http://l7-filter.sourceforge.net/protocols
http://protocolinfo.org/wiki/Skype
Check these links out for Skype.
I have no proof it works 100% but without using these my Skype quality is poor. When L7 in use the quality is much better and much less connection cuts.
Skype to Skype works better though then Phone to Skype or Skype to phone.
I tested with client that was downloading, at the same time remote desktop and the communication between us was done with SkypeToSkype. Clear lines and sometimes connections for more then 20mins while network was busy too...
It is still happening at times that some connections in general (over all my users) are dropped but that is more because my network has only 2.2Mb upload capacity and with 100+ Skype clients, several P2P users and some remote desktop and VOIP users at times the ceiling is hit and some connections suffer.
After all, Skype and VOIP and Remote Desktop (SSL) traffic all get the same high priority which at times does not have enough capacity.
And sometimes when I have a Skype conversation with family in absolute slack time on my network connections still drop. This must be Skype or general internet performance related...
But using L7 improved things for me....
P.S.
Not to forget; have, and your users too, the latest version of Skype installed. It makes a difference! (Special against versions older then 6 months.)