1. Introduction
Section 2.9 of the MPLS architecture [2] defines a label distribution protocol as a set of procedures by which one Label Switched Router (LSR) informs another of the meaning of labels used to forward traffic between and through them. The MPLS architecture does not assume a single label distribution protocol. This document is a specification of extensions to RSVP for establishing label switched paths (LSPs) in MPLS networks. Several of the new features described in this document were motivated by the requirements for traffic engineering over MPLS (see [3]). In particular, the extended RSVP protocol supports the instantiation of explicitly routed LSPs, with or without resource reservations. It also supports smooth rerouting of LSPs, preemption, and loop detection. The LSPs created with RSVP can be used to carry the "Traffic Trunks" described in [3]. The LSP which carries a traffic trunk and a traffic trunk are distinct though closely related concepts. For example, two LSPs between the same source and destination could be load shared to carry a single traffic trunk. Conversely several traffic trunks could be carried in the same LSP if, for instance, the LSP were capable of carrying several service classes. The applicability of these extensions is discussed further in [10]. Since the traffic that flows along a label-switched path is defined by the label applied at the ingress node of the LSP, these paths can be treated as tunnels, tunneling below normal IP routing and filtering mechanisms. When an LSP is used in this way we refer to it as an LSP tunnel. LSP tunnels allow the implementation of a variety of policies related to network performance optimization. For example, LSP tunnels can be automatically or manually routed away from network failures, congestion, and bottlenecks. Furthermore, multiple parallel LSP tunnels can be established between two nodes, and traffic between the two nodes can be mapped onto the LSP tunnels according to local policy. Although traffic engineering (that is, performance optimization of operational networks) is expected to be an important application of this specification, the extended RSVP protocol can be used in a much wider context. The purpose of this document is to describe the use of RSVP to establish LSP tunnels. The intent is to fully describe all the objects, packet formats, and procedures required to realize interoperable implementations. A few new objects are also defined that enhance management and diagnostics of LSP tunnels. The document also describes a means of rapid node failure detection via a new HELLO message. All objects and messages described in this specification are optional with respect to RSVP. This document discusses what happens when an object described here is not supported by a node. Throughout this document, the discussion will be restricted to unicast label switched paths. Multicast LSPs are left for further study.
1.1. Background
Hosts and routers that support both RSVP [1] and Multi-Protocol Label Switching [2] can associate labels with RSVP flows. When MPLS and RSVP are combined, the definition of a flow can be made more flexible. Once a label switched path (LSP) is established, the traffic through the path is defined by the label applied at the ingress node of the LSP. The mapping of label to traffic can be accomplished using a number of different criteria. The set of packets that are assigned the same label value by a specific node are said to belong to the same forwarding equivalence class (FEC) (see [2]), and effectively define the "RSVP flow." When traffic is mapped onto a label-switched path in this way, we call the LSP an "LSP Tunnel". When labels are associated with traffic flows, it becomes possible for a router to identify the appropriate reservation state for a packet based on the packet's label value. The signaling protocol model uses downstream-on-demand label distribution. A request to bind labels to a specific LSP tunnel is initiated by an ingress node through the RSVP Path message. For this purpose, the RSVP Path message is augmented with a LABEL_REQUEST object. Labels are allocated downstream and distributed (propagated upstream) by means of the RSVP Resv message. For this purpose, the RSVP Resv message is extended with a special LABEL object. The procedures for label allocation, distribution, binding, and stacking are described in subsequent sections of this document. The signaling protocol model also supports explicit routing capability. This is accomplished by incorporating a simple EXPLICIT_ROUTE object into RSVP Path messages. The EXPLICIT_ROUTE object encapsulates a concatenation of hops which constitutes the explicitly routed path. Using this object, the paths taken by label-switched RSVP-MPLS flows can be pre-determined, independent of conventional IP routing. The explicitly routed path can be administratively specified, or automatically computed by a suitable entity based on QoS and policy requirements, taking into consideration the prevailing network state. In general, path computation can be control-driven or data-driven. The mechanisms, processes, and algorithms used to compute explicitly routed paths are beyond the scope of this specification. One useful application of explicit routing is traffic engineering. Using explicitly routed LSPs, a node at the ingress edge of an MPLS domain can control the path through which traffic traverses from itself, through the MPLS network, to an egress node. Explicit routing can be used to optimize the utilization of network resources and enhance traffic oriented performance characteristics. The concept of explicitly routed label switched paths can be generalized through the notion of abstract nodes. An abstract node is a group of nodes whose internal topology is opaque to the ingress node of the LSP. An abstract node is said to be simple if it contains only one physical node. Using this concept of abstraction, an explicitly routed LSP can be specified as a sequence of IP prefixes or a sequence of Autonomous Systems. The signaling protocol model supports the specification of an explicit path as a sequence of strict and loose routes. The combination of abstract nodes, and strict and loose routes significantly enhances the flexibility of path definitions. An advantage of using RSVP to establish LSP tunnels is that it enables the allocation of resources along the path. For example, bandwidth can be allocated to an LSP tunnel using standard RSVP reservations and Integrated Services service classes [4]. While resource reservations are useful, they are not mandatory. Indeed, an LSP can be instantiated without any resource reservations whatsoever. Such LSPs without resource reservations can be used, for example, to carry best effort traffic. They can also be used in many other contexts, including implementation of fall-back and recovery policies under fault conditions, and so forth.
1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC2119 [6]. The reader is assumed to be familiar with the terminology in [1], [2] and [3]. Abstract Node A group of nodes whose internal topology is opaque to the ingress node of the LSP. An abstract node is said to be simple if it contains only one physical node. Explicitly Routed LSP An LSP whose path is established by a means other than normal IP routing. Label Switched Path The path created by the concatenation of one or more label switched hops, allowing a packet to be forwarded by swapping labels from an MPLS node to another MPLS node. For a more precise definition see [2]. LSP A Label Switched Path LSP Tunnel An LSP which is used to tunnel below normal IP routing and/or filtering mechanisms. Traffic Engineered Tunnel (TE Tunnel) A set of one or more LSP Tunnels which carries a traffic trunk. Traffic Trunk A set of flows aggregated by their service class and then placed on an LSP or set of LSPs called a traffic engineered tunnel. For further discussion see [3].