## 1 Introduction  
Optical burst switching (OBS) [1, 2] is considered as an efficient switching candidate for the core of IP over optical networks. OBS avoids the inefficient resource utilization of optical circuit switching (OCS) and the requirements of buffers, optical logic processing and synchronization problems of optical packet switching (OPS). OBS is widely presented as combining the merits of both OCS and OPS while avoiding their respective shortcomings. In OBS the basic transport unit is the burst, an aggregate message that can be considered as an optical \"super packet\" containing multiple IP packets going to the same egress node and (if used) classified by some quality of service (QoS) criteria. Bursts are assembled at the ingress nodes and their transmission is preceded by dedicated setup messages, transmitted on a dedicated control channel with the purpose of reserving bandwidth along the path for the upcoming data bursts. Based on the information carried by each setup message, the intermediate nodes reserve switching resources along a certain path, providing an optical channel through which a data burst can be transmitted, after an adequate delay, from source to final destination without any optical-electrical-optical (OEO) conversion [2,3]. OBS, like other switching paradigms, does not perform well in overloaded scenarios and can present low reliability since it generally uses one-way reservation protocols, where data bursts are transmitted without confirmation that resources along the path will be successfully reserved to establish the required end-to-end transparent connection. Consequently, whenever the number of simultaneous reservation attempts exceed the number of available resources, some will fail and owing to the lack of sophisticated optical buffers will result in burst loss. Burst loss degrades the global network performance since dropping may lead to significant IP data losses or rescheduling of lost data with significant impact on any end-to-end delay sensitive applications running in the network layers above. Therefore, minimizing burst loss is a key factor for the practical implementation of OBS networks and any related QoS support. Considerable effort has been devoted to the study of different methods to handle contention, including, burst scheduling, optical buffering, burst segmentation, wavelength conversion, and deflection routing [4,5]. These are mainly reactive mechanisms driven by burst contention and requiring extra hardware and/or software components at each core node, significantly increasing their cost and complexity, leading to scalability impairments. A simple and cost efficient solution is to deploy contention mechanisms at the edge nodes. This approach has been followed by using burst assembly mechanisms [6,7], by path selection and wavelength assignment [8,9] or by balancing the traffic load between alternate paths [10, 11, 12]. Path selection mechanisms at the ingress nodes can alleviate contention when compared with typical shortest path (SP) routing approaches. In fact, although successfully used in both circuit switching and packet switching networks, SP routing does not take into consideration the traffic load offered to the network, and it often causes certain links to become congested while other links remain underutilized [10]. This is highly undesirable in bufferless OBS networks, since a few highly congested links can lead to unacceptably high burst loss values for the entire network, corroborating contention avoidance strategies as an important feature in the OBS field. The aim of this paper is to discuss three static routing strategies intended to minimize the global network contention and the overall burst loss as a preliminary foundation for the development of end-to-end QoS policies. The algorithms presented here (already proposed as ongoing research work in [13,14]) are discussed together for the first time in the context of the QoS provisioning technologies.