fast extension.mht

关于ＢＴ的协议文档

<P>Let <I>k</I> denote the final number of pieces in the allowed fast =
set. </P><PRE> x =3D 0xFFFFFF00 &amp; ip                           (1)
 x.append(infohash)                            (2)
 while |a| &lt; k:=20
   x =3D SHA1(x)                                 (3)
   for i in [0:5] and |a| &lt; k:                 (4)
     j =3D i*4                                   (5)
     y =3D x[j:j+4]                              (6)
     index =3D y % sz                            (7)
     if index not in a:                        (8)
       add index to a                          (9)
</PRE>
<P>Step (1) selects the most significant octets in peer <I>P'</I>s ip =
address.=20
We do this to prevent a user that obtains more than one IP address on =
the same=20
network from obtaining more than one <I>allowed fast set.</I> Use of =
three bytes=20
is heuristic and historical. </P>
<P>Step (3) generates a 20-byte random number on each call. By =
performing a=20
SHA-1 hash on the previous iteration's hash, we can generate an =
arbitrarily long=20
pseudorandom sequence. </P>
<P>Steps (4) through (9) partition the 20-byte hash into piece indices =
and add=20
them to the allowed fast set. </P></DIV><A =
name=3DExample_Implementation></A>
<H2>Example Implementation </H2>
<P>The following C++ implementation was provided by CacheLogic: =
</P><PRE>void generate_fast_set(
  uint32 k,     // number of pieces in set
  uint32 sz,    // number of pieces in torrent
  const char infohash[20], // infohash of torrent
  uint32 ip, // in host byte order, ie localhost is 0x7f000001
  std::vector&lt;uint32&gt; &amp;a) // generated set of piece indices
{
   a.clear();
   std::string x;
   char buf[4];
   *(uint32*)buf =3D htonl(ip &amp; 0xffffff00);
   x.assign(buf, 4);
   x.append(infohash, 20); // (3)
   while (a.size()&lt;k) {
     x =3D SHA1(x); // (4)
     for ( int i=3D0&nbsp;; i&lt;5 &amp;&amp; a.size()&lt;k&nbsp;; i++ ) =
{ // (5)
       int j =3D i*4; // (6)
       uint32 y =3D ntohl(*(uint32*)(x.data()+j)); // (7)
       uint32 index =3D y % sz; // (8)
       if (std::find(a.begin(), a.end(), index)=3D=3Da.end()) { // (9)
         a.push_back(index); // (10)
       }
     }
   }
}
</PRE>
<P>Example results generated by this function: </P><PRE>7 piece allowed =
fast set for torrent with 1313 pieces and hex infohash  =20
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa for node with IP 80.4.4.200:
  1059,431,808,1217,287,376,1188
9 piece allowed fast set for torrent with 1313 pieces and hex infohash   =
 =20
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa for node with IP 80.4.4.200:
  1059,431,808,1217,287,376,1188,353,508
</PRE></DIV><!-- ### End Content ### --></DIV>
<DIV></DIV>
<DIV id=3Dfooter>
<HR>

<P>Copyright 2006 BitTorrent.org</P></DIV></BODY></HTML>

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