Weaving%20a%20Tapestry

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Weaving a Tapestry

• Distributed Algorithms for Secure Node
  Integration, Routing and Fault Handling
• Ben Y. Zhao (John Kubiatowicz, Anthony Joseph)
• Fault-tolerant Computing
• Fall 2000

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Why Tapestry?

• Distributed systems scaling to WAN
• Larger scale ? frequent component faults
• More data centralization ? performance
  bottleneck
• Dynamic environment ? manageability complexity
• More principals ? attacks on system (e.g. DoS)
  more likely
• Tapestry
• Decentralized approach to location and routing
  focusing on fault-resilience and adaptability
• Builds on previous work Plaxton trees

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Background Tapestry/Plaxton

• Decentralized Routing
• Local neighbor pointers
• Finite storage overhead
• Wide-area Location
• Each obj maps to root node
• Store backpointers en route to root node
• Exploits locality

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New Tapestry Mechanisms

• Sibling Mesh
• Nodes in Sibling Mesh of level N share common
  suffix of length N
• Neighbors of level N1 are siblings of level N
• Clusters connected w/in, possibly disconnected
  between regions
• Gateway Nodes
• Nodes that also serve as integration points
• Integrates new nodes w/in coverage area

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Issues

• Routing to non-existent node Ids
• Node integration into Tapestry
• Populate neighbor maps, divert relevant traffic
• Goals
• Low latency
• Limit stress on system/nodes
• Prevent/Limit Denial of Service attacks
• Approximate optimal mapping
• Fault-handling
• Fast detection, avoidance, and recovery

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Surrogate Routing

• Messages to non-existent node go to surrogate
• When routing hits null entry in neighbor map
• Plaxton algorithm
• Find global set of all nodes matching on most
  of suffix bits to destination ID
• Use global ordering to choose 1 determinstically
• One hop to surrogate
• Tapestry distributed algorithm
• Find local set of routes matching on most suffix
  bits
• Deterministically (via pseudo-random hash) choose
  an existing alternate route on
• Terminate when local node is only entry in
  neighbor map

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Surrogate Overhead

• Additional hops after Plaxton version terminates
• Function of how even sparseness spreads through
  the namespace
• Probabilistic reasoning
• Assumption even ID distribution in space
• If perfectly evenly spaced names in namespace
  overhead lt 1
• Overhead function of skew, with high
  probability is very small constant

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Node Weaving Algorithm

• Phase I
• Given desired Guid g, nearby gateway
• Ask gateway to route to g, return hops
• Router for hop i1 is sibling at level i
• For the i th hop router
• Copy i th neighbor map
• For each entry E in neighbor list,
• Look at Es higher order siblings
• Traverse sibling mesh until local optima reached

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Node Weaving Phase I
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Node Weaving Phase II
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• Notify nearby nodes to divert traffic
• Recognize Neighbor(i1) Sibling(i)
• For each route level
• Send NotifyMsg to neighbors
• Msg forwarded w/ small TTL
• Intuition
• Only local nodes need notification
• Probability dictates distant nodes have better
  alternative ? no notifynecessary

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Denial of Service

• Scenario 1
• Generate large of Guids, weave each in
• Soln Attach cost function to use of each Guid
• Key Bit Sequence L such that
• SHA-1(GuidLS) 0000000
• S random string refreshed periodically per
  server
• Solving inv(SHA-1) is costly
• Showing L with request to integrate is enough
• Scenario 2
• Weave same Guid in using large of Gateways
• Soln Allow Gateways to arbitrarily limit
  coverage area

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Fault Handling

• Detection
• Routing heartbeats to immediate neighbors
• Location inventory beats to routers
• Resilience
• Use secondary neighbors/siblings to route around
  failed links/servers
• Repair
• Failed nodes marked with invalid flag
• Periodic probes (piggyback msgs)
• Success ? Change back to active status

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In Progress

• Simulations
• Java Simulation of dynamic Tapestry
• C Simulation of perfect topology with SGB
  library
• Algorithms
• Theoretical analysis
• Expected insertion latencies
• Optimality of dynamic insertions
• Optimality of overlay network distance
• Routing
• Better routing using landmarks