Under Development

Beyond P2P with WAND

The Wide Area Network Directory (WAND) from PlanetLab could revolutionise the Internet's data management system, says Vinutha V

Peer-to-Peer (P2P) systems are getting all the buzz, be it Skype for voice or Kaaza for file-sharing. Locating data elements in a pure P2P system that has no central server or any other form of centralised control is a challenge. To facilitate browsing and retrieval from a wide-area P2P network, a file system is required'one that is robust and retains its structure even when nodes enter and leave the network on the fly.

To evolve and maintain a global file system structure over a wide-area P2P network, a team of four academicians from the Indian Institute of Information Technology-Bangalore (IIIT-B) have developed a new model called Wide Area Network Directory (WAND). This system has a novel approach to managing the root directory, and an innovative mechanism for propagating changes using a simple form for metadata'a file system tree.

How it works

In a WAND network, peer systems (meaning PCs) share one or more directories from their local file system. WAND provides a file system tree where peers can mount their shared directories. The global file system structure is implemented in such a way that each peer is in charge of one or more mounted directories.

A peer can enter a WAND network by either connecting to an existing network or building its own network using the buildroot function. When a node executes buildroot, a horizontal cache (root cache) is created containing the contents of the root directory. The file system comprises a single virtual folder corresponding to the root directory that is owned by the peer performing buildroot. The node then has to create at least one virtual directory under the root directory. It can mount its shared directories under these virtual directories at the root level. The node performing buildroot is termed as the WAND node of its network. When a new node wants to join, it requests a copy of the horizontal cache from any node in an existing network. The host node can be either a root level or a sub-directory level node.

Handling networked partitions

The WAND directory structure is resilient to network partitions. These are handled by splitting the file system tree into two or more sub-trees. The splitting algorithm is such that a node tries its best to remain in its file system tree. But if it cannot contact any node even after trying its best, it decides to walk away and form its own network containing the same sub-tree to which it belongs. It does not matter whether the network is really partitioned'the objective is to maintain consistent sub-trees when the tree splits.

Tested at PlanetLab

The WAND model has been piloted through the global PlanetLab project, which is a test bed for researchers and academicians to experiment with network applications and services that benefit from distribution across a wide geographic area.

A prototype of WAND has been implemented over Java and RMI, and the implementation is sufficient to test the usability of WAND. The performance of the WAND network is dependent on the number of messages being passed between nodes, the delay in message-passing due to geographic distribution of nodes, and consistency in the network view that each node has with respect to the actual network topology.

Says Dr Srinath Srinivasa, a professor at IIIT-B who is also in charge of PlanetLab at the institute, 'We are hoping that WAND will become an open source tool and a resilient file system for various applications in fields where data sharing is required.'

WAND is expected to become a comprehensive tool for data sharing without having to maintain a Website or using a domain name. It will be useful in the areas of education and in the corporate world, and wherever sharing of files or online meetings or anything to do with co-ordination is required.