[bittorrent] Choice of listener ports

Simon Spero ses at unc.edu
Sun Mar 27 17:10:11 EST 2005


Kenneth Porter wrote:

> [Porter proposes circumventing local firewalls by using ports outside 
> the 6881-6889 range]
>
> What problems would you foresee from doing this?

The first problem is that bittorrent connections can easily be 
identified by examining the first 48 bytes transmitted by the session 
initiator.   The first 28 bytes tell you it's  bittorrent; the next 20 
bytes identify the torrent.

The second problem is that bittorrent has a rather distinctive traffic 
pattern; even if connections are encrypted,  it's pretty obvious what's 
going on.  Any actions to disguise these traffic patterns will severely 
impact thruput.  It's also  hard to disguise the sheer volume of traffic 
associated with a busy torrent.

The final problem is that listen ports are chosen by individual clients; 
although the user can chose his own advertised listen port, unless all 
other members of the swarm use that port,  this move is futile.  I 
believe there are some trackers that enforce non-standard listen ports 
for precisely this reason.

Simon







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