Title: Spray and Wait: An Efficient Routing Scheme for Intermittently Connected Mobile Networks
1 Spray 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 2 Routing 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
send multiple copies
to whom and when
1 12 D 13 S 14 2 16 11 15 3 7 4 5 8 10 3 Existing 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 4 Existing 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
No existing protocol has both low transmissions and low delay!
5 Efficient Routing Design Goals
perform significantly fewer transmissions than flooding-based schemes under all conditions
better delay than existing single and multi-copy schemes close to optimal
scalability good performance under a wide range of values for various parameters (e.g. number of nodes)
simplicity require little knowledge about the network
Our Approach Spray and Wait
Optimizing Spray and Wait
7 Spray 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)
8 Spraying 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
9 Spraying 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!
10 Delay 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 11 Performance 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 12 Performance of Spray and WaitDelay of Spray Phase
D S Tight if L ltlt M 13 Choosing The Right Number of Copies (L) Minimum L such that EDsw a EDopt
Minimum L to achieve expected delay a times the optimal (M 100) 14 What If Network Parameters Are UnknownOnline 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 15 Scalability 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!
16 Simulations (contention waypoint model)
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 17 Scenario A Effect of Traffic Load (500x500 grid M 100 nodes Tx Range 10) increasing traffic 18 Scenario 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
19 Limitations 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)
20 Work 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) 21 Conclusions and Future Work
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
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
22 The End
23 Target 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.
Ad hoc networks for low cost Internet provision to remote areas/communities
Africa Saami etc.
extend the idea of Internet to space
Ad-hoc military networks
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