Network Working Group                                        Kohei Shiomoto (NTT)
  Internet-Draft                              Dimitri Papadimitriou (Alcatel-Lucent)
  Intended Status: Informational                   Jean-Louis Le Roux (France Telecom)
                                                Martin Vigoureux (Alcatel-Lucent)
                                                         Deborah Brungard (AT&T)
  Expires: February 2008                                              August 2007
                   Requirements for GMPLS-based multi-region and
                         multi-layer networks (MRN/MLN)
  Status of this Memo
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     Copyright Notice
     Copyright (C) The IETF Trust (2007).
     Most of the initial efforts to utilize Generalized MPLS (GMPLS) have been
     related to environments hosting devices with a single switching capability. The
     complexity raised by the control of such data planes is similar to that seen in
     classical IP/MPLS networks.
     By extending MPLS to support multiple switching technologies, GMPLS provides a
     comprehensive framework for the control of a multi-layered network of either a
     single switching technology or multiple switching technologies.
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     In GMPLS, a switching technology domain defines a region, and a network of
     multiple switching types is referred to in this document as a Multi-Region
     Network (MRN). When referring in general to a layered network, which may consist
     of either a single or multiple regions, this document uses the term, Multi-Layer
     Network (MLN). This document defines a framework for GMPLS based multi-
     region/multi-layer networks and lists a set of functional requirements.
  Table of Contents
     1. Introduction.....................................................2
     2. Conventions Used in this Document....................................4
     2.1. List of acronyms................................................4
     3. Positioning......................................................5
     3.1. Data Plane Layers and Control Plane Regions..........................5
     3.2. Service layer networks...........................................6
     3.3. Vertical and Horizontal interaction and integration...................6
     4. Key Concepts of GMPLS-Based MLNs and MRNs.............................8
     4.1. Interface Switching Capability....................................8
     4.2. Multiple Interface Switching Capabilities...........................8
     4.2.1. Networks with Multi-Switching-Type-Capable Hybrid Nodes..............9
     4.3. Integrated Traffic Engineering (TE) and Resource Control..............10
     4.3.1. Triggered Signaling...........................................10
     4.3.2. FA-LSPs.....................................................11
     4.3.3. Virtual Network Topology (VNT)..................................11
     5. Requirements....................................................12
     5.1. Handling Single-Switching and Multi-Switching-Type-Capable Nodes.......12
     5.2. Advertisement of the Available Adaptation Resource...................12
     5.3. Scalability...................................................13
     5.4. Stability.....................................................13
     5.5. Disruption Minimization.........................................14
     5.6. LSP Attribute Inheritance........................................14
     5.7. Computing Paths With and Without Nested Signaling....................15
     5.8. LSP Resource Utilization........................................15
     5.8.1. FA-LSP Release and Setup.......................................16
     5.8.2. Virtual TE-Links.............................................16
     5.9. Verification of the LSPs........................................17
     6. Security Considerations...........................................18
     7. IANA Considerations..............................................18
     8. References......................................................18
     8.1. Normative Reference.............................................18
     8.2. Informative References..........................................18
     9. Authors' Addresses...............................................19
     10. Contributors' Addresses..........................................20
     11. Intellectual Property Considerations...............................20
     12. Full Copyright Statement.........................................20
  1. Introduction
     Generalized MPLS (GMPLS) extends MPLS to handle multiple switching technologies:
     packet switching, layer-2 switching, TDM switching, wavelength switching, and
     fiber switching (see [RFC3945]). The Interface Switching Capability (ISC)
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     concept is introduced for these switching technologies and is designated as
     follows: PSC (packet switch capable), L2SC (Layer-2 switch capable), TDM (Time
     Division Multiplex capable), LSC (lambda switch capable), and FSC (fiber switch
     The representation, in a GMPLS control plane, of a switching technology domain
     is referred to as a region [RFC4206]. A switching type describes the ability of
     a node to forward data of a particular data plane technology, and uniquely
     identifies a network region. A layer describes a data plane switching
     granularity level (e.g., VC4, VC-12). A data plane layer is associated with a
     region in the control plane (e.g., VC4 is associated with TDM, MPLS is
     associated with PSC). However, more than one data plane layer can be associated
     with the same region (e.g., both VC4 and VC12 are associated with TDM). Thus, a
     control plane region, identified by its switching type value (e.g., TDM), can be
     sub-divided into smaller granularity component networks based on "data plane
     switching layers". The Interface Switching Capability Descriptor (ISCD)
     [RFC4202], identifying the interface switching capability (ISC), the encoding
     type, and the switching bandwidth granularity, enables the characterization of
     the associated layers.
     In this document, we define a Multi Layer Network (MLN) to be a  TE domain
     comprising multiple data plane switching layers either of the same ISC (e.g.
     TDM) or different ISC (e.g. TDM and PSC) and controlled by a single GMPLS
     control plane instance. We further define a particular case of MLNs. A Multi
     Region Network (MRN) is defined as a TE domain supporting at least two different
     switching technologies (e.g. PSC + TDM) hosted on the same device (referred to
     as multi-switching-type-capable LSRs, see below) and under the control of a
     single GMPLS control plane instance.
     MLNs can be further categorized according to the distribution of the ISCs among
     the LSRs:
     - Each LSR may support just one ISC.
       Such LSRs are known as single-switching-type-capable LSRs.
       The MLN may comprise a set of single-switching-type-capable LSRs
       that support different ISCs.
     - Each LSR may support more than one ISC at the same time.
       Such LSRs are known as multi-switching-type-capable LSRs, and
       can be further classified as either ‘‘simplex’’ or hybrid’’ nodes
       as defined in Section 4.2.
     - The MLN may be constructed from any combination of single-switching-type-
       capable LSRs and multi-switching-type-capable LSRs.
     Since GMPLS provides a comprehensive framework for the control of different
     switching capabilities, a single GMPLS instance controlling the MLN/MRN enables
     rapid service provisioning and efficient traffic engineering across all
     switching capabilities. In such networks, TE Links are consolidated into a
     single Traffic Engineering Database (TED). Since this TED contains the
     information relative to all the different regions and layers existing in the
     network, a path across multiple regions or layers can be computed using this TED.
     Thus optimization of network resources can be achieved across the whole MLN/MRN.
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     Consider, for example, a MRN consisting of packet-switch capable routers and TDM
     cross-connects. Assume that a packet LSP is routed between source and
     destination packet-switch capable routers, and that the LSP can be routed across
     the PSC-region (i.e., utilizing only resources of the packet region topology).
     If the performance objective for the packet LSP is not satisfied, new TE links
     may be created between the packet-switch capable routers across the TDM-region
     (for example, VC-12 links) and the LSP can be routed over those TE links.
     Further, even if the LSP can be successfully established across the PSC-region,
     TDM hierarchical LSPs across the TDM region between the packet-switch capable
     routers may be established and used if doing so is necessary to meet the
     operator's objectives for network resources availability (e.g., link bandwidth,
     or adaptation ports between regions) across the regions. The same considerations
     hold when VC4 LSPs are provisioned to provide extra flexibility for the VC12
     and/or VC11 layers in an MLN.
  1.1 Scope
     This document describes the requirements to support multi-region/multi-layer
     networks. There is no intention to specify solution-specific and/or protocol
     elements in this document. The applicability of existing GMPLS protocols and any
     protocol extensions to the MRN/MLN is addressed in separate documents [MRN-EVAL].
     This document covers the elements of a single GMPLS control plane instance
     controlling multiple layers within a given TE domain. A control plane instance
     can serve one, two or more layers. Other possible approaches such as having
     multiple control plane instances serving disjoint sets of layers are outside the
     scope of this document.
     For such TE domain to interoperate with edge nodes/domains supporting interfaces
     by other SDOs e.g. ITU-T and OIF, an interworking function may be needed.
     Location and specification of this function are outside the scope of this
     document (because interworking aspects are strictly under the responsibility of
     the interworking function.)
     This document assumes that the interconnection of adjacent MRN/MLN TE domains
     makes use of [RFC4726] when their edges also support inter-domain GMPLS RSVP-TE
  2. Conventions Used in this Document
     Although this is not a protocol specification, the key words "MUST", "MUST NOT",
     and "OPTIONAL" are used in this document to highlight requirements, and are to
     be interpreted as described in RFC 2119 [RFC2119].
  2.1.List of acronyms
     MLN: Multi-Layer Network
     MRN: Multi-Region Network
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     ISC: Interface Switching Capability
     ISCD: Interface Switching Capability Descriptor
     PSC: Packet Switching Capable
     L2SC: Layer-2 Switching Capable
     TDM: Time-Division Switch Capable
     LSC: Lambda Switching Capable
     FSC: Fiber Switching Capable
     SRLG: Shared Risk Ling Group
     VNT: Virtual Network Topology
     FA: Forwarding Adjacency
     FA-LSP: Forwarding Adjacency Label Switched Path
     TE: Traffic Engineering
     TED: Traffic Engineering Database
     LSP: Label Switched Path
     LSR: Label Switching Router
  3. Positioning
     A multi-region network (MRN) is always a multi-layer network (MLN) since the
     network devices on region boundaries bring together different ISCs. A MLN,
     however, is not necessarily a MRN since multiple layers could be fully contained
     within a single region. For example, VC12, VC4, and VC4-4c are different layers
     of the TDM region.
  3.1. Data Plane Layers and Control Plane Regions
     A data plane layer is a collection of network resources capable of terminating
     and/or switching data traffic of a particular format [RFC4397]. These resources
     can be used for establishing LSPs for traffic delivery. For example, VC-11 and
     VC4-64c represent two different layers.
     From the control plane viewpoint, an LSP region is defined as a set of one or
     more data plane layers that share the same type of switching technology, that is,
     the same switching type. For example, VC-11, VC-4, and VC-4-7v layers are part
     of the same TDM region. The regions that are currently defined are: PSC, L2SC,
     TDM, LSC, and FSC. Hence, an LSP region is a technology domain (identified by
     the ISC type) for which data plane resources (i.e., data links) are represented
     into the control plane as an aggregate of TE information associated with a set
     of links (i.e., TE links). For example VC-11 and VC4-64c capable TE links are
     part of the same TDM region. Multiple layers can thus exist in a single region
     Note also that the region may produce a distinction within the control plane.
     Layers of the same region share the same switching technology and, therefore,
     use the same set of technology-specific signaling objects and technology-
     specific value setting of TE link attributes within the control plane, but
     layers from different regions may use different technology-specific objects and
     TE attribute values. This means that it may not be possible to simply forward
     the signaling message between LSR hosting different switching technologies
     because change in some of the signaling objects (for example, the traffic
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     parameters)  when crossing a region boundary even if a single control plane
     instance is used to manage the whole MRN. We may solve the issue by using
     triggered signaling (See 4.3.1).
  3.2. Service layer networks
     A service provider's network may be divided into different service layers. The
     customer's network is considered from the provider's perspective as the highest
     service layer. It interfaces to the highest service layer of the service
     provider's network. Connectivity across the highest service layer of the service
     provider's network may be provided with support from successively lower service
     layers. Service layers are realized via a hierarchy of network layers located
     generally in several regions and commonly arranged according to the switching
     capabilities of network devices.
     For instance some customers purchase Layer 1 (i.e., transport) services from the
     service provider, some Layer 2 (e.g., ATM), while others purchase Layer 3
     (IP/MPLS) services. The service provider realizes the services by a stack of
     network layers located within one or more network regions. The network layers
     are commonly arranged according to the switching capabilities of the devices in
     the networks. Thus, a customer network may be provided on top of the GMPLS-based
     multi-region/multi-layer network. For example, a Layer 1 service (realized via
     the network layers of TDM, and/or LSC, and/or FSC regions) may support a Layer 2
     network (realized via ATM VP/VC) which may itself support a Layer 3 network
     (IP/MPLS region). The supported data plane relationship is a data plane client-
     server relationship where the lower layer provides a service for the higher
     layer using the data links realized in the lower layer.
     Services provided by a GMPLS-based multi-region/multi-layer network are referred
     to as "Multi-region/Multi-layer network services". For example, legacy IP and
     IP/MPLS networks can be supported on top of multi-region/multi-layer networks.
     It has to be emphasized that delivery of such diverse services is a strong
     motivator for the deployment of multi-region/multi-layer networks.
     A customer network may be provided on top of a server GMPLS-based MRN/MLN which
     is operated by a service provider. For example, a pure IP and/or an IP/MPLS
     network can be provided on top of GMPLS-based packet over optical networks
     [MPLS-GMPLS]. The relationship between the networks is a client/server
     relationship and, such services are referred to as "MRN/MLN services". In this
     case, the customer network may form part of the MRN/MLN, or may be partially
     separated, for example to maintain separate routing information but retain
     common signaling.
  3.3. Vertical and Horizontal interaction and integration
     Vertical interaction is defined as the collaborative mechanisms within a network
     element that is capable of supporting more than one layer or region and of
     realizing the client/server relationships between the layers or regions.
     Protocol exchanges between two network controllers managing different regions or
     layers are also a vertical interaction. Integration of these interactions as
     part of the control plane is referred to as vertical integration. Thus, this
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     refers to the collaborative mechanisms within a single control plane instance
     driving multiple network layers part of the same region or not. Such a concept
     is useful in order to construct a framework that facilitates efficient network
     resource usage and rapid service provisioning in carrier networks that are based
     on multiple layers, switching technologies, or ISCs.
     Horizontal interaction is defined as the protocol exchange between network
     controllers that manage transport nodes within a given layer or region. For
     instance, the control plane interaction between two TDM network elements
     switching at OC-48 is an example of horizontal interaction. GMPLS protocol
     operations handle horizontal interactions within the same routing area. The case
     where the interaction takes place across a domain boundary, such as between two
     routing areas within the same network layer, is evaluated as part of the inter-
     domain work [RFC4726], and is referred to as horizontal integration. Thus,
     horizontal integration refers to the collaborative mechanisms between network
     partitions and/or administrative divisions such as routing areas or autonomous
     This distinction needs further clarification when administrative domains match
     layer/region boundaries. Horizontal interaction is extended to cover such cases.
     For example, the collaborative mechanisms in place between two lambda switching
     capable areas relate to horizontal integration. On the other hand, the
     collaborative mechanisms in place between a packet switching capable (e.g.
     IP/MPLS) domain over a different time division switching capable (eg VC4 SDH)
     domain is part of the horizontal integration while it can be seen as a first
     step towards vertical integration.
     The applicability of GMPLS to multiple switching technologies provides the
     unified control management approach for both LSP provisioning and recovery.
     Indeed, one of the main motivations for unifying the capabilities and operations
     GMPLS control plane is the desire to support multi LSP-region [RFC4206] routing
     and Traffic Engineering (TE) capability. For instance, this enables effective
     network resource utilization of both the Packet/Layer2 LSP regions and the Time
     Division Multiplexing (TDM) or Lambda LSP regions in high capacity networks.
     The rationales for GMPLS controlled multi-layer/multi-region networks context
     are summarized here below:
     - The maintenance of multiple instances of the control plane on devices hosting
       more than one switching capability not only increases the complexity of their
       interactions but also increases the total amount of processing individual
       instances would handle.
     - The unification of the addressing spaces helps in avoiding multiple
       identification for the same object (a link for instance or more generally any
       network resource), on the other hand such aggregation does not impact the
       separation between the control and the data plane.
     - By maintaining a single routing protocol instance and a single TE database
       per LSR, a unified control plane model prevents from maintaining a dedicated
       routing topology per layer and therefore does not mandate a full mesh of
       routing adjacencies as it is the case with overlaid control planes.
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     - The collaboration between associated control planes (packet/framed data
       planes) and non-associated control planes (SONET/SDH, G.709, etc.) is
       facilitated due to the capability of hooking the associated in-band signaling
       to the IP terminating interfaces of the control plane.
     - Resource management and policies to be applied at the edges of such
       environment is facilitated (less control to management interactions) and more
       scalable (through the use of aggregated information).
     - Multi-region/multi-layer traffic engineering is facilitated as TE-links from
       distinct regions/layers are stored within the same TE Database.
  4. Key Concepts of GMPLS-Based MLNs and MRNs
     A network comprising transport nodes with multiple data plane layers of either
     the same ISC or different ISCs, controlled by a single GMPLS control plane
     instance, is called a Multi-Layer Network (MLN). A sub-set of MLNs consists of
     networks supporting LSPs of different switching technologies (ISCs). A network
     supporting more than one switching technology is called a Multi-Region Network
  4.1. Interface Switching Capability
     The Interface Switching Capability (ISC) is introduced in GMPLS to support
     various kinds of switching technology in a unified way [RFC4202]. An ISC is
     identified via a switching type.
     A switching type (also referred to as the switching capability type) describes
     the ability of a node to forward data of a particular data plane technology, and
     uniquely identifies a network region. The following ISC types (and, hence,
     regions) are defined: PSC, L2SC, TDM, LSC, and FSC. Each end of a data link
     (more precisely, each interface connecting a data link to a node) in a GMPLS
     network is associated with an ISC.
     The ISC value is advertised as a part of the Interface Switching Capability
     Descriptor (ISCD) attribute (sub-TLV) of a TE link end associated with a
     particular link interface [RFC4202]. Apart from the ISC, the ISCD contains
     information including the encoding type, the bandwidth granularity, and the
     unreserved bandwidth on each of eight priorities at which LSPs can be
     established. The ISCD does not "identify" network layers, it uniquely
     characterizes information associated to one or more network layers.
     TE link end advertisements may contain multiple ISCDs. This can be interpreted
     as advertising a multi-layer (or multi-switching-capable) TE link end. That is,
     the TE link end (and therefore the TE link) is present in multiple layers.
  4.2. Multiple Interface Switching Capabilities
     In an MLN, network elements may be single-switching-type-capable or multi-
     switching-type-capable nodes. Single-switching-type-capable nodes advertise the
     same ISC value as part of their ISCD sub-TLV(s) to describe the termination
     capabilities of each of their TE Link(s). This case is described in [RFC4202].
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     Multi-switching-type-capable LSRs are classified as "simplex" or "hybrid" nodes.
     Simplex and hybrid nodes are categorized according to the way they advertise
     these multiple ISCs:
     - A simplex node can terminate data links with different switching capabilities
       where each data link is connected to the node by a separate link interface.
       So, it advertises several TE Links each with a single ISC value carried in
       its ISCD sub-TLV. For example, an LSR with PSC and TDM links each of which is
       connected to the LSR via a separate interface.
     - A hybrid node can terminate data links with different switching capabilities
       where the data links are connected to the node by the same interface. So, it
       advertises a single TE Link containing more than one ISCD each with a
       different ISC value. For example, a node may terminate PSC and TDM data links
       and interconnect those external data links via internal links. The external
       interfaces connected to the node have both PSC and TDM capabilities.
     Additionally, TE link advertisements issued by a simplex or a hybrid node may
     need to provide information about the node's internal adaptation capabilities
     between the switching technologies supported. That is, the node's capability to
     perform layer border node functions.
  4.2.1. Networks with Multi-Switching-Type-Capable Hybrid Nodes
     This type of network contains at least one hybrid node, zero or more simplex
     nodes, and a set of single-switching-type-capable nodes.
     Figure 1 shows an example hybrid node. The hybrid node has two switching
     elements (matrices), which support, for instance, TDM and PSC switching
     respectively. The node terminates a PSC and a TDM link (Link1 and Link2
     respectively). It also has an internal link connecting the two switching
     The two switching elements are internally interconnected in such a way that it
     is possible to terminate some of the resources of, say, Link2 and provide
     adaptation for PSC traffic received/sent over the PSC interface (#b). This
     situation is modeled in GMPLS by connecting the local end of Link2 to the TDM
     switching element via an additional interface realizing the
     termination/adaptation function. There are two possible ways to set up PSC LSPs
     through the hybrid node. Available resource advertisement (i.e., Unreserved and
     Min/Max LSP Bandwidth) should cover both of these methods.
                               Network element
                          :            --------       :
                          :           |  PSC   |      :
              Link1 -------------<->--|#a      |      :
                          :  +--<->---|#b      |      :
                          :  |         --------       :
              TDM         :  |        ----------      :
               +PSC       :  +--<->--|#c  TDM   |     :
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              Link2 ------------<->--|#d        |     :
                          :           ----------      :
                               Figure 1. Hybrid node.
  4.3. Integrated Traffic Engineering (TE) and Resource Control
     In GMPLS-based multi-region/multi-layer networks, TE Links may be consolidated
     into a single Traffic Engineering Database (TED) for use by the single control
     plane instance. Since this TED contains the information relative to all the
     layers of all regions in the network, a path across multiple layers (possibly
     crossing multiple regions) can be computed using the information in this TED.
     Thus, optimization of network resources across the multiple layers of the same
     region and across multiple regions can be achieved.
     These concepts allow for the operation of one network layer over the topology
     (that is, TE links) provided by other network layers (for example, the use of a
     lower layer LSC LSP carrying PSC LSPs). In turn, a greater degree of control and
     inter-working can be achieved, including (but not limited too):
     - Dynamic establishment of Forwarding Adjacency (FA) LSPs
       [RFC4206] (see Sections 4.3.2 and 4.3.3).
     - Provisioning of end-to-end LSPs with dynamic triggering of FA
     Note that in a multi-layer/multi-region network that includes multi-switching-
     type-capable nodes, an explicit route used to establish an end-to-end LSP can
     specify nodes that belong to different layers or regions. In this case, a
     mechanism to control the dynamic creation of FA LSPs may be required (see
     Sections 4.3.2 and 4.3.3).
     There is a full spectrum of options to control how FA LSPs are dynamically
     established. The process can be subject to the control of a policy, which may be
     set by a management component, and which may require that the management plane
     is consulted at the time that the FA LSP is established. Alternatively, the FA
     LSP can be established at the request of the control plane without any
     management control.
  4.3.1. Triggered Signaling
     When an LSP crosses the boundary from an upper to a lower layer, it may be
     nested into a lower layer FA LSP that crosses the lower layer. From a signaling
     perspective, there are two alternatives to establish the lower layer FA LSP:
     static (pre-provisioned) and dynamic (triggered).  A pre-provisioned FA-LSP may
     be initiated either by the operator or automatically using features like TE
     auto-mesh [AUTO-MESH]. If such a lower layer LSP does not already exist, the LSP
     may be established dynamically. Such a mechanism is referred to as "triggered
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  4.3.2. FA-LSPs
     Once an LSP is created across a layer from one layer border node to another, it
     can be used as a data link in an upper layer.
     Furthermore, it can be advertised as a TE-link, allowing other nodes to consider
     the LSP as a TE link for their path computation [RFC4206]. An LSP created either
     statically or dynamically by one instance of the control plane and advertised as
     a TE link into the same instance of the control plane is called a Forwarding
     Adjacency LSP (FA-LSP). The FA-LSP is advertised as a TE link, and that TE link
     is called a Forwarding Adjacency (FA). An FA has the special characteristic of
     not requiring a routing adjacency (peering) between its end points yet still
     guaranteeing control plane connectivity between the FA-LSP end points based on a
     signaling adjacency. An FA is a useful and powerful tool for improving the
     scalability of GMPLS Traffic Engineering (TE) capable networks since multiple
     higher layer LSPs may be nested (aggregated) over a single FA-LSP.
     The aggregation of LSPs enables the creation of a vertical (nested) LSP
     Hierarchy. A set of FA-LSPs across or within a lower layer can be used during
     path selection by a higher layer LSP. Likewise, the higher layer LSPs may be
     carried over dynamic data links realized via LSPs (just as they are carried over
     any "regular" static data links). This process requires the nesting of LSPs
     through a hierarchical process [RFC4206]. The TED contains a set of LSP
     advertisements from different layers that are identified by the ISCD contained
     within the TE link advertisement associated with the LSP [RFC4202].
     If a lower layer LSP is not advertised as an FA, it can still be used to carry
     higher layer LSPs across the lower layer. For example, if the LSP is set up
     using triggered signaling, it will be used to carry the higher layer LSP that
     caused the trigger. Further, the lower layer remains available for use by other
     higher layer LSPs arriving at the boundary.
     Under some circumstances it may be useful to control the advertisement of LSPs
     as FAs during the signaling establishment of the LSPs [DYN-HIER].
  4.3.3. Virtual Network Topology (VNT)
     A set of one or more of lower-layer LSPs provides information for efficient path
     handling in upper-layer(s) of the MLN, or, in other words, provides a virtual
     network topology (VNT) to the upper-layers. For instance, a set of LSPs, each of
     which is supported by an LSC LSP, provides a virtual network topology to the
     layers of a PSC region, assuming that the PSC region is connected to the LSC
     region. Note that a single lower-layer LSP is a special case of the VNT. The
     virtual network topology is configured by setting up or tearing down the lower
     layer LSPs. By using GMPLS signaling and routing protocols, the virtual network
     topology can be adapted to traffic demands.
     A lower-layer LSP appears as a TE-link in the VNT. Whether the diversely-routed
     lower-layer LSPs are used or not, the routes of lower-layer LSPs are hidden from
     the upper layer in the VNT. Thus, the VNT simplifies the upper-layer routing and
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     traffic engineering decisions by hiding the routes taken by the lower-layer LSPs.
     However hiding the routes of the lower-layer LSPs may lose important information
     that is needed to make the higher-layer LSPs reliable. For instance, the routing
     and traffic engineering in the IP/MPLS layer does not usually consider how the
     IP/MPLS TE links are formed from optical paths that are routed in the fiber
     layer. Two optical paths may share the same fiber link in the lower-layer and
     therefore they may both fail if the fiber link is cut. Thus the shared risk
     properties of the TE links in the VNT must be made available to the higher layer
     during path computation. Further, the topology of the VNT should be designed so
     that any single fiber cut does not bisect the VNT. These issues are addressed
     later in this document.
     Reconfiguration of the virtual network topology may be triggered by traffic
     demand changes, topology configuration changes, signaling requests from the
     upper layer, and network failures. For instance, by reconfiguring the virtual
     network topology according to the traffic demand between source and destination
     node pairs, network performance factors, such as maximum link utilization and
     residual capacity of the network, can be optimized. Reconfiguration is performed
     by computing the new VNT from the traffic demand matrix and optionally from the
     current VNT. Exact details are outside the scope of this document. However, this
     method may be tailored according to the service provider's policy regarding
     network performance and quality of service (delay, loss/disruption, utilization,
     residual capacity, reliability).
  5.1.Handling Single-Switching and Multi-Switching-Type-Capable Nodes
     The MRN/MLN can consist of single-switching-type-capable and multi-switching-
     type-capable nodes. The path computation mechanism in the MLN SHOULD be able to
     compute paths consisting of any combination of such nodes.
     Both single-switching-type-capable and multi-switching-type-capable (simplex or
     hybrid) nodes could play the role of layer boundary. MRN/MLN Path computation
     SHOULD handle TE topologies built of any combination of nodes
  5.2. Advertisement of the Available Adaptation Resource
     A hybrid node SHOULD maintain resources on its internal links (the links
     required for vertical (layer) integration) and SHOULD advertise the resource
     information for those links. Likewise, path computation elements SHOULD be
     prepared to use the availability of termination/adaptation resources as a
     constraint in MRN/MLN path computations to reduce the higher layer LSP setup
     blocking probability caused by the lack of necessary termination/ adaptation
     resources in the lower layer(s).
     The advertisement of the adaptation capability to terminate LSPs of lower-region
     and forward traffic in the upper-region is REQUIRED, as it provides critical
     information when performing multi-region path computation.
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     The mechanism SHOULD cover the case where the upper-layer links which are
     directly connected to upper-layer switching element and the ones which are
     connected through internal links between upper-layer element and lower-layer
     element coexist (See section 4.2.1).
  5.3. Scalability
     The MRN/MLN relies on a unified traffic engineering and routing model.
     - Unified routing model: by maintaining a single routing protocol instance and
       a single TE database per LSR, a unified control plane model prevents from
       maintaining a dedicated routing topology per layer and therefore does not
       mandate a full mesh of routing adjacencies per layer.
     - Unified TE model: the TED in each LSR is populated with TE-links from all
       layers of all regions (TE links interfaces on multiple-switching capability
       LSR can be advertised with multiple ISCD). This may lead to a large amount of
       information that has to be flooded and stored within the network.
     Furthermore, path computation times, which may be of great importance during
     restoration, will depend on the size of the TED.
     Thus MRN/MLN routing mechanisms MUST be designed to scale well with an increase
     of any of the following:
      - Number of nodes
      - Number of TE-links (including FA-LSPs)
      - Number of LSPs
      - Number of regions and layers
      - Number of ISCDs per TE-link.
     Further, design of the routing protocols MUST NOT prevent TE information
     filtering based on ISCDs. The path computation mechanism and the signaling
     protocol SHOULD be able to operate on partial TE information.
     Since TE Links can advertise multiple Interface Switching Capabilities (ISC),
     the number of links can be limited (by combination) by using specific
     topological maps referred to as VNT (Virtual Network Topologies). The
     introduction of virtual topological maps leads us to consider the concept of
     emulation of data plane overlays.
     Path computation is dependent on the network topology and associated link state.
     The path computation stability of an upper layer may be impaired if the VNT
     changes frequently and/or if the status and TE parameters (the TE metric, for
     instance) of links in the VNT changes frequently. In this context, robustness of
     the VNT is defined as the capability to smooth changes that may occur and avoid
     their propagation into higher layers. Changes to the VNT may be caused by the
     creation, deletion, or modification of LSPs.
     Creation, deletion, and modification of LSPs MAY be triggered by adjacent layers
     or through operational actions to meet traffic demand changes, topology changes,
     signaling requests from the upper layer, and network failures. Routing
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     robustness SHOULD be traded with adaptability with respect to the change of
     incoming traffic requests.
  5.5.Disruption Minimization
     When reconfiguring the VNT according to a change in traffic demand, the upper-
     layer LSP might be disrupted. Such disruption to the upper layers MUST be
     When residual resource decreases to a certain level, some lower layer LSPs MAY
     be released according to local or network policies. There is a trade-off between
     minimizing the amount of resource reserved in the lower layer and disrupting
     higher layer traffic (i.e. moving the traffic to other TE-LSPs so that some LSPs
     can be released). Such traffic disruption MAY be allowed, but MUST be under the
     control of policy that can be configured by the operator. Any repositioning of
     traffic MUST be as non-disruptive as possible (for example, using make-before-
  5.6.LSP Attribute Inheritance
     TE-Link parameters SHOULD be inherited from the parameters of the LSP that
     provides the TE-link, and so from the TE-links in the lower layer that are
     traversed by the LSP.
     These include:
     - Interface Switching Capability
     - TE metric
     - Maximum LSP bandwidth per priority level
     - Unreserved bandwidth for all priority levels
     - Maximum Reservable bandwidth
     - Protection attribute
     - Minimum LSP bandwidth (depending on the Switching Capability)
     - SRLG
     Inheritance rules MUST be applied based on specific policies. Particular
     attention should be given to the inheritance of TE metric (which may be other
     than a strict sum of the metrics of the component TE links at the lower layer),
     protection attributes, and SRLG.
     As described earlier, hiding the routes of the lower-layer LSPs may lose
     important information necessary to make LSPs in the higher layer network
     reliable. SRLGs may be used to identify which lower-layer LSPs share the same
     failure risk so that the potential risk of the VNT becoming disjoint can be
     minimized, and so that resource disjoint protection paths can be set up in the
     higher layer. How to inherit the SRLG information from the lower layer to the
     upper layer needs more discussion and is out of scope of this document.
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  5.7.Computing Paths With and Without Nested Signaling
     Path computation MAY take into account LSP region and layer boundaries when
     computing a path for an LSP. For example, path computation MAY restrict the path
     taken by an LSP to only the links whose interface switching capability is PSC.
     Interface switching capability is used as a constraint in path computation. For
     example, a TDM-LSP is routed over the topology composed of TE links of the same
     TDM layer. In calculating the path for the LSP, the TED MAY be filtered to
     include only links where both end include requested LSP switching type. In this
     way hierarchical routing is done by using a TED filtered with respect to
     switching capability (that is, with respect to particular layer).
     If triggered signaling is allowed, the path computation mechanism MAY produce a
     route containing multiple layers/regions. The path is computed over the multiple
     layers/regions even if the path is not "connected" in the same layer as the
     endpoints of the path exist. Note that here we assume that triggered signaling
     will be invoked to make the path "connected", when the upper-layer signaling
     request arrives at the boundary node.
     The upper-layer signaling request may contain an ERO that includes only hops in
     the upper layer, in which case the boundary node is responsible for triggered
     creation of the lower-layer FA-LSP using a path of its choice, or for the
     selection of any available lower layer LSP as a data link for the higher layer.
     This mechanism is appropriate for environments where the TED is filtered in the
     higher layer, where separate routing instances are used per layer, or where
     administrative policies prevent the higher layer from specifying paths through
     the lower layer.
     Obviously, if the lower layer LSP has been advertised as a TE link (virtual or
     real) into the higher layer, then the higher layer signaling request may contain
     the TE link identifier and so indicate the lower layer resources to be used. But
     in this case, the path of the lower layer LSP can be dynamically changed by the
     lower layer at any time.
     Alternatively, the upper-layer signaling request may contain an ERO specifying
     the lower layer FA-LSP route. In this case, the boundary node is responsible for
     decision as to which it should use the path contained in the strict ERO or it
     should re-compute the path within in the lower-layer.
     Even in case the lower-layer FA-LSPs are already established, a signaling
     request may also be encoded as loose ERO. In this situation, it is up to the
     boundary node to decide whether it should a new lower-layer FA-LSP or it should
     use the existing lower-layer FA-LSPs.
     The lower-layer FA-LSP can be advertised just as an FA-LSP in the upper-layer or
     an IGP adjacency can be brought up on the lower-layer FA-LSP.
  5.8. LSP Resource Utilization
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     It MUST be possible to utilize network resources efficiently. Particularly,
     resource usage in all layers SHOULD be optimized as a whole (i.e., across all
     layers), in a coordinated manner, (i.e., taking all layers into account). The
     number of lower-layer LSPs carrying upper-layer LSPs SHOULD be minimized (note
     that multiple LSPs MAY be used for load balancing). Lower-layer LSPs that could
     have their traffic re-routed onto other LSPs are unnecessary and SHOULD be
  5.8.1. FA-LSP Release and Setup
     Statistical multiplexing can only be employed in PSC and L2SC regions. A PSC or
     L2SC LSP may or may not consume the maximum reservable bandwidth of the TE link
     (FA LSP) that carries it. On the other hand, a TDM, or LSC LSP always consumes a
     fixed amount of bandwidth as long as it exists (and is fully instantiated)
     because statistical multiplexing is not available.
     If there is low traffic demand, some FA LSPs that do not carry any higher-layer
     LSP MAY be released so that lower-layer resources are released and can be
     assigned to other uses. Note that if a small fraction of the available bandwidth
     of an FA-LSP is still in use, the nested LSPs can also be re-routed to other FA-
     LSPs (optionally using the make-before-break technique) to completely free up
     the FA-LSP. Alternatively, unused FA LSPs MAY be retained for future use.
     Release or retention of underutilized FA LSPs is a policy decision.
     As part of the re-optimization process, the solution MUST allow rerouting of an
     FA LSP while keeping interface identifiers of corresponding TE links unchanged.
     Further, this process MUST be possible while the FA LSP is carrying traffic
     (higher layer LSPs) with minimal disruption to the traffic.
     Additional FA LSPs MAY also be created based on policy, which might consider
     residual resources and the change of traffic demand across the region. By
     creating the new FA LSPs, the network performance such as maximum residual
     capacity may increase.
     As the number of FA LSPs grows, the residual resource may decrease. In this case,
     re-optimization of FA LSPs MAY be invoked according to policy.
     Any solution MUST include measures to protect against network destabilization
     caused by the rapid setup and teardown of LSPs as traffic demand varies near a
     Signaling of lower-layer LSPs SHOULD include a mechanism to rapidly advertise
     the LSP as a TE link and to coordinate into which routing instances the TE link
     should be advertised.
  5.8.2. Virtual TE-Links
     It may be considered disadvantageous to fully instantiate (i.e. pre-provision)
     the set of lower layer LSPs that provide the VNT since this might reserve
     bandwidth that could be used for other LSPs in the absence of upper-layer
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     However, in order to allow path computation of upper-layer LSPs across the
     lower-layer, the lower-layer LSPs MAY be advertised into the upper-layer as
     though they had been fully established, but without actually establishing them.
     Such TE links that represent the possibility of an underlying LSP are termed
     "virtual TE-links." It is an implementation choice at a layer boundary node
     whether to create real or virtual TE-links, and the choice if available in an
     implementation MUST be under the control of operator policy. Note that there is
     no requirement to support the creation of virtual TE-links, since real TE-links
     (with established LSPs) may be used, and even if there are no TE-links (virtual
     or real) advertised to the higher layer, it is possible to route a higher layer
     LSP into a lower layer on the assumptions that proper hierarchical LSPs in the
     lower layer will be dynamically created (triggered) as needed.
     If an upper-layer LSP that makes use of a virtual TE-Link is set up, the
     underlying LSP MUST be immediately signaled in the lower layer.
     If virtual TE-Links are used in place of pre-established LSPs, the TE-links
     across the upper-layer can remain stable using pre-computed paths while wastage
     of bandwidth within the lower-layer and unnecessary reservation of adaptation
     ports at the border nodes can be avoided.
     The solution SHOULD provide operations to facilitate the build-up of such
     virtual TE-links, taking into account the (forecast) traffic demand and
     available resource in the lower-layer.
     Virtual TE-links MAY be added, removed or modified dynamically (by changing
     their capacity) according to the change of the (forecast) traffic demand and the
     available resource in the lower-layer. The maximum number of virtual TE links
     that can be defined SHOULD be configurable.
     Any solution MUST include measures to protect against network destabilization
     caused by the rapid changes in the virtual network topology as traffic demand
     varies near a threshold.
     The concept of the VNT can be extended to allow the virtual TE-links to form
     part of the VNT. The combination of the fully provisioned TE-links and the
     virtual TE-links defines the VNT provided by the lower layer. The VNT can be
     changed by setting up and/or tearing down virtual TE links as well as by
     modifying real links (i.e. the fully provisioned LSPs). How to design the VNT
     and how to manage it are out of scope of this document.
  5.9. Verification of the LSPs
     When a lower layer LSP is established for use as a data link by a higher layer,
     the LSP MAY be verified for correct connectivity and data integrity. Such
     mechanisms are data technology-specific and are beyond the scope of this
     document, but may be coordinated through the GMPLS control plane.
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  6. Security Considerations
     The current version of this document does not introduce any new security
     considerations as it only lists a set of requirements.
     It is expected that solution documents will include a full analysis of the
     security issues that any protocol extensions introduce.
  7. IANA Considerations
     This informational document makes no requests to IANA for action.
  8. References
  8.1. Normative Reference
     [RFC2119]        Bradner, S., "Key words for use in RFCs to Indicate
                      Requirement Levels", BCP 14, RFC 2119, March 1997.
     [RFC4202]   K.Kompella and Y.Rekhter, "Routing Extensions in Support of
                   Generalized Multi-Protocol Label Switching (GMPLS)," RFC4202,
                   October 2005.
     [RFC4726]   A.Farrel, J-P. Vasseur, and A.Ayyangar, "A Framework for Inter-
                   Domain Multiprotocol Label Switching  Traffic Engineering", RFC
                   4726, November 2006.
     [RFC4206]   K.Kompella and Y.Rekhter, "Label Switched Paths (LSP) Hierarchy
                   with Generalized Multi-Protocol Label Switching (GMPLS) Traffic
                   Engineering (TE),"  RFC4206, Oct. 2005.
     [RFC3945]   E.Mannie (Ed.), "Generalized Multi-Protocol Label Switching
                   (GMPLS) Architecture", RFC 3945, October 2004.
     [RFC4397]   I.Bryskin and A. Farrel, "A Lexicography for the Interpretation of
                   Generalized Multiprotocol     Label Switching (GMPLS)
                   Terminology within the Context of the ITU-T's Automatically
                   Switched Optical Network (ASON) Architecture", RFC 4397,
                   February 2006.
  8.2. Informative References
     [MRN-EVAL]   Le Roux, J.L., Brungard, D., Oki, E., Papadimitriou, D., Shiomoto,
                   K., Vigoureux, M.,"Evaluation of existing GMPLS Protocols
                   against Multi Layer and Multi Region Networks (MLN/MRN)",
                   draft-ietf-ccamp-gmpls-mrn-eval, work in progress.
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     [MPLS-GMPLS] K. Kumaki (Editor), "Interworking Requirements to Support
                   operation of MPLS-TE over GMPLS networks",             draft-
                   ietf-ccamp-mpls-gmpls-interwork-reqts, work in progress.
     [DYN-HIER] Shiomoto, K., Rabbat, R., Ayyangar, A., Farrel, A. and Ali, Z.,
                   "Procedures for Dynamically Signaled Hierarchical Label
                   Switched Paths", draft-ietf-ccamp-lsp-hierarchy-bis, work in
     [AUTO-MESH]    Vasseur, JP., Le Roux, JL., et al., "Routing extensions for
                   discovery of Multiprotocol (MPLS) Label Switch Router (LSR)
                   Traffic Engineering (TE) mesh membership", draft-ietf-ccamp-
                   automesh, work in progress.
  9. Authors' Addresses
     Kohei Shiomoto
     NTT Network Service Systems Laboratories
     3-9-11 Midori-cho,
     Musashino-shi, Tokyo 180-8585, Japan
     Dimitri Papadimitriou
     Copernicuslaan 50,
     B-2018 Antwerpen, Belgium
     Phone : +32 3 240 8491
     Jean-Louis Le Roux
     France Telecom R&D,
     Av Pierre Marzin,
     22300 Lannion, France
     Martin Vigoureux
     Route de Nozay, 91461 Marcoussis cedex, France
     Phone: +33 (0)1 69 63 18 52
     Deborah Brungard
     Rm. D1-3C22 - 200
     S. Laurel Ave., Middletown, NJ 07748, USA
     Phone: +1 732 420 1573
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  10.Contributors' Addresses
     Eiji Oki
     NTT Network Service Systems Laboratories
     3-9-11 Midori-cho,
     Tokyo 180-8585,
     Phone: +81 422 59 3441
     Ichiro Inoue
     NTT Network Service Systems Laboratories
     3-9-11 Midori-cho,
     Tokyo 180-8585,
     Phone: +81 422 59 3441
     Emmanuel Dotaro
     Route de Nozay,
     91461 Marcoussis cedex,
     Phone : +33 1 6963 4723
  11. Intellectual Property Considerations
     The IETF takes no position regarding the validity or scope of any Intellectual
     Property Rights or other rights that might be claimed to pertain to the
     implementation or use of the technology described in this document or the extent
     to which any license under such rights might or might not be available; nor does
     it represent that it has made any independent effort to identify any such rights.
     Information on the procedures with respect to rights in RFC documents can be
     found in BCP 78 and BCP 79.
     Copies of IPR disclosures made to the IETF Secretariat and any assurances of
     licenses to be made available, or the result of an attempt made to obtain a
     general license or permission for the use of such proprietary rights by
     implementers or users of this specification can be obtained from the IETF on-
     line IPR repository at
     The IETF invites any interested party to bring to its attention any copyrights,
     patents or patent applications, or other proprietary rights that may cover
     technology that may be required to implement this standard.  Please address the
     information to the IETF at
  12. Full Copyright Statement
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     Copyright (C) The IETF Trust (2007). This document is subject to the rights,
     licenses and restrictions contained in BCP 78, and except as set forth therein,
     the authors retain all their rights.
     This document and the information contained herein are provided on an "AS IS"
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