[bittorrent] question about the peer exchange in BitTorrent

Anwar Al-Hamra Anwar.Al_Hamra at sophia.inria.fr
Mon Oct 16 19:54:56 EDT 2006

I am trying to understand the peer exchange protocol PEX in BitTorrent.
I believe I know the basic idea: When a new client joins the torrent, it 
gets from the tracker the IP addresses of 40 nodes. Then, the new client 
contacts these 40 nodes and performs a PEX with them. Is it really 40 

Here are my questions:
1- Does the new client have a limit on the number of neighbors? If yes, 
how much is this limit? And does this mean that additional IP addresses 
obtained with the PEX are simply discared?
2- Is the PEX performed only once at the joining time or periodically?
3- Suppose that the new client performs a PEX with an existing node A. 
Does the new client provide also that node A with its list of neighbors? 
is the PEX bi-directional?

Sorry for bothering.
Anwar Al Hamra.

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