dynamic networks & evolving graphs afonso ferreira cnrs i3s & inria sophia antipolis...
TRANSCRIPT
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Dynamic Networks
&
Evolving Graphs
Afonso FerreiraCNRS
I3S & INRIA Sophia Antipolis
With Aubin Jarry
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Dynamic Networks
• Mobile Wireless Networks (eg, Ad-hoc)• Fixed Packet Networks (eg, Internet)• Fixed Connected Networks (eg, WDM)• Fixed Schedule Fixed Networks
– (eg, Sensors, Transport)
• Fixed Schedule Mobile Networks(eg, LEO Satellites, Robots)
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T1
T2
T3
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T1
T2
T3
T4
Distance = 3
= 4
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T1
T2
T3
T4
Distance= 3 hops / 1 TU
= 1 hop / 4 TU
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Outline
• Motivation: grasp dynamic networks• The Evolving Graph• Distances, Paths, Journeys, Connectivity,
...• Old questions - New insights• A direct application• Conclusions
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2003 NSF report onFundamental Research in
Networking• Understanding about networks
– Needs: Substantial innovation and paradigm shifts
• Scalable design and control of networks– Needs: Fundamental understanding
• Reproducibility of experiments – Needs: Reference models and benchmarks
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1,2,31,3
1,4
1,3,4
2
23
14
The Evolving Graph
4
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1,2,31,3
1,4
1,3,4
2
23
14
The Evolving Graph
4
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Evolving Graphs
• Given a graph G(V,E) and an ordered sequence of
its subgraphs, SSG=Gt0, Gt1, ..., GtT.
The system EG = (G, SSG) is called an evolving
graph.
• Input coding: list of presence intervals for each edge and for each vertex (this can be evidently relaxed in case of a valid mobility model, eg)
• Dynamics: – Size of edge and node lists.
[Algotel’02]
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Journeys in EGs
• Sequence of edges {e1, e2, …, ek} of G called a Route R(u,v) (= a path in G).
• A schedule s respecting EG and R, defines a journey J(u,v, s).
• Observations:– Journeys cannot go to the past– A round journey is J(u,u, s). Like a usual cycle,
but not quite.
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1,2,31,3
1,4
1,3,4
2
23
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The Evolving Graph
4
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Routing Issues
• Minimum hop count = Distance– shortest journey
• Timed evolving graphs (TEGs): – traversal time on the edges.
• Minimum arrival date = Earliest arrival date– foremost journey
• Minimum journey time = Delay – fastest journey
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Algorithm for Foremost Journeys
• Delete root of heap into x.• For each open neighbor v of x:
– Compute first valid edge schedule time greater or equal to current time step
– Insert v in the heap if it was not there already.
• If needed, update distance to v and its key.• Update the heap.• Close x. Insert it in the ‘shortest paths’ tree.
(TEGs are complex: Prefix journeys of foremost journeys are not necessarily foremost.)
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2/3/5/9 1/2/4/106/8
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2/3/6
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Algorithm
Source:
0
Time: 12345678
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Analysis
• For each closed vertex, the algorithm performs O(log + log N) operations per neighbor.
• Total number of operations is
O(vV [| +(v) | (log + log N)]) =
O(M (log + log N)).• Bounded by the actual size of the schedule
lists, which measures the number of changes in the network topology.
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1/3/5/92/6
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2/3/6
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Algorithm for fading memory
Source:
0 2/3/5/6/10
Time: 12345678
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Analysis
• O(M (log + log N)) operations.• Again bounded by the actual dynamics
of the evolving graph.
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Journey Issues
• Minimum arrival date = Earliest arrival date– O(M (log + log N))
• Minimum hop count = Usual distance– O(NM log )
• Minimum journey time = Delay – O(NM 2 )
• Many others to explore
[WiOpt’03, IJFCS 03]
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Connectivity Issues
• An EG is said to be connected if for every pair (u,v) there is a journey from u to v and a journey from v to u.
• A connected component of EG is defined as a maximal subset U of V, such that for every pair (u,v) there is a journey from u to v and a journey from v to u.
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Example I: CC
24
11
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Example I: CC
24
11
CC:
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Example II: CC
24
11
CC:
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Example II: o-CC
24
11
O-CC:
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Complexity result
• Computing (o-)CCs is NP-Complete.– It is in NP: computing journeys is
polynomial.– Reduction from Clique
[Ad-Hoc Now 03]
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The GadgetGiven G =(V,E) and integer k, create an EG:For each ui in V create a vi and a hii.• Time step 1:
– Create a CC connecting all h-nodes.
• Time step 2:– Create edges {vi,hii}, – For each edge {ui, uj} in E, create edges {vi,hij}.
• Time step 3:– For each edge {ui, uj} in E, create edges {hij,vj}.
• Time step 4:– Create a CC connecting all h-nodes.
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1,2,31,3
1,4
1,3,4
2
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A Key Issue?
4
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An application
• Minimum Energy Broadcast & Range Assignment Problem (STACS 97, Infocom 00)
• E~d2
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The MEB&RAP
• NP- Complete• 12-Approximation
– Direct computation of the MST of the underlying weighted complete graph
– Analysis using geometric arguments
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The MEB&RAP
• Static x (Low) Dynamic
• What is a MST over time??• Take an Ad-Hoc network where nodes do
not move, but can alternate sleep/awake modes according to a predefined schedule:– A MST over time is a rooted MST allowing for
journeys from the root to the leafs in the corresponding weighted evolving graph
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Computing a MST over time
• NP-Complete (Reduction from Steiner)• Precludes the use of the MST-based
heuristic to solve the MEB&RAP in dynamic networks
[WiOpt 04]
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Current & Future Work
• Rooted MST is NP-Complete– But Local Minima RST is Polynomial!
• Flows in EGs
• Algorithms for EGs (eg Connected Components)
• Distributed algorithms for EGs– Competitive analysis of protocols
• Harness Dynamic Networks
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Related Combinatorial Models
used in Dynamic Networks• Graphs• Random Graphs & Adversaries [Scheideler’02]
• Dynamic Graph Algorithms [Frigioni et al’00]
• Time-Expanded Graphs [FoFu’58]
• MERIT [FaSy’01]
– A sequence of historic network snapshots– Competitive analysis of protocols
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NSF report onFundamental Research in
Networking• Understanding about networks
– Needs: Substantial innovation and paradigm shifts
• Scalable design and control of networks– Needs: Fundamental understanding
• Reproducibility of experiments – Needs: Reference models and benchmarks
![Page 39: Dynamic Networks & Evolving Graphs Afonso Ferreira CNRS I3S & INRIA Sophia Antipolis Afonso.Ferreira@sophia.inria.fr With Aubin Jarry](https://reader030.vdocuments.site/reader030/viewer/2022032707/56649e315503460f94b2211e/html5/thumbnails/39.jpg)
Conclusion
• Evolving Graphs– Graphs + Time Domain– A model for complexity, combinatorics, algorithms
• Old questions - New insights– Hardness induced by time
• Many applications in Dynamic Networks– Wireless nets, evolving request matrices,
transports...
• Many new ways to explore!
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The idea
vi
hii hik
hki hkk
vk
3
1,4
1,4
1,4
1,4
22
223
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Networks
• Valued graphs• Weights = costs, distance, traversal time, etc.
31
4
1
2
23
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4
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Networks
• Road networks, railway systems– Traversal times are arbitrary but finite– Arcs are closed at certain periods– Parking is allowed at vertices whenever
possible• Computing shortest paths in loaded
networks– Earliest Arrival Times [HP’74,D’66]– Dijkstra-like, no complexity analysis
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Networks
• Weights = traversal time: Time-expanded graphs [FF’58]
1 1
2 1 1
1 12
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Networks
• Weights = traversal time: Time-expanded graphs [FF’58]
• Complexity increases– Pseudo-polynomial (weights must be
integers)• Time-dependent networks [CH’66]
– Weights depend on the number of flow units entering the link
– Computation of quickest flows (ESA’02, SODA’02)
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2/3/5/9 1/2/4/106/8
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2/3/6
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Algorithm
Source:
0
Time: 12345678
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Fading memory
Source:
0 2/3/5/6/10
Time: 12345678
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1,2,31,3
1,2,4
1,3,4
2
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Example of Journeys
4
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22h00
18h00
17h00 16h00
13h00
12h00
07h0010h00
10h0011h0013h0015h00
Fixed-Schedule Dynamic Networks
07h00
10h00
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Models for Dynamic Networks
• Graphs• Random Graphs • Dynamic Graphs
– Discrete step is one link/node change– Focus on data-structures & amortized
analyses– Time is not an issue
Motivation: Formalize the notion of time in graphs
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EGs & Dynamic Networks
• Fixed-Schedule– Satellite constellations– Transportation networks– Robot networks
• History– Competitive analysis– MERIT
• Stochastic?– Mobility model
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Some Issues in Dynamic Networks
• Property maintenance– E.g., Minimum Spanning Tree
• Fault tolerance– Link/node failure
• Congestion avoidance– Time dependency
• Topology prediction – The Web
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Combinatorial Models for Dynamic Networks
.
. .
.