P2P
=
“Structured Overlay Networks
for Peer-to-Peer systems”
Luigi Liquori,
• 97 Ph.D. Università degli studi di Torino
• 07 H.d.R. “Habilitation à diriger les recherches”, École Nat Sup des Mines de Nancy
• 08 “Habilité au fonctions de professeurs de Universités’’ 2008-2012
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• 10 Research Director, Équipe LogNet, INRIA Sophia Antipolis Méditerranée
Course setting
o COURSE = P2P : “Future Internet and Overlay Networks”
o HH = around 40h (20h by me and +20h by Prof. Legout)
o MODULE = “Structured Overlay Networks P2P systems”
o Exams = SW project
o PRE = General notions of systems and networks
o OPT = “Computability”, “Data-bases”, “Logics”, “Security”
o POST = “Design, analyze and implement p2p networks and
overlay-based applications”
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Topics : structured overlay networks for P2P
o CHORD (Stoica & al), lessons
o Academic and pedagogical a sort of “PASCAL/BASIC” in overlay networks
o KADEMLIA (Maymounkov and Mazières), lessons
o Academic with free implementations and widely used (emule)
o BIT TORRENT, (Cohen) ½ of the course <<<< A. LEGOUT
o Non academic, with free implementations and widely used
o SKYPE (Zennström & Friis), lesson
o Non academic : open, use Kademlia, very widely used for VOIP
o INTERCONNECTING OVERLAY NETW (Liquori), lesson
o Academic but practical : allow different overlay protocols to communicate
o NAT TRAVERSAL, lessons
o How stablish and maintain TCP/IP/UDP connections traversing gateways
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Other issues (micro survey) in the course
o Publish/subscribe paradigm
o Content-centric routing (Jacobson & al. CONEXT 09)
o Ontologies for internet computability
o Coordination languages to deal with algorithmic aspects
o Trust and reputation issues
o Denial of service attacks
o “Inter netting” overlay networks
o Principles of “Internet computability”
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“Spot on”
1. A quick window of the module (lesson 1)
o
o
o
o
Preludio : some internet facts
Course vision : “Computer scale-up to internet”
Step 1 : Reference model of future internet
Step 2 : Reference model of internet computer
2. Inside submodule “Structured overlay networks for p2p” (lessons 3-4)
o
o
Chord, lecture
o Topology, routing, and churn
Kademlia, lecture
o Topology
o Routing (put, get),
o Churn (join, leave)
3. Inter-netting structured overlay networks (lesson 7)
o
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Dealing with network partitions
Preludio : some internet facts
o Internet traffic : ~80% is P2P and ~20% is Web
o Some leading p2p protocols :
o Some leading p2p class of applications : file exchange
o
In progress: VOIP, TVIP, STREAMIP, CLOUD
o General p2p anarchy : no coordination, no cooperation
o Total p2p heterogeneity : protocols, topology, security, devices, users ..
o P2P “inter-routing” is almost impossible
o
Often with the same purpose but ≠ routing and topologies
o Actor 1 : Resource discovery
o Actor 3 : Peers organization
o Actor 2 : Resource coordination
o Actor 4 : Peers reputation
o “ …les ingredients pour … un modèle de calcul pour l’internet ! ”
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Course vision : “Computer scale-up to internet”
o 1946. von Neumann. “Principle of large scale computing machines”
o
“Large Scale” in 1946 means ENIAC
o 1946-2010. From ENIAC to Cray XT5 Jaguar and G5K via iPhone
o 20XX. “Large scale” means “Internet scale”
o Von Neumann architecture
does not scale-up to internet
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Step 1 : reference model of future internet
o “Inter-netting” heterogeneous overlay networks
o
The “Cerf & Kahn ’77” cannot lead to a standardized p2p communication layer
o Backward compatibility of all existent p2p protocols
o
P2P inventors are often next door computer scientists or users’ communities
o Competition vs. Collaboration
o Interconnecting etherogeneous ON
o
Exaustive routing is almost achieved
o Content-based routing (Jacobson)
o
o
Logical payload
Hybrid topologies and underlay networks
o Peers organization via social-based & reputation primitives
o Genericity : add many services on the top of the ON
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Step 2 : reference model of an internet computer
o Internet Computer (IC) : abstraction on top of an overlay network
o
o
Peers are physically connected via IP/adhoc/MANET
Peers are logically organized in a Virtual Organization
o IC Reference model
o
o
o
Bus
Memory
CPU
= Internet and routing
= ΣkK DHTk
(distributed hash table)
= ΣkK CPUk
(distributed central units)
o IC Programming model
o
o
o
o
Language
Word
Pointer
Type
=
=
=
=
Protocol
Packet
Address
Port
o
o
o
o
Virtual intermittence
Resource discovery
Reputation
Orchestration
o Universality, Genericity, Polymorphism, “Turing completeness”
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“Spot on”
1. A quick window of the module (lesson 1)
o
o
o
o
Preludio : some internet facts
Course vision : “Computer scale-up to internet”
Step 1 : Reference model of future internet
Step 2 : Reference model of internet computer
2. Inside submodule “Structured overlay networks for p2p” (lessons 3-4)
o
o
o
Key figures (reminder)
Chord, lecture
o Topology, routing, and churn
Kademlia, lecture
o Topology
o Routing (put, get),
o Churn (join, leave)
3. Inter-netting structured overlay networks (lesson 7)
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General picture of overlay networks
Treat n hops
through IP network
as m (less than m)
hops in an overlay
network
B
A
C
Overlay Network
Physical Network
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Key figures in SON (reminder)
o Data discovery is deterministic : a.k.a. 2nd generation overlays
o Distributed Hash Table (DHT) : stores (key, value) pairs in
nodes
o Key-based routing : N.lookup(K) route from the node N
generator of the lookup to the node M that owns the key K via
a routing path of “closer” nodes (according to a given metric
distance in a logical key space)
o Routing table : local table that maintain links to other nodes
o Churn : rate of node joins and leaves in a p2p network
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Key figures in SON (cont’d)
o Overlay topologies
o Exhaustive lookup with logarithmic complexity
o Uniformity vs. proximity of key storage
o Consistent hashing of keys and IPs via SHA-1
o Peer join
o Peer leave
o Getting a logic ID
o Positioning into the overlay structure
o Stabilize the overlay (maintenance)
o Faulty routing tables
o Fair play vs. non fair play
leaving
o Opportunistic vs. Active maintenance of routing tables
o Bootstrapping of an overlay network
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Chord 1 : Consistent Hashing
o SHA-1: {IP} U {KEYS} -> NAT
o SHA-1(IP) = NIP
o SHA-1(fookey) = Kfoo
o Node Nx stores
all keys Ky such that
Nx ≤ Ky < pred(Nx)
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Chord 2 : (Local) Finger Tables
o On every node N
o finger : array[1...m]
o 2m is the logical space
o finger[k] = succ(N + 2k-1) mod 2m
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with 1 ≤ k ≤ m
Chord 3 : Recursive routing
8 < finger[6] ≤ 54
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Chord 4 : Churn
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Chord 5 : Bootstrapping
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Chord 6 : Stabilization
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Kademlia 1
o Peer-to-peer (key,value) storage and lookup system
o A number of desirable features not simultaneously offered by
any previous peer-to-peer system
o It minimizes the number of messages to learn topology
o Stabilization spreads automatically during key lookup
o Nodes can route queries through low-latency paths
o Parallel, asynchronous msg to avoid timeout delays from failed nodes
o Basic mechanisms to resists to certain basic denial of service attacks
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Kademlia 2
o Keys are “opaque”, 160-bit quantities
o Participating computers each have a node ID in the 160-bit
key space. (key, value) pairs are stored on nodes with IDs
“close” to the key for some notion of closeness
o A node-ID-based routing algorithm lets anyone locate servers
near a destination
o XOR metric for distance between points in the key space
o XOR is symmetric, allowing Kademlia participants to receive
lookup queries from precisely the same distribution of nodes
contained in their routing tables
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XOR metric 1
o Given two 160-bit identifiers, x and y, Kademlia defines the
distance between them as their bitwise exclusive or (XOR)
interpreted as an integer, i.e. d (x, y) = x ⊕ y
o d (x, x) = 0
o d (x, y) > 0 if x ≠ y,
o For all x, y. d (x, y) = d (y, x)
o d (x, y) + d (y, z) ≧ d (x, z)
o d (x, y) ⊕ d (y, z) = d (x, z)
o For all a≧0, b≧0. a + b ≧ a ⊕ b
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XOR metric 2
o XOR is unidirectional
o For any given point x and distance △ > 0, there is exactly one
point y such that d (x, y) = △
o Unidirectionality ensures that all lookups for the same key
converge along the same path, regardless of the originating
node
o Caching (key, value) pairs along the lookup path alleviates hot
spots
o XOR topology is also symmetric
o d (x, y) = d (y, x) for all x and y
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XOR : do it ….
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Topology : do it ….
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Node state 1
o For each 0 ≦ i < 160, every node keeps a list of
(IP address, UDP port, Node ID) triples for nodes of distance
between 2i and 2i+1 from itself
XOR distance
Bucket
i
0
[20, 21)
(IP address,UDP port,Node ID) 0-1
......
(IP address,UDP port,Node ID) 0-k
1
[21, 22)
(IP address,UDP port,Node ID) 1-1
......
(IP address,UDP port,Node ID) 1-k
o We call these lists “k-buckets”
o The size is not fixed a priori but
chosen such that any given k
nodes are very unlikely to fail
within an hour of each other
(for example k = 20)
is
2
[22, 23)
(IP address,UDP port,Node ID) 2-1
......
(IP address,UDP port,Node ID) 2-k
[2i, 2i+1)
(IP address,UDP port,Node ID) i-1
......
(IP address,UDP port,Node ID) i-k
[2159, 2160)
(IP address,UDP port,Node ID) 159-1
......
(IP address,UDP port,Node ID) 159-k
…
i
…
159
i-bucket
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Node state 2
o Each k-bucket is kept sorted by time last seen
o Least-recently seen node at the head
o Most-recently seen at the tail
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Build buckets : do it ….
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Node state 3
o When a Kademlia node receives any message (request or
reply) from another node, it updates the appropriate k-bucket
for the sender’s node ID
o If the sending node already exists in the recipient’s k-bucket,
the recipient moves it to the tail of the list
o If the node is not already in the appropriate k-bucket and the
bucket has fewer than k entries, then the recipient just inserts
the new sender at the tail of the list
o If the appropriate k-bucket is full, then the recipient pings the
k-bucket’s least-recently seen node to decide what to do
o If the least-recently seen node fails to respond, it is evicted from the
o
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k-bucket and the new sender inserted at the tail
If the least-recently seen node responds, it is moved to the tail of the
list, and the new sender’s contact is discarded
Routing and upgrade buckets : do it ….
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Kademlia in the P2P system 1
o In the eMule,a P2P file exchange software, Kademlia network has two
table : the Key words table and the Data index table
To Find: Wii tips and tricks.pdf
Key words: Wii, tricks
Hash(Wii)
Key
Key words table
Value
Hash(Wii)
(data name, data length, Hash(data))
Hash()=>SHA-1,160bit
(I Love Wii.txt, 30, 1011…001)
(Wii tips and tricks.pdf, 375, 1110…101)
Hash(trick)
Key
Value
Hash(tricks)
(data name, data length, Hash(data))
(Wii tips and tricks.pdf, 375, 1110…101)
(Card trick.mpg, 65000, 1000…100)
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Kademlia in the P2P system 2
o Data index table:
Data index table
To Find: 1011…001
1011…001
Key
Value
1011…001
(IP, UDP Port, node ID)
(218.164.185.90, 3347, 1011…001)
(125.230.122.183, 3475, 1011…011)
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Nodes in Kademlia
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Files in Kademlia
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Keywords in Kademlia
Hash of “break” only !
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“Spot on”
1. A quick window of the module (lesson 1)
o
o
o
o
Preludio : some internet facts
Course vision : “Computer scale-up to internet”
Step 1 : Reference model of future internet
Step 2 : Reference model of internet computer
2. Inside submodule “Structured overlay networks for p2p” (lessons 3-4)
o
o
Chord (previous lecture)
o Topology, routing, and churn
Kademlia (this lecture)
o Topology
o Routing (put, get),
o Churn (join, leave)
3. Inter-netting structured overlay networks (lesson 7)
o
36
Dealing with network partitions
Inter netting structured overlay networks
o Example 1: Two DHT-based overlay networks (key,value)
o
o
o
o
One pair is stored in DHT1 and searched for in DHT2
Many pairs stored in both DHTs can be found
Two companies wishing to share/aggregate information
Better fault-tolerance, and data availability
o Example 2: An Overlay Network get some nodes isolated
o
o
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So called “network partitions”
Alternative physical routing via ON inter-routing
(Techi) Inside Protocols
o VIP: SREG
(Id,Mode,FromCard,ToCard, Payload)
o VIP: SUPD
(Id,Mode,FromCard,ToCard, Payload)
o RDP: SREQ
(Id,Mode,FromCard,ToCard, Payload
o RDP: SRESP
(Id,Mode,FromCard,ToCard, Payload)
o RDP: SNOTIF
(Id,Mode,FromCard,ToCard)
o Mode  {LOGIN, LOGOUT, ACCEPT, REJECT, LOOP, ☺, ☠,…}
o Card = (IP-PORT-PKI)
o Service ::= HumW | RunW | StockW | ProgW | DataW | LinkW
o Payload ::= ORi=1..m(ANDj=1..n
j)i
where  ::= (Service,Constraints*) | NOT()
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