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

Document Type Active Internet-Draft (pce WG)
Authors Mike Koldychev  , Siva Sivabalan  , Colby Barth  , Shuping Peng  , Hooman Bidgoli 
Last updated 2021-03-08
Replaces draft-barth-pce-segment-routing-policy-cp
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PCE Working Group                                           M. Koldychev
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                            S. Sivabalan
Expires: September 9, 2021                             Ciena Corporation
                                                                C. Barth
                                                  Juniper Networks, Inc.
                                                                 S. Peng
                                                     Huawei Technologies
                                                              H. Bidgoli
                                                                   Nokia
                                                           March 8, 2021

    PCEP extension to support Segment Routing Policy Candidate Paths
              draft-ietf-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, endpoint> tuple.  An SR policy can
   contain one or more candidate paths where each candidate path is
   identified in PCEP by its uniquely assigned 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

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   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 September 9, 2021.

Copyright Notice

   Copyright (c) 2021 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
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   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  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     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 . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Multiple Optimization Objectives and Constraints  . . . .   7
   5.  SR Policy Association . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Association Parameters  . . . . . . . . . . . . . . . . .   8
     5.2.  Association Information . . . . . . . . . . . . . . . . .   9
       5.2.1.  SR Policy Name TLV  . . . . . . . . . . . . . . . . .  10
       5.2.2.  SR Policy Candidate Path Identifiers TLV  . . . . . .  10
       5.2.3.  SR Policy Candidate Path Name TLV . . . . . . . . . .  11
       5.2.4.  SR Policy Candidate Path Preference TLV . . . . . . .  12
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  PCC Initiated SR Policy with single candidate-path  . . .  13
     6.2.  PCC Initiated SR Policy with multiple candidate-paths . .  13
     6.3.  PCE Initiated SR Policy with single candidate-path  . . .  14
     6.4.  PCE Initiated SR Policy with multiple candidate-paths . .  14
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     7.1.  Association Type  . . . . . . . . . . . . . . . . . . . .  15
     7.2.  PCEP TLV Type Indicators  . . . . . . . . . . . . . . . .  15
     7.3.  PCEP Errors . . . . . . . . . . . . . . . . . . . . . . .  15

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   8.  Implementation Status . . . . . . . . . . . . . . . . . . . .  16
     8.1.  Cisco . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     8.2.  Juniper . . . . . . . . . . . . . . . . . . . . . . . . .  17
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  18
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     11.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Appendix A.  Contributors . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   Path Computation Element (PCE) Communication Protocol (PCEP)
   [RFC5440] enables the communication between a Path Computation Client
   (PCC) and a Path Computation 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 [RFC8664] 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
   [RFC8697] 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 SR Policy 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

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   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.

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 [RFC8697], 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 [RFC8697], the ASSOCIATION
      object could also include other TLVs based on the association
      types, that provides non-key information.

   SRPAG:  SR Policy Association Group.

   SRPAT:  SR Policy Association Type.

   SRPAT ASSOCIATION:  ASSOCIATION object of type SR Policy Association.

   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].

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3.  Motivation

   The SR Policy Association and its TLVs, defined in this document,
   allow PCEP speakers to exchange additional information about SR
   Policy Candidate Paths and their container SR Policy.

3.1.  Group Candidate Paths belonging to the same SR policy

   Since each SR Policy Candidate Path appears as a different Tunnel
   (identified via a PLSP-ID) in PCEP, it is useful to group together
   all the SR Policy Candidate Paths that belong to the same SR Policy.
   Furthermore, it is useful for the PCE to have knowledge of the SR
   Policy related information such as color, endpoint, protocol origin,
   discriminator, and preference.

3.2.  Instantiation of SR policy candidate paths

   A PCE needs to instantiate one or more SR Policy Candidate Paths on
   the PCC, as specified in [RFC8281].  Each SR Policy Candidate Path is
   identified by the tuple <headend, color, endpoint, originator,
   discriminator, preference>.  This draft provides a mechanism to
   signal this information in PCEP.

3.3.  Avoid computing lower preference candidate paths

   When a PCE knows that a given set of SR Policy 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.

3.4.  Minimal signaling overhead

   When an SR Policy contains multiple SR Policy 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
   SR Policy Candidate Path correspond to a unique Tunnel (identified
   via PLSP-ID).  For example, if an SR Policy has 4 SR Policy Candidate
   Paths, then if the PCE wants to update one of those, only one set of
   PCUpd and PCRpt messages needs to be exchanged.

4.  Procedure

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4.1.  Overview

   As per [RFC8697], 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 Policy
   Candidate Paths and PCEP Associations naturally map to SR Policies.

   The mapping between PCEP Associations and SR Policies is always one-
   to-one.  However, the mapping between PCEP Tunnels and SR Policy
   Candidate Paths may be either one-to-one, or many-to-one, see
   Section 4.2.

   Each SR Policy Candidate Path contains one or more Segment Lists.
   The subject of encoding multiple Segment Lists within an SR Policy
   Candidate Path is described in [I-D.koldychev-pce-multipath].

   This document defines a new Association Type called "SR Policy
   Association", of value TBD1, based on the generic ASSOCIATION object.
   The new Association Type is also called "SRPAT", for "SR Policy
   Association Type".  We say "SRPAT ASSOCIATION" to mean "ASSOCIATION
   object of type SR Policy Association".  The group of LSPs that are
   part of the SR Policy Association is called "SRPAG", for "SR Policy
   Association Group".

   An SRPAT ASSOCIATION carries three pieces of information: SR Policy
   Identifiers, SR Policy Candidate Path Identifiers, and SR Policy
   Candidate Path Attributes.

   SR Policy Identifiers uniquely identify the SR policy within the
   context of the headend.  SR Policy Identifiers MUST be the same for
   all SR Policy Candidate Paths in the same SRPAG.  SR Policy
   Identifiers MUST NOT change for a given SR Policy Candidate Path
   during its lifetime.  SR Policy Identifiers MUST be different for
   different SRPAGs.  SR Policy Identifiers consist of:

   o  Headend router where the SR Policy originates.

   o  Color of SR Policy.

   o  Endpoint of SR Policy.

   SR Policy Candidate Path Identifiers uniquely identify the SR Policy
   Candidate Path within the context of an SR Policy.  SR Policy
   Candidate Path Identifiers MUST NOT change for a given LSP during its
   lifetime.  SR Policy 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-Type = 26

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   "Association Error", Error Value = TBD8 "SR Policy Candidate Path
   Identifiers Mismatch".  SR Policy Candidate Path Identifiers consist
   of:

   o  Protocol Origin.

   o  Originator.

   o  Discriminator.

   SR Policy Candidate Path Attributes carry non-key information about
   the candidate path and MAY change during the lifetime of the LSP.  SR
   Policy Candidate Path Attributes consist of:

   o  Preference.

   o  Optionally, the SR Policy Candidate Path name.

   o  Optionally, the SR Policy name.

   As per the processing rules specified in section 5.4 of [RFC8697], if
   a PCEP speaker does not support the SRPAT, it MUST return a PCErr
   message with Error-Type = 26 "Association Error", Error-Value = 1
   "Association-type is not supported".

   A given LSP MUST belong to at most one SRPAG, since an SR Policy
   Candidate Path cannot belong to multiple SR Policies.  If a PCEP
   speaker receives a PCEP message with more than one SRPAT ASSOCIATION
   for the same LSP, then the PCEP speaker MUST send a PCErr message
   with Error-Type = 26 "Association Error", Error-Value = 7 "Cannot
   join the association group".

4.2.  Multiple Optimization Objectives and Constraints

   In certain scenarios, it is desired for each SR Policy 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 Policy Candidate Paths.  This means that multiple PCEP
   Tunnels are allocated for each SR Policy Candidate Path.  Each PCEP
   Tunnel has its own optimization objective and constraints.  When a
   single SR Policy 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 5.2.2.

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5.  SR Policy Association

   Two ASSOCIATION object types for IPv4 and IPv6 are defined in
   [RFC8697].  The ASSOCIATION object includes "Association Type"
   indicating the type of the association group.  This document adds a
   new Association Type = TBD1 "SR Policy Association".  This
   Association Type is dynamic in nature, thus operator-configured
   Association Range MUST NOT be set for this Association type and MUST
   be ignored.

5.1.  Association Parameters

   As per [I-D.ietf-spring-segment-routing-policy], an SR Policy is
   identified through the tuple <headend, color, endpoint>. the headend
   is encoded as the Association Source in the ASSOCIATION object and
   the color and endpoint are encoded as part of Extended Association ID
   TLV.

   The Association Parameters (see Section 2) consist of:

   o  Association Type: set to TBD1 "SR Policy Association".

   o  Association Source (IPv4/IPv6): set to the headend IP address.

   o  Association ID (16-bit): set to "1".

   o  Extended Association ID TLV: encodes the Color and Endpoint of the
      SR Policy.

   The Association Source MUST be set to the headend value of the SR
   Policy, as defined in [I-D.ietf-spring-segment-routing-policy]
   Section 2.1.  If the PCC receives a PCInit message for a non-existent
   SR Policy, where the Association Source is set not to the headend
   value but to some globally unique IP address that the PCC owns, then
   the PCC SHOULD accept the PCInit message and create the SR Policy
   Association with the Association Source that was sent in the PCInit
   message.

   The 16-bit Association ID field in the ASSOCIATION object MUST be set
   to the value of "1".

   The Extended Association ID TLV MUST be included and it MUST be in
   the following format:

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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 = 31           |       Length = 8 or 20        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             Color                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                           Endpoint                            ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 1: Extended Association ID TLV format

   Type: Extended Association ID TLV, type = 31.

   Length: Either 8 or 20, depending on whether IPv4 or IPv6 address is
   encoded in the Endpoint.

   Color: SR Policy color value.

   Endpoint: can be either IPv4 or IPv6, depending on whether the policy
   endpoint is IPv4 or IPv6.  This value MAY be different from the one
   contained in the END-POINTS object, or in the LSP IDENTIFIERS TLV of
   the LSP object.  This value is part of the tuple <color, endpoint>
   that identifies the SR Policy on a given headend.

   If the PCEP speaker receives an SRPAT ASSOCIATION whose Association
   Parameters do not follow the above specification, then the PCEP
   speaker MUST send PCErr message with Error-Type = 26 "Association
   Error", Error-Value = TBD7 "SR Policy Identifiers Mismatch".

   The purpose of choosing the Association Parameters in this way is to
   guarantee that there is no possibility of a race condition when
   multiple PCEP speakers want to create the same SR Policy at the same
   time.  By adhering to this format, all PCEP speakers come up with the
   same Association Parameters independently of each other.  Thus, there
   is no chance that different PCEP speakers will come up with different
   Association Parameters for the same SR Policy.

5.2.  Association Information

   The SRPAT ASSOCIATION contains the following TLVs:

   o  SRPOLICY-POL-NAME TLV: (optional) encodes SR Policy Name string.

   o  SRPOLICY-CPATH-ID TLV: (mandatory) encodes SR Policy Candidate
      Path Identifiers.

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   o  SRPOLICY-CPATH-NAME TLV: (optional) encodes SR Policy Candidate
      Path string name.

   o  SRPOLICY-CPATH-PREFERENCE TLV: (optional) encodes SR Policy
      Candidate Path preference value.

   Of these new TLVs, SRPOLICY-CPATH-ID TLV is mandatory.  When a
   mandatory TLV is missing from the SRPAT ASSOCIATION object, the PCE
   MUST send a PCErr message with Error-Type = 6 "Mandatory Object
   Missing", Error-Value = TBD6 "Missing Mandatory TLV".

5.2.1.  SR Policy Name TLV

   The SRPOLICY-POL-NAME TLV is an optional TLV for the SRPAT
   ASSOCIATION.  At most one SRPOLICY-POL-NAME TLV SHOULD be encoded by
   the sender 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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                       SR Policy Name                          ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 2: The SRPOLICY-POL-NAME TLV format

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

   Length: indicates the length of the value portion 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.

   SR Policy Name: SR 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.

5.2.2.  SR Policy Candidate Path Identifiers TLV

   The SRPOLICY-CPATH-ID TLV is a mandatory TLV for the SRPAT
   ASSOCIATION.  Only one SRPOLICY-CPATH-ID TLV SHOULD be encoded by the
   sender 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: TBD3 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.

5.2.3.  SR Policy Candidate Path Name TLV

   The SRPOLICY-CPATH-NAME TLV is an optional TLV for the SRPAT
   ASSOCIATION.  At most one SRPOLICY-CPATH-NAME TLV SHOULD be encoded
   by the sender 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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                 SR Policy Candidate Path Name                 ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 4: The SRPOLICY-CPATH-NAME TLV format

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

   Length: indicates the length of the value portion 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.

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

5.2.4.  SR Policy Candidate Path Preference TLV

   The SRPOLICY-CPATH-PREFERENCE TLV is an optional TLV for the SRPAT
   ASSOCIATION.  Only one SRPOLICY-CPATH-PREFERENCE TLV SHOULD be
   encoded by the sender 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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Preference                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 5: 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.

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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 SRPAT
       ASSOCIATION and TLVs in the PCReq message.

   2.  PCE returns the path in PCRep message, and echoes back the SRPAT
       ASSOCIATION.

   PCRpt and PCUpd messages are exchanged in the following sequence:

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

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

   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:

   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 SRPAT ASSOCIATION 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.

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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 SRPAT ASSOCIATION.
       The Association Source and the Association ID are set as
       described in Section 5.1.

   2.  PCC uses the color, endpoint and preference from the SRPAT
       ASSOCIATION 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 SRPAT
       ASSOCIATION.

   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.  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 SRPAT ASSOCIATION is sent for
       every LSP in the PCInitiate message.  The Association Source and
       the Association ID are set as described in Section 5.1.

   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 SRPAT
       ASSOCIATION.  The Association Source and the Association ID are
       set as described in Section 5.1.

   A candidate path is deleted using the following steps:

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   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.
   IANA is requested to make the following codepoint assignment in the
   "ASSOCIATION Type Field" subregistry [RFC8697] within the "Path
   Computation Element Protocol (PCEP) Numbers" registry:

   +-----------+-------------------------------------------+-----------+
   | Type      | Name                                      | Reference |
   +-----------+-------------------------------------------+-----------+
   | TBD1      | SR Policy Association                     | This.I-D  |
   +-----------+-------------------------------------------+-----------+

7.2.  PCEP TLV Type Indicators

   This document defines four 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:

   +-----------+-------------------------------------------+-----------+
   | Value     | Description                               | Reference |
   +-----------+-------------------------------------------+-----------+
   | TBD2      | SRPOLICY-POL-NAME                         | This.I-D  |
   +-----------+-------------------------------------------+-----------+
   | TBD3      | SRPOLICY-CPATH-ID                         | This.I-D  |
   +-----------+-------------------------------------------+-----------+
   | TBD4      | SRPOLICY-CPATH-NAME                       | This.I-D  |
   +-----------+-------------------------------------------+-----------+
   | TBD5      | SRPOLICY-CPATH-PREFERENCE                 | This.I-D  |
   +-----------+-------------------------------------------+-----------+

7.3.  PCEP Errors

   This document defines one new Error-Value within the "Mandatory
   Object Missing" Error-Type and two 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:

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   +------------+------------------+-----------------------+-----------+
   | Error-Type | Meaning          | Error-value           | Reference |
   +------------+------------------+-----------------------+-----------+
   | 6          | Mandatory Object |                       | [RFC5440] |
   |            | Missing          |                       |           |
   +------------+------------------+-----------------------+-----------+
   |            |                  | TBD6: SR Policy       | This.I-D  |
   |            |                  | Missing Mandatory TLV |           |
   +------------+------------------+-----------------------+-----------+
   | 26         | Association      |                       | [RFC8697] |
   |            | Error            |                       |           |
   +------------+------------------+-----------------------+-----------+
   |            |                  | TBD7: SR Policy       | This.I-D  |
   |            |                  | Identifers Mismatch   |           |
   +------------+------------------+-----------------------+-----------+
   |            |                  | TBD8: SR Policy       | This.I-D  |
   |            |                  | Candidate Path        |           |
   |            |                  | Identifiers Mismatch  |           |
   +------------+------------------+-----------------------+-----------+

8.  Implementation Status

   [Note to the RFC Editor - remove this section before publication, as
   well as remove the reference to RFC 7942.]

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to [RFC7942], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

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8.1.  Cisco

   o  Organization: Cisco Systems

   o  Implementation: IOS-XR PCC and PCE.

   o  Description: An experimental code-point is currently used.

   o  Maturity Level: Proof of concept.

   o  Coverage: Full.

   o  Contact: mkoldych@cisco.com

8.2.  Juniper

   o  Organization: Juniper Networks

   o  Implementation: Head-end and controller.

   o  Description: An experimental code-point is currently used.

   o  Maturity Level: Proof of concept.

   o  Coverage: Partial.

   o  Contact: cbarth@juniper.net

9.  Security Considerations

   This document defines one new type for association, which do not add
   any new security concerns beyond those discussed in [RFC5440],
   [RFC8231], [RFC8664], [I-D.ietf-pce-segment-routing-ipv6] and
   [RFC8697] in itself.

   The information carried in the SRPAT ASSOCIATION, 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 SRPAT
   ASSOCIATION could provide an adversary with the opportunity to
   eavesdrop on the relationship between the LSPs.  Thus securing the
   PCEP session using Transport Layer Security (TLS) [RFC8253], as per
   the recommendations and best current practices in [RFC7525], is
   RECOMMENDED.

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10.  Acknowledgement

   Would like to thank Stephane Litkowski, Praveen Kumar and Tom Petch
   for review comments.

11.  References

11.1.  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>.

   [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>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-09 (work in progress),
              November 2020.

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   [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>.

   [RFC8664]  Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "Path Computation Element Communication
              Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
              DOI 10.17487/RFC8664, December 2019,
              <https://www.rfc-editor.org/info/rfc8664>.

   [I-D.koldychev-pce-operational]
              Koldychev, M., Sivabalan, S., Negi, M., Achaval, D., and
              H. Kotni, "PCEP Operational Clarification", draft-
              koldychev-pce-operational-02 (work in progress), August
              2020.

   [I-D.koldychev-pce-multipath]
              Koldychev, M., Sivabalan, S., Saad, T., Beeram, V.,
              Bidgoli, H., Yadav, B., and S. Peng, "PCEP Extensions for
              Signaling Multipath Information", draft-koldychev-pce-
              multipath-04 (work in progress), October 2020.

11.2.  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>.

   [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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   [I-D.ietf-pce-segment-routing-ipv6]
              Li, C., Negi, M., Sivabalan, S., Koldychev, M.,
              Kaladharan, P., and Y. Zhu, "PCEP Extensions for Segment
              Routing leveraging the IPv6 data plane", draft-ietf-pce-
              segment-routing-ipv6-08 (work in progress), November 2020.

Appendix A.  Contributors

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

   Email: dhruv.ietf@gmail.com

   Cheng Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing, 10095
   China

   Email: chengli13@huawei.com

Authors' Addresses

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

   Email: mkoldych@cisco.com

   Siva Sivabalan
   Ciena Corporation
   385 Terry Fox Dr.
   Kanata, Ontario  K2K 0L1
   Canada

   Email: ssivabal@ciena.com

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   Colby Barth
   Juniper Networks, Inc.

   Email: cbarth@juniper.net

   Shuping Peng
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095
   China

   Email: pengshuping@huawei.com

   Hooman Bidgoli
   Nokia

   Email: hooman.bidgoli@nokia.com

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