Exploring Tradeoffs in Failure Detection in P2P Networks Shelley Zhuang, Ion Stoica, Randy Katz Sahara Retreat June 4-6, 2003

Problem Statement One of the key challenges to achieve robustness in overlay networks: quickly detect a node failure Canonical solution: each node periodically pings its neighbors Propose keep-alive techniques Study the fundamental limitations and tradeoffs between detection time, control overhead, and probability of false positives

Network Model and Assumptions P2P system with n nodes Each node A knows d other nodes Average path length = l Node up-time ~ i.i.d. T = exponential(λ f ) Failstop failures If a neighbor is lost, a node can use another neighbor to route the packet w/o affecting the path length

Packet Loss Probability δ = average time it takes a node to detect that a neighbor has failed Probability that a node forwards a packet to a neighbor that has failed is 1- e -λ f δ  δλ f P(T-t  δ | T  t) = P(T<=δ) Probability that the packet is lost is p l  lδλ f δ T pdf

Aliveness Techniques Baseline –Each node sends a ping message to each of its neighbors every Δ seconds A BC D

Aliveness Techniques Information Sharing –Piggyback failures of neighbors in acknowledgement messages –Best case: completely connected graph of degree d BC DA

Aliveness Techniques Information Sharing with Boosting –When a node detects failure of a neighbor, D, it announces to all other nodes that have D as their neighbor –Best case: completely connected graph of degree d BC DA

Loss Rate – Gnutella Trace Loss Rate = # Lookup timeouts / # Lookups 20 lookups per second Boosting (simple) –No additional state - Constant probing overhead - Small difference due to boosts -Boosting w/ bptr 1.23 times the baseline - Send backpointers every 10 probe acks

False Positive – Analysis & Planetlab Packet losses are independent over time Packet losses are independent across network paths Propagation of positive information

Conclusion Examined three keep-alive techniques in Chord with Gnutella join/leave trace By carefully designing keep-alive algorithms, it is possible to significantly reduce packet loss probability w/o additional control overhead False positive analysis and experiments for boosting with backpointer