Refactor Relay for Symmetric NAT

[joshuakarp]

Created by @CMCDragonkai

The following discussion from 'Refactoring Network Module' MR should be addressed:

• @Zachaccino: (+2 comments)

  Nat Type and Strategy

  Full Cone NAT

  Once an internal address (iAddr:iPort) is mapped to an external address (eAddr:ePort), any packets from iAddr:iPort are sent through eAddr:ePort.

  Any external host can send packets to iAddr:iPort by sending packets to eAddr:ePort.

  Single sided hole punching would be sufficient.

  Address Restricted Cone NAT

  Once an internal address (iAddr:iPort) is mapped to an external address (eAddr:ePort), any packets from iAddr:iPort are sent through eAddr:ePort.

  An external host (hAddr:any) can send packets to iAddr:iPort by sending packets to eAddr:ePort only if iAddr:iPort has previously sent a packet to hAddr:any. "Any" means the port number doesn't matter.

  Require coordinated 2 sided hole punching.

  Port Restricted Cone NAT

  Once an internal address (iAddr:iPort) is mapped to an external address (eAddr:ePort), any packets from iAddr:iPort are sent through eAddr:ePort.

  An external host (hAddr:hPort) can send packets to iAddr:iPort by sending packets to eAddr:ePort only if iAddr:iPort has previously sent a packet to hAddr:hPort.

  Require coordinated 2 sided hole punching.

  Symmetric NAT

  Each request from the same internal IP address and port to a specific destination IP address and port is mapped to a unique external source IP address and port; if the same internal host sends a packet even with the same source address and port but to a different destination, a different mapping is used.

  Only an external host that receives a packet from an internal host can send a packet back.

  Not Punchable, Relay is needed.

[CMCDragonkai]

I beleive @joshuakarp is the best person to help work on this.

[CMCDragonkai]

This is superseded by #182.