I work on software for p2p networking and I'm always researching strange things because of it. One thing that I've discovered is that SIP has a mechanism to rel

I work on software for p2p networking and I'm always researching strange things because of it. One thing that I've discovered is that SIP has a mechanism to relay a single message to a single IP. I know this might not sound like much but I think that theoretically it might offer a way to reach nodes behind symmetric NATs (which is the most restrictive type of NAT.) The idea is that you would have a list of N SIP servers who you send packets to on sip_ip:sip_port. This creates a stateful rule allowing back packets for the server in the router. Now anyone is able to use that SIP server to relay messages to you so long as you listen on the right ports. This could be useful for highly restrictive NATs like symmetric NATs that only reuse external mappings if an inbound connection uses the same IP and port (more applicable for UDP.) If you can get one, just ONE message to a peer then you can use it to exchange information on strategies to connect directly to them. E.g. TCP hole punching. There are a metric crap load of SIP servers out there and any one of them would effectively enable you to exchange information with a symmetric NAT and certain firewalls. I think I did basic tests for this ages ago between multiple Internet connections and it seemed to work. So I think this has potential in p2p networking and decentralized apps.