Add support for connection tracking.

Add support for connection tracking.

This patch adds a new action and fields to OVS that allow connection
tracking to be performed. This support works in conjunction with the
Linux kernel support merged into the Linux-4.3 development cycle.

Packets have two possible states with respect to connection tracking:
Untracked packets have not previously passed through the connection
tracker, while tracked packets have previously been through the
connection tracker. For OpenFlow pipeline processing, untracked packets
can become tracked, and they will remain tracked until the end of the
pipeline. Tracked packets cannot become untracked.

Connections can be unknown, uncommitted, or committed. Packets which are
untracked have unknown connection state. To know the connection state,
the packet must become tracked. Uncommitted connections have no
connection state stored about them, so it is only possible for the
connection tracker to identify whether they are a new connection or
whether they are invalid. Committed connections have connection state
stored beyond the lifetime of the packet, which allows later packets in
the same connection to be identified as part of the same established
connection, or related to an existing connection - for instance ICMP
error responses.

The new 'ct' action transitions the packet from "untracked" to
"tracked" by sending this flow through the connection tracker.
The following parameters are supported initally:

- "commit": When commit is executed, the connection moves from
  uncommitted state to committed state. This signals that information
  about the connection should be stored beyond the lifetime of the
  packet within the pipeline. This allows future packets in the same
  connection to be recognized as part of the same "established" (est)
  connection, as well as identifying packets in the reply (rpl)
  direction, or packets related to an existing connection (rel).
- "zone=[u16|NXM]": Perform connection tracking in the zone specified.
  Each zone is an independent connection tracking context. When the
  "commit" parameter is used, the connection will only be committed in
  the specified zone, and not in other zones. This is 0 by default.
- "table=NUMBER": Fork pipeline processing in two. The original instance
  of the packet will continue processing the current actions list as an
  untracked packet. An additional instance of the packet will be sent to
  the connection tracker, which will be re-injected into the OpenFlow
  pipeline to resume processing in the specified table, with the
  ct_state and other ct match fields set. If the table is not specified,
  then the packet is submitted to the connection tracker, but the
  pipeline does not fork and the ct match fields are not populated. It
  is strongly recommended to specify a table later than the current
  table to prevent loops.

When the "table" option is used, the packet that continues processing in
the specified table will have the ct_state populated. The ct_state may
have any of the following flags set:

- Tracked (trk): Connection tracking has occurred.
- Reply (rpl): The flow is in the reply direction.
- Invalid (inv): The connection tracker couldn't identify the connection.
- New (new): This is the beginning of a new connection.
- Established (est): This is part of an already existing connection.
- Related (rel): This connection is related to an existing connection.

For more information, consult the ovs-ofctl(8) man pages.

Below is a simple example flow table to allow outbound TCP traffic from
port 1 and drop traffic from port 2 that was not initiated by port 1:

    table=0,priority=1,action=drop
    table=0,arp,action=normal
    table=0,in_port=1,tcp,ct_state=-trk,action=ct(commit,zone=9),2
    table=0,in_port=2,tcp,ct_state=-trk,action=ct(zone=9,table=1)
    table=1,in_port=2,ct_state=+trk+est,tcp,action=1
    table=1,in_port=2,ct_state=+trk+new,tcp,action=drop

Based on original design by Justin Pettit, contributions from Thomas
Graf and Daniele Di Proietto.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>