If a client respond to a query with query hits that each one has a URN then a lot of bandwidth is beeing wasted.

What i want to say is that for some files a client should not respond with a URN/Query Hit.
For example text files usually are small files which can be downloaded again in case of an incomplete download. There is no need to apply a URN for a text file. Not just .TXT files but also .HTM, .XML, .DOC and other text files. Another reason is that , if you just alter a text file (ex: put a comma) then the HASH changes. So many users can have (almost) the same (text) file but not the exactly identical and sending diferrent URNs.
URNs should be used only for binary files. But not just for every binary file. For small binary files (800~1000KB) a client should not reply with a URN.

Also when a client searches for the same file in other servers should use and the size of the file, not just the HASH. This way we can use an algorithm smaller than SHA1. If you have two binary files with different file size then there is no chance to be the same (or one of them is corrupted).