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Versions: (draft-litkowski-pce-association-diversity) 00 01 02 03 04 05 06 07

PCE Working Group                                           S. Litkowski
Internet-Draft                                                    Orange
Intended status: Standards Track                            S. Sivabalan
Expires: December 6, 2019                            Cisco Systems, Inc.
                                                                C. Barth
                                                        Juniper Networks
                                                                 M. Negi
                                                     Huawei Technologies
                                                            June 4, 2019


  Path Computation Element communication Protocol (PCEP) extension for
                   signaling LSP diversity constraint
                draft-ietf-pce-association-diversity-07

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 disjoint-group, thus the PCE knows that
   LSPs in the same group needs 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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on December 6, 2019.

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.  Protocol extension  . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Association group . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Disjoint TLVs . . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  Relationship to SVEC  . . . . . . . . . . . . . . . . . .  10
     4.4.  Disjointness objective functions  . . . . . . . . . . . .  10
     4.5.  P-flag considerations . . . . . . . . . . . . . . . . . .  12
     4.6.  Disjointness computation issues . . . . . . . . . . . . .  14
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     6.1.  Association Type  . . . . . . . . . . . . . . . . . . . .  16
     6.2.  PCEP TLVs . . . . . . . . . . . . . . . . . . . . . . . .  16
     6.3.  Objective Functions . . . . . . . . . . . . . . . . . . .  17
     6.4.  NO-PATH-VECTOR Bit Flags  . . . . . . . . . . . . . . . .  17
     6.5.  PCEP-ERROR codes  . . . . . . . . . . . . . . . . . . . .  18
   7.  Manageability Considerations  . . . . . . . . . . . . . . . .  18
     7.1.  Control of Function and Policy  . . . . . . . . . . . . .  18
     7.2.  Information and Data Models . . . . . . . . . . . . . . .  18
     7.3.  Liveness Detection and Monitoring . . . . . . . . . . . .  18
     7.4.  Verify Correct Operations . . . . . . . . . . . . . . . .  19
     7.5.  Requirements On Other Protocols . . . . . . . . . . . . .  19
     7.6.  Impact On Network Operations  . . . . . . . . . . . . . .  19
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Appendix A.  Contributor Addresses  . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22








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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 particular
   group of LSPs 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 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.

   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.



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   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
   IP/MPLS core.  The customer may use those paths as primary/backup or
   active/active.

   Different level 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).














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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, consider that the customer wants to have two
   disjoint paths between CE1/CE2 and CE3/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 4.3 describes the relationship between the disjoint
   association group 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 uses 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 PCEP, the PCC could indicate that the disjoint path computation
   is required, such indication should include disjointness parameters
   such as the type of disjointness, the disjoint group identifiers, and
   any customization parameters according to the configured local
   policy.  As mentioned previously, the extension described in




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   [I-D.ietf-pce-association-group] is well suited to associate a set of
   LSPs with a particular disjoint-group.

   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 should allow 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,
   association source could be the PCE address or any other unique value
   to identify the disjoint association group.



             Initiate Disjoint LSPs
                      |
                      |                       PCReq/PCRpt
                      V                   {Disjoint-group Y}
                   +-----+                ----------------> +-----+
        _ _ _ _ _ _| PCE |               |                  | PCE |
       |           +-----+               |      ----------> +-----+
       | PCInitiate                      |     |    PCReq/PCRpt
       |{Disjoint-group X}               |     | {Disjoint-group Y}
       |                                 |     |
       |              .-----.            |     |         .-----.
       |             (       )           |  +----+      (       )
       |         .--(         )--.       |  |PE 1|--.--(         )--.
       V        (                 )      |  +----+ (                 )
     +---+     (                  )      |        (                  )
     |PCC|----(   (G)MPLS network )    +----+    ( (G)MPLS network   )
     +---+     (                  )    |PE 3|-----(                  )
   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 may have two purpose:





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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.

4.  Protocol extension

4.1.  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.  The Association parameters, as described in
   [I-D.ietf-pce-association-group] as the combination of the mandatory
   fields Association type, Association ID and Association Source in the
   ASSOCIATION object, that uniquely identify the association group
   belonging to this association.  If the optional TLVs - Global
   Association Source or Extended Association ID are included, then they
   are included in combination with mandatory fields to uniquely
   identifying the association group.  This document defines a new
   Association type, based on the generic Association object -

   o  Association type = TBD1 ("Disjointness Association Type") for
      Disjoint Association Group (DAG).

   [I-D.ietf-pce-association-group] specify 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 association type
   described in this document (i.e.  Disjointness Association Type) MUST
   be done before using the disjointness association.  Thus the PCEP
   speaker MUST include the Disjointness Association Type (TBD1) in the
   ASSOC-Type-List TLV before using the disjoint association group (DAG)
   in the PCEP messages.

   This association type is considered to be both dynamic and operator-
   configured in nature.  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
   [I-D.ietf-pce-association-group].)




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   A disjoint group can have two or more LSPs.  But a PCE may be limited
   in how many 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 is informed to the PCC.

   Associating a particular LSP to multiple disjoint groups is
   authorized from a protocol perspective, however there is no assurance
   that the PCE will be able to compute properly the multi-disjointness
   constraint.

4.2.  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 4.4.

   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 = TBD2           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Flags                               |T|P|S|N|L|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type: TBD2.

   Length: Fixed value of 4 bytes.



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      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 an LSP with P flag set should be
         placed as if the disjointness constraint has not been
         configured, while the other LSP 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 others LSPs in the
         group that have the P-flag unset.

      *  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, PCE is allowed to relax disjointness
         by using either applying a requested objective function or any
         other behavior if no objective function is requested (e.g.:
         using a lower disjoint type (link instead of node) or relaxing
         disjointness constraint fully).

      *  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-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 flags
   L, N, and S are set based if the computed path meet the disjointness



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   criteria.  The flag P is set to indicate that the computed path is
   the shortest and the flag T has no meaning in the DISJOINTNESS-
   STATUS-TLV and MUST NOT be set while sending and ignored on receipt.

   Any new flag defined for the DISJOINTNESS-CONFIGURATION-TLV is be
   automatically applicable to the DISJOINTNESS-STATUS-TLV.

4.3.  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 identify 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 request 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 SVEC and
   ASSOCIATION object to identify the related path computation request
   as well as the diversity association group.  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 SVEC object.  The PCE would consider both the objects as per
   the processing rules and aim to find a path that meets both these
   constraints.  In case no such path is possible (or the constraints
   are incompatible), the PCE MUST send a path computation reply (PCRep)
   with NO-PATH object indcating path computation failure as per
   [RFC5440].

4.4.  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.  The PCEP OF-List TLV allow 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:



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   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.

   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.

   MSN

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

   * Objective Function Code:  TBD6

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

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

   [RFC5440] uses SVEC diversity flag for node, link or SRLG to describe
   the potential disjointness between the set of path computation
   requests used in PCEP protocol.

   This document defines three new OF codes to maximize diversity as
   much as possible, in other words, minimize the common shared
   resources (Node,Link or SRLG) between a set of paths.

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

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





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   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.

4.5.  P-flag considerations

   As mentioned in Section 4.2, 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 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 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
   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



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   In the figure above, a customer has two dual homed sites (CE1/CE3 and
   CE2/CE4).  Consider, 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
   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 a simple expression 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 PE->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.










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            _________________________________________
           /                                         \
          /        +------+                           \
         |         | 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.

   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.

4.6.  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-bit is set (Strict disjointness requested), if
   disjointness cannot be ensured for one or more LSPs, the PCE SHOULD
   reply with a PCUpd message containing an empty ERO.  In addition to



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   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-bit is unset, the PCE is allowed to reduce the required
   level of disjointness.  The actual level of disjointness 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 expected level of disjointness required
   by configuration, the DISJOINTNESS-STATUS-TLV defines the actual
   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.

   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 SHOULD NOT try to add the LSPs in disjoint group and SHOULD
   reply with a PCErr with Error-type 26 (Association Error) and Error-
   Value 6 (Association information mismatch).

5.  Security Considerations

   This document defines one new type for association, which do not add
   any new security concerns beyond those discussed in [RFC5440],
   [RFC8231] and [I-D.ietf-pce-association-group] in itself.

   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 provides an adversary with the
   opportunity to eavesdrop on the relationship between the LSPs.  Thus
   securing the PCEP session using Transport Layer Security (TLS)



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   [RFC8253], as per the recommendations and best current practices in
   [RFC7525], is RECOMMENDED.

6.  IANA Considerations

6.1.  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:

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

6.2.  PCEP TLVs

   This document defines 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







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          +------------+-------------------------+-------------+
          | 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

6.3.  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)                                  |             |
   +------------+----------------------------------------+-------------+

6.4.  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






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6.5.  PCEP-ERROR codes

   This document defines new Error-Type and Error-Value 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:

   +----------+-------------------------+------------------------------+
   | 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 codes   |                              |
   +----------+-------------------------+------------------------------+

7.  Manageability Considerations

7.1.  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.

7.2.  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.

7.3.  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].



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7.4.  Verify Correct Operations

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

7.5.  Requirements On Other Protocols

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

7.6.  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 disjoint association group.

8.  Acknowledgments

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

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>.

   [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, "PCEP Extensions for
              Establishing Relationships Between Sets of LSPs", draft-
              ietf-pce-association-group-09 (work in progress), April
              2019.

9.2.  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-11 (work in progress), March 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
   Orange

   EMail: stephane.litkowski@orange.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: mahendrasingh@huawei.com











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