Internet Engineering Task Force
                                              Daniel O. Awduche
Expiration Date: January 2001                 UUNET (Worldcom)

                                              Yakov Rekhter
                                              Cisco Systems

                                              John Drake
                                              Calient Networks

                                              Rob Coltun
                                              Redback Networks
                                              July 2000

                    Multi-Protocol Lambda Switching:
 Combining MPLS Traffic Engineering Control With Optical Crossconnects


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 except that the right to
   produce derivative works is not granted.

   Internet-Drafts are working documents of the Internet Engineering
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     The list of current Internet-Drafts can be accessed at

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   This document describes an approach to the design of control planes
   for optical crossconnects (OXCs), which leverages existing control
   plane techniques developed for MPLS Traffic Engineering.  The
   proposed approach combines recent advances in MPLS traffic
   engineering control plane constructs with OXC technology to:  (1)
   provide a framework for real-time provisioning of optical channels in
   automatically switched optical networks, (2) foster the expedited
   development and deployment of a new class of versatile OXCs, and (3)
   allow the use of uniform semantics for network management and
   operations control in hybrid networks consisting of OXCs and label
   switching routers (LSRs).  The proposed approach is particularly
   advantageous for OXCs intended for data-centric optical
   internetworking systems. In such environments, it will help to
   simplify network administration.  This approach also paves the way
   for the eventual incorporation of DWDM multiplexing capabilities in
   IP routers.

1. Introduction

   This document describes an approach to the design of control planes
   for optical crossconnects (OXCs), which is based on the Multiprotocol
   Label Switching (MPLS) traffic engineering control plane model. In
   this approach, the main idea it to leverage recent advances in
   control plane technology developed for MPLS traffic engineering (see
   [1,2,3,4,8,9,10]). This approach is driven by pragmatic
   considerations, as it exploits an existing technology base to foster
   rapid development and deployment of a new class of versatile OXCs
   that address the optical transport needs of the rapidly evolving
   Internet. This approach will assist in optical channel layer
   bandwidth management, dynamic provisioning of optical channels, and
   network survivability through enhanced protection and restoration
   capabilities in the optical domain.

   As used in this document, an OXC is a path switching element in an
   optical transport network that establishes routed paths for optical
   channels by locally connecting an optical channel from an input port
   (fiber) to an output port (fiber) on the switch element. Additional
   characteristics of OXCs, as used in this document, are discussed in
   Section 4.1.

   The proposed OXC control plane uses the IGP extensions for MPLS
   traffic engineering (with additional enhancements) to distribute
   relevant optical transport network state information, including
   topology state information. This state information is subsequently

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   used by a constraint-based routing system to compute paths for
   point-to-point optical channels through the optical transport
   network.  The proposed OXC control plane also uses an MPLS signaling
   protocol (see [3,4]) to instantiate point-to-point optical channels
   between access points in the optical transport network.

   This document does not specify the details of the extensions and
   domain specific adaptations required to map the MPLS traffic
   engineering control plane model onto the optical domain. These
   aspects will be covered in a number of supplementary documents that
   will follow. However, in Section 7 of this memo, we provide a high
   level overview of the architectural issues involved in making such

2. Advantages

   The advantages of the proposed approach are numerous.

      - It offers a framework for optical bandwidth management
        and the real-time provisioning of optical channels in
        automatically switched optical networks.

      - It exploits recent advances in MPLS control plane technology
        and also leverages accumulated operational experience with IP
        distributed routing control.

      - It obviates the need to reinvent a new class of control
        protocols for optical transport networks and allows reuse
        of software artifacts originally developed for the MPLS
        traffic engineering application. Consequently, it fosters
        the rapid development and deployment of a new class of
        versatile OXCs.

      - It facilitates the introduction of control coordination concepts
        between data network elements and optical network elements.

      - It simplifies network administration in facilities based service
        provider networks by providing uniform semantics for network
        management and control in both the data and optical domains.

      - It paves the way for the eventual introduction of DWDM
        multiplexing capabilities on IP routers

      - Lastly, it establishes a preliminary framework for the notion
        of an optical Internet.

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3. Background

   The growth, performance, and survivability requirements of the
   Internet (which is also becoming very mission critical) are mandating
   IP-centric networks to be cost effective, survivable, scalable, and
   to provide control capabilities that facilitate network performance
   optimization. Some of these requirements are being addressed by the
   Multiprotocol Label Switching (MPLS) traffic engineering capabilities
   under development by the IETF [1,2,3,4].

   The underlying optical transport network also needs to be versatile,
   reconfigurable, cost effective, and to support a variety of
   protection and restoration capabilities due to the rapidly changing
   requirements of the Internet.

   A result of these trends, therefore, is the evolution of optical
   transport networks from simple linear and ring topologies to mesh
   topologies. By a mesh (not necessarily fully meshed) topology, we
   mean a connected (not necessarily fully connected) network of
   arbitrary topology in which the node degree is typically more than
   two. In mesh optical networks, optical crossconnects engender
   versatility and reconfigurability by performing switching and
   rearrangement functions.

   Underscoring the importance of versatile networking capabilities in
   the optical domain, a number of standardization organizations and
   interoperability forums have initiated work items to study the
   requirements and architectures for reconfigurable optical networks.
   Refer, for example, to ITU-T recommendation G.872 [5].  This document
   defines a functional architecture for an optical transport network
   (OTN) that supports the transport of digital client signals.  ITU-T
   G.872 speaks of an OTN as "a transport network bounded by optical
   channel access points"[5].  The ITU-T G.872 OTN architecture is based
   on a layered structure, which includes:

     (a) an optical channel (OCh) layer network,
     (b) an optical multiplex section (OMS) layer network, and
     (c) an optical transmission section (OTS) layer network.

   The optical channel layer is the most relevant to the discussions in
   this document. The optical channel layer network supports end-to-end
   networking of optical channel trails between access points. The OCh
   layer network provides the following functions: routing, monitoring,
   grooming, and protection and restoration of optical channels.  In
   this situation, programmable Optical crossconnects, with
   rearrangeable switch fabrics and relatively smart control planes,
   will be critical to the realization of the OCh layer functions,
   especially in mesh optical networks. In the data network domain,

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   routing, monitoring, and survivability are crucial functions
   performed by the MPLS traffic engineering control plane (see

   Note: Although we mention the ITU-T recommendation G.872, the OXC
   control plane design approach described here is not restricted to
   G.872 compliant networks. Instead, it can be applied to any mesh
   optical network that uses programmable and reconfigurable OXCs.

   Other standards organizations and interoperability forums that are
   actively pursuing projects related to dynamically reconfigurable
   optical networks include the ANSI T1X1.5 committee, the Optical
   Internetworking Forum (OIF), and now by virtue of this memo the IETF.

   Recent contributions to ANSI T1X1.5 emphasize the need for automation
   of the OCh layer network by using appropriate signaling protocols to
   establish optical channels in real time (see [12] and [13]).

   The Optical Internetworking Forum (, an
   international organization engaged in the development and promotion
   of interoperability agreements for optical internetworking systems,
   is also evaluating architectural and signaling options related to the
   internetworking of data network elements with reconfigurable optical
   networks -- to facilitate rapid provisioning, efficient
   protection/restoration, and other services in optical internetworking

   In all these cases, the successful realization of the vision of
   versatile optical networking depends very much on the synthesis of
   appropriate control plane technologies with programmable and
   reconfigurable OXCs.

4. OXCs, LSRs, Optical Trails, and Explicit LSPs

   Consider a hybrid, IP-centric optical internetworking environment
   consisting of both label switching routers (LSRs) and OXCs, where the
   OXCs are programmable and support wavelength conversion/translation.

   At a level of abstraction, an LSR and an OXC exhibit a number of
   isomorphic relations. It is important to enumerate these relations
   because they help to expose the reusable software artifacts from the
   MPLS traffic engineering control plane model. Architecturally, both
   LSRs and OXCs emphasize problem decomposition by decoupling the
   control plane from the data plane.

   The data plane of an LSR uses the label swapping paradigm to transfer
   a labeled packet from an input port to an output port (see e.g.,

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   [6,7]). The data plane of an OXC uses a switching matrix to connect
   an optical channel trail from an input port to an output port.

   An LSR performs label switching by first establishing a relation
   between an <input port, input label> tuple and an <output port,
   output label> tuple. Likewise, an OXC provisions optical channel
   trails by first establishing a relation between an <input port, input
   optical channel> tuple and an <output port, output optical channel>
   tuple. These relations are determined by the control plane of the
   respective network elements, and are locally instantiated on the
   device through a switch controller. In the LSR, the next hop label
   forwarding entry (NHLFE) maintains the input-output relations. In the
   OXC, the switch controller reconfigures the internal interconnection
   fabric to establish the relations.  These relations cannot be altered
   by the payload of the data plane.

   The functions of the control plane (for both LSRs and OXCs) include
   resource discovery, distributed routing control, and connection
   management.  In particular, the control plane of the LSR is used to
   discover, distribute, and maintain relevant state information
   associated with the MPLS network, and to instantiate and maintain
   label switched paths (LSPs) under various MPLS traffic engineering
   rules and policies.  An LSP is the path through one or more LSRs
   followed by a specific forwarding equivalence class (FEC) (see [7]).
   An explicit LSP is one whose route is defined at its origination

   The control plane of the OXC, on the other hand, is used to discover,
   distribute, and maintain relevant state information associated with
   the OTN, and to establish and maintain optical channel trails under
   various optical internetworking traffic engineering rules and
   policies. An optical channel trail provides a point-to-point optical
   connection between two access points.  An optical channel trail may
   consist of just one wavelength or a concatenation of multiple
   wavelengths. If an optical trail consists of just one wavelength,
   then it is said to satisfy the "wavelength continuity property." At
   each intermediate OXC along the route of an optical channel trail,
   OXC switch fabric connects the trail from an input port to an output

   A distinction between the current generation of OXCs and LSRs is that
   the former do not perform packet level processing in the data plane,
   while the later are datagram devices which may perform certain packet
   level operations in the data plane.  A significant conceptual
   difference is that with LSRs the forwarding information is carried
   explicitly as part of the labels appended to data packets, while with
   OXCs the switching information is implied from the wavelength or
   optical channel.

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4.1 Review of Relevant OXC Characteristics

   The following section contains a brief overview of relevant OXC
   characteristics, focusing on the switching functions and underlying

   As used in this document, the switching function of an OXC may be
   electrical or optical. If the switching fabric is purely electrical,
   then the crossconnect is typically referred to as a digital
   crossconnect (DXC), or a broadband digital cross-connect (BBDXC) --
   if the capacity and port density are sufficiently high.  Optical-
   Electrical-Optical (OEO) conversion is required in BBDXCs.  A BBDXC
   may or may not have WDM multiplexing capabilities. If a BBDXC has WDM
   multiplexing capabilities, then it may be connected directly to other
   compatible WDM devices through optical fiber links that carry
   multiple wavelengths per fiber. If a BBDXC does not have WDM
   multiplexing capabilities, then it may be connected to an external
   DWDM multiplexer through a set of discrete fibers, where each fiber
   carries only one wavelength. A BBDXC may also perform regeneration,
   reshaping, and re-timing functions.

   If the switching fabric of an OXC is completely photonic, then we
   refer to the cross-connect as a pure OXC. If the granularity of
   channel switching is the wavelength, then the OXC is called a
   wavelength routing switch (WRS), or simply a wavelength router. The
   problem of establishing optical channel trails using WRS is called
   the "Routing and Wavelength Assignment problem" (RWA) [11].

   An OXC may also be equipped with both electrical and optical
   switching capabilities. In this situation, some channels may be
   switched in the electrical domain and others in the optical domain.

   Other terms commonly used within the context of optical transport
   network switching elements include: wavelength selective
   crossconnects (WSXC) and wavelength interchanging crossconnects

   In this document, we use the generic term OXC to refer to all
   categories of programmable and reconfigurable crossconnects for
   optical transport networks, irrespective of the technologies that
   underlie them.

   The OXC control plane design approach described in this document is
   independent of the underlying OXC switch technologies. It is also
   independent of specific OXC implementation details. Local adaptation
   mechanisms can be used to tailor the control plane onto various OXC
   implementations with different hardware capabilities. As an example,
   a local adaption function can map a channel/port input/output

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   relation into specialized low level instructions to actuate a
   rearrangement of the crossconnect switch fabric such that the
   required input/output relation is realized.

   A number of forthcoming supplementary documents will describe in some
   detail the extensions needed to adapt the control plane approach
   described in this memo to various OXC technologies and optical
   transport network contexts.

4.2 Explicit LSPs and Optical Channel Trails

   At a conceptual level, explicit LSPs and optical channel trails
   exhibit certain commonalities. Essentially, they are both
   fundamentally unidirectional, point-to-point virtual path connection
   abstractions.  An explicit LSP provides a parameterized packet
   forwarding path (traffic-trunk) between an ingress LSR and an egress
   LSR. Correspondingly, an optical channel trail provides a (possibly
   parameterized) optical channel between two endpoints for the
   transport of client digital signals.

   The payload carried by both LSPs and optical trails are transparent
   to intermediate nodes along their respective paths. Both LSPs and
   optical trails can be parameterized to stipulate their performance,
   behavioral, and survivability requirements from the network. A set of
   LSPs induce a virtual graph on a data network topology, while a set
   of optical trails induce a virtual graph on the topology of a fiber

   A constraint-based routing scheme can be used to select appropriate
   paths for both LSPs and optical trails. Generally such paths may
   satisfy some demands and policy requirements subject to some
   constraints imposed by the operational environment.

   There are also commonalities in the allocation of labels to LSPs and
   in the allocation of wavelengths to optical trails. Two different
   LSPs that traverse through a given LSR port or interface cannot be
   allocated the same label.  The exception is for LSP aggregation using
   label merge or label stacking.  Similarly, two different optical
   trails that traverse through a given OXC port cannot be allocated the
   same wavelength.

5. Generic Requirements for the OXC Control Plane

   The following section contains the requirements for the OXC control
   plane, with emphasis on the routing components of these requirements.
   There are three key aspects to these requirements:

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     (1) the capability to establish optical channel trails
         expeditiously, (in seconds or even milliseconds rather
         than days or months).

     (2) the capability to support traffic engineering functions,
          (see note below)

     (3) the capability to support various protection and restoration

   Note: the introduction of DWDM and automatically switched optical
   networks is unlikely to eliminate the need for traffic engineering.
   Instead, it will simply mandate OXCs to also support some traffic
   engineering capabilities.

   Historically, the "control plane" of optical transport networks has
   been implemented via network management.  This approach has the
   following drawbacks:

     (1) It leads to relatively slow convergence following failure
         events (typical restoration times are measured in minutes,
         or even days and weeks especially in systems that require
         explicit manual intervention). The only way to expedite
         service recovery in such environments is to pre-provision
         dedicated protection channels.

     (2) It complicates the task of interworking equipment from
         different manufacturers, especially at the management level
         (generally, a custom "umbrella network management system
         -NMS- or operations support system -OSS-" is required to
         integrate otherwise incompatible Element Management Systems
         from different vendors)

     (3) It precludes the use of distributed dynamic routing control
         capabilities in such environments.

      (4) It complicates the task of inter-network provisioning (due
          to the lack of EDI between operator NMSs).

   Thus, another important motivation for the approach described in this
   document is to improve the responsiveness of the optical transport
   network, and to increase the level of interoperability within and
   between service provider networks.

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6. MPLS Traffic Engineering as a Generic Control Plane for OXCs

   The requirements for the OXC control plane described in the previous
   section have already been addressed by the MPLS Traffic Engineering
   control plane, under development by the IETF (see e.g.,

6.1 Overview of the MPLS Traffic Engineering Control Plane

   Let us now discuss the components of the MPLS traffic engineering
   control plane model. The MPLS traffic engineering control plane is a
   synthesis of new concepts in IP traffic engineering (enabled by MPLS)
   and the conventional IP network layer control plane. The high level
   requirements for traffic engineering over MPLS were articulated in
   RFC-2702 [1]. It is the combination of the notions defined in RFC-
   2702 (including relevant extensions) along with the conventional IP
   control plane constructs that effectively establishes a framework for
   the MPLS traffic engineering control plane model [1] (see also [2]).

   The components of the MPLS traffic engineering control plane model
   include the following modules:

    - Resource discovery

    - State information dissemination, which is used to distribute
      relevant information concerning the state of the  network,
      including topology and resource availability information.
      In the MPLS context, this is accomplished by extending
      conventional IP link state interior gateway protocols to carry
      additional information in their link state advertisements
      (see [8,9]).

    - Path selection, which is used to select an appropriate route
      through the MPLS network for explicit routing. It is implemented
      by introducing the concept of constraint-based routing which is
      used to compute paths that satisfy certain specifications subject
      to certain constraints, including constraints imposed by the
      operational environment (see [1]).

    - Path management, which includes label distribution, path
      placement, path maintenance, and path revocation. These are
      used to establish, maintain, and tear down LSPs in the MPLS
      context. The label distribution, path placement, and path
      revocation functions are implemented through a signaling
      protocol, such as the RSVP extensions [3] or through CR-LDP [4].

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   These components of the MPLS traffic engineering control plane are
   separable, and independent of each other. This is a very attractive
   feature because it allows an MPLS control plane to be implemented
   using a composition or synthesis of best of breed modules.

   In RFC-2702 [1], several new MPLS control plane capabilities were
   proposed that allow various traffic engineering policies to be
   actualized in MPLS networks.  Many of these capabilities are also
   relevant and applicable to automatically switched optical transport
   networks with reconfigurable OXCs.

   We summarize some of these capabilities below, focusing on the set of
   attributes that can be associated with traffic-trunks. A traffic-
   trunk is an aggregation of traffic belonging to the same class which
   are forwarded through a common path. In general, the traffic-trunk
   concept is a technology independent abstraction. In [1], it was used
   within the context of MPLS and allowed certain attributes of the
   traffic transported through LSPs to be parameterized. The traffic-
   trunk concept can also be extended, in an obvious manner, to the
   optical transport network.

   As stipulated in RFC-2702 [1], the attributes that can can be
   associated with traffic-trunks include: (1) traffic parameters which
   indicate the bandwidth requirements of the traffic-trunk, (2)
   adaptivity attributes which specify the sensitivity of the traffic-
   trunk to changes in the state of the network and in particular
   indicates whether the traffic-trunk can be re-routed when "better"
   paths become available, (3) priority attributes which impose a
   partial order on the set of traffic-trunks and allow path selection
   and path placement operations to be prioritized, (4) preemption
   attributes which indicate whether a traffic-trunk can preempt an
   existing traffic-trunk from its path, (5) resilience attributes which
   stipulate the survivability requirements of the traffic-trunk and in
   particular the response of the system to faults that impact the path
   of the traffic-trunk, and (6) resource class affinity attributes
   which further restrict route selection to specific subsets of
   resources and in particular allow generalized inclusion and exclusion
   policies to be implemented. Other policy attributes and options are
   also defined by Awduche et al in RFC-2702 [1] for traffic-trunks,
   including policing attributes [1] (policing is irrelevant in the OXC
   context). Concepts of subscription (booking) factors are also
   supported to either bound the utilization of network resources
   through under-subscription or to exploit statistical multiplexing
   gain through over-subscription (this aspect is also not very relevant
   in the OXC context).

   It should be clear that a subset of these capabilities can be mapped
   onto an optical transport network by substituting the term "traffic-

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   trunk" with the term "optical channel trail."

   The MPLS control plane also supports the notion of abstract nodes.
   An abstract node is essentially a set of nodes (e.g., a subnet, an
   autonomous system, etc) whose internal topology is opaque to the
   origination node of an explicit LSP. So, in the most general manner,
   the route of an explicit LSP (or traffic-trunk) can be specified as a
   sequence of single hops and/or as a sequence of abstract nodes.

   The MPLS control plane is very general and is also oblivious of the
   specifics of the data plane technology. In this regard, the MPLS
   control plane can be used in conjunction with a data plane that (a)
   does not necessarily process IP packet headers and (b) does not know
   about IP packet boundaries. For an existence proof, note that the
   MPLS control plane has been implemented on IP-LSRs, ATM-LSRs, and
   Frame Relay-LSRs.

   The MPLS control plane may also be implemented on OXCs as discussed
   in this document.

6.2 Synthesizing the MPLS Traffic Engineering Control Plane with OXCs

   Given that that both OXCs and LSRs require control planes, one option
   would be to have two separate, independent, and incompatible control
   planes - one for OXCs, and another for LSRs. To understand the
   drawbacks of this approach, especially in IP-centric optical
   internetworking systems, one need to look no further than the
   experience with IP over ATM, where IP has its own control plane (BGP,
   IS-IS, OSPF), and ATM its own control plane (PNNI) [12]. For some of
   the drawbacks see [1,2].

   Given that the control planes for both OXCs and LSRs have relatively
   similar requirements, an alternative approach is to develop a
   coherent control plane technology that can be used for LSRs and for
   OXCs. Such a uniform control plane will eliminate the administrative
   complexity of managing hybrid optical internetworking systems with
   separate, dissimilar control and operational semantics.
   Specializations may be introduced in the control plane, as necessary,
   to account for inherent peculiarities of the underlying technologies
   and networking contexts.

   All of the above observations suggest, therefore, that the MPLS
   Traffic Engineering control plane (with some minor extensions) would
   be very suitable as the control plane for OXCs. An OXC that uses the
   MPLS traffic engineering control plane would effectively become an IP
   addressable device. Thus, this proposition also solves the problem of
   addressing for OXCs. The distribution of topology state information,

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   establishment of optical channel trails, OTN traffic engineering
   functions, and protection and restoration capabilities would be
   faciliated by the MPLS Traffic Engineering control plane.

   An out-of-band IP communications system can be used to carry and
   distribute control traffic between the control planes of OXCs,
   perhaps through dedicated supervisory channels (using e.g., dedicated
   wavelengths or channels, or an independent out-of-band IP network).
   In this environment, SNMP, or some other network management
   technology, could be used for element management. From the
   perspective of control semantics, an OXC with an MPLS Traffic
   Engineering control plane would resemble a Label Switching Router.

   If the OXC is a wavelength routing switch, then the physical fiber
   between a pair of OXCs would represent a single link in the OTN
   network topology. Individual wavelengths or channels would be
   analogous to labels.  If there are multiple fibers between a pair of
   OXCs, then as an option, these multiple fibers could be logically
   grouped together through a process called bundling and represented as
   a single link in the OTN network topology.

   If a fiber terminates on a device that functions as both an OXC and
   an IP router, then the following situation  may be possible:

    - A subset of optical channels within the fiber may be uncommitted.
      That is, they are not currently in use and hence are available for

    - A second subset of channels may already be committed for transit
      purposes. That is, they are already cross-connected by the OXC
      element to other out-bound optical channels and thus are not
      immediately available for allocation.

    - Another subset of optical channels (within the same fiber) could
      in use as terminal channels. That is, they are already allocated
      but terminate on the local OXC/router device, for example, as

   In the above scenario one way to represent the fiber in the OTN
   network topology is to depict it is as several links, where one of
   these links would represent the set of uncommitted channels which
   constitute the residual capacity of the fiber; while each terminal
   channel that terminates on the OXC/router could be represented as an
   individual link.

   In the control plane model described here, IS-IS or OSPF, with

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   extensions for traffic engineering ([8] or [9]) and possibly
   additional optical network specific extensions would be used to
   distribute information about the optical transport network topology
   and information about available bandwidth and available channels per
   fiber, as well as other OTN network topology state information. This
   information will then be used to compute explicit routes for optical
   channel trails. An MPLS signaling protocol, such as RSVP extensions
   (see [RSVP]), will be used to instantiate the optical channel trails.
   Using the RSVP extensions, for example, the wavelength information or
   optical channel information (as the case may be) will be carried in
   the LABEL object, which will be used to control and reconfigure the

   The use of a uniform control plane technology for both LSRs and OXCs
   introduces a number of interesting (and potentially advantageous)
   architectural possibilities.  One such possibility is that a single
   control plane (MPLS Traffic Engineering) may be able to span both
   routers and OXCs. In such an environment a Label Switched Path could
   traverse an intermix of routers and OXCs, or could span just routers,
   or just OXCs.  This offers the potential for real bandwidth-on-demand
   networking, in which an IP router may dynamically request bandwidth
   services from the optical transport network. Another possibility is
   that OXCs and LSRs may run different instances of the control plane
   which are decoupled with little or no interaction between the control
   plane instances.

   To bootstrap the system, OXCs must be able to exchange control
   information.  One way to support this is to pre-configure a dedicated
   control wavelength between each pair of adjacent OXCs, or between an
   OXC and a router, and to use this wavelength as a supervisory channel
   for exchange of control traffic. Another possibility, which has
   already been mentioned, is to construct a dedicated out of band IP
   network for the distribution of control traffic.

   Even though an OXC equipped with an MPLS traffic engineering control
   plane would (from a control perspective) resemble a Label Switching
   Router, there are some important distinctions and limitations.  One
   distinction concerns the fact that there are no analogs of label
   merging in the optical domain. This implies that an OXC cannot merge
   several wavelengths into one wavelength.  Another distinction is that
   an OXC cannot perform the equivalent of label push and pop operations
   in the optical domain. This is because the analog of a label in the
   OXC is a wavelength or an optical channel, and the concept of pushing
   and popping wavelengths is infeasible with contemporary commercial
   optical technologies.

   In the proposed control plane approach, an OXC will maintain a WFIB
   (Wavelength Forwarding Information Base) per interface (or per

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   fiber). This is because lambdas and/or channels (labels) are specific
   to a particular interface (fiber), and the same lambda and/or channel
   (label) could be used concurrently on multiple interfaces (fibers).

   The MPLS traffic engineering control plane is already being
   implemented on data plane technologies that exhibit some of the
   aforementioned distinctions. For example, an ATM-LSR supports only a
   subset of the MPLS functionality.  In particular, most ATM-LSRs are
   incapable of merging Label Switching Paths, and may not be able to
   perform label push/pop operations as well. Also, similar to the
   approach proposed here for OXCs, ATM-LSRs have per interface LFIB
   (Label Forwarding Information Base).

   Yet another important distinction concerns the granularity of
   resource allocation. An MPLS Label Switching Router which operates in
   the electrical domain can potentially support an arbitrary number of
   LSPs with arbitrary bandwidth reservation granularities (bounded by
   the maximum reservable bandwidth per interface, the label space, and
   the amount of required control overhead). In sharp contrast, an OXC
   can only support a relatively small number of optical channel trails
   (this may change as the technology evolves), each of which will have
   coarse discrete bandwidth granularities (e.g.,OC-12, OC-48, OC-192,
   and OC-768). A special degenerate case occurs when the control plane
   is used to establish optical channel trails which all have a fixed
   bandwidth (e.g., OC-48).

   If the bandwidth associated with an LSP is small relative to the
   capacity of an optical channel trail, then very inefficient
   utilization of network resources could result if only one LSP is
   mapped onto a given optical channel trail. To improve utilization of
   resources, therefore, it is necessary to be able to map several low
   bandwidth LSPs onto a relatively high capacity optical channel trail.
   For this purpose, a generalized notion of "nested LSPs" may be used.
   Note that since an OXC cannot perform label push/pop operations, the
   start/end of a nested LSP has to be on a router (as nesting requires
   label push/pop). Also note that in this nesting situation, it is the
   wavelength of the "container" optical channel trail itself that
   effectively constitutes the outermost label.

   The transparency and multiprotocol properties of the MPLS Control
   Plane approach would allow an OXC to route optical channel trails
   carrying various types of digital payloads (including IP, ATM, Sonet,
   etc) in a coherent and uniform way.

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7. Control Adaptation

   This section provides a high level overview of the architectural
   considerations involved in tailoring the MPLS traffic engineering
   control plane model to the optical domain. More detailed discussions
   of these issues will be provided in a number of supplementary
   documents to follow.

   In adapting the MPLS traffic engineering control plane model to OXCs,
   a number of critical issues should be considered. One critical issue
   concerns the development of OTN specific domain models which
   abstracts and captures relevant characteristics of the OTN. The
   domain models help to delineate the design space for the control
   plane problem in OXCs, and to construct domain specific software
   reference architectures.

   A domain model includes functional and structural aspects. For the
   purpose of the present discussions, however, we have grouped the
   considerations pertaining to OTN domain models into two broad
   categories: (1) a horizontal dimension and (2) a vertical dimension.
   The horizontal dimension pertains to the specialized networking
   requirements of the OTN environment. It indicates the enhancements
   needed to the MPLS TE control plane model to address the peculiar OTN
   networking requirements.  The vertical dimension pertains to
   localized hardware and software characteristics of the OXCs, which
   helps to determine the device specific adaptations and support
   mechanisms needed to port and reuse the MPLS TE control plane
   software artifacts on an OXC.

   Horizontal dimension considerations include the following aspects:

     - What type of OTN state information should be discovered and
       disseminated to support path selection for optical channel
       trails?  Such state information may include domain specific
       characteristics of the OTN (encoded as metrics), such as
       attenuation, dispersion (chromatic, PMD), etc. This aspect will
       determine the type of additional extensions that are required
       to IGP link state advertisements to allow distribute such

     - What infrastructure will be used to propagate the control

     - How are constrained paths computed for optical channel trails
       which fulfill a set of performance and policy requirements
       subject to a set of system constraints?

     - What are the domain specific requirements for setting up optical

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       channel trails and what are the enhancements needed to existing
       MPLS signaling protocols to address these requirements?

   Vertical dimension considerations include the aspects required to
   practically port MPLS control plane software onto an OXC.  In terms
   of vertical dimensions, a candidate system architecture for an OXC
   equipped with an MPLS control plane model is shown in Figure 1 below.

      |                                |
      |       -------------------      |
      |      |                   |     |
      |      | MPLS Control Plane|     |
      |      |                   |     |
      |       -------------------      |
      |               |                |
      |       -------------------      |
      |      |                   |     |
      |      |Control Adaptation |     |
      |       -------------------      |
      |      |    OXC Switch     |     |
      |      |    Controller     |     |
      |       -------------------      |
      |               |                |
      |       -------------------      |
      |      |                   |     |
      |      | OXC Switch Fabric |     |
      |      | OXC Data Plane    |     |
      |       -------------------      |
      |                                |

          Figure 1: Candidate OXC systems architecture

8. Architectural Considerations for Deployment in Operational Networks

   This section provides a high level overview of the architectural
   considerations for deployment of the proposed control plane in
   operational networks consisting of LSRs and OXCs. These architectural
   issues have implications on the degree of control isolation, control
   coupling, and control cohesion between LSRs and OXCs.

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   Essentially, there are two extremal architectural options for
   deployment of the proposed control plane in an operational context
   consisting of LSRs and OXCs.

    - Overlay Option: One option is to use different instances of the
      control plane in the OTN (OXC) and IP (LSR) domains. In this
      situation, each instance of the control plane will operate
      independent of the other. Interworking (including control
      coordination) between the two domains can be established through
      static configuration or through some other procedures that are
      outside the scope of this document. This partitioned and
      explicitely decoupled deployment option allows maximal
      control isolation between the OTN and IP domains. This scheme is
      conceptually similar to the model in use today, whereby the OTN
      simply provides point-to-point channels between IP network
      elements with very minimal control interaction between the two

    - Peer Option: Another option is to use a single instance of the
      control plane that subsumes and spans LSRs and OXCs.

   Other architectural options are also possible which allow various
   degrees of control isolation and control integration between the OXCs
   and LSRs.

   To improve scalability the control plane may use routing hierarchy
   (e.g., routing areas). Hierarchy may be applied in either of the
   situations mentioned above. Furthermore, in the overlay option with
   different control plane instances for OXCs and LSRs, hierarchy could
   be enabled for each control plane instance independent of the other.

   In the deployment option with a single instance of the control plane,
   each routing area may maintain a link state database that contains:
   (1) physical LSPs (fiber links), (2) optical LSPs (optical channel
   trails), and (3) logical LSPs (conventional label switched paths). As
   a general rule, all of these path-oriented connection entities could
   simply be considered as LSPs with different characteristics. The
   origination LSR (the head-end) of each LSP entity may locally decide
   whether to advertise the LSP (with appropriate attributes), so that
   other LSRs could use it as a link for subsequent path computations.

   There are significant tradeoffs to the above deployment options,
   including aspects related to scalability and fault isolation.
   Additional documents to follow may elaborate on some of these

   One of the advantages of the control plane design approach described
   in this memo is that it potentially allows network administrators the

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   leeway to make these deployment architectural decisions based on
   their specific objectives, network contexts, and service models.

9. Summary

   This document outlined how the MPLS Traffic Engineering control plane
   could be adapted and reused as the control plane for optical
   crossconnects.  Such a control plane would be used to distribute
   optical transport network topology state information and to setup
   optical channel trails. Such a control plane would support various
   traffic engineering functions in the optical domain, and enable a
   variety of protection and restoration capabilities.  Furthermore,
   such a control plane technology would expedite the development and
   deployment of a new class of versatile data-centric OXCs.
   Additionally, the proposed control plane approach would simplify
   integration of OXCs and label switching routers. Finally, the
   proposed control plane approach would provide coherent semantics for
   network management and operations control in hybrid optical
   internetworking systems consisting of LSRs and OXCs.


   [1] D. Awduche, J. Malcolm, J. Agogbua, M. O'Dell, and J.
   McManus,"Requirements for Traffic Engineering Over MPLS," RFC-
   2702,September 1999.

   [2] D. Awduche, "MPLS and Traffic Engineering in IP Networks," IEEE
   Communications Magazine, December, 1999.

   [3] D. Awduche, L. Berger, D. Gan, T. Li, G. Swallow, and V.
   Srinivasan, "Extensions to RSVP for LSP Tunnels," Internet Draft,Work
   in Progress, 1999

   [4] B. Jamoussi et al, "Constraint-Based LSP Setup using
   LDP,"Internet Draft, Work in Progress, 1999

   [5] ITU-T G.872, "Architecture for Optical Transport Networks," 1999.

   [6] R. Callon, P. Doolan, N. Feldman, A. Fredette, G. Swallow,A.
   Viswanathan, "A Framework for Multiprotocol Label Switching,"Internet
   Draft, Work in Progress, 1999

   [7] E. Rosen, A. Viswanathan, R. Callon, "Multiprotocol Label

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   Switching Architecture," Internet Draft, Work in Progress, 1999

   [8] H. Smit and T. Li, "IS-IS extensions for Traffic
   Engineering,"Internet Draft, Work in Progress, 1999

   [9] D. Katz, D. Yeung, "Traffic Engineering Extensions to
   OSPF,"Internet Draft, Work in Progress, 1999

   [10] Y. Rekhter and T. Li, "A Border Gateway Protocol 4 (BGP-
   4),"RFC-1171, March 1995

   [11] B. Mukherjee, "Optical Communications Networks," McGraw-Hill,

   [12] G. Newsome and P. Bonenfant, "The Automatic Switched Optical
   Network," Contribution to T1 STANDARDS PROJECT - T1X1.5,  1999.

   [13] P. Bonenfant and and X. Liu, "A Proposal for Automatically
   Switched Optical Channel Networks (ASON)," Contribution to T1
   STANDARDS PROJECT - T1X1.5, 1999.

   [14] "T. Worster, "General Switch Management Protocol," Internet
   Draft, Work in Progress, 1999.

   [15] E. Crawley, R. Nair, B. Rajagopalan, and H. Sandick, "A
   Framework for QoS-Based Routing in the Internet," RFC-2386, August,

Security Considerations

   It is imperative to guarantee the integrity and confidentiality of
   control information used by the proposed OXC control plane. This can
   be accomplished by using existing security mechanisms for the various
   components of the MPLS traffic engineering control plane model.

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Author's Addresses

   Daniel O. Awduche
   UUNET (Worldcom)
   22001 Loudoun County Parkway
   Ashburn, VA 20147
   Phone: 703-886-5277

   Yakov Rehkter
   Cisco Systems
   170 W. Tasman Dr.
   San Jose, CA 95134

   John Drake
   Calient Networks
   5853 Rue Ferrari
   San Jose, CA 95138
   Phone: (408) 972-3720

   Rob Coltun
   Redback Networks
   300 Ferguson Drive
   Mountain View, CA 94043
   Phone: (650) 390-9030

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