Pastry (DHT) - Overview

Overview

Although the distributed hash table functionality of Pastry is almost identical to other DHTs, what sets it apart is the routing overlay network built on top of the DHT concept. This allows Pastry to realize the scalability and fault tolerance of other networks, while reducing the overall cost of routing a packet from one node to another by avoiding the need to flood packets. Because the routing metric is supplied by an external program based on the IP address of the target node, the metric can be easily switched to shortest hop count, lowest latency, highest bandwidth, or even a general combination of metrics.

The hash table's key-space is taken to be circular, like the key-space in the Chord system, and node IDs are 128-bit unsigned integers representing position in the circular key-space. Node IDs are chosen randomly and uniformly so peers who are adjacent in node ID are geographically diverse. The routing overlay network is formed on top of the hash table by each peer discovering and exchanging state information consisting of a list of leaf nodes, a neighborhood list, and a routing table. The leaf node list consists of the L/2 closest peers by node ID in each direction around the circle.

In addition to the leaf nodes there is also the neighborhood list. This represents the M closest peers in terms of the routing metric. Although it is not used directly in the routing algorithm, the neighborhood list is used for maintaining locality principals in the routing table.

Finally there is the routing table itself. It contains one entry for each address block assigned to it. To form the address blocks, the 128-bit key is divided up into digits with each digit being b bits long, yielding a numbering system with base 2b. This partitions the addresses into distinct levels from the viewpoint of the client, with level 0 representing a zero-digit common prefix between two addresses, level 1 a one-digit common prefix, and so on. The routing table contains the address of the closest known peer for each possible digit at each address level, except for the digit that belongs to the peer itself at that particular level. This results in the storage of contacts per level, with the number of levels scaling as . Values of and represent operating values on a typical network.