RelayCast: Scalable Multicast Routing in Delay Tolerant Networks

Uichin Lee, Soon Young Oh, Kang-Won Lee*, Mario Gerla

*

DTN Multicast Routing Delay tolerant networking:

Suitable for non-interactive, delay tolerant apps Ranging from connected wireless nets to wireless mobile nets with

disruptions (delay tolerant networks) Provides reliable data multicast even with disruptions DTN multicast routing methods:

Tree/mesh (+ mobility), ferry/mule, epidemic dissemination DTN multicast questions: Throughput/delay/buffer bounds? Focus: dissemination; upper bound of all cases

S

R1

R2

Dissemination

mobility

S

R2

R3

R1R4

SR3

R1R4

R2mobility

Disrupted node

Disrupted node

Tree Mesh

S

F

R1

F

Ferry/mule

Disrupted nodes

Access Point

Access Point

Bluetooth-based pocket switched networking Figure from http://www.dtnrg.org

InternetCity

Remotevillage

Connecting remote villagers

DTN Model Pair-wise inter-contact time: interval between two contact

points

Common assumption: exponential inter-contact time Random direction, random waypoint, etc. Real world traces also have “exponential” tails [Karagiannis07]

Exponential inter-contact time Inter-contact rate: λ ~ speed x radio range [Groenevelt05]

Assumption: n nodes in 1x1 unit area; radio range: O(1/√n) and speed: O(1/√n) meeting rate: λ=O(1/n)

T1T2 T3

Contact points between two nodes: i and j

Tic(n): pair-wise inter-contact time

time

Tic(1)

T1

Tic(2)

T2

Tic(3)

T3

2-Hop Relay: DTN Unicast Routing Each source has a random destination (n source-destination pairs) 2-hop relay protocol:

1. Source sends a packet to a relay node2. Relay node delivers a packet to the corresponding receiver

2-hop Relay by Grossglauser and Tse

Source

Relay

Destination Source is also mobile

2-Hop Relay: Throughput/Delay Throughput is determined by aggregate meeting rate

[Src relay nodes], [Dest relay nodes] 2-hop relay throughput: Θ(nλ)

G&T’s results: Θ(nλ)=Θ(1) for λ=1/n (i.e., speed=radio=1/√n) 2-hop relay delay: Θ(1/λ)

Avg. time for a relay to meet a dest (~exp dist!): 1/λ Ex) For λ=1/n, avg. delay is Θ(n) (Neely&Modiano)

Source to Relay Relay to Destination

Sou

rce

Des

tinat

ion

Agg. meeting rate: nλ

Agg. meeting rate: nλ

Avg. delay = Avg. inter-contact time

Food (=source)

Ants (=relays)

RelayCast: DTN Multicast Routing 2-hop relay based multicast:

1. Source sends a packet to a relay node

2. Relay node delivers the packet to ALL multicast receivers

RelayCast: 2-hop relay based multicast

S

Source SR1

R2

R3

Relay Nodes

Phase 1: S→Ri Phase 2: Ri →{D1,D2,D3}

D1

D2

D3

D1

D2

D3

D1

D2

D3

D1

D2

D3

Source

Relay

Destinations

D2

D1

D3

NB. Source & Destinations are also mobile

RelayCast: Throughput Analysis RelayCast throughput: Θ(nλ/nx)

ns srcs, each of which associated with nd random dests

Multiple srcs may choose the same node as a dest Avg. # of competing sources per receiver: nx

S1

S2

S3

S4

D1

D2

D3

D4

Each src chooses 3 rand dests

ns srcs dests

D1D1

D2

D3

To D2

To D3

To D1

λ

λ

λ

λλ

λ

nx srcs to D1

λ/nx

λ/nxnx srcs to D2

nx srcs to D3

λ/nx

Relay Node in RelayCast

S1

S2R D1

nx competing

sources

Relay

nx=ns*nd/n

Destinations are also mobile

RelayCast: Delay Analysis Relay node delivers a packet to ALL destinations nx competing srcs per dest: individual rate is split to λ/nx

RelayCast avg. delay: Θ(nx/λ(log nd+γ)) where γ = Euler constant

Avg. delay = ∑nx/kλ = nx/λ∑1/k = nx/λ (log nd + γ)

To D1

D1To D1

To D1

λ/nxS1

S2

S3

λ/nx

λ/nx

λ/nx

λ/nx

λ/nx

nx c

om

pet

ing

sr

cs t

o D

1

nx sub-queues to D1

D1

D2

D3

To D2

To D3

To D1

λ

λ

λ

λλ

λ

nx srcs to D1

λ/nx

λ/nxnx srcs to D2

nx srcs to D3

λ/nx

D2

D1

D3 nd nd-1 1 0

ndλ/nx λ/nx

Markov Chain for pkt delivery* State = # of remaining nodes

(nd-1)λ/nx

RelayCast: Buffer Requirement Little’s law: buffer = (rate) x (delay) Buffer per source = Θ(nnd)

Avg. sub-queue length: λ/nx*nx/λ = Θ(1) by Little’s law Each src has nd dest: packet is replicated to nd copies Per src buffer at a relay = Θ(nd) n relays: buffer = Θ(nnd)

Buffer upper bound per source = Θ(n2)

D1

D2

D3

To D2

To D3

To D1

λ

λ

λ

λλ

λ

nx srcs to D1

λ/nx

λ/nxnx srcs to D2

nx srcs to D3

λ/nx

Relay Node

To nd destTo D1

D1To D1

To D1

S1

S2

S3

λ/nx

nx c

om

pet

ing

sr

cs t

o D

1

nx sub-queues to D1

λ/nx

λ/nx

λ/nx

λ/nx

λ/nx

Comparison with Previous Results

Θ(n/log n)Θ(1)

nn log

1

Θ(1)

n

1

Uni

cast

Reg

ime

Multicast Regime

Assumptions; n fixed, and r = √logn/n for G&K; r=1/√n for 2-hop relay Throughput scaling with ns= Θ(n); nx = nsnd/n = nd RelayCast = Θ(1/ nd) Better throughput than conventional multi-hop multicast (w/ r=√logn/n)

Multi-hopUnicast

nn log

1 Gupta & Kumar 2001

dnnn

1

log

1 Multi-hopMulticast

Shakkottai, Liu, Srikant 2006 Li, Tang, & Frieder 2007

Tavli 2006 Keshavarz-Haddad, Ribeiro, Riedi 2006

n

1Broadcast

Grossglauser & Tse 2001Delay Tolerant Networks

Two-hop Relay 1

RelayCast: Delay Tolerant Networks

RelayCast

dn

1

# of multicast receivers (nd) per source

Per

no

de

thro

ug

hp

ut

wit

h n

s= Θ

(n)

Simulation ResultsRelayCast throughput with varying # of relay nodes

DTN with fixed λ: throughput linearly increases RelayCast throughput = Θ(nλ) for nsnd ≤ n

As # node increases, interference comes in; throughput is tapered off

Number of relay nodes in the network

Thr

ough

put

per

node

(K

bps)

QualNet v3.9.5Network: 5000mx5000mRandom waypoint802.11b: 2Mbps 250m radio rangeTraffic : ns=1, nd=# of relay nodes

Simulation ResultsComparison with conventional multicast protocol

Number of sources

Per

nod

e th

roug

hput

(kb

ps)

Traffic: 5 rand dest per srcTotal 100 nodes in the net

Replication reduces delay at the cost of throughput decrement

RelayCast is scalable; ODMRP’s throughput decreases significantly, as # sources increases

But delay has significantly increased; RelayCast ~ 2000s vs. ODMRP < 1s

Simulation ResultsAverage delay with varying # of receivers

RelayCast delay = Θ(nx/λ(log nd+γ)) Delay increases as # of receivers increases

Number of multicast receivers

Ave

rage

del

ay (

s)

30m/s

20m/s

Conclusion RelayCast:

Provides reliable multicast even with disruption Achieves the maximum throughput bound of DTN

multicast routing DTN routing protocol design and comparison

must consider throughput/delay/buffer trade-offs Future work

Analysis of other DTN routing strategies Impact of correlated motion patterns: i.e., power-law

head and exponential tail inter-contact time distribution