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Versions: (draft-busibel-ccamp-path-computation-api) 00 01 02 03 draft-ietf-teas-yang-path-computation

TEAS Working Group                                     Italo Busi (Ed.)
Internet Draft                                                   Huawei
Intended status: Informational                     Sergio Belotti (Ed.)
Expires: December 2017                                            Nokia
                                                           Victor Lopez
                                                 Oscar Gonzalez de Dios
                                                             Telefonica
                                                          Anurag Sharma
                                                               Infinera
                                                                Yan Shi
                                                           China Unicom
                                                         Ricard Vilalta
                                                                   CTTC
                                                     Karthik Sethuraman
                                                                    NEC

                                                          June 30, 2017





               Yang model for requesting Path Computation
            draft-busibel-teas-yang-path-computation-03.txt




Status of this Memo

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   http://www.ietf.org/ietf/1id-abstracts.txt




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

   This Internet-Draft will expire on December 30, 2016.

Copyright Notice

   Copyright (c) 2017 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
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Abstract

   There are scenarios, typically in a hierarchical SDN context, in
   which an orchestrator may not have detailed information to be able
   to perform an end-to-end path computation and would need to request
   lower layer/domain controllers to calculate some (partial) feasible
   paths.

   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).

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

   This document also proposes a yang model for a stateless RPC which
   complements the stateful solution defined in [TE-TUNNEL].

Table of Contents


   1. Introduction...................................................3
   2. Use Cases......................................................4


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      2.1. IP-Optical integration....................................5
         2.1.1. Inter-layer path computation.........................6
         2.1.2. Route Diverse IP Services............................8
      2.2. Multi-domain TE Networks..................................8
      2.3. Data center interconnections..............................9
   3. Interactions with TE Topology.................................11
      3.1. TE Topology Aggregation using the "virtual link model"...11
      3.2. TE Topology Abstraction..................................19
      3.3. Complementary use of TE topology and path computation....20
   4. Motivation for a YANG Model...................................22
      4.1. Benefits of common data models...........................22
      4.2. Benefits of a single interface...........................23
      4.3. Extensibility............................................23
   5. Path Computation for multiple LSPs............................24
   6. YANG Model for requesting Path Computation....................25
      6.1. Stateless and Stateful Path Computation..................25
      6.2. YANG model for stateless TE path computation.............26
         6.2.1. YANG Tree...........................................26
         6.2.2. YANG Module.........................................34
   7. Security Considerations.......................................40
   8. IANA Considerations...........................................41
   9. References....................................................41
      9.1. Normative References.....................................41
      9.2. Informative References...................................42
   10. Acknowledgments..............................................42

1. Introduction

   There are scenarios, typically in a hierarchical SDN context, in
   which an orchestrator may not have detailed information to be able
   to perform an end-to-end path computation and would need to request
   lower layer/domain controllers to calculate some (partial) feasible
   paths.

   When we are thinking to this type of scenarios we have in mind
   specific level of interfaces on which this request can be applied.

   We can reference ABNO Control Interface [RFC7491] in which an
   Application Service Coordinator can request ABNO controller to take
   in charge path calculation (see Figure 1 in the RFC) and/or ACTN
   [ACTN-frame],where 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).[ACTN-



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   Info] describes an information model for the Path Computation
   request.

   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. [L1-TOPO] for Layer-1 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.

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

   This document also proposes a yang model for a stateless RPC which
   complements the stateful solution defined in [TE-TUNNEL].

2. Use Cases

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

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





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2.1. IP-Optical integration

   In these use cases, an Optical domain is used to provide
   connectivity between IP routers which are connected with the Optical
   domains using access links (see Figure 1).


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                    Figure 1 - IP+Optical Use Cases

   It is assumed that the Optical domain controller provides to the
   orchestrator an abstracted view of the Optical network. A possible
   abstraction shall be representing the optical domain as one "virtual
   node" with "virtual ports" connected to the access links.

   The path computation request helps the orchestrator to know which
   are the real connections that can be provided at the optical domain.












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               Figure 2 - IP+Optical Topology Abstraction

2.1.1. Inter-layer path computation

   In this use case, the orchestrator needs to setup an optimal path
   between two IP routers R1 and R2.

   As depicted in Figure 2, the Orchestrator 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 orchestrator 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



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   which one of these potential paths to use to setup the optimal e2e
   path crossing optical network.


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             Figure 3 - IP+Optical Path Computation Example

   For example, in Figure 3, the Orchestrator can request the Optical
   domain 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
   domain controller when requested by the Orchestrator is no longer
   valid when the Orchestrator requests it to be setup up.

   It is worth noting that with the approach proposed in this document,
   the likelihood for this issue to happen can be quite small since the
   time window between the path computation request and the path setup
   request should be quite short (especially if compared with the time
   that would be needed to update the information of a very detailed
   abstract connectivity matrix).

   If this risk is still not acceptable, the Orchestrator may also
   optionally request the Optical domain controller not only to compute
   the path but also to keep track of its resources (e.g., these
   resources can be reserved to avoid being used by any other
   connection). In this case, some mechanism (e.g., a timeout) needs to
   be defined to avoid having stranded resources within the Optical
   domain.

   These issues and solutions can be fine-tuned during the design of
   the YANG model for requesting Path Computation.




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2.1.2. Route Diverse IP Services

   This is for further study.

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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           Figure 4 - Multi-domain multi-link interconnection

   In order to setup an end-to-end multi-domain TEpath (e.g., between
   nodes A and H), the orchestrator 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-specific optical
   attributes (which may be different in the two domains if they are
   provided by different vendors).




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   In order to setup a multi-domain TE path (e.g., between nodes A and
   H), Orchestrator 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 Orchestrator 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 Orchestrator 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 Orchestrator 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 Orchestrator)

  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 domain controllers to limit the number of
  potential optimal end-to-end paths and then request path computation
  to fewer domain controllers in order to decide what the optimal path
  within this limited set is.

  For more details, see section 3.3.

2.3. Data center interconnections

   In these use case, there is an 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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            Figure 5 - Data Center Interconnection Use Case

   In this use case, a virtual machine within Data Center 1 (DC1) needs
   to transfer data to another virtual machine that can reside either
   in DC2 or in DC3.

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

   The Cloud Orchestrator 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 orchestrator can request to the TE domain controller to
   compute the cost of the possible TE paths (e.g., DC1-DC2 and DC1-
   DC3) and to the DC controller to compute the cost of the computing
   power (DC resources) within DC2 and DC3 and then it can take the
   decision about the optimal solution based on this information and
   its policy.






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3. 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 the orchestrator thus allowing the Path Computation
   Element (PCE) within the Orchestrator to perform multi-domain path
   computation by its own, without requesting path computations to the
   underlying SDN controllers.

   This section analyzes the need for an orchestrator to request
   underlying SDN controllers for path computation even in these
   scenarios 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 the Orchestrator's PCE to take optimal
   path computation decisions by its own versus requesting the
   Orchestrator to ask to too many underlying SDN Domain Controllers a
   set of feasible optimal intra-domain TE paths.

3.1. TE Topology Aggregation using the "virtual link model"

   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", which extends the
   "connectivity matrix", defined in [RFC7446], with specific TE
   attributes (e.g., delay, SRLGs and summary TE metrics).

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

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



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

   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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      Figure 6 - IP+Optical Path Computation Example with multiple choices

   Reporting all the information, as in Figure 6, using the "detailed
   abstract 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.

   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.







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   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
              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.




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              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 [PCEP-Service-Aware], 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
              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


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              objective functions of [RFC5541] as they are conceived
              only for optimization)*5 = 30 cardinality.

   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
              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,


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               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
      connectivity matrix (whose size we are estimating), no other
      topological details are available, therefore the size of the
      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 connectivity matrix exported.

   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
      connectivity matrixes to be estimated here are the ones relevant
      to the abstract TE nodes exported to the Orchestrator. These
      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 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


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   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
   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 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.

   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 "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 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 abstract connectivity
   matrix" is even more dynamic since the establishment of another
   optical path may change some of the parameters (e.g., delay or


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   available bandwidth) in the "detailed abstract connectivity matrix"
   while not changing the feasibility of the path.

   "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
   "connectivity matrix" updated which means that there another
   tradeoff between the accuracy (i.e., providing "all" the information
   that might be needed by the Orchestrator'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 Orchestrator's PCE computes paths using not updated
   information.

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

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

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





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   o  If only the VP1-VP4 path with available bandwidth of 10 Gb/s and
      cost 60 is reported, the Orchestrator'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.

   Instead, using the approach proposed in this document, the
   Orchestrator, 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. 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, which are abstracting 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.

   In order to setup a multi-domain TE path (e.g., between nodes A and
   H), the Orchestrator 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:


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   o  Orchestrator'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 hierarchical controller computed is not good and need
      to re-start the path computation from scratch

  As discussed in section 3.1, providing more extensive abstract
  information from the TE domain controllers to the multi-domain
  Orchestator 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.

3.3. 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.

   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.



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      --------------------------------------------------------------------
      I                                                                  I
      I                                                                  I
      I                                                                  I
      I                Multi-domain with many domains                    I
      I                      (Topology information)                      I
      I                                                                  I
      I                                                                  I
      I                     (only in PDF version)                        I
      I                                                                  I
      I                                                                  I
      I                                                                  I
      --------------------------------------------------------------------

     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 Orchestrator 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 Orchestrator can understand by
   its own that:

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

   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 Orchestrator 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 Orchestrator 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).




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      --------------------------------------------------------------------
      I                                                                  I
      I                                                                  I
      I                                                                  I
      I                Multi-domain with many domains                    I
      I                (Path Computation information)                    I
      I                                                                  I
      I                                                                  I
      I                                                                  I
      I                                                                  I
      I                     (only in PDF version)                        I
      I                                                                  I
      I                                                                  I
      I                                                                  I
      --------------------------------------------------------------------

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

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

4. Motivation for a YANG Model

4.1. Benefits of common data models

   Path computation requests should be 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]. Otherwise, an error-prone mapping or
   correlation of information would be required. For instance, there is
   benefit in using the same endpoint identifiers in path computation
   requests and in the topology modeling. Also, the attributes used in
   path computation constraints could use the same or similar data
   models. As a result, there are many benefits in aligning path
   computation requests with YANG models for TE topology information
   and TE Tunnels configuration and management.






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4.2. Benefits of a single interface

   A typical use case for path computation requests is the interface
   between an orchestrator and a domain controller. The system
   integration effort is typically lower if a single, consistent
   interface is used between such systems, 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.

4.3. 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.

   Adding support for path computation requests to YANG models would
   seamlessly complement with [TE-TOPO] and [TE-TUNNEL] in the use
   cases where YANG-based protocols (e.g., NETCONF or RESTCONF) are
   used.

5. Path Computation for multiple LSPs

   There are use cases, where path computation is required for multiple
   Traffic Engineering Label Switched Paths (TE LSPs) through a network
   or through a network domain. It may be advantageous to request the
   new paths for a set of LSPs in one single path computation request
   [RFC5440] that also includes information regarding the desired
   objective function, see [RFC5541].

   In the context of abstraction and control of TE networks (ACTN), as
   defined in [ACTN-Frame], when a MDSC receives a vitual network (VN)
   request from a CNC, the MDSC needs to perform path computation for
   multiple LSPs as a typical VN is constructed by a set of multiple
   paths also called end-to-end tunnels. The MDSC may send a single
   path computation request to the PNC for multiple LSPs, i.e. between
   the VN end points (access points in ACTN terminology).

   In a more general context, when a MDSC needs to send multiple path
   provisioning requests to the PNC, the MDSC may also group these path
   provisioning requests together and send them in a single message to
   the PNC instead of sending separet requests for each path.






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6. YANG Model for requesting Path Computation

   The TE Tunnel YANG model has been extended to 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.

   The need also for a stateless solution, based on an RPC, has been
   recognized, as outlined in section 6.1.

   A proposal for a stateless RPC to request path computation is
   provided in section 6.2.

6.1. Stateless and Stateful Path Computation

   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 is not a major issue in case the time between path computation
   and path setup is short.

   The RPC response must be provided synchronously and, if
   collaborative computations are time consuming, it may not be
   possible to immediate reply to client.

   In this case, the client can define a maximum time it can wait for
   the reply, such that if the computation does not complete in time,
   the server will abort the path computation and reply to the client
   with an error. It may be possible that the server has tighter timing



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   constraints than the client: in this case the path computation is
   aborted earlier than the time specified by the client.

   Note - The RPC response issue (slow RPC server) is not specific to
   the path computation RPC case so, it may be worthwhile, evaluating
   whether a more generic solution applicable to any YANG RPC can be
   used instead.

   In case the stateless solution is not sufficient, a stateful
   solution, based on "compute-only" TE tunnel, could be used to
   support asynchronous operations and/or 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, it does not guaranteed that because notifications may
   not be reliable or delivered on time.

   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

6.2. YANG model for stateless TE path computation

6.2.1. 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
       +--rw paths
       |  +--ro path* [path-id]
       |     +--ro _telink* [link-ref]
       |     |  +--ro link-ref       ->
   /nd:networks/network[nd:network-id=current()/../network-
   ref]/lnk:link/link-id



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       |     |  +--ro network-ref?   -> /nd:networks/network/network-id
       |     +--ro path-constraints
       |     |  +--ro path-metric-bound* [metric-type]
       |     |  |  +--ro metric-type    identityref
       |     |  |  +--ro upper-bound?   uint64
       |     |  +--ro topology-id?         te-types:te-topology-id
       |     |  +--ro ignore-overload?     boolean
       |     |  +--ro bandwidth-generic
       |     |  |  +--ro te-bandwidth
       |     |  |     +--ro (technology)?
       |     |  |        +--:(psc)
       |     |  |        |  +--ro psc?       rt-types:bandwidth-ieee-
   float32
       |     |  |        +--:(otn)
       |     |  |        |  +--ro otn* [rate-type]
       |     |  |        |     +--ro rate-type    identityref
       |     |  |        |     +--ro counter?     uint16
       |     |  |        +--:(lsc)
       |     |  |        |  +--ro wdm* [spectrum slot]
       |     |  |        |     +--ro spectrum    identityref
       |     |  |        |     +--ro slot        int16
       |     |  |        |     +--ro width?      uint16
       |     |  |        +--:(generic)
       |     |  |           +--ro generic?   te-bandwidth
       |     |  +--ro disjointness?        te-types:te-path-
   disjointness
       |     |  +--ro setup-priority?      uint8
       |     |  +--ro hold-priority?       uint8
       |     |  +--ro signaling-type?      identityref
       |     |  +--ro path-affinities
       |     |  |  +--ro constraint* [usage]
       |     |  |     +--ro usage    identityref
       |     |  |     +--ro value?   admin-groups
       |     |  +--ro path-srlgs
       |     |     +--ro usage?    identityref
       |     |     +--ro values*   srlg
       |     +--ro path-id             yang-types:uuid
       +--ro pathComputationService
          +--ro _path-ref*          -> /paths/path/path-id



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          +--ro _servicePort
          |  +--ro source?          inet:ip-address
          |  +--ro destination?     inet:ip-address
          |  +--ro src-tp-id?       binary
          |  +--ro dst-tp-id?       binary
          |  +--ro bidirectional
          |     +--ro association
          |        +--ro id?              uint16
          |        +--ro source?          inet:ip-address
          |        +--ro global-source?   inet:ip-address
          |        +--ro type?            identityref
          |        +--ro provisioing?     identityref
          +--ro path-constraints
          |  +--ro path-metric-bound* [metric-type]
          |  |  +--ro metric-type    identityref
          |  |  +--ro upper-bound?   uint64
          |  +--ro topology-id?         te-types:te-topology-id
          |  +--ro ignore-overload?     boolean
          |  +--ro bandwidth-generic
          |  |  +--ro te-bandwidth
          |  |     +--ro (technology)?
          |  |        +--:(psc)
          |  |        |  +--ro psc?       rt-types:bandwidth-ieee-
   float32
          |  |        +--:(otn)
          |  |        |  +--ro otn* [rate-type]
          |  |        |     +--ro rate-type    identityref
          |  |        |     +--ro counter?     uint16
          |  |        +--:(lsc)
          |  |        |  +--ro wdm* [spectrum slot]
          |  |        |     +--ro spectrum    identityref
          |  |        |     +--ro slot        int16
          |  |        |     +--ro width?      uint16
          |  |        +--:(generic)
          |  |           +--ro generic?   te-bandwidth
          |  +--ro disjointness?        te-types:te-path-disjointness
          |  +--ro setup-priority?      uint8
          |  +--ro hold-priority?       uint8
          |  +--ro signaling-type?      identityref



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          |  +--ro path-affinities
          |  |  +--ro constraint* [usage]
          |  |     +--ro usage    identityref
          |  |     +--ro value?   admin-groups
          |  +--ro path-srlgs
          |     +--ro usage?    identityref
          |     +--ro values*   srlg
          +--ro optimizations
             +--ro (algorithm)?
                +--:(metric) {path-optimization-metric}?
                |  +--ro optimization-metric* [metric-type]
                |  |  +--ro metric-type    identityref
                |  |  +--ro weight?        uint8
                |  +--ro tiebreakers
                |     +--ro tiebreaker* [tiebreaker-type]
                |        +--ro tiebreaker-type    identityref
                +--:(objective-function) {path-optimization-objective-
   function}?
                   +--ro objective-function
                      +--ro objective-function-type?   identityref
     augment /te:tunnels-rpc/te:input/te:tunnel-info:
       +---- request-list* [request-id-number]
       |  +---- request-id-number    uint32
       |  +---- servicePort*
       |  |  +---- source?          inet:ip-address
       |  |  +---- destination?     inet:ip-address
       |  |  +---- src-tp-id?       binary
       |  |  +---- dst-tp-id?       binary
       |  |  +---- bidirectional
       |  |     +---- association
       |  |        +---- id?              uint16
       |  |        +---- source?          inet:ip-address
       |  |        +---- global-source?   inet:ip-address
       |  |        +---- type?            identityref
       |  |        +---- provisioing?     identityref
       |  +---- path-constraints
       |  |  +---- path-metric-bound* [metric-type]
       |  |  |  +---- metric-type    identityref
       |  |  |  +---- upper-bound?   uint64



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       |  |  +---- topology-id?         te-types:te-topology-id
       |  |  +---- ignore-overload?     boolean
       |  |  +---- bandwidth-generic
       |  |  |  +---- te-bandwidth
       |  |  |     +---- (technology)?
       |  |  |        +--:(psc)
       |  |  |        |  +---- psc?       rt-types:bandwidth-ieee-
   float32
       |  |  |        +--:(otn)
       |  |  |        |  +---- otn* [rate-type]
       |  |  |        |     +---- rate-type    identityref
       |  |  |        |     +---- counter?     uint16
       |  |  |        +--:(lsc)
       |  |  |        |  +---- wdm* [spectrum slot]
       |  |  |        |     +---- spectrum    identityref
       |  |  |        |     +---- slot        int16
       |  |  |        |     +---- width?      uint16
       |  |  |        +--:(generic)
       |  |  |           +---- generic?   te-bandwidth
       |  |  +---- disjointness?        te-types:te-path-disjointness
       |  |  +---- setup-priority?      uint8
       |  |  +---- hold-priority?       uint8
       |  |  +---- signaling-type?      identityref
       |  |  +---- path-affinities
       |  |  |  +---- constraint* [usage]
       |  |  |     +---- usage    identityref
       |  |  |     +---- value?   admin-groups
       |  |  +---- path-srlgs
       |  |     +---- usage?    identityref
       |  |     +---- values*   srlg
       |  +---- optimizations
       |     +---- (algorithm)?
       |        +--:(metric) {path-optimization-metric}?
       |        |  +---- optimization-metric* [metric-type]
       |        |  |  +---- metric-type    identityref
       |        |  |  +---- weight?        uint8
       |        |  +---- tiebreakers
       |        |     +---- tiebreaker* [tiebreaker-type]
       |        |        +---- tiebreaker-type    identityref



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       |        +--:(objective-function) {path-optimization-objective-
   function}?
       |           +---- objective-function
       |              +---- objective-function-type?   identityref
       +---- synchronization* [synchronization-index]
          +---- synchronization-index    uint32
          +---- svec
          |  +---- relaxable?           boolean
          |  +---- link-diverse?        boolean
          |  +---- node-diverse?        boolean
          |  +---- srlg-diverse?        boolean
          |  +---- request-id-number*   uint32
          +---- path-constraints
             +---- path-metric-bound* [metric-type]
             |  +---- metric-type    identityref
             |  +---- upper-bound?   uint64
             +---- topology-id?         te-types:te-topology-id
             +---- ignore-overload?     boolean
             +---- bandwidth-generic
             |  +---- te-bandwidth
             |     +---- (technology)?
             |        +--:(psc)
             |        |  +---- psc?       rt-types:bandwidth-ieee-
   float32
             |        +--:(otn)
             |        |  +---- otn* [rate-type]
             |        |     +---- rate-type    identityref
             |        |     +---- counter?     uint16
             |        +--:(lsc)
             |        |  +---- wdm* [spectrum slot]
             |        |     +---- spectrum    identityref
             |        |     +---- slot        int16
             |        |     +---- width?      uint16
             |        +--:(generic)
             |           +---- generic?   te-bandwidth
             +---- disjointness?        te-types:te-path-disjointness
             +---- setup-priority?      uint8
             +---- hold-priority?       uint8
             +---- signaling-type?      identityref



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             +---- path-affinities
             |  +---- constraint* [usage]
             |     +---- usage    identityref
             |     +---- value?   admin-groups
             +---- path-srlgs
                +---- usage?    identityref
                +---- values*   srlg
     augment /te:tunnels-rpc/te:output/te:result:
       +--ro response* [response-index]
          +--ro response-index     uint32
          +--ro (response-type)?
             +--:(no-path-case)
             |  +--ro no-path
             +--:(path-case)
                +--ro pathCompService
                   +--ro _path-ref*          -> /paths/path/path-id
                   +--ro _servicePort
                   |  +--ro source?          inet:ip-address
                   |  +--ro destination?     inet:ip-address
                   |  +--ro src-tp-id?       binary
                   |  +--ro dst-tp-id?       binary
                   |  +--ro bidirectional
                   |     +--ro association
                   |        +--ro id?              uint16
                   |        +--ro source?          inet:ip-address
                   |        +--ro global-source?   inet:ip-address
                   |        +--ro type?            identityref
                   |        +--ro provisioing?     identityref
                   +--ro path-constraints
                   |  +--ro path-metric-bound* [metric-type]
                   |  |  +--ro metric-type    identityref
                   |  |  +--ro upper-bound?   uint64
                   |  +--ro topology-id?         te-types:te-topology-
   id
                   |  +--ro ignore-overload?     boolean
                   |  +--ro bandwidth-generic
                   |  |  +--ro te-bandwidth
                   |  |     +--ro (technology)?
                   |  |        +--:(psc)



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                   |  |        |  +--ro psc?       rt-types:bandwidth-
   ieee-float32
                   |  |        +--:(otn)
                   |  |        |  +--ro otn* [rate-type]
                   |  |        |     +--ro rate-type    identityref
                   |  |        |     +--ro counter?     uint16
                   |  |        +--:(lsc)
                   |  |        |  +--ro wdm* [spectrum slot]
                   |  |        |     +--ro spectrum    identityref
                   |  |        |     +--ro slot        int16
                   |  |        |     +--ro width?      uint16
                   |  |        +--:(generic)
                   |  |           +--ro generic?   te-bandwidth
                   |  +--ro disjointness?        te-types:te-path-
   disjointness
                   |  +--ro setup-priority?      uint8
                   |  +--ro hold-priority?       uint8
                   |  +--ro signaling-type?      identityref
                   |  +--ro path-affinities
                   |  |  +--ro constraint* [usage]
                   |  |     +--ro usage    identityref
                   |  |     +--ro value?   admin-groups
                   |  +--ro path-srlgs
                   |     +--ro usage?    identityref
                   |     +--ro values*   srlg
                   +--ro optimizations
                      +--ro (algorithm)?
                         +--:(metric) {path-optimization-metric}?
                         |  +--ro optimization-metric* [metric-type]
                         |  |  +--ro metric-type    identityref
                         |  |  +--ro weight?        uint8
                         |  +--ro tiebreakers
                         |     +--ro tiebreaker* [tiebreaker-type]
                         |        +--ro tiebreaker-type    identityref
                         +--:(objective-function) {path-optimization-
   objective-function}?
                            +--ro objective-function
                               +--ro objective-function-type?
   identityref



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                  Figure 9 - TE path computation tree

6.2.2. YANG Module

   <CODE BEGINS>file " ietf-te-path-computation.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-network-topology {
       prefix "nt";
     }

     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 "2016-10-10" {
       description "Initial revision";
       reference "YANG model for stateless TE path computation";
     }

     /*
      * Features
      */

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


     /*
      * Groupings
      */

     grouping Path {
       list _telink {
         key 'link-ref';
         config false;
         uses nt:link-ref;
         description "List of telink refs.";
       }
       uses te-types:generic-path-constraints;



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       leaf path-id {
         type yang-types:uuid;
         config false;
         description "path-id ref.";
       }
       description "Path is described by an ordered list of TE Links.";
     }

     grouping PathCompServicePort {
       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.";
       }
       uses te:bidir-assoc-properties;
       description "Path Computation Service Port grouping.";
     }

     grouping PathComputationService {
       leaf-list _path-ref {
         type leafref {
           path '/paths/path/path-id';
         }
         config false;
         description "List of previously computed path references.";
       }



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       container _servicePort {
         uses PathCompServicePort;
         description "Path Computation Service Port.";
       }
       uses te-types:generic-path-constraints;
       uses te-types:generic-path-optimization;

       description "Path computation service.";
     }




     grouping synchronization-info {
       description "Information for sync";
       list synchronization {
         key "synchronization-index";
         description "sync list";
         leaf synchronization-index {
           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.";
           }
           leaf link-diverse {
             type boolean;
             default false;
             description "link-diverse";
           }
           leaf node-diverse {



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             type boolean;
             default false;
             description "node-diverse";
           }
           leaf srlg-diverse {
             type boolean;
             default false;
             description "srlg-diverse";
           }
           leaf-list request-id-number {
             type uint32;
             description
              "This list reports the set of M path computation requests
   that must be synchronized.";
           }
         }
         uses te-types:generic-path-constraints;
       }
     }

     grouping no-path-info {
       description "no-path-info";
       container no-path {
         description "no-path container";
       }
     }

     /*
      * Root container
      */
     container paths {
       list path {
         key "path-id";
         config false;
         uses Path;

         description "List of previous computed paths.";
       }
       description "Root container for path-computation";



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     }

     container pathComputationService {
       config false;
       uses PathComputationService;
       description "Service for computing paths.";
     }



     /**
      * AUGMENTS TO TE RPC
      */

     augment "/te:tunnels-rpc/te:input/te:tunnel-info" {
       description "statelessComputeP2PPath input";
       list request-list {
         key "request-id-number";
         description "request-list";
         leaf request-id-number {
           type uint32;
           mandatory true;
           description "Each path computation request is uniquely
   identified by the request-id-number.
             It must be present also in rpcs.";
         }
         list servicePort {
           min-elements 1;
           uses PathCompServicePort;
           description "List of service ports.";
         }
         uses te-types:generic-path-constraints;
         uses te-types:generic-path-optimization;

       }
       uses synchronization-info;
     }

     augment "/te:tunnels-rpc/te:output/te:result" {



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       description "statelessComputeP2PPath output";
       list response {
         key response-index;
         config false;
         description "response";
         leaf response-index {
           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 {
             uses no-path-info;
           }
           case path-case {
             container pathCompService {
               uses PathComputationService;
               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 [ACTN-frame]. 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 [RFC6020], [RFC7950], [RFC7491] and [ACTN-
   frame].




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   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 [RFC6020] apply for this
   document as well.

8. IANA Considerations

   This section is for further study: to be completed when the YANG
   model is more stable.

9. References

9.1. Normative References

   [RFC6020] Bjorklund, M., "YANG - A Data Modeling Language for the
             Network Configuration Protocol (NETCONF)", RFC 6020,
             October 2010.

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

   [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.

   [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.

   [ACTN-Frame]   Ceccarelli, D., Lee, Y. et al., "Framework for
             Abstraction and Control of Traffic Engineered Networks"
             draft-ietf-actn-framework, work in progress.



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

9.2. Informative References

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

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

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

   [ACTN-Info] Lee, Y., Belotti, S., Dhody, D.,  Ceccarelli, D.,
             "Information Model for Abstraction and Control of
             Transport Networks", draft-leebelotti-actn-info, work in
             progress.

   [PCEP-Service-Aware] Dhody, D. et al., " Extensions to the Path
             Computation Element Communication Protocol (PCEP) to
             compute service aware Label Switched Path (LSP)", draft-
             ietf-pce-pcep-service-aware, work in progress.

10. Acknowledgments

   The authors would like to thank Igor Bryskin and Xian Zhang for
   participating in discussions 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.

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








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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
   Infinera
   Email: AnSharma@infinera.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@nokia.com


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














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