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PCE Working Group                                           M. Koldychev
Internet-Draft                                              S. Sivabalan
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: May 4, 2020                                            C. Barth
                                                  Juniper Networks, Inc.
                                                                   C. Li
                                                     Huawei Technologies
                                                              H. Bidgoli
                                                                   Nokia
                                                       November 01, 2019


    PCEP extension to support Segment Routing Policy Candidate Paths
              draft-barth-pce-segment-routing-policy-cp-04

Abstract

   This document introduces a mechanism to specify a Segment Routing
   (SR) policy, as a collection of SR candidate paths.  An SR policy is
   identified by <headend, color, end-point> tuple.  An SR policy can
   contain one or more candidate paths where each candidate path is
   identified in PCEP via an PLSP-ID.  This document proposes extension
   to PCEP to support association among candidate paths of a given SR
   policy.  The mechanism proposed in this document is applicable to
   both MPLS and IPv6 data planes of SR.

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.

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."



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   This Internet-Draft will expire on May 4, 2020.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Group Candidate Paths belonging to the same SR policy . .   5
     3.2.  Instantiation of SR policy candidate paths  . . . . . . .   5
     3.3.  Avoid computing lower preference candidate paths  . . . .   5
     3.4.  Minimal signaling overhead  . . . . . . . . . . . . . . .   5
   4.  Procedure . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Choice of Association Parameters  . . . . . . . . . . . .   7
     4.3.  Multiple Optimization Objectives and Constraints  . . . .   8
   5.  SR Policy Association Group . . . . . . . . . . . . . . . . .   8
     5.1.  SR Policy Identifiers TLV . . . . . . . . . . . . . . . .   9
     5.2.  SR Policy Name TLV  . . . . . . . . . . . . . . . . . . .  10
     5.3.  SR Policy Candidate Path Identifiers TLV  . . . . . . . .  10
     5.4.  SR Policy Candidate Path Preference TLV . . . . . . . . .  11
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  PCC Initiated SR Policy with single candidate-path  . . .  12
     6.2.  PCC Initiated SR Policy with multiple candidate-paths . .  12
     6.3.  PCE Initiated SR Policy with single candidate-path  . . .  13
     6.4.  PCE Initiated SR Policy with multiple candidate-paths . .  14
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Association Type  . . . . . . . . . . . . . . . . . . . .  14
     7.2.  PCEP Errors . . . . . . . . . . . . . . . . . . . . . . .  15
     7.3.  SRPAG TLVs  . . . . . . . . . . . . . . . . . . . . . . .  15
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   9.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  16
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  16



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     10.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Contributors . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   Path Computation Element (PCE) Communication Protocol (PCEP)
   [RFC5440] 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 the Stateful PCE Model [RFC8231] describes a set
   of extensions to PCEP to enable active control of Multiprotocol Label
   Switching Traffic Engineering (MPLS-TE) and Generalized MPLS (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 dynamic centralized
   control of a network.

   PCEP Extensions for Segment Routing [I-D.ietf-pce-segment-routing]
   specifies extensions to the Path Computation Element Protocol (PCEP)
   that allow a stateful PCE to compute and initiate Traffic Engineering
   (TE) paths, as well as a PCC to request a path subject to certain
   constraint(s) and optimization criteria in SR networks.

   PCEP Extensions for Establishing Relationships Between Sets of LSPs
   [I-D.ietf-pce-association-group] introduces a generic mechanism to
   create a grouping of LSPs 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
   stateful PCE (active and passive modes) and stateless PCE.

   Segment Routing Policy for Traffic Engineering
   [I-D.ietf-spring-segment-routing-policy] details the concepts of SR
   Policy and approaches to steering traffic into an SR Policy.

   An SR policy contains one or more candidate paths where one or more
   such paths can be computed via PCE.  This document specifies PCEP
   extensions to signal additional information to map candidate paths to
   their SR policies.  Each candidate path maps to a unique PLSP-ID in
   PCEP.  By associating multiple candidate paths together, a PCE
   becomes aware of the hierarchical structure of an SR policy.  Thus
   the PCE can take computation and control decisions about the
   candidate paths, with the additional knowledge that these candidate
   paths belong to the same SR policy.  This is accomplished via the use
   of the existing PCEP Association object, by defining a new
   association type specifically for associating SR candidate paths into
   a single SR policy.



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   [Editor's Note- Currently it is assumed that each candidate path has
   only one ERO (SID-List) within the scope of this document.  A future
   update or another document will deal with a way to allow multiple
   ERO/SID-Lists for a candidate path within PCEP.]

2.  Terminology

   The following terminologies are used in this document:

   Endpoint:  The IPv4 or IPv6 endpoint address of the SR policy in
      question, as described in
      [I-D.ietf-spring-segment-routing-policy].

   Association parameters:  As described in
      [I-D.ietf-pce-association-group], the combination of the mandatory
      fields Association type, Association ID and Association Source in
      the ASSOCIATION object uniquely identify the association group.
      If the optional TLVs - Global Association Source or Extended
      Association ID are included, then they MUST be included in
      combination with mandatory fields to uniquely identify the
      association group.

   Association information:  As described in
      [I-D.ietf-pce-association-group], the ASSOCIATION object could
      also include other optional TLVs based on the association types,
      that provides 'information' related to the association type.

   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 Protocol.

   PCEP Tunnel:  The entity identified by the PLSP-ID, as per
      [I-D.koldychev-pce-operational].

3.  Motivation

   The new Association Type (SR Policy Association) and the new TLVs for
   the ASSOCIATION object, defined in this document, allow a PCEP peer
   to exchange additional parameters of SR candidate paths and of their
   parent SR policy.  For the SR policy, the parameters are: color and
   endpoint.  For the candidate path, the parameters are: protocol
   origin, originator, discriminator and preference.



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   [I-D.ietf-spring-segment-routing-policy] describes the concept of SR
   Policy and these parameters.

   The motivation for signaling these parameters is summarized in the
   following subsections.

3.1.  Group Candidate Paths belonging to the same SR policy

   Since each candidate path of an SR policy appears as a different LSP
   (identified via a PLSP-ID) in PCEP, it is useful to group together
   all the candidate paths that belong to the same SR policy.
   Furthermore, it is useful for the PCE to have knowledge of the SR
   candidate path parameters such as color, protocol origin,
   discriminator, and preference.

3.2.  Instantiation of SR policy candidate paths

   A PCE may want to instantiate one or more candidate paths on the PCC,
   as specified in [RFC8281].  In this scenario, the PCE needs to signal
   to a PCC <headend, color, end-point, originator, discriminator,
   preference> tuple using which the PCC can instantiate a candidate
   path for the SR policy identified.  Current PCEP standards (as of the
   time of this writing) do not provide a way to signal color and
   preference.  Although end-point can be signaled via the PCEP END-
   POINTS object, this object may not be suitable because the end-point
   to which the path is computed is not required to be the same IPv4/
   IPv6 address as the actual endpoint of the SR policy.  Thus, a
   separate way to specify SR policy's end-point is provided in this
   document.

3.3.  Avoid computing lower preference candidate paths

   When a PCE knows that a given set of candidate paths all belong to
   the same SR policy, then path computation MAY be done on only the
   highest preference candidate-path(s).  Path computation for lower
   preference paths is not necessary if one or two higher preference
   paths are already computed.  Since computing their paths will not
   affect traffic steering, it MAY be postponed until the higher
   preference paths become invalid, thus saving computation resources on
   the PCE.

3.4.  Minimal signaling overhead

   When an SR policy contains multiple candidate paths computed by a
   PCE, such candidate paths can be created, updated and deleted
   independently of each other.  This is achieved by making each
   candidate path correspond to a unique LSP (identified via PLSP-ID).
   For example, if an SR policy has 4 candidate paths, then if the PCE



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   wants to update one of those candidate paths, only one set of PCUpd
   and PCRpt messages needs to be exchanged.

4.  Procedure

4.1.  Overview

   As per [I-D.ietf-pce-association-group], LSPs are placed into an
   association group.  As per [I-D.koldychev-pce-operational], LSPs are
   contained in PCEP Tunnels and a PCEP Tunnel is contained in an
   Association if all of its LSPs are in that Association.

   PCEP Tunnels naturally map to SR Candidate Paths and PCEP
   Associations naturally map to SR Policies.  Definition of these
   mappings is the central purpose of this document.

   The mapping between PCEP Associations and SR Policies is always one-
   to-one.  However, the mapping between PCEP Tunnels and SR Candidate
   Paths may be either one-to-one, or many-to-one.  The mapping is one-
   to-one when the SR Candidate Path has only a single constraint and
   optimization objective.  The mapping is many-to-one when the SR
   Candidate Path has multiple constraints and optimization objectives.
   For more details on multiple optimization objectives and constraints,
   see Section 4.3.

   [Editor's Note - Segment-lists within a candidate path are not
   represented by different PCEP Tunnels.  The subject of encoding
   multiple segment lists within a candidate path is left to a future
   document and is not specified in this document.  It is not a good
   idea to have each segment-list correspond to a different Tunnel,
   because when the PCC wants to get a path, it must know in advance how
   many multipaths (i.e., segment-lists) there will be and create that
   many Tunnels.  For example, if the PCC supports 32 multipaths, then
   it must delegate 32 Tunnels for every candidate path, which may not
   be scalable.]

   A new Association Type is defined in this document, based on the
   generic ASSOCIATION object.  Association type = TBD1 "SR Policy
   Association Type" for SR Policy Association Group (SRPAG).  The SRPAG
   Association is only meant to be used for SR LSPs and with PCEP peers
   which advertise SR capability.

   An Association object of SRPAG group contains TLVs that carry
   Association Information.  The association information can be
   subdivided into three parts: Policy identifiers, Candidate path
   identifiers, and Candidate path attributes.





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   Policy Identifiers uniquely identify the SR policy to which a given
   LSP belongs, within the context of the head-end.  Policy Identifiers
   MUST be the same for all candidate paths in the same SRPAG.  Policy
   Identifiers MUST NOT change for a given LSP during its lifetime.
   Policy Identifiers MUST be different for different SRPAG
   associations.  When these rules are not satisfied, the PCE MUST send
   a PCErr message with Error Code = 26 "Association Error", Error Type
   = TBD6 "Conflicting SRPAG TLV".  Policy Identifiers consist of:

   o  Color of SR policy.

   o  End-point of SR policy.

   o  Optionally, the policy name.

   Candidate Path Identifiers uniquely identify the SR candidate path
   within the context of an SR policy.  Candidate path Identifiers MUST
   NOT change for a given LSP during its lifetime.  Candidate path
   Identifiers MUST be different for different LSPs within the same
   SRPAG.  When these rules are not satisfied, the PCE MUST send a PCErr
   message with Error Code = 26 "Association Error", Error Type = TBD6
   "Conflicting SRPAG TLV".  Candidate path Identifiers consist of:

   o  Protocol Origin of candidate path.

   o  Originator of candidate path.

   o  Discriminator of candidate path.

   Candidate Path Attributes MUST NOT be used to identify the candidate
   path.  Candidate path attributes carry additional information about
   the candidate path and MAY change during the lifetime of the LSP.
   Candidate path Attributes consist of:

   o  Preference of candidate path.

   As per the processing rules specified in section 5.4 of
   [I-D.ietf-pce-association-group], if a PCEP speaker does not support
   the SRPAG association type, it MUST return a PCErr message with
   Error-Type 26 (Early allocation by IANA) "Association Error" and
   Error-Value 1 "Association-type is not supported".  Please note that
   the corresponding PCEP session is not reset.

4.2.  Choice of Association Parameters

   The Association Parameters (see Section 2) uniquely identify the
   Association.  In this section, we describe how these are to be set.




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   The Association Source MUST be set to the PCC's address.  This
   applies for both PCC-initiated and PCE-initiated candidate paths.
   The reasoning for this is that if different PCEs could set their own
   Association Source, then the candidate paths instantiated by
   different PCEs would by definition be in different PCEP Associations,
   which contradicts our requirement that the SR Policy is represented
   by an Association.

   The Association ID MUST be chosen by the PCC when the SR policy is
   allocated.  In PCRpt messages from the PCC, the Association ID MUST
   be set to the unique value that was allocated by the PCC at the time
   of policy creation.  In PCInit messages from the PCE, the Association
   ID MUST be set to the reserved value 0xFFFF, which indicates that the
   PCE is asking the PCC to choose an ID value.  The PCE MUST NOT send
   the Extended Association ID TLV in the PCInit messages.

   If the PCC receives a PCInit message with Association Source not
   equal to the PCCs address, or with Association ID not equal to
   0xFFFF, or with Extended Association ID TLV present, the PCC SHOULD
   ignore the given ASSOCIATION object.

4.3.  Multiple Optimization Objectives and Constraints

   In certain scenarios, it is desired for each SR Candidate Path to
   contain multiple sub-candidate paths, each of which has a different
   optimization objective and constraints.  Traffic is then sent ECMP or
   UCMP among these sub-candidate paths.

   This is represented in PCEP by a many-to-one mapping between PCEP
   Tunnels and SR Candidate Paths.  This means that multiple PCEP
   Tunnels are allocated for each SR Candidate Path.  Each PCEP Tunnel
   has its own optimization objective and constraints.  When a single SR
   Candidate Path contains multiple PCEP Tunnels, each of these PCEP
   Tunnels MUST have identical values of Candidate Path Identifiers, as
   encoded in SRPOLICY-CPATH-ID TLV, see Section 5.3.

5.  SR Policy Association Group

   Two ASSOCIATION object types for IPv4 and IPv6 are defined in
   [I-D.ietf-pce-association-group].  The ASSOCIATION object includes
   "Association type" indicating the type of the association group.
   This document adds a new Association type.

   Association type = TBD1 "SR Policy Association Type" for SR Policy
   Association Group (SRPAG).

   The operator configured Association Range SHOULD NOT be set for this
   association type and MUST be ignored.



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   SRPAG MUST carry additional TLVs to communicate Association
   Information.  This document specifies four new TLVs to carry
   Association Information: SRPOLICY-POL-ID TLV, SRPOLICY-POL-NAME TLV,
   SRPOLICY-CPATH-ID TLV, SRPOLICY-CPATH-PREFERENCE TLV.  These four
   TLVs encode the Policy Identifiers, Policy name, Candidate path
   Identifiers and Candidate path Preference, respectively.  When any of
   the mandatory TLVs are missing from the SRPAG association object, the
   PCE MUST send a PCErr message with Error Code = 26 "Association
   Error", Error Type = TBD7 "Missing mandatory SRPAG TLV".

   A given LSP MUST belong to at most one SRPAG, since a candidate path
   cannot belong to multiple SR policies.  If a PCEP speaker receives a
   PCEP message with more than one SRPAG for an LSP, then the PCEP
   speaker MUST send a PCErr message with Error-Type 26 "Association
   Error" and Error-Value TBD8 "Multiple SRPAG for one LSP".  If the
   message is a PCRpt message, then the PCEP speaker MUST close the PCEP
   connection.  Closing the PCEP connection is necessary because
   ignoring PCRpt messages may lead to inconsistent LSP DB state between
   the two PCEP peers.

   If the PCEP speaker receives the SRPAG association when the SR
   capability (as per [I-D.ietf-pce-segment-routing] or
   [I-D.negi-pce-segment-routing-ipv6]) was not exchanged, the PCEP
   speaker MUST send a PCErr message with Error-Type 26 "Association
   Error" and Error-Value TBD9 "Use of SRPAG without SR capability
   exchange".  If the Path Setup Type (PST) of the LSP in SRPAG is not
   set to SR or SRv6, then the PCEP speaker MUST send a PCErr message
   with Error-Type 26 "Association Error" and Error-Value TBD10 "non-SR
   LSP in SRPAG".

5.1.  SR Policy Identifiers TLV

   The SRPOLICY-POL-ID TLV is a mandatory TLV for the SRPAG Association.
   Only one SRPOLICY-POL-ID TLV can be carried and only the first
   occurrence is processed and any others MUST be ignored.

       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              |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             Color                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                           End-point                           ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 1: The SRPOLICY-POL-ID TLV format




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   Type: TBD2 for "SRPOLICY-POL-ID" TLV.

   Length: 8 or 20, depending on length of End-point (IPv4 or IPv6)

   Color: any unsigned 32-bit number.

   End-point: can be either IPv4 or IPv6, depending on whether the
   policy endpoint has IPv4 or IPv6 address.  This value may be
   different from the one contained in the END-POINTS object, or in the
   LSP IDENTIFIERS TLV of the LSP object.  Endpoint is meant to strictly
   correspond to the endpoint of the SR policy, as it is defined in
   [I-D.ietf-spring-segment-routing-policy].

5.2.  SR Policy Name TLV

   The SRPOLICY-POL-NAME TLV is an optional TLV for the SRPAG
   Association.  At most one SRPOLICY-POL-NAME TLV can be carried and
   only the first occurrence is processed and any others MUST be
   ignored.

       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              |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                          Policy Name                          ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 2: The SRPOLICY-POL-NAME TLV format

   Type: TBD3 for "SRPOLICY-POL-NAME" TLV.

   Length: indicates the total length of the TLV in octets and MUST be
   greater than 0.  The TLV MUST be zero-padded so that the TLV is
   4-octet aligned.

   Policy Name: Policy name, as defined in
   [I-D.ietf-spring-segment-routing-policy].  It SHOULD be a string of
   printable ASCII characters, without a NULL terminator.

5.3.  SR Policy Candidate Path Identifiers TLV

   The SRPOLICY-CPATH-ID TLV is a mandatory TLV for the SRPAG
   Association.  Only one SRPOLICY-CPATH-ID TLV can be carried and only
   the first occurrence is processed and any others MUST be ignored.




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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Type              |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Proto. Origin |                    Reserved                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Originator ASN                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                       Originator Address                      |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Discriminator                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 3: The SRPOLICY-CPATH-ID TLV format

   Type: TBD4 for "SRPOLICY-CPATH-ID" TLV.

   Length: 28.

   Protocol Origin: 8-bit value that encodes the protocol origin, as
   specified in [I-D.ietf-spring-segment-routing-policy] Section 2.3.

   Reserved: MUST be set to zero on transmission and ignored on receipt.

   Originator ASN: Represented as 4 byte number, part of the originator
   identifier, as specified in [I-D.ietf-spring-segment-routing-policy]
   Section 2.4.

   Originator Address: Represented as 128 bit value where IPv4 address
   are encoded in lowest 32 bits, part of the originator identifier, as
   specified in [I-D.ietf-spring-segment-routing-policy] Section 2.4.

   Discriminator: 32-bit value that encodes the Discriminator of the
   candidate path.

5.4.  SR Policy Candidate Path Preference TLV

   The SRPOLICY-CPATH-PREFERENCE TLV is an optional TLV for the SRPAG
   Association.  Only one SRPOLICY-CPATH-PREFERENCE TLV can be carried
   and only the first occurrence is processed and any others MUST be
   ignored.






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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Type              |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Preference                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 4: The SRPOLICY-CPATH-PREFERENCE TLV format

   Type: TBD5 for "SRPOLICY-CPATH-PREFERENCE" TLV.

   Length: 4.

   Preference: Numerical preference of the candidate path, as specified
   in [I-D.ietf-spring-segment-routing-policy] Section 2.7.

   If the TLV is missing, a default preference of 100 as specified in
   [I-D.ietf-spring-segment-routing-policy] is used.

6.  Examples

6.1.  PCC Initiated SR Policy with single candidate-path

   PCReq and PCRep messages are exchanged in the following sequence:

   1.  PCC sends PCReq message to the PCE, encoding the SRPAG
       ASSOCIATION object and TLVs in the PCReq message.

   2.  PCE returns the path in PCRep message, and echoes back the SRPAG
       object that was used in the computation.

   PCRpt and PCUpd messages are exchanged in the following sequence:

   1.  PCC sends PCRpt message to the PCE, including the LSP object and
       the SRPAG ASSOCIATION object.

   2.  PCE computes path, possibly making use of the Association
       Information from the SRPAG ASSOCIATION object.

   3.  PCE updates the SR policy candidate path's ERO using PCUpd
       message.

6.2.  PCC Initiated SR Policy with multiple candidate-paths

   PCRpt and PCUpd messages are exchanged in the following sequence:





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   1.  For each candidate path of the SR Policy, the PCC generates a
       different PLSP-ID and symbolic-name and sends multiple PCRpt
       messages (or one message with multiple LSP objects) to the PCE.
       Each LSP object is followed by SRPAG ASSOCIATION object with
       identical Color and Endpoint values.  The Association Source is
       set to the IP address of the PCC and the Association ID is set to
       a number that PCC locally chose to represent the SR Policy.

   2.  PCE takes into account that all the LSPs belong to the same SR
       policy.  PCE prioritizes computation for the highest preference
       LSP and sends PCUpd message(s) back to the PCC.

   3.  If a new candidate path is added on the PCC by the operator, then
       a new PLSP-ID and symbolic name is generated for that candidate
       path and a new PCRpt is sent to the PCE.

   4.  If an existing candidate path is removed from the PCC by the
       operator, then that PLSP-ID is deleted from the PCE by sending
       PCRpt with the R-flag in the LSP object set.

6.3.  PCE Initiated SR Policy with single candidate-path

   A candidate-path is created using the following steps:

   1.  PCE sends PCInitiate message, containing the SRPAG Association
       object.  The Association Source is set to the IP address of the
       PCC and the Association ID is set to 0xFFFF, as described in
       Section 4.2.

   2.  PCC uses the color, endpoint and preference from the SRPAG object
       to create a new candidate path.  If no SR policy exists to hold
       the candidate path, then a new SR policy is created to hold the
       new candidate-path.  The Originator of the candidate path is set
       to be the address of the PCE that is sending the PCInitiate
       message.

   3.  PCC sends a PCRpt message back to the PCE to report the newly
       created Candidate Path.  The PCRpt message contains the SRPAG
       Association object.  The Association Source is set to the IP
       address of the PCC and the Association ID is set to a number that
       PCC locally chose to represent the SR Policy.

   A candidate-path is deleted using the following steps:

   1.  PCE sends PCInitiate message, setting the R-flag in the LSP
       object.





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   2.  PCC uses the PLSP-ID from the LSP object to find the candidate
       path and delete it.  If this is the last candidate path under the
       SR policy, then the containing SR policy is deleted as well.

6.4.  PCE Initiated SR Policy with multiple candidate-paths

   A candidate-path is created using the following steps:

   1.  PCE sends a separate PCInitiate message for every candidate path
       that it wants to create, or it sends multiple LSP objects within
       a single PCInitiate message.  The SRPAG Association object is
       sent for every LSP in the PCInitiate message.  The Association
       Source is set to the IP address of the PCC and the Association ID
       is set to 0xFFFF, as described in Section 4.2.

   2.  PCC creates multiple candidate paths under the same SR policy,
       identified by Color and Endpoint.

   3.  PCC sends a PCRpt message back to the PCE to report the newly
       created Candidate Path.  The PCRpt message contains the SRPAG
       Association object.  The Association Source is set to the IP
       address of the PCC and the Association ID is set to a number that
       PCC locally chose to represent the SR Policy.

   A candidate path is deleted using the following steps:

   1.  PCE sends PCInitiate message, setting the R-flag in the LSP
       object.

   2.  PCC uses the PLSP-ID from the LSP object to find the candidate
       path and delete it.

7.  IANA Considerations

7.1.  Association Type

   This document defines a new association type: SR Policy Association
   Group (SRPAG).  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        |
   | Value                |                         |                  |
   +----------------------+-------------------------+------------------+
   | TBD1                 | SR Policy Association   | This document    |
   +----------------------+-------------------------+------------------+




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7.2.  PCEP Errors

   This document defines three new Error-Values within the "Association
   Error" Error-Type.  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 | Error    | Meaning                     | Reference        |
   | Type  | Value    |                             |                  |
   +-------+----------+-----------------------------+------------------+
   | 29    | TBD6     | Conflicting SRPAG TLV       | This document    |
   +-------+----------+-----------------------------+------------------+
   | 29    | TBD7     | Missing mandatory SRPAG TLV | This document    |
   +-------+----------+-----------------------------+------------------+
   | 29    | TBD8     | Multiple SRPAG for one LSP  | This document    |
   +-------+----------+-----------------------------+------------------+
   | 29    | TBD9     | Use of SRPAG without SR     | This document    |
   |       |          | capability exchange         |                  |
   +-------+----------+-----------------------------+------------------+
   | 29    | TBD10    | non-SR LSP in SRPAG         | This document    |
   +-------+----------+-----------------------------+------------------+


7.3.  SRPAG TLVs

   This document defines three new TLVs for carrying additional
   information about SR policy and SR candidate paths.  IANA is
   requested to make the assignment of a new value for the existing
   "PCEP TLV Type Indicators" registry as follows:

   +------------+-----------------------------------+------------------+
   | TLV Type   | TLV Name                          | Reference        |
   | Value      |                                   |                  |
   +------------+-----------------------------------+------------------+
   | TBD2       | SRPOLICY-POL-ID                   | This document    |
   +------------+-----------------------------------+------------------+
   | TBD3       | SRPOLICY-POL-NAME                 | This document    |
   +------------+-----------------------------------+------------------+
   | TBD4       | SRPOLICY-CPATH-ID                 | This document    |
   +------------+-----------------------------------+------------------+
   | TBD5       | SRPOLICY-CPATH-PREFERENCE         | This document    |
   +------------+-----------------------------------+------------------+








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8.  Security Considerations

   This document defines one new type for association, which do not add
   any new security concerns beyond those discussed in [RFC5440],
   [RFC8231], [I-D.ietf-pce-segment-routing],
   [I-D.negi-pce-segment-routing-ipv6] and
   [I-D.ietf-pce-association-group] in itself.

   The information carried in the SRPAG Association object, as per this
   document is related to SR Policy.  It often reflects information that
   can also be derived from the SR Database, but association provides a
   much easier grouping of related LSPs and messages.  The SRPAG
   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) [RFC8253], as per
   the recommendations and best current practices in [RFC7525], is
   RECOMMENDED.

9.  Acknowledgement

10.  References

10.1.  Normative References

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

   [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>.

   [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>.








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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-spring-segment-routing-policy]
              Filsfils, C., Sivabalan, S., daniel.voyer@bell.ca, d.,
              bogdanov@google.com, b., and P. Mattes, "Segment Routing
              Policy Architecture", draft-ietf-spring-segment-routing-
              policy-03 (work in progress), May 2019.

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

   [I-D.ietf-pce-segment-routing]
              Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "PCEP Extensions for Segment Routing",
              draft-ietf-pce-segment-routing-16 (work in progress),
              March 2019.

   [I-D.koldychev-pce-operational]
              Koldychev, M., Sivabalan, S., Negi, M., Achaval, D., and
              H. Kotni, "PCEP Operational Clarification", draft-
              koldychev-pce-operational-00 (work in progress), July
              2019.

10.2.  Informative References

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

   [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>.







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   [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>.

   [I-D.negi-pce-segment-routing-ipv6]
              Negi, M., Li, C., Sivabalan, S., and P. Kaladharan, "PCEP
              Extensions for Segment Routing leveraging the IPv6 data
              plane", draft-negi-pce-segment-routing-ipv6-04 (work in
              progress), February 2019.

Appendix A.  Contributors

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

   Email: dhruv.ietf@gmail.com

Authors' Addresses

   Mike Koldychev
   Cisco Systems, Inc.
   2000 Innovation Drive
   Kanata, Ontario  K2K 3E8
   Canada

   Email: mkoldych@cisco.com


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

   Email: msiva@cisco.com


   Colby Barth
   Juniper Networks, Inc.

   Email: cbarth@juniper.net





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   Cheng Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095
   China

   Email: chengli13@huawei.com


   Hooman Bidgoli
   Nokia

   Email: hooman.bidgoli@nokia.com






































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