When I call via Skype and the technical info during my call shows that the call is being relayed through 4 relays, my SessionIn and SessionOut is sometimes UDP and sometimes RELAY_UDP. That's why I would like to know the difference between UDP and RELAY_UDP because in both cases, the call is actually relayed through 4 relay nodes.
Thanks.

UDP is good.
RELAY_UDP is UDP transmitted/received via a third party.



No, it means there are 4 available relay nodes. The traffic is relayed only when the session type is RELAY_UDP.

Ok. And why then the Skype Technical Info sometimes shows "Relays: 0" and sometimes "Relays: 4" when, in both cases, the SessionIn and SessionOut is UDP?

the Relays field in the tech info display only tells you how many relay nodes are AVAILABLE for your client to use, not whether they are being used or not -- as Spud indicated, for relaying to be in use, SessionIn or SessionOut must include the word 'Relay'

btw, you will never see an indication for more than 4 Relays -- at most only 1 relay node is used for each direction, and the other 2 are "spares"

Thank you for your explanation. I just don't understand one last thing. Why during calls with some people the number of available relay nodes is always 4, and with others the number of available relay nodes is always 0?

I was not aware that this was ever the case -- this is only a guess, but perhaps the relay nodes are reserved on the basis of how Skype has ranked the peer-to-peer friendliness of routers