Or they are centrally/corporate-controlled and do not allow hole punching.
https://github.com/tbocek/qotp and https://github.com/qh-project/qh
The main idea is to have a simple encryption (ed25519/chacha20+poly1305) for encryption in the transport layer, on top of that then qh, where certs are use for signing content.
With out of band key exchange, you can establish a connection after you successfully punched a hole.
However, its not QUIC compatible in any way (https://xkcd.com/927)
... how does that work when the network disallows UDP altogether?
If you're really really desperate you can send UDP packets with fake TCP headers (i.e. you aren't actually doing any congestion control or retransmission) but you have to control both ends of the connection for that.
And there's ICMP.
"Cannot" is a strong word:
> UDP hole punching will not work with symmetric NAT devices (also known as bi-directional NAT) which tend to be found in large corporate networks. In symmetric NAT, the NAT's mapping associated with the connection to the known STUN server is restricted to receiving data from the known server, and therefore the NAT mapping the known server sees is not useful information to the endpoint.
* https://en.wikipedia.org/wiki/UDP_hole_punching#Overview
I've also heard lots of people complain about how they're stuck behind CG-NAT and various P2P things do not work.
This link is 404.
QOTP looks really cool. Like what QUIC would be if DJB were in charge of it.
> Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN)
> Abstract
> If a host is located behind a NAT, then in certain situations it can be impossible for that host to communicate directly with other hosts (peers). In these situations, it is necessary for the host to use the services of an intermediate node that acts as a communication relay. This specification defines a protocol, called TURN (Traversal Using Relays around NAT), that allows the host to control the operation of the relay and to exchange packets with its peers using the relay.
The tldr is having a TURN server enables more hole-punching methods to be attempted, but that's not the same thing as a guarantee the session will succeed in switching to P2P.
"Unfortunately, no matter how hard you try, there is a certain percentage of nodes for whom hole punching will never work. This is because their NAT behaves in an unpredictable way. While most NATs are well-behaved, some aren’t. This is one of the sad facts of life that network engineers have to deal with."
In this scenario, the article goes on to describe a convention relay-based approach.
I would guess that most consumer routers are very cooperative as far as hole punching because it's pretty critical functionality for bittorrent and many online games. Corporate firewalls wouldn't be as motivated to care about those use-cases or may want to actively block them.
I think parents point is a bit like "you can't disallow lock picking"; the term "hole punching" being used to describe techniques that are intentionally trying to bypass whatever thing others (particularly corporations) try to put in the way, sometimes for good reasons and sometimes for kind of shit reasons.
Try doing it over a network that only allows connections through a SOCKS/Squid proxy, or on a network that uses CG-NAT (i.e., double-NAT).
See also:
> UDP hole punching will not work with symmetric NAT devices (also known as bi-directional NAT) which tend to be found in large corporate networks. In symmetric NAT, the NAT's mapping associated with the connection to the known STUN server is restricted to receiving data from the known server, and therefore the NAT mapping the known server sees is not useful information to the endpoint.
As I understand it, most consumer devices will set up a port mapping which is completely independent of the destination's IP and port. It's just "incoming packet for $wanip:567 goes to $internal:123, outgoing packet from $internal:123 get rewritten to appear from $wanip:567". This allows any packet towards $wanip:567 to reach the internal host - both the original server the client initiated the connection to, and any other random host on the internet. Do this on two clients, have the server tell them each other's mappings, and they can do P2P comms: basic hole punching. I believe this is usually called "Full Cone NAT".
However, nothing is stopping you from setting up destination-dependent mapping, where it becomes "incoming packet from $server:443 to $wanip:456 goes to $internal:123, outgoing packet from $internal:123 to $server:443 gets rewritten to appear from $wanip:567". This would still work totally fine for regular client-to-server communication, but that mapping would only work for that specific server. A packet heading towards $wanip:456 would get dropped because the source isn't $server:443 - or it could even get forwarded to another host on the NATed network. This would block traditional hole punching. I believe this is called "Address Restricted Cone NAT" if it filters only on source IP, or "Port Restricted Cone NAT" if it filters on both source IP and source port.
And there's a lot of other considerations; chances are your NAT won't be happy if you send all those probe packets at once, and your user may not be either. It's probably only worth it to do exhaustive probing if the connection is long lived, and proxying is expensive (in dollars because of bandwidth or in latency)
[1] https://github.com/danderson/nat-birthday-paradox/tree/maste...
If you can manage to bump it up to 65536 probes without getting blocked, hitting a NAT limit, or causing the user to fall asleep waiting, then it should hit the same success rate :D. I'm not sure many would like to use that P2P service though, at that point just pay for the TURN server.
If you need to send 64k probes to get p2p and you want to make a 15 minute call, it probably doesn't make sense, but it's probably worth trying a bit in case you catch an easy case. Not that p2p is always better than going through a relay, but it's often less expensive.
For wireguard that might be fine because you likely control the head end and opening ~65k NAT sessions is something you can opt to do if you tune things accordingly. Of course, in that case, you can also just opt to use the more lenient form of NAT at your head end and just use attempt with 256 ports instead.
ISPs are increasingly putting customers behind CGNAT, so wireguard at home doesn't imply control over NAT policies. Especially new entrants and fixed wireless ISPs don't tend to have the resources to get an IP (v4) for every customer, and some of them don't offer v6 either, so having some form of hope would be nice.