Gnutella
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Gnutella (pronounced /nʌˈtɛlə/ or /gnʌˈtɛlə/) is a file sharing network used primarily to exchange files. As of December 2005, Gnutella is the third-most-popular file sharing network in the Internet, following eDonkey 2000 and FastTrack. While figures vary from hour to hour and day to day, Gnutella is thought to host on average approximately 2.2 million users, although around 400 000-500 000 are on at any given moment. [link]
History
The first client was developed by Justin Frankel and Tom Pepper of Nullsoft, in early 2000, soon after the company's acquisition by AOL. On March 14, the program was made available for download on Nullsoft's servers. The event was prematurely announced on Slashdot, and thousands downloaded the program that day. The source code was to be released later, supposedly under the GNU General Public License (GPL).
The next day, AOL stopped the availability of the program over legal concerns and restrained Nullsoft from doing any further work on the project. This did not stop Gnutella; after a few days, the protocol had been reverse engineered, and compatible open-source clones began to appear. This parallel development of different clients by different groups remains the modus operandi of Gnutella development today.
The Gnutella network is a fully distributed alternative to such semi-centralized systems as FastTrack (KaZaA) and such centralized systems as Napster. Initial popularity of the network was spurred on by Napster's threatened legal demise in early 2001. This growing surge in popularity revealed the limits of the initial protocol's scalability. In early 2001, variations on the protocol (first implemented in closed-source clients) allowed somewhat of an improvement in scalability. Instead of treating every user as client and server, some users were now treated as "ultrapeers", routing search requests and responses for users connected to them.
This allowed the network to grow in popularity. In late 2001, the Gnutella client LimeWire became open source. In February 2002, Morpheus, a commercial file-sharing group, abandoned its FastTrack-based peer-to-peer software and released a new client based on the open source Gnutella client Gnucleus.
The word "Gnutella" refers not to any one project or piece of software, but to the open protocol used by the various clients. Since various parties are developing new clients, and the protocol will likely continue to evolve, it is hard to say what the word 'Gnutella' will come to mean in the future.
The name is a portmanteau of GNU and Nutella: supposedly, Frankel and Pepper ate a lot of Nutella working on the original project, and intended to license their finished program under the GNU General Public License. Gnutella is not associated with the GNU project; see GNUnet for the GNU project's equivalent.
How it works
To envision how Gnutella originally worked, imagine a large circle of users (called nodes), who each have Gnutella client software. On initial startup, the client software must bootstrap and find at least one other node. Different methods have been used for this, including a pre-existing address list of possibly working node shipped with the software, using updated web caches of known nodes (called GWebCaches), UDP host caches and rarely even IRC. Once connected, the client will request a list of working addresses. The client will try to connect to the nodes it was shipped with as well as nodes it receives from other clients until it reaches a certain quota. It will only connect to that many nodes, locally cache the addresses it has not yet tried and discarding addresses it tried which were invalid.Now, when the user wanted to do a search, the client would send the request to each node it is actively connected to. The number of actively connected nodes for a client was usually quite small (around 5), so each node then forwards the request to all the nodes it is connected to, and they in turn forward the request, and so on, until the packet was a predetermined number of "hops" from the sender.
If a search request turns up a result, the node that had the result needs to contact the searcher. In the classic Gnutella protocol response messages were always send back along the route the query came in through, as the query itself did not contain identifying information of the node. This scheme was later revised, so that search results are delivered over UDP directly to the node which initiated the search respectively a proxying peer usually an ultrapeer of the node. The queries do therefore carry the IP address and port number of either node. This lowers the amount of traffic routed through the Gnutella network significantly making it more scalable.
If the user decides to download the file, they negotiate the file transfer. If the node which has the requested file is not firewalled, the querying node can connect to it directly. However, if the node is firewalled, stopping the source node from receiving incoming connections, the client wanting to download a file will send it a so called "push request" to the server for the remote client to initiate the connection instead (to "push" the file). At first, these push requests were routed along the original chain it used to send the query. This was however rather unreliable because routes would often break and routed packets are always subject to flow control. Therefore so called "push proxies" were introduced. These are usually the ultrapeers of a leaf node and they are announced in search results. The client connects to one of these "push proxies" using a HTTP request and the proxy sends a "push request" to leaf on behalf of the client. Normally, it is also possible to send a push request over UDP to the push proxy which is more efficient than using TCP. Push proxies have two advantages: First, ultrapeer-leaf connections are more stable than routes which makes push requests much more reliable. Second, it reduces the amount of traffic routed through the Gnutella network.
Finally, when a user disconnects, the client software saves the list of nodes that it was actively connected to and those collected from pong packets for use the next time it attempts to connect so that it becomes independent from any kind of bootstrap services.
In practice, this method of searching on the Gnutella network was often unreliable. Each node is a regular computer user; as such, they are constantly connecting and disconnecting, so the network is never completely stable. Also, the bandwidth cost of searching on Gnutella would grow exponentially to the number of connected users [link], often saturating connections rendering slower nodes useless. Therefore, search requests would often be dropped, and most queries reached only a very small percentage of the network. This observation identified the Gnutella network as an unscalable distributed system, and inspired the development of distributed hash tables, which are much more scalable but support only exact-match, rather than keyword, search.
To address the problems of bottlenecks, Gnutella developers implemented a tiered system of ultrapeers and leaves. Instead of all nodes being considered equal, nodes entering into the network were kept at the 'edge' of the network as a leaf, not responsible for any routing, and nodes which were capable of routing messages were promoted to ultrapeers, which would accept leaf connections and route searches and network maintenance messages. This allowed searches to propagate further through the network, and allowed for numerous alterations in the topology which have improved the efficiency and scalability greatly.
The real benefit of having Gnutella so decentralized is to make it very difficult to shut the network down. Unlike Napster, where the entire network relied on the central server, Gnutella cannot be shut down by shutting down any one node.
Protocol features and extensions
Gnutella operates on a query flooding protocol. The outdated Gnutella version 0.4 network protocol employs five different packet types, namely
- ping: discover hosts on network
- pong: reply to ping
- query: search for a file
- query hit: reply to query
- push: download request for firewalled servents
The development of the Gnutella protocol is currently led by the GDF (Gnutella Developer Forum). Many protocol extensions have been and are being developed by the software vendors and free Gnutella developers of the GDF. These extensions include intelligent query routing, SHA-1 checksums, query hit transmission via UDP, querying via UDP, dynamic queries via TCP, file transfers via UDP, XML meta data, source exchange (also known as "the download mesh") and parallel downloading in slices (swarming).
There are efforts to finalize these protocol extensions in the Gnutella 0.6 specification at the Gnutella protocol development website. The Gnutella 0.4 standard, although being still the latest protocol specification since all extensions only exist as proposals so far, is outdated. In fact, it is hard to impossible to connect today with the 0.4 handshake.
The Gnutella protocol remains under development and in spite of attempts to make a clean break with the complexity inherited from the old Gnutella 0.4 and to design a clean new message architecture (such as Gnutella2), it is still one of the most successful file-sharing protocols to date.
Software
See also
- Gnutella2, a reworked network based on Gnutella
- Bitzi, an open content file catalog integrated with some Gnutella clients
- [[Wikisource:GnuFU|GnuFU]], Gnutella For Users: A description of the inner workings of the Gnutella network in User-Friendly Style
- Gnutella crawler, a program used to gather information from the Gnutella network
- WASTE, a different network developed by Justin Frankel
- Freenet, a different network which focuses on anonymization
- MUTE, a different network which focuses on anonymization, routing network traffic according to an algorithm adopted from ant behaviour.
External links
- [Gnutelliums] - A list of Gnutella clients for Windows, Linux/Unix, and Macintosh
- [Gnutella Forums]
- [GnuFU: Gnutella For Users] – A guide to Gnutella in Userfriendly style
- [Pytella] – A collection of bits of Gnutella 0.4 related Python code
- [CPCD] - Gnutella host cache software
- [GWebCache] - Gnutella host cache software
Papers on Gnutella and Filesharing
- [The 5th annual Passive & Active Measurement Workshop]
- [Advanced Peer-Based Technology Business Models]. Ghosemajumder, Shuman. MIT Sloan School of Management, 2002.
- [Music Downloads: Pirates- or Customers?]. Silverthorne, Sean. Harvard Business School Working Knowledge, 2004.
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