RFC 6163
Framework for GMPLS and Path Computation Element (PCE) Control of Wavelength Switched Optical Networks (WSONs)
Pages: 51
Informational
Informational
Part 2 of 2 – Pages 26 to 51
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
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
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
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.
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.
+--+ +--+ +--+ +--------+ +-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 Environment5.1.1. 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.
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):
+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 | |
+--+---+---+---+---+---+---+---+---+---+---+---+---+
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.
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.
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 -> R25.5. 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.
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.
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.
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
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
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)
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
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:
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.
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.
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.
[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.
[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.
[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
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
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