How to Easily Make Existing Java Software
Work on a Peer-to-Peer Network
Brad GNUberg, bkn3@columbia.edu

Summary
This tutorial describes how to easily port over existing Java network software which depends on java.net.Socket, java.net.ServerSocket, and java.net.InetAddress to work on the Jxta peer-to-peer network

Questions? Comments?
See the P2P Sockets Homepage or contact Brad Neuberg at bkn3@columbia.edu.  Feel free to call him at 1-510-938-3263 (Pacific Standard Time, San Francisco)
Monday through Friday, not including weekends.  Also see his weblog, www.codinginparadise.org, for Mozilla, Java, Jxta, and P2P news.

What is the P2P Sockets Project?
Are you interested in:
If you answered yes to any of the above, then welcome to the Peer-to-Peer Sockets project!  The Peer-to-Peer Sockets Project reimplements Java's standard Socket, ServerSocket, and InetAddress classes to work on a peer-to-peer network rather than on the standard TCP/IP network.  

"Aren't standard TCP/IP sockets and server sockets already peer-to-peer?" some might ask.  Standard TCP/IP sockets and server sockets are theoretically peer-to-peer but in practice aren't for several important reasons.  First, many of the peers on the Internet are given dynamic IP addresses through DHCP, shared or filtered IP addresses through NAT devices, or IP addresses that are very hard to reach due to firewalls.  Creating server sockets under such conditions is either impossible or requires elaborate application-level ruses to bypass these restrictions.  Second, TCP/IP sockets and server sockets depend on the Domain Name System (DNS) to resolve human-friendly host names into IP addresses.  DNS is theoretically semi-decentralized, but on an administrative level it is centralized under ICANN, an unresponsive, non-innovative organization.  Further, standard DNS does not deal well with edge-peers that have filtered or dynamic IP addresses, and updates take too long to propagate and have no presence information.  Developers who wish to create exciting, new applications that extend DNS into new directions, such as storing instant messaging usernames and presence info in DNS, are stymied by the DNS systems technical and political issues.

The P2P Sockets project deals with these issues by re-implementing the standard java.net classes on top of the Jxta peer-to-peer network.  Jxta is an open-source project that creates a peer-to-peer overlay network that sits on top of TCP/IP.  Ever peer on the network is given an IP-address like number, even if they are behind a firewall or don't have a stable IP address.  Super-peers on the Jxta network run application-level routers which store special information such as how to reach peers, how to join sub-groups of peers, and what content peers are making available.  Jxta application-level relays can proxy requests between peers that would not normally be able to communicate due to firewalls or NAT devices.  Peers organize themselves into Peer Groups, which scope all search requests and act as natural security containers.  Any peer can publish and create a peer group in a decentralized way, and other peers can search for and discover these peer groups using other super-peers.  Peers communicate using Pipes, which are very similar to Unix pipes.  Pipes abstract the exact way in which two peers communicate, allowing peers to communicate using other peers as intermediaries if they normally would not be able to communicate due to network partitioning.

Jxta is an extremely powerful framework.  However, it is not an easy framework to learn, and porting existing software to work on Jxta is not for the faint-of-heart.  P2P Sockets effectively hides Jxta by creating a thin illusion that the peer-to-peer network is actually a standard TCP/IP network.   If a peer wishes to become a server they simply create a P2P server socket with the domain name they want and the port other peers should use to contact them.  P2P clients open socket connections to hosts that are running services on given ports.  Hosts can be resolved either by domain name, such as "www.nike.laborpolicy", or by IP address, such as "44.22.33.22".  Behind the scenes these resolve to JXTA primitives rather than being resolved through DNS or TCP/IP.  For example, the host name "www.nike.laborpolicy" is actually the NAME field of a Jxta Peer Group Advertisement.  P2P sockets and server sockets work exactly the same as normal TCP/IP sockets and server sockets.  For the technically inclined and those who already understand Jxta, a table exists illustrating how the standard TCP/IP concepts, such as host name, IP address, etc., map to their Jxta equivalents.

The benefits of taking this approach are many-fold.  First, programmers can easily leverage their knowledge of standard TCP/IP sockets and server sockets to work on the Jxta peer-to-peer network without having to learn about Jxta.  Second, all of the P2P Sockets code subclasses standard java.net objects, such as java.net.Socket, so existing network applications can quickly be ported to work on a peer-to-peer network.  The P2P Sockets project already includes a large amount of software ported to use the peer-to-peer network, including a web server (Jetty) that can receive requests and serve content over the peer-to-peer network; a servlet and JSP engine (Jetty and Jasper) that allows existing servlets and JSPs to serve P2P clients; an XML-RPC client and server (Apache XML-RPC) for accessing and exposing P2P XML-RPC endpoints; an HTTP/1.1 client (Apache Commons HTTP-Client) that can access P2P web servers; a gateway (Smart Cache) to make it possible for existing browsers to access P2P web sites; and a WikiWiki (JSPWiki) that can be used to host WikiWikis on your local machine that other peers can access and edit through the P2P network. Even better, all of this software works and looks exactly as it did before being ported.  The P2P Sockets abstraction is so strong that porting each of these pieces of software took as little as 30 minutes to several hours.  Everything included in the P2P sockets project is open-source, mostly under BSD-type licenses, and cross-platform due to being written in Java.

Because P2P Sockets are based on Jxta, they can easily do things that ordinary server sockets and sockets can't handle.  First, creating server sockets that can fail-over and scale is easy with P2P Sockets.  Many different peers can start server sockets for the same host name and port, such as "www.nike.laborpolicy" on port 80.  When a client opens a P2P socket to "www.nike.laborpolicy" on port 80, they will randomly connect to one of the machines that is hosting this port.  All of these server peers might be hosting the same web site, for example, making it very easy to partition client requests across different server peers or to recover from losing one server peer.  This is analagous to DNS round-robining, where one host name will resolve to many different IP addresses to help with load-balancing.  Second, since P2P Sockets don't use the DNS system, host names can be whatever you wish them to.  You can create your own fanciful endings, such as "www.boobah.cat" or "www.cynthia.goddess", or application-specific host names, such as "Brad GNUberg" or "Fidget666" for an instant messaging system.  Third, the service ports for a given host name can be distributed across many different peers around the world.  For example, imagine that you have a virtual host for "www.nike.laborpolicy".  One peer could be hosting port 80, to serve web pages; another could be hosting port 2000, for instant messaging, and a final peer could be hosting port 3000 for peers to subscribe to real-time RSS updates.  Hosts now become decentralized coalitions of peers working together to serve requests.

Requirements and Configuration
You must download and install the following software to develop and work with P2P Sockets:
Install and configure the JDK and Ant, and make sure both are in your path so they can be run from the command-line.  Unzip the P2PSockets zip file into the top-level of your hard-drive; spaces are not allowed in the directory names, or the P2P Sockets build files will not work correctly.

The P2P Sockets directory already includes two different directories, test/clientpeer and test/serverpeer, that have Jxta configuration information already set up (these are in the hidden directories test/clientpeer/.jxta and test/serverpeer/.jxta, respectively).  If you want to learn more about how to configure Jxta, read about the Jxta Configurator.  The two test peers have already been configured for the worst possible case, which is that you are behind a firewall and a NAT device, which means you must use other intermediate peers to relay your requests; this configuration will work even if you are not under these conditions.  One of the nice aspects about Jxta, though, is that this will all be transparent and hidden from you as you program and use the system.

When testing the examples in this tutorial, you must be connected to the Internet.  This is for two reasons; first, the examples use a public Jxta server Sun has set up that helps bootstrap peers into the Jxta network; and second, on some operating systems, such as on Windows XP by default, the network subsystem shuts down if you are not connected to the Internet, preventing client peers and server peers that are running on the same machine from finding or communicating with each other.

Overview of P2P Socket Classes
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What to Change in your Code
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How to Test your Changes
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Limitations and Security Concerns
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Resources

About the Author
Image of Brad Neuberg as a childBrad "GNUberg" Neuberg is an open-source developer living in San Francisco.  He is currently involved with the BlackConnect project, an effort to allow programmers to write Mozilla XPCOM components in Java; the Java-Mozilla Integration project, an umbrella effort for open-source developers to work on turning Java into a full-fledged member of the Mozilla platform; the P2P Sockets project, an initiative to port the existing web onto a peer-to-peer foundation and make working with Jxta easier; the Paper Airplane project, a browser plugin that empowers everone to be a writer as well as a reader; and submissions to the Jxta and Mozilla projects.  In the past he has worked on the OpenPortal project and the Flash/Java Integration project.  He is an avid Java, C++, JavaScript, and Mozilla Platform hacker who is a strong believer in usability.  He has a B.A. in Computer Science from Columbia University and experience building enterprise trading systems for the securities industry using J2EE technologies.  He is now slightly older than he was when the picture on the left was taken.  Occasionally, after enough beers, he'll start tap dancing.



This work is licensed under the Creative Commons Attribution-NonCommercial License by Bradley Keith Neuberg. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc/1.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.