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RFC 6163

 
 
 

Framework for GMPLS and Path Computation Element (PCE) Control of Wavelength Switched Optical Networks (WSONs)

Part 2 of 2, p. 26 to 51
Prev RFC Part

 


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4.  Routing and Wavelength Assignment and the Control Plane

   From a control plane perspective, a wavelength-convertible network
   with full wavelength-conversion capability at each node can be
   controlled much like a packet MPLS-labeled network or a circuit-
   switched Time Division Multiplexing (TDM) network with full-time slot
   interchange capability is controlled.  In this case, the path

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   selection process needs to identify the Traffic Engineered (TE) links
   to be used by an optical path, and wavelength assignment can be made
   on a hop-by-hop basis.

   However, in the case of an optical network without wavelength
   converters, an optical path needs to be routed from source to
   destination and must use a single wavelength that is available along
   that path without "colliding" with a wavelength used by any other
   optical path that may share an optical fiber.  This is sometimes
   referred to as a "wavelength continuity constraint".

   In the general case of limited or no wavelength converters, the
   computation of both the links and wavelengths is known as RWA.

   The inputs to basic RWA are the requested optical path's source and
   destination, the network topology, the locations and capabilities of
   any wavelength converters, and the wavelengths available on each
   optical link.  The output from an algorithm providing RWA is an
   explicit route through ROADMs, a wavelength for optical transmitter,
   and a set of locations (generally associated with ROADMs or switches)
   where wavelength conversion is to occur and the new wavelength to be
   used on each component link after that point in the route.

   It is to be noted that the choice of a specific RWA algorithm is out
   of the scope of this document.  However, there are a number of
   different approaches to dealing with RWA algorithms that can affect
   the division of effort between path computation/routing and
   signaling.

4.1.  Architectural Approaches to RWA

   Two general computational approaches are taken to performing RWA.
   Some algorithms utilize a two-step procedure of path selection
   followed by wavelength assignment, and others perform RWA in a
   combined fashion.

   In the following sections, three different ways of performing RWA in
   conjunction with the control plane are considered.  The choice of one
   of these architectural approaches over another generally impacts the
   demands placed on the various control plane protocols.  The
   approaches are provided for reference purposes only, and other
   approaches are possible.

4.1.1.  Combined RWA (R&WA)

   In this case, a unique entity is in charge of performing routing and
   wavelength assignment.  This approach relies on a sufficient
   knowledge of network topology, of available network resources, and of

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   network nodes' capabilities.  This solution is compatible with most
   known RWA algorithms, particularly those concerned with network
   optimization.  On the other hand, this solution requires up-to-date
   and detailed network information.

   Such a computational entity could reside in two different places:

   o  In a PCE that maintains a complete and updated view of network
      state and provides path computation services to nodes

   o  In an ingress node, in which case all nodes have the R&WA
      functionality and network state is obtained by a periodic flooding
      of information provided by the other nodes

4.1.2.  Separated R and WA (R+WA)

   In this case, one entity performs routing while a second performs
   wavelength assignment.  The first entity furnishes one or more paths
   to the second entity, which will perform wavelength assignment and
   final path selection.

   The separation of the entities computing the path and the wavelength
   assignment constrains the class of RWA algorithms that may be
   implemented.  Although it may seem that algorithms optimizing a joint
   usage of the physical and wavelength paths are excluded from this
   solution, many practical optimization algorithms only consider a
   limited set of possible paths, e.g., as computed via a k-shortest
   path algorithm.  Hence, while there is no guarantee that the selected
   final route and wavelength offer the optimal solution, reasonable
   optimization can be performed by allowing multiple routes to pass to
   the wavelength selection process.

   The entity performing the routing assignment needs the topology
   information of the network, whereas the entity performing the
   wavelength assignment needs information on the network's available
   resources and specific network node capabilities.

4.1.3.  Routing and Distributed WA (R+DWA)

   In this case, one entity performs routing, while wavelength
   assignment is performed on a hop-by-hop, distributed manner along the
   previously computed path.  This mechanism relies on updating of a
   list of potential wavelengths used to ensure conformance with the
   wavelength continuity constraint.

   As currently specified, the GMPLS protocol suite signaling protocol
   can accommodate such an approach.  GMPLS, per [RFC3471], includes
   support for the communication of the set of labels (wavelengths) that

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   may be used between nodes via a Label Set.  When conversion is not
   performed at an intermediate node, a hop generates the Label Set it
   sends to the next hop based on the intersection of the Label Set
   received from the previous hop and the wavelengths available on the
   node's switch and ongoing interface.  The generation of the outgoing
   Label Set is up to the node local policy (even if one expects a
   consistent policy configuration throughout a given transparency
   domain).  When wavelength conversion is performed at an intermediate
   node, a new Label Set is generated.  The egress node selects one
   label in the Label Set that it received; additionally, the node can
   apply local policy during label selection.  GMPLS also provides
   support for the signaling of bidirectional optical paths.

   Depending on these policies, a wavelength assignment may not be
   found, or one may be found that consumes too many conversion
   resources relative to what a dedicated wavelength assignment policy
   would have achieved.  Hence, this approach may generate higher
   blocking probabilities in a heavily loaded network.

   This solution may be facilitated via signaling extensions that ease
   its functioning and possibly enhance its performance with respect to
   blocking probability.  Note that this approach requires less
   information dissemination than the other techniques described.

   The first entity may be a PCE or the ingress node of the LSP.

4.2.  Conveying Information Needed by RWA

   The previous sections have characterized WSONs and optical path
   requests.  In particular, high-level models of the information used
   by RWA process were presented.  This information can be viewed as
   either relatively static, i.e., changing with hardware changes
   (including possibly failures), or relatively dynamic, i.e., those
   that can change with optical path provisioning.  The time requirement
   in which an entity involved in RWA process needs to be notified of
   such changes is fairly situational.  For example, for network
   restoration purposes, learning of a hardware failure or of new
   hardware coming online to provide restoration capability can be
   critical.

   Currently, there are various methods for communicating RWA relevant
   information.  These include, but are not limited to, the following:

   o  Existing control plane protocols, i.e., GMPLS routing and
      signaling.  Note that routing protocols can be used to convey both
      static and dynamic information.

   o  Management protocols such as NetConf, SNMPv3, and CORBA.

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   o  Methods to access configuration and status information such as a
      command line interface (CLI).

   o  Directory services and accompanying protocols.  These are
      typically used for the dissemination of relatively static
      information.  Directory services are not suited to manage
      information in dynamic and fluid environments.

   o  Other techniques for dynamic information, e.g., sending
      information directly from NEs to PCEs to avoid flooding.  This
      would be useful if the number of PCEs is significantly less than
      the number of WSON NEs.  There may be other ways to limit flooding
      to "interested" NEs.

   Possible mechanisms to improve scaling of dynamic information
   include:

   o  Tailoring message content to WSON, e.g., the use of wavelength
      ranges or wavelength occupation bit maps

   o  Utilizing incremental updates if feasible

5.  Modeling Examples and Control Plane Use Cases

   This section provides examples of the fixed and switched optical node
   and wavelength constraint models of Section 3 and use cases for WSON
   control plane path computation, establishment, rerouting, and
   optimization.

5.1.  Network Modeling for GMPLS/PCE Control

   Consider a network containing three routers (R1 through R3), eight
   WSON nodes (N1 through N8), 18 links (L1 through L18), and one OEO
   converter (O1) in a topology shown in Figure 7.

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                       +--+    +--+             +--+       +--------+
                  +-L3-+N2+-L5-+  +--------L12--+N6+--L15--+   N8   +
                  |    +--+    |N4+-L8---+      +--+       ++--+---++
                  |            |  +-L9--+|                  |  |   |
      +--+      +-+-+          ++-+     ||                  | L17 L18
      |  ++-L1--+   |           |      ++++      +----L16---+  |   |
      |R1|      | N1|           L7     |R2|      |             |   |
      |  ++-L2--+   |           |      ++-+      |            ++---++
      +--+      +-+-+           |       |        |            +  R3 |
                  |    +--+    ++-+     |        |            +-----+
                  +-L4-+N3+-L6-+N5+-L10-+       ++----+
                       +--+    |  +--------L11--+ N7  +
                               +--+             ++---++
                                                 |   |
                                                L13 L14
                                                 |   |
                                                ++-+ |
                                                |O1+-+
                                                +--+

        Figure 7.  Routers and WSON Nodes in a GMPLS and PCE Environment

5.1.1.  Describing the WSON Nodes

   The eight WSON nodes described in Figure 7 have the following
   properties:

   o  Nodes N1, N2, and N3 have FOADMs installed and can therefore only
      access a static and pre-defined set of wavelengths.

   o  All other nodes contain ROADMs and can therefore access all
      wavelengths.

   o  Nodes N4, N5, N7, and N8 are multi-degree nodes, allowing any
      wavelength to be optically switched between any of the links.
      Note, however, that this does not automatically apply to
      wavelengths that are being added or dropped at the particular
      node.

   o  Node N4 is an exception to that: this node can switch any
      wavelength from its add/drop ports to any of its output links (L5,
      L7, and L12 in this case).

   o  The links from the routers are only able to carry one wavelength,
      with the exception of links L8 and L9, which are capable to
      add/drop any wavelength.

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   o  Node N7 contains an OEO transponder (O1) connected to the node via
      links L13 and L14.  That transponder operates in 3R mode and does
      not change the wavelength of the signal.  Assume that it can
      regenerate any of the client signals but only for a specific
      wavelength.

   Given the above restrictions, the node information for the eight
   nodes can be expressed as follows (where ID = identifier, SCM =
   switched connectivity matrix, and FCM = fixed connectivity matrix):

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      +ID+SCM                    +FCM                    +
      |  |   |L1 |L2 |L3 |L4 |   |   |L1 |L2 |L3 |L4 |   |
      |  |L1 |0  |0  |0  |0  |   |L1 |0  |0  |1  |0  |   |
      |N1|L2 |0  |0  |0  |0  |   |L2 |0  |0  |0  |1  |   |
      |  |L3 |0  |0  |0  |0  |   |L3 |1  |0  |0  |1  |   |
      |  |L4 |0  |0  |0  |0  |   |L4 |0  |1  |1  |0  |   |
      +--+---+---+---+---+---+---+---+---+---+---+---+---+
      |  |   |L3 |L5 |   |   |   |   |L3 |L5 |   |   |   |
      |N2|L3 |0  |0  |   |   |   |L3 |0  |1  |   |   |   |
      |  |L5 |0  |0  |   |   |   |L5 |1  |0  |   |   |   |
      +--+---+---+---+---+---+---+---+---+---+---+---+---+
      |  |   |L4 |L6 |   |   |   |   |L4 |L6 |   |   |   |
      |N3|L4 |0  |0  |   |   |   |L4 |0  |1  |   |   |   |
      |  |L6 |0  |0  |   |   |   |L6 |1  |0  |   |   |   |
      +--+---+---+---+---+---+---+---+---+---+---+---+---+
      |  |   |L5 |L7 |L8 |L9 |L12|   |L5 |L7 |L8 |L9 |L12|
      |  |L5 |0  |1  |1  |1  |1  |L5 |0  |0  |0  |0  |0  |
      |N4|L7 |1  |0  |1  |1  |1  |L7 |0  |0  |0  |0  |0  |
      |  |L8 |1  |1  |0  |1  |1  |L8 |0  |0  |0  |0  |0  |
      |  |L9 |1  |1  |1  |0  |1  |L9 |0  |0  |0  |0  |0  |
      |  |L12|1  |1  |1  |1  |0  |L12|0  |0  |0  |0  |0  |
      +--+---+---+---+---+---+---+---+---+---+---+---+---+
      |  |   |L6 |L7 |L10|L11|   |   |L6 |L7 |L10|L11|   |
      |  |L6 |0  |1  |0  |1  |   |L6 |0  |0  |1  |0  |   |
      |N5|L7 |1  |0  |0  |1  |   |L7 |0  |0  |0  |0  |   |
      |  |L10|0  |0  |0  |0  |   |L10|1  |0  |0  |0  |   |
      |  |L11|1  |1  |0  |0  |   |L11|0  |0  |0  |0  |   |
      +--+---+---+---+---+---+---+---+---+---+---+---+---+
      |  |   |L12|L15|   |   |   |   |L12|L15|   |   |   |
      |N6|L12|0  |1  |   |   |   |L12|0  |0  |   |   |   |
      |  |L15|1  |0  |   |   |   |L15|0  |0  |   |   |   |
      +--+---+---+---+---+---+---+---+---+---+---+---+---+
      |  |   |L11|L13|L14|L16|   |   |L11|L13|L14|L16|   |
      |  |L11|0  |1  |0  |1  |   |L11|0  |0  |0  |0  |   |
      |N7|L13|1  |0  |0  |0  |   |L13|0  |0  |1  |0  |   |
      |  |L14|0  |0  |0  |1  |   |L14|0  |1  |0  |0  |   |
      |  |L16|1  |0  |1  |0  |   |L16|0  |0  |1  |0  |   |
      +--+---+---+---+---+---+---+---+---+---+---+---+---+
      |  |   |L15|L16|L17|L18|   |   |L15|L16|L17|L18|   |
      |  |L15|0  |1  |0  |0  |   |L15|0  |0  |0  |1  |   |
      |N8|L16|1  |0  |0  |0  |   |L16|0  |0  |1  |0  |   |
      |  |L17|0  |0  |0  |0  |   |L17|0  |1  |0  |0  |   |
      |  |L18|0  |0  |0  |0  |   |L18|1  |0  |1  |0  |   |
      +--+---+---+---+---+---+---+---+---+---+---+---+---+

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5.1.2.  Describing the Links

   For the following discussion, some simplifying assumptions are made:

   o  It is assumed that the WSON node supports a total of four
      wavelengths, designated WL1 through WL4.

   o  It is assumed that the impairment feasibility of a path or path
      segment is independent from the wavelength chosen.

   For the discussion of RWA operation, to build LSPs between two
   routers, the wavelength constraints on the links between the routers
   and the WSON nodes as well as the connectivity matrix of these links
   need to be specified:

   +Link+WLs supported    +Possible output links+
   | L1 | WL1             | L3                  |
   +----+-----------------+---------------------+
   | L2 | WL2             | L4                  |
   +----+-----------------+---------------------+
   | L8 | WL1 WL2 WL3 WL4 | L5 L7 L12           |
   +----+-----------------+---------------------+
   | L9 | WL1 WL2 WL3 WL4 | L5 L7 L12           |
   +----+-----------------+---------------------+
   | L10| WL2             | L6                  |
   +----+-----------------+---------------------+
   | L13| WL1 WL2 WL3 WL4 | L11 L14             |
   +----+-----------------+---------------------+
   | L14| WL1 WL2 WL3 WL4 | L13 L16             |
   +----+-----------------+---------------------+
   | L17| WL2             | L16                 |
   +----+-----------------+---------------------+
   | L18| WL1             | L15                 |
   +----+-----------------+---------------------+

   Note that the possible output links for the links connecting to the
   routers is inferred from the switched connectivity matrix and the
   fixed connectivity matrix of the Nodes N1 through N8 and is shown
   here for convenience; that is, this information does not need to be
   repeated.

5.2.  RWA Path Computation and Establishment

   The calculation of optical impairment feasible routes is outside the
   scope of this document.  In general, optical impairment feasible
   routes serve as an input to an RWA algorithm.

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   For the example use case shown here, assume the following feasible
   routes:

    +Endpoint 1+Endpoint 2+Feasible Route        +
    |  R1      | R2       | L1 L3 L5 L8          |
    |  R1      | R2       | L1 L3 L5 L9          |
    |  R1      | R2       | L2 L4 L6 L7 L8       |
    |  R1      | R2       | L2 L4 L6 L7 L9       |
    |  R1      | R2       | L2 L4 L6 L10         |
    |  R1      | R3       | L1 L3 L5 L12 L15 L18 |
    |  R1      | N7       | L2 L4 L6 L11         |
    |  N7      | R3       | L16 L17              |
    |  N7      | R2       | L16 L15 L12 L9       |
    |  R2      | R3       | L8 L12 L15 L18       |
    |  R2      | R3       | L8 L7 L11 L16 L17    |
    |  R2      | R3       | L9 L12 L15 L18       |
    |  R2      | R3       | L9 L7 L11 L16 L17    |

   Given a request to establish an LSP between R1 and R2, an RWA
   algorithm finds the following possible solutions:

    +WL  + Path          +
    | WL1| L1 L3 L5 L8   |
    | WL1| L1 L3 L5 L9   |
    | WL2| L2 L4 L6 L7 L8|
    | WL2| L2 L4 L6 L7 L9|
    | WL2| L2 L4 L6 L10  |

   Assume now that an RWA algorithm yields WL1 and the path L1 L3 L5 L8
   for the requested LSP.

   Next, another LSP is signaled from R1 to R2.  Given the established
   LSP using WL1, the following table shows the available paths:

    +WL  + Path          +
    | WL2| L2 L4 L6 L7 L9|
    | WL2| L2 L4 L6 L10  |

   Assume now that an RWA algorithm yields WL2 and the path L2 L4 L6 L7
   L9 for the establishment of the new LSP.

   An LSP request -- this time from R2 to R3 -- cannot be fulfilled
   since the four possible paths (starting at L8 and L9) are already in
   use.

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5.3.  Resource Optimization

   The preceding example gives rise to another use case: the
   optimization of network resources.  Optimization can be achieved on a
   number of layers (e.g., through electrical or optical multiplexing of
   client signals) or by re-optimizing the solutions found by an RWA
   algorithm.

   Given the above example again, assume that an RWA algorithm should
   identify a path between R2 and R3.  The only possible path to reach
   R3 from R2 needs to use L9.  L9, however, is blocked by one of the
   LSPs from R1.

5.4.  Support for Rerouting

   It is also envisioned that the extensions to GMPLS and PCE support
   rerouting of wavelengths in case of failures.

   For this discussion, assume that the only two LSPs in use in the
   system are:

   LSP1: WL1 L1 L3 L5 L8

   LSP2: WL2 L2 L4 L6 L7 L9

   Furthermore, assume that the L5 fails.  An RWA algorithm can now
   compute and establish the following alternate path:

   R1 -> N7 -> R2

   Level 3 regeneration will take place at N7, so that the complete path
   looks like this:

   R1 -> L2 L4 L6 L11 L13 -> O1 -> L14 L16 L15 L12 L9 -> R2

5.5.  Electro-Optical Networking Scenarios

   In the following subsections, various networking scenarios are
   considered involving regenerators, OEO switches, and wavelength
   converters.  These scenarios can be grouped roughly by type and
   number of extensions to the GMPLS control plane that would be
   required.

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5.5.1.  Fixed Regeneration Points

   In the simplest networking scenario involving regenerators,
   regeneration is associated with a WDM link or an entire node and is
   not optional; that is, all signals traversing the link or node will
   be regenerated.  This includes OEO switches since they provide
   regeneration on every port.

   There may be input constraints and output constraints on the
   regenerators.  Hence, the path selection process will need to know
   the regenerator constraints from routing or other means so that it
   can choose a compatible path.  For impairment-aware routing and
   wavelength assignment (IA-RWA), the path selection process will also
   need to know which links/nodes provide regeneration.  Even for
   "regular" RWA, this regeneration information is useful since
   wavelength converters typically perform regeneration, and the
   wavelength continuity constraint can be relaxed at such a point.

   Signaling does not need to be enhanced to include this scenario since
   there are no reconfigurable regenerator options on input, output, or
   processing.

5.5.2.  Shared Regeneration Pools

   In this scenario, there are nodes with shared regenerator pools
   within the network in addition to the fixed regenerators of the
   previous scenario.  These regenerators are shared within a node and
   their application to a signal is optional.  There are no
   reconfigurable options on either input or output.  The only
   processing option is to "regenerate" a particular signal or not.

   In this case, regenerator information is used in path computation to
   select a path that ensures signal compatibility and IA-RWA criteria.

   To set up an LSP that utilizes a regenerator from a node with a
   shared regenerator pool, it is necessary to indicate that
   regeneration is to take place at that particular node along the
   signal path.  Such a capability does not currently exist in GMPLS
   signaling.

5.5.3.  Reconfigurable Regenerators

   This scenario is concerned with regenerators that require
   configuration prior to use on an optical signal.  As discussed
   previously, this could be due to a regenerator that must be
   configured to accept signals with different characteristics, for
   regenerators with a selection of output attributes, or for
   regenerators with additional optional processing capabilities.

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   As in the previous scenarios, it is necessary to have information
   concerning regenerator properties for selection of compatible paths
   and for IA-RWA computations.  In addition, during LSP setup, it is
   necessary to be able to configure regenerator options at a particular
   node along the path.  Such a capability does not currently exist in
   GMPLS signaling.

5.5.4.  Relation to Translucent Networks

   Networks that contain both transparent network elements such as
   Reconfigurable Optical Add/Drop Multiplexers (ROADMs) and electro-
   optical network elements such as regenerators or OEO switches are
   frequently referred to as translucent optical networks.

   Three main types of translucent optical networks have been discussed:

   1.  Transparent "islands" surrounded by regenerators.  This is
       frequently seen when transitioning from a metro optical
       subnetwork to a long-haul optical subnetwork.

   2.  Mostly transparent networks with a limited number of OEO
       ("opaque") nodes strategically placed.  This takes advantage of
       the inherent regeneration capabilities of OEO switches.  In the
       planning of such networks, one has to determine the optimal
       placement of the OEO switches.

   3.  Mostly transparent networks with a limited number of optical
       switching nodes with "shared regenerator pools" that can be
       optionally applied to signals passing through these switches.
       These switches are sometimes called translucent nodes.

   All three types of translucent networks fit within the networking
   scenarios of Sections 5.5.1 and 5.5.2.  Hence, they can be
   accommodated by the GMPLS extensions envisioned in this document.

6.  GMPLS and PCE Implications

   The presence and amount of wavelength conversion available at a
   wavelength switching interface have an impact on the information that
   needs to be transferred by the control plane (GMPLS) and the PCE
   architecture.  Current GMPLS and PCE standards address the full
   wavelength conversion case, so the following subsections will only
   address the limited and no wavelength conversion cases.

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6.1.  Implications for GMPLS Signaling

   Basic support for WSON signaling already exists in GMPLS with the
   lambda (value 9) LSP encoding type [RFC3471] or for G.709-compatible
   optical channels, the LSP encoding type (value = 13) "G.709 Optical
   Channel" from [RFC4328].  However, a number of practical issues arise
   in the identification of wavelengths and signals and in distributed
   wavelength assignment processes, which are discussed below.

6.1.1.  Identifying Wavelengths and Signals

   As previously stated, a global-fixed mapping between wavelengths and
   labels simplifies the characterization of WDM links and WSON devices.
   Furthermore, a mapping like the one described in [RFC6205] provides
   fixed mapping for communication between PCE and WSON PCCs.

6.1.2.  WSON Signals and Network Element Processing

   As discussed in Section 3.3.2, a WSON signal at any point along its
   path can be characterized by the (a) modulation format, (b) FEC, (c)
   wavelength, (d) bitrate, and (e) G-PID.

   Currently, G-PID, wavelength (via labels), and bitrate (via bandwidth
   encoding) are supported in [RFC3471] and [RFC3473].  These RFCs can
   accommodate the wavelength changing at any node along the LSP and can
   thus provide explicit control of wavelength converters.

   In the fixed regeneration point scenario described in Section 5.5.1,
   no enhancements are required to signaling since there are no
   additional configuration options for the LSP at a node.

   In the case of shared regeneration pools described in Section 5.5.2,
   it is necessary to indicate to a node that it should perform
   regeneration on a particular signal.  Viewed another way, for an LSP,
   it is desirable to specify that certain nodes along the path perform
   regeneration.  Such a capability does not currently exist in GMPLS
   signaling.

   The case of reconfigurable regenerators described in Section 5.5.3 is
   very similar to the previous except that now there are potentially
   many more items that can be configured on a per-node basis for an
   LSP.

   Note that the techniques of [RFC5420] that allow for additional LSP
   attributes and their recording in a Record Route Object (RRO) could
   be extended to allow for additional LSP attributes in an Explicit
   Route Object (ERO).  This could allow one to indicate where optional

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   3R regeneration should take place along a path, any modification of
   LSP attributes such as modulation format, or any enhance processing
   such as performance monitoring.

6.1.3.  Combined RWA/Separate Routing WA support

   In either the combined RWA case or the separate routing WA case, the
   node initiating the signaling will have a route from the source to
   destination along with the wavelengths (generalized labels) to be
   used along portions of the path.  Current GMPLS signaling supports an
   Explicit Route Object (ERO), and within an ERO, an ERO Label
   subobject can be used to indicate the wavelength to be used at a
   particular node.  In case the local label map approach is used, the
   label subobject entry in the ERO has to be interpreted appropriately.

6.1.4.  Distributed Wavelength Assignment: Unidirectional, No Converters

   GMPLS signaling for a unidirectional optical path LSP allows for the
   use of a Label Set object in the Resource Reservation Protocol -
   Traffic Engineering (RSVP-TE) path message.  Processing of the Label
   Set object to take the intersection of available lambdas along a path
   can be performed, resulting in the set of available lambdas being
   known to the destination, which can then use a wavelength selection
   algorithm to choose a lambda.

6.1.5.  Distributed Wavelength Assignment: Unidirectional, Limited
        Converters

   In the case of wavelength converters, nodes with wavelength
   converters would need to make the decision as to whether to perform
   conversion.  One indicator for this would be that the set of
   available wavelengths that is obtained via the intersection of the
   incoming Label Set and the output links available wavelengths is
   either null or deemed too small to permit successful completion.

   At this point, the node would need to remember that it will apply
   wavelength conversion and will be responsible for assigning the
   wavelength on the previous lambda-contiguous segment when the RSVP-TE
   RESV message is processed.  The node will pass on an enlarged label
   set reflecting only the limitations of the wavelength converter and
   the output link.  The record route option in RSVP-TE signaling can be
   used to show where wavelength conversion has taken place.

6.1.6.  Distributed Wavelength Assignment: Bidirectional, No Converters

   There are cases of a bidirectional optical path that require the use
   of the same lambda in both directions.  The above procedure can be
   used to determine the available bidirectional lambda set if it is

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   interpreted that the available Label Set is available in both
   directions.  According to [RFC3471], Section 4.1, the setup of
   bidirectional LSPs is indicated by the presence of an upstream label
   in the path message.

   However, until the intersection of the available Label Sets is
   determined along the path and at the destination node, the upstream
   label information may not be correct.  This case can be supported
   using current GMPLS mechanisms but may not be as efficient as an
   optimized bidirectional single-label allocation mechanism.

6.2.  Implications for GMPLS Routing

   GMPLS routing [RFC4202] currently defines an interface capability
   descriptor for "Lambda Switch Capable" (LSC) that can be used to
   describe the interfaces on a ROADM or other type of wavelength
   selective switch.  In addition to the topology information typically
   conveyed via an Interior Gateway Protocol (IGP), it would be
   necessary to convey the following subsystem properties to minimally
   characterize a WSON:

   1.  WDM link properties (allowed wavelengths)

   2.  Optical transmitters (wavelength range)

   3.  ROADM/FOADM properties (connectivity matrix, port wavelength
       restrictions)

   4.  Wavelength converter properties (per network element, may change
       if a common limited shared pool is used)

   This information is modeled in detail in [WSON-Info], and a compact
   encoding is given in [WSON-Encode].

6.2.1.  Electro-Optical Element Signal Compatibility

   In network scenarios where signal compatibility is a concern, it is
   necessary to add parameters to our existing node and link models to
   take into account electro-optical input constraints, output
   constraints, and the signal-processing capabilities of an NE in path
   computations.

   Input constraints:

   1.  Permitted optical tributary signal classes: A list of optical
       tributary signal classes that can be processed by this network
       element or carried over this link (configuration type)

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   2.  Acceptable FEC codes (configuration type)

   3.  Acceptable bitrate set: a list of specific bitrates or bitrate
       ranges that the device can accommodate.  Coarse bitrate info is
       included with the optical tributary signal-class restrictions.

   4.  Acceptable G-PID list: a list of G-PIDs corresponding to the
       "client" digital streams that is compatible with this device

   Note that the bitrate of the signal does not change over the LSP.
   This can be communicated as an LSP parameter; therefore, this
   information would be available for any NE that needs to use it for
   configuration.  Hence, it is not necessary to have "configuration
   type" for the NE with respect to bitrate.

   Output constraints:

   1.  Output modulation: (a) same as input, (b) list of available types

   2.  FEC options: (a) same as input, (b) list of available codes

   Processing capabilities:

   1.  Regeneration: (a) 1R, (b) 2R, (c) 3R, (d) list of selectable
       regeneration types

   2.  Fault and performance monitoring: (a) G-PID particular
       capabilities, (b) optical performance monitoring capabilities.

   Note that such parameters could be specified on (a) a network-
   element-wide basis, (b) a per-port basis, or (c) a per-regenerator
   basis.  Typically, such information has been on a per-port basis; see
   the GMPLS interface switching capability descriptor [RFC4202].

6.2.2.  Wavelength-Specific Availability Information

   For wavelength assignment, it is necessary to know which specific
   wavelengths are available and which are occupied if a combined RWA
   process or separate WA process is run as discussed in Sections 4.1.1
   and 4.1.2.  This is currently not possible with GMPLS routing.

   In the routing extensions for GMPLS [RFC4202], requirements for
   layer-specific TE attributes are discussed.  RWA for optical networks
   without wavelength converters imposes an additional requirement for
   the lambda (or optical channel) layer: that of knowing which specific
   wavelengths are in use.  Note that current DWDM systems range from 16
   channels to 128 channels, with advanced laboratory systems with as
   many as 300 channels.  Given these channel limitations, if the

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   approach of a global wavelength to label mapping or furnishing the
   local mappings to the PCEs is taken, representing the use of
   wavelengths via a simple bitmap is feasible [Gen-Encode].

6.2.3.  WSON Routing Information Summary

   The following table summarizes the WSON information that could be
   conveyed via GMPLS routing and attempts to classify that information
   according to its static or dynamic nature and its association with
   either a link or a node.

     Information                         Static/Dynamic       Node/Link
     ------------------------------------------------------------------
     Connectivity matrix                 Static               Node
     Per-port wavelength restrictions    Static               Node(1)
     WDM link (fiber) lambda ranges      Static               Link
     WDM link channel spacing            Static               Link
     Optical transmitter range           Static               Link(2)
     Wavelength conversion capabilities  Static(3)            Node
     Maximum bandwidth per wavelength    Static               Link
     Wavelength availability             Dynamic(4)           Link
     Signal compatibility and processing Static/Dynamic       Node

   Notes:

   1.  These are the per-port wavelength restrictions of an optical
       device such as a ROADM and are independent of any optical
       constraints imposed by a fiber link.

   2.  This could also be viewed as a node capability.

   3.  This could be dynamic in the case of a limited pool of converters
       where the number available can change with connection
       establishment.  Note that it may be desirable to include
       regeneration capabilities here since OEO converters are also
       regenerators.

   4.  This is not necessarily needed in the case of distributed
       wavelength assignment via signaling.

   While the full complement of the information from the previous table
   is needed in the Combined RWA and the separate Routing and WA
   architectures, in the case of Routing + Distributed WA via Signaling,
   only the following information is needed:

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     Information                         Static/Dynamic       Node/Link
     ------------------------------------------------------------------
     Connectivity matrix                 Static               Node
     Wavelength conversion capabilities  Static(3)            Node

   Information models and compact encodings for this information are
   provided in [WSON-Info], [Gen-Encode], and [WSON-Encode].

6.3.  Optical Path Computation and Implications for PCE

   As previously noted, RWA can be computationally intensive.  Such
   computationally intensive path computations and optimizations were
   part of the impetus for the PCE architecture [RFC4655].

   The Path Computation Element Communication Protocol (PCEP) defines
   the procedures necessary to support both sequential [RFC5440] and
   Global Concurrent Optimization (GCO) path computations [RFC5557].
   With some protocol enhancement, the PCEP is well positioned to
   support WSON-enabled RWA computation.

   Implications for PCE generally fall into two main categories: (a)
   optical path constraints and characteristics, (b) computation
   architectures.

6.3.1.  Optical Path Constraints and Characteristics

   For the varying degrees of optimization that may be encountered in a
   network, the following models of bulk and sequential optical path
   requests are encountered:

   o  Batch optimization, multiple optical paths requested at one time
      (PCE-GCO)

   o  Optical path(s) and backup optical path(s) requested at one time
      (PCEP)

   o  Single optical path requested at a time (PCEP)

   PCEP and PCE-GCO can be readily enhanced to support all of the
   potential models of RWA computation.

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   Optical path constraints include:

   o  Bidirectional assignment of wavelengths

   o  Possible simultaneous assignment of wavelength to primary and
      backup paths

   o  Tuning range constraint on optical transmitter

6.3.2.  Electro-Optical Element Signal Compatibility

   When requesting a path computation to PCE, the PCC should be able to
   indicate the following:

   o  The G-PID type of an LSP

   o  The signal attributes at the transmitter (at the source): (i)
      modulation type, (ii) FEC type

   o  The signal attributes at the receiver (at the sink): (i)
      modulation type, (ii) FEC type

   The PCE should be able to respond to the PCC with the following:

   o  The conformity of the requested optical characteristics associated
      with the resulting LSP with the source, sink, and NE along the LSP

   o  Additional LSP attributes modified along the path (e.g.,
      modulation format change)

6.3.3.  Discovery of RWA-Capable PCEs

   The algorithms and network information needed for RWA are somewhat
   specialized and computationally intensive; hence, not all PCEs within
   a domain would necessarily need or want this capability.  Therefore,
   it would be useful to indicate that a PCE has the ability to deal
   with RWA via the mechanisms being established for PCE discovery
   [RFC5088].  [RFC5088] indicates that a sub-TLV could be allocated for
   this purpose.

   Recent progress on objective functions in PCE [RFC5541] would allow
   operators to flexibly request differing objective functions per their
   need and applications.  For instance, this would allow the operator
   to choose an objective function that minimizes the total network cost
   associated with setting up a set of paths concurrently.  This would
   also allow operators to choose an objective function that results in
   the most evenly distributed link utilization.

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   This implies that PCEP would easily accommodate a wavelength
   selection algorithm in its objective function to be able to optimize
   the path computation from the perspective of wavelength assignment if
   chosen by the operators.

7.  Security Considerations

   This document does not require changes to the security models within
   GMPLS and associated protocols.  That is, the OSPF-TE, RSVP-TE, and
   PCEP security models could be operated unchanged.

   However, satisfying the requirements for RWA using the existing
   protocols may significantly affect the loading of those protocols.
   This may make the operation of the network more vulnerable to denial-
   of-service attacks.  Therefore, additional care maybe required to
   ensure that the protocols are secure in the WSON environment.

   Furthermore, the additional information distributed in order to
   address RWA represents a disclosure of network capabilities that an
   operator may wish to keep private.  Consideration should be given to
   securing this information.  For a general discussion on MPLS- and
   GMPLS-related security issues, see the MPLS/GMPLS security framework
   [RFC5920].

8.  Acknowledgments

   The authors would like to thank Adrian Farrel for many helpful
   comments that greatly improved the contents of this document.

9.  References

9.1.  Normative References

   [RFC3471]     Berger, L., Ed., "Generalized Multi-Protocol Label
                 Switching (GMPLS) Signaling Functional Description",
                 RFC 3471, January 2003.

   [RFC3473]     Berger, L., Ed., "Generalized Multi-Protocol Label
                 Switching (GMPLS) Signaling Resource ReserVation
                 Protocol-Traffic Engineering (RSVP-TE) Extensions", RFC
                 3473, January 2003.

   [RFC3945]     Mannie, E., Ed., "Generalized Multi-Protocol Label
                 Switching (GMPLS) Architecture", RFC 3945, October
                 2004.

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   [RFC4202]     Kompella, K., Ed., and Y. Rekhter, Ed., "Routing
                 Extensions in Support of Generalized Multi-Protocol
                 Label Switching (GMPLS)", RFC 4202, October 2005.

   [RFC4328]     Papadimitriou, D., Ed., "Generalized Multi-Protocol
                 Label Switching (GMPLS) Signaling Extensions for G.709
                 Optical Transport Networks Control", RFC 4328, January
                 2006.

   [RFC4655]     Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
                 Computation Element (PCE)-Based Architecture", RFC
                 4655, August 2006.

   [RFC5088]     Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and
                 R. Zhang, "OSPF Protocol Extensions for Path
                 Computation Element (PCE) Discovery", RFC 5088, January
                 2008.

   [RFC5212]     Shiomoto, K., Papadimitriou, D., Le Roux, JL.,
                 Vigoureux, M., and D. Brungard, "Requirements for
                 GMPLS-Based Multi-Region and Multi-Layer Networks
                 (MRN/MLN)", RFC 5212, July 2008.

   [RFC5557]     Lee, Y., Le Roux, JL., King, D., and E. Oki, "Path
                 Computation Element Communication Protocol (PCEP)
                 Requirements and Protocol Extensions in Support of
                 Global Concurrent Optimization", RFC 5557, July 2009.

   [RFC5420]     Farrel, A., Ed., Papadimitriou, D., Vasseur, JP., and
                 A. Ayyangarps, "Encoding of Attributes for MPLS LSP
                 Establishment Using Resource Reservation Protocol
                 Traffic Engineering (RSVP-TE)", RFC 5420, February
                 2009.

   [RFC5440]     Vasseur, JP., Ed., and JL. Le Roux, Ed., "Path
                 Computation Element (PCE) Communication Protocol
                 (PCEP)", RFC 5440, March 2009.

   [RFC5541]     Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of
                 Objective Functions in the Path Computation Element
                 Communication Protocol (PCEP)", RFC 5541, June 2009.

9.2.  Informative References

   [Gen-Encode]  Bernstein, G., Lee, Y., Li, D., and W. Imajuku,
                 "General Network Element Constraint Encoding for GMPLS
                 Controlled Networks", Work in Progress, December 2010.

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   [G.652]       ITU-T Recommendation G.652, "Characteristics of a
                 single-mode optical fibre and cable", November 2009.

   [G.653]       ITU-T Recommendation G.653, "Characteristics of a
                 dispersion-shifted single-mode optical fibre and
                 cable", July 2010.

   [G.654]       ITU-T Recommendation G.654, "Characteristics of a cut-
                 off shifted single-mode optical fibre and cable", July
                 2010.

   [G.655]       ITU-T Recommendation G.655, "Characteristics of a non-
                 zero dispersion-shifted single-mode optical fibre and
                 cable", November 2009.

   [G.656]       ITU-T Recommendation G.656, "Characteristics of a fibre
                 and cable with non-zero dispersion for wideband optical
                 transport", July 2010.

   [G.671]       ITU-T Recommendation G.671, "Transmission
                 characteristics of optical components and subsystems",
                 January 2009.

   [G.694.1]     ITU-T Recommendation G.694.1, "Spectral grids for WDM
                 applications: DWDM frequency grid", June 2002.

   [G.694.2]     ITU-T Recommendation G.694.2, "Spectral grids for WDM
                 applications: CWDM wavelength grid", December 2003.

   [G.698.1]     ITU-T Recommendation G.698.1, "Multichannel DWDM
                 applications with single-channel optical interfaces",
                 November 2009.

   [G.698.2]     ITU-T Recommendation G.698.2, "Amplified multichannel
                 dense wavelength division multiplexing applications
                 with single channel optical interfaces ", November
                 2009.

   [G.707]       ITU-T Recommendation G.707, "Network node interface for
                 the synchronous digital hierarchy (SDH)", January 2007.

   [G.709]       ITU-T Recommendation G.709, "Interfaces for the Optical
                 Transport Network (OTN)", December 2009.

   [G.872]       ITU-T Recommendation G.872, "Architecture of optical
                 transport networks", November 2001.

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   [G.959.1]     ITU-T Recommendation G.959.1, "Optical transport
                 network physical layer interfaces", November 2009.

   [G.Sup39]     ITU-T Series G Supplement 39, "Optical system design
                 and engineering considerations", December 2008.

   [Imajuku]     Imajuku, W., Sone, Y., Nishioka, I., and S. Seno,
                 "Routing Extensions to Support Network Elements with
                 Switching Constraint", Work in Progress, July 2007.

   [RFC6205]     Otani, T., Ed. and D. Li, Ed., "Generalized Labels of
                 Lambda-Switch Capable (LSC) Label Switching Routers",
                 RFC 6205, March 2011.

   [RFC5920]     Fang, L., Ed., "Security Framework for MPLS and GMPLS
                 Networks", RFC 5920, July 2010.

   [WSON-Encode] Bernstein, G., Lee, Y., Li, D., and W. Imajuku,
                 "Routing and Wavelength Assignment Information Encoding
                 for Wavelength Switched Optical Networks", Work in
                 Progress, March 2011.

   [WSON-Imp]    Lee, Y., Bernstein, G., Li, D., and G. Martinelli, "A
                 Framework for the Control of Wavelength Switched
                 Optical Networks (WSON) with Impairments", Work in
                 Progress, April 2011.

   [WSON-Info]   Bernstein, G., Lee, Y., Li, D., and W. Imajuku,
                 "Routing and Wavelength Assignment Information Model
                 for Wavelength Switched Optical Networks", Work in
                 Progress, July 2008.

Contributors

   Snigdho Bardalai
   Fujitsu
   EMail: Snigdho.Bardalai@us.fujitsu.com

   Diego Caviglia
   Ericsson
   Via A. Negrone 1/A 16153
   Genoa
   Italy
   Phone: +39 010 600 3736
   EMail: diego.caviglia@marconi.com, diego.caviglia@ericsson.com

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   Daniel King
   Old Dog Consulting
   UK
   EMail: daniel@olddog.co.uk

   Itaru Nishioka
   NEC Corp.
   1753 Simonumabe, Nakahara-ku
   Kawasaki, Kanagawa 211-8666
   Japan
   Phone: +81 44 396 3287
   EMail: i-nishioka@cb.jp.nec.com

   Lyndon Ong
   Ciena
   EMail: Lyong@Ciena.com

   Pierre Peloso
   Alcatel-Lucent
   Route de Villejust, 91620 Nozay
   France
   EMail: pierre.peloso@alcatel-lucent.fr

   Jonathan Sadler
   Tellabs
   EMail: Jonathan.Sadler@tellabs.com

   Dirk Schroetter
   Cisco
   EMail: dschroet@cisco.com

   Jonas Martensson
   Acreo
   Electrum 236
   16440 Kista
   Sweden
   EMail: Jonas.Martensson@acreo.se

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Authors' Addresses

   Young Lee (editor)
   Huawei Technologies
   1700 Alma Drive, Suite 100
   Plano, TX 75075
   USA

   Phone: (972) 509-5599 (x2240)
   EMail: ylee@huawei.com


   Greg M. Bernstein (editor)
   Grotto Networking
   Fremont, CA
   USA

   Phone: (510) 573-2237
   EMail: gregb@grotto-networking.com


   Wataru Imajuku
   NTT Network Innovation Labs
   1-1 Hikari-no-oka, Yokosuka, Kanagawa
   Japan

   Phone: +81-(46) 859-4315
   EMail: wataru.imajuku@ieee.org