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PCE Working Group                                            Xian Zhang
Internet Draft                                            Haomian Zheng
Category: Standards track                           Huawei Technologies
                                               Oscar Gonzales de Dios
                                                            Victor Lopez
                                                          Telefonica I+D
                                                             Yunbin Xu

Expires: April 15, 2021                              October 15, 2020

  Extensions to the Path Computation Element Protocol (PCEP) to Support
                Resource Sharing-based Path Computation



   Resource sharing in a network means two or more Label Switched Paths
   (LSPs) use common pieces of resource along their paths. This can
   help save network resources and is useful in scenarios such as LSP
   recovery or when two LSPs do not need to be active at the same time.
   A Path Computation Element (PCE) is responsible for path computation
   with such requirement.

   Existing extensions to the Path Computation Element Protocol (PCEP)
   allow one path computation request for an LSP to be associated with
   other (existing) LSPs through the use of the PCEP Association

   This document extends PCEP in order to support resource-sharing-
   based path computation as another use of the Association Object to
   enable better efficiency in the computation and in the resultant
   paths and network resource usage.

Status of this Memo

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

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

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

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on  April 15, 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1. Introduction and Motivation ................................. 3
      1.1. Requirements Language .................................. 4
   2. Motivation .................................................. 5
      2.1. Single Domain Use Case.................................. 5
      2.2. Multiple Layers/Domains Use Case ....................... 6
      2.3. Bulk Path Computation Use Case ......................... 8
   3. Extensions to PCEP ......................................... 10
      3.1. Association Group and Type............................. 10
      3.2. Resource Sharing TLV .................................. 10
      3.3. Processing Rules ...................................... 11
   4. Implementation Status ...................................... 12
   5. Manageability Considerations ............................... 12
      5.1. Control of Function and Policy ........................ 13
      5.2. Information and Data Models ........................... 13
      5.3. Liveness Detection and Monitoring ..................... 13
      5.4. Verify Correct Operations ............................. 13
      5.5. Requirements on Other Protocols ....................... 13

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      5.6. Impact on Network Operations .......................... 13
   6. Security Considerations .................................... 13
   7. IANA Considerations ........................................ 14
      7.1. Association Object Type Indicators .................... 14
      7.2. PCEP TLV Definitions .................................. 14
   8. References ................................................. 15
      8.1. Normative References .................................. 15
      8.2. Informative References ................................ 16
   9. Acknowledgements ........................................... 17
   10. Contributor's Address ..................................... 17
   11. Authors' Addresses ........................................ 17

1. Introduction and Motivation

   A Path Computation Element (PCE) is a way to provide path
   computation function, and it is especially useful in the scenarios
   where complex constraints and/or a demanding amount of computation
   resource are required [RFC4655]. The development of PCE
   standardization has evolved from stateless to stateful. A stateful
   PCE has access to the LSP database information of the networks it
   serves as a computation engine [RFC8231]. Unless specified, this
   document assumes a PCE mentioned is a stateful PCE.

   Resource sharing denotes that two or more Label Switched Paths
   (LSPs) share common pieces of resource, (such as a common time slot
   of a link in an Optical Transport Network (OTN)). This is usually
   useful in the scenario where only one of the LSPs is active and the
   benefit is to save network resources. A simple example of this is
   dynamically calculating a recovery LSP for an existing LSP
   undergoing a link failure. Note that resource sharing can be worked
   out using a stateless PCE, but the mechanism may be complex and is
   out the scope of this document.

   This document considers the requirement that a new LSP may request
   for resource sharing with one or multiple existing LSPs. Furthermore,
   if there is resource sharing between a new LSP and existing an LSP,
   the two LSPs cannot be used to carry traffic simultaneously, the new
   LSP will take over the traffic from the existing LSP.

   In a single domain, this is a common requirement in the recovery
   cases especially in order to increase traffic resilience against
   failure while reducing the amount of network resource used for
   recovery purposes [RFC4428].

   The current protocol supporting the communication between a PCE and
   a Path Computation Client (PCC), i.e. PCE Protocol (PCEP), allows

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   for re-optimization of an existing LSP [RFC5440]. This is achieved
   by setting the R bit in the Request Parameter (RP) object, together
   with some additional information if applicable, in the Path
   Computation Request (PCReq) message sent from a PCC to the PCE. To
   support this type of resource sharing, a PCC needs to ask a PCE to
   compute a new path with the constraints of sharing resource with one
   or multiple existing LSPs. It is worth noting the "resource sharing"
   in this draft not only means one LSP re-using the same links of
   another LSP, but also the same slice of bandwidth in the network.
   This may occur when an LSP is required for re-routing, or online re-
   optimization. Current PCEP specifications do not provide such
   function. More specifically, this document describes the resource
   sharing issue during the procedure when a new LSP is required to
   replace an existing LSP for use together with Make-before-break
   (MBB) described in [RFC3209].

   As mentioned in [RFC8231], the PLSP-ID provides a unique identifier
   for an LSP during a PCEP session between PCC and PCE. Such
   identification is helpful in supporting the resource sharing
   requirement for stateful PCEs because it greatly simplifies the
   operation of a PCC. Instead of the PCC determining all the resources
   to be shared, the PCC can request that the PCE share the resources
   of a specific LSP: the stateful PCE is able to determine those
   resource itself.

   Resource sharing can also be required in an inter-layer PCEP
   session. This is similar to the previous requirement. However, it is
   more complex and therefore deserves a more detailed explanation

   In a multi-layer network, LSPs in a lower layer are used to carry
   higher-layer LSPs across the lower-layer network [RFC5623].
   Therefore, the resource sharing constraints in the higher layer
   might actually relate to resource sharing in the lower layer. Thus,
   it is useful to consider how this can be achieved and whether
   additional extensions are needed using the models defined in

   In the next sections, use cases are provided to show what
   information needs to be exchanged to fulfill these requirements.
   This memo then provides extensions to PCEP to enable this function.

1.1. Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

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   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2. Motivation

2.1. Single Domain Use Case

   There are two potential cases that request resource to be shared:
   restoration and re-optimization. Figure 1 shows a single domain
   network with a stateful PCE, and is used as an example for the
   resource sharing application.

                 |              |
                 | Stateful PCE |
                 |              |

            +------+          +------+          +------+
            |  N1  +----------+  N2  +-----X---+  N3  |
            +--+---+          +---+--+          +---+--+
               |                  |                 |
               |                  +---------+       |
               |                            |       |
               |     +------+          +------+     |
               +-----+  N5  +----------+  N4  +-----+
                     +------+          +------+

               Figure 1: A Single Domain Example

   LSP0 (existing): N1-N2-N3
   LSP1 (restoration): N1-N2-N4-N3
   LSP2 (re-optimization): N1-N5-N4-N3

   For the failure restoration, we can assume a working LSP (LSP0)
   exists in the network. When there is failure on the link N2-N3, it
   is desired to set up a restoration path for this working LSP.
   Suppose N1 serves as the PCC and sends a request to the stateful PCE
   for such an LSP. Besides the head-end and tail-end node of the
   working LSP, N1 may also need to check what policy should be applied
   for the restoration. For example, it may evaluate resource sharing
   and prefer to share as much resource with the working LSP as
   possible and specify this policy as a special object in the PCReq
   message.  Given such policy, a probable outcome from the path

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   computation would be LSP1, which shares the link 'N1-N2' with the
   existing LSP. The LSP1 will be set up by PCC via either PCInitiate
   of RSVP.

   Re-optimization does not usually result from a specific failure in
   the network, but takes place on a stable network when more optimal
   paths may have become available. Thus switching from the existing
   LSP to the new LSP happens with live traffic. An example can be
   found in Figure 1 without failure on the link N2-N3. Instead, an
   online re-optimization is needed for the working LSP (LSP0) from the
   stateful PCE. In such cases, the best choice is to set up a backup
   LSP for the working LSP with totally separate routing (for example,
   LSP2), and move the traffic to that backup LSP. After that, the
   working LSP can be torn down, which will not result in any
   interruption during the optimization procedure. This can actually be
   implemented with existing PCEP mechanisms. However, if there is no
   such separate path, existing PCEP mechanisms will return an error. A
   secondary option for this case is to set up an LSP and complete re-
   optimization with resource sharing, even if some interruption is

   In the example from Figure 1 it is assumed that the restored LSP or
   re-optimized LSP have the same source and destination nodes. But in
   some applications there is no restriction for this assumption, i.e.,
   after an LSP is failed, it can be restored as a new LSP with
   different source/destination.

   In the use cases above it is also assumed that the characteristics
   of the restored LSP or re-optimized LSP are unchanged. However, it
   is possible to have parameter changes during the resource sharing
   computation. For example, the bandwidth of the request LSP may be
   different from the existing LSP, while resource sharing is still
   preferred by the PCC. The PCE should consider the sharing request
   together with the policy and available resources in the network.
   Details can be found in Section 3.3.

   Conversely to resource sharing, it may also be required to apply a
   disjoint constraint for the path computation. [ietf-pce-association-
   diversity] discusses the solution under such a scenario, which is a
   companion work to this document.

2.2. Multiple Layers/Domains Use Case

   As Discussed in Section 3 of [RFC5623], there are three models for
   inter-layer path computation. They are single PCE computation,
   multiple PCE with inter-PCE communication, and multiple PCE without

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   inter-PCE communication. For the single PCE computation, the process
   would be similar to that of the use case in Section 2.1.

   An inter-layer path computation example is shown in Figure 2. Assume
   an LSP (LSP1: H2-H3) has been established already, visible as H2-H3
   from the view of the higher-layer PCE, and as H2-L1-L2-H3 from the
   global view (or from the view of the lower-layer PCE). A new request
   is received by H2 to establish a new LSP (LSP2: from H2 to H5),
   given the constraint that it can share resources with LSP1. This
   requirement is possible if only one of the LSPs needs to be active
   and resource sharing is the target.

                             .................................| LSR |
                           .:                                 | H5  |
                         .:                                   /-----
                       .:                                    /   |
       -----    -----.:                       -----    -----/    |
      | LSR |--| LSR |.......................| LSR |--| LSR |   /
      | H1  |  | H2  |                       | H3  |  | H4  |  /
       -----    -----\                       /-----    -----  /
                      \                     /                /
                       \                   /                /
                        \                 /                /
                         \               /                /
                          \-----   -----/                /
                          | LSR |-| LSR |               /
                          | L1  | | L2  |              /
                           -----   -----\             /
                             |           \           /
                             |            \         /
                             |             \       /
                           -----            \-----/
                          | LSR |-----------| LSR |
                          | L3  |           | L4  |
                           -----             -----
               Figure 2: A Two-layer Network Example

   If the model of multiple PCEs with inter-PCE communication is
   employed, the path computation request sent by H2 to higher-layer
   PCE will be forwarded to lower-layer PCE since there is no resource
   readily available in the higher layer. So it leaves the lower-layer
   PCE to compute a path in the lower layer in order to support the
   higher layer request. In this case, the lower-layer PCE is required
   to compute a path between H2 and H5 under the constraint that it can
   share the resource with that of LSP1. At this moment the lower-layer
   PCE has knowledge of the mapping relationship between the higher-

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   layer link H2-H3 and the lower layer link L1-L2, and therefore can
   convert the resource to be shared from higher layer to lower layer.
   So when the lower-layer PCE computes the path for LSP2, it can
   consider the resource used by L1-L2 as available with higher
   priority. For example, the lower-layer PCE may choose H2-L1-L2-L4-H5
   as the computation result. On the other hand, if the path
   computation policy is to have a separate path with LSP1, the lower-
   layer PCE may choose H2-L1-L3-L4-H5.

   During this procedure the higher-layer PCE can only use information
   about LSP1 (such as its five-tuple LSP information). An issue to
   solve is how the lower-layer PCE can resolve this information to the
   actual resource usage in its own layer, i.e. the lower layer. This
   could be solved by the edge LSR (L1) reporting this higher-lower LSP
   correlation to the lower-layer PCE as part of the LSP information
   during the LSP state synchronization process. If needed, it can be
   updated later when there is a change in this information.
   Alternatively, the lower-layer PCE can get this information from
   other sources, such as a network management system, where this
   information should be stored.

   If the model of multiple PCEs without inter-PCE communication is
   employed, the path computation request in the lower layer will be
   initiated by the border LSR node, i.e., L1. The process would be
   similar to that of the previous scenario. A point worth noting is
   that the border LSR node may be able to resolve the higher layer LSP
   information itself, such as by mapping it to the corresponding LSP
   in the lower layer, in this way the lower-layer PCE does not need to
   perform this function. Otherwise, the mapping method mentioned above
   can still be used.

2.3. Bulk Path Computation Use Case

   There is a potential need for resource sharing during bulk path
   computation, especially the processing of the "sticky resources" in
   [RFC7399]. It would be useful to specify the resources that can be
   shared among different paths, i.e., the bandwidth information.

   Considering the H-PCE architecture in [RFC8751], when the parent PCE
   asks for a single path across a few domains, such a request may
   become a bulk path computation to a certain child PCE. Figure 3
   shows an example of 3 domains. The parent PCE will select one of
   these path for establishment.

                             /| P-PCE |\

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                            / +---+---+ \
                           /       |     \
                          /        |      \
                         /         |       \
                        /          |        \
                       /           |         \
                      /            |          \
               +-----/+        +---+---+      +\------+
               |C-PCE1|        |C-PCE2 |      |C-PCE3 |
               +------+        +-------+      +-------+
                /                  |                  \
    ---------------      -----------------------       -------------
   /                \   /                       \    /              \
   | +---+     +---+ |  |  +---+   +---+   +---+ |  | +---+    +---+ |
   | | A +-----+ B +-+--+--+ D +---+ E +---+ H +-+--+-+ J +----+ L | |
   | +-\-+     +---+ |  |  +---+   +---+   +--\+ |  | +---+    +-/-+ |
   |     \           |  |          /           \ |  |           /    |
   |      \          |  |         /             \|  |          /     |
   |        \  +---+ |  |  +---+ /               |\\|    +---+/      |
   |          \+ C +-+--+--+ G +/                |  |----| K |       |
   \           +---+/   \  +---+                /    \   +---+      /
    ----------------     -----------------------      --------------
           Figure 3: Bulk Request example with Hierarchical PCEs

   A 3-domain example is shown in Figure 3, with the hierarchical PCE
   architecture. In this example nodes A/B/C belong to domain 1, nodes
   D/E/G/H belong to domain 2, and nodes J/K/L belong to domain 3.
   Inter-domain links are B-D/C-G between domains 1 and 2, and H-J/H-K
   between domains 2 and 3. Given a path computation request from A to
   L, a bulk request from P-PCE would be helpful to understand whether
   it is possible to have different combinations on the inter-domain
   links. However, the resources on some specific links become 'sticky'
   and have to be indicated as 'sharing allowed' to avoid unnecessary
   resource competition. For example, both the route A-B-D-E-H-J-L and
   A-C-G-E-H-K-L are qualified, but these routes are competing for the
   resource on the link E-H and cannot be established simultaneously,
   so there must be one route failed to be reported to P-PCE. Given the
   indication of allowing resource sharing on the link E-H, both of
   these routes can be reported for P-PCE's decision, and there will
   not be any competition as the P-PCE understands that only one path
   needs to be set up.

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3. Extensions to PCEP

3.1. Association Group and Type

   According to the definition in [RFC8697], the association group is
   used to associate multiple LSPs into one group for further path
   computation considerations, such as disjointness and resource
   sharing, in the messages when requesting path computation. An
   association ID will be used to identify the resource sharing group.
   An association type that described disjointness has been defined in
   [ietf-pce-association-diversity]. In this document, a new
   association type is defined as follows:

      Association type = TBD1 ("Sharing Association Type").

   A sharing group should have multiple LSPs. The number of LSPs and
   the criteria for how LSPs share among each other are dependent on
   local policy.

3.2. Resource Sharing TLV

   The PCEP Resource Sharing group MAY carry the following TLV. It MAY
   be carried within a PCReq message from the network element (or other
   PCCs) so as to indicate the desired resource sharing requirements to
   be applied by the stateful PCE during path computation.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |         Type = [TBD2]         |            Length             |
    |                 Flags                                 |B|S|N|L|
    |                          Optional TLVs                        |

   The following flags are defined:

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   *  L (Link share) bit: when set, this flag indicates that the PCE
   should prioritize the links shared by existing LSPs within the
   sharing group for path computation. The existing LSP identifier and
   its available link identifiers can be contained in the optional

   *  N (Node share) bit: when set, this flag indicates that the PCE
   should prioritize the nodes shared by existing LSPs within the
   sharing group for path computation. The existing LSP identifier and
   its available node identifiers can be contained in the optional

   *  S (SRLG share) bit: when set, this flag indicates that the PCE
   should set the SRLG (Shared Risk Link Group) of the computed LSP to
   the same as existing LSPs within the sharing group for path
   computation. The existing LSP identifier and SRLG information can be
   contained in the optional TLVs.

   *  B (Bandwidth share) bit: when set, this flag indicates that the
   PCE should prioritize the bandwidth to be shared by LSPs within the
   sharing group for bulk path computation. The LSP identifiers can be
   contained in the optional TLVs.

   It is worth noting that there can be multiple flags set which may
   conflict with each other. In this scenario, the result for path
   computation may not be unique, and is dependent on the
   implementation. The selection among multiple computation results is
   out of the scope of this document.

3.3. Processing Rules

   To request a path allowing resource sharing with one or multiple
   existing LSPs, a PCC includes a Resource Sharing TLV in the
   Association Group Object in any kind of path computation request
   message, such as the PCReq, PCUpd, or PCInitiate messages specified
   in [RFC8231] and [RFC8281].

   On receipt of a PCEP message with a Resource Sharing TLV, a stateful
   PCE MUST proceed as follows:

     - If the Resource Sharing TLV is unknown/unsupported, the PCE will
     follow procedures defined in [RFC5440].  That is, the PCE sends a
     PCErr message with error type 26 (Association Error) and error
     value 6 (Association Information Mismatch), and the related path
     computation request is discarded.

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     - If the Resource Sharing TLV is extracted correctly, the PCE MUST
     apply the requested resource sharing requirement, i.e., try to
     share as much resource as possible with the LSP specified in
     Resource Sharing TLV.

   The procedure of setting flags follows the rules defined in Section
   3.1. The flags in the Resource Sharing TLV may be locally configured
   on the requesting nodes via external entities, such as a network
   management system or the entity that imposes the resource sharing

   It is worth noting that the Resource Sharing TLV can be used
   together with other path indication objects like the IRO/XRO, with
   different objectives. The first difference is, the use of the
   Resource Sharing TLV is to set up an alternative path, instead a new
   path. It is also dependent on the knowledge held be the PCC, e.g.,
   if the PCC has full knowledge of the path information and has a
   strong preference on the route, it may send the request message with
   an IRO to specify the route. On the other hand, if the PCC does not
   know how the path should go but just wants to set up a new LSP to
   replace the old one, it may use the Resource Sharing TLV instead of
   an IRO. The second difference is that the Resource Sharing TLV is a
   loose requirement. For example, if the constraint specified in an
   IRO/XRO in an A-Z path computation request cannot be satisfied, the
   reply message from PCE to PCC would be unsuccessful. However it is
   still possible to have a path from the A-Z. If the target
   node/link/SRLG/Bandwidth is set in the Resource Sharing TLV rather
   than an IRO, the PCE may feedback a path from A-Z that does not
   share the target specified in the Resource Sharing TLV.

4. Implementation Status

   [Note to the RFC Editor - remove this section before publication, as
   well as remove the reference to [RFC7942].

   Currently the authors are not aware of any implementations.

5. Manageability Considerations

   All manageability requirements and considerations listed in
   [RFC5440] and [RFC8231] apply to the PCEP protocol extensions
   defined in this document.  In addition, requirements and
   considerations listed in this section apply.

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5.1. Control of Function and Policy

   A PCE or PCC implementation MUST allow operator-configured
   associations and SHOULD allow setting of the resource sharing TLV
   (Section 3.4) as described in this document.

5.2. Information and Data Models

   An implementation SHOULD allow the operator to view the resource
   sharing configured or created dynamically.  Further implementation
   SHOULD allow to view resource sharing associations reported by each
   peer, and the current set of LSPs in the association. The PCEP YANG
   module [ietf-pce-pcep-yang] includes association groups information.

5.3. Liveness Detection and Monitoring

   Mechanisms defined in this document do not imply any new liveness
   detection and monitoring requirements in addition to those already
   listed in [RFC5440].

5.4. Verify Correct Operations

   Mechanisms defined in this document do not imply any new operation
   verification requirements in addition to those already listed in
   [RFC5440] and [RFC8231].

5.5. Requirements on Other Protocols

   Mechanisms defined in this document do not imply any new
   requirements on other protocols. The configuration on local policy
   may be accomplished by other protocols, such as Netconf.

5.6. Impact on Network Operations

   Mechanisms defined in [RFC5440] and [RFC8231] also apply to PCEP
   extensions defined in this document.

6. Security Considerations

   Security of PCEP is discussed in [RFC5440] and [RFC6952]. The
   extensions in this document do not change the fundamentals of
   security for PCEP.

   However, the introduction of the Resource Sharing TLV in the
   Association Group Object provides a vector that may be used to probe
   for information from a network. For example, a PCC that wants to
   discover the path of an LSP with which it is not involved can issue

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   a request message with a Resource Sharing TLV and may be able to get
   back quite a lot of information about the path of the LSP through
   issuing multiple such requests for different endpoints and analyzing
   the received results. To protect against this, a PCE SHOULD be
   configured with access and authorization controls such that only
   authorized PCCs (for example, those within the network) can make
   computation requests, only specifically authorized PCCs can make
   requests for resource sharing, and such requests relating to
   specific LSPs are further limited to a select few PCCs. How such
   access controls and authorization is managed is outside the scope of
   this document, but it will at the least include Access Control

   Furthermore, a PCC must be aware that setting up an LSP that shares
   resources with another LSP may be a way of attacking the other LSP,
   for example by depriving it of the resources it needs to operate
   correctly. Thus it is important that, both in PCEP and the
   associated signaling protocols, only authorized resource sharing is

7. IANA Considerations

7.1. Association Object Type Indicators

   IANA maintains a registry called the "Path Computation Element
   Protocol (PCEP) Numbers" registry with a subregistry called the
   "Association Type Field" subregistry.  IANA is requested to make an
   assignment from that subregistry as follows:

   Object    Name              Object          Reference
   Class                       Type

    TBD1    Sharing-group     Association Type   [this document]

7.2. PCEP TLV Definitions

   This document defines the following TLVs to support the resource
   sharing scenario:

   Value    Name                      Reference

    TBD2    Resource-sharing TLV     [this document]

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   IANA is requested to allocate the following bit numbers in the flag
   spaces of Resource-sharing TLV:

   Bit      Flag name                         Reference

    31      Link Share                      [this document]

    30      Node Share                      [this document]

    29      SRLG Share                      [this document]

    28      Bandwidth Share                 [this document]

8. References

8.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to indicate
             requirements levels", RFC 2119, March 1997.

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

   [RFC5440] Vasseur, J.-P., and Le Roux, JL., "Path Computation
             Element (PCE) Communication Protocol (PCEP)", RFC 5440,
             March 2009. <https://www.rfc-editor.org/info/rfc5440>.

   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
             May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8231] Crabbe, E., Medved, J., Minei, I., and R. Varga, "PCEP
             Extensions for Stateful PCE", RFC8231, June 2017.

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

   [RFC8697] Minei, I., Crabbe E., Sivabalan S., Ananthakrishnan H.,
             Dhody D., Tanaka Y., "PCEP Extensions for Establishing
             Relationships Between Sets of LSPs", RFC8697, January
             2020. <https://www.rfc-editor.org/info/rfc8697>.

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   [ietf-pce-association-diversity] Litkowski, S., Sivabalan, S.,
             Barth, C., Dhody, D., "Path Computation Element
             communication Protocol extension for signaling LSP
             diversity constraint", work in progress.

8.2. Informative References

   [RFC4428] Papadimitriou, D., Mannie., E., "Analysis of Generalized
             Multi-Protocol Label Switching (GMPLS)-based Recovery
             Mechanisms (including Protection and Restoration)",
             RFC4428, March 2006. <https://www.rfc-

   [RFC4655] Farrel, A., Vasseur, J.-P., and Ash, J., "A Path
             Computation Element (PCE)-Based Architecture", RFC 4655,
             August 2006. <https://www.rfc-editor.org/info/rfc4655>.

   [RFC5623] Oki., E., Takeda, T., Le Roux, JL., Farrel, A., "Framework
             for PCE-Based Inter-Layer MPLS and GMPLS Traffic
             Engineering", RFC5623, September 2009. <https://www.rfc-

   [RFC6952] Jethanandani, M., Patel, K., Zheng, L., "Analysis of BGP,
             LDP, PCEP, and MSDP Issues According to the Keying and
             Authentication for Routing Protocols (KARP) Design Guide",
             RFC6952, May 2013. <https://www.rfc-

   [RFC7399] Farrel, A., King, D., "Unanswered Questions in the Path
             Computation Element Architecture", RFC7399, October 2014.

   [RFC7942] Sheffer, Y., Farrel, A., "Improving Awareness of Running
             Code: The Implementation Status Section", RFC7942, July
             2016. <https://www.rfc-editor.org/info/rfc7942>.

   [RFC8751] Dhody, D., Lee, Y., Ceccarelli, D., Shin, J., King, D.,
             Gonzalez de Dios, O., "Hierarchical Stateful Path
             Computation Element (PCE)", RFC8751, March 2020.

   [ietf-pce-pcep-yang] Dhody, D., Hardwick, J., Beeram, V., Tantsura,
             J., "A YANG Data Model for Path Computation Element
             Communications Protocol(PCEP)", work in progress.

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9. Acknowledgements

   The authors would like to thank Adrian Farrel for his review and
   valuable comments.

10. Contributor's Address

   Dhruv Dhody
   Huawei Technologies
   Email: dhruv.dhody@huawei.com

   Igor Bryskin
   Huawei Technologies
   Email: Igor.Bryskin@huawei.com

11. Authors' Addresses

   Xian Zhang
   Huawei Technologies
   Email: zhang.xian@huawei.com

   Haomian Zheng
   Huawei Technologies
   Email: zhenghaomian@huawei.com

   Oscar Gonzalez de Dios
   Telefonica I+D/gCTIO
   Distrito Telefonica
   E-28050 Madrid, Spain
   EMail: oscar.gonzalezdedios@telefonica.com

   Victor Lopez
   Telefonica I+D/gCTIO
   Distrito Telefonica
   E-28050 Madrid, Spain
   EMail: victor.lopezalvarez@telefonica.com

   Yunbin Xu

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