Re: Video chat, voice over IP, video conferencing, and the use of NAT.

[Conrad Newton <conrad.newton@broadpark.no>]

>From Herman Robak on Monday, 2005-11-07 at 00:46:33 +0100:
> 
>  Actually, for UDP traffic there are more options, most of them dirty hacks.
> <http://en.wikipedia.org/wiki/UDP_hole_punching>
> 
> One of them, STUN, even has an RFC <http://www.ietf.org/rfc/rfc3489.txt>

Skype uses a variant of STUN, according to this analysis:

http://arxiv.org/pdf/cs.NI/0412017

7. CONCLUSION 
  
  Skype is the first VoIP client based on peer-to-peer technology.
  We think that three factors are responsible for its increasing
  popularity. First, it provides better voice quality than MSN and 
  Yahoo IM clients; second, it can work almost seamlessly behind 
  NATs and firewalls; and third, it is extremely easy to install and 
  use. We believe that Skype client uses its version of STUN [1]
  protocol to determine the type of NAT or firewall it is behind. The
  NAT and firewall traversal techniques of Skype are similar to
  many existing applications such as network games. It is by the
  random selection of sender and listener ports, the use of TCP as
  voice streaming protocol, and the peer-to-peer nature of the Skype
  network, that not only a SC traverses NATs and firewalls but it 
  does so withhout any explicit NAT or firewall traversal server. 
  Skype uses TCP for signaling. It uses wide band codecs and has
  probably licensed them from GlobalIPSound [10]. Skype
  communication is encrypted.

  The underlying search technique that Skype uses for user search is
  still not clear. Our guess is that it uses a combination of hashing
  and periodic controlled flooding to gain information about the
  online Skype users.

  Skype has a central login server which stores the login name and
  password of each user. Since Skype packets are encrypted, it is
  not possible to say with certainty what other information is stored
  on the login server. However, during our experiments we did not 
  observe any subsequent exchange of information with the login 
  server after a user logged onto the Skype network.
    

Conrad