CS262B Reading Summary

Pastry: Scalable, decentralized object location and routing for large-scale peer-to-peer systems

Summary by Feng Zhou
3/17/04

Strong points of the paper are:

1. Compared to Chord, Pastry utilizes locality information using PRR-tree like routing structure. This reduces routing distance greatly. It also reduces overall network bandwidth consumption because most hops are localized. This is at the cost of larger routing tables (O(log^2 n) instead of O(log n)) and potentially more messages to maintain the routing table.
   
2. Another feature of Pastry is the leaf set. It's conceptually and functionally similar to the successor and predecessor in Chord. The significance of the leaf set is at least two fold: 1) It ensures provable fault-tolerance of the routing algorithm, i.e. tolerating simultaneous failures of at least |L|/2 of the |L| leaf set nodes. 2) By doing so, it greatly reduces the need to keep the main PRR routing table up-to-date. It acts as an slower fall-back mechanism that essentially renders the main routing table an optimization.  Now the routing efficient vs. route maintaining overhead becomes a tunable parameter.
   

The evaluation of node joining and leaving behavior is simplistic.  Only batching joining/leaving is simulated.  In real network the dynamics will be much more complex.  It is important to study how the network behaves under continuous node arrival, leaving and failures.