Protocol versions/extensions

[KyroMaster]

I´m very interested in writing my own gnutella client but at first there´s a thing which I don´t understand.
Each gnutella client supports a different range of features like Ultrapeers, NAT traversal and so on, perhaps only the 0.60 protocol.
AFAIK all these clients can talk to each other and use all the features available on both clients. So even older clients can download from new LimeWire clients, but only with the 0.6 protocol. Does the gnutella protocol support enhancements natively. And what is when a client receives a request for a non-supported feature? Or how does it work?

Thanks in advance for your help.