Pastry Antony Rowstron and Peter Druschel

1
PastryAntony Rowstron and Peter Druschel

• Presented By
• David Deschenes

2
Introduction

• General substrate for P2P applications
• Provides object location and routing
• Characteristics
• Completely decentralized
• Fault-resilient
• Scalable
• Reliability

3
Design

• Circular Node Space
• Nodes randomly assigned unique 128 bit
  identifiers
• Uniform distribution of identifiers
• Identifiers indicate position in space
• Messages and Keys
• Messages assigned N-bit keys
• Each message routed to the node whose identifier
  is numerically closest to its assigned key
• Key generation is application specific

4
Node State

• Leaf Set
• L closest nodes in the node space
• Routing Table
• Identifies 2b-1 hosts whose identifiers match the
  current node identifier in n bits
• Neighborhood Set
• M closest nodes according to a proximity metric

5
Network Organization

• Node Arrival
• New node acquires the identifier X and routes a
  join message with key X through node A
• New nodes initialize their state based upon state
  shared by nodes that route its join message
• Node Departure
• If a member of a nodes leaf set is lost, the
  leaf set is expanded outward to find a new node
• If a routing table entry is lost a node in the
  same row is consulted for a replacement

6
Maintaining Locality

• Proximity based on a scalar metric
• E.g. IP routing hops, geographic distance
• Metric determined by application
• Assumed that the triangulation inequality holds
  for the metric
• Connections to nodes with greater proximity are
  desired
• Facilitated by the neighborhood set

7
Message Routing

• Check the leaf set
• If destination present route to it directly
• Consult the routing table
• Forward to a node whose identifier matches the
  message key in at least one more bit
• Forward to a node whose identifier is numerically
  closer to the message key

8
Evaluation

• Number of routing hops scales with the network
  size (log2bN) Figure 4.
• Routes are only 30 to 40 longer despite minimal
  routing tables Figure 6.
• Able to locate one of the top two nodes 92 of
  the time
• Minimally impacted by node failure Figure 10.