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+Internet Engineering Task Force (IETF)                       Y. Lee, Ed.
+Request for Comments: 6566                                        Huawei
+Category: Informational                                G. Bernstein, Ed.
+ISSN: 2070-1721                                        Grotto Networking
+                                                                   D. Li
+                                                                  Huawei
+                                                           G. Martinelli
+                                                                   Cisco
+                                                              March 2012
+
+
+                     A Framework for the Control of
+     Wavelength Switched Optical Networks (WSONs) with Impairments
+
+Abstract
+
+   As an optical signal progresses along its path, it may be altered by
+   the various physical processes in the optical fibers and devices it
+   encounters.  When such alterations result in signal degradation,
+   these processes are usually referred to as "impairments".  These
+   physical characteristics may be important constraints to consider
+   when using a GMPLS control plane to support path setup and
+   maintenance in wavelength switched optical networks.
+
+   This document provides a framework for applying GMPLS protocols and
+   the Path Computation Element (PCE) architecture to support
+   Impairment-Aware Routing and Wavelength Assignment (IA-RWA) in
+   wavelength switched optical networks.  Specifically, this document
+   discusses key computing constraints, scenarios, and architectural
+   processes: routing, wavelength assignment, and impairment validation.
+   This document does not define optical data plane aspects; impairment
+   parameters; or measurement of, or assessment and qualification of, a
+   route; rather, it describes the architectural and information
+   components for protocol solutions.
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+Lee, et al.                   Informational                     [Page 1]
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+RFC 6566            Framework for Optical Impairments         March 2012
+
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+Status of This Memo
+
+   This document is not an Internet Standards Track specification; it is
+   published for informational purposes.
+
+   This document is a product of the Internet Engineering Task Force
+   (IETF).  It represents the consensus of the IETF community.  It has
+   received public review and has been approved for publication by the
+   Internet Engineering Steering Group (IESG).  Not all documents
+   approved by the IESG are a candidate for any level of Internet
+   Standard; see Section 2 of RFC 5741.
+
+   Information about the current status of this document, any errata,
+   and how to provide feedback on it may be obtained at
+   http://www.rfc-editor.org/info/rfc6566.
+
+Copyright Notice
+
+   Copyright (c) 2012 IETF Trust and the persons identified as the
+   document authors.  All rights reserved.
+
+   This document is subject to BCP 78 and the IETF Trust's Legal
+   Provisions Relating to IETF Documents
+   (http://trustee.ietf.org/license-info) in effect on the date of
+   publication of this document.  Please review these documents
+   carefully, as they describe your rights and restrictions with respect
+   to this document.  Code Components extracted from this document must
+   include Simplified BSD License text as described in Section 4.e of
+   the Trust Legal Provisions and are provided without warranty as
+   described in the Simplified BSD License.
+
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+Lee, et al.                   Informational                     [Page 2]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+Table of Contents
+
+   1. Introduction ....................................................3
+   2. Terminology .....................................................4
+   3. Applicability ...................................................6
+   4. Impairment-Aware Optical Path Computation .......................7
+      4.1. Optical Network Requirements and Constraints ...............8
+           4.1.1. Impairment-Aware Computation Scenarios ..............9
+           4.1.2. Impairment Computation and
+                  Information-Sharing Constraints ....................10
+           4.1.3. Impairment Estimation Process ......................11
+      4.2. IA-RWA Computation and Control Plane Architectures ........13
+           4.2.1. Combined Routing, WA, and IV .......................15
+           4.2.2. Separate Routing, WA, or IV ........................15
+           4.2.3. Distributed WA and/or IV ...........................16
+      4.3. Mapping Network Requirements to Architectures .............16
+   5. Protocol Implications ..........................................19
+      5.1. Information Model for Impairments .........................19
+      5.2. Routing ...................................................20
+      5.3. Signaling .................................................21
+      5.4. PCE .......................................................21
+           5.4.1. Combined IV & RWA ..................................21
+           5.4.2. IV-Candidates + RWA ................................22
+           5.4.3. Approximate IA-RWA + Separate Detailed-IV ..........24
+   6. Manageability and Operations ...................................25
+   7. Security Considerations ........................................26
+   8. References .....................................................27
+      8.1. Normative References ......................................27
+      8.2. Informative References ....................................27
+   9. Contributors ...................................................29
+
+1.  Introduction
+
+   Wavelength Switched Optical Networks (WSONs) are constructed from
+   subsystems that may include wavelength division multiplexed links,
+   tunable transmitters and receivers, Reconfigurable Optical Add/Drop
+   Multiplexers (ROADMs), wavelength converters, and electro-optical
+   network elements.  A WSON is a Wavelength Division Multiplexing
+   (WDM)-based optical network in which switching is performed
+   selectively based on the center wavelength of an optical signal.
+
+   As an optical signal progresses along its path, it may be altered by
+   the various physical processes in the optical fibers and devices it
+   encounters.  When such alterations result in signal degradation,
+   these processes are usually referred to as "impairments".  Optical
+   impairments accumulate along the path (without 3R regeneration
+   [G.680]) traversed by the signal.  They are influenced by the type of
+   fiber used, the types and placement of various optical devices, and
+
+
+
+Lee, et al.                   Informational                     [Page 3]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
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+   the presence of other optical signals that may share a fiber segment
+   along the signal's path.  The degradation of the optical signals due
+   to impairments can result in unacceptable bit error rates or even a
+   complete failure to demodulate and/or detect the received signal.
+
+   In order to provision an optical connection (an optical path) through
+   a WSON, a combination of path continuity, resource availability, and
+   impairment constraints must be met to determine viable and optimal
+   paths through the network.  The determination of appropriate paths is
+   known as Impairment-Aware Routing and Wavelength Assignment (IA-RWA).
+
+   Generalized Multi-Protocol Label Switching (GMPLS) [RFC3945] provides
+   a set of control plane protocols that can be used to operate networks
+   ranging from packet switch capable networks to those networks that
+   use time division multiplexing and WDM.  The Path Computation Element
+   (PCE) architecture [RFC4655] defines functional computation
+   components that can be used in cooperation with the GMPLS control
+   plane to compute and suggest appropriate paths.  [RFC4054] provides
+   an overview of optical impairments and their routing (path selection)
+   implications for GMPLS.  This document uses [G.680] and other ITU-T
+   Recommendations as references for the optical data plane aspects.
+
+   This document provides a framework for applying GMPLS protocols and
+   the PCE architecture to the control and operation of IA-RWA for
+   WSONs.  To aid in this evaluation, this document provides an overview
+   of the subsystems and processes that comprise WSONs and describes
+   IA-RWA models based on the corresponding ITU-T Recommendations, so
+   that the information requirements for use by GMPLS and PCE systems
+   can be identified.  This work will facilitate the development of
+   protocol extensions in support of IA-RWA within the GMPLS and PCE
+   protocol families.
+
+2.  Terminology
+
+   ADM: Add/Drop Multiplexer.  An optical device used in WDM networks
+      and composed of one or more line side ports and, typically, many
+      tributary ports.
+
+   Black Links: Black links refer to tributary interfaces where only
+      link characteristics are defined.  This approach enables
+      transverse compatibility at the single-channel point using a
+      direct wavelength-multiplexing configuration.
+
+   CWDM: Coarse Wavelength Division Multiplexing
+
+   DGD: Differential Group Delay
+
+   DWDM: Dense Wavelength Division Multiplexing
+
+
+
+Lee, et al.                   Informational                     [Page 4]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
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+   FOADM: Fixed Optical Add/Drop Multiplexer
+
+   GMPLS: Generalized Multi-Protocol Label Switching
+
+   IA-RWA: Impairment-Aware Routing and Wavelength Assignment
+
+   Line Side: In a WDM system, line side ports and links typically can
+      carry the full multiplex of wavelength signals, as compared to
+      tributary (add or drop ports), which typically carry a few
+      (typically one) wavelength signals.
+
+   NEs: Network Elements
+
+   OADMs: Optical Add/Drop Multiplexers
+
+   OSNR: Optical Signal-to-Noise Ratio
+
+   OXC: Optical Cross-Connect.  An optical switching element in which a
+      signal on any input port can reach any output port.
+
+   PCC: Path Computation Client.  Any client application requesting that
+      a path computation be performed by the Path Computation Element.
+
+   PCE: Path Computation Element.  An entity (component, application, or
+      network node) that is capable of computing a network path or route
+      based on a network graph and application of computational
+      constraints.
+
+   PCEP: PCE Communication Protocol.  The communication protocol between
+      a Path Computation Client and Path Computation Element.
+
+   PXC: Photonic Cross-Connect
+
+   Q-Factor: The Q-factor provides a qualitative description of the
+      receiver performance.  It is a function of the optical signal-to-
+      noise ratio.  The Q-factor suggests the minimum SNR (Signal-to-
+      Noise Ratio) required to obtain a specific bit error rate (BER)
+      for a given signal.
+
+   ROADM: Reconfigurable Optical Add/Drop Multiplexer.  A wavelength-
+      selective switching element featuring input and output line side
+      ports as well as add/drop tributary ports.
+
+   RWA: Routing and Wavelength Assignment
+
+   Transparent Network: A Wavelength Switched Optical Network that does
+      not contain regenerators or wavelength converters.
+
+
+
+
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+RFC 6566            Framework for Optical Impairments         March 2012
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+   Translucent Network:  A Wavelength Switched Optical Network that is
+      predominantly transparent but may also contain limited numbers of
+      regenerators and/or wavelength converters.
+
+   Tributary: A link or port on a WDM system that can carry
+      significantly less than the full multiplex of wavelength signals
+      found on the line side links/ports.  Typical tributary ports are
+      the add and drop ports on an ADM, and these support only a single
+      wavelength channel.
+
+   Wavelength Conversion/Converters: The process of converting an
+      information-bearing optical signal centered at a given wavelength
+      to information with "equivalent" content centered at a different
+      wavelength.  Wavelength conversion can be implemented via an
+      optical-electronic-optical (OEO) process or via a strictly optical
+      process.
+
+   WDM: Wavelength Division Multiplexing
+
+   Wavelength Switched Optical Networks (WSONs): WDM-based optical
+      networks in which switching is performed selectively based on the
+      center wavelength of an optical signal.
+
+3.  Applicability
+
+   There are deployment scenarios for WSONs where not all possible paths
+   will yield suitable signal quality.  There are multiple reasons;
+   below is a non-exhaustive list of examples:
+
+   o  WSONs are evolving and are using multi-degree optical cross-
+      connects in such a way that network topologies are changing from
+      rings (and interconnected rings) to general mesh.  Adding network
+      equipment such as amplifiers or regenerators to ensure that all
+      paths are feasible leads to an over-provisioned network.  Indeed,
+      even with over-provisioning, the network could still have some
+      infeasible paths.
+
+   o  Within a given network, the optical physical interface may change
+      over the network's life; e.g., the optical interfaces might be
+      upgraded to higher bitrates.  Such changes could result in paths
+      being unsuitable for the optical signal.  Moreover, the optical
+      physical interfaces are typically provisioned at various stages of
+      the network's life span, as needed, by traffic demands.
+
+   o  There are cases where a network is upgraded by adding new optical
+      cross-connects to increase network flexibility.  In such cases,
+      existing paths will have their feasibility modified while new
+      paths will need to have their feasibility assessed.
+
+
+
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+RFC 6566            Framework for Optical Impairments         March 2012
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+   o  With the recent bitrate increases from 10G to 40G and 100G over a
+      single wavelength, WSONs will likely be operated with a mix of
+      wavelengths at different bitrates.  This operational scenario will
+      impose impairment constraints due to different physical behavior
+      of different bitrates and associated modulation formats.
+
+   Not having an impairment-aware control plane for such networks will
+   require a more complex network design phase that needs to take into
+   account the evolving network status in terms of equipment and traffic
+   at the beginning stage.  In addition, network operations such as path
+   establishment will require significant pre-design via non-control-
+   plane processes, resulting in significantly slower network
+   provisioning.
+
+   It should be highlighted that the impact of impairments and use in
+   determination of path viability is not sufficiently well established
+   for general applicability [G.680]; it will depend on network
+   implementations.  The use of an impairment-aware control plane, and
+   the set of information distributed, will need to be evaluated on a
+   case-by-case scenario.
+
+4.  Impairment-Aware Optical Path Computation
+
+   The basic criterion for path selection is whether one can
+   successfully transmit the signal from a transmitter to a receiver
+   within a prescribed error tolerance, usually specified as a maximum
+   permissible BER.  This generally depends on the nature of the signal
+   transmitted between the sender and receiver and the nature of the
+   communications channel between the sender and receiver.  The optical
+   path utilized (along with the wavelength) determines the
+   communications channel.
+
+   The optical impairments incurred by the signal along the fiber and at
+   each optical network element along the path determine whether the BER
+   performance or any other measure of signal quality can be met for a
+   signal on a particular end-to-end path.
+
+   Impairment-aware path calculation also needs to take into account
+   when regeneration is used along the path.  [RFC6163] provides
+   background on the concept of optical translucent networks that
+   contain transparent elements and electro-optical elements such as OEO
+   regenerations.  In such networks, a generic light path can go through
+   a number of regeneration points.
+
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+
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+RFC 6566            Framework for Optical Impairments         March 2012
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+   Regeneration points could happen for two reasons:
+
+    (i) Wavelength conversion is performed in order to assist RWA in
+        avoiding wavelength blocking.  This is the impairment-free case
+        covered by [RFC6163].
+
+   (ii) The optical signal without regeneration would be too degraded to
+        meet end-to-end BER requirements.  This is the case when RWA
+        takes into consideration impairment estimation covered by this
+        document.
+
+   In the latter case, an optical path can be seen as a set of
+   transparent segments.  The calculation of optical impairments needs
+   to be reset at each regeneration point so each transparent segment
+   will have its own impairment evaluation.
+
+         +---+    +----+   +----+     +-----+     +----+    +---+
+         | I |----| N1 |---| N2 |-----| REG |-----| N3 |----| E |
+         +---+    +----+   +----+     +-----+     +----+    +---+
+
+         |<----------------------------->|<-------------------->|
+                    Segment 1                    Segment 2
+
+         Figure 1.  Optical Path as a Set of Transparent Segments
+
+   For example, Figure 1 represents an optical path from node I to
+   node E with a regeneration point, REG, in between.  This is feasible
+   from an impairment validation perspective if both segments (I, N1,
+   N2, REG) and (REG, N3, E) are feasible.
+
+4.1.  Optical Network Requirements and Constraints
+
+   This section examines the various optical network requirements and
+   constraints under which an impairment-aware optical control plane may
+   have to operate.  These requirements and constraints motivate the
+   IA-RWA architectural alternatives presented in Section 4.2.
+   Different optical network contexts can be broken into two main
+   criteria: (a) the accuracy required in the estimation of impairment
+   effects and (b) the constraints on the impairment estimation
+   computation and/or sharing of impairment information.
+
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+Lee, et al.                   Informational                     [Page 8]
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+RFC 6566            Framework for Optical Impairments         March 2012
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+4.1.1.  Impairment-Aware Computation Scenarios
+
+   A. No Concern for Impairments or Wavelength Continuity Constraints
+
+      This situation is covered by existing GMPLS with local wavelength
+      (label) assignment.
+
+   B. No Concern for Impairments, but Wavelength Continuity Constraints
+
+      This situation is applicable to networks designed such that every
+      possible path is valid for the signal types permitted on the
+      network.  In this case, impairments are only taken into account
+      during network design; after that -- for example, during optical
+      path computation -- they can be ignored.  This is the case
+      discussed in [RFC6163] where impairments may be ignored by the
+      control plane and only optical parameters related to signal
+      compatibility are considered.
+
+   C. Approximated Impairment Estimation
+
+      This situation is applicable to networks in which impairment
+      effects need to be considered but where there is a sufficient
+      margin such that impairment effects can be estimated via such
+      approximation techniques as link budgets and dispersion [G.680]
+      [G.Sup39].  The viability of optical paths for a particular class
+      of signals can be estimated using well-defined approximation
+      techniques [G.680] [G.Sup39].  This is generally known as the
+      linear case, where only linear effects are taken into account.
+      Note that adding or removing an optical signal on the path should
+      not render any of the existing signals in the network non-viable.
+      For example, one form of non-viability is the occurrence in
+      existing links of transients of sufficient magnitude to impact the
+      BER of existing signals.
+
+      Much work at ITU-T has gone into developing impairment models at
+      this level and at more detailed levels.  Impairment
+      characterization of network elements may be used to calculate
+      which paths are conformant with a specified BER for a particular
+      signal type.  In such a case, the impairment-aware (IA) path
+      computation can be combined with the RWA process to permit more
+      optimal IA-RWA computations.  Note that the IA path computation
+      may also take place in a separate entity, i.e., a PCE.
+
+
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+RFC 6566            Framework for Optical Impairments         March 2012
+
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+   D. Accurate Impairment Computation
+
+      This situation is applicable to networks in which impairment
+      effects must be more accurately computed.  For these networks, a
+      full computation and evaluation of the impact to any existing
+      paths need to be performed prior to the addition of a new path.
+      Currently, no impairment models are available from ITU-T, and this
+      scenario is outside the scope of this document.
+
+4.1.2.  Impairment Computation and Information-Sharing Constraints
+
+   In GMPLS, information used for path computation is standardized for
+   distribution amongst the elements participating in the control plane,
+   and any appropriately equipped PCE can perform path computation.  For
+   optical systems, this may not be possible.  This is typically due to
+   only portions of an optical system being subject to standardization.
+   In ITU-T Recommendations [G.698.1] and [G.698.2], which specify
+   single-channel interfaces to multi-channel DWDM systems, only the
+   single-channel interfaces (transmit and receive) are specified, while
+   the multi-channel links are not standardized.  These DWDM links are
+   referred to as "black links", since their details are not generally
+   available.  However, note that the overall impact of a black link at
+   the single-channel interface points is limited by [G.698.1] and
+   [G.698.2].
+
+   Typically, a vendor might use proprietary impairment models for DWDM
+   spans in order to estimate the validity of optical paths.  For
+   example, models of optical nonlinearities are not currently
+   standardized.  Vendors may also choose not to publish impairment
+   details for links or a set of network elements, in order not to
+   divulge their optical system designs.
+
+   In general, the impairment estimation/validation of an optical path
+   for optical networks with black links in the path could not be
+   performed by a general-purpose IA computation entity, since it would
+   not have access to or understand the black-link impairment
+   parameters.  However, impairment estimation (optical path validation)
+   could be performed by a vendor-specific IA computation entity.  Such
+   a vendor-specific IA computation entity could utilize standardized
+   impairment information imported from other network elements in these
+   proprietary computations.
+
+   In the following, the term "black links" will be used to describe
+   these computation and information-sharing constraints in optical
+   networks.  From the control plane perspective, the following options
+   are considered:
+
+
+
+
+
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+RFC 6566            Framework for Optical Impairments         March 2012
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+   1. The authority in control of the black links can furnish a list of
+      all viable paths between all viable node pairs to a computation
+      entity.  This information would be particularly useful as an input
+      to RWA optimization to be performed by another computation entity.
+      The difficulty here is that such a list of paths, along with any
+      wavelength constraints, could get unmanageably large as the size
+      of the network increases.
+
+   2. The authority in control of the black links could provide a
+      PCE-like entity a list of viable paths/wavelengths between two
+      requested nodes.  This is useful as an input to RWA optimizations
+      and can reduce the scaling issue previously mentioned.  Such a
+      PCE-like entity would not need to perform a full RWA computation;
+      i.e., it would not need to take into account current wavelength
+      availability on links.  Such an approach may require PCEP
+      extensions for both the request and response information.
+
+   3. The authority in control of the black links provides a PCE that
+      performs full IA-RWA services.  The difficulty here is that this
+      option requires the one authority to also become the sole source
+      of all RWA optimization algorithms.
+
+   In all of the above cases, it would be the responsibility of the
+   authority in control of the black links to import the shared
+   impairment information from the other NEs via the control plane or
+   other means as necessary.
+
+4.1.3.  Impairment Estimation Process
+
+   The impairment estimation process can be modeled through the
+   following functional blocks.  These blocks are independent of any
+   control plane architecture; that is, they can be implemented by the
+   same or by different control plane functions, as detailed in the
+   following sections.
+
+
+
+
+
+
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+RFC 6566            Framework for Optical Impairments         March 2012
+
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+                                               +-----------------+
+        +------------+        +-----------+    |  +------------+ |
+        |            |        |           |    |  |            | |
+        | Optical    |        | Optical   |    |  | Optical    | |
+        | Interface  |------->| Impairment|--->|  | Channel    | |
+        | (Transmit/ |        | Path      |    |  | Estimation | |
+        |  Receive)  |        |           |    |  |            | |
+        +------------+        +-----------+    |  +------------+ |
+                                               |        ||       |
+                                               |        ||       |
+                                               |    Estimation   |
+                                               |        ||       |
+                                               |        \/       |
+                                               |  +------------+ |
+                                               |  |  BER/      | |
+                                               |  |  Q Factor  | |
+                                               |  +------------+ |
+                                               +-----------------+
+
+   Starting from the functional block on the left, the optical interface
+   represents where the optical signal is transmitted or received and
+   defines the properties at the path endpoints.  Even the impairment-
+   free case, such as scenario B in Section 4.1.1, needs to consider a
+   minimum set of interface characteristics.  In such a case, only a few
+   parameters used to assess the signal compatibility will be taken into
+   account (see [RFC6163]).  For the impairment-aware case, these
+   parameters may be sufficient or not, depending on the accepted level
+   of approximation (scenarios C and D).  This functional block
+   highlights the need to consider a set of interface parameters during
+   the impairment validation process.
+
+   The "Optical Impairment Path" block represents the types of
+   impairments affecting a wavelength as it traverses the networks
+   through links and nodes.  In the case of a network where there are no
+   impairments (scenario A), this block will not be present.  Otherwise,
+   this function must be implemented in some way via the control plane.
+   Architectural alternatives to accomplish this are provided in
+   Section 4.2.  This block implementation (e.g., through routing,
+   signaling, or a PCE) may influence the way the control plane
+   distributes impairment information within the network.
+
+   The last block implements the decision function for path feasibility.
+   Depending on the IA level of approximation, this function can be more
+   or less complex.  For example, in the case of no IA approximation,
+   only the signal class compatibility will be verified.  In addition to
+   a feasible/not-feasible result, it may be worthwhile for decision
+   functions to consider the case in which paths would likely be
+   feasible within some degree of confidence.  The optical impairments
+
+
+
+Lee, et al.                   Informational                    [Page 12]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
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+   are usually not fixed values, as they may vary within ranges of
+   values according to the approach taken in the physical modeling
+   (worst-case, statistical, or based on typical values).  For example,
+   the utilization of the worst-case value for each parameter within the
+   impairment validation process may lead to marking some paths as not
+   feasible, while they are very likely to be, in reality, feasible.
+
+4.2.  IA-RWA Computation and Control Plane Architectures
+
+   From a control plane point of view, optical impairments are
+   additional constraints to the impairment-free RWA process described
+   in [RFC6163].  In IA-RWA, there are conceptually three general
+   classes of processes to be considered: Routing (R), Wavelength
+   Assignment (WA), and Impairment Validation (IV), i.e., estimation.
+
+   Impairment validation may come in many forms and may be invoked at
+   different levels of detail in the IA-RWA process.  All of the
+   variations of impairment validation discussed in this section are
+   based on scenario C ("Approximated Impairment Estimation") as
+   discussed in Section 4.1.1.  From a process point of view, the
+   following three forms of impairment validation will be considered:
+
+   o  IV-Candidates
+
+      In this case, an IV process furnishes a set of paths between two
+      nodes along with any wavelength restrictions, such that the paths
+      are valid with respect to optical impairments.  These paths and
+      wavelengths may not actually be available in the network, due to
+      its current usage state.  This set of paths could be returned in
+      response to a request for a set of at most K valid paths between
+      two specified nodes.  Note that such a process never directly
+      discloses optical impairment information.  Note also that this
+      case includes any paths between the source and destination that
+      may have been "pre-validated".
+
+      In this case, the control plane simply makes use of candidate
+      paths but does not have any optical impairment information.
+      Another option is when the path validity is assessed within the
+      control plane.  The following cases highlight this situation.
+
+   o  IV-Approximate Verification
+
+      Here, approximation methods are used to estimate the impairments
+      experienced by a signal.  Impairments are typically approximated
+      by linear and/or statistical characteristics of individual or
+      combined components and fibers along the signal path.
+
+
+
+
+
+Lee, et al.                   Informational                    [Page 13]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+   o  IV-Detailed Verification
+
+      In this case, an IV process is given a particular path and
+      wavelength through an optical network and is asked to verify
+      whether the overall quality objectives for the signal over this
+      path can be met.  Note that such a process never directly
+      discloses optical impairment information.
+
+   The next two cases refer to the way an impairment validation
+   computation can be performed from a decision-making point of view.
+
+   o  IV-Centralized
+
+      In this case, impairments to a path are computed at a single
+      entity.  The information concerning impairments, however, may
+      still be gathered from network elements.  Depending on how
+      information is gathered, this may put additional requirements on
+      routing protocols.  This topic will be detailed in later sections.
+
+   o  IV-Distributed
+
+      In the distributed IV process, approximate degradation measures
+      such as OSNR, dispersion, DGD, etc., may be accumulated along the
+      path via signaling.  Each node on the path may already perform
+      some part of the impairment computation (i.e., distributed).  When
+      the accumulated measures reach the destination node, a decision on
+      the impairment validity of the path can be made.  Note that such a
+      process would entail revealing an individual network element's
+      impairment information, but it does not generally require
+      distributing optical parameters to the entire network.
+
+   The control plane must not preclude the possibility of concurrently
+   performing one or all of the above cases in the same network.  For
+   example, there could be cases where a certain number of paths are
+   already pre-validated (IV-Candidates), so the control plane may set
+   up one of those paths without requesting any impairment validation
+   procedure.  On the same network, however, the control plane may
+   compute a path outside the set of IV-Candidates for which an
+   impairment evaluation can be necessary.
+
+   The following subsections present three major classes of IA-RWA path
+   computation architectures and review some of their respective
+   advantages and disadvantages.
+
+
+
+
+
+
+
+
+Lee, et al.                   Informational                    [Page 14]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+4.2.1.  Combined Routing, WA, and IV
+
+   From the point of view of optimality, reasonably good IA-RWA
+   solutions can be achieved if the PCE can conceptually/algorithmically
+   combine the processes of routing, wavelength assignment, and
+   impairment validation.
+
+   Such a combination can take place if the PCE is given (a) the
+   impairment-free WSON information as discussed in [RFC6163] and (b)
+   impairment information to validate potential paths.
+
+4.2.2.  Separate Routing, WA, or IV
+
+   Separating the processes of routing, WA, and/or IV can reduce the
+   need for the sharing of different types of information used in path
+   computation.  This was discussed for routing, separate from WA, in
+   [RFC6163].  In addition, as was discussed in Section 4.1.2, some
+   impairment information may not be shared, and this may lead to the
+   need to separate IV from RWA.  In addition, if IV needs to be done at
+   a high level of precision, it may be advantageous to offload this
+   computation to a specialized server.
+
+   The following conceptual architectures belong in this general
+   category:
+
+   o  R + WA + IV
+      separate routing, wavelength assignment, and impairment
+      validation.
+
+   o  R + (WA & IV)
+      routing separate from a combined wavelength assignment and
+      impairment validation process.  Note that impairment validation is
+      typically wavelength dependent.  Hence, combining WA with IV can
+      lead to improved efficiency.
+
+   o  (RWA) + IV
+      combined routing and wavelength assignment with a separate
+      impairment validation process.
+
+   Note that the IV process may come before or after the RWA processes.
+   If RWA comes first, then IV is just rendering a yes/no decision on
+   the selected path and wavelength.  If IV comes first, it would need
+   to furnish a list of possible (valid with respect to impairments)
+   routes and wavelengths to the RWA processes.
+
+
+
+
+
+
+
+Lee, et al.                   Informational                    [Page 15]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+4.2.3.  Distributed WA and/or IV
+
+   In the non-impairment RWA situation [RFC6163], it was shown that a
+   distributed WA process carried out via signaling can eliminate the
+   need to distribute wavelength availability information via an
+   interior gateway protocol (IGP).  A similar approach can allow for
+   the distributed computation of impairment effects and avoid the need
+   to distribute impairment characteristics of network elements and
+   links by routing protocols or by other means.  Therefore, the
+   following conceptual options belong to this category:
+
+   o  RWA + D(IV)
+      combined routing and wavelength assignment and distributed
+      impairment validation.
+
+   o  R + D(WA & IV)
+      routing separate from a distributed wavelength assignment and
+      impairment validation process.
+
+   Distributed impairment validation for a prescribed network path
+   requires that the effects of impairments be calculated by approximate
+   models with cumulative quality measures such as those given in
+   [G.680].  The protocol encoding of the impairment-related information
+   from [G.680] would need to be agreed upon.
+
+   If distributed WA is being done at the same time as distributed IV,
+   then it is necessary to accumulate impairment-related information for
+   all wavelengths that could be used.  The amount of information is
+   reduced somewhat as potential wavelengths are discovered to be in use
+   but could be a significant burden for lightly loaded networks with
+   high channel counts.
+
+4.3.  Mapping Network Requirements to Architectures
+
+   Figure 2 shows process flows for the three main architectural
+   alternatives to IA-RWA that apply when approximate impairment
+   validation is sufficient.  Figure 3 shows process flows for the two
+   main architectural alternatives that apply when detailed impairment
+   verification is required.
+
+
+
+
+
+
+
+
+
+
+
+
+Lee, et al.                   Informational                    [Page 16]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+                  +-----------------------------------+
+                  |   +--+     +-------+     +--+     |
+                  |   |IV|     |Routing|     |WA|     |
+                  |   +--+     +-------+     +--+     |
+                  |                                   |
+                  |        Combined Processes         |
+                  +-----------------------------------+
+                                  (a)
+
+           +--------------+      +----------------------+
+           | +----------+ |      | +-------+    +--+    |
+           | |    IV    | |      | |Routing|    |WA|    |
+           | |Candidates| |----->| +-------+    +--+    |
+           | +----------+ |      |  Combined Processes  |
+           +--------------+      +----------------------+
+                                  (b)
+
+            +-----------+        +----------------------+
+            | +-------+ |        |    +--+    +--+      |
+            | |Routing| |------->|    |WA|    |IV|      |
+            | +-------+ |        |    +--+    +--+      |
+            +-----------+        | Distributed Processes|
+                                 +----------------------+
+                                  (c)
+
+    Figure 2.  Process Flows for the Three Main Approximate Impairment
+                        Architectural Alternatives
+
+   The advantages, requirements, and suitability of these options are as
+   follows:
+
+   o  Combined IV & RWA process
+
+      This alternative combines RWA and IV within a single computation
+      entity, enabling highest potential optimality and efficiency in
+      IA-RWA.  This alternative requires that the computation entity
+      have impairment information as well as non-impairment RWA
+      information.  This alternative can be used with black links but
+      would then need to be provided by the authority controlling the
+      black links.
+
+   o  IV-Candidates + RWA process
+
+      This alternative allows separation of impairment information into
+      two computation entities while still maintaining a high degree of
+      potential optimality and efficiency in IA-RWA.  The IV-Candidates
+      process needs to have impairment information from all optical
+      network elements, while the RWA process needs to have
+
+
+
+Lee, et al.                   Informational                    [Page 17]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+      non-impairment RWA information from the network elements.  This
+      alternative can be used with black links, but the authority in
+      control of the black links would need to provide the functionality
+      of the IV-Candidates process.  Note that this is still very
+      useful, since the algorithmic areas of IV and RWA are very
+      different and conducive to specialization.
+
+   o  Routing + Distributed WA and IV
+
+      In this alternative, a signaling protocol may be extended and
+      leveraged in the wavelength assignment and impairment validation
+      processes.  Although this doesn't enable as high a potential
+      degree of optimality as (a) or (b), it does not require
+      distribution of either link wavelength usage or link/node
+      impairment information.  Note that this is most likely not
+      suitable for black links.
+
+             +-----------------------------------+     +------------+
+             | +-----------+  +-------+    +--+  |     | +--------+ |
+             | |    IV     |  |Routing|    |WA|  |     | |  IV    | |
+             | |Approximate|  +-------+    +--+  |---->| |Detailed| |
+             | +-----------+                     |     | +--------+ |
+             |        Combined Processes         |     |            |
+             +-----------------------------------+     +------------+
+                                      (a)
+
+       +--------------+      +----------------------+     +------------+
+       | +----------+ |      | +-------+    +--+    |     | +--------+ |
+       | |    IV    | |      | |Routing|    |WA|    |---->| |  IV    | |
+       | |Candidates| |----->| +-------+    +--+    |     | |Detailed| |
+       | +----------+ |      |  Combined Processes  |     | +--------+ |
+       +--------------+      +----------------------+     |            |
+                                      (b)                 +------------+
+
+        Figure 3.  Process Flows for the Two Main Detailed Impairment
+                       Validation Architectural Options
+
+      The advantages, requirements, and suitability of these detailed
+      validation options are as follows:
+
+   o  Combined Approximate IV & RWA + Detailed-IV
+
+      This alternative combines RWA and approximate IV within a single
+      computation entity, enabling the highest potential optimality and
+      efficiency in IA-RWA while keeping a separate entity performing
+      detailed impairment validation.  In the case of black links, the
+      authority controlling the black links would need to provide all
+      functionality.
+
+
+
+Lee, et al.                   Informational                    [Page 18]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+   o  IV-Candidates + RWA + Detailed-IV
+
+      This alternative allows separation of approximate impairment
+      information into a computation entity while still maintaining a
+      high degree of potential optimality and efficiency in IA-RWA;
+      then, a separate computation entity performs detailed impairment
+      validation.  Note that detailed impairment estimation is not
+      standardized.
+
+5.  Protocol Implications
+
+   The previous IA-RWA architectural alternatives and process flows make
+   differing demands on a GMPLS/PCE-based control plane.  This section
+   discusses the use of (a) an impairment information model, (b) the PCE
+   as computation entity assuming the various process roles and
+   consequences for PCEP, (c) possible extensions to signaling, and
+   (d) possible extensions to routing.  This document is providing this
+   evaluation to aid protocol solutions work.  The protocol
+   specifications may deviate from this assessment.  The assessment of
+   the impacts to the control plane for IA-RWA is summarized in
+   Figure 4.
+
+       +--------------------+-----+-----+------------+---------+
+       | IA-RWA Option      | PCE | Sig | Info Model | Routing |
+       +--------------------+-----+-----+------------+---------+
+       |          Combined  | Yes | No  |    Yes     |   Yes   |
+       |          IV & RWA  |     |     |            |         |
+       +--------------------+-----+-----+------------+---------+
+       |     IV-Candidates  | Yes | No  |    Yes     |   Yes   |
+       |         + RWA      |     |     |            |         |
+       +--------------------+-----+-----+------------+---------+
+       |    Routing +       | No  | Yes |    Yes     |   No    |
+       |Distributed IV, RWA |     |     |            |         |
+       +--------------------+-----+-----+------------+---------+
+
+     Figure 4.  IA-RWA Architectural Options and Control Plane Impacts
+
+5.1.  Information Model for Impairments
+
+   As previously discussed, most IA-RWA scenarios rely, to a greater or
+   lesser extent, on a common impairment information model.  A number of
+   ITU-T Recommendations cover both detailed and approximate impairment
+   characteristics of fibers, a variety of devices, and a variety of
+   subsystems.  An impairment model that can be used as a guideline for
+   optical network elements and assessment of path viability is given
+   in [G.680].
+
+
+
+
+
+Lee, et al.                   Informational                    [Page 19]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+   It should be noted that the current version of [G.680] is limited to
+   networks composed of a single WDM line system vendor combined with
+   OADMs and/or PXCs from potentially multiple other vendors.  This is
+   known as "situation 1" and is shown in Figure 1-1 of [G.680].  It is
+   planned in the future that [G.680] will include networks
+   incorporating line systems from multiple vendors, as well as OADMs
+   and/or PXCs from potentially multiple other vendors.  This is known
+   as "situation 2" and is shown in Figure 1-2 of [G.680].
+
+   For the case of distributed IV, this would require more than an
+   impairment information model.  It would need a common impairment
+   "computation" model.  In the distributed IV case, one needs to
+   standardize the accumulated impairment measures that will be conveyed
+   and updated at each node.  Section 9 of [G.680] provides guidance in
+   this area, with specific formulas given for OSNR, residual
+   dispersion, polarization mode dispersion/polarization-dependent loss,
+   and effects of channel uniformity.  However, specifics of what
+   intermediate results are kept and in what form would need to be
+   standardized for interoperability.  As noted in [G.680], this
+   information may possibly not be sufficient, and in such a case, the
+   applicability would be network dependent.
+
+5.2.  Routing
+
+   Different approaches to path/wavelength impairment validation give
+   rise to different demands placed on GMPLS routing protocols.  In the
+   case where approximate impairment information is used to validate
+   paths, GMPLS routing may be used to distribute the impairment
+   characteristics of the network elements and links based on the
+   impairment information model previously discussed.
+
+   Depending on the computational alternative, the routing protocol may
+   need to advertise information necessary to the impairment validation
+   process.  This can potentially cause scalability issues, due to the
+   high volume of data that need to be advertised.  Such issues can be
+   addressed by separating data that need to be advertised only rarely
+   from data that need to be advertised more frequently, or by adopting
+   other forms of awareness solutions as described in previous sections
+   (e.g., a centralized and/or external IV entity).
+
+   In terms of scenario C in Section 4.1.1, the model defined by [G.680]
+   will apply, and the routing protocol will need to gather information
+   required for such computations.
+
+   In the case of distributed IV, no new demands would be placed on the
+   routing protocol.
+
+
+
+
+
+Lee, et al.                   Informational                    [Page 20]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+5.3.  Signaling
+
+   The largest impacts on signaling occur in the cases where distributed
+   impairment validation is performed.  In this case, it is necessary to
+   accumulate impairment information, as previously discussed.  In
+   addition, since the characteristics of the signal itself, such as
+   modulation type, can play a major role in the tolerance of
+   impairments, this type of information will need to be implicitly or
+   explicitly signaled so that an impairment validation decision can be
+   made at the destination node.
+
+   It remains for further study whether it may be beneficial to include
+   additional information to a connection request, such as desired
+   egress signal quality (defined in some appropriate sense) in
+   non-distributed IV scenarios.
+
+5.4.  PCE
+
+   In Section 4.2, a number of computational architectural alternatives
+   were given that could be used to meet the various requirements and
+   constraints of Section 4.1.  Here, the focus is on how these
+   alternatives could be implemented via either a single PCE or a set of
+   two or more cooperating PCEs, and the impacts on the PCEP.  This
+   document provides this evaluation to aid solutions work.  The
+   protocol specifications may deviate from this assessment.
+
+5.4.1.  Combined IV & RWA
+
+   In this situation, shown in Figure 2(a), a single PCE performs all of
+   the computations needed for IA-RWA.
+
+   o  Traffic Engineering (TE) Database requirements: WSON topology and
+      switching capabilities, WSON WDM link wavelength utilization, and
+      WSON impairment information.
+
+   o  PCC to PCE Request Information: Signal characteristics/type,
+      required quality, source node, and destination node.
+
+   o  PCE to PCC Reply Information: If the computations completed
+      successfully, then the PCE returns the path and its assigned
+      wavelength.  If the computations could not complete successfully,
+      it would be potentially useful to know why.  At a minimum, it is
+      of interest to know if this was due to lack of wavelength
+      availability, impairment considerations, or both.  The information
+      to be conveyed is for further study.
+
+
+
+
+
+
+Lee, et al.                   Informational                    [Page 21]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+5.4.2.  IV-Candidates + RWA
+
+   In this situation, as shown in Figure 2(b), two separate processes
+   are involved in the IA-RWA computation.  This requires two
+   cooperating path computation entities: one for the IV-Candidates
+   process and another for the RWA process.  In addition, the overall
+   process needs to be coordinated.  This could be done with yet another
+   PCE, or this functionality could be added to one of a number of
+   previously defined entities.  This later option requires that the RWA
+   entity also act as the overall process coordinator.  The roles,
+   responsibilities, and information requirements for these two
+   entities, when instantiated as PCEs, are given below.
+
+   RWA and Coordinator PCE (RWA-Coord PCE):
+
+      The RWA-Coord PCE is responsible for interacting with the PCC and
+      for utilizing the IV-Candidates PCE as needed during RWA
+      computations.  In particular, it needs to know that it is to use
+      the IV-Candidates PCE to obtain a potential set of routes and
+      wavelengths.
+
+      o  TE Database requirements: WSON topology and switching
+         capabilities, and WSON WDM link wavelength utilization (no
+         impairment information).
+
+      o  PCC to RWA PCE request: same as in the combined case.
+
+      o  RWA PCE to PCC reply: same as in the combined case.
+
+      o  RWA PCE to IV-Candidates PCE request: The RWA PCE asks for a
+         set of at most K routes, along with acceptable wavelengths
+         between nodes specified in the original PCC request.
+
+      o  IV-Candidates PCE reply to RWA PCE: The IV-Candidates PCE
+         returns a set of at most K routes, along with acceptable
+         wavelengths between nodes specified in the RWA PCE request.
+
+   IV-Candidates PCE:
+
+      The IV-Candidates PCE is responsible for impairment-aware path
+      computation.  It need not take into account current link
+      wavelength utilization, but this is not prohibited.  The
+      IV-Candidates PCE is only required to interact with the RWA PCE as
+      indicated above, and not the initiating PCC.  Note: The
+      RWA-Coord PCE is also a PCC with respect to the IV-Candidate.
+
+
+
+
+
+
+Lee, et al.                   Informational                    [Page 22]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+      o  TE Database requirements: WSON topology and switching
+         capabilities, and WSON impairment information (no information
+         link wavelength utilization required).
+
+   Figure 5 shows a sequence diagram for the possible interactions
+   between the PCC, RWA-Coord PCE, and IV-Candidates PCE.
+
+      +---+                +-------------+          +-----------------+
+      |PCC|                |RWA-Coord PCE|          |IV-Candidates PCE|
+      +-+-+                +------+------+          +---------+-------+
+        ...___     (a)            |                           |
+        |     ````---...____      |                           |
+        |                   ```-->|                           |
+        |                         |                           |
+        |                         |--..___    (b)             |
+        |                         |       ```---...___        |
+        |                         |                   ```---->|
+        |                         |                           |
+        |                         |                           |
+        |                         |           (c)       ___...|
+        |                         |       ___....---''''      |
+        |                         |<--''''                    |
+        |                         |                           |
+        |                         |                           |
+        |          (d)      ___...|                           |
+        |      ___....---'''      |                           |
+        |<--'''                   |                           |
+        |                         |                           |
+        |                         |                           |
+
+     Figure 5.  Sequence Diagram for the Interactions between the PCC,
+                   RWA-Coord PCE, and IV-Candidates PCE
+
+   In step (a), the PCC requests a path that meets specified quality
+   constraints between two nodes (A and Z) for a given signal
+   represented either by a specific type or a general class with
+   associated parameters.  In step (b), the RWA-Coord PCE requests up to
+   K candidate paths between nodes A and Z, and associated acceptable
+   wavelengths.  The term "K candidate paths" is associated with the k
+   shortest path algorithm.  It refers to an algorithm that finds
+   multiple k short paths connecting the source and the destination in a
+   graph allowing repeated vertices and edges in the paths.  See details
+   in [Eppstein].
+
+
+
+
+
+
+
+
+Lee, et al.                   Informational                    [Page 23]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+   In step (c), the IV-Candidates PCE returns this list to the
+   RWA-Coord PCE, which then uses this set of paths and wavelengths as
+   input (e.g., a constraint) to its RWA computation.  In step (d), the
+   RWA-Coord PCE returns the overall IA-RWA computation results to
+   the PCC.
+
+5.4.3.  Approximate IA-RWA + Separate Detailed-IV
+
+   Previously, Figure 3 showed two cases where a separate detailed
+   impairment validation process could be utilized.  It is possible to
+   place the detailed validation process into a separate PCE.  Assuming
+   that a different PCE assumes a coordinating role and interacts with
+   the PCC, it is possible to keep the interactions with this separate
+   IV-Detailed PCE very simple.  Note that, from a message flow
+   perspective, there is some inefficiency as a result of separating the
+   IV-Candidates PCE from the IV-Detailed PCE in order to achieve a high
+   degree of potential optimality.
+
+   IV-Detailed PCE:
+
+   o  TE Database requirements: The IV-Detailed PCE will need optical
+      impairment information, WSON topology, and, possibly, WDM link
+      wavelength usage information.  This document puts no restrictions
+      on the type of information that may be used in these computations.
+
+   o  RWA-Coord PCE to IV-Detailed PCE request: The RWA-Coord PCE will
+      furnish signal characteristics, quality requirements, path, and
+      wavelength to the IV-Detailed PCE.
+
+   o  IV-Detailed PCE to RWA-Coord PCE reply: The reply is essentially a
+      yes/no decision as to whether the requirements could actually be
+      met.  In the case where the impairment validation fails, it would
+      be helpful to convey information related to the cause or to
+      quantify the failure, e.g., so that a judgment can be made
+      regarding whether to try a different signal or adjust signal
+      parameters.
+
+   Figure 6 shows a sequence diagram for the interactions corresponding
+   to the process shown in Figure 3(b).  This involves interactions
+   between the PCC, RWA PCE (acting as coordinator), IV-Candidates PCE,
+   and IV-Detailed PCE.
+
+   In step (a), the PCC requests a path that meets specified quality
+   constraints between two nodes (A and Z) for a given signal
+   represented either by a specific type or a general class with
+   associated parameters.  In step (b), the RWA-Coord PCE requests up to
+   K candidate paths between nodes A and Z, and associated acceptable
+   wavelengths.  In step (c), the IV-Candidates PCE returns this list to
+
+
+
+Lee, et al.                   Informational                    [Page 24]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+   the RWA-Coord PCE, which then uses this set of paths and wavelengths
+   as input (e.g., a constraint) to its RWA computation.  In step (d),
+   the RWA-Coord PCE requests a detailed verification of the path and
+   wavelength that it has computed.  In step (e), the IV-Detailed PCE
+   returns the results of the validation to the RWA-Coord PCE.  Finally,
+   in step (f), the RWA-Coord PCE returns the final results (either a
+   path and wavelength, or a cause for the failure to compute a path and
+   wavelength) to the PCC.
+
+                +----------+      +--------------+      +------------+
+    +---+       |RWA-Coord |      |IV-Candidates |      |IV-Detailed |
+    |PCC|       |   PCE    |      |     PCE      |      |    PCE     |
+    +-+-+       +----+-----+      +------+-------+      +-----+------+
+      |.._   (a)     |                   |                    |
+      |   ``--.__    |                   |                    |
+      |          `-->|                   |                    |
+      |              |        (b)        |                    |
+      |              |--....____         |                    |
+      |              |          ````---.>|                    |
+      |              |                   |                    |
+      |              |         (c)  __..-|                    |
+      |              |     __..---''     |                    |
+      |              |<--''              |                    |
+      |              |                                        |
+      |              |...._____          (d)                  |
+      |              |         `````-----....._____           |
+      |              |                             `````----->|
+      |              |                                        |
+      |              |                 (e)          _____.....+
+      |              |          _____.....-----'''''          |
+      |              |<----'''''                              |
+      |     (f)   __.|                                        |
+      |    __.--''   |
+      |<-''          |
+      |              |
+
+     Figure 6.  Sequence Diagram for the Interactions between the PCC,
+           RWA-Coord PCE, IV-Candidates PCE, and IV-Detailed PCE
+
+6.  Manageability and Operations
+
+   The issues concerning manageability and operations are beyond the
+   scope of this document.  The details of manageability and operational
+   issues will have to be deferred to future protocol implementations.
+
+
+
+
+
+
+
+Lee, et al.                   Informational                    [Page 25]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+   On a high level, the GMPLS-routing-based architecture discussed in
+   Section 5.2 may have to deal with how to resolve potential scaling
+   issues associated with disseminating a large amount of impairment
+   characteristics of the network elements and links.
+
+   From a scaling point of view, the GMPLS-signaling-based architecture
+   discussed in Section 5.3 would be more scalable than other
+   alternatives, as this architecture would avoid the dissemination of a
+   large amount of data to the networks.  This benefit may come,
+   however, at the expense of potentially inefficient use of network
+   resources.
+
+   The PCE-based architectures discussed in Section 5.4 would have to
+   consider operational complexity when implementing options that
+   require the use of multiple PCE servers.  The most serious case is
+   the option discussed in Section 5.4.3 ("Approximate IA-RWA + Separate
+   Detailed-IV").  The combined IV & RWA option (which was discussed in
+   Section 5.4.1), on the other hand, is simpler to operate than are
+   other alternatives, as one PCE server handles all functionality;
+   however, this option may suffer from a heavy computation and
+   processing burden compared to other alternatives.
+
+   Interoperability may be a hurdle to overcome when trying to agree on
+   some impairment parameters, especially those that are associated with
+   the black links.  This work has been in progress in ITU-T and needs
+   some more time to mature.
+
+7.  Security Considerations
+
+   This document discusses a number of control plane architectures that
+   incorporate knowledge of impairments in optical networks.  If such an
+   architecture is put into use within a network, it will by its nature
+   contain details of the physical characteristics of an optical
+   network.  Such information would need to be protected from
+   intentional or unintentional disclosure, similar to other network
+   information used within intra-domain protocols.
+
+   This document does not require changes to the security models within
+   GMPLS and associated protocols.  That is, the OSPF-TE, RSVP-TE, and
+   PCEP security models could be operated unchanged.  However,
+   satisfying the requirements for impairment information dissemination
+   using the existing protocols may significantly affect the loading of
+   those protocols and may make the operation of the network more
+   vulnerable to active attacks such as injections, impersonation, and
+   man-in-the-middle attacks.  Therefore, additional care may be
+   required to ensure that the protocols are secure in the impairment-
+   aware WSON environment.
+
+
+
+
+Lee, et al.                   Informational                    [Page 26]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+   Furthermore, the additional information distributed in order to
+   address impairment information represents a disclosure of network
+   capabilities that an operator may wish to keep private.
+   Consideration should be given to securing this information.  For a
+   general discussion on MPLS- and GMPLS-related security issues, see
+   the MPLS/GMPLS security framework [RFC5920] and, in particular, text
+   detailing security issues when the control plane is physically
+   separated from the data plane.
+
+8.  References
+
+8.1.  Normative References
+
+   [G.680]     ITU-T Recommendation G.680, "Physical transfer functions
+               of optical network elements", July 2007.
+
+   [RFC3945]   Mannie, E., Ed., "Generalized Multi-Protocol Label
+               Switching (GMPLS) Architecture", RFC 3945, October 2004.
+
+   [RFC4655]   Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
+               Computation Element (PCE)-Based Architecture", RFC 4655,
+               August 2006.
+
+8.2.  Informative References
+
+   [Eppstein]  Eppstein, D., "Finding the k shortest paths", 35th IEEE
+               Symposium on Foundations of Computer Science, Santa Fe,
+               pp. 154-165, 1994.
+
+   [G.698.1]   ITU-T Recommendation G.698.1, "Multichannel DWDM
+               applications with single-channel optical interfaces",
+               November 2009.
+
+   [G.698.2]   ITU-T Recommendation G.698.2, "Amplified multichannel
+               dense wavelength division multiplexing applications with
+               single channel optical interfaces", November 2009.
+
+   [G.Sup39]   ITU-T Series G Supplement 39, "Optical system design and
+               engineering considerations", February 2006.
+
+   [RFC4054]   Strand, J., Ed., and A. Chiu, Ed., "Impairments and Other
+               Constraints on Optical Layer Routing", RFC 4054,
+               May 2005.
+
+
+
+
+
+
+
+
+Lee, et al.                   Informational                    [Page 27]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+   [RFC5920]   Fang, L., Ed., "Security Framework for MPLS and GMPLS
+               Networks", RFC 5920, July 2010.
+
+   [RFC6163]   Lee, Y., Ed., Bernstein, G., Ed., and W. Imajuku,
+               "Framework for GMPLS and Path Computation Element (PCE)
+               Control of Wavelength Switched Optical Networks (WSONs)",
+               RFC 6163, April 2011.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Lee, et al.                   Informational                    [Page 28]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+9.  Contributors
+
+   Ming Chen
+   Huawei Technologies Co., Ltd.
+   F3-5-B R&D Center, Huawei Base
+   Bantian, Longgang District
+   Shenzhen  518129
+   P.R. China
+
+   Phone: +86-755-28973237
+   EMail: mchen@huawei.com
+
+
+   Rebecca Han
+   Huawei Technologies Co., Ltd.
+   F3-5-B R&D Center, Huawei Base
+   Bantian, Longgang District
+   Shenzhen  518129
+   P.R.China
+
+   Phone: +86-755-28973237
+   EMail: hanjianrui@huawei.com
+
+
+   Gabriele Galimberti
+   Cisco
+   Via Philips 12
+   20052 Monza
+   Italy
+
+   Phone: +39 039 2091462
+   EMail: ggalimbe@cisco.com
+
+
+   Alberto Tanzi
+   Cisco
+   Via Philips 12
+   20052 Monza
+   Italy
+
+   Phone: +39 039 2091469
+   EMail: altanzi@cisco.com
+
+
+
+
+
+
+
+
+
+Lee, et al.                   Informational                    [Page 29]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+   David Bianchi
+   Cisco
+   Via Philips 12
+   20052 Monza
+   Italy
+
+   EMail: davbianc@cisco.com
+
+
+   Moustafa Kattan
+   Cisco
+   Dubai  500321
+   United Arab Emirates
+
+   EMail: mkattan@cisco.com
+
+
+   Dirk Schroetter
+   Cisco
+
+   EMail: dschroet@cisco.com
+
+
+   Daniele Ceccarelli
+   Ericsson
+   Via A. Negrone 1/A
+   Genova - Sestri Ponente
+   Italy
+
+   EMail: daniele.ceccarelli@ericsson.com
+
+
+   Elisa Bellagamba
+   Ericsson
+   Farogatan 6
+   Kista  164 40
+   Sweden
+
+   EMail: elisa.bellagamba@ericsson.com
+
+
+   Diego Caviglia
+   Ericsson
+   Via A. Negrone 1/A
+   Genova - Sestri Ponente
+   Italy
+
+   EMail: diego.caviglia@ericsson.com
+
+
+
+Lee, et al.                   Informational                    [Page 30]
+
+RFC 6566            Framework for Optical Impairments         March 2012
+
+
+Authors' Addresses
+
+   Young Lee (editor)
+   Huawei Technologies
+   5340 Legacy Drive, Building 3
+   Plano, TX  75024
+   USA
+
+   Phone: (469) 277-5838
+   EMail: leeyoung@huawei.com
+
+
+   Greg M. Bernstein (editor)
+   Grotto Networking
+   Fremont, CA
+   USA
+
+   Phone: (510) 573-2237
+   EMail: gregb@grotto-networking.com
+
+
+   Dan Li
+   Huawei Technologies Co., Ltd.
+   F3-5-B R&D Center, Huawei Base
+   Bantian, Longgang District
+   Shenzhen  518129
+   P.R. China
+
+   Phone: +86-755-28973237
+   EMail: danli@huawei.com
+
+
+   Giovanni Martinelli
+   Cisco
+   Via Philips 12
+   20052 Monza
+   Italy
+
+   Phone: +39 039 2092044
+   EMail: giomarti@cisco.com
+
+
+
+
+
+
+
+
+
+
+
+Lee, et al.                   Informational                    [Page 31]
+