Uploading on torrent applications

Question

Once i had asked about how uploading a file actually works. Based on responses given to me, i was convinced that uploading a file always consists of an initiation process from a private network to outside as it happens on downloading, with a single difference. rather than GET command, POST is used.

But, consider that i have launched a torrent client and lets assume i completed downloading a file. I am gonna seed the file and share with others now.

1) Does not this mean 'uploading' ? which invalidates the case above?

2) If this is 'uploading', isn't it 'a connection/several connections opened from the outside to my private network thus, torrent client requires port forwarding', not a request which we have made?

  In short, who launches the connection during upload process? 

Answer 1

Seeding torrents no longer requires forwarding ports. There are many advanced techniques to “circumvent” NAT, e.g. NAT Hole Punching. Naturally, most of these need a fully connected third party to mutually negotiate.

That being said, in the simplest case, with a port forwarded, the remote side initiates the connection.

You also need to keep in mind that upload and download is just a matter of perspective. This is of course completely unrelated to NAT and whatnot. If you download something from a server, the server uploads something to you. TCP connections work both ways, both the local and remote end send and receive data.

Answer 2

Uploading works as follows, regardless of the technique used.

First you have a client and a server. The server just listens for any incoming signal.

The client connects to the server saying: "hi, I want to use you." The server responds: "sure, go ahead, and start a connection to this port" (the client now becomes the sender part, instead of the initial job of the server).

The client initiates a command stating to the server that it wants to upload anything, say... a file.

The server accepts and prepares to receive the file.

The client now starts sending the file as how TCP/IP would do it: sending 1 packet waiting for the server to respond that it has received it, then sending the next 2, waiting for the response, etc. This process is continued until the file has been transmitted. This stage is seen as uploading from a client's perspective, and downloading from a server's perspective.

In a torrent it works the same way. You first download the file from different clients who upload their data to you then you start uploading to other clients and they download from you.

A server in a torrent is a machine who keeps track of who has which bits of the file and where someone can download from. Instead of telling to the client, you can download from me, it says, you can download from client x, the same way as it will refer other clients to your client.

Note that with a torrent, you do not start uploading when your download is complete, but rather as soon as you have something to offer. Say you downloaded 10kb of a 1mb file, then someone else can already download those 10kb from you.