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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Sounds promising.

Is it possible to resolve SHA1 from TTH? One could probably get SHA1 for a specific peer and use this SHA1 to request a file from this peer only.

Well, maybe, but this in fact means that one have to spend time calculating SHA1 just because it is used for identification. Calculating sha1 is even more difficult when you don't have the files (they are remote, distributed through DC userbase). Websites like (...deleted...) and also lots of intranet dc portals can only transit to Gnutella given that Gnutella can work without SHA1.

With regards to local files I'd better spend time calculating md5 when idle. Multiple non-p2p-aware sites provide md5 for file verification. Multiple tools (cli and gui) use md5. This is not gonna change in the foreseeable future.

This is a very important moment. I thought it is supposed to be done automatically. Of course most people won't edit magnet links. In eDonkey modern clients include aich by default; at least, it is highly recommended to include AICH in ed2k links, and it's hard to meet non-aich ed2k link. In eDonkey, however, both ed2k and aich hashes allow partial file verify.

Is there Gnutella clients around that generate TTHful magnet links by default? bitprint is also fine, Direct Connect clients are aware of it.

Another relevant question: Is is possible to look for file sources without knowing SHA1?

Can anybody download this file in Gnutella?

magnet:?xt=urn:tree:tiger:GMRI7CX6MRUGEGN4BITB25UG G3QUANYMPO33SLA&xl=1388544&dn=msvbvm60.dll

(remove the space, the space is inserted by forum engine, I can't get rid of it)

It is very popular in Direct Connect. Lots of stuff depend on it. There should be no problem to locate it in Gnutella given that TTH searches are implemented.