Path Computation Element Communication Protocol (PCEP) Extension for LSP Diversity Constraint Signaling
draft-ietf-pce-association-diversity-12

Document Type Active Internet-Draft (pce WG)
Last updated 2019-10-31 (latest revision 2019-10-28)
Replaces draft-dhody-pce-of-diverse, draft-litkowski-pce-association-diversity
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PCE Working Group                                           S. Litkowski
Internet-Draft                                              S. Sivabalan
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: April 30, 2020                                         C. Barth
                                                        Juniper Networks
                                                                 M. Negi
                                                     Huawei Technologies
                                                        October 28, 2019

Path Computation Element Communication Protocol (PCEP) Extension for LSP
                     Diversity Constraint Signaling
                draft-ietf-pce-association-diversity-12

Abstract

   This document introduces a simple mechanism to associate a group of
   Label Switched Paths (LSPs) via an extension to the Path Computation
   Element (PCE) communication Protocol (PCEP) with the purpose of
   computing diverse paths for those LSPs.  The proposed extension
   allows a Path Computation Client (PCC) to advertise to a PCE that a
   particular LSP belongs to a particular disjoint-group, thus the PCE
   knows that the LSPs in the same group need to be disjoint from each
   other.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on April 30, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Protocol Extension  . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Association Group . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Disjoint TLVs . . . . . . . . . . . . . . . . . . . . . .   8
     5.3.  Disjointness Objective Functions  . . . . . . . . . . . .  10
     5.4.  Relationship to SVEC  . . . . . . . . . . . . . . . . . .  12
       5.4.1.  SVEC and OF . . . . . . . . . . . . . . . . . . . . .  12
     5.5.  P Flag Considerations . . . . . . . . . . . . . . . . . .  13
     5.6.  Disjointness Computation Issues . . . . . . . . . . . . .  16
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     7.1.  Association Type  . . . . . . . . . . . . . . . . . . . .  17
     7.2.  PCEP TLVs . . . . . . . . . . . . . . . . . . . . . . . .  18
     7.3.  Objective Functions . . . . . . . . . . . . . . . . . . .  19
     7.4.  NO-PATH-VECTOR Bit Flags  . . . . . . . . . . . . . . . .  19
     7.5.  PCEP-ERROR Codes  . . . . . . . . . . . . . . . . . . . .  19
   8.  Manageability Considerations  . . . . . . . . . . . . . . . .  20
     8.1.  Control of Function and Policy  . . . . . . . . . . . . .  20
     8.2.  Information and Data Models . . . . . . . . . . . . . . .  20
     8.3.  Liveness Detection and Monitoring . . . . . . . . . . . .  20
     8.4.  Verification of Correct Operations  . . . . . . . . . . .  20
     8.5.  Requirements on Other Protocols . . . . . . . . . . . . .  21
     8.6.  Impact on Network Operations  . . . . . . . . . . . . . .  21
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  21
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     10.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Appendix A.  Contributor Addresses  . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

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1.  Introduction

   [RFC5440] describes the Path Computation Element communication
   Protocol (PCEP) which enables the communication between a Path
   Computation Client (PCC) and a Path Control Element (PCE), or between
   two PCEs based on the PCE architecture [RFC4655].

   PCEP Extensions for Stateful PCE Model [RFC8231] describes a set of
   extensions to PCEP to enable active control of MPLS-TE and GMPLS
   tunnels.  [RFC8281] describes the setup and teardown of PCE-initiated
   LSPs under the active stateful PCE model, without the need for local
   configuration on the PCC, thus allowing for a dynamic network.

   [I-D.ietf-pce-association-group] introduces a generic mechanism to
   create a grouping of LSPs in the context of a PCE which can then be
   used to define associations between a set of LSPs and a set of
   attributes (such as configuration parameters or behaviors) and is
   equally applicable to the active and passive modes of a stateful PCE
   [RFC8231] or a stateless PCE [RFC5440].

   This document specifies a PCEP extension to signal that a set of LSPs
   in a particular group should use diverse paths, including the
   requested type of diversity.  A PCC can use this extension to signal
   to a PCE that a particular LSP belongs to a particular disjoint-
   group.  When a PCE receives LSP states belonging to the same
   disjoint-group from some PCCs, the PCE should ensure that the LSPs
   within the group are disjoint from each other.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Terminology

   The following terminology is used in this document.

   DAT:  Disjoint Association Type.

   DAG:  Disjoint Association Group.

   MPLS:  Multiprotocol Label Switching.

   OF:  Objective Function.

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   PCC:  Path Computation Client.  Any client application requesting a
      path computation to be performed by a Path Computation Element.

   PCE:  Path Computation Element.  An entity (component, application,
      or network node) that is capable of computing a network path or
      route based on a network graph and applying computational
      constraints.

   PCEP:  Path Computation Element communication Protocol.

   SRLG:  Shared Risk Link Group.

3.  Motivation

   Path diversity is a very common use case in today's IP/MPLS networks
   especially for layer 2 transport over MPLS.  A customer may request
   that the operator provide two end-to-end disjoint paths across the
   operator's IP/MPLS core.  The customer may use these paths as
   primary/backup or active/active configuration.

   Different levels of disjointness may be offered:

   o  Link disjointness: the paths of the associated LSPs should transit
      different links (but may use common nodes or different links that
      may have some shared fate).

   o  Node disjointness: the paths of the associated LSPs should transit
      different nodes (but may use different links that may have some
      shared fate).

   o  SRLG disjointness: the paths of the associated LSPs should transit
      different links that do not share fate (but may use common transit
      nodes).

   o  Node+SRLG disjointness: the paths of the associated LSPs should
      transit different links that do not have any common shared fate
      and should transit different nodes.

   The associated LSPs may originate from the same or from different
   head-end(s) and may terminate at the same or different tail-end(s).

4.  Applicability

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            _________________________________________
           /                                         \
          /        +------+                           \
         |         | PCE  |                            |
         |         +------+                            |
         |                                             |
         |          ***********************>           |
         | +------+           10             +------+  |
   CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2
         | +------+       |        |         +------+  |
         |                |        |                   |
         |                |        |                   |
         | +------+       |        |         +------+  |
   CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4
         | +------+ ***********************> +------+  |
         |                                             |
          \                                           /
           \_________________________________________/

     Figure 1 - Disjoint paths with different head-ends and tail-ends

   In the figure above, let us consider that the customer wants to have
   two disjoint paths, one between CE1 and CE2 and one between CE3 and
   CE4.  From an IP/MPLS network point view, in this example, the CEs
   are connected to different PEs to maximize their disjointness.  When
   LSPs originate from different head-ends, distributed computation of
   diverse paths can be difficult, whereas, computation via a
   centralized PCE ensures path disjointness, correctness and
   simplicity.

   Section 5.4 describes the relationship between the Disjoint
   Association Group (DAG) and Synchronization VECtor (SVEC) object.

   The PCEP extension for stateful PCE [RFC8231] defined new PCEP
   messages - Path Computation Report (PCRpt), Path Computation Update
   (PCUpd) and Path Computation Initiate (PCInitiate) [RFC8281].  These
   messages use PLSP-ID in the LSP object for identification.  Moreover
   to allow diversity between LSPs originating from different PCCs, the
   generic mechanism to create a grouping of LSPs is described in
   [I-D.ietf-pce-association-group] (that is equally applicable to the
   active and passive modes of a stateful PCE).

   Using the extension to PCEP defined in this document, the PCC uses
   the [I-D.ietf-pce-association-group] extension to indicate that a
   group of LSPs are required to be disjoint; such indication should
   include disjointness parameters such as the type of disjointness, the

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   disjoint group identifiers, and any customization parameters
   according to the configured local policy.

   The management of the disjoint group IDs will be a key point for the
   operator as the Association ID field is limited to 65535.  The local
   configuration of IPv4/IPv6 association source, or Global Association
   Source/Extended Association ID allows to overcome this limitation as
   described in [I-D.ietf-pce-association-group].  When a PCC or PCE
   initiates all the LSPs in a particular disjoint-group, it can set the
   IPv4/IPv6 association source as one of its own IP address.  When
   disjoint LSPs are initiated from different head-ends, the association
   source could be the PCE address or any other unique value to identify
   the DAG.

          Initiate Disjoint LSPs
                   |
                   |                       PCReq/PCRpt
                   V                   {Disjoint-group Y}
                +-----+               ----------------> +-----+
     _ _ _ _ _ _| PCE |               |                  | PCE |
    |           +-----+               |      ----------> +-----+
    | PCInitiate                      |     |    PCReq/PCRpt
    |{Disjoint-group X}               |     | {Disjoint-group Y}
    |                                 |     |
    |              .-----.            |     |         .-----.
    |             (       )           | +-----+      (       )
    |         .--(         )--.       | |PCC 2|--.--(         )--.
    V        (                 )      | +-----+ (                 )
  +---+     (                  )      |        (                  )
  |PCC|----(   (G)MPLS network )    +-----+    ( (G)MPLS network  )
  +---+     (                  )    |PCC 1|-----(                 )
  Disjoint-group X            )     +-----+      (                )
             '--(         )--'                 '--(           )--'
                 (       )                          (        )
                  '-----'                             '-----'

  Case 1: Disjointness initiated by    Case 2: Disjointness initiated by
         PCE and enforced by PCC              PCC and enforced by PCE

     Figure 2 - Sample use-cases for carrying disjoint-group over PCEP
                                  session

   Using the disjoint-group within a PCEP messages is used for:

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   o  Configuration: Used to communicate the configured disjoint
      requirement to a PCEP peer.

   o  Status: Used to communicate the status of the computed
      disjointness.

5.  Protocol Extension

5.1.  Association Group

   As per [I-D.ietf-pce-association-group], LSPs are associated with
   other LSPs with which they interact by adding them to a common
   association group.  As described in [I-D.ietf-pce-association-group]
   the association group is uniquely identified by the combination of
   these fields in the ASSOCIATION object: Association Type, Association
   ID, Association Source, and (if present) Global Association Source or
   Extended Association ID.

   This document defines a new Association type, based on the generic
   Association object:

   o  Association type = TBD1 Disjoint Association Type (DAT).

   [I-D.ietf-pce-association-group] specifies the mechanism for the
   capability advertisement of the association types supported by a PCEP
   speaker by defining a ASSOC-Type-List TLV to be carried within an
   OPEN object.  This capability exchange for the Disjointness
   Association Type (TBD1) MUST be done before using the disjointness
   association.  Thus the PCEP speaker MUST include the Disjointness
   Association Type in the ASSOC-Type-List TLV before using the Disjoint
   Association Group (DAG) in PCEP messages.

   This association type is considered to be both dynamic and operator-
   configured in nature.  As per [I-D.ietf-pce-association-group], the
   association group could be created by the operator manually on the
   PCEP peers and the LSPs belonging to this associations is conveyed
   via PCEP messages to the PCEP peer; or the association group could be
   created dynamically by the PCEP speaker and both the association
   group information and the LSPs belonging to the association group is
   conveyed to the PCEP peer.  The Operator-configured Association Range
   MUST be set for this association-type to mark a range of association
   identifiers that are used for operator-configured associations to
   avoid any association identifier clash within the scope of the
   association source.  (Refer to [I-D.ietf-pce-association-group].)

   A disjoint group can have two or more LSPs, but a PCE may be limited
   in the number of LSPs it can take into account when computing
   disjointness.  If a PCE receives more LSPs in the group than it can

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   handle in its computation algorithm, it SHOULD apply disjointness
   computation to only a subset of LSPs in the group.  The subset of
   disjoint LSPs will be decided by PCE as a local policy.  Local
   polices MAY define the computational behavior for the other LSPs in
   the group.  For example, the PCE may provide no path, a shortest
   path, or a constrained path based on relaxing disjointness, etc.  The
   disjoint status of the computed path is informed to the PCC via
   DISJOINTNESS-STATUS-TLV (see Section 5.2).

   There are differet types of disjointness identified by the flags (T,
   S, N, L) in the DISJOINTNESS-CONFIGURATION-TLV (see Section 5.2).
   All LSPs in a particular disjoint group MUST use the same combination
   of T, S, N, L flags in the DISJOINTNESS-CONFIGURATION-TLV.  If a PCEP
   peer receives a PCEP messages for LSPs belonging to the same disjoint
   group but having an inconsistent combination of T, S, N, L flags, the
   PCEP peer MUST NOT try to add the LSPs in disjoint group and MUST
   reply with a PCErr with Error-type 26 (Association Error) and Error-
   Value 6 (Association information mismatch).

   A particular LSP MAY be associated to multiple disjoint groups, but
   in that case, the PCE SHOULD try to consider all the disjoint groups
   during path computation if possible.  Otherwise a local policy MAY
   define the computational behavior.  If a PCE does not support such a
   path computation it MUST NOT add the LSP into association group and
   return a PCErr with Error-type 26 (Association Error) and Error-Value
   7 (Cannot join the association group).

5.2.  Disjoint TLVs

   The disjoint group MUST carry the following TLV:

   o  DISJOINTNESS-CONFIGURATION-TLV: Used to communicate some
      disjointness configuration parameters.

   In addition, the disjoint group MAY carry the following TLV:

   o  DISJOINTNESS-STATUS-TLV: Used to communicate the status of the
      computed disjointness.  This is applicable for messages from PCE
      to PCC (PCUpd, PCInitiate or PCRep message).

   o  VENDOR-INFORMATION-TLV: Used to communicate arbitrary vendor-
      specific behavioral information, described in [RFC7470].

   o  OF-List TLV: Used to communicate the disjointness objective
      function.  See Section 5.3.

   The DISJOINTNESS-CONFIGURATION-TLV is shown in the following figure:

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    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                               |T|P|S|N|L|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type: TBD2.

   Length: Fixed value of 4 bytes.

      Flags:

      *  L (Link diverse) bit: when set, this indicates that the
         computed paths within the disjoint group MUST NOT have any link
         in common.

      *  N (Node diverse) bit: when set, this indicates that the
         computed paths within the disjoint group MUST NOT have any node
         in common.

      *  S (SRLG diverse) bit: when set, this indicates that the
         computed paths within the disjoint group MUST NOT share any
         SRLG (Shared Risk Link Group).

      *  P (Shortest path) bit: when set, this indicates that the
         computed path of the LSP SHOULD satisfy all the constraints and
         objective functions first without considering the diversity
         constraint, this means that all of the LSPs with P flag set in
         the association group are computed first as if the disjointness
         constraint has not been configured, and then with those LSPs
         fixed, the other LSPs with P flag unset in the association
         group are computed by taking into account the disjointness
         constraint.  The role of P flag is further described with
         examples in Section 5.5.

      *  T (Strict disjointness) bit: when set, if disjoint paths cannot
         be found, PCE SHOULD return no path for LSPs that could not be
         be disjoint.  When unset, the PCE is allowed to relax
         disjointness by Section 5.5 either applying a requested
         objective function (cf.  Section 5.3 below) or using the local
         policy if no objective function is requested (e.g. using a
         lower disjoint type (link instead of node) or fully relaxing
         disjointness constraint).  Further see Section 5.6 for details.

      *  Unassigned bits are considered reserved.  They MUST be set to 0
         on transmission and MUST be ignored on receipt.

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   If a PCEP speaker receives a disjoint-group without DISJOINTNESS-
   CONFIGURATION-TLV, it SHOULD reply with a PCErr Error-type=6
   (Mandatory Object missing) and Error-value=TBD8 (DISJOINTNESS-
   CONFIGURATION-TLV missing).

   The DISJOINTNESS-STATUS-TLV uses the same format as the DISJOINTNESS-
   CONFIGURATION-TLV with a different type TBD3 (in the TLV).  The L, N,
   and S flags are set if the respective disjointness criterion was
   requested and the computed paths meet it.  The P flag indicates that
   the computed path is the shortest path (computed first without taking
   disjointness constraints into consideration, but considering other
   constraints).

   The T flag has no meaning in the DISJOINTNESS-STATUS-TLV and MUST NOT
   be set while sending and MUST be ignored on receipt.

   Any document defining a new flag for the DISJOINTNESS-CONFIGURATION-
   TLV automatically defines a new flag with the same name and in the
   same location in DISJOINTNESS-STATUS-TLV; the semantics of the flag
   in DISJOINTNESS-STATUS-TLV MUST be specified in the document that
   specifies the flag in DISJOINTNESS-CONFIGURATION-TLV.

5.3.  Disjointness Objective Functions

   An objective function (OF) MAY be applied to the disjointness
   computation to drive the PCE computation behavior.  In this case, the
   OF-List TLV (defined in ([RFC5541]) is used as an optional TLV in the
   Association Group Object.  Whereas the PCEP OF-List TLV allows
   multiple OF-codes inside the TLV, a sender SHOULD include a single
   OF-code in the OF-List TLV when included in the Association Group,
   and the receiver MUST consider the first OF-code only and ignore
   others if included.

   To minimize the common shared resources (Node, Link or SRLG) between
   a set of paths during path computation three new OF-codes are
   proposed:

   MSL

   * Name:  Minimize the number of shared (common) Links.

   * Objective Function Code:  TBD4

   * Description:  Find a set of paths such that it passes through the
      least number of shared (common) links.

      *  A network comprises a set of N links {Li, (i=1...N)}.

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      *  A path P passes through K links {Lpi,(i=1...K)}.

      *  A set of paths {P1...Pm} have L links that are common to more
         than one path {Lpi,(i=1...L)}.

      *  Find a set of paths such that the value of L is minimized.

   MSS

   * Name:  Minimize the number of shared (common) SRLGs.

   * Objective Function Code:  TBD5

   * Description:  Find a set of paths such that it passes through the
      least number of shared (common) SRLGs.

      *  A network comprises a set of N links {Li, (i=1...N)}.

      *  A path P passes through K links {Lpi,(i=1...K)} belonging to
         unique M SRLGs {Spi,(i=1..M)}.

      *  A set of paths {P1...Pm} have L SRLGs that are common to more
         than one path {Spi,(i=1...L)}.

      *  Find a set of paths such that the value of L is minimized.

   MSN

   * Name:  Minimize the number of shared (common) Nodes.

   * Objective Function Code:  TBD6

   * Description:  Find a set of paths such that they pass through the
      least number of shared (common) nodes.

      *  A network comprises a set of N nodes {Ni, (i=1...N)}.

      *  A path P passes through K nodes {Npi,(i=1...K)}.

      *  A set of paths {P1...Pm} have L nodes that are common to more
         than one path {Npi,(i=1...L)}.

      *  Find a set of paths such that the value of L is minimized.

   If the OF-list TLV is included in the Association Object, the OF-code
   inside the OF Object MUST include one of the disjoint OFs defined in
   this document.  If this condition is not met, the PCEP speaker MUST

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   respond with a PCErr message with Error-Type=10 (Reception of an
   invalid object) and Error-Value=TBD9 (Incompatible OF code).

5.4.  Relationship to SVEC

   [RFC5440] defines a mechanism for the synchronization of a set of
   path computation requests by using the SVEC object, that specifies
   the list of synchronized requests that can either be dependent or
   independent.  The SVEC object identifies the relationship between the
   set of path computation requests, identified by 'Request-ID-number'
   in RP (Request Parameters) object.  [RFC6007] further clarified the
   use of the SVEC list for synchronized path computations when
   computing dependent requests as well as described a number of usage
   scenarios for SVEC lists within single-domain and multi-domain
   environments.

   The SVEC object includes a Flags field that indicates the potential
   dependency between the set of path computation requests in a similar
   way as the Flags field in the TLVs defined in this document.  The
   path computation request in the PCReq message MAY use both the SVEC
   and ASSOCIATION objects to identify the related path computation
   request as well as the DAG.  The PCE MUST try to find a path that
   meets both the constraints.  It is possible that the diversity
   requirement in the association group is different from the one in the
   SVEC object.  The PCE MUST consider both the objects as per the
   processing rules and aim to find a path that meets both of these
   constraints.  In case no such path is possible, the PCE MUST send a
   path computation reply (PCRep) with a NO-PATH object indicating path
   computation failure as per [RFC5440].  It should be noted that the
   LSPs in the association group can be fully same or partially
   overlapping with the LSPs grouped by the SVEC object in PCReq
   message.

   Some examples of usage are listed below:

   o  PCReq with SVEC object with node-diverse bit=1 (LSP1,LSP2) and DAG
      with S=1 (LSP1,LSP2) - both node and SRLG diverse path between
      LSP1, LSP2.

   o  PCReq with SVEC object with link-diverse bit=1 (LSP1,LSP2) and DAG
      with L=1 (LSP1,LSP3) - link diverse paths between LSP1, LSP2,
      LSP3.  But any future change in LSP2 will have no impact.

5.4.1.  SVEC and OF

   This document defines three new OF-codes Section 5.3 to maximize
   diversity as much as possible, in other words, new OF-codes allow

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   specification of minimization of common shared resources (Node, Link
   or SRLG) among a set of paths during path computation.

   It may be interesting to note that the diversity flags in the SVEC
   object and OF for diversity can be used together.  Some examples of
   usage are listed below:

   o  SVEC object with node-diverse bit=1 - ensure full node-diversity.

   o  SVEC object with node-diverse bit=1 and OF=MSS - full node diverse
      with as much as SRLG-diversity as possible.

   o  SVEC object with domain-diverse bit=1;link diverse bit=1 and
      OF=MSS - full domain and node diverse path with as much as SRLG-
      diversity as possible.

   o  SVEC object with node-diverse bit=1 and OF=MSN - ensure full node-
      diversity.

   In the last example above, it is interesting to note that "OF"
   becomes redundant as "SVEC object" ensures full node-diversity,
   however this specification does not prohibit redundant constraints
   while using "SVEC object" and "OF" together for diversity.

5.5.  P Flag Considerations

   As mentioned in Section 5.2, the P flag (when set) indicates that the
   computed path of the LSP SHOULD satisfies all constraints and
   objective functions first without considering the diversity
   constraint.

   This means that an LSP with P flag set should be placed first as if
   the disjointness constraint has not been configured, while the other
   LSPs in the association with P flag unset should be placed by taking
   into account the disjointness constraint.  Setting P flag changes the
   relationship between LSPs to a one-sided relationship (LSP 1 with P=0
   depends of LSP 2 with P=1, but LSP 2 with P=1 does not depend of LSP
   1 with P=0).  Multiple LSPs in the same disjoint group may have the P
   flag set.  In such a case, those LSPs may not be disjoint from each
   other but will be disjoint from other LSPs in the group that have the
   P flag unset.

   This could be required in some primary/backup scenarios where the
   primary path should use the more optimal path available (taking into
   account the other constraints).  When disjointness is computed, it is
   important for the algorithm to know that it should try to optimize
   the path of one or more LSPs in the disjoint group (for instance the
   primary path) while other paths are allowed to be costlier (compared

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   to a similar path without the disjointness constraint).  Without such
   a hint, the disjointness algorithm may set a path for all LSPs that
   may not completely fulfill the customer's requirement.

            _________________________________________
           /                                         \
          /        +------+                           \
         |         | PCE  |                            |
         |         +------+                            |
         |                                             |
         |                                             |
         | +------+           10             +------+  |
   CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2
         | +------+       |        |         +------+  |
         |                |        |                   |
         |                |        |                   |
         | +------+       |        |         +------+  |
   CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4
         | +------+ \     |               /  +------+  |
         |           \    |     10       /             |
          \           +-- R5 --------- R6             /
           \_________________________________________/

   Cost of all the links is 1, unless explicitly marked otherwise.

                                 Figure 3

   In the figure above, a customer has two dual homed sites (CE1/CE3 and
   CE2/CE4).  Let us consider that this customer wants two disjoint
   paths between the two sites.  Due to physical meshing, the customer
   wants to use CE1 and CE2 as primary (and CE3 and CE4 are hosted in a
   remote site for redundancy purpose).

   Without any hint (constraint) provided, the PCE may compute the two
   disjoint LSPs together, leading to PE1->PE2 using a path
   PE1->R1->R2->PE2 and PE3->PE4 using PE3->R3->R4->PE4.  In this case,
   even if the disjointness constraint is fulfilled, the path from PE1
   to PE2 does not use the best optimal path available in the network
   (path delay may be higher): the customer requirement is thus not
   completely fulfilled.

   The usage of the P flag allows the PCE to know that a particular LSP
   should be tied to the best path as if the disjointness constraint was
   not requested.

   In our example, if the P flag is set to the LSP PE1->PE2, the PCE
   should use the path PE1->R1->R3->R4->R2->PE2 for this LSP, while the

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   other LSP should be disjoint from this path.  The second LSP will be
   placed on PE3->R5->R6->PE4 as it is allowed to be costlier.

   Driving the PCE disjointness computation may be done in other ways,
   for instance setting a metric boundary reflecting an path delay
   boundary.  Other constraints may also be used.

   The P flag allows to simply express that the disjointness constraint
   should not make the LSP worst.

   Any constraint added to a path disjointness computation may reduce
   the chance to find suitable paths.  The usage of the P flag, as any
   other constraint, may prevent to find a disjoint path.  In the
   example above, if we consider that the router R5 is down, if PE1->PE2
   has the P flag set, there is no room available to place PE3->PE4 (the
   disjointness constraint cannot be fulfilled).  If PE1->PE2 has the P
   flag unset, the algorithm may be able to place PE1->PE2 on R1->R2
   link leaving a room for PE3->PE4 using the R3->R4 link.  When using P
   flag or any additional constraint on top of the disjointness
   constraint, the user should be aware that there is less chance to
   fulfill the disjointness constraint.

            _________________________________________
           /                                         \
          /        +------+                           \
         |         | PCE  |                            |
         |         +------+                            |
         |                                             |
         |                                             |
         | +------+           10             +------+  |
   CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2
         | +------+       |  \     |         +------+  |
         |                |   \2   |                   |
         |                |    \   |                   |
         | +------+       |     \  |         +------+  |
   CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4
         | +------+                          +------+  |
         |                                             |
          \                                           /
           \_________________________________________/

   Cost of all the links is 1, unless explicitly marked otherwise.

                                 Figure 4

   In the figure above, we still consider the same previous
   requirements, so PE1->PE2 LSP should be optimized (P flag set) while
   PE3->PE4 should be disjoint and may use a costlier path.

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   Regarding PE1->PE2, there are two paths that are satisfying the
   constraints (ECMP): PE1->R1->R4->R2->PE2 (path 1) and
   PE1->R1->R3->R4->R2->PE2 (path 2).  An implementation may choose one
   of the paths.

   If the implementation elects only one path, there is a chance that
   picking up one path may prevent disjointness.  In our example, if
   path 2 is used for PE1->PE2, there is no room left for PE3->PE4 while
   if path 1 is used, PE3->PE4 can be placed on R3->R4 link.

   When P flag is set for an LSP and when ECMPs are available, an
   implementation should aim to select a path that allows disjointness.

5.6.  Disjointness Computation Issues

   There may be some cases where the PCE is not able to provide a set of
   disjoint paths for one or more LSPs in the association.

   When the T flag is set (Strict disjointness requested), if
   disjointness cannot be ensured for one or more LSPs, the PCE MUST
   reply to a Path Computation Request (PCReq) with a Path Computation
   Reply (PCRep) message containing a NO-PATH object.  In case of PCRpt
   message, the PCE MUST return a PCErr message with Error-Type 26
   "Association Error" and Error-Value 7 "Cannot join the association
   group".

   In case of network event leading to an impossible strict
   disjointness, the PCE MUST send a PCUpd message containing an empty
   ERO to the corresponding PCCs.  In addition to the empty ERO Object,
   the PCE MAY add the NO-PATH-VECTOR TLV ([RFC5440]) in the LSP Object.

   This document adds new bits in the NO-PATH-VECTOR TLV:

      bit "TBD7": when set, the PCE indicates that it could not find a
      disjoint path for this LSP.

      bit "TBD8": when set, the PCE indicates that it does not support
      the requested disjointness computation.

   When the T flag is unset, the PCE is allowed to relax disjointness by
   applying a requested objective function (Section 5.3) if specified.
   Otherwise, if no objective function is specified, the PCE is allowed
   to reduce the required level of disjointness as it deems fit.  The
   actual level of disjointness of the paths computed by the PCE can be
   reported through the DISJOINTNESS-STATUS-TLV by setting the
   appropriate flags in the TLV.  While the DISJOINTNESS-CONFIGURATION-
   TLV defines the desired level of disjointness required by

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   configuration, the DISJOINTNESS-STATUS-TLV defines the achieved level
   of disjointness computed.

   There are some cases where the PCE may need to completely relax the
   disjointness constraint in order to provide a path to all the LSPs
   that are part of the association.  A mechanism that allows the PCE to
   fully relax a constraint is considered by the authors as more global
   to PCEP rather than linked to the disjointness use case.  As a
   consequence, it is considered as out of scope of the document.

6.  Security Considerations

   This document defines one new PCEP association type, which on itself
   does not add any new security concerns beyond those discussed in
   [RFC5440], [RFC8231] and [I-D.ietf-pce-association-group].  But,
   adding of a spurious LSP into the disjointness association group
   could lead to re-computation and set-up of all LSPs in the group,
   that could be used to overwhelm the PCE and the network.

   A spurious LSP can have flags that are inconsistent with those of the
   legitimate LSPs of the group and thus cause LSP allocation for the
   legitimate LSPs to fail with an error.

   Also, as stated in [I-D.ietf-pce-association-group], much of the
   information carried in the Disjointness Association object, as per
   this document is not extra sensitive.  It often reflects information
   that can also be derived from the LSP Database, but association
   provides a much easier grouping of related LSPs and messages.  The
   disjointness association could provide an adversary with the
   opportunity to eavesdrop on the relationship between the LSPs.

   Thus securing the PCEP session using Transport Layer Security (TLS)
   [RFC8253], as per the recommendations and best current practices in
   BCP 195 [RFC7525], is RECOMMENDED.

7.  IANA Considerations

7.1.  Association Type

   This document defines a new Association type, originally described in
   [I-D.ietf-pce-association-group].  IANA is requested to make the
   assignment of a new value for the sub-registry "ASSOCIATION Type
   Field" (request to be created in [I-D.ietf-pce-association-group]),
   as follows:

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     +------------------+-----------------------------+-------------+
     | Association type | Association Name            | Reference   |
     +------------------+-----------------------------+-------------+
     | TBD1             | Disjoint-group Association  | [This.I-D]  |
     +------------------+-----------------------------+-------------+

7.2.  PCEP TLVs

   This document defines the following new PCEP TLVs and the IANA is
   requested to make the assignment of new values for the existing "PCEP
   TLV Type Indicators" registry as follows:

       +----------+---------------------------------+-------------+
       | TLV Type | TLV Name                        | Reference   |
       +----------+---------------------------------+-------------+
       | TBD2     | Disjointness Configuration TLV  | [This.I-D]  |
       | TBD3     | Disjointness Status TLV         | [This.I-D]  |
       +----------+---------------------------------+-------------+

   This document requests that a new sub-registry, named "Disjointness
   Configuration TLV Flag Field", is created within the "Path
   Computation Element Protocol (PCEP) Numbers" registry to manage the
   Flag field in the Disjointness Configuration TLV.  New values are to
   be assigned by Standards Action [RFC8126].  Each bit should be
   tracked with the following qualities:

   o  Bit number (count from 0 as the most significant bit)

   o  Flag Name

   o  Reference

          +------------+-------------------------+-------------+
          | Bit Number | Name                    | Reference   |
          +------------+-------------------------+-------------+
          | 31         | L - Link Diverse        | [This.I-D]  |
          | 30         | N - Node Diverse        | [This.I-D]  |
          | 29         | S - SRLG Diverse        | [This.I-D]  |
          | 28         | P - Shortest Path       | [This.I-D]  |
          | 27         | T - Strict Disjointness | [This.I-D]  |
          +------------+-------------------------+-------------+

                  Table 1: Disjointness Configuration TLV

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7.3.  Objective Functions

   Three new Objective Functions have been defined in this document.
   IANA is requested to make the following allocations from the PCEP
   "Objective Function" sub-registry:

   +------------+----------------------------------------+-------------+
   | Code Point | Name                                   | Reference   |
   +------------+----------------------------------------+-------------+
   | TBD4       | Minimize the number of shared Links    | [This.I-D]  |
   |            | (MSL)                                  |             |
   | TBD5       | Minimize the number of shared SRLGs    | [This.I-D]  |
   |            | (MSS)                                  |             |
   | TBD6       | Minimize the number of shared Nodes    | [This.I-D]  |
   |            | (MSN)                                  |             |
   +------------+----------------------------------------+-------------+

7.4.  NO-PATH-VECTOR Bit Flags

   This documents defines new bits for the NO-PATH-VECTOR TLV in the
   "NO-PATH-VECTOR TLV Flag Field" sub-registry of the "Path Computation
   Element Protocol (PCEP) Numbers" registry.  IANA is requested to make
   the following allocation:

   +------------+-----------------------------------------+------------+
   | Bit Number | Name                                    | Reference  |
   +------------+-----------------------------------------+------------+
   | TBD7       | Disjoint path not found                 | [This.I-D] |
   | TBD8       | Requested disjoint computation not      | [This.I-D] |
   |            | supported                               |            |
   +------------+-----------------------------------------+------------+

                        Table 2: NO-PATH-VECTOR TLV

7.5.  PCEP-ERROR Codes

   This document defines new Error-Value within existing Error-Type
   related to path protection association.  IANA is requested to
   allocate new error values within the "PCEP-ERROR Object Error Types
   and Values" sub-registry of the PCEP Numbers registry, as follows:

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   +----------+-------------------------+------------------------------+
   | Error-   | Meaning                 | Reference                    |
   | Type     |                         |                              |
   +----------+-------------------------+------------------------------+
   | 6        | Mandatory Object        | [I-D.ietf-pce-association-gr |
   |          | missing                 | oup]                         |
   |          | Error-value=TBD8:       | [This.I-D]                   |
   |          | DISJOINTNESS-           |                              |
   |          | CONFIGURATION TLV       |                              |
   |          | missing                 |                              |
   | 10       | Reception of an invalid | [RFC5440]                    |
   |          | object                  |                              |
   |          | Error-value=TBD9:       | [This.I-D]                   |
   |          | Incompatible OF code    |                              |
   +----------+-------------------------+------------------------------+

8.  Manageability Considerations

8.1.  Control of Function and Policy

   An operator SHOULD be allowed to configure the disjointness
   association groups and disjoint parameters at the PCEP peers and
   associate it with the LSPs.  The Operator-configured Association
   Range MUST be allowed to be set by the operator.  Operator SHOULD be
   allowed to set the local policies to define various disjoint
   computational behavior at the PCE.

8.2.  Information and Data Models

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

8.3.  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].

8.4.  Verification of Correct Operations

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

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8.5.  Requirements on Other Protocols

   Mechanisms defined in this document do not imply any new requirements
   on other protocols.

8.6.  Impact on Network Operations

   Mechanisms defined in [RFC5440], Section 8.6 also apply to PCEP
   extensions defined in this document.  Additionally, a PCEP
   implementation SHOULD allow a limit to be placed on the number of
   LSPs that can belong to a DAG.

9.  Acknowledgments

   A special thanks to authors of [I-D.ietf-pce-association-group], this
   document borrow some of the text from it.  Authors would also like to
   thank Adrian Farrel and Julien Meuric for the valuable comments.

   Thanks to Emmanuel Baccelli for RTGDIR reviews.

   Thanks to Dale Worley for a detailed GENART review.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [RFC5541]  Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of
              Objective Functions in the Path Computation Element
              Communication Protocol (PCEP)", RFC 5541,
              DOI 10.17487/RFC5541, June 2009,
              <https://www.rfc-editor.org/info/rfc5541>.

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   [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., Minei, I., Medved, J., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for Stateful PCE", RFC 8231,
              DOI 10.17487/RFC8231, September 2017,
              <https://www.rfc-editor.org/info/rfc8231>.

   [I-D.ietf-pce-association-group]
              Minei, I., Crabbe, E., Sivabalan, S., Ananthakrishnan, H.,
              Dhody, D., and Y. Tanaka, "Path Computation Element
              Communication Protocol (PCEP) Extensions for Establishing
              Relationships Between Sets of Label Switched Paths
              (LSPs)", draft-ietf-pce-association-group-10 (work in
              progress), August 2019.

10.2.  Informative References

   [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,
              <https://www.rfc-editor.org/info/rfc4655>.

   [RFC6007]  Nishioka, I. and D. King, "Use of the Synchronization
              VECtor (SVEC) List for Synchronized Dependent Path
              Computations", RFC 6007, DOI 10.17487/RFC6007, September
              2010, <https://www.rfc-editor.org/info/rfc6007>.

   [RFC7470]  Zhang, F. and A. Farrel, "Conveying Vendor-Specific
              Constraints in the Path Computation Element Communication
              Protocol", RFC 7470, DOI 10.17487/RFC7470, March 2015,
              <https://www.rfc-editor.org/info/rfc7470>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC8253]  Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
              "PCEPS: Usage of TLS to Provide a Secure Transport for the
              Path Computation Element Communication Protocol (PCEP)",
              RFC 8253, DOI 10.17487/RFC8253, October 2017,
              <https://www.rfc-editor.org/info/rfc8253>.

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   [RFC8281]  Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for PCE-Initiated LSP Setup in a Stateful PCE
              Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
              <https://www.rfc-editor.org/info/rfc8281>.

   [I-D.ietf-pce-pcep-yang]
              Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A
              YANG Data Model for Path Computation Element
              Communications Protocol (PCEP)", draft-ietf-pce-pcep-
              yang-12 (work in progress), July 2019.

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Appendix A.  Contributor Addresses

   Dhruv Dhody
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore, Karnataka  560066
   India

   EMail: dhruv.ietf@gmail.com

Authors' Addresses

   Stephane Litkowski
   Cisco Systems, Inc.

   EMail: slitkows.ietf@gmail.com

   Siva Sivabalan
   Cisco Systems, Inc.
   2000 Innovation Drive
   Kanata, Ontario  K2K 3E8
   Canada

   EMail: msiva@cisco.com

   Colby Barth
   Juniper Networks

   EMail: cbarth@juniper.net

   Mahendra Singh Negi
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore, Karnataka  560066
   India

   EMail: mahend.ietf@gmail.com

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