NAT traversal

One side behind NAT

In this scenario, we assume one host has a public IP while the other doesn't. We are assuming that the side behind NAT is a receiver. If it were a sender only, the situation is trivial and doesn't need any workaround.

The idea behind the approach described below is that the computer behind NAT initializes the connection. It is analogous to TCP in a sense that the NAT keeps track of the "connection" state even for UDP. Client behind the NAT is required to send keepalive data to keep the UDP hole open the whole time (even if it acts as a receiver only).

In easiest scenario, we create an UDP hole from inside and send to it from outside. We are assuming use of default ports (5004 for video, 5006 for audio). Assuming that A has public IP while B is behind NAT (doesn't have a public IP address).

A ---------------- B NAT --- B
^                     ^      ^
|                     |       \
A IP (pub)      NAT IP (pub)   B IP (private)

This approach utilizes a concept of UDP hole punching. Follows the setup of both machines.

Behind NAT

If we act in a full-duplex way (both sender and receiver), you can use simply:

uv -t decklink -c JPEG -d gl A

Please note that both -t and -d are required.

If you want to receive only, use something like:

uv -t testcard:32:32:10:UYVY -d gl A

to generate a dummy keepalive traffic (to keep the connection open).

Outside NAT

In easiest case you can run UltraGrid as usual (assuming you now B's NAT IP address):

uv -t decklink -d gl NAT

This is sufficient if you send and receive, if outside UltraGrid is sender only, use:

uv -t decklink -c JPEG -P 5004:5004

This is because if UltraGrid acts as a sender only, it uses random source port which may confuse NAT.

This works as long as you know the NAT address and NAT keeps port numbers unchanged. If not, you may obtain the address of the B's NAT and source port by runnig:

sudo jnettop -i eth0 -x 'udp port 5004'

There you should see address and src port (in jnettop the column right next to address) of the NAT.

With this knowledge, you can send to B using the NAT address and port:

uv -t decklink -P 5004:NAT_RX NAT_IP

Audio

Audio setup is not described to details for simplicity but is analogous to video - we need always send data from inside the NAT (host B) to keep the UDP hole opened.

If the NAT changes port numbers you will also need to obtain the audio source port used by NAT (can be done by jnettop - see above). Full commands may look like:

B$ uv -t decklink -s analog A

A$ uv -t decklink -s alsa -P 5004:NAT_VID_RX:5006:NAT_AUD_RX NAT_IP

or simply (if NAT keeps port numbers):

A$ uv -t decklink -s alsa NAT_IP