Title: Spray and Wait: An Efficient Routing Scheme for Intermittently Connected Mobile Networks
1Spray and Wait An Efficient Routing Scheme for
Intermittently Connected Mobile Networks
Thrasyvoulos Spyropoulos, Kostantinos Psounis,
and Cauligi S. Raghavendra EE Department,
USC spyropou, kpsounis, raghu_at_usc.edu
2Routing in Intermittently Connected Mobile
Networks (ICMN)
Network Characteristics
Routing
- Network is sparse and partitioned
- Nodes follow stochastic mobility model
- Mobility is not enforced (e.g. Zhao et al. 04)
- Mobility is not predictable (e.g. Jain et al. 04)
- Exploit node mobility
- store-carry-and-forward?
- Replicate message
- send multiple copies
- to whom and when?
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3Existing Proposals
- Flooding everyone gets a copy (Epidemic Routing
- Vahdat et al. 00) - Note optimal delay only when traffic is very
low! - Reducing the overhead of flooding
- Randomized Flooding (Y. Tseng et al. 02)
handover a copy with probability p lt 1 - Utility-based Flooding (A. Lindgren et al. 03)
handover a copy to a node with a utility at least
Uth higher than current - Can use p and Uth to tradeoff transmissions for
delay, BUT
Dilemma low p / high Uth? significant delay
increase high p / low Uth? degenerates to
flooding
4Existing Proposals (contd)
- Single-copy solutions (Spyropoulos et al. 04)
- Only one copy per message at any time
- Randomized, utility-based, hybrid, etc.
- Significantly reduced transmissions, BUT high
delay - Summarizing
- No existing protocol has both low transmissions
and low delay!
5Efficient Routing Design Goals
- Performance goals
- perform significantly fewer transmissions than
flooding-based schemes under all conditions - better delay than existing single and multi-copy
schemes close to optimal - Additional goals
- scalability good performance under a wide range
of values for various parameters (e.g. number of
nodes) - simplicity require little knowledge about the
network
6Outline
- Our Approach Spray and Wait
- Optimizing Spray and Wait
- Simulations
- Conclusion
7Spray and Wait
- Redundant copies reduce delay
- Too much redundancy is wasteful and often
disastrous! - Spray and Wait send only L copies
- Spray phase spread L message copies to L
distinct relays - Wait phase wait until any of the L relays
finds the destination (direct transmission)
- Important questions to be answered
- How are copies distributed? How many?
- What is the effect on delay?
- Can all our design goals be met? (e.g.
scalability)
8Spraying Matters
- Source Spraying Slowest
- source distributes all L copies one by one
- Binary Spraying Optimal
- source starts with L copies
- whenever a node with n gt 1 copies finds a new
node, it hands over half of the copies that it
carries - proof of optimality see paper
- intuition when movement is I.I.D., any two nodes
will find on average an equal number of potential
relays in the same amount of time
9Spraying Matters!
(analysis)
100x100 network with 100 nodes
- Efficient spraying becomes more important for
large L - Few copies suffice to achieve a delay only 2x the
optimal!
10Delay of Spray and WaitAn Upper Bound
- Assume independent random walks/random waypoint
and no contention - To keep things simple
- Some DTN applications are close to this model
- e.g. taxis forming a content distribution
network for exchanging traffic conditions, clips,
etc. - Exact delay can be calculated using a system of
recursive equations, but is not in closed form
Derive a bound
Probability a wait phase is needed
Wait Phase
Spray Phase
M number nodes, L number of copies
11Performance of Spray and WaitDelay of Wait Phase
Expected Meeting Time (from stationarity)
D
EDdt expected delay of Direct Transmission,
which is known (Spyropoulos et al. 04)
S
L relays
12Performance of Spray and WaitDelay of Spray Phase
D
S
Tight if L ltlt M
13Choosing The Right Number of Copies (L)
Minimum L such that EDsw a EDopt
- Method 1) Calculate L from bound
- Method 2) Approximate HM-L (Taylor series) and
solve resulting 3rd degree polynomial equation
Minimum L to achieve expected delay a times the
optimal (M 100)
14What If Network Parameters Are Unknown?Online
Parameter Estimation
IDEA use meeting times statistics
Method
Estimator
Note Optimal L depends on M only
Applies to any mobility model with exponential
meeting times
15Scalability of Spray and Wait
a 2
Number of Copies L (M 100)
a 5
a 10
- Spray and Wait M? ? ?
- In contrast in flooding, transmissions grow
linearly with M - Less than 10 need a copy to achieve 2x delay!
16Simulations (contention, waypoint model)
- Simulated schemes
- Epidemic routing (epidemic)
- Randomized flooding (random-flood)
- Utility-based flooding (utility-flood)
- Spray and Wait (spray wait(L xx) )
- Seek and Focus (seek focus) Spyropoulos et al.
04 - Simple slotted collision detection MAC protocol
to handle contention
Adjusted individual protocol parameters per
scenario to achieve a good transmissions-delay
tradeoff
17Scenario A Effect of Traffic Load
(500x500 grid, M 100 nodes, Tx Range 10)
increasing traffic
18Scenario B Effect of Connectivity
(500x500 grid, M 200, medium traffic)
Spray and Wait is better with respect to both
metrics under all load and connectivity scenarios
considered !
- Spray and Wait clearly outperforms all schemes
for all connectivity levels, in terms of both
transmissions and delay - Spray and Wait is considerably more scalable
- Performance of other schemes varies greatly with
connectivity - Spray and wait (i) fixed transmissions, (ii)
decreasing delay
19Limitations of Spray and WaitRestricted Mobility
- So far weve assumed that every node may go
anywhere inside the network - But, Spray and Wait may get in trouble if
- nodes mobility is restricted inside a local area
- nodes mobility is unrestricted but nodes move
extremely slow - Solution? Spray and Focus (work in progress)
- Spray L copies to L relays
- Route each copy using a single copy utility-based
scheme (instead of direct transmission)
20Work in Progress Performance of Spraying Schemes
in a Very Localized Scenario
Lessons learned Case 1 - highly mobile
nodes Spray and Wait is adequate (close to
optimal) Case 2 slow moving nodes/local
movement Spray and Focus is the winner (utility
contains a lot of information here)
21Conclusions and Future Work
- Conclusion
- Spray and Wait yields lower delay than existing
flooding and utility-based schemes, and
significantly reduces transmissions - delays close to the optimal can be achieved with
few copies - theory and simulations prove that it is scalable
- It is simple can be optimized with little
knowledge about the network - Future Work
- Performance of all protocols under more realistic
mobility models that exhibit correlation in space
and/or time - Preliminary simulations show Spray and Focus
performs well - Good utility function at the focus phase is the
key - Extend theory for such scenarios (non-exponential
meeting times) - Extend theory to model contention
22The End
23Target Applications(Delay Tolerant Networks)
- Sensor networks for habitat monitoring and
wildlife tracking - ZebraNet sensor nodes attached on zebras,
collecting information about movement patterns,
speed, herd size, etc. - Boatnet
- Ad hoc networks for low cost Internet provision
to remote areas/communities - Africa, Saami, etc.
- Inter-planetary networks
- extend the idea of Internet to space
- Ad-hoc military networks