Support multiple UDP source ports (multiport)

[wadey]

The goal of this work is to send packets between two hosts using more than one 5-tuple. When running on networks like AWS where the underlying network driver and overlay fabric makes routing, load balancing, and failover decisions based on the flow hash, this enables more than one flow between pairs of hosts.

Multiport spreads outgoing UDP packets across multiple UDP send ports, which allows nebula to work around any issues on the underlay network. Some example issues this could work around:

• UDP rate limits on a per flow basis.
• Partial underlay network failure in which some flows work and some don't

Agreement is done during the handshake to decide if multiport mode will be used for a given tunnel (one side must have tx_enabled set, the other side must have rx_enabled set)

NOTE: you cannot use multiport on a host if you are relying on UDP hole punching to get through a NAT or firewall.

NOTE: Linux only (uses raw sockets to send). Also currently only works with IPv4 underlay network remotes.

This is implemented by opening a raw socket and sending packets with a source port that is based on a hash of the overlay source/destiation port. For ICMP and Nebula metadata packets, we use a random source port.

Example configuration:

multiport:
  # This host support sending via multiple UDP ports.
  tx_enabled: false

  # This host supports receiving packets sent from multiple UDP ports.
  rx_enabled: false

  # How many UDP ports to use when sending. The lowest source port will be
  # listen.port and go up to (but not including) listen.port + tx_ports.
  tx_ports: 100

  # NOTE: All of your hosts must be running a version of Nebula that supports
  # multiport if you want to enable this feature. Older versions of Nebula
  # will be confused by these multiport handshakes.
  #
  # If handshakes are not getting a response, attempt to transmit handshakes
  # using random UDP source ports (to get around partial underlay network
  # failures).
  tx_handshake: false

  # How many unresponded handshakes we should send before we attempt to
  # send multiport handshakes.
  tx_handshake_delay: 2

[brad-defined]

Branch now has conflicts, needs updating