Orangeman Burns - Sriram Sridharan's Blog

BIT TORRENT PROTOCOL

The search for newer protocols with lesser overhead and user exposure lead to Bit Torrent protocol, now familiarly known as “Torrents”.

When we download a file from the generic decentralized network, the P2P client or the program downloads the file from a completed and available source. Here’s where the BT protocol is advantageous. The BT tracker (The server that tracks file availability) not only gives you parts from the completed source (Seeder), but also from the partially complete sources (Leecher).

To put the whole thing in perspective, here’s an example:

Peer A and Peer B require a file named illegal.mp3. There’s one source: Seeder A.

The file is made up of the following segments:

illegal.mp3 = SEGMENT 1 + SEGMENT 2 + SEGMENT 3+ SEGMENT 4

This is what the Seeder A has.

Peer A = SEGMENT 1 + SEGMENT 3

Peer B = SEGMENT 2 + SEGMENT 4

In generic Decentralized configuration, Peer A downloads SEGMENT 2 and SEGMENT 4 from Seeder A and Peer B downloads SEGMENT 1 and SEGMENT 3 from Seeder A.

When we analyze this protocol deeply, we find that this is very inefficient. Peer A and Peer B have segments that each other wants. So instead of utilizing their network resources, they waste Seeder A’s resources. This is exactly what the Bit Torrent protocol avoids. By having the users to upload and download and exchange with peers at the same time, network congestion is eased.

The Bit Torrent downloads have two components:

1. The .torrent file
2. The client

The .torrent file is found by searching in a torrent search engine (much like Google) and downloading it to the end user’s PC. Then the torrent client like Azureus or utorrent is started up and the .torrent file is loaded into the program. The program talks to the tracker, finds out what pieces are available in which peers and downloads the same.

image courtesy HowStuffWorks

Cable Labs, the research organization of the North American cable industry, estimates that Bit Torrent represents 18% of all broadband traffic. In 2004, Cache Logic put that number at roughly 35% of all traffic on the Internet. So the ISP’s decided to cut down on this protocol by throttling the frequently used ports. The users retorted by assigning random ports to the client and by encrypting their traffic.

These are two major bonus points in favor of Bit Torrent sites:

1. They just host the index (hash) on the servers and the copyrighted material is not stored on them.
2. Means of finding the torrent file and downloading it are not linked.

There’re many programs like Peer Guardian 2 and Safe Peer that maintain a list of Government, MPAA/RIAA/Industry watchdog, University Nodes that are to be avoided. These programs simply block these IP’s and disallow traffic between these peers.

Bit Torrent does not offer its users anonymity. It is possible to obtain the IP addresses of all current, and possibly previous, participants in a swarm from the tracker. This may expose users with insecure systems to attacks. But encryption solves that, provided the client used supports it.

Even Bit Torrent is now in it’s dying days and a new protocol is emerging called Dark Net. This operates on the principle that all the peers are trusted and the trusted peers cannot be penetrated. This is analogous to having a group of friends and assuming none of your friends betray you. This protocol is still very nascent and we’ll see its mainstream emergence soon.