Download - Peer-to-peer: research questions
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Peer-to-peer: research questions
Antony Rowstron
Microsoft Research, Cambridge, UK.
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Peer-to-peer systems
• Nodes have symmetric roles– No centralised components
• Two basic types– Unstructured
• E.g. Gnutella
– Structured• CAN, Chord, Kademlia, Pastry, Tapestry, SkipGraphs,
SkipNets, Viceroy
• Different usage scenarios:– Internet, enterprises, clusters
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Structured versus unstructured
• Urban myths?– Structured overlays (DHTs) can’t deal with
high churn rates– Unstructured overlays have lower overheads
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Structured P2P overlay networks
• structured overlay network maps keys to nodes• routes messages to keys; can implement hash table
overlay network with N nodes
k,v
[CAN, Chord, Kademlia, Pastry, Skipnets, Tapestry, Viceroy]
route(“insert v”, k)
route(“lookup”, k) v
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Mapping keys to nodes
• large id space (128-bit integers)
• nodeIds picked randomly from space
• keys picked randomly from space
• key is managed by its root node:
• live node with id closest to the key
• key is replicated by its replica roots:
• r nodes with ids closest to keyroot nodefor key
id space
nodeIdkey
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Pastry: routing
• prefix matching: each hop resolves an extra key digit• neighbor set used to find root node in last hop• properties: log16 N hops with low delay routes
d46a1c
d462ba
d4213f
d13da3
65a1fc
nodeId
key
route(m,d46a1c)
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Structured overlays (Pastry)Unstructured overlays (Gnutella-like)
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Unstructured overlays (Gnutella-like)
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Unstructured overlays (Flooding)
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Unstructured overlays (Random Walk)
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Structured overlays (Random Walk)
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Structured overlays (Random Walk)
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Structured overlays (Flood)
0x
1x
2x
3x
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Maintenance costs
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60 70
Time (Hours)
Msg
s p
er s
eco
nd
per
no
de
RW MIN 6 MAX 100RW MIN 4 MAX 100FLOOD MIN 6 MAX 100FLOOD MIN 4 MAX 100Pastry 20%Pastry 20% (leafset 6)
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Security questions
• Gnutella and robustness– Information propagation
• Structured overlays– Initial work (OSDI) – Byzantine faults– Potentially expensive - Constraints– Assumed high percentage of colluding nodes
(25%)
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Underlay meets overlay
• Should overlays exploit network locality?– What heuristics can be used?
• Exploiting network services in overlays
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Using P2P infrastructure
• Richer searching in DHT’s– Peer-to-Peer Information Retrieval Using Self-Organizing Semantic Overlay Networks.Chunqiang Tang (U Rochester), Zhichen Xu
(HP Labs), Sandhya Dwarkadas (U Rochester) Peer-to-Peer Information Retrieval Using Self-Organizing Semantic Overlay Networks.Chunqiang Tang (U Rochester), Zhichen Xu (HP Labs), Sandhya Dwarkadas (U Rochester)
• Interesting applications/different approaches– Content streaming
• SplitStream
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Content Streaming: SplitStream
Problems with a single multicast tree in a p2p environment:
Interior nodes must invest upstream bandwidth many times the downstream bandwidth (unfair)
Peers acting as interior nodes may fail (poor reliability)
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SplitStream: basic approach
Stripe content across multiple independent multicast trees
Fairness: interior node in one tree is leaf in other trees
Reliability: failure of one tree causes drop in quality (at worst)
Erasure coding or multiple description video codecs
Flexibility: can accommodate low bandwidth nodes
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SplitStream: Multiple multicast trees
B
C
EF
DA
G D
E G
B A F C
A
B C
F G D E
N kb/sec
N/2 kb/sec N/2 kb/sec
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SplitStream: Multiple multicast trees
B
C
EF
DA
G D
E G
B A F C
A
B C
F G D E
N kb/sec
N/2 kb/sec N/2 kb/sec
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Thank you
• FreePastry available from Rice University
http://www.research.microsoft.com/~antr/Pastry
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Further information• R. Mahajan, M. Castro and A. Rowstron, "Controlling the Cost of Reliability in Peer-to-peer
Overlays", IPTPS'03, Berkeley, CA, February 2003.
• M. Castro, P. Druschel, A. Ganesh, A. Rowstron, and D. S. Wallach, "Security for structured peer-to-peer overlay networks". In Proceedings of the Fifth Symposium on Operating Systems Design and Implementation (OSDI'02), Boston, MA, December 2002.
• M. Castro, P. Druschel, Y. C. Hu and A. Rowstron, "Exploiting network proximity in peer-to-peer overlay networks", Technical report MSR-TR-2002-82, 2002.
• S. Iyer, A. Rowstron and P. Druschel, "SQUIRREL: A decentralized, peer-to-peer web cache", 12th ACM Symposium on Principles of Distributed Computing (PODC 2002), Monterey, California, USA, July 2002.
• “The Impact of DHT Routing Geometry on Resilience and Proximity” .Krishna P. Gummadi (U Washington), Ramakrishna Gummadi (USC), Steven D. Gribble (U Washington), Sylvia Ratnasamy (Intel), Scott Shenker (ICSI), Ion Stoica (UC Berkeley), SIGCOMM 2003
• Internet Indirection Infrastructure. Ion Stoica, Daniel Adkins, Shelley Zhuang, Scott Shenker, Sonesh Surana (University of California, Berkeley), SIGCOMM 2003
• Workshop on Economics of Peer-to-Peer Systems http://www.sims.berkeley.edu/research/conferences/p2pecon/program.html
• IPTPS’03 http://iptps03.cs.berkeley.edu/