seedtorrent.mht

关于ＢＴ的协议文档

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      size=3D1><B>&nbsp;&nbsp;&nbsp;&nbsp;<A name=3D(none)>Proposed =
Addition to=20
      .torrent Files</A> </B></FONT></TD>
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    <TD vAlign=3Dtop align=3Dleft bgColor=3D#ffffff><FONT =
face=3DVerdana,Arial=20
      size=3D2>&nbsp;<BR>I propose a new key/value in the .torrent =
metadata file=20
      but not part of the "info" section.=20
      <P>'url-list' =3D either a single item or a list of normal URLs.=20
      <P>For example (on multiple lines for readability): <BR>d=20
      <BR>8:announce27:http://tracker.com/announce <BR>13:creation=20
      datei1128487910e =
<BR><B>8:url-list26:http://mirror.com/file.exe</B>=20
      <BR>4:info...=20
      <P>If the "url-list" URL ends in a slash, "<B>/</B>" the client =
should add=20
      the "name" from the torrent to make the full URL. This should make =
it=20
      easier on Torrent generators and let them treat this field same =
for single=20
      file and multi-file torrents.=20
      <P>
      <HR>

      <P>For <B>Multi-File</B> torrents, this gets a bit more =
interesting.=20
      Normally, BitTorrent clients use the "name" from the .torrent info =
section=20
      to make a folder, then use the "path/file" items from the info =
section=20
      within that folder. For the case of Multi-File torrents, the =
'url-list'=20
      should be a root folder where a client could add the same "name" =
and=20
      "path/file" to create the URL for the request. <BR>...=20
      <BR><B>8:url-list22:<FONT =
color=3D#ff0000>http://mirror.com/pub/</FONT></B>=20
      <BR>4:infod5:filesld6:lengthi949e4:pathl10:<B><FONT=20
      color=3D#008000>Readme.txt</FONT></B>e <BR>e4:name7:<B><FONT=20
      color=3D#0000ff>michael</FONT></B>=20
      <P>So a client would use all that to build a url: <B><FONT=20
      color=3D#ff0000>http://mirror.com/pub/</FONT><FONT=20
      color=3D#0000ff>michael/</FONT><FONT =
color=3D#008000>Readme.txt</FONT></B>=20
      <BR>&nbsp; </FONT></P></TD>
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      bgColor=3D#0e58d1><FONT face=3DVerdana color=3D#ffffff=20
      size=3D1><B>&nbsp;&nbsp;&nbsp;&nbsp;<A =
name=3D(none)>Implementation Notes for=20
      Clients</A> </B></FONT></TD>
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    <TD vAlign=3Dtop align=3Dleft bgColor=3D#ffffff><FONT =
face=3DVerdana,Arial=20
      size=3D2>&nbsp;<BR>GetRight was already able to download the same =
file from=20
      several HTTP/FTP mirrors at once, a feature common in many =
download=20
      managers. When adding the BitTorrent protocols into GetRight, it =
made=20
      sense to just consider BitTorrent another protocol and be able to =
do the=20
      same.=20
      <P>HTTP/FTP are streaming protocols, and don't have BitTorrent's =
concept=20
      of blocks. For HTTP you can use byte-ranges to resume anywhere or =
download=20
      specific ranges you specify, but with FTP you can only say where =
to start=20
      the download. So I wanted to have big "gaps" in the data =
downloaded from=20
      BitTorrent peers so a HTTP/FTP connection would have big spaces to =
fill=20
      in. You could use the byte-ranges with HTTP to request individual=20
      blocks--but each request will show in the server's logs, and =
somebody is=20
      going to think your DoSing them if it shows 100's of connections =
in their=20
      log. So I made a couple changes to the usual "rarest first"=20
      piece-selection method to better allow "gaps" to develop between =
pieces.=20
      That way there are longer spaces in the file for HTTP and FTP =
threads to=20
      fill. They can start at the beginning of a gap and download until =
they get=20
      to the end.=20
      <P><I>This actually could be implemented differently. You could =
use the=20
      HTTP byte-ranges to request specific pieces and not worry about =
any=20
      server's logs. This method just fit in very well with all the code =
I'd=20
      already done for GetRight's accelerating files. Plus does minimize =
some=20
      connections and restarts.</I>=20
      <DL>
        <DT>How I'm defining Gaps in a file:=20
        <DD>
        <LI>Gaps are spaces of multiple pieces in a row that I don't =
have. </LI>
        <DD>
        <LI>Given a bitfield <B>"YYnnnnYnnY"</B> where Y is pieces it =
has and n=20
        are ones it doesn't, there are two gaps <B>"YY<FONT=20
        color=3D#008800>nnnn</FONT>Y<FONT =
color=3D#0000ff>nn</FONT>Y"</B>, one of=20
        <FONT color=3D#008800><B>4</B></FONT> pieces, and another of =
<FONT=20
        color=3D#0000ff><B>2</B></FONT>. </LI>
        <DD>
        <LI>If everything else is more-or-less equivalent, it's better =
to pick a=20
        piece to do from the gap of <FONT =
color=3D#0000ff><B>2</B></FONT> when=20
        requesting a piece from a BitTorrent peer. </LI>
        <DD>
        <LI>In any gap, it will fill in from the end (ie, the highest =
piece=20
        number). That way a HTTP/FTP connection could be cruising along =
filling=20
        in pieces from the beginning and I don't have to do any =
reconnects for=20
        it until they meet in the middle.=20
        <P></P></LI>
        <DT>Change the "rarest first" piece selection to a "pretty rare =
with=20
        biggest distance from another completed piece".=20
        <DD>
        <LI>When scanning for the rarest piece, if the distance from =
another=20
        completed piece is less than that for the current rarest piece, =
it must=20
        be "rare-X". Or if the distance is greater than the current =
piece's, it=20
        can be rare+X to be picked as the next piece. (For no better =
reason than=20
        it seemed to make sense and scale, X is the square root of the =
number of=20
        peers minus 1.) </LI>
        <DD>
        <LI>So if 3 peers had the current rarest piece, the normal =
algorithm=20
        would pick a piece where 2 peers had it...my changed algorithm =
would=20
        require that only 1 peer has the piece if that piece's distance =
from a=20
        complete piece was less than the gap for the current rarest =
piece. </LI>
        <DD>
        <LI>If the gap is bigger and the piece is the same "rareness" or =
the=20
        usual "rare-1" that piece is selected. (So if the gap is bigger, =
a piece=20
        with either 2 or 3 peers would be chosen.) </LI>
        <DD>
        <LI>So given <B>"YYnnn<FONT color=3D#008800>1</FONT>Yn<FONT=20
        color=3D#0000ff>2</FONT>Y"</B>, unless <FONT =
color=3D#008800><B>1</B></FONT>=20
        is a lot more rare than <FONT color=3D#0000ff><B>2</B></FONT>, =
it's better=20
        to pick piece <FONT color=3D#0000ff><B>2</B></FONT>. </LI>
        <DD>
        <LI>I always do better explaining or understanding with some =
code logic=20
        to look at...=20
        <UL><PRE>X =3D sqrt(Peers) - 1;
for (i=3D0; i&lt;maxpieces; i++) {
   if (*IDon'tHavePiece*) {
      Gap++;
      if (*PeerHasPiece*) {
         PieceRareness =3D *Number of peers with the piece*;
         if (PieceRareness&lt;(CurRareness-X) || =
(PieceRareness&lt;=3D(CurRareness+X) &amp;&amp; Gap&gt;CurGap)) {
            CurRareness =3D PieceRareness;
            CurGap =3D Gap;
            NextPiece =3D i;
         }
      }
   } else {
      Gap =3D 0;
   }
}
</PRE></UL>
        <P></P></LI>
        <DT>Added an extra "fill-in-the-gaps" piece selection method.=20
        <DD>
        <LI>This check is only done If a file is more than 50% =
complete--so you=20
        should have a large number of pieces that other peers will want =
to=20
        download. </LI>
        <DD>
        <LI>Every few pieces (randomly, 1 in 10), it will pick the piece =
with=20
        the smallest gap from a complete piece. For the bitfield=20
        <B>"YYnnnnYnnY"</B> it would select the <FONT =
color=3D#880000>#</FONT> one=20
        <B>"YYnnnnYn<FONT color=3D#880000>#</FONT>Y"</B>=20
        <P></P></LI>
        <DT>For the HTTP/FTP download part...=20