PCEP LSP diversity: Path Computation Element Communication Protocol (PCEP) Extension for Label Switched Path (LSP) Diversity Constraint Signaling

Internet Engineering Task Force (IETF)                      S. Litkowski
Request for Comments: 8800                           Cisco Systems, Inc.
Category: Standards Track                                   S. Sivabalan
ISSN: 2070-1721                                        Ciena Corporation
                                                                C. Barth
                                                        Juniper Networks
                                                                 M. Negi
                                                           RtBrick India
                                                               July 2020


  Path Computation Element Communication Protocol (PCEP) Extension for
        Label Switched Path (LSP) Diversity Constraint Signaling

Abstract

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

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8800.

Copyright Notice

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

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   described in the Simplified BSD License.

Table of Contents

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

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].

   The 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.

   [RFC8697] introduces a generic mechanism to create a grouping of LSPs
   in the context of a PCE that 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
   [RFC4655].

   This document specifies a PCEP extension to signal that a set of LSPs
   in a particular group should use diverse (disjointed) paths,
   including the requested type of diversity.  Sections 3 and 4 describe
   the property and use of a Disjoint Association Group.  A PCC can use
   this extension to signal to a PCE that a particular LSP belongs to a
   particular Disjoint Association Group.  When a PCE receives LSP
   states belonging to the same Disjoint Association 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

   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

   PLSP-ID:  PCEP-specific identifier for the LSP

   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:

   *  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).

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

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

   *  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 different head
   end(s) and may terminate at the same or different tail end(s).

4.  Applicability

            _________________________________________
           /                                         \
          /        +------+                           \
         |         | 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 a 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 that is
   equally applicable to the active and passive modes of a stateful PCE
   is described in [RFC8697].

   Using the extension to PCEP defined in this document, the PCC uses
   the extension defined in [RFC8697] to indicate that a group of LSPs
   are required to be disjoint; such indication should include
   disjointness parameters like the type of disjointness, the Disjoint
   Association Group identifiers, and any customization parameters
   according to the configured local policy.

   The management of the Disjoint Association Group IDs will be a key
   point for the operator as the Association ID field is limited to
   65535.  The local configuration of the IPv4/IPv6 Association Source,
   or Global Association Source/Extended Association ID, can overcome
   this limitation, as described in [RFC8697].  When a PCC or PCE
   initiates all the LSPs in a particular Disjoint Association 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                        {DAG Y}
                 +-----+                ----------------> +-----+
      _ _ _ _ _ _| PCE |               |                  | PCE |
     |           +-----+               |      ----------> +-----+
     | PCInitiate                      |     |    PCReq/PCRpt
     |{DAG X}                          |     | {DAG Y}
     |                                 |     |
     |              .-----.            |     |         .-----.
     |             (       )           | +-----+      (       )
     |         .--(         )--.       | |PCC 2|--.--(         )--.
     V        (                 )      | +-----+ (                 )
   +---+     (                  )      |        (                  )
   |PCC|----(   (G)MPLS network )   +-----+    ( (G)MPLS network   )
   +---+    (                   )   |PCC 1|-----(                  )
   {DAG 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 Association
                          Group over PCEP Session

   The Disjoint Association Group within a PCEP messages is used for:

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

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

5.  Protocol Extension

5.1.  Association Group

   As per [RFC8697], LSPs are associated with other LSPs with which they
   interact by adding them to a common association group.  As described
   in [RFC8697], the association group is uniquely identified by the
   combination of the following 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, called "Disjoint
   Association" (2), based on the generic ASSOCIATION object.  This new
   Association type is also called "DAT", for "Disjoint Association
   Type".

   [RFC8697] specifies the mechanism for the capability advertisement of
   the Association types supported by a PCEP speaker by defining an
   ASSOC-Type-List TLV to be carried within an OPEN object.  This
   capability exchange for the DAT (2) MUST be done before using the
   disjoint association.  Thus, the PCEP speaker MUST include the DAT in
   the ASSOC-Type-List TLV and MUST receive the same from the PCEP peer
   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 [RFC8697], the association group could
   be manually created by the operator on the PCEP peers, and the LSPs
   belonging to this association are conveyed via PCEP messages to the
   PCEP peer; alternately, 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 are
   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 [RFC8697].)

   A Disjoint Association 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 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 the DISJOINTNESS-STATUS TLV (see Section 5.2).

   There are different types of disjointness identified by the flags (T,
   S, N, and L) in the DISJOINTNESS-CONFIGURATION TLV (see Section 5.2).
   All LSPs in a particular Disjoint Association Group MUST use the same
   combination of T, S, N, and L flags in the DISJOINTNESS-CONFIGURATION
   TLV.  If a PCEP peer receives a PCEP message for LSPs belonging to
   the same Disjoint Association Group but having an inconsistent
   combination of T, S, N, and L flags, the PCEP peer MUST NOT add the
   LSPs to the Disjoint Association 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 Association
   Groups, but in that case, the PCE SHOULD try to consider all the
   Disjoint Association 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 the association group and MUST return a PCErr with Error-
   Type 26 (Association Error) and Error-value 7 (Cannot join the
   association group).

5.2.  Disjoint TLVs

   The Disjoint Association Group (ASSOCIATION object with Association
   type = 2 for DAT) MUST carry the following TLV:

   *  DISJOINTNESS-CONFIGURATION TLV: Used to communicate some
      disjointness configuration parameters.  This is applicable for all
      PCEP messages that include DAG.

   In addition, the Disjoint Association Group (ASSOCIATION object with
   Association type = 2 for DAT) MAY carry the following TLVs:

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

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

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

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

    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 = 46             |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Flags                               |T|P|S|N|L|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 3: DISJOINTNESS-CONFIGURATION TLV

   Type:  46

   Length:  Fixed value of 4 bytes.

   Flags:

      L (Link Diverse) bit:  When set, this indicates that the computed
         paths within the Disjoint Association Group MUST NOT have any
         link in common.

      N (Node Diverse) bit:  When set, this indicates that the computed
         paths within the Disjoint Association Group MUST NOT have any
         node in common.

      S (SRLG Diverse) bit:  When set, this indicates that the computed
         paths within the Disjoint Association 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; 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, the PCE MUST return no path for LSPs that could not
         be disjoint.  When unset, the PCE is allowed to relax
         disjointness by either applying a requested objective function
         (cf. Section 5.3) 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).
         See Section 5.6 for further details.

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

   If a PCEP speaker receives a Disjoint Association Group (ASSOCIATION
   object with Association type = 2 for DAT) without the DISJOINTNESS-
   CONFIGURATION TLV, it SHOULD reply with a PCErr Error-Type 6
   (Mandatory Object missing) and Error-value 15 (DISJOINTNESS-
   CONFIGURATION TLV missing).

   The DISJOINTNESS-STATUS TLV uses the same format as the DISJOINTNESS-
   CONFIGURATION TLV with a different type 47 (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 the DISJOINTNESS-STATUS TLV MUST be specified in the document that
   specifies the flag in the 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 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
   defined:

   MSL

      Name:  Minimize the number of Shared (common) Links.
      Objective Function Code:  15
      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)}.
         -  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 {Lci,(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:  16
      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 {Sci,(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:  17
      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 {Nci,(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 first
   OF-code inside the OF object MUST be one of the disjoint OFs defined
   in this document.  If this condition is not met, the PCEP speaker
   MUST respond with a PCErr message with Error-Type 10 (Reception of an
   invalid object) and Error-value 32 (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, which specifies
   the list of synchronized requests that can be either dependent or
   independent.  The SVEC object identifies the relationship between the
   set of path computation requests, identified by 'Request-ID-number'
   in the RP (Request Parameters) object.  [RFC6007] further clarifies
   the use of the SVEC list for synchronized path computations when
   computing dependent requests and describes 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 Path Computation Request (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 (including the flags set inside 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:

   *  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 and LSP2.

   *  PCReq with SVEC object with link-diverse bit=1 (LSP1,LSP2) and DAG
      with L=1 (LSP1,LSP3) - link-diverse paths between LSP1 and LSP2
      and between LSP1 and LSP3.  If the DAG is part of the stateful
      database, any future change in LSP3 will have an impact on LSP1.
      But any future change in LSP2 will have no impact on LSP1, as LSP2
      is part of SVEC object (which is considered once on receipt of the
      PCReq message only).

5.4.1.  SVEC and OF

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

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

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

   *  SVEC object with domain-diverse bit=1 [RFC8685]; node-diverse
      bit=1, and OF=MSS - full domain and node diversity with as much
      SRLG diversity as possible.

   *  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 satisfy all constraints and objective
   functions first without considering the diversity constraint.

   This means that an LSP with the P flag set should be placed first, as
   if the disjointness constraint has not been configured, while the
   other LSPs in the association with the P flag unset should be placed
   by taking into account the disjointness constraint.  Setting the P
   flag changes the relationship between LSPs to a one-sided
   relationship (LSP 1 with P=0 depends on LSP 2 with P=1, but LSP 2
   with P=1 does not depend on LSP 1 with P=0).  Multiple LSPs in the
   same Disjoint Association 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 Association Group (for
   instance, the primary path), while other paths are allowed to be
   costlier (compared 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             /
           \_________________________________________/

            Figure 4: Example Topology with Six Internal Routers

   Note: In Figure 4, the cost of all the links is 1, unless explicitly
   marked otherwise.

   In the figure above, a customer has two dual-homed sites (CE1/CE3 and
   CE2/CE4).  Let us consider that this customer wants two link disjoint
   paths between the two sites.  Due to physical meshing, the customer
   wants to use CE1 and CE2 as the 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
   link disjoint LSPs together, leading to PE1->PE2 using 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
   other LSP should be link 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 a 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 finding a disjoint path.  In the
   example above, if we consider that router R5 is down and if PE1->PE2
   has the P flag set, there is no room available to place PE3->PE4 (the
   link disjointness constraint cannot be fulfilled).  If PE1->PE2 has
   the P flag unset, the algorithm may be able to place PE1->PE2 on the
   R1->R2 link leaving room for PE3->PE4 using the R3->R4 link.  When
   using the 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
         | +------+                          +------+  |
         |                                             |
          \                                           /
           \_________________________________________/

           Figure 5: Example Topology with Four Internal Routers

   Note: In Figure 5, the cost of all the links is 1, unless explicitly
   marked otherwise.

   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 link disjoint and may use a costlier path.

   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 link 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 the 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), if disjointness cannot
   be ensured for one or more LSPs, the PCE MUST reply to a PCReq with a
   PCRep message containing a NO-PATH object.  In case of a 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 a network event leading to an impossible strict
   disjointness, the PCE MUST send a PCUpd message containing an empty
   Explicit Route Object (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 Flags field of the NO-PATH-VECTOR
   TLV:

   *  bit 11: When set, the PCE indicates that it could not find a
      disjoint path for this LSP.

   *  bit 10: 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
   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 out of scope of the document.  See
   [PCE-OPTIONAL] for a proposed mechanism.

6.  Security Considerations

   This document defines one new PCEP Association type, which by itself
   does not add any new security concerns beyond those discussed in
   [RFC5440], [RFC8231], [RFC7470], and [RFC8697].  But adding of a
   spurious LSP into the Disjoint Association Group could lead to
   recomputation and setup of all LSPs in the group, which 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, certain combinations of
   flags (notably, the 'T' bit) can result in conflicts that cannot be
   resolved.

   Also, as stated in [RFC8697], much of the information carried in the
   ASSOCIATION object reflects information that can also be derived from
   the LSP database, but association provides a much easier grouping of
   related LSPs and messages.  This holds true for the DAT as well;
   thus, this could provide an adversary with the opportunity to
   eavesdrop on the relationship between the LSPs and understand the
   network topology.

   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
   [RFC8697].  IANA has assigned the following new value in the
   "ASSOCIATION Type Field" subregistry [RFC8697] within the "Path
   Computation Element Protocol (PCEP) Numbers" registry:

                +======+======================+===========+
                | Type | Name                 | Reference |
                +======+======================+===========+
                | 2    | Disjoint Association | RFC 8800  |
                +------+----------------------+-----------+

                      Table 1: ASSOCIATION Type Field

7.2.  PCEP TLVs

   This document defines two new PCEP TLVs.  IANA has assigned the
   following values in the "PCEP TLV Type Indicators" subregistry within
   the "Path Computation Element Protocol (PCEP) Numbers" registry:

           +==========+============================+===========+
           | TLV Type | TLV Name                   | Reference |
           +==========+============================+===========+
           | 46       | DISJOINTNESS-CONFIGURATION | RFC 8800  |
           +----------+----------------------------+-----------+
           | 47       | DISJOINTNESS-STATUS        | RFC 8800  |
           +----------+----------------------------+-----------+

                     Table 2: PCEP TLV Type Indicators

   IANA has created a new subregistry, named "DISJOINTNESS-CONFIGURATION
   TLV Flag Field", within the "Path Computation Element Protocol (PCEP)
   Numbers" registry to manage the Flags 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:

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

   *  Flag Name

   *  Reference

   The initial contents of this subregistry are shown below:

              +======+=========================+===========+
              | Bit  | Name                    | Reference |
              +======+=========================+===========+
              | 31   | L - Link Diverse        | RFC 8800  |
              +------+-------------------------+-----------+
              | 30   | N - Node Diverse        | RFC 8800  |
              +------+-------------------------+-----------+
              | 29   | S - SRLG Diverse        | RFC 8800  |
              +------+-------------------------+-----------+
              | 28   | P - Shortest Path       | RFC 8800  |
              +------+-------------------------+-----------+
              | 27   | T - Strict Disjointness | RFC 8800  |
              +------+-------------------------+-----------+
              | 0-26 | Unassigned              |           |
              +------+-------------------------+-----------+

                 Table 3: DISJOINTNESS-CONFIGURATION TLV
                                Flag Field

7.3.  Objective Functions

   This document defines three new objective functions.  IANA has made
   the following allocations in the "Objective Function" subregistry
   within the "Path Computation Element Protocol (PCEP) Numbers"
   registry:

            +============+=======================+===========+
            | Code Point | Name                  | Reference |
            +============+=======================+===========+
            | 15         | Minimize the number   | RFC 8800  |
            |            | of Shared Links (MSL) |           |
            +------------+-----------------------+-----------+
            | 16         | Minimize the number   | RFC 8800  |
            |            | of Shared SRLGs (MSS) |           |
            +------------+-----------------------+-----------+
            | 17         | Minimize the number   | RFC 8800  |
            |            | of Shared Nodes (MSN) |           |
            +------------+-----------------------+-----------+

                       Table 4: Objective Function

7.4.  NO-PATH-VECTOR Bit Flags

   This document defines new bits for the NO-PATH-VECTOR TLV in the "NO-
   PATH-VECTOR TLV Flag Field" subregistry of the "Path Computation
   Element Protocol (PCEP) Numbers" registry.  IANA has made the
   following allocations:

          +============+===========================+===========+
          | Bit Number | Name                      | Reference |
          +============+===========================+===========+
          | 11         | Disjoint path not found   | RFC 8800  |
          +------------+---------------------------+-----------+
          | 10         | Requested disjoint        | RFC 8800  |
          |            | computation not supported |           |
          +------------+---------------------------+-----------+

                  Table 5: NO-PATH-VECTOR TLV Flag Field

7.5.  PCEP-ERROR Codes

   This document defines two new Error-values within existing Error-
   Types related to disjoint association.  IANA has allocated the
   following new Error-values in the "PCEP-ERROR Object Error Types and
   Values" subregistry within the "Path Computation Element Protocol
   (PCEP) Numbers" registry:

    +============+===========+============================+===========+
    | Error-Type | Meaning   | Error-value                | Reference |
    +============+===========+============================+===========+
    | 6          | Mandatory |                            | [RFC5440] |
    |            | Object    |                            |           |
    |            | missing   |                            |           |
    +------------+-----------+----------------------------+-----------+
    |            |           | 15: DISJOINTNESS-          | RFC 8800  |
    |            |           | CONFIGURATION TLV missing  |           |
    +------------+-----------+----------------------------+-----------+
    | 10         | Reception |                            | [RFC5440] |
    |            | of an     |                            |           |
    |            | invalid   |                            |           |
    |            | object    |                            |           |
    +------------+-----------+----------------------------+-----------+
    |            |           | 32: Incompatible OF code   | RFC 8800  |
    +------------+-----------+----------------------------+-----------+

             Table 6: PCEP-ERROR Object Error Types and Values

8.  Manageability Considerations

8.1.  Control of Function and Policy

   An operator SHOULD be allowed to configure the Disjoint Association
   Groups and disjoint parameters at the PCEP peers and associate them
   with the LSPs.  The operator MUST be allowed to set the Operator-
   configured Association Range.  The 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.  Furthermore,
   implementations SHOULD allow to view disjoint associations reported
   by each peer and the current set of LSPs in this association.  The
   PCEP YANG module [PCEP-YANG] includes association group 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

   Apart from the operation verification requirements already listed in
   [RFC5440], a PCEP implementation SHOULD provide parameters related to
   disjoint path computation, such as number of DAG, number of disjoint
   path computation failures, etc.  A PCEP implementation SHOULD log
   failure events (e.g., incompatible Flags).

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 Section 8.6 of [RFC5440] 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.  References

9.1.  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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [RFC8685]  Zhang, F., Zhao, Q., Gonzalez de Dios, O., Casellas, R.,
              and D. King, "Path Computation Element Communication
              Protocol (PCEP) Extensions for the Hierarchical Path
              Computation Element (H-PCE) Architecture", RFC 8685,
              DOI 10.17487/RFC8685, December 2019,
              <https://www.rfc-editor.org/info/rfc8685>.

   [RFC8697]  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)", RFC 8697, DOI 10.17487/RFC8697, January 2020,
              <https://www.rfc-editor.org/info/rfc8697>.

9.2.  Informative References

   [PCE-OPTIONAL]
              Li, C., Zheng, H., and S. Litkowski, "Extension for
              Stateful PCE to allow Optional Processing of PCEP
              Objects", Work in Progress, Internet-Draft, draft-dhody-
              pce-stateful-pce-optional-06, 9 July 2020,
              <https://tools.ietf.org/html/draft-dhody-pce-stateful-pce-
              optional-06>.

   [PCEP-YANG]
              Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A
              YANG Data Model for Path Computation Element
              Communications Protocol (PCEP)", Work in Progress,
              Internet-Draft, draft-ietf-pce-pcep-yang-14, 7 July 2020,
              <https://tools.ietf.org/html/draft-ietf-pce-pcep-yang-14>.

   [RFC4655]  Farrel, A., Vasseur, J.-P., 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>.

   [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>.

   [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>.

Acknowledgments

   A special thanks to the authors of [RFC8697]; this document borrows
   some text from it.  The authors would also like to thank Adrian
   Farrel and Julien Meuric for the valuable comments.

   Thanks to Emmanuel Baccelli for the RTGDIR review.

   Thanks to Dale Worley for a detailed GENART review.

   Thanks to Alvaro Retana, Benjamin Kaduk, Suresh Krishnan, Roman
   Danyliw, Alissa Cooper, and Éric Vyncke for the IESG review.

Contributors

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

   Email: dhruv.ietf@gmail.com


Authors' Addresses

   Stephane Litkowski
   Cisco Systems, Inc.

   Email: slitkows.ietf@gmail.com


   Siva Sivabalan
   Ciena Corporation

   Email: msiva282@gmail.com


   Colby Barth
   Juniper Networks

   Email: cbarth@juniper.net


   Mahendra Singh Negi
   RtBrick India
   N-17L, Floor-1, 18th Cross Rd, HSR Layout Sector-3
   Bangalore 560102
   Karnataka
   India

   Email: mahend.ietf@gmail.com