Presence is a service that allows a user to be informed about the reachability, availability, and willingness of communication of another user. Presence service has become a key enabler for many popular applications such as instant messaging and push-to-talk. Traffic to a presence server is resulted by user behaviors such as login/logout, online status modification, and automatic status refresh by client application. Mathematical models are proposed in this paper to study the user behaviors associated with a presence server during the time of a day such that traffic characteristics to a presence server can be inferred. The correctness of our estimation and analysis are verified by extensive simulations using Matlab. This study on the relationship between user behaviors and traffic to a presence server can help network operators to plan network capacity, optimize server performance and detect traffic anomaly.

Presence is a service that allows a user to be informed about the reachability, availability, and willingness to communicate of another user. Presence service has become a key enabler for many popular applications such as instant messaging and push-to-talk. However a study shows that presence service can account for 50% or more of the total signalling traffic the IMS core network handles (C. Urrutia-Valds et al., 2006). This is quite a burden for a real IMS network and need to be tackled. In this paper, a thorough analysis of the traffic load distribution reveals that notify messages account for the largest portion of the traffic load on a presence server. We propose a mathematical model of a queueing system with batch arrival and controlled vacation to describe the processing of notify messages inside a presence server. The model is then used to calculate the optimal value of the timer that controls the notify queue, which is an essential parameter to adjust in network sizing upon deploying a presence server. An approximation of the model is also provided so that the performance of a presence server can be tuned in an online manner based on real traffic statistics.

Handover between various networks or cells of the same network is an important problem faced by mobile nodes. With so many handover algorithms available, how to evaluate and select them become critical. A mathematical model for wrong decision probability and handover probability is given in this paper such that they can be used as criteria to evaluate handover algorithms. Explicit expressions for these two parameters are vigorously studied via a general bandwidth based handover algorithm. A WDP predication based handover algorithm is also proposed to improve the handover performance.

In mobile ad hoc networks(MANET), the limited wireless spectrum, distributed multiple access control, low complexity, and high mobility together impose significant challenges for MAC protocol design to provide reliable wireless communications with high data rates. In contention based MAC protocols, such as 802.11 MAC, each multi-hop data flow encounters contentions not only from the transmissions of itself, but also from other flows that bypassing the neighborhood. This paper presents a Dynamic Token Ring based MAC protocol(DRP) for mobile ad hoc networks, which aims to solve the intra-flow and inter-flow contention problems in MAC layer. It is called dynamic, since the token ring in a cluster can be a low-priority ring or a high priority ring. The node can join high priority ring dynamically according to transmission status. Our simulation results show that DRP protocol has superior performance in improving data delivery efficiency and avoiding collisions. It is a good solution for inter-flow/intra-flow contention problems as well as hidden/exposed terminal problems.

For network fault management, we present a new technique that is based on on-line monitoring of networks with link state routing protocols, such as OSPF (open shortest path first) and integrated IS-IS. Our approach employs an agent that monitors the on-line information of the network link state database, analyzes the events generated by network faults for event correlation, and detects and localizes the faults. We apply our method to a real network topology with various types of network faults. Experimental results show that our approach can detect and localize the faults in a timely manner, yet without disrupting normal network operations.

This work has been motivated by the need to test interoperability of systems carrying voice calls over the IP network. The voice over IP (VoIP) systems must be integrated and interoperate with the existing public switched telephone network (PSTN) before they are widely adopted. Standards have been developed to address this problem, but unfortunately different standards bodies and commercial consortiums have defined different standards. Furthermore, the prevailing VoIP standard such as H.323 is incomplete, complex, and presents the implementers with "vendors latitudes". As a result, there is no guarantee that the integrated VoIP systems would interoperate properly even if the implementations are all H.323-compliant. Thus interoperability testing has become indispensable. We want to test all the system interoperations by exercising all the required patterns of "interoperating behaviors". On the other hand, test execution in real environment is expensive, and we want to minimize the number of tests while maintaining the coverage. We present a general method for automatic generation of test cases, which cover all the required system interoperations and contain a minimal number of tests. We study data structures and efficient test generation algorithms, which take time proportional to the total test case size. Finally, we report experimental results on VoIP systems.

BGP, a path vector protocol, is the de facto inter-domain routing protocol. However, slow convergence problem, the bigotry of path vector protocol, has demonstrated a significant impact on the performance of BGP. In this paper, we propose an enhancement to path vector protocol to alleviate the impact of slow convergence process. We compare the convergence time of the legacy path vector protocol and the improved one for complete AS graph, meanwhile the upper and lower bound of convergence time for any AS graph are given for the improved protocol. Simulation results reveal that the improved path vector protocol has a much better performance than the original one.

Passive testing is a process of detecting faults in a system under test by passively observing its input/output behaviors only without interrupting its normal operations, and proves to be a promising technique for network fault management. We study passive testing of data portions of network protocols and present two algorithms, using an event-driven extended finite state machine model. Experimental results on the Internet routing protocol OSPF (open shortest path first) are reported.

Voice over IP (VoIP) systems have to be integrated and interoperate with the existing telephone network before they become widely adopted. However, different standards have been proposed and the prevailing standard (H.323) is incomplete, complex, and provides the implementers with "vendors latitudes". As a result, there is no guarantee that even H.323-compliant systems would interoperate properly. Interoperability testing has become indispensable. We present techniques for automatic generation of a minimal set of test cases, covering all the required system interoperations. Our algorithms have been implemented and incorporated in a software tool-ITIS. We report experimental results on VoIP systems interoperability testing, using ITIS.

SOCRATES is a software system for testing correctness of implementations of IP routing protocols such as RIP, OSPF and BGP. It uses a probabilistic algorithm to automatically construct random network topologies. For each generated network topology, it checks the correctness of routing table calculation and the IP packet forwarding behavior. For OSPF, it also checks the consistency between network topologies and the link-state databases of router under test. For BGP, it further checks the BGP update redistribution. Being different than commercial testing tools, which select their test cases in an ad-hoc manner, SOCRATES chooses test cases with a guaranteed fault coverage.

In this paper we present a security framework for Bond, a message-oriented distributed object middleware for network computing. Bond Security Framework, BSF, allows developers to exercise performance-security tradeoffs and use the security model best suited for a specific application and for a given environment. BSF consists of an extensible core and a set of well-defined security interfaces. Any Bond object can become a secure object when extended with a dynamic property called bondSecurityContext.

This paper presents a new method and its implementation called TUGEN. This method integrates the features of data-flow and control-flow test. TUGEN is based on a formal model called EBE that specifies only the external behavior of a protocol. We introduce the unique input/output (UIO) sequence algorithm to determine the state and the concept of adaptive input/output (AIO) tree to solve the nondeterminism of a protocol. We also use the protocol constraint set to control the complexity of exploring space. Furthermore, we can optimally generate the test suite by simulant testing.

The formal test execution method is an important research field in formal protocol conformance testing. We propose a formal test execution approach that is based on the test notation of the TTCN's (tree and tabular combined notation) operational semantics, and describe its execution process by using the input-output transition system (IOTS). We also present a practical design of this formal approach. This formal TTCN-based test execution method is very suitable for the construction of a general protocol test system, and also as a means of automatic test suite verification.