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Interworking of GMPLS Control and Centralized Controller Systems
draft-ietf-teas-gmpls-controller-inter-work-14

Document Type Active Internet-Draft (teas WG)
Authors Haomian Zheng , Yi Lin , Yang Zhao , Yunbin Xu , Dieter Beller
Last updated 2024-06-20 (Latest revision 2024-06-11)
Replaces draft-zheng-teas-gmpls-controller-inter-work
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draft-ietf-teas-gmpls-controller-inter-work-14
TEAS Working Group                                         Haomian Zheng 
Internet Draft                                                    Yi Lin 
Category: Informational                              Huawei Technologies 
                                                               Yang Zhao 
                                                            China Mobile 
                                                               Yunbin Xu 
                                                                   CAICT 
                                                           Dieter Beller 
                                                                   Nokia 
Expires: December 13, 2024                                 June 11, 2024 
                                    
                                    
    Interworking of GMPLS Control and Centralized Controller Systems 
                                    
              draft-ietf-teas-gmpls-controller-inter-work-14 

Abstract 
 
   Generalized Multi-Protocol Label Switching (GMPLS) control allows 
   each network element (NE) to perform local resource discovery, 
   routing and signaling in a distributed manner.  

   On the other hand, with the development of software-defined 
   transport networking technology, a set of NEs can be controlled via 
   centralized controller hierarchies to address the issues from multi-
   domain, multi-vendor, and multi-technology. An example of such 
   centralized architecture is Abstraction and Control of Traffic 
   Engineered Networks (ACTN) controller hierarchy described in RFC 
   8453.   

   Instead of competing with each other, both the distributed and the 
   centralized control plane have their own advantages, and should be 
   complementary in the system. This document describes how the GMPLS 
   distributed control plane can interwork with a centralized 
   controller system in a transport network. 

Status of this Memo 

   This Internet-Draft is submitted to IETF in full conformance with   
   the provisions of BCP 78 and BCP 79. 

   Internet-Drafts are working documents of the Internet Engineering   
   Task Force (IETF), its areas, and its working groups.  Note that   
   other groups may also distribute working documents as Internet-   
   Drafts. 

   Internet-Drafts are draft documents valid for a maximum of six 
   months and may be updated, replaced, or obsoleted by other documents 
   at any time.  It is inappropriate to use Internet-Drafts as 
   reference material or to cite them other than as "work in progress." 
 
 
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Internet-Draft        GMPLS and Controller Interwork           June 2024 

   The list of current Internet-Drafts can be accessed at   
   http://www.ietf.org/ietf/1id-abstracts.txt. 

   The list of Internet-Draft Shadow Directories can be accessed at   
   http://www.ietf.org/shadow.html. 

   This Internet-Draft will expire on December 13, 2024. 

Copyright Notice 

   Copyright (c) 2024 IETF Trust and the persons identified as the    
   document authors.  All rights reserved. 

   This document is subject to BCP 78 and the IETF Trust's Legal 
   Provisions Relating to IETF Documents 
   (http://trustee.ietf.org/license-info) in effect on the date of 
   publication of this document. Please review these documents 
   carefully, as they describe your rights and restrictions with 
   respect to this document. Code Components extracted from this 
   document must include Simplified BSD License text as described in 
   Section 4.e of the Trust Legal Provisions and are provided without 
   warranty as described in the Simplified BSD License. 

Table of Contents 

   1. Introduction ................................................... 3 
   2. Overview ....................................................... 4 
      2.1. Overview of GMPLS Control Plane ........................... 4 
      2.2. Overview of Centralized Controller System ................. 4 
      2.3. GMPLS Control Interworking with a Centralized Controller 
      System ......................................................... 5 
   3. Discovery Options .............................................. 7 
      3.1. LMP ....................................................... 7 
   4. Routing Options ................................................ 7 
      4.1. OSPF-TE ................................................... 8 
      4.2. ISIS-TE ................................................... 8 
      4.3. NETCONF/RESTCONF .......................................... 8 
   5. Path Computation ............................................... 8 
      5.1. Controller-based Path Computation ......................... 8 
      5.2. Constraint-based Path Computing in GMPLS Control .......... 9 
      5.3. Path Computation Element (PCE) ............................ 9 
   6. Signaling Options .............................................. 9 
      6.1. RSVP-TE .................................................. 10 
   7. Interworking Scenarios ........................................ 10 
      7.1. Topology Collection & Synchronization .................... 10 
      7.2. Multi-domain Service Provisioning ........................ 10 
      7.3. Multi-layer Service Provisioning ......................... 14 
         7.3.1. Multi-layer Path Computation ........................ 14 
         7.3.2. Cross-layer Path Creation ........................... 17 
         7.3.3. Link Discovery ...................................... 18 
 
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      7.4. Recovery ................................................. 18 
         7.4.1. Span Protection ..................................... 19 
         7.4.2. LSP Protection ...................................... 19 
         7.4.3. Single-domain LSP Restoration ....................... 19 
         7.4.4. Multi-domain LSP Restoration ........................ 20 
         7.4.5. Fast Reroute ........................................ 24 
      7.5. Controller Reliability ................................... 24 
   8. Manageability Considerations .................................. 25 
   9. Security Considerations ....................................... 25 
   10. IANA Considerations........................................... 25 
   11. References ................................................... 25 
      11.1. Normative References .................................... 25 
      11.2. Informative References .................................. 27 
   12. Contributors ................................................. 30 
   13. Authors' Addresses ........................................... 30 
   Adknowledgements ................................................. 30 
 
 
1. Introduction 

   Generalized Multi-Protocol Label Switching (GMPLS) [RFC3945] extends 
   MPLS to support different classes of interfaces and switching 
   capabilities such as Time-Division Multiplex Capable (TDM), Lambda 
   Switch Capable (LSC), and Fiber-Switch Capable (FSC). Each network 
   element (NE) running a GMPLS control plane collects network 
   information from other NEs and supports service provisioning through 
   signaling in a distributed manner. A more generic description of 
   Traffic-engineering networking information exchange can be found in 
   [RFC7926].  

   On the other hand, Software-Defined Networking (SDN) technologies 
   have been introduced to control the transport network centrally. 
   Centralized controllers can collect network information from each 
   node and provision services on corresponding nodes. One example is 
   the Abstraction and Control of Traffic Engineered Networks (ACTN) 
   [RFC8453], which defines a hierarchical architecture with 
   Provisioning Network Controller (PNC), Multi-domain Service 
   Coordinator (MDSC) and Customer Network Controller (CNC) as 
   centralized controllers for different network abstraction levels. A 
   Path Computation Element (PCE) based approach has been proposed as 
   Application-Based Network Operations (ABNO) in [RFC7491].  

   GMPLS can be applied for the Network Element (NE) level control in 
   such centralized controller architectures. A centralized controller 
   may support GMPLS enabled domains and interact with a GMPLS enabled 
   domain where the GMPLS control plane does the service provisioning 
   from ingress to egress. In this case the centralized controller 
   sends the request to the ingress node and does not have to configure 
   all NEs along the path through the domain from ingress to egress, 

 
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   thus leveraging the GMPLS control plane. This document describes how 
   the GMPLS control plane interworks with a centralized controller 
   system in a transport network. 

2. Overview 

   This section provides an overview of the GMPLS control plane and 
   centralized controller systems and the interactions between the 
   GMPLS control plane and centralized controllers, for transport 
   networks.  

   A transport network [RFC5654] is a server-layer network designed to 
   provide connectivity services for client-layer connectivity. This 
   facilitates client traffic to be carried transparently across the 
   server-layer network resources. 

2.1. Overview of GMPLS Control Plane 

   GMPLS separates the control plane and the data plane to support 
   time-division, wavelength, and spatial switching, which are 
   significant in transport networks. For the NE level control in 
   GMPLS, each node runs a GMPLS control plane instance. 
   Functionalities such as service provisioning, protection, and 
   restoration can be performed via GMPLS communication among multiple 
   NEs. At the same time, the GMPLS control plane instance can also 
   collect information about node and link resources in the network to 
   construct the network topology and compute routing paths for serving 
   service requests. 

   Several protocols have been designed for the GMPLS control plane 
   [RFC3945], including link management [RFC4204], signaling [RFC3471], 
   and routing [RFC4202] protocols. The GMPLS control plane instances 
   applying these protocols communicate with each other to exchange 
   resource information and establish Label Switched Paths (LSPs). In 
   this way, GMPLS control plane instances in different nodes in the 
   network have the same view of the network topology and provision 
   services based on local policies. 

2.2. Overview of Centralized Controller System 

   With the development of SDN technologies, a centralized controller 
   architecture has been introduced to transport networks. One example 
   architecture can be found in ACTN [RFC8453]. In such systems, a 
   controller is aware of the network topology and is responsible for 
   provisioning incoming service requests.  

   Multiple hierarchies of controllers are designed at different levels 
   to implement different functions. This kind of architecture enables 
   multi-vendor, multi-domain, and multi-technology control. For 
   example, a higher-level controller coordinates several lower-level 
 
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   controllers controlling different domains, for topology collection 
   and service provisioning. Vendor-specific features can be abstracted 
   between controllers, and a standard API (e.g., generated from 
   RESTCONF [RFC8040] / YANG [RFC7950]) may be used.  

2.3. GMPLS Control Interworking with a Centralized Controller System 

   Besides GMPLS and the interactions among the controller hierarchies, 
   it is also necessary for the controllers to communicate with the 
   network elements. Within each domain, GMPLS control can be applied 
   to each NE. The bottom-level centralized controller can act as an NE 
   to collect network information and initiate LSPs. Figure 1 shows an 
   example of GMPLS interworking with centralized controllers (ACTN 
   terminologies are used in the figure). 

                           +-------------------+ 
                           |    Orchestrator   | 
                           |       (MDSC)      | 
                           +-------------------+ 
                             ^       ^       ^ 
                             |       |       | 
               +-------------+       |       +-------------+ 
               |                     |RESTCONF/YANG models | 
               V                     V                     V 
         +-------------+      +-------------+       +-------------+ 
         |Controller(N)|      |Controller(G)|       |Controller(G)| 
         |    (PNC)    |      |    (PNC)    |       |    (PNC)    | 
         +-------------+      +-------------+       +-------------+ 
              ^  ^                  ^  ^                  ^  ^ 
              |  |                  |  |                  |  | 
       NETCONF|  |PCEP       NETCONF|  |PCEP       NETCONF|  |PCEP 
        /YANG |  |            /YANG |  |            /YANG |  | 
              V  V                  V  V                  V  V 
          .----------.  Inter-  .----------.  Inter-  .----------. 
         /            \ domain /            \ domain /            \ 
        |              | link |     LMP      | link |     LMP      | 
        |              |======|   OSPF-TE    |======|   OSPF-TE    | 
        |              |      |   RSVP-TE    |      |   RSVP-TE    | 
         \            /        \            /        \            / 
          `----------`          `----------`          `----------` 
       Non-GMPLS domain 1      GMPLS domain 2        GMPLS domain 3 

      Controller(N): A domain controller controlling a non-GMPLS domain 
      Controller(G): A domain controller controlling a GMPLS domain  

     Figure 1: Example of GMPLS/non-GMPLS interworking with Controllers 

   Figure 1 shows the scenario with two GMPLS domains and one non-GMPLS 
   domain. This system supports the interworking among non-GMPLS 
   domain, GMPLS domain and the controller hierarchies.  
 
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   For domain 1, the network elements were not enabled with GMPLS so 
   the control is purely from the controller, via Network Configuration 
   Protocol (NETCONF) [RFC6241] / YANG and/or PCE Communication 
   Protocol (PCEP) [RFC5440].  

   For domains 2 and 3: 

   -  Each domain has the GMPLS control plane enabled at the physical 
      network level. The Provisioning Network Controller (PNC) can 
      exploit GMPLS capabilities implemented in the domain to listen to 
      the IGP routing protocol messages (OSPF LSAs, for example) that 
      the GMPLS control plane instances are disseminating into the 
      network and thus learn the network topology. For path computation 
      in the domain with PNC implementing a PCE, Path Computation 
      Clients (PCCs) (e.g. NEs, other controller/PCE) use PCEP to ask 
      the PNC for a path and get replies. The Multi-Domain Service 
      Coordinator (MDSC) communicates with PNCs using, for example 
      REST/RESTCONF based on YANG data models. As a PNC has learned its 
      domain topology, it can report the topology to the MDSC. When a 
      service arrives, the MDSC computes the path and coordinates PNCs 
      to establish the corresponding LSP segment; 

   -  Alternatively, the NETCONF protocol can be used to retrieve 
      topology information utilizing the [RFC8795] Yang model and the 
      technology-specific YANG model augmentations required for the 
      specific network technology. The PNC can retrieve topology 
      information from any NE (the GMPLS control plane instance of each 
      NE in the domain has the same topological view), construct the 
      topology of the domain, and export an abstract view to the MDSC. 
      Based on the topology retrieved from multiple PNCs, the MDSC can 
      create a topology graph of the multi-domain network, and can use 
      it for path computation. To set up a service, the MDSC can exploit 
      the [TE-Tunnel] YANG model together with the technology-specific 
      YANG model augmentations.  

   This document focuses on the interworking between GMPLS and the 
   centralized controller system, including: 

   -  The interworking between the GMPLS domains and the centralized 
      controllers (including the orchestrator, if it exists) controlling 
      the GMPLS domains; 

   -  The interworking between a non-GMPLS domain (which is controlled 
      by a centralized controller system) and a GMPLS domain, through 
      the controller hierarchy architecture. 

   For convenience, this document uses the following terminologies for 
   the controller and the orchestrator:  

 
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   -  Controller(G): A domain controller controlling a GMPLS domain (the 
      controller(G) of the GMPLS domains 2 and 3 in Figure 1); 

   -  Controller(N): A domain controller controlling a non-GMPLS domain 
      (the controller(N) of the non-GMPLS domain 1 in Figure 1); 

   -  H-Controller(G): A domain controller controlling the higher-layer 
      GMPLS domain, in the context of multi-layer networks; 

   -  L-Controller(G): A domain controller controlling the lower-layer 
      GMPLS domain, in the context of multi-layer networks; 

   -  H-Controller(N): A domain controller controlling the higher-layer 
      non-GMPLS domain, in the context of multi-layer networks; 

   -  L-Controller(N): A domain controller controlling the lower-layer 
      non-GMPLS domain, in the context of multi-layer networks; 

   -  Orchestrator(MD): An orchestrator used to orchestrate the multi-
      domain networks; 

   -  Orchestrator(ML): An orchestrator used to orchestrate the multi-
      layer networks. 

3. Discovery Options 

   In GMPLS control, the link connectivity must be verified between 
   each pair of nodes. In this way, link resources, which are 
   fundamental resources in the network, are discovered by both ends of 
   the link. 

3.1. LMP 

   Link management protocol (LMP) [RFC4204] runs between nodes and 
   manages TE links. In addition to the setup and maintenance of 
   control channels, LMP can be used to verify the data link 
   connectivity and correlate the link properties.  

4. Routing Options 

   In GMPLS control, link state information is flooded within the 
   network as defined in [RFC4202]. Each node in the network can build 
   the network topology according to the flooded link state 
   information. Routing protocols such as OSPF-TE [RFC4203] and ISIS-TE 
   [RFC5307] have been extended to support different interfaces in 
   GMPLS. 

   In a centralized controller system, the centralized controller can 
   be placed in the GMPLS network and passively receives the IGP 

 
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   information flooded in the network. In this way, the centralized 
   controller can construct and update the network topology.  

4.1. OSPF-TE 

   OSPF-TE is introduced for TE networks in [RFC3630]. OSPF extensions 
   have been defined in [RFC4203] to enable the capability of link 
   state information for the GMPLS network. Based on this work, OSPF 
   has been extended to support technology-specific routing. The 
   routing protocol for Optical Transport Network (OTN), Wavelength 
   Switched Optical Network (WSON) and optical flexi-grid networks are 
   defined in [RFC7138], [RFC7688] and [RFC8363], respectively. 

4.2. ISIS-TE 

   ISIS-TE is introduced for TE networks in [RFC5305] and is extended 
   to support GMPLS routing functions [RFC5307], and has been updated 
   to [RFC7074] to support the latest GMPLS switching capability and 
   Types fields. 

4.3. NETCONF/RESTCONF 

   NETCONF [RFC6241] and RESTCONF [RFC8040] protocols are originally 
   used for network configuration. These protocols can also utilize 
   topology-related YANG models, such as [RFC8345] and [RFC8795]. These 
   protocols provide a powerful mechanism for notification of topology 
   changes to the client.  

5. Path Computation 

5.1. Controller-based Path Computation 

   Once a controller learns the network topology, it can utilize the 
   available resources to serve service requests by performing path 
   computation. Due to abstraction, the controllers may not have 
   sufficient information to compute the optimal path. In this case, 
   the controller can interact with other controllers by sending, for 
   example, YANG-based Path Computation requests [PAT-COMP] or PCEP, to 
   compute a set of potential optimal paths and then, based on its 
   constraints, policy, and specific knowledge (e.g. cost of access 
   link) can choose the more feasible path for end-to-end (E2E) service 
   path setup.  

   Path computation is one of the key objectives in various types of 
   controllers. In the given architecture, it is possible for different 
   components that have the capability to compute the path.  

 
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5.2. Constraint-based Path Computing in GMPLS Control 

   In GMPLS control, a routing path may be computed by the ingress node 
   ([RFC3473]) based on the ingress node Traffic Engineering Database 
   (TED). In this case, constraint-based path computation is performed 
   according to the local policy of the ingress node. 

5.3. Path Computation Element (PCE) 

   The PCE has been introduced in [RFC4655] as a functional component 
   that provides services to compute paths in a network. In [RFC5440], 
   the path computation is accomplished by using the TED, which 
   maintains a view of the link resources in the network. The emergence 
   of PCE efficiently improves the quality of network planning and 
   offline computation. However, there is a risk that the computed path 
   may be infeasible if there is a diversity requirement, because 
   stateless PCE has no knowledge about the former computed paths.  

   To address this issue, stateful PCE has been proposed in [RFC8231]. 
   Besides the TED, an additional LSP Database (LSP-DB) is introduced 
   to archive each LSP computed by the PCE. This way, PCE can easily 
   determine the relationship between the computing path and former 
   computed paths. In this approach, PCE provides computed paths to 
   PCC, and then PCC decides which path is deployed and when to be 
   established.  

   With PCE-Initiated LSPs [RFC8281], PCE can trigger the PCC to 
   perform setup, maintenance, and teardown of the PCE-initiated LSP 
   under the stateful PCE model. This would allow a dynamic network 
   that is centrally controlled and deployed. 

   In a centralized controller system, the PCE can be implemented in a 
   centralized controller, and the centralized controller performs path 
   computation according to its local policies. On the other hand, the 
   PCE can also be placed outside of the centralized controller. In 
   this case, the centralized controller acts as a PCC to request path 
   computation to the PCE through PCEP. One of the reference 
   architecture can be found in [RFC7491].  

6. Signaling Options 

   Signaling mechanisms are used to set up LSPs in GMPLS control. 
   Messages are sent hop by hop between the ingress node and the egress 
   node of the LSP to allocate labels. Once the labels are allocated 
   along the path, the LSP setup is accomplished. Signaling protocols 
   such as Resource Reservation Protocol - Traffic Engineering (RSVP-
   TE) [RFC3473] have been extended to support different interfaces in 
   GMPLS. 

 
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6.1. RSVP-TE 

   RSVP-TE is introduced in [RFC3209] and extended to support GMPLS 
   signaling in [RFC3473]. Several label formats are defined for a 
   generalized label request, a generalized label, a suggested label 
   and label sets. Based on [RFC3473], RSVP-TE has been extended to 
   support technology-specific signaling. The RSVP-TE extensions for 
   OTN, WSON, and optical flexi-grid network are defined in [RFC7139], 
   [RFC7689], and [RFC7792], respectively. 

7. Interworking Scenarios 

7.1. Topology Collection & Synchronization 

   Topology information is necessary on both network elements and 
   controllers. The topology on a network element is usually raw 
   information, while the topology used by the controller can be either 
   raw, reduced, or abstracted. Three different abstraction methods 
   have been described in [RFC8453], and different controllers can 
   select the corresponding method depending on the application.  

   When there are changes in the network topology, the impacted network 
   elements need to report changes to all the other network elements, 
   together with the controller, to sync up the topology information. 
   The inter-NE synchronization can be achieved via protocols mentioned 
   in Sections 3 and 4. The topology synchronization between NEs and 
   controllers can either be achieved by routing protocols OSPF-
   TE/PCEP-LS in [PCEP-LS] or NETCONF protocol notifications with YANG 
   model.  

7.2. Multi-domain Service Provisioning 

   Service provisioning can be deployed based on the topology 
   information on controllers and network elements. Many methods have 
   been specified for single-domain service provisioning, such as the 
   PCEP and RSVP-TE, methods.  

   Multi-domain service provisioning would require coordination among 
   the controller hierarchies. Given the service request, the end-to-
   end delivery procedure may include interactions at any level (i.e. 
   interface) in the hierarchy of the controllers (e.g. MPI and SBI for 
   ACTN). The computation for a cross-domain path is usually completed 
   by controllers who have a global view of the topologies. Then the 
   configuration is decomposed into lower-level controllers, to 
   configure the network elements to set up the path.  

   A combination of centralized and distributed protocols may be 
   necessary to interact between network elements and controllers. 
   Several methods can be used to create the inter-domain path: 

 
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   1) With end-to-end RSVP-TE session:  

   In this method, all the domains need to support the RSVP-TE protocol 
   and thus need to be GMPLS domains. The Controller(G) of the source 
   domain triggers the source node to create the end-to-end RSVP-TE 
   session, and the assignment and distribution of the labels on the 
   inter-domain links are done by the border nodes of each domain, 
   using RSVP-TE protocol. Therefore, this method requires the 
   interworking of RSVP-TE protocols between different domains. 

   There are two possible methods: 

   1.1) One single end-to-end RSVP-TE session 

   In this method, an end-to-end RSVP-TE session from the source node 
   to the destination node will be used to create the inter-domain 
   path. A typical example would be the PCE Initiation scenario, in 
   which a PCE message (PCInitiate) is sent from the controller(G) to 
   the source node, and then trigger an RSVP procedure along the path. 
   Similarly, the interaction between the controller and the source 
   node of the source domain can be achieved by using the NETCONF 
   protocol with corresponding YANG models, and then it can be 
   completed by running RSVP among the network elements.  

   1.2) LSP Stitching 

   The LSP stitching method defined in [RFC5150] can also create the 
   E2E LSP. I.e., when the source node receives an end-to-end path 
   creation request (e.g., using PCEP or NETCONF protocol), the source 
   node starts an end-to-end RSVP-TE session along the endpoints of 
   each LSP segment (refers to S-LSP in [RFC5150]) of each domain, to 
   assign the labels on the inter-domain links between each pair of 
   neighbor S-LSPs, and stitch the end-to-end LSP to each S-LSP. See 
   Figure 2 as an example. Note that the S-LSP in each domain can be 
   either created by its Controller(G) in advance, or created 
   dynamically triggered by the end-to-end RSVP-TE session. 

 
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                       +------------------------+ 
                       |    Orchestrator(MD)    | 
                       +-----------+------------+ 
                                   | 
      +---------------+     +------V-------+     +---------------+ 
      | Controller(G) |     | Controller(G)|     | Controller(G) | 
      +-------+-------+     +------+-------+     +-------+-------+ 
              |                    |                     | 
     +--------V--------+   +-------V--------+   +--------V--------+ 
     |Client           |   |                |   |           Client| 
     |Signal   Domain 1|   |    Domain 2    |   |Domain 3   Signal| 
     |  |              |   |                |   |              |  | 
     |+-+-+            |   |                |   |            +-+-+| 
     || | |  +--+  +--+|   |+--+  +--+  +--+|   |+--+  +--+  | | || 
     || | |  |  |  |  ||   ||  |  |  |  |  ||   ||  |  |  |  | | || 
     || ******************************************************** || 
     ||   |  |  |  |  ||   ||  |  |  |  |  ||   ||  |  |  |  |   || 
     |+---+  +--+  +--+|   |+--+  +--+  +--+|   |+--+  +--+  +---+| 
     +-----------------+   +----------------+   +-----------------+ 
      |   .           .     .              .     .           .   | 
      |   .<-S-LSP 1->.     .<- S-LSP 2 -->.     .<-S-LSP 3->.   | 
      |               .     .              .     .               | 
      |-------------->.---->.------------->.---->.-------------->| 
      |<--------------.<----.<-------------.<----.<--------------| 
      |       End-to-end RSVP-TE session for LSP stitching       | 

                          Figure 2: LSP stitching 

   2) Without end-to-end RSVP-TE session: 

   In this method, each domain can be a GMPLS domain or a non-GMPLS 
   domain. Each controller (may be a Controller(G) or a Controller(N)) 
   is responsible for creating the path segment within its domain. The 
   border node does not need to communicate with other border nodes in 
   other domains for the distribution of labels on inter-domain links, 
   so end-to-end RSVP-TE session through multiple domains is not 
   required, and the interworking of RSVP-TE protocol between different 
   domains is not needed.  

   Note that path segments in the source domain and the destination 
   domain are "asymmetrical" segments, because the configuration of 
   client signal mapping into server layer tunnel is needed at only one 
   end of the segment, while configuration of server layer cross-
   connect is needed at the other end of the segment. See the example 
   in Figure 3. 

 
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                       +------------------------+ 
                       |    Orchestrator(MD)    | 
                       +-----------+------------+ 
                                   | 
      +---------------+     +------V-------+     +---------------+ 
      |  Controller   |     |  Controller  |     |  Controller   | 
      +-------+-------+     +------+-------+     +-------+-------+ 
              |                    |                     | 
     +--------V--------+   +-------V--------+   +--------V--------+ 
     |Client   Domain 1|   |    Domain 2    |   | Domain 3  Client| 
     |Signal           |   |                |   |           Signal| 
     |  |  Server layer|   |                |   |              |  | 
     |  |     tunnel   |   |                |   |              |  | 
     |+-+-+       ^    |   |                |   |            +-+-+| 
     || | |  +--+ |+--+|   |+--+  +--+  +--+|   |+--+  +--+  | | || 
     || | |  |  | ||  ||   ||  |  |  |  |  ||   ||  |  |  |  | | || 
     || ******************************************************** || 
     ||   |  |  |  |  || . ||  |  |  |  |  || . ||  |  |  |  |   || 
     |+---+  +--+  +--+| . |+--+  +--+  +--+| . |+--+  +--+  +---+| 
     +-----------------+ . +----------------+ . +-----------------+ 
      .                  .                    .                  . 
      .<-Path Segment 1->.<--Path Segment 2-->.<-Path Segment 3->. 

              Figure 3: Example of asymmetrical path segment 

   The PCEP / GMPLS protocols should support the creation of such 
   asymmetrical segments. 

   Note also that mechanisms to assign the labels in the inter-domain 
   links also need to be considered. There are two possible methods: 

   2.1) Inter-domain labels assigned by NEs: 

   The concept of Stitching Label that allows stitching local path 
   segments was introduced in [RFC5150] and [sPCE-ID], in order to form 
   the inter-domain path crossing several different domains. It also 
   describes the Backward-Recursive PCE-Based Computation (BRPC) and 
   Hierarchical Path Computation Element (H-PCE) PCInitiate procedure, 
   i.e., the ingress node of each downstream domain assigns the 
   stitching label for the inter-domain link between the downstream 
   domain and its upstream neighbor domain, and this stitching label 
   will be passed to the upstream neighbor domain by PCE protocol, 
   which will be used for the path segment creation in the upstream 
   neighbor domain. 

   2.2) Inter-domain labels assigned by controller: 

   If the resources of inter-domain links are managed by the 
   orchestrator(MD), each domain controller can provide to the 
   orchestrator(MD) the list of available labels (e.g. timeslots if OTN 
 
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   is the scenario) using the IETF Topology YANG model and related 
   technology specific extension. Once the orchestrator(MD) has 
   computed the E2E path, RSVP-TE or PCEP can be used in the different 
   domains to set up the related segment tunnel consisting of label 
   inter-domain information, e.g. for PCEP, the label Explicit Route 
   Object (ERO) can be included in the PCInitiate message to indicate 
   the inter-domain labels, so that each border node of each domain can 
   configure the correct cross-connect within itself. 

7.3. Multi-layer Service Provisioning 

   GMPLS can interwork with centralized controller systems in multi-
   layer networks.  

   +----------------+ 
   |Orchestrator(ML)| 
   +------+--+------+ 
          |  |                            Higher-layer Network 
          |  |                           .--------------------. 
          |  |                          /                      \ 
          |  |   +--------------+      |   +--+   Link   +--+   | 
          |  +-->| H-Controller +----->|   |  |**********|  |   | 
          |      +--------------+      |   +--+          +--+   | 
          |                             \    .            .    / 
          |                               `--.------------.---` 
          |                                  .            . 
          |                              .---.------------.---. 
          |                             /    .            .    \ 
          |      +--------------+      |   +--+   +--+   +--+   | 
          +----->| L-Controller +----->|   | ============== |   | 
                 +--------------+      |   +--+   +--+   +--+   | 
                                        \         H-LSP        / 
                                          `-------------------` 
                                           Lower-layer Network 
    
      Figure 4: GMPLS-controller interworking in multi-layer networks 

   An example with two layers of network is shown in Figure 4. In this 
   example, the GMPLS control plane is enabled in at least one layer 
   network (otherwise, it is out of the scope of this document) and 
   interworks with the controller of its domain (H-Controller and L-
   Controller, respectively). The Orchestrator(ML) is used to 
   coordinate the control of the multi-layer network. 

7.3.1. Multi-layer Path Computation 

   [RFC5623] describes three inter-layer path computation models and 
   four inter-layer path control models:  

   -  3 Path computation: 
 
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      o  Single PCE path computation model 

      o  Multiple PCE path computation with inter-PCE communication 
         model 

      o  Multiple PCE path computation without inter-PCE communication 
         model  

   -  4 Path control: 

      o  PCE-Virtual Network Topology Manager (PCE-VNTM) cooperation 
         model 

      o  Higher-layer signaling trigger model 

      o  Network Management System-VNTM (NMS-VNTM) cooperation model 
         (integrated flavor) 

      o  NMS-VNTM cooperation model (separate flavor) 

   Section 4.2.4 of [RFC5623] also provides all the possible 
   combinations of inter-layer path computation and inter-layer path 
   control models. 

   To apply [RFC5623] in multi-layer network with GMPLS-controller 
   interworking, the H-Controller and the L-Controller can act as the 
   PCE Hi and PCE Lo respectively, and typically, the Orchestrator(ML) 
   can act as a VNTM because it has the abstracted view of both the 
   higher-layer and lower-layer networks.  

   Table 1 shows all possible combinations of path computation and path 
   control models in multi-layer network with GMPLS-controller 
   interworking: 

    

    

 
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     Table 1: Combinations of path computation and path control models 

         --------------------------------------------------------- 
        | Path computation    |Single PCE | Multiple  | Multiple  | 
        |      \              |   (Not    | PCE with  | PCE w/o   | 
        | Path control        |applicable)| inter-PCE | inter-PCE | 
        |---------------------+-----------+-----------+-----------| 
        | PCE-VNTM            |  ......   |           |           | 
        | cooperation         |  . -- .   |   Yes     |   Yes     | 
        |                     |  .    .   |           |           | 
        |---------------------+--.----.---+-----------+-----------| 
        | Higher-layer        |  .    .   |           |           | 
        | signaling trigger   |  . -- .   |   Yes     |   Yes     | 
        |                     |  .    .   |           |           | 
        |---------------------+--.----.---+-----------+-----------| 
        | NMS-VNTM            |  .    .   |  .........|.......    | 
        | cooperation         |  . -- .   |  .Yes     |   No .    | 
        | (integrated flavor) |  .    .   |  .        |      .    | 
        |---------------------+--.----.---+--.--------+------.----| 
        | NMS-VNTM            |  .    .   |  .        |      .    | 
        | cooperation         |  . -- .   |  .No      |   Yes.    | 
        | (separate flavor)   |  ......   |  .........|.......    | 
         ---------------------+----|------+--------|--+----------- 
                                   V               V 
                 Not applicable because   Typical models to be used 
                 there are multiple PCEs 

   Note that: 

   -  Since there is one PCE in each layer network, the path computation 
      model "Single PCE path computation" is not applicable. 

   -  For the other two path computation models "Multiple PCE with 
      inter-PCE" and "Multiple PCE w/o inter-PCE", the possible 
      combinations are the same as defined in [RFC5623]. More 
      specifically: 

      o  The path control models "NMS-VNTM cooperation (integrated 
         flavor)" and "NMS-VNTM cooperation (separate flavor)" are the 
         typical models to be used in multi-layer network with GMPLS-
         controller interworking. This is because in these two models, 
         the path computation is triggered by the NMS or VNTM. And in 
         the centralized controller system, the path computation 
         requests are typically from the Orchestrator(ML) (acts as 
         VNTM).  

      o  For the other two path control models "PCE-VNTM cooperation" 
         and "Higher-layer signaling trigger", the path computation is 
         triggered by the NEs, i.e., NE performs PCC functions. These 

 
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         two models are still possible to be used, although they are not 
         the main methods. 

7.3.2. Cross-layer Path Creation 

   In a multi-layer network, a lower-layer LSP in the lower-layer 
   network can be created, which will construct a new link in the 
   higher-layer network. Such lower-layer LSP is called Hierarchical 
   LSP, or H-LSP for short, see [RFC6107]. 

   The new link constructed by the H-LSP can then be used by the 
   higher-layer network to create new LSPs. 

   As described in [RFC5212], two methods are introduced to create the 
   H-LSP: the static (pre-provisioned) method and the dynamic 
   (triggered) method. 

   1) Static (pre-provisioned) method 

   In this method, the H-LSP in the lower-layer network is created in 
   advance. After that, the higher-layer network can create LSPs using 
   the resource of the link constructed by the H-LSP. 

   The Orchestrator(ML) is responsible to decide the creation of H-LSP 
   in the lower-layer network if it acts as a VNTM. It then requests 
   the L-Controller to create the H-LSP via, for example, MPI interface 
   under the ACTN architecture. See Section 3.3.2 of [TE-Tunnel]. 

   If the lower-layer network is a GMPLS domain, the L-Controller(G) 
   can trigger the GMPLS control plane to create the H-LSP. As a 
   typical example, the PCInitiate message can be used for the 
   communication between the L-Controller and the source node of the H-
   LSP. And the source node of the H-LSP can trigger the RSVP-TE 
   signaling procedure to create the H-LSP, as described in [RFC6107]. 

   If the lower-layer network is a non-GMPLS domain, other methods may 
   be used by the L-Controller(N) to create the H-LSP, which is out of 
   scope of this document. 

   2) Dynamic (triggered) method 

   In this method, the signaling of LSP creation in the higher-layer 
   network will trigger the creation of H-LSP in the lower-layer 
   network dynamically, if it is necessary. Therefore, both the higher-
   layer and lower-layer networks need to support the RSVP-TE protocol 
   and thus need to be GMPLS domains. 

   In this case, after the cross-layer path is computed, the 
   Orchestrator(ML) requests the H-Controller(G) for the cross-layer 

 
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   LSP creation. As a typical example, the MPI interface under the ACTN 
   architecture could be used. 

   The H-Controller(G) can trigger the GMPLS control plane to create 
   the LSP in the higher-layer network. As a typical example, the 
   PCInitiate message can be used for the communication between the H-
   Controller(G) and the source node of the Higher-layer LSP, as 
   described in Section 4.3 of [RFC8282]. At least two sets of ERO 
   information should be included to indicate the routes of higher-
   layer LSP and lower-layer H-LSP. 

   The source node of the Higher-layer LSP follows the procedure 
   defined in Section 4 of [RFC6001], to trigger the GMPLS control 
   plane in both higher-layer network and lower-layer network to create 
   the higher-layer LSP and the lower-layer H-LSP. 

   On success, the source node of the H-LSP should report the 
   information of the H-LSP to the L-Controller(G) via, for example, 
   PCRpt message. 

7.3.3. Link Discovery 

   If the higher-layer network and the lower-layer network are under 
   the same GMPLS control plane instance, the H-LSP can be a Forwarding 
   Adjacency LSP (FA-LSP). Then the information of the link constructed 
   by this FA-LSP, called Forwarding Adjacency (FA), can be advertised 
   in the routing instance, so that the H-Controller can be aware of 
   this new FA. [RFC4206] and the following updates to it (including 
   [RFC6001] and [RFC6107]) describe the detailed extensions to support 
   advertisement of an FA. 

   If the higher-layer network and the lower-layer network are under 
   separate GMPLS control plane instances, or one of the layer networks 
   is a non-GMPLS domain, after an H-LSP is created in the lower-layer 
   network, the link discovery procedure will be triggered in the 
   higher-layer network to discover the information of the link 
   constructed by the H-LSP. LMP protocol defined in [RFC4204] can be 
   used if the higher-layer network supports GMPLS. The information of 
   this new link will be advertised to the H-Controller. 

7.4. Recovery  

   The GMPLS recovery functions are described in [RFC4426]. Span 
   protection, end-to-end protection and restoration, are discussed 
   with different protection schemes and message exchange requirements. 
   Related RSVP-TE extensions to support end-to-end recovery is 
   described in [RFC4872]. The extensions in [RFC4872] include 
   protection, restoration, preemption, and rerouting mechanisms for an 
   end-to-end LSP. Besides end-to-end recovery, a GMPLS segment 
   recovery mechanism is defined in [RFC4873], which also intends to be 
 
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   compatible with Fast Reroute (FRR) (see [RFC4090] which defines 
   RSVP-TE extensions for the FRR mechanism, and [RFC8271] which 
   described the updates of GMPLS RSVP-TE protocol for FRR of GMPLS TE-
   LSPs). 

7.4.1. Span Protection 

   Span protection refers to the protection of the link between two 
   neighboring switches. The main protocol requirements include: 

   -  Link management: Link property correlation on the link protection 
      type; 

   -  Routing: announcement of the link protection type; 

   -  Signaling: indication of link protection requirement for that LSP. 

   GMPLS already supports the above requirements, and there are no new 
   requirements in the scenario of interworking between GMPLS and 
   centralized controller system. 

7.4.2. LSP Protection 

   The LSP protection includes end-to-end and segment LSP protection. 
   For both cases: 

   -  In the provisioning phase: 

      In both single-domain and multi-domain scenarios, the disjoint 
      path computation can be done by the centralized controller system, 
      as it has the global topology and resource view. And the path 
      creation can be done by the procedure described in Section 7.2. 

   -  In the protection switchover phase: 

      In both single-domain and multi-domain scenarios, the existing 
      standards provide the distributed way to trigger the protection 
      switchover. For example, data plane Automatic Protection Switching 
      (APS) mechanism described in [G.808.1], [RFC7271] and [RFC8234], 
      or GMPLS Notify mechanism described in [RFC4872] and [RFC4873]. In 
      the scenario of interworking between GMPLS and centralized 
      controller system, using these distributed mechanisms rather than 
      centralized mechanism (i.e., the controller triggers the 
      protection switchover) can significantly shorten the protection 
      switching time. 

7.4.3. Single-domain LSP Restoration 

   -  Pre-planned LSP protection (including shared-mesh restoration):  

 
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      In pre-planned protection, the protecting LSP is established only 
      in the control plane in the provisioning phase, and will be 
      activated in the data plane once failure occurs. 

      In the scenario of interworking between GMPLS and centralized 
      controller system, the route of protecting LSP can be computed by 
      the centralized controller system. This takes the advantage of 
      making better use of network resource, especially for the resource 
      sharing in shared-mesh restoration. 

   -  Full LSP rerouting:  

      In full LSP rerouting, the normal traffic will be switched to an 
      alternate LSP that is fully established only after failure 
      occurrence. 

      As described in [RFC4872] and [RFC4873], the alternate route can 
      be computed on demand when failure occurrence, or pre-computed and 
      stored before failure occurrence.  

      In a fully distributed scenario, the pre-computation method offers 
      faster restoration time, but has the risk that the pre-computed 
      alternate route may become out of date due to the changes of the 
      network. 

      In the scenario of interworking between GMPLS and centralized 
      controller system, the pre-computation of the alternate route 
      could take place in the centralized controller (and may be stored 
      in the controller or the head-end node of the LSP). In this way, 
      any changes in the network can trigger the refreshment of the 
      alternate route by the centralized controller. This makes sure 
      that the alternate route will not become out of date. 

7.4.4. Multi-domain LSP Restoration 

   A working LSP may traverse multiple domains, each of which may or 
   may not support GMPLS distributed control plane. 

   If all the domains support GMPLS, both the end-to-end rerouting 
   method and the domain segment rerouting method could be used. 

   If only some domains support GMPLS, the domain segment rerouting 
   method could be used in those GMPLS domains. For other domains which 
   do not support GMPLS, other mechanisms may be used to protect the 
   LSP segments, which are out of scope of this document. 

   1) End-to-end rerouting: 

   In this scenario, a failure on the working LSP inside any domain or 
   on the inter-domain links will trigger the end-to-end restoration. 
 
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   In both pre-planned and full LSP rerouting, the end-to-end 
   protecting LSP could be computed by the centralized controller 
   system, and could be created by the procedure described in Section 
   7.2. Note that the end-to-end protecting LSP may traverse different 
   domains from the working LSP, depending on the result of multi-
   domain path computation for the protecting LSP. 

                       +----------------+ 
                       |Orchestrator(MD)| 
                       +-------.--------+ 
          ............................................ 
          .             .              .             . 
     +----V-----+  +----V-----+   +----V-----+  +----V-----+ 
     |Controller|  |Controller|   |Controller|  |Controller| 
     |  (G) 1   |  |  (G) 2   |   |  (G) 3   |  |  (G) 4   | 
     +----.-----+  +-------.--+   +-------.--+  +----.-----+ 
          .                .              .          . 
     +----V--------+     +-V-----------+  .  +-------V-----+ 
     |  Domain 1   |     |  Domain 2   |  .  |  Domain 4   | 
     |+---+   +---+|     |+---+   +---+|  .  |+---+   +---+| 
     || ===/~/======/~~~/================================ || 
     |+-*-+   +---+|     |+---+   +---+|  .  |+---+   +-*-+| 
     |  *          |     +-------------+  .  |          *  | 
     |  *          |                      .  |          *  | 
     |  *          |     +-------------+  .  |          *  | 
     |  *          |     |  Domain 3   <...  |          *  | 
     |+-*-+   +---+|     |+---+   +---+|     |+---+   +-*-+| 
     || ************************************************* || 
     |+---+   +---+|     |+---+   +---+|     |+---+   +---+| 
     +-------------+     +-------------+     +-------------+ 

     ====: Working LSP   ****: Protecting LSP   /~/: Failure 

                     Figure 5: End-to-end restoration 

   2) Domain segment rerouting: 

   2.1) Intra-domain rerouting:  

   If failure occurs on the working LSP segment in a GMPLS domain, the 
   segment rerouting ([RFC4873]) could be used for the working LSP 
   segment in that GMPLS domain. Figure 6 shows an example of intra-
   domain rerouting. 

   The intra-domain rerouting of a non-GMPLS domain is out of scope of 
   this document. 

    

    
 
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                       +----------------+ 
                       |Orchestrator(MD)| 
                       +-------.--------+ 
          ............................................ 
          .             .              .             . 
     +----V-----+  +----V-----+   +----V-----+  +----V-----+ 
     |Controller|  |Controller|   |Controller|  |Controller| 
     |  (G) 1   |  |(G)/(N) 2 |   |(G)/(N) 3 |  |(G)/(N) 4 | 
     +----.-----+  +-------.--+   +-------.--+  +----.-----+ 
          .                .              .          . 
     +----V--------+     +-V-----------+  .  +-------V-----+ 
     |  Domain 1   |     |  Domain 2   |  .  |  Domain 4   | 
     |+---+   +---+|     |+---+   +---+|  .  |+---+   +---+| 
     || ===/~/=========================================== || 
     |+-*-+   +-*-+|     |+---+   +---+|  .  |+---+   +---+| 
     |  *       *  |     +-------------+  .  |             | 
     |  *       *  |                      .  |             | 
     |  *       *  |     +-------------+  .  |             | 
     |  *       *  |     |  Domain 3   <...  |             | 
     |+-*-+   +-*-+|     |+---+   +---+|     |+---+   +---+| 
     || ********* ||     ||   |   |   ||     ||   |   |   || 
     |+---+   +---+|     |+---+   +---+|     |+---+   +---+| 
     +-------------+     +-------------+     +-------------+ 

     ====: Working LSP  ****: Rerouting LSP segment  /~/: Failure 

                 Figure 6: Intra-domain segment rerouting 

   2.2) Inter-domain rerouting:  

   If intra-domain segment rerouting failed (e.g., due to lack of 
   resource in that domain), or if failure occurs on the working LSP on 
   an inter-domain link, the centralized controller system may 
   coordinate with other domain(s), to find an alternative path or path 
   segment to bypass the failure, and then trigger the inter-domain 
   rerouting procedure. Note that the rerouting path or path segment 
   may traverse different domains from the working LSP. 

   The domains involved in the inter-domain rerouting procedure need to 
   be GMPLS domains, which support the RSVP-TE signaling for the 
   creation of rerouting LSP segment.  

   For inter-domain rerouting, the interaction between GMPLS and 
   centralized controller system is needed: 

   -  Report of the result of intra-domain segment rerouting to its 
      Controller(G), and then to the Orchestrator(MD). The former one 
      could be supported by the PCRpt message in [RFC8231], while the 
      latter one could be supported by the MPI interface of ACTN. 

 
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   -  Report of inter-domain link failure to the two Controllers (e.g., 
      Controller(G) 1 and Controller(G) 2 in Figure 7) by which the two 
      ends of the inter-domain link are controlled respectively, and 
      then to the Orchestrator(MD). The former one could be done as 
      described in Section 7.1 of this document, while the latter one 
      could be supported by the MPI interface of ACTN. 

   -  Computation of rerouting path or path segment crossing multi-
      domains by the centralized controller system (see [PAT-COMP]); 

   -  Creation of rerouting LSP segment in each related domain. The 
      Orchestrator(MD) can send the LSP segment rerouting request to the 
      source Controller(G) (e.g., Controller(G) 1 in Figure 7) via MPI 
      interface, and then the Controller(G) can trigger the creation of 
      rerouting LSP segment through multiple GMPLS domains using GMPLS 
      rerouting signaling. Note that the rerouting LSP segment may 
      traverse a new domain which the working LSP does not traverse 
      (e.g., Domain 3 in Figure 7). 

                            +----------------+ 
                            |Orchestrator(MD)| 
                            +-------.--------+ 
           .................................................. 
           .               .                .               . 
     +-----V------+  +-----V------+   +-----V------+  +-----V------+ 
     | Controller |  | Controller |   | Controller |  | Controller | 
     |   (G) 1    |  |   (G) 2    |   |   (G) 3    |  | (G)/(N) 4  | 
     +-----.------+  +------.-----+   +-----.------+  +-----.------+ 
           .                .               .               . 
     +-----V-------+   +----V--------+      .        +------V------+ 
     |  Domain 1   |   |  Domain 2   |      .        |  Domain 4   | 
     |+---+   +---+|   |+---+   +---+|      .        |+---+   +---+| 
     ||   |   |   ||   ||   |   |   ||      .        ||   |   |   || 
     || ============/~/========================================== || 
     || * |   |   ||   ||   |   | * ||      .        ||   |   |   || 
     |+-*-+   +---+|   |+---+   +-*-+|      .        |+---+   +---+| 
     |  *          |   +----------*--+      .        |             | 
     |  *          |              *****     .        |             | 
     |  *          |       +----------*-----V----+   |             | 
     |  *          |       |          *Domain 3  |   |             | 
     |+-*-+   +---+|       |+---+   +-*-+   +---+|   |+---+   +---+| 
     || * |   |   ||       ||   |   | * |   |   ||   ||   |   |   || 
     || ******************************* |   |   ||   ||   |   |   || 
     ||   |   |   ||       ||   |   |   |   |   ||   ||   |   |   || 
     |+---+   +---+|       |+---+   +---+   +---+|   |+---+   +---+| 
     +-------------+       +---------------------+   +-------------+ 

      ====: Working LSP  ****: Rerouting LSP segment  /~/: Failure 

                 Figure 7: Inter-domain segment rerouting 
 
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7.4.5. Fast Reroute 

   [RFC4090] defines two methods of fast reroute, the one-to-one backup 
   method and the facility backup method. For both methods: 

   1) Path computation of protecting LSP: 

   In Section 6.2 of [RFC4090], the protecting LSP (detour LSP in one-
   to-one backup, or bypass tunnel in facility backup) could be 
   computed by the Point of Local Repair (PLR) using, for example, 
   Constraint-based Shortest Path First (CSPF) computation. In the 
   scenario of interworking between GMPLS and centralized controller 
   system, the protecting LSP could also be computed by the centralized 
   controller system, as it has the global view of the network 
   topology, resource and information of LSPs. 

   2) Protecting LSP creation: 

   In the scenario of interworking between GMPLS and centralized 
   controller system, the Protecting LSP could still be created by the 
   RSVP-TE signaling protocol as described in [RFC4090] and [RFC8271].  

   In addition, if the protecting LSP is computed by the centralized 
   controller system, the Secondary Explicit Route Object defined in 
   [RFC4873] could be used to explicitly indicate the route of the 
   protecting LSP. 

   3) Failure detection and traffic switchover: 

   If a PLR detects that failure occurs, it may significantly shorten 
   the protection switching time by using the distributed mechanisms 
   described in [RFC4090] to switch the traffic to the related detour 
   LSP or bypass tunnel, rather than in a centralized way. 

7.5. Controller Reliability  

   Given the important role in the network, the reliability of 
   controller is critical. If the controller is shut down or 
   disconnected from the network, it is highly desirable that all of 
   the services currently provisioned in the network continue to 
   function and carry traffic. Furthermore, protection switching to 
   pre-established paths should also function. Additionally, it is 
   desirable to provide protection mechanisms, such as redundancy, so 
   that full operational control can be maintained even when one 
   instance of the controller fails. This can be either achieved by 
   controller back up or functionality back up. There are several of 
   controller backup or federation mechanisms in the literature. It is 
   also more reliable to have some function back up in the network 
   element, to guarantee the performance in the network.  

 
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8. Manageability Considerations 

   Each network entity, including controllers and network elements, 
   should be managed properly and with the relevant trust and security 
   policies applied, as they will interact with other entities. The 
   manageability considerations in controller hierarchies and network 
   elements still apply, respectively. The overall manageability of the 
   protocols applied in the network should also be a key consideration.  

   The responsibility of each entity should be clarified. The control 
   of function and policy among different controllers should be 
   consistent via a proper negotiation process.   

9. Security Considerations 

   This document provides the interworking between the GMPLS and 
   controller hierarchies. The security requirements in both systems 
   still apply respectively. Protocols referenced in this document also 
   have various security considerations, which are expected to be 
   satisfied, with known risks detailed in their core specifications 
   and referenced earlier in this document.  

   Other considerations on the controller and the network element 
   interface are also important. Such security includes the functions 
   to authenticate and authorize the control access to the controller 
   from multiple network elements. Security mechanisms on the 
   controller are also required to safeguard the underlying network 
   elements against attacks on the control plane and/or unauthorized 
   usage of data transport resources.  

10. IANA Considerations 

   This document requires no IANA actions. 

11. References 

11.1. Normative References 

   [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., 
             and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP 
             Tunnels", RFC 3209, December 2001. 

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

   [RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering 
             (TE) Extensions to OSPF Version 2", RFC 3630, September 
             2003. 
 
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   [RFC3945] Mannie, E., Ed., "Generalized Multi-Protocol Label 
             Switching (GMPLS) Architecture", RFC 3945, October 2004. 

   [RFC4090] Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast 
             Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090, 
             May 2005. 

   [RFC4203] Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in 
             Support of Generalized Multi-Protocol Label Switching 
             (GMPLS)", RFC 4203, October 2005. 

   [RFC4206] Kompella, K. and Rekhter Y., "Label Switched Paths (LSP) 
             Hierarchy with Generalized Multi-Protocol Label Switching 
             (GMPLS) Traffic Engineering (TE)", RFC 4206, October 2005. 

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

   [RFC4872] Lang, J., Ed., Rekhter, Y., Ed., and D. Papadimitriou, 
             Ed., "RSVP-TE Extensions in Support of End-to-End 
             Generalized Multi-Protocol Label Switching (GMPLS) 
             Recovery", RFC 4872, May 2007. 

   [RFC4873] Berger, L., Bryskin, I., Papadimitriou, D., and A. Farrel, 
             "GMPLS Segment Recovery", RFC 4873, May 2007. 

   [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic 
             Engineering", RFC 5305, October 2008. 

   [RFC5307] Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions 
             in Support of Generalized Multi-Protocol Label Switching 
             (GMPLS)", RFC 5307, October 2008. 

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

   [RFC6001] Papadimitriou D., Vigoureux M., Shiomoto K., Brungard D. 
             and Le Roux JL., "Generalized MPLS (GMPLS) Protocol 
             Extensions for Multi-Layer and Multi-Region Networks 
             (MLN/MRN)", RFC 6001, October 2010. 

   [RFC6107] Shiomoto K. and Farrel A., "Procedures for Dynamically 
             Signaled Hierarchical Label Switched Paths", RFC 6107, 
             February 2011. 

   [RFC6241] Enns, R., Bjorklund, M., Schoenwaelder J., Bierman A., 
             "Network Configuration Protocol (NETCONF)", RFC 6241, June 
             2011. 

 
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   [RFC7074] Berger, L. and J. Meuric, "Revised Definition of the GMPLS 
             Switching Capability and Type Fields", RFC 7074, November 
             2013. 

   [RFC7491] King, D., Farrel, A., "A PCE-Based Architecture for 
             Application-Based Network Operations", RFC7491, March 
             2015. 

   [RFC7926] Farrel, A., Drake, J., Bitar, N., Swallow, G., Ceccarelli, 
             D. and Zhang, X., "Problem Statement and Architecture for 
             Information Exchange between Interconnected Traffic-
             Engineered Networks", RFC7926, July 2016.  

   [RFC7950] Bjorklund, M., "The YANG 1.1 Data Modeling Language", 
             RFC7950, August 2016. 

   [RFC8040] Bierman, A., Bjorklund, M., Watsen, K., "RESTCONF 
             Protocol", RFC 8040, January 2017. 

   [RFC8271] Taillon M., Saad T., Gandhi R., Ali Z. and Bhatia M., 
             "Updates to the Resource Reservation Protocol for Fast 
             Reroute of Traffic Engineering GMPLS Label Switched 
             Paths", RFC 8271, October 2017. 

   [RFC8282] Oki E., Takeda T., Farrel A. and Zhang F., "Extensions to 
             the Path Computation Element Communication Protocol (PCEP) 
             for Inter-Layer MPLS and GMPLS Traffic Engineering", RFC 
             8282, December 2017. 

   [RFC8453] Ceccarelli, D. and Y. Lee, "Framework for Abstraction and 
             Control of Traffic Engineered Networks", RFC 8453, August 
             2018. 

   [RFC8795] Liu, X., Bryskin, I., Beeram, V., Saad, T., Shah, H., 
             Gonzalez De Dios, O., "YANG Data Model for Traffic 
             Engineering (TE) Topologies", RFC8795, August 2020.  

    

11.2.  Informative References 

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

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

 
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   [RFC4204] Lang, J., Ed., "Link Management Protocol (LMP)", RFC 4204, 
             October 2005. 

   [RFC4426] Lang, J., Ed., Rajagopalan, B., Ed., and D. Papadimitriou, 
             Ed., "Generalized Multi-Protocol Label witching (GMPLS) 
             Recovery Functional Specification", RFC 4426, March 2006. 

   [RFC5150] Ayyangar, A., Kompella, K., Vasseur, J.P., Farrel, A., 
             "Label Switched Path Stitching with Generalized 
             Multiprotocol Label Switching Traffic Engineering (GMPLS 
             TE)", RFC 5150, February, 2008.  

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

   [RFC5623] Oki E., Takeda T., Le Roux JL. and Farrel A., "Framework 
             for PCE-Based Inter-Layer MPLS and GMPLS Traffic 
             Engineering", RFC 5623, September 2009. 

   [RFC5654] Niven-Jenkins B., Ed., Brungard D., Ed., Betts M., Ed., 
             Sprecher N., Ueno S., "Requirements of an MPLS Transport 
             Profile", RFC 5654, September 2009. 

   [RFC7138] Ceccarelli, D., Ed., Zhang, F., Belotti, S., Rao, R., and 
             J. Drake, "Traffic Engineering Extensions to OSPF for 
             GMPLS Control of Evolving G.709 Optical Transport 
             Networks", RFC 7138, March 2014. 

   [RFC7139] Zhang, F., Ed., Zhang, G., Belotti, S., Ceccarelli, D., 
             and K. Pithewan, "GMPLS Signaling Extensions for Control 
             of Evolving G.709 Optical Transport Networks", RFC 7139, 
             March 2014. 

   [RFC7271] Ryoo, J., Ed., Gray, E., Ed., van Helvoort, H., 
             D'Alessandro, A., Cheung, T., and Osborne, E., "MPLS 
             Transport Profile (MPLS-TP) Linear Protection to Match the 
             Operational Expectations of Synchronous Digital Hierarchy, 
             Optical Transport Network, and Ethernet Transport Network 
             Operators", RFC 7271, June 2014. 

   [RFC7688] Lee, Y., Ed. and G. Bernstein, Ed., "GMPLS OSPF 
             Enhancement for Signal and Network Element Compatibility 
             for Wavelength Switched Optical Networks", RFC 7688, 
             November 2015. 

   [RFC7689] Bernstein, G., Ed., Xu, S., Lee, Y., Ed., Martinelli, G., 
             and H. Harai, "Signaling Extensions for Wavelength 
             Switched Optical Networks", RFC 7689, November 2015. 
 
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   [RFC7792] Zhang, F., Zhang, X., Farrel, A., Gonzalez de Dios, O., 
             and D. Ceccarelli, "RSVP-TE Signaling Extensions in 
             Support of Flexi-Grid Dense Wavelength Division 
             Multiplexing (DWDM) Networks", RFC 7792, March 2016. 

   [RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path 
             Computation Element Communication Protocol (PCEP) 
             Extensions for Stateful PCE", RFC 8231, September 2017. 

   [RFC8234] Ryoo, J., Cheung, T., van Helvoort, H., Busi, I. and Wen 
             G., "Updates to MPLS Transport Profile (MPLS-TP) Linear 
             Protection in Automatic Protection Switching (APS) Mode", 
             RFC 8234, August 2017. 

   [RFC8281] Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "PCEP 
             Extensions for PCE-initiated LSP Setup in a Stateful PCE 
             Model", RFC 8281, October 2017. 

   [RFC8345] Clemm, A., Medved, J., Varga, R., Bahadur, N., 
             Ananthakrishnan, H., Liu, X., "A YANG Data Model for 
             Network Topologies", RFC 8345, March 2018.  

   [RFC8363] Zhang, X., Zheng, H., Casellas, R., Dios, O., and D. 
             Ceccarelli, "GMPLS OSPF-TE Extensions in support of Flexi-
             grid DWDM networks", RFC8363, February 2017. 

   [PAT-COMP] Busi, I., Belotti, S., Lopez, V., Gonzalez de Dios, O., 
             Sharma, A., Shi, Y., Vilalta, R., Setheraman, K., "Yang 
             model for requesting Path Computation", draft-ietf-teas-
             yang-path-computation, work in progress.  

   [PCEP-LS] Dhody, D., Lee, Y., Ceccarelli, D., "PCEP Extensions for 
             Distribution of Link-State and TE Information", draft-
             dhodylee-pce-pcep-ls, work in progress. 

   [TE-Tunnel] Saad, T. et al., "A YANG Data Model for Traffic 
             Engineering Tunnels and Interfaces", draft-ietf-teas-yang-
             te, work in progress. 

   [sPCE-ID] Dugeon, O. et al., "PCEP Extension for Stateful Inter-
             Domain Tunnels", draft-ietf-pce-stateful-interdomain, work 
             in progress. 

   [G.808.1] ITU-T, "Generic protection switching - Linear trail and 
             subnetwork protection", G.808.1, May 2014. 

 
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12. Contributors 

   Xianlong Luo 
   Huawei Technologies 
   G1, Huawei Xiliu Beipo Village, Songshan Lake 
   Dongguan 
   Guangdong, 523808 China 
   Email: luoxianlong@huawei.com 
    

   Sergio Belotti  
   Nokia  
   Email: sergio.belotti@nokia.com 
    
    
13. Authors' Addresses 

   Haomian Zheng 
   Huawei Technologies 
   H1, Huawei Xiliu Beipo Village, Songshan Lake 
   Dongguan 
   Guangdong, 523808 China 
   Email: zhenghaomian@huawei.com 
    
   Yunbin Xu 
   CAICT 
   Email: xuyunbin@caict.ac.cn 
    
   Yang Zhao 
   China Mobile 
   Email: zhaoyangyjy@chinamobile.com 
     
   Dieter Beller 
   Nokia 
   Email: Dieter.Beller@nokia.com 
    
   Yi Lin 
   Huawei Technologies 
   H1, Huawei Xiliu Beipo Village, Songshan Lake 
   Dongguan 
   Guangdong, 523808 China 
   Email: yi.lin@huawei.com 
    
    
Adknowledgements 

   The authors would like to thank Jim Guichard, AD Director of IETF 
   Routing Area, and Vishnu Pavan Beeram, Chair of TEAS WG, for their 
   reviews and comments on this document. 

 
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