On the Benefit of Random Linear Coding for Unicast Applications in Disruption Tolerant Networks - PowerPoint PPT Presentation

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On the Benefit of Random Linear Coding for Unicast Applications in Disruption Tolerant Networks

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Disaster relief team/military ad hoc network. Resource constrained: power, bandwidth, storage ... meet (RLC: in proportion to ranks) No copy when no token left ... – PowerPoint PPT presentation

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Title: On the Benefit of Random Linear Coding for Unicast Applications in Disruption Tolerant Networks


1
On the Benefit of Random Linear Coding for
Unicast Applications in Disruption Tolerant
Networks
  • Xiaolan Zhang, Giovanni Neglia
  • Jim Kurose, Don Towsley

2
Disruption/Delay Tolerant Network (DTN)
  • Intermittent connectivity
  • Limited/no infrastructure gt ad hoc network
  • Mobility and sparse settings gt frequent
  • partition
  • Examples
  • Vehicular network DakNet, UMassDieselNet
  • Sparse mobile sensor networks ZebraNet, under
    water sensor networks
  • Disaster relief team/military ad hoc network
  • Resource constrained power, bandwidth, storage
  • We focus on DTN with random contact

3
Routing unicast packets in DTN
  • Trade-off delivery delay versus num. of copies
    made for each pkt
  • Why limit copies ?
  • Transmission power affect network lifetime
  • Bandwidth consumption affect network throughput

src/dest transmission
Probabilistic forwarding
Average Delay
2-hop forwarding
epidemic routing
Average Copies Sent
4
Network Coding Benefit
  • Previous work studied network coding benefit in
    wireless network (including DTN)
  • Broadcast/multicast network coding increases
    energy efficiency Fragouli06, Widmer05, lun04,
    Wu04
  • Unicast network coding increases throughput by
    leveraging broadcast nature of wireless medium
    katti et al 05, Wu et al 04
  • Our contribution network coding (RLC) improves
    delay vs. energy efficiency tradeoff for unicast
    application in DTN.

5
Outline
  • Background
  • Random Linear Coding, Network Setting
  • Benefit under single generation case
  • Summary and Future Work

6
Random Linear Coding Ho et al 03
  • Packet a vector of symbols in Fq a finite field
    of size q
  • Pkts form generations of size k m1,m2,,mk
  • Random linear combination of pkt m1,m2,,mk
  • Network nodes
  • Collect and store encoded pkts
  • Forward random combinations of currently stored
    encoded pkts, together with coefficients
  • Decodable on collecting k independent
    combinations

encoded pkt
7
Network Setting
  • A DTN of N mobile nodes running unicast app.
  • Pair-wise Poisson meeting process with rate ß
    groenevlt05
  • Bandwidth constraint b pkts/per contact in each
    direction
  • Buffer constraint node can store limited num. of
    relay pkts

8
Forwarding/Coding schemes compared
  • Schemes
  • random uniformly randomly selects pkt to forward
    (and drop)
  • RR_random src selects pkt to send in round robin
    manner relay nodes perform random scheduling
  • RLC scheme apply RLC to a block of K pkt
  • Common delivery notification mechanism
  • On first delivery of pkts/generations,
    pkt/gen-delivered info generated, propagated
  • Performance metrics
  • Block delivery delay (Dblock) time to deliver
    the last pkt (other metrics avg. pkt delay,
    in-order delay)
  • Num. of transmissions made

9
Outline
  • Background
  • Random Linear Coding, Network Setting
  • Benefit under single generation case
  • Summary and Future Work

10
Single Generation bandwidth- constrained case
  • Single block of K pkts
  • Generated at same time
  • Sent from a source to a dest
  • Contact graph
  • Vertex network nodes
  • Edge contact between nodes, labeled with contact
    time b directed edges for bandwidth b contact
  • Time-respective path 1-gt3-gt4, 1-gt2-gt4

N4, b1 pkt/contact/dir
1
t7
t3.5
t1.2
3
2
t10.2
t23
4
Minimum block delivery delay time to have K
edge-disjoint time-respective paths from src to
dest
11
Single Generation Case (contd)
  • Can this minimum delay be achieved ? For this
    example,
  • RR_random scheme achieves min. delay with prob.
    0.5
  • RLC achieves min. delay with prob 1-1/q (q size
    of the finite field)
  • RR_random chooses from K packets RLC chooses
    from qK-1 different combs
  • Similar in spirit to algebraic gossip Deb and
    Medard 04

m1
m2
m2
m2
m1
m2
1
c1
t7
c2
c3
t3.5
t1.2
3
2
m2
c3
c12
t10.2
t23
Nothing to send !
4
With prob. 1-1/q, c12 and c3 independent !
12
Different delay metrics
  • Simulation result
  • N101,K10,q701, ß0.0049

RR_random avg. delay
RR_randomblock delay
RR_randomin-order
RLC
RLC tradeoff average pkt delay for block delivery
delay
13
Improvement in block delivery delay
  • Larger coding benefit under smaller bandwidth

N101, K10, q701, ß0.0049
14
Num. of transmissions made
  • RLC makes more transmissions
  • More often to have info. to exchange when nodes
    meet
  • Delivery notification starts later
  • Can RLC decrease block delay without increasing
    num. of transmissions ?

N101, K10, q701, ß0.0049
15
Limiting Copies token scheme
  • Token scheme src assigns fixed num. of tokens to
    pkt/generation
  • Decrease tokens after each transmission
  • Reallocate tokens when nodes meet (RLC in
    proportion to ranks)
  • No copy when no token left
  • Spray and wait spyropoulos05, small05

N101, K10, q701, ß0.0049
RLC achieves smaller block delivery delay than
non-coding scheme with similar transmission num
16
Buffer-constrained Case
N101,K10,bw1,q701
  • When relay buffer size decreases
  • RR_random/random delay increases substantially
  • RLC delay increases slightly
  • More copies made under RLC
  • under token scheme, RLC achieves better delay vs.
    transmission tradeoff than non-coding scheme

Buffer2
17
Outline
  • Background Motivation
  • Random Linear Coding, Network Setting
  • Benefit under single generation case
  • Summary and Future Work

18
Summary
  • Benefit of RLC for unicast app. in DTN
  • Achieves smaller block delay for given num. of
    copies, especially with limited buffers
  • Other results
  • Applied RLC to a single block of pkts
  • From diff. src to same dest
  • From diff src to diff dest
  • Multiple generation cases up to 22.5 reduction
    in block delivery delay
  • Insight benefit due to increased randomness of
    RLC (coupon collector problem)

19
Future Work
  • Practical issues
  • RLC overhead
  • Small application message size
  • Quantify coding benefit analytically
  • RLC benefit under real mobility traces
  • Can RLC increase network throughput for this
    network setting ?

20
Questions ? Comments ?
  • Thanks !

21
Benefit of RLC
  • Num. of ways of choosing K linearly independent
    combination of K pkts in a finite field is (deb
    and Medrad04)
  • Lemma 2.1

22
Multiple Generation Case
  • Uniform traffic load
  • N flows with each node being src of a flow and
    dest of one other flow
  • Each flow generates blocks of K10 pkts according
    to Poisson process with rate ?
  • Randomized scheduling
  • non-coding randomly select pkts the other node
    does not have
  • RLC randomly select gen. that has useful info.
    to other node
  • Randomized buffer management
  • Non-coding randomly select pkts to drop
  • RLC randomly select gen. to compress

23
Without limiting transmissions
  • Under low traffic rate (?)
  • RLC achieves smaller block delay than non-coding
  • Under higher traffic rate (?), RLC incurs larger
    block delay than non-coding
  • More pkts to choose from for non-coding
  • Higher contention for bandwidth under RLC
    randomized scheduling does not favor new gen.

N101,K10, ?0.00045,bw1
24
The need to limit copies
N101,K10, ?0.00045,bw1
  • One solution limit copies when network is loaded
  • Optimal per-packet token num. under certain
    traffic rate
  • Too small gt some contacts not utilized
  • Too big gt contention
  • With higher traffic rate, non-coding benefits
    from limiting copies too
  • Up to 22.5 reduction of block delivery delay

Buffer5
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