What percentage of users are behind symmetric NATs, such that "p2p" traffic needs to be relayed?

Question

We're implementing a SIP-based solution and have configured the setup to work with RTPProxy. Right now, we're routing everything through RTPProxy as we were having some issues with media transport relying on ICE. If we're not mistaken, a central relay server is necessary for relaying streaming data between two clients if they're behind symmetric NATs. In practice, is this a large percentage of all consumer users? How much bandwidth woudl we save if we implemented proper routing to skip the relay server when not necessary. Are there better solutions we're missing?

Answer 1

In falling order of usefulness:

• There is a direct connection between the two endpoints in both directions. You just connect and you are essentially done.
• There is a direct connection between the two endpoints in one direction. In that case you just connect via the right direction by trying both.
• Both parties are behind NATs of some kind.
  • Luckily, UPnP works in one end, you can then upgrade the connection to the above scheme
  • UPnP doesn't work, but STUN does. Use it to punch a hole in the NAT. There are a couple of different protocols but the general trick is to negotiate via a middle man that coordinates the NAT-piercing.
  • You fall back to let another node on the network act as a relaying proxy.

If you implement the full list above, then you have to give up very few connections and don't have to spend much time on bandwidth utilization at proxies. The BitTorrent protocol, of which I am somewhat familiar, usually stops at UPnP, but provides a built-in test to test for connectivity through the NAT.

One really wonders why IPv6 did not get implemented earlier - this is a waste of programmers time.