I've been looking into Skype and how it actually works.
The reason why I investigated all this is that I started logging all outgoing connections through my router and inccoming connections to my port forwarded skype port (this is a randomly chosen port which appears in the skype settings pages). I saw loads of connections from IP addresses all over the world and wondered what was going on. It was only after investigating this did a realise that skype generates lots of traffic all over the place. Although in my case it seems only to use a very small amount of bandwidth less than 1K bits per second. This traffic occurs even when not in a call and the traffic isn't from anyone you know in your contacts it can be anyone.
Users behind port closed NATs or firewalls can still receive calls from other Skype users even though the system is decentralized. This works by such users initiating outgoing connections to other (randomly chosen) users with public IPS that can accept incomming connections (super nodes).
Call signalling is then routed through the open outgoing connection from the cient to the Super Node. So the user can open an outgoing connection to the user calling them. So the super node just carries the signaling in this case. However if both parties in the call are behind a NAT or firewall then neither of them can receive connections from anyone. This means that their outgoing connections to Super Nodes carry the call signaling and the actual call or file transfer traffic.
This means that if you use skype other peoples calls can eat away at your bandwidth. You could close your port to incomming connections but this could be counter productive as you may end up now getting calls through super nodes which could effect latency and quality?? Of course if everyone did this the whole system would not work at all.
You can tranfer files through skype too and the same would apply as call traffic. Your bandwidth could be used as a go between for a file transfer.
So, what if the file transfer happens to be some illegal file? Or a copyrighted song/video track? You will be a party to the tranfer of that file. Your IP will show up at some point as the source/destination of the file in question. It may look like you are uploading that file when infact someone else is uploading it through your clients connection somewhere else.
We know that the RIAA have been sending out take down notices and fining people for swapping files through napster and bitorrent. It is easy for them to see the IPs of people sharing files in a torrent stream.
I wonder what the legality is if someone downloads a file but through your IP using a P2P program like skype or winmx this happens when you are a super node in skype or "primary connection" in WinMX which I think works in a similar way.
WIth Skype the signaling and data is all encrypted but somewhere along the link it must be unencrptable so that the skype clients know what they are doing. The RIAA could reverse engineer the system and snoop on all the traffic, there is lots of stuff about this on Google eg the autorities "could" make super nodes with modified skype clients that monitor calls and transfers, someone commented that you would only need 100 or so clients to capture a lot of the traffic (signaling at least).
With WinMX actual files are trasfered and there is a search facility. So if the RIAA detect a copyrighted file download is happening through your primary connection could you face prosectution for that? You of course would be unknowingly doing so, it could look like an upload or download from your IP. At the very least you could be the one to get the letter from them demanding a out of court settlement of thousands. Which you would have to deal with. Even though you only actually use winmx yourself to download or share non copyright legal stuff?
Could this be a reason to never be a primary winmx client or a skype super node (actually skype doesn't let you choose this or not but if you close your firewall to incomming connections you cant be a super node).
Could this be a reason not to ever use any kind of P2P client including Skype and go for a SIP based system instead?