I would like to understand networking services with a large user base a bit better so that I know how to approach a project I am busy with.
The following statements that I make may be incorrect but they still lead to the question that I want to ask...
Please consider Skype and TeamViewer clients. It seems that both keep persistent network connections open to their respective servers. They use these persistent connections to initiate additional connections. Some of these connections are created by means of Hole Punching if the clients are behind NATs. They are then used for direct Peer-to-Peer communications.
Now according to http://expandedramblings.com/index.php/skype-statistics/ there are 300 million users using Skype and 4.9 million daily active users. I would assume that most of that 4.9 million users will most probably have their client apps running most of the day. That is a lot of connections to the Skype servers that are open at any given time.
So to my question; Is this feasible or at least acceptable? I mean, wouldn't it be better to not have a network connection open while idle and aspecially when there are so many connections open to the servers at once? The only reason I can think is that it would be the only way to properly do Hole Punching. Techically, how is this achieved on the server side?
Is this feasible or at least acceptable?
Feasible it certainly is, you mention already two popular apps that do it, so it is very doable in practice.
As for acceptable, to start no internet authority (e.g. IETF) has ever said it is unacceptable to have long-lived connections even with low traffic.
Furthermore, the only components for which this matters are network elements that keep connection/flow state. These are for sure the endpoints and so-called middleboxes like NAT and firewalls. For the client this is only one connection, the server is usually fine tuned by the application developers (who made this choice) themselves, so for these it is acceptable. For middleboxes it's simple: they have no choice, they're designed to just work with all kind of flows, including long-lived persistent connections.
I mean, wouldn't it be better to not have a network connection open while idle and aspecially when there are so many connections open to the servers at once?
Not at all. First of all, that could be 'much' slower as you'd need to set up a full connection before each control-plane call. This is especially noticeable if your RTT is big or if the servers do some complicated connection proxying/redirection for load-balancing/localization purposes.
Next to that this would historically make incoming calls difficult for a huge amount of users. Many ISP's block/blocked unknown incoming connections from the internet by means of a firewall. Similar, if you are behind a NAT device that does not support UPnP or PCP you can't open a port to listen on for your public IP address. So you need it even aside from hole-punching.
The only reason I can think is that it would be the only way to properly do Hole Punching. Techically, how is this achieved on the server side?
Technically you can't do proper hole-punching as soon as the NAT devices maintain a full <src-ip,src-port,dest-ip,dest-port,protocol> (classical 5-tuple) flow match. Then the best you can do with 'hole punching' is set up a proxy between peers.
What hole-punching relies on is that the NAT flow lookup is only looking at <src-ip,src-port,protocol> upstream and <dest-ip,dest-port,protocol> downstream to do the translation. In that case both clients just set up a connection to the server, their ip and port gets translated and the server passes this to the other client. The other client can now start sending packets to that translated <ip,port> combination which should work because NAT ignores the server's ip/port. But even if the particular NAT would work like this, some security device (e.g. stateful firewall) might detect session hi-jacking and drop this anyway.
Nowadays you rather use UPnP to open up a port to listen on your public IP which is much easier if supported.