Internet Draft

IPO and MPLS                                      N. Chandhok
                                                  Ohio State University
                                                  A. Durresi
                                                  Ohio State University
                                                  R. Jagannathan,
                                                  Ohio State University
                                                  R. Jain
                                                  Nayna Networks
                                                  S. Seetharaman
                                                  Ohio State University
                                                  K. Vinodkrishnan
                                                  Ohio State University
Internet Draft
Document: draft-osu-ipo-mpls-issues-00.txt        July 2000
Category: Informational


             IP over Optical Networks: A Summary of Issues


Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time.  It is inappropriate to use Internet- Drafts as
   reference material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

   This draft presents a summary of issues related to transmission of
   IP packets over optical networks.  This is a compilation of many
   drafts presented so far in IETF.  The goal is to create a common
   document, which by including all the views and proposals will serve
   as a better reference point for further discussion.  The novelty of
   this draft is that we try to cover all the main areas of integration
   and deployment of IP and optical networks including architecture,
   routing, signaling, management, and survivability.



Many Authors     Informational _ Expires January 2001     Page 1 of 51
             IP over Optical Networks: Summary of Issues     July 2000


   Several existing and proposed network architectures are discussed.
   The two-layer model, which aims at a tighter integration between IP
   and optical layers, offers a series of important advantages over the
   current multi-layer architecture.  The benefits include more
   flexibility in handling higher capacity networks, better network
   scalability, more efficient operations and better traffic
   engineering.

   Multiprotocol Label Switching (MPLS) has been proposed as the
   integrating structure between IP and optical layers. Routing in the
   non-optical and optical parts of the hybrid IP network needs to be
   coordinated.  Several models have been proposed including overlay,
   augmented, and peer-to-peer models.  These models and the required
   enhancements to IP routing protocols, such as, OSPF and IS-IS are
   provided.

   Control in the IP over Optical networks is facilitated by MPLS
   control plane.  Each node consists of an integrated IP router and
   optical layer crossconnect (OLXC).  The interaction between the
   router and OLXC layers is defined.  Signaling among various nodes is
   achieved using CR-LDP and RSVP protocols.

   The management functionality in optical networks is still being
   developed.  The issues of link initialization and performance
   monitoring are summarized in this document.

   With the introduction of IP in telecommunications networks, there is
   tremendous focus on reliability and availability of the new IP-
   optical hybrid infrastructures.  Automated establishment and
   restoration of end to end paths in such networks require
   standardized signaling and routing mechanisms.  Layering models that
   facilitate fault restoration are discussed.  A better integration
   between IP and optical will provide opportunities to implement a
   better fault restoration.



Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC-2119.

Contents:

   1.  Overview
        1.1 Introduction
        1.2 Network Models
   2.  Optical Switch Architecture
        2.1 Isomorphic Relations Between OXCs And LSRs
        2.2 Distinctions Between OXCs And LSRs
        2.3 Isomorphic Relations between LSPs And Lightpaths
        2.4 Distinction Between LSPs And Lightpaths

Various Authors  Informational _ Expires January 2001                2
             IP over Optical Networks: Summary of Issues     July 2000


        2.5 General Requirements For The OXC Control Plane
                2.5.1 Overview of The MPLS Traffic Engineering Control
                2.5.2 OXC Enhancements to Support MPLS Control Plane
                2.5.3 MPLS Control Plane Enhancements
        2.6 MPLS Traffic Engineering Control Plane With OXCs

   3.  Routing in Optical Networks
        3.1 Models for IP-Optical Network Interaction
                3.1.1 Overlay model
                3.1.2 Integrated/Augmented model
                3.1.3 Peer model
        3.2 The Lightpath routing solution
                3.2.1 What is an IGP?
                3.2.2 How does MPLS come into the picture?
                3.2.3 Lightpath Selection
        3.3 IS-IS/OSPF enhancements
                3.3.1 Link Type
                3.3.2 Link Media Type (LMT)
                3.3.3 Link ID
                3.3.4 Local Interface IP address
                3.3.5 Remote Interface IP address.
                3.3.6 TE Metric
                3.3.7 Path TLV
                3.3.8 Shared Risk Link Group TLV
        3.4 Control Channels, Data Channels and IP links
                3.4.1 Excluding Data Traffic from Control Channels
                3.4.2 Forwarding Adjacencies
                3.4.3 Two way Connectivity
                3.4.4 Optical LSAs
        3.5 Open Questions
   4.  Control
        4.1 MPLS Control Plane
        4.2 Addressing
        4.3 Path Setup
                4.3.1 Basic Path Setup Procedure
                4.3.2 CR-LDP Extensions for Path Setup
                4.3.3 RSVP Extensions for Path Setup
        4.4 Resource Discovery and Maintenance
        4.5 Configuration Control Using GSMP
        4.6 Resource Discovery Using NHRP

   5.  Optical Network Management
        5.1 Link Initialization
                5.1.1 Control Channel Management
                5.1.2 Verifying Link Connectivity
                5.1.3 Fault Localization
        5.2 Optical Performance Monitoring (OPM)

   6.  Fault restoration in Optical networks
        6.1 Layering
                6.1.1 Layer 1 Protection
                6.1.2 Layer 0 Protection
        6.2 MPLS in Protection

Various Authors  Informational _ Expires January 2001                3
             IP over Optical Networks: Summary of Issues     July 2000


                6.2.1 Motivations
                6.2.2 Goals
        6.3 Protection options
                6.3.1 Dynamic Protection
                6.3.2 Pre-negotiated Protection
                6.3.3 End to end repair
                6.3.4 Local repair
                6.3.5 Link protection
                6.3.6 Path protection
                6.3.7 Revertive Mode
                6.3.8 Non-revertive Mode
                6.3.9 1+1 Protection
                6.3.10 1:1, 1:n, and n:m  Protection
                6.3.11 Recovery Granularity
        6.4.  Failure detection
        6.5 Failure Notification
                6.5.1 Reverse Notification Tree (RNT)
        6.6.  Timing
                6.6.1 Protection Switching Interval Timer
                6.6.2 Inter-FIS Packet Timer
                6.6.3 Maximum FIS Duration Timer
                6.6.4 Protection switching Dampening Timer
                6.6.5 Liveness Message Send interval
                6.6.6 Failure Indication Hold-off Timer
                6.6.7 Lost Liveness Message Threshold
        6.7 Signaling Requirements related to restoration
        6.8 RSVP/CR-LDP Support for Restoration
                6.8.1 Proposed Extensions for Protection Paths
        6.9 Fast restoration of MPLS LSPs
                6.9.1 L1/L2/L3 Integration
                6.9.2 An example
        6.10 LMP's Fault localization mechanism
   7.  Security Considerations
   8.  Acronyms
   9.  Terminology
   10. References
   11. Author's Addresses


1.  Overview

1.1.  Introduction

   Challenges presented by the exponential growth of the Internet have
   resulted in the intense demand for broadband services.  In
   satisfying the increasing demand for bandwidth, optical network
   technologies represent an unique opportunity because of their almost
   unlimited potential bandwidth.

   Recent developments in wavelength-division multiplexing (WDM)
   technology have dramatically increased the traffic capacities of
   optical networks. Research is ongoing to introduce more intelligence
   in the control plane of the optical transport systems, which will

Various Authors  Informational _ Expires January 2001                4
             IP over Optical Networks: Summary of Issues     July 2000


   make them more survivable, flexible, controllable and open for
   traffic engineering.  Some of the essential desirable attributes of
   optical transport networks include real-time provisioning of optical
   channel trails, providing capabilities that enhance network
   survivability, providing interoperability functionality between
   vendor-specific optical sub-networks, and enabling protection and
   restoration capabilities in operational contexts.  The research
   efforts now are focusing on the efficient internetworking of higher
   layers, primarily IP with WDM layer.

   Along with this WDM network, IP networks, SONET networks, ATM
   backbones shall all coexist.  Various standardization bodies have
   been involved in determining an architectural framework for the
   interoperability of all these systems.

   One approach for sending IP traffic on WDM networks would use a
   multi-layered architecture comprising of IP/MPLS layer over ATM over
   SONET over WDM.  If an appropriate interface is designed to provide
   access to the optical network, multiple higher layer protocols can
   request lightpaths to peers connected across the optical network.
   This architecture has 4 management layers.  One can also use a
   packet over SONET approach, doing away with the ATM layer, by
   putting IP/PPP/HDLC into SONET framing.  This architecture has 3
   management layers.  A few problems of such multi layered
   architectures have been studied.



        +---------------+
        |               |
        |   IP / MPLS   |
        |               |
        +---------------+   +---------------+
        |               |   |               |
        |     ATM       |   |    IP/ MPLS   |
        |               |   |               |
        +---------------+   +---------------+    +-------------+
        |               |   |               |    |             |
        |    SONET      |   |    SONET      |    |  IP / MPLS  |
        |               |   |               |    |             |
        +---------------+   +---------------+    +-------------+
        |               |   |               |    |             |
        |     WDM       |   |      WDM      |    |     WDM     |
        |               |   |               |    |             |
        +---------------+   +---------------+    +-------------+

            (4 LAYERS)         (3 LAYERS)           (2 LAYERS)


              Fig.1: Layering Architectures Possible

   The fact that it supports multiple protocols, will increase
   complexity for IP-WDM integration because of various edge-

Various Authors  Informational _ Expires January 2001                5
             IP over Optical Networks: Summary of Issues     July 2000


   interworkings required to route, map and protect client signals
   across WDM subnetworks.  The existence of separate optical layer
   protocols will increase management costs for service providers.

   One of the main goals of the integration architecture is to make
   optical channel provisioning driven by IP data paths and traffic
   engineering mechanisms.  This will require a tight cooperation of
   routing and resource management protocols at the two layers.  The
   multi-layered protocols architecture can complicate the timely flow
   of the possibly large amount of topological and resource
   information.

   Another problem is with respect to survivability.  There are various
   proposals stating that the optical layer itself should provide
   restoration/protection capabilities of some form.  This will require
   careful coordination with the mechanisms of the higher layers such
   as the SONET Automatic Protection Switching (APS) and the IP re-
   routing strategies.  Hold-off timers have been proposed to inhibit
   higher layers backup mechanisms.

   Problems can also arise from the high level of multiplexing done.
   The optical fiber links contain a large number of higher layer flows
   such as SONET / SDH, IP flows or ATM VCs.  Since these have their
   own mechanisms, a flooding of alarm messages can take place.

   Hence, a much closer IP/WDM integration is required.  The
   discussions, henceforth in this document, shall be of such an
   architecture.  There exist, clouds of IP networks, clouds of WDM
   networks.  Transfer of packets from a source IP router to a
   destination is required.  How the combination does signaling to find
   an optimal path, route the packet, and ensure survivability are the
   topics of discussion.

   Multi-Protocol Label Switching (MPLS) for IP packets is believed to
   be the best integrating structure between IP and WDM.  MPLS brings
   two main advantages.  First, it can be used as a powerful instrument
   for traffic engineering.  Second, it fits naturally to WDM when
   wavelengths are used as labels.  This extension of the MPLS is
   called the Multi-protocol lambda switching.

   This document starts off with a description of the optical network
   model.  Section 2 describes the correspondence between the optical
   network model and the MPLS architecture and how it can bring about
   the inter-working.  Section 3 is on routing in this architecture.
   It also describes 3 models for looking at the IP cloud and the
   Optical cloud namely the Overlay model, the augmented model and the
   peer model.  Sections 4 and 5 are on control, signaling and
   management, respectively.  Section 6 is on restoration. Acronyms and
   glossary are defined in Sections 8 and 9.

1.2 Network Model



Various Authors  Informational _ Expires January 2001                6
             IP over Optical Networks: Summary of Issues     July 2000


   The network model consists of IP routers attached to an optical core
   network.  The optical network consists of multiple optical
   crossconnects (OXCs) interconnected by optical links.  Each OXC is
   capable of switching a data stream using a switching function,
   controlled by appropriately configuring a crossconnect table.  Thus,
   in this document, the term OXC is used to denote the hybrid node
   consisting of switching element referred to as optical layer
   crossconnect (OLXC) and a control plane.  The switching within the
   OXC can be accomplished either in the electrical domain, or in the
   optical domain.  In this network model, a switched lightpath is
   established between IP routers.  Designing an IP-based control plane
   should include designing standard signaling and routing protocols
   for coherent end-to-end provisioning and restoration of lightpaths
   across multiple optical sub-networks, and determining IP
   reachability and seamless establishment of paths from one IP end-
   point to another over an optical core network.

   Several standards organizations and interoperability forums have
   initiated work items to study the requirements and architectures for
   reconfigurable optical networks, under-scoring the importance of
   versatile networking capabilities in the optical domain.  ITU-T
   recommendation G.872, for example, defines a functional architecture
   for an optical transport network (OTN) that supports the transport
   of digital client signals.  It defines OTN as "a transport network
   bounded by optical channel access points".  The architecture of
   G.872's OTN is based on a layered structure, which includes:
   (a) an optical channel (OCh) layer network : The optical channel
   layer network supports end-to-end networking of optical channel
   trails providing functionalities like routing, monitoring, grooming,
   and protection and restoration of optical channels.

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

   (c) an optical transmission section (OTS) layer network : This
   provides functionality for transmission of the optical signals on
   optical media of different types.

   To realize the functions in the OCh layer, an optical crossconnect
   with rearrangeable switch fabrics and a control plane will be
   critical.  In the existing IP-centric data network domain, the
   functionalities of the OCh layer are performed by the MPLS traffic
   engineering control plane.  Thus, there is a similarity between the
   IP/MPLS over WDM and the ITU recommendation.

   In the following section, we stress on the relations that exist
   between the all-optical crossconnects of the optical networks and
   the label switch routers of the MPLS networks and identify how the
   control plane model of MPLS traffic engineering (TE) can be applied
   to that of optical transport networks.  Before we propose a control

Various Authors  Informational _ Expires January 2001                7
             IP over Optical Networks: Summary of Issues     July 2000


   plane model for the optical networks based on the MPLS control plane
   traffic engineering, we discuss how the similarities can help to
   expose the reusable software artifacts from the MPLS traffic
   engineering control plane model.  Consider an IP-centric hybrid
   optical internetworking environment, which consist of both LSRs and
   OXCs.  Let us assume that OXCs are programmable and support
   wavelength conversion.


2.  Optical Switch Architecture

   Multiprotocol Label Switching is a switching method in which a label
   field in the incoming packets is used to determine the next hop. At
   each hop, the incoming label is replaced by another label that is
   used at the next hop. The path thus realized is called a Label
   Switched Path (LSP).  Each LSP has a set of criteria associated with
   it, which describes the traffic that traverses the LSP.  This set of
   criteria groups the incoming traffic into classed called _Forwarding
   Equivalence Classes (FECs)._ LSPs are setup using signaling
   protocols like RSVP or CR-LDP.  A device that can classify traffic
   into FECs is called a label edge router (LER) while the devices
   which base their forwarding decision only on the basis of the
   incoming labels (and ports) are called Label Switched Routers
   (LSRs).

   Here we consider a hybrid, IP-centric optical internetworking
   environment consisting of both label switching routers (LSRs) and
   OXCs.  The OXCs are programmable and support wavelength conversion
   and translation.  It is important here to enumerate the relations
   and distinctions between OXCs and LSRs to expose the reusable
   software artifacts from the MPLS traffic engineering control plane
   model.  Both OXCs and LSRs emphasize problem decomposition by
   architecturally decoupling the control plane from the data plane.

2.1 Isomorphic Relations between OXCs And LSRs

   While an LSR's data plane uses the label swapping paradigm to
   transfer a labeled packet from an input port to an output port, the
   data plane of an OXC uses a switch matrix to provision an optical
   channel trail from an input port to an output port.  An LSR performs
   label switching by establishing a relation between an  tuple and an  tuple.
   Similarly, OXC provisions optical channel trail by establishing a
   relation between an  tuple and an
    tuple.  The functions of the
   control plane of both LSRs and OXCs include resource discovery,
   distributed routing control, and connection management.  LSR's
   control plane is used to discover, distribute, and maintain relevant
   state information related to the MPLS network, and to instantiate
   and maintain label switched paths (LSPs) under various MPLS traffic
   engineering rules and policies.  OXC's control plane is used to
   discover, distribute, and maintain relevant state information
   associated with the OTN, and to establish and maintain optical

Various Authors  Informational _ Expires January 2001                8
             IP over Optical Networks: Summary of Issues     July 2000


   channel trails under various optical internetworking traffic
   engineering rules and policies [Awuduche99].


2.2 Distinctions Between OXCs And LSRs

   Current generation of OXCs and LSRs differ in certain
   characteristics.  While LSRs are datagram devices which can perform
   certain packet level operations in the data plane, OXCs cannot.  It
   cannot perform packet level processing in the data plane.
   Conceptually another difference is there, which is that the
   forwarding information is carried explicitly in LSRs as part of the
   labels appended to the data packets, while in the OXCs switching
   information is not appended to the data packet, rather it is implied
   from the wavelength or the optical channel.

2.3 Isomorphic Relations Between Explicit LSPs And Optical Channel
Trails

   Both the explicit LSPs and optical channel trails (OCTs) exhibit
   certain commonalties.  For example, both of them are the
   abstractions of unidirectional, point-to-point virtual path
   connections.  An explicit LSP provides a parameterized packet
   forwarding path (traffic-trunk)  between an ingress LSR and an
   egress LSR, while an optical channel trail provides a optical
   channel between two endpoints for the transport of digital client
   signals [Awuduche99].  Another commonality is that the payload
   carried by both LSPs and optical trails are transparent along their
   respective paths.  They can be parameterized to stipulate their
   performance, behavioral, and survivability requirements from the
   network.  Paths that satisfy some demands and policy requirements
   subject to some constraints imposed by the operational environment
   can be selected using constraint-based routing scheme.  There are
   certain similarities in the allocation of labels to LSPs and in the
   allocation of wavelengths to optical trails.

2.4 Distinction between LSPs and Optical Channel Trails

   There is one major distinction between LSPs and OCTs in that LSPs
   support label stacking, but the concept similar to label stacking,
   ie, wavelength stacking doesn't exist in the optical domain at this
   time.

2.5 General Requirements for the OXC Control Plane

   This section has some of the requirements for the OXC control plane
   emphasizing on the routing components of these requirements.  Some
   of the key aspects to these requirements are:(a) to expedite the
   capability to establish optical channel trails.(b) to support
   traffic engineering functions.(c) to support various protection and
   restoration schemes.  Since the historical implementation of the
   "control plane" of optical transport networks via network management
   has detrimental effects like slow restoration, preclusion of

Various Authors  Informational _ Expires January 2001                9
             IP over Optical Networks: Summary of Issues     July 2000


   distributed dynamic routing control, etc., motivation is to improve
   the responsiveness of the optical transport network and to increase
   the level of interoperability within and between service provider
   networks.

   In the following sections, we summarize the enhancements that are
   required in the OXCs to support the MPLS TE as well as the changes
   required in the MPLS control plane to adapt to the OXCs.  The next
   section gives a brief overview of MPLS traffic engineering.

2.5.1 Overview Of The MPLS Traffic Engineering Control

   In this section, we discuss the components of the MPLS traffic
   engineering control plane model, which include the following modules
   [Awuduche99]:

   (a) Resource discovery.

   (b) State information dissemination to distribute relevant
   information concerning the state of the network.  The state of the
   network includes topology and resource availability information.
   This can be accomplished by extending conventional interior gateway
   protocols (IGPs) to carry additional information in their link state
   advertisements.

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

   (d) Path management, which includes label distribution, path
   placement, path maintenance, and path revocation.  These functions
   are implemented through a signaling protocol, such as the RSVP
   extensions or through CR-LDP.The above components of the MPLS
   traffic engineering control plane are separable, and independent of
   each other, and hence it allows an MPLS control plane to be
   implemented using a composition of best of breed modules.

2.5.2 Required Enhancements To OXCs To Support MPLS Control Plane

   This section discusses some of the enhancements to OXCs to support
   MPL(ambda)S.(a) There should be a mechanism to exchange control
   information between OXCs, and between OXCs and other LSRs.  This can
   be accomplished in-band or quasi-in-band using the same links that
   are used to carry data-plane traffic, or out-of-band via a separate
   network.(b) An OXC must be able to provide the MPLS traffic
   engineering control plane with pertinent information regarding the
   state of individual fibers attached to that OXC, as well the state
   of individual lightpaths or optical channel trails within each
   fiber.(c) Even when an edge LSR does not have WDM capabilities, it
   should still have the capability to exchange control information
   with the OXCs in the domain.

Various Authors  Informational _ Expires January 2001               10
             IP over Optical Networks: Summary of Issues     July 2000



2.5.3 Required Enhancements To Current MPLS Control Plane

   This section discusses the enhancements that are to be made in the
   MPLS control plane to support MPL(ambda)S [Basak99].

   An MPLS domain may consist of links with different properties
   depending upon the type of network elements at the endpoints of the
   links.  Within the context of MPL(ambda)S, the properties of a link
   consisting of a fiber with WDM that interconnects two OXCs are
   different form that of a SONET link that interconnects two LSRs.  As
   an example, a conventional LSP cannot be terminated on a link
   connected to a pure OXC.  However, a conventional LSP can be
   certainly be terminated on a link connected to a frame-based LSR.
   These differences should be taken into account when performing path
   computations to determine an explicit route for an LSP.  It is also
   feasible to have the capability to restrict the path of some LSPs to
   links with certain characteristics.  Path computation algorithms may
   then take this information into account when computing paths LSPs.

   There must be procedures to associate control channels to bearer
   channels between a pair of adjacent OXCs, if there exists multiple
   control channels and bearer channels between them.  Procedures are
   required to demultiplex the control traffic for different bearer
   channels if a control channel is associated with multiple bearer
   channels.  Procedures are also needed to activate and deactivate
   bearer channels, to identify the bearer channels associated with any
   given physical link, to identify spare bearer channels for
   protection purposes, and to identify impaired bearer channels,
   particularly, in the situation where the physical links carrying the
   bearer channel are not impaired.

   Signaling protocols (RSVP and CR-LDP) need to be extended with
   objects that can provide sufficient details to establish
   reconfiguration parameters for OXC switch elements.  IGP should be
   extended to carry information about the physical diversity of the
   fibers.  IGP should be able to distribute information regarding the
   allocatable bandwidth granularity of any particular link.

2.6 Synthesizing The MPLS Traffic Engineering Control Plane With OXCs

   In IP-centric optical interworking systems, given that both OXCs and
   LSRs require control planes, one option would be to have two
   separate and independent control planes [Awuduche99].  Another
   option is to develop a uniform control plane that can be used for
   both LSRs and OXCs.  This option of having a uniform control plane
   will eliminate the administrative complexity of managing hybrid
   optical internetworking systems with separate, dissimilar control
   and operational semantics.  Specialization may be introduced in the
   control plane, as necessary, to account for inherent peculiarities
   of the underlying technologies and networking contexts.  A single
   control plane would be able to span both routers and OXCs.  In such
   an environment a LSP could traverse an intermix of routers and OXCs,

Various Authors  Informational _ Expires January 2001               11
             IP over Optical Networks: Summary of Issues     July 2000


   or could span just routers, or just OXCs.  This offers the potential
   for real bandwidth-on-demand networking, in which an IP router may
   dynamically request bandwidth services from the optical transport
   network.

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

   Though an OXC equipped with an MPLS traffic engineering control
   plane would resemble a Label Switching Router, there are some
   important distinctions and limitations.  The distinction concerns
   the fact that there are no analogs of label merging in the optical
   domain, which implies that an OXC cannot merge several wavelengths
   into one wavelength.  Another major distinction is that an OXC
   cannot perform the equivalent of label push and pop operation in the
   optical domain.  This is due to lack of the concept of pushing and
   popping wavelengths is infeasible with contemporary commercial
   optical technologies.  Finally, there is another important
   distinction, which is concerned with the granularity of resource
   allocation.  An MPLS router operating in the electrical domain can
   potentially support an arbitrary number of LSPs with arbitrary
   bandwidth reservation granularities, whereas an OXC can only support
   a relatively small number of optical channel trails, each of which
   will have coarse discrete bandwidth granularities.

3.  Routing in Optical Networks

   The optical network model considered in this draft consists of
   multiple Optical Crossconnects (OXCs) interconnected by optical
   links in a general topology (referred to as an "optical mesh
   network").  Each OXC is assumed to be capable of switching a data
   stream from a given input port to a given output port.  This
   switching function is controlled by appropriately configuring a
   crossconnect table.  Conceptually, the crossconnect table consists
   of entries of the form , indicating
   that data stream entering input port i will be switched to output
   port j.  An "lightpath" from an ingress port in an OXC to an egress
   port in a remote OXC is established by setting up suitable
   crossconnects 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.  Lightpaths are assumed to be bi-directional, i.e.,
   the return path from the egress port to the ingress port follows the
   same path as the forward path.

   It is assumed that one or more control channels exist between
   neighboring OXCs for signaling purposes.

   3.1 Models for IP-Optical Network Interaction

Various Authors  Informational _ Expires January 2001               12
             IP over Optical Networks: Summary of Issues     July 2000



   Some of the proposed models for interaction between IP and optical
   components in a hybrid network are [Luciani00]:

   (1) Overlay model
   (2) Integrated / Augmented model
   (3) Peer model.

   The key consideration in deciding which model is whether there is a
   single/separate monolithic routing and signaling protocol spanning
   the IP and the Optical domains.  If there are separate instances of
   routing protocols running for each domain then 1) What is the
   interface defined between the two protocol instances?  2) What kind
   of information can be leaked from one protocol instance to the
   other?  3) Would one label switching protocol run on both domains?
   If that were to be the case then how would labels map to
   wavelengths? Also, how would IP QoS parameters be mapped into the
   optical domain?

3.1.1 Overlay model

   Under the overlay model, IP is more or less independent of the
   optical subnetwork.  That is IP acts as a client to the Optical
   domain.  In this scenario, the optical network provides point to
   point connection to the IP domain.  The IP/ MPLS routing protocols
   are independent of the routing and signaling protocols of the
   optical layer.  The overlay model may be divided into 2 parts:

   a) Static overlay model: In this model path endpoints are specified
   through a network management system (NMS) though the paths may be
   laid out statically by the NMS or dynamically by the network
   elements. This would be similar to ATM permanent virtual circuits
   (PVCs) and ATM Soft PVCs (SPVCs).

   b) Signaled overlay model: The path end-points are specified through
   signaling via a User to Network Interface (UNI).  Paths must be laid
   out dynamically since they are specified by signaling.  This is
   similar to ATM switched virtual circuits (SVCs).  The Optical Domain
   Services Interoperability (ODSI) forum and Optical Internetworking
   Forum (OIF) are also defining similar standards for the Optical UNI.
   In these models user devices which reside on the edge of the optical
   network can signal and request bandwidth dynamically.  These models
   use IP/optical layering.  Endpoints are specified using a port
   number/IP address tuple. PPP is used for service discovery wherein a
   user device can discover whether it can use ODSI or OIF protocols to
   connect to an optical port.  Unlike MPLS there are no labels to be
   setup.  The resulting bandwidth connection will look like a leased
   line.

3.1.2 The Integrated/ Augmented model

   In the integrated model, the MPLS/IP layers act as peers of the
   optical transport network, such that a single routing protocol

Various Authors  Informational _ Expires January 2001               13
             IP over Optical Networks: Summary of Issues     July 2000


   instance runs over both the IP/MPLS and optical domains.  A common
   IGP like OSPF or IS-IS, with appropriate extensions will be used to
   distribute topology information.  Also this model assumes a common
   address space for the optical and IP domain.  In the augmented
   model, there are actually separate routing instances in the IP and
   optical domains but information from one routing instance is leaked
   into the other routing instance.  For example IP addresses could be
   assigned to optical network elements and carried by optical routing
   protocols to allow reachability information to be shared with the IP
   domain to support some degree of automated discovery.

3.1.3 The peer model

   The peer model is somewhat similar to the integrated model in that
   the IP reachability information might be passed around within the
   optical routing protocol but the actual flow will be terminated at
   the edge of the optical network and will only be reestablished upon
   reaching a non-peer capable node at the edge of the optical domain
   or at the edge of the domain, which implements both the peer and the
   overlay models.


3.2 The Lightpath routing solution

3.2.1 What is an IGP?

   An IGP is a interior gateway routing protocol.  Examples of IGPs
   would be OSPF and IS-IS.  IGPs are used to exchange state
   information within a specified administrative domain and for
   topology discovery.  This exchange of information inside the domain
   is done by advertising the Link state information periodically.
   Please refer to [OSPF] and [IS-IS] for more details.

3.2.2 How does MPLS fit into the picture?

   While the idea of bandwidth-on-demand is certainly not new, existing
   networks do not support instantaneous service provisioning.  Current
   provisioning of bandwidth is painstakingly static.  Activation of
   large pipes of bandwidth takes anything from weeks to months.  The
   imminent introduction of photonic switches in the transport networks
   opens new perspectives.  Combining the bandwidth provisioning
   capabilities of photonic switches with the traffic engineering
   capabilities of MPLS, will allow routers and ATM switches to request
   bandwidth where and when they need it.

         +--------+       +--------+       +--------+
         |        |<----->|Path    |       |OSPF    |
         | CR-LDP |   +-->|Selector|       |        |
         |        |   |   |        |       |TE EXT  |
         +--------+   |   +---+----+   +---|OPT EXT |
                      |       |        |   +--------+
                      |       v        |
                      |   +--------+   |   +--------+

Various Authors  Informational _ Expires January 2001               14
             IP over Optical Networks: Summary of Issues     July 2000


         +--------+   |   |TE      |   |   |IS-IS   |
         |        |   |   |Database|   |   |        |
         | RSVP   |<--+   |        |   |   |TE EXT  |
         |        |       |        |<--+---|OPT EXT |
         +--------+       +--------+       +--------+

                Fig.2: Lightpath Selection


3.2.3 Lightpath Selection

   The lightpath routing system is based on the MPLS Constraint based
   routing model.  Fig.2 illustrates lightpath selection.  These
   systems use CR-LDP or RSVP to signal MPLS paths.  These protocols
   can source route by consulting a traffic engineering database which
   is maintained along with the IGP database.  This information is
   carried opaquely by the IGP for constraint based routing.  If RSVP
   or CR-LDP is used solely for label provisioning, the IP router
   functionality must be present at every label switch hop along the
   way.  Once the label has been provisioned by the protocol then at
   each hop the traffic is switched using the native capabilities of
   the device to the eventual egress LSR.  To exchange information
   using IGP protocols like OSPF and IS-IS, certain extensions need to
   be made to both of these to support MPL(ambda) switching.

3.3 IS-IS/ OSPF enhancements

   OSPF defined in [OSPF] and IS-IS defined in [IS-IS] are the commonly
   deployed routing protocols in large networks. OSPF/IS-IS have been
   extended to include traffic engineering capability [Katz99],
   [ISISTE]. There is a need to add the optical link state
   advertisement (LSA) to OSPF/IS-IS to support lightpath routing
   computation. The optical LSA would include a number of new elements,
   called TLVs (type-length-value) because of the way they are coded.
   The following sections describe some of the proposed TLVs.

3.3.1 Link Type

   A network may have a link with many different characteristics. A
   link type TLV allows to identify a particular type of link. One way
   to describe the links would be [WANG00]:
   a) Service transparent: Service transparent is a point-to-point
      physical link.
   b) Service aware: A service aware link is a point-to-point logical
   optical link.

   Another way of classifying the links is based on the types of end
   nodes [Kompella00-a].  Nodes that can switch individual packets are
   called packet switch capable (PSC).  Nodes that can transmit/receive
   SONET payloads are called time division multiplex (TDM) capable.
   Nodes that can switch individual wavelengths are called lambda
   switch capable (LSC). Finally, nodes that switch entire contents of
   one fiber into other are called fiber switch capable (FSC).

Various Authors  Informational _ Expires January 2001               15
             IP over Optical Networks: Summary of Issues     July 2000



   Links can be either physical (one hop) links or logical links
   consisting of multiple hop connections. Logical links are called
   _Forwarding Adjacencies (FAs)._ This leads to the following types of
   links:

   a) PSC links end (terminate or egress) on PSC nodes.   Depending
      upon the hierarchy of LSPs tunneled within LSPs, several
      different types of PSC links can be defined.

   b) TDM links end on TDM nodes and carry SONET/SDH payloads.

   c) LSC links end on LSC nodes and consist of wavelengths.

   d) FSC links end on FSC nodes and consist of fibers.

   e) Forwarding Adjacency PSC (FA-PSC) links are forwarding
   adjacencies whose egress nodes are packet switching.

   f) FA-TDM, FA-LSC, and FA-LSP are forwarding adjacencies whose
   egress nodes are TDM, LSC, and LSP capable, respectively.


3.3.2 Link Media Type (LMT)/ Link resource

   A link may support a set of media types depending on resource
   availability and capacity of link. Such TLVs may have two fields of
   which the first one defines the media type, and the second field
   defines the lowest priority at which the media is available
   [Kompella00-a]. Link Media Types present a new constraint for LSP
   path computation. Specifically when a LSP is setup and it includes
   one or more subsequences of links which carry the LMT TLV then for
   all the links within each subsequence the encoding has to be the
   same and the bandwidth has to be at least the LSP's specified
   bandwidth. The total classified bandwidth available over one link
   can be classified using a resource component TLV [WANG00]. This TLV
   represents a group of lambdas with the same line encoding rate and
   total current available bandwidth over these lambdas. This TLV
   describes all lambdas that can be used on this link in this
   direction grouped by encoding protocol. There is one resource
   component per encoding type per fiber.  If multiple fibers are used
   per link there will be a resource component per fiber to support
   fiber bundling.

3.3.3 Link ID

   An identifier which identifies the optical link exactly as the point
   to point case for TE extensions.

3.3.4 Local Interface IP address

   The interface address may be omitted in which case it defaults to
   the router address of the local node.

Various Authors  Informational _ Expires January 2001               16
             IP over Optical Networks: Summary of Issues     July 2000



3.3.5 Remote Interface IP address

   This address may be specified as an IP address on the remote node or
   the router address of the remote node .

3.3.6 Traffic Engineering (TE) Metric

   This metric value can be assigned for path selection.

3.3.7 Path sub-TLV

   When an LSP advertises a forwarding adjacency into an IGP, it may be
   desirable to carry the information about the path taken by this
   adjacency. This information may be used for path calculation by
   other LSRs.

3.3.8 Shared Risk Link group TLV

   A set of links may constitute a 'shared risk link group' (SRLG) if
   they share a resource whose failure may affect all links in the set.
   An example would be two fibers in the same conduit. Also, a fiber
   may be part of more than one SRLG.


3.4 Control Channels, Data Channels, and IP Links

   A pair of OXCs are said to be neighbors from the MPLS point of view
   if they are connected by one or more logical or physical channels.
   If several fibers share the same TE characteristic then a single
   control channel would suffice for all of them.  From the IGP point
   of view this control channel along with all its fibers form a single
   IP link. Sometimes fibers may need to be divided into sets that
   share the same TE characteristic. Corresponding to each such set,
   there must be a logical control channel to form an IP link. All of
   the multiple logical control channels could be realized via one
   common control channel. When an adjacency is established over a
   logical control channel that is part of an IP link formed by the
   channel and a set of fibers, this link is announced into IS- IS/OSPF
   as a "normal" link; the fiber characteristics are represented as TE
   parameters of that link. If there are more than one fibers in the
   set, the set is announced using bundling techniques discussed in
   [Kompella00-b].

3.4.1 Excluding data traffic from control channels

   The control channels between OXCs or between an OXC and a router are
   generally meant for low bandwidth control traffic. These control
   channels are advertised as normal IP links. However if regular
   traffic is forwarded on these links the channel capacity will soon
   be exhausted. To avoid this, if we assume that data traffic is sent
   over BGP destinations and control traffic is sent to IGP
   destinations. Ways to do this are discussed in [KOMPELLA00-a].

Various Authors  Informational _ Expires January 2001               17
             IP over Optical Networks: Summary of Issues     July 2000



3.4.2 Forwarding adjacencies

   An LSR at the head of an LSP may advertise this LSP as a link into a
   link state IGP. When this LSP is advertised into the same instance
   of the IGP as the one that determines the route taken this adjacency
   then we call such a link a "forwarding adjacency".  Such an LSP is
   referred to as a "forwarding adjacency LSP" or just FA-LSP.
   Forwarding adjacencies may be statically provisioned or created
   dynamically.  Forwarding adjacencies are by definition
   unidirectional.

   When a forwarding adjacency is statically provisioned, the
   parameters that can be configured are: the head-end address, the
   tail-end address, bandwidth, and resource color constraints. The
   path taken by the FA-LSP can be computed by the Constrained Shortest
   Path Formulation (CSPF) mechanism or MPLS TE or by explicit
   configuration. When a forwarding adjacency is created dynamically
   its parameters are inherited by the LSP which induced its creation.
   Note that the bandwidth of the FA-LSP must be at least as big as the
   LSP that induced it.

   When an FA-LSP is advertised into IS-IS/OSPF, the link type
   associated with this LSP is the link type of the last link in the
   FA-LSP. Some of the attributes of this link can be derived from the
   FA-LSP but others need to be configured.  Configuration of the
   attributes of statically provisioned FAs is straightforward, but for
   dynamically provisioned FAs a policy-based mechanism may be needed.

   The link media type of the FA is the most restrictive of the link
   media types of the component links of the forwarding adjacency. FAs
   will not be used to establish peering relationships between routers
   at the end of the adjacencies but will only be used for CSPF
   computation.

3.4.3 Two way connectivity

   CSPF shouldn't perform any two way connectivity check on links used
   by CSPF.  This is because some of the links are unidirectional and
   may be associated with forwarding adjacencies.

3.4.4 Optical LSAs

   There needs to be a way of controlling the protocol overhead
   introduced by optical LSAs. One way to do this is to make sure that
   a Link State Advertisement happens only when there is a significant
   change in the value of metrics since the last advertisement. A
   definition of  significant change is when the difference between the
   currently available bandwidth and last advertised bandwidth crosses
   a threshold[WANG00]. The frequency of these updates can be decreased
   dramatically using event driven feedback.

3.5 Some Open Questions

Various Authors  Informational _ Expires January 2001               18
             IP over Optical Networks: Summary of Issues     July 2000



   Some issues that have not been resolved so far are: How to ensure
   that end-to-end information is propagated across as an optical
   network? How to accommodate proprietary optimizations within optical
   sub-networks for provisioning and restoration of lightpaths? Whether
   dynamic and precomputed information can be used and if so what is
   the interaction between them? What QOS related parameters need to be
   defined? How to ensure fault tolerant operation at protocol level
   when hardware does not support fault tolerance?  How to address
   scalability issues? What additional modifications are required to
   support a network for routing control traffic?


4.  Signaling & Control

   Signaling means to intimate any particular element of certain
   characteristics or services.  This section discusses a few of the
   signaling procedures.  It is assumed that there exists some default
   communication mechanism between routers prior to using any of the
   routing and signaling mechanisms.

4.1 MPLS Control Plane

        A candidate system architecture for an OXC equipped with an
   MPLS control plane model is shown in Fig.3

          --------------------------------
         |   OXC WITH MPLS CONTROL PLANE  |
         |                                |
         |       -------------------      |
         |      |                   |     |
         |      | MPLS Control Plane|     |
         |      |                   |     |
         |       -------------------      |
         |               |                |
         |       -------------------      |
         |      |                   |     |
         |      |Control Adaptation |     |
         |       -------------------      |
         |      |    OXC Switch     |     |
         |      |    Controller     |     |
         |       -------------------      |
         |               |                |
         |       -------------------      |
         |      |                   |     |
         |      | OXC Switch Fabric |     |
         |      | OXC Data Plane    |     |
         |       -------------------      |
         |                                |
          --------------------------------

                Fig.3: OXC Architecture


Various Authors  Informational _ Expires January 2001               19
             IP over Optical Networks: Summary of Issues     July 2000



   The salient feature of the network architecture is that every node
   in the network consists of an IP router and a reconfigurable OLXC.
   The IP router is responsible for all non-local management functions,
   including the management of optical resources, configuration and
   capacity management, addressing, routing, traffic engineering,
   topology discovery, exception handling and restoration.  In general,
   the router may be traffic bearing as proposed in, or it may function
   purely as a controller for the optical network and carry no IP data
   traffic.  Although the IP router performs all management and control
   functions, lightpaths may carry arbitrary types of traffic.


                      +-------------------------------+
                      |                               |
                      |        Router   module        |
                      |                               |
                      |                               |
                      |  |                         |  |
                      |  |        Primitives       |  |
                      |  |        /|\   |          |  |
                      |  |         |    |          |  |
                      +--+---------+----+----------+--+  Control
   Channel            |  |         |    |          |  |
            ----------+--+         |    |          +--+---------
            ----------+----------  |   \|/  ----------+---------
            ----------+----------           ----------+---------
            ----------+----------           ----------+---------
            ----------+----------           ----------+---------
                      |             OLXC              |
                      |                               |
                      |                               |
                      +-------------------------------+

                        Fig.4: Control Plane Architecture


   The IP router implements the necessary IP protocols and uses IP for
   signaling to establish lightpaths.  Specifically, optical resource
   management requires resource availability per link to be propagated,
   implying link state protocols such as OSPF.  On each link within the
   network, one channel is assigned as the default routed (one hop)
   lightpath.  The routed lightpath provides router to router
   connectivity over this link.  These routed lightpaths reflect (and
   are thus identical to) the physical topology.  The assignment of
   this default lightpath is by convention, e.g.  the 'first' channel.
   All traffic using this lightpath is IP traffic and is forwarded by
   the router.  All control messages are sent in-band on a routed
   lightpath as regular IP datagrams, potentially mixed with other data
   but with the highest forwarding priority.  It is assumed multiple
   channels on each link, a fraction of which is reserved at any given
   time for restoration.  The default-routed lightpath is restored on
   one of these channels.  Therefore, we can assume that as long as the

Various Authors  Informational _ Expires January 2001               20
             IP over Optical Networks: Summary of Issues     July 2000


   link is functional, there is a default routed lightpath on that
   link.

   The IP router communicates with the OLXC device through a logical
   interface.  The interface defines a set of basic primitives to
   configure the OLXC, and to enable the OLXC to convey information to
   the router.  The mediation device translates the logical primitives
   to and from the proprietary controls of the OLXC.  Ideally, this
   interface is both explicit and open.  We recognize that a particular
   realization may integrate the router and the OLXC into a single box
   and use a proprietary interface implementation.  Fig.4 illustrates
   this implementation.

   The following interface primitives are examples of a proposal for
   communication between the router and the OLXC within a node:

   a) connect(input link, input channel, output link, output channel):
   Commands sent from the router to the OLXC requesting that the OLXC
   crossconnect input channel on the input link to the output channel
   on the output link.

   b) disconnect(input link, input channel, output link, output
   channel):  Command sent from the router to the OLXC requesting that
   it disconnect the output channel on the output link from the
   connected input channel on the input link.

   c) bridge(input link, input channel, output link, output channel):
   Command sent from the router controller to the OLXC requesting the
   bridging of a connected input channel on input link to another
   output channel on output link.

   d) switch(old input link, old input channel, new input link, new
   input channel, output link, output channel): Switch output port from
   the currently connected input channel on the input link to the new
   input channel on the new input link.  The switch primitive is
   equivalent to atomically implementing a disconnect(old input
   channel, old input link, output channel, output link) followed by a
   connect(new input link, new input channel, output link, output
   channel).

   e) alarm(exception, object):
   Command sent from the OLXC to the router informing it of a failure
   detected by the OLXC.  The object represents the element for which
   the failure has been detected.

   For all of the above interfaces, the end of the connection can also
   be a drop port.


   4.2 Addressing

   Every network addressable element must have an IP address.
   Typically these elements include each node and every optical link

Various Authors  Informational _ Expires January 2001               21
             IP over Optical Networks: Summary of Issues     July 2000


   and IP router port.  When it is desirable to have the ability to
   address individual optical channels those are assigned IP addresses
   as well.  The IP addresses must be globally unique if the element is
   globally addressable.  Otherwise domain unique addresses suffice.  A
   client must also have an IP address by which it is identified.
   However, optical lightpaths could potentially be established between
   devices that do not support IP (i.e., are not IP aware), and
   consequently do not have IP addresses.  This could be handled either
   by assigning an IP address to the device, or by assigning an address
   to the OLXC port to which the device is attached.  Whether or not a
   client is IP aware can be discovered by the network using
   traditional IP mechanisms.


4.3 Path setup

   This section describes a protocol proposed for setting up an end-to-
   end lightpath for a channel.  This proposal uses the concept of
   softness of lightpaths.  This implies that the lightpath expires
   unless refreshed periodically bu the source. The first-hop router
   must periodically resend the lightpath setup request.  If the state
   of a lightpath expires at a particular node, the state is locally
   removed and all resources allocated to the lightpath are reclaimed.


4.3.1 Basic Path Setup Procedure

   Techniques for link provisioning depend upon whether the OXCs do or
   do not have wavelength conversion. Both these cases are discussed
   below.

4.3.1.1 Network with Wavelength Converters

   In an optical network with wavelength conversion, channel allocation
   can be performed independently on different links along a route. A
   lightpath request from a source is received by the first-hop router.
   (The term router here denotes the routing entity in the optical
   nodes or OXCs)  A sample format for the setup request has been
   defined in [Chaudhuri00].  The first-hop router creates a lightpath
   setup message and sends it towards the destination of the lightpath
   where it is received by the last-hop router.  The lightpath setup is
   sent from the first-hop router on the default routed lightpath as
   the payload of a normal IP packet with router alert.  A router alert
   ensures that the packet is processed by every router in the path.  A
   channel is allocated for the lightpath on the downstream link at
   every node traversed by the setup.  The identifier of the allocated
   channel is written to the setup message.

   Note that the lightpath is established over the links traversed by
   the lightpath setup packet.  After a channel has been allocated at a
   node, the router communicates with the OLXC to reconfigure the OLXC
   to provide the desired connectivity.  After processing the setup,
   the destination (or the last-hop router) returns an acknowledgement

Various Authors  Informational _ Expires January 2001               22
             IP over Optical Networks: Summary of Issues     July 2000


   to the source.  The acknowledgment indicates that a channel has been
   allocated on each hop of the lightpath.  It does not, however,
   confirm that the lightpath has been successfully implemented (or
   configured).

   If no channel is available on some link, the setup fails, and a
   message is returned to the first-hop router informing it that the
   lightpath cannot be established.  If the setup fails, the first-hop
   router issues a release message to release resources allocated for
   the partially constructed lightpath.  Upon failure, the first-hop
   router may attempt to establish the lightpath over an alternate
   route, before giving up on satisfying the original user request.
   The first-hop router is obligated to establish the complete path.
   Only if it fails on all possible routes does it give a failure
   notification to the true source.

4.3.1.2 Network without wavelength converters

   However, if wavelength converters are not available, then a common
   wavelength must be located on each link along the entire route,
   which requires some degree of coordination between different nodes
   in choosing an appropriate wavelength.

   Sections of a network that do not have wavelength converters are
   thus referred to as being wavelength continuous.  A common
   wavelength must be chosen on each link along a wavelength continuous
   section of a lightpath.  Whatever wavelength is chosen on the first
   link defines the wavelength allocation along the rest of the
   section.  A wavelength assignment algorithm must thus be used to
   choose this wavelength.  Wavelength selection within the network
   must be performed within a subset of client wavelengths.

   Optical non-linearities, chromatic dispersion, amplifier spontaneous
   emission and other factors together may limit the scalability of an
   all-optical network.  Routing in such networks will then have to
   take into account noise accumulation and dispersion to ensure that
   lightpaths are established with adequate signal qualities.  Hence,
   all routes become geographically constrained so that they will have
   adequate signal quality, and physical layer attributes can be
   ignored during routing and wavelength assignment.

   One approach to provisioning in a network without wavelength
   converters would be to propagate information throughout the network
   about the state of every wavelength on every link in the network.
   However, the state required and the overhead involved in maintaining
   this information would be excessive.  By not propagating individual
   wavelength availability information around the network, we must
   select a route and wavelength upon which to establish a new
   lightpath, without detailed knowledge of wavelength availability.

   A probe message can be used to determine available wavelengths along
   wavelength continuous routes.  A vector of the same size as the
   number of wavelengths on the first link is sent out to each node in

Various Authors  Informational _ Expires January 2001               23
             IP over Optical Networks: Summary of Issues     July 2000


   turn along the desired route.  This vector represents wavelength
   availability, and is set at the first node to the wavelength
   availability on the first link along the wavelength continuous
   section.  If a wavelength on a link is not available or does not
   exist, then this is noted in the wavelength availability vector
   (i.e.  the wavelength is set to being unavailable ).  Once the
   entire route has been traversed, the wavelength availability vector
   will denote the wavelengths that are available on every link along
   the route.  The vector is returned to the source OXC, and a
   wavelength is chosen from amongst the available wavelengths using an
   arbitrary wavelength assignment scheme, such as first-fit.

   The construction of a bi-directional lightpath differs from the
   construction of a uni-directional lightpath above only in that upon
   receiving the setup request, the last-hop router returns the setup
   message using the reverse of the explicit route of the forward path.
   Both directions of a bi-directional lightpath share the same
   characteristics, i.e., set of nodes, bandwidth and restoration
   requirements.  For more general bi-directional connectivity, a user
   simply requests multiple individual lightpaths.

   A lightpath must be removed when it is no longer required.  To
   achieve this, an explicit release request is sent by the first-hop
   router along the lightpath route.  Each router in the path processes
   the release message by releasing the resources allocated to the
   lightpath, and removing the associated state.  It is worth noting
   that the release message is an optimization and need not be sent
   reliably, as if it is lost or never issued (e.g., due to customer
   premise equipment failure) the softness of the lightpath state
   ensures that it will eventually expire and be released.

4.3.2 CR-LDP Extensions for Path Setup

   Label Distribution Protocol (LDP) is defined for distribution of
   labels inside one MPLS domain.  CR-LDP is the contraint-based
   extesion of LDP. One of the most important services that may be
   offered using MPLS in general and CR-LDP in particular is support
   for constraint-based routing of lightpaths across the routed
   network.  Constraint-based routing offers the opportunity to extend
   the information used to setup paths beyond what is available for the
   routing protocol.  For instance, an LSP can be setup based on
   explicit route constraints, QoS constraints, and other constraints.
   Constraint-based routing (CR) is a mechanism used to meet traffic
   engineering requirements that have been proposed.

   Automated establishment of lightpaths involves setting up the
   crossconnect table entries in the appropriate OLXCs in a coordinated
   manner such that the desired physical path is realized.  The request
   to establish an lightpath may arise either from a router (or some
   other device) connected to the OXCs or from a management system.
   Such a request must identify the ingress and the egress OXC as
   endpoints of the lightpath.  In addition, it may also optionally
   specify the input and output ports, wavelengths, and TDM channels.

Various Authors  Informational _ Expires January 2001               24
             IP over Optical Networks: Summary of Issues     July 2000


   The request may also include bandwidth parameters and channel type,
   reliability parameters, restoration options, setup and holding
   priorities for the path etc.  On receipt of the request, the ingress
   node computes a suitable route for the requested path, following
   applicable policies and constraints.  Once the route has been
   computed, the ingress node invokes CR-LDP to set up the path.

   In optical networks, label mapping corresponds to the assignment of
   input or output ports for paths by optical switches and the
   communication of this information to the appropriate neighbors.

   A Label Request message is used by an upstream LSR to request a
   label binding from the downstream LSR for a specified FEC and CR-
   LSP.  In optical networks, a Label Request message may be used by
   the upstream OXC to request a port (and wavelength) assignment from
   the downstream OXC for the lightpath being established.  Using
   downstream-on-demand and ordered control mode, a Label Request
   message is initially generated at the ingress OXC and is propagated
   to the egress OXC.  Also, a protocol is required to determine the
   port mappings.

   To incorporate the above mentioned constraints, the following
   extensions to current version of CR-LDP have been proposed:

   * Inclusion of Signaling Port ID: This field specifies ports to be
   assigned for setting up the path.  Such a "label" (wavelength) must
   be assigned in a coordinated manner by a pair of adjacent OXCs,
   since the "label" at one OXC is tied to a specific "label" at a
   neighboring OXC based on physical connectivity.

   * Signaling Optical Switched Path Identifier: This field identifies
   the lightpath being established.  This provides the flexibility of
   establishing LSPs on the top of an lightpath already setup.

   * Signaling the two end points of the path being set up: These
   fields indicate the two end-points at the port level of the
   lightpath. The port selected for the egress node is propagated to
   the egress node.

   * Signaling requirements for both span and path protection: This
   field signals the protection levels required for both span (or
   local) and path protection.  Examples of span (or local) protection
   include SONET 1+1 and 1:N APS.  Examples of path protection include
   various levels regarding how an alternate path is shared such as in
   a style of 1+1 or 1:N analogous to span protection.

   * Recording the precise route of the path being established: This is
   done by letting each OXC insert its node ID and the both output and
   input port selected for the path in the Label Mapping message.  The
   message received by the ingress OXC will have the complete route at
   the port level.  This information is useful for network management
   functions.


Various Authors  Informational _ Expires January 2001               25
             IP over Optical Networks: Summary of Issues     July 2000


4.3.3 Extensions to RSVP for Path Setup

   Resource reSerVation Protocol (RSVP) is a unicast and multicast
   signaling protocol, designed to install and maintain reservation
   state information at each routing engine along a path [Luciani00].
   The key characteristics of RSVP are that it is simplex, receiver-
   oriented and soft.  It makes reservations for unidirectional data
   flows. The receiver of a data flow generally initiates and maintains
   the resource reservation used for that flow. It maintains "soft"
   state in routing engines. The _path_ messages are propagated from
   the source towards potential receipients. The receivers interested
   in communicating with the source send the _Resv_ messages.

   The following extensions to RSVP have been proposed to support path
   setup [Jonathan00]:

   - Reduction of trail establishment latency
   - Establishment of bi-directional lightpaths
   - Fast failure notification
   - Bundling of notifications

   These extensions are described below.

4.3.3.1 Reduction Of Trail Establishment Latency

   Currently due to receiver-oriented nature of RSVP, the internal
   configuration of an OXC in the downstream direction cannot be
   initiated until it receives the Resv message from the downstream
   node.  The ability to begin configuring an OXC before receiving a
   Label Object in the Resv message can provide a significant reduction
   in the setup latency, especially in OXCs with non-negligible
   configuration time.  To accomplish this, a new approach has been
   proposed in which an upstream OXC suggest a (fiber, lambda) label
   for the downstream node to use by including the suggested Label
   object in the Label Request object of the Path message.  The Label
   object will contain the downstream node's Label for the bearer
   channel, which can be obtained through the Link Management Protocol
   (LMP).  This will allow the upstream OXC to begin its internal
   configuration before receiving the Resv message from the downstream
   node.

4.3.3.2 Establishment Of Bi-directional Lightpaths

   In the new approach that is proposed, a Label Object is added to the
   Path message in the downstream direction.  In this way, the upstream
   direction of the bi-directional path is established on the first
   pass from the source to destination, reducing the latency of the
   reservation process.  For bi-directional trails, if a label
   suggestion is also used, there will be two Labels in the Path
   message: the upstream Label in the Label object and the suggested
   Label in the Label_Request object.

4.3.3.3 Failure Notification

Various Authors  Informational _ Expires January 2001               26
             IP over Optical Networks: Summary of Issues     July 2000



   A new RSVP message, called the Notify message, can be used to notify
   RSVP nodes when failures occur.  The Notify message will be
   transmitted with the router alert option turned off so that
   intermediate nodes will not process or modify the message, but only
   perform standard IP forwarding of the message.

4.3.3.4 Bundling of Notifications

   Another extension to RSVP has been recently proposed to allow the
   use of bundle messages in order to reduce the overall message-
   handling load.  An RSVP bundle message consists of a bundle header
   followed by a body consisting of a variable number of standard RSVP
   messages.  Support for the bundle message is optional, and
   currently, bundle messages can only be sent to adjacent RSVP nodes.
   In order to effectively restore a network to a stable state, nodes
   that are running restoration algorithms should consider as many
   failed trails as possible before making restoration decisions.  To
   improve performance and ensure that the nodes are provided with as
   many of the affected paths as possible, it is useful to include the
   entire set of Notify messages in a single bundle message and send it
   to the responsible RSVP node directly, without message processing by
   the intermediate RSVP nodes.  This can be accomplished by addressing
   the bundle message to the source RSVP nodes and turning off the
   router alert option in the IP header.  Intermediate RSVP nodes then
   should perform standard IP forwarding of this message.

4.4 Resource Discovery and Maintenance

   Topology information is distributed and maintained using standard
   routing algorithms, e.g., OSPF and IS-IS.  On boot, each network
   node goes through neighbor discovery.  By combining neighbor
   discovery with local configuration, each node creates an inventory
   of local resources and resource hierarchies, namely: channels,
   channel capacity, wavelengths, and links.

   For optical networks, the following information need to be stored at
   each node and propagated throughout the network as OSPF link-state
   information:

   - Representation of the current network topology and the link states
   (which reflect the wavelength availability).  This can be achieved
   by associating with the link state,
        - total number of active channels
        - number of allocated non-preemptable channels
        - number of allocated preemptable channels
        - number of reserved protection channels

   - Optional physical layer parameters for each link.  These
   parameters are not expected to be required in a network with 3R
   signal regeneration, but may be used in all-optical networks.



Various Authors  Informational _ Expires January 2001               27
             IP over Optical Networks: Summary of Issues     July 2000


   All of the above information is obtained via OSPF updates, and is
   propagated throughout the network.  In networks with OXCs without
   wavelength converters, decisions at the first-hop router are made
   without knowledge of wavelength availability.  This is done to
   reduce the state information that needs to be propagated within the
   network.


4.5 Configuration Control using GSMP

   In a general mesh network where the OXCs do not participate in
   topology distribution protocols, General Switch Management Protocol
   (GSMP) is used to  communicate crossconnect information.  This
   ensures that the OXCs on the lightpath maintain appropriate
   databases.  The first hop router having complete knowledge of LP, L2
   and L3 topology acts as the "controller" to the OXCs in the
   lightpath.

   GSMP is a master-slave protocol [GSMP].  The controller issues
   request messages to the switch.  Each request message indicates
   whether a response is required from the switch (and contains a
   transaction identifier to enable the response to be associated with
   the request).  The switch replies with a response message indicating
   either a successful result or a failure.  There are six classes of
   GSMP request-response message:
        Connection Management
        Reservation Management
        Port Management
        State and Statistics
        Configuration, and
        Quality of Service

   The switch may also generate asynchronous Event messages to inform
   the controller of asynchronous events.

4.6 Resource Discovery Using NHRP

   The Next Hop Resolution Protocol (NHRP) allows a source station (a
   host or router), wishing to communicate over a Non-Broadcast, Multi-
   Access (NBMA)  subnetwork, to determine the internetworking layer
   addresses and NBMA addresses of suitable "NBMA next hops" toward a
   destination station [NHRP].  A subnetwork can be non-broadcast
   either because it technically doesn't support broadcasting (e.g., an
   X.25 subnetwork) or because broadcasting is not feasible for one
   reason or another (e.g., a Switched Multi-megabit Data Service
   multicast group or an extended Ethernet would be too large).

   If the destination is connected to the NBMA subnetwork, then the
   NBMA next hop is the destination station itself.  Otherwise, the
   NBMA next hop is the egress router from the NBMA subnetwork that is
   "nearest" to the destination station.  NHRP is intended for use in a
   multiprotocol internetworking layer environment over NBMA


Various Authors  Informational _ Expires January 2001               28
             IP over Optical Networks: Summary of Issues     July 2000


   subnetworks.  NHRP functions are performed by two types of logical
   entities:
        Next Hop Server (NHS) - implemented in routers
        Next Hop Client (NHC) - implemented in routers or NBMA-attached
   hosts.

   In short, NHRP may be applied as a resource discovery to find the
   egress OXC in an optical network. To request this information, the
   existing packet format for the NHRP Resolution Request would be used
   with a new extension in the form of a modified Forward Transit NHS
   Extension.  The extension would include enough information at each
   hop (including the source and destination)

   * to uniquely identify which wavelength.
   * to use when bypassing each routed/forwarded hop and which port
   that the request was received on.

   Essentially a shortcut is setup from ingress to egress using this
   protocol.


5.  Optical Network Management

   The management functionality in all-optical networks are still in
   the rudimentary phase.  Management in a system refers to set of
   functionalities like performance monitoring, link initialization and
   other network diagnostics to verify safe and continued operation of
   the network.  The wavelengths in the optical domain will require
   routing, add/drop, and protection functions, which can only be
   achieved through the implementation of network-wide management and
   monitoring capabilities. Current proposals for link initialization
   and performance monitoring are summarized below.

5.1 Link Initialization

   The links between OXCs will carry a number of user bearer channels
   and possibly one or more associated control channels.  This section
   describes a link management protocol (LMP) that can be run between
   neighboring OXCs and can be used for both link provisioning and
   fault isolation.  A unique feature of LMP is that it is able to
   isolate faults independent of the encoding scheme used for the
   bearer channels.  LMP will be used to maintain control channel
   connectivity, verify bearer channel connectivity, and isolate link,
   fiber, or channel failures within the optical network.

5.1.1 Control Channel Management

   For LMP, it is essential that a control channel is always available
   for a link, and in the event of a control channel failure, an
   alternate (or backup) control channel must be made available to
   reestablish communication with the neighboring OXC.  If the control
   channel cannot be established on the primary (fiber, wavelength)
   pair, then a backup control channel should be tried.  The control

Various Authors  Informational _ Expires January 2001               29
             IP over Optical Networks: Summary of Issues     July 2000


   channel of a link can be either explicitly configured or
   automatically selected. The control channel can be used to exchange:

   a)MPLS control-plane information such as link provisioning and fault
   isolation information (implemented using a messaging protocol such
   as LMP, proposed in this section),

   b)path management and label distribution information (implemented
   using a signaling protocol such as RSVP-TE or CR-LDP), and

   c)topology and state distribution information (implemented using
   traffic engineering extended protocols such as OSPF and IS-IS).

Once a control channel is configured between two OXCs, a Hello protocol
can be used to establish and maintain connectivity between the OXCs and
to detect link failures.  The Hello protocol of LMP is intended to be a
lightweight keep-alive mechanism that will react to control channel
failures rapidly.  A protocol similar to the HDLC frame exchange is
used to continue the handshake. [Lang00]

5.1.2 Verifying link connectivity

   In this section, we describe the mechanism used to verify the
   physical connectivity of the bearer channels.  This will be done
   initially when a link is established, and subsequently, on a
   periodic basis for all free bearer channels on the link.  To ensure
   proper verification of bearer channel connectivity, it is required
   that until the bearer channels are allocated, they must be opaque.

   As part of the link verification protocol, the control channel is
   first verified, and connectivity maintained, using the Hello
   protocol discussed in Section 5.1.1.  Once the control channel has
   been established between the two OXCs, bearer channel connectivity
   is verified by exchanging Ping-type Test messages over all of the
   bearer channels specified in the link.  It should be noted that all
   messages except for the Test message are exchanged over the control
   channel and that Hello messages continue to be exchanged over the
   control channel during the bearer channel verification process.  The
   Test message is sent over the bearer channel that is being verified.
   Bearer channels are tested in the transmit direction as they are
   uni-directional, and as such, it may be possible for both OXCs to
   exchange the Test messages simultaneously. [Lang00]


5.1.3 Fault localization

   Fault detection is delegated to the physical layer (i.e., loss of
   light or optical monitoring of the data) instead of the layer 2 or
   layer 3.  Hence, detection must be handled at the layer closest to
   the failure; for optical networks, this is the physical (optical)
   layer.  One measure of fault detection at the physical layer is
   simply detecting loss of light (LOL).  Other techniques for
   monitoring optical signals are still being developed.

Various Authors  Informational _ Expires January 2001               30
             IP over Optical Networks: Summary of Issues     July 2000



   A link connecting two OXCs consists of a control channel and a
   number of bearer channels.  If bearer channels fail between two
   OXCs, a  mechanism must be used to rapidly locate the failure so
   that appropriate protection/restoration mechanisms can be initiated.
   This is discussed further in Section 6.10.

5.2 Optical Performance Monitoring (OPM)

   Current-generation DWDM networks are monitored, managed and
   protected within the digital domain, using SONET and its associated
   support systems.  However, to leverage the full potential of
   wavelength-based networking, the provisioning, switching, management
   and monitoring functions have to move from the digital to the
   optical domain.

   The information generated by the performance monitoring operation
   can be used to ensure safe operation of the optical network.  In
   addition to verifying the service level provided by the network to
   the user, performance monitoring is also necessary to ensure that
   the users of the network comply with the requirements that were
   negotiated between them and the network operator.  For example, one
   function may be to monitor the wavelength and power levels of
   signals being input to the network to ensure that they meet the
   requirements imposed by the network.  Current performance monitoring
   in optical networks requires termination of a channel at an optical-
   electrical-optical conversion point to detect bits related to BER of
   the payload or frame (e.g., SONET LTE monitoring).  However, while
   these bits indicate if errors have occurred, they do not supply
   channel-performance data.  This makes it very difficult to assess
   the actual cause of the degraded performance.

   Fast and accurate determination of the various performance measures
   of a wavelength channel implies that measurements have to be done
   while leaving it in optical format.  One possible way of achieving
   this is by tapping a portion of the optical power from the main
   channel using a low loss tap of about 1%.  In this scenario, the
   most basic form of monitoring will utilize a power-averaging
   receiver to detect loss of signal at the optical power tap point.
   Existing DWDM systems use optical time-domain reflectometers to
   measure the parameters of the optical links.

   Another problem lies in determining the threshold values for the
   various parameters at which alarms must be declared.  Very often
   these values depend on the bit rate on the channel and should
   ideally be set depending on the bit rate.  In addition, since a
   signal is not terminated at an intermediate node, if a wavelength
   fails, all nodes along the path downstream of the failed wavelength
   could trigger an alarm.  This can lead to a large number of alarms
   for a single failure, and makes it somewhat more complicated to
   determine the cause of the alarm (alarm correlation).  A list of
   such optical parameters to be monitored periodically have been


Various Authors  Informational _ Expires January 2001               31
             IP over Optical Networks: Summary of Issues     July 2000


   proposed [Ceuppens00].  Optical cross talk, dispersion, insertion
   loss are key parameters to name a few.

   Care needs to be taken in exchanging these performance parameters.
   The vast majority of existing telecommunication networks uses
   framing and data formatting overhead as the means to communicate
   between network elements and management systems.  It is worth
   mentioning that while the signaling is used to communicate all
   monitoring results, the monitoring itself is done on the actual data
   channel, or some range of bandwidth around the channel.  Therefore,
   all network elements must be guaranteed to pass this bandwidth in
   order for monitoring to happen at any point in the network.

   One of the options being considered for transmitting the information
   is the framing and formatting bits of the SONET interface.  But, it
   hampers transparency.  It is clear that truly transparent and open
   photonic networks can only be built with transparent signaling
   support.  The MPLS control plane architecture suggested can be
   extended beyond simple bandwidth provisioning to include optical
   performance monitoring.


6.  Fault restoration in Optical networks

   Telecom networks have traditionally been designed with rapid fault
   detection, rapid fault isolation and recovery. With the introduction
   of IP and WDM in these networks, these features need to be provided
   in the IP and WDM layers also. Automated establishment and
   restoration of end-to-end paths in such networks requires
   standardized signaling, routing, and restoration mechanisms.

6.1 Layering

   Clearly the layering and architecture for service restoration is a
   major component for IP to optical internetworking.  This section
   summarizes some schemes, which aid in optical protection at the
   lower layers, SONET and Optical.

6.1.1 SONET Protection

   The SONET standards specify an end-to-end two-way availability
   objective of 99.98% for inter office applications (0.02%
   unavailability or 105 minutes/year maximum down time) and 99.99 %
   for loop transport between the central office and the customer's
   premises.  To conform to these standards, failure/restoration times
   have to be short.  For both, point-to-point and ring systems,
   automatic protection switching (APS) is used, the network performs
   failure restoration in tens of milliseconds (approximately 50
   milliseconds).

   Architectures composed of SONET add-drop multiplexers (ADMs)
   interconnected in a ring  provide an a method of APS that allows
   facilities to be shared while protecting  traffic within an

Various Authors  Informational _ Expires January 2001               32
             IP over Optical Networks: Summary of Issues     July 2000


   acceptable restoration time.  There are 2 possible ring
   architectures:

   * UPSR:  Unidirectional path switched ring architecture is a 1+1
   single-ended, unidirectional, SONET path layer dedicated protection
   architecture.  The nodes are connected in a ring configuration with
   one fiber pair connecting adjacent nodes.  One fiber on a link is
   used as the working and other is protection.  They operate in
   opposite directions.  So there is a working ring in one direction
   and a protection ring in the opposite direction. The optical signal
   is sent on both outgoing fibers.  The receiver compares the 2
   signals and selects the better of the two based on signal quality.
   This transmission on both fibers is called 1+1 protection.

   * BLSR: In bidirectional line switched ring architecture, a
   bidirectional connection between 2 nodes traverses the same
   intermediate nodes and links in opposite directions.  In contrast to
   the UPSR, where the protection capacity is dedicated, the BLSR
   shares protection capacity among all spans on the ring.  They are
   also called Shared Protection ring (SPRing) architectures.  In BLSR
   architecture, switching is coordinated by the nodes on either side
   of a failure in the ring, so that a signaling protocol is required
   to perform a line switch and to restore the network.  These
   architectures are more difficult to operate than UPSRs where no
   signaling is required.

6.1.2 Optical layer Protection

   The concept of SONET ring architectures can be extended to WDM self-
   healing optical rings (SHRs). As in SONET, WDM SHRs can be either
   path switched or line switched.  In recent testbed experiments,
   lithium niobate protection switches have been used to achieve 10
   micro-seconds restoration times in WDM Shared protection Rings.
   Multiwavelength systems add extra complexity to the restoration
   problem.  Under these circumstances, a simple ring architecture may
   not suffice.  Hence, arbitrary mesh architectures become important.
   Usually, for such architectures, restoration is usually performed
   after evaluation at the higher layer.  But this takes a lot of time.

   Optical protection techniques for mesh architectures have also been
   proposed.  They operate on a line rather than path protection basis.
   The fundamental unit being protected is a transmission link rather
   than an end to end connection.  The methodology is a generalization
   of that is used in SPRings.  The system requires 100% redundancy.
   Fault recovery decisions are made locally in a distributed fashion
   and independent of the state of activity of the network.  The
   implementation uses simple and reliable protection switches in each
   network node so that protection is accomplished without significant
   processing, transmission, and propagation delays.

   Thus, the main advantage of lower layer mechanisms is the fast
   restoration.  When we talk of the IP/MPLS over WDM architecture, we
   may seal off SONET APS protection from the discussion and the WDM

Various Authors  Informational _ Expires January 2001               33
             IP over Optical Networks: Summary of Issues     July 2000


   optical layer can provide the same kind of restoration capabilities
   at the lower layer.  Thus there has to be interaction only between
   the MPLS and optical layers.

6.2 MPLS in Protection

   Although the current routing algorithms are very robust and
   survivable, the amount of time they take to recover from a failure
   can be significant, on the order of several seconds or minutes,
   causing serious disruption of service in the interim.  This is
   unacceptable to many organizations that aim to provide a highly
   reliable service, and thus require recovery times on the order of
   tens of milliseconds.

   Since MPLS binds packets to a route (or path) via the labels, it is
   imperative that MPLS be able to provide protection and restoration
   of traffic.  In fact, a protection priority could be used as a
   differentiating mechanism for premium services that require high
   reliability.

6.2.1 Motivations

   The need for MPLS layer protection and for open standards in
   protection arises because of the following:

   1.  Layer 3 or IP rerouting may be too slow for a core MPLS network
   that needs to support high reliability/availability.

   2.  Layer 0 (for example, optical layer) or Layer 1 (for example,
   SONET) mechanisms may be limited to ring topologies and may not
   include mesh protection.

   3.  Layer 0 or Layer 1 mechanisms may have no visibility into higher
   layer operations.  Thus, while they may provide link protection for
   example, they cannot easily provide MPLS path protection.

   4.  Establishing interoperability of protection mechanisms between
   multi-vendor LSRs in core MPLS networks is urgently required to
   enable the adoption of MPLS as a viable core transport technology.

6.2.2 Goals

   Based on our motivations, we can state some goals for MPLS based
   protection:

   1.  MPLS-based recovery mechanisms should facilitate fast (10As of
   ms) recovery times.

   2.  MPLS-based recovery techniques should be applicable for
   protection of traffic at various granularities.  For example, it
   should be possible to specify MPLS- based recovery for a portion of
   the traffic on an individual path, for all traffic on an individual
   path, or for all traffic on a group of paths.

Various Authors  Informational _ Expires January 2001               34
             IP over Optical Networks: Summary of Issues     July 2000



   3.  MPLS-based recovery techniques may be applicable for an entire
   end-to-end path or for segments of an end-to-end path.

   4.  MPLS-based recovery mechanisms should be able to take into
   consideration the recovery actions of other layers.

   5.  MPLS-based recovery actions should avoid network-layering
   violations.  That is, defects in MPLS-based mechanisms should not
   trigger lower layer protection switching.

   6.  MPLS-based recovery mechanisms should minimize the loss of data
   and packet reordering during recovery operations.

   7.  MPLS-based recovery mechanisms should minimize the state
   overhead incurred for each recovery path maintained.

   8.  MPLS-based recovery mechanisms should be able to preserve the
   constraints on traffic after switchover, if desired.  That is, if
   desired, the recovery path should meet the resource requirements of,
   and achieve the same performance characteristics, as the working
   path.

6.3 Protection options

6.3.1 Dynamic Protection

   These protection mechanisms dynamically create protection entities
   for restoring traffic, based upon failure information, bandwidth
   allocation and optimized reroute assignment.  Thus, upon detecting
   failure, the LSPs crossing a failed link or LSR are broken at the
   point of failure and reestablished using signaling.  These methods
   may increase resource utilization because capacity or bandwidth is
   not reserved beforehand and because it is available for use by other
   (possibly lower priority) traffic, when the protection path does not
   require this capacity.  They may, however, require longer
   restoration times, since it is difficult to instantaneously switch
   over to a protection entity, following the detection of a failure.

6.3.2 Pre-negotiated Protection

   These are dedicated protection mechanisms, where for each working
   path there exists a pre-established protection path, which is node
   and link disjoint with the primary/working path, but may merge with
   other working paths that are disjoint with the primary.  The
   resources (bandwidth, buffers, processing) on the backup entity may
   be either pre-determined and reserved beforehand (and unused), or
   may be allocated dynamically by displacing lower priority traffic
   that was allowed to use them in the absence of a failure on the
   working path.

6.3.3 End-to-end Repair


Various Authors  Informational _ Expires January 2001               35
             IP over Optical Networks: Summary of Issues     July 2000


   In end-to-end repair, upon detection of a failure on the primary
   path, an alternate or backup path is re-established starting at the
   source.  Thus, protection is always activated on an end-to-end
   basis, irrespective of where along a working path a failure occurs.
   This method might be slower than the local repair method discussed
   below, since the failure information has to propagate all the way
   back to the source before a protection switch is accomplished.

6.3.4 Local Repair

   In local repair, upon detecting a failure on the primary path, an
   alternate path is re-established starting from the point of failure.
   Thus protection is activated by each LSR along the path in a
   distributed fashion on an as-needed basis.  While this method has an
   advantage in terms of the time taken to react to a fault, it
   introduces the complication that every LSR along a working path may
   now have to function as a path splitting LSR (PSL).

6.3.5 Link Protection

   The intent is to protect against a single link failure.  For
   example, the protection path may be configured to route around
   certain links deemed to be potentially risky.  If static
   configuration is used, several protection paths may be
   preconfigured, depending on the specific link failure that each
   protects against.  Alternatively, if dynamic configuration is used,
   upon the occurrence of a failure on the working path, the protection
   path is rebuilt such that it detours around the failed link.

6.3.6 Path Protection

   The intention is to protect against any link or node failure on the
   entire working path.  This has the advantage of protecting against
   multiple simultaneous failures on the working path, and possibly
   being more bandwidth efficient than link protection.

6.3.7 Revertive Mode

   In the revertive mode of operation, the traffic is automatically
   restored to the working path once repairs have been affected, and
   the PSL(s) are informed that the working path is up.  This is
   useful, since once traffic is switched to the protection path it is,
   in general, unprotected.  Thus, revertive switching ensures that the
   traffic remains unprotected only for the shortest amount of time.
   This could have the disadvantage, however, of producing oscillation
   of traffic in the network, by altering link loads.

6.3.8 Non-revertive Mode

   In the non-revertive mode of operation, traffic once switched to the
   protection path is not automatically restored to the working path,
   even if the working path is repaired.  Thus, some form of
   administrative intervention is needed to invoke the restoration

Various Authors  Informational _ Expires January 2001               36
             IP over Optical Networks: Summary of Issues     July 2000


   action.  The advantage is that only one protection switch is needed
   per working path.  A disadvantage is that the protection path
   remains unprotected until administrative action (or manual
   reconfiguration) is taken to either restore the traffic back to the
   working path or to configure a backup path for the protection path.

6.3.9 1+1 Protection

   In 1+1 protection, the resources (bandwidth, buffers, processing
   capacity) on the backup path are fully reserved to carry only
   working traffic.  In MPLS, this bandwidth may be considered wasted.
   Alternately, this bandwidth could be used to transmit an exact copy
   of the working traffic, with a selection between the traffic on the
   working and protection paths being made at the path merge LSR (PML).

6.3.10 1:1, 1:n, and n:m  Protection

   In 1:1 protection, the resources (bandwidth, buffers, and processing
   capacity) allocated on the protection path are fully available to
   preemptable low priority traffic when the protection path is not in
   use by the working traffic.  In other words, in 1:1 protection, the
   working traffic normally travels only on the working path, and is
   switched to the protection path only when the working entity is
   unavailable.  Once the protection switch is initiated, all the low
   priority traffic being carried on the protection path is discarded
   to free resources for the working traffic.  This method affords a
   way to make efficient use of the backup path, since resources on the
   protection path are not locked and can be used by other traffic when
   the backup path is not being used to carry working traffic.

   Similarly, in 1:n protection, up to n working paths are protected
   using only one backup path, while in m:n protection, up to n working
   paths are protected using up to m backup paths.

6.3.11 Recovery Granularity

   Another dimension of recovery considers the amount of traffic
   requiring protection.  This may range from a fraction of a path to a
   bundle of paths.

6.3.11.1 Selective Traffic Recovery

   This option allows for the protection of a fraction of traffic
   within the same path.  The portion of the traffic on an individual
   path that requires protection is called a protected traffic portion
   (PTP).  A single path may carry different classes of traffic, with
   different protection requirements.  The protected portion of this
   traffic may be identified by its class, as for example, via the EXP
   bits in the MPLS shim header or via the cell loss priority (CLP) bit
   in the ATM header.

6.3.11.2 Bundling


Various Authors  Informational _ Expires January 2001               37
             IP over Optical Networks: Summary of Issues     July 2000


   Bundling is a technique used to group multiple working paths
   together in order to recover them simultaneously.  The logical
   bundling of multiple working paths requiring protection, each of
   which is routed identically between a PSL and a PML, is called a
   protected path group (PPG).  When a fault occurs on the working path
   carrying the PPG, the PPG as a whole can be protected either by
   being switched to a bypass tunnel or by being switched to a recovery
   path.

6.4.  Failure detection

   Loss of Signal (LOS) is a lower layer impairment that arises when a
   signal is not detected at an interface, for example, a SONET LOS.
   In this case, enough time should be provided for the lower layer to
   detect LOS and take corrective action.

   A Loss of Function (LF) is declared when the link probing mechanism
   fails.  An example of a probing mechanism is the Liveness message
   that is exchanged periodically along the working path between peer
   LSRs.  A LF is detected when a certain number k of consecutive
   Liveness messages are either not received from a peer LSR or are
   received in error.

   A Loss of Packets (LOP) occurs when there is excessive discarding of
   packets at an LSR interface, either due to label mismatches or due
   to time-to-live (TTL) errors.  LOP due to label mismatch may be
   detected simply by counting the number of packets dropped at an
   interface because an incoming label did not match any label in the
   forwarding table.  Likewise, LOP due to invalid TTL may be detected
   by counting the number of packets that were dropped at an interface
   because the TTL decrements to zero.

6.5 Failure Notification

   Protection switching relies on rapid notification of failures.  Once
   a failure is detected, the node that detected the failure must send
   out a notification of the failure by transmitting a fault indication
   signal (FIS) to those of its upstream LSRs that were sending traffic
   on the working path that is affected by the failure.  This
   notification is relayed hop-by-hop by each subsequent LSR to its
   upstream neighbor, until it eventually reaches a PSL.

   The PSL is the LSR that originates both the working and protection
   paths, and the LSR that is the termination point of both the FIS and
   the fault restoration signal (FRS).  Note that the PSL need not be
   the origin of the working LSP.

   The PML is the LSR that terminates both the working path and its
   corresponding protection path.  Depending on whether or not the PML
   is a destination, it may either pass the traffic on to the higher
   layers or may merge the incoming traffic on to a single outgoing
   LSR.  Thus, the PML need not be the destination of the working LSP.


Various Authors  Informational _ Expires January 2001               38
             IP over Optical Networks: Summary of Issues     July 2000


   An LSR that is neither a PSL nor a PML is called an intermediate
   LSR.  The intermediate LSR could be either on the working or the
   protection path, and could be a merging LSR (without being a PML).

6.5.1 Reverse Notification Tree (RNT)

   Since the LSPs are unidirectional entities and protection requires
   the notification of failures, the failure indication and the failure
   recovery notification both need to travel along a reverse path of
   the working path from the point of failure back to the PSL(s).  When
   label merging occurs, the working paths converge to form a
   multipoint-to-point tree, with the PSLs as the leaves and the PML as
   the root.  The reverse notification tree is a point-multipoint tree
   rooted at the PML along which the FIS and the FRS travel, and which
   is an exact mirror image of the converged working paths

   The establishment of the protection path requires identification of
   the working path, and hence the protection domain.  In most cases,
   the working path and its corresponding protection path would be
   specified via administrative configuration, and would be established
   between the two nodes at the boundaries of the protection domain
   (the PSL and PML) via explicit (or source) routing using LDP , RSVP,
   signaling (alternatively, using manual configuration).

   The RNT is used for propagating the FIS and the FRS, and can be
   created very easily by a simple extension to the LSP setup process.
   During the establishment of the working path, the signaling message
   carries with it the identity (address) of the upstream node that
   sent it.  Each LSR along the path simply remembers the identity of
   its immediately prior upstream neighbor on each incoming link.  The
   node then creates an inverse crossconnect table that for each
   protected outgoing LSP maintains a list of the incoming LSPs that
   merge into that outgoing LSP, together with the identity of the
   upstream node that each incoming LSP comes from.  Upon receiving an
   FIS, an LSR extracts the labels contained in it (which are the
   labels of the protected LSPs that use the outgoing link that the FIS
   was received on) consults its inverse crossconnect table to
   determine the identity of the upstream nodes that the protected LSPs
   come from, and creates and transmits an FIS to each of them.

6.6.  Timing

   There are a number of timing parameters that need to be specified
   for proper restoration of the IP/WDM networks. Some of these are
   described below.

6.6.1 Protection Switching Interval Timer T1:

   Controls the maximum duration within which a protection switch must
   be accomplished, following the detection of a failure.

6.6.2 Inter-FIS Packet Timer T2:


Various Authors  Informational _ Expires January 2001               39
             IP over Optical Networks: Summary of Issues     July 2000


   Interval at which successive FIS packets are transmitted by a LSR to
   its upstream neighbor.

6.6.3 Maximum FIS duration timer T3:

   Max.  time for which FIS packets are transmitted by an LSR to is
   upstream peer.

6.6.4 Protection switching dampening timer T4:

   Time interval between receipt of a protection switch trigger and the
   initiation of the protection switch.  The purpose of this timer is
   to minimize misordering of packets at a PML following a protection
   (restoration) switch from the working (backup) to the backup
   (working) path.  This is because packets buffered on the working
   (backup) path may continue to arrive at the PML even as working
   traffic begins to arrive on the protection (working) path.
   Therefore, forcing the PSL to hold off the protection (or
   restoration) switching action, gives the buffers on the working
   (protection) path time to clear before data on the protection
   (working) path begins to arrive.

6.6.5 Liveness Message Send interval T5:

   Interval at which successive Liveness messages are sent by an LSR to
   peer LSRs that have a working path (and RNT) through this LSR.

6.6.6 Failure Indication Hold-off Timer T6:

   Interval between the detection of a failure at an LSR, and the
   generation of the first FIS message, to allow time for lower layer
   protection to take effect.

6.6.7 Lost Liveness Message Threshold K:

   No.  of Liveness messages that can be lost before an LSR will
   declare LF and generate the FIS.

   For proper operation, it is required that T1 >> T2 > T3 and T1> T4


6.7.  Signaling Requirements related to restoration

   Signaling mechanisms for optical networks should be tailored to the
   needs of optical networking.

   Some signaling requirements directed towards restoration in optical
   networks are:

   1.  Signaling mechanisms must minimize the need for manual
   configuration of relevant information, such as local topology.



Various Authors  Informational _ Expires January 2001               40
             IP over Optical Networks: Summary of Issues     July 2000


   2.  Lightpaths are fixed bandwidth pipes.  There is no need to
   convey complex traffic characterization or other QoS parameters in
   signaling messages.  On the other hand, new service related
   parameters such as restoration priority, protection scheme desired,
   etc., may have to be conveyed.

   3.  Signaling for path establishment must be quick and reliable.  It
   is especially important to minimize signaling delays during
   restoration.

   4.  Lightpaths are typically bidirectional.  Both directions of the
   path should generally be established along the same physical route.

   5.  OXCs are subject to high reliability requirements.  A transient
   failure that does not affect the data plane of the established paths
   should not result in these paths being torn down.

   6.  Restoration schemes in mesh networks rely on sharing backup path
   among many primary paths.  Signaling protocols should support this
   feature.

   7.  The interaction between path establishment signaling and
   automatic protection schemes must be well-defined to avoid false
   restoration attempts during path set-up or tear down.

6.8 RSVP/CR-LDP Support for Restoration

   Special requirements for protecting and restoring lightpaths and the
   extensions to RSVP and CR-LDP have been identified. Some of the
   proposed extensions are as follows:

   a. A new SESSION_ATTRIBUTE object has been proposed, which indicates
      whether the path is unidirectional/bi-directional,
      primary/backup. Local protection 1+1 or1:N can also be specified.
   b. Setup Priority: The priority of the session with respect to
      taking resources. The Setup Priority is used in deciding whether
      this session can preempt another session.
   Holding Priority: The priority of the session with respect to
   holding resources.  Holding Priority is used in deciding whether
   this session can be preempted by another session.

   Note that for the shared backup paths the crossconnects can not be
   setup during the signaling for the backup path since multiple backup
   paths may share the same resource and can over-subscribe it.  The
   idea behind shared backups is to make soft reservations and to claim
   the resource only when it is required.


6.9 Fast restoration of MPLS LSPs

   Fast recovery in MPLS is hampered by the fact that detecting an LSP
   failure at the ingress LSR can take a long time.  After a break in


Various Authors  Informational _ Expires January 2001               41
             IP over Optical Networks: Summary of Issues     July 2000


   an LSP hop, Notification messages are propagated along the LSP
   intermediate nodes back to the ingress LSR.

   The fastest detection occurs at the local end of a link failure.
   Schemes that try to mend connections at the point of failure are
   known as "local repair" schemes.

   A problem with single L2 link failure is that multiple LSPs can be
   affected and many (hundreds) ingress points must be informed.  Just
   as a single L2 failure can affect multiple LSPs, a single L1 failure
   can affect multiple L2 links.

   As noted earlier, L1 failure detection is fast due to physical
   methods (loss of light, loss of carrier signal).  This is an
   attractive property.  Further, in a TDM, optical mux (SONET), or
   optical cross connect network, when a link fails all of the paths
   (at that layer) which use the link go down.

   Unlike higher layers, the endpoints of those paths detect the
   failure quickly because the signaling of the failure is very fast
   (e.g., AIS signals in SONET) and because the signaling is sent to
   each channel of the failed link.  So in L1 networks, the detection
   of a failed connection is fast and scales well for all connections
   on the failed link.

   A key to the solution for fast detection is the alignment of L1, L2,
   and L3 capabilities into a single node.  This architecture and its
   impacts on the ability to detect LSP failure are now described.

6.9.1 L1/L2/L3 Integration

   As was noted earlier, in MPLS LSRs, the alignment of the L3 and L2
   topology brings some advantages in the speed at which the network
   can react to a link failure.  This integration is extended to
   encompass L1 components in order to realize further speed
   advantages.

   An L1/L2/L3 switch is defined as an LSR combined with an L1 cross
   connect switch.  This could be a SONET Add/Drop Mux, an optical
   cross connect, or traditional TDM switch.  The integrated switch is
   able to originate and terminate IP traffic from the L1 cross
   connects.  Conceptually, this is done over dedicated L1 channels
   between the L1 cross connect and the pure IP router function of the
   integrated switch.

   Two L1/L2/L3 nodes are connected by a physical L1 link.  A channel
   in that link is used as a router-router IP link.  For example, an
   OC-3 channel of an OC-48 link with PPP over SONET for the framing.
   This is analogous to the L2 control channel between two MPLS
   switches connected over an ATM interface.

   A key difference between this type of network and L2/L3 networks,
   which are overlayed on L1 networks, is that the L1/L2/L3 network

Various Authors  Informational _ Expires January 2001               42
             IP over Optical Networks: Summary of Issues     July 2000


   does not have any L1 paths, which act as router-router links.  In an
   integrated network, the L3 routing protocol has a view of both the
   L2 and L1 topology since those layers are now aligned.

   Here, in an L1/L2/L3 network, an L1 path has an LSR at every cross
   connect point.  To use an L1 path, treat it as if were an LSP, or
   overlay an LSP onto this path.  That is, consider the L1 path as a
   cut-through.  When an incoming IP packet is matched to a Forwarding
   Equivalence Class associated with this L1 cut- through, the IP
   forwarding table entry points to the start of this L1 path.  As with
   L2 cut-throughs, an L2 header is added.  The packet is sent to this
   path and is then L1 switched until it reaches the end of the path.
   At the termination point, the packet could be L2 switched or L3
   forwarded.

6.9.2 An example

   Using L1 cut-throughs in an L1/L2/L3 network enables fast detection
   of LSP failure.  Consider two LSPs that are L1 cut-throughs:
      LSR1-LSR2-LSR3-LSR4 and
      LSR5-LSR2-LSR3-LSR6

   If L1 link LSR2-LSR3 goes down, all nodes in both LSRs can detect
   the path failure based on L1 physical methods.  For example, loss of
   light (Alarm Indication Signal in SONET) or carrier signal (TDM).
   In particular, the LSP endpoints can determine that the LSP is down
   much faster than the protocol based method in LDP of Notification
   messages which is processed at each LSR on the paths back to the
   ingress and egress.  For example, propagation of the physical
   failure is about 5 microseconds per kilometer.

   Not only is the failure detection fast, but it scales for all LSPs
   that are affected by a single L1 failure.  In the example above, two
   LSPs are notified, but if there were 192 paths in an OC192 link,
   then all of their endpoints could detect the link failure within a
   short period of time (a few milliseconds).

   When an LSP failure is detected, the LSR can reroute the traffic to
   a backup LSP.  This backup LSP could be pre-defined to be link
   disjoint from the primary LSP, and could also be set up in advance.
   To avoid wasting dedicated bandwidth (i.e., a dedicated backup L1
   cut-through), the backup LSP for the L1 cut-through could be an LSP
   created over L2 connections which share bandwidth (e.g., ATM UBR
   VC).

   Assuming that a backup LSP is already set up, restoration of a
   failed LSP that is overlayed on an L1 cut-through could be
   implemented with similar performance to SONET Line and Ring
   restoration.

   For LSR which provide L3 connectionless forwarding, traffic from the
   failed LSP could also be immediately handled by L3 forwarding if a
   backup path LSP is not provided.

Various Authors  Informational _ Expires January 2001               43
             IP over Optical Networks: Summary of Issues     July 2000




6.10 LMP's Fault localization mechanism

   If bearer channels fail between two OXCs, the power monitoring
   system in all of the downstream nodes will detect loss of light
   (LOL) and indicate a failure.  As part of the fault localization, a
   monitoring window can be used in each node to determine if a single
   bearer channel has failed or if multiple bearer channels have
   failed.

   As part of the fault localization, a downstream node that detects
   bearer channel failures across a link will send a Channel_Fail
   message to its upstream neighbor (bundling together the notification
   of all of the failed bearer channels) and the node will put the
   ports associated with the failed bearer channels into the standby
   state.  An upstream node that receives the Channel_Fail message will
   correlate the failure to see if there is a failure on the
   corresponding input and output ports for the optical trail(s).  If
   there is also a failure on the input channel(s) of the upstream
   node, the node will return a Channel_Fail_Ack message to the
   downstream node (bundling together the notification of all the
   channels), indicating that it too has detected a failure.  If,
   however, the fault is CLEAR in the upstream node (i.e., there is no
   LOL on the corresponding input channels), then the upstream node
   will have localized the failure and will return a Channel_Fail_Nack
   message to the downstream node, and initiate protection/restoration
   procedures.  The protection channels may be preconfigured or they
   may be dynamically selected by the OXC on the transmit side.

   In Fig. 5, a sample network is shown where four OXCs are connected
   in a linear array configuration.  The control channels are bi-
   directional and are labeled with a "c".  All optical trails are uni-
   directional going left to right.  In the example there is a failure
   on a single bearer channel between OXC2 and OXC3.  Both OXC3 and
   OXC4 will detect the failure and each node will send a Channel_Fail
   message to the corresponding upstream node (OXC3 will send a message
   to OXC2 and OXC4 will send a message to OXC3).  When OXC3 receives
   the Channel_Fail message from OXC4, it will correlate the failure
   and return a Channel_Fail_Ack message back to OXC4.  Upon receipt of
   the Channel_Fail_Ack message, OXC4 will move the associated ports
   into a standby state.  When OXC2 receives the Channel_Fail message
   from OXC3, it will correlate the failure, verify that it is CLEAR,
   localize the failure to the bearer channel between OXC2 and OXC3,
   and send a Channel_Fail_Nack message back to OXC3.


       +-------+        +-------+        +-------+        +-------+
       + OXC 1 +        + OXC 2 +        + OXC 3 +        + OXC 4 +
       +       +-- c ---+       +-- c ---+       +-- c ---+       +
   ----+-------+--------+---\   +        +       +        +       +
       +       +        +    \--+---##---+--\    +        +       +
       +       +        +       +   (A)  +   \   +        +       +

Various Authors  Informational _ Expires January 2001               44
             IP over Optical Networks: Summary of Issues     July 2000


   ----+--\    +        +       +        +    \--+--------+-------+--->
       +   \   +        +    /--+--------+---\   +        +       +
       +    \--+--------+---/   +        +    \--+--------+-------+--->
       +       +        +       +        +       +        +       +
       +-------+        +-------+        +-------+        +-------+

   Fig.5 : Example - We show one type of bearer channel failures
   (indicated by ## in the figure): a single bearer channel fails
   between two OXCs


7.  Security Considerations

   This document raises no new security issues for MPL(ambda) Switching
   implementation over optical networks.  Security considerations are
   for future study.


8.  Acronyms

   CR-LPD - Constraint-Based Routing Setup using LDP
   CSPF - Constraint Shortest Path First
   FIS  - Failure Indication Signal
   FRS  - Failure Recovery Signal
   GSMP - General Switch Management Protocol
   LDP  - Label Distribution Protocol
   LF   - Link Failure
   LMP  - Link Management Protocol
   LOP  - Loss of Packets
   LOS  - Loss Of Signal
   LP   - Lightpath
   LSR  - Label Switched Router
   MPLS - Multi-Protocol Lambda Switching
   MTG  - MPLS Traffic Group
   NBMA - Non-Broadcast Multi-Access
   NHRP - Next Hop Resolution Protocol
   OCT  - Optical Channel Trail
   OLXC - Optical layer crossconnect
   OPM  - Opical Performance Monitoring
   OSPF - Open Shortest Path First
   OTN  - Optical Transport Network
   OXC  - Optical Crossconnect
   PML  - Protection Merge LSR
   PMTG - Protected MPLS Traffic Group
   PMTP - Protected MPLS Traffic Portion
   PSL  - Protection Switch LSR
   PXC  - Photonic Crossconnect
   RNT  - Reverse Notification Tree
   RSVP - Resource reSerVation Protocol
   SPRing - Shared Protection ring
   TLV  - Type Length Value

9.  Terminology

Various Authors  Informational _ Expires January 2001               45
             IP over Optical Networks: Summary of Issues     July 2000



   Channel:
   A channel is a uni-directional optical tributary connecting two
   OLXCs.  Multiple channels are multiplexed optically at the WDM
   system.  One direction of an OC- 48/192 connecting two immediately
   neighboring OLXCs is an example of a channel.  A channel can
   generally be associated with a specific wavelength in the WDM
   system.  A single wavelength may transport multiple channels
   multiplexed in the time domain.

   Downstream node:
   In an uni-directional lightpath, this is the next node closer to
   destination.

   Failure Indication Signal:
   A signal that indicates that a failure has been detected at a peer
   LSR.  It consists of a sequence of failure indication packets
   transmitted by a downstream LSR to an upstream LSR repeatedly.  It
   is relayed by each intermediate LSR to its upstream neighbor, until
   it reaches an LSR that is setup to perform a protection switch.

   Failure Recovery Signal:
   A signal that indicates that a failure along the path of an LSP has
   been repaired.  It consists of a sequence of recovery indication
   packets that are transmitted by a downstream LSR to its upstream
   LSR, repeatedly.  Again, like the failure indication signal, it is
   relayed by each intermediate LSR to its upstream neighbor, until is
   reaches the LSR that performed the original protection switch.

   First-hop router:
   The first router within the domain of concern along the lightpath
   route.  If the source is a router in the network, it is also its own
   first-hop router.

   Intermediate LSR:
   LSR on the working or protection path that is neither a PSL nor a
   PML.

   Last-hop router:
   The last router within the domain of concern along the lightpath
   route.  If the destination is a router in the network, it is also
   its own last-hop router.

   Lightpath:
   This denotes an Optical Channel Trail in the context of this
   document.

   Link Failure:
   A link failure is defined as the failure of the link probing
   mechanism, and is indicative of the failure of either the underlying
   physical link between adjacent LSRs or a neighbor LSR itself.  (In
   case of a bidirectional link implemented as two unidirectional


Various Authors  Informational _ Expires January 2001               46
             IP over Optical Networks: Summary of Issues     July 2000


   links, it could mean that either one or both unidirectional links
   are damaged.)

   Liveness Message:
   A message exchanged periodically between two adjacent LSRs that
   serves as a link probing mechanism.  It provides an integrity check
   of the forward and the backward directions of the link between the
   two LSRs as well as a check of neighbor aliveness.

   Loss of Signal:
   A lower layer impairment that occurs when a signal is not detectedat
   an interface.  This may be communicated to the MPLS layer by the
   lower layer.

   Loss of Packet:
   An MPLS layer impairment that is local to the LSR and consists of
   excessive discarding of packets at an interface, either due to label
   mismatch or due to TTL errors.  Working or Active LSP established to
   carry traffic from a source LSR to a destination LSR under normal
   conditions, that is, in the absence of failures.  In other words, a
   working LSP is an LSP that contains streams that require protection.

   MPLS Traffic Group:
   A logical bundling of multiple, working LSPs, each of which is
   routed identically between a PSL and a PML.  Thus, each LSP in a
   trafficgroup shares the same redundant routing between the PSL and
   the PML.

   MPLS Protection Domain:
   The set of LSRs over which a working path and its corresponding
   protection path are routed.  The protection domain is denoted as:
   (working path, protection path).

   Non-revertive:
   A switching option in which streams are not automatically switched
   back from a protection path to its corresponding working path upon
   the restoration of the working path to a fault-free condition.

   Opaque:
   Used to denote a bearer channel characteristic where it is capable
   of being terminated.

   Optical Channel Trail:
   The elementary abstraction of optical layer connectivity between two
   end points is a uni-directional Optical Channel Trail.  A Optical
   Channel Trail is a fixed bandwidth connection between two network
   elements established via the OLXCs.  A bi-directional Optical
   Channel Trail consists of two associated Optical Channel Trails in
   opposite directions routed over the same set of nodes.

   Optical layer crossconnect (OLXC):



Various Authors  Informational _ Expires January 2001               47
             IP over Optical Networks: Summary of Issues     July 2000


   A switching element which connects an optical channel from an input
   port to an output port. The switching fabric in an OLXC may be
   either electronic or optical.

   Protected MPLS Traffic Group (PMTG):
   An MPLS traffic group that requires protection.

   Protected MPLS Traffic Portion:
   The portion of the traffic on an individual LSP that requires
   protection.  A single LSP may carry different classes of traffic,
   with different protection requirements.  The protected portion of
   this traffic may be identified by its class, as for example, via the
   EXP bits in the MPLS shim header or via the priority bit in the ATM
   header.

   Protection Merge LSR:
   LSR that terminates both a working path and its corresponding
   protection path, and either merges their traffic into a single
   outgoing LSP, or, if it is itself the destination, passes the
   traffic on to the higher layer protocols.

   Protection Switch LSR:
   LSR that is the origin of both the working path and its
   corresponding protection path.  Upon learning of a failure, either
   via the FIS or via its own detection mechanism, the protection
   switch LSR switches protected traffic from the working path to the
   corresponding backup path.

   Protection or Backup LSP (or Protection or Backup Path):
   A LSP established to carry the traffic of a working path (or paths)
   following a failure on the working path (or on one of the working
   paths, if more than one)  and a subsequent protection switch by the
   PSL.  A protection LSP may protect either a segment of a working LSP
   (or a segment of a PMTG) or an entire working LSP (or PMTG).  A
   protection path is denoted by the sequence of LSRs that it
   traverses.

   Reverse Notification Tree:
   A point-to-multipoint tree that is rooted at a PML and follows
   theexact reverse path of the multipoint-to-point tree formed by
   merging ofworking paths (due to label merging).  The reverse
   notification tree allows the FIS to travel along its branches
   towards the PSLs, which perform the protection switch.

   Revertive:
   A switching option in which streams are automatically switched
   backfrom the protection path to the working path upon the
   restoration of the working path to a fault-free condition.

   Soft state:
   It has an associated time-to-live, and expires and may be discarded
   once that time is passed.  To avoid expiration the state must be
   periodically refreshed.  To reduce the overhead of the state

Various Authors  Informational _ Expires January 2001               48
             IP over Optical Networks: Summary of Issues     July 2000


   maintenance, the expiration period may be increased exponentially
   over time to a predefined maximum.  This way the longer a state has
   survived the fewer the number of refresh messages that are required.

   Traffic Trunk:
   An abstraction of traffic flow that follows the same path between
   two access points which allows some characteristics and attributes
   of the traffic to be parameterized.

   Upstream node:
   In an uni-directional lightpath, this is the node closer to the
   source.

   Working or Active Path:
   The portion of a working LSP that requires protection.  (A working
   path can be a segment of an LSP (or a segment of a PMTG) or a
   complete LSP (or PMTG).) The working path is denoted by the sequence
   of LSRs that it tranverses.



10.  References

   [Awduche99] D.  Awduche, Y.  Rekhter, J.  Drake, R.  Coltun, "Multi-
   Protocol Lambda Switching: Combining MPLS Traffic Engineering
   Control With Optical Crossconnects", Internet Draft draft-awduche-
   mpls-te-optical-01.txt, Work in Progress, November 1999.

   [Basak99] Debashis Basak, D.  Awduche, J.  Drake, Y.  Rekhter,
   "Multi-Protocol Lambda Switching: Issues in Combining MPLS Traffic
   Engineering Control With Optical Crossconnects", Internet Draft
   draft-basak-mpls-oxc-issues-01.txt, Work in Progress, February 2000.

   [Ceuppens00] L.  Ceuppens, et.  al.  "Performance Monitoring in
   Photonic Networks in support of MPL(ambda)S," Internet Draft draft-
   ceuppens-mpls-optical-00.txt, March 2000.

   [Chaudhuri00] S.  Chaudhuri, et.  al.  "Control of Lightpaths in an
   Optical Network," Internet Draft draft-chaudhuri-ip-olxc-control-
   00.txt, February 2000.

   [CRLDP] B.  Jamoussi, et.  al.  "Constraint-Based LSP Setup using
   LDP," Internet Draft draft-ietf-mpls-cr-ldp-03.txt, February 1999.

   [GSMP] A.  Doria, et.  al.  "General Switch Management Protocol
   V3,"Internet Draft draft-ietf-gsmp-05.txt, April 2000.

   [ISIS] ISO 10589, "Intermediate System to Intermediate System Intra-
   Domain Routing Exchange Protocol for use in Conjunction with the
   Protocol for Providing the Connectionless-mode Network Service".

   [ISISTE] Henk Smit, Tony Li, "IS-IS extensions for Traffic
   Engineering," Internet Draft, draft-ietf-isis-traffic-01.txt, work

Various Authors  Informational _ Expires January 2001               49
             IP over Optical Networks: Summary of Issues     July 2000


   in progress, May 1999

   [Jonathan00] J.P.  Lang, K.  Mitra, J.  Drake,, "Extensions to RSVP
   for Optical Networking", Internet Draft draft-lang-mpls-rsvp-oxc-
   00.txt, Work in Progress, March 2000.

   [Jamoussi99] L.  Andersson, A.  Fredette, B.  Jamoussi, et al.,
   "Constraint- Based LSP Setup using LDP", Internet Draft draft-tang-
   crldp-optical-00.txt, Work in Progress, January 1999.

   [Katz99] Katz, D.  and Yeung, D., "Traffic Engineering Extensions to
   OSPF," Internet Draft dtaft-katz-yeung-traffic-01.txt, Work in
   progress, October 1999.

   [KOMPELLA00-a] K.  Kompella et.  al., "Extensions to IS-IS/OSPF and
   RSVP  in support of MPL(ambda)S", Internet Draft draft-kompella-
   mpls-optical-00.txt, Work in Progress, February 2000 .

   [Kompella00-b] Kompella, K., Rekhter, Y., "Link Bundling in MPLS
   Traffic Engineering", draft-kompella-mpls-bundle-00.txt, February
   2000.

   [Lang00] J.P.  Lang, "Link Management Protocol (LMP)," Internet
   Draft draft-lang- mpls-lmp-00.txt, March 2000.

   [Luciani00] J.  Luciani, B.  Rajagopalan, D.  Awuduche, B.  Cain,
   Bilel Jamoussi, "IP Over Optical Networks - A Framework", Internet
   Draft draft-ip-optical-framework-00.txt, Work in Progress, March
   2000.

   [NHRP] Luciani, et.  al.  "NBMA Next Hop Resolution Protocol
   (NHRP)," RFC 2332, April 1998.

   [ODSI00] G.Bernstein et.  al., " Optical Domain Service Interconnect
   (ODSI) Functional Specification", ODSI Coalition, April 2000.

   [OSPF] Moy, J., _OSPF Version 2, RFC 1583, March 1994

   [Tang00] Z.B.  Tang et.  al.  "Extensions to CR-LDP for Path
   Establishment in Optical Networks," Internet Draft draft-tang-crldp-
   optical-00.txt, March 2000

   [WANG] G.Wang et.  al., "Extensions to OSPF/IS-IS for Optical
   Routing", Internet Draft draft-wang-ospf-isis-lambda-te-routing-
   00.txt, Work in Progress, March 2000.




11.  Author's Addresses




Various Authors  Informational _ Expires January 2001               50
             IP over Optical Networks: Summary of Issues     July 2000


   N. Chandhok, A. Durresi, R. Jagannathan, S. Seetharaman, K.
   vinodkrishnan
   Department of Computer and Information Science
   The Ohio State University
   2015 Neil Avenue, Columbus, OH 43210-1277, USA
   Phone: (614)-292-3989
   Email: {chandhok, durresi, rjaganna, seethara, vinodkri}@cis.ohio-
   state.edu

   Raj Jain
   Nayna Networks, Inc.
   157 Topaz Street
   Milpitas, CA 95035
   Phone: (408)-956-8000X309
   Email: raj@nayna.com


Full Copyright Statement

   "Copyright (C) The Internet Society (date).  All Rights Reserved.
   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implmentation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into.






















Various Authors  Informational _ Expires January 2001               51