ON standard, such a rerouting service is performed on a per-routing domain basis, i.e., the rerouting operation takes place between the edges of the routing domain and is entirely contained within it. This assumption does not exclude requests for an end-to-end rerouting service. Naturally, if restoration mechanism is foreseen to increase the network availability, the spare capacity has to be suitably distributed in the network SO to permit restoration after the desired class of failures. Hard and soft rerouting services can be distinguished. The first one is a failure recovery mechanism and is always triggered by a failure event. Soft rerouting is associated with such operations as path optimization, network maintenance, or planned engineering works, and is usually activated by the management plane. In soft rerouting the original connection is removed after creation of the rerouting con- nection, while in hard rerouting, the original connection segment is released prior to cre- ation of a new alternative segment. 8.3 GMPLS Architecture If ASON is a reference architecture that is intended to assure functionalities standard- ization and interoperability to different control and management plane implementations, GMPLS is the IETF-driven effort to standardize a specific control plane architecture that extends the concepts of MPLS to a multilayer network including TDM and WDM layers implementing physical circuit switching. Within the GMPLS architecture, different GMPLS models are defined to comply with different levels of GMPLS implementation and with gradual control plane introduction in the carrier networks. The first model, intended mainly for the first implementation of the control plane, is the so-called overlay model. In this model the packet layer, implementing Telecommunication Networks MPLS as control plane and constituted by LSRs, is considered as a client of the underly- ing transport network that can be either a TDM network (like SDH or SONET) or a WDM network. GMPLS is used in the overlay model as a contr