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What is the mechanism behind allowing BitTorrent to work with only outbound connections?

That is, without port forwarding.

Brian's BitTorrent FAQ and Guide says:

BitTorrent will usually work fine in a NAT (network address translation) environment, since it can function with only outbound connections.

Do other peers work as rendezvous servers in this case?

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3 Answers 3

As far as I know, It works using reverse connections - basically, your client does an outgoing connection which is kept open by the remote machine and they pump data through that same connection.

If everyone used this and no one had ports setup, it would most likely fail. When I have used Bittorent with ports disabled, I get horrendous speeds, but it does work which is why I think it works like this.

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Normally, people will connect to you to ask for data from you, and you will connect to people asking for data from them.

If incoming connections is impossible, your client will also go out and actively ask to give data to other people (and may do this if incoming connections are possible as well).

You do not upload as much this way, but it's not much of a concern on public trackers.

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There was a recent podcast on IT Conversations titled How Your ISP Plans to "Help" You, and Break the Internet where Dr. Cullen Jennings talked about various mechanisms for NAT traversals. It's only 15 minutes long and worth a listen.

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