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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)

                    draft-ietf-ccamp-gmpls-mln-reqs-05.txt


Status of this Memo

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   This Internet-Draft will expire in February 2008.

Abstract

   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 ....................................................
   1.1. Scope .........................................................
   2. Conventions Used in this Document ...............................
   2.1. List of Acronyms ..............................................
   3. Positioning .....................................................
   3.1. Data Plane Layers and Control Plane Regions ...................
   3.2. Service Layer Networks .. .....................................
   3.3. Vertical and Horizontal Interaction and Integration ...........
   3.4. Motivation ....................................................
   4. Key Concepts of GMPLS-Based MLNs and MRNs .......................
   4.1. Interface Switching Capability ................................
   4.2. Multiple Interface Switching Capabilities .....................
   4.2.1. Networks with Multi-Switching-Type-Capable Hybrid Nodes .....
   4.3. Integrated Traffic Engineering (TE) and Resource Control ......
   4.3.1. Triggered Signaling .........................................
   4.3.2. FA-LSPs .....................................................
   4.3.3. Virtual Network Topology (VNT) ..............................
   5. Requirements ....................................................
   5.1. Handling Single-Switching and Multi-Switching-Type-Capable
          Nodes .......................................................
   5.2. Advertisement of the Available Adaptation Resource ............
   5.3. Scalability ...................................................
   5.4. Stability .....................................................
   5.5. Disruption Minimization .......................................
   5.6. LSP Attribute Inheritance .....................................
   5.7. Computing Paths With and Without Nested Signaling .............
   5.8. LSP Resource Utilization ......................................
   5.8.1. FA-LSP Release and Setup ....................................
   5.8.2. Virtual TE-Links ............................................
   5.9. Verification of the LSPs ......................................
   6. Security Considerations .........................................
   7. IANA Considerations ............................................
   8. Acknowledgements ................................................
   9. References ......................................................
   9.1. Normative Reference ...........................................
   9.2. Informative References ........................................
   10. Authors' Addresses .............................................
   11. Contributors' Addresses ........................................
   12. Intellectual Property Considerations ...........................
   13. Full Copyright Statement .......................................

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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) 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 capable).

   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 and TDM) hosted on the same devices (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 some of
     which 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.

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   - 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 may be used
   to control the MLN/MRN. This 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. 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.

   Furthermore, 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. It is most probable that such a MLN or MRN would be
   operated by a single Service Provider, but this document does not
   exclude the possibility of two layers (or regions) being under
   different administrative control (for example, by different Service

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   Providers that share a single control plane instance) where the
   administrative domains are prepared to share a limited amount of
   information.

   For such TE domain to interoperate with edge nodes/domains supporting
   non-GMPLS interfaces (such as those defined by other SDOs), 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 extensions.

2. Conventions Used in this Document

   Although this is not a protocol specification, the key words "MUST",
   "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
   "RECOMMENDED",  "MAY", 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

   FA: Forwarding Adjacency
   FA-LSP: Forwarding Adjacency Label Switched Path
   FSC: Fiber Switching Capable
   ISC: Interface Switching Capability
   ISCD: Interface Switching Capability Descriptor
   L2SC: Layer-2 Switching Capable
   LSC: Lambda Switching Capable
   LSP: Label Switched Path
   LSR: Label Switching Router
   MLN: Multi-Layer Network
   MRN: Multi-Region Network
   PSC: Packet Switching Capable
   SRLG: Shared Risk Ling Group
   TDM: Time-Division Switch Capable
   TE: Traffic Engineering
   TED: Traffic Engineering Database
   VNT: Virtual Network Topology

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

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   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 network.

   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 parameters) when crossing a region boundary even
   if a single control plane instance is used to manage the whole MRN.
   We may solve this issue by using triggered signaling (see Section
   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

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   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 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

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   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 systems.

   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 and a separate time division switching capable
   (e.g., VC4 SDH) domain over which it operates are part of the
   horizontal integration while it can also be seen as a first step
   towards vertical integration.

3.4. Motivation

   The applicability of GMPLS to multiple switching technologies
   provides a unified control and management approach for both LSP
   provisioning and recovery. Indeed, one of the main motivations for
   unifying the capabilities and operations of the GMPLS control plane
   is the desire to support multi-LSP-region [RFC4206] routing and
   Traffic Engineering (TE) capabilities. 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
   are summarized 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
     identifiers 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
     plane and the data plane.

   - By maintaining a single routing protocol instance and a single TE
     database per LSR, a unified control plane model removes the
     requirement to maintain a dedicated routing topology per layer and
     therefore does not mandate a full mesh of routing adjacencies as is

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     the case with overlaid control planes.

   - The collaboration between technology layers where the control
     channel is associated with the data channel (e.g. packet/framed
     data planes) and technology layers where the control channel is not
     directly associated with the data channel (SONET/SDH, G.709, etc.)
     is facilitated by the capability within GMPLS to associate in-band
     control plane signaling to the IP terminating interfaces of the
     control plane.

   - Resource management and policies to be applied at the edges of such
     a MRN/MLN is made more simple (fewer 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 (MRN).

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.

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   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].

   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. This information allows path computation to select an end-
   to-end multi-layer or multi-region path that includes links of
   different switching capabilities that are joined by LSRs that can
   adapt the signal between the links.

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

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   connecting the two switching elements.

   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         :  +--<->--|#c  TDM   |     :
             +PSC       :          |          |     :
            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).


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   - Provisioning of end-to-end LSPs with dynamic triggering of FA
     LSPs.

   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
   [RFC4972]. If such a lower layer LSP does not already exist, the LSP
   may be established dynamically. Such a mechanism is referred to as
   "triggered signaling".

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

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   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 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

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   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. Requirements

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.

   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).





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5.3. Scalability

   The MRN/MLN relies on unified routing and traffic engineering
   models.

   - Unified routing model: By maintaining a single routing protocol
     instance and a single TE database per LSR, a unified control plane
     model removes the requirement to maintain 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 link interfaces on multiple-
     switching-capability LSRs can be advertised with multiple ISCDs).
     This may lead to an increase in the 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 (ISCs), 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.

5.4. Stability

   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

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   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 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 minimized.

   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-break).

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

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   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.

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.

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   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

   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 avoided.

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.


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   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 threshold.

   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 traffic.

   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

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   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.

6. Security Considerations

   The MLN/MRN architecture does not introduce any new security
   requirements over the general GMPLS architecture described in
   [RFC3945]. Additional security considerations form MPLS and GMPLS
   networks are described in [MPLS-SEC].

   However, where the separate layers of a MLN/MRN network are operated
   as different administrative domains, additional security
   considerations may be given to the mechanisms for allowing inter-
   layer LSP setup, for triggering lower-layer LSPs, or for VNT
   management. Similarly, consideration may be given to the amount of
   information shared between administrative domains, and the trade-off
   between multi-layer TE and confidentiality of information belonging
   to each administrative domain.

   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. Acknowledgements

   The authors would like to thank Adrian Farrel and the participants of
   ITU-T Study Group 15 Question 14 for their careful review.

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9. References

9.1. Normative Reference

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.


   [RFC4202] Kompella, K., and Rekhter, Y., "Routing Extensions in
             Support of Generalized Multi-Protocol Label Switching
             (GMPLS)," RFC4202, October 2005.

   [RFC4726] Farrel, A., Vasseur, JP., and Ayyangar, A., "A Framework
             for Inter-Domain Multiprotocol Label Switching  Traffic
             Engineering", RFC 4726, November 2006.

   [RFC4206] Kompella, K., and Rekhter, Y., "Label Switched Paths (LSP)
             Hierarchy with Generalized Multi-Protocol Label Switching
             (GMPLS) Traffic Engineering (TE),"  RFC4206, Oct. 2005.

   [RFC3945] E. Mannie (Editor), "Generalized Multi-Protocol Label
             Switching (GMPLS) Architecture", RFC 3945, October 2004.

   [RFC4397] Bryskin, I., and Farrel, A., "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.

9.2. 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
                Network (MLN/MRN) Requirements", draft-ietf-ccamp-gmpls-
                mln-eval, work in progress.

   [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 progress.

   [MPLS-SEC]   Fang, L., et al., " Security Framework for MPLS and
                GMPLS Networks", draft-fang-mpls-gmpls-security-
                framework, work in progress.

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   [RFC4972]    Vasseur, JP., Le Roux, JL., et al., "Routing Extensions
                for Discovery of Multiprotocol (MPLS) Label Switch
                Router (LSR) Traffic Engineering (TE) Mesh Membership",
                RFC 4972, July 2007.

10. Authors' Addresses

   Kohei Shiomoto
   NTT Network Service Systems Laboratories
   3-9-11 Midori-cho,
   Musashino-shi, Tokyo 180-8585, Japan
   Email: shiomoto.kohei@lab.ntt.co.jp

   Dimitri Papadimitriou
   Alcatel-Lucent
   Copernicuslaan 50,
   B-2018 Antwerpen, Belgium
   Phone : +32 3 240 8491
   Email: dimitri.papadimitriou@alcatel-lucent.be

   Jean-Louis Le Roux
   France Telecom R&D,
   Av Pierre Marzin,
   22300 Lannion, France
   Email: jeanlouis.leroux@orange-ft.com

   Martin Vigoureux
   Alcatel-Lucent
   Route de Nozay, 91461 Marcoussis cedex, France
   Phone: +33 (0)1 69 63 18 52
   Email: martin.vigoureux@alcatel-lucent.fr

   Deborah Brungard
   AT&T
   Rm. D1-3C22 - 200
   S. Laurel Ave., Middletown, NJ 07748, USA
   Phone: +1 732 420 1573
   Email: dbrungard@att.com

11. Contributors' Addresses

   Eiji Oki
   NTT Network Service Systems Laboratories
   3-9-11 Midori-cho,
   Musashino-shi,
   Tokyo 180-8585,
   Japan
   Phone: +81 422 59 3441
   Email: oki.eiji@lab.ntt.co.jp


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   Ichiro Inoue
   NTT Network Service Systems Laboratories
   3-9-11 Midori-cho,
   Musashino-shi,
   Tokyo 180-8585,
   Japan
   Phone: +81 422 59 3441
   Email: ichiro.inoue@lab.ntt.co.jp

   Emmanuel Dotaro
   Alcatel-Lucent
   Route de Nozay,
   91461 Marcoussis cedex,
   France
   Phone : +33 1 6963 4723
   Email: emmanuel.dotaro@alcatel-lucent.fr

12. 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
   http://www.ietf.org/ipr.

   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
   ietf-ipr@ietf.org.

13. Full Copyright Statement

   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.



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   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST
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