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Transmission Control Protocol (TCP) is still used by vast majority of Internet applications. However, the huge increase in bandwidth availability and consumption during the last decade has stimulated the evolution of TCP and introduction of new versions that are more suited for high speed networks. Many factors can influence the performance of TCP protocol, starting from scarcity of network resources, through client or server misconfiguration, to internal limitations of applications. Proper identification of the TCP performance bottlenecks is therefore an important challenge for network operators. In the paper we proposed the methodology for finding root causes of througput degradation in TCP connections based on passive measurements. This methodology was verified by experiments conducted in a live network with 4G wireless Internet access.

This paper deals with optimal load balancing in telecommunication networks based on multiple spanning tree routing. This is the case in switched Ethernet networks where the operator configures different routing spanning trees and assigns each demand VLAN to one of the spanning trees. We consider modeling and solving three load balancing objectives: (i) minimization of the average link load with a guaranteed optimal worst case link load, (ii) minimization of the worst case link load with a guaranteed optimal average link load and (iii) the min-max optimization of link loads. We also propose heuristic techniques to compute both feasible solutions and lower bounds for the addressed optimization problems. Finally, we assess both the efficiency and the efficacy of the different solution techniques and compare the quality of each problem solution taking into account the optimization criteria of the other problems.

The post-failure convergence of the shortest path routing (SPR) protocols used in IP networks can be too slow to meet the restrictive requirements (i.e., maximum allowable delay, jitter, etc.) of the multimedia services and therefore new restoration mechanisms combined with IP routing are of interest. The paper addresses optimization of three potential rerouting mechanisms based on the IP fast reroute mechanism proposed by Shand and Bryant [1]. The first mechanism takes advantage of equal-cost multiple (shortest) paths (ECMP) where two or more ECMP paths outgoing from one router can be used to protect one another in the IP fast reroute mechanism. Due to a limited number of the ECMP paths, the ECMP protection cannot be used as a stand-alone rerouting mechanism which assures protection against all link failures. Therefore, two other mechanisms, called loop-free alternate (LFA) and multi-hop repair path (MHRP) are considered. The LFA protection consists in determining an alternative next-hop address used in the case of a link failure. MHRP is a generalization of LFA which uses multi-hop tunnels to redirect packets from the failing link to a router that is able to send them to the destination based on a shortest path based forwarding. For each of the mechanisms we formulate an appropriate optimization problem as a mixed integer program (MIP). Moreover, we consider a combined approach where protection is assured through ECMP paths, LFA next-hop addresses, or MHRP paths. Thanks to the variety of protection mechanisms, the IP fast reroute technique is able to provide protection for any single link failure. The associated optimization problem (consisting in a simultaneous optimization of a weight system, LFA alternative next-hop addresses and MHRP paths) is difficult and is thus approached with a heuristic method. In our numerical experiments we evaluate effectiveness of this method. (Less)

In this paper we address link dimensioning and routing problems related to the area of resilient network design. We present two network design problems that assume different flow restoration schemes used to cope with network failures. In both cases we allow bifurcation of traffic flows in the normal (failure-less) network state. In the case of a failure, we assume that affected primary flows (i.e., flows applied in the normal state) are restored using assigned protection paths and that the primary flows are restored (also in a bifurcated manner) in a separate pool of (protection) capacity, distinct from the basic capacity used in the normal network operation state. The two presented models differ in the way protection paths are used to protect primary flows against different failure states. The first model, called state-independent flow restoration, assumes that once the backup path is assigned it must be used in every state in which the protected primary path fails. The second model allows different protection paths to be used in different network failure states and is called state-dependent flow restoration. The considered problems are formulated as linear programming (LP) problems using link-path (L-P) notation of multi-commodity flow network optimization. As the L-P notation is useful only when an effective column generation scheme is known, we discuss the applicability of this method on the basis of the theory of duality of LP. The paper presents and compares three different approaches and evaluates their usefulness for solving problem instances of practical size.

Intra-domain routing in IP networks is based on the shortest path principle by assigning administrative weights (costs) to links. The resulting least-cost paths determine routes between pairs of routers. If several such equal-cost paths exist between a pair of routers, it may not be clear which of them is actually used to route traffic. This makes it difficult to predict the network traffic flow distribution. Therefore, the selected link costs should assure uniqueness of the shortest paths. On top of that, the link costs can be optimized with respect to some traffic objective. The resulting optimization problem, referred to as SSPP, turns out to be NP-hard. SSPP can be formulated as a mixed-integer programming problem and, as such, solved with branch-and- bound (B&B). In this paper, we consider three methods for SSPP. Two of them are exact methods based on B&B, namely branch- and-cut and constraint programming. Since the exact solutions of SSPP may require excessive computation time and may not always be effective when applied to practical networks, we also study a fast heuristic method. Finally, in a numerical study, we compare the effectiveness of the three approaches.

In this paper we address intra-domain routing optimization issues in Internet networks related to a combination of open shortest path first (OSPF) and multi protocol label switching (MPLS) routing protocols. Since MPLS can be combined with any routing protocol there has appeared an idea of coupling IP streams routed via the OSPF protocol and MPLS label switched paths (LSPs) in the same network. Such combined routing is potentially more flexible and efficient than OSPF itself from the traffic engineering (TE) viewpoint. In the paper we present the exact approach to the considered hybrid routing design using a strict mathematical formulation of the underlying optimization problem, and also a heuristic approach decomposing the problem into two subproblems solved with evolutionary algorithm and simulated allocation. The effectiveness of both approaches is illustrated by means of a numerical study.

In the paper we address traffic engineering problems related to optimization of routing in IP networks applying destination-based shortest path routing (SPR) of the OSPF type. An SPR routing pattern is determined by a system of (administrative) weights defined over the set of IP links: the routes for IP forwarding are determined as the shortest paths computed locally at the nodes using the current link weights. When the shortest path from a certain node (node v, say) to a particular destination (destination t, say) is not unique, the traffic routed from node v to destination t is split equally among all links outgoing from v that belong to the shortest paths to destination t, i.e., according to the Equal Cost Multiple-Paths (ECMP) rule. The basic problem considered in this paper consists in finding a resilient link weight system generating a routing scheme that satisfies given traffic demands and does not lead to link overloads both in the normal network state of operation and in all considered failure states when certain IP links are failed. We assume that if a failure occurs then the weight system is modified by assigning infinite weights to the failed links, and not altering the weights of the remaining, operating links. We consider the traffic engineering goal related to minimization, over all failure states, of the maximal link overload. We formulate the considered problem as a mixed integer programme (MIP) and propose a heuristic algorithm based on the tabu search metaheuristic. The efficiency of the proposed weight optimization method is illustrated by means of a numerical study.

In this paper we address the problem of routing optimization in IP networks. We assume that traffic is routed along the shortest paths computed with respect to administrative link metrics. Metrics are distributed in a network by open shortest path first (OSPF) or a similar routing protocol. If it happens that the shortest path is not unique then equal-cost multi-path (ECMP) load balancing principle is applied. It means that the demand traffic destined to specific node is split among all the shortest paths to that node. The problem considered here is to determine the shortest-path routing pattern satisfying traffic demands, and to find appropriate link metrics while link capacities are not exceeded. Besides that many traffic engineering criterias can be used as objective function of the problem, we assume that the residual capacity volume is maximized. In this paper we formulate the problem as a mixed integer programme (MIP) and propose some combinatorial separation cuts for the problem and give an effective method for deriving such cuts.

Titanium boride is used here for contact formation in scaled down devices. Properties of deposited films show strong dependence on electron beam deposition and annealing conditions. Thin TiB/sub 2/ films show stable composition in high temperature RTP. Ohmic contacts are formed on p type silicon both by using as deposited and annealed layers. On n-type Si, large thermal budget processes result in dopant outdiffusion and formation of junctions significantly more shallow than in implantation. Recrystallization of TiB/sub 2/, observed after high temperature annealing, depends on films deposition conditions and interface perfection. Material characterization techniques including RBS, XPS, SIMS, and TEM are used to explain results of electrical measurement of test structures.

Future generations of CMOS integrated circuits (IC) face challenging requirements to lower parasitic resistances associated with the source and drain (SID) regions. New solutions are necessary as the current technology cannot provide parameters of ultra shallow junctions and their contacts to would allow avoiding device degradation. We studied properties of titanium boride layers and process integration issues for potential application of such films in the SID engineering focused on both junction and contact formation. Controlled dopant outdiffusion, obtained by RTP conditions, resulted in either diode fabrication on n-type silicon or ohmic contacts on p-type substrates. Complementary analytical techniques including RBS, XPS, SIMS, and TEM as well as various electrical measurements on test structnres allowed for identification of the most difficult process constrains.