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RFC 7698

 
 
 

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

Part 2 of 3, p. 14 to 31
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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.

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   ---------------+                             +-----------------
                  |                             |
         +--------+                             +--------+
         |        |                             |        |  +---------
     ---o|        ===============================        o--|
         |        |             Fiber           |        |  | --\  /--
     ---o|        |                             |        o--|    \/
         |        |                             |        |  |    /\
     ---o|        ===============================        o--| --/  \--
         | Filter |                             | Filter |  |
         |        |                             |        |  +---------
         +--------+                             +--------+
                  |                             |
               |------- Basic Media Channel  ---------|
   ---------------+                             +-----------------


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

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   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.

   ----------+       +------------------------------+       +---------
             |       |                              |       |
      +------+       +------+                +------+       +------+
      |      |       |      |  +----------+  |      |       |      |
   --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  ---------------->

   ------+                  +---------------+                  +------
         |------------------|               |------------------|
    LSR  |       TE link    |      LSR      |   TE link        |  LSR
         |------------------|               |------------------|
   ------+                  +---------------+                  +------

                     Figure 9: Extended Media Channel

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   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.

   ----------+       +------------------------------+       +---------
             |       |                              |       |
      +------+       +------+                +------+       +------+
      |      |       |      |  +----------+  |      |       |      |
   --o|      =========      o--|          |--o      =========      o--
      |      | Fiber |      |  | --\  /-- |  |      | Fiber |      |
   --o|      |       |      o--|    \/    |--o      |       |      o--
      |      |       |      |  |    /\    |  |      |       |      |
   --o|      =========      o--***********|--o      =========      o--
      |Filter|       |Filter|  |          |  |Filter|       |Filter|
      |      |       |      |                |      |       |      |
      +------+       +------+                +------+       +------+
             |       |                              |       |
   ----------+       +------------------------------+       +---------

          <------------------------  Media Channel  ----------->

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

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   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.

   ---------+       +--------------------------------+       +--------
            |       |                                |       |
     +------+       +------+                  +------+       +------+
     |      |       |      |   +----------+   |      |       |      |
   -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

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   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)

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   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)

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   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.

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   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.

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   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.

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   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

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      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

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   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

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   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

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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.

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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,

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   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

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   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



(page 31 continued on part 3)

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