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RFC 7698

Informational
Pages: 42
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Framework and Requirements for GMPLS-Based Control of Flexi-Grid Dense Wavelength Division Multiplexing (DWDM) Networks

Part 1 of 3, p. 1 to 13
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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.

Page 2 
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.

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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

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   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.

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   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

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   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].

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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.

            -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].

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   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.

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   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.

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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.

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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.

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   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

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   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).


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