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  op‐
              tion  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.

       n-threads N
              In certain situations, it may  be  desirable  to  enable  paral‐
              lelization  on  a  system  to decrease latency (at the potential
              cost of increasing CPU usage).

              This option will cause ovn-northd to use N threads when building
              logical flows, when N is within [2-256]. If N is 1, paralleliza‐
              tion is disabled (default behavior). If N is less than 1, then N
              is set to 1,  parallelization  is  disabled  and  a  warning  is
              logged.  If  N  is  more  than 256, then N is set to 256, paral‐
              lelization is enabled  (with  256  threads)  and  a  warning  is
              logged.

              ovn-northd-ddlog does not support this option.

       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  de‐
              taches.  Otherwise, invoking the daemon from a carelessly chosen
              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, re‐
                   spectively. (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-ap‐‐
            pctl(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  lo‐‐
            cal0  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  ad‐
                   dress 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 ex‐
                   tra precaution needs to be taken into account, for example,
                   syslog daemon needs to be configured to listen on the spec‐
                   ified UDP port, accidental iptables rules could  be  inter‐
                   fering  with  local syslog traffic and there are some secu‐
                   rity 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, be‐
                     cause 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  southbound
                     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 ex‐
                     cept for the northbound database client.

              set-n-threads N
                     Set the number  of  threads  used  for  building  logical
                     flows.  When  N is within [2-256], parallelization is en‐
                     abled. When N is 1 parallelization is disabled. When N is
                     less than 1 or more than 256, an error  is  returned.  If
                     ovn-northd  fails to start parallelization (e.g. fails to
                     setup semaphores, parallelization is disabled and an  er‐
                     ror is returned.

              get-n-threads
                     Return  the  number  of threads used for building logical
                     flows.

              inc-engine/show-stats
                     Display ovn-northd engine counters. For each engine  node
                     the following counters have been added:

                     •      recomputecomputeabort

              inc-engine/show-stats engine_node_name counter_name
                     Display  the  ovn-northd engine counter(s) for the speci‐
                     fied engine_node_name. counter_name is optional  and  can
                     be one of recompute, compute or abort.

              inc-engine/clear-stats
                     Reset ovn-northd engine counters.

       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  en‐
                   abled  (even if it was later disabled), the output also in‐
                   cludes a CPU time profile. See Profiling inside the  tutor‐
                   ial  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,  be‐
       cause  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 check

       Ingress table 0 contains these logical flows:

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

              •      For  each  disabled  logical port, a priority 100 flow is
                     added which matches on all packets and applies the action
                     REGBIT_PORT_SEC_DROP" = 1; next;" so that the packets are
                     dropped in the next stage.

              •      For each (enabled) vtep logical port, a priority 70  flow
                     is added which matches on all packets and applies the ac‐
                     tion  next(pipeline=ingress, table=S_SWITCH_IN_L2_LKUP) =
                     1; to skip most stages of ingress  pipeline  and  go  di‐
                     rectly to ingress L2 lookup table to determine the output
                     port.  Packets from VTEP (RAMP) switch should not be sub‐
                     jected to any ACL checks. Egress pipeline will do the ACL
                     checks.

              •      For each enabled logical port configured with qdisc queue
                     id  in  the  options:qdisc_queue_id   column   of   Logi‐‐
                     cal_Switch_Port,  a  priority  70  flow  is  added  which
                     matches  on  all   packets   and   applies   the   action
                     set_queue(id);           REGBIT_PORT_SEC_DROP"          =
                     check_in_port_sec(); next;".

              •      A priority 1 flow is added which matches on  all  packets
                     for  all  the  logical  ports and applies the action REG‐‐
                     BIT_PORT_SEC_DROP" = check_in_port_sec(); next; to evalu‐
                     ate the port security. The action  check_in_port_sec  ap‐
                     plies  the  port security rules defined in the port_secu‐‐
                     rity column of Logical_Switch_Port table.

     Ingress Table 1: Ingress Port Security - Apply

       This table drops the packets if the port security check failed  in  the
       previous stage i.e the register bit REGBIT_PORT_SEC_DROP is set to 1.

       Ingress table 1 contains these logical flows:

              •      A  priority-50 fallback flow that drops the packet if the
                     register bit REGBIT_PORT_SEC_DROP is set to 1.

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

     Ingress Table 2: 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 for
       logical  switch VIF ports whose port security is disabled and ’unknown’
       address  setn  as  well  as  for  localnet  ports  with  option  local‐
       net_learn_fdb.  A localnet port entry does not overwrite a VIF port en‐
       try.

              •      For each such VIF logical port p whose port  security  is
                     disabled  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;

              •      For each such localnet logical port p following  flow  is
                     added.

                     •      Priority  100  flow with the match inport == p and
                            action   flags.localnet   =    1;    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 3: Learn MAC of unknown ports.

       This table learns the MAC addresses seen on the VIF logical ports whose
       port  security  is disabled and ’unknown’ address set as well as on lo‐
       calnet ports with localnet_learn_fdb option set if the  lookup_fdb  ac‐
       tion  returned  false  in  the previous table. For localnet ports (with
       flags.localnet = 1), lookup_fdb returns true if (port, mac) is found or
       if a mac is found for a port of type vif.

              •      For each such VIF logical port p whose port  security  is
                     disabled and ’unknown’ address set and localnet port fol‐
                     lowing 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 ad‐
                            vances the packet to the next table.

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

     Ingress Table 4: 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.  Multicast,  IPv6
       Neighbor  Discovery  and MLD traffic also skips stateful ACLs. For "al‐
       low-stateless" ACLs, a flow is added to bypass  setting  the  hint  for
       connection tracker processing when there are stateful ACLs or LB rules;
       REGBIT_ACL_STATELESS is set for traffic matching stateless ACL flows.

       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 col‐
       umn of NB_Global table.

     Ingress Table 5: 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 two
       priority-110 flows to move multicast, IPv6 Neighbor Discovery  and  MLD
       traffic  to  the next table. It also contains two priority-110 flows to
       move stateless traffic, i.e traffic for which  REGBIT_ACL_STATELESS  is
       set,  to  the  next  table. If load balancing rules with virtual IP ad‐
       dresses (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  table  Pre-stateful  to send IP packets to the connection
       tracker for packet de-fragmentation (and to possibly do  DNAT  for  al‐
       ready established 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 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

       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 also has priority-120 flows that punt all  IGMP/MLD  packets
       to  ovn-controller  if the switch is an interconnect switch with multi‐
       cast snooping enabled.

       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  col‐
       umn 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 6: 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_mark; as the action so that  the  al‐
                     ready  established  traffic destined to the load balancer
                     VIP gets DNATted. These flows  match  each  VIPs  IP  and
                     port.  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  that  don’t  match
                     the  above  flows  to  connection tracker based on a hint
                     provided by the previous tables (with a match for reg0[2]
                     == 1) by using the ct_lb_mark; action.

              •      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; ac‐
                     tion.

     Ingress Table 7: 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 8: from-lport ACLs before LB

       Logical flows in this table closely reproduce those in the ACL table in
       the OVN_Northbound database for the from-lport  direction  without  the
       option apply-after-lb set or set to false. 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 priorities.

              •      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 la‐
                     bel  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_re‐‐
                     set { output ->gt;>gt; inport;  next(pipeline=egress,table=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 associa‐
                     tions.

              •      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 options:default_acl_drop column of NB_Global is false  or  not  set.
       Otherwise  the  flow action is set to drop; to implement a default drop
       behavior.

       A priority-65532 flow is added to  allow  IPv6  Neighbor  solicitation,
       Neighbor  discover,  Router  solicitation, Router advertisement and MLD
       packets regardless of other ACLs defined.

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

              •      If  options:default_acl_drop column of NB_Global is false
                     or not set, a priority-1 flow that sets the hint to  com‐
                     mit  IP  traffic that is not part of established sessions
                     to the connection  tracker  (with  action  reg0[1]  =  1;
                     next;).  This  is needed for the default allow policy be‐
                     cause, 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.

              •      If options:default_acl_drop column of NB_Global is  true,
                     a  priority-1 flow that drops IP traffic that is not part
                     of established sessions.

              •      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 be‐
                     cause, 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  re‐
                     ply 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_mark.blocked set.
                     We only handle traffic in the reply  direction  here  be‐
                     cause  we want all packets going in the request direction
                     to still go through the flows  that  implement  the  cur‐
                     rently  defined  policy based on ACLs. If a connection is
                     no longer allowed by policy, ct_mark.blocked will get set
                     and packets in the reply direction will no longer be  al‐
                     lowed,  either.  This  flow also clears the register bits
                     reg0[9] and reg0[10] and sets register bit  reg0[17].  If
                     ACL  logging  and  logging of related packets is enabled,
                     then a companion priority-65533 flow  will  be  installed
                     that  accomplishes the same thing but also logs the traf‐
                     fic.

              •      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_mark.blocked set. This flow also applies  NAT  to
                     the  related  traffic  so that ICMP headers and the inner
                     packet have correct addresses. If ACL logging and logging
                     of related packets is enabled, then  a  companion  prior‐
                     ity-65533  flow  will  be installed that accomplishes the
                     same thing but also logs the traffic.

              •      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_mark.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.

       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   column  of
                     NB_Global table.

     Ingress Table 9: 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 10: 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 11: Load balancing affinity check

       Load  balancing  affinity  check  table  contains the following logical
       flows:

              •      For all the configured load balancing rules for a  switch
                     in  OVN_Northbound  database  where  a  positive affinity
                     timeout is specified in options column, that  includes  a
                     L4  port  PORT of protocol P and IP address VIP, a prior‐
                     ity-100 flow is added. For IPv4 VIPs,  the  flow  matches
                     ct.new &&&& ip &&&& ip4.dst == VIP &&&& P.dst == PORT. For IPv6
                     VIPs, the flow matches ct.new &&&& ip &&&& ip6.dst == VIP&&&& P
                     &&&&  P.dst  ==   PORT.  The  flow’s  action  is  reg9[6] =
                     chk_lb_aff(); next;.

              •      A priority 0 flow is added which matches on  all  packets
                     and applies the action next;.

     Ingress Table 12: LB

              •      For  all the configured load balancing rules for a switch
                     in OVN_Northbound  database  where  a  positive  affinity
                     timeout  is  specified in options column, that includes a
                     L4 port PORT of protocol P and IP address VIP,  a  prior‐
                     ity-150  flow  is  added. For IPv4 VIPs, the flow matches
                     reg9[6] == 1 &&&& ct.new &&&& ip &&&& ip4.dst == VIP  &&&&  P.dst
                     == PORT . For IPv6 VIPs, the flow matches reg9[6] == 1 &&&&
                     ct.new  &&&&  ip &&&& ip6.dst ==  VIP &&&& P &&&& P.dst ==  PORT.
                     The flow’s action is ct_lb_mark(args),  where  args  con‐
                     tains  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.

              •      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.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_mark(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. The
                     above flow is created even if the load  balancer  is  at‐
                     tached to a logical router connected to the current logi‐
                     cal  switch and the install_ls_lb_from_router variable in
                     options is set to true.

              •      For all the configured load balancing rules for a  switch
                     in  OVN_Northbound  database that includes just an IP ad‐
                     dress 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_mark(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. The
                     above flow is created even if the load  balancer  is  at‐
                     tached to a logical router connected to the current logi‐
                     cal  switch and the install_ls_lb_from_router variable in
                     options is set to true.

              •      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  re‐
                     ceived for this load-balancer. Please note using --reject
                     option will disable empty_lb SB controller event for this
                     load balancer.

     Ingress Table 13: Load balancing affinity learn

       Load  balancing  affinity  learn  table  contains the following logical
       flows:

              •      For all the configured load balancing rules for a  switch
                     in  OVN_Northbound  database  where  a  positive affinity
                     timeout T is specified in options column, that includes a
                     L4 port PORT of protocol P and IP address VIP,  a  prior‐
                     ity-100  flow  is  added. For IPv4 VIPs, the flow matches
                     reg9[6] == 0 &&&& ct.new &&&& ip &&&& ip4.dst == VIP  &&&&  P.dst
                     ==  PORT. For IPv6 VIPs, the flow matches ct.new &&&& ip &&&&
                     ip6.dst == VIP &&&& P &&&& P.dst == PORT . The flow’s  action
                     is  commit_lb_aff(vip  =  VIP:PORT, backend = backend ip:
                     backend port, proto = P, timeout = T); .

              •      A priority 0 flow is added which matches on  all  packets
                     and applies the action next;.

     Ingress Table 14: 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 15: 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 16: Hairpin

              •      If logical switch has attached  logical  switch  port  of
                     vtep  type, then for each distributed gateway router port
                     RP attached to this logical switch and has chassis  redi‐
                     rect  port  cr-RP, a priority-2000 flow is added with the
                     match .IP
                     reg0[14] == 1 &&&& is_chassis_resident(cr-RP)

                     and action next;.

                     reg0[14] register bit is set in the ingress L2 port secu‐
                     rity check table for traffic received from HW VTEP (ramp)
                     ports.

              •      If logical switch has attached  logical  switch  port  of
                     vtep  type,  then  a  priority-1000  flow that matches on
                     reg0[14] register bit for the traffic  received  from  HW
                     VTEP  (ramp) ports. This traffic is passed to ingress ta‐
                     ble ls_in_l2_lkup.

              •      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 17: from-lport ACLs after LB

       Logical flows in this table closely reproduce those in the ACL table in
       the OVN_Northbound database for the from-lport direction with  the  op‐
       tion apply-after-lb set to true. 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 priorities.

              •      allow apply-after-lb ACLs translate  into  logical  flows
                     with  the  next;  action.  If there are any stateful ACLs
                     (including both before-lb  and  after-lb  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  apply-after-lb ACLs translate into logical
                     flows with the ct_commit(ct_label=0/1); next; actions for
                     new connections and reg0[1] = 1; next; for existing  con‐
                     nections. 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-stateless apply-after-lb ACLs translate into  logi‐
                     cal flows with the next; action.

              •      reject  apply-after-lb  ACLs translate into logical flows
                     with    the    tcp_reset    {    output    ->gt;>gt;    inport;
                     next(pipeline=egress,table=5);}  action  for  TCP connec‐
                     tions,icmp4/icmp6  action  for   UDP   connections,   and
                     sctp_abort         {output         -%gt;         inport;
                     next(pipeline=egress,table=5);} action for SCTP  associa‐
                     tions.

              •      Other  apply-after-lb  ACLs translate to drop; for new or
                     untracked connections  and  ct_commit(ct_label=1/1);  for
                     known connections. Setting ct_label marks a connection as
                     one  that was previously allowed, but should no longer be
                     allowed due to a policy change.

              •      One priority-65532 flow matching  packets  with  reg0[17]
                     set  (either  replies to existing sessions or traffic re‐
                     lated to existing sessions) and allows these by advancing
                     to the next table.

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

     Ingress Table 18: Stateful

              •      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 19: 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  de‐
       fault.  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 re‐
       ceiving ARP requests externally on a L2 gateway port. In this case, the
       hypervisor acting as an L2 gateway, responds to the ARP request on  be‐
       half of a destination VM.

       Note  that  ARP requests received from localnet logical inports can ei‐
       ther 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 re‐
       solve 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 advances directly to the next table. ARP
                     requests sent to localnet ports can be received by multi‐
                     ple 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  flows  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))
                     inport == P &&&& &&&& ((nd_ns &&&& ip6.dst == {VIP, NS_MULTICAST_ADDR} &&&& nd.target == VIP) || (nd_na &&&& nd.target == 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  op‐‐
                     tions:virtual-ip  and NS_MULTICAST_ADDR is solicited-node
                     multicast address corresponding to the VIP.

              •      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 ad‐
                     dress 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 ’un‐
                     known’ 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 re‐
                     spond 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 ’un‐
                     known’ 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 at‐
                     tempt 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 20: 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 ad‐
                     vances 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 21: 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 22 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 23 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 24 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 == ex
                     ternal  &&&&  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 25 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   column  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.

              •      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_L2 multicast group,
                     which ovn-northd populates with  all  non-router  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 en‐
                     abled 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 un‐
                     registered 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  op‐‐
                     tions :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  op‐‐
                     tions :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-75  flows  for  each port connected to a logical
                     router matching  self  originated  ARP  request/RARP  re‐
                     quest/ND  packets.  These  packets  are  flooded  to  the
                     MC_FLOOD_L2 which contains 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 ad‐
                     dress  against  eth.dst.  Action of this flow outputs the
                     packet to the single associated output port if it is  en‐
                     abled. drop; action is applied if LSP is disabled.

                     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 26 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.

              •      Priority 50 flow with the match outport == P is added for
                     each disabled Logical Switch Port P. This flow has action
                     drop;.

              •      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 ac‐
                     tion.

     Egress Table 0: 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  col‐
       umn 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 1: 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
       two priority-110 flows to move multicast, IPv6 Neighbor  Discovery  and
       MLD  traffic  to  the next table. If any load balancing rules exist for
       the datapath, a priority-100 flow is added with a match of ip  and  ac‐
       tion  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 committed 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  col‐
       umn of NB_Global table.

     Egress Table 2: Pre-stateful

       This  is similar to ingress table Pre-stateful. This table adds the be‐
       low 3 logical flows.

              •      A Priority-120 flow that send the packets  to  connection
                     tracker  using  ct_lb_mark; as the action so that the al‐
                     ready 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_mark 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; ac‐
                     tion.

              •      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.

       Similar to ingress table, a priority-65532 flow is added to allow  IPv6
       Neighbor  solicitation,  Neighbor discover, Router solicitation, Router
       advertisement and MLD packets regardless of other ACLs defined.

       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 al‐
                     low the DHCPv6 reply packet from the  Ingress  Table  18:
                     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 20: 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 column 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 - check

       This  is similar to the port security logic in table Ingress Port Secu‐‐
       rity check except that action check_out_port_sec is used to  check  the
       port security rules. This table adds the below logical flows.

              •      A  priority 100 flow which matches on the multicast traf‐
                     fic and applies the  action  REGBIT_PORT_SEC_DROP"  =  0;
                     next;" to skip the out port security checks.

              •      A  priority  0 logical flow is added which matches on all
                     the packets and applies the action  REGBIT_PORT_SEC_DROP"
                     =     check_out_port_sec();     next;".     The    action
                     check_out_port_sec applies the port security rules  based
                     on  the  addresses defined in the port_security column of
                     Logical_Switch_Port table before delivering the packet to
                     the outport.

     Egress Table 9: Egress Port Security - Apply

       This is similar to the ingress port security logic in ingress  table  A
       Ingress Port Security - Apply. This table drops the packets if the port
       security  check  failed in the previous stage i.e the register bit REG‐‐
       BIT_PORT_SEC_DROP is set to 1.

       The following flows are added.

              •      For each localnet port configured with egress qos in  the
                     options:qdisc_queue_id  column  of Logical_Switch_Port, a
                     priority 100 flow is added which matches on the  localnet
                     outport and applies the action set_queue(id); output;".

                     Please remember to mark the corresponding physical inter‐
                     face with ovn-egress-iface set to true in external_ids.

              •      A  priority-50 flow that drops the packet if the register
                     bit REGBIT_PORT_SEC_DROP is set to 1.

              •      A priority-0 flow that outputs the packet to the outport.

   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. If LRP’s logical switch has attached LSP  of
                     vtep type, the is_chassis_resident() part is not added to
                     lflow  to allow traffic originated from logical switch to
                     reach LR services (LBs, NAT).

                     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.  If
                     the  port is also configured with options:gateway_mtu_by‐‐
                     pass then another flow is added, with priority-55, to by‐
                     pass the check_pkt_larger flow. This is useful for  traf‐
                     fic  that  normally doesn’t need to be fragmented and for
                     which check_pkt_larger, which might not  be  offloadable,
                     is not really needed. One such example is TCP traffic.

              •      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 distributed  gateway  port
                     corresponding  to  the  NAT rule (specified or inferred),
                     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.

              •      A  priority-0  logical  flow that matches all packets not
                     already handled (match 1) and drops them (action drop;).

       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  be‐
                     longs to subnet S with prefix length L, if the option al‐‐
                     ways_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 re‐
                     quest) 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   al‐‐
                     ways_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-95 flow with the match nd_ns &&&& (ip6.src ==  0
                     || nd.sll == 0) and applies the action next;

              •      A priority-90 flow with the match arp and applies the ac‐
                     tion put_arp(inport, arp.spa, arp.sha); next;

              •      A  priority-95  flow with the match nd_na  &&&& nd.tll == 0
                     and  applies   the   action   put_nd(inport,   nd.target,
                     eth.src); 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;

              •      A  priority-0  logical  flow that matches all packets not
                     already handled (match 1) and drops them (action drop;).

     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  each dnat_and_snat NAT rule on a distributed logical
                     routers or gateway routers with gateway  port  configured
                     with  options:gateway_mtu  to  a valid integer value M, a
                     priority-160 flow with the match inport ==  LRP  &&&&  REG‐‐
                     BIT_PKT_LARGER  &&&& REGBIT_EGRESS_LOOPBACK == 0, where LRP
                     is the logical router port and applies the following  ac‐
                     tion for ipv4 and ipv6 respectively:

                     icmp4_error {
                         icmp4.type = 3; /* Destination Unreachable. */
                         icmp4.code = 4;  /* Frag Needed and DF was Set. */
                         icmp4.frag_mtu = M;
                         eth.dst = eth.src;
                         eth.src = E;
                         ip4.dst = ip4.src;
                         ip4.src = I;
                         ip.ttl = 255;
                         REGBIT_EGRESS_LOOPBACK = 1;
                         REGBIT_PKT_LARGER 0;
                         outport = LRP;
                         flags.loopback = 1;
                         output;
                     };
                     icmp6_error {
                         icmp6.type = 2;
                         icmp6.code = 0;
                         icmp6.frag_mtu = M;
                         eth.dst = eth.src;
                         eth.src = E;
                         ip6.dst = ip6.src;
                         ip6.src = I;
                         ip.ttl = 255;
                         REGBIT_EGRESS_LOOPBACK = 1;
                         REGBIT_PKT_LARGER 0;
                         outport = LRP;
                         flags.loopback = 1;
                         output;
                     };


                     where  E  and  I are the NAT rule external mac and IP re‐
                     spectively.

              •      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 ap‐
                     plies the following action  for  ipv4  and  ipv6  respec‐
                     tively:

                     icmp4_error {
                         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_error {
                         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(s)) of type snat, a  pri‐
                     ority-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 corresponding to the  NAT
                     entry  (specified  or  inferred) 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: Priority-120 flows allows IGMP and
                     MLD packets if the router has logical ports that have op‐‐
                     tions :mcast_flood=’’true’’.

              •      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  ad‐
                     dress 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  re‐
                     quest). For each A that is an IPv6 address, a priority-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  re‐
                     questor’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  re‐‐
                     side-on-redirect-chassis set (which is centralized),  the
                     above  flows  are only programmed on the gateway port in‐
                     stance on the gateway chassis (if the logical router  has
                     a distributed gateway port). This behavior avoids genera‐
                     tion  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,  so‐
                     licited  node address S, and Ethernet address E, a prior‐
                     ity-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 in‐
                     stance on the gateway chassis. This behavior avoids  gen‐
                     eration  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  ad‐
                     dress 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  fol‐
                     lowing 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  VIP is reachable from any logical router port of the
                     logical router.

                     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  ad‐
                     dress or a load balancer IPv6 VIP A (if the VIP is reach‐
                     able from any logical router port of the logical router),
                     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 be‐
                            havior 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  up‐
                            stream  MAC learning to point to the correct chas‐
                            sis.

              •      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 op‐‐
                     tions:mcast_relay=’true’, otherwise drops it.

              •      UDP port unreachable.  Priority-80  flows  generate  ICMP
                     port  unreachable  messages in reply to UDP datagrams di‐
                     rected 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.

              •      TCP  reset. Priority-80 flows generate TCP reset messages
                     in reply to TCP datagrams directed to the router’s IP ad‐
                     dress, except in the special case of gateways, which  ac‐
                     cept  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 or if it is used as a load balancer
                     VIP.

       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.

              •      Avoid ICMP time exceeded  for  multicast.  A  priority-32
                     flow  with  match  ip.ttl  == {0, 1} &&&& !ip.later_frag &&&&
                     (ip4.mcast || ip6.mcast) and actions drop;  drops  multi‐
                     cast  packets  whose TTL has expired without sending ICMP
                     time exceeded.

              •      ICMP time exceeded. For each router port P, whose IP  ad‐
                     dress  is A, a priority-31 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 respectively:

                     icmp4 {
                         icmp4.type = 11; /* Time exceeded. */
                         icmp4.code = 0;  /* TTL exceeded in transit. */
                         ip4.dst = ip4.src;
                         ip4.src = A;
                         ip.ttl = 254;
                         next;
                     };
                     icmp6 {
                         icmp6.type = 3; /* Time exceeded. */
                         icmp6.code = 0;  /* TTL exceeded in transit. */
                         ip6.dst = ip6.src;
                         ip6.src = A;
                         ip.ttl = 254;
                         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 next;.

                     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, two priority-100 flows are added.

                     If  the  NAT rule cannot be handled in a distributed man‐
                     ner, then the below  priority-100  flows  are  only  pro‐
                     grammed on the gateway chassis.

                     •      The  first  flow matches ip &&&& ip4.dst == B &&&& in‐‐
                            port == GW &&&& flags.loopback == 0 or ip &&&& ip6.dst
                            == B &&&& inport == GW &&&& flags.loopback == 0  where
                            GW  is  the distributed gateway port corresponding
                            to the NAT rule (specified or inferred),  with  an
                            action  ct_snat_in_czone;  to unSNAT in the common
                            zone. If the NAT rule is of type dnat_and_snat and
                            has stateless=true in the options, then the action
                            would be next;.

                            If the NAT entry is of type snat, then there is an
                            additional match is_chassis_resident(cr-GW)
                             where cr-GW is the chassis resident port of GW.

                     •      The second flow matches ip &&&& ip4.dst == B &&&&  in‐‐
                            port   ==   GW   &&&&   flags.loopback   ==   1   &&&&
                            flags.use_snat_zone == 1 or ip &&&& ip6.dst == B  &&&&
                            inport   ==   GW   &&&&   flags.loopback   ==  0  &&&&
                            flags.use_snat_zone == 1 where GW is the  distrib‐
                            uted  gateway  port  corresponding to the NAT rule
                            (specified or inferred), with an  action  ct_snat;
                            to  unSNAT in the snat zone. 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 entry is of type snat, then there is an
                            additional match is_chassis_resident(cr-GW)
                             where cr-GW is the chassis resident port of GW.

                     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-100 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 actions chk_ecmp_nh_mac(); ct_next or
       chk_ecmp_nh(); ct_next to send IP packets to table 76 or to table 77 in
       order to check if source info are already stored by OVN and then to the
       connection tracker for  packet  de-fragmentation  and  tracking  before
       sending it to the next table.

       If load balancing rules are configured in OVN_Northbound database for a
       Gateway  router,  a priority 50 flow that matches icmp || icmp6 with an
       action of ct_dnat;, this allows potentially  related  ICMP  traffic  to
       pass through CT.

     Ingress Table 6: Load balancing affinity check

       Load  balancing  affinity  check  table  contains the following logical
       flows:

              •      For all the configured load balancing rules for a logical
                     router where a positive affinity timeout is specified  in
                     options  column, that includes a L4 port PORT of protocol
                     P and IPv4 or IPv6 address VIP, a priority-100 flow  that
                     matches  on  ct.new  &&&&  ip  &&&&  reg0  ==  VIP  &&&&  P  &&&&
                     reg9[16..31] ==  PORT (xxreg0 == VIP
                      in  the  IPv6  case)  with  an  action  of   reg9[6]   =
                     chk_lb_aff(); next;

              •      A  priority  0 flow is added which matches on all packets
                     and applies the action next;.

     Ingress Table 7: 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 7: 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.

              •      For all the configured load balancing rules for a logical
                     router where a positive affinity timeout is specified  in
                     options  column, that includes a L4 port PORT of protocol
                     P and IPv4 or IPv6 address VIP, a priority-150 flow  that
                     matches on reg9[6] == 1 &&&& ct.new &&&& ip &&&& reg0 == VIP &&&&
                     P  &&&&  reg9[16..31]  ==   PORT (xxreg0 == VIP in the IPv6
                     case) with an action of  ct_lb_mark(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  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 &&&& !ct.rel &&&& ip &&&& reg0 == VIP &&&& P &&&&
                     reg9[16..31] ==
                      PORT (xxreg0 == VIP
                      in the IPv6 case) with an  action  of  ct_lb_mark(args),
                     where  args  contains  comma  separated  IPv4 or IPv6 ad‐
                     dresses (and optional port numbers) to load  balance  to.
                     If  the  router is configured to force SNAT any load-bal‐
                     anced packets, the  above  action  will  be  replaced  by
                     flags.force_snat_for_lb  =  1;  ct_lb_mark(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_mark(args);. If  health
                     check  is enabled, then args will only contain those end‐
                     points whose service monitor status entry  in  OVN_South‐‐
                     bound 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 &&&& !ct.rel &&&& 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  ad‐
                     dress  VIP  to match on, a priority-110 flow that matches
                     on ct.new &&&& !ct.rel &&&& ip4  &&&&  reg0  ==  VIP  (ip6  and
                     xxreg0  ==  VIP  in  the  IPv6  case)  with  an action of
                     ct_lb_mark(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_mark(args);. If the load balancing rule is  config‐
                     ured with skip_snat set to true, the above action will be
                     replaced      by      flags.skip_snat_for_lb     =     1;
                     ct_lb_mark(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  ad‐
                     dress  VIP  to match on, a priority-110 flow that matches
                     on ct.est &&&& !ct.rel &&&& 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  re‐
                     ceived for this load-balancer. Please note using --reject
                     option will disable empty_lb SB controller event for this
                     load balancer.

              •      For  the related traffic, a priority 50 flow that matches
                     ct.rel &&&& !ct.est &&&& !ct.new  with an action  of  ct_com‐‐
                     mit_nat;, if the router has load balancer assigned to it.
                     Along with two priority 70 flows that match skip_snat and
                     force_snat flags.

       Ingress Table 7: 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 ac‐
                     tion  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  ==  al
                     lowed_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: 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 corresponding to the NAT rule (speci‐
                     fied or inferred), 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  ==  al
                     lowed_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 8: Load balancing affinity learn

       Load  balancing  affinity  learn  table  contains the following logical
       flows:

              •      For all the configured load balancing rules for a logical
                     router where a positive affinity timeout T  is  specified
                     in options
                      column,  that  includes a L4 port PORT of protocol P and
                     IPv4 or  IPv6  address  VIP,  a  priority-100  flow  that
                     matches on reg9[6] == 0 &&&& ct.new &&&& ip &&&& reg0 == VIP &&&&
                     P  &&&&  reg9[16..31]  ==  PORT (xxreg0 == VIP  in the IPv6
                     case) with an action  of  commit_lb_aff(vip  =  VIP:PORT,
                     backend  = backend ip: backend port, proto = P, timeout =
                     T);.

              •      A priority 0 flow is added which matches on  all  packets
                     and applies the action next;.

     Ingress Table 9: 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_mark.ecmp_reply_port   =   K;};   com‐‐
                     mit_ecmp_nh(); next;
                      to commit the connection and  storing  eth.src  and  the
                     ECMP  reply port binding tunnel key K in the ct_label and
                     the traffic pattern to table 76 or 77.

     Ingress Table 10: 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 11: 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 12: IP Routing Pre

       If  a packet arrived at this table from Logical Router Port P which has
       options:route_table value set, a logical flow with match inport ==  "P"
       with  priority  100  and  action  setting unique-generated per-datapath
       32-bit value (non-zero) in OVS register 7.  This  register’s  value  is
       checked  in  next  table.  If packet didn’t match any configured inport
       (<lt;main>gt; route table), register 7 value is set to 0.

       This table contains the following logical flows:

              •      Priority-100 flow with match inport ==  "LRP_NAME"  value
                     and action, which set route table identifier in reg7.

                     A priority-0 logical flow with match 1 has actions reg7 =
                     0; next;.

     Ingress Table 13: 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 with a priority-10300 rule if the traffic going out the ECMP
       route  is  reply traffic, and the ECMP route was configured to use sym‐
       metric replies. Instead, the stored values  in  conntrack  is  used  to
       choose  the destination. The ct_label.ecmp_reply_eth tells the destina‐
       tion  MAC  address  to  which  the   packet   should   be   sent.   The
       ct_mark.ecmp_reply_port  tells  the  logical  router  port on which the
       packet should be sent. These values saved to the conntrack fields  when
       the initial ingress traffic is received over the ECMP route and commit‐
       ted  to  conntrack.  If REGBIT_KNOWN_ECMP_NH is set, the priority-10300
       flows in this stage set the outport, while the eth.dst is set by  flows
       at the ARP/ND Resolution stage.

       This table contains the following logical flows:

              •      Priority-10550  flow  that  drops  IPv6  Router Solicita‐
                     tion/Advertisement packets that  were  not  processed  in
                     previous tables.

              •      Priority-10550  flows that drop IGMP and MLD packets with
                     source MAC address owned by the router. These are used to
                     prevent looping statically forwarded IGMP and MLD packets
                     for which TTL is not decremented (it is always 1).

              •      Priority-10500 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  in‐
                     terested attached logical switches. The flows also decre‐
                     ment TTL.

              •      Priority-10460  flows  that  match  IGMP  and MLD control
                     packets, set 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.

              •      Priority-10450 flow that matches unregistered  IP  multi‐
                     cast  traffic  decrements  TTL  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 each static route the reg7 == id &&&&  is  prefixed  in
                     logical  flow  match portion. For routes with route_table
                     value set a unique non-zero id is used. For routes within
                     main>gt;>gt; route table (no route table set), this id value is
                     0.

                     For each connected route (route to the LRP’s subnet CIDR)
                     the logical flow match portion has no reg7 == id &&&&  pre‐
                     fix to have route to LRP’s subnets in all routing tables.

              •      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 ac‐
                     tions:

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


              •      A  priority-0  logical  flow that matches all packets not
                     already handled (match 1) and drops them (action drop;).

     Ingress Table 14: 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;


              •      A  priority-0  logical  flow that matches all packets not
                     already handled (match 1) and drops them (action drop;).

     Ingress Table 15: 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 ac‐
                     tions:

                     [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 16: 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.

              •      A  priority-0  logical  flow that matches all packets not
                     already handled (match 1) and drops them (action drop;).

     Ingress Table 17: 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.

              •      Priority-200 flows that match ECMP reply traffic for  the
                     routes  configured to use symmetric replies, with actions
                     push(xxreg1);    xxreg1    =    ct_label;    eth.dst    =
                     xxreg1[32..79];  pop(xxreg1);  next;. xxreg1 is used here
                     to avoid masked access to ct_label, to make the flow  HW-
                     offloading friendly.

              •      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:dy‐‐
                     namic_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 &&&& xxreg0/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  &&&&  xxreg0/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, below two flows are pro‐
                     grammed:

                     A priority-100 flow with the match outport == P  &&&&  reg0
                     ==  A has actions eth.dst = E; next;, where P is the dis‐
                     tributed logical router port, E is the  Ethernet  address
                     if  set  in  the  external_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  exter‐‐
                     nal_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.

                     Corresponding to the above flow, a priority-150 flow with
                     the match inport == P &&&& outport == P &&&& ip4.dst == A has
                     actions  drop;  to exclude packets that have gone through
                     DNAT/unSNAT stage but failed to convert the  destination,
                     to avoid loop.

                     For IPv6 NAT entries, same flows are added, but using the
                     register xxreg0 and field ip6 for the match.

              •      If the router datapath runs a port with redirect-type set
                     to  bridged,  for  each distributed NAT rule with IP A in
                     the logical_ip column and logical port  P  in  the  logi‐‐
                     cal_port column of NAT table, a priority-90 flow with the
                     match  outport  ==  Q &&&& ip.src === A &&&& is_chassis_resi‐‐
                     dent(P), where Q is the distributed logical  router  port
                     and  action  get_arp(outport,  reg0);  next; for IPv4 and
                     get_nd(outport, xxreg0); next; for IPv6.

              •      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 un‐
                     SNAT 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-2  logical  flow  with  match  ip4.dst = {..}
                     matches on traffic destined  to  router  owned  IPv4  ad‐
                     dresses  which  are  also  SNAT IPs. This flow has action
                     drop;.

                     A priority-2 logical  flow  with  match  ip6.dst  =  {..}
                     matches  on  traffic  destined  to  router owned IPv6 ad‐
                     dresses which are also SNAT IPs.  This  flow  has  action
                     drop;.

                     A  priority-0  logical  that flow matches all packets not
                     already handled (match 1) and drops them (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 18: 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.

       If the port is also configured with options:gateway_mtu_bypass then an‐
       other flow is added, with priority-55, to bypass  the  check_pkt_larger
       flow.

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

     Ingress Table 19: 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 20: 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 distributed gate‐
                     way port specified in the NAT rule and P is the NAT logi‐
                     cal port. IP traffic matching the above rule will be man‐
                     aged locally setting reg1 to C and eth.src to D, where  C
                     is NAT external ip and D is NAT external mac.

              •      For  each  dnat_and_snat NAT rule with stateless=true and
                     allowed_ext_ips configured, a priority-75  flow  is  pro‐
                     grammed  with match ip4.dst == B and action outport = CR;
                     next; where B is the NAT rule external IP and CR  is  the
                     chassisredirect  port  representing  the  instance of the
                     logical router distributed gateway port  on  the  gateway
                     chassis.  Moreover  a priority-70 flow is programmed with
                     same match and action drop;. For each  dnat_and_snat  NAT
                     rule with stateless=true and exempted_ext_ips configured,
                     a  priority-75 flow is programmed with match ip4.dst == B
                     and action drop; where B is the NAT rule external  IP.  A
                     similar flow is added for IPv6 traffic.

              •      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 21: 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  ac‐
                     tions 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 ac‐
                     tions output;.

     Egress Table 0: Check DNAT local

       This  table  checks  if  the  packet  needs  to be DNATed in the router
       ingress table lr_in_dnat after it is SNATed  and  looped  back  to  the
       ingress  pipeline.  This check is done only for routers configured with
       distributed gateway ports and NAT entries. This check is done  so  that
       SNAT and DNAT is done in different zones instead of a common zone.

              •      For  each  NAT  rule  in the OVN Northbound database on a
                     distributed router, a priority-50 logical flow with match
                     ip4.dst == E &&&& is_chassis_resident(P), where  E  is  the
                     external  IP address specified in the NAT rule, GW is the
                     logical   router   distributed    gateway    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 actions: REGBIT_DST_NAT_IP_LOCAL = 1; next;

              •      A priority-0 logical flow with match 1 has  actions  REG‐‐
                     BIT_DST_NAT_IP_LOCAL = 0; next;.

       This  table  also  installs a priority-50 logical flow for each logical
       router that has NATs configured on it. The flow has match ip &&&&  ct_la‐‐
       bel.natted  == 1 and action REGBIT_DST_NAT_IP_LOCAL = 1; next;. This is
       intended to ensure that traffic that was DNATted  locally  will  use  a
       separate  conntrack  zone  for  SNAT  if  SNAT is required later in the
       egress pipeline. Note that this flow checks the value of  ct_label.nat‐‐
       ted,  which  is set in the ingress pipeline. This means that ovn-northd
       assumes that this value is carried over from the  ingress  pipeline  to
       the  egress  pipeline and is not altered or cleared. If conntrack label
       values are ever changed to be cleared between the  ingress  and  egress
       pipelines,  then  the match conditions of this flow will be updated ac‐
       cordingly.

     Egress Table 1: 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.

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

     Egress Table 1: UNDNAT on Gateway Routers

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

     Egress Table 1: UNDNAT on Distributed Routers

              •      For all the configured load balancing rules for a  router
                     with  gateway  port  in  OVN_Northbound database that in‐
                     cludes an IPv4 address VIP, for every  backend  IPv4  ad‐
                     dress  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_in_czone;. If the backend
                     IPv4 address B is also configured with L4  port  PORT  of
                     protocol  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_in_czone;.  If  the   NAT   rule   is   of   type
                     dnat_and_snat and has stateless=true in the options, then
                     the action would be next;.

                     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 with the action ct_dnat_in_czone.

                     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.

     Egress Table 2: Post UNDNAT

              •      A priority-50 logical flow is added that commits any  un‐
                     tracked  flows  from the previous table lr_out_undnat for
                     Gateway routers. 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 3: 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 through conntrack.

       Egress Table 3: 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  ap‐
                     plied 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   op‐‐
                     tions:lb_force_snat_ip=router_ip),  then for each logical
                     router port P attached to the Gateway  router,  a  prior‐
                     ity-110 flow matches flags.force_snat_for_lb == 1 &&&& out‐‐
                     port == 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 ad‐
                     dress of a packet that belongs to network A to B, a  flow
                     matches  ip  &&&& ip4.src == A &&&& (!ct.trk || !ct.rpl) with
                     an action ct_snat(B);. The priority of the flow is calcu‐
                     lated based on the mask of A, with matches having  larger
                     masks  getting  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  == al
                     lowed_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 3: 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 ad‐
                     dress of a packet that belongs to network  A  to  B,  two
                     flows are added. The priority P of these flows are calcu‐
                     lated  based on the mask of A, with matches having larger
                     masks getting higher priorities.

                     If the NAT rule cannot be handled in a  distributed  man‐
                     ner,  then  the  below  flows  are only programmed on the
                     gateway chassis increasing flow priority by 128 in  order
                     to be run first.

                     •      The first flow is added with the calculated prior‐
                            ity  P  and match ip &&&& ip4.src == A &&&& outport ==
                            GW, where GW is the logical router  gateway  port,
                            with  an  action ct_snat_in_czone(B); to SNATed in
                            the common zone.  If  the  NAT  rule  is  of  type
                            dnat_and_snat  and  has  stateless=true in the op‐
                            tions, then the action would be ip4/6.src=(B).

                     •      The second flow is added with the calculated  pri‐
                            ority  P + 1  and match ip &&&& ip4.src == A &&&& out‐‐
                            port == GW &&&& REGBIT_DST_NAT_IP_LOCAL == 0,  where
                            GW is the logical router gateway port, with an ac‐
                            tion  ct_snat(B); to SNAT in the snat zone. 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 can be handled in a distributed manner,
                     then there is an additional action (for both  the  flows)
                     eth.src  =  EA;, where EA is the ethernet address associ‐
                     ated 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  ==  al
                     lowed_ext_ips.

                     If  the  NAT rule has exempted_ext_ips set, then there is
                     an additional flow configured at the priority P +  2   of
                     corresponding  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 4: 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 distributed gateway  port  corre‐
                     sponding  to  the  NAT  rule (specified or inferred). 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;
                         flags.use_snat_zone = REGBIT_DST_NAT_IP_LOCAL;
                         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 ad‐
                     dress check against the router address.

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

     Egress Table 5: 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 en‐
                     abled logical router port, with action output;.

              •      A priority-0 logical flow that matches  all  packets  not
                     already handled (match 1) and drops them (action drop;).

DROP SAMPLING
       As  described  in  the previous section, there are several places where
       ovn-northd might decided to drop a packet by explicitly creating a Log‐‐
       ical_Flow with the drop; action.

       When debug drop-sampling has been cofigured in the OVN Northbound data‐
       base, the ovn-northd will replace all the drop;  actions  with  a  sam‐‐
       ple(priority=65535,         collector_set=id,        obs_domain=obs_id,
       obs_point=@cookie) action, where:

              •      id is the value the debug_drop_collector_set option  con‐
                     figured in the OVN Northbound.

              •      obs_id  has  it’s  8  most  significant bits equal to the
                     value of  the  debug_drop_domain_id  option  in  the  OVN
                     Northbound  and  it’s  24 least significant bits equal to
                     the datapath’s tunnel key.

OVN 22.12.3                       ovn-northd                     ovn-northd(8)