CRESCENDOCRESCENDO

GEORGE S. NOMIKOS

Advisor: Dr. George Xylomenos

ATHENS UNIVERSITY OF ECONOMICS AND BUSINESSDEPARTMENT OF INFORMATICS

MSc IN COMPUTER SCIENCE

Implementation of Hierarchical Chord (Crescendo), according to Canon paradigm and evaluation, via simulation over realistic network topologies, of the Crescendo's advantages in comparison to Chord.

ATHENS UNIVERSITY OF ECONOMICS AND BUSINESS

Motivation

● All current DHTs are “Flat”, non-hierarchical structures.● No single point of failure, homogeneity.● Decentralization, Scalability.

● Why Hierarchical Design?● Hierarchies exist! Physical Network has hierarchy.● Fault Isolation, Security.● Efficient caching and bandwidth usage.● Adaptation to the underlying physical network.● Hierarchical storage of content.● Hierarchical access control.

● Goal: Inherit the best of both designs Hierarchical DHTs.

Motivation

● Hierarchical DHTs● Maintain all the advantages of flat DHTs and add

even more.

● Simulators offering hierarchical DHT support● None of the current simulators offer hierarchical

DHT support. No other module exists.

● This is the first and the only hierarchical DHT implementation available. Implemented for the simulation environment OMNeT++/OverSim.

Chord

● Hash function (SHA-1) assigns each node and key an m-bit identifier.

● Node keys are arranged in a circle.

● Each node:

● Has links to it's previous node (predecessor) and to it's next node (successor).

● Maintains a routing table with up to m entries called the finger table.

● Runs a stabilization protocol periodically to find newly joined and failed nodes.

● Responsible node for a key, is the successor of the key.

● Finger table contains records like the following:

● finger[k] = first node that succeeds (n + 2k-1) mod 2m,

where 1 ≤ k ≤ m, n is the current node.

● Example Chord Ring

● 10 Nodes, 5 Keys

● m = 6 bits

Canon

Networks are hierarchical

Canon adapts to the underlying network hierarchy

● Canon Paradigm

● Recursive Structure.

● Construct bottom-up.

● Merge smaller DHTs.

● Multi-level hierarchies.

● A global ring at the top level containing all the small rings.

Crescendo

● Crescendo, Hierarchical Chord using Canon Paradigm.

● Merge Multiple Rings.

● All original links are retained.

● Each node n in one ring creates a link to a node n' in another ring if and only if:

(a) n' is the closest node that is at least distance 2k away for some 0 ≤ k < m

(b) and n' is closer to n than any node in n` s ring.

● Node state:● Chord Node State x Level of hierarchy.

● Finger Tables after the lowest level of hierarchyhave very few links, according to (a), (b)

CrescendoMerging Procedure

● Crescendo Merging example for nodes 0 (Ring A) and 8 (Ring B).

● Node 0 links to node 2.

● Node 8 links to node 10 and node 12.

● Ring A and Ring B

● Chord Rings.

● m = 4 bits

● Fingers distance (+1, +2, +4, +8)

● After merging procedure:

● One Crescendo Ring, containing both A and B Rings.

CrescendoFinger Tables

● Merging Procedure, including full Finger Tables.

● Comparison if instead of Crescendo, we had two Chord rings, one local and one global.

● Two Chord rings need 32 extra links.

● Crescendo needs only 12 extra links.

● Crescendo uses almost 3 times less links than the local, global Chord rings.

CrescendoTwo Crucial Properties

Crescendo has two crucial properties

● Locality of intra-domain paths.

● When the node which starts the lookup and the destination node are in the same domain, then the lookup never leaves this domain.

● Convergence of inter-domain paths.

● When different nodes from one domain A route to the same node in another domain, all the different routes exit the domain A through the same node. This node is the closest predecessor of the target node's identifier in the domain A.

CrescendoKey Responsibility

Crescendo changes key responsibility. Responsible node for a key is it's predecessor.

Realistic Network Topologies

● OMNeT++/OverSim offers physical network topology with limitations.

● No Autonomous Systems Identification Numbers (ASIDs).

● No routing policy weights.

● Crescendo requires realistic physical network topology.

● Autonomous Systems Identification Numbers (ASIDs) required. With ASIDs nodes can calculate the relation among themselves and other nodes.

● Routing policy weights required.

● Solution: Extend BRITE topology generator export tool and OMNeT++.

● Support for Realistic Network Topologies based on GT-ITM model.

● Support for Autonomous Systems Identification Numbers (ASIDs).

● Support for Routing Policy Weights.

● Realistic network topologies can also be used as a standalone solution for other modules that require this level of reality.

Crescendo Implementation

● More than 7000 Lines of Code (including comments)

● Fully documented code

● Doxygen documentation

● NED documentation

● Crescendo implemented as a new autonomous module.

● Realistic Topologies based on theGT-ITM Model.

● KBRTestApp extended to export analytical statistics.

● Supports two levels hierarchy.

● Low level: Local AS.

● High level: All AS merged.

OverSim Architecture. In blue boxes the sections where new code added.

Crescendo Implementation

● Crescendo node join procedure.● Node sends join local ring request to

the bootstrap node with same ASID.

● Node joins local ring.

● Node sends join global ring request to it's local predecessor.

● Node joins global ring.

● Join procedure ends.

● Predecessor node is the node that forwards the lookup to the next level of the hierarchy.

Request to join the next level ring:(a) To any node except predecessor, extra hops.(b) To predecessor, minimum hops.

Crescendo Correctness

● Simulation to check that this implementation has the Crescendo's two crucial properties.

● Key length 16 bits, 1000 Nodes, 4 ASIDs.

● Lookup for key 2390.

ASIDs Keys

Crescendo Evaluation

● Simulation Parameters

● Two topologies

– 112 ASIDs, 784 Access Routers, 28 Backbone Routers.

– 196 ASIDs, 1372 Access Routers, 28 Backbone Routers.

– Two runs for each topology with different seeds.

● Parameter: Number of nodes in the overlay.

– 256, 512, 1024, 2048, 4096.

– 4500 for the multicast simulation.

● Link latency.

– Transit-Transit 100ms, Transit-Stub 20ms, Stub-Stub 5 ms.

● Extended KBRTestApp for statistics.

– Physical network hops, direct and overlay path.

– Lookup latency.

– ASIDs path.

– Overlay hops.

● More than 50 hours of simulation.

Crescendo Simulation ResultsAverage Lookup Latency

● Chord flat design. Lookups do many inter-domain hops until final destination.

● Crescendo adapts to the underlying network hierarchy.

● Crescendo key responsibility, many times needed one less hop than Chord.

● Crescendo, intra-domain paths locality, inter-domain paths convergence.

Crescendo Simulation ResultsPhysical Network Routing Stretch

● Chord flat design. Lookups do many inter-domain hops until final destination.

● Crescendo adapts to the underlying network hierarchy.

● Crescendo key responsibility, many times needed one less hop than Chord.

● Crescendo, intra-domain paths locality, inter-domain paths convergence.

● Crescendo, more “cheap” intra-domain hops, fewer “expensive” inter-domain hops.

Crescendo Simulation ResultsAverage Overlay Network Hops

● Chord flat design. Lookups do many inter-domain hops until final destination.

● Crescendo adapts to the underlying network hierarchy.

● Crescendo key responsibility, many times needed one less hop than Chord.

● Crescendo, intra-domain paths locality, inter-domain paths convergence.

● Crescendo, when very few nodes in network (e.g. 1 per AS), Chord like behavior.

Crescendo Simulation ResultsAverage Physical Network Hops

● Chord flat design. Lookups do many inter-domain hops until final destination.

● Crescendo adapts to the underlying network hierarchy.

● Crescendo key responsibility, many times needed one less hop than Chord.

● Crescendo, intra-domain paths locality, inter-domain paths convergence.

● Crescendo, more “cheap” intra-domain hops, fewer “expensive” inter-domain hops.

Crescendo Simulation ResultsEfficient bandwidth usage during multicast

● Chord flat design. Lookups do many inter-domain hops until final destination.

● Crescendo adapts to the underlying network hierarchy.

● Crescendo key responsibility, many times needed one less hop than Chord.

● Crescendo, intra-domain paths locality, inter-domain paths convergence.

● Crescendo, more “cheap” intra-domain hops, fewer “expensive” inter-domain hops.

● 112 ASIDs Network Topology.

● 4500 Participating Nodes.

● 5.267.079 Hops Processed.

● Lookup for the same key.

Overlay Network Inter-domain Hops Intra-domain Hops

Chord 5.213.545 53.534

Crescendo 2.950.470 2.316.609

Conclusion - Future Work

● Crescendo has many more advantages than Chord without extra disadvantages.

● Locality: Fault isolation, Security, Efficiency.● Convergence: Caching, Bandwidth savings.

● Future Work

● More simulations for multicast behavior over Crescendo.● Comparison of Crescendo with other DHTs.● Implementation of hierarchical structure for other DHTs.

Thanks

THANK YOU

George S. Nomikos

Mobile Multimedia Laboratory

ATHENS UNIVERSITY OF ECONOMICS AND BUSINESS

Backup Slides

Distributed Hash Table (DHT)

● Hash table distributed among a set of nodes.● Partition key space by node ID.

● Each node responsible for a part of the key space.● SHA-1 Hash function, 160 bits identifiers.

● Flat overlay network.● Basic actions.

● Insert (Key, Value)● Lookup (Key, Value)

Chord

● Finger Table example for Node 8.

● Lookup Examples.

● Simple Lookup.

● Normal Lookup, using Finger Tables.

Simple Lookup O(N)

Normal Lookup O(logN)

Chord

Definition of variables for node n, using m-bit identifiers

Notation Definition

finger[k] First node on circle that succeeds (n + 2k-1) mod 2m ,1 ≤ k ≤ m

successor The next node on the identifier circle, finger[1].node

predecessor The previous node on the identifier circle

Node Join Procedure Example:

Node 26 joins the system between nodes 21 and 32. The arcs represent the successor relationship.

(a) Initial state, node 21 points to node 32. Node 26 appears and finds it's successor node 32.

(b) Node 26 points to it's found successor node 32.

(c) Node 26 copies all keys less than 26 from node 32.

(d) The stabilize procedure updates the successor of node 21 to node 26.

Chord

● Stabilization protocol● Runs periodically at each node.

● Handles newly joined nodes and failed nodes.

● Stabilization Protocol procedure:● Periodically ask your successor who it's predecessor is.

● If the predecessor is immediately after you then it's your successor.

● Notify this found successor and update self.

● Repeat this procedure.

Crescendo

● Due to the previous two properties Crescendo offers:

● Fault Isolation.● Security.● Efficient caching and bandwidth usage.● Adaptation to the underlying physical network.● Hierarchical Storage of content.● Hierarchical access control.● Less network stretch.● Less lookup latency.

Crescendo Implementation

● Crescendo Core Structure.

● Crescendo (simple module).

● CrescendoFingerTable (simple module).

● CrescendoSuccessorList (simple module).

● CrescendoModules (compound module).

● CrescendoMessages.

● New code added to the following core OMNeT++ structures.

● cTopology: Routing Policy Weights support.

● New code added to the following core OverSim structures.

● INETUnderlay: Routing Policy Weights, ASIDs, Realistic Network Topologies.

● GlobalNodeList, BootstrapList: ASIDs, Hierarchy support.

● Common API, BaseOverlay: ASIDs, Hierarchy support.

● KBRTestApp: Heavily extended to export analytical statistics.

● Bash and Java parsers: To parse the simulation results.

CrescendoModules Structure

Software Used

● OMNeT++ 4● OverSim● BRITE● GT-ITM● INET for OverSim● Arch Linux● Doxygen

Realistic Network Topologies