Re: Updated PFSP specifications

Quoting Arne Babenhauserheide <[email protected]> from ml.gnutella.dev-forum:
:The texts overally looks good, but the extension is problematic for Phex: 
:
:Phex uses the X-Available-Ranges to compute the most common and least common 
:segments, so it can start the download with the least common segment, and the 
:chance of getting the whole file when some uploaders drop out increases. 
:
:Your change will make this pointless, since we can't know the rarest segment 
:anymore, as soon as even one segment is omitted. 
:
:Is there another motivation for this change besides saving bandwidth? 

The sole motivation of this change is simply to save bandwidth when "bad"
clients request a highly fragmented file without honouring the Retry-After
indication given for that file.  Some clients will retry every minute even
though the Retry-After header states 20 minutes.

Note that gtk-gnutella has been omitting chunks from X-Available-Ranges
since the beginning of PFSP, but this was not documented, and I hadn't
invented the X-Available header yet...

I do not believe this is problematic for Phex, in that it is trivial to
see that a client has limited its X-Available-Range emission (by comparing
the sum of advertized bytes there with the ones in the X-Available header),
and successive requests to the same server will likely return different
chunks (selection of returned chunks is random).  After some requests, you
will have complete knowledge of the available ranges.

I suggess you leave out the servents from which you have only an incomplete
knowledge of the available chunks for the purpose of the rarest chunk
compuation, and you'll be fine.

Raphael