3G networks are currently facing severe traffic overload problems caused by excessive demands of mobile users. Offloading part of the 3G traffic through other forms of networks, such as Delay Tolerant Networks (DTNs), WiFi hotspots, and Femtocells, is a promising solution. However, since these networks can only provide intermittent and opportunistic connectivity to mobile users, utilizing them for 3G traffic offloading may result in a non-negligible delay. As the delay increases, the users' satisfaction decreases. In this paper, we investigate the tradeoff between the amount of traffic being offloaded and the users' satisfaction. We provide a novel incentive framework to motivate users to leverage their delay tolerance for 3G traffic offloading. To minimize the incentive cost given an offloading target, users with high delay tolerance and large offloading potential should be prioritized for traffic offloading. To effectively capture the dynamic characteristics of users' delay tolerance, our incentive framework is based on reverse auction to let users proactively express their delay tolerance by submitting bids. We further take DTN as a case study to illustrate how to predict the offloading potential of the users by using stochastic analysis. Extensive trace-driven simulations verify the efficiency of our incentive framework for 3G traffic offloading.

The recent popularization of hand-held mobile devices, such as smartphones, enables the inter-connectivity among mobile users without the support of Internet infrastructure. When mobile users move and contact each other opportunistically, they form a Delay Tolerant Network (DTN), which can be exploited to share data among them. Data replication is one of the common techniques for such data sharing. However, the unstable network topology and limited contact duration in DTNs make it difficult to directly apply traditional data replication schemes. Although there are a few existing studies on data replication in DTNs, they generally ignore the contact duration limits. In this paper, we recognize the deficiency of existing data replication schemes which treat the complete data item as the replication unit, and propose to replicate data at the packet level. We analytically formulate the contact duration aware data replication problem and give a centralized solution to better utilize the limited storage buffers and the contact opportunities. We further propose a practical contact Duration Aware Replication Algorithm (DARA) which operates in a fully distributed manner and reduces the computational complexity. Extensive simulations on both synthetic and realistic traces show that our distributed scheme achieves close-to-optimal performance, and outperforms other existing replication schemes.

Data access is an important issue in Delay Tolerant Networks (DTNs), and a common technique to improve the performance of data access is cooperative caching. However, due to the unpredictable node mobility in DTNs, traditional caching schemes cannot be directly applied. In this paper, we propose DAC, a novel caching protocol adaptive to the challenging environment of DTNs. Specifically, we exploit the social community structure to combat the unstable network topology in DTNs. We propose a new centrality metric to evaluate the caching capability of each node within a community, and solutions based on this metric are proposed to determine where to cache. More importantly, we consider the impact of the contact duration limitation on cooperative caching, which has been ignored by the existing works. We prove that the marginal caching benefit that a node can provide diminishes when more data is cached. We derive an adaptive caching bound for each mobile node according to its specific contact patterns with others, to limit the amount of data it caches. In this way, both the storage space and the contact opportunities are better utilized. To mitigate the coupon collector's problem, network coding techniques are used to further improve the caching efficiency. Extensive trace-driven simulations show that our cooperative caching protocol can significantly improve the performance of data access in DTNs.

Vehicular Ad hoc NETworks (VANETs), which provide vehicles with an easy access to exchange the up-to-date traffic status and various kinds of data, have become a promising application of mobile ad-hoc networks. In the life-critical VANETs, security issues are considered as a focal topic. One challenging problem among these issues is the insider misbehavior since it bypasses the traditional security mechanisms such as authentication. The existing works focusing on this problem do not take the privacy issue into account. Their presented solutions are paralyzed in the anonymous VANETs, where the drivers' real identity is protected. In this paper, we propose SLEP and PRP, two novel protocols to efficiently remove the misbehaving insiders from the anonymous VANETs. Through analysis and extensive simulations, we demonstrate that these two protocols can achieve high reaction speed and accuracy for both the local eviction and the permanent revocation to the misbehaving vehicles, at an acceptable cost.

The session initiation protocol (SIP) is one popular signaling protocol used in many collaborative applications like VoIP, instant messaging and presence. In this paper, we evaluate one well-known SIP proxy server (i.e. OpenSER) on two multi-core platforms: SUN Niagara and Intel Clovertown, which are installed with Solaris OS and Linux OS respectively. Through the evaluation, we identify three factors that determine the performance scalability of OpenSER server. One is inside the OSes: overhead from the coarse-grained locks used in the UDP socket layer. Others are specific to the multi-process programming model: 1. overhead caused by passing socket descriptors among processes; 2. overhead brought by sharing transaction objects among processes. To remedy these problems, we propose several incremental optimizations, including out-of-box dispatcher, light-weight connection dispatcher and dataset partition, and achieve significant improvements: for UDP and TCP transport, on SUN Niagara, speedup (ideal is 8) are improved from 1.5 to 5.8 and from 2.2 to 6.2, respectively; on Intel Clovertown, speedup (ideal is 8) are improved from 1.2 to 3.1 and from 2.6 to 4.8, respectively.

Session Initiation Protocol (SIP) is one building block for IP Multimedia Subsystem (IMS) and Next Generation Network (NGN), and is becoming ubiquitous in Internet telephony services. Therefore, the performance issue of SIP servers is becoming crucial. In this paper, SIP stack processing is identified to be CPU-bound in nature and dominates the system overhead for common SIP server scenarios. These facts provide motivations for the idea of SIP offload. We also find that current protocol offload models have not considered program parallelism and can not analyze throughput and latency in together, which makes them difficult to apply to SIP offload. Therefore, a new model is proposed based on 7 ratios capturing fundamental properties of the protocol stack and offload scheme. The model can be applied in prediction of performance gains, selection of parameters for adaptive offloading schemes, and estimation of software or hardware characteristics using observed performance results.

Based on a new representation of GF(2/sup n/), we present two multipliers for all irreducible trinomials. Space complexities of the multipliers match the best results. The time complexity of one multiplier is T/sub A/ + (1 + [log/sub 2/ n])T/sub X/ for all irreducible trinomials, where T/sub A/ and T/sub X/ are the delay of one 2-input AND and XOR gates, respectively.