Closer look at P2P technology

Napster used a proprietary P2P protocol to organise the downloading between peers and added a central server to register and validate users and store an index of what files were available on which clients.

It then wrapped it all up in an easy-to-use application that enabled users to search for a particular file or even chat to other Napster users.

Napster wasn’t ‘true’ P2P, but a centralised P2P system, because it relied on a central server for most administrative tasks, and didn’t fully use the computing power of the peers.

A ‘pure’ P2P architecture doesn’t need any central servers at all, sharing all the indexing and admin tasks between the clients as well. Such an architecture is often called a decentralised (or fully distributed) P2P network. Examples that are still going strong are the Gnutella and Edonkey networks. The once popular Kazaa, created by the inventors of Skype, was also a decentralised network.

As we’ll see below, Bittorrent falls between these extremes, adding new and unique twists that make it a hybrid between ‘pure’ P2P and a traditional client-server architecture.

Naming names
In any discussion of P2P, it’s easy to get muddled between application and protocol names, so let’s clear a few names out the way first. Napster was an application based on its own proprietary protocol, Kazaa software used the Fasttrack protocol and Gnutella is the name of the protocol. Edonkey is also a protocol.

For any particular protocol, many client applications are usually available, offering a variety of different features. For example, Gnutella clients include Limewire, Bearshare and Morpheus. Emule is the most popular Edonkey client.

Confusingly, Bittorrent is the name of the file-sharing protocol, the company, the associated website and the ‘official’ free software client, but there are dozens of alternative Bittorrent clients available, such as Utorrent and Azureus. For the purposes of this feature we’ll use ‘Bittorrent’ for the protocol or client and ‘Bittorrent.com’ for the company.

Torrents, seeds and trackers
From day one, Cohen designed Bittorrent and its interface to be easy to use, reliable, give fast downloads and avoid the problem of unfair P2P behaviour. In a normal P2P network, once a client has downloaded a file, it has no further incentive to make that file available to other clients. So the P2P network becomes reliant on a few generous clients and becomes very slow and inefficient.

In Bittorrent terminology, clients that are downloading are called ‘leechers’, whereas clients that are actively uploading are called ‘seeds’. A P2P network with only leechers wouldn’t work for obvious reasons.

Bittorrent tries to prevent this behaviour by forcing clients to do simultaneous downloads and uploads and using other tit-for-tat tricks in the protocol. The easiest way to explain this is to look at how you go about downloading a file in practice.

To download a file via Bittorrent, you first need a Bittorrent client. Probably the best one to start with is the official free client from Bittorrent.com. Although it’s a commercial website, you don’t have to register and there’s lots of free content that you can use to see how it all works.