ADC protocol extensions supported:

STATUS: **** Not yet implemented.

the 'AUT0' extension (network connection auto detection extension).
Rationale: Detect if client is capable of initiating active connections.

After logging in:

Client -> 'HCHK 12345 ABCD'.

Server sends a UDP packet containing token to the client's IP address at the given port as shown in the INF message.

Server -> 'ICHK ABCD'

The server should send it from a UDP socket bound to the same port as the TCP server, which means the server will
have to listen to both TCP and UDP.

If client receives the packet, it knows it can receive UDP connections, and will advertise it in the INF message as
a supported feature.

If the client does not receive any UDP packets within a few seconds, it MAY try to reconfigure the router using 
UPnP/ZeroConf, and issue a HCHK command to the server again.

If the client still doesn't receive any UDP packets, hole punching can be tried.
The server will send a UDP packet to the hub (using the port of the TCP server), and reissue the HCHK command.
The UDP packet SHOULD be echoed by the hub.
This UDP packet should contain simply 'HECH {cid} {token}' (Hub echo).

The hub should send a packet containing the token back:
'IECH {token} {host:port}', as well as the same message via TCP.

If the client receives the message via UDP, it should now be able to determine the type of NAT.
If the client receives the message via TCP only it knows it has a firewall blocking incoming communication.
If the client does not receive the message, it should assume a firewall is blocking all UDP communication,
and resume in passive mode.

Requirements:
'AUT0' in the extensions message (SUP) for client and hub.
Server will also listen to UDP messages on the same port as the TCP port.

The server MUST now respond to the 'HCHK' command via TCP
The server MUST now respond to the 'HECH' command via UDP.

The client will always initiate communication.

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Syntax: HCHK {port} {token}
- port is a 16-bit port number where the client is listening for packets.
- token is 4 bytes base32 encoded data specified by the client.

Example:
Client: 'HCHK 1511 BACD' (tcp)
Server: 'ICHK BACD' (udp)

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Syntax: HECH {cid} {token}
- cid is the client ID.
- token is 4 bytes base32 encoded data specified by the client.

Example:
Client: 'HECH 3NGFVJUDBRHX2CYRJGQ5HACRDM5CTNLGZ36M64I BACD' (udp)
Server: 'IECH BACD 192.168.0.1:1512' (udp)
Server: 'IECH BACD 192.168.0.1:1512' (tcp)

Security considerations:
The hub must verify that IP address where the HECH command originated from
matches the IP address where the user with the given CID is connected from.
If the CID and IP does not match, or the CID is not used the hub MUST ignore
the request.