RFC3717: IP over Optical Networks: A Framework

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Network Working Group                                     B. Rajagopalan
Request for Comments: 3717                                    Consultant
Category: Informational                                       J. Luciani
                                                  Marconi Communications
                                                              D. Awduche
                                                              March 2004

                 IP over Optical Networks: A Framework

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   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
   document 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.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology and Concepts . . . . . . . . . . . . . . . . . . .  4
   3.  The Network Model. . . . . . . . . . . . . . . . . . . . . . .  8
       3.1.  Network Interconnection. . . . . . . . . . . . . . . . .  8
       3.2.  Control Structure. . . . . . . . . . . . . . . . . . . . 11
   4.  IP over Optical Service Models and Requirements. . . . . . . . 13
       4.1.  Domain Services Model. . . . . . . . . . . . . . . . . . 13
       4.2.  Unified Service Model. . . . . . . . . . . . . . . . . . 14
       4.3.  Which Service Model? . . . . . . . . . . . . . . . . . . 15
       4.4.  What are the Possible Services?. . . . . . . . . . . . . 16
   5.  IP transport over Optical Networks . . . . . . . . . . . . . . 16
       5.1.  Interconnection Models . . . . . . . . . . . . . . . . . 17
       5.2.  Routing Approaches . . . . . . . . . . . . . . . . . . . 18
       5.3.  Signaling-Related. . . . . . . . . . . . . . . . . . . . 21
       5.4.  End-to-End Protection Models . . . . . . . . . . . . . . 23
   6.  IP-based Optical Control Plane Issues. . . . . . . . . . . . . 25
       6.1.  Addressing . . . . . . . . . . . . . . . . . . . . . . . 25
       6.2.  Neighbor Discovery . . . . . . . . . . . . . . . . . . . 27
       6.3.  Topology Discovery . . . . . . . . . . . . . . . . . . . 28
       6.4.  Protection and Restoration Models. . . . . . . . . . . . 29
       6.5.  Route Computation. . . . . . . . . . . . . . . . . . . . 30
       6.6.  Signaling Issues . . . . . . . . . . . . . . . . . . . . 32
       6.7.  Optical Internetworking. . . . . . . . . . . . . . . . . 34
   7.  Other Issues . . . . . . . . . . . . . . . . . . . . . . . . . 35
       7.1.  WDM and TDM in the Same Network. . . . . . . . . . . . . 35
       7.2.  Wavelength Conversion. . . . . . . . . . . . . . . . . . 36
       7.3.  Service Provider Peering Points. . . . . . . . . . . . . 36
       7.4.  Rate of Lightpath Set-Up . . . . . . . . . . . . . . . . 36
       7.5.  Distributed vs. Centralized Provisioning . . . . . . . . 37
       7.6.  Optical Networks with Additional Configurable
             Components . . . . . . . . . . . . . . . . . . . . . . . 38
       7.7.  Optical Networks with Limited Wavelength Conversion
             Capability . . . . . . . . . . . . . . . . . . . . . . . 38
   8.  Evolution Path for IP over Optical Architecture. . . . . . . . 39
   9.  Security Considerations. . . . . . . . . . . . . . . . . . . . 41
       9.1.  General Security Aspects . . . . . . . . . . . . . . . . 42
       9.2.  Security Considerations for Protocol Mechanisms. . . . . 43
   10. Summary and Conclusions. . . . . . . . . . . . . . . . . . . . 44
   11. Informative References . . . . . . . . . . . . . . . . . . . . 44
   12. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 45
   13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 46
   14. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 47
   15. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 48

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1.  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 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 switched optical networks (including 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

   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 plane 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 optical channels 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 and adapt the IP-based protocols.
   This is especially the case for restoration, and for routing and
   signaling in all-optical networks.  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
   relative merits.

   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

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   second is the transport of IP traffic through an optical network
   together with the control and coordination issues that arise

   This document 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 document therefore advocates the definition of
   "capability sets" that define the evolution of functionality at the
   interfaces as more sophisticated operational requirements arise.

   This document is organized as follows.  In the next section,
   terminology covering some basic concepts related to this framework
   are described.  The definitions are specific to this framework and
   may have other connotations elsewhere.  In Section 3, 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 4.  This section also
   considers some general requirements.  Section 5 considers the
   architectural models for IP-optical interworking, describing the
   relative merits of each model.  It should be noted that it is not the
   intent of this document 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 6 describes IP-centric control plane mechanisms
   for optical networks, covering signaling and routing issues in
   support of provisioning and restoration.  The approaches described in
   Section 5 and 6 range from the relatively simple to the
   sophisticated.  Section 7 describes a number of specialized issues in
   relation to IP over optical networks.  Section 8 describes a possible
   evolution path for IP over optical networking capabilities in terms
   of increasingly sophisticated functionality that may be supported as
   the need arises.  Section 9 considers security issues pertinent to
   this framework.  Finally, the summary and conclusion are presented in
   Section 10.

2.  Terminology and Concepts

   This section introduces  terminology pertinent to this framework and
   some related concepts.  The definitions are specific to this
   framework and may have other interpretations elsewhere.

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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 optical fiber and 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 dense
   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 and type of nodes through which the signals pass before
      reaching 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 from an input port to a output port.  Such a switch may
   utilize optical-electrical conversion at the input port and
   electrical-optical conversion at the output port, or it may be all-
   optical.  An OXC is assumed to have a control-plane processor that
   implements the signaling and routing protocols necessary for
   computing and instantiating optical channel connectivity in the
   optical domain.

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   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 document,
   the term "lightpath" is used interchangeably with optical channel

   Optical mesh sub-network

   An optical sub-network, as used 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 mesh topology. The
   following sub-layers may be associated with this network:

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

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

   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 used in document, is a topologically
   connected collection of optical sub-networks whose node degree may
   exceed 2.  Such an optical network is assumed to be under the purview
   of a single administrative entity.  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 such an
   environment, abstraction can be used to hide the internal details of
   each autonomous optical cloud from external clouds.

   Optical internetwork

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

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   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.

   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 optical
   signals are transported from transmitter to receiver entirely in the
   optical domain without OEO conversion.  Generally, intermediate
   switching nodes in a transparent optical network do not have access
   to the payload carried by the optical signals.

   Note that amplification  of signals at transit nodes is permitted in
   transparent optical networks (e.g., using Erbium Doped Fiber
   Amplifiers << EDFAs).

   On the other hand, in opaque optical networks,  transit nodes may
   manipulate  optical signals traversing through them.  An example of
   such manipulation would be OEO conversion which may involve 3R
   operations (reshaping, retiming, regeneration, and perhaps

   Trust domain

   A trust domain is a network under a single technical administration
   in which adequate security measures are established to prevent
   unauthorized intrusion from outside the domain.  Hence, it may be
   assumed that most nodes in the domain are deemed to be secure or
   trusted in some fashion.  Generally, the rule for "single"
   administrative control over a trust domain 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 document, it will be assumed, without loss of
   generality, that the term trust domain applies to a single
   administrative entity with appropriate security policies.  It should
   be noted that within a trust domain, any subverted node can send
   control messages which can compromise the entire network.

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   In this document, the term flow will be used to signify the smallest
   non-separable stream of data, from the point of view of an endpoint
   or termination point (source or destination node).  The reader should
   note that the term flow is heavily overloaded in contemporary
   networking literature.  In this document, we will consider a
   wavelength to be 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 using time
   division multiplexing (TDM), 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 traversing the same
   path between two access points which allows some characteristics and
   attributes of the traffic to be parameterized.

3.  The Network Model

3.1.  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 optical channels.  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 of which may be administered by a different entity.
   Each optical network consists of sub-networks interconnected by
   optical fiber 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
   switches from a single vendor.  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.

   In this environment, 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
   implemented through compatible physical interfaces, with suitable

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   optical-electrical conversions where necessary.  The routers that
   have direct physical connectivity with the optical network are
   referred to as "edge routers" with respect to the optical network. As
   shown in Figure 1, other client networks (e.g., ATM) may also 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
   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 interface cards as the forward path, but
   this may not be the case under all circumstances.

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                           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

   Multiple traffic streams exiting from an OXC may be multiplexed onto
   a fiber optic 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 later 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

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   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 the routers can transfer user
   traffic among themselves.  A lightpath between IP routers may
   traverse multiple optical networks and be subject to different
   provisioning and restoration procedures in each network.

   The IP-based control plane issue for optical networks pertains to the
   design of standard signaling and routing protocols for provisioning
   and restoration of lightpaths across multiple optical networks.
   Similarly, IP transport over optical networks involves establishing
   IP reachability and seamlessly constructing forwarding paths from one
   IP endpoint to another over an optical network.

3.2.  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 an optical network (between OXCs in
   different sub-networks), and the external node-to-node interface
   between nodes in different optical networks.  These interfaces are
   also referred to as the User-Network Interface (UNI), the internal
   NNI (INNI), and the external NNI (ENNI), respectively.

   The distinction between these interfaces arises out of the type and
   amount of control information flow across them.  The client-optical
   internetwork interface (UNI) represents a service boundary between
   the client (e.g., IP router) and the optical network.  The client and
   server (optical network) 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 client-optical internetwork
   interface 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 4.  The NNIs represent vendor-independent
   standardized interfaces for 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.

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   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 the distinction between them.  On the other
   hand, it may be required to minimize flow of control 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 document, these interfaces are treated as

   The client-optical internetwork interface can be categorized as
   public or private depending upon context and service models.  Routing
   information (i.e., topology state information) can be exchanged
   across a private client-optical internetwork interface.  On the other
   hand, such information is not exchanged across a public client-
   optical internetwork interface, 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 5) may occur across private and public logical

   The physical control structure used to realize these logical
   interfaces may vary.  For instance, for the client-optical
   internetwork interface, 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 with respect to the control plane.  This
      situation is shown in Figure 2.  The type of routing and signaling
      information exchanged across  the direct interface may vary
      depending on the service definition.  This issue is addressed in
      the next section.  Some choices for  the routing protocol are OSPF
      or ISIS (with traffic engineering extensions and additional
      enhancements 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 document.

   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

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      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.

   +---------------------------+    +---------------------------+
   |                           |    |                           |
   | +---------+   +---------+ |    | +---------+   +---------+ |
   | |         |   |         | |    | |         |   |         | |
   | | 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.

4.  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.

4.1.  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

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   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.

   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 5.

4.2.  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

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   interface from a routing and signaling point of view.  It is assumed
   that this control plane is IP-based, for example leveraging the
   traffic engineering extensions for MPLS or GMPLS, 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 5).

   Under the unified service model and within the context of a GMPLS
   network, optical network services are obtained implicitly during
   end-to-end GMPLS signaling.  Specifically, an edge router can create
   a lightpath with specified attributes, or delete and modify
   lightpaths as it creates GMPLS 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 packet-based 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, the lightpath 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 Sections 5 and 6.

4.3.  Which Service Model?

   The relative merits 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 models.  As noted
   above, signaling for service requests 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 Sections 5 and 6.

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4.4.  What are the Possible Services?

   Specialized services may be built atop the point-to-point
   connectivity service offered by the optical network.  For example,
   optical virtual private networks and bandwidth on demand are some of
   the services that can be envisioned.

4.4.1.  Optical Virtual Private Networks (OVPNs)

   Given that the data plane links between IP routers over an optical
   network amounts to a virtual topology which is an overlay over the
   fiber optic network, it is easy to envision a virtual private network
   of lightpaths that interconnect routers (or any other set of clients)
   belonging to a single entity or a group of related entities across a
   public optical network.  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 client networks
   (i.e., VPN service).

5.  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.  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 with respect to
   the control plane, especially for routing protocols that permit the
   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 IP-based control plane [1] is used, such as
   GMPLS.  Depending on the service model(Section 4), however, the
   control planes in the IP and optical networks can be loosely or
   tightly coupled.  This coupling determines the following

   o  The details of the topology and routing information advertised by
      the optical network across the client interface;

   o  The level of control that IP routers can exercise in selecting
      explicit paths for connections across the optical network;

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

   The following interconnection models are then possible:

5.1.  Interconnection Models

5.1.1.  The Peer Model

   Under the peer model, the IP control plane acts as a peer of the
   optical transport network control plane.  This implies that a single
   instance of the control plane is deployed over the IP and optical
   domains.  When there is a single optical network involved and the IP
   and optical domains belong to the same entity, then 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.  Many of
   these extensions are occurring within the context of GMPLS.

   When an optical internetwork with multiple optical networks is
   involved (e.g., spanning different administrative domains), a single
   instance of an intra-domain routing protocol is not attractive or
   even realistic.  In this case, inter-domain routing and signaling
   protocols 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 in both IP and optical domains.  Thus, the optical
   network elements become IP addressable entities as noted in [1].

5.1.2.  The Overlay Model

   Under the overlay model, the IP layer routing, topology distribution,
   and signaling protocols are independent of the routing, topology
   distribution, and signaling protocols within the optical domain.
   This model is conceptually similar to the classical IP over ATM or
   MPOA models, but applied to an optical internetwork instead.  In the
   overlay model, a separate instance of the control plane (especially
   the routing and signaling protocols) would have to be deployed in the
   optical domain, independent of what exists in the IP domain.  In
   certain circumstances, it may also be feasible to statically
   configure the optical channels that provide connectivity for the IP
   domain in the overlay model.  Static configuration can be effected
   through network management functions.  Static configuration, however,

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   is unlikely to scale in very large networks, and may not support the
   rapid connection provisioning requirements of  future highly
   competitive networking environments.

5.1.3.  The Augmented Model

   Under the augmented model, there are separate routing instances in
   the IP and optical domains, but certain types of information from one
   routing instance can be passed through to 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.

5.2.  Routing Approaches

5.2.1.  Integrated Routing

   This routing approach supports the peer model within a single
   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) may be
   identical, but not necessarily.  This approach 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) in a GMPLS environment.  Such an
   LSP can be established using GMPLS signaling, e.g., RSVP-TE or CR-LDP
   with appropriate extensions.  In this case, the signaling protocol
   will establish a lightpath between two edge routers.  This lightpath
   is in essence a tunnel across the optical network, and may have
   capacity much larger than the bandwidth required to support the first
   LSP.  Thus, it is essential that other routers in the network realize
   the availability of excess capacity within the lightpath so that
   subsequent LSPs between the routers can use it rather than
   instantiating a new lightpath.  The lightpath may therefore be
   advertised as a virtual link in the topology as a means to address
   this issue.

   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

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   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.

5.2.2.  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
   would allow 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

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   network to others.  For instance, edge routers and OXCs can run
   exterior BGP (EBGP).  Within the optical internetwork, interior BGP
   (IBGP) is may be used between border optical switches, and EBGP may
   be used between different networks (over ENNI, Figure 1).

   Under this scheme, it may be necessary to identify the egress points
   in the optical internetwork corresponding to externally reachable IP
   addresses.  To see this, suppose an edge router intends to establish
   an LSP to a destination node across the optical internetwork.  It may
   request a direct lightpath to that destination, without explicitly
   specifying the egress optical port for the lightpath because the
   optical internetwork has knowledge of externally reachable IP
   addresses.  However, if the same edge router were to establish
   another LSP to a different external destination, then for efficiency
   reasons, it may first need to determine whether there is an existing
   lightpath (with sufficient residual capacity) to the target
   destination.  For this purpose, it may be 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.  When optical VPNs are
   implemented, the address prefixes advertised by the border OXCs may
   be accompanied by some VPN specific identification.

   There are however, some potential negative effects that could result
   from domain-specific routing using BGP in an IPO environment:

   o  The amount of information that optical nodes will have to maintain
      will not be bound by the size of the optical network anymore, but
      will have to include external routes as well.

   o  The stability of the optical network control plane will no longer
      be dictated solely by the dynamics emanating within the optical
      network, but may be affected by the dynamics originating from
      external routing domains from which  external reachability
      information is received.

5.2.3.  Overlay Routing

   The overlay routing approach supports the overlay interconnection
   model.  Under this approach, an overlay mechanism that allows edge
   routers to register 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

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   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
   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.

5.3.  Signaling-Related

5.3.1.  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, can be appropriately extended and used
   for signaling lightpath requests.  These protocols can be adapted for
   client/server signaling in the case of the domain services model, and
   for end-to-end integrated signaling in the case of the unified
   services model.

5.3.2.  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].

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   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

   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.

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                        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 [10].

5.4.  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

5.4.1.  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

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   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

5.4.2.  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

5.4.3.  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

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   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.

6.  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
   and across sub-networks.  But this is simply a recommendation within
   this framework document, rather than an imperative requirement. Thus,
   in the following, signaling and routing across sub-networks is
   considered first, followed by a discussion of similar issues across

6.1.  Addressing

   For interoperability across optical sub-networks using an IP-centric
   control plane, one of the fundamental issues 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 high level guidelines on this issue.

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   Identifiable entities in optical networks include 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 [10].

   Another entity that must be identified is the SRLG [11].  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
   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).

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   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 [12].  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

6.2.  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

6.3.  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 6.7.  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 [12]. 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 [12].  Also, the SRLGs corresponding to
      each optical link in the bundle may be included as a parameter.

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   o  The link state information should capture restoration-related
      parameters for optical links.  Specifically, with shared
      protection (Section 6.5), the link state updates must have
      information that allows the computation of shared protection

   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.

6.4.  Protection and 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 (or network segment)
   between two OXCs across the INNI when a failure affects the primary
   link (or primary network segment) over which the (protected)
   lightpath is 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.  Under this
   scenario this, the affected lightpath may be rerouted over an
   alternate diverse path to circumvent failed resources.  For end-to-
   end restoration, alternate paths may be pre-computed to expedite the
   recovery time.  End to end restoration may also be mixed with local
   recovery in various ways depending on acceptable tradeoffs between
   utilization of network resources and recovery times.

   End-to-end protection 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

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   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
   across the INNI is described in Section 6.6.3.  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.

6.5.  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 [13].

   Route computation with constraints may be accomplished using a number
   of algorithms [14].  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

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   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
         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 [15].  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 computed together in one
      step, for example, using Suurbaale's algorithm [16].  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

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   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
   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 [11].

6.6.  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 6.7.  In the former case, signaling
   messages may carry strict explicit route information, while in the
   latter case the route information  should be loose, at the level of
   abstraction of sub-networks.  Once a route is determined for a
   lightpath, each OXC along the path must appropriately configure their
   cross-connects in a coordinated fashion.  This coordination is
   conceptually analogous to selecting incoming and outgoing labels in a
   label-switched environment.  Thus, protocols like RSVP-TE [9] may be
   adapted and used across the INNI for this purpose.  The adaptation of
   IP-based signaling protocols must take into account a number of
   peculiar attributes of optical networks.

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6.6.1.  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.

6.6.2.  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
   by adjacent nodes, for example, through a keepalive mechanism between
   signaling peers, must not result in these optical paths being torn

   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

6.6.3.  Restoration

   Signaling for restoration has two distinct 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.

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   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 among themselves.
   The signaling message must itself indicate shared reservation.  The
   sharing rule is as described in Section 6.4: back-up paths
   corresponding to physically diverse primary paths may share the same
   network resources.  It may therefore be necessary for the signaling
   message to carry adequate information that allows an OXC to verify
   that appropriateness of having a set of back-up paths sharing

   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 expedited
   response.  With out-of-band control, it may be 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 may therefore
   be necessary to develop a notion of prioritization for restoration
   signaling and incorporate appropriate mechanisms into existing
   signaling protocols to achieve this.  Alternatively, a new signaling
   mechanism may be developed exclusively for activating protection
   paths during restoration.

6.7.  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.

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   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.

6.7.1.  Neighbor Discovery

   Neighbor discovery procedure, as described in Section 6.2, can be
   used for this.  Indeed, a single protocol should be standardized for
   neighbor discovery within and across networks.

6.7.2.  Addressing and Routing Model

   The addressing mechanisms described in Section 6.1 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.

6.7.3.  Restoration

   Local restoration across the ENNI is similar to that across INNI
   described in Section 6.6.3.  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 6.4.

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7.  Other Issues

7.1.  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.

7.2.  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.

7.3.  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 over
   optical networks, peering relationships between different trust
   domains will eventually have to occur at the IP layer, on IP routing

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   elements, even though non-IP paths may exist between the peering

7.4.  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

   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 document
   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 concurrently.

   Another possible short term reason for dynamic shortcut lightpath
   setup would be to quickly pre-provision paths based on some criteria
   (e.g., a corporate executive wants a high bandwidth reliable
   connection, etc.).  In this scenario, a set of paths can be pre-
   provisioned, but not actually instantiated until the customer

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   initiates an authenticated and authorized setup requests, which is
   consistent with existing agreements between the provider and the
   customer.  In a sense, the 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.

7.5.  Distributed vs. Centralized Provisioning

   This document 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., [9]) 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.

7.6.  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

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   parameterize the characteristics of these components and advertise
   them  within the control plane.  This aspect is left for further

7.7.  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
   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 [17].  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
   dispersion and polarization mode dispersion), 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 [17] in a generic fashion, so
   that they can be used for path computation.

8.  Evolution Path for IP over Optical Architecture

   The architectural models described in Section 5 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.

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   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.

   Using a common signaling framework (based on GMPLS) from the
   beginning facilitates this type of evolution.  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.

   From a routing point of view, the use of Network Management Systems
   (NMS) for static connection management is prevalent in legacy optical
   networks.  Going forward, it can be expected that connection routing
   using the control plane will be gradually introduced and integrated
   into operational infrastructures.  The introduction of routing
   capabilities can be expected to occur in a phased approach.

   It is likely that in the first phase, service providers will either
   upgrade existing local element management (EMS) software with
   additional control plane capabilities (and perhaps the hardware as
   well), or upgrade the NMS software in order to introduce some degree
   of automation within each optical subnetwork.  For this reason, it
   may be desirable to partition the network into subnetworks and
   introduce IGP interoperability within each subnetwork (i.e., at the
   I-NNI level), and employ either static or signaled interoperability
   between subnetworks.  Consequently, it can be envisioned that the
   first phase in the evolution towards network level control plane
   interoperability in IP over Optical networks will be organized around
   a system of optical subnetworks which are interconnected statically
   (or dynamically in a signaled configuration).  During this phase, an
   overlay interconnection model will be used between the optical
   network itself and external IP and MPLS routers (as described in
   Section 5.2.3).

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   Progressing with this phased approach to IPO routing
   interoperabibility evolution, the next level of integration will be
   achieved when a single carrier provides dynamic optical routing
   interoperability between subnetworks and between domains.  In order
   to become completely independent of the network switching capability
   within subnetworks and across domains, routing information exchange
   may need to be enabled at the UNI level.  This would constitute a
   significant evolution: even if the routing instances are kept
   separate and independent, it would still be possible to dynamically
   exchange reachability and other types of routing information. Another
   more sophisticated step during this phase is to introduce dynamic
   routing at the E-NNI level.  This means that any neighboring networks
   (independent of internal switching capability) would be capable of
   exchanging routing information with peers across the E-NNI.

   Another alternative would be for private networks to bypass these
   intermediate steps and directly consider an integrated routing model
   from the onset.  This direct evolution strategy is realistic, but is
   more likely to occur in operational contexts where both the IP (or
   MPLS) and optical networks are built simultaneously, using equipment
   from a single source or from multiple sources that are closely
   affiliated.  In any case, due to the current lack of operational
   experience in managing this degree of control plane interaction in a
   heterogeneous network (these issues may exist even if the hardware
   and software originate from the same vendor), an augmented model is
   likely to be the most viable initial option.  Alternatively, a very
   modular or hierarchical peer model may be contemplated.  There may be
   other challenges (not just of a technical, but also administrative
   and even political issues) that may need to be resolved in order to
   achieve full a peer model at the routing level in a multi-technology
   and multi-vendor environment.  Ultimately, the main technical
   improvement would likely arise from efficiencies derived from the
   integration of traffic-engineering capabilities in the dynamic
   inter-domain routing environments.

9.  Security Considerations

   The architectural framework described in this document requires a
   number of different protocol mechanisms for its realization.
   Specifically, the role of neighbor discovery, routing, and signaling
   protocols were highlighted in previous sections.  The general
   security issues that arise with these protocols include:

   o  The authentication of entities exchanging information (e.g.,
      signaling, routing, or link management) across a control

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   o  Ensuring the integrity of the information exchanged across the

   o  Protection of the control mechanisms from intrusions and other
      modes of outside interference.

   Because optical connections may carry high volumes of traffic and are
   generally quite expensive, mechanisms are required to safeguard
   optical networks against intrusions and unauthorized utilization of
   network resources.

   In addition to the security aspects relating to the control plane,
   the data plane must also be protected from external interference.

   An important consideration in optical networks is the separation of
   control channels from data channels.  This decoupling implies that
   the state of the bearer channels carrying user traffic cannot be
   inferred from the state of the control channels.  Similarly, the
   state of the control channels cannot be inferred from the state of
   the data channels.  The potential security implications of this
   decoupling should be taken into account in the design of pertinent
   control protocols and in the operation of IPO networks.

   Another issue in IPO networks concerns the fact that the underlying
   optical network elements may be invisible to IP client nodes,
   especially in the overlay model.  This means that traditional IP
   tools such as traceroute cannot be used by client IP nodes to detect
   attacks within the optical domain.

   For the aforementioned reasons, the output of the routing protocol
   security (RPSEC) efforts within the IETF should be considered in the
   design of control protocols for optical networks.

   In Section 2, the concept of a trust domain was defined as a network
   under a single technical administration in which adequate security
   measures are established to prevent unauthorized intrusion from
   outside the domain.  It should be strongly noted that within a trust
   domain, any subverted node can send control messages which can
   compromise the entire network.

9.1.  General security aspects

   Communication protocols usually require two main security mechanisms:
   authentication and confidentiality.  Authentication mechanisms ensure
   data origin verification and message integrity so that intrusions and
   unauthorized operations can be detected and mitigated.  For example,
   with reference to Figure 1, message authentication can prevent a
   malicious IP client from mounting a denial of service attack against

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   the optical network by invoking an excessive number of connection
   creation requests across the UNI interface.  Another important
   security consideration is the need to reject replayed control
   packets.  This capability can assist in countering some forms of
   denial of service attacks.  Replay protection provides a form of
   partial sequence integrity, and can be implemented in conjunction
   with an authentication mechanism.

   Confidentiality of signaling messages is also desirable, especially
   in scenarios where message attributes between communicating entities
   include sensitive or private information.  Examples of such
   attributes include account numbers, contract identification
   information, and similar types of private data.

   The case of equipment that are not co-located presents increased
   security threats.  In such scenarios, the communicating entities
   engaged in protocol message transactions may be connected over an
   external network.  Generally, the external network may be outside the
   span of control of the optical network (or client IP network)
   administrators.  As a result, the protocol messages may be subject to
   increased security threats, such as address spoofing, eavesdropping,
   and intrusion.  To mitigate such threats, appropriate security
   mechanisms must be employed to protect the control channels and
   associated signaling and routing messages.

   Requests for optical connections from client networks must also be
   filtered using appropriate policies to protect against security
   infringements and excess resource consumption.  Additionally, there
   may be a need for confidentiality of SRLGs in some circumstances.

   Optical networks may also be subject to subtle forms of denial of
   service attacks.  An example of this would be requests for optical
   connections with explicit routes that induce a high degree of
   blocking for subsequent requests.  This aspect might require some
   global coordination of resource allocation.

   Another related form of subtle denial of service attack could occur
   when improbable optical paths are requested (i.e., paths within the
   network for which resources are insufficiently provisioned).  Such
   requests for improbable paths may consume ports on optical switching
   elements within the network resulting in denial of service for
   subsequent connection requests.

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9.2.  Security Considerations for Protocol Mechanisms

   The security requirements for IP-centric control protocols employed
   in the control plane of optical networks would depend on the specific
   characteristics of the protocols and the security risks that exist in
   a particular operational context.  Such details relating to
   particular operational contexts are beyond the scope of this document
   and hence are not considered further.  Nevertheless, it must be
   stated that such control protocols must take into account the issues
   associated with the separation of control channels from data channels
   in switched optical networks, and the magnitude and extent of service
   interruptions  within the IP domain that could result from outages
   emanating from the optical domain.

10.  Summary and Conclusions

   The objective of this document was to define a framework for IP over
   optical networks, considering the service models, and routing and
   signaling issues.  There are a diversity of choices for IP-optical
   control interconnection, service models, and protocol mechanisms. The
   approach advocated in this document was to support different service
   models which allow for future enhancements, and define complementary
   signaling and routing mechanisms to enable these capabilities.  An
   evolutionary scenario, based on a common signaling framework (e.g.,
   based on GMPLS) was suggested, with the capability to increase the
   complexity of interworking functionality as the requirements become
   more sophisticated.  A key aspect of this evolutionary principle is
   that the IP-optical control and service interaction is first based on
   the domain services model with overlay interconnection that will
   eventually evolve to support full peer interaction.

11.  Informative References

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

   [2]   Lang, J., et al., "Link Management Protocol", Work in progress.

   [3]   Kompella, K. and Y. Rekhter, "LSP Hierarchy with MPLS TE",
         Internet Draft, Work in progress.

   [4]   Berger, L., Ed., "Generalized Multi-Protocol Label Switching
         (GMPLS) Signaling Functional Description", RFC 3471, January

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   [5]   Rajagopalan, B., "Documentation of IANA Assignments for Label
         Distribution Protocol (LDP), Resource ReSeVation Protocol
         (RSVP), and Resource ReSeVation Protocol-Traffic Engineering
         (RSVP-TE) Extensions for Optical UNI Signaling", RFC 3476,
         March 2003.

   [6]   The Optical Interworking Forum, "UNI 1.0 Signaling
         Specification", December 2001.

   [7]   Kompella, K., et al., "OSPF Extensions in Support of
         Generalized MPLS," Work in Progress.

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

   [9]   Berger, L., Ed., "Generalized Multi-Protocol Label Switching
         (GMPLS) Signaling Resource ReSeVation Protocol-Traffic
         Engineering (RSVP-TE) Extensions", RFC 3473, January 2003.

   [10]  Mannie, E., "GMPLS Extensions for SONET/SDH Control", Work in

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

   [12]  Kompella, K., et al., "Link Bundling in MPLS Traffic
         Engineering", Work in Progress.

   [13]  Ramamurthy, S., Bogdanowicz, Z., Samieian, S., et al.,
         "Capacity Performance of Dynamic Provisioning in Optical
         Networks", Journal of Lightwave Technology, January 2001.

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

   [15]  Awduche, D., Berger, L., Gan, D., Li, T., Swallow, G. and V.
         Srinivasan, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC
         3209, December 2001.

   [16]  Suurballe, J., "Disjoint Paths in a Network", Networks, vol. 4,

   [17]  Chiu, A., et al., "Impairments and Other Constraints On Optical
         Layer Routing", Work in Progress.

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12.  Acknowledgments

   We would like to thank Zouheir Mansourati (Movaz Networks), Ian
   Duncan (Nortel Networks), Dimitri Papadimitriou (Alcatel), and
   Dimitrios Pendarakis (Tellium) for their contributions to this
   document.  The Security Considerations section was revised to reflect
   input from Scott Bradner and Steve Bellovin.

13.  Contributors

   Contributors are listed alphabetically.

   Brad Cain
   Cereva Networks
   3 Network Dr.
   Marlborough, MA 01752

   EMail: bcain@cereva.com

   Bilel Jamoussi
   Nortel Networks
   600 Tech Park
   Billerica, MA 01821

   Phone: 978-288-4734
   EMail: jamoussi@nortelnetworks.com

   Debanjan Saha

   EMail: debanjan@acm.org

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14.  Authors' Addresses

   Bala Rajagopalan
   Tellium, Inc.
   2 Crescent Place
   P.O. Box 901
   Oceanport, NJ 07757-0901

   EMail: braja@tellium.com

   James V. Luciani
   Marconi Communications
   2000 Marconi Dr.
   Warrendale, PA 15086

   EMail: james_luciani@mindspring.com

   Daniel O. Awduche
   22001 Loudoun County Parkway
   Ashburn, VA 20147

   Phone: 703-886-1753
   EMail: awduche@awduche.com

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15.  Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78 and
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   This document and the information contained herein are provided on an

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