Virtual Segment: Store-Carry-Forward Relay-Based Support for Wide-Area Non-Real-time Data Exchange

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1 2009 International Conference on Intelligent Networking and Collaborative Systems Virtual Segment: Store-Carry-Forward Relay-Based Support for Wide-Area Non-Real-time Data Exchange Shinya Yamamura Akira Nagata Service Platform Architecture Research Center National Institute of Information and Communications Technology Tokyo, Japan Masato Tsuru Hitomi Tamura Network Design Research Center, Kyushu Institute of Technology Fukuoka, Japan Abstract In the Internet of the future, a flexible, dynamic combination of wireless and wired access networks is expected to be a key driver for enlarging the broadband communication service area. However, in general, the number of broadband wireless access points is limited due to their cost, and hence, they cannot cover the whole network. On the other hand, various message delivery schemes based on store-carry-forward routing with some extensions have been studied to support nonreal-time communication in sparse or intermittent networks. However, communication services based solely on the storecarry-forward schemes covering a large area are inefficient because the delay time for message delivery increases in proportion to the size of the service area. Therefore, to extend the communication service area with controllable performance in a cost-effective manner, the authors have proposed a concept of Virtual Segment (VS) in which global communication service is provided by combining a store-carry-forward scheme with broadband wireless/wired network infrastructures connected to the Internet to deploy the supplementary infrastructure. Along this line, in the present paper, the issues on the addressing and the message transfer methods in VS approach are considered which are essential for designing a practical framework for non-real-time, asynchronous data communication, especially for large-sized message transfer. A logical architecture for implementation is also presented. Finally, the message transmission methods are examined using computer simulation. Keywords-DTN; store-carry-forward routing schemes; wireless communication; addressing; retransmission; I. INTRODUCTION The Internet Research Task Force (IRTF) Disruption Tolerant Networks Research Group (DTNRG) and related research work have proposed different types of store-carryforwarding routing schemes. Epidemic routing [1][5] is the basic concept, where the Throw Box[2], Data Mules[3], and Message Ferry[4] are taken from currently available solutions for accelerating the store-carry-forwarding routing schemes. The authors have proposed a concept of Virtual Segment[7], based on an idea of combining broadband network infrastructures (connected to the Internet) that have a high data rate but a small coverage area with a storecarry-forward routing scheme that can roughly plug the gaps in broadband service areas. We are aiming to design and develop a practical framework that supports asynchronous Relay Node R1 Figure 1. IBN Virtual Segment «««««Virtual Segment 1 R1 Correspondent Node CN1 Core Network Egress Base Node Virtual Segment 2 «road Virtual Segment 3 Virtual Segment 4 A conceptual example of the proposed system large-sized data exchange among users or from/to the Internet by segmenting the entire communication area so as that a store-carry-forward routing scheme could be efficiently adopted within each segmented area. In particular, the possibility of building a new network infrastructure with vehicles that have wireless communication devices is considered. Figure 1 shows a conceptual example of the proposed system in our previous work[7]. The basic idea is to carefully divide the entire service area into a certain number of properly formed segments along the road and regard them as virtual hot-spots, which are denoted as virtual segments in Figure 1. These virtual segments are then persistently connected to wired/wireless networks to form a core network, as shown in Figure 1. The three types of communication nodes are: a correspondent node (CN) representing each stationary or mobile communication user; a relay node (RN) representing a vehicle that traverses roads and has the ability to store, carry, and forward messages from/to CNs; and a base node (BN) representing a stationary gateway that is normally located at the edge of a virtual segment and communicates with and controls the relay nodes passing R /09 $ IEEE DOI /INCOS

2 near the BN and also stores the messages to be collected and delivered. Each BN has two roles. The first role is as an Ingress BN that collects and distributes messages from the Internet to the CN and asks the relay nodes to carry the messages. The second role is as an Egress BN that collects the messages from relay nodes and may store them for retransmission. The proposed framework was evaluated using a simple computer simulation [7] and field trials, from which it was confirmed that the proposal could be scaled by making a trade-off between the cost and the allowable delivery delay time. We should note that the proposed framework aims at the supplementary infrastructure for non-real-time, asynchronous but global (i.e., Internet-wide) information services. For instance, the asynchronous RSS delivery to many CNs in many segments should be archived in our framework, involving a question how to efficiently and reliably exchange a number of variable-sized inter-dependent messages. Therefore, in contrast to a variety of existing store-carry-forward routing studies, which often focus on efficiency in simply exchanging a number of independent messages inside a closed region, more sophisticated message transmission methods are required, including the retransmission control and buffer management taking account of a unique structure composed of three distinct roles: BN, RN, and CN. For example, as BNs can cooperatively maintain a complete list of messages exchanged in the segment and mostly confirm the delivery in VS, unnecessary copy and forwarding of a message may be avoided. Note that it is necessary to combine dynamic detection (identification) in contacting with neighboring nodes, addressing, and message transmission scheduling with fairness among users (destinations). Mansy et al.[6] investigated a detailed retransmission mechanism with data fragmentation wherein messages were carried between two nodes along a road by a vehicle passing through the road. This mechanism can be used in transferring a message between a CN and a BN, but not directly be applied to in exchanging messages among multiple BNs and multiple CNs in a VS by relay nodes. We should tackle the following issues. (i) Arrangement of the segments (locations of BNs), (ii) Message transmission methods between BN and CN, as well as between CN and RN; (iii) Retransmission mechanisms for undelivered messages; (iv) Intersegment communication schemes (addressing and routing); and (v) Control of authentication, authorization, and accounting (or other security-related issues). This paper will consider the issues raised in points (ii), (iii), and (iv). For points (ii) and (iii), the message transmission methods will be examined with a particular emphasize on the efficiency in delivering messages to multiple CNs within a closed region using the store-carry-forward routing scheme. For (iii), an addressing scheme for nodes in virtual segments with a late binding approach is proposed. The rest of the paper is organized as follows. Section 2 presents the addressing method in the virtual segment, while Section 3 considers efficient transmission in the store-carryforward routing scheme and shows the evaluation result obtained from computer simulations. Section 4 presents the proposed implemental architecture. Finally, Section 5 presents the key conclusions of this paper. II. ADDRESSING A global DTN world in which each end-node (source and destination of messages) has a unique global identifier called End-point ID (EID) is assumed. Hence, each correspondent node can be identified by its EID. In the proposed virtual segment, two types of address resolution are needed. The first type of address resolution is addressing in a segment, which is needed so that the relay node may forward the message to the CN. This addressing is achieved by the segment management node (SMN), which maintains mapping information between the CN s EID and the IP address, as well as additional information necessary for the relay node to communicate with the CN. The second type of address resolution is addressing for message delivery among segments. This addressing is required to allow the SMN to forward a message to the CN in another segment. This is achieved by composing the system on the backbone core network to manage the mapping between the EID of the CN and the EID of the SMN that manages the segment consisting of the CN. Figure 2 shows the addressing and relation in VS, DTN region, and the Internet. BNs that belong to each virtual segment are managed by the SMN. The SMN manages the global identifier EID of the CN in the segment and its corresponding IP address. This mapping information is called Local Binding. The root node (root) manages the correspondence of the global identifier of the CN and the global identifier of the SMN of the virtual segment that consists of the CN. This mapping information is called Segment Binding. The root node is the most significant node in the hierarchical mapping in the proposed method, and it functions as a gateway to other regions. Similarly to Domain Name System (DNS) in the Internet, layering and load-balancing can be done on the basis of the topological structure of all of the segments. Due to the nature of storecarry-forward routing, the CN is not necessarily active when a message to the CN is sent. Therefore, the message that cannot be addressed at a given moment is kept by the root node for a certain period. The application gateway (AGW) is a node that becomes a gateway when the node communicates with existing Internet applications. Now, the detailed addressing mechanism of each node that exists in the segment is explained. The conventional IEEE Wireless LAN is assumed for communication among nodes. Relay node addressing The relay node operates as a client of wireless communications. The relay node is assumed to know the common 367

3 BN Figure 2. Local binding SMN Private address space RN segment CN Segment binding BN Root BN AGW Global address space «bone SMN Private address space RN segment CN BN Internet Addressing and relation in VS, DTN region, and the Internet Service Set Identifier (SSID) and security information of all wireless access points where the virtual segment is composed beforehand by negotiating a contract with the service provider. When the relay node accesses a wireless access point of the BN, the IP address is automatically distributed by the DHCP. The IP address space (subnet) needs neither to be global nor be specific to each segment. The BN and CN notify a global identifier and IP address as link information to the relay node when coming in contact with it. Note that the relay node receives some messages to be delivered within the segment from an Ingress BN when it enters the segment, and forwards all messages received from the CN to the Egress BN when it exits the segment. CN addressing The CN operates as a wireless access point and dynamically detects the relay node. No static link information is set beforehand at the CN, and the route table is set such that the CN may forward all messages to the relay node when the two come in contact. The CN transmits all messages to the relay node, regardless of the address of the actual message requested by the application. BN addressing The BN also operates as a wireless access point, and it recognizes the relay node on the basis of segment registration after establishing the communication link from the relay node. The BN returns the message to the CN as a response to the segment registration message. III. MESSAGE TRANSMISSION METHODS A. Message transmission mode of the store-carry-forward routing in the DTN2 implementation The DTN Research Group (DTNRG) of IRTF has opened the reference implementation (DTN2) for the store-carryforward routing scheme to the public. Flooding type and static-routing type are implemented as the basic message transmission modes. For instance, the transmission mode of flooding type is used in epidemic routing. This method transmits the copy of a message to all the nodes when a link is established. The buffered message is deleted when the timer expires. Since this method is not aware of the properties of messages, there is no chance to control priority or fairness based on a property (e.g., destination nodes on buffering the message to multiple nodes). Static routing type is used by which a message is transmitted based on its destination address when the interface link related to the send queue connects. Both of two types are insufficient to implement the transmission mechanisms for the VS nodes. B. Requirements of transmission methods in the Virtual Segment The requirements for message transmission by the two types of nodes are shown below. Ingress Base Node (IBN) The IBN should forward buffered messages bound for multiple CNs to an arbitrary relay node (RN) that is dynamically detected. After transmitting a message, the method may not delete the message from the buffer for retransmission. On the other hand, because the number of buffered messages increases with time, an appropriate replacement of the buffered messages is necessary to forward them to a RN during a limited contact time. Since the address information of the next RN is not known in advance, static routing cannot be applied. Relay Node The relay node looks for and contacts with CNs by switching the wireless link to each CN along with traversing the road. When multiple CNs are very closely located, the relay node should contact with and transmit messages to those CNs simultaneously. In such a case, there is a possibility that the transmission of the following messages to other CNs fails if a large-sized proceeding message to a certain CN is transmitted, leading to unfairness. C. Transmission methods for the Virtual Segment Neither simple flooding nor static routing is suitable for message transmission in the virtual segment, as explained in the previous section. Thus, three new methods that can be adapted to the virtual segment are proposed and evaluated for effectiveness using computer simulation. 1) Simple First In, First Out (SFIFO): In this method, when the message is transmitted to the next hop, the message is simply deleted from the buffer. The node has one send queue, and it monitors the link to the address of the head message of the send queue. If it is possible to communicate with the address, the node detaches the message from the head of the send queue and transmits the message to the link. When communication is interrupted before completing the transmission, the node attaches the remainder of the message to the head of the send queue. Next, when it encounters another node, the node restarts the transmission of the message from the breakpoint. 368

4 2) Periodical Queue Search (PQS): In this method, the node has one send queue, and it sequentially detaches the message from the queue head at a constant interval. If the link to the addresses of the detached message is active, the node transmits the message to the address. When the link is inactive, the node attaches the message to the tail of the send queue. When communication is interrupted, the node attaches the remainder of the message to the tail of the send queue. 3) Periodical Queue Search with Round Robin (PQSRR): In this method, the node has the send queue of each address, which can be dynamically changing, and a physical destination. The node refers to all the send queues in a round-robin manner and processes the message with FIFO in each queue. The PQSRR has a virtual queue for each address based on the node, unlike in PQS. The fairness of message sending can be expected to be better because it will change during processing to other nodes, even if there is a large-sized data message to a certain specific node. D. Outline of the simulation A computer simulation was performed to investigate and evaluate the three transmission methods. For simplicity, message loss is not considered at this stage. The outline of the simulation is as follows: The communication in one virtual segment is simulated. Each message arrives at a BN, and is delivered to a CN in the segment via RN, that is, the download scenario is examined. The simulation is time-driven and its unit time is one second. The transmission and reception processing of the message data at each node is executed within the range of the communication. The volume of the transmitted message is calculated from the transmission rate of wireless communication and the length of time during which the two nodes are within the communication range. The operation of the bundle layer conforms to the DTN2 implementation. The message loss rate is zero. The remainder of the message that is not completely delivered is received by an EBN and distributed to other BNs as IBN. Figure 3 shows the screen shot of the simulation visualization tool. A dotted circle shows the wireless communication range of the RN. A circle shows the wireless communication range of the CN or the BN. An arrow shows the message request from the BN to CN, and dotted arrow shows the physical message transmission form the RN to the CN. E. Evaluation of the transmission methods The three proposed transmission methods were simulated for different combinations of base nodes and relay nodes that forwarded to them using the conditions listed in Table I. BN_01 Figure 3. RN_02 CN_01 RN_01 segment length CN_02 distance from the road BN_02 The screen shot of the simulation visualization tool Figure 4. Comparison of the average delay time as a function of the number of transmission methods Each BN or CN can exchange 25 MB of data with a relay node during one contact. Thus, an encounter with one relay node is needed for delivering a 5-MB message, while at least four relay nodes are needed for a 100-MB message. Figure 4 shows the simulation result. From the simulation result, it can be inferred that the delay time increases when the relay node that uses the SFIFO transmission method. However, when both the base nodes and relay nodes implement SFIFO, the difference in delay time is no different from any other Figure 5. The average delay time variation as a function of the message size distribution 369

5 Table I SIMULATION CONDITIONS Condition Segment length Simulation execution time Final message send time The number of correspondent nodes Traffic of vehicle Vehicle speed Average message interval for CN Message size Value 1000 (m) 7200 (s) 1800 (s) 10 (units) 100 (units/hour) 10 (m/s) 300 (s) mixedwith5mb(90%) and 100 MB (10%) Traffic intensity combination of transmission methods. As a result, it can be observed that the relay node should implement a transmission method other than SFIFO, although the transmission method implemented by IBN does not have a big impact on the average delay time. Thus, three combinations of the same transmission method were evaluated in greater detail using the same conditions except for message size. Figure 5 gives the comparative result for the variation in fixation, the Poisson distribution, and the exponential distribution of the message size when the average data size of the message to the CN is 30 MB. Figure6 shows the results as a function of the arrival rate of the message when the message size comes from an exponential distribution with a mean size of 30 MB. The average message interval value range is from 550 (sec) to 950 (sec). The average delay time for PQS and PQSRR is small, and as the data size increases, it is clear that the effect of PQS and PQSRR is quite noticeable when compared to SFIFO. There is a slight difference between PQS and PQSRR, in that when the message size range is wide, PQSRR is more effective. As for SFIFO, when the average message arrival rate increases, the average delay time increases considerably. As a result, it can be confirmed that a time-sharing approach, for example in a round-robin manner, is necessary for message delivery to multiple addresses in the virtual segment. The average communication delay time based on the variation of the message fragment size and the location distribution of the CNs was considered. A brief summary of the results follows. In general, it is effective to maximize the fragment size within a range such that fairness is not compromised. It should be noted that further research should be done to determine the optimum message size. As well, the average communication delay time increases when there are a considerable number of correspondent nodes in the segment. Thus, it was found that to realize efficient delivery scheduling of the message from the IBN to the relay node, the location of the CN along the road within the segment should be considered. Figure 6. The average delay time variation as a function of the average arrival rate of the message IV. LOGICAL ARCHITECTURE PROPOSAL This paper considered address resolution and a message transmission method in the virtual segment and carried out a preliminarily evaluation of the proposed method. On the basis of the evaluation results, a logical node architecture is suggested for the proposed method. Figure 7 shows the node architecture used in the virtual segment. The architecture is common for three types of nodes. The message decoder is a functional block where the message fragmented by SME is restored, and the ARQ/fragment manager is a functional block where retransmission is controlled and the message is fragmented. The received message is detached from the receiving queue of the bundle layer with FIFO, and if the message is addressed to the receiving node itself, the message is passed to the message decoder, and the ARQ/fragment manager reassembles the fragmented message. Otherwise, the node passes the message to the scheduler to forward it to other nodes. The logical send queue is the same as a virtual queue of the PQRSS transmission method. The scheduler decides the transmission schedule on the basis of the segment information notified from the segment manager, as in the example using segment information as described in the previous section. The logical send queue can function as a hash table during actual implementation, and it is likely to become more complex since it uses the ARQ method for retransmission, although a simple queue was used in the simulation conducted for the evaluation. The segment manager is a functional block for address resolution, and it dynamically sets the link and the route to the bundle layer by exchanging a pair of IP address and EID between nodes according to its role, as previously explained in this paper. It is necessary to enhance the information exchange for this address resolution of the segment manager for ARQ and achieve efficient transmission scheduling. For 370

6 SME Segment Management Extension Bundle layer receiving queue Application layer registration delivery Message decoder request queue reassemble queue segment information ARQ/Fragment manager Segment manager ack. vector segment information logical rend queue exchange Scheduler segment information physical send queue Router/Forwarder dynamic setting Transport layer [3] R. Shah, S. Roy, S. Jain, W. Brunette, Data MULEs: Modeling a Three-Tier Architecture for Sparse Sensor Networks., In Proceedings of the First IEEE International Workshop on Sensor Network Protocols and Applications, 2003, pp [4] W. Zhao, M. Ammar, Message Ferrying: Proactive Routing in Highly-partitioned Wireless Ad Hoc Networks., In Proceedings of Ninth IEEE Workshop on Future Trends., 2003, pp [5] T. Matsuda, T. Takine, (p,q)-epidemic Routing for Sparsely Populated Mobile Ad Hock Networks., Selected Areas in Communications, IEEE Journal, vol.28, 2008, pp [6] A. Mansy, M. Ammar, E. Zegura, Reliable roadside-toroadside data transfer using vehicle traffic., In Mobile Ad hoc Sensor Systems, MASS 2007, pp Figure 7. Node architecture in the Virtual Segment instance, the log of the heartbeat between the correspondent node and the relay node or vector-based ACK notification is considered as enhanced information. [7] S. Yamamura, A. Nagata, M. Uchida, and M. Tsuru, Virtual Segment: Segmentation Method for Store-carry-forward Routing Schemes., In Proceedings of the First Workshop on Wireless Broadband Access for Communities and Rural Developing Regions, 2008, pp V. CONCLUSIONS The issues on the addressing and the message transfer methods in Virtual Segment approach were discussed, for designing a practical framework for non-real-time, asynchronous, and large-sized data communication. We considered three transmission methods in which address resolution is combined with schedule management. They were evaluated through computer simulation by examining how suitable to store-carry-forward routing-based message delivery to multiple correspondent nodes that exist in a closed region, e.g., a virtual segment. Evaluation results gave us some insight for further designing. A logical node architecture for the virtual segment was also provided. Future work will consider more effective transmission methods with detailed implementations, which includes considering packet loss on the wireless link and retransmission to multiple correspondent nodes. We will also prepare real experiments using prototypical implementations of the three node types. ACKNOWLEDGMENT This work was supported in part by the Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (S) (No ). REFERENCES [1] A. Vahdat and D. Becker, Epidemic routing for partially connected ad hoc networks., Technical Report CS , [2] W. Zhao, Y. Chen, M. Ammar, M. Corner, B. Levine, and E. Zegura, Capacity Enhancement using throwboxes in mobile tolerant network., SCS Technical report GIT-CSS-06-04,