[BitTorrent] Re: KEY tracker extension (and peer ids)

smilindog2000 bill at viasic.com
Thu Dec 30 09:35:59 EST 2004

--- In BitTorrent at yahoogroups.com, John Hoffman <theshadow at s...> wrote:
> The concept behind the "key" extension in the tracker announce is very 
> simple.  The peer chooses it randomly and will send the same value with 
> each announce.  This allows the tracker to verify the contactee is the 
> same peer even if its IP has changed, and lets the tracker update that 
> information.

I'm trying to figure this stuff out now.  If we get some good
descriptions here, I'll try to add them to the wiki.

The peer_id is enough for the tracker to identify a contactee, even if
the IP has changed, so that's not what the key is for.  I think the
key flag is probably meant to keep other contactees from pretending to
be you during a download session.  You never tell anyone else the key,
but you do tell them your peer id.  So, the only secret information
you tell the tracker is your key for the download session.  That makes
it the only reliable way to identify you.

I think from the client point of view, you just generate the random
key at the same time you generate the peer id, but the only time you
ever use the key is when talking to the tracker.

I think there's a bit more to know about peer id's, too.

>From the protocol spec, you'd think that you could use the same peer
id over and over if you wanted, and never change it.  However, the
trackers don't seem to expect clients to reuse peer ids.  Some wont
let me re-download, since they figure I've already tried it once. 
But, if I use a new peer id, I get through.  This may seem like
reasonable behavior, but users simply have to have the ability to
re-download.  Thus, it seems that peer-ids can't actually be preserved.


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