Generalized Multiprotocol Label Switching: An Overview of Routing and Management Enhancements <s> INTRODUCTION <s> This document specifies the architecture for Multiprotocol Label ::: Switching (MPLS). [STANDARDS-TRACK] <s> BIB001 </s> Generalized Multiprotocol Label Switching: An Overview of Routing and Management Enhancements <s> INTRODUCTION <s> This article describes an approach to the design of control planes for optical crossconnects which leverages existing control plane techniques developed for MPLS traffic engineering. The proposed approach combines recent advances in MPLS traffic engineering control plane constructs with OXC technology to provide a framework for real-time provisioning of optical channels, foster development and deployment of a new class of OXCs, and allow the use of uniform semantics for network management and operations control in hybrid networks consisting of OXCs and label switching routers. The proposed approach is particularly advantageous for OXCs intended for data-centric optical internetworking systems. <s> BIB002
Today's synchronous optical network/synchronous digital hierarchy (SONET/SDH) transport network infrastructure provides a guaranteed level of performance and reliability for voice calls and leased lines, the predominant traffic types prior to 1995. Since 1995, however, there has been a dramatic increase in data traffic, driven primarily by the explosive growth of the Internet as well as the proliferation of virtual private networks (VPNs). Just before the turn of the millennium, the amount of data traffic worldwide surpassed voice traffic and will continue to outpace voice for years to come. At the same time, there is increasingly strong demand from customers to keep the cost of networking down. The need to carry more traffic, combined with the need to minimize the cost of carrying this traffic, results in a situation where service providers (SPs) need solutions that enable them to carry a large volume of traffic in the most costefficient manner. Delivering solutions that enable SPs to carry a large volume of traffic in a cost-efficient manner has proven to be a challenge within the current data network architecture. Today's data networks typically have four layers: IP for carrying applications and services, asynchronous transfer mode (ATM) for traffic engineering, SONET/SDH for transport, and dense wavelength-division multiplexing (DWDM) for capacity ( Fig. 1 ). This architecture has been slow to scale for very large volumes of traffic, and at the same time fairly cost-ineffective. Multilayer architectures typically suffer from the lowest common denominator effect where any one layer can limit the scalability of the entire network, as well as add to the cost of the entire network. Effective transport should optimize the cost of data multiplexing as well as data switching over a wide range of traffic volumes. DWDM is a cost-efficient multiplexing technique that offers significant technical advantages. DWDM increases the bandwidth-carrying capacity of a single optical fiber by effectively creating multiple virtual fibers, each carrying multigigabits of traffic per second, on a single fiber. This provides multifold increase in bandwidth while leveraging the existing fiber infrastructure. Likewise, optical cross-connects (OXCs) are likely to emerge as the preferred option for switching multigigabit or even terabit data streams, since electronic per-packet processing is avoided. It is widely expected that the predominant traffic carried over data networks will be IPbased, which suggests that the development of fast router technologies is essential for the aggregation of slower data streams into streams suitable for OXCs. Likewise, IP packet-based statistical multiplexing is likely to be the predominant multiplexing technology for data streams smaller than those suitable for DWDM. As the capabilities of both routers and OXCs grow rapidly, the high data rates of optical transport suggest the distinct possibility of bypassing the SONET/SDH and ATM layers. In order to bypass these layers their necessary functions must move directly to the routers, OXCs, and DWDMs. In the end, this results in a simpler, more cost-efficient network that will transport a wide range of data streams and very large volumes of traffic. Operationally, this new optical architecture can be viewed from two vantage points, best described as an overlay model and a peer model. The overlay model (Fig. 2a) hides details of the internal network, resulting in two separate control planes with minimal interaction between them. One control plane operates within the core optical network, and the other between the core and the surrounding edge devices (called the user-network interface, UNI). The edge devices support lightpaths that are either dynamically signaled across the core optical network or statically provisioned without seeing inside the core's topology. This is very similar to current combined IP/ATM networks. The overlay model imposes administrative control boundaries between the core and edge by effectively hiding the contents of the core. In the peer model (Fig. 2b) , a single instance of the control plane spans an administrative domain consisting of the core optical network and the surrounding edge devices. This allows the SP edge devices to see the topology of the core network. Although an O(N 2 ) mesh of point-to-point connections is still required if full connectivity between the edge devices is needed, it is used exclusively for the purpose of data forwarding. As far as the routing protocols are concerned, each edge device is adjacent to the photonic switch to which it is attached, rather than to the other edge devices. With O(N) routing adjacencies a full mesh of O(N 2 ) forwarding paths can be supported. This allows the routing protocols to scale to a much larger network. Another approach is a hybrid model that combines both the peer and overlay models. Some edge devices serve as peers to the core network and share the same instance of a common control plane with the core network. Other edge devices could have their own control plane (or a separate instance of the control plane used by the core network), and interface with the core network through the UNI. This represents the highly desirable solution offering carriers and SPs substantial flexibility to deploy the most cost-effective model for their needs, be it peer, overlay, or some hybrid of these models. Functionally, the peer model forms a superset of the overlay model. That is, the set of functions required to support the overlay model is a subset of the set of functions required to support the peer model. An overlay model can be derived from a peer model by administratively disabling topology sharing while preserving the connection signaling functions. This observation suggests that rather than having one set of protocols to support the overlay model and another to support the peer model, one suite of control plane protocols with enough flexibility to support both models would be the most efficient approach. Sustaining the advantages of flexible control models are the challenges of rapid provisioning, routing, monitoring, and efficient restoration. Reliability requirements make these challenges of paramount importance to photonic networks. While several vendors are developing proprietary routing and signaling protocols to enable automatic provisioning, such implementations are unlikely to interoperate in multivendor deployments. Providers of "best-of-breed" optical solutions require the integration of photonic switches into a heterogeneous optical network, which will eventually combine next-generation equipment with legacy equipment. A common standardized control plane must be used to communicate between the various elements. Fortunately, such a standard is well known in the industry. Over the last few years, IP routing has evolved to include new functionality under the umbrella of multiprotocol label switching (MPLS) BIB001 , and recent work has been done on extending MPLS as a control plane that can be used not merely with routers, but also with legacy equipment (e.g., SONET, ADMs) and newer devices like OXCs BIB002 . These efforts offer the necessary standardized common control plane, an essential component in the evolution of open and interoperable optical networks. First, a common control plane simplifies operations and management, which reduces the cost of operations. Second, a common control plane provides a wide range of deployment scenarios ranging from overlay to peer, where the overlay model is realized by using just a subset of the functionality provided by the peer model. A common control plane allows the choice of peer or overlay (or a combination of both) to be driven by business considerations, rather than constrained by technology. At the same time, building the common control plane from proven signaling and routing avoids "reinventing the wheel" for protocol development, thereby minimizing risk while reducing time to market. Some modifications and additions are required to the MPLS routing and signaling protocols to adapt to the peculiarities of photonic switches. These are being standardized by the Internet Engineering Task Force (IETF) under the umbrella of generalized MPLS, which can be summarized as follows: designed to address issues related to link management in optical networks using photonic switches  • Enhancements to the Open Shortest Path First/Intermediate System to Intermediate System (OSPF/IS-IS) routing protocols to advertise availability of optical resources in the network (e.g., generalized representation of various link types, bandwidth on wavelengths, link protection type, fiber identifiers)   • Enhancements to the Resource Reservation Protocol (RSVP)/Constraint-Based Routing Label-Distributed Protocol (CR-LDP) signaling protocols for traffic engineering purposes that allow a label-switched path (LSP) to be explicitly specified across the optical core  • Scalability enhancements such as hierarchical LSP formation, link bundling, and unnumbered links This article deals with enhancements made to the routing and link management protocols in support of generalized MPLS (GMPLS). The reader is referred to  for the enhancements to the signaling protocols.