How Gnutella Works

Gnutella Clients

The original Napster had one piece of "client software" -- the software that users ran on their machines to access the Napster servers. Gnutella has dozens of clients available. Some of the popular Gnutella clients include:

How a Gnutella client finds a song

Given that there is no central server to store the names and locations of all the available files, how does the Gnutella software on your machine find a song on someone else's machine? The process goes something like this:

  • You type in the name of the song or file you want to find.
  • Your machine knows of at least one other Gnutella machine somewhere on the network. It knows this because you've told it the location of the machine by typing in the IP address, or because the software has an IP address for a Gnutella host pre-programmed in. Your machine sends the song name you typed in to the Gnutella machine(s) it knows about.
  • These machines search to see if the requested file is on the local hard disk. If so, they send back the file name (and machine IP address) to the requester.
  • At the same time, all of these machines send out the same request to the machines they are connected to, and the process repeats.
  • A request has a TTL (time to live) limit placed on it. A request might go out six or seven levels deep before it stops propagating. If each machine on the Gnutella network knows of just four others, that means that your request might reach 8,000 or so other machines on the Gnutella network if it propagates seven levels deep.

It is an extremely simple and clever way of distributing a query to thousands of machines very quickly.

This approach has one big advantage -- Gnutella works all the time. As long as you can get to at least one other machine running Gnutella software, you are able to query the network. No court order is going to shut this system down, because there is no one machine that controls everything. However, Gnutella has at least three disadvantages:

  • There is no guarantee that the file you want is on any of the 8,000 machines you can reach.
  • Queries for files can take some time to get a complete response. It might be a minute or more before all of the responses, seven levels deep, come in.
  • Your machine is part of this network. It is answering requests and passing them along, and in the process routing back responses as well. You give up some amount of your bandwidth to handle requests from all the other users.

Apparently, these disadvantages are minor, because people have downloaded hundreds of millions of copies of Gnutella clients.