A Novel Framework of Message Scheduling in
Delay Tolerant Networks (DTNs) with Buffer
Limitation
Dr. Pin-Han Ho
Associate Professor
University of Waterloo, Canada
Outline
• Delay Tolerant Networks – An overview
• SAURP – a self-adaptive utility based routing
protocol for DTNs
• Bound Analysis of SAURP
• Performance Evaluation
• Conclusions
Motivation of DTNs
• The usability of the Internet depends on some important
assumptions
- Continuous and bidirectional e2e (end-to-end) path
- Short round-trips: relatively consistent network delay in
sending data packets and receiving corresponding ACK.
- Symmetric data rates: relatively consistent data rates in
both directions between source and destination.
- Low error rates: relatively little loss or corruption of data on
each link.
Solution: Delay-Tolerant Networks
• What happens if the above does not hold?
- due to user mobility and device constraints
- conventional Internet protocols do not work or become
significantly ineffective.
•
Refer to a wide range of challenged networks, where
- End-to-End connection cannot be assumed to exist, and
network partitioning and delay/disruption is frequent
ex: ad hoc networks with high user mobility
• DTN is one of the solutions to confront the Internet’s
underlying assumptions.
Characteristics of DTN
•
Unique connectivity concepts
- can occur when a node is in power save mode/out-of-range
due to mobility.
- Scheduled link with a priori knowledge on nodes’ schedule
e.g., bus networks, periodic flights.
- Opportunistic link, relies on a link ‘by chance’.
• Very long or variable delay, or high BER
- due to long propagation delay, variable queuing delay, and
fragmented data transmission.
The Bundle/Message layer
• Bundle/message: Application-meaningful message
– Contains all necessary info packed inside one “bundle” (atomic
message)
– Next hop has immediate knowledge of storage and
capacity/service requirements of the message
• Store-and-forward message switching is implemented by overlaying a
new protocol layer called the bundle layer, on top of heterogeneous
region-specific lower layers.
• DTN routers form an overlay network with its own addressing,
signaling, and routing protocol.
Store-and-Forward Message Switching
• Whole messages or pieces of such messages are forwarded from a
storage place on one node to a storage place on another node, along a
path that eventually reaches the destination.
• These storage places are persistent storage; necessary because:
- A communication link to the next hop may not be available for a
long time.
- A message, once transmitted, may need to be retransmitted if an
error occurs.
Custody Transfers (1/2)
• DTN supports node-to-node retransmission of lost or corrupted data at
both the transport layer and the bundle layer.
• End-to-end reliability implemented at the bundle/message layer
- source requests custody transfer and return receipt. The source
must retain a copy of the message until receiving a return receipt.
Custody Transfers (2/2)
• A bundle/message custodian must store a bundle until either (1)
another node accepts the custody, or (2) expiration of the bundle’s
time-to-live.
time-to-live >> time-to-acknowledge
• When the current bundle layer custodian sends a bundle to the next
node, it requests a custody transfer and starts a time-to-ack
retransmission timer.
• If the next-hop bundle layer accepts custody, it returns an
acknowledgement to the previous custodian.
• If no ack is returned before the sender’s time-to-ack expires, the
sender retransmits the bundle.
Types of Contacts
• Precise (Scheduled) contacts
– E.g. satellite links, message ferry via public transits
– All info known
• Opportunistic contacts
– Not known before it occurs
– E.g. a tourist car that happens to drive by the village
• Predicted contacts
- Use the statistics about the nodal contacts for mobility exploitation
- Those statistics could be: based on a mobility model, past observation
+ prediction via the history of encounters (# of encounters), time
elapsed since last encounters, and inter-contact time.
Routing Solutions - Replication
• Single copy schemes: the source launches a single copy
of each message
• Multi-copy schemes: distribute multiple and identical
data copies to its contacts to increase delivery ratio (or
delivery delay
– Flooding (unlimited contacts)
– Heuristics: random forwarding, history-based forwarding,
predication-based forwarding, etc.
– Routing performance (delivery rate, latency, etc.) heavily
dependent on “deliverability” of these contacts (or predictability
of heuristics)
Routing Solutions - Replication
Eg. Spray and Focus (S&F)
- A two-hop scheme
- First stage: spray the L message tokens in the networks
- Second stage: hand over the last token to an
encountered node if it has a better utility
- should be done intelligently to make the message
delivery as fast as possible
- Compromise between consumed network resources
and performance
SAURP - Self Adaptive Contention Aware Routing
Protocol for DTNs
Introduction of SAURP (1/2)
• SAURP - Self Adaptive Contention Aware Routing
Protocol for DTNs
- based on spray and focus
- design for rather dense ad hoc networks with miniature
devices such as smart phones
=> resource contention becomes frequent
=> some nodes cannot accept a custody even if it stands
a good chance to delivery the message
- exchange contact history and nodal capacity dynamic
information (such as buffer status and congestion) during
each nodal contact
- each node performs the two stages of tasks based on
correlated utility
Introduction of SAURP (2/2)
- aiming to optimize message delivery ratio or message
delivery delay
- The utility function is dynamically determined at each
node via a window-based update rule
novel
-- collect the statistics during each time window
-- Predicting inter-contact time of each pair of nodes via a
transitivity update rule
-- Determine the utility function for each buffered message
jointly based on the inter-contact time, buffer status of
nodes, and network congestion
The SAURP Architecture
-
Contact Statistics (CS(i) ): obtained between each node pair regarding their total nodal
contact duration, channel condition, and buffer occupancy state.
T (i ) : the inter-contact time between any node pair.
Utility-function Calculation and Update Module (UCUM): to perform smooth transfer
between two consecutive time windows.
Transitivity Update Module (TUM)
Forwarding Strategy Module (FSM) : applied at the custodian node as a decision
making process
SAURP Architecture
Contact Statistics (CS):
- regarding to each node
Tfree: amount of time the channel is available during W
Tbusy: amount of time the channel is busy or the buffer is full during W
Ttotal: total contact time between an node pair during W
Utility-function Calculation and Update Module (UCUM)
Calculation of inter-contact time:
Tcs(i()A, B ) 
W
Tfree
, nc 
, where Tp is the time required to transmit at
nc
Tp
least one message
Time-window Transfer Update : T((Ai , B1))  TCS(i )( A, B)  (1   )T((Ai ), B)

| T((Ai ), B )  Tcs(i()A, B ) |
max( T((Ai ), B ) , Tcs(i()A, B ) )
, T((Ai ), B ) , Tcs(i()A, B )  0
SAURP Architecture
The Transitivity Update Module (TUM)

T((Ai ), D) new  T((Ai ), D)  (1   ) T((Ai ), B)  T((Bi ), D)
where  
T((Ai ), B )  T((Bi ), D )
T((Ai ), D )

, T((Ai ), D )  T((Ai ), B )  T((Bi ), D )
The Forwarding Strategy Module (FSM)
The Weighted Copy Rule for Spraying
Node A hands over Msgs copies to B according to:


B  



N



A


N
T((Ai ), D )




(i )
( A, D )  
T((Bi ), D )  T
where NA is the number of message tokens that node A has.
SAURP Architecture
The Forwarding Strategy Module (FSM): when # token = 1, perform
message forwarding in the second stage of Focus
Analytical Model
•
Evaluating the expected message delay and delivery
ratio
• Assumption:
- Node mobility is independent and heterogeneous
- Assume each node in the network maintains at least
one forwarding path to every other node.
- Each node follows a route of belongs to a single
community at a time, and the residing time on a
community is proportional to its physical size.
Analytical Model
•
Analyzing message delivery ratio
Analytical Model
where PRSD and PRl are random variables representing the
delivery probability in case of direct message delivery between
S and D, and through one of L − 1 paths, respectively.
Examination of the Proposed Analytical Model
•
•
•
•
Sc1: no contention
Sc2: with limited network resources
50 nodes are launched
according to communitybased mobility model in a
300x300 network area.
The transmission range is set
to 30 meters to enable
moderate network
connectivity with respect to
the considered network size.
The traffic load is varied from
a low traffic load (i.e., 20
messages generated per node
in 40,000 time units) to high
traffic load (i.e., 80 messages
generated per node in 40,000
time units).
A source node randomly
chosen a destination and
generates messages to it.
Simulation
• 110 nodes are launched according to the community-based
mobility model in a 600 x 600 meter network.
• 40,000 time units are simulated, with the message inter-arrival time
of each node pair uniformly distributed in such a way that the
traffic can be varied from low (10 messages per node in 40,000 time
units) to high (70 messages per node in 40,000 time units).
• TTL = 9,000 time units.
• Each source node selects a random destination node, begins
generating messages to it during simulation time.
• The following schemes are used to compare the proposed SAURP
- Epidemic routing (epidemic)
- Spray and Focus (S&F)
- Most mobile first (MMF)
- Delegation forwarding (DF)
Effect of Buffer Size
(b) Transmissions
(a) Average Delay
9000
30000
DF
8000
S&F
SAURP
MMF
25000
20000
6000
Transmissions
Delay (Time Units)
7000
5000
15000
4000
10000
3000
2000
Epidemic
DF
1000
S&F
SAURP
5000
0
MMF
0
5
5
10
20
50
100
200
Buffer Size
Traffic 70, Transmission Range = 30
10
20
Buffer Size
50
100
200
Effect of Traffic Load
(a) Average Delivery Delay
(b)Total Transmissions
4000
3000
40000
DF
S&F
SAURP
MMF
35000
Transmissions
3500
Delay(Time units)
45000
DF
S&F
SAURP
MMF
2500
2000
1500
1000
30000
25000
20000
15000
10000
5000
500
0
20
0
20
30
50
Traffic Load
60
30
50
70
Traffic Load
Buffer Size = 2000, Transmission Range = 70, BW = 1
60
70
Effect of Traffic Load
(a) Average Delivery Delay
(b)Total Transmissions
7000
25000
Tr=20
Tr=30
Tr=40
Tr=50
Tr=60
Tr=70
20000
5000
Transmissions
Delay(Time units)
6000
4000
3000
2000
1000
DF
S&F
SAURP
MMF
15000
10000
5000
0
0
20
30
40
50
60
70
Epidemic
DF
Traffic load
Buffer Size = 10, transmission range = 70
S&F
SAURP
MMF
Effect of Connectivity
(a)Average Delivery Delay
Epidemic
S&F
MMF
7000
DF
SAURP
60000
50000
DF
S&F
SAURP
MMF
Transmissions
Delay (Time units)
6000
(b)Total Transmissions
5000
40000
4000
30000
3000
20000
2000
10000
1000
0
0
k=20
k=30
k=40 k =50 k=60 k=70
Transmission range
k= 80
k=90
Traffic = 60, Buffer = 10
k=20
k=30
k=40
k=50
k=60
Transmission Range
k=70
k=80
k=90
Conclusive Remarks
• A DTN routing protocol SAURP is introduced.
- Following the spray and forward approach, the best carrier is
determined using a novel contact model by jointly considering
wireless link condition and nodal buffer availability.
- Analytical model for SAURP is provided, whose correctness was
further verified via simulation.
- SAURP can achieve shorter delivery delays than all the existing
spraying and flooding based schemes when the network
experiences considerable contention on wireless links and/or
buffer space.
• When nodal contact does not solely serve as the major
performance factor, the DTN routing performance can be
significantly improved by further considering other resource
limitations in the utility function and message weighting/forwarding
process.