Constraint-based routing gradually becomes an essential enabling mechanism for a variety of emerging network services such as virtual private networking and QoS support. A number of previous works have recognized its significance and investigated many aspects of the operation of constraint-based routing and in particular its variant concerned with determining paths for requests with specific QoS requirements, known as QoS routing. In this work we build on previous results on the cost of QoS routing and investigate the performance/cost trade-offs involved in the operation of a representative QoS routing architecture, elaborate on the constituents of this cost, and identify the main methods for containing the cost that QoS routing incurs on routers. Our results show that the cost of QoS routing is not excessive and that there indeed exist operational configurations, which can achieve reasonable performance gains with only a minimal increase in processing cost when compared to conventional best-effort routing.

Network survivability, especially in the context of optical networks, has been a hotbed of research for a long time. The steady growth in the use of the Internet for business-critical applications is the primary driver behind this increased interest in network protection and restoration. The turbulent political environment following the events of September 11, 2001 has also helped drive the issue of disaster recovery to the mainstream. Service providers deploy some measure of survivability in their networks, because there is the issue of customer satisfaction and expectation. Also, in today's networked world network survivability is important because of the revenue losses to businesses and consequent negative publicity from outages. The first applications of network survivability were developed specifically for network operations in telephony networks. In designing a survivable networks, especiaily when multiple layers in the network can provide restoration services, it is not desirable for routers to detect and react to a failure if the optical layer is also attempting to survive the failure at the same time. Situations like this are the driving force behind the study of multilayer survivability.