Mikko Pitkänen

Message fragmentation in opportunistic DTNs

Delay-tolerant networking is used for communication in challenged environments such as MANETs, in which links are unstable and end-to-end paths between communicating nodes may not exist. Messages may be significantly larger than packets in IP networks. Large messages lead to longer transfer times rendering it more likely that a link breaks in the middle of a message transfer. This motivates investigating how to support partial message transfers through fragmentation. In this paper, we formulate fragmentation independent of routing algorithms, introduce several fragmentation strategies, and evaluate these by simulations to derive recommendations for using fragmentation in DTNs.

Redundancy and distributed caching in mobile DTNs

Delay tolerant networking (DTN) allows endpoints to exchange information in networks where end-to-end path may not exist at any given time. In this opportunistic model, routing and forwarding functionality in intermediate nodes enables data transfer following the store, carry, and forward paradigm. Thereby, even in sparsely populated settings, node-to-node contacts can be exploited for communications where network infrastructure does not exist or is not viable to use. Beyond plain message forwarding, the increasing amount of storage in modern devices enables nodes to hold messages for an extended period of time. This feature can then be utilized to create an opportunistic cooperative storage. Having established how to leverage DTN routing nodes for distributed content retrieval, caching, and storage in previous work, in this paper, we investigate applying redundancy schemes to improve message delivery and content retrieval.

Adaptive routing in mobile opportunistic networks

In this paper, we study how to adapt the routing according to dynamic network conditions in wireless ad hoc networks. We present a method that dynamically chooses routing agent between ad hoc on-demand distance vector routing protocol (AODV) with TCP (end-to-end transport) and delay-tolerant networking (DTN) routing and bundle protocol (hop-by-hop transport). We use simulations to confirm that DTN routing and the bundle protocol leads to significantly shorter end-to-end delays and higher message delivery ratios than AODV and end-to-end TCP when the wireless node density is low. However, with high node density, DTN routing, especially epidemic routing, suffers from multiple bundle copies and simultaneous transmissions that lead into collisions and retransmissions at the wireless link (MAC) layer. Thus, we propose a simple adaptive scheme that uses only local information to transmit the messages from source to destination using either AODV or DTN routing, depending on current node density, message size, and path length to destination.

DTN-based Content Storage and Retrieval

Delay-tolerant networking (DTN) enables nodes to communicate by means of asynchronous messaging witout the need for an end-to-end path. Suitably designed application protocols may operate in DTNs by minimizing end-to-end interactions and using self-contained messages for communication. The store-carry-and-forward operation and message replication of many DTN routing protocols may yield multiple copies of messages spread across many nodes for an axtended period of time. We leverage these properties for application support in (mobile) intermediate DTN nodes which act as ad-hoc routers. We add explicit application hints to messages that are visible to each node, allowing them, e.g., to cache content, act as distributed storage, or perform application-specific forwarding.

Searching for content in mobile DTNs

Delay-tolerant Networking (DTN) provides a platform for applications in environments where end-to-end paths may be highly unreliable or do not exist at all. In many applications such as distributed wikis or photo sharing, users need to be able to find content even when they do not know an unambiguous identifier. In order do bring these applications to the domain of DTNs, a search scheme is required that works despite the unreliable network conditions. In this paper, we introduce a search scheme that makes no assumptions about the underlying routing protocols and the format of search requests. We evaluate different algorithms for forwarding and terminating search queries, using simulations with different classes of DTN routing protocols for different mobility scenarios.

Opportunistic web access via WLAN hotspots

Mobile phones are becoming commonplace for consuming Internet content and services. However, availability, affordability, and quality of the supposedly ubiquitous cellular network infrastructure may be limited, so that delay-tolerant web access via WLAN hotspots becomes an interesting alternative, even in urban areas. In this paper we explore mobile web access using asynchronous messaging via WLAN hotspots: for nodes directly connected to an access point and nodes relying on others for message forwarding. We investigate different routing and caching approaches using real-world access point locations in Helsinki. We find that a significant number of requests can be satisfied without requiring an always-on infrastructure, provided that users are willing to tolerate some response delay; this allows offloading traffic from the cellular network. We also report on our prototype implementation of mobile DTN-based web browsing.

Erasure Codes for Increasing the Availability of Grid Data Storage

In this paper, we describe the design of a highlyavailable Grid data storage system. Increased availability is ensured by data redundancy and file striping. Redundant data is computed using Reed-Solomon (RS) codes. The level of availability can be chosen for each individual file. Storage overhead is minimal compared to traditional redundancy strategies based on complete replication. Our prototype uses existing Grid data management tools (GridFTP) for communication, and the RS encoding and decoding is embedded in the client software. Performance measurements in wide area network prove the efficiency of our design for high-availability and striped file transfers.

Initalizing Mobile User's Identity from Federated Security Infrastructure

Mobile terminals are increasingly used as a part of our every day information access and management. While being mobile, the users network connectivity may be unavailable for various reasons. At the same time, sophisticated security infrastructures are developed to allow service composition across multiple administrative domains. In this paper we discuss how security mechanisms which require frequent client-server interaction can be adapted to mobile environments to enable operation at the time of disconnections. We illustrate applicability of the proposed solution by enabling an identity service for a commonly used mobile platform.