Add p2p(peer to peer) discovery spec

[liugenping]

The discovery specification does not meet the P2P download image now , this PR is for download images by P2P.
The proposal is rkt/rkt#1751 and the PR for rkt is rkt/rkt#1869

[jonboulle]

Hey @liugenping, thanks for the patch. I believe that the discovery semantics as they are today are sufficient to facilitate P2P. Since we do not prescribe a protocol (scheme) for the returned URL, discovery something can simply return something like a magnet URI - for example:

  <meta name="ac-discovery" content="example.com magnet:?xt=urn:btih:28ddf5885bc6cae5ckkc5ace36c70a4392f3fcda85&dn=Some+Cool+Thing&tr=udp%3A%2F%2Ftracker.example.com%3A80"

then a client that understands magnet links (and in this particular case, torrents) can choose to retrieve the discovered artefact via that method.

Does that make sense?

[liugenping]

@jonboulle I think is ok.
1.Peer can get metadata(file name, pieces, pieces length, and hash) from other peers using bittrorrent extension( http://bittorrent.org/beps/bep_0009.html)
2. "tr=" should be retured in magnet URI as DHT is slowly

[jonboulle]

Are you happy for us to close this out then?
Alternatively, perhaps instead of this PR you could add an example of using the current meta tags to return a torrent reference?