nection method, as described in ovsdb(7).

   Daemon Options
       --pidfile[=pidfile]
              Causes a file (by default, program.pid) to be created indicating
              the PID of the running process. If the pidfile argument  is  not
              specified, or if it does not begin with /, then it is created in
              .

              If --pidfile is not specified, no pidfile is created.

       --overwrite-pidfile
              By default, when --pidfile is specified and the  specified  pid‐
              file already exists and is locked by a running process, the dae‐
              mon refuses to start. Specify --overwrite-pidfile to cause it to
              instead overwrite the pidfile.

              When --pidfile is not specified, this option has no effect.

       --detach
              Runs  this  program  as a background process. The process forks,
              and in the child it starts a new session,  closes  the  standard
              file descriptors (which has the side effect of disabling logging
              to the console), and changes its current directory to  the  root
              (unless  --no-chdir is specified). After the child completes its
              initialization, the parent exits.

       --monitor
              Creates an additional process to monitor  this  program.  If  it
              dies  due  to a signal that indicates a programming error (SIGA
              BRT, SIGALRM, SIGBUS, SIGFPE, SIGILL, SIGPIPE, SIGSEGV, SIGXCPU,
              or SIGXFSZ) then the monitor process starts a new copy of it. If
              the daemon dies or exits for another reason, the monitor process
              exits.

              This  option  is  normally used with --detach, but it also func‐
              tions without it.

       --no-chdir
              By default, when --detach is specified, the daemon  changes  its
              current  working  directory  to  the  root  directory  after  it
              detaches. Otherwise, invoking the daemon from a carelessly  cho‐
              sen  directory  would  prevent the administrator from unmounting
              the file system that holds that directory.

              Specifying --no-chdir suppresses this behavior,  preventing  the
              daemon  from changing its current working directory. This may be
              useful for collecting core files, since it is common behavior to
              write core dumps into the current working directory and the root
              directory is not a good directory to use.

              This option has no effect when --detach is not specified.

       --no-self-confinement
              By default this daemon will try to self-confine itself  to  work
              with  files  under  well-known  directories  determined at build
              time. It is better to stick with this default behavior  and  not
              to  use  this  flag  unless some other Access Control is used to
              confine daemon. Note that in contrast to  other  access  control
              implementations  that  are  typically enforced from kernel-space
              (e.g. DAC or MAC), self-confinement is imposed  from  the  user-
              space daemon itself and hence should not be considered as a full
              confinement strategy, but instead should be viewed as  an  addi‐
              tional layer of security.

       --user=user:group
              Causes  this  program  to  run  as a different user specified in
              user:group, thus dropping most of  the  root  privileges.  Short
              forms  user  and  :group  are also allowed, with current user or
              group assumed, respectively. Only daemons started  by  the  root
              user accepts this argument.

              On   Linux,   daemons   will   be   granted   CAP_IPC_LOCK   and
              CAP_NET_BIND_SERVICES before dropping root  privileges.  Daemons
              that  interact  with  a  datapath, such as ovs-vswitchd, will be
              granted three  additional  capabilities,  namely  CAP_NET_ADMIN,
              CAP_NET_BROADCAST  and  CAP_NET_RAW.  The capability change will
              apply even if the new user is root.

              On Windows, this option is not currently supported. For security
              reasons,  specifying  this  option will cause the daemon process
              not to start.

   Logging Options
       -v[spec]
       --verbose=[spec]
            Sets logging levels. Without any spec,  sets  the  log  level  for
            every  module and destination to dbg. Otherwise, spec is a list of
            words separated by spaces or commas or colons, up to one from each
            category below:

            ·      A  valid module name, as displayed by the vlog/list command
                   on ovs-appctl(8), limits the log level change to the speci‐
                   fied module.

            ·      syslog,  console, or file, to limit the log level change to
                   only to the system log, to  the  console,  or  to  a  file,
                   respectively.  (If --detach is specified, the daemon closes
                   its standard file descriptors, so logging  to  the  console
                   will have no effect.)

                   On  Windows  platform,  syslog is accepted as a word and is
                   only useful along with the --syslog-target option (the word
                   has no effect otherwise).

            ·      off,  emer,  err,  warn,  info,  or dbg, to control the log
                   level. Messages of the given severity  or  higher  will  be
                   logged,  and  messages  of  lower severity will be filtered
                   out. off filters out all messages. See ovs-appctl(8) for  a
                   definition of each log level.

            Case is not significant within spec.

            Regardless  of the log levels set for file, logging to a file will
            not take place unless --log-file is also specified (see below).

            For compatibility with older versions of OVS, any is accepted as a
            word but has no effect.

       -v
       --verbose
            Sets  the  maximum  logging  verbosity level, equivalent to --ver
            bose=dbg.

       -vPATTERN:destination:pattern
       --verbose=PATTERN:destination:pattern
            Sets  the  log  pattern  for  destination  to  pattern.  Refer  to
            ovs-appctl(8) for a description of the valid syntax for pattern.

       -vFACILITY:facility
       --verbose=FACILITY:facility
            Sets  the RFC5424 facility of the log message. facility can be one
            of kern, user, mail, daemon, auth, syslog, lpr, news, uucp, clock,
            ftp,  ntp,  audit,  alert, clock2, local0, local1, local2, local3,
            local4, local5, local6 or local7. If this option is not specified,
            daemon  is  used  as  the  default for the local system syslog and
            local0 is used while sending a message to the target provided  via
            the --syslog-target option.

       --log-file[=file]
            Enables  logging  to a file. If file is specified, then it is used
            as the exact name for the log file. The default log file name used
            if file is omitted is /usr/local/var/log/ovn/program.log.

       --syslog-target=host:port
            Send  syslog messages to UDP port on host, in addition to the sys‐
            tem syslog. The host must be a numerical IP address, not  a  host‐
            name.

       --syslog-method=method
            Specify  method  as  how  syslog messages should be sent to syslog
            daemon. The following forms are supported:

            ·      libc, to use the libc syslog() function. Downside of  using
                   this  options  is that libc adds fixed prefix to every mes‐
                   sage before it is actually sent to the syslog  daemon  over
                   /dev/log UNIX domain socket.

            ·      unix:file, to use a UNIX domain socket directly. It is pos‐
                   sible to specify arbitrary message format with this option.
                   However,  rsyslogd  8.9  and  older versions use hard coded
                   parser function anyway that limits UNIX domain socket  use.
                   If  you  want  to  use  arbitrary message format with older
                   rsyslogd versions, then use  UDP  socket  to  localhost  IP
                   address instead.

            ·      udp:ip:port,  to  use  a UDP socket. With this method it is
                   possible to use arbitrary message format  also  with  older
                   rsyslogd.  When  sending  syslog  messages  over UDP socket
                   extra precaution needs to be taken into account, for  exam‐
                   ple,  syslog daemon needs to be configured to listen on the
                   specified UDP port,  accidental  iptables  rules  could  be
                   interfering  with  local  syslog traffic and there are some
                   security considerations that apply to UDP sockets,  but  do
                   not apply to UNIX domain sockets.

            ·      null, to discard all messages logged to syslog.

            The  default is taken from the OVS_SYSLOG_METHOD environment vari‐
            able; if it is unset, the default is libc.

   PKI Options
       PKI configuration is required in order to use SSL for  the  connections
       to the Northbound and Southbound databases.

              -p privkey.pem
              --private-key=privkey.pem
                   Specifies  a  PEM  file  containing the private key used as
                   identity for outgoing SSL connections.

              -c cert.pem
              --certificate=cert.pem
                   Specifies a PEM file containing a certificate  that  certi‐
                   fies the private key specified on -p or --private-key to be
                   trustworthy. The certificate must be signed by the certifi‐
                   cate  authority  (CA) that the peer in SSL connections will
                   use to verify it.

              -C cacert.pem
              --ca-cert=cacert.pem
                   Specifies a PEM file containing the CA certificate for ver‐
                   ifying certificates presented to this program by SSL peers.
                   (This may be the same certificate that  SSL  peers  use  to
                   verify the certificate specified on -c or --certificate, or
                   it may be a different one, depending on the PKI  design  in
                   use.)

              -C none
              --ca-cert=none
                   Disables  verification  of  certificates  presented  by SSL
                   peers. This introduces a security risk,  because  it  means
                   that  certificates  cannot be verified to be those of known
                   trusted hosts.

   Other Options
       --unixctl=socket
              Sets the name of the control socket on which program listens for
              runtime  management  commands  (see RUNTIME MANAGEMENT COMMANDS,
              below). If socket does not begin with /, it  is  interpreted  as
              relative  to  .  If  --unixctl  is  not used at all, the default
              socket is /program.pid.ctl, where pid is program’s process ID.

              On Windows a local named pipe is used to listen for runtime man‐
              agement  commands.  A  file  is  created in the absolute path as
              pointed by socket or if --unixctl is not used at all, a file  is
              created  as  program in the configured OVS_RUNDIR directory. The
              file exists just to mimic the behavior of a Unix domain socket.

              Specifying none for socket disables the control socket feature.



       -h
       --help
            Prints a brief help message to the console.

       -V
       --version
            Prints version information to the console.

RUNTIME MANAGEMENT COMMANDS
       ovs-appctl can send commands to a running ovn-northd process. The  cur‐
       rently supported commands are described below.

              exit   Causes ovn-northd to gracefully terminate.

              pause  Pauses ovn-northd. When it is paused, ovn-northd receives
                     changes  from  the  Northbound  and  Southbound  database
                     changes  as  usual,  but  it does not send any updates. A
                     paused ovn-northd also drops database locks, which allows
                     any other non-paused instance of ovn-northd to take over.

              resume Resumes  the  ovn-northd  operation to process Northbound
                     and Southbound database  contents  and  generate  logical
                     flows.  This  will also instruct ovn-northd to aspire for
                     the lock on SB DB.

              is-paused
                     Returns "true" if ovn-northd is currently paused, "false"
                     otherwise.

              status Prints  this  server’s status. Status will be "active" if
                     ovn-northd has acquired OVSDB lock on SB DB, "standby" if
                     it has not or "paused" if this instance is paused.

              sb-cluster-state-reset
                     Reset  southbound  database cluster status when databases
                     are destroyed and rebuilt.

                     If all databases in a clustered southbound  database  are
                     removed from disk, then the stored index of all databases
                     will be reset to zero. This will cause ovn-northd  to  be
                     unable  to  read  or  write  to  the southbound database,
                     because it will always detect the data as stale. In  such
                     a  case,  run  this command so that ovn-northd will reset
                     its local index so that it can interact with  the  south‐
                     bound database again.

              nb-cluster-state-reset
                     Reset  northbound  database cluster status when databases
                     are destroyed and rebuilt.

                     This performs the  same  task  as  sb-cluster-state-reset
                     except for the northbound database client.

       Only ovn-northd-ddlog supports the following commands:

              enable-cpu-profiling
              disable-cpu-profiling
                   Enables or disables profiling of CPU time used by the DDlog
                   engine. When CPU profiling is enabled, the profile  command
                   (see  below) will include DDlog CPU usage statistics in its
                   output. Enabling CPU profiling will slow  ovn-northd-ddlog.
                   Disabling  CPU  profiling  does  not  clear  any previously
                   recorded statistics.

              profile
                   Outputs a profile of the current and peak sizes of arrange‐
                   ments  inside  DDlog. This profiling data can be useful for
                   optimizing DDlog code.  If  CPU  profiling  was  previously
                   enabled  (even  if  it was later disabled), the output also
                   includes a CPU time profile. See Profiling inside the tuto‐
                   rial in the DDlog repository for an introduction to profil‐
                   ing DDlog.

ACTIVE-STANDBY FOR HIGH AVAILABILITY
       You may run ovn-northd more than once in an OVN deployment.  When  con‐
       nected  to  a  standalone or clustered DB setup, OVN will automatically
       ensure that only one of them is active at a time. If multiple instances
       of  ovn-northd  are running and the active ovn-northd fails, one of the
       hot standby instances of ovn-northd will automatically take over.

   Active-Standby with multiple OVN DB servers
       You may run multiple OVN DB servers in an OVN deployment with:

              ·      OVN DB servers deployed in active/passive mode  with  one
                     active and multiple passive ovsdb-servers.

              ·      ovn-northd  also  deployed on all these nodes, using unix
                     ctl sockets to connect to the local OVN DB servers.

       In such deployments, the ovn-northds on the passive nodes will  process
       the  DB  changes  and  compute  logical  flows  to be thrown out later,
       because write transactions  are  not  allowed  by  the  passive  ovsdb-
       servers. It results in unnecessary CPU usage.

       With  the  help  of  runtime  management  command  pause, you can pause
       ovn-northd on these nodes. When a passive node becomes master, you  can
       use  the  runtime management command resume to resume the ovn-northd to
       process the DB changes.

LOGICAL FLOW TABLE STRUCTURE
       One of the main purposes of ovn-northd is to populate the  Logical_Flow
       table  in  the  OVN_Southbound  database.  This  section  describes how
       ovn-northd does this for switch and router logical datapaths.

   Logical Switch Datapaths
     Ingress Table 0: Admission Control and Ingress Port Security - L2

       Ingress table 0 contains these logical flows:

              ·      Priority 100 flows to drop packets with VLAN tags or mul‐
                     ticast Ethernet source addresses.

              ·      Priority  50  flows  that implement ingress port security
                     for each enabled logical port. For logical ports on which
                     port  security is enabled, these match the inport and the
                     valid eth.src address(es) and advance only those  packets
                     to  the  next flow table. For logical ports on which port
                     security is not enabled, these advance all  packets  that
                     match the inport.

              ·      For  logical  ports  of type vtep, the above logical flow
                     will also apply the action REGBIT_FROM_RAMP = 1; to indi‐
                     cate  that  the packet is coming from a RAMP (controller-
                     vtep) device. Later pipelines will use  this  information
                     to skip sending the packet to the conntrack. Packets from
                     vtep logical ports should go though ingress pipeline only
                     to  determine the output port and they should not be sub‐
                     jected to any ACL checks. Egress pipeline will do the ACL
                     checks.

       There  are no flows for disabled logical ports because the default-drop
       behavior of logical flow tables causes packets that ingress  from  them
       to be dropped.

     Ingress Table 1: Ingress Port Security - IP

       Ingress table 1 contains these logical flows:

              ·      For  each  element in the port security set having one or
                     more IPv4 or IPv6 addresses (or both),

                     ·      Priority 90 flow to allow IPv4 traffic if  it  has
                            IPv4  addresses  which  match  the  inport,  valid
                            eth.src and valid ip4.src address(es).

                     ·      Priority 90 flow  to  allow  IPv4  DHCP  discovery
                            traffic  if it has a valid eth.src. This is neces‐
                            sary since DHCP discovery messages are  sent  from
                            the  unspecified  IPv4 address (0.0.0.0) since the
                            IPv4 address has not yet been assigned.

                     ·      Priority 90 flow to allow IPv6 traffic if  it  has
                            IPv6  addresses  which  match  the  inport,  valid
                            eth.src and valid ip6.src address(es).

                     ·      Priority 90 flow  to  allow  IPv6  DAD  (Duplicate
                            Address  Detection)  traffic  if  it  has  a valid
                            eth.src. This is is necessary  since  DAD  include
                            requires  joining  an  multicast group and sending
                            neighbor  solicitations  for  the  newly  assigned
                            address.  Since  no address is yet assigned, these
                            are sent from the unspecified IPv6 address (::).

                     ·      Priority 80 flow to drop IP (both IPv4  and  IPv6)
                            traffic which match the inport and valid eth.src.

              ·      One priority-0 fallback flow that matches all packets and
                     advances to the next table.

     Ingress Table 2: Ingress Port Security - Neighbor discovery

       Ingress table 2 contains these logical flows:

              ·      For each element in the port security set,

                     ·      Priority 90 flow to allow ARP traffic which  match
                            the  inport  and valid eth.src and arp.sha. If the
                            element has one or more IPv4  addresses,  then  it
                            also matches the valid arp.spa.

                     ·      Priority  90 flow to allow IPv6 Neighbor Solicita‐
                            tion and Advertisement  traffic  which  match  the
                            inport,  valid  eth.src  and nd.sll/nd.tll. If the
                            element has one or more IPv6  addresses,  then  it
                            also  matches  the valid nd.target address(es) for
                            Neighbor Advertisement traffic.

                     ·      Priority 80 flow to drop  ARP  and  IPv6  Neighbor
                            Solicitation and Advertisement traffic which match
                            the inport and valid eth.src.

              ·      One priority-0 fallback flow that matches all packets and
                     advances to the next table.

     Ingress Table 3: Lookup MAC address learning table

       This  table looks up the MAC learning table of the logical switch data‐
       path to check if the port-mac pair is present or  not.  MAC  is  learnt
       only  for  logical switch VIF ports whose port security is disabled and
       ’unknown’ address set.

              ·      For each such logical port p whose port security is  dis‐
                     abled and ’unknown’ address set following flow is added.

                     ·      Priority  100  flow with the match inport == p and
                            action  reg0[11]  =  lookup_fdb(inport,  eth.src);
                            next;

              ·      One priority-0 fallback flow that matches all packets and
                     advances to the next table.

     Ingress Table 4: Learn MAC of unknown ports.

       This table learns the MAC addresses seen on  the  logical  ports  whose
       port  security  is disabled and ’unknown’ address set if the lookup_fdb
       action returned false in the previous table.

              ·      For each such logical port p whose port security is  dis‐
                     abled and ’unknown’ address set following flow is added.

                     ·      Priority  100  flow  with the match inport == p &&&&
                            reg0[11] == 0 and action put_fdb(inport, eth.src);
                            next;  which stores the port-mac in the mac learn‐
                            ing table  of  the  logical  switch  datapath  and
                            advances the packet to the next table.

              ·      One priority-0 fallback flow that matches all packets and
                     advances to the next table.

     Ingress Table 5: from-lport Pre-ACLs

       This table prepares flows  for  possible  stateful  ACL  processing  in
       ingress  table  ACLs.  It  contains a priority-0 flow that simply moves
       traffic to the next table. If stateful ACLs are  used  in  the  logical
       datapath, a priority-100 flow is added that sets a hint (with reg0[0] =
       1; next;) for table Pre-stateful to send IP packets to  the  connection
       tracker  before  eventually advancing to ingress table ACLs. If special
       ports such as route ports or localnet ports can’t use  ct(),  a  prior‐
       ity-110 flow is added to skip over stateful ACLs. IPv6 Neighbor Discov‐
       ery and MLD traffic also skips  stateful  ACLs.  For  "allow-stateless"
       ACLs, a flow is added to bypass setting the hint for connection tracker
       processing.

       This table has a priority-110 flow with the match REGBIT_FROM_RAMP == 1
       for all logical switch datapaths to resubmit traffic to the next table.
       REGBIT_FROM_RAMP indicates that packet was received from  vtep  logical
       ports  and  it  can  be skipped from the stateful ACL processing in the
       ingress pipeline.

       This table also has a priority-110 flow with the match eth.dst == E for
       all logical switch datapaths to move traffic to the next table. Where E
       is the service monitor mac defined in the options:svc_monitor_mac colum
       of NB_Global table.

     Ingress Table 6: Pre-LB

       This table prepares flows for possible stateful load balancing process‐
       ing in ingress table LB and Stateful. It  contains  a  priority-0  flow
       that  simply  moves  traffic  to the next table. Moreover it contains a
       priority-110 flow to move IPv6 Neighbor Discovery and  MLD  traffic  to
       the  next table. If load balancing rules with virtual IP addresses (and
       ports) are configured in OVN_Northbound database for a  logical  switch
       datapath, a priority-100 flow is added with the match ip to match on IP
       packets and sets the action reg0[2] = 1; next; to act as a hint for ta‐
       ble  Pre-stateful  to  send  IP  packets  to the connection tracker for
       packet de-fragmentation (and to possibly do  DNAT  for  already  estab‐
       lished  load  balanced  traffic) before eventually advancing to ingress
       table Stateful. If controller_event has been enabled and load balancing
       rules with empty backends have been added in OVN_Northbound, a 130 flow
       is added to trigger ovn-controller events whenever the chassis receives
       a  packet  for  that particular VIP. If event-elb meter has been previ‐
       ously created, it will be associated to the empty_lb logical flow

       Prior to OVN 20.09 we were setting the reg0[0] = 1 only if the IP  des‐
       tination  matches  the  load  balancer VIP. However this had few issues
       cases where a logical switch doesn’t have any ACLs  with  allow-related
       action.  To  understand  the  issue  lets  a take a TCP load balancer -
       10.0.0.10:80=10.0.0.3:80. If a logical port - p1  with  IP  -  10.0.0.5
       opens a TCP connection with the VIP - 10.0.0.10, then the packet in the
       ingress pipeline of ’p1’ is sent to the p1’s conntrack zone id and  the
       packet is load balanced to the backend - 10.0.0.3. For the reply packet
       from the backend lport, it is not sent  to  the  conntrack  of  backend
       lport’s  zone  id. This is fine as long as the packet is valid. Suppose
       the backend lport sends an invalid TCP packet (like incorrect  sequence
       number),  the packet gets delivered to the lport ’p1’ without unDNATing
       the packet to the VIP - 10.0.0.10. And this causes the connection to be
       reset by the lport p1’s VIF.

       We can’t fix this issue by adding a logical flow to drop ct.inv packets
       in the egress pipeline since it will drop  all  other  connections  not
       destined  to  the  load  balancers.  To fix this issue, we send all the
       packets to the conntrack in the ingress pipeline if a load balancer  is
       configured. We can now add a lflow to drop ct.inv packets.

       This table has a priority-110 flow with the match REGBIT_FROM_RAMP == 1
       for all logical switch datapaths to resubmit traffic to the next table.
       REGBIT_FROM_RAMP  indicates  that packet was received from vtep logical
       ports and it can be skipped from the load balancer  processing  in  the
       ingress pipeline.

       This table also has a priority-110 flow with the match eth.dst == E for
       all logical switch datapaths to move traffic to the next table. Where E
       is the service monitor mac defined in the options:svc_monitor_mac colum
       of NB_Global table.

       This table also has a priority-110 flow with the match inport == I  for
       all logical switch datapaths to move traffic to the next table. Where I
       is the peer of a logical router port. This flow is added  to  skip  the
       connection tracking of packets which enter from logical router datapath
       to logical switch datapath.

     Ingress Table 7: Pre-stateful

       This table prepares flows for all possible stateful processing in  next
       tables.  It contains a priority-0 flow that simply moves traffic to the
       next table.

              ·      Priority-120 flows that send the  packets  to  connection
                     tracker  using  ct_lb;  as the action so that the already
                     established traffic destined to  the  load  balancer  VIP
                     gets  DNATted  based  on  a hint provided by the previous
                     tables (with a match for reg0[2] == 1  and  on  supported
                     load  balancer  protocols and address families). For IPv4
                     traffic the flows also load the original  destination  IP
                     and  transport  port in registers reg1 and reg2. For IPv6
                     traffic the flows also load the original  destination  IP
                     and transport port in registers xxreg1 and reg2.

              ·      A  priority-110  flow  sends  the  packets  to connection
                     tracker based on a hint provided by the  previous  tables
                     (with  a  match  for  reg0[2]  ==  1) by using the ct_lb;
                     action. This flow is added to handle the traffic for load
                     balancer  VIPs  whose protocol is not defined (mainly for
                     ICMP traffic).

              ·      A priority-100  flow  sends  the  packets  to  connection
                     tracker  based  on a hint provided by the previous tables
                     (with a match for reg0[0] == 1)  by  using  the  ct_next;
                     action.

     Ingress Table 8: from-lport ACL hints

       This  table  consists of logical flows that set hints (reg0 bits) to be
       used in the next stage, in the ACL processing table, if  stateful  ACLs
       or  load  balancers  are  configured. Multiple hints can be set for the
       same packet. The possible hints are:

              ·      reg0[7]: the packet might match an allow-related ACL  and
                     might have to commit the connection to conntrack.

              ·      reg0[8]:  the packet might match an allow-related ACL but
                     there will be no need to commit the  connection  to  con‐
                     ntrack because it already exists.

              ·      reg0[9]: the packet might match a drop/reject.

              ·      reg0[10]:  the  packet  might match a drop/reject ACL but
                     the connection was previously allowed so it might have to
                     be committed again with ct_label=1/1.

       The table contains the following flows:

              ·      A priority-65535 flow to advance to the next table if the
                     logical switch has no ACLs configured, otherwise a prior‐
                     ity-0 flow to advance to the next table.

              ·      A priority-7 flow that matches on packets that initiate a
                     new session. This flow sets reg0[7] and reg0[9] and  then
                     advances to the next table.

              ·      A priority-6 flow that matches on packets that are in the
                     request direction of an already existing session that has
                     been  marked  as  blocked.  This  flow  sets  reg0[7] and
                     reg0[9] and then advances to the next table.

              ·      A priority-5 flow that matches  untracked  packets.  This
                     flow  sets  reg0[8]  and reg0[9] and then advances to the
                     next table.

              ·      A priority-4 flow that matches on packets that are in the
                     request direction of an already existing session that has
                     not been marked as blocked. This flow  sets  reg0[8]  and
                     reg0[10] and then advances to the next table.

              ·      A priority-3 flow that matches on packets that are in not
                     part of established sessions. This flow sets reg0[9]  and
                     then advances to the next table.

              ·      A  priority-2  flow that matches on packets that are part
                     of  an  established  session  that  has  been  marked  as
                     blocked.  This flow sets reg0[9] and then advances to the
                     next table.

              ·      A priority-1 flow that matches on packets that  are  part
                     of  an  established  session  that has not been marked as
                     blocked. This flow sets reg0[10] and then advances to the
                     next table.

     Ingress table 9: from-lport ACLs

       Logical flows in this table closely reproduce those in the ACL table in
       the OVN_Northbound database for the from-lport direction. The  priority
       values  from  the ACL table have a limited range and have 1000 added to
       them to leave room for OVN default flows at both higher and lower  pri‐
       orities.

              ·      allow  ACLs  translate  into logical flows with the next;
                     action. If there are any stateful ACLs on this  datapath,
                     then allow ACLs translate to ct_commit; next; (which acts
                     as a hint for the next tables to commit the connection to
                     conntrack).  In  case  the  ACL  has a label then reg3 is
                     loaded with the label value and reg0[13] bit is set to  1
                     (which  acts  as a hint for the next tables to commit the
                     label to conntrack).

              ·      allow-related ACLs translate into logical flows with  the
                     ct_commit(ct_label=0/1);  next;  actions  for new connec‐
                     tions and reg0[1] = 1; next; for existing connections. In
                     case  the  ACL  has  a label then reg3 is loaded with the
                     label value and reg0[13] bit is set to 1 (which acts as a
                     hint  for  the  next  tables  to commit the label to con‐
                     ntrack).

              ·      allow-stateless ACLs translate into  logical  flows  with
                     the next; action.

              ·      reject   ACLs  translate  into  logical  flows  with  the
                     tcp_reset { output ->gt;>gt;  inport;  next(pipeline=egress,ta
                     ble=5);}  action  for  TCP connections,icmp4/icmp6 action
                     for  UDP  connections,  and  sctp_abort  {output   -%gt;
                     inport;  next(pipeline=egress,table=5);}  action for SCTP
                     associations.

              ·      Other ACLs translate to drop; for new or  untracked  con‐
                     nections  and  ct_commit(ct_label=1/1); for known connec‐
                     tions. Setting ct_label marks a connection  as  one  that
                     was  previously  allowed, but should no longer be allowed
                     due to a policy change.

       This table contains a priority-65535 flow to advance to the next  table
       if  the  logical  switch has no ACLs configured, otherwise a priority-0
       flow to advance to the  next  table  so  that  ACLs  allow  packets  by
       default.

       If  the logical datapath has a stateful ACL or a load balancer with VIP
       configured, the following flows will also be added:

              ·      A priority-1 flow that sets the hint to commit IP traffic
                     to  the  connection  tracker  (with  action  reg0[1] = 1;
                     next;). This is  needed  for  the  default  allow  policy
                     because, while the initiator’s direction may not have any
                     stateful rules, the server’s  may  and  then  its  return
                     traffic would not be known and marked as invalid.

              ·      A  priority-65532  flow  that  allows  any traffic in the
                     reply direction for a connection that has been  committed
                     to  the  connection tracker (i.e., established flows), as
                     long as the committed flow does not have ct_label.blocked
                     set.  We  only handle traffic in the reply direction here
                     because we want all packets going in the  request  direc‐
                     tion  to  still  go  through the flows that implement the
                     currently defined policy based on ACLs. If  a  connection
                     is no longer allowed by policy, ct_label.blocked will get
                     set and packets in the reply direction will no longer  be
                     allowed, either.

              ·      A  priority-65532  flow  that  allows any traffic that is
                     considered related to a committed flow in the  connection
                     tracker  (e.g.,  an ICMP Port Unreachable from a non-lis‐
                     tening UDP port), as long as the committed flow does  not
                     have ct_label.blocked set.

              ·      A  priority-65532  flow  that drops all traffic marked by
                     the connection tracker as invalid.

              ·      A priority-65532 flow that drops all traffic in the reply
                     direction with ct_label.blocked set meaning that the con‐
                     nection should no longer  be  allowed  due  to  a  policy
                     change. Packets in the request direction are skipped here
                     to let a newly created ACL re-allow this connection.

              ·      A priority-65532 flow that allows IPv6 Neighbor solicita‐
                     tion,  Neighbor  discover,  Router  solicitation,  Router
                     advertisement and MLD packets.

       If the logical datapath has any ACL or a load balancer with VIP config‐
       ured, the following flow will also be added:

              ·      A  priority  34000 logical flow is added for each logical
                     switch datapath with the match eth.dst = E to  allow  the
                     service  monitor  reply packet destined to ovn-controller
                     with the action next, where E is the service monitor  mac
                     defined in the options:svc_monitor_mac colum of NB_Global
                     table.

     Ingress Table 10: from-lport QoS Marking

       Logical flows in this table closely reproduce those in  the  QoS  table
       with  the  action  column  set  in  the OVN_Northbound database for the
       from-lport direction.

              ·      For every qos_rules entry in a logical switch  with  DSCP
                     marking  enabled,  a  flow  will be added at the priority
                     mentioned in the QoS table.

              ·      One priority-0 fallback flow that matches all packets and
                     advances to the next table.

     Ingress Table 11: from-lport QoS Meter

       Logical  flows  in  this table closely reproduce those in the QoS table
       with the bandwidth column set in the OVN_Northbound  database  for  the
       from-lport direction.

              ·      For every qos_rules entry in a logical switch with meter‐
                     ing enabled, a flow will be added at  the  priority  men‐
                     tioned in the QoS table.

              ·      One priority-0 fallback flow that matches all packets and
                     advances to the next table.

     Ingress Table 12: Stateful

              ·      For all the configured load balancing rules for a  switch
                     in  OVN_Northbound  database that includes a L4 port PORT
                     of protocol P and IP address VIP, a priority-120 flow  is
                     added.  For  IPv4 VIPs , the flow matches ct.new &&&& ip &&&&
                     ip4.dst == VIP &&&& P &&&& P.dst == PORT. For IPv6 VIPs,  the
                     flow matches ct.new &&&& ip &&&& ip6.dst == VIP &&&& P &&&& P.dst
                     == PORT. The flow’s action is ct_lb(args)  ,  where  args
                     contains  comma separated IP addresses (and optional port
                     numbers) to load balance to. The address family of the IP
                     addresses  of  args  is the same as the address family of
                     VIP. If health check is enabled, then args will only con‐
                     tain  those  endpoints whose service monitor status entry
                     in OVN_Southbound db is either online or empty. For  IPv4
                     traffic  the  flow also loads the original destination IP
                     and transport port in registers reg1 and reg2.  For  IPv6
                     traffic  the  flow also loads the original destination IP
                     and transport port in registers xxreg1 and reg2.

              ·      For all the configured load balancing rules for a  switch
                     in  OVN_Northbound  database  that  includes  just  an IP
                     address VIP to match on, OVN adds  a  priority-110  flow.
                     For  IPv4  VIPs, the flow matches ct.new &&&& ip &&&& ip4.dst
                     == VIP. For IPv6 VIPs, the flow matches ct.new &&&&  ip  &&&&
                     ip6.dst  ==  VIP. The action on this flow is ct_lb(args),
                     where args contains comma separated IP addresses  of  the
                     same  address  family  as  VIP. For IPv4 traffic the flow
                     also loads the original destination IP and transport port
                     in  registers  reg1  and  reg2. For IPv6 traffic the flow
                     also loads the original destination IP and transport port
                     in registers xxreg1 and reg2.

              ·      If  the load balancer is created with --reject option and
                     it has no active backends, a TCP reset segment (for  tcp)
                     or an ICMP port unreachable packet (for all other kind of
                     traffic) will be sent  whenever  an  incoming  packet  is
                     received   for  this  load-balancer.  Please  note  using
                     --reject option will disable empty_lb SB controller event
                     for this load balancer.

              ·      A  priority 100 flow is added which commits the packet to
                     the conntrack and sets the most  significant  32-bits  of
                     ct_label  with  the reg3 value based on the hint provided
                     by previous tables (with a match  for  reg0[1]  ==  1  &&&&
                     reg0[13]  ==  1).  This is used by the ACLs with label to
                     commit the label value to conntrack.

              ·      For ACLs without label, a second priority-100  flow  com‐
                     mits packets to connection tracker using ct_commit; next;
                     action based on a hint provided by  the  previous  tables
                     (with a match for reg0[1] == 1 &&&& reg0[13] == 0).

              ·      A  priority-0  flow that simply moves traffic to the next
                     table.

     Ingress Table 13: Pre-Hairpin

              ·      If the logical switch has  load  balancer(s)  configured,
                     then  a  priority-100  flow is added with the match ip &&&&
                     ct.trk to check if the packet needs to be hairpinned  (if
                     after  load  balancing  the  destination  IP  matches the
                     source IP) or not by  executing  the  actions  reg0[6]  =
                     chk_lb_hairpin();  and reg0[12] = chk_lb_hairpin_reply();
                     and advances the packet to the next table.

              ·      A priority-0 flow that simply moves traffic to  the  next
                     table.

     Ingress Table 14: Nat-Hairpin

              ·      If  the  logical  switch has load balancer(s) configured,
                     then a priority-100 flow is added with the  match  ip  &&&&
                     ct.new &&&& ct.trk &&&& reg0[6] == 1 which hairpins the traf‐
                     fic by NATting source IP to the load balancer VIP by exe‐
                     cuting  the action ct_snat_to_vip and advances the packet
                     to the next table.

              ·      If the logical switch has  load  balancer(s)  configured,
                     then  a  priority-100  flow is added with the match ip &&&&
                     ct.est &&&& ct.trk &&&& reg0[6] == 1 which hairpins the traf‐
                     fic by NATting source IP to the load balancer VIP by exe‐
                     cuting the action ct_snat and advances the packet to  the
                     next table.

              ·      If  the  logical  switch has load balancer(s) configured,
                     then a priority-90 flow is added with  the  match  ip  &&&&
                     reg0[12]  == 1 which matches on the replies of hairpinned
                     traffic (i.e., destination IP is VIP, source  IP  is  the
                     backend IP and source L4 port is backend port for L4 load
                     balancers) and executes ct_snat and advances  the  packet
                     to the next table.

              ·      A  priority-0  flow that simply moves traffic to the next
                     table.

     Ingress Table 15: Hairpin

              ·      A priority-1 flow that hairpins traffic matched  by  non-
                     default  flows  in  the Pre-Hairpin table. Hairpinning is
                     done at L2, Ethernet addresses are swapped and the  pack‐
                     ets are looped back on the input port.

              ·      A  priority-0  flow that simply moves traffic to the next
                     table.

     Ingress Table 16: ARP/ND responder

       This table implements ARP/ND responder in a logical  switch  for  known
       IPs. The advantage of the ARP responder flow is to limit ARP broadcasts
       by locally responding to ARP requests without the need to send to other
       hypervisors. One common case is when the inport is a logical port asso‐
       ciated with a VIF and the broadcast is responded to on the local hyper‐
       visor  rather  than broadcast across the whole network and responded to
       by the destination VM. This behavior is proxy ARP.

       ARP requests arrive from VMs from  a  logical  switch  inport  of  type
       default.  For  this case, the logical switch proxy ARP rules can be for
       other VMs or logical router ports. Logical switch proxy ARP  rules  may
       be  programmed  both  for  mac binding of IP addresses on other logical
       switch VIF ports (which are of the default logical  switch  port  type,
       representing connectivity to VMs or containers), and for mac binding of
       IP addresses on logical switch router type  ports,  representing  their
       logical  router  port  peers. In order to support proxy ARP for logical
       router ports, an IP address must be configured on  the  logical  switch
       router  type port, with the same value as the peer logical router port.
       The configured MAC addresses must match as well. When a VM sends an ARP
       request  for  a  distributed logical router port and if the peer router
       type port of the attached logical switch does not have  an  IP  address
       configured,  the  ARP  request will be broadcast on the logical switch.
       One of the copies of the ARP request will go through the logical switch
       router  type  port  to  the  logical router datapath, where the logical
       router ARP responder will generate a reply. The MAC binding of  a  dis‐
       tributed  logical router, once learned by an associated VM, is used for
       all that VM’s communication needing routing. Hence, the action of a  VM
       re-arping  for  the  mac  binding  of the logical router port should be
       rare.

       Logical switch ARP responder proxy ARP  rules  can  also  be  hit  when
       receiving  ARP  requests externally on a L2 gateway port. In this case,
       the hypervisor acting as an L2 gateway, responds to the ARP request  on
       behalf of a destination VM.

       Note  that  ARP requests received from localnet or vtep logical inports
       can either go directly to VMs, in which case the VM responds or can hit
       an  ARP  responder  for  a logical router port if the packet is used to
       resolve a logical router port next hop address. In either case, logical
       switch  ARP  responder rules will not be hit. It contains these logical
       flows:

              ·      Priority-100 flows to skip the ARP responder if inport is
                     of  type  localnet  or  vtep and advances directly to the
                     next table. ARP requests sent to localnet or  vtep  ports
                     can be received by multiple hypervisors. Now, because the
                     same mac binding rules are downloaded to all hypervisors,
                     each  of the multiple hypervisors will respond. This will
                     confuse L2 learning on the source of  the  ARP  requests.
                     ARP requests received on an inport of type router are not
                     expected to hit any logical switch ARP  responder  flows.
                     However,  no  skip flows are installed for these packets,
                     as there would be some additional flow cost for this  and
                     the value appears limited.

              ·      If  inport V is of type virtual adds a priority-100 logi‐
                     cal flow for each P configured  in  the  options:virtual-
                     parents column with the match

                     inport == P &&&& &&&& ((arp.op == 1 &&&& arp.spa == VIP &&&& arp.tpa == VIP) || (arp.op == 2 &&&& arp.spa == VIP))


                     and applies the action

                     bind_vport(V, inport);


                     and advances the packet to the next table.

                     Where  VIP  is  the  virtual  ip configured in the column
                     options:virtual-ip.

              ·      Priority-50 flows that match ARP requests to  each  known
                     IP  address  A  of every logical switch port, and respond
                     with ARP replies  directly  with  corresponding  Ethernet
                     address E:

                     eth.dst = eth.src;
                     eth.src = E;
                     arp.op = 2; /* ARP reply. */
                     arp.tha = arp.sha;
                     arp.sha = E;
                     arp.tpa = arp.spa;
                     arp.spa = A;
                     outport = inport;
                     flags.loopback = 1;
                     output;


                     These  flows  are  omitted  for logical ports (other than
                     router ports or localport ports) that  are  down  (unless
                     ignore_lsp_down  is  configured as true in options column
                     of NB_Global table of the Northbound database), for logi‐
                     cal  ports  of  type  virtual,  for  logical  ports  with
                     ’unknown’ address set and for logical ports of a  logical
                     switch configured with other_config:vlan-passthru=true.

                     The  above  ARP responder flows are added for the list of
                     IPv4 addresses if defined in options:arp_proxy column  of
                     Logical_Switch_Port  table  for  logical  switch ports of
                     type router.

              ·      Priority-50 flows that match IPv6 ND  neighbor  solicita‐
                     tions  to each known IP address A (and A’s solicited node
                     address) of every logical  switch  port  except  of  type
                     router, and respond with neighbor advertisements directly
                     with corresponding Ethernet address E:

                     nd_na {
                         eth.src = E;
                         ip6.src = A;
                         nd.target = A;
                         nd.tll = E;
                         outport = inport;
                         flags.loopback = 1;
                         output;
                     };


                     Priority-50 flows that match IPv6 ND  neighbor  solicita‐
                     tions  to each known IP address A (and A’s solicited node
                     address) of logical  switch  port  of  type  router,  and
                     respond with neighbor advertisements directly with corre‐
                     sponding Ethernet address E:

                     nd_na_router {
                         eth.src = E;
                         ip6.src = A;
                         nd.target = A;
                         nd.tll = E;
                         outport = inport;
                         flags.loopback = 1;
                         output;
                     };


                     These flows are omitted for  logical  ports  (other  than
                     router  ports  or  localport ports) that are down (unless
                     ignore_lsp_down is configured as true in  options  column
                     of NB_Global table of the Northbound database), for logi‐
                     cal ports of type virtual  and  for  logical  ports  with
                     ’unknown’ address set.

              ·      Priority-100  flows  with match criteria like the ARP and
                     ND flows above, except that they only match packets  from
                     the  inport  that owns the IP addresses in question, with
                     action next;. These flows prevent OVN from  replying  to,
                     for  example,  an ARP request emitted by a VM for its own
                     IP address. A VM only  makes  this  kind  of  request  to
                     attempt  to  detect a duplicate IP address assignment, so
                     sending a reply will prevent the VM from accepting the IP
                     address that it owns.

                     In  place  of  next;, it would be reasonable to use drop;
                     for the flows’ actions. If everything is working as it is
                     configured,  then  this would produce equivalent results,
                     since no host should reply to the request. But ARPing for
                     one’s  own  IP  address  is intended to detect situations
                     where the network is not working as configured, so  drop‐
                     ping the request would frustrate that intent.

              ·      For  each  SVC_MON_SRC_IP  defined  in  the  value of the
                     ip_port_mappings:ENDPOINT_IP column of Load_Balancer  ta‐
                     ble,  priority-110  logical  flow is added with the match
                     arp.tpa == SVC_MON_SRC_IP &&&& &&&& arp.op == 1  and  applies
                     the action

                     eth.dst = eth.src;
                     eth.src = E;
                     arp.op = 2; /* ARP reply. */
                     arp.tha = arp.sha;
                     arp.sha = E;
                     arp.tpa = arp.spa;
                     arp.spa = A;
                     outport = inport;
                     flags.loopback = 1;
                     output;


                     where  E is the service monitor source mac defined in the
                     options:svc_monitor_mac column in  the  NB_Global  table.
                     This mac is used as the source mac in the service monitor
                     packets for the load balancer endpoint IP health checks.

                     SVC_MON_SRC_IP is used as the source ip  in  the  service
                     monitor  IPv4  packets  for the load balancer endpoint IP
                     health checks.

                     These flows are required if an ARP request  is  sent  for
                     the IP SVC_MON_SRC_IP.

              ·      For  each  VIP configured in the table Forwarding_Group a
                     priority-50 logical flow is added with the match  arp.tpa
                     == vip &&&& &&&& arp.op == 1
                      and applies the action

                     eth.dst = eth.src;
                     eth.src = E;
                     arp.op = 2; /* ARP reply. */
                     arp.tha = arp.sha;
                     arp.sha = E;
                     arp.tpa = arp.spa;
                     arp.spa = A;
                     outport = inport;
                     flags.loopback = 1;
                     output;


                     where  E  is  the  forwarding  group’s mac defined in the
                     vmac.

                     A is used as either the destination ip for load balancing
                     traffic  to child ports or as nexthop to hosts behind the
                     child ports.

                     These flows are required to respond to an ARP request  if
                     an ARP request is sent for the IP vip.

              ·      One priority-0 fallback flow that matches all packets and
                     advances to the next table.

     Ingress Table 17: DHCP option processing

       This table adds the DHCPv4 options to a DHCPv4 packet from the  logical
       ports  configured  with  IPv4 address(es) and DHCPv4 options, and simi‐
       larly for DHCPv6 options. This table also adds flows  for  the  logical
       ports of type external.

              ·      A  priority-100  logical  flow is added for these logical
                     ports which matches the IPv4 packet with udp.src = 68 and
                     udp.dst  =  67  and  applies the action put_dhcp_opts and
                     advances the packet to the next table.

                     reg0[3] = put_dhcp_opts(offer_ip = ip, options...);
                     next;


                     For DHCPDISCOVER and  DHCPREQUEST,  this  transforms  the
                     packet  into  a DHCP reply, adds the DHCP offer IP ip and
                     options to the packet, and stores  1  into  reg0[3].  For
                     other  kinds  of  packets, it just stores 0 into reg0[3].
                     Either way, it continues to the next table.

              ·      A priority-100 logical flow is added  for  these  logical
                     ports  which  matches  the IPv6 packet with udp.src = 546
                     and udp.dst = 547 and applies the action  put_dhcpv6_opts
                     and advances the packet to the next table.

                     reg0[3] = put_dhcpv6_opts(ia_addr = ip, options...);
                     next;


                     For  DHCPv6  Solicit/Request/Confirm packets, this trans‐
                     forms the packet into a DHCPv6 Advertise/Reply, adds  the
                     DHCPv6  offer IP ip and options to the packet, and stores
                     1 into reg0[3]. For  other  kinds  of  packets,  it  just
                     stores  0  into  reg0[3]. Either way, it continues to the
                     next table.

              ·      A priority-0 flow that matches all packets to advances to
                     table 16.

     Ingress Table 18: DHCP responses

       This  table implements DHCP responder for the DHCP replies generated by
       the previous table.

              ·      A priority 100 logical flow  is  added  for  the  logical
                     ports  configured  with DHCPv4 options which matches IPv4
                     packets with udp.src == 68 &&&& udp.dst == 67 &&&& reg0[3] ==
                     1  and  responds  back to the inport after applying these
                     actions. If reg0[3] is set to 1, it means that the action
                     put_dhcp_opts was successful.

                     eth.dst = eth.src;
                     eth.src = E;
                     ip4.src = S;
                     udp.src = 67;
                     udp.dst = 68;
                     outport = P;
                     flags.loopback = 1;
                     output;


                     where  E  is  the  server MAC address and S is the server
                     IPv4 address defined in the  DHCPv4  options.  Note  that
                     ip4.dst field is handled by put_dhcp_opts.

                     (This  terminates  ingress  packet processing; the packet
                     does not go to the next ingress table.)

              ·      A priority 100 logical flow  is  added  for  the  logical
                     ports  configured  with DHCPv6 options which matches IPv6
                     packets with udp.src == 546 &&&& udp.dst == 547 &&&&  reg0[3]
                     == 1 and responds back to the inport after applying these
                     actions. If reg0[3] is set to 1, it means that the action
                     put_dhcpv6_opts was successful.

                     eth.dst = eth.src;
                     eth.src = E;
                     ip6.dst = A;
                     ip6.src = S;
                     udp.src = 547;
                     udp.dst = 546;
                     outport = P;
                     flags.loopback = 1;
                     output;


                     where  E  is  the  server MAC address and S is the server
                     IPv6 LLA address generated from the server_id defined  in
                     the  DHCPv6  options and A is the IPv6 address defined in
                     the logical port’s addresses column.

                     (This terminates packet processing; the packet  does  not
                     go on the next ingress table.)

              ·      A priority-0 flow that matches all packets to advances to
                     table 17.

     Ingress Table 19 DNS Lookup

       This table looks up and resolves the DNS  names  to  the  corresponding
       configured IP address(es).

              ·      A priority-100 logical flow for each logical switch data‐
                     path if it is configured with DNS records, which  matches
                     the  IPv4  and IPv6 packets with udp.dst = 53 and applies
                     the action dns_lookup and advances the packet to the next
                     table.

                     reg0[4] = dns_lookup(); next;


                     For  valid DNS packets, this transforms the packet into a
                     DNS reply if the DNS name can be resolved, and  stores  1
                     into reg0[4]. For failed DNS resolution or other kinds of
                     packets, it just stores 0 into reg0[4].  Either  way,  it
                     continues to the next table.

     Ingress Table 20 DNS Responses

       This  table  implements  DNS responder for the DNS replies generated by
       the previous table.

              ·      A priority-100 logical flow for each logical switch data‐
                     path  if it is configured with DNS records, which matches
                     the IPv4 and IPv6 packets with udp.dst = 53 &&&& reg0[4] ==
                     1  and  responds  back to the inport after applying these
                     actions. If reg0[4] is set to 1, it means that the action
                     dns_lookup was successful.

                     eth.dst ->gt;>gt; eth.src;
                     ip4.src ->gt;>gt; ip4.dst;
                     udp.dst = udp.src;
                     udp.src = 53;
                     outport = P;
                     flags.loopback = 1;
                     output;


                     (This  terminates  ingress  packet processing; the packet
                     does not go to the next ingress table.)

     Ingress table 21 External ports

       Traffic from the external logical  ports  enter  the  ingress  datapath
       pipeline via the localnet port. This table adds the below logical flows
       to handle the traffic from these ports.

              ·      A priority-100 flow is added for  each  external  logical
                     port  which  doesn’t  reside  on  a  chassis  to drop the
                     ARP/IPv6 NS request to the router IP(s) (of  the  logical
                     switch) which matches on the inport of the external logi‐
                     cal port and the valid eth.src address(es) of the  exter
                     nal logical port.

                     This  flow  guarantees  that  the  ARP/NS  request to the
                     router IP address from the external ports is responded by
                     only  the chassis which has claimed these external ports.
                     All the other chassis, drops these packets.

                     A priority-100 flow is added for  each  external  logical
                     port which doesn’t reside on a chassis to drop any packet
                     destined to the router mac - with  the  match  inport  ==
                     external  &&&&  eth.src  ==  E &&&& eth.dst == R &&&& !is_chas
                     sis_resident("external") where E is the external port mac
                     and R is the router port mac.

              ·      A priority-0 flow that matches all packets to advances to
                     table 20.

     Ingress Table 22 Destination Lookup

       This table implements switching behavior.  It  contains  these  logical
       flows:

              ·      A  priority-110  flow with the match eth.src == E for all
                     logical switch datapaths  and  applies  the  action  han
                     dle_svc_check(inport). Where E is the service monitor mac
                     defined in the options:svc_monitor_mac colum of NB_Global
                     table.

              ·      A  priority-100  flow  that punts all IGMP/MLD packets to
                     ovn-controller if multicast snooping is  enabled  on  the
                     logical switch. The flow also forwards the IGMP/MLD pack‐
                     ets  to  the  MC_MROUTER_STATIC  multicast  group,  which
                     ovn-northd populates with all the logical ports that have
                     options :mcast_flood_reports=true.

              ·      Priority-90 flows that forward  registered  IP  multicast
                     traffic  to  their  corresponding  multicast group, which
                     ovn-northd creates based on  learnt  IGMP_Group  entries.
                     The  flows  also  forward packets to the MC_MROUTER_FLOOD
                     multicast group, which ovn-nortdh populates with all  the
                     logical  ports that are connected to logical routers with
                     options:mcast_relay=’true’.

              ·      A priority-85 flow that forwards all IP multicast traffic
                     destined  to  224.0.0.X  to the MC_FLOOD multicast group,
                     which  ovn-northd  populates  with  all  enabled  logical
                     ports.

              ·      A priority-85 flow that forwards all IP multicast traffic
                     destined to reserved multicast IPv6 addresses (RFC  4291,
                     2.7.1,  e.g.,  Solicited-Node  multicast) to the MC_FLOOD
                     multicast group,  which  ovn-northd  populates  with  all
                     enabled logical ports.

              ·      A priority-80 flow that forwards all unregistered IP mul‐
                     ticast traffic to the MC_STATIC  multicast  group,  which
                     ovn-northd populates with all the logical ports that have
                     options  :mcast_flood=true.  The  flow  also   forwards
                     unregistered IP multicast traffic to the MC_MROUTER_FLOOD
                     multicast group, which ovn-northd populates with all  the
                     logical  ports  connected  to  logical  routers that have
                     options :mcast_relay=true.

              ·      A priority-80 flow that drops all unregistered IP  multi‐
                     cast  traffic  if  other_config  :mcast_snoop=true  and
                     other_config  :mcast_flood_unregistered=false  and  the
                     switch  is  not  connected  to  a logical router that has
                     options :mcast_relay=true and the switch  doesn’t  have
                     any logical port with options :mcast_flood=true.

              ·      Priority-80  flows  for  each  IP address/VIP/NAT address
                     owned by a router port connected  to  the  switch.  These
                     flows  match ARP requests and ND packets for the specific
                     IP addresses. Matched packets are forwarded only  to  the
                     router  that  owns  the IP address and to the MC_FLOOD_L2
                     multicast group which  contains  all  non-router  logical
                     ports.

              ·      Priority-90  flows  for  each  IP address/VIP/NAT address
                     configured outside its owning router port’s subnet. These
                     flows  match ARP requests and ND packets for the specific
                     IP  addresses.  Matched  packets  are  forwarded  to  the
                     MC_FLOOD  multicast  group  which  contains all connected
                     logical ports.

              ·      Priority-75 flows for each port connected  to  a  logical
                     router  matching  self originated ARP request/ND packets.
                     These packets are flooded to the MC_FLOOD_L2  which  con‐
                     tains all non-router logical ports.

              ·      A  priority-70 flow that outputs all packets with an Eth‐
                     ernet broadcast or multicast eth.dst to the MC_FLOOD mul‐
                     ticast group.

              ·      One  priority-50  flow  that  matches each known Ethernet
                     address against eth.dst and outputs  the  packet  to  the
                     single associated output port.

                     For the Ethernet address on a logical switch port of type
                     router, when that logical switch port’s addresses  column
                     is  set  to  router and the connected logical router port
                     has a gateway chassis:

                     ·      The flow for the connected logical  router  port’s
                            Ethernet address is only programmed on the gateway
                            chassis.

                     ·      If the logical router has rules specified  in  nat
                            with  external_mac,  then those addresses are also
                            used to populate the switch’s  destination  lookup
                            on the chassis where logical_port is resident.

                     For the Ethernet address on a logical switch port of type
                     router, when that logical switch port’s addresses  column
                     is  set  to  router and the connected logical router port
                     specifies a reside-on-redirect-chassis  and  the  logical
                     router to which the connected logical router port belongs
                     to has a distributed gateway LRP:

                     ·      The flow for the connected logical  router  port’s
                            Ethernet address is only programmed on the gateway
                            chassis.

                     For each  forwarding  group  configured  on  the  logical
                     switch  datapath,  a  priority-50  flow  that  matches on
                     eth.dst == VIP
                      with an action  of  fwd_group(childports=args  ),  where
                     args  contains comma separated logical switch child ports
                     to load balance to. If liveness is enabled,  then  action
                     also includes  liveness=true.

              ·      One  priority-0  fallback  flow  that matches all packets
                     with the action outport =  get_fdb(eth.dst);  next;.  The
                     action  get_fdb  gets the port for the eth.dst in the MAC
                     learning table of the logical switch datapath.  If  there
                     is  no  entry for eth.dst in the MAC learning table, then
                     it stores none in the outport.

     Ingress Table 24 Destination unknown

       This table handles the packets whose destination was not found  or  and
       looked  up in the MAC learning table of the logical switch datapath. It
       contains the following flows.

              ·      If the logical switch has logical  ports  with  ’unknown’
                     addresses set, then the below logical flow is added

                     ·      Priority  50  flow  with the match outport == none
                            then outputs  them  to  the  MC_UNKNOWN  multicast
                            group, which ovn-northd populates with all enabled
                            logical  ports  that  accept  unknown  destination
                            packets.  As  a  small optimization, if no logical
                            ports   accept   unknown   destination    packets,
                            ovn-northd  omits this multicast group and logical
                            flow.

                     If the logical switch has no logical ports with ’unknown’
                     address set, then the below logical flow is added

                     ·      Priority  50  flow  with the match outport == none
                            and drops the packets.

              ·      One priority-0 fallback flow that outputs the  packet  to
                     the  egress  stage  with  the outport learnt from get_fdb
                     action.

     Egress Table 0: Pre-LB

       This table is similar to ingress table Pre-LB. It contains a priority-0
       flow  that simply moves traffic to the next table. Moreover it contains
       a priority-110 flow to move IPv6 Neighbor Discovery traffic to the next
       table.  If  any  load  balancing rules exist for the datapath, a prior‐
       ity-100 flow is added with a match of ip and action  of  reg0[2]  =  1;
       next; to act as a hint for table Pre-stateful to send IP packets to the
       connection tracker for packet de-fragmentation and  possibly  DNAT  the
       destination  VIP  to  one  of the selected backend for already commited
       load balanced traffic.

       This table also has a priority-110 flow with the match eth.src == E for
       all logical switch datapaths to move traffic to the next table. Where E
       is the service monitor mac defined in the options:svc_monitor_mac colum
       of NB_Global table.

     Egress Table 1: to-lport Pre-ACLs

       This is similar to ingress table Pre-ACLs except for to-lport traffic.

       This table also has a priority-110 flow with the match eth.src == E for
       all logical switch datapaths to move traffic to the next table. Where E
       is the service monitor mac defined in the options:svc_monitor_mac colum
       of NB_Global table.

       This table also has a priority-110 flow with the match outport == I for
       all logical switch datapaths to move traffic to the next table. Where I
       is the peer of a logical router port. This flow is added  to  skip  the
       connection  tracking  of  packets which will be entering logical router
       datapath from logical switch datapath for routing.

     Egress Table 2: Pre-stateful

       This is similar to ingress table  Pre-stateful.  This  table  adds  the
       below 3 logical flows.

              ·      A  Priority-120  flow that send the packets to connection
                     tracker using ct_lb; as the action so  that  the  already
                     established traffic gets unDNATted from the backend IP to
                     the load balancer VIP based on a  hint  provided  by  the
                     previous  tables  with  a  match for reg0[2] == 1. If the
                     packet was not DNATted earlier, then ct_lb functions like
                     ct_next.

              ·      A  priority-100  flow  sends  the  packets  to connection
                     tracker based on a hint provided by the  previous  tables
                     (with  a  match  for  reg0[0] == 1) by using the ct_next;
                     action.

              ·      A priority-0 flow that matches all packets to advance  to
                     the next table.

     Egress Table 3: from-lport ACL hints

       This is similar to ingress table ACL hints.

     Egress Table 4: to-lport ACLs

       This is similar to ingress table ACLs except for to-lport ACLs.

       In addition, the following flows are added.

              ·      A  priority  34000 logical flow is added for each logical
                     port which has DHCPv4 options defined to allow the DHCPv4
                     reply  packet  and  which  has  DHCPv6 options defined to
                     allow the DHCPv6 reply packet from the Ingress Table  16:
                     DHCP responses.

              ·      A  priority  34000 logical flow is added for each logical
                     switch datapath configured  with  DNS  records  with  the
                     match udp.dst = 53 to allow the DNS reply packet from the
                     Ingress Table 18: DNS responses.

              ·      A priority 34000 logical flow is added for  each  logical
                     switch  datapath  with the match eth.src = E to allow the
                     service monitor  request  packet  generated  by  ovn-con
                     troller with the action next, where E is the service mon‐
                     itor mac defined in the options:svc_monitor_mac colum  of
                     NB_Global table.

     Egress Table 5: to-lport QoS Marking

       This  is  similar  to  ingress  table  QoS marking except they apply to
       to-lport QoS rules.

     Egress Table 6: to-lport QoS Meter

       This is similar to  ingress  table  QoS  meter  except  they  apply  to
       to-lport QoS rules.

     Egress Table 7: Stateful

       This  is  similar  to  ingress  table Stateful except that there are no
       rules added for load balancing new connections.

     Egress Table 8: Egress Port Security - IP

       This is similar to the port security logic in table Ingress Port  Secu
       rity - IP except that outport, eth.dst, ip4.dst and ip6.dst are checked
       instead of inport, eth.src, ip4.src and ip6.src

     Egress Table 9: Egress Port Security - L2

       This is similar to the ingress port security  logic  in  ingress  table
       Admission  Control  and  Ingress Port Security - L2, but with important
       differences. Most obviously, outport and eth.dst are checked instead of
       inport  and eth.src. Second, packets directed to broadcast or multicast
       eth.dst are always accepted instead of being subject to the port  secu‐
       rity  rules;  this  is  implemented  through  a  priority-100 flow that
       matches on eth.mcast with action output;. Moreover, to ensure that even
       broadcast  and  multicast packets are not delivered to disabled logical
       ports, a priority-150 flow for each disabled logical outport  overrides
       the  priority-100  flow  with a drop; action. Finally if egress qos has
       been enabled on a localnet port, the outgoing queue id is  set  through
       set_queue  action.  Please  remember to mark the corresponding physical
       interface with ovn-egress-iface set to true in external_ids

   Logical Router Datapaths
       Logical router datapaths will only exist for Logical_Router rows in the
       OVN_Northbound database that do not have enabled set to false

     Ingress Table 0: L2 Admission Control

       This  table drops packets that the router shouldn’t see at all based on
       their Ethernet headers. It contains the following flows:

              ·      Priority-100 flows to drop packets with VLAN tags or mul‐
                     ticast Ethernet source addresses.

              ·      For each enabled router port P with Ethernet address E, a
                     priority-50 flow that matches inport == P  &&&&  (eth.mcast
                     || eth.dst == E), stores the router port ethernet address
                     and advances to next table, with  action  xreg0[0..47]=E;
                     next;.

                     For  the  gateway  port  on  a distributed logical router
                     (where one of the logical router ports specifies a  gate‐
                     way  chassis),  the  above  flow matching eth.dst == E is
                     only programmed on the gateway port instance on the gate‐
                     way chassis.

                     For  a  distributed  logical router or for gateway router
                     where the port is configured with options:gateway_mtu the
                     action   of   the   above   flow   is   modified   adding
                     check_pkt_larger in order to mark the packet setting REG
                     BIT_PKT_LARGER if the size is greater than the MTU.

              ·      For  each  dnat_and_snat NAT rule on a distributed router
                     that specifies an external Ethernet address E,  a  prior‐
                     ity-50  flow  that  matches inport == GW &&&& eth.dst == E,
                     where GW is the logical router gateway port, with  action
                     xreg0[0..47]=E; next;.

                     This flow is only programmed on the gateway port instance
                     on the chassis where the logical_port  specified  in  the
                     NAT rule resides.

       Other packets are implicitly dropped.

     Ingress Table 1: Neighbor lookup

       For  ARP and IPv6 Neighbor Discovery packets, this table looks into the
       MAC_Binding records to determine if OVN needs to learn  the  mac  bind‐
       ings. Following flows are added:

              ·      For  each  router  port  P  that owns IP address A, which
                     belongs to subnet S with prefix length L, if  the  option
                     always_learn_from_arp_request  is true for this router, a
                     priority-100 flow is added which matches inport ==  P  &&&&
                     arp.spa == S/L &&&& arp.op == 1 (ARP request) with the fol‐
                     lowing actions:

                     reg9[2] = lookup_arp(inport, arp.spa, arp.sha);
                     next;


                     If the option always_learn_from_arp_request is false, the
                     following two flows are added.

                     A priority-110 flow is added which matches inport == P &&&&
                     arp.spa == S/L &&&& arp.tpa  ==  A  &&&&  arp.op  ==  1  (ARP
                     request) with the following actions:

                     reg9[2] = lookup_arp(inport, arp.spa, arp.sha);
                     reg9[3] = 1;
                     next;


                     A priority-100 flow is added which matches inport == P &&&&
                     arp.spa == S/L &&&& arp.op == 1 (ARP request) with the fol‐
                     lowing actions:

                     reg9[2] = lookup_arp(inport, arp.spa, arp.sha);
                     reg9[3] = lookup_arp_ip(inport, arp.spa);
                     next;


                     If  the  logical  router  port P is a distributed gateway
                     router port, additional  match  is_chassis_resident(cr-P)
                     is added for all these flows.

              ·      A  priority-100  flow  which matches on ARP reply packets
                     and    applies    the    actions    if     the     option
                     always_learn_from_arp_request is true:

                     reg9[2] = lookup_arp(inport, arp.spa, arp.sha);
                     next;


                     If the option always_learn_from_arp_request is false, the
                     above actions will be:

                     reg9[2] = lookup_arp(inport, arp.spa, arp.sha);
                     reg9[3] = 1;
                     next;


              ·      A priority-100 flow which matches on IPv6  Neighbor  Dis‐
                     covery  advertisement  packet  and applies the actions if
                     the option always_learn_from_arp_request is true:

                     reg9[2] = lookup_nd(inport, nd.target, nd.tll);
                     next;


                     If the option always_learn_from_arp_request is false, the
                     above actions will be:

                     reg9[2] = lookup_nd(inport, nd.target, nd.tll);
                     reg9[3] = 1;
                     next;


              ·      A  priority-100  flow which matches on IPv6 Neighbor Dis‐
                     covery solicitation packet and applies the actions if the
                     option always_learn_from_arp_request is true:

                     reg9[2] = lookup_nd(inport, ip6.src, nd.sll);
                     next;


                     If the option always_learn_from_arp_request is false, the
                     above actions will be:

                     reg9[2] = lookup_nd(inport, ip6.src, nd.sll);
                     reg9[3] = lookup_nd_ip(inport, ip6.src);
                     next;


              ·      A priority-0 fallback flow that matches all  packets  and
                     applies  the  action  reg9[2]  =  1;  next; advancing the
                     packet to the next table.

     Ingress Table 2: Neighbor learning

       This table adds flows to learn the mac bindings from the ARP  and  IPv6
       Neighbor  Solicitation/Advertisement  packets if it is needed according
       to the lookup results from the previous stage.

       reg9[2] will be 1 if the lookup_arp/lookup_nd in the previous table was
       successful  or  skipped,  meaning no need to learn mac binding from the
       packet.

       reg9[3] will be 1 if the lookup_arp_ip/lookup_nd_ip in the previous ta‐
       ble  was  successful  or skipped, meaning it is ok to learn mac binding
       from the packet (if reg9[2] is 0).

              ·      A priority-100 flow  with  the  match  reg9[2]  ==  1  ||
                     reg9[3] == 0 and advances the packet to the next table as
                     there is no need to learn the neighbor.

              ·      A priority-90 flow with the match  arp  and  applies  the
                     action put_arp(inport, arp.spa, arp.sha); next;

              ·      A  priority-90  flow with the match nd_na and applies the
                     action put_nd(inport, nd.target, nd.tll); next;

              ·      A priority-90 flow with the match nd_ns and  applies  the
                     action put_nd(inport, ip6.src, nd.sll); next;

     Ingress Table 3: IP Input

       This table is the core of the logical router datapath functionality. It
       contains the following flows to implement very basic IP host  function‐
       ality.

              ·      For  distributed  logical routers or gateway routers with
                     gateway port configured  with  options:gateway_mtu  to  a
                     valid  integer  value, a priority-150 flow with the match
                     inport == LRP &&&& REGBIT_PKT_LARGER &&&& REGBIT_EGRESS_LOOP
                     BACK  ==  0,  where  LRP  is  the logical router port and
                     applies the following action for ipv4  and  ipv6  respec‐
                     tively:

                     icmp4 {
                         icmp4.type = 3; /* Destination Unreachable. */
                         icmp4.code = 4;  /* Frag Needed and DF was Set. */
                         icmp4.frag_mtu = M;
                         eth.dst = E;
                         ip4.dst = ip4.src;
                         ip4.src = I;
                         ip.ttl = 255;
                         REGBIT_EGRESS_LOOPBACK = 1;
                         REGBIT_PKT_LARGER 0;
                         next(pipeline=ingress, table=0);
                     };
                     icmp6 {
                         icmp6.type = 2;
                         icmp6.code = 0;
                         icmp6.frag_mtu = M;
                         eth.dst = E;
                         ip6.dst = ip6.src;
                         ip6.src = I;
                         ip.ttl = 255;
                         REGBIT_EGRESS_LOOPBACK = 1;
                         REGBIT_PKT_LARGER 0;
                         next(pipeline=ingress, table=0);
                     };


              ·      For  each NAT entry of a distributed logical router (with
                     distributed gateway router port) of type snat,  a  prior‐
                     ity-120  flow  with the match inport == P &&&& ip4.src == A
                     advances the packet to the next pipeline, where P is  the
                     distributed  logical router port and A is the external_ip
                     set in the NAT entry. If  A  is  an  IPv6  address,  then
                     ip6.src is used for the match.

                     The  above  flow is required to handle the routing of the
                     East/west NAT traffic.

              ·      For each BFD port the two  following  priority-110  flows
                     are added to manage BFD traffic:

                     ·      if  ip4.src  or ip6.src is any IP address owned by
                            the router port and udp.dst == 3784 ,  the  packet
                            is advanced to the next pipeline stage.

                     ·      if  ip4.dst  or ip6.dst is any IP address owned by
                            the router port and udp.dst ==  3784  ,  the  han
                            dle_bfd_msg action is executed.

              ·      L3  admission  control: A priority-100 flow drops packets
                     that match any of the following:

                     ·      ip4.src[28..31] == 0xe (multicast source)

                     ·      ip4.src == 255.255.255.255 (broadcast source)

                     ·      ip4.src == 127.0.0.0/8 || ip4.dst  ==  127.0.0.0/8
                            (localhost source or destination)

                     ·      ip4.src == 0.0.0.0/8 || ip4.dst == 0.0.0.0/8 (zero
                            network source or destination)

                     ·      ip4.src or ip6.src is any IP address owned by  the
                            router,  unless the packet was recirculated due to
                            egress   loopback    as    indicated    by    REG
                            BIT_EGRESS_LOOPBACK.

                     ·      ip4.src is the broadcast address of any IP network
                            known to the router.

              ·      A priority-100 flow parses DHCPv6 replies from IPv6  pre‐
                     fix  delegation  routers  (udp.src  ==  547 &&&& udp.dst ==
                     546). The handle_dhcpv6_reply is used to send IPv6 prefix
                     delegation messages to the delegation router.

              ·      ICMP  echo reply. These flows reply to ICMP echo requests
                     received for the router’s IP address.  Let  A  be  an  IP
                     address  owned by a router port. Then, for each A that is
                     an IPv4 address, a priority-90 flow matches on ip4.dst ==
                     A  and  icmp4.type  ==  8  &&&&  icmp4.code == 0 (ICMP echo
                     request). For each A that is an IPv6  address,  a  prior‐
                     ity-90 flow matches on ip6.dst == A and icmp6.type == 128
                     &&&& icmp6.code == 0 (ICMPv6 echo request). The port of the
                     router  that  receives  the echo request does not matter.
                     Also, the ip.ttl  of  the  echo  request  packet  is  not
                     checked,  so  it complies with RFC 1812, section 4.2.2.9.
                     Flows for ICMPv4 echo requests use the following actions:

                     ip4.dst ->gt;>gt; ip4.src;
                     ip.ttl = 255;
                     icmp4.type = 0;
                     flags.loopback = 1;
                     next;


                     Flows for ICMPv6 echo requests use the following actions:

                     ip6.dst ->gt;>gt; ip6.src;
                     ip.ttl = 255;
                     icmp6.type = 129;
                     flags.loopback = 1;
                     next;


              ·      Reply to ARP requests.

                     These flows reply to ARP requests for the router’s own IP
                     address.  The  ARP  requests  are  handled  only  if  the
                     requestor’s IP belongs to the same subnets of the logical
                     router  port. For each router port P that owns IP address
                     A, which belongs to subnet S with prefix  length  L,  and
                     Ethernet  address E, a priority-90 flow matches inport ==
                     P &&&& arp.spa == S/L &&&& arp.op == 1 &&&& arp.tpa ==  A  (ARP
                     request) with the following actions:

                     eth.dst = eth.src;
                     eth.src = xreg0[0..47];
                     arp.op = 2; /* ARP reply. */
                     arp.tha = arp.sha;
                     arp.sha = xreg0[0..47];
                     arp.tpa = arp.spa;
                     arp.spa = A;
                     outport = inport;
                     flags.loopback = 1;
                     output;


                     For  the  gateway  port  on  a distributed logical router
                     (where one of the logical router ports specifies a  gate‐
                     way  chassis), the above flows are only programmed on the
                     gateway port instance on the gateway chassis. This behav‐
                     ior avoids generation of multiple ARP responses from dif‐
                     ferent chassis, and allows upstream MAC learning to point
                     to the gateway chassis.

                     For the logical router port with the option reside-on-re
                     direct-chassis set  (which  is  centralized),  the  above
                     flows are only programmed on the gateway port instance on
                     the gateway chassis (if the logical router has a distrib‐
                     uted  gateway  port).  This behavior avoids generation of
                     multiple ARP responses from different chassis, and allows
                     upstream MAC learning to point to the gateway chassis.

              ·      Reply  to  IPv6 Neighbor Solicitations. These flows reply
                     to Neighbor Solicitation requests for  the  router’s  own
                     IPv6  address and populate the logical router’s mac bind‐
                     ing table.

                     For  each  router  port  P  that  owns  IPv6  address  A,
                     solicited  node address S, and Ethernet address E, a pri‐
                     ority-90 flow matches inport == P &&&& nd_ns &&&& ip6.dst  ==
                     {A, E} &&&& nd.target == A with the following actions:

                     nd_na_router {
                         eth.src = xreg0[0..47];
                         ip6.src = A;
                         nd.target = A;
                         nd.tll = xreg0[0..47];
                         outport = inport;
                         flags.loopback = 1;
                         output;
                     };


                     For  the  gateway  port  on  a distributed logical router
                     (where one of the logical router ports specifies a  gate‐
                     way  chassis),  the above flows replying to IPv6 Neighbor
                     Solicitations are only programmed  on  the  gateway  port
                     instance  on  the  gateway  chassis. This behavior avoids
                     generation of multiple replies  from  different  chassis,
                     and  allows upstream MAC learning to point to the gateway
                     chassis.

              ·      These flows reply to ARP requests or IPv6 neighbor solic‐
                     itation  for  the  virtual IP addresses configured in the
                     router for NAT (both DNAT and SNAT) or load balancing.

                     IPv4: For a  configured  NAT  (both  DNAT  and  SNAT)  IP
                     address  or  a  load balancer IPv4 VIP A, for each router
                     port P  with  Ethernet  address  E,  a  priority-90  flow
                     matches  arp.op  ==  1 &&&& arp.tpa == A (ARP request) with
                     the following actions:

                     eth.dst = eth.src;
                     eth.src = xreg0[0..47];
                     arp.op = 2; /* ARP reply. */
                     arp.tha = arp.sha;
                     arp.sha = xreg0[0..47];
                     arp.tpa ->gt;>gt; arp.spa;
                     outport = inport;
                     flags.loopback = 1;
                     output;


                     IPv4: For a configured load balancer IPv4 VIP, a  similar
                     flow is added with the additional match inport == P.

                     If  the  router  port  P  is a distributed gateway router
                     port, then the is_chassis_resident(P) is  also  added  in
                     the match condition for the load balancer IPv4 VIP A.

                     IPv6:  For  a  configured  NAT  (both  DNAT  and SNAT) IP
                     address or a load balancer IPv6  VIP  A,  solicited  node
                     address  S,  for each router port P with Ethernet address
                     E, a priority-90 flow matches inport ==  P  &&&&  nd_ns  &&&&
                     ip6.dst  ==  {A,  S} &&&& nd.target == A with the following
                     actions:

                     eth.dst = eth.src;
                     nd_na {
                         eth.src = xreg0[0..47];
                         nd.tll = xreg0[0..47];
                         ip6.src = A;
                         nd.target = A;
                         outport = inport;
                         flags.loopback = 1;
                         output;
                     }


                     If the router port P  is  a  distributed  gateway  router
                     port,  then  the  is_chassis_resident(P) is also added in
                     the match condition for the load balancer IPv6 VIP A.

                     For the gateway port on a distributed logical router with
                     NAT  (where  one  of the logical router ports specifies a
                     gateway chassis):

                     ·      If the corresponding NAT rule cannot be handled in
                            a  distributed  manner, then a priority-92 flow is
                            programmed on the gateway  port  instance  on  the
                            gateway  chassis.  A priority-91 drop flow is pro‐
                            grammed on the other chassis when ARP  requests/NS
                            packets  are  received  on  the gateway port. This
                            behavior  avoids  generation   of   multiple   ARP
                            responses   from  different  chassis,  and  allows
                            upstream MAC learning  to  point  to  the  gateway
                            chassis.

                     ·      If  the corresponding NAT rule can be handled in a
                            distributed manner, then this flow  is  only  pro‐
                            grammed  on  the  gateway  port instance where the
                            logical_port specified in the NAT rule resides.

                            Some of the actions are different for  this  case,
                            using  the  external_mac specified in the NAT rule
                            rather than the gateway port’s Ethernet address E:

                            eth.src = external_mac;
                            arp.sha = external_mac;


                            or in the case of IPv6 neighbor solicition:

                            eth.src = external_mac;
                            nd.tll = external_mac;


                            This behavior avoids generation  of  multiple  ARP
                            responses   from  different  chassis,  and  allows
                            upstream MAC learning  to  point  to  the  correct
                            chassis.

              ·      Priority-85  flows  which drops the ARP and IPv6 Neighbor
                     Discovery packets.

              ·      A priority-84 flow explicitly allows IPv6 multicast traf‐
                     fic  that is supposed to reach the router pipeline (i.e.,
                     router solicitation and router advertisement packets).

              ·      A priority-83 flow explicitly drops IPv6 multicast  traf‐
                     fic that is destined to reserved multicast groups.

              ·      A   priority-82  flow  allows  IP  multicast  traffic  if
                     options:mcast_relay=’true’, otherwise drops it.

              ·      UDP port unreachable.  Priority-80  flows  generate  ICMP
                     port  unreachable  messages  in  reply  to  UDP datagrams
                     directed to the router’s IP address, except in  the  spe‐
                     cial case of gateways, which accept traffic directed to a
                     router IP for load balancing and NAT purposes.

                     These flows should not match IP  fragments  with  nonzero
                     offset.

              ·      TCP  reset. Priority-80 flows generate TCP reset messages
                     in reply to TCP datagrams directed  to  the  router’s  IP
                     address,  except  in  the special case of gateways, which
                     accept traffic directed to a router IP for load balancing
                     and NAT purposes.

                     These  flows  should  not match IP fragments with nonzero
                     offset.

              ·      Protocol or address unreachable. Priority-70 flows gener‐
                     ate  ICMP  protocol  or  address unreachable messages for
                     IPv4 and IPv6 respectively in reply to  packets  directed
                     to  the  router’s  IP  address on IP protocols other than
                     UDP, TCP, and ICMP, except in the special case  of  gate‐
                     ways,  which  accept  traffic directed to a router IP for
                     load balancing purposes.

                     These flows should not match IP  fragments  with  nonzero
                     offset.

              ·      Drop  other  IP  traffic to this router. These flows drop
                     any other traffic destined  to  an  IP  address  of  this
                     router  that  is  not already handled by one of the flows
                     above, which amounts to ICMP (other than  echo  requests)
                     and fragments with nonzero offsets. For each IP address A
                     owned by the router, a priority-60 flow  matches  ip4.dst
                     ==  A or ip6.dst == A and drops the traffic. An exception
                     is made and the above flow is not  added  if  the  router
                     port’s  own  IP  address  is used to SNAT packets passing
                     through that router.

       The flows above handle all of the traffic that might be directed to the
       router  itself.  The following flows (with lower priorities) handle the
       remaining traffic, potentially for forwarding:

              ·      Drop Ethernet local broadcast. A  priority-50  flow  with
                     match  eth.bcast drops traffic destined to the local Eth‐
                     ernet  broadcast  address.  By  definition  this  traffic
                     should not be forwarded.

              ·      ICMP  time  exceeded.  For  each  router port P, whose IP
                     address is A, a priority-40 flow with match inport  ==  P
                     &&&&  ip.ttl  ==  {0,  1} &&&& !ip.later_frag matches packets
                     whose TTL has expired, with the following actions to send
                     an  ICMP  time  exceeded  reply for IPv4 and IPv6 respec‐
                     tively:

                     icmp4 {
                         icmp4.type = 11; /* Time exceeded. */
                         icmp4.code = 0;  /* TTL exceeded in transit. */
                         ip4.dst = ip4.src;
                         ip4.src = A;
                         ip.ttl = 255;
                         next;
                     };
                     icmp6 {
                         icmp6.type = 3; /* Time exceeded. */
                         icmp6.code = 0;  /* TTL exceeded in transit. */
                         ip6.dst = ip6.src;
                         ip6.src = A;
                         ip.ttl = 255;
                         next;
                     };


              ·      TTL discard. A priority-30 flow with match ip.ttl ==  {0,
                     1}  and  actions  drop; drops other packets whose TTL has
                     expired, that should not receive a ICMP error reply (i.e.
                     fragments with nonzero offset).

              ·      Next  table.  A  priority-0  flows match all packets that
                     aren’t already handled and uses  actions  next;  to  feed
                     them to the next table.

     Ingress Table 4: UNSNAT

       This  is  for  already  established connections’ reverse traffic. i.e.,
       SNAT has already been done in egress pipeline and now  the  packet  has
       entered the ingress pipeline as part of a reply. It is unSNATted here.

       Ingress Table 4: UNSNAT on Gateway and Distributed Routers

              ·      If the Router (Gateway or Distributed) is configured with
                     load balancers, then below lflows are added:

                     For each IPv4 address A defined as load balancer VIP with
                     the  protocol  P  (and the protocol port T if defined) is
                     also present as an external_ip in the NAT table, a prior‐
                     ity-120  logical  flow  is  added  with  the match ip4 &&&&
                     ip4.dst == A &&&& P with the action next;  to  advance  the
                     packet to the next table. If the load balancer has proto‐
                     col port B defined, then the match also has P.dst == B.

                     The above flows are also added for IPv6 load balancers.

       Ingress Table 4: UNSNAT on Gateway Routers

              ·      If the Gateway router has been configured to  force  SNAT
                     any  previously DNATted packets to B, a priority-110 flow
                     matches ip &&&& ip4.dst == B or ip &&&& ip6.dst == B with  an
                     action ct_snat; .

                     If    the    Gateway    router    is    configured   with
                     lb_force_snat_ip=router_ip then for every logical  router
                     port  P attached to the Gateway router with the router ip
                     B, a priority-110 flow is added with the match inport  ==
                     P  &&&& ip4.dst == B or inport == P &&&& ip6.dst == B with an
                     action ct_snat; .

                     If the Gateway router has been configured to  force  SNAT
                     any previously load-balanced packets to B, a priority-100
                     flow matches ip &&&& ip4.dst == B or ip  &&&&  ip6.dst  ==  B
                     with an action ct_snat; .

                     For  each  NAT  configuration in the OVN Northbound data‐
                     base, that asks to change the  source  IP  address  of  a
                     packet  from  A  to  B,  a priority-90 flow matches ip &&&&
                     ip4.dst == B or  ip  &&&&  ip6.dst  ==  B  with  an  action
                     ct_snat;  .  If the NAT rule is of type dnat_and_snat and
                     has stateless=true in the options, then the action  would
                     be ip4/6.dst= (B).

                     A priority-0 logical flow with match 1 has actions next;.

       Ingress Table 4: UNSNAT on Distributed Routers

              ·      For  each  configuration  in the OVN Northbound database,
                     that asks to change the source IP  address  of  a  packet
                     from A to B, a priority-100 flow matches ip &&&& ip4.dst ==
                     B &&&& inport == GW or ip &&&& ip6.dst == B &&&& inport  ==  GW
                     where  GW  is  the  logical  router gateway port, with an
                     action ct_snat;. If the NAT rule is of type dnat_and_snat
                     and  has  stateless=true  in the options, then the action
                     would be ip4/6.dst= (B).

                     If the NAT rule cannot be handled in a  distributed  man‐
                     ner,  then the priority-100 flow above is only programmed
                     on the gateway chassis.

                     A priority-0 logical flow with match 1 has actions next;.

     Ingress Table 5: DEFRAG

       This is to send packets to connection tracker for tracking and  defrag‐
       mentation.  It  contains a priority-0 flow that simply moves traffic to
       the next table.

       If load balancing rules with only virtual IP addresses  are  configured
       in OVN_Northbound database for a Gateway router, a priority-100 flow is
       added for each configured virtual IP address VIP.  For  IPv4  VIPs  the
       flow  matches  ip &&&& ip4.dst == VIP. For IPv6 VIPs, the flow matches ip
       &&&& ip6.dst == VIP. The flow applies the action reg0 = VIP; ct_dnat; (or
       xxreg0  for  IPv6)  to  send  IP  packets to the connection tracker for
       packet de-fragmentation and to dnat the destination IP for the  commit‐
       ted connection before sending it to the next table.

       If load balancing rules with virtual IP addresses and ports are config‐
       ured in OVN_Northbound database for a Gateway  router,  a  priority-110
       flow  is  added  for  each  configured virtual IP address VIP, protocol
       PROTO and port PORT. For IPv4 VIPs the flow matches ip  &&&&  ip4.dst  ==
       VIP  &&&&  PROTO &&&& PROTO.dst == PORT. For IPv6 VIPs, the flow matches ip
       &&&& ip6.dst == VIP &&&& PROTO &&&& PROTO.dst == PORT. The flow  applies  the
       action  reg0  =  VIP; reg9[16..31] = PROTO.dst; ct_dnat; (or xxreg0 for
       IPv6) to send IP packets to the connection tracker for packet  de-frag‐
       mentation  and  to dnat the destination IP for the committed connection
       before sending it to the next table.

       If ECMP routes with symmetric reply are configured  in  the  OVN_North
       bound  database  for a gateway router, a priority-300 flow is added for
       each router port on which symmetric replies are configured. The  match‐
       ing  logic for these ports essentially reverses the configured logic of
       the ECMP route. So for instance, a route  with  a  destination  routing
       policy  will  instead match if the source IP address matches the static
       route’s prefix. The flow uses the action ct_next to send IP packets  to
       the  connection tracker for packet de-fragmentation and tracking before
       sending it to the next table.

     Ingress Table 6: DNAT

       Packets enter the pipeline with destination IP address that needs to be
       DNATted  from a virtual IP address to a real IP address. Packets in the
       reverse direction needs to be unDNATed.

       Ingress Table 6: Load balancing DNAT rules

       Following load balancing DNAT flows are added  for  Gateway  router  or
       Router  with gateway port. These flows are programmed only on the gate‐
       way chassis. These flows do not get programmed for load balancers  with
       IPv6 VIPs.

              ·      If  controller_event has been enabled for all the config‐
                     ured load balancing rules for a Gateway router or  Router
                     with  gateway  port  in OVN_Northbound database that does
                     not have configured  backends,  a  priority-130  flow  is
                     added to trigger ovn-controller events whenever the chas‐
                     sis  receives  a  packet  for  that  particular  VIP.  If
                     event-elb  meter  has been previously created, it will be
                     associated to the empty_lb logical flow

              ·      For all the configured load balancing rules for a Gateway
                     router  or  Router  with  gateway  port in OVN_Northbound
                     database that includes a L4 port PORT of protocol  P  and
                     IPv4  or  IPv6  address  VIP,  a  priority-120  flow that
                     matches  on  ct.new  &&&&  ip  &&&&  reg0  ==  VIP  &&&&  P  &&&&
                     reg9[16..31]  ==   PORT  (xxreg0 == VIP in the IPv6 case)
                     with an action of ct_lb(args), where args contains  comma
                     separated  IPv4 or IPv6 addresses (and optional port num‐
                     bers) to load balance to. If the router is configured  to
                     force  SNAT  any  load-balanced packets, the above action
                     will  be  replaced  by   flags.force_snat_for_lb   =   1;
                     ct_lb(args);.  If  the  load balancing rule is configured
                     with skip_snat set to true,  the  above  action  will  be
                     replaced  by flags.skip_snat_for_lb = 1; ct_lb(args);. If
                     health check is enabled,  then  args  will  only  contain
                     those  endpoints  whose  service  monitor status entry in
                     OVN_Southbound db is either online or empty.

                     The previous table lr_in_defrag sets  the  register  reg0
                     (or  xxreg0  for IPv6) and does ct_dnat. Hence for estab‐
                     lished traffic, this table just advances  the  packet  to
                     the next stage.

              ·      For  all the configured load balancing rules for a router
                     in OVN_Northbound database that includes a L4  port  PORT
                     of  protocol  P  and  IPv4  or IPv6 address VIP, a prior‐
                     ity-120 flow that matches on ct.est &&&& ip4 &&&& reg0 == VIP
                     &&&&  P  &&&& reg9[16..31] ==  PORT (ip6 and xxreg0 == VIP in
                     the IPv6 case) with an action of next;. If the router  is
                     configured  to  force SNAT any load-balanced packets, the
                     above action will be replaced by  flags.force_snat_for_lb
                     = 1; next;. If the load balancing rule is configured with
                     skip_snat set to true, the above action will be  replaced
                     by flags.skip_snat_for_lb = 1; next;.

                     The  previous  table  lr_in_defrag sets the register reg0
                     (or xxreg0 for IPv6) and does ct_dnat. Hence  for  estab‐
                     lished  traffic,  this  table just advances the packet to
                     the next stage.

              ·      For all the configured load balancing rules for a  router
                     in  OVN_Northbound  database  that  includes  just  an IP
                     address VIP to match on, a priority-110 flow that matches
                     on ct.new &&&& ip4 &&&& reg0 == VIP (ip6 and xxreg0 == VIP in
                     the IPv6 case) with an action of ct_lb(args), where  args
                     contains  comma  separated IPv4 or IPv6 addresses. If the
                     router is configured  to  force  SNAT  any  load-balanced
                     packets,   the   above   action   will   be  replaced  by
                     flags.force_snat_for_lb = 1; ct_lb(args);.  If  the  load
                     balancing  rule is configured with skip_snat set to true,
                     the    above    action    will     be     replaced     by
                     flags.skip_snat_for_lb = 1; ct_lb(args);.

                     The  previous  table  lr_in_defrag sets the register reg0
                     (or xxreg0 for IPv6) and does ct_dnat. Hence  for  estab‐
                     lished  traffic,  this  table just advances the packet to
                     the next stage.

              ·      For all the configured load balancing rules for a  router
                     in  OVN_Northbound  database  that  includes  just  an IP
                     address VIP to match on, a priority-110 flow that matches
                     on  ct.est  &&&&  ip4  &&&& reg0 == VIP (or ip6 and xxreg0 ==
                     VIP) with an action of next;. If the router is configured
                     to force SNAT any load-balanced packets, the above action
                     will be replaced by flags.force_snat_for_lb =  1;  next;.
                     If  the  load balancing rule is configured with skip_snat
                     set to  true,  the  above  action  will  be  replaced  by
                     flags.skip_snat_for_lb = 1; next;.

                     The  previous  table  lr_in_defrag sets the register reg0
                     (or xxreg0 for IPv6) and does ct_dnat. Hence  for  estab‐
                     lished  traffic,  this  table just advances the packet to
                     the next stage.

              ·      If the load balancer is created with --reject option  and
                     it  has no active backends, a TCP reset segment (for tcp)
                     or an ICMP port unreachable packet (for all other kind of
                     traffic)  will  be  sent  whenever  an incoming packet is
                     received  for  this  load-balancer.  Please  note   using
                     --reject option will disable empty_lb SB controller event
                     for this load balancer.

       Ingress Table 6: DNAT on Gateway Routers

              ·      For each configuration in the  OVN  Northbound  database,
                     that  asks  to  change  the  destination  IP address of a
                     packet from A to B, a priority-100  flow  matches  ip  &&&&
                     ip4.dst  ==  A  or  ip  &&&&  ip6.dst  ==  A with an action
                     flags.loopback = 1; ct_dnat(B);. If the Gateway router is
                     configured  to  force  SNAT  any DNATed packet, the above
                     action will be replaced by flags.force_snat_for_dnat = 1;
                     flags.loopback  =  1;  ct_dnat(B);. If the NAT rule is of
                     type dnat_and_snat and has stateless=true in the options,
                     then the action would be ip4/6.dst= (B).

                     If  the  NAT  rule  has  allowed_ext_ips configured, then
                     there is an additional match ip4.src == allowed_ext_ips .
                     Similarly,   for   IPV6,   match   would  be  ip6.src  ==
                     allowed_ext_ips.

                     If the NAT rule has exempted_ext_ips set, then  there  is
                     an  additional  flow configured at priority 101. The flow
                     matches if source ip is an exempted_ext_ip and the action
                     is next; . This flow is used to bypass the ct_dnat action
                     for a packet originating from exempted_ext_ips.

              ·      A priority-0 logical flow with match 1 has actions next;.

       Ingress Table 6: DNAT on Distributed Routers

       On distributed routers, the DNAT table only handles packets with desti‐
       nation IP address that needs to be DNATted from a virtual IP address to
       a real IP address. The unDNAT processing in the  reverse  direction  is
       handled in a separate table in the egress pipeline.

              ·      For  each  configuration  in the OVN Northbound database,
                     that asks to change  the  destination  IP  address  of  a
                     packet  from  A  to  B, a priority-100 flow matches ip &&&&
                     ip4.dst == B &&&& inport == GW, where  GW  is  the  logical
                     router  gateway  port,  with  an  action ct_dnat(B);. The
                     match will include ip6.dst == B in the IPv6 case. If  the
                     NAT  rule is of type dnat_and_snat and has stateless=true
                     in the options, then the action would be ip4/6.dst=(B).

                     If the NAT rule cannot be handled in a  distributed  man‐
                     ner,  then the priority-100 flow above is only programmed
                     on the gateway chassis.

                     If the NAT  rule  has  allowed_ext_ips  configured,  then
                     there is an additional match ip4.src == allowed_ext_ips .
                     Similarly,  for  IPV6,  match   would   be   ip6.src   ==
                     allowed_ext_ips.

                     If  the  NAT rule has exempted_ext_ips set, then there is
                     an additional flow configured at priority 101.  The  flow
                     matches if source ip is an exempted_ext_ip and the action
                     is next; . This flow is used to bypass the ct_dnat action
                     for a packet originating from exempted_ext_ips.

                     A priority-0 logical flow with match 1 has actions next;.

     Ingress Table 7: ECMP symmetric reply processing

              ·      If ECMP routes with symmetric reply are configured in the
                     OVN_Northbound database for a gateway  router,  a  prior‐
                     ity-100  flow is added for each router port on which sym‐
                     metric replies are configured.  The  matching  logic  for
                     these  ports essentially reverses the configured logic of
                     the ECMP route. So for instance, a route with a  destina‐
                     tion  routing  policy will instead match if the source IP
                     address matches the static route’s prefix. The flow  uses
                     the   action   ct_commit   {   ct_label.ecmp_reply_eth  =
                     eth.src;" " ct_label.ecmp_reply_port  =  K;};  next;   to
                     commit  the  connection  and storing eth.src and the ECMP
                     reply port binding tunnel key K in the ct_label.

     Ingress Table 8: IPv6 ND RA option processing

              ·      A priority-50 logical flow  is  added  for  each  logical
                     router  port  configured  with  IPv6  ND RA options which
                     matches IPv6 ND Router Solicitation  packet  and  applies
                     the  action put_nd_ra_opts and advances the packet to the
                     next table.

                     reg0[5] = put_nd_ra_opts(options);next;


                     For a valid IPv6 ND RS packet, this transforms the packet
                     into  an  IPv6 ND RA reply and sets the RA options to the
                     packet and stores 1 into  reg0[5].  For  other  kinds  of
                     packets,  it  just  stores 0 into reg0[5]. Either way, it
                     continues to the next table.

              ·      A priority-0 logical flow with match 1 has actions next;.

     Ingress Table 9: IPv6 ND RA responder

       This table implements IPv6 ND RA responder for the IPv6 ND  RA  replies
       generated by the previous table.

              ·      A  priority-50  logical  flow  is  added for each logical
                     router port configured with  IPv6  ND  RA  options  which
                     matches  IPv6 ND RA packets and reg0[5] == 1 and responds
                     back to the  inport  after  applying  these  actions.  If
                     reg0[5]   is   set   to  1,  it  means  that  the  action
                     put_nd_ra_opts was successful.

                     eth.dst = eth.src;
                     eth.src = E;
                     ip6.dst = ip6.src;
                     ip6.src = I;
                     outport = P;
                     flags.loopback = 1;
                     output;


                     where E is the MAC address and I is the IPv6  link  local
                     address of the logical router port.

                     (This  terminates  packet processing in ingress pipeline;
                     the packet does not go to the next ingress table.)

              ·      A priority-0 logical flow with match 1 has actions next;.

     Ingress Table 10: IP Routing

       A packet that arrives at this table is an  IP  packet  that  should  be
       routed  to  the address in ip4.dst or ip6.dst. This table implements IP
       routing, setting reg0 (or xxreg0 for IPv6) to the next-hop  IP  address
       (leaving ip4.dst or ip6.dst, the packet’s final destination, unchanged)
       and advances to the next table for ARP resolution. It  also  sets  reg1
       (or  xxreg1)  to  the  IP  address  owned  by  the selected router port
       (ingress table ARP Request will generate an  ARP  request,  if  needed,
       with  reg0 as the target protocol address and reg1 as the source proto‐
       col address).

       For ECMP routes, i.e. multiple static routes with same policy and  pre‐
       fix  but different nexthops, the above actions are deferred to next ta‐
       ble. This table, instead, is responsible for determine the  ECMP  group
       id and select a member id within the group based on 5-tuple hashing. It
       stores group id in reg8[0..15] and member id in reg8[16..31]. This step
       is  skipped  if  the traffic going out the ECMP route is reply traffic,
       and the ECMP route was configured to use  symmetric  replies.  Instead,
       the  stored ct_label value is used to choose the destination. The least
       significant 48 bits of the ct_label tell the destination MAC address to
       which  the  packet  should  be  sent. The next 16 bits tell the logical
       router port on which the packet should be sent.  These  values  in  the
       ct_label  are set when the initial ingress traffic is received over the
       ECMP route.

       This table contains the following logical flows:

              ·      Priority-550  flow  that  drops  IPv6  Router   Solicita‐
                     tion/Advertisement  packets  that  were  not processed in
                     previous tables.

              ·      Priority-500 flows that match IP multicast  traffic  des‐
                     tined  to  groups  registered  on  any  of  the  attached
                     switches and sets outport  to  the  associated  multicast
                     group  that  will  eventually  flood  the  traffic to all
                     interested attached  logical  switches.  The  flows  also
                     decrement TTL.

              ·      Priority-450  flow that matches unregistered IP multicast
                     traffic and  sets  outport  to  the  MC_STATIC  multicast
                     group,  which ovn-northd populates with the logical ports
                     that have options :mcast_flood=true. If no router ports
                     are configured to flood multicast traffic the packets are
                     dropped.

              ·      IPv4 routing table. For each route to IPv4 network N with
                     netmask  M, on router port P with IP address A and Ether‐
                     net address E, a logical flow with match ip4.dst ==  N/M,
                     whose priority is the number of 1-bits in M, has the fol‐
                     lowing actions:

                     ip.ttl--;
                     reg8[0..15] = 0;
                     reg0 = G;
                     reg1 = A;
                     eth.src = E;
                     outport = P;
                     flags.loopback = 1;
                     next;


                     (Ingress table 1 already verified that ip.ttl--; will not
                     yield a TTL exceeded error.)

                     If  the route has a gateway, G is the gateway IP address.
                     Instead, if the route is from a configured static  route,
                     G is the next hop IP address. Else it is ip4.dst.

              ·      IPv6 routing table. For each route to IPv6 network N with
                     netmask M, on router port P with IP address A and  Ether‐
                     net address E, a logical flow with match in CIDR notation
                     ip6.dst == N/M, whose priority is the integer value of M,
                     has the following actions:

                     ip.ttl--;
                     reg8[0..15] = 0;
                     xxreg0 = G;
                     xxreg1 = A;
                     eth.src = E;
                     outport = inport;
                     flags.loopback = 1;
                     next;


                     (Ingress table 1 already verified that ip.ttl--; will not
                     yield a TTL exceeded error.)

                     If the route has a gateway, G is the gateway IP  address.
                     Instead,  if the route is from a configured static route,
                     G is the next hop IP address. Else it is ip6.dst.

                     If the address A is in the link-local  scope,  the  route
                     will be limited to sending on the ingress port.

              ·      For  ECMP  routes, they are grouped by policy and prefix.
                     An unique id (non-zero) is assigned to  each  group,  and
                     each  member  is  also  assigned  an unique id (non-zero)
                     within each group.

                     For each IPv4/IPv6 ECMP group with group id GID and  mem‐
                     ber  ids  MID1,  MID2,  ..., a logical flow with match in
                     CIDR notation ip4.dst == N/M, or ip6.dst  ==  N/M,  whose
                     priority  is  the  integer  value of M, has the following
                     actions:

                     ip.ttl--;
                     flags.loopback = 1;
                     reg8[0..15] = GID;
                     select(reg8[16..31], MID1, MID2, ...);


     Ingress Table 11: IP_ROUTING_ECMP

       This table implements the second part of IP  routing  for  ECMP  routes
       following  the  previous table. If a packet matched a ECMP group in the
       previous table, this table matches the group id and  member  id  stored
       from the previous table, setting reg0 (or xxreg0 for IPv6) to the next-
       hop IP address (leaving ip4.dst or ip6.dst, the packet’s final destina‐
       tion,  unchanged) and advances to the next table for ARP resolution. It
       also sets reg1 (or xxreg1) to the IP  address  owned  by  the  selected
       router port (ingress table ARP Request will generate an ARP request, if
       needed, with reg0 as the target protocol address and reg1 as the source
       protocol address).

       This  processing is skipped for reply traffic being sent out of an ECMP
       route if the route was configured to use symmetric replies.

       This table contains the following logical flows:

              ·      A priority-150 flow that matches reg8[0..15]  ==  0  with
                     action   next;  directly  bypasses  packets  of  non-ECMP
                     routes.

              ·      For each member with ID MID in each ECMP  group  with  ID
                     GID, a priority-100 flow with match reg8[0..15] == GID &&&&
                     reg8[16..31] == MID has following actions:

                     [xx]reg0 = G;
                     [xx]reg1 = A;
                     eth.src = E;
                     outport = P;


     Ingress Table 12: Router policies

       This table adds flows for the logical router policies configured on the
       logical   router.   Please   see   the  OVN_Northbound  database  Logi
       cal_Router_Policy table documentation in ovn-nb for supported actions.

              ·      For each router policy configured on the logical  router,
                     a  logical  flow  is added with specified priority, match
                     and actions.

              ·      If the policy action is reroute with 2 or  more  nexthops
                     defined,  then the logical flow is added with the follow‐
                     ing actions:

                     reg8[0..15] = GID;
                     reg8[16..31] = select(1,..n);


                     where GID is the ECMP group id  generated  by  ovn-northd
                     for  this  policy and n is the number of nexthops. select
                     action selects one of the nexthop member id, stores it in
                     the  register reg8[16..31] and advances the packet to the
                     next stage.

              ·      If the policy action is reroute  with  just  one  nexhop,
                     then  the  logical  flow  is  added  with  the  following
                     actions:

                     [xx]reg0 = H;
                     eth.src = E;
                     outport = P;
                     reg8[0..15] = 0;
                     flags.loopback = 1;
                     next;


                     where H is the nexthop  defined in the router  policy,  E
                     is  the  ethernet address of the logical router port from
                     which the nexthop is  reachable  and  P  is  the  logical
                     router port from which the nexthop is reachable.

              ·      If  a  router policy has the option pkt_mark=m set and if
                     the action is not drop, then  the  action  also  includes
                     pkt.mark = m to mark the packet with the marker m.

     Ingress Table 13: ECMP handling for router policies

       This  table  handles  the  ECMP for the router policies configured with
       multiple nexthops.

              ·      A priority-150 flow is added to advance the packet to the
                     next  stage  if the ECMP group id register reg8[0..15] is
                     0.

              ·      For each ECMP reroute router policy  with  multiple  nex‐
                     thops,  a  priority-100  flow is added for each nexthop H
                     with the match reg8[0..15] == GID &&&&  reg8[16..31]  ==  M
                     where  GID  is  the  router  policy group id generated by
                     ovn-northd and M is the member id of the nexthop H gener‐
                     ated  by  ovn-northd.  The following actions are added to
                     the flow:

                     [xx]reg0 = H;
                     eth.src = E;
                     outport = P
                     "flags.loopback = 1; "
                     "next;"


                     where H is the nexthop  defined in the router  policy,  E
                     is  the  ethernet address of the logical router port from
                     which the nexthop is  reachable  and  P  is  the  logical
                     router port from which the nexthop is reachable.

     Ingress Table 14: ARP/ND Resolution

       Any  packet that reaches this table is an IP packet whose next-hop IPv4
       address is in reg0 or IPv6 address is in xxreg0.  (ip4.dst  or  ip6.dst
       contains  the final destination.) This table resolves the IP address in
       reg0 (or xxreg0) into an output port in outport and an Ethernet address
       in eth.dst, using the following flows:

              ·      A  priority-500  flow  that  matches IP multicast traffic
                     that was allowed in the routing pipeline. For  this  kind
                     of  traffic  the outport was already set so the flow just
                     advances to the next table.

              ·      Static MAC bindings. MAC bindings can be known statically
                     based  on data in the OVN_Northbound database. For router
                     ports connected to logical switches, MAC bindings can  be
                     known  statically  from the addresses column in the Logi
                     cal_Switch_Port table.  For  router  ports  connected  to
                     other  logical  routers, MAC bindings can be known stati‐
                     cally from the mac  and  networks  column  in  the  Logi
                     cal_Router_Port  table.  (Note: the flow is NOT installed
                     for the IP addresses that belong to  a  neighbor  logical
                     router    port    if   the   current   router   has   the
                     options:dynamic_neigh_routers set to true)

                     For each IPv4 address A whose host is known to have  Eth‐
                     ernet  address  E  on  router port P, a priority-100 flow
                     with match outport === P &&&& reg0 == A has actions eth.dst
                     = E; next;.

                     For  each  virtual  ip  A configured on a logical port of
                     type virtual and its virtual parent  set  in  its  corre‐
                     sponding  Port_Binding record and the virtual parent with
                     the Ethernet address E and the virtual  ip  is  reachable
                     via  the  router  port  P, a priority-100 flow with match
                     outport === P &&&& reg0 ==  A  has  actions  eth.dst  =  E;
                     next;.

                     For  each  virtual  ip  A configured on a logical port of
                     type virtual and its virtual parent not set in its corre‐
                     sponding  Port_Binding  record  and  the  virtual ip A is
                     reachable via the router port P, a priority-100 flow with
                     match  outport  ===  P &&&& reg0 == A has actions eth.dst =
                     00:00:00:00:00:00; next;. This flow is added so that  the
                     ARP is always resolved for the virtual ip A by generating
                     ARP request and not consulting the MAC_Binding  table  as
                     it can have incorrect value for the virtual ip A.

                     For  each IPv6 address A whose host is known to have Eth‐
                     ernet address E on router port  P,  a  priority-100  flow
                     with  match  outport  ===  P  &&&&  xxreg0 == A has actions
                     eth.dst = E; next;.

                     For each logical router port with an IPv4 address A and a
                     mac  address of E that is reachable via a different logi‐
                     cal router port P, a priority-100 flow with match outport
                     === P &&&& reg0 == A has actions eth.dst = E; next;.

                     For each logical router port with an IPv6 address A and a
                     mac address of E that is reachable via a different  logi‐
                     cal router port P, a priority-100 flow with match outport
                     === P &&&& xxreg0 == A has actions eth.dst = E; next;.

              ·      Static MAC bindings from NAT entries.  MAC  bindings  can
                     also  be  known  for  the entries in the NAT table. Below
                     flows are programmed for distributed logical routers  i.e
                     with a distributed router port.

                     For  each row in the NAT table with IPv4 address A in the
                     external_ip column of NAT table, a priority-100 flow with
                     the  match outport === P &&&& reg0 == A has actions eth.dst
                     = E; next;, where P is  the  distributed  logical  router
                     port,  E  is  the  Ethernet  address if set in the exter
                     nal_mac column of NAT table for  of  type  dnat_and_snat,
                     otherwise the Ethernet address of the distributed logical
                     router port. Note that if the external_ip is not within a
                     subnet  on  the owning logical router, then OVN will only
                     create ARP resolution flows if the  options:add_route  is
                     set  to  true. Otherwise, no ARP resolution flows will be
                     added.

                     For IPv6 NAT entries, same flows are added, but using the
                     register xxreg0 for the match.

              ·      Traffic  with  IP  destination  an  address  owned by the
                     router  should  be  dropped.  Such  traffic  is  normally
                     dropped in ingress table IP Input except for IPs that are
                     also shared with SNAT rules. However,  if  there  was  no
                     unSNAT  operation  that  happened successfully until this
                     point in the pipeline  and  the  destination  IP  of  the
                     packet  is  still  a  router owned IP, the packets can be
                     safely dropped.

                     A priority-1 logical  flow  with  match  ip4.dst  =  {..}
                     matches   on   traffic  destined  to  router  owned  IPv4
                     addresses which are also SNAT IPs. This flow  has  action
                     drop;.

                     A  priority-1  logical  flow  with  match  ip6.dst = {..}
                     matches  on  traffic  destined  to  router   owned   IPv6
                     addresses  which  are also SNAT IPs. This flow has action
                     drop;.

              ·      Dynamic MAC bindings. These flows resolve MAC-to-IP bind‐
                     ings  that  have  become known dynamically through ARP or
                     neighbor discovery. (The ingress table ARP  Request  will
                     issue  an  ARP or neighbor solicitation request for cases
                     where the binding is not yet known.)

                     A priority-0 logical flow  with  match  ip4  has  actions
                     get_arp(outport, reg0); next;.

                     A  priority-0  logical  flow  with  match ip6 has actions
                     get_nd(outport, xxreg0); next;.

              ·      For a distributed gateway LRP with redirect-type  set  to
                     bridged,   a  priority-50  flow  will  match  outport  ==
                     "ROUTER_PORT" and !is_chassis_resident ("cr-ROUTER_PORT")
                     has  actions  eth.dst = E; next;, where E is the ethernet
                     address of the logical router port.

     Ingress Table 15: Check packet length

       For distributed logical routers or gateway routers  with  gateway  port
       configured  with options:gateway_mtu to a valid integer value, this ta‐
       ble adds a priority-50 logical flow with the match outport  ==  GW_PORT
       where  GW_PORT  is  the  gateway  router  port  and  applies the action
       check_pkt_larger and advances the packet to the next table.

       REGBIT_PKT_LARGER = check_pkt_larger(L); next;


       where L is the packet length to check for. If the packet is larger than
       L, it stores 1 in the register bit REGBIT_PKT_LARGER. The value of L is
       taken from options:gateway_mtu column of Logical_Router_Port row.

       This table adds one priority-0 fallback flow that matches  all  packets
       and advances to the next table.

     Ingress Table 16: Handle larger packets

       For  distributed  logical  routers or gateway routers with gateway port
       configured with options:gateway_mtu to a valid integer value, this  ta‐
       ble  adds  the  following  priority-150  logical  flow for each logical
       router port with the match inport == LRP &&&& outport == GW_PORT &&&&  REG
       BIT_PKT_LARGER  &&&&  !REGBIT_EGRESS_LOOPBACK,  where  LRP is the logical
       router port and GW_PORT is the gateway port and applies  the  following
       action for ipv4 and ipv6 respectively:

       icmp4 {
           icmp4.type = 3; /* Destination Unreachable. */
           icmp4.code = 4;  /* Frag Needed and DF was Set. */
           icmp4.frag_mtu = M;
           eth.dst = E;
           ip4.dst = ip4.src;
           ip4.src = I;
           ip.ttl = 255;
           REGBIT_EGRESS_LOOPBACK = 1;
           REGBIT_PKT_LARGER = 0;
           next(pipeline=ingress, table=0);
       };
       icmp6 {
           icmp6.type = 2;
           icmp6.code = 0;
           icmp6.frag_mtu = M;
           eth.dst = E;
           ip6.dst = ip6.src;
           ip6.src = I;
           ip.ttl = 255;
           REGBIT_EGRESS_LOOPBACK = 1;
           REGBIT_PKT_LARGER = 0;
           next(pipeline=ingress, table=0);
       };


              ·      Where  M  is the (fragment MTU - 58) whose value is taken
                     from options:gateway_mtu  column  of  Logical_Router_Port
                     row.

              ·      E is the Ethernet address of the logical router port.

              ·      I is the IPv4/IPv6 address of the logical router port.

       This  table  adds one priority-0 fallback flow that matches all packets
       and advances to the next table.

     Ingress Table 17: Gateway Redirect

       For distributed logical routers where one or more of the logical router
       ports specifies a gateway chassis, this table redirects certain packets
       to the distributed gateway port instances  on  the  gateway  chassises.
       This table has the following flows:

              ·      For each NAT rule in the OVN Northbound database that can
                     be handled in a distributed manner, a priority-100  logi‐
                     cal  flow  with  match  ip4.src  == B &&&& outport == GW &&
                     is_chassis_resident(P), where GW is  the  logical  router
                     distributed  gateway  port and P is the NAT logical port.
                     IP traffic  matching  the  above  rule  will  be  managed
                     locally  setting  reg1  to C and eth.src to D, where C is
                     NAT external ip and D is NAT external mac.

              ·      For each NAT rule in the OVN Northbound database that can
                     be handled in a distributed manner, a priority-80 logical
                     flow with drop action if the NAT logical port is  a  vir‐
                     tual port not claimed by any chassis yet.

              ·      A  priority-50  logical flow with match outport == GW has
                     actions outport = CR; next;,  where  GW  is  the  logical
                     router  distributed  gateway  port  and  CR  is the chas
                     sisredirect port representing the instance of the logical
                     router distributed gateway port on the gateway chassis.

              ·      A priority-0 logical flow with match 1 has actions next;.

     Ingress Table 18: ARP Request

       In  the  common  case where the Ethernet destination has been resolved,
       this table outputs the packet. Otherwise, it composes and sends an  ARP
       or IPv6 Neighbor Solicitation request. It holds the following flows:

              ·      Unknown MAC address. A priority-100 flow for IPv4 packets
                     with match eth.dst == 00:00:00:00:00:00 has the following
                     actions:

                     arp {
                         eth.dst = ff:ff:ff:ff:ff:ff;
                         arp.spa = reg1;
                         arp.tpa = reg0;
                         arp.op = 1;  /* ARP request. */
                         output;
                     };


                     Unknown  MAC  address. For each IPv6 static route associ‐
                     ated with the router with the nexthop  IP:  G,  a  prior‐
                     ity-200  flow  for  IPv6  packets  with  match eth.dst ==
                     00:00:00:00:00:00 &&&&  xxreg0  ==  G  with  the  following
                     actions is added:

                     nd_ns {
                         eth.dst = E;
                         ip6.dst = I
                         nd.target = G;
                         output;
                     };


                     Where E is the multicast mac derived from the Gateway IP,
                     I is the solicited-node multicast  address  corresponding
                     to the target address G.

                     Unknown MAC address. A priority-100 flow for IPv6 packets
                     with match eth.dst == 00:00:00:00:00:00 has the following
                     actions:

                     nd_ns {
                         nd.target = xxreg0;
                         output;
                     };


                     (Ingress  table  IP  Routing initialized reg1 with the IP
                     address owned by outport and (xx)reg0 with  the  next-hop
                     IP address)

                     The  IP  packet  that triggers the ARP/IPv6 NS request is
                     dropped.

              ·      Known MAC address. A priority-0 flow  with  match  1  has
                     actions output;.

     Egress Table 0: UNDNAT

       This  is  for  already  established connections’ reverse traffic. i.e.,
       DNAT has already been done in ingress pipeline and now the  packet  has
       entered  the  egress  pipeline as part of a reply. This traffic is unD‐
       NATed here.

              ·      For all the configured load balancing rules for a  router
                     with   gateway   port  in  OVN_Northbound  database  that
                     includes an IPv4 address  VIP,  for  every  backend  IPv4
                     address B defined for the VIP a priority-120 flow is pro‐
                     grammed on gateway chassis that matches ip &&&& ip4.src  ==
                     B  &&&& outport == GW, where GW is the logical router gate‐
                     way port with an action ct_dnat;.  If  the  backend  IPv4
                     address  B is also configured with L4 port PORT of proto‐
                     col P, then the match also includes P.src == PORT.  These
                     flows are not added for load balancers with IPv6 VIPs.

                     If  the  router is configured to force SNAT any load-bal‐
                     anced  packets,  above  action  will   be   replaced   by
                     flags.force_snat_for_lb = 1; ct_dnat;.

              ·      For  each  configuration  in  the OVN Northbound database
                     that asks to change  the  destination  IP  address  of  a
                     packet  from an IP address of A to B, a priority-100 flow
                     matches ip &&&& ip4.src == B &&&& outport == GW, where GW  is
                     the logical router gateway port, with an action ct_dnat;.
                     If the NAT rule is of type dnat_and_snat and  has  state
                     less=true  in  the  options,  then  the  action  would be
                     ip4/6.src= (B).

                     If the NAT rule cannot be handled in a  distributed  man‐
                     ner,  then the priority-100 flow above is only programmed
                     on the gateway chassis.

                     If the NAT rule can be handled in a  distributed  manner,
                     then  there  is an additional action eth.src = EA;, where
                     EA is the ethernet address associated with the IP address
                     A  in  the NAT rule. This allows upstream MAC learning to
                     point to the correct chassis.

              ·      For all IP packets, a priority-50  flow  with  an  action
                     flags.loopback = 1; ct_dnat;.

              ·      A priority-0 logical flow with match 1 has actions next;.

     Egress Table 1: Post UNDNAT

              ·      A  priority-50  logical  flow  is  added that commits any
                     untracked flows from the  previous  table  lr_out_undnat.
                     This flow matches on ct.new &&&& ip with action ct_commit {
                     } ; next; .

              ·      A priority-0 logical flow with match 1 has actions next;.

     Egress Table 2: SNAT

       Packets that are configured to be SNATed get their  source  IP  address
       changed based on the configuration in the OVN Northbound database.

              ·      A  priority-120 flow to advance the IPv6 Neighbor solici‐
                     tation packet to next table to skip  SNAT.  In  the  case
                     where  ovn-controller  injects an IPv6 Neighbor Solicita‐
                     tion packet (for nd_ns action) we don’t want  the  packet
                     to go throught conntrack.

       Egress Table 2: SNAT on Gateway Routers

              ·      If  the Gateway router in the OVN Northbound database has
                     been configured to force SNAT a  packet  (that  has  been
                     previously  DNATted)  to  B,  a priority-100 flow matches
                     flags.force_snat_for_dnat ==  1  &&&&  ip  with  an  action
                     ct_snat(B);.

              ·      If  a  load  balancer  configured  to  skip snat has been
                     applied to the Gateway router  pipeline,  a  priority-120
                     flow  matches  flags.skip_snat_for_lb  == 1 &&&& ip with an
                     action next;.

              ·      If the Gateway router in the OVN Northbound database  has
                     been  configured  to  force  SNAT a packet (that has been
                     previously   load-balanced)   using   router   IP    (i.e
                     options:lb_force_snat_ip=router_ip),  then for each logi‐
                     cal router port P attached to the Gateway router, a  pri‐
                     ority-110  flow  matches  flags.force_snat_for_lb == 1 &&&&
                     outport == P
                      with an action ct_snat(R); where R is the IP  configured
                     on  the  router  port.  If  R is an IPv4 address then the
                     match will also include ip4 and if it is an IPv6 address,
                     then the match will also include ip6.

                     If  the logical router port P is configured with multiple
                     IPv4 and multiple IPv6 addresses, only the first IPv4 and
                     first IPv6 address is considered.

              ·      If  the Gateway router in the OVN Northbound database has
                     been configured to force SNAT a  packet  (that  has  been
                     previously  load-balanced)  to  B,  a  priority-100  flow
                     matches flags.force_snat_for_lb == 1 &&&& ip with an action
                     ct_snat(B);.

              ·      For  each  configuration  in the OVN Northbound database,
                     that asks to change the source IP  address  of  a  packet
                     from  an  IP  address  of  A  or  to change the source IP
                     address of a packet that belongs to network  A  to  B,  a
                     flow   matches   ip  &&&&  ip4.src  ==  A  with  an  action
                     ct_snat(B);. The priority of the flow is calculated based
                     on  the  mask of A, with matches having larger masks get‐
                     ting higher priorities.  If  the  NAT  rule  is  of  type
                     dnat_and_snat and has stateless=true in the options, then
                     the action would be ip4/6.src= (B).

              ·      If the NAT  rule  has  allowed_ext_ips  configured,  then
                     there is an additional match ip4.dst == allowed_ext_ips .
                     Similarly,  for  IPV6,  match   would   be   ip6.dst   ==
                     allowed_ext_ips.

              ·      If  the  NAT rule has exempted_ext_ips set, then there is
                     an additional flow configured at the priority + 1 of cor‐
                     responding  NAT  rule. The flow matches if destination ip
                     is an exempted_ext_ip and the action is next; . This flow
                     is  used  to bypass the ct_snat action for a packet which
                     is destinted to exempted_ext_ips.

              ·      A priority-0 logical flow with match 1 has actions next;.

       Egress Table 2: SNAT on Distributed Routers

              ·      For each configuration in the  OVN  Northbound  database,
                     that  asks  to  change  the source IP address of a packet
                     from an IP address of  A  or  to  change  the  source  IP
                     address  of  a  packet  that belongs to network A to B, a
                     flow matches ip &&&& ip4.src == A &&&& outport ==  GW,  where
                     GW  is  the  logical  router gateway port, with an action
                     ct_snat(B);. The priority of the flow is calculated based
                     on  the  mask of A, with matches having larger masks get‐
                     ting higher priorities.  If  the  NAT  rule  is  of  type
                     dnat_and_snat and has stateless=true in the options, then
                     the action would be ip4/6.src= (B).

                     If the NAT rule cannot be handled in a  distributed  man‐
                     ner,  then the flow above is only programmed on the gate‐
                     way chassis increasing flow priority by 128 in  order  to
                     be run first

                     If  the  NAT rule can be handled in a distributed manner,
                     then there is an additional action eth.src =  EA;,  where
                     EA is the ethernet address associated with the IP address
                     A in the NAT rule. This allows upstream MAC  learning  to
                     point to the correct chassis.

                     If  the  NAT  rule  has  allowed_ext_ips configured, then
                     there is an additional match ip4.dst == allowed_ext_ips .
                     Similarly,   for   IPV6,   match   would  be  ip6.dst  ==
                     allowed_ext_ips.

                     If the NAT rule has exempted_ext_ips set, then  there  is
                     an additional flow configured at the priority + 1 of cor‐
                     responding NAT rule. The flow matches if  destination  ip
                     is an exempted_ext_ip and the action is next; . This flow
                     is used to bypass the ct_snat action for a flow which  is
                     destinted to exempted_ext_ips.

              ·      A priority-0 logical flow with match 1 has actions next;.

     Egress Table 3: Egress Loopback

       For  distributed  logical routers where one of the logical router ports
       specifies a gateway chassis.

       While UNDNAT and SNAT processing have already occurred by  this  point,
       this  traffic  needs  to be forced through egress loopback on this dis‐
       tributed gateway port instance, in order for UNSNAT and DNAT processing
       to  be applied, and also for IP routing and ARP resolution after all of
       the NAT processing, so that the packet can be forwarded to the destina‐
       tion.

       This table has the following flows:

              ·      For  each  NAT  rule  in the OVN Northbound database on a
                     distributed router,  a  priority-100  logical  flow  with
                     match  ip4.dst  == E &&&& outport == GW &&&& is_chassis_resi
                     dent(P), where E is the external IP address specified  in
                     the  NAT rule, GW is the logical router distributed gate‐
                     way port. For dnat_and_snat NAT rule, P  is  the  logical
                     port specified in the NAT rule. If logical_port column of
                     NAT table is NOT set, then P is the chassisredirect  port
                     of GW with the following actions:

                     clone {
                         ct_clear;
                         inport = outport;
                         outport = "";
                         flags = 0;
                         flags.loopback = 1;
                         reg0 = 0;
                         reg1 = 0;
                         ...
                         reg9 = 0;
                         REGBIT_EGRESS_LOOPBACK = 1;
                         next(pipeline=ingress, table=0);
                     };


                     flags.loopback  is set since in_port is unchanged and the
                     packet may return back to that port after NAT processing.
                     REGBIT_EGRESS_LOOPBACK  is  set  to  indicate that egress
                     loopback has occurred, in order to  skip  the  source  IP
                     address check against the router address.

              ·      A priority-0 logical flow with match 1 has actions next;.

     Egress Table 4: Delivery

       Packets that reach this table are ready for delivery. It contains:

              ·      Priority-110  logical flows that match IP multicast pack‐
                     ets on each enabled logical router port  and  modify  the
                     Ethernet  source  address  of the packets to the Ethernet
                     address of the port and then execute action output;.

              ·      Priority-100 logical flows that  match  packets  on  each
                     enabled logical router port, with action output;.



OVN 21.09.90                      ovn-northd                     ovn-northd(8)