To give i bit more information (I was a bit brief):

Currently about 95% of the live Gnutella nodes support TTH (Limewire/Frostwire, Phex, gtk-gnutella, ...).

Sha1 is being used to identify a file (to request it), while TTH is being used to verify each chunk. A request is a simple GET request with /uri-res/N2R?urn:sha1:<sha1> as path (and for this sha1 is perfectly suited, because it just identifies the file).

And you can add TTH to magnets, it's just not done by most people.

I think it was something like “xt=urn:tth:<hash>” (replace <hash>). Or bitprint…

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