A Software Engineering Perspective on Context-Awareness in Ad Hoc Mobile Networks

1
A Software Engineering Perspective on
Context-Awareness in Ad Hoc Mobile Networks

• Christine Julien
• Mobile Computing Laboratory
• Washington University in Saint Louis
• January 9, 2012

2
Motivation

• Simplify mobile application development
• Individually address application-specific needs
• Dynamically gather context information from the
  network
• Seamlessly adapt to environmental changes
• Transparently help programmers cope with problems
  inherent in ad hoc networks

3
Ad Hoc Network Challenges

• Form opportunistically
• Change rapidly in response to mobility
• Rely on no fixed infrastructure
• Provide transient interactions
• Foster decoupled computing
• Embody open environments with limited guarantees

4
Computational Model

• Ad hoc network
• Host (mobile or stationary)
• Defines location in physical space
• Agents/Application Components
• Unit of mobility residing on a host
• Data
• Owned and generated by each agent

Host 2
Host 1
Host 3
5
Overview

• Context-aware computing
• EgoSpaces conceptual model
• EgoSpaces middleware
• Network abstractions
• Conclusions and future work

6
Context-Aware ComputingThe Need for Context

• Software and hardware constantly move and change
• Applications must opportunistically adapt
• Perception of context varies by application
• Current solutions
• Use basic types of context information in
  specific ways
• Fieldwork tools, guide tools
• Provide access to only a single components
  sensors at a time
• Context Toolkit, Context Fabric
• Support only local or symmetric interactions
• MARS, LIME

7
Context-Aware ComputingRethinking Applications
(1)
8
Context-Aware ComputingRethinking Applications
(2)
9
Context-Aware ComputingRethinking Applications
(3)
Charlie
Angie
10
Context-Aware ComputingNovel Notion of Context

• Flexible adaptation in dynamic environments
• Scope that extends beyond the local host
• Generalized interaction with context types
• Context needs vary by task and situation
• Asymmetric context interaction
• Multiple contexts evolving over time
• Context computation must be scalable
• Explicit control over the scope, size, and cost

11
Overview

• Context-aware computing
• EgoSpaces conceptual model
• EgoSpaces middleware
• Network abstractions
• Conclusions

12
EgoSpaces Conceptual ModelThe View Concept

• Maximal context contains all available data
• A view is a projection of the maximal context
• Egocentric abstraction of operating context
• Tailored to an applications individual needs
• Allow agent to control scope of views
• Ease program development
• Minimize performance penalties

Host 2
Host 1
Reference agents view yellow data within one hop
Host 3
13
EgoSpaces Conceptual ModelDeclarative View
Specification

• Allows simple yet meaningful view definitions
• Restrict context based on properties of the
  network, hosts, agents, and data
• All location data owned by collision warning
  agents on cars within 100 meters of my current
  location
• Facilitate transparent context maintenance
• Rely on underlying middleware for context
  maintenance tasks

Roman, G.-C., Julien, C., and Murphy, A. L., "A
Declarative Approach to Agent-Centered
Context-Aware Computing in Ad Hoc Wireless
Environments, Software Engineering for
Large-Scale Multi-Agent Systems, LNCS 2603, April
2003.
14
EgoSpaces Conceptual ModelView Interaction

• Present view contents as tuple space
• Allows use of traditional Linda-like operations
• Content-based pattern matching for tuple
  retrieval
• retrieve tuples (rd)
• remove tuples (in)
• Can affect overlapping views
• Atomic blocking and probing operations

Julien, C., and Roman, G.-C., "Egocentric
Context-Aware Programming in Ad Hoc Mobile
Environments," in Proceedings of FSE-10, November
2002.
15
EgoSpaces Conceptual ModelConsistency Concerns

• Transactional semantics (e.g., money transfers)
• Strong application guarantees
• Can be expensive
• Use atomic Linda-like operations
• Best-effort semantics (e.g., traffic)
• Variety of implementations
• Application chooses semantics based on its
  particular situation
• Use scattered probing operations

16
EgoSpaces Conceptual ModelReactive Views

• Facilitate behavioral adaptation
• Agents respond to presence of certain tuples
• Associate application-level reaction to the
  appearance of a tuple
• Scheduling modalities correspond to consistency
  requirements
• Eager semantics provide transactional guarantees
• Lazy semantics provide fewer guarantees
• Allow more flexible implementation
• Increase performance

17
EgoSpaces Conceptual ModelActive Views

• Capture natural context interactions
• Transparent data migration
• Automatic duplication
• Event capture
• Extensibility
• Simplify programming
• Programming
• abstractions
• Reduced code size

migrate
Julien, C. and Roman, G.-C., "Active Coordination
in Ad Hoc Networks," to appear in Proceedings of
Coordination 2004.
18
EgoSpaces Conceptual ModelAccess Control
Requirements

• Ad hoc networks are open environments, yet agents
  may need to restrict data access
• No permanent trusted central server
• Current work
• Administrative domains
• Mobile Ambients, SEAL
• Specialized type systems
• KLAIM
• Fine-grained access control
• SECOS
• Must scale to large systems
• Must address needs of individualized agents

19
EgoSpaces Conceptual ModelAccess Control
Provision

• Agent specified access control function
• Limits access based on agent properties
• Allows fine-grained access control
• Individualized to particular applications needs
• Reference agent provides credentials and intended
  operations
• Access control functions evaluated on a per-tuple
  basis to determine inclusion in the view
• Can be used to define administrative domains

20
Overview

• Context-aware computing
• EgoSpaces conceptual model
• EgoSpaces middleware
• Network abstractions
• Conclusions

21
EgoSpaces MiddlewareWhy Middleware?

• Speed up application development in ad hoc
  networks
• Enhance level of programming abstraction
• Provide generic and flexible context operations
• Shift complex tasks to underlying infrastructure
• Facilitate rapid dissemination and adoption
• Proven solutions for other domains
• Distributed object systems
• CORBA, Jini, DENO
• Mobile agent systems
• Aglets, DAgents, ?Code
• Coordination systems
• LIME, MARS, Jedi

22
EgoSpaces MiddlewareEgoSpaces Middleware

• Programming interface simplifies application
  development burden
• Allows declarative view specification
• Presents view contents as tuple space
• Provides standard Linda tuple space operations on
  view definitions
• Includes light-weight implementations of tuple
  space operations
• Provides tailored access control

23
EgoSpaces MiddlewareEgoSpaces Architecture

• Application provides declarative view
  specifications
• Middleware
• Discovers neighbors
• Monitors context sensors
• Provides data contents of context to application
  through view abstraction

application
EgoSpaces
ELights
network abstractions
sensor monitoring
discovery
message
sensing
ad hoc physical network
24
EgoSpaces MiddlewareDemo Application

• Application-specific view restriction
• Hop-count based metric
• File-size restriction
• Content-based selection from view
• Songs displayed based on desired properties
• Reactive programming allows downloads to resume

25
EgoSpaces MiddlewareSimplifying Application
Development

• Abstracts network for application programmer
• Removes concern for low-level network and
  communication concerns
• Provides high-level, abstract definition of needs
• Maintains contents of views as the environment
  changes
• Programmer does not have to persistently poll
• Shifts developer focus to key components of
  applications
• User interface design
• Local data storage and representation

26
Overview

• Context-aware computing
• EgoSpaces conceptual model
• EgoSpaces middleware
• Network abstractions
• Conclusions

27
Network AbstractionsRestricting the Network

• Defining a view requires limiting participating
  hosts
• e.g., all hosts within five miles or within three
  hops
• Consider the overhead in defining such a network
• Deal with volatility of ad hoc networks
• Unannounced disconnection
• Unpredictable reconnection
• Heterogeneous devices

Roman, G.-C., Julien, C., and Huang, Q., "Network
Abstractions for Context Aware Mobile Computing,"
in Proceedings of ICSE, May 2002.
28
Network AbstractionsSolution Overview

• Represent the ad hoc network as a graph
• Abstract properties of nodes and links to weights
  on edges in this graph
• Calculate the cost of paths from the reference
  node
• Determine the shortest path to each node and
  build a tree
• Limit this tree using a bound, D

29
Network AbstractionsWeight Assignment

• Individualized for an application
• Abstracts varied properties of hosts and links
• Host properties form ?i link properties form ?ij
• ?i includes battery power, CPU power, location,
  etc.
• ?ij includes physical distance, bandwidth,
  throughput, etc.
• Edge weights combine link properties and the
  properties of the connected nodes
• mij ?(?i, ?j, ?ij)

30
Network AbstractionsComputing Path Cost

• Assuming each link has a single weight (mij), an
  application can define a generalized cost
  function
• ?v0(Pk) Cost(?v0(Pk-1), mk-1,k)
• ?v0(?v0?) 0
• Cost function must be strictly increasing along a
  path

31
Network AbstractionsLimiting the Scope
3

• Define a subtree of the network by placing a
  bound on the cost of each path
• Bound is useful only if shortest path cost is
  strictly increasing
• Allows reference to restrict computation to a
  subnet

1
2
2
1
1
0
1
2
1
1
2
1
Note All links have a weight of 1
32
Network AbstractionsContext Calculation Protocol

• Service new queries
• Respond at the application level
• Remember cost, parent, sequence number, and
  information about the computation
• Propagate the query
• Service shorter distances (same sequence number)
• Remember cost and new parent
• Propagate query
• Disregard longer distances (same sequence number)
• But remember cost and parent if within bound
• Halt when computation reaches boundary
• Based on traditional ad hoc routing protocols

33
Network AbstractionsProtocol Example
Query (source, new cost, ) State cost,
parent, ... (neighbor, cost through neighbor),

Bound 6 Cost additive
0, null,
r
4
2
1
a
b
5
1
4
c
d
34
Network AbstractionsContext Maintenance Protocol

• Persistent queries require context maintenance
• React to increase in link weight
• If parent, adjust cost and propagate information
• Otherwise, update local information
• React to decrease in link weight
• If non-parent link, recalculate cost for neighbor
  on the other end
• Handle count-to-infinity problem
• Store entire shortest path at each hop
• Regain resources when context is no longer used

35
Network AbstractionsMaintenance Example
Query (source, source cost, new cost,
) State cost, parent, (neighbor, cost
through neighbor),
Bound 6 Cost additive
0, null,
r
4
2
6
1
a
b
5
1
4
d
c
36
Network AbstractionsEvaluation

• Simulation using ns-2 network simulator
• 100 node network in 1000x1000m2 space
• Vary network density by changing transmission
  range
• Using random waypoint mobility model adapted to
  avoid speed degradation
• Metrics
• Context consistency
• Settling time (at most 35ms)
• Percentage of useful broadcasts

37
Network AbstractionsContext Building
Consistency (1)

• Good consistency for reasonably sized contexts

one-hop context
two-hop context
Percentage of context receiving message
three-hop context
four-hop context
Transmission range (meters)
38
Network AbstractionsContext Building
Consistency (2)

• Consistency gt80 with increased network load

five context definitions
one context definition
Percentage of context receiving message
ten context definitions
Transmission range (meters)
39
Network AbstractionsUseful Broadcasts

• Efficiency decreases with increasing density

one-hop context
two-hop context
Percentage of useful broadcasts
three-hop context
four-hop context
Transmission range (meters)
40
Conclusions

• Melding of context-awareness and ad hoc networks
• Novel notion of context
• Individualized declarative specification
• Includes general facets of the environment
• Extended in scope to remote components
• Asymmetric context interactions
• EgoSpaces model and middleware
• Supporting algorithms and protocols

41
Contributions (1)

• Algorithms for ad hoc mobile computing
• Network abstractions
• Consistent group membership

R
R
R
a
a
r
b
b
Huang, Q., Julien, C., and Roman, G.-C., "Relying
on Safe Distance to Achieve Strong Partitionable
Group Membership in Ad Hoc Networks," to appear
in IEEE Transactions on Mobile Computing.
42
Contributions (2)

• Middleware for context-awareness
• Addressing software engineering concerns of
  mobile environments
• Facilitating rapid development of mobile
  applications
• Bringing application development to the hands of
  novice programmers

Handorean, R., Payton, J., Julien C., and Roman,
G.-C., "Coordination Middleware Supporting Rapid
Deployment of Ad Hoc Mobile Systems", in
Proceedings of the 1st ICDCS Workshop on Mobile
Computing Middleware (MCM03), May 2003.
43
Contributions (3)

• Formal models of context-aware and mobile
  computing
• Specification and refinement using Mobile UNITY
• Building Context UNITY for direct reasoning about
  context-aware systems

• Roman, G.-G., Julien, C., and Payton, J., "A
  Formal Treatment of Context-Awareness," (invited
  paper) to appear in Proceedings of FASE 2004.
• Julien, C., Payton, J., and Roman, G.-C.,
  "Reasoning About Context-Awareness in the
  Presence of Mobility," in Proceedings of FOCLASA,
  September 2003.
• Roman, G.-C., Julien, C., and Huang, Q., "Formal
  Specification and Design of Mobile Systems," in
  Proceedings of FMPPTA'2002, April 2002.

44
Future Plans

• Addressing ad hoc network and communication
  concerns in software engineering
• Transactional programming in ad hoc networks
• Tailored communication and coordination paradigms
  for applications in ad hoc networks
• Context-aware routing and communication
• Applying formal methods to context-aware
  environments
• Reasoning about mobile and context-aware
  applications through formal notation and proof
  logic

45
Questions?

• http//www.cse.wustl.edu/julien
• julien_at_wustl.edu

46
Publications www.cse.wustl.edu/julien

• Roman, G.-G., Julien, C., and Payton, J., "A
  Formal Treatment of Context-Awareness," (invited
  paper) to appear in Proceedings of Fundamental
  Approaches to Software Engineering 2004.
• Julien, C. and Roman, G.-C., "Active Coordination
  in Ad Hoc Networks," to appear in Proceedings of
  Coordination 2004.
• Huang, Q., Julien, C., and Roman, G.-C., "Relying
  on Safe Distance to Achieve Strong Partitionable
  Group Membership in Ad Hoc Networks," to appear
  in IEEE Transactions on Mobile Computing.
• Julien, C., Payton, J., and Roman, G.-C.,
  "Reasoning About Context-Awareness in the
  Presence of Mobility," in Proceedings of the 2nd
  International Workshop on Foundations of
  Coordination Languages and Software Architectures
  (FOCLASA03), September 2003.
• Handorean, R., Payton, J., Julien C., and Roman,
  G.-C., "Coordination Middleware Supporting Rapid
  Deployment of Ad Hoc Mobile Systems", in
  Proceedings of the First International ICDCS
  Workshop on Mobile Computing Middleware (MCM03),
  May 2003.
• Roman, G.-C., Julien, C., and Murphy, A. L., "A
  Declarative Approach to Agent-Centered
  Context-Aware Computing in Ad Hoc Wireless
  Environments (extended version)," Software
  Engineering for Large-Scale Multi-Agent Systems,
  A. Garcia et. al. (editors), LNCS 2603, April
  2003.
• Julien, C., and Roman, G.-C., "Egocentric
  Context-Aware Programming in Ad Hoc Mobile
  Environments," in Proceedings of FSE-10, November
  2002, pp. 21-30.
• Roman, G.-C., Julien, C., and Huang, Q., "Network
  Abstractions for Context Aware Mobile Computing,"
  in Proceedings of the 24th International
  Conference on Software Engineering (ICSE'02),
  Orlando, FL (USA), May 2002, pp. 363-373.
• Roman, G.-C., Julien, C., and Murphy, A. L., "A
  Declarative Approach to Agent-Centered
  Context-Aware Computing in Ad Hoc Wireless
  Environments," in the 1st International Workshop
  on Software Engineering for Large-Scale
  Multi-Agent Systems (SELMAS'2002), May 2002.
• Roman, G.-C., Julien, C., and Huang, Q., "Formal
  Specification and Design of Mobile Systems," in
  Proceedings of FMPPTA'2002, April 2002.