Thanks Guy, now it's OK.
 >> 3. Why Ethereal's protocol dissector doesn't recognize Yahoo
 >> Messenger protocol (YMSG) packets, despite of its presence in
 >> protocol list?
 er> Because those packets either
 er> 1) aren't being sent to or from the port the dissector thinks
 er> is     the Yahoo port (port 5050)
 Packets ARE being sent to port 5050.
 er> 2) aren't at the beginning of TCP segments or don't begin
 er> with     "YPNS" or "YHOO"
 Yes they are beginning with "YMSG", that means "Yahoo
 Messenger". And I couldn't find in internet a description of any
 other Yahoo Messenger protocol, whose packets to begin with "YPNS"
 or "YHOO". Maybe these ones are too old?
 >>And if "YMSG" means they
 >>begin with YMSG, then the reason why the dissector rejects them is
 >>that nobody's added to the Yahoo Messenger dissector the ability to
 >>dissect the version of the Yahoo Messenger protocol that uses YMSG.
 Has anybody in developers team intention to do this? Or one has to do
 it by himself?
                WBR, LV.