Link Bundle in Wavelength Switched Optical Networks

After Link Bundle is applied to WSON, much important information will be lost. The case where several TE links are advertised for one TE Link without link bundling is further for study.

In term of [RFC4201], all component links in a bundled link should have the same Link Type, the same Traffic Engineering metric, the same set of resource classes at each end of the links, and must begin and end on the same pair of LSRs. There should be not any other restrictions on WSON bundling. For each WDM node, not all the fibers can necessarily be connected to any other fibers. So different ports connected to different separate component links which will be bundled into one TE Link always have different connectivity in the same node. For example, Fiber1 on the west side can be connected to Fiber3 and Fiber4 on the east side, but Fiber2 on the west side can not be connected to Fiber 3 and Fiber4 on the east side. If Fiber1 and Fiber2 are necessary to be bundled into one TE link, they could still be bundled though they have different connectivity. The connectivity constraint and wavelength conversion capability/availability information between the bundled links and fibers on the east side should be a union (maximum) of connectivity constraint and wavelength conversion capability/availability information between the component links and fibers on the east side. So the bundled link can be connected to Fiber3 and Fiber4 though there is not any connectivity between the component link (Fiber2) and other fibers on east side. If one GMPLS control plane instance only controls the LSC network, each wavelength should be considered as a label but not as a TE Link. Link bundling should be done on the level of fiber. In MRN/MLN (e.g., SDH/SONET over ODUk over LSC, ODUk over LSC and PSC over LSC), service provider may deal with wavelengths as links or component link from the perspective of a client layer. The case where link bundle is applied in MRN/MLN is further for consideration. The scope of this draft is cases where wavelengths are viewed as labels.

Different ports connected to different component links which will be bundled into one TE Link always have different connectivity in the same node in WSON. Although these component links can be bundled into a TE Link, connectivity constraint information and wavelength conversion capability/availability information in one node have to be affected. They should be summarized after using link bundle.

Figure 1 shows a typical WDM node, which consists of a Wavelength Switch module, a Multiplexer/DeMultiplexer module, and a Wavelength Converter module. In this example, there are six directions on the line side. For example, in an 80-wavelength system, each fiber of the line side contains 80 wavelengths, and traffic can be carried by each wavelength. The wavelength can be switched to a different direction (or a different fiber) at the WDM node. But there may be a limitation for wavelength conversion, so that not every wavelength can be switched to any other wavelength on any other fiber. Some scenarios are assumed as followings: There is connectivity between (Fiber1, Fiber4), (Fiber2, Fiber6) and (Fiber3, Fiber5). Lambda 1 on an incoming fiber 1 might only be converted to lambda 2, lambda 3 and lambda 4 on outgoing fiber4. Lambda 2 on an incoming fiber 1 might only be converted to lambda 7 and lambda 8 on outgoing fiber4. Other wavelengths on Fiber1 can only be connected to the same wavelengths on Fiber4. Lambda 1 on an incoming fiber 2 might only be converted to lambda 4 and lambda 5 on outgoing fiber6. Other wavelengths on Fiber2 can only be connected to the same wavelengths on Fiber6. Lambda 3 on an incoming fiber 3 might only be converted to lambda 7 and lambda 8 on outgoing fiber5. Lambda 1 on an incoming fiber 3 might only be converted to lambda 5 and lambda 6 on outgoing fiber5. Other wavelengths on Fiber3 can only be connected to the same wavelengths on Fiber5. So node information used by path computation entity is as followings: Connectivity Constraint: [Fiber1--->Fiber4] [Fiber2--->Fiber6] [Fiber3--->Fiber5] Wavelength Conversion Capability: [Fiber1:lambda1----> Fiber4:lambda2, lambda3, lambda4] [Fiber1:lambda2----> Fiber4:lambda7, lambda8] [Fiber2:lambda1----> Fiber6:lambda4, lambda5] [Fiber3:lambda1----> Fiber5:lambda5, lambda6] [Fiber3:lambda3----> Fiber5:lambda7, lambda8] Wavelength Conversion Availability (same as wavelength conversion capability): [Fiber1:lambda1----> Fiber4:lambda2, lambda3, lambda4] [Fiber1:lambda2----> Fiber4:lambda7, lambda8] [Fiber2:lambda1----> Fiber6:lambda4, lambda5] [Fiber3:lambda1----> Fiber5:lambda5, lambda6] [Fiber3:lambda3----> Fiber5:lambda7, lambda8] When Fiber1, Fiber2 and Fiber3 are bundled into one TE Link (Link1), there should be a maximum connectivity between (Link1, Fiber4), (Link1, Fiber5) and (Link1, Fiber6) which must be known to path computation entity. If path computation entity keep having the connectivity information between (Fiber1, Fiber4), (Fiber2, Fiber6) and (Fiber3, Fiber5), it would be better for centralized RWA. The connectivity constraint and wavelength conversion capability/availability information among bundled/unbundled links is the union of connectivity constraint and wavelength conversion capability/availability information among component links. Any connectivity constraint information and wavelength conversion capability/availability information among component links and other TE Links (unbundled) should not be flooded any more. Node information used by path computation entity after using Link Bundle is as followings: Connectivity Constraint: [Link1--->Fiber4] [Link1--->Fiber5] [Link1--->Fiber6] Wavelength Conversion Capability: [Link1:lambda1----> Fiber4:lambda2, lambda3, lambda4] [Link1:lambda2----> Fiber4:lambda7, lambda8] [Link1:lambda1----> Fiber6:lambda4, lambda5] [Link1:lambda3----> Fiber5:lambda7, lambda8] [Link1:lambda1----> Fiber5:lambda5, lambda6] Wavelength Conversion Availability: [Link1:lambda1----> Fiber4:lambda2, lambda3, lambda4] [Link1:lambda2----> Fiber4:lambda7, lambda8] [Link1:lambda1----> Fiber6:lambda4, lambda5] [Link1:lambda3----> Fiber5:lambda7, lambda8] [Link1:lambda1----> Fiber5:lambda5, lambda6] If Fiber5 and Fiber6 are also bundled into one TE Link (Link2), node information used by path computation entity after using Link Bundle again is as followings: Connectivity Constraint: [Link1--->Fiber4] [Link1--->Link2] Wavelength Conversion Capability: [Link1:lambda1----> Fiber4:lambda2, lambda3, lambda4] [Link1:lambda2----> Fiber4:lambda7, lambda8] [Link1:lambda1----> Link2:lambda4, lambda5, lambda6] [Link1:lambda3----> Link2:lambda7, lambda8] Wavelength Conversion Availability: [Link1:lambda1----> Fiber4:lambda2, lambda3, lambda4] [Link1:lambda2----> Fiber4:lambda7, lambda8] [Link1:lambda1----> Link2:lambda4, lambda6] [Link1:lambda1----> Link2:lambda5, lambda5] (Two available wavelength conversions) [Link1:lambda3----> Link2:lambda7, lambda8] It is assumed that a wavelength switch path is along the fiber3 of Link1 and fiber5 of Link2 using the wavelength conversion between lambda1 and lambda5. So wavelength conversion availability used by path computation entity again is as followings: [Link1:lambda1----> Fiber4:lambda2, lambda3, lambda4] [Link1:lambda2----> Fiber4:lambda7, lambda8] [Link1:lambda1----> Link2:lambda4, lambda6] [Link1:lambda1----> Link2:lambda5] (Remain an available wavelength conversion) [Link1:lambda3----> Link2:lambda7, lambda8] Upon the above scenario, if one of the available wavelength conversions among component links is occupied, the available wavelength conversions among the associated bundled links must be summarized and advertised again. Another WSON node which connected to the same bundled link must summary its connectivity constraint information and wavelength conversion capability/availability information by the same means and principle as the peer node. If path computation entity (e.g., PCE) get the connectivity constraint and wavelength conversion capability/availability information via IGP, the summarized information should be flooded in terms of [WSON-ENCODE] and [WSON-INFO] (i.e., Connectivity Matrix and OEOWavelength ConverterInfo/AvailableWavelengthConverters).

Link Bundle is accomplished by performing information aggregation/abstraction. So the available wavelength information and the wavelength constraint information will be lost after component links are bundled into a TE Link. If the computational entity performing routing computation and wavelength assignment without the available wavelength information on component links, they could not perform efficient and accurate path computation. There are several RWA computation architectures: Fully Centralized: In the case where a centralized PCE is responsible for both routing and wavelength assignment or separate centralized PCEs perform routing and wavelength assignment, they all require PCEs to have detailed link wavelength utilization information. They also require PCEs to have detailed node connectivity constraint information and wavelength conversion capability/availability information. [WSON-INFO] provides efficient encodings of information needed by RWA and extends GMPLS IGP to collect this information. Obviously, PCEs only has aggregate wavelength usage (bandwidth) information in term of [RFC4201] after component links bundled into TE links without any IGP extension or any configuration of TED. In this architecture, there are two kinds of component link selection: Distributed Component Link Selection: If component links are selected hop-by-hop by each node along the path, wavelength assignment has to be performed at the level of bundled link. An LSP originator can control and record the resource by an explicit route, i.e., ERO and RRO. Bundled link and the wavelength have to be specified in the ERO objects. The selection of the component link within the bundle link is a local decision, but the nodes must select an appropriate component link where there are acceptable wavelengths specified in ERO. If PCEs have no any wavelength availability information on any component links, it can easily lead to blocking problems during the process of component links selection. Centralized Component Link Selection: If component link selection and wavelength assignment are performed within one centralized PCE, the LSP originator must explicitly specify resource at the level of (Bundled Link, Component Link, and Wavelength/Label). [EXPLICIT-CONTROL-BUNDLE] defines extensions to perform explicit label/resource control over bundled TE link by ERO and RRO subobject. So the LSP originator should use the ERO and RRO subobject to specify the component link identifier for resource control and recording over TE link bundles. If PCEs have to know information of component links via IGP, it is not significant to apply Link Bundle in this case. How PCEs get the information of component links and nodes without IGP is further for study. There are some approaches which have been described in [PCE-TED-ALTERNATIVE]. In all above cases it requires PCEs to have the wavelength availability information and wavelength constraint information on component links and connectivity constraint information and wavelength conversion capability/availability information in nodes. But this information of component links will be hided after component links are bundled into one TE link. It can be resolved by the IGP extension to get this information defined in this draft. PCE can create the TED via IGP. It also can be created by the alternative approaches which have been described in [PCE-TED-ALTERNATIVE]. Centralized Routing + Distributed Wavelength Assignment: In the case where a centralized PCE was assumed to compute paths with distributed wavelength assignment, it is not necessary for the PCE to have the wavelength availability information and wavelength constraint information on component links and connectivity constraint information and wavelength conversion capability/availability information in nodes, but this information of component links is still hided in bundled links. In this case component link selection and wavelength assignment should be simultaneously done hop-by-hop to reduce higher blocking probability. But in some case service provider have to explicitly specify which component link within a bundled TE link should be used for a given LSP for administrative purpose. For example, [PC-SPC-EXT] defines a conversion between PC (Permanent Connections) and SPC (Soft Permanent Connections) in a GMPLS enabled transport network. When PC whose path is passing some bundled links is converted to SPC, we should explicitly specify which component links should be used on bundled links via ERO subobjects defined in [EXPLICIT-CONTROL-BUNDLE]. Otherwise the component links which are selected by nodes along the path may be inconsistent with the ones which actually be used by PC. In order to reduce higher blocking probability, wavelengths (labels) and component links should be simultaneously specified. If component links and wavelengths have to be explicitly specified via ERO subobjects define in [EXPLICIT-CONTROL-BUNDLE], PCE should have the wavelength utilization information and wavelength conversion capability/availability information. PCEs can get this information of component links and nodes via IGP or without IGP. Fully Distributed: In the case fully distributed routing computation and wavelength assignment, it has the highest blocking probability among all the RWA computation architectures with limited or no wavelength conversions. Crankback defined in RFC4920 is a scheme where the node unable to progress the connection setup due to blocked resource returns information to source node to allow new connection setup attempts to be made avoiding the blocked resources. But it will still lead to crankback frequently and be inefficient in the signaling protocol. In this case, it is not necessary for PCEs to have the wavelength utilization information. As the previous case, component link selections and wavelength assignment should be simultaneously done hop-by-hop to reduce higher blocking probability. In all above RWA computation architectures, if the computational entities have the wavelength utilization information, it can calculate accurate paths and reduce blocking probability.

In the general case of limited or no wavelength conversion, wavelength availability information is essential to perform efficient and accurate path computation. PCE can get this information via IGP. [WSON-INFO] provides efficient encodings of information needed by RWA process in WSON and extends GMPLS IGP. [PCE-TED-ALTERNATIVE] also give other alternative methods where nodes can send this information to PCEs without IGP. There should be a fact in both means of TED creation and maintenance that wavelengths utilization information will be lost after component links are bundled into a TE link in terms of [RFC4201]. Hence, this document defines the IGP extensions to Link Bundle [RFC4201] to summarize the available wavelengths information and wavelengths constraint information of component links. This summarized information is one of the traffic parameters to be advertised for a bundled link. The most important thing in this IGP extension to [RFC4201] is that any other information about component links must not flooded except the summarized wavelengths availability information and wavelength constraint information. Following IGP extension defined in this document is limited to WSON.

[WSON-INFO] extends GMPLS IGP to provide efficient encodings of information (e.g., wavelength conversion constraints and wavelength connectivity information of node, wavelength availability information and wavelength constraint information of Link) needed by RWA process in WSON. [WSON-ENCODE] defines a Port Wavelength Restriction sub-TLV for encoding of wavelength constraint information and a Wavelength Set sub-TLV for encoding of available wavelengths information. This document extends Link Bundle [RFC4201] to make wavelength constraint information be a traffic parameter of bundled link. A new traffic parameter which is encoded with Port Wavelength Restriction sub-TLV defined in [WSON-ENCODE] can be introduced to extend [RFC4201] to carry wavelength constraint information of a bundled link. This information must be summarized from all component links on a bundled link and should be a union of all wavelengths constraint information on all component links. The wavelengths constraint information of a bundled link is the union of the wavelengths constraint information of all the component links. How to calculate this summarization is left as a local decision and out of this document. The wavelength that an optical fiber can support is pre-configured, static information. This information needs to be advertised only once in the general case, because it is not expected to change frequently when the network is running. But any change in wavelengths constraint information of a component link results in a change in the wavelengths constraint information of the bundled link. The summarization must be recomputed and flooded again.

This document extends Link Bundle [RFC4201] to make wavelength availability information a traffic parameter of bundled link. A new traffic parameter which is encoded with Wavelength Set Sub-TLV defined in [WSON-ENCODE] can be introduced to extend [RFC4201] to carry wavelength availability information of a bundled link. This information must be summarized from all component links on a bundled link and should be a union of all wavelengths availability information on all component links. For example, one fiber (Fiber1) can support lambda 1 to lambda 5 and lambda 8 to lambda 11, another fiber (Fiber2) can support lambda4 to lambda 6 and lambda 9 to lambda 12. The wavelength constraint information should be as followings: Fiber 1: [lambda1-lambda5], [lambda8-lambda11] Fiber 2: [lambda4-lambda6], [lambda9-lambda12] The wavelength availability information should be as followings (same as wavelength constraint): Fiber 1: [lambda1-lambda5], [lambda8-lambda11] Fiber 2: [lambda4-lambda6], [lambda9-lambda12] After Fiber 1 and Fiber2 are bundled into one TE Link (Link1), the wavelength constraint information of bundled link should be as followings: Link1: [lambda1-lambda6], [lambda8-lambda12] The wavelength availability information should be as followings: Link1: [lambda1-lambda3] Link1: (lambda4, lambda4) (two lambda4 wavelengths are available) Link1: (lambda5, lambda5) (two lambda5 wavelengths are available) Link1: [lambda6-lambda8] Link1: (lambda9, lambda9) (two lambda9 wavelengths are available) Link1: [lambda10-lambda12] If one wavelength is available in more than one component link, available numbers of this wavelength are the summation from all component links. This wavelength will be unavailable on a bundled link after this wavelength is unavailable on all component links. How to calculate this summarization is left as a local decision and out of this document. In WSON, the status information of a certain wavelength in a fiber should be refreshed following any change. When a wavelength is assigned to set up a wavelength LSP or released when a wavelength LSP is torn down, the status information of this wavelength on each link along the path should be updated. This information is dynamic information and needs to be distributed to all computation points. So any change in wavelengths availability information of a component link results in a change in the wavelengths availability information of the bundled link. The summarization must be recomputed and flooded again. If one of the component links goes down, the associated bundled link remains up and continues to be advertised, provided that at least one component link associated with the bundled link is up. The available wavelength of the component link that is down is set to zero, and wavelengths availability information of the bundle must be recomputed. If all the component links associated with a given bundled link are down, the bundled link MUST not be advertised into OSPF/IS-IS in terms of [RFC4201].

Although link bundling is not used, several TE links also can be advertised for one TE Link. Path computation entity should still have the detailed wavelength availability information on this link to perform RWA. This case is further for study.

Base on the requirement of carriers for simplified management to reduce the OPEX, they would like to create an end-to-end path that uses the same component links and the same wavelength on both directions of each bundled link. For example, service provider may require that original service should not be affected after unbundling the bundled link into several component links (unbundled links). If the LSP uses different component links on different directions of a bundled link, it is very difficult in keeping the RSVP-TE signaling state on different TE link of separate directions for this LSP after the bundled link is unbundled. Using the same component link on both directions of a bundled link is sometimes required to control not only WSON but also other switching capability network. Sometimes, with limited or no wavelength conversion, the wavelength could not be available on the same component link on both directions. So it may fail to select the same component on both directions in a bundled link. If there are several component links where there are some wavelengths available on both directions, how to select a component link is a local decision. It may looks at all available component links for the path then chooses from those available at random. Also all available component links are ordered (e.g., order by Traffic Engineering metric), it may chose the first available on all component link. There are two component link selection architectures.

In the case of a centralized PCE performs component link selection, PCE should select the same component link on both directions of a bundled link along the path. The component link selected by a centralized PCE should be explicitly specified by the LSP originator via ERO subobjects defined in [EXPLICIT-CONTROL-BUNDLE]. In order to reduce blocking probability, wavelength assignment and component link selection should be done simultaneously within one PCE. PCE is responsible for using same component link and wavelength on both directions of a bundle link. So in this case wavelengths and component links have to be explicitly specified via ERO object and ERO subobject. In the general case of limited or no wavelength conversion, because it may actually be the case that the lambda is only available in one direction on one component link, and the other direction is only available on a different component link, PCE may fail to calculate such path to meet the requirement of same component link and wavelength on both direction of a bundle link.

In the case of distributed component link selection which is done hop-by-hop by nodes along path, how to select the component link could be the default policy of a node, but different nodes may apply different policies in the interoperability environment. So each node should be explicitly notified of bidirectional same component link path request. [WSON-SIGNALING] introduces a bit in Attributes Flags TLV of LSP_ATTRIBUTES object defined in [RFC4420] to indicate that the bidirectional same wavelength path should be created. To meet the requirement of the bidirectional same component link path request, this document add a similar bit in Attributes Flags TLV of LSP_ATTRIBUTES object. In this component link selection architecture, there are two wavelength assignment approaches.

If a centralized PCE performs wavelength assignment, wavelength assignment has to be performed at the level of bundled link. Bundled link and the wavelength have to be specified in the ERO objects by the LSP originator. The centralized PCE has to assign the same wavelength on both directions of a bundled link or non-bundled link. The LSP originator can only specify resource at the level of (Bundle Link or Non-Bundled Link, Wavelength/Label). Ingress node adds the bidirectional same component link path request in a LSP_ATTRIBUTES object of Path message. The path setup procedure is described below: Ingress node adds the bidirectional same component link request in a LSP_ATTRIBUTES object of Path message. If this Path message is to be sent for a downstream bundled TE link, the node must select the same component link where there are acceptable wavelengths specified in ERO objects. If there are not acceptable wavelengths on non-bundled links or component links of a bundled link, Path message will be terminated and error information is generated. On reception of a Path message containing bidirectional same component link indication in a LSP_ATTRIBUTES object, and next hop Path message is to be sent for a downstream bundled TE link, the node must select the same component link where there are acceptable wavelengths specified in ERO objects. If there are not acceptable wavelengths on non-bundled links or component links of a bundled link, the Path message will be terminated, and a PathErr message with a "Component Link Selection Error" indication will be generated.

If wavelength assignment should be simultaneously done hop-by-hop, it is necessary to notify each node the request of bidirectional same component link on both directions of a bundle link and same wavelength on such a component link. The path setup procedure is described below: Ingress node adds the request of the bidirectional same component link on both direction of a bundled link and same wavelength on such a component link in a LSP_ATTRIBUTES object of Path message. If this Path message is to be sent for a downstream bundled TE link, the node must select one component link where there are acceptable same wavelengths available on both directions, then copy them into Label Set object and forward the Path message to downstream node. If there aren¡¯t acceptable wavelengths on non-bundled links or component links of a bundled link, Path message will be terminated and error information is generated. On reception of a Path message containing the above indication in a LSP_ATTRIBUTES object and Label Set object, there will be two cases: If this Path message is to be sent for a downstream bundled TE link, the node must select one component link where the Label Set TLVs are acceptable and available on both directions in it. If there are acceptable same wavelengths on both directions on such a component link, then copy the values of them into new Label Set TLVs, and forward the Path message to the downstream node. Otherwise the Path message will be terminated, and a PathErr message with a "Routing problem/Label Set" indication will be generated. If this Path message is to be sent for a downstream non-bundled link, the receiver of message checks if the Label Set TLVs are acceptable on both directions. If there are acceptable same wavelengths on both directions, then copy the values of them into new Label Set TLVs, and forward the Path message to the downstream node. Otherwise the Path message will be terminated, and a PathErr message with a "Routing problem/Label Set" indication will be generated. On reception of a Path message containing the above indication in a LSP_ATTRIBUTES object and Label Set object, the egress node verifies whether the Label Set TLVs are acceptable, if one or more wavelengths are available on both directions in a component link, then any one available wavelength could be selected. A Resv message is generated and propagated to upstream node. On reception of a Resv message containing the above indication in a LSP_ATTRIBUTES object, the intermediate node allocates the label to interfaces on both directions in the component link, and then configures the local ROADM or OXC on both directions.