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 -- collect the statistics during each time window -- Predicting inter-contact time of each pair of nodes via a novel 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. : 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

Contact Statistics (CS): - regarding to each node T free : amount of time the channel is available during W T busy : amount of time the channel is busy or the buffer is full during W T total : total contact time between an node pair during W Utility-function Calculation and Update Module (UCUM) Calculation of inter-contact time: where T p is the time required to transmit at least one message Time-window Transfer Update : SAURP Architecture

The Transitivity Update Module (TUM) where The Forwarding Strategy Module (FSM) The Weighted Copy Rule for Spraying Node A hands over Msgs copies to B according to: where N A 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 SAURP Architecture

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 PR SD and PR l 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 50 nodes are launched according to community- based 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. Sc1: no contention Sc2: with limited network resources

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 Traffic 70, Transmission Range = 30

Effect of Traffic Load Buffer Size = 2000, Transmission Range = 70, BW = 1

Effect of Traffic Load Buffer Size = 10, transmission range = 70

Effect of Connectivity Traffic = 60, Buffer = 10

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.