CODEWITHSUNDEEP
socketsnetwork-programmingtcpp2pbittorrent
Thomas Vale
Thomas Vale

Reputation: 45

Understanding the requisites that allow bittorrent peers to connect to each other via TCP

BitTorrent peers connect with each other via TCP (mainly). When a peer A tries to connect to peer B, does peer B also needs to try to connect with A simultaneously so the TCP 3-way handshake happens and they form a connection? If not, why?

Also, I have been studying three bittorrent client implementations. While they start TCP connections with the obtained peers, I noticed none of them opens a TCP socket to listen on the port they are announcing to the tracker. Does it mean no one can initiate connection to them? Shouldn't they create such TCP socket?

Upvotes: 1

Views: 746

Answers (1)

the8472
the8472

Reputation: 43125

When a peer A tries to connect to peer B, does peer B also needs to try to connect with A simultaneously so the TCP 3-way handshake happens and they form a connection? If not, why?

Connection setup is a general TCP feature, not specific to bittorrent. One side initiates the connection by calling connect on an unconnected socket and the other side has a listening socket configured on which it calls accept in a loop to create create connection-specific sockets for each accepted incoming connection.

There is a simultaneous open flow for connection setup but that's rarely relevant and the connect/accept flow is used by bittorrent clients.

I noticed none of them opens a TCP socket to listen on the port they are announcing to the tracker.

They generally do and should unless process privileges are insufficient to bind a particular port or another process is already listening on it, in which case they should log a warning at least.

If you used a portscan then you may be seeing firewall or NATs getting in the way rather than the client not having a listening socket open. Instead you could use something like netstat (may need some additional arguments, depending on OS) to show listening sockets.

If they truly do not have a listening socket open then yes, that would be a problem since they could not accept incoming connections and only talk to a more limited set of clients (those that do). Bittorrent being a peer-to-peer protocol means that clients should be equals (peers) which means they should be equally capable of initiating and accepting connections.

Upvotes: 0

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