On 25 Sep 2003, at 7:08 am, Stephen Boulet wrote:
On Tuesday 23 September 2003 09:01 am, Terry Churchill wrote:Absoloutely, but I don't agree that bittorrent is the way to go.
But - it should be an option for those who do want to use it, and are willing to open the required ports on firewalls, etc.
[snip]
You only have to open specific ports on firewalls if you are serving a
torrent. Others jumping on it have no problem, and are encouraged to do so,
since they make everyone's download rate go up.
I think you're mistaken. As far as BitTorrent is concerned there should be no difference between someone "serving" a torrent (IE: the original individual with the complete file) and other peer with parts to share. Someone who is NATted or firewalled but who has the complete file may contribute very little (if at all - I don't know how BT implements this) to other peers.
For anyone else reading who is unfamiliar with BitTorrent, background is available at http://bitconjurer.org/BitTorrent/introduction.html
I've been using BitTorrent quite a bit recently, and this is what I can surmise:
- BitTorrent relies on all peers serving the file simultaneously as downloading it.
- If you are behind NAT or a firewall, and have not opened the appropriate ports then other peers cannot connect to you. They can, however, share the file with you if you can connect to them.
- The problem with this is that the VAST majority of users are NATted or firewalled & do NOT open these ports.
- Consequently, the burden of sharing is shifted largely onto the few peers who do have their ports open correctly.
- All NATted / firewalled peers (without ports open) share a total download bandwidth supplied by the total upload of peers with their ports open correctly. It is more desirable that all peers share a download bandwidth equalling the total upload of ALL peers (which is many times more).
- If you open your ports, all the incorrectly configured clients can then connect to you, and you benefit from their total upload bandwidth, too.
Whether this is important is a matter of scaling. I tend to deal mostly, at present, with .torrents with a shortish life-span (of a week or three) and total peers in the order of 10 - 200.
My theory is that when a .torrent is fresh, there a relatively high number of "sophisticated" peers connected: that is to say peers who know what they're doing & have their ports open; if just one or two of these peers are on a fast connection (say 3mbit or a university connection) then they can largely compensate for all the "unsophisticated" peers (without their ports open). It is still possible for the unsophisticated peers to download, say, a 650meg file in a matter of a few hours, especially as an unspohisticated peer may get a chunk from one sophisticated peer and share it with several others.
When there are only a dozen or two peers, however, and only one of them has his ports open, the download rate for NATted/firewalled peers slows to trickle - if the sophisticated peer has only 512/256 ADSL, then his meagre upload is shared between ALL of the unsophisticated peers. The sophisticated peer can get chunks from all of the other peers, however, and may still be able to saturate his bandwidth. A characteristic of this scenario is that you may experience very fast download rates up to a certain point, and only then does the trickle become noticeable.
Conclusion: it may not always matter to you if you're NATted or firewalled when you BitTorrent, but if you are then you become part of the problem. Users opening their BitTorrent ports (6881 - 6889 are a good start) may experience speed increases of a factor of 10 or more. It is my experience that for any reasonably well-seeded (or even well-peered) file then BitTorrent should be able to saturate my 512k ADSL download bandwidth - I can download a 650meg file in 3 - 6 hours, when NATted / firewalled peers might show ETAs of several days for the same file.
I like this discussion of BitTorrent's ports & firewall requirements, but it seems to be down at the time of writing: http://www.dessent.net/btfaq/#ports
I would be interested to discuss this matter further, especially if you can point out any flaws in my reasoning.
Stroller.
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