NAT type detection

Determine in advance if you can complete NAT punchthrough
NAT punchthrough success rates is based on what kind of algorithm the NAT uses.

Full cone NAT: Accepts any datagrams to a port that has been previously used. Will accept the first datagram from the remote peer.

Address-Restricted cone NAT: Accepts datagrams to a port as long as the datagram source IP address is a system we have already sent to. Will accept the first datagram if both systems send simultaneously. Otherwise, will accept the first datagram after we have sent one datagram.

Port-Restricted cone NAT: Same as address-restricted cone NAT, but we had to send to both the correct remote IP address and correct remote port. The same source address and port to a different destination uses the same mapping.

Symmetric NAT: A different port is chosen for every remote destination. The same source address and port to a different destination uses a different mapping. Since the port will be different, the first external punchthrough attempt will fail. For this to work it requires port-prediction (MAX_PREDICTIVE_PORT_RANGE>1) and that the router chooses ports sequentially.

Success Graph

Router Type | Full cone NAT | Address-Restricted cone NAT | Port-Restricted cone NAT | Symmetric NAT
Full cone NAT | YES | YES | YES | YES
Address-Restricted cone NAT | YES | YES | YES | YES
Port-Restricted cone NAT | YES | YES | YES | NO
Symmetric NAT | YES | YES | NO | NO

The NatTypeDetection plugins allow you to determine which type of NAT you have, and therefore if NAT punchthrough is likely to complete or not. This can be determined in advance of joining a game.

NAT Type Detection algorithm

1. Client opens two ports on the same IP address. In NatTypeDetectionClient, the RakNet socket is the first port, c2 is the socket on the second port, created in NatTypeDetectionClient::DetectNATType()
2. Server opens two ports on one IP address, and one port on three additional IP addresses. This is done in NatTypeDetectionServer::Startup(). The first port on the first IP address is the normal RakNet socket. The second port on the first IP address is s1p2. The other three addresses are bound to the sockets s2p3, s3p4, and s4p5.
3. Client connects to the server on the first IP address normally
4. Client requests NAT type detection begins
5. Server attempts to send to client's second port. This was a port that was never opened previously on the client, therefore if received, then the client is not behind a NAT. This is done in NatTypeDetectionServer::Update(), defined by STATE_TESTING_NONE_1 and STATE_TESTING_NONE_2. The reason for 2 attempts is that every attempt occurs twice. The time for each attempt is the ping * 3 + 50 milliseconds. s4p5 is used for this.
6. Server sends from a different IP address to client's first port, the port that RakNet is connected on. If received, then the client will accept datagrams from any source IP address to a port already in use. This is therefore full-cone NAT. s2p3 is used for this.
7. Server sends from the second port on the already-connected IP address, s1p2. If received, then the client is using address-restricted cone NAT.
8. Client sends to another IP address on the server, from its first (already connected) port. If the IP address and port is the same, then the client uses the same external IP address and port to all connections from the same source address. This is port-restricted NAT.
9. Else symmetric NAT.

Client Implementation

1. Create an instance of the plugin: NatTypeDetectionServer nayTypeDetectionClient;
2. Attach the plugin to an instance of RakPeerInterface: rakPeer->AttachPlugin(&nayTypeDetectionClient);
3. Connect to the server, and wait for ID_CONNECTION_REQUEST_ACCEPTED. Use the following line to use the free server provided by RakNet: rakPeer->Connect("8.17.250.34", 60481, 0, 0);
4. Call DetectNATType with the SystemAddress of the server.
5. Wait for ID_NAT_TYPE_DETECTION_RESULT
6. Byte 1 contains the type of NAT that you have. See the enumeration NATTypeDetectionResult in NATTypeDetectionCommon.h
7. Various utility functions are provided for this enumeration: CanConnect(), NATTypeDetectionResultToString(), NATTypeDetectionResultToStringFriendly()

Server Implementation

1. Host a server somewhere, not using NAT / e.g. behind a firewall. (RakNet provides a free one at 8.17.250.34:60481, however you may wish to host your own for consistent uptime). The server must have enough external IP addresses, as described in NAT Type Detection Algorithm.
2. Create an instance of the plugin: NatTypeDetectionServernatTypeDetectionServer;
3. Attach the plugin: rakPeer->AttachPlugin(&natTypeDetectionServer);
4. Get a list of IP addresses on your system
   char ipList[ MAXIMUM_NUMBER_OF_INTERNAL_IDS ][ 16 ];
   unsigned int binaryAddresses[MAXIMUM_NUMBER_OF_INTERNAL_IDS];
   SocketLayer::Instance()->GetMyIP( ipList, binaryAddresses );
5. Call natTypeDetectionServer.Startup(ip2,ip3,ip4); // ip2,ip3,ip4 must be ip addresses not already in use. If you bound RakNet to ip1 in the call to RakPeer::Startup(), then use the 2nd to 4th indices in ipList.

Sample
See the sample \Samples\NATCompleteClient

This is also implemented for the server in \Samples\NATCompleteServer

See Also

Index NAT Punchthrough