[bittorrent] Open letter to the BBC

Elliott Mitchell ehem at m5p.com
Sun Jun 19 23:54:58 EDT 2005


>From: larytet.8708132 at bloglines.com
> regarding 50%
> overhead. i witnessed the traffic in my firewall on dayly basis when downloading
> from big swarms with small number of seeds. overall traffic reported by my
> firewall in some cases was more than double of what actual data download i
> got. IP sniffer in that time reported up to 10 "I have " packets for evey
> "useful" data packet. 

You may have your client badly configured as the previous poster
suggested, but more likely you're just plain looking numbers that say one
thing and misinterpreting them to say another. You could have too many
peers. Generally in discussions 50 peers is used as the generic number
because 50 peers is ludicrously high!

There is a very simple and very incorrect assumption in your statement
though. Mainly that packets equal data. Beyond that, your statement is
saying "packets" without specifying what layer. Are these IP packets?
(generally what "packets" would suggest) Are these BT messages? (the
layer at which "haves" are significant)

By BT's design there are going to be lots of HAVE messages being sent
around, those do not directly equate to overhead though. A HAVE message
is a grand total of 9 bytes, a REQUEST message is 17 bytes, a PIECE
message will tend to run around 16K. Even if one hundred HAVE messages
are sent for every PIECE message, the bandwidth overhead is less than
10% while latency isn't an issue for a properly tuned client.

> previous post
> suggested to limit access to the BT trackers - it means proprietary BT and
> proprietary BT client. Because if BBC lets Azureus clients to connect to the

Flat wrong. There are plenty of ways to do this. The simplest is to
simply limit access to the .torrent file. Without the hashes, a client
cannot connect to the swarm (and you're not going to guess them). Further
the request URL can be one that has an embedded key, also easily allowing
the tracker to control access to the swarm.


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