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Versions: (draft-busibel-teas-yang-path-computation) 00 01 02 03 04

TEAS Working Group                                     Italo Busi (Ed.)
Internet Draft                                                   Huawei
Intended status: Standard Track                    Sergio Belotti (Ed.)
Expires: May 2019                                                 Nokia
                                                           Victor Lopez
                                                 Oscar Gonzalez de Dios
                                                             Telefonica
                                                          Anurag Sharma
                                                                 Google
                                                                Yan Shi
                                                           China Unicom
                                                         Ricard Vilalta
                                                                   CTTC
                                                     Karthik Sethuraman
                                                                    NEC

                                                       November 4, 2018





                Yang model for requesting Path Computation
               draft-ietf-teas-yang-path-computation-04.txt


Status of this Memo

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   This Internet-Draft will expire on May 4, 2019.

Copyright Notice

   Copyright (c) 2018 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.

Abstract

   There are scenarios, typically in a hierarchical SDN context, where
   the topology information provided by a TE network provider may not
   be sufficient for its client to perform end-to-end path computation.
   In these cases the client would need to request the provider to
   calculate some (partial) feasible paths.

   This document defines a YANG data model for a stateless RPC to
   request path computation. This model complements the stateful
   solution defined in [TE-TUNNEL].

   Moreover this document describes some use cases where a path
   computation request, via YANG-based protocols (e.g., NETCONF or
   RESTCONF), can be needed.

Table of Contents


   1. Introduction...................................................3
      1.1. Terminology...............................................4
   2. Use Cases......................................................5
      2.1. Packet/Optical Integration................................5
      2.2. Multi-domain TE Networks.................................10
      2.3. Data center interconnections.............................12
   3. Motivations...................................................14
      3.1. Motivation for a YANG Model..............................14
         3.1.1. Benefits of common data models......................14


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         3.1.2. Benefits of a single interface......................15
         3.1.3. Extensibility.......................................15
      3.2. Interactions with TE Topology............................16
         3.2.1. TE Topology Aggregation.............................17
         3.2.2. TE Topology Abstraction.............................20
         3.2.3. Complementary use of TE topology and path computation21
      3.3. Stateless and Stateful Path Computation..................24
   4. Path Computation and Optimization for multiple paths..........25
   5. YANG Model for requesting Path Computation....................26
      5.1. Synchronization of multiple path computation requests....27
      5.2. Returned metric values...................................29
   6. YANG model for stateless TE path computation..................30
      6.1. YANG Tree................................................30
      6.2. YANG Module..............................................39
   7. Security Considerations.......................................49
   8. IANA Considerations...........................................50
   9. References....................................................50
      9.1. Normative References.....................................50
      9.1. Informative References...................................51
   10. Acknowledgments..............................................52
   Appendix A.    Examples of dimensioning the "detailed connectivity
   matrix"        53

1. Introduction

   There are scenarios, typically in a hierarchical SDN context, where
   the topology information provided by a TE network provider may not
   be sufficient for its client to perform end-to-end path computation.
   In these cases the client would need to request the provider to
   calculate some (partial) feasible paths, complementing his topology
   knowledge, to make his end-to-end path computation feasible.

   This type of scenarios can be applied to different interfaces in
   different reference architectures:

   o  ABNO control interface [RFC7491], in which an Application Service
      Coordinator can request ABNO controller to take in charge path
      calculation (see Figure 1 in [RFC7491]).

   o  ACTN [RFC8453], where a controller hierarchy is defined, the need
      for path computation arises on both interfaces CMI (interface
      between Customer Network Controller (CNC) and Multi Domain
      Service Coordinator (MDSC)) and/or MPI (interface between MSDC-
      PNC). [RFC8454] describes an information model for the Path
      Computation request.


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   Multiple protocol solutions can be used for communication between
   different controller hierarchical levels. This document assumes that
   the controllers are communicating using YANG-based protocols (e.g.,
   NETCONF or RESTCONF).

   Path Computation Elements, Controllers and Orchestrators perform
   their operations based on Traffic Engineering Databases (TED). Such
   TEDs can be described, in a technology agnostic way, with the YANG
   Data Model for TE Topologies [TE-TOPO]. Furthermore, the technology
   specific details of the TED are modeled in the augmented TE topology
   models (e.g. [OTN-TOPO] for OTN ODU technologies).

   The availability of such topology models allows providing the TED
   using YANG-based protocols (e.g., NETCONF or RESTCONF). Furthermore,
   it enables a PCE/Controller performing the necessary abstractions or
   modifications and offering this customized topology to another
   PCE/Controller or high level orchestrator.

   Note: This document assumes that the client of the YANG data model
   defined in this document may not implement a "PCE" functionality, as
   defined in [RFC4655].

   The tunnels that can be provided over the networks described with
   the topology models can be also set-up, deleted and modified via
   YANG-based protocols (e.g., NETCONF or RESTCONF) using the TE-Tunnel
   Yang model [TE-TUNNEL].

   This document proposes a YANG model for a path computation request
   defined as a stateless RPC, which complements the stateful solution
   defined in [TE-TUNNEL].

   Moreover, this document describes some use cases where a path
   computation request, via YANG-based protocols (e.g., NETCONF or
   RESTCONF), can be needed.

1.1. Terminology

   TED: The traffic engineering database is a collection of all TE
   information about all TE nodes and TE links in a given network.

   PCE: A Path Computation Element (PCE) is an entity that is capable
   of computing a network path or route based on a network graph, and
   of applying computational constraints during the computation.  The
   PCE entity is an application that can be located within a network
   node or component, on an out-of-network server, etc.  For example, a


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   PCE would be able to compute the path of a TE LSP by operating on
   the TED and considering bandwidth and other constraints applicable
   to the TE LSP service request. [RFC4655]

2. Use Cases

   This section presents different use cases, where a client needs to
   request underlying SDN controllers for path computation.

   The presented uses cases have been grouped, depending on the
   different underlying topologies: a) Packet-Optical integration; b)
   Multi-domain Traffic Engineered (TE) Networks; and c) Data center
   interconnections.

2.1. Packet/Optical Integration

   In this use case, an Optical network is used to provide connectivity
   to some nodes of a Packet network (see Figure 1).





























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                                +----------------+
                                |                |
                                | Packet/Optical |
                                |  Coordinator   |
                                |                |
                                +---+------+-----+
                                    |      |
                       +------------+      |
                       |                   +-----------+
                +------V-----+                         |
                |            |                  +------V-----+
                | Packet     |                  |            |
                | Network    |                  | Optical    |
                | Controller |                  | Network    |
                |            |                  | Controller |
                +------+-----+                  +-------+----+
                       |                                |
              .........V.........................       |
              :          Packet Network         :       |
          +----+                               +----+   |
          | R1 |= = = = = = = = = = = = = = = =| R2 |   |
          +-+--+                               +--+-+   |
            | :                                 : |     |
            | :................................ : |     |
            |                                     |     |
            |               +-----+               |     |
            |    ...........| Opt |...........    |     |
            |    :          |  C  |          :    |     |
            |    :         /+--+--+\         :    |     |
            |    :        /    |    \        :    |     |
            |    :       /     |     \       :    |     |
            |   +-----+ /   +--+--+   \ +-----+   |     |
            |   | Opt |/    | Opt |    \| Opt |   |     |
            +---|  A  |     |  D  |     |  B  |---+     |
                +-----+\    +--+--+    /+-----+         |
                 :      \      |      /      :          |
                 :       \     |     /       :          |
                 :        \ +--+--+  / Optical<---------+
                 :         \| Opt |/  Network:
                 :..........|  E  |..........:
                            +-----+

              Figure 1  - Packet/Optical Integration Use Case




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   Figure 1 as well as Figure 2 below only show a partial view of the
   packet network connectivity, before additional packet connectivity
   is provided by the Optical network.

   It is assumed that the Optical network controller provides to the
   packet/optical coordinator an abstracted view of the Optical
   network. A possible abstraction could be to represent the whole
   optical network as one "virtual node" with "virtual ports" connected
   to the access links, as shown in Figure 2.

   It is also assumed that Packet network controller can provide the
   packet/optical coordinator the information it needs to setup
   connectivity between packet nodes through the Optical network (e.g.,
   the access links).

   The path computation request helps the coordinator to know the real
   connections that can be provided by the optical network.






























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                       ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,.
                      ,  Packet/Optical Coordinator view          ,
                     ,                              +----+       , .
                    ,                               |    |      ,
                   ,                                | R2 |     ,   .
                  ,  +----+  +------------ +       /+----+    ,
                 ,   |    |  |             |/-----/ / /      ,     .
                ,    | R1 |--O VP1     VP4 O       / /      ,
               ,     |    |\ |             | /----/ /      ,       .
              ,      +----+ \|             |/      /      ,
             ,        /      O VP2     VP5 O      /      ,         .
            ,        /       |             |  +----+    ,
           ,        /        |             |  |    |   ,           .
          ,        /         O VP3     VP6 O--| R4 |  ,
         ,     +----+ /-----/|_____________|  +----+ ,             .
        ,      |    |/       +------------ +        ,
       ,       | R3 |                              ,               .
      ,        +----+                             ,,,,,,,,,,,,,,,,,
     ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,                ,.
     . Packet Network Controller view               +----+       ,
       only packet nodes and packet links           |    |      ,  .
     . with access links to the optical network     | R2 |     ,
                  ,  +----+                        /+----+    ,    .
     .           ,   |    |                 /-----/ / /      ,
                ,    | R1 |---                     / /      ,      .
     .         ,     +----+\                 /----/ /      ,
              ,        /    \               /      /      ,        .
     .       ,        /                           /      ,
            ,        /                        +----+    ,          .
     .     ,        /                         |    |   ,
          ,        /                       ---| R4 |  ,            .
     .   ,     +----+ /-----/                 +----+ ,
        ,      |    |/                              ,              .
     . ,       | R3 |                              ,
      ,        +----+                             ,,,,,,,,,,,,,,,,,.
     .,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,                ,
       Optical Network Controller view                           , .
     . only optical nodes,        +--+                          ,
       optical links and         /|OF|                         ,   .
     . access links from the  +--++--+             /          ,
       packet network         |OA|    \     /-----/ /        ,     .
     .          ,          ---+--+--\  +--+/       /        ,
               ,           \   |  \  \-|OE|-------/        ,       .
     .        ,             \  |   \ /-+--+               ,
             ,               \+--+  X    |               ,         .


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     .      ,                 |OB|-/ \   |              ,
           ,                  +--+-\  \+--+            ,           .
     .    ,                  /   \  \--|OD|---        ,
         ,            /-----/     +--+ +--+          ,             .
     .  ,            /            |OC|/             ,
       ,                          +--+             ,               .
     .,                                           ,,,,,,,,,,,,,,,,,,
      ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,                ,
     . Actual Physical View                         +----+       ,
                    ,             +--+              |    |      ,
     .             ,             /|OF|              | R2 |     ,
                  ,  +----+   +--++--+             /+----+    ,
     .           ,   |    |   |OA|    \     /-----/ / /      ,
                ,    | R1 |---+--+--\  +--+/       / /      ,
     .         ,     +----+\   |  \  \-|OE|-------/ /      ,
              ,        /    \  |   \ /-+--+        /      ,
     .       ,        /      \+--+  X    |        /      ,
            ,        /        |OB|-/ \   |    +----+    ,
     .     ,        /         +--+-\  \+--+   |    |   ,
          ,        /         /   \  \--|OD|---| R4 |  ,
     .   ,     +----+ /-----/     +--+ +--+   +----+ ,
        ,      |    |/            |OC|/             ,
     . ,       | R3 |             +--+             ,
      ,        +----+                             ,
     .,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

            Figure 2 - Packet and Optical Topology Abstractions

   In this use case, the coordinator needs to setup an optimal
   underlying path for an IP link between R1 and R2.

   As depicted in Figure 2, the coordinator has only an "abstracted
   view" of the physical network, and it does not know the feasibility
   or the cost of the possible optical paths (e.g., VP1-VP4 and VP2-
   VP5), which depend from the current status of the physical resources
   within the optical network and on vendor-specific optical
   attributes.

   The coordinator can request the underlying Optical domain controller
   to compute a set of potential optimal paths, taking into account
   optical constraints. Then, based on its own constraints, policy and
   knowledge (e.g. cost of the access links), it can choose which one
   of these potential paths to use to setup the optimal end-to-end path
   crossing optical network.



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                    ............................
                    :                          :
                    O VP1                  VP4 O
           cost=10 /:\                        /:\ cost=10
                  / : \----------------------/ : \
          +----+ /  :         cost=50          :  \ +----+
          |    |/   :                          :   \|    |
          | R1 |    :                          :    | R2 |
          |    |\   :                          :   /|    |
          +----+ \  :  /--------------------\  :  / +----+
                  \ : /       cost=55        \ : /
            cost=5 \:/                        \:/ cost=5
                    O VP2                  VP5 O
                    :                          :
                    :..........................:

            Figure 3  - Packet/Optical Path Computation Example

   For example, in Figure 3, the Coordinator can request the Optical
   network controller to compute the paths between VP1-VP4 and VP2-VP5
   and then decide to setup the optimal end-to-end path using the VP2-
   VP5 Optical path even this is not the optimal path from the Optical
   domain perspective.

   Considering the dynamicity of the connectivity constraints of an
   Optical domain, it is possible that a path computed by the Optical
   network controller when requested by the Coordinator is no longer
   valid/available when the Coordinator requests it to be setup up.
   This is further discussed in section 3.3.

2.2. Multi-domain TE Networks

   In this use case there are two TE domains which are interconnected
   together by multiple inter-domains links.

   A possible example could be a multi-domain optical network.











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                            +--------------+
                            | Multi-domain |
                            | Controller   |
                            +---+------+---+
                                |      |
                   +------------+      |
                   |                   +-----------+
            +------V-----+                         |
            |            |                         |
            | TE Domain  |                  +------V-----+
            | Controller |                  |            |
            |      1     |                  | TE Domain  |
            +------+-----+                  | Controller |
                   |                        |      2     |
                   |                        +------+-----+
          .........V..........                     |
          :                  :                     |
         +-----+             :                     |
         |     |             :            .........V..........
         |  X  |             :            :                  :
         |     |          +-----+      +-----+                :
         +-----+          |     |      |     |               :
          :               |  C  |------|  E  |               :
      +-----+    +-----+ /|     |      |     |\ +-----+    +-----+
      |     |    |     |/ +-----+      +-----+ \|     |    |     |
      |  A  |----|  B  |     :            :     |  G  |----|  H  |
      |     |    |     |\    :            :    /|     |    |     |
      +-----+    +-----+ \+-----+      +-----+/ +-----+    +-----+
          :               |     |      |     |               :
          :               |  D  |------|  F  |               :
          :               |     |      |     |          +-----+
          :               +-----+      +-----+          |     |
          :                  :            :             |  Y  |
          :                  :            :             |     |
          :    Domain 1      :            : Domain 2    +-----+
          :..................:            :.................:

            Figure 4  - Multi-domain multi-link interconnection

   In order to setup an end-to-end multi-domain TE path (e.g., between
   nodes A and H), the multi-domain controller needs to know the
   feasibility or the cost of the possible TE paths within the two TE
   domains, which depend from the current status of the physical
   resources within each TE network. This is more challenging in case
   of optical networks because the optimal paths depend also on vendor-


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   specific optical attributes (which may be different in the two
   domains if they are provided by different vendors).

   In order to setup a multi-domain TE path (e.g., between nodes A and
   H), the multi-domain controller can request the TE domain
   controllers to compute a set of intra-domain optimal paths and take
   decisions based on the information received. For example:

   o  The multi-domain controller asks TE domain controllers to provide
      set of paths between A-C, A-D, E-H and F-H

   o  TE domain controllers return a set of feasible paths with the
      associated costs: the path A-C is not part of this set(in optical
      networks, it is typical to have some paths not being feasible due
      to optical constraints that are known only by the optical domain
      controller)

   o  The multi-domain controller will select the path A-D-F-H since it
      is the only feasible multi-domain path and then request the TE
      domain controllers to setup the A-D and F-H intra-domain paths

   o  If there are multiple feasible paths, the multi-domain controller
      can select the optimal path knowing the cost of the intra-domain
      paths (provided by the TE domain controllers) and the cost of the
      inter-domain links (known by the multi-domain controller)

   This approach may have some scalability issues when the number of TE
   domains is quite big (e.g. 20).

   In this case, it would be worthwhile using the abstract TE topology
   information provided by the TE domain controllers to limit the
   number of potential optimal end-to-end paths and then request path
   computation to fewer TE domain controllers in order to decide what
   the optimal path within this limited set is.

   For more details, see section 3.2.3.

2.3. Data center interconnections

   In these use case, there is a TE domain which is used to provide
   connectivity between data centers which are connected with the TE
   domain using access links.





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                        +--------------+
                        | Cloud Network|
                        | Orchestrator |
                        +--------------+
                          |  |  |  |
            +-------------+  |  |  +------------------------+
            |                |  +------------------+        |
            |       +--------V---+                 |        |
            |       |            |                 |        |
            |       | TE Network |                 |        |
     +------V-----+ | Controller |          +------V-----+  |
     | DC         | +------------+          | DC         |  |
     | Controller |     |                   | Controller |  |
     +------------+     |   +-----+         +------------+  |
          |         ....V...|     |........         |       |
          |         :       |  P  |       :         |       |
     .....V.....    :      /+-----+\      :    .....V.....  |
     :         :  +-----+ /    |    \ +-----+  :         :  |
     :  DC1 || :  |     |/     |     \|     |  :  DC2 || :  |
     :    ||||----| PE1 |      |      | PE2 |----   |||| :  |
     : _|||||| :  |     |\     |     /|     |  : _|||||| :  |
     :         :  +-----+ \ +-----+ / +-----+  :         :  |
     :.........:    :      \|     |/      :    :.........:  |
                    :.......| PE3 |.......:                 |
                            |     |                         |
                            +-----+               +---------V--+
                          .....|.....             | DC         |
                          :         :             | Controller |
                          :  DC3 || :             +------------+
                          :    |||| :                  |
                          : _|||||| <------------------+
                          :         :
                          :.........:

             Figure 5  - Data Center Interconnection Use Case

   In this use case, there is need to transfer data from Data Center 1
   (DC1) to either DC2 or DC3 (e.g. workload migration).

   The optimal decision depends both on the cost of the TE path (DC1-
   DC2 or DC1-DC3) and of the data center resources within DC2 or DC3.

   The cloud network orchestrator needs to make a decision for optimal
   connection based on TE Network constraints and data centers



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   resources. It may not be able to make this decision because it has
   only an abstract view of the TE network (as in use case in 2.1).

   The cloud network orchestrator can request to the TE network
   controller to compute the cost of the possible TE paths (e.g., DC1-
   DC2 and DC1-DC3) and to the DC controller to provide the information
   it needs about the required data center resources within DC2 and DC3
   and then it can take the decision about the optimal solution based
   on this information and its policy.

3. Motivations

   This section provides the motivation for the YANG model defined in
   this document.

   Section 3.1 describes the motivation for a YANG model to request
   path computation.

   Section 3.2 describes the motivation for a YANG model which
   complements the TE Topology YANG model defined in [TE-TOPO].

   Section 3.3 describes the motivation for a stateless YANG RPC which
   complements the TE Tunnel YANG model defined in [TE-TUNNEL].

3.1. Motivation for a YANG Model

3.1.1. Benefits of common data models

   The YANG data model for requesting path computation is closely
   aligned with the YANG data models that provide (abstract) TE
   topology information, i.e., [TE-TOPO] as well as that are used to
   configure and manage TE Tunnels, i.e., [TE-TUNNEL].

   There are many benefits in aligning the data model used for path
   computation requests with the YANG data models used for TE topology
   information and for TE Tunnels configuration and management:

   o  There is no need for an error-prone mapping or correlation of
      information.

   o  It is possible to use the same endpoint identifiers in path
      computation requests and in the topology modeling.

   o  The attributes used for path computation constraints are the same
      as those used when setting up a TE Tunnel.


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3.1.2. Benefits of a single interface

   The system integration effort is typically lower if a single,
   consistent interface is used by controllers, i.e., one data modeling
   language (i.e., YANG) and a common protocol (e.g., NETCONF or
   RESTCONF).

   Practical benefits of using a single, consistent interface include:

   1. Simple authentication and authorization: The interface between
      different components has to be secured. If different protocols
      have different security mechanisms, ensuring a common access
      control model may result in overhead. For instance, there may be
      a need to deal with different security mechanisms, e.g.,
      different credentials or keys. This can result in increased
      integration effort.

   2. Consistency: Keeping data consistent over multiple different
      interfaces or protocols is not trivial. For instance, the
      sequence of actions can matter in certain use cases, or
      transaction semantics could be desired. While ensuring
      consistency within one protocol can already be challenging, it is
      typically cumbersome to achieve that across different protocols.

   3. Testing: System integration requires comprehensive testing,
      including corner cases. The more different technologies are
      involved, the more difficult it is to run comprehensive test
      cases and ensure proper integration.

   4. Middle-box friendliness: Provider and consumer of path
      computation requests may be located in different networks, and
      middle-boxes such as firewalls, NATs, or load balancers may be
      deployed. In such environments it is simpler to deploy a single
      protocol. Also, it may be easier to debug connectivity problems.

   5. Tooling reuse: Implementers may want to implement path
      computation requests with tools and libraries that already exist
      in controllers and/or orchestrators, e.g., leveraging the rapidly
      growing eco-system for YANG tooling.

3.1.3. Extensibility

   Path computation is only a subset of the typical functionality of a
   controller. In many use cases, issuing path computation requests
   comes along with the need to access other functionality on the same


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   system. In addition to obtaining TE topology, for instance also
   configuration of services (setup/modification/deletion) may be
   required, as well as:

   1. Receiving notifications for topology changes as well as
      integration with fault management

   2. Performance management such as retrieving monitoring and
      telemetry data

   3. Service assurance, e.g., by triggering OAM functionality

   4. Other fulfilment and provisioning actions beyond tunnels and
      services, such as changing QoS configurations

   YANG is a very extensible and flexible data modeling language that
   can be used for all these use cases.

3.2. Interactions with TE Topology

   The use cases described in section 2 have been described assuming
   that the topology view exported by each underlying SDN controller to
   the orchestrator is aggregated using the "virtual node model",
   defined in [RFC7926].

   TE Topology information, e.g., as provided by [TE-TOPO], could in
   theory be used by an underlying SDN controllers to provide TE
   information to its client thus allowing a PCE available within its
   client to perform multi-domain path computation by its own, without
   requesting path computations to the underlying SDN controllers.

   In case the client does not implement a PCE function, as discussed
   in section 1, it could not perform path computation based on TE
   Topology information and would instead need to request path
   computation to the underlying controllers to get the information it
   needs to compute the optimal end-to-end path.

   This section analyzes the need for a client to request underlying
   SDN controllers for path computation even in case it implements a
   PCE functionality, as well as how the TE Topology information and
   the path computation can be complementary.

   In nutshell, there is a scalability trade-off between providing all
   the TE information needed by PCE, when implemented by the client, to
   take optimal path computation decisions by its own versus sending


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   too many requests to underlying SDN Domain Controllers to compute a
   set of feasible optimal intra-domain TE paths.

3.2.1. TE Topology Aggregation

   Using the TE Topology model, as defined in [TE-TOPO], the underlying
   SDN controller can export the whole TE domain as a single abstract
   TE node with a "detailed connectivity matrix".

   The concept of a "detailed connectivity matrix" is defined in [TE-
   TOPO] to provide specific TE attributes (e.g., delay, SRLGs and
   summary TE metrics) as an extension of the "basic connectivity
   matrix", which is based on the "connectivity matrix" defined in
   [RFC7446].

   The information provided by the "detailed connectivity matrix" would
   be equivalent to the information that should be provided by "virtual
   link model" as defined in [RFC7926].

   For example, in the Packet/Optical integration use case, described
   in section 2.1, the Optical network controller can make the
   information shown in Figure 3 available to the Coordinator as part
   of the TE Topology information and the Coordinator could use this
   information to calculate by its own the optimal path between R1 and
   R2, without requesting any additional information to the Optical
   network Controller.

   However, when designing the amount of information to provide within
   the "detailed connectivity matrix", there is a tradeoff to be
   considered between accuracy (i.e., providing "all" the information
   that might be needed by the PCE available to Orchestrator) and
   scalability.

   Figure 6 below shows another example, similar to Figure 3, where
   there are two possible Optical paths between VP1 and VP4 with
   different properties (e.g., available bandwidth and cost).











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                    ............................
                    :  /--------------------\  :
                    : /       cost=65        \ :
                    :/    available-bw=10G    \:
                    O VP1                  VP4 O
           cost=10 /:\                        /:\ cost=10
                  / : \----------------------/ : \
          +----+ /  :         cost=50          :  \ +----+
          |    |/   :     available-bw=2G      :   \|    |
          | R1 |    :                          :    | R2 |
          |    |\   :                          :   /|    |
          +----+ \  :  /--------------------\  :  / +----+
                  \ : /       cost=55        \ : /
            cost=5 \:/    available-bw=3G     \:/ cost=5
                    O VP2                  VP5 O
                    :                          :
                    :..........................:

     Figure 6  - Packet/Optical Path Computation Example with multiple
                                  choices

   Reporting all the information, as in Figure 6, using the "detailed
   connectivity matrix", is quite challenging from a scalability
   perspective. The amount of this information is not just based on
   number of end points (which would scale as N-square), but also on
   many other parameters, including client rate, user
   constraints/policies for the service, e.g. max latency < N ms, max
   cost, etc., exclusion policies to route around busy links, min OSNR
   margin, max preFEC BER etc. All these constraints could be different
   based on connectivity requirements.

   Examples of how the "detailed connectivity matrix" can be
   dimensioned are described in Appendix A.

   It is also worth noting that the "connectivity matrix" has been
   originally defined in WSON, [RFC7446], to report the connectivity
   constrains of a physical node within the WDM network: the
   information it contains is pretty "static" and therefore, once taken
   and stored in the TE data base, it can be always being considered
   valid and up-to-date in path computation request.

   Using the "basic connectivity matrix" with an abstract node to
   abstract the information regarding the connectivity constraints of
   an Optical domain, would make this information more "dynamic" since
   the connectivity constraints of an Optical domain can change over


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   time because some optical paths that are feasible at a given time
   may become unfeasible at a later time when e.g., another optical
   path is established. The information in the "detailed connectivity
   matrix" is even more dynamic since the establishment of another
   optical path may change some of the parameters (e.g., delay or
   available bandwidth) in the "detailed connectivity matrix" while not
   changing the feasibility of the path.

   The "connectivity matrix" is sometimes confused with optical reach
   table that contain multiple (e.g. k-shortest) regen-free reachable
   paths for every A-Z node combination in the network. Optical reach
   tables can be calculated offline, utilizing vendor optical design
   and planning tools, and periodically uploaded to the Controller:
   these optical path reach tables are fairly static. However, to get
   the connectivity matrix, between any two sites, either a regen free
   path can be used, if one is available, or multiple regen free paths
   are concatenated to get from src to dest, which can be a very large
   combination. Additionally, when the optical path within optical
   domain needs to be computed, it can result in different paths based
   on input objective, constraints, and network conditions. In summary,
   even though "optical reachability table" is fairly static, which
   regen free paths to build the connectivity matrix between any source
   and destination is very dynamic, and is done using very
   sophisticated routing algorithms.

   There is therefore the need to keep the information in the "detailed
   connectivity matrix" updated which means that there another tradeoff
   between the accuracy (i.e., providing "all" the information that
   might be needed by the client's PCE) and having up-to-date
   information. The more the information is provided and the longer it
   takes to keep it up-to-date which increases the likelihood that the
   client's PCE computes paths using not updated information.

   It seems therefore quite challenging to have a "detailed
   connectivity matrix" that provides accurate, scalable and updated
   information to allow the client's PCE to take optimal decisions by
   its own.

   Instead, if the information in the "detailed connectivity matrix" is
   not complete/accurate, we can have the following drawbacks
   considering for example the case in Figure 6:






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   o  If only the VP1-VP4 path with available bandwidth of 2 Gb/s and
      cost 50 is reported, the client's PCE will fail to compute a 5
      Gb/s path between routers R1 and R2, although this would be
      feasible;

   o  If only the VP1-VP4 path with available bandwidth of 10 Gb/s and
      cost 60 is reported, the client's PCE will compute, as optimal,
      the 1 Gb/s path between R1 and R2 going through the VP2-VP5 path
      within the Optical domain while the optimal path would actually
      be the one going thought the VP1-VP4 sub-path (with cost 50)
      within the Optical domain.

   Using the approach proposed in this document, the client, when it
   needs to setup an end-to-end path, it can request the Optical domain
   controller to compute a set of optimal paths (e.g., for VP1-VP4 and
   VP2-VP5) and take decisions based on the information received:

   o  When setting up a 5 Gb/s path between routers R1 and R2, the
      Optical domain controller may report only the VP1-VP4 path as the
      only feasible path: the Orchestrator can successfully setup the
      end-to-end path passing though this Optical path;

   o  When setting up a 1 Gb/s path between routers R1 and R2, the
      Optical domain controller (knowing that the path requires only 1
      Gb/s) can report both the VP1-VP4 path, with cost 50, and the
      VP2-VP5 path, with cost 65. The Orchestrator can then compute the
      optimal path which is passing thought the VP1-VP4 sub-path (with
      cost 50) within the Optical domain.

3.2.2. TE Topology Abstraction

   Using the TE Topology model, as defined in [TE-TOPO], the underlying
   SDN controller can export an abstract TE Topology, composed by a set
   of TE nodes and TE links, representing the abstract view of the
   topology controlled by each domain controller.

   Considering the example in Figure 4, the TE domain controller 1 can
   export a TE Topology encompassing the TE nodes A, B, C and D and the
   TE Link interconnecting them. In a similar way, TE domain controller
   2 can export a TE Topology encompassing the TE nodes E, F, G and H
   and the TE Link interconnecting them.

   In this example, for simplicity reasons, each abstract TE node maps
   with each physical node, but this is not necessary.



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   In order to setup a multi-domain TE path (e.g., between nodes A and
   H), the multi-domain controller can compute by its own an optimal
   end-to-end path based on the abstract TE topology information
   provided by the domain controllers. For example:

   o  Multi-domain controller's PCE, based on its own information, can
      compute the optimal multi-domain path being A-B-C-E-G-H, and then
      request the TE domain controllers to setup the A-B-C and E-G-H
      intra-domain paths

   o  But, during path setup, the domain controller may find out that
      A-B-C intra-domain path is not feasible (as discussed in section
      2.2, in optical networks it is typical to have some paths not
      being feasible due to optical constraints that are known only by
      the optical domain controller), while only the path A-B-D is
      feasible

   o  So what the multi-domain controller computed is not good and need
      to re-start the path computation from scratch

  As discussed in section 3.2.1, providing more extensive abstract
  information from the TE domain controllers to the multi-domain
  controller may lead to scalability problems.

  In a sense this is similar to the problem of routing and wavelength
  assignment within an Optical domain. It is possible to do first
  routing (step 1) and then wavelength assignment (step 2), but the
  chances of ending up with a good path is low. Alternatively, it is
  possible to do combined routing and wavelength assignment, which is
  known to be a more optimal and effective way for Optical path setup.
  Similarly, it is possible to first compute an abstract end-to-end
  path within the multi-domain Orchestrator (step 1) and then compute
  an intra-domain path within each Optical domain (step 2), but there
  are more chances not to find a path or to get a suboptimal path that
  performing per-domain path computation and then stitch them.

3.2.3. Complementary use of TE topology and path computation

   As discussed in section 2.2, there are some scalability issues with
   path computation requests in a multi-domain TE network with many TE
   domains, in terms of the number of requests to send to the TE domain
   controllers. It would therefore be worthwhile using the TE topology
   information provided by the domain controllers to limit the number
   of requests.



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   An example can be described considering the multi-domain abstract
   topology shown in Figure 7. In this example, an end-to-end TE path
   between domains A and F needs to be setup. The transit domain should
   be selected between domains B, C, D and E.

                          .........B.........
                          : _ _ _ _ _ _ _ _ :
                          :/               \:
                      +---O  NOT FEASIBLE   O---+
                cost=5|   :                 :   |
    ......A......     |   :.................:   |     ......F......
    :           :     |                         |     :           :
    :           O-----+   .........C.........   +-----O           :
    :           :         : /-------------\ :         :           :
    :           :         :/               \:         :           :
    :  cost<=20 O---------O   cost <= 30    O---------O cost<=20  :
    :          /: cost=5  :                 : cost=5  :\          :
    :  /------/ :         :.................:         : \------\  :
    : /         :                                     :         \ :
    :/ cost<=25 :         .........D.........         : cost<=25 \:
    O-----------O-------+ : /-------------\ : +-------O-----------O
    :\          : cost=5| :/               \: |cost=5 :          /:
    : \         :       +-O   cost <= 30    O-+       :         / :
    :  \------\ :         :                 :         : /------/  :
    : cost>=30 \:         :.................:         :/ cost>=30 :
    :           O-----+                         +-----O           :
    :...........:     |   .........E.........   |     :...........:
                      |   : /-------------\ :   |
                cost=5|   :/               \:   |cost=5
                      +---O   cost >= 30    O---+
                          :                 :
                          :.................:

     Figure 7 - Multi-domain with many domains (Topology information)

   The actual cost of each intra-domain path is not known a priori from
   the abstract topology information. The Multi-domain controller only
   knows, from the TE topology provided by the underlying domain
   controllers, the feasibility of some intra-domain paths and some
   upper-bound and/or lower-bound cost information. With this
   information, together with the cost of inter-domain links, the
   Multi-domain controller can understand by its own that:

   o  Domain B cannot be selected as the path connecting domains A and
      E is not feasible;


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   o  Domain E cannot be selected as a transit domain since it is know
      from the abstract topology information provided by domain
      controllers that the cost of the multi-domain path A-E-F (which
      is 100, in the best case) will be always be higher than the cost
      of the multi-domain paths A-D-F (which is 90, in the worst case)
      and A-E-F (which is 80, in the worst case)

   Therefore, the Multi-domain controller can understand by its own
   that the optimal multi-domain path could be either A-D-F or A-E-F
   but it cannot known which one of the two possible option actually
   provides the optimal end-to-end path.

   The Multi-domain controller can therefore request path computation
   only to the TE domain controllers A, D, E and F (and not to all the
   possible TE domain controllers).

                          .........B.........
                          :                 :
                      +---O                 O---+
    ......A......     |   :.................:   |     ......F......
    :           :     |                         |     :           :
    :           O-----+   .........C.........   +-----O           :
    :           :         : /-------------\ :         :           :
    :           :         :/               \:         :           :
    :  cost=15  O---------O    cost = 25    O---------O  cost=10  :
    :          /: cost=5  :                 : cost=5  :\          :
    :  /------/ :         :.................:         : \------\  :
    : /         :                                     :         \ :
    :/ cost=10  :         .........D.........         : cost=15  \:
    O-----------O-------+ : /-------------\ : +-------O-----------O
    :           : cost=5| :/               \: |cost=5 :           :
    :           :       +-O    cost = 15    O-+       :           :
    :           :         :                 :         :           :
    :           :         :.................:         :           :
    :           O-----+                         +-----O           :
    :...........:     |   .........E.........   |     :...........:
                      |   :                 :   |
                      +---O                 O---+
                          :.................:

       Figure 8  - Multi-domain with many domains (Path Computation
                               information)

   Based on these requests, the Multi-domain controller can know the
   actual cost of each intra-domain paths which belongs to potential


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   optimal end-to-end paths, as shown in Figure 8, and then compute the
   optimal end-to-end path (e.g., A-D-F, having total cost of 50,
   instead of A-C-F having a total cost of 70).

3.3. Stateless and Stateful Path Computation

   The TE Tunnel YANG model, defined in [TE-TUNNEL], can support the
   need to request path computation.

   It is possible to request path computation by configuring a
   "compute-only" TE tunnel and retrieving the computed path(s) in the
   LSP(s) Record-Route Object (RRO) list as described in section 3.3.1
   of [TE-TUNNEL].

   This is a stateful solution since the state of each created
   "compute-only" TE tunnel needs to be maintained and updated, when
   underlying network conditions change.

   It is very useful to provide options for both stateless and stateful
   path computation mechanisms. It is suggested to use stateless
   mechanisms as much as possible and to rely on stateful path
   computation when really needed.

   Stateless RPC allows requesting path computation using a simple
   atomic operation and it is the natural option/choice, especially
   with stateless PCE.

   Since the operation is stateless, there is no guarantee that the
   returned path would still be available when path setup is requested:
   this does not cause major issues in case the time between path
   computation and path setup is short (especially if compared with the
   time that would be needed to update the information of a very
   detailed connectivity matrix).

   In most of the cases, there is even no need to guarantee that the
   path that has been setup is the exactly same as the path that has
   been returned by path computation, especially if has the same or
   even better metrics. Depending on the abstraction level applied by
   the server, the client may also not know the actual computed path.

   The most important requirement is that the required global
   objectives (e.g., multi-domain path metrics and constraints) are
   met. For this reason a path verification phase is necessary to
   verify that the actual path that has been setup meets the global
   objectives (for example in a multi-domain network, the resulting


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   end-to-end path meets the required end-to-end metrics and
   constraints).

   In most of the cases, even if the setup path is not exactly the same
   as the path returned by path computation, its metrics and
   constraints are "good enough" (the path verification passes
   successfully). In the few corner cases where the path verification
   fails, it is possible repeat the whole process (path computation,
   path setup and path verification).

   In case the stateless solution is not sufficient, a stateful
   solution, based on "compute-only" TE tunnel, could be used to get
   notifications in case the computed path has been changed.

   It is worth noting that also the stateful solution, although
   increasing the likelihood that the computed path is available at
   path setup, does not guaranteed that because notifications may not
   be reliable or delivered on time. Path verification is needed also
   when stateful path computation is used.

   The stateful path computation has also the following drawbacks:

   o  Several messages required for any path computation

   o  Requires persistent storage in the provider controller

   o  Need for garbage collection for stranded paths

   o  Process burden to detect changes on the computed paths in order
      to provide notifications update

4. Path Computation and Optimization for multiple paths

   There are use cases, where it is advantageous to request path
   computation for a set of paths, through a network or through a
   network domain, using a single request [RFC5440].

   In this case, sending a single request for multiple path
   computations, instead of sending multiple requests for each path
   computation, would reduce the protocol overhead and it would consume
   less resources (e.g., threads in the client and server).

   In the context of a typical multi-domain TE network, there could
   multiple choices for the ingress/egress points of a domain and the
   Multi-domain controller needs to request path computation between


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   all the ingress/egress pairs to select the best pair. For example,
   in the example of section 2.2, the Multi-domain controller needs to
   request the TE network controller 1 to compute the A-C and the A-D
   paths and to the TE network controller 2 to compute the E-H and the
   F-H paths.

   It is also possible that the Multi-domain controller receives a
   request to setup a group of multiple end to end connections. The
   multi-domain controller needs to request each TE domain controller
   to compute multiple paths, one (or more) for each end to end
   connection.

   There are also scenarios where it can be needed to request path
   computation for a set of paths in a synchronized fashion.

   One example could be computing multiple diverse paths. Computing a
   set of diverse paths in a not-synchronized fashion, leads to the
   possibility of not being able to satisfy the diversity requirement.
   In this case, it is preferable to compute a sub-optimal primary path
   for which a diversely routed secondary path exists.

   There are also scenarios where it is needed to request optimizing a
   set of paths using objective functions that apply to the whole set
   of paths, see [RFC5541], e.g. to minimize the sum of the costs of
   all the computed paths in the set.

5. YANG Model for requesting Path Computation

   This document define a YANG stateless RPC to request path
   computation as an "augmentation" of tunnel-rpc, defined in [TE-
   TUNNEL]. This model provides the RPC input attributes that are
   needed to request path computation and the RPC output attributes
   that are needed to report the computed paths.

     augment /te:tunnels-rpc/te:input/te:tunnel-info:
       +---- path-request* [request-id]
       ...........

     augment /te:tunnels-rpc/te:output/te:result:
       +--ro response* [response-id]
          +--ro response-id      uint32
          +--ro (response-type)?
             +--:(no-path-case)



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             |  +--ro no-path!
             +--:(path-case)
                +--ro computed-path
                   ...........

   This model extensively re-uses the grouping defined in [TE-TUNNEL]
   to ensure maximal syntax and semantics commonality.

5.1. Synchronization of multiple path computation requests

   The YANG model permits to synchronize a set of multiple path
   requests (identified by specific request-id) all related to a "svec"
   container emulating the syntax of "SVEC" PCEP object [RFC5440].

       +---- synchronization* [synchronization-id]
          +---- synchronization-id    uint32
          +---- svec
          |  +---- relaxable?           boolean
          |  +---- disjointness?        te-types:te-path-disjointness
          |  +---- request-id-number*   uint32
          +---- svec-constraints
          |  +---- path-metric-bound* [metric-type]
          |     +---- metric-type    identityref
          |     +---- upper-bound?   uint64
          +---- path-srlgs-values
          |  +---- usage?    identityref
          |  +---- values*   srlg
          +---- path-srlgs-names
          |  +---- path-srlgs-name* [usage]
          |     +---- usage        identityref
          |     +---- srlg-name* [name]
          |        +---- name    string
          +---- exclude-objects
          ...........
          +---- optimizations
             +---- (algorithm)?
                +--:(metric)
                |  +---- optimization-metric* [metric-type]
                |     +---- metric-type    identityref
                |     +---- weight?        uint8



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                +--:(objective-function)
                   +---- objective-function
                      +---- objective-function-type?   identityref

   The model, in addition to the metric types, defined in [TE-TUNNEL],
   which can be applied to each individual path request, defines
   additional specific metrics types that apply to a set of
   synchronized requests, as referenced in [RFC5541].

     identity svec-metric-type {
       description
         "Base identity for svec metric type";
     }

     identity svec-metric-cumul-te {
       base svec-metric-type;
       description
         "TE cumulative path metric";
     }

     identity svec-metric-cumul-igp {
       base svec-metric-type;
       description
         "IGP cumulative path metric";
     }

     identity svec-metric-cumul-hop {
       base svec-metric-type;
       description
         "Hop cumulative path metric";
     }

     identity svec-metric-aggregate-bandwidth-consumption {
       base svec-metric-type;
       description
         "Cumulative bandwith consumption of the set of
          synchronized paths";
     }

     identity svec-metric-load-of-the-most-loaded-link {


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       base svec-metric-type;
       description
         "Load of the most loaded link";
     }

5.2. Returned metric values

   This YANG model provides a way to return the values of the metrics
   computed by the path computation in the output of RPC, together with
   other important information (e.g. srlg, affinities, explicit route),
   emulating the syntax of the "C" flag of the "METRIC" PCEP object
   [RFC5440]:

     augment /te:tunnels-rpc/te:output/te:result:
       +--ro response* [response-id]
          +--ro response-id      uint32
          +--ro (response-type)?
             +--:(no-path-case)
             |  +--ro no-path!
             +--:(path-case)
                +--ro computed-path
                   +--ro path-id?           yang-types:uuid
                   +--ro path-properties
                      +--ro path-metric* [metric-type]
                      |  +--ro metric-type           identityref
                      |  +--ro accumulative-value?   uint64
                      +--ro path-affinities-values
                      |  +--ro path-affinities-value* [usage]
                      |     +--ro usage    identityref
                      |     +--ro value?   admin-groups
                      +--ro path-affinity-names
                      |  +--ro path-affinity-name* [usage]
                      |     +--ro usage            identityref
                      |     +--ro affinity-name* [name]
                      |        +--ro name    string
                      +--ro path-srlgs-values
                      |  +--ro usage?    identityref
                      |  +--ro values*   srlg
                      +--ro path-srlgs-names
                      |  +--ro path-srlgs-name* [usage]



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                      |     +--ro usage        identityref
                      |     +--ro srlg-name* [name]
                      |        +--ro name    string
                      +--ro path-route-objects
                      ...........

   It also allows to request in the input of RPC which information
   (metrics, srlg and/or affinities) should be returned:

   module: ietf-te-path-computation
     augment /te:tunnels-rpc/te:input/te:tunnel-info:
       +---- path-request* [request-id]
       |  +---- request-id                uint32
           ...........
       |  +---- requested-metrics* [metric-type]
       |  |  +---- metric-type    identityref
       |  +---- return-srlgs?             boolean
       |  +---- return-affinities?        boolean
           ...........

   This feature is essential for using a stateless path computation in
   a multi-domain TE network as described in section 2.2. In this case,
   the metrics returned by a path computation requested to a given TE
   network controller must be used by the client to compute the best
   end-to-end path. If they are missing the client cannot compare
   different paths calculated by the TE network controllers and choose
   the best one for the optimal e2e path.

6. YANG model for stateless TE path computation

6.1. YANG Tree

   Figure 9 below shows the tree diagram of the YANG model defined in
   module ietf-te-path-computation.yang.

   module: ietf-te-path-computation
     augment /te:tunnels-rpc/te:input/te:tunnel-info:
       +---- path-request* [request-id]
       |  +---- request-id                uint32
       |  +---- te-topology-identifier
       |  |  +---- provider-id?   te-types:te-global-id



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       |  |  +---- client-id?     te-types:te-global-id
       |  |  +---- topology-id?   te-types:te-topology-id
       |  +---- source?                   inet:ip-address
       |  +---- destination?              inet:ip-address
       |  +---- src-tp-id?                binary
       |  +---- dst-tp-id?                binary
       |  +---- bidirectional?            boolean
       |  +---- encoding?                 identityref
       |  +---- switching-type?           identityref
       |  +---- explicit-route-objects
       |  |  +---- route-object-exclude-always* [index]
       |  |  |  +---- index            uint32
       |  |  |  +---- (type)?
       |  |  |     +--:(num-unnum-hop)
       |  |  |     |  +---- num-unnum-hop
       |  |  |     |     +---- node-id?      te-types:te-node-id
       |  |  |     |     +---- link-tp-id?   te-types:te-tp-id
       |  |  |     |     +---- hop-type?     te-hop-type
       |  |  |     |     +---- direction?    te-link-direction
       |  |  |     +--:(as-number)
       |  |  |     |  +---- as-number-hop
       |  |  |     |     +---- as-number?   binary
       |  |  |     |     +---- hop-type?    te-hop-type
       |  |  |     +--:(label)
       |  |  |        +---- label-hop
       |  |  |           +---- te-label
       |  |  |              +---- (technology)?
       |  |  |              |  +--:(generic)
       |  |  |              |     +---- generic?
       |  |  |              |             rt-types:generalized-label
       |  |  |              +---- direction?   te-label-direction
       |  |  +---- route-object-include-exclude* [index]
       |  |     +---- explicit-route-usage?   identityref
       |  |     +---- index                   uint32
       |  |     +---- (type)?
       |  |        +--:(num-unnum-hop)
       |  |        |  +---- num-unnum-hop
       |  |        |     +---- node-id?      te-types:te-node-id
       |  |        |     +---- link-tp-id?   te-types:te-tp-id



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       |  |        |     +---- hop-type?     te-hop-type
       |  |        |     +---- direction?    te-link-direction
       |  |        +--:(as-number)
       |  |        |  +---- as-number-hop
       |  |        |     +---- as-number?   binary
       |  |        |     +---- hop-type?    te-hop-type
       |  |        +--:(label)
       |  |        |  +---- label-hop
       |  |        |     +---- te-label
       |  |        |        +---- (technology)?
       |  |        |        |  +--:(generic)
       |  |        |        |     +---- generic?
       |  |        |        |             rt-types:generalized-label
       |  |        |        +---- direction?   te-label-direction
       |  |        +--:(srlg)
       |  |           +---- srlg
       |  |              +---- srlg?   uint32
       |  +---- path-constraints
       |  |  +---- te-bandwidth
       |  |  |  +---- (technology)?
       |  |  |     +--:(generic)
       |  |  |        +---- generic?   te-bandwidth
       |  |  +---- setup-priority?           uint8
       |  |  +---- hold-priority?            uint8
       |  |  +---- signaling-type?           identityref
       |  |  +---- path-metric-bounds
       |  |  |  +---- path-metric-bound* [metric-type]
       |  |  |     +---- metric-type    identityref
       |  |  |     +---- upper-bound?   uint64
       |  |  +---- path-affinities-values
       |  |  |  +---- path-affinities-value* [usage]
       |  |  |     +---- usage    identityref
       |  |  |     +---- value?   admin-groups
       |  |  +---- path-affinity-names
       |  |  |  +---- path-affinity-name* [usage]
       |  |  |     +---- usage            identityref
       |  |  |     +---- affinity-name* [name]
       |  |  |        +---- name    string
       |  |  +---- path-srlgs-values



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       |  |  |  +---- usage?    identityref
       |  |  |  +---- values*   srlg
       |  |  +---- path-srlgs-names
       |  |  |  +---- path-srlgs-name* [usage]
       |  |  |     +---- usage        identityref
       |  |  |     +---- srlg-name* [name]
       |  |  |        +---- name    string
       |  |  +---- disjointness?             te-types:te-path-
   disjointness
       |  +---- optimizations
       |  |  +---- (algorithm)?
       |  |     +--:(metric) {path-optimization-metric}?
       |  |     |  +---- optimization-metric* [metric-type]
       |  |     |  |  +---- metric-type
   identityref
       |  |     |  |  +---- weight?                           uint8
       |  |     |  |  +---- explicit-route-exclude-objects
       |  |     |  |  |  +---- route-object-exclude-object* [index]
       |  |     |  |  |     +---- index            uint32
       |  |     |  |  |     +---- (type)?
       |  |     |  |  |        +--:(num-unnum-hop)
       |  |     |  |  |        |  +---- num-unnum-hop
       |  |     |  |  |        |     +---- node-id?      te-types:te-
   node-id
       |  |     |  |  |        |     +---- link-tp-id?   te-types:te-
   tp-id
       |  |     |  |  |        |     +---- hop-type?     te-hop-type
       |  |     |  |  |        |     +---- direction?    te-link-
   direction
       |  |     |  |  |        +--:(as-number)
       |  |     |  |  |        |  +---- as-number-hop
       |  |     |  |  |        |     +---- as-number?   binary
       |  |     |  |  |        |     +---- hop-type?    te-hop-type
       |  |     |  |  |        +--:(label)
       |  |     |  |  |        |  +---- label-hop
       |  |     |  |  |        |     +---- te-label
       |  |     |  |  |        |        +---- (technology)?
       |  |     |  |  |        |        |  +--:(generic)
       |  |     |  |  |        |        |     +---- generic?



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       |  |     |  |  |        |        |             rt-
   types:generalized-label
       |  |     |  |  |        |        +---- direction?
       |  |     |  |  |        |                te-label-direction
       |  |     |  |  |        +--:(srlg)
       |  |     |  |  |           +---- srlg
       |  |     |  |  |              +---- srlg?   uint32
       |  |     |  |  +---- explicit-route-include-objects
       |  |     |  |     +---- route-object-include-object* [index]
       |  |     |  |        +---- index            uint32
       |  |     |  |        +---- (type)?
       |  |     |  |           +--:(num-unnum-hop)
       |  |     |  |           |  +---- num-unnum-hop
       |  |     |  |           |     +---- node-id?      te-types:te-
   node-id
       |  |     |  |           |     +---- link-tp-id?   te-types:te-
   tp-id
       |  |     |  |           |     +---- hop-type?     te-hop-type
       |  |     |  |           |     +---- direction?    te-link-
   direction
       |  |     |  |           +--:(as-number)
       |  |     |  |           |  +---- as-number-hop
       |  |     |  |           |     +---- as-number?   binary
       |  |     |  |           |     +---- hop-type?    te-hop-type
       |  |     |  |           +--:(label)
       |  |     |  |              +---- label-hop
       |  |     |  |                 +---- te-label
       |  |     |  |                    +---- (technology)?
       |  |     |  |                    |  +--:(generic)
       |  |     |  |                    |     +---- generic?
       |  |     |  |                    |             rt-
   types:generalized-label
       |  |     |  |                    +---- direction?
       |  |     |  |                            te-label-direction
       |  |     |  +---- tiebreakers
       |  |     |     +---- tiebreaker* [tiebreaker-type]
       |  |     |        +---- tiebreaker-type    identityref
       |  |     +--:(objective-function)
       |  |              {path-optimization-objective-function}?



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       |  |        +---- objective-function
       |  |           +---- objective-function-type?   identityref
       |  +---- requested-metrics* [metric-type]
       |  |  +---- metric-type    identityref
       |  +---- return-srlgs?             boolean
       |  +---- return-affinities?        boolean
       |  +---- path-in-segment!
       |  |  +---- label-restrictions
       |  |     +---- label-restriction* [index]
       |  |        +---- restriction?    enumeration
       |  |        +---- index           uint32
       |  |        +---- label-start
       |  |        |  +---- te-label
       |  |        |     +---- (technology)?
       |  |        |     |  +--:(generic)
       |  |        |     |     +---- generic?     rt-types:generalized-
   label
       |  |        |     +---- direction?   te-label-direction
       |  |        +---- label-end
       |  |        |  +---- te-label
       |  |        |     +---- (technology)?
       |  |        |     |  +--:(generic)
       |  |        |     |     +---- generic?     rt-types:generalized-
   label
       |  |        |     +---- direction?   te-label-direction
       |  |        +---- label-step
       |  |        |  +---- (technology)?
       |  |        |     +--:(generic)
       |  |        |        +---- generic?   int32
       |  |        +---- range-bitmap?   binary
       |  +---- path-out-segment!
       |     +---- label-restrictions
       |        +---- label-restriction* [index]
       |           +---- restriction?    enumeration
       |           +---- index           uint32
       |           +---- label-start
       |           |  +---- te-label
       |           |     +---- (technology)?
       |           |     |  +--:(generic)



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       |           |     |     +---- generic?     rt-types:generalized-
   label
       |           |     +---- direction?   te-label-direction
       |           +---- label-end
       |           |  +---- te-label
       |           |     +---- (technology)?
       |           |     |  +--:(generic)
       |           |     |     +---- generic?     rt-types:generalized-
   label
       |           |     +---- direction?   te-label-direction
       |           +---- label-step
       |           |  +---- (technology)?
       |           |     +--:(generic)
       |           |        +---- generic?   int32
       |           +---- range-bitmap?   binary
       +---- synchronization* [synchronization-id]
          +---- synchronization-id    uint32
          +---- svec
          |  +---- relaxable?           boolean
          |  +---- disjointness?        te-types:te-path-disjointness
          |  +---- request-id-number*   uint32
          +---- svec-constraints
          |  +---- path-metric-bound* [metric-type]
          |     +---- metric-type    identityref
          |     +---- upper-bound?   uint64
          +---- path-srlgs-values
          |  +---- usage?    identityref
          |  +---- values*   srlg
          +---- path-srlgs-names
          |  +---- path-srlgs-name* [usage]
          |     +---- usage        identityref
          |     +---- srlg-name* [name]
          |        +---- name    string
          +---- exclude-objects
          |  +---- excludes* [index]
          |     +---- index            uint32
          |     +---- (type)?
          |        +--:(num-unnum-hop)
          |        |  +---- num-unnum-hop



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          |        |     +---- node-id?      te-types:te-node-id
          |        |     +---- link-tp-id?   te-types:te-tp-id
          |        |     +---- hop-type?     te-hop-type
          |        |     +---- direction?    te-link-direction
          |        +--:(as-number)
          |        |  +---- as-number-hop
          |        |     +---- as-number?   binary
          |        |     +---- hop-type?    te-hop-type
          |        +--:(label)
          |           +---- label-hop
          |              +---- te-label
          |                 +---- (technology)?
          |                 |  +--:(generic)
          |                 |     +---- generic?
          |                 |             rt-types:generalized-label
          |                 +---- direction?   te-label-direction
          +---- optimizations
             +---- (algorithm)?
                +--:(metric)
                |  +---- optimization-metric* [metric-type]
                |     +---- metric-type    identityref
                |     +---- weight?        uint8
                +--:(objective-function)
                   +---- objective-function
                      +---- objective-function-type?   identityref
     augment /te:tunnels-rpc/te:output/te:result:
       +--ro response* [response-id]
          +--ro response-id      uint32
          +--ro (response-type)?
             +--:(no-path-case)
             |  +--ro no-path!
             +--:(path-case)
                +--ro computed-path
                   +--ro path-id?           yang-types:uuid
                   +--ro path-properties
                      +--ro path-metric* [metric-type]
                      |  +--ro metric-type           identityref
                      |  +--ro accumulative-value?   uint64
                      +--ro path-affinities-values



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                      |  +--ro path-affinities-value* [usage]
                      |     +--ro usage    identityref
                      |     +--ro value?   admin-groups
                      +--ro path-affinity-names
                      |  +--ro path-affinity-name* [usage]
                      |     +--ro usage            identityref
                      |     +--ro affinity-name* [name]
                      |        +--ro name    string
                      +--ro path-srlgs-values
                      |  +--ro usage?    identityref
                      |  +--ro values*   srlg
                      +--ro path-srlgs-names
                      |  +--ro path-srlgs-name* [usage]
                      |     +--ro usage        identityref
                      |     +--ro srlg-name* [name]
                      |        +--ro name    string
                      +--ro path-route-objects
                         +--ro path-route-object* [index]
                            +--ro index            uint32
                            +--ro (type)?
                               +--:(num-unnum-hop)
                               |  +--ro num-unnum-hop
                               |     +--ro node-id?      te-types:te-
   node-id
                               |     +--ro link-tp-id?   te-types:te-
   tp-id
                               |     +--ro hop-type?     te-hop-type
                               |     +--ro direction?    te-link-
   direction
                               +--:(as-number)
                               |  +--ro as-number-hop
                               |     +--ro as-number?   binary
                               |     +--ro hop-type?    te-hop-type
                               +--:(label)
                                  +--ro label-hop
                                     +--ro te-label
                                        +--ro (technology)?
                                        |  +--:(generic)
                                        |     +--ro generic?



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                                        |             rt-
   types:generalized-label
                                        +--ro direction?
                                                te-label-direction

                 Figure 9  - TE path computation YANG tree

6.2. YANG Module

   <CODE BEGINS>file "ietf-te-path-computation@2018-10-23.yang"
   module ietf-te-path-computation {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-te-path-computation";
     // replace with IANA namespace when assigned

     prefix "tepc";

     import ietf-inet-types {
       prefix "inet";
     }

     import ietf-yang-types {
       prefix "yang-types";
     }

     import ietf-te {
       prefix "te";
     }

     import ietf-te-types {
       prefix "te-types";
     }

     organization
       "Traffic Engineering Architecture and Signaling (TEAS)
        Working Group";

     contact
       "WG Web:   <http://tools.ietf.org/wg/teas/>
        WG List:  <mailto:teas@ietf.org>


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        WG Chair: Lou Berger
                  <mailto:lberger@labn.net>

        WG Chair: Vishnu Pavan Beeram
                  <mailto:vbeeram@juniper.net>

      ";

     description "YANG model for stateless TE path computation";

     revision "2018-10-23" {
       description
         "Initial revision";
       reference
         "draft-ietf-teas-yang-path-computation";
     }

     /*
      * Features
      */

     feature stateless-path-computation {
       description
         "This feature indicates that the system supports
          stateless path computation.";
     }


     /*
      * Groupings
      */

     grouping path-info {
       leaf path-id {
         type yang-types:uuid;
         config false;
         description "path-id ref.";
       }



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       uses te-types:generic-path-properties;
       description "Path computation output information";
     }

     grouping requested-info {
       description
         "This grouping defines the information (e.g., metrics)
          which must be returned in the response";
       list requested-metrics {
         key 'metric-type';
         description
           "The list of the requested metrics
            The metrics listed here must be returned in the response.
            Returning other metrics in the response is optional.";
         leaf metric-type {
           type identityref {
             base te-types:path-metric-type;
           }
           description
             "The metric that must be returned in the response";
         }
       }
       leaf return-srlgs {
         type boolean;
         default false;
         description
           "If true, path srlgs must be returned in the response.
            If false, returning path srlgs in the response optional.";
       }
       leaf return-affinities {
         type boolean;
         default false;
         description
         "If true, path affinities must be returned in the response.
          If false, returning path affinities in the response is
          optional.";
       }
     }




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     identity svec-metric-type {
       description
         "Base identity for svec metric type";
     }

     identity svec-metric-cumul-te {
       base svec-metric-type;
       description
         "TE cumulative path metric";
     }

     identity svec-metric-cumul-igp {
       base svec-metric-type;
       description
         "IGP cumulative path metric";
     }

     identity svec-metric-cumul-hop {
       base svec-metric-type;
       description
         "Hop cumulative path metric";
     }

     identity svec-metric-aggregate-bandwidth-consumption {
       base svec-metric-type;
       description
         "Cumulative bandwith consumption of the set of
          synchronized paths";
     }

     identity svec-metric-load-of-the-most-loaded-link {
       base svec-metric-type;
       description
         "Load of the most loaded link";
     }

     grouping svec-metrics-bounds_config {
       description
         "TE path metric bounds grouping for computing a set of



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          synchronized requests";
       leaf metric-type {
         type identityref {
           base svec-metric-type;
         }
         description "TE path metric type usable for computing a set of
            synchronized requests";
       }
       leaf upper-bound {
         type uint64;
         description "Upper bound on end-to-end svec path metric";
       }
     }

     grouping svec-metrics-optimization_config {
       description
         "TE path metric bounds grouping for computing a set of
          synchronized requests";

       leaf metric-type {
         type identityref {
           base svec-metric-type;
         }
         description "TE path metric type usable for computing a set of
            synchronized requests";
       }
       leaf weight {
         type uint8;
         description "Metric normalization weight";
       }
     }

     grouping svec-exclude {
       description "List of resources to be excluded by all the paths
         in the SVEC";
       container exclude-objects {
         description "resources to be excluded";
         list excludes {
           key index;



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           description
             "List of explicit route objects to always exclude
              from synchronized path computation";
           leaf index {
             type uint32;
             description "XRO subobject index";
           }
           uses te-types:explicit-route-hop;
         }
       }
     }

     grouping synchronization-constraints {
       description "Global constraints applicable to synchronized
         path computation";
       container svec-constraints {
         description "global svec constraints";
         list path-metric-bound {
           key metric-type;
           description "list of bound metrics";
           uses svec-metrics-bounds_config;
         }
       }
       uses te-types:generic-path-srlgs;
       uses svec-exclude;
     }

     grouping synchronization-optimization {
         description "Synchronized request optimization";
       container optimizations {
         description
           "The objective function container that includes attributes
            to impose when computing a synchronized set of paths";

         choice algorithm {
           description "Optimizations algorithm.";
           case metric {
             list optimization-metric {
               key "metric-type";



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               description "svec path metric type";
               uses svec-metrics-optimization_config;
             }
           }
           case objective-function {
             container objective-function {
               description
                 "The objective function container that includes
                  attributes to impose when computing a TE path";
               uses te-types:path-objective-function_config;
             }
           }
         }
       }
     }

     grouping synchronization-info {
       description "Information for sync";
       list synchronization {
         key "synchronization-id";
         description "sync list";
         leaf synchronization-id {
           type uint32;
           description "index";
         }
         container svec {
           description
            "Synchronization VECtor";
           leaf relaxable {
             type boolean;
             default true;
             description
              "If this leaf is true, path computation process is
               free to ignore svec content.
               Otherwise, it must take into account this svec.";
           }
           uses te-types:generic-path-disjointness;
           leaf-list request-id-number {
             type uint32;



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             description
               "This list reports the set of path computation
                requests that must be synchronized.";
           }
         }
         uses synchronization-constraints;
         uses synchronization-optimization;
       }
     }

     grouping no-path-info {
       description "no-path-info";
       container no-path {
         presence "Response without path information, due to failure
           performing the path computation";
         description "if path computation cannot identify a path,
           rpc returns no path.";
       }
     }

     /*
      * These groupings should be removed when defined in te-types
      */

     grouping encoding-and-switching-type {
       description
         "Common grouping to define the LSP encoding and
          switching types";

       leaf encoding {
         type identityref {
           base te-types:lsp-encoding-types;
         }
         description "LSP encoding type";
         reference "RFC3945";
       }
       leaf switching-type {
         type identityref {
           base te-types:switching-capabilities;



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         }
         description "LSP switching type";
         reference "RFC3945";
       }
     }

     grouping end-points {
       description
         "Common grouping to define the TE tunnel end-points";

       leaf source {
         type inet:ip-address;
         description "TE tunnel source address.";
       }
       leaf destination {
         type inet:ip-address;
         description "P2P tunnel destination address";
       }
       leaf src-tp-id {
         type binary;
         description
           "TE tunnel source termination point identifier.";
       }
       leaf dst-tp-id {
         type binary;
         description
           "TE tunnel destination termination point identifier.";
       }
       leaf bidirectional {
         type boolean;
         default 'false';
         description "TE tunnel bidirectional";
       }
     }

     /**
      * AUGMENTS TO TE RPC
      */




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     augment "/te:tunnels-rpc/te:input/te:tunnel-info" {
       description "statelessComputeP2PPath input";
       list path-request {
         key "request-id";
         description "request-list";
         leaf request-id {
           type uint32;
           mandatory true;
           description
             "Each path computation request is uniquely identified
              by the request-id-number.";
         }
         uses te-types:te-topology-identifier;
         uses end-points;
         uses encoding-and-switching-type;
         uses te-types:path-route-objects;
         uses te-types:generic-path-constraints;
         uses te-types:generic-path-optimization;
         uses requested-info;
         uses te:path-access-segment-info;
       }
       uses synchronization-info;
     }

     augment "/te:tunnels-rpc/te:output/te:result" {
       description "statelessComputeP2PPath output";
       list response {
         key response-id;
         config false;
         description "response";
         leaf response-id {
           type uint32;
           description
             "The list key that has to reuse request-id-number.";
         }
         choice response-type {
           config false;
           description "response-type";
           case no-path-case {



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             uses no-path-info;
           }
           case path-case {
             container computed-path {
               uses path-info;
               description "Path computation service.";
             }
           }
         }
       }
     }
   }
   <CODE ENDS>

               Figure 10   - TE path computation YANG module

7. Security Considerations

   This document describes use cases of requesting Path Computation
   using YANG models, which could be used at the ABNO Control Interface
   [RFC7491] and/or between controllers in ACTN [RFC8453]. As such, it
   does not introduce any new security considerations compared to the
   ones related to YANG specification, ABNO specification and ACTN
   Framework defined in [RFC7950], [RFC7491] and [RFC8453].

   The YANG module defined in this draft is designed to be accessed via
   the NETCONF protocol [RFC6241] or RESTCONF protocol [RFC8040]. The
   lowest NETCONF layer is the secure transport layer, and the
   mandatory-to-implement secure transport is Secure Shell (SSH)
   [RFC6242]. The lowest RESTCONF layer is HTTPS, and the mandatory-to-
   implement secure transport is TLS [RFC8446].

   This document also defines common data types using the YANG data
   modeling language. The definitions themselves have no security
   impact on the Internet, but the usage of these definitions in
   concrete YANG modules might have. The security considerations
   spelled out in the YANG specification [RFC7950] apply for this
   document as well.

   The NETCONF access control model [RFC8341] provides the means to
   restrict access for particular NETCONF or RESTCONF users to a
   preconfigured subset of all available NETCONF or RESTCONF protocol
   operations and content.


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   Note - The security analysis of each leaf is for further study.

8. IANA Considerations

   This document registers the following URIs in the IETF XML registry
   [RFC3688]. Following the format in [RFC3688], the following
   registration is requested to be made.

   URI: urn:ietf:params:xml:ns:yang:ietf-te-path-computation
   XML: N/A, the requested URI is an XML namespace.

   This document registers a YANG module in the YANG Module Names
   registry [RFC7950].

   name: ietf-te-path-computation
   namespace: urn:ietf:params:xml:ns:yang:ietf-te-path-computation
   prefix: tepc

9. References

9.1. Normative References

   [RFC3688] Mealling, M., "The IETF XML Registry", RFC 3688, January
             2004.

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

   [RFC5541] Le Roux, JL. et al., " Encoding of Objective Functions in
             the Path Computation Element Communication Protocol
             (PCEP)", RFC 5541, June 2009.

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

   [RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure
             Shell (SSH)", RFC 6242, June 2011.

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




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   [RFC8341] Bierman, A., and M. Bjorklund, "Network Configuration
             Access Control Model", RFC 8341, March 2018.

   [RFC7491] Farrel, A., King, D., "A PCE-Based Architecture for
             Application-Based Network Operations", RFC 7491, March
             2015.

   [RFC7926] Farrel, A. et al., "Problem Statement and Architecture for
             Information Exchange Between Interconnected Traffic
             Engineered Networks", RFC 7926, July 2016.

   [RFC7950] Bjorklund, M., "The YANG 1.1 Data Modeling Language", RFC
             7950, August 2016.

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

   [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
             Version 1.3", RFC 8446, August 2018.

   [RFC8453] Ceccarelli, D., Lee, Y. et al., "Framework for Abstraction
             and Control of TE Networks (ACTN)", RFC8453, August 2018.

   [RFC8454] Lee, Y. et al., "Information Model for Abstraction and
             Control of TE Networks (ACTN)", RFC8454, September 2018.

   [TE-TOPO] Liu, X. et al., "YANG Data Model for TE Topologies",
             draft-ietf-teas-yang-te-topo, 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.

9.1. Informative References

   [RFC4655] Farrel, A. et al., "A Path Computation Element (PCE)-Based
             Architecture", RFC 4655, August 2006.

   [RFC7139] Zhang, F. et al., "GMPLS Signaling Extensions for Control
             of Evolving G.709 Optical Transport Networks", RFC 7139,
             March 2014.

   [RFC7446] Lee, Y. et al., "Routing and Wavelength Assignment
             Information Model for Wavelength Switched Optical
             Networks", RFC 7446, February 2015.


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   [RFC8233] Dhody, D. et al., "Extensions to the Path Computation
             Element Communication Protocol (PCEP) to Compute Service-
             Aware Label Switched Paths (LSPs)", RFC 8233, September
             2017

   [OTN-TOPO] Zheng, H. et al., "A YANG Data Model for Optical
             Transport Network Topology", draft-ietf-ccamp-otn-topo-
             yang, work in progress.

   [ITU-T G.709-2016]   ITU-T Recommendation G.709 (06/16), "Interface
             for the optical transport network", June 2016.

10. Acknowledgments

   The authors would like to thank Igor Bryskin and Xian Zhang for
   participating in the initial discussions that have triggered this
   work and providing valuable insights.

   The authors would like to thank the authors of the TE Tunnel YANG
   model [TE-TUNNEL], in particular Igor Bryskin, Vishnu Pavan Beeram,
   Tarek Saad and Xufeng Liu, for their inputs to the discussions and
   support in having consistency between the Path Computation and TE
   Tunnel YANG models.

   The authors would like to thank Adrian Farrel, Dhruv Dhody, Igor
   Bryskin, Julien Meuric and Lou Berger for their valuable input to
   the discussions that has clarified that the path being setup is not
   necessarily the same as the path that have been previously computed
   and, in particular to Dhruv Dhody, for his suggestion to describe
   the need for a path verification phase to check that the actual path
   being setup meets the required end-to-end metrics and constraints.

   This document was prepared using 2-Word-v2.0.template.dot.














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Appendix A. Examples of dimensioning the "detailed connectivity matrix"

   In the following table, a list of the possible constraints,
   associated with their potential cardinality, is reported.

   The maximum number of potential connections to be computed and
   reported is, in first approximation, the multiplication of all of
   them.

   Constraint  Cardinality
   ----------  -------------------------------------------------------

   End points N(N-1)/2 if connections are bidirectional (OTN and WDM),
              N(N-1) for unidirectional connections.

   Bandwidth  In WDM networks, bandwidth values are expressed in GHz.

              On fixed-grid WDM networks, the central frequencies are
              on a 50GHz grid and the channel width of the transmitters
              are typically 50GHz such that each central frequency can
              be used, i.e., adjacent channels can be placed next to
              each other in terms of central frequencies.

              On flex-grid WDM networks, the central frequencies are on
              a 6.25GHz grid and the channel width of the transmitters
              can be multiples of 12.5GHz.

              For fixed-grid WDM networks typically there is only one
              possible bandwidth value (i.e., 50GHz) while for flex-
              grid WDM networks typically there are 4 possible
              bandwidth values (e.g., 37.5GHz, 50GHz, 62.5GHz, 75GHz).

              In OTN (ODU) networks, bandwidth values are expressed as
              pairs of ODU type and, in case of ODUflex, ODU rate in
              bytes/sec as described in section 5 of [RFC7139].

              For "fixed" ODUk types, 6 possible bandwidth values are
              possible (i.e., ODU0, ODU1, ODU2, ODU2e, ODU3, ODU4).

              For ODUflex(GFP), up to 80 different bandwidth values can
              be specified, as defined in Table 7-8 of [ITU-T G.709-
              2016].

              For other ODUflex types, like ODUflex(CBR), the number of
              possible bandwidth values depends on the rates of the


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              clients that could be mapped over these ODUflex types, as
              shown in Table 7.2 of [ITU-T G.709-2016], which in theory
              could be a countinuum of values. However, since different
              ODUflex bandwidths that use the same number of TSs on
              each link along the path are equivalent for path
              computation purposes, up to 120 different bandwidth
              ranges can be specified.

              Ideas to reduce the number of ODUflex bandwidth values in
              the detailed connectivity matrix, to less than 100, are
              for further study.

              Bandwidth specification for ODUCn is currently for
              further study but it is expected that other bandwidth
              values can be specified as integer multiples of 100Gb/s.

              In IP we have bandwidth values in bytes/sec. In
              principle, this is a countinuum of values, but in
              practice we can identify a set of bandwidth ranges, where
              any bandwidth value inside the same range produces the
              same path.
              The number of such ranges is the cardinality, which
              depends on the topology, available bandwidth and status
              of the network. Simulations (Note: reference paper
              submitted for publication) show that values for medium
              size topologies (around 50-150 nodes) are in the range 4-
              7 (5 on average) for each end points couple.

   Metrics    IGP, TE and hop number are the basic objective metrics
              defined so far. There are also the 2 objective functions
              defined in [RFC5541]: Minimum Load Path (MLP) and Maximum
              Residual Bandwidth Path (MBP). Assuming that one only
              metric or objective function can be optimized at once,
              the total cardinality here is 5.

              With [RFC8233], a number of additional metrics are
              defined, including Path Delay metric, Path Delay
              Variation metric and Path Loss metric, both for point-to-
              point and point-to-multipoint paths. This increases the
              cardinality to 8.

   Bounds     Each metric can be associated with a bound in order to
              find a path having a total value of that metric lower
              than the given bound. This has a potentially very high
              cardinality (as any value for the bound is allowed). In


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              practice there is a maximum value of the bound (the one
              with the maximum value of the associated metric) which
              results always in the same path, and a range approach
              like for bandwidth in IP should produce also in this case
              the cardinality. Assuming to have a cardinality similar
              to the one of the bandwidth (let say 5 on average) we
              should have 6 (IGP, TE, hop, path delay, path delay
              variation and path loss; we don't consider here the two
              objective functions of [RFC5541] as they are conceived
              only for optimization)*5 = 30 cardinality.

   Technology
   constraints For further study

   Priority   We have 8 values for setup priority, which is used in
              path computation to route a path using free resources
              and, where no free resources are available, resources
              used by LSPs having a lower holding priority.

   Local prot It's possible to ask for a local protected service, where
              all the links used by the path are protected with fast
              reroute (this is only for IP networks, but line
              protection schemas are available on the other
              technologies as well). This adds an alternative path
              computation, so the cardinality of this constraint is 2.

   Administrative
   Colors     Administrative colors (aka affinities) are typically
              assigned to links but when topology abstraction is used
              affinity information can also appear in the detailed
              connectivity matrix.

              There are 32 bits available for the affinities. Links can
              be tagged with any combination of these bits, and path
              computation can be constrained to include or exclude any
              or all of them. The relevant cardinality is 3 (include-
              any, exclude-any, include-all) times 2^32 possible
              values. However, the number of possible values used in
              real networks is quite small.

   Included Resources

              A path computation request can be associated to an
              ordered set of network resources (links, nodes) to be
              included along the computed path. This constraint would


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              have a huge cardinality as in principle any combination
              of network resources is possible. However, as far as the
              Orchestrator doesn't know details about the internal
              topology of the domain, it shouldn't include this type of
              constraint at all (see more details below).

   Excluded Resources

               A path computation request can be associated to a set of
               network resources (links, nodes, SRLGs) to be excluded
               from the computed path. Like for included resources,
               this constraint has a potentially very high cardinality,
               but, once again, it can't be actually used by the
               Orchestrator, if it's not aware of the domain topology
               (see more details below).
   As discussed above, the Orchestrator can specify include or exclude
   resources depending on the abstract topology information that the
   domain controller exposes:

   o  In case the domain controller exposes the entire domain as a
      single abstract TE node with his own external terminations and
      detailed connectivity matrix (whose size we are estimating), no
      other topological details are available, therefore the size of
      the detailed connectivity matrix only depends on the combination
      of the constraints that the Orchestrator can use in a path
      computation request to the domain controller. These constraints
      cannot refer to any details of the internal topology of the
      domain, as those details are not known to the Orchestrator and so
      they do not impact size of the detailed connectivity matrix
      exported.
















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   o  Instead in case the domain controller exposes a topology
      including more than one abstract TE nodes and TE links, and their
      attributes (e.g. SRLGs, affinities for the links), the
      Orchestrator knows these details and therefore could compute a
      path across the domain referring to them in the constraints. The
      detailed connectivity matrixes, whose size need to be estimated
      here, are the ones relevant to the abstract TE nodes exported to
      the Orchestrator. These detailed connectivity matrixes and
      therefore theirs sizes, while cannot depend on the other abstract
      TE nodes and TE links, which are external to the given abstract
      node, could depend to SRLGs (and other attributes, like
      affinities) which could be present also in the portion of the
      topology represented by the abstract nodes, and therefore
      contribute to the size of the related detailed connectivity
      matrix.

   We also don't consider here the possibility to ask for more than one
   path in diversity or for point-to-multi-point paths, which are for
   further study.

   Considering for example an IP domain without considering SRLG and
   affinities, we have an estimated number of paths depending on these
   estimated cardinalities:

   Endpoints = N*(N-1), Bandwidth = 5, Metrics = 6, Bounds = 20,
   Priority = 8, Local prot = 2

   The number of paths to be pre-computed by each IP domain is
   therefore 24960 * N(N-1) where N is the number of domain access
   points.

   This means that with just 4 access points we have nearly 300000
   paths to compute, advertise and maintain (if a change happens in the
   domain, due to a fault, or just the deployment of new traffic, a
   substantial number of paths need to be recomputed and the relevant
   changes advertised to the upper controller).

   This seems quite challenging. In fact, if we assume a mean length of
   1K for the json describing a path (a quite conservative estimate),
   reporting 300000 paths means transferring and then parsing more than
   300 Mbytes for each domain. If we assume that 20% (to be checked) of
   this paths change when a new deployment of traffic occurs, we have
   60 Mbytes of transfer for each domain traversed by a new end-to-end
   path. If a network has, let say, 20 domains (we want to estimate the
   load for a non-trivial domain setup) in the beginning a total


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   initial transfer of 6Gigs is needed, and eventually, assuming 4-5
   domains are involved in mean during a path deployment we could have
   240-300 Mbytes of changes advertised to the higher order controller.

   Further bare-bone solutions can be investigated, removing some more
   options, if this is considered not acceptable; in conclusion, it
   seems that an approach based only on the information provided by the
   detailed connectivity matrix is hardly feasible, and could be
   applicable only to small networks with a limited meshing degree
   between domains and renouncing to a number of path computation
   features.




































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Contributors

   Dieter Beller
   Nokia
   Email: dieter.beller@nokia.com


   Gianmarco Bruno
   Ericsson
   Email: gianmarco.bruno@ericsson.com


   Francesco Lazzeri
   Ericsson
   Email: francesco.lazzeri@ericsson.com


   Young Lee
   Huawei
   Email: leeyoung@huawei.com


   Carlo Perocchio
   Ericsson
   Email: carlo.perocchio@ericsson.com


Authors' Addresses

   Italo Busi (Editor)
   Huawei
   Email: italo.busi@huawei.com


   Sergio Belotti (Editor)
   Nokia
   Email: sergio.belotti@nokia.com


   Victor Lopez
   Telefonica
   Email: victor.lopezalvarez@telefonica.com





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   Oscar Gonzalez de Dios
   Telefonica
   Email: oscar.gonzalezdedios@telefonica.com


   Anurag Sharma
   Google
   Email: ansha@google.com


   Yan Shi
   China Unicom
   Email: shiyan49@chinaunicom.cn


   Ricard Vilalta
   CTTC
   Email: ricard.vilalta@cttc.es


   Karthik Sethuraman
   NEC
   Email: karthik.sethuraman@necam.com


   Michael Scharf
   Nokia
   Email: michael.scharf@gmail.com


   Daniele Ceccarelli
   Ericsson
   Email: daniele.ceccarelli@ericsson.com














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