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Versions: 00 01 02 03 04 05 rfc3717                                     
                                             Bala Rajagopalan
Internet Draft                                 Tellium, Inc.
draft-ietf-ipo-framework-00.txt              James Luciani
Expires on: 1/13/2002                          Crescent Networks
                                             Daniel Awduche
                                               Movaz Networks
                                             Brad Cain
                                               Cereva Networks
                                             Bilel Jamoussi
                                               Nortel Networks
                                             Debanjan Saha
                                               Tellium, Inc.

                   IP over Optical Networks: A Framework

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-
   Drafts. Internet-Drafts are draft documents valid for a maximum of
   six months and may be updated, replaced, or obsoleted by other
   documents at any time. It is inappropriate to use Internet- Drafts
   as reference material or to cite them other than as "work in

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

1. Abstract

   The Internet transport infrastructure is moving towards a model of
   high-speed routers interconnected by optical core networks. The
   architectural choices for the interaction between IP and optical
   network layers, specifically, the routing and signaling aspects, are
   maturing. At the same time, a consensus has emerged in the industry
   on utilizing IP-based protocols for the optical control plane. This
   draft defines a framework for IP over Optical networks, considering
   both the IP-based control plane for optical networks as well as IP-
   optical network interactions (together referred to as "IP over
   optical networks").

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                             Table of Contents

   1. Abstract........................................................1
   2. Conventions used in this document...............................3
   3. Introduction....................................................3
   4. Terminology and Concepts........................................4
   5. The Network Model...............................................8
    5.1  Network Interconnection.....................................8
    5.2  Control Structure..........................................10
   6. IP over Optical Service Models and Requirements................12
    6.1  Domain Services Model......................................12
    6.2  Unified Service Model......................................13
    6.3  Which Service Model?.......................................14
    6.4 What are the Possible Services?.............................14
   7. IP transport over Optical Networks.............................15
    7.1 Interconnection Models......................................15
    7.2 Routing Approaches..........................................16
    7.3 Signaling-Related...........................................19
   8. IP-based Optical Control Plane Issues..........................22
    8.1  Addressing.................................................23
    8.2  Neighbor Discovery.........................................24
    8.3  Topology Discovery.........................................25
    8.4  Restoration Models.........................................26
    8.5  Route Computation..........................................27
    8.6  Signaling Issues...........................................29
    8.7   Optical Internetworking...................................31
   9. Other Issues...................................................32
    9.1   WDM and TDM in the Same Network...........................32
    9.2   Wavelength Conversion.....................................32
    9.3   Service Provider Peering Points...........................32
    9.4   Rate of Lightpath Set-Up..................................33
    9.5   Distributed vs. Centralized Provisioning..................34
    9.6   Optical Networks with Additional Configurable Components..34
    9.7   Optical Networks with Limited Wavelength Conversion
   10.  Evolution Path for IP over Optical Architecture..............35
   11. Security Considerations.......................................36
   12. Summary and Conclusions.......................................36
   13. References....................................................36
   14. Acknowledgments...............................................38
   15. Author's Addresses............................................38

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2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   this document are to be interpreted as described in RFC 2119.

3. Introduction

   Optical network technologies are evolving rapidly in terms of
   functions and capabilities. The increasing importance of optical
   networks is evidenced by the copious amount of attention focused on
   IP over optical networks and related photonic and electronic
   interworking issues by all the major network service providers,
   telecommunications equipment vendors, and standards organizations.
   In this regard, the term "optical network" is used generically in
   practice to refer to both SONET/SDH-based transport networks, as
   well as transparent all-optical networks.

   It has been realized that optical networks must be survivable,
   flexible, and controllable. There is, therefore, an ongoing trend to
   introduce intelligence in the control plane of optical networks to
   make them more versatile [1]. An essential attribute of intelligent
   optical networks is the capability to instantiate and route optical
   layer connections in real-time or near real-time, and to provide
   capabilities that enhance network survivability. Furthermore, there
   is a need for multi-vendor optical network interoperability, when an
   optical network may consist of interconnected vendor-specific
   optical sub-networks.

   The optical network must also be versatile because some service
   providers may offer generic optical layer services that may not be
   client-specific. It would therefore be necessary to have an optical
   network control layer that can handle such generic optical services.

   There is general consensus in the industry that the optical network
   control plane should utilize IP-based protocols for dynamic
   provisioning and restoration of lightpaths within and across optical
   sub-networks. This is based on the practical view that signaling and
   routing mechanisms developed for IP traffic engineering applications
   could be re-used in optical networks. Nevertheless, the issues and
   requirements that are specific to optical networking must be
   understood to suitably adopt the IP-based protocols. This is
   especially the case for restoration.  Also, there are different
   views on the model for interaction between the optical network and
   client networks, such as IP networks. Reasonable architectural
   alternatives in this regard must be supported, with an understanding
   of their pros and cons.

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   Thus, there are two fundamental issues related to IP over optical
   networks. The first is the adaptation and reuse of IP control plane
   protocols within the optical network control plane, irrespective of
   the types of digital clients that utilize the optical network. The
   second is the transport of IP traffic through an optical network
   together with the control and coordination issues that arise

   This draft defines a framework for IP over optical networks covering
   the requirements and mechanisms for establishing an IP-centric
   optical control plane, and the architectural aspects of IP transport
   over optical networks. In this regard, it is recognized that the
   specific capabilities required for IP over optical networks would
   depend on the services expected at the IP-optical interface as well
   as the optical sub-network interfaces.  Depending on the specific
   operational requirements, a progression of capabilities is possible,
   reflecting increasingly sophisticated interactions at these
   interfaces. This draft therefore advocates the definition of
   "capability sets" that define the evolution of functionality at the
   interfaces as more sophisticated operational requirements arise.

   This draft is organized as follows. In the next section, terminology
   covering certain concepts related to this framework are described.
   In Section 5, the network model pertinent to this framework is
   described. The service model and requirements for IP-optical, and
   multi-vendor optical internetworking are described in Section 6.
   This section presently considers certain general requirements.
   Specific operational requirements may be accommodated in this
   section as they arise.  Section 7 considers the architectural models
   for IP-optical interworking, describing the pros and cons of each
   model. It should be noted that it is not the intent of this draft to
   promote any particular model over the others. However, particular
   aspects of the models that may make one approach more appropriate
   than another in certain circumstances are described. Section 8
   describes IP-centric control plane mechanisms for optical networks,
   covering signaling and routing issues in support of provisioning and
   restoration. Section 9 describes certain specialized issues in
   relation to IP over optical networks.  The approaches described in
   Section 7 and 8 range from the relatively simple to the
   sophisticated. Section 10 describes a possible evolution path for IP
   over optical networking capabilities in terms of increasingly
   sophisticated functionality supported. Section 11 considers security
   aspects. Finally, summary and conclusion are presented in Section

4. Terminology and Concepts

   This section introduces the terminology pertinent to this framework
   and some related concepts.

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   Wavelength Division Multiplexing (WDM) is a technology that allows
   multiple optical signals operating at different wavelengths to be
   multiplexed onto a single fiber so that they can be transported in
   parallel through the fiber. In general, each optical wavelength may
   carry digital client payloads at a different data rate (e.g., OC-3c,
   OC-12c, OC- 48c, OC-192c, etc.) and in a different format (SONET,
   Ethernet, ATM, etc.) For example, there are many commercial WDM
   networks in existence today that support a mix of SONET signals
   operating at OC-48c (approximately 2.5 Gbps) and OC-192
   (approximately  10 Gbps) over a single optical fiber. An optical
   system with WDM capability can achieve parallel transmission of
   multiple wavelengths gracefully while maintaining high system
   performance and reliability.  In the near future, commercial WDM
   systems are expected to concurrently carry  more than 160
   wavelengths at data rates of OC-192c and above, for a total of 1.6
   Tbps or more. The term WDM will be used  in this document to refer
   to both WDM and DWDM (Dense WDM).

   In general, it is worth noting that WDM links are affected by the
   following factors, which may introduce impairments into the optical
   signal path:

   1. The number of wavelengths on a single fiber.
   2. The serial bit rate per wavelength.
   3. The type of fiber.
   4. The amplification mechanism.
   5. The number of nodes through which the signal passes before
      it reaches the egress node or before regeneration.

   All these factors and others not mentioned here constitute domain
   specific features of optical transport networks. As noted in [1],
   these features should be taken into account in developing standards
   based solutions for IP over optical networks.

   Optical cross-connect (OXC)

   An OXC is a space-division switch that can switch an optical data
   stream on an input port to a output port. Such a switch may have
   optical-electrical conversion at the input port and electrical-
   optical conversion at the output port, or it can be all-optical. An
   OXC is assumed to have a control-plane processor that implements
   signaling and routing protocols necessary for realizing an optical

   Optical channel trail or Lightpath

   An optical channel trail is a point-to-point optical layer
   connection between two access points in an optical network. In this

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   draft, the term "lightpath" is used interchangeably with optical
   channel trail.

   Optical mesh sub-network

   An optical sub-network, as discussed in this framework, is a network
   of OXCs that supports end-to-end networking of optical channel
   trails  providing functionality like routing, monitoring, grooming,
   and protection and restoration of optical channels. The
   interconnection of OXCs in this network can be based on a general
   topology (also called "mesh" topology) Underlying this network could
   be the following:

   (a) An optical multiplex section (OMS) layer network : The optical
       multiplex section layer provides the transport of the optical
       channels.  The information contained in this layer is a data
       stream comprising a set of n optical channels, which have a
       defined aggregate bandwidth.

   (b) An optical transmission section (OTS) layer network : This
       provides functionality for transmission of the optical signal on
       optical media of different types.

   This framework does not address the interaction between the optical
   sub-network and the OMS, or between the OMS and OTS layer networks.

   Mesh optical network (or simply, "optical network")

   A mesh  optical network, as considered in draft, is a mesh-connected
   collection of optical sub-networks. Such an optical network is
   assumed to be under a single administration. It is also possible to
   conceive of a large scale global mesh optical network consisting of
   the voluntary interconnection of autonomous optical networks, each
   of which is owned and administered by an independent entity. In this
   circumstance, abstraction can be used to hide the internal details
   of each autonomous optical cloud from the remainder of the network.

   Optical internetwork

   An optical internetwork is a mesh-connected collection of optical
   networks. Each of these networks may be under different

   Wavelength continuity property

   A lightpath is said to satisfy the wavelength continuity property if
   it is transported over the same wavelength end-to-end. Wavelength
   continuity is required in optical  networks with no wavelength
   conversion feature.

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   Wavelength path

   A lightpath that satisfies the wavelength continuity property is
   called a wavelength path.

   Opaque vs. transparent optical networks

   A transparent optical network is an optical network in which each
   transit node along a path passes optical transmission without using
   transducers  to convert the optical signal into an electrical signal
   and back again to an optical signal.  More generally, all
   intermediate nodes in a transparent optical network will pass
   optical signals without performing retiming and reshaping and thus
   such nodes are unaware of many characteristics of the carried
   signals.  One could, for example, carry analog signals together with
   digital signals (potentially of varying bit rate) on different
   wavelengths over such a network.

   Note that amplification  of signals at transit nodes is
   permitted in transparent optical networks.  This is a result of the
   availability of all optical amplification devices such as Erbium
   Doped Fiber Amplifiers (EDFAs).

   In opaque optical networks, by comparison, transit nodes will
   manipulate  the optical signals as the signals traverse through
   them.   An example of such manipulation would be 3R (reshaping,
   retiming, regeneration/amplification).

   Trust domain

   A trust domain is a network under a single technical administration
   in which most nodes in the network are considered to be secure or
   trusted in some fashion.  An example of a trust domain is a campus
   or corporate network in which all routing protocol packets are
   considered to be authentic, without the need for additional security
   schemes to prevent unauthorized access to the network
   infrastructure.  Generally, the "single" administrative control rule
   may be relaxed in practice if a set of administrative entities agree
   to trust one another to form an enlarged heterogeneous trust domain.
   However, to simplify the discussions in this draft, it will be
   assumed, without loss of generality, that the term trust domain
   applies to a single administrative entity.


   For the purpose of this document, the term flow will be used to
   represent the smallest non-separable stream of data, as seen by an
   endpoint (source or destination node).  It is to be noted that the
   term flow is heavily overloaded in the networking literature. Within

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   the context of this document, it may be convenient to consider a
   wavelength as a flow under certain circumstances. However, if there
   is a method to partition the bandwidth of the wavelength, then each
   partition may be considered a flow, for example by quantizing time
   into some nicely manageable intervals, it may be feasible to
   consider    each quanta of time within a given wavelength as a flow.

   Traffic Trunk

   A traffic trunk is an abstraction of traffic flow that follows the
   same path between two access points which allows some
   characteristics and attributes of the traffic to be parameterized.

5. The Network Model

5.1  Network Interconnection

   The network model considered in this memo consists of IP routers
   attached to an optical core internetwork, and connected to their
   peers over dynamically established switched lightpaths. The optical
   core itself is assumed to be incapable of processing individual IP
   packets in the data plane.

   The optical internetwork is assumed to consist of multiple optical
   networks, each may possibly be administered by a different entity.
   Each optical network consists of sub-networks interconnected by
   optical links in a general topology (referred to as an optical mesh
   network). This network may contain re-configurable optical equipment
   from a single vendor or from multiple vendors. In the near term, it
   may be expected that each sub-network will consist of a single
   vendor switches. In the future, as standardization efforts mature,
   each optical sub-network may in fact contain optical switches from
   different vendors. In any case, each sub-network itself is assumed
   to be mesh-connected internally. In general, it can be expected that
   topologically adjacent OXCs in an optical mesh network will be
   connected via multiple, parallel (bi-directional) optical links.
   This network model is shown in Figure 1.

   Here, an optical sub-network may consist entirely of all-optical
   OXCs or OXCs with optical-electrical-optical (OEO) conversion.
   Interconnection between sub-networks is assumed to be through
   compatible physical interfaces, with suitable optical-electrical
   conversions where necessary. The routers that have direct physical
   connectivity with the optical network are referred to as "edge
   routers". As shown in the figure, other client networks (e.g., ATM)
   may connect to the optical network.

   The switching function in an OXC is controlled by appropriately
   configuring the cross-connect fabric. Conceptually, this may be
   viewed as setting up a cross-connect table whose entries are of the
   form <input port i, output port j>, indicating that the data stream
   entering input port i will be switched to output port j.  In the

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   context of a wavelength selective cross-connect (generally referred
   to as a WXC), the cross-connect tables may also indicate the input
   and output wavelengths along with the input and output ports. A
   lightpath from an ingress port in an OXC to an egress port in a
   remote OXC is established by setting up suitable cross-connects in
   the ingress, the egress and a set of intermediate OXCs such that a
   continuous physical path exists from the ingress to the egress port.
   Optical paths tend  to be bi-directional, i.e., the return path from
   the egress port to the ingress port is typically routed along the
   same set of intermediate ports as the forward path, but this may not
   be the case under all circumstances.

                              Optical Network
                           |                                        |
                           |           Optical Subnetwork           |
      +--------------+     | +------------------------------------+ |
      |              |     | |  +-----+      +-----+      +-----+ | |
      |   IP         |     | |  |     |      |     |      |     | | |
      |   Network    +--UNI--+--+ OXC +------+ OXC +------+ OXC + | |
      |              |     | |  |     |      |     |      |     | | |
      +--------------+     | |  +--+--+      +--+--+      +--+--+ | |
                           | +-----|------------|------------|----+ |
                           |       |            |            |      |
                           |      INNI         INNI         INNI    |
      +--------------+     |       |            |            |      |
      |              |     | +-----+------+     |    +-------+----+ |
      |   IP         +--UNI--|            +-----+    |            | |
      |   Network    |     | |   Optical  |          |   Optical  | |
      |              |     | | Subnetwork +---INNI---+ Subnetwork | |
      +--------------+     | |            |          |            | |
                           | +------+-----+          +------+-----+ |
                           |        |                       |       |
                                    |                       |
                                   ENNI                    ENNI
                                    |                       |
                           |                                        |
                           |            Optical Network             |
                           |                                        |
                                    |                       |
                                   UNI                     UNI
                                    |                       |
                             +------+-------+        +------+-----+
                             |              |        |            |
                             | Other Client |        |Other Client|
                             |   Network    |        |   Network  |
                             | (e.g., ATM)  |        |            |
                             +--------------+        +------------+

                   Figure 1: Optical Internetwork Model

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   Multiple data streams output from an OXC may be multiplexed onto an
   optical link using WDM technology. The WDM functionality may exist
   outside of the OXC, and be transparent to the OXC. Or, this function
   may be built into the OXC. In the latter case, the cross-connect
   table   (conceptually) consists of pairs of the form, <{input port
   i, Lambda(j)}, {output port k, Lambda(l)}>. This indicates that the
   data stream received on wavelength Lambda(j) over input port i is
   switched to output port k on Lambda(l). Automated establishment of
   lightpaths involves setting up the cross-connect table entries in
   the appropriate OXCs in a coordinated manner such that the desired
   physical path is realized.

   Under this network model, a switched lightpath must be established
   between a pair of IP routers before they can communicate. This
   lightpath might traverse multiple optical networks and be subject to
   different provisioning and restoration procedures in each
   network.  The IP-based control plane issue is that of designing
   standard signaling and routing protocols for coherent end-to-end
   provisioning and restoration of lightpaths across multiple optical
   networks. Similarly, IP transport over such an optical network
   involves determining IP reachability and seamless establishment of
   paths from one IP endpoint to another over an optical core

5.2  Control Structure

   There are three logical control interfaces identified in Figure 1.
   These are the client-optical internetwork interface, the internal
   node-to-node interface within a network (between OXCs in different
   sub-networks), and the external node-to-node interface between nodes
   in different networks. These interfaces are also referred to as the
   User-Network Interface (UNI), the internal NNI (INNI), and the
   external NNI, respectively. The distinction between these interfaces
   arises out of the type and amount of control information flow across
   them. The UNI represents a service  boundary between the client and
   optical networks.  The client and server are essentially two
   different roles: the client role requests a service connection from
   a server; the server role establishes the connection to fulfill the
   service request -- provided all relevant admission control
   conditions are satisfied. Thus, the control flow across the UNI is
   dependent on  the set of services defined across it and the manner
   in which the services may be accessed. The service models are
   described in Section 7. The NNIs represent vendor-independent
   standardized control flow between nodes. The distinction between the
   INNI and the ENNI is that the former is an interface within a given
   network under a single technical administration, while the later
   indicates an interface at the administrative boundary between
   networks. The INNI and ENNI may thus differ in the policies that
   restrict  control flow between nodes. Security, scalability,
   stability, and information hiding are important considerations in
   the specification of the ENNI. It is possible in principle to
   harmonize the control flow across the UNI and the NNI and eliminate

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   the distinction between them. On the other hand, it may be required
   to minimize control flow information, especially routing-related
   information, over the UNI; and even over the ENNI.  In this case,
   UNI and NNIs may look different in some respects. In this draft,
   these interfaces are treated as distinct.

   The UNI can be categorized as public or private depending upon
   context and service models. Routing information (ie, topology state
   information) can be exchanged across a private UNI. On the other
   hand, such information is not exchanged across a public UNI, or such
   information may be exchanged with very explicit restrictions
   (including for example abstraction, filtration, etc). Thus,
   different relationships (e.g., peer or over-lay, Section 7) may
   occur across private and public logical interfaces.

   The physical control structure used to realize these logical
   interfaces may vary. For instance, for the UNI, some of the
   possibilities are:

   1.Direct interface: An in-band or out-of-band IP control channel
     (IPCC) may be implemented between an edge router and each OXC
     to  which it is connected.  This control channel is used for
     exchanging signaling and routing messages between the router and
     the OXC. With a direct interface, the edge router and the OXC it
     connects to are peers in the control plane. This is shown in
     Figure 2. The type of routing and signaling information exchanged
     across  the direct interface would vary depending on the service
     definition. This issue is dealt with in the next section. Some
     choices for  the routing protocol are OSPF/ISIS (with traffic
     engineering Extensions and additional extensions to deal with the
     peculiar characteristics of optical networks) or BGP, or some
     other protocol. Other directory-based routing information
     exchanges are also possible. Some of the signaling protocol
     choices are adaptations of RSVP-TE or CR-LDP. The details of how
     the IP control channel is realized is outside the scope of this

   2.Indirect interface: An out-of-band IP control channel may be
     implemented between the client and a device in the optical network
     to signal service requests and responses. For instance, a
     management system or a server in the optical network may receive
     service requests from clients. Similarly, out-of-band signaling
     may be used between management systems in client and optical
     networks to signal service requests. In these cases, there is no
     direct control interaction between clients and respective
      OXCs. One reason to have an indirect interface would be that the
      OXCs and/or clients do not support a direct signaling interface.

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   +-----------------------------+      +-----------------------------+
   |                             |      |                             |
   |  +---------+   +---------+  |      |  +---------+   +---------+  |
   |  |         |   |         |  |      |  |         |   |         |  |
   |  | Routing |   |Signaling|  |      |  | Routing |   |Signaling|  |
   |  | Protocol|   |Protocol |  |      |  | Protocol|   |Protocol |  |
   |  |         |   |         |  |      |  |         |   |         |  |
   |  +-----+---+   +---+-----+  |      |  +-----+---+   +---+-----+  |
   |        |           |        |      |        |           |        |
   |        |           |        |      |        |           |        |
   |     +--+-----------+---+    |      |     +--+-----------+---+    |
   |     |                  |    |      |     |                  |    |
   |     |     IP Layer     +......IPCC.......+     IP Layer     |    |
   |     |                  |    |      |     |                  |    |
   |     +------------------+    |      |     +------------------+    |
   |                             |      |                             |
   |         Edge Router         |      |             OXC             |
   +-----------------------------+      +-----------------------------+

                            Figure 2: Direct Interface

   3. Provisioned interface: In this case, the optical network services
      are manually provisioned and there is no control interactions
      between the client and the optical network.

   Although different control structures are possible, further
   descriptions in this framework assume direct interfaces for IP-
   optical and optical sub-network control interactions.

6. IP over Optical Service Models and Requirements

   In this section, the service models and requirements at the UNI and
   the NNIs are considered. Two general models have emerged for the
   services at the UNI (which can also be applied at the NNIs). These
   models are as follows.

6.1  Domain Services Model

   Under the domain services model, the optical network primarily
   offers high bandwidth connectivity in the form of lightpaths.
   Standardized signaling across the UNI (Figure 1) is used to invoke
   the following

  1. Lightpath creation: This service allows a lightpath with the
     specified attributes to be created between a pair of termination
     points in the optical network. Lightpath creation may be subject
     to network-defined policies (e.g., connectivity restrictions) and
     security procedures.

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   2. Lightpath deletion: This service allows an existing lightpath to
      be deleted.

   3. Lightpath modification: This service allows certain parameters of
      the lightpath to be modified.

   4. Lightpath status enquiry: This service allows the status of
      certain parameters of the lightpath (referenced by its ID) to be
      queried by the router that created the lightpath.

   An end-system discovery procedure may be used over the UNI to verify
   local port connectivity between the optical and client devices, and
   allows each device to bootstrap the UNI control channel. Finally, a
   "service discovery" procedure may be employed as a precursor to
   obtaining UNI services. Service discovery allows a client to
   determine the static parameters of the interconnection with the
   optical network, including the UNI signaling protocols supported.
   The protocols for neighbor and service discovery are different from
   the UNI signaling protocol itself (for example, see LMP [2]).

   Because a small set of well-defined services is offered across the
   UNI, the signaling protocol requirements are minimal. Specifically,
   the signaling protocol is required to convey a few messages with
   certain attributes in a point-to-point manner between the router and
   the optical network. Such a protocol may be based on RSVP-TE or LDP,
   for example.

   The optical domain services model does not deal with the type and
   nature of routing protocols within and across optical networks.

   The optical domain services model would result in the establishment
   of a lightpath topology between routers at the edge of the optical
   network. The resulting overlay model for IP over optical networks
   is discussed in Section 7.

6.2  Unified Service Model

   Under this model, the IP and optical networks are treated together
   as a single integrated network from a control plane point of view.
   In this regard, the OXCs are treated just like any other router as
   far as the control plane is considered. Thus, in principle, there is
   no distinction between the UNI, NNIs and any other router-to-router
   interface from a routing and signaling point of view. It is assumed
   that this control plane is MPLS-based, as described in [1]. The
   unified service model has so far been discussed only in the context
   of a single administrative domain. A unified control plane is
   possible even when there are administrative boundaries within an
   optical internetwork, but some of the integrated routing
   capabilities may  not be practically attractive or even feasible in
   this case (see Section 7).

   Under the unified service model, optical network services are
   obtained implicitly during end-to-end MPLS signaling. Specifically,

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   an edge router can create a lightpath with specified attributes, or
   delete and modify lightpaths as it creates MPLS label-switched paths
   (LSPs). In this regard, the services obtained from the optical
   network are similar to the domain services model. These services,
   however, may be invoked in a more seamless manner as compared to the
   domain services model. For instance, when routers are attached to a
   single optical network (i.e., there are no ENNIs), a remote router
   could compute an end-to-end path across the optical internetwork. It
   can then establish an LSP across the optical internetwork. But the
   edge routers must still recognize that an LSP  across the optical
   internetwork is a lightpath, or a conduit for multiple LSPs. The
   concept of "forwarding adjacency" can be used to specify virtual
   links across optical internetworks in routing protocols such as OSPF
   [3]. In essence, once a lightpath is established across an optical
   internetwork between two edge routers, it can be advertised as a
   forwarding adjacency (a virtual link) between these routers.  Thus,
   from a data plane point of view, the lightpaths result in a virtual
   overlay between edge routers. The decisions as to when to create
   such lightpaths, and the bandwidth management for these lightpaths
   is identical in both the domain services model and the unified
   service model. The routing and signaling models for unified services
   is described in Section 7.

6.3  Which Service Model?

   The pros and cons of the above service models can be debated at
   length, but the approach recommended in this framework is to define
   routing and signaling mechanisms in support of both. As pointed out
   above, signaling for service request can be unified to cover both
   models. The developments in GMPLS signaling [4] for the unified
   service model and its adoption for UNI signaling [5, 6] under the
   domain services model essentially supports this view. The
   significant difference between the service models, however, is in
   routing protocols, as described in Section 7.

6.4 What are the Possible Services?

   Specialized services may be built atop the point-to-point
   connectivity service offered by the optical network. For example,

6.4.1  Virtual Private Networks (VPNs)

   Given that the data plane between IP routers over an optical network
   amounts to  a virtual topology which is an overlay over the optical
   network, it is easy to imagine a virtual private network of
   lightpaths that interconnect routers (or any other set of clients).
   Indeed, in the case where the optical network provides connectivity
   for multiple sets of external client networks, there has to be a
   way to enforce routing policies that ensure routing separation
   between different sets of clients (i.e., VPN service).

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7. IP transport over Optical Networks

   To examine the architectural alternatives for IP over optical
   networks, it is important to distinguish between the data and
   control planes over the UNI. As described in Section 6, the optical
   network provides a service to external entities in the form of fixed
   bandwidth transport pipes (optical paths). IP routers at the edge of
   the optical networks must necessarily have  such paths established
   between them before communication at the IP layer can commence.
   Thus, the IP data plane over optical networks is realized over a
   virtual topology of optical paths. On the other hand, IP routers and
   OXCs can have a peer relation on the control plane, especially for
   the implementation of a routing protocol that allows dynamic
   discovery of IP endpoints attached to the optical network. The IP
   over optical network architecture is defined essentially by the
   organization of the control plane. The assumption in this framework
   is that an MPLS-based control plane [1] is used. Depending on the
   service model(Section 6), however, the control planes in the IP and
   optical networks can be loosely or tightly coupled. This coupling

   o the details of the topology and routing information advertised by
     the optical network across UNI;

   o Level of control that IP routers can exercise in selecting
     specific paths for connections across the optical network; and

   o Policies regarding the dynamic provisioning of optical paths
     between routers. This includes access control, accounting and
     security issues.

   The following interconnection models are then possible:

7.1 Interconnection Models

7.1.1 The Peer Model

   Under the peer model, the IP/MPLS layers act as peers of the optical
   transport network, such that a single control plane runs
   over both the IP/MPLS and optical domains. When there is a single
   optical network involved, presumably a common IGP
   such as OSPF or IS-IS, with appropriate extensions, can be used to
   distribute topology information [7] over the integrated IP-optical
   network. In the case of OSPF, opaque LSAs can be used to advertise
   topology state information. In the case of IS-IS, extended TLVs will
   have to be defined to propagate topology state information. When an
   optical internetwork with multiple optical networks is involved
   (that  spans different administrative boundaries), a single instance
   of an intra-domain routing protocol is not practically attractive or
   even feasible. In this case, inter-domain routing and signaling are
   needed. In either case, a tacit assumption is that a common
   addressing scheme will be used for the optical and IP networks. A
   common address space can be trivially realized by using IP addresses

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   in both IP and optical domains. Thus, the optical network elements
   become IP addressable entities as noted in [1].

7.1.2 The Overlay Model

   Under the overlay model, the IP/MPLS routing, topology distribution,
   and signaling protocols are independent of the routing, topology
   distribution, and signaling protocols at the optical layer. This
   model is conceptually similar to the classical IP over ATM or MPOA
   models, but applied to an optical internetwork directly. In the
   overlay model, topology distribution, path computation and signaling
   protocols would have to be defined for the optical domain. In
   certain circumstances, it may also be feasible to statically
   configure the optical channels that provide connectivity in the
   overlay model through network management. Static configuration is,
   however, unlikely to scale in very large networks, nor support the
   rapid connection provisioning required in todayÆs competitive
   networking environment.

7.1.3  The Augmented Model

   Under the augmented model, there are actually separate routing
   instances in the IP and optical domains, but information from one
   routing instance is passed through the other routing instance. For
   example, external IP addresses could be carried within the optical
   routing protocols to allow reachability information to be passed to
   IP clients.

   The routing approaches corresponding to these interconnection models
   are described below.

7.2 Routing Approaches

7.2.1 Integrated Routing

   This routing approach supports the peer model within  an
   administrative domain. Under this approach, the IP and optical
   networks are assumed to run the same instance of an IP routing
   protocol, e.g., OSPF with suitable "optical" extensions.  These
   extensions must capture optical link parameters, and any constraints
   that are specific to optical networks. The topology and link state
   information maintained by all nodes (OXCs and routers) is identical.
   This permits a router to compute an end-to-end path to another
   router across the optical network. Suppose the path computation is
   triggered by the need to route a label switched path (LSP). Such an
   LSP can be established using MPLS signaling, e.g., RSVP-TE or CR-
   LDP. When the LSP is routed over the optical network, a lightpath
   must be established between two edge routers. This lightpath is in
   essence a tunnel across the optical network, and may have capacity
   much larger than that required to route the first LSP. Thus, it is
   essential that other routers in the network realize the availability
   of resources in the lightpath for other LSPs to be routed over it.

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   The lightpath may  therefore be advertised as a virtual link in the

   The notion of "forwarding adjacency" (FA) described in [3] is
   essential in propagating existing lightpath information to other
   routers. An FA is essentially a virtual link advertised into a link
   state routing protocol. Thus, an FA could be described by the same
   parameters that define resources in any regular link. While it is
   necessary to specify the mechanism for creating an FA, it is not
   necessary to specify how an FA is used by the routing scheme. Once
   an FA is advertised in a link state protocol, its usage for routing
   LSPs is defined by the route computation and traffic engineering
   algorithms implemented.

   It should be noted that at the IP-optical interface, the physical
   ports over which routers are connected to OXCs constrain the
   connectivity and resource availability. Suppose a router R1 is
   connected to OXC O1 over two ports, P1 and P2. Under integrated
   routing, the connectivity between R1 and O1 over the two ports would
   have been captured in the link state representation of the network.
   Now, suppose an FA at full port bandwidth is created from R1 to
   another router R2 over port P1. While this FA is advertised as a
   virtual link between R1 and R2, it is also necessary to remove the
   link R1-O1 (over P1) from the link state representation since that
   port is no longer available for creating a lightpath. Thus, as FAs
   are created, an overlaid set of virtual links is introduced into the
   link state representation, replacing the links previously advertised
   at the IP-Optical interface. Finally, the details of the optical
   network captured in the link state representation is replaced by a
   network of FAs. The above scheme is one way to tackle the problem.
   Another approach is to associate appropriate dynamic attributes with
   link state information, so that a link that cannot be used to
   establish a particular type of connection will be appropriately
   tagged.   Generally, however, there is a great deal of similarity
   between integrated routing   and domain-specific routing (described
   next). Both ultimately deal with the creation of  a virtual
   lightpath topology (which is overlaid over the optical network) to
   meet certain traffic engineering objectives.

7.2.2 Domain-Specific Routing

   The domain-specific routing approach supports the augmented
   interconnection model. Under this approach, routing within the
   optical and IP domains are separated, with a standard routing
   protocol running between domains. This is similar to the IP inter-
   domain routing model. A specific  approach for this is considered
   next. It is to be noted that other approaches are equally possible.  Domain-Specific Routing using BGP

   The inter-domain IP routing protocol, BGP [8], may be adapted for
   exchanging routing information between IP and optical domains. This

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   would allow the routers to advertise IP address prefixes within
   their  network to the optical internetwork and to receive external
   IP address prefixes from the optical internetwork. The optical
   internetwork transports the reachability information from one IP
   network to others. For instance, edge routers and OXCs can run
   exterior BGP (EBGP).  Within the optical internetwork, interior BGP
   (IBGP) is used between border OXCs within the same network, and EBGP
   is used between networks (over ENNI, Figure 1).

   Under this scheme, it is necessary to identify the egress points in
   the optical internetwork corresponding to externally reachable IP
   addresses. This is due to the following. Suppose an edge router
   desires to establish an LSP to a destination reachable across the
   optical internetwork. It could directly request a lightpath to that
   destination, without explicitly specifying the egress optical port
   for the lightpath as the optical internetwork has knowledge of
   externally reachable IP addresses. However, if the same edge router
   has to establish another LSP to a different external destination, it
   must first determine whether there is a lightpath already available
   (with sufficient residual capacity) that leads to that destination.
   To identify this, it is necessary for edge routers to keep track of
   which egress ports in the optical internetwork lead to which
   external destinations. Thus, a border OXC receiving external IP
   prefixes from an edge router through EBGP must include its own IP
   address as the egress point before propagating these prefixes to
   other border OXCs or   edge routers. An edge router receiving this
   information need not propagate the egress address further, but it
   must keep the association   between external IP addresses and egress
   OXC addresses. Specific BGP mechanisms for propagating egress OXC
   addresses are to be determined,  considering prior examples
   described in [9]. When VPNs are implemented, the address prefixes
   advertised by the border OXCs may be accompanied by some VPN
   identification (for example, VPN IPv4 addresses, as defined in [9],
   may be used).

7.2.3  Overlay Routing

   The overlay routing approach supports the overlay interconnection
   model.Under this approach, an overlay mechanism that allows edge
   routers toregister and query for external addresses is implemented.
   This is conceptually similar to the address resolution mechanism
   used for IP over ATM. Under this approach, the optical network could
   implement a registry that allows edge routers to register IP
   addresses and VPN identifiers. An edge router may be allowed to
   query for external addresses belonging to the same set of VPNs it
   belongs to. A successful query would return the address of the
   egress optical port through which the external destination can be

   Because IP-optical interface connectivity is limited, the
   determination of how many lightpaths must be established and to what
   endpoints are traffic engineering decisions. Furthermore, after an

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   initial set of such lightpaths are established, these may be used as
   adjacencies within VPNs for a VPN-wide routing scheme, for example,
   OSPF. With this approach, an edge router could first determine other
   edge routers of interest by querying the registry. After it obtains
   the appropriate addresses, an initial overlay lightpath topology may
   be formed. Routing adjacencies may then be established across the
   lightpaths and further routing information may be exchanged to
   establish VPN-wide routing.

7.3 Signaling-Related

7.3.1 The Role of MPLS

   It is possible to model wavelengths, and potentially TDM channels
   within a wavelength as "labels". This concept was proposed in [1],
   and "generalized" MPLS (GMPLS) mechanisms for realizing this are
   described in [4]. MPLS signaling protocols with traffic engineering
   extensions, such as RSVP-TE and CR-LDP can be used for signaling
   lightpath requests. In the case of the domain services model, these
   protocols can be adapted for UNI signaling as well [5, 6]. In the
   case of the unified services model, lightpath establishment occurs
   to support end-to-end LSP establishment using these protocols (with
   suitable GMPLS enhancements [10, 11]).

7.3.2 Signaling Models

   With the domain-services model, the signaling control plane in the
   IP and optical network are completely separate as shown in Figure 3
   below. This separation also implies the separation of IP and optical
   address spaces (even though the optical network would be using
   internal IP addressing). While RSVP-TE and LDP can be adapted for
   UNI signaling, the full functionality of these protocols will not be
   used. For example, UNI signaling does not require the specification
   of explicit routes. On the other hand, based on the service
   attributes, new objects need to be signaled using these protocols as
   described in [5, 6].

   MPLS Signaling      UNI Signaling     MPLS or other signaling
   +-----------------------------+  |   +-----------------------------+
   |         IP Network          |  |   |       Optical Internetwork  |
   |  +---------+   +---------+  |  |   |  +---------+   +---------+  |
   |  |         |   |         |  |  |   |  |         |   |         |  |
   |  | Router  +---+ Router  +-----+------+  OXC    +---+   OXC   |  |
   |  |         |   |         |  |  |   |  |         |   |         |  |
   |  +-----+---+   +---+-----+  |  |   |  +-----+---+   +---+-----+  |
   +-----------------------------+  |   +-----------------------------+
              Completely Separated Addressing and Control Planes

                 Figure 3: Domain Services Signaling Model

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   With the unified services model, the addressing is common in the IP
   network and optical internetwork and the respective signaling
   control are related, as shown in Figure 4. It is understood that
   GMPLS signaling is implemented in the IP and optical domains, using
   suitably enhanced RSVP-TE or  CR-LDP protocols. But the semantics of
   services within the optical internetwork may be different from that
   in the IP network. As an example, the protection services offered in
   the optical internetwork may be different from the end-to-end
   protection services offered by the IP network. Another example is
   with regard to bandwidth. While the IP network may offer a continuum
   of bandwidths, the optical internetwork will offer only discrete
   bandwidths. Thus, the signaling attributes and services are defined
   independently for IP and optical domains. The routers at the edge of
   the optical internetwork must therefore identify service boundaries
   and perform suitable translations in the signaling messages crossing
   the IP-optical boundary. This may still occur even though the
   signaling control plane in both networks are GMPLS-based and there
   is tighter coupling of the control plane as compared to the domain
   services model.

                        Service Boundary         Service Boundary
                              |                       |
   IP Layer GMPLS Signaling   | Optical Layer GMPLS   | IP Layer GMPLS
                              |                       |
      +--------+  +--------+  |  +-------+  +-------+ |  +--------+
      |        |  |        |  |  |       |  |       | |  |        |
      | IP LSR +--+ IP LSR +--+--+Optical+--+Optical+-+--+ IP LSR +---
      |        |  |        |  |  |  LSR  |  |  LSR  | |  |        |
      +-----+--+  +---+----+  |  +-----+-+  +---+---+ |  +--------+

                     Common Address Space, Service Translation

               Figure 4: Unified Services Signaling Model

   Thus, as illustrated in Figure 4, the signaling in the case of
   unified services is actually multi-layered. The layering is based on
   the technology and functionality. As an example, the specific
   adaptations of GMPLS signaling for SONET layer (whose functionality
   is transport) are described in [12].

7.4  End-to-End Protection Models

   Suppose an LSP is established from an ingress IP router to an egress
   router across an ingress IP network, a transit optical internetwork
   and an egress IP network. If this LSP is to be afforded protection
   in the IP layer, how is the service coordinated between the IP and
   optical layers?

   Under this scenario, there are two approaches to end-to-end

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7.4.1 Segment-Wise Protection

   The protection services in the IP layer could utilize optical layer
   protection services for the LSP segment that traverses the optical
   internetwork. Thus, the end-to-end LSP would be treated as a
   concatenation of three LSP segments from the protection point of
   view: a segment in the ingress IP network, a segment in the optical
   internetwork and a segment in the egress IP network. The protection
   services at the IP layer for an end-to-end LSP must be mapped onto
   suitable protection services offered by the optical internetwork.
   Suppose that 1+1 protection is offered to LSPs at the IP layer,
   i.e., each protected LSP has a pre-established hot stand-by in a 1+1
   or 1:1 configuration. In case of a failure of the primary LSP,
   traffic can be immediately switched to the stand-by. This type of
   protection can be realized end-to-end as follows. With reference to
   Figure 5, let an LSP originate at (ingress) router interface A and
   terminate at (egress) router interface F. Under the first protection
   option, a primary path for the LSP must be established first. Let
   this path be as shown in   Figure 5, traversing router interface B
   in the ingress network, optical ports C (ingress) and D (egress),
   and router interface E in the egress network. Next, 1+1 protection
   is realized separately in each network by establishing a protection
   path between points A and B, C and D and E and F. Furthermore, the
   segments B-C and D-E must themselves be 1+1 protected, using drop-
   side protection. For the segment between C and D, the optical
   internetwork must offer a 1+1 service similar to that offered in the
   IP networks.

      +----------------+    +------------------+    +---------------+
      |                |    |                  |    |               |
      A Ingress IP Net B----C Optical Internet D----E Egress IP Net F
      |                |    |                  |    |               |
      +----------------+    +------------------+    +---------------+

                  Figure 5: End-to-End Protection Example

7.4.2 Single-Layer Protection

   Under this model, the protection services in the IP layer do not
   rely on any protection services offered in the optical internetwork.
   Thus, with reference to Figure 5, two SRLG-disjoint LSPs are
   established between A and F. The corresponding segments in the
   optical internetwork are treated as independent lightpaths in the
   optical internetwork. These lightpaths may be unprotected in the
   optical internetwork.

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7.4.3 Differences

   A distinction between these two choices is as follows. Under the
   first choice, the optical internetwork is actively involved in end-
   to-end protection, whereas under the second choice, any protection
   service offered in the optical internetwork is not utilized                                                                directly
   by client IP network. Also, under the first choice, the protection
   in the optical internetwork may apply collectively to a number of IP
   LSPs. That is, with reference to Figure 5, many LSPs may be
   aggregated into a single lightpath between C and D. The optical
   internetwork protection may then be applied to all of them at once
   leading to some gain in scalability. Under the second choice, each
   IP LSP must be separately protected. Finally, the first choice
   allows different restoration signaling to be implemented in the IP
   and optical internetwork. These restoration protocols are "patched
   up" at the service boundaries to realize end-to-end protection. A
   further advantage of this is that restoration is entirely contained
   within the network where the failure occurs, thereby improving the
   restoration latency, and perhaps network stability as a fault within
   an optical domain is contained and corrected within the domain. For
   instance, if there is a failure in the optical internetwork, optical
   network protocols restore the affected internal segments.  Under the
   second choice, restoration signaling is always end-to-end between IP
   routers, essentially by-passing the optical internetwork. A result
   of this is that restoration latency could be higher.  In addition,
   restoration protocols in the IP layer must run transparently over
   the optical internetwork in the overlay mode. IP based recovery
   techniques may however be more resource efficient, as it may be
   possible to convey traffic through the redundant capacity under
   fault-free scenarios. In particular, it may be possible to utilize
   classification, scheduling, and concepts of forwarding equivalence
   class  to route lower class traffic over protect facilities and then
   possibly preempt them to make way for high priority traffic when
   faults occur.

8. IP-based Optical Control Plane Issues

   Provisioning and restoring lightpaths end-to-end between IP networks
   requires protocol and signaling support within optical sub-networks,
   and across the INNI and ENNI. In this regard, a distinction is made
   between control procedures within an optical sub-network (Figure 1),
   between sub-networks, and between networks. The general guideline
   followed in this framework is to separate these cases, and allow the
   possibility that different control procedures are followed inside
   different sub-networks, while a common set of procedures are
   followed across sub-networks and networks.

   The control plane procedures within a single vendor sub-network need
   not be defined since these can be proprietary. Clearly, it is
   possible to follow the same control procedures inside a sub-network
   as defined for control across sub-networks. But this is left as a
   recommendation û even choice û within this framework document,

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   rather than as an imperative requirement. Thus, in the following,
   signaling and routing across sub-networks is considered first,
   followed by a discussion of similar issues across networks.

8.1  Addressing

   For interoperability across optical sub-networks using an IP-centric
   control plane, the fundamental issue is that of addressing. What
   entities should be identifiable from a signaling and routing point
   of view? How should they be addressed? This section presents some
   guidelines on this issue.

   Identifiable entities in optical networks includes OXCs, optical
   links, optical channels and sub-channels, Shared Risk Link Groups
   (SRLGs), etc. An issue here is how granular the identification
   should be as far as the establishment of optical trails are
   concerned. The scheme for identification must accommodate the
   specification of the termination points in the optical network with
   adequate granularity when establishing optical trails. For instance,
   an OXC could have many ports, each of which may in turn terminate
   many optical channels, each of which contain many sub-channels etc.
   It is perhaps not reasonable to assume that every sub-channel or
   channel termination, or even OXC ports could be assigned a unique IP
   address. Also, the routing of an optical trail within the network
   does not depend on the precise termination point information, but
   rather only on the terminating OXC.   Thus, finer granularity
   identification of termination points is of relevance only to the
   terminating OXC and not to intermediate OXCs (of course, resource
   allocation at each intermediate point would depend on the
   granularity of resources requested). This suggests an identification
   scheme whereby OXCs are identified by a unique IP address and a
   "selector" identifies further fine-grain information of relevance at
   an OXC. This, of course, does not preclude the identification of
   these termination points directly with IP addresses(with a null
   selector). The selector can be formatted to have adequate number of
   bits and a structure that expresses port, channel, sub-channel, etc,

   Within the optical network, the establishment of trail segments
   between adjacent OXCs require the identification of specific port,
   channel, sub-channel, etc. With a GMPLS control plane, a label
   serves this function. The structure of the label must be such that
   it can encode the required information [12].

   Another entity that must be identified is the SRLG [13]. An
   SRLG is an identifier assigned to a group of optical links that
   share a physical resource. For instance, all optical channels routed
   over the same fiber could belong to the same SRLG. Similarly, all
   fibers routed over a conduit could belong to the same SRLG. The
   notable characteristic of SRLGs is that a given link could belong to
   more than   one SRLG, and two links belonging to a given SRLG may
   individually belong to two other SRLGs. This is illustrated in
   Figure 6. Here, the   links 1,2,3 and 4 may belong to SRLG 1, links

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   1,2 and 3 could belong to SRLG 2 and link 4 could belong to SRLG 3.
   Similarly, links 5 and 6 could belong to SRLG 1, and links 7 and 8
   could belong to SRLG 4. (In this example, the same SRLG, i.e., 1,
   contains links from two different adjacencies).

   While the classification of physical resources into SRLGs is a
   manual operation, the assignment of unique identifiers to these
   SRLGs    within an optical network is essential to ensure correct
   SRLG-disjoint path computation for protection. SRLGs could be
   identified with a flat identifier (e.g., 32 bit integer).

   Finally, optical links between adjacent OXCs may be bundled for
   advertisement into a link state protocol [14]. A bundled interface
   may be numbered or unnumbered. In either case, the component links
   within the bundle must be identifiable. In concert with SRLG
   identification, this information is necessary for correct path

8.2  Neighbor Discovery

   Routing within the optical network relies on knowledge of network
   topology and resource availability. This information may be gathered
   and used by a centralized system, or by a distributed link state
   routing protocol. In either case, the first step towards network-
   wide link state determination is the discovery of the status of
   local links to all neighbors by each OXC.  Specifically, each OXC
   must determine the up/down status of each optical link, the
   bandwidth and other parameters of the link, and the identity of the
   remote end of the link (e.g., remote port number). The last piece of
   information is used to specify an appropriate label when signaling
   for lightpath provisioning. The determination of these parameters
   could be based on a combination of manual configuration and an
   automated protocol running between adjacent OXCs. The
   characteristics of such a protocol would depend on the type of OXCs
   that are adjacent (e.g., transparent or opaque).

   Neighbor discovery would typically require in-band communication on
   the bearer channels to determine local connectivity and link status.
   In the case of opaque OXCs with SONET termination, one instance of a
   neighbor discovery protocol (e.g., LMP [2]) would run on each OXC
   port, communicating with the corresponding protocol instance at the
   neighboring OXC. The protocol would utilize the SONET overhead bytes
   to transmit the (configured) local attributes periodically to the
   neighbor. Thus, two neighboring switches can automatically determine
   the identities of each other and the local connectivity, and also
   keep track of the up/down status of local links. Neighbor discovery
   with transparent OXCs is described in [2].

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       +--------------+          +------------+         +------------+
       |              +-1:OC48---+            +-5:OC192-+            |
       |              +-2:OC48---+            +-6:OC192-+            |
       |    OXC1      +-3:OC48---+     OXC2   +-7:OC48--+     OXC3   |
       |              +-4:OC192--+            +-8:OC48--+            |
       |              |          |            |  +------+            |
       +--------------+          +----+-+-----+  | +----+------+-----+
                                      | |        | |          |
                                      | |        | |          |
       +--------------+               | |        | |          |
       |              |          +----+-+-----+  | |   +------+-----+
       |              +----------+            +--+ |   |            |
       |     OXC4     +----------+            +----+   |            |
       |              +----------+    OXC5    +--------+     OXC6   |
       |              |          |            +--------+            |
       +--------------+          |            |        |            |
                                 +------+-----+        +------+-----+

                Figure 6: Mesh Optical Network with SRLGs

8.3  Topology Discovery

   Topology discovery is the procedure by which the topology and
   resource state of all the links in a network are determined. This
   procedure may be done as part of a link state routing protocol
   (e.g., OSPF, ISIS), or it can be done via the management plane (in
   the case of centralized path computation). The implementation of a
   link state protocol within a network (i.e., across sub-network
   boundaries) means that the same protocol runs in OXCs in every sub-
   network. If this assumption does not hold then interworking of
   routing between sub-networks is required. This is similar to inter-
   network routing discussed in Section 8.7. The focus in the following
   is therefore on standardized link state routing.

   In general, most of the link state routing functionality is
   maintained when applied to optical networks. However, the
   representation of optical links, as well as some link parameters,
   are changed in this setting. Specifically,

   o The link state information may consist of link bundles [14].
     Each link bundle is represented as an abstract link in the network
     topology. Different bundling representations are possible. For
     instance, the parameters of the abstract link may include the
     number, bandwidth and the type of optical links contained in the
     underlying link bundle [14]. Also, the SRLGs corresponding to each
     optical link in the bundle may be included as a parameter.

   o The link state information should capture restoration-related
     parameters for optical links. Specifically, with shared protection
     (Section 8.5), the link state updates must have information that
     allows the computation of shared protection paths.

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   o A single routing adjacency could be maintained between neighbors
     which may have multiple optical links (or even multiple link
     bundles) between them. This reduces the protocol messaging

   o Since link availability information changes dynamically, a
     flexible policy for triggering link state updates based on
     availability thresholds may be implemented. For instance, changes
     in availability of links of a given bandwidth (e.g., OC-48) may
     trigger updates only after the availability figure changes by a
     certain percentage.

   These concepts are relatively well-understood. On the other hand,
   the resource representation models and the topology discovery
   process for hierarchical routing (e.g., OSPF with multiple areas)
   are areas that need further work.

8.4  Restoration Models

   Automatic restoration of lightpaths is a service offered by optical
   networks. There could be local and end-to-end mechanisms for
   restoration of lightpaths within a network (across the INNI). Local
   mechanisms are used to select an alternate link between two adjacent
   OXCs across the INNI when a failure affects the primary link over
   which the (protected) lightpath is being routed. Local restoration
   does not affect the end-to-end route of the lightpath. When local
   restoration is not possible (e.g., no alternate link is available
   between the adjacent OXCs in question), end-to-end restoration may
   be performed. With this, the affected lightpath may be rerouted over
   an alternate path that completely avoids the OXCs or the link
   segment where the failure occurred. For end-to-end restoration,
   alternate paths are typically pre-computed. Such back-up paths may
   have to be physically diverse from the corresponding primary paths.

   End-to-end restoration may be based on two types of protection
   schemes; "1 + 1" protection or shared protection. Under 1 + 1
   protection, a back-up path is established for the protected primary
   path along a physically diverse route. Both paths are active and the
   failure along the primary path results in an immediate switch-over
   to the back-up path. Under shared protection, back-up paths
   corresponding to physically diverse primary paths may share the same
   network resources. When a failure affects a primary path, it is
   assumed that the same failure will not affect the other primary
   paths whose back-ups share resources.

   It is possible that different restoration schemes may be implemented
   within optical sub-networks. It is therefore necessary to consider a
   two-level restoration mechanism. Path failures within an optical
   sub-network could be handled using procedures specific to the
   sub-network. If this fails, end-to-end restoration across sub-
   networks could be invoked. The border OXC that is the ingress to a
   sub-network can act as the source for restoration procedures within
   a sub-network. The signaling for invoking end-to-end restoration

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   across the INNI is described in Section 8.6.3. The computation of
   the back-up path for end-to-end restoration may be based on various
   criteria. It is assumed that the back-up path is computed by the
   source OXC, and signaled using standard methods.

8.5  Route Computation

   The computation of a primary route for a lightpath within an optical
   network is essentially a constraint-based routing problem. The
   constraint is typically the bandwidth required for the lightpath,
   perhaps along with administrative and policy constraints. The
   objective of path computation could be to minimize the total
   capacity required for routing lightpaths [15].

   Route computation with constraints may be accomplished using a
   number of algorithms [16]. When 1+1 protection is used, a back-up
   path that does not traverse on any link which is part of the same
   SRLG as links in the primary path must be computed. Thus, it is
   essential that the SRLGs in the primary path be known during
   alternate path computation, along with the availability of resources
   in links that belong to other SRLGs. This requirement has certain
   implications on optical link bundling. Specifically, a bundled LSA
   must include adequate information such that a remote OXC can
   determine the resource availability under each SRLG that the bundled
   link refers to, and the    relationship between links belonging to
   different SRLGs in the bundle.   For example, considering Figure 3,
   if links 1,2,3 and 4 are bundled together in an LSA, the bundled LSA
   must indicate that there are three   SRLGs which are part of the
   bundle (i.e., 1, 2 and 3), and that links in SRLGs 2 and 3 are also
   part of SRLG 1.

   To encode the SRLG relationships in a link bundle LSA, only links
   which belong to exactly the same set of SRLGs must be bundled
   together. With reference to Figure 3, for example, two bundles can
   be advertised for links between OXC1 and OXC2, with the following

     Bundle No.     SRLGs    Link Type   Number   Other Info
     ----------     -----    ---------   ------   ----------
       1             1,2       OC-48       3          ---
       2             1,3       OC-192      1          ---

   Assuming that the above information is available for each bundle at
   every node, there are several approaches possible for path
   computation.  For instance,

   1. The primary path can be computed first, and the (exclusive
      or shared) back-up is computed next based on the SRLGs chosen
      for the primary path.  In this regard,

   o    The primary path computation procedure can output a series of
        bundles the path  is routed over. Since a bundle is uniquely

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        identified with a set of SRLGs, the alternate path can be
        computed right away based on this knowledge. In this case, if
        the primary path set up does not succeed for lack of resources
        in a chosen bundle, the primary and backup paths must be

   o    It might be desirable to compute primary paths without choosing
        a specific bundle apriori. That is, resource availability over
        all bundles between a node pair is taken into account rather
        than specific bundle information. In this case, the primary
        path computation procedure would output a series of nodes the
        path traverses.  Each OXC in the path would have the freedom to
        choose the particular bundle to route that segment of the
        primary path. This procedure would increase the chances of
        successfully setting up the primary path when link state
        information is not up to date everywhere. But the specific
        bundle chosen, and hence the SRLGs in the primary path, must be
        captured during primary path set-up, for example, using the
        RSVP-TE Route Record Object [17].  This SRLG information is
        then used for computing the back-up path. The back-up path may
        also be established specifying only which SRLGs to AVOID in a
        given segment, rather than which bundles to use. This would
        maximize the chances of establishing the back-up path.

    2. The primary path and the back-up path are comptuted together in
       one step, for example, using Suurbaale's algorithm [18]. In this
       case, the paths must be computed using specific bundle

    To summarize, it is essential to capture sufficient information in
    link bundle LSAs to accommodate different path computation
    procedures    and to maximize the chances of successful path
    establishment. Depending on the path computation procedure used,
    the type of support needed during path establishment (e.g., the
    recording of link group or SRLG information during path
    establishment) may differ.

   When shared protection is used, the route computation algorithm must
   take into account the possibility of sharing links among multiple
   back-up paths. Under shared protection, the back-up paths
   corresponding to SRLG-disjoint primary paths can be assigned the
   same  links. The assumption here is that since the primary paths are
   not routed over links that have the same SRLG, a given failure will
   affect   only one of them. Furthermore, it is assumed that multiple
   failure events affecting links belonging to more than one SRLG will
   not occur    concurrently. Unlike the case of 1+1 protection, the
   back-up paths are not established apriori. Rather, a failure event
   triggers the establishment of a single back-up path corresponding to
   the affected primary path.

   The distributed implementation of route computation for shared back-
   up paths require knowledge about the routing of all primary and
   back-up paths at every node. This raises scalability concerns. For

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   this reason, it may be practical to consider the centralization of
   the route computation algorithm in a route server that has complete
   knowledge of the link state and path routes. Heuristics for fully
   distributed route computation without complete knowledge of path
   routes are to be determined. Path computation for restoration is
   further described in [13].

8.6  Signaling Issues

   Signaling within an optical network for lightpath provisioning
   is a relatively simple operation if a standard procedure is
   implemented within all sub-networks. Otherwise, proprietary
   signaling may be implemented within sub-networks, but converted back
   to standard signaling across the INNI. This is similar to signaling
   across the ENNI, as described in Section 8.7. In the former case,
   signaling messages could carry a strict explicit route in signaling
   messages, while in the latter case the route should be loose, at the
   level of sub-networks. Once a route is determined for a lightpath,
   each OXC in the path must establish appropriate cross-connects in a
   coordinated fashion. This coordination is akin to selecting incoming
   and outgoing labels in a label-switched environment. Thus, protocols
   like RSVP-TE [11] and CR-LDP [10] can be used across the INNI for
   this. A few new concerns, however, must be addressed.

8.6.1 Bi-Directional Lightpath Establishment

   Lightpaths are typically bi-directional. That is, the output port
   selected at an OXC for the forward direction is also the input port
   for the reverse direction of the path. Since signaling for optical
   paths may be autonomously initiated by different nodes, it is
   possible   that two path set-up attempts are in progress at the same
   time. Specifically, while setting up an optical path, an OXC A may
   select output port i which is connected to input port j of the
   "next" OXC B.    Concurrently, OXC B may select output port j for
   setting up a different optical path, where the "next" OXC is A. This
   results in a "collision". Similarly, when WDM functionality is built
   into OXCs, a collision occurs when adjacent OXCs choose directly
   connected output ports and the same wavelength for two different
   optical paths. There are two ways to deal with such collisions.
   First, collisions may be detected and the involved paths may be torn
   down and re-established. Or, collisions may be avoided altogether.

8.6.2  Failure Recovery

   The impact of transient partial failures must be minimized in an
   optical network. Specifically, optical paths that are not directly
   affected by a failure must not be torn down due to the failure. For
   example, the control processor in an OXC may fail, affecting
   signaling   and other internodal control communication. Similarly,
   the control channel between OXCs may be affected temporarily by a
   failure. These failure may not affect already established optical
   paths passing through the OXC fabric. The detection of such failures

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   by adjacent nodes, for example, through a keepalive mechanism
   between signaling peers, must not result in these optical paths
   being torn down.

   It is likely that when the above failures occur, a backup processor
   or a backup control channel will be activated. The signaling
   protocol must be designed such that it is resilient to transient
   failures. During failure recovery, it is desirable to recover local
   state at the concerned OXC with least disruption to existing optical

8.6.3 Restoration

   Signaling for restoration has two distict phases. There is a
   reservation phase in which capacity for the protection path is
   established. Then, there is an activation phase in which the
   back-up path is actually put in service. The former phase typically
   is not subject to strict time constraints, while the latter is.

   Signaling to establish a "1+1" back-up path is relatively straight-
   forward. This signaling is very similar to signaling used for
   establishing the primary path. Signaling to establish a shared back-
   up   path is a little bit different. Here, each OXC must understand
   which back-up paths can share resources. The signaling message must
   itself indicate shared reservation. The sharing rule is as described
   in Section 8.4: back-up paths corresponding to physically diverse
   primary   paths may share the same network resources. It is
   therefore necessary    for the signaling message to carry adequate
   information that allows an   OXC to verify that back-up paths that
   share a certain resources are allowed to do so.

   Under both 1+1 and shared protection, the activation phase has two
   parts: propagation of failure information to the source OXC from the
   point of failure, and activation of the back-up path. The signaling
   for these two phases must be very fast in order to realize response
   times in the order of tens of milliseconds. When optical links are
   SONET-based, in-band signals may be used, resulting in quick
   response.   With out-of-band control, it is necessary to consider
   fast signaling over the control channel using very short IP packets
   and prioritized processing. While it is possible to use RSVP or CR-
   LDP for activating protection paths, these protocols do not provide
   any means to give priority to restoration signaling as opposed to
   signaling for provisioning. For instance, it is possible for a
   restoration-related RSVP message to be queued behind a number of
   provisioning messages thereby delaying restoration. It is therefore
   necessary to develop a definition of QoS for restoration signaling
   and incorporate mechanisms   in existing signaling protocols to
   achieve this. Or, a new signaling protocol may be developed
   exclusively for activating protection paths during restoration [19].

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8.7   Optical Internetworking

   Within an optical internetwork, it must be possible to dynamically
   provision and restore lightpaths across optical networks. Therefore:

   o A standard scheme for uniquely identifying lightpath end-points in
     different networks is required.

   o A protocol is required for determining reachability of end-points
     across networks.

   o A standard signaling protocol is required for provisioning
     lightpaths across networks.

   o A standard procedure is required for the restoration of lightpaths
     across networks.

   o Support for policies that affect the flow of control information
   across networks will be required.

   The IP-centric control architecture for optical networks can be
   extended to satisfy the functional requirements of optical
   internetworking. Routing and signaling interaction between optical
   networks can be standardized across the ENNI (Figure 1). The
   functionality provided across ENNI is as follows.

8.7.1 Neighbor Discovery

   Neighbor discovery procedure, as described in Section 8.2, can be
   used for this. Indeed, a single protocol should be standardized for
   neighbor discovery within and across networks.

8.7.2 Addressing and Routing Model

   The addressing mechanisms described in Section 8.1 can be used to
   identify OXCs, ports, channels and sub-channels in each network.
   It is essential that the OXC IP addresses are unique within the

   Provisioning an end-to-end lightpath across multiple networks
   involves the establishment of path segments in each network
   sequentially. Thus, a path segment is established from the source
   OXC to a border OXC in the source network. From this border OXC,
   signaling across NNI is used to establish a path segment to a border
   OXC in the next network. Provisioning then continues in the next
   network and so on until the destination OXC is reached. The usage of
   protocols like BGP for this purpose need to be explored.

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8.7.3 Restoration

   Local restoration across the ENNI is similar to that across INNI
   described in Section 8.6.3. End-to-end restoration across networks
   is likely to be either of the 1+1 type, or segmented within each
   network, as described in Section 8.4.

9. Other Issues

9.1   WDM and TDM in the Same Network

   A practical assumption would be that if SONET (or some other TDM
   mechanism that is capable partitioning the bandwidth of a
   wavelength) is used, then TDM is leveraged as an additional method
   to differentiate between "flows."  In such cases, wavelengths and
   time intervals (sub-channels) within a wavelength become analogous
   to labels (as noted in [1]) which can be used to make switching
   decisions. This would be somewhat akin to using VPI (e.g.,
   wavelength) and VCI (e.g., TDM sub-channel) in ATM networks. More
   generally, this will be akin to label stacking and to LSP nesting
   within the context of Multi-Protocol Lambda Switching [1]. GMPLS
   signaling [4] supports this type of multiplexing.

9.2   Wavelength Conversion

   Some form of wavelength conversion may exist at some switching
   elements. This however may not be the case in some pure optical
   switching elements.  A switching element is essentially anything
   more sophisticated than a simple repeater, that is capable of
   switching and converting a wavelength Lambda(k) from an input port
   to a wavelength  Lambda(l) on an output port.  In this display, it
   is not necessarily the case that Lambda(k) = Lambda(l), nor is it
   necessarily the case that the data carried on Lambda(k) is switched
   through the device without being examined or modified.

   It is not necessary to have a wavelength converter at every
   switching element.  A number of studies have attempted to address
   the issue of the value of wavelength conversion in an optical
   network. Such studies typically use the blocking probability (the
   probability that a lightpath cannot be established because the
   requisite wavelengths are not available) as a metric to adjudicate
   the effectiveness of wavelength conversion.  The IP over optical
   architecture must take into account hybrid networks with some OXCs
   capable of wavelength conversion and others incapable of this. The
   GMPLS "label set" mechanism [4] supports the selection of the same
   label (i.e., wavelength) across an NNI.

9.3   Service Provider Peering Points

   There are proposed inter-network interconnect models which allow
   certain types of peering relationships to occur at the optical
   layer. This is consistent with the need to support optical layer
   services independent of higher layers payloads. In the context of IP

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   over optical networks, peering relationships between different trust
   domains will eventually have to occur at the IP layer, on IP routing
   elements, even though non-IP paths may exist between the peering

9.4   Rate of Lightpath Set-Up

   Dynamic establishment of optical channel trails and lightpaths is
   quite desirable in IP over optical networks, especially when such
   instantiations are driven by a stable traffic engineering control
   system, or in response to authenticated and authorized requests from
   client          s.

   However, there are many proposals suggesting the use of dynamic,
   data-driven shortcut-lightpath setups in IP over optical networks.
   The arguments put forth in such proposals are quite reminiscent of
   similar discussions regarding ATM deployment in the core of IP
   networks.  Deployment of highly dynamic data driven shortcuts within
   core networks has not been widely adopted by carriers and ISPs for a
   number   of reasons: possible CPU overhead in core network elements,
   complexity   of proposed solutions, stability concerns, and lack of
   true economic drivers for this type of service.  This draft assumes
   that this paradigm will not change and that highly dynamic, data-
   driven shortcut lightpath setups are for future investigation.
   Instead, the optical channel trails and lightpaths that are expected
   to be widely used at the initial phases in the evolution of IP over
   optical networks will include the following:

   o Dynamic connections for control plane traffic and default path
     routed data traffic,

   o Establishment and re-arrangement of arbitrary virtual topologies
     over rings and other physical layer topologies.

   o Use of stable traffic engineering control systems to engineer
     lightpath connections to enhance network performance, either for
     explicit demand based QoS reasons or for load balancing).

   Other issues surrounding dynamic connection setup within the core
   center around  resource usage at the edge of the optical domain.
   One potential issue pertains to the number of flows that can be
   processed by an ingress or egress network element either because of
   aggregate bandwidth limitations or because of a limitation on the
   number of flows (e.g., lightpaths) that can be processed

   Another possible short term reason for dynamic shortcut lightpath
   setup would be to quickly pre-provisioned paths based on some
   criteria (TOD, CEO wants a high BW reliable connection, etc.).  In
   this scenario, a set of paths is pre-provisioned, but not actually
   instantiated until the customer initiates an authenticated and
   authorized setup requests, which is consistent with existing
   agreements between the provider and the customer.   In a sense, the

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   provider may have already agreed to supply this service, but will
   only instantiate it by setting up a lightpath when the customer
   submits an explicit request.

9.5   Distributed vs. Centralized Provisioning

   This draft has mainly dealt with a distributed model for lightpath
   provisioning, in which all nodes maintain a synchronized topology
   database, and advertise topology state information to maintain and
   refresh the database. A constraint-based routing entity in each node
   then uses the information in the topology database and other
   relevant details to compute appropriate paths through the optical
   domain. Once a path is computed, a signaling protocol (e.g., [11])
   is used to instantiate the lightpath.

   Another provisioning model is to have a centralized server which has
   complete knowledge of the physical topology, the available
   wavelengths, and where applicable, relevant time domain information.
   A corresponding client will reside on each network element that can
   source or sink a lightpath.  The source client would query the
   server in order to set up a lightpath from the source to the
   destination.  The server would then check to see if such a lightpath
   can be established based on prevailing conditions. Furthermore,
   depending on the specifics of the model, the server may either setup
   the lightpath on behalf of the client or provide the necessary
   information to the client or to some other entity to allow the
   lightpath to be instantiated.

   Centralization aids in implementing complex capacity optimization
   schemes, and may be the near-term provisioning solution in optical
   networks with interconnected multi-vendor optical sub-networks. In
   the long term, however, the distributed solution with centralization
   of some control procedures (e.g., traffic engineering) is likely to
   be the approach followed.

9.6   Optical Networks with Additional Configurable Components

   Thus far, this memo has focused mainly on IP over optical networks
   where the cross-connect is the basic dynamically re-configurable
   device in the optical network. Recently, as a consequence of
   technology evolution, various types of re-configurable optical
   components are now available, including tunable lasers, tunable
   filters, etc. Under certain circumstances, it may be necessary to
   parameterize the characteristics of these components and advertise
   them  within the control plane. This aspect is left for further

9.7   Optical Networks with Limited Wavelength Conversion Capability

   At the time of the writing of this document, the majority of optical
   networks being deployed are "opaque".  In this context the term
   opaque means that each link is optically isolated by transponders

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   doing optical-electrical-optical conversions. Such conversions have
   the added benefit of permitting 3R regeneration.  The 3Rs refer to
   re-power, signal retiming and reshaping. Unfortunately, this
   regeneration requires that the underlying optical equipment be aware
   of both the bit rate and frame format of the carried signal. These
   transponders are quite expensive and their lack of transparency
   constrains the rapid introduction of new services [20].  Thus there
   are strong motivators to introduce "domains of transparency" wherein
   all-optical networking equipment would transport data unfettered by
   these drawbacks.

   Thus, the issue of IP over optical networking in all optical sub-
   networks, and sub-networks with limited wavelength conversion
   capability merits special attention.  In such networks, transmission
   impairments resulting from the peculiar characteristics of optical
   communications complicate the process of path selection. These
   transmission impairments include loss, noise (due primarily to
   amplifier spontaneous emission - ASE), dispersion (chromatic and
   PMD), cross-talk, and non-linear effects. In such networks, the
   feasibility of a path between two nodes is no longer simply a
   function of topology and resource availability but will also depend
   on the accumulation of impairments along the path. If the impairment
   accumulation is excessive, the optical signal to noise ratio (OSNR)
   and hence the electrical bit error rate (BER) at the destination
   node may exceed prescribed thresholds, making the resultant optical
   channel unusable for data communication.  The challenge in the
   development of IP-based control plane for optical networks is to
   abstract these peculiar characteristics of the optical layer [20] in
   a generic fashion, so that they can be used for path computation.

10.  Evolution Path for IP over Optical Architecture

   The architectural models described in Section 7 imply a certain
   degree of implementation complexity. Specifically, the overlay
   model was described as the least complex for near term deployment
   and the peer model the most complex. Nevertheless, each model has
   certain advantages and this raises the question as to the evolution
   path for IP over optical network architectures.

   The evolution approach recommended in this framework is the
   definition of capability sets that start with simpler functionality
   in the beginning and include more complex functionality later. In
   this regard, it is realistic to expect that initial IP over optical
   deployments will be based on the domain services model (with overlay
   interconnection), with no routing exchange between the IP and
   optical domains. Under this model, direct signaling between IP
   routers and optical networks is likely to be triggered by offline
   traffic engineering decisions. The next step in the evolution of IP-
   optical interaction is the introduction of reachability information
   exchange between the two domains. This would potentially allow
   lightpaths to be established as part of end-to-end LSP set-up. The
   final phase is the support for the full peer model with more
   sophisticated routing interaction between IP and optical domains.

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   Using a common signaling framework (based on GMPLS) from the
   beginning facilitates this type of evolution. For the domain
   services model, implementation agreement based on GMPLS UNI
   signaling is being developed in the Optical Interworking Forum (OIF)
   [5, 6]. This agreement is aimed at near term deployment and this
   could be the precursor to a future peer model architecture. In this
   evolution, the signaling capability and semantics at the IP-optical
   boundary would become more sophisticated, but the basic structure of
   signaling would remain. This would allow incremental developments as
   the interconnection model becomes more sophisticated, rather than
   complete re-development of signaling capabilities.

11. Security Considerations


12. Summary and Conclusions

   The objective of this draft was to define a framework for IP over
   optical networks, considering the service models, routing and
   signaling issues. There are a diversity of choices, as described
   in this draft, for IP-optical interconnection, service models
   and protocol mechanisms. The approach advocated in this draft
   was to allow different service models and proprietary enhancements
   in optical networks, and define complementary signaling and
   routing mechanisms that would support these. An evolution scenario,
   based on a common GMPLS-based signaling framework with increasing
   interworking functionality was suggested. Under this scenario, the
   IP-optical interaction is first based on the domain services model
   with overlay interconnection that eventually evolves to support full
   peer interaction.

13. References

   1. D. Awduche and Y. Rekhter, , "Multi-Protocol
      Lambda Switching: Combining MPLS Traffic Engineering Control With
      Optical Crossconnects," IEEE Communications Magazine, March 2001.

   2. J. P. Lang, et. al., "Link Management Protocol," draft-ietf-mpls-
      lmp-03.txt, Internet Draft, Work in progress.

   3. K. Kompella and Y. Rekhter, "LSP Hierarchy with MPLS TE," draft-
      ietf-mpls-lsp-hierarchy-01.txt, Work in Progress, November, 2000.

   4. P. Ashwood-Smith et. al, "Generalized MPLS - Signaling Functional
      Description", draft-ietf-mpls-generalized-signaling-04.txt,
      Internet Draft, Work in Progress.

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   5. E. Gray, et. al., "RSVP Extensions in Support of OIF Optical UNI
      Signaling," draft-gray-mpls-rsvp-oif-uni-ext-00.txt, Internet
      Draft, Work in Progress, October, 2000.

   6. O. Abul-Magd, et. al., "LDP Extensions for Optical UNI
      Signaling," draft-ietf-mpls-ldp-optical-uni-00.txt, Internet
      Draft, Work in Progress, October, 2000.

   7. K. Kompella et al, "OSPF Extensions in Support of Generalized
      MPLS," draft-kompella-ospf-gmpls-extensions-00.txt, Work in
      Progress, November, 2000.

   8. Y. Rekhter and T. Li, "A Border Gateway Protocol 4 (BGP4)",RFC
      1771, March, 1995.

   9. E. Rosen and Y. Rekhter, "BGP/MPLS VPNs", RFC 2547, March, 1999.

   10.  P. Ashwood-Smith, et. al., "Generalized MPLS - CR-LDP Signaling
      Functional Description," draft-ietf-mpls-generalized-cr-ldp-
      03.txt, Internet Draft, Work in Progress.

   11.  P. Ashwood-Smith, et. al., "Generalized MPLS - RSVP-TE
      Signaling Functional Description," draft-ietf-mpls-generalized-
      rsvp-te-03.txt, Internet Draft, Work in Progress.

   12.  , et. al., "Enhancements to GMPLS Signaling for Optical
      Technologies," draft-mack-crane-gmpls-signaling-enchancements-
      00.txt, Internet Draft, Work in Progress, November, 2000.

   13.  B. Doshi, S. Dravida, P. Harshavardhana, et. al, "Optical
      Network Design and Restoration," Bell Labs Technical Journal,
      Jan-March, 1999.

   14.  K. Kompella, et al., "Link Bundling in MPLS Traffic
      Engineering," draft-kompella-mpls-bundle-05.txt, Internet Draft,
      Work in Progress.

   15.  S. Ramamurthy, Z. Bogdanowicz, S. Samieian, et al., "Capacity
      Performance of Dynamic Provisioning in Optical Networks", to
      appear in J. of Lightwave Technology.

   16.  E. Crawley, R. Nair, B. Rajagopalan and H. Sandick, "A
      Framework for QoS-based Routing in the Internet," RFC 2386,
      August, 1998.

   17.  D. Awduche, L. Berger, Der-Hwa Gan, T. Li, G. Swallow, V.
      Srinivasan, "RSVP-TE: Extensions to RSVP for LSP Tunnels,"draft-
      ietf-mpls-rsvp-lsp-tunnel-08.txt, Internet Draft, Work in

   18.  J. Suurballe, "Disjoint Paths in a Network," Networks, vol. 4,

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   19.  B. Rajagopalan, D. Saha, G. Bernstein and V. Sharma, "Signaling
      for fast restoration in optical mesh networks," draft-bala-
      restoration-signaling-00.txt, Internet Draft, Work in Progress.

   20.  A. Chie et al., "Impairments And Other Constraints On Optical
      Layer Routing", draft-ietf-ipo-impairments-00.txt, Internet
      Draft, Work in Progress.

14. Acknowledgments

   We would like to thank Zouheir Mansourati of Movaz Networks and Ian
   Duncan of Nortel Networks for their comments on this draft.

15. Author's Addresses

      Bala Rajagopalan            James V. Luciani
      Debanjan Saha               Crescent Networks
      Tellium, Inc.               900 Chelmsford St.
      2 Crescent Place            Lowell, MA 01851
      P.O. Box 901                Email: jluciani@CrescentNetworks.com
      Oceanport, NJ 07757-0901
      Email: {braja, dsaha}@tellium.com

      Daniel O. Awduche          Bilel Jamoussi
      Movaz Networks             Nortel Networks
      7926 Jones Branch Dr.      600 Tech Park
      McLean, VA 22102           Billerica, MA 01821
      Phone: 703-847-7350        Phone: 978-288-4734
      Email: awduche@movaz.com   Email: jamoussi @nortelnetworks.com

      Brad Cain
      Cereva Networks
      3 Network Dr.
      Marlborough, MA 01752
      Email: bcain@cereva.com

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