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rfc:rfc7698

Internet Engineering Task Force (IETF) O. Gonzalez de Dios, Ed. Request for Comments: 7698 Telefonica I+D Category: Informational R. Casellas, Ed. ISSN: 2070-1721 CTTC

                                                              F. Zhang
                                                                Huawei
                                                                 X. Fu
                                                             Stairnote
                                                         D. Ceccarelli
                                                              Ericsson
                                                            I. Hussain
                                                              Infinera
                                                         November 2015
         Framework and Requirements for GMPLS-Based Control
of Flexi-Grid Dense Wavelength Division Multiplexing (DWDM) Networks

Abstract

 To allow efficient allocation of optical spectral bandwidth for
 systems that have high bit-rates, the International Telecommunication
 Union Telecommunication Standardization Sector (ITU-T) has extended
 its Recommendations G.694.1 and G.872 to include a new Dense
 Wavelength Division Multiplexing (DWDM) grid by defining a set of
 nominal central frequencies, channel spacings, and the concept of the
 "frequency slot".  In such an environment, a data-plane connection is
 switched based on allocated, variable-sized frequency ranges within
 the optical spectrum, creating what is known as a flexible grid
 (flexi-grid).
 Given the specific characteristics of flexi-grid optical networks and
 their associated technology, this document defines a framework and
 the associated control-plane requirements for the application of the
 existing GMPLS architecture and control-plane protocols to the
 control of flexi-grid DWDM networks.  The actual extensions to the
 GMPLS protocols will be defined in companion documents.

Gonzalez de Dios, et al. Informational [Page 1] RFC 7698 GMPLS Flexi-Grid Framework November 2015

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/rfc7698.

Copyright Notice

 Copyright (c) 2015 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.

Gonzalez de Dios, et al. Informational [Page 2] RFC 7698 GMPLS Flexi-Grid Framework November 2015

Table of Contents

 1. Introduction ....................................................4
 2. Terminology .....................................................5
    2.1. Requirements Language ......................................5
    2.2. Abbreviations ..............................................5
 3. Overview of Flexi-Grid Networks .................................6
    3.1. Flexi-Grid in the Context of OTN ...........................6
    3.2. Flexi-Grid Terminology .....................................6
         3.2.1. Frequency Slots .....................................7
         3.2.2. Media-Layer Elements ................................9
         3.2.3. Media Channels .....................................10
         3.2.4. Optical Tributary Signals ..........................10
         3.2.5. Composite Media Channels ...........................11
    3.3. Hierarchy in the Media Layer ..............................11
    3.4. Flexi-Grid Layered Network Model ..........................12
         3.4.1. DWDM Flexi-Grid Enabled Network Element Models .....13
 4. GMPLS Applicability ............................................14
    4.1. General Considerations ....................................14
    4.2. Consideration of TE Links .................................14
    4.3. Consideration of LSPs in Flexi-Grid .......................17
    4.4. Control-Plane Modeling of Network Elements ................22
    4.5. Media Layer Resource Allocation Considerations ............22
    4.6. Neighbor Discovery and Link Property Correlation ..........26
    4.7. Path Computation, Routing and Spectrum Assignment (RSA) ...27
         4.7.1. Architectural Approaches to RSA ....................28
    4.8. Routing and Topology Dissemination ........................29
         4.8.1. Available Frequency Ranges (Frequency
                Slots) of DWDM Links ...............................29
         4.8.2. Available Slot Width Ranges of DWDM Links ..........29
         4.8.3. Spectrum Management ................................29
         4.8.4. Information Model ..................................30
 5. Control-Plane Requirements .....................................31
    5.1. Support for Media Channels ................................31
         5.1.1. Signaling ..........................................32
         5.1.2. Routing ............................................32
    5.2. Support for Media Channel Resizing ........................33
    5.3. Support for Logical Associations of Multiple Media
         Channels ..................................................33
    5.4. Support for Composite Media Channels ......................33
    5.5. Support for Neighbor Discovery and Link Property
         Correlation ...............................................34
 6. Security Considerations ........................................34
 7. Manageability Considerations ...................................35

Gonzalez de Dios, et al. Informational [Page 3] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 8. References .....................................................36
    8.1. Normative References ......................................36
    8.2. Informative References ....................................37
 Acknowledgments ...................................................39
 Contributors ......................................................39
 Authors' Addresses ................................................41

1. Introduction

 The term "flexible grid" ("flexi-grid" for short), as defined by the
 International Telecommunication Union Telecommunication
 Standardization Sector (ITU-T) Study Group 15 in the latest version
 of [G.694.1], refers to the updated set of nominal central
 frequencies (a frequency grid), channel spacing, and optical spectrum
 management and allocation considerations that have been defined in
 order to allow an efficient and flexible allocation and configuration
 of optical spectral bandwidth for systems that have high bit-rates.
 A key concept of flexi-grid is the "frequency slot": a variable-sized
 optical frequency range that can be allocated to a data connection.
 As detailed later in the document, a frequency slot is characterized
 by its nominal central frequency and its slot width, which, as per
 [G.694.1], is constrained to be a multiple of a given slot width
 granularity.
 Compared to a traditional fixed-grid network, which uses fixed-size
 optical spectrum frequency ranges or frequency slots with typical
 channel separations of 50 GHz, a flexible-grid network can select its
 media channels with a more flexible choice of slot widths, allocating
 as much optical spectrum as required.
 From a networking perspective, a flexible-grid network is assumed to
 be a layered network [G.872] [G.800] in which the media layer is the
 server layer and the optical signal layer is the client layer.  In
 the media layer, switching is based on a frequency slot, and the size
 of a media channel is given by the properties of the associated
 frequency slot.  In this layered network, a media channel can
 transport more than one Optical Tributary Signal (OTSi), as defined
 later in this document.
 A Wavelength Switched Optical Network (WSON), addressed in [RFC6163],
 is a term commonly used to refer to the application/deployment of a
 GMPLS-based control plane for the control (e.g., provisioning and
 recovery) of a fixed-grid Wavelength Division Multiplexing (WDM)
 network in which media (spectrum) and signal are jointly considered.

Gonzalez de Dios, et al. Informational [Page 4] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 This document defines the framework for a GMPLS-based control of
 flexi-grid enabled Dense Wavelength Division Multiplexing (DWDM)
 networks (in the scope defined by ITU-T layered Optical Transport
 Networks [G.872]), as well as a set of associated control-plane
 requirements.  An important design consideration relates to the
 decoupling of the management of the optical spectrum resource and the
 client signals to be transported.

2. Terminology

 Further terminology specific to flexi-grid networks can be found in
 Section 3.2.

2.1. Requirements Language

 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 [RFC2119].
 While [RFC2119] describes interpretations of these key words in terms
 of protocol specifications and implementations, they are used in this
 document to describe design requirements for protocol extensions.

2.2. Abbreviations

 FS: Frequency Slot
 FSC: Fiber-Switch Capable
 LSR: Label Switching Router
 NCF: Nominal Central Frequency
 OCC: Optical Channel Carrier
 OCh: Optical Channel
 OCh-P: Optical Channel Payload
 OTN: Optical Transport Network
 OTSi: Optical Tributary Signal
 OTSiG: OTSi Group is a set of OTSi
 PCE: Path Computation Element

Gonzalez de Dios, et al. Informational [Page 5] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 ROADM: Reconfigurable Optical Add/Drop Multiplexer
 SSON: Spectrum-Switched Optical Network
 SWG: Slot Width Granularity

3. Overview of Flexi-Grid Networks

3.1. Flexi-Grid in the Context of OTN

 [G.872] describes, from a network level, the functional architecture
 of an OTN.  It is decomposed into independent-layer networks with
 client/layer relationships among them.  A simplified view of the OTN
 layers is shown in Figure 1.
                          +----------------+
                          | Digital Layer  |
                          +----------------+
                          | Signal Layer   |
                          +----------------+
                          |  Media Layer   |
                          +----------------+
                    Figure 1: Generic OTN Overview
 In the OTN layering context, the media layer is the server layer of
 the optical signal layer.  The optical signal is guided to its
 destination by the media layer by means of a network media channel.
 In the media layer, switching is based on a frequency slot.
 In this scope, this document uses the term "flexi-grid enabled DWDM
 network" to refer to a network in which switching is based on
 frequency slots defined using the flexible grid.  This document
 mainly covers the media layer, as well as the required adaptations
 from the signal layer.  The present document is thus focused on the
 control and management of the media layer.

3.2. Flexi-Grid Terminology

 This section presents the definitions of the terms used in flexi-grid
 networks.  More details about these terms can be found in ITU-T
 Recommendations [G.694.1], [G.872], [G.870], [G.8080], and
 [G.959.1-2013].
 Where appropriate, this document also uses terminology and
 lexicography from [RFC4397].

Gonzalez de Dios, et al. Informational [Page 6] RFC 7698 GMPLS Flexi-Grid Framework November 2015

3.2.1. Frequency Slots

 This subsection is focused on the frequency slot and related terms.
 o  Frequency Slot [G.694.1]: The frequency range allocated to a slot
    within the flexible grid and unavailable to other slots.  A
    frequency slot is defined by its nominal central frequency and its
    slot width.
 o  Nominal Central Frequency: Each of the allowed frequencies as per
    the definition of the flexible DWDM grid in [G.694.1].  The set of
    nominal central frequencies can be built using the following
    expression:
    f = 193.1 THz + n x 0.00625 THz
    where 193.1 THz is the ITU-T "anchor frequency" for transmission
    over the C-band and 'n' is a positive or negative integer
    including 0.
  1. 5 -4 -3 -2 -1 0 1 2 3 4 5 ← values of n

…+–+–+–+–+–+–+–+–+–+–+-

                          ^
                          193.1 THz <- anchor frequency
   Figure 2: Anchor Frequency and Set of Nominal Central Frequencies
 o  Nominal Central Frequency Granularity: The spacing between allowed
    nominal central frequencies.  It is set to 6.25 GHz [G.694.1].
 o  Slot Width Granularity (SWG): 12.5 GHz, as defined in [G.694.1].

Gonzalez de Dios, et al. Informational [Page 7] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 o  Slot Width: Determines the "amount" of optical spectrum,
    regardless of its actual "position" in the frequency axis.  A slot
    width is constrained to be m x SWG (that is, m x 12.5 GHz),
    where 'm' is an integer greater than or equal to 1.
               Frequency Slot 1     Frequency Slot 2
                -------------     -------------------
                |           |     |                 |
            -3 -2 -1  0  1  2  3  4  5  6  7  8  9 10 11
        ...--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--...
                -------------     -------------------
                      ^                    ^
            Slot NCF = 193.1 THz    Slot NCF = 193.14375 THz
            Slot width = 25 GHz     Slot width = 37.5 GHz
              n = 0, m = 2            n = 7, m = 3
                   Figure 3: Example Frequency Slots
  • The symbol '+' represents the allowed nominal central

frequencies.

  • The '–' represents the nominal central frequency granularity

in units of 6.25 GHz.

  • The '^' represents the slot nominal central frequency.
  • The number on the top of the '+' symbol represents the 'n' in

the frequency calculation formula.

  • The nominal central frequency is 193.1 THz when n equals zero.
 o  Effective Frequency Slot [G.870]: That part of the frequency slots
    of the filters along the media channel that is common to all of
    the filters' frequency slots.  Note that both the terms "frequency
    slot" and "effective frequency slot" are applied locally.

Gonzalez de Dios, et al. Informational [Page 8] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 o  Figure 4 shows the effect of combining two filters along a
    channel.  The combination of Frequency Slot 1 and Frequency Slot 2
    applied to the media channel is the effective frequency slot
    shown.
                Frequency Slot 1
                 -------------
                 |           |
       -3 -2 -1  0  1  2  3  4  5  6  7  8  9 10 11
       ..--+--+--+--+--X--+--+--+--+--+--+--+--+--+--+--+--...
               Frequency Slot 2
              -------------------
              |                 |
       -3 -2 -1  0  1  2  3  4  5  6  7  8  9 10 11
       ..--+--+--+--+--X--+--+--+--+--+--+--+--+--+--+--+--...
    ===============================================
            Effective Frequency Slot
                 -------------
                 |           |
       -3 -2 -1  0  1  2  3  4  5  6  7  8  9 10 11
       ..--+--+--+--+--X--+--+--+--+--+--+--+--+--+--+--+--...
                  Figure 4: Effective Frequency Slot

3.2.2. Media-Layer Elements

 o  Media Element: A media element directs an optical signal or
    affects the properties of an optical signal.  It does not modify
    the properties of the information that has been modulated to
    produce the optical signal [G.870].  Examples of media elements
    include fibers, amplifiers, filters, and switching matrices.
 o  Media Channel Matrix: The media channel matrix provides flexible
    connectivity for the media channels.  That is, it represents a
    point of flexibility where relationships between the media ports
    at the edge of a media channel matrix may be created and broken.
    The relationship between these ports is called a "matrix channel".
    (Network) media channels are switched in a media channel matrix.

Gonzalez de Dios, et al. Informational [Page 9] RFC 7698 GMPLS Flexi-Grid Framework November 2015

3.2.3. Media Channels

 This section defines concepts such as the (network) media channel;
 the mapping to GMPLS constructs (i.e., LSP) is detailed in Section 4.
 o  Media Channel: A media association that represents both the
    topology (i.e., path through the media) and the resource
    (frequency slot) that it occupies.  As a topological construct, it
    represents a frequency slot (an effective frequency slot)
    supported by a concatenation of media elements (fibers,
    amplifiers, filters, switching matrices...).  This term is used to
    identify the end-to-end physical-layer entity with its
    corresponding (one or more) frequency slots local at each link
    filter.
 o  Network Media Channel: Defined in [G.870] as a media channel that
    transports a single OTSi (defined in the next subsection).

3.2.4. Optical Tributary Signals

 o  Optical Tributary Signal (OTSi): The optical signal that is placed
    within a network media channel for transport across the optical
    network.  This may consist of a single modulated optical carrier
    or a group of modulated optical carriers or subcarriers.  To
    provide a connection between the OTSi source and the OTSi sink,
    the optical signal must be assigned to a network media channel
    (see also [G.959.1-2013]).
 o  OTSi Group (OTSiG): The set of OTSi that are carried by a group of
    network media channels.  Each OTSi is carried by one network media
    channel.  From a management perspective, it SHOULD be possible to
    manage both the OTSiG and a group of network media channels as
    single entities.

Gonzalez de Dios, et al. Informational [Page 10] RFC 7698 GMPLS Flexi-Grid Framework November 2015

3.2.5. Composite Media Channels

 o  It is possible to construct an end-to-end media channel as a
    composite of more than one network media channel.  A composite
    media channel carries a group of OTSi (i.e., OTSiG).  Each OTSi is
    carried by one network media channel.  This OTSiG is carried over
    a single fiber.
 o  In this case, the effective frequency slots may be contiguous
    (i.e., there is no spectrum between them that can be used for
    other media channels) or non-contiguous.
 o  It is not currently envisaged that such composite media channels
    may be constructed from slots carried on different fibers whether
    those fibers traverse the same hop-by-hop path through the network
    or not.
 o  Furthermore, it is not considered likely that a media channel may
    be constructed from a different variation of slot composition on
    each hop.  That is, the slot composition (i.e., the group of OTSi
    carried by the composite media channel) must be the same from one
    end of the media channel to the other, even if the specific slot
    for each OTSi and the spacing among slots may vary hop by hop.
 o  How the signal is carried across such groups of network media
    channels is out of scope for this document.

3.3. Hierarchy in the Media Layer

 In summary, the concept of the frequency slot is a logical
 abstraction that represents a frequency range, while the media layer
 represents the underlying media support.  Media channels are media
 associations, characterized by their respective (effective) frequency
 slots, and media channels are switched in media channel matrices.
 From the control and management perspective, a media channel can be
 logically split into network media channels.

Gonzalez de Dios, et al. Informational [Page 11] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 In Figure 5, a media channel has been configured and dimensioned to
 support two network media channels, each of them carrying one OTSi.
                           Media Channel Frequency Slot
   +-------------------------------X------------------------------+
   |                                                              |
   |       Frequency Slot                  Frequency Slot         |
   |   +-----------X-----------+       +----------X-----------+   |
   |   |         OTSi          |       |         OTSi         |   |
   |   |           o           |       |          o           |   |
   |   |           |           |       |          |           |   |
  -4  -3  -2  -1   0   1   2   3   4   5   6   7  8   9  10  11  12
 --+---+---+---+---+---+---+---+---+---+---+---+--+---+---+---+---+--
        <- Network Media Channel ->    <- Network Media Channel ->
    <------------------------ Media Channel ----------------------->
       X - Frequency Slot Central Frequency
       o - Signal Central Frequency
    Figure 5: Example of Media Channel, Network Media Channels, and
                      Associated Frequency Slots

3.4. Flexi-Grid Layered Network Model

 In the OTN layered network, the network media channel transports a
 single OTSi (see Figure 6).
   |                            OTSi                                 |
   O - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - O
   |                                                                 |
   | Channel Port         Network Media Channel         Channel Port |
   O - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - O
   |                                                                 |
 +--------+                 +-----------+                   +--------+
 |  \ (1) |                 |    (1)    |                   | (1)  / |
 |   \----|-----------------|-----------|-------------------|-----/  |
 +--------+ Link Channel    +-----------+  Link Channel     +--------+
   Media Channel            Media Channel                Media Channel
   Matrix                   Matrix                       Matrix
 The symbol (1) indicates a matrix channel
              Figure 6: Simplified Layered Network Model

Gonzalez de Dios, et al. Informational [Page 12] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 Note that a particular example of OTSi is the OCh-P.  Figure 7 shows
 this specific example as defined in G.805 [G.805].
  OCh AP                     Trail (OCh)                    OCh AP
   O- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - O
   |                                                              |
  --- OCh-P                                                OCh-P ---
  \ / source                                               sink  \ /
   +                                                              +
   | OCh-P               OCh-P Network Connection           OCh-P |
   O TCP - - - - - - - - - - - - - - - - - - - - - - - - - - -TCP O
   |                                                              |
   |Channel Port          Network Media Channel      Channel Port |
   O - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  O
   |                                                              |
 +--------+                 +-----------+                 +---------+
 |  \ (1) |  OCh-P LC       |    (1)    |  OCh-P LC       |  (1)  / |
 |   \----|-----------------|-----------|-----------------|------/  |
 +--------+ Link Channel    +-----------+  Link Channel   +---------+
 Media Channel              Media Channel                Media Channel
   Matrix                     Matrix                        Matrix
 The symbol (1) indicates a matrix channel
 "LC" indicates a link connection
          Figure 7: Layered Network Model According to G.805

3.4.1. DWDM Flexi-Grid Enabled Network Element Models

 A flexible-grid network is constructed from subsystems that include
 WDM links, tunable transmitters, and receivers (i.e., media elements
 including media-layer switching elements that are media matrices), as
 well as electro-optical network elements.  This is just the same as
 in a fixed-grid network, except that each element has flexible-grid
 characteristics.
 As stated in Clause 7 of [G.694.1], the flexible DWDM grid has a
 nominal central frequency granularity of 6.25 GHz and a slot width
 granularity of 12.5 GHz.  However, devices or applications that make
 use of the flexible grid might not be capable of supporting every
 possible slot width or position.  In other words, applications may be
 defined where only a subset of the possible slot widths and positions
 is required to be supported.  For example, an application could be
 defined where the nominal central frequency granularity is 12.5 GHz
 (by only requiring values of n that are even) and where slot widths
 are a multiple of 25 GHz (by only requiring values of m that are
 even).

Gonzalez de Dios, et al. Informational [Page 13] RFC 7698 GMPLS Flexi-Grid Framework November 2015

4. GMPLS Applicability

 The goal of this section is to provide an insight into the
 application of GMPLS as a control mechanism in flexi-grid networks.
 Specific control-plane requirements for the support of flexi-grid
 networks are covered in Section 5.  This framework is aimed at
 controlling the media layer within the OTN hierarchy and controlling
 the required adaptations of the signal layer.  This document also
 defines the term "Spectrum-Switched Optical Network" (SSON) to refer
 to a flexi-grid enabled DWDM network that is controlled by a GMPLS or
 PCE control plane.
 This section provides a mapping of the ITU-T G.872 architectural
 aspects to GMPLS and control-plane terms and also considers the
 relationship between the architectural concept or construct of a
 media channel and its control-plane representations (e.g., as a TE
 link, as defined in [RFC3945]).

4.1. General Considerations

 The GMPLS control of the media layer deals with the establishment of
 media channels that are switched in media channel matrices.  GMPLS
 labels are used to locally represent the media channel and its
 associated frequency slot.  Network media channels are considered a
 particular case of media channels when the endpoints are transceivers
 (that is, the source and destination of an OTSi).

4.2. Consideration of TE Links

 From a theoretical point of view, a fiber can be modeled as having a
 frequency slot that ranges from minus infinity to plus infinity.
 This representation helps us understand the relationship between
 frequency slots and ranges.
 The frequency slot is a local concept that applies within a component
 or element.  When applied to a media channel, we are referring to its
 effective frequency slot as defined in [G.872].
 The association sequence of the three components (i.e., a filter, a
 fiber, and a filter) is a media channel in its most basic form.  From
 the control-plane perspective, this may be modeled as a (physical)
 TE link with a contiguous optical spectrum.  This can be represented
 by saying that the portion of spectrum available at time t0 depends
 on which filters are placed at the ends of the fiber and how they
 have been configured.  Once filters are placed, we have a one-hop
 media channel.  In practical terms, associating a fiber with the
 terminating filters determines the usable optical spectrum.

Gonzalez de Dios, et al. Informational [Page 14] RFC 7698 GMPLS Flexi-Grid Framework November 2015

  1. ————–+ +—————–

| |

       +--------+                             +--------+
       |        |                             |        |  +---------
   ---o|        ===============================        o--|
       |        |             Fiber           |        |  | --\  /--
   ---o|        |                             |        o--|    \/
       |        |                             |        |  |    /\
   ---o|        ===============================        o--| --/  \--
       | Filter |                             | Filter |  |
       |        |                             |        |  +---------
       +--------+                             +--------+
                |                             |
             |------- Basic Media Channel  ---------|
 ---------------+                             +-----------------
  1. ——-+ +——–

|————————————–|

      LSR    |               TE link                |  LSR
             |--------------------------------------|
     --------+                                      +--------
              Figure 8: (Basic) Media Channel and TE Link
 Additionally, when a cross-connect for a specific frequency slot is
 considered, the resulting media support of joining basic media
 channels is still a media channel, i.e., a longer association
 sequence of media elements and its effective frequency slot.  In
 other words, it is possible to "concatenate" several media channels
 (e.g., patch on intermediate nodes) to create a single media channel.

Gonzalez de Dios, et al. Informational [Page 15] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 The architectural construct resulting from the association sequence
 of basic media channels and media-layer matrix cross-connects can be
 represented as (i.e., corresponds to) a Label Switched Path (LSP)
 from a control-plane perspective.
  1. ———+ +——————————+ +———

| | | |

    +------+       +------+                +------+       +------+
    |      |       |      |  +----------+  |      |       |      |
 --o|      =========      o--|          |--o      =========      o--
    |      | Fiber |      |  | --\  /-- |  |      | Fiber |      |
 --o|      |       |      o--|    \/    |--o      |       |      o--
    |      |       |      |  |    /\    |  |      |       |      |
 --o|      =========      o--***********|--o      =========      o--
    |Filter|       |Filter|  |          |  |Filter|       |Filter|
    |      |       |      |                |      |       |      |
    +------+       +------+                +------+       +------+
           |       |                              |       |
       <- Basic Media ->    <- Matrix ->       <- Basic Media ->
           |Channel|           Channel            |Channel|
 ----------+       +------------------------------+       +---------
       <--------------------  Media Channel  ---------------->
  1. —–+ +—————+ +——

|——————| |——————|

  LSR  |       TE link    |      LSR      |   TE link        |  LSR
       |------------------|               |------------------|
 ------+                  +---------------+                  +------
                   Figure 9: Extended Media Channel

Gonzalez de Dios, et al. Informational [Page 16] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 Furthermore, if appropriate, the media channel can also be
 represented as a TE link or Forwarding Adjacency (FA) [RFC4206],
 augmenting the control-plane network model.
  1. ———+ +——————————+ +———

| | | |

    +------+       +------+                +------+       +------+
    |      |       |      |  +----------+  |      |       |      |
 --o|      =========      o--|          |--o      =========      o--
    |      | Fiber |      |  | --\  /-- |  |      | Fiber |      |
 --o|      |       |      o--|    \/    |--o      |       |      o--
    |      |       |      |  |    /\    |  |      |       |      |
 --o|      =========      o--***********|--o      =========      o--
    |Filter|       |Filter|  |          |  |Filter|       |Filter|
    |      |       |      |                |      |       |      |
    +------+       +------+                +------+       +------+
           |       |                              |       |
 ----------+       +------------------------------+       +---------
        <------------------------  Media Channel  ----------->
  1. —–+ +—–

|——————————————————|

  LSR  |                               TE link                | LSR
       |------------------------------------------------------|
 ------+                                                      +-----
            Figure 10: Extended Media Channel TE Link or FA

4.3. Consideration of LSPs in Flexi-Grid

 The flexi-grid LSP is a control-plane representation of a media
 channel.  Since network media channels are media channels, an LSP may
 also be the control-plane representation of a network media channel
 (without considering the adaptation functions).  From a control-plane
 perspective, the main difference (regardless of the actual effective
 frequency slot, which may be dimensioned arbitrarily) is that the LSP
 that represents a network media channel also includes the endpoints
 (transceivers), including the cross-connects at the ingress and
 egress nodes.  The ports towards the client can still be represented
 as interfaces from the control-plane perspective.

Gonzalez de Dios, et al. Informational [Page 17] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 Figure 11 shows an LSP routed between three nodes.  The LSP is
 terminated before the optical matrix of the ingress and egress nodes
 and can represent a media channel.  This case does not (and cannot)
 represent a network media channel because it does not include (and
 cannot include) the transceivers.
  1. ——–+ +——————————–+ +——–

| | | |

   +------+       +------+                  +------+       +------+
   |      |       |      |   +----------+   |      |       |      |
 -o|      =========      o---|          |---o      =========      o-
   |      | Fiber |      |   | --\  /-- |   |      | Fiber |      |
 -o|>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>o-
   |      |       |      |   |    /\    |   |      |       |      |
 -o|      =========      o---***********|---o      =========      o-
   |Filter|       |Filter|   |          |   |Filter|       |Filter|
   |      |       |      |                  |      |       |      |
   +------+       +------+                  +------+       +------+
          |       |                                |       |
 ---------+       +--------------------------------+       +--------
        >>>>>>>>>>>>>>>>>>>>>>>>>>>> LSP >>>>>>>>>>>>>>>>>>>>>>>>
   -----+                  +---------------+                +-----
        |------------------|               |----------------|
   LSR  |       TE link    |     LSR       |      TE link   | LSR
        |------------------|               |----------------|
   -----+                  +---------------+                +-----
 Figure 11: Flexi-Grid LSP Representing a Media Channel That Starts at
  the Filter of the Outgoing Interface of the Ingress LSR and Ends at
        the Filter of the Incoming Interface of the Egress LSR

Gonzalez de Dios, et al. Informational [Page 18] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 In Figure 12, a network media channel is represented as terminated at
 the network side of the transceivers.  This is commonly named an
 OTSi-trail connection.
 |--------------------- Network Media Channel ----------------------|
      +----------------------+           +----------------------+
      |                                  |                      |
      +------+        +------+           +------+        +------+
      |      | +----+ |      |           |      | +----+ |      |OTSi
  OTSi|      o-|    |-o      |  +-----+  |      o-|    |-o      |sink
  src |      | |    | |      ===+-+ +-+==|      | |    | |      O---|R
 T|***o******o********************************************************
      |      | |\  /| |         | | | |  |      | |\  /| |      |
      |      o-| \/ |-o      ===| | | |==|      o-| \/ |-o      |
      |      | | /\ | |      |  +-+ +-+  |      | | /\ | |      |
      |      o-|/  \|-o      |  |  \/ |  |      o-|/  \|-o      |
      |Filter| |    | |Filter|  |  /\ |  |Filter| |    | |Filter|
      +------+ |    | +------+  +-----+  +------+ |    | +------+
      |        |    |        |           |        |    |        |
      +----------------------+           +----------------------+
                                    LSP
 <------------------------------------------------------------------->
                                    LSP
  <------------------------------------------------------------------>
       +-----+                   +--------+                +-----+
  o--- |     |-------------------|        |----------------|     |---o
       | LSR |       TE link     |  LSR   |   TE link      | LSR |
       |     |-------------------|        |----------------|     |
       +-----+                   +--------+                +-----+
   Figure 12: LSP Representing a Network Media Channel (OTSi Trail)

Gonzalez de Dios, et al. Informational [Page 19] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 In a third case, a network media channel is terminated on the filter
 ports of the ingress and egress nodes.  This is defined in G.872 as
 an OTSi Network Connection.  As can be seen from the figures, from a
 GMPLS modeling perspective there is no difference between these
 cases, but they are shown as distinct examples to highlight the
 differences in the data plane.
   |---------------------  Network Media Channel --------------------|
   +------------------------+               +------------------------+
   +------+        +------+                 +------+          +------+
   |      | +----+ |      |                 |      | +----+ |      |
   |      o-|    |-o      |    +------+     |      o-|    |-o      |
   |      | |    | |      =====+-+  +-+=====|      | |    | |      |
 T-o******o********************************************************O-R
   |      | |\  /| |           | |  | |     |      | |\  /| |      |
   |      o-| \/ |-o      =====| |  | |=====|      o-| \/ |-o      |
   |      | | /\ | |      |    +-+  +-+     |      | | /\ | |      |
   |      o-|/  \|-o      |    |  \/  |     |      o-|/  \|-o      |
   |Filter| |    | |Filter|    |  /\  |     |Filter| |    | |Filter|
   +------+ |    | +------+    +------+     +------+ |    | +------+
   |        |    |        |                 |        |    |        |
   +----------------------+                 +----------------------+
   <----------------------------------------------------------------->
                                  LSP
                                   LSP
   <-------------------------------------------------------------->
    +-----+                    +--------+                   +-----+
 o--|     |--------------------|        |-------------------|     |--o
    | LSR |       TE link      |  LSR   |      TE link      | LSR |
    |     |--------------------|        |-------------------|     |
    +-----+                    +--------+                   +-----+
          Figure 13: LSP Representing a Network Media Channel
                       (OTSi Network Connection)

Gonzalez de Dios, et al. Informational [Page 20] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 Applying the notion of hierarchy at the media layer, by using the LSP
 as an FA (i.e., by using hierarchical LSPs), the media channel
 created can support multiple (sub-)media channels.
 +--------------+                      +--------------+
 | Media Channel|           TE         | Media Channel|  Virtual TE
 |              |          link        |              |    link
 |    Matrix    |o- - - - - - - - - - o|    Matrix    |o- - - - - -
 +--------------+                      +--------------+
                |     +---------+      |
                |     |  Media  |      |
                |o----| Channel |-----o|
                      |         |
                      | Matrix  |
                      +---------+
              Figure 14: Topology View with TE Link or FA
 Note that there is only one media-layer switch matrix (one
 implementation is a flexi-grid ROADM) in SSON, while a signal-layer
 LSP (network media channel) is established mainly for the purpose of
 management and control of individual optical signals.  Signal-layer
 LSPs with the same attributes (such as source and destination) can be
 grouped into one media-layer LSP (media channel); this has advantages
 in spectral efficiency (reduced guard band between adjacent OChs in
 one FSC channel) and LSP management.  However, assuming that some
 network elements perform signal-layer switching in an SSON, there
 must be enough guard band between adjacent OTSi in any media channel
 to compensate for the filter concatenation effects and other effects
 caused by signal-layer switching elements.  In such a situation, the
 separation of the signal layer from the media layer does not bring
 any benefit in spectral efficiency or in other aspects, and it makes
 the network switching and control more complex.  If two OTSi must be
 switched to different ports, it is better to carry them via different
 FSC channels, and the media-layer switch is enough in this scenario.
 As discussed in Section 3.2.5, a media channel may be constructed
 from a composite of network media channels.  This may be achieved in
 two ways using LSPs.  These mechanisms may be compared to the
 techniques used in GMPLS to support inverse multiplexing in Time
 Division Multiplexing (TDM) networks and in OTN [RFC4606] [RFC6344]
 [RFC7139].
 o  In the first case, a single LSP may be established in the control
    plane.  The signaling messages include information for all of the
    component network media channels that make up the composite media
    channel.

Gonzalez de Dios, et al. Informational [Page 21] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 o  In the second case, each component network media channel is
    established using a separate control-plane LSP, and these LSPs are
    associated within the control plane so that the endpoints may see
    them as a single media channel.

4.4. Control-Plane Modeling of Network Elements

 Optical transmitters and receivers may have different tunability
 constraints, and media channel matrices may have switching
 restrictions.  Additionally, a key feature of their implementation is
 their highly asymmetric switching capability, which is described in
 detail in [RFC6163].  Media matrices include line-side ports that are
 connected to DWDM links and tributary-side input/output ports that
 can be connected to transmitters/receivers.
 A set of common constraints can be defined:
 o  Slot widths: The minimum and maximum slot width.
 o  Granularity: The optical hardware may not be able to select
    parameters with the lowest granularity (e.g., 6.25 GHz for nominal
    central frequencies or 12.5 GHz for slot width granularity).
 o  Available frequency ranges: The set or union of frequency ranges
    that have not been allocated (i.e., are available).  The relative
    grouping and distribution of available frequency ranges in a fiber
    are usually referred to as "fragmentation".
 o  Available slot width ranges: The set or union of slot width ranges
    supported by media matrices.  It includes the following
    information:
  • Slot width threshold: The minimum and maximum slot width

supported by the media matrix. For example, the slot width

       could be from 50 GHz to 200 GHz.
  • Step granularity: The minimum step by which the optical filter

bandwidth of the media matrix can be increased or decreased.

       This parameter is typically equal to slot width granularity
       (i.e., 12.5 GHz) or integer multiples of 12.5 GHz.

4.5. Media Layer Resource Allocation Considerations

 A media channel has an associated effective frequency slot.  From the
 perspective of network control and management, this effective slot is
 seen as the "usable" end-to-end frequency slot.  The establishment of
 an LSP is related to the establishment of the media channel and the
 configuration of the effective frequency slot.

Gonzalez de Dios, et al. Informational [Page 22] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 A "service request" is characterized (at a minimum) by its required
 effective slot width.  This does not preclude the request from adding
 additional constraints, such as also imposing the nominal central
 frequency.  A given effective frequency slot may be requested for the
 media channel in the control-plane LSP setup messages, and a specific
 frequency slot can be requested on any specific hop of the LSP setup.
 Regardless of the actual encoding, the LSP setup message specifies a
 minimum effective frequency slot width that needs to be fulfilled in
 order to successfully establish the requested LSP.
 An effective frequency slot must equally be described in terms of a
 central nominal frequency and its slot width (in terms of usable
 spectrum of the effective frequency slot).  That is, it must be
 possible to determine the end-to-end values of the n and m
 parameters.  We refer to this by saying that the "effective frequency
 slot of the media channel or LSP must be valid".
 In GMPLS, the requested effective frequency slot is represented to
 the TSpec present in the RSVP-TE Path message, and the effective
 frequency slot is mapped to the FlowSpec carried in the RSVP-TE Resv
 message.
 In GMPLS-controlled systems, the switched element corresponds to the
 'label'.  In flexi-grid, the switched element is a frequency slot,
 and the label represents a frequency slot.  Consequently, the label
 in flexi-grid conveys the necessary information to obtain the
 frequency slot characteristics (i.e., central frequency and slot
 width: the n and m parameters).  The frequency slot is locally
 identified by the label.
 The local frequency slot may change at each hop, given hardware
 constraints and capabilities (e.g., a given node might not support
 the finest granularity).  This means that the values of n and m may
 change at each hop.  As long as a given downstream node allocates
 enough optical spectrum, m can be different along the path.  This
 covers the issue where media matrices can have different slot width
 granularities.  Such variations in the local value of m will appear
 in the allocated label that encodes the frequency slot as well as in
 the FlowSpec that describes the flow.
 Different operational modes can be considered.  For Routing and
 Spectrum Assignment (RSA) with explicit label control, and for
 Routing and Distributed Spectrum Assignment (R+DSA), the GMPLS
 signaling procedures are similar to those described in Section 4.1.3
 of [RFC6163] for Routing and Wavelength Assignment (RWA) and for
 Routing and Distributed Wavelength Assignment (R+DWA).  The main
 difference is that the label set specifies the available nominal
 central frequencies that meet the slot width requirements of the LSP.

Gonzalez de Dios, et al. Informational [Page 23] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 The intermediate nodes use the control plane to collect the
 acceptable central frequencies that meet the slot width requirement
 hop by hop.  The tail-end node also needs to know the slot width of
 an LSP to assign the proper frequency resource.  Except for
 identifying the resource (i.e., fixed wavelength for WSON, and
 frequency resource for flexible grids), the other signaling
 requirements (e.g., unidirectional or bidirectional, with or without
 converters) are the same as for WSON as described in Section 6.1 of
 [RFC6163].
 Regarding how a GMPLS control plane can assign n and m hop by hop
 along the path of an LSP, different cases can apply:
 a.  n and m can both change.  It is the effective frequency slot that
     matters; it needs to remain valid along the path.
 b.  m can change, but n needs to remain the same along the path.
     This ensures that the nominal central frequency stays the same,
     but the width of the slot can vary along the path.  Again, the
     important thing is that the effective frequency slot remains
     valid and satisfies the requested parameters along the whole path
     of the LSP.
 c.  n and m need to be unchanging along the path.  This ensures that
     the frequency slot is well known from end to end and is a simple
     way to ensure that the effective frequency slot remains valid for
     the whole LSP.
 d.  n can change, but m needs to remain the same along the path.
     This ensures that the effective frequency slot remains valid but
     also allows the frequency slot to be moved within the spectrum
     from hop to hop.
 The selection of a path that ensures n and m continuity can be
 delegated to a dedicated entity such as a Path Computation Element
 (PCE).  Any constraint (including frequency slot and width
 granularities) can be taken into account during path computation.
 Alternatively, A PCE can compute a path, leaving the actual frequency
 slot assignment to be done, for example, with a distributed
 (signaling) procedure:
 o  Each downstream node ensures that m is >= requested_m.
 o  A downstream node cannot foresee what an upstream node will
    allocate.  A way to ensure that the effective frequency slot is
    valid along the length of the LSP is to ensure that the same value
    of n is allocated at each hop.  By forcing the same value of n, we

Gonzalez de Dios, et al. Informational [Page 24] RFC 7698 GMPLS Flexi-Grid Framework November 2015

    avoid cases where the effective frequency slot of the media
    channel is invalid (that is, the resulting frequency slot cannot
    be described by its n and m parameters).
 o  This may be too restrictive, since a node (or even a centralized/
    combined RSA entity) may be able to ensure that the resulting
    end-to-end effective frequency slot is valid, even if n varies
    locally.  That means that the effective frequency slot that
    characterizes the media channel from end to end is consistent and
    is determined by its n and m values but that the effective
    frequency slot and those values are logical (i.e., do not map
    "direct" to the physically assigned spectrum) in the sense that
    they are the result of the intersection of locally assigned
    frequency slots applicable at local components (such as filters),
    each of which may have different frequency slots assigned to them.
 As shown in Figure 15, the effective slot is made valid by ensuring
 that the minimum m is greater than the requested m.  The effective
 slot (intersection) is the lowest m (bottleneck).
                          C                B                A
           |Path(m_req)   |                ^                |
           |--------->    |                #                |
           |              |                #                ^
          -^--------------^----------------#----------------#--
 Effective #              #                #                #
 FS n, m   # . . . . . . .#. . . . . . . . # . . . . . . . .# <-fixed
           #              #                #                #   n
          -v--------------v----------------#----------------#---
           |              |                #                v
           |              |                #          Resv  |
           |              |                v        <------ |
           |              |                |FlowSpec(n, m_a)|
           |              |       <--------|                |
           |              |  FlowSpec(n,   |
                 <--------|      min(m_a, m_b))
           FlowSpec(n,    |
             min(m_a, m_b, m_c))
             m_a, m_b, m_c: Selected frequency slot widths
     Figure 15: Distributed Allocation with Different m and Same n

Gonzalez de Dios, et al. Informational [Page 25] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 In Figure 16, the effective slot is made valid by ensuring that it is
 valid at each hop in the upstream direction.  The intersection needs
 to be computed; otherwise, invalid slots could result.
                         C                B                 A
           |Path(m_req)  ^                |                 |
           |--------->   #                |                 |
           |             #                ^                 ^
          -^-------------#----------------#-----------------#--------
 Effective #             #                #                 #
 FS n, m   #             #                #                 #
           #             #                #                 #
          -v-------------v----------------#-----------------#--------
           |             |                #                 v
           |             |                #           Resv  |
           |             |                v         <------ |
           |             |                |FlowSpec(n_a, m_a)
           |             |       <--------|                 |
           |             |  FlowSpec(FSb [intersect] FSa)
                <--------|
          FlowSpec([intersect] FSa,FSb,FSc)
           n_a: Selected nominal central frequency by node A
           m_a: Selected frequency slot widths by node A
           FSa, FSb, FSc: Frequency slot at each hop A, B, C
  Figure 16: Distributed Allocation with Different m and Different n
 Note that when a media channel is bound to one OTSi (i.e., is a
 network media channel), the effective FS must be the frequency slot
 of the OTSi.  The media channel set up by the LSP may contain the
 effective FS of the network media channel effective FS.  This is an
 endpoint property; the egress and ingress have to constrain the
 effective FS to be the OTSi effective FS.

4.6. Neighbor Discovery and Link Property Correlation

 There are potential interworking problems between fixed-grid DWDM
 nodes and flexi-grid DWDM nodes.  Additionally, even two flexi-grid
 nodes may have different grid properties, leading to link property
 conflict and resulting in limited interworking.
 Devices or applications that make use of flexi-grid might not be able
 to support every possible slot width.  In other words, different
 applications may be defined where each supports a different grid
 granularity.  In this case, the link between two optical nodes with

Gonzalez de Dios, et al. Informational [Page 26] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 different grid granularities must be configured to align with the
 larger of both granularities.  Furthermore, different nodes may have
 different slot width tuning ranges.
 In summary, in a DWDM link between two nodes, at a minimum, the
 following properties need to be negotiated:
 o  Grid capability (channel spacing) - Between fixed-grid and
    flexi-grid nodes.
 o  Grid granularity - Between two flexi-grid nodes.
 o  Slot width tuning range - Between two flexi-grid nodes.

4.7. Path Computation, Routing and Spectrum Assignment (RSA)

 In WSON, if there is no (available) wavelength converter in an
 optical network, an LSP is subject to the "wavelength continuity
 constraint" (see Section 4 of [RFC6163]).  Similarly, in flexi-grid,
 if the capability to shift or convert an allocated frequency slot is
 absent, the LSP is subject to the "spectrum continuity constraint".
 Because of the limited availability of spectrum converters (in what
 is called a "sparse translucent optical network"), the spectrum
 continuity constraint always has to be considered.  When available,
 information regarding spectrum conversion capabilities at the optical
 nodes may be used by RSA mechanisms.
 The RSA process determines a route and frequency slot for an LSP.
 Hence, when a route is computed, the spectrum assignment process
 determines the central frequency and slot width based on the
 following:
 o  the requested slot width
 o  the information regarding the transmitter and receiver
    capabilities, including the availability of central frequencies
    and their slot width granularity
 o  the information regarding available frequency slots (frequency
    ranges) and available slot widths of the links traversed along
    the route

Gonzalez de Dios, et al. Informational [Page 27] RFC 7698 GMPLS Flexi-Grid Framework November 2015

4.7.1. Architectural Approaches to RSA

 Similar to RWA for fixed grids [RFC6163], different ways of
 performing RSA in conjunction with the control plane can be
 considered.  The approaches included in this document are provided
 for reference purposes only; other possible options could also be
 deployed.
 Note that all of these models allow the concept of a composite media
 channel supported by a single control-plane LSP or by a set of
 associated LSPs.

4.7.1.1. Combined RSA (R&SA)

 In this case, a computation entity performs both routing and
 frequency slot assignment.  The computation entity needs access to
 detailed network information, e.g., the connectivity topology of the
 nodes and links, available frequency ranges on each link, and node
 capabilities.
 The computation entity could reside on a dedicated PCE server, in
 the provisioning application that requests the service, or on the
 ingress node.

4.7.1.2. Separated RSA (R+SA)

 In this case, routing computation and frequency slot assignment are
 performed by different entities.  The first entity computes the
 routes and provides them to the second entity.  The second entity
 assigns the frequency slot.
 The first entity needs the connectivity topology to compute the
 proper routes.  The second entity needs information about the
 available frequency ranges of the links and the capabilities of the
 nodes in order to assign the spectrum.

4.7.1.3. Routing and Distributed SA (R+DSA)

 In this case, an entity computes the route, but the frequency slot
 assignment is performed hop by hop in a distributed way along the
 route.  The available central frequencies that meet the spectrum
 continuity constraint need to be collected hop by hop along the
 route.  This procedure can be implemented by the GMPLS signaling
 protocol.

Gonzalez de Dios, et al. Informational [Page 28] RFC 7698 GMPLS Flexi-Grid Framework November 2015

4.8. Routing and Topology Dissemination

 In the case of the combined RSA architecture, the computation entity
 needs the detailed network information, i.e., connectivity topology,
 node capabilities, and available frequency ranges of the links.
 Route computation is performed based on the connectivity topology and
 node capabilities, while spectrum assignment is performed based on
 the available frequency ranges of the links.  The computation entity
 may get the detailed network information via the GMPLS routing
 protocol.
 For WSON, the connectivity topology and node capabilities can be
 advertised by the GMPLS routing protocol (refer to Section 6.2 of
 [RFC6163]).  Except for wavelength-specific availability information,
 the information for flexi-grid is the same as for WSON and can
 equally be distributed by the GMPLS routing protocol.
 This section analyzes the necessary changes to link information
 required by flexible grids.

4.8.1. Available Frequency Ranges (Frequency Slots) of DWDM Links

 In the case of flexible grids, channel central frequencies span from
 193.1 THz towards both ends of the C-band spectrum with a granularity
 of 6.25 GHz.  Different LSPs could make use of different slot widths
 on the same link.  Hence, the available frequency ranges need to be
 advertised.

4.8.2. Available Slot Width Ranges of DWDM Links

 The available slot width ranges need to be advertised in combination
 with the available frequency ranges, so that the computing entity can
 verify whether an LSP with a given slot width can be set up or not.
 This is constrained by the available slot width ranges of the media
 matrix.  Depending on the availability of the slot width ranges, it
 is possible to allocate more spectrum than what is strictly needed by
 the LSP.

4.8.3. Spectrum Management

 The total available spectrum on a fiber can be described as a
 resource that can be partitioned.  For example, a part of the
 spectrum could be assigned to a third party to manage, or parts of
 the spectrum could be assigned by the operator for different classes
 of traffic.  This partitioning creates the impression that the
 spectrum is a hierarchy in view of the management plane and the
 control plane: each partition could itself be partitioned.  However,

Gonzalez de Dios, et al. Informational [Page 29] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 the hierarchy is created purely within a management system; it
 defines a hierarchy of access or management rights, but there is no
 corresponding resource hierarchy within the fiber.
 The end of the fiber is a link end and presents a fiber port that
 represents all of the spectrum available on the fiber.  Each spectrum
 allocation appears as a Link Channel Port (i.e., frequency slot port)
 within the fiber.  Thus, while there is a hierarchy of ownership (the
 Link Channel Port and corresponding LSP are located on a fiber and
 therefore are associated with a fiber port), there is no continued
 nesting hierarchy of frequency slots within larger frequency slots.
 In its way, this mirrors the fixed-grid behavior where a wavelength
 is associated with a fiber port but cannot be subdivided even though
 it is a partition of the total spectrum available on the fiber.

4.8.4. Information Model

 This section defines an information model to describe the data that
 represents the capabilities and resources available in a flexi-grid
 network.  It is not a data model and is not intended to limit any
 protocol solution such as an encoding for an IGP.  For example,
 information required for routing and path selection may be the set of
 available nominal central frequencies from which a frequency slot of
 the required width can be allocated.  A convenient encoding for this
 information is left for further study in an IGP encoding document.
 Fixed DWDM grids can also be described via suitable choices of slots
 in a flexible DWDM grid.  However, devices or applications that make
 use of the flexible grid may not be capable of supporting every
 possible slot width or central frequency position.  Thus, the
 information model needs to enable:
 o  the exchange of information to enable RSA in a flexi-grid network
 o  the representation of a fixed-grid device participating in a
    flexi-grid network
 o  full interworking of fixed-grid and flexible-grid devices within
    the same network
 o  interworking of flexible-grid devices with different capabilities

Gonzalez de Dios, et al. Informational [Page 30] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 The information model is represented using the Routing Backus-Naur
 Format (RBNF) as defined in [RFC5511].
 <Available Spectrum> ::=
   <Available Frequency Range-List>
   <Available NCFs>
   <Available Slot Widths>
 where
 <Available Frequency Range-List> ::=
   <Available Frequency Range> [<Available Frequency Range-List>]
 <Available Frequency Range> ::=
   ( <Start NCF> <End NCF> ) |
   <FS defined by (n, m) containing contiguous available NCFs>
 and
 <Available NCFs> ::=
   <Available NCF Granularity> [<Offset>]
   -- Subset of supported n values given by p x n + q
   -- where p is a positive integer
   -- and q (offset) belongs to 0,..,p-1.
 and
 <Available Slot Widths> ::=
   <Available Slot Width Granularity>
   <Min Slot Width>
   -- given by j x 12.5 GHz, with j a positive integer
   <Max Slot Width>
   -- given by k x 12.5 GHz, with k a positive integer (k >= j)
                 Figure 17: Routing Information Model

5. Control-Plane Requirements

 The control of flexi-grid networks places additional requirements on
 the GMPLS protocols.  This section summarizes those requirements for
 signaling and routing.

5.1. Support for Media Channels

 The control plane SHALL be able to support media channels,
 characterized by a single frequency slot.  The representation of the
 media channel in the GMPLS control plane is the so-called "flexi-grid
 LSP".  Since network media channels are media channels, an LSP may

Gonzalez de Dios, et al. Informational [Page 31] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 also be the control-plane representation of a network media channel.
 Consequently, the control plane will also be able to support network
 media channels.

5.1.1. Signaling

 The signaling procedure SHALL be able to configure the nominal
 central frequency (n) of a flexi-grid LSP.
 The signaling procedure SHALL allow a flexible range of values for
 the frequency slot width (m) parameter.  Specifically, the control
 plane SHALL allow setting up a media channel with frequency slot
 width (m) ranging from a minimum of m = 1 (12.5 GHz) to a maximum of
 the entire C-band (the wavelength range 1530 nm to 1565 nm, which
 corresponds to the amplification range of erbium-doped fiber
 amplifiers) with a slot width granularity of 12.5 GHz.
 The signaling procedure SHALL be able to configure the minimum width
 (m) of a flexi-grid LSP.  In addition, the signaling procedure SHALL
 be able to configure local frequency slots.
 The control-plane architecture SHOULD allow for the support of the
 L-band (the wavelength range 1565 nm to 1625 nm) and the S-band (the
 wavelength range 1460 nm to 1530 nm).
 The signaling process SHALL be able to collect the local frequency
 slot assigned at each link along the path.
 The signaling procedures SHALL support all of the RSA architectural
 models (R&SA, R+SA, and R+DSA) within a single set of protocol
 objects, although some objects may only be applicable within one of
 the models.

5.1.2. Routing

 The routing protocol will support all functions described in
 [RFC4202] and extend them to a flexi-grid data plane.
 The routing protocol SHALL distribute sufficient information to
 compute paths to enable the signaling procedure to establish LSPs as
 described in the previous sections.  This includes, at a minimum, the
 data described by the information model in Figure 17.
 The routing protocol SHALL update its advertisements of available
 resources and capabilities as the usage of resources in the network
 varies with the establishment or teardown of LSPs.  These updates
 SHOULD be amenable to damping and thresholds as in other traffic
 engineering routing advertisements.

Gonzalez de Dios, et al. Informational [Page 32] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 The routing protocol SHALL support all of the RSA architectural
 models (R&SA, R+SA, and R+DSA) without any configuration or change of
 behavior.  Thus, the routing protocols SHALL be agnostic to the
 computation and signaling model that is in use.

5.2. Support for Media Channel Resizing

 The signaling procedures SHALL allow the resizing (growing or
 shrinking) of the frequency slot width of a media channel or network
 media channel.  The resizing MAY imply resizing the local frequency
 slots along the path of the flexi-grid LSP.
 The routing protocol SHALL update its advertisements of available
 resources and capabilities as the usage of resources in the network
 varies with the resizing of LSPs.  These updates SHOULD be amenable
 to damping and thresholds as in other traffic engineering routing
 advertisements.

5.3. Support for Logical Associations of Multiple Media Channels

 A set of media channels can be used to transport signals that have a
 logical association between them.  The control-plane architecture
 SHOULD allow multiple media channels to be logically associated.  The
 control plane SHOULD allow the co-routing of a set of media channels
 that are logically associated.

5.4. Support for Composite Media Channels

 As described in Sections 3.2.5 and 4.3, a media channel may be
 composed of multiple network media channels.
 The signaling procedures SHOULD include support for signaling a
 single control-plane LSP that includes information about multiple
 network media channels that will comprise the single compound media
 channel.
 The signaling procedures SHOULD include a mechanism to associate
 separately signaled control-plane LSPs so that the endpoints may
 correlate them into a single compound media channel.
 The signaling procedures MAY include a mechanism to dynamically vary
 the composition of a composite media channel by allowing network
 media channels to be added to or removed from the whole.
 The routing protocols MUST provide sufficient information for the
 computation of paths and slots for composite media channels using any
 of the three RSA architectural models (R&SA, R+SA, and R+DSA).

Gonzalez de Dios, et al. Informational [Page 33] RFC 7698 GMPLS Flexi-Grid Framework November 2015

5.5. Support for Neighbor Discovery and Link Property Correlation

 The control plane MAY include support for neighbor discovery such
 that a flexi-grid network can be constructed in a "plug-and-play"
 manner.  Note, however, that in common operational practice,
 validation processes are used rather than automatic discovery.
 The control plane SHOULD allow the nodes at opposite ends of a link
 to correlate the properties that they will apply to the link.  Such a
 correlation SHOULD include at least the identities of the nodes and
 the identities that they apply to the link.  Other properties, such
 as the link characteristics described for the routing information
 model in Figure 17, SHOULD also be correlated.
 Such neighbor discovery and link property correlation, if provided,
 MUST be able to operate in both an out-of-band and an out-of-fiber
 control channel.

6. Security Considerations

 The control-plane and data-plane aspects of a flexi-grid system are
 fundamentally the same as a fixed-grid system, and there is no
 substantial reason to expect the security considerations to be any
 different.
 A good overview of the security considerations for a GMPLS-based
 control plane can be found in [RFC5920].
 [RFC6163] includes a section describing security considerations for
 WSON, and it is reasonable to infer that these considerations apply
 and may be exacerbated in a flexi-grid SSON system.  In particular,
 the detailed and granular information describing a flexi-grid network
 and the capabilities of nodes in that network could put stress on the
 routing protocol or the out-of-band control channel used by the
 protocol.  An attacker might be able to cause small variations in the
 use of the network or the available resources (perhaps by modifying
 the environment of a fiber) and so trigger the routing protocol to
 make new flooding announcements.  This situation is explicitly
 mitigated in the requirements for the routing protocol extensions
 where it is noted that the protocol must include damping and
 configurable thresholds as already exist in the core GMPLS routing
 protocols.

Gonzalez de Dios, et al. Informational [Page 34] RFC 7698 GMPLS Flexi-Grid Framework November 2015

7. Manageability Considerations

 GMPLS systems already contain a number of management tools:
 o  MIB modules exist to model the control-plane protocols and the
    network elements [RFC4802] [RFC4803], and there is early work to
    provide similar access through YANG.  The features described in
    these models are currently designed to represent fixed-label
    technologies such as optical networks using the fixed grid;
    extensions may be needed in order to represent bandwidth,
    frequency slots, and effective frequency slots in flexi-grid
    networks.
 o  There are protocol extensions within GMPLS signaling to allow
    control-plane systems to report the presence of faults that affect
    LSPs [RFC4783], although it must be carefully noted that these
    mechanisms do not constitute an alarm mechanism that could be used
    to rapidly propagate information about faults in a way that would
    allow the data plane to perform protection switching.  These
    mechanisms could easily be enhanced with the addition of
    technology-specific reason codes if any are needed.
 o  The GMPLS protocols, themselves, already include fault detection
    and recovery mechanisms (such as the PathErr and Notify messages
    in RSVP-TE signaling as used by GMPLS [RFC3473]).  It is not
    anticipated that these mechanisms will need enhancement to support
    flexi-grid, although additional reason codes may be needed to
    describe technology-specific error cases.
 o  [RFC7260] describes a framework for the control and configuration
    of data-plane Operations, Administration, and Maintenance (OAM).
    It would not be appropriate for the IETF to define or describe
    data-plane OAM for optical systems, but the framework described in
    RFC 7260 could be used (with minor protocol extensions) to enable
    data-plane OAM that has been defined by the originators of the
    flexi-grid data-plane technology (the ITU-T).
 o  The Link Management Protocol (LMP) [RFC4204] is designed to allow
    the two ends of a network link to coordinate and confirm the
    configuration and capabilities that they will apply to the link.
    LMP is particularly applicable to optical links, where the
    characteristics of the network devices may considerably affect how
    the link is used and where misconfiguration or mis-fibering could
    make physical interoperability impossible.  LMP could easily be
    extended to collect and report information between the endpoints
    of links in a flexi-grid network.

Gonzalez de Dios, et al. Informational [Page 35] RFC 7698 GMPLS Flexi-Grid Framework November 2015

8. References

8.1. Normative References

 [G.694.1]  International Telecommunication Union, "Spectral grids for
            WDM applications: DWDM frequency grid", ITU-T
            Recommendation G.694.1, February 2012,
            <https://www.itu.int/rec/T-REC-G.694.1/en>.
 [G.800]    International Telecommunication Union, "Unified functional
            architecture of transport networks", ITU-T
            Recommendation G.800, February 2012,
            <http://www.itu.int/rec/T-REC-G.800/>.
 [G.805]    International Telecommunication Union, "Generic functional
            architecture of transport networks", ITU-T
            Recommendation G.805, March 2000,
            <https://www.itu.int/rec/T-REC-G.805-200003-I/en>.
 [G.8080]   International Telecommunication Union, "Architecture for
            the automatically switched optical network", ITU-T
            Recommendation G.8080/Y.1304, February 2012,
            <https://www.itu.int/rec/T-REC-G.8080-201202-I/en>.
 [G.870]    International Telecommunication Union, "Terms and
            definitions for optical transport networks", ITU-T
            Recommendation G.870/Y.1352, October 2012,
            <https://www.itu.int/rec/T-REC-G.870/en>.
 [G.872]    International Telecommunication Union, "Architecture of
            optical transport networks", ITU-T Recommendation G.872,
            October 2012,
            <http://www.itu.int/rec/T-REC-G.872-201210-I>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC3945]  Mannie, E., Ed., "Generalized Multi-Protocol Label
            Switching (GMPLS) Architecture", RFC 3945,
            DOI 10.17487/RFC3945, October 2004,
            <http://www.rfc-editor.org/info/rfc3945>.
 [RFC4202]  Kompella, K., Ed., and Y. Rekhter, Ed., "Routing
            Extensions in Support of Generalized Multi-Protocol Label
            Switching (GMPLS)", RFC 4202, DOI 10.17487/RFC4202,
            October 2005, <http://www.rfc-editor.org/info/rfc4202>.

Gonzalez de Dios, et al. Informational [Page 36] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 [RFC4206]  Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
            Hierarchy with Generalized Multi-Protocol Label Switching
            (GMPLS) Traffic Engineering (TE)", RFC 4206,
            DOI 10.17487/RFC4206, October 2005,
            <http://www.rfc-editor.org/info/rfc4206>.
 [RFC5511]  Farrel, A., "Routing Backus-Naur Form (RBNF): A Syntax
            Used to Form Encoding Rules in Various Routing Protocol
            Specifications", RFC 5511, DOI 10.17487/RFC5511,
            April 2009, <http://www.rfc-editor.org/info/rfc5511>.

8.2. Informative References

 [G.959.1-2013]
            International Telecommunication Union, "Optical transport
            network physical layer interfaces", Update to ITU-T
            Recommendation G.959.1, 2013.
 [RFC3473]  Berger, L., Ed., "Generalized Multi-Protocol Label
            Switching (GMPLS) Signaling Resource ReserVation Protocol-
            Traffic Engineering (RSVP-TE) Extensions", RFC 3473,
            DOI 10.17487/RFC3473, January 2003,
            <http://www.rfc-editor.org/info/rfc3473>.
 [RFC4204]  Lang, J., Ed., "Link Management Protocol (LMP)", RFC 4204,
            DOI 10.17487/RFC4204, October 2005,
            <http://www.rfc-editor.org/info/rfc4204>.
 [RFC4397]  Bryskin, I. and A. Farrel, "A Lexicography for the
            Interpretation of Generalized Multiprotocol Label
            Switching (GMPLS) Terminology within the Context of the
            ITU-T's Automatically Switched Optical Network (ASON)
            Architecture", RFC 4397, DOI 10.17487/RFC4397,
            February 2006, <http://www.rfc-editor.org/info/rfc4397>.
 [RFC4606]  Mannie, E. and D. Papadimitriou, "Generalized
            Multi-Protocol Label Switching (GMPLS) Extensions for
            Synchronous Optical Network (SONET) and Synchronous
            Digital Hierarchy (SDH) Control", RFC 4606,
            DOI 10.17487/RFC4606, August 2006,
            <http://www.rfc-editor.org/info/rfc4606>.
 [RFC4783]  Berger, L., Ed., "GMPLS - Communication of Alarm
            Information", RFC 4783, DOI 10.17487/RFC4783,
            December 2006, <http://www.rfc-editor.org/info/rfc4783>.

Gonzalez de Dios, et al. Informational [Page 37] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 [RFC4802]  Nadeau, T., Ed., Farrel, A., and , "Generalized
            Multiprotocol Label Switching (GMPLS) Traffic Engineering
            Management Information Base", RFC 4802,
            DOI 10.17487/RFC4802, February 2007,
            <http://www.rfc-editor.org/info/rfc4802>.
 [RFC4803]  Nadeau, T., Ed., and A. Farrel, Ed., "Generalized
            Multiprotocol Label Switching (GMPLS) Label Switching
            Router (LSR) Management Information Base", RFC 4803,
            DOI 10.17487/RFC4803, February 2007,
            <http://www.rfc-editor.org/info/rfc4803>.
 [RFC5920]  Fang, L., Ed., "Security Framework for MPLS and GMPLS
            Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
            <http://www.rfc-editor.org/info/rfc5920>.
 [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, DOI 10.17487/RFC6163, April 2011,
            <http://www.rfc-editor.org/info/rfc6163>.
 [RFC6344]  Bernstein, G., Ed., Caviglia, D., Rabbat, R., and H. van
            Helvoort, "Operating Virtual Concatenation (VCAT) and the
            Link Capacity Adjustment Scheme (LCAS) with Generalized
            Multi-Protocol Label Switching (GMPLS)", RFC 6344,
            DOI 10.17487/RFC6344, August 2011,
            <http://www.rfc-editor.org/info/rfc6344>.
 [RFC7139]  Zhang, F., Ed., Zhang, G., Belotti, S., Ceccarelli, D.,
            and K. Pithewan, "GMPLS Signaling Extensions for Control
            of Evolving G.709 Optical Transport Networks", RFC 7139,
            DOI 10.17487/RFC7139, March 2014,
            <http://www.rfc-editor.org/info/rfc7139>.
 [RFC7260]  Takacs, A., Fedyk, D., and J. He, "GMPLS RSVP-TE
            Extensions for Operations, Administration, and Maintenance
            (OAM) Configuration", RFC 7260, DOI 10.17487/RFC7260,
            June 2014, <http://www.rfc-editor.org/info/rfc7260>.

Gonzalez de Dios, et al. Informational [Page 38] RFC 7698 GMPLS Flexi-Grid Framework November 2015

Acknowledgments

 The authors would like to thank Pete Anslow for his insights and
 clarifications, and Matt Hartley and Jonas Maertensson for their
 reviews.
 This work was supported in part by the FP-7 IDEALIST project under
 grant agreement number 317999.

Contributors

 Adrian Farrel
 Old Dog Consulting
 Email: adrian@olddog.co.uk
 Daniel King
 Old Dog Consulting
 Email: daniel@olddog.co.uk
 Xian Zhang
 Huawei
 Email: zhang.xian@huawei.com
 Cyril Margaria
 Juniper Networks
 Email: cmargaria@juniper.net
 Qilei Wang
 ZTE
 Ruanjian Avenue, Nanjing, China
 Email: wang.qilei@zte.com.cn
 Malcolm Betts
 ZTE
 Email: malcolm.betts@zte.com.cn
 Sergio Belotti
 Alcatel-Lucent
 Optics CTO
 Via Trento 30 20059 Vimercate (Milano) Italy
 Phone: +39 039 686 3033
 Email: sergio.belotti@alcatel-lucent.com
 Yao Li
 Nanjing University
 Email: wsliguotou@hotmail.com

Gonzalez de Dios, et al. Informational [Page 39] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 Fei Zhang
 Huawei
 Email: zhangfei7@huawei.com
 Lei Wang
 Email: wang.lei@bupt.edu.cn
 Guoying Zhang
 China Academy of Telecom Research
 No.52 Huayuan Bei Road, Beijing, China
 Email: zhangguoying@ritt.cn
 Takehiro Tsuritani
 KDDI R&D Laboratories Inc.
 2-1-15 Ohara, Fujimino, Saitama, Japan
 Email: tsuri@kddilabs.jp
 Lei Liu
 UC Davis, United States
 Email: leiliu@ucdavis.edu
 Eve Varma
 Alcatel-Lucent
 Phone: +1 732 239 7656
 Email: eve.varma@alcatel-lucent.com
 Young Lee
 Huawei
 Jianrui Han
 Huawei
 Sharfuddin Syed
 Infinera
 Rajan Rao
 Infinera
 Marco Sosa
 Infinera
 Biao Lu
 Infinera
 Abinder Dhillon
 Infinera

Gonzalez de Dios, et al. Informational [Page 40] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 Felipe Jimenez Arribas
 Telefonica I+D
 Andrew G. Malis
 Huawei
 Email: agmalis@gmail.com
 Huub van Helvoort
 Hai Gaoming BV
 The Netherlands
 Email: huubatwork@gmail.com

Authors' Addresses

 Oscar Gonzalez de Dios (editor)
 Telefonica I+D
 Ronda de la Comunicacion s/n
 Madrid  28050
 Spain
 Phone: +34 91 312 96 47
 Email: oscar.gonzalezdedios@telefonica.com
 Ramon Casellas (editor)
 CTTC
 Av. Carl Friedrich Gauss n.7
 Castelldefels  Barcelona
 Spain
 Phone: +34 93 645 29 00
 Email: ramon.casellas@cttc.es
 Fatai Zhang
 Huawei
 Huawei Base, Bantian, Longgang District
 Shenzhen  518129
 China
 Phone: +86 755 28972912
 Email: zhangfatai@huawei.com

Gonzalez de Dios, et al. Informational [Page 41] RFC 7698 GMPLS Flexi-Grid Framework November 2015

 Xihua Fu
 Stairnote
 No.118, Taibai Road, Yanta District
 Xi'An
 China
 Email: fu.xihua@stairnote.com
 Daniele Ceccarelli
 Ericsson
 Via Calda 5
 Genova
 Italy
 Phone: +39 010 600 2512
 Email: daniele.ceccarelli@ericsson.com
 Iftekhar Hussain
 Infinera
 140 Caspian Ct.
 Sunnyvale, CA  94089
 United States
 Phone: 408 572 5233
 Email: ihussain@infinera.com

Gonzalez de Dios, et al. Informational [Page 42]

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