ChordNet: A Chord-based self-organizing super-peer network Dennis - PDF document

ChordNet: A Chord-based self-organizing super-peer network Dennis Schwerdel , Matthias Priebe, Paul Müller Dennis Schwerdel University of Kaiserslautern Department of Computer Science Integrated Communication Systems ICSY http://www.icsy.de Outline � Introduction � Overlay structure � Improvements

Introduction � Peer to peer overlay network � Based on Chord DHT � Inherits basic structure � Lots of improvements � General purpose � Not only for searching/locating content � Also for transfering content Dennis Schwerdel, University of Kaiserslautern 3 Chord � Peer to peer overlay � For storing key/value pairs (DHT) � All peers arranged in a ring sorted by IDs � Connections to predecessor and successort � Shortcuts to nodes with ID + 2^n (”Fingers”) � Routing to an ID uses farest finger preceeding the destination ID � Keys are mapped to IDs using hash functions � Nodes store all key/value pairs for Ids between them and their successor 4 Dennis Schwerdel, University of Kaiserslautern

Chord: Example Dennis Schwerdel, University of Kaiserslautern 5 Chord Summary � Efficient message delivery � O(log n) � Only unicast, no multicast, no broadcast � Scales well with millions of nodes � All nodes must be reachable � Problems with Firewalls/NAT � Does not consider latency 6 Dennis Schwerdel, University of Kaiserslautern

ChordNet Structure � Separation: Super-Peers/Edge- Peers � Super-Peer � Reachable � Inside the Chord ring � Has list of its edge-peers � Edge-Peer � Behind Firewall/NAT, not reachable � Connected to preceding and succeeding super-peer � Communicates through its super- peers Dennis Schwerdel, University of Kaiserslautern 7 ChordNet routing � Only considering super-peers � d Fingers in both directions � Finger � i to nearest node to (ID � 2^(i-1)) mod N � Successor is Finger 1, Predecessor is Finger -1 � Only slightly increases connection count � Routing in both directions possible � Routing can overshoot � When fingers are optimal � Average case: log(n)/3 hops (33% reduction) � Worst case: log(n)/2 hops (50% reduction) 8 Dennis Schwerdel, University of Kaiserslautern

Multicast/Broadcast � Multicast destination = ID list � Broadcast destination = ID range � Routing � split list/range at each hop when needed � Global broadcast has n-1 messages Dennis Schwerdel, University of Kaiserslautern 9 Routing improvements � Chord: Routing only based on IDs � Routing will take expensive hops because of ID gain � Better: Calculate routing gain (PRS) � Latency can measured on connections � Formula: gain = (old-dist – new-dist) / latency-costs � Even better: gain = (1/new-dist – 1/old-dist) / latency-costs � Chord: Decision is made by looking at own fingers � Better: Look at finger tables of finger peers � 2-Hop greedy routing (NoN) � Additional messages needed for finger table exchange � Both improvements can be combined 10 Dennis Schwerdel, University of Kaiserslautern

Routing improvement impact Dennis Schwerdel, University of Kaiserslautern 11 Maintenance method � Chord searches for fingers with routed requests � Better: exchange finger-table with peers Needed for NoN rounting anyway � � Allows for quick start � Combine predecessor and successor finger tables 12 Dennis Schwerdel, University of Kaiserslautern

Shortcut connections � Problem: multiple hops are inefficient for big data transfers DHTs have no data transfers, new problem � Traffic influences latencies, automatic load � balancing with PRS � Idea: establish shortcut connections � Nodes detect high traffic routes using a traffic- matrix � When HTR is detected, notification is sent to both peers � Peers establish shortcut connection � Shortcut connections are treated as fingers � Result: HTR path length reduces automatically � HTR peers are connected directly after a short while Dennis Schwerdel, University of Kaiserslautern 13 Shortcut impact 14 Dennis Schwerdel, University of Kaiserslautern

ChordNet status � Protocol specification � Reference implementation in Java � Able to run in simulation mode � Addition features (work in progress) � Publish/Subscribe API � MessageQueue API � Datagram API � TCP-like API Dennis Schwerdel, University of Kaiserslautern 15 ��������������������������� ������������������������������������� ���������������������������� ������������������������������ ��������� !"# �$%&%' ��������������� �(���) *"#�+!,% -�.!'$.%�" /��) *"#�+!,% -�.!'$ !�'% 0����) ��(1�����2���������3����$3���� ��������) (���)44111��������