PCE Working Group                                           M. Koldychev
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                            S. Sivabalan
Expires: 28 January 2022                               Ciena Corporation
                                                                 T. Saad
                                                               V. Beeram
                                                  Juniper Networks, Inc.
                                                              H. Bidgoli
                                                                   Nokia
                                                                B. Yadav
                                                                   Ciena
                                                                 S. Peng
                                                     Huawei Technologies
                                                            27 July 2021


          PCEP Extensions for Signaling Multipath Information
                      draft-ietf-pce-multipath-01

Abstract

   Current PCEP standards allow only one intended and/or actual path to
   be present in a PCEP message.  Applications that require multipath
   support such as SR Policy require an extension to allow signaling
   multiple intended and/or actual paths within a single PCEP message.
   This document introduces such an extension.  Encoding of multiple
   intended and/or actual paths is done by encoding multiple Explicit
   Route Objects (EROs) and/or multiple Record Route Objects (RROs).  A
   special separator object is defined in this document, to facilitate
   this.  This mechanism is applicable to SR-TE and RSVP-TE and is
   dataplane agnostic.

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 28 January 2022.



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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 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
     2.1.  Terms and Abbreviations . . . . . . . . . . . . . . . . .   4
   3.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Signaling Multiple Segment-Lists of an SR
           Candidate-Path  . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Splitting of Requested Bandwidth  . . . . . . . . . . . .   4
     3.3.  Providing Backup path for Protection  . . . . . . . . . .   4
   4.  Protocol Extensions . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Multipath Capability TLV  . . . . . . . . . . . . . . . .   5
     4.2.  Path Attributes Object  . . . . . . . . . . . . . . . . .   6
     4.3.  Multipath Weight TLV  . . . . . . . . . . . . . . . . . .   6
     4.4.  Multipath Backup TLV  . . . . . . . . . . . . . . . . . .   7
     4.5.  Multipath Opposite Direction Path TLV . . . . . . . . . .   8
     4.6.  Composite Candidate Path  . . . . . . . . . . . . . . . .   9
   5.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Signaling Multiple Paths for Loadbalancing  . . . . . . .  10
     5.2.  Signaling Multiple Paths for Protection . . . . . . . . .  11
   6.  PCEP Message Extensions . . . . . . . . . . . . . . . . . . .  12
   7.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  SR Policy Candidate-Path with Multiple Segment-Lists  . .  12
     7.2.  Two Primary Paths Protected by One Backup Path  . . . . .  13
     7.3.  Composite Candidate Path  . . . . . . . . . . . . . . . .  14
     7.4.  Opposite Direction Tunnels  . . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  PCEP Object . . . . . . . . . . . . . . . . . . . . . . .  17
     8.2.  PCEP TLV  . . . . . . . . . . . . . . . . . . . . . . . .  17
     8.3.  PCEP-Error Object . . . . . . . . . . . . . . . . . . . .  17
     8.4.  Flags in the Multipath Capability TLV . . . . . . . . . .  18
     8.5.  Flags in the Path Attribute Object  . . . . . . . . . . .  18
     8.6.  Flags in the Multipath Backup TLV . . . . . . . . . . . .  18
     8.7.  Flags in the Multipath Opposite Direction Path TLV  . . .  19



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   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   10. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  19
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  19
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     12.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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 that 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 for a PCC to request a path subject to certain constraint(s) and
   optimization criteria in SR networks.

   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.  In
   particular, it describes the SR candidate-path as a collection of one
   or more Segment-Lists.  The current PCEP standards only allow for
   signaling of one Segment-List per Candidate-Path.  PCEP extension to
   support Segment Routing Policy Candidate Paths
   [I-D.ietf-pce-segment-routing-policy-cp] specifically avoids defining
   how to signal multipath information, and states that this will be
   defined in another document.

   This document defines the required extensions that allow the
   signaling of multipath information via PCEP.









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2.  Terminology

   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.

2.1.  Terms and Abbreviations

   The following terms are used in this document:

   PCEP Tunnel:

      The object identified by the PLSP-ID, see
      [I-D.koldychev-pce-operational] for more details.

3.  Motivation

   This extension is motivated by the use-cases described below.

3.1.  Signaling Multiple Segment-Lists of an SR Candidate-Path

   The Candidate-Path of an SR Policy is the unit of report/update in
   PCEP, see [I-D.ietf-pce-segment-routing-policy-cp].  Each Candidate-
   Path can contain multiple Segment-Lists and each Segment-List is
   encoded by one ERO.  However, each PCEP LSP can contain only a single
   ERO (containing multiple SR-ERO subobject), which prevents us from
   encoding multiple Segment- Lists within the same SR Candidate-Path.

   With the help of the protocol extensions defined in this document,
   this limitation is overcome.

3.2.  Splitting of Requested Bandwidth

   A PCC may request a path with 80 Gbps of bandwidth, but all links in
   the network have only 50 Gbps capacity.  The PCE can return two
   paths, that can together carry 80 Gbps.  The PCC can then equally or
   unequally split the incoming 80 Gbps of traffic among the two paths.
   Section 4.3 introduces a new TLV that carries the path weight that
   allows for distribution of incoming traffic on to the multiple paths.

3.3.  Providing Backup path for Protection

   It is desirable for the PCE to compute and signal to the PCC a backup
   path that is used to protect a primary path within the multipaths in
   a given LSP.




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   Note that [RFC8745] specify the Path Protection association among
   LSPs.  The use of [RFC8745] with multipath is out of scope of this
   document and is for future study.

   When multipath is used, a backup path may protect one or more primary
   paths.  For this reason, primary and backup path identifiers are
   needed to indicate which backup path(s) protect which primary
   path(s).  Section 4.4 introduces a new TLV that carries the required
   information.

4.  Protocol Extensions

4.1.  Multipath Capability TLV

   We define the MULTIPATH-CAP TLV that MAY be present in the OPEN
   object and/or the LSP object.  The purpose of this TLV is two-fold:

   1.  From PCC: it tells how many multipaths per PCEP Tunnel, the PCC
       can install in forwarding.

   2.  From PCE: it tells that the PCE supports this standard and how
       many multipaths per PCEP Tunnel, the PCE can compute.

   Only the first instance of this TLV can be processed, subsequent
   instances SHOULD be ignored.

   Section 5 specify the usage of this TLV with Open message (within the
   OPEN object) and other PCEP messages (within the LSP object).

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Number of Multipaths      |            Flags        |O|B|W|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 1: MULTIPATH-CAP TLV format

   Type: TBD1 for "MULTIPATH-CAP" TLV.

   Length: 4.

   Number of Multipaths: the maximum number of multipaths per PCEP
   Tunnel.  The value 0 indicates unlimited number.

   W-flag: whether MULTIPATH-WEIGHT-TLV is supported.




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   B-flag: whether MULTIPATH-BACKUP-TLV is supported.

   O-flag: whether MULTIPATH-OPPDIR-PATH-TLV is supported.

4.2.  Path Attributes Object

   We define the PATH-ATTRIB object that is used to carry per-path
   information and to act as a separator between several ERO/RRO objects
   in the <intended-path>/<actual-path> RBNF element.  The PATH-ATTRIB
   object always precedes the ERO/RRO that it applies to.  If multiple
   ERO/RRO objects are present, then each ERO/RRO object MUST be
   preceded by an PATH-ATTRIB object that describes it.

   The PATH-ATTRIB Object-Class value is TBD2.

   The PATH-ATTRIB Object-Type value is 1.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Flags                           |  O  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Path ID                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                          Optional TLVs                        ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 2: PATH-ATTRIB object format

   O (Operational - 3 bits): operational state of the path, same values
   as the identically named field in the LSP object [RFC8231].

   Path ID: 4-octet identifier that identifies a path in the set of
   multiple paths.  It uniquely identifies a path (encoded in the ERO/
   RRO) within the set of multiple paths under the PCEP LSP.  Once a
   path changes, a new Path ID is assigned.  Value 0x0 is reserved to
   indicate the absense of a Path ID.

   TLVs that may be included in the PATH-ATTRIB object are described in
   the following sections.  Other optional TLVs could be defined by
   future documents to be included within the PATH-ATTRIB object body.

4.3.  Multipath Weight TLV

   We define the MULTIPATH-WEIGHT TLV that MAY be present in the PATH-
   ATTRIB object.





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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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                             Weight                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 3: MULTIPATH-WEIGHT TLV format

   Type: TBD3 for "MULTIPATH-WEIGHT" TLV.

   Length: 4.

   Weight: weight of this path within the multipath, if W-ECMP is
   desired.  The fraction of flows a specific ERO/RRO carries is derived
   from the ratio of its weight to the sum of all other multipath ERO/
   RRO weights.

   When the MULTIPATH-WEIGHT TLV is absent from the PATH-ATTRIB object,
   or the PATH-ATTRIB object is absent from the <intended-path>/<actual-
   path>, then the Weight of the corresponding path is taken to be "1".

4.4.  Multipath Backup TLV

   This document introduces a new MULTIPATH-BACKUP TLV that is optional
   and can be present in the PATH-ATTRIB object.

   This TLV is used to indicate the presence of a backup path that is
   used for protection in case of failure of the primary path.  The
   format of the MULTIPATH-BACKUP TLV is:

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Backup Path Count       |             Flags           |B|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Backup Path ID 1                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Backup Path ID 2                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Backup Path ID n                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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                   Figure 4: MULTIPATH-BACKUP TLV format

   Type: TBD4 for "MULTIPATH-BACKUP" TLV

   Length: 4 + (N * 4) (where N is the Backup Path Count)

   Backup Path Count: Number of backup path(s).

   B: If set, indicates a pure backup path.  This is a path that only
   carries rerouted traffic after the protected path fails.  If this
   flag is not set, or if the MULTIPATH-BACKUP TLV is absent, then the
   path is assumed to be primary that carries normal traffic.

   Backup Path ID(s): a series of 4-octet identifier(s) that identify
   the backup path(s) in the set that protect this primary path.

4.5.  Multipath Opposite Direction Path TLV

   This document introduces a new MULTIPATH-OPPDIR-PATH TLV that is
   optional and can be present in the PATH-ATTRIB object.

   This TLV is used to indicate whether the given path is a forward path
   or a reverse path in its PCEP Tunnel, as well as give information
   about the opposite-direction path(s) of the given path.

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Reserved (MBZ)         |             Flags       |L|N|R|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Opposite Direction Path ID                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 5: MULTIPATH-OPPDIR-PATH TLV format

   Type: TBD9 for "MULTIPATH-OPPDIR-PATH" TLV

   Length: 16.

   R (Reverse path): If set, indicates this path is reverse, i.e., it
   originates on the Tunnel destination and terminates on the Tunnel
   source (usually the PCC headend itself).  Paths with this flag set
   MUST NOT be installed into forwarding, they serve only informational
   purposes.





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   N (Node co-routed): If set, indicates this path is node co-routed
   with its opposite direction path, specified in this TLV.  Two
   opposite direction paths are node co-routed if they traverse the same
   nodes, but MAY traverse different links.

   L (Link co-routed): If set, indicates this path is link co-routed
   with its opposite directions path, specified in this TLV.  Two
   opposite direction paths are link co-routed if they traverse the same
   links (but in the opposite directions).

   Opposite Direction Path ID: Identifies the path that goes in the
   opposite direction to this path.  If no such path exists, then this
   field MUST be set to 0x0, which is reserved to indicate the absense
   of a Path ID.

   Multiple instances of this TLV present in the same PATH-ATTRIB object
   indicate that there are multiple opposite-direction paths
   corresponding to the given path.  This allows for many-to-many
   relationship among the paths of two opposite direction Tunnels.

   Whenever path A references another path B as being the opposite-
   direction path, then path B typically also reference path A as its
   own opposite-direction path.

   See Section 7.4 for an example of usage.

4.6.  Composite Candidate Path

   SR Policy Architecture [I-D.ietf-spring-segment-routing-policy]
   defines the concept of a Composite Candidate Path.  Unlike a Non-
   Composite Candidate Path, which contains Segment Lists, the Composite
   Candidate Path contains Colors of other policies.  The traffic that
   is steered into a Composite Candidate Path is split among the
   policies that are identified by the Colors contained in the Composite
   Candidate Path.  The split can be either ECMP or UCMP by adjusting
   the weight of each color in the Composite Candidate Path, in the same
   manner as the weight of each Segment List in the Non-Composite
   Candidate Path is adjusted.

   To signal the Composite Candidate Path, we make use of the COLOR TLV,
   defined in [I-D.peng-pce-te-constraints].  For a Composite Candidate
   Path, the COLOR TLV is included in the PATH-ATTRIB Object, thus
   allowing each Composite Candidate Path to do ECMP/UCMP among SR
   Policies or Tunnels identified by its constituent Colors.  Only one
   COLOR TLV SHOULD be included into the PATH-ATTRIB object.  If
   multiple COLOR TLVs are contained in the PATH-ATTRIB object, only the
   first one MUST be processed and the others SHOULD be ignored.




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   An empty SR-ERO/SR-RRO object MUST be included as per the existing
   RBNF, i.e., SR-ERO/SR-RRO MUST contain no sub-objects.  If the head-
   end receives a non-empty SR-ERO/SR-RRO, then it MUST send PCError
   message with Error-Type 19 ("Invalid Operation") and Error-Value =
   TBD8 ("Non-empty path").

   See Section 7.3 for an example of the encoding.

5.  Operation

   When the PCC wants to indicate to the PCE that it wants to get
   multipaths for a PCEP Tunnel, instead of a single path, it can do (1)
   or both (1) and (2) of the following:

   (1) Send the MULTIPATH-CAP TLV in the OPEN object during session
   establishment.  This applies to all PCEP Tunnels on the PCC, unless
   overridden by PCEP Tunnel specific information.

   (2) Additionally send the MULTIPATH-CAP TLV in the LSP object for a
   particular PCEP Tunnel in the PCRpt or PCReq message.  This applies
   to the specified PCEP Tunnel and overrides the information from the
   OPEN object.

   When PCE computes the path for a PCEP Tunnel, it MUST NOT return more
   multipaths than the corresponding value of "Number of Multipaths"
   from the MULTIPATH-CAP TLV.  If this TLV is absent (from both OPEN
   and LSP objects), then the "Number of Multipaths" is assumed to be 1.

   If the PCE supports this standard, then it MUST include the
   MULTIPATH-CAP TLV in the OPEN object.  This tells the PCC that it can
   report multiple ERO/RRO objects per PCEP Tunnel to this PCE.  If the
   PCE does not include the MULTIPATH-CAP TLV in the OPEN object, then
   the PCC MUST assume that the PCE does not support this standard and
   fall back to reporting only a single ERO/RRO.  The PCE MUST NOT
   include MULTIPATH-CAP TLV in the LSP object in any other PCEP message
   towards the PCC and the PCC MUST ignore it if received.

   The Path ID of each ERO/RRO MUST be unique within that LSP.  If a
   PCEP speaker detects that there are two paths with the same Path ID,
   then the PCEP speaker SHOULD send PCError message with Error-Type = 1
   ("Reception of an invalid object") and Error-Value = TBD5
   ("Conflicting Path ID").

5.1.  Signaling Multiple Paths for Loadbalancing

   The PATH-ATTRIB object can be used to signal multiple path(s) and
   indicate (un)equal loadbalancing amongst the set of multipaths.  In
   this case, the PATH-ATTRIB is populated for each ERO as follows:



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   1.  The PCE assigns a unique Path ID to each ERO path and populates
       it inside the PATH-ATTRIB object.  The Path ID is unique within
       the context of a PLSP or PCEP Tunnel.

   2.  The MULTIPATH-WEIGHT TLV MAY be carried inside the PATH-ATTRIB
       object.  A weight is populated to reflect the relative loadshare
       that is to be carried by the path.  If the MULTIPATH-WEIGHT is
       not carried inside a PATH-ATTRIB object, the default weight 1
       MUST be assumed when computing the loadshare.

   3.  The fraction of flows carried by a specific primary path is
       derived from the ratio of its weight to the sum of all other
       multipath weights.

5.2.  Signaling Multiple Paths for Protection

   The PATH-ATTRIB object can be used to describe a set of backup
   path(s) protecting a primary path within a PCEP Tunnel.  In this
   case, the PATH-ATTRIB is populated for each ERO as follows:

   1.  The PCE assigns a unique Path ID to each ERO path and populates
       it inside the PATH-ATTRIB object.  The Path ID is unique within
       the context of a PLSP or PCEP Tunnel.

   2.  The MULTIPATH-BACKUP TLV MUST be added inside the PATH-ATTRIB
       object for each ERO that is protected.  The backup path ID(s) are
       populated in the MULTIPATH-BACKUP TLV to reflect the set of
       backup path(s) protecting the primary path.  The Length field and
       Backup Path Number in the MULTIPATH-BACKUP are updated according
       to the number of backup path ID(s) included.

   3.  The MULTIPATH-BACKUP TLV MAY be added inside the PATH-ATTRIB
       object for each ERO that is unprotected.  In this case,
       MULTIPATH-BACKUP does not carry any backup path IDs in the TLV.
       If the path acts as a pure backup - i.e. the path only carries
       rerouted traffic after the protected path(s) fail- then the B
       flag MUST be set.

   Note that if a given path has the B-flag set, then there MUST be some
   other path within the same LSP that uses the given path as a backup.
   If this condition is violated, then the PCEP speaker SHOULD send a
   PCError message with Error-Type = 10 ("Reception of an invalid
   object") and Error-Value = TBD6 ("No primary path for pure backup").








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   Note that a given PCC may not support certain backup combinations,
   such as a backup path that is itself protected by another backup
   path, etc.  If a PCC is not able to implement a requested backup
   scenario, the PCC SHOULD send a PCError message with Error-Type = 19
   ("Invalid Operation") and Error-Value = TBD7 ("Not supported path
   backup").

6.  PCEP Message Extensions

   The RBNF of PCReq, PCRep, PCRpt, PCUpd and PCInit messages currently
   use a combination of <intended-path> and/or <actual-path>.  As
   specified in Section 6.1 of [RFC8231], <intended-path> is represented
   by the ERO object and <actual-path> is represented by the RRO object:

      <intended-path> ::= <ERO>

      <actual-path> ::= <RRO>

   In this standard, we extend these two elements to allow multiple ERO/
   RRO objects to be present in the <intended-path>/<actual-path>:

      <intended-path> ::= (<ERO>|
                          (<PATH-ATTRIB><ERO>)
                          [<intended-path>])


      <actual-path> ::= (<RRO>|
                         (<PATH-ATTRIB><RRO>)
                         [<actual-path>])

7.  Examples

7.1.  SR Policy Candidate-Path with Multiple Segment-Lists

   Consider the following sample SR Policy, taken from
   [I-D.ietf-spring-segment-routing-policy].

           SR policy POL1 <headend, color, endpoint>
               Candidate-path CP1 <protocol-origin = 20, originator =
       100:1.1.1.1, discriminator = 1>
                   Preference 200
                   Weight W1, SID-List1 <SID11...SID1i>
                   Weight W2, SID-List2 <SID21...SID2j>
               Candidate-path CP2 <protocol-origin = 20, originator =
       100:2.2.2.2, discriminator = 2>
                   Preference 100
                   Weight W3, SID-List3 <SID31...SID3i>
                   Weight W4, SID-List4 <SID41...SID4j>



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   As specified in [I-D.ietf-pce-segment-routing-policy-cp], CP1 and CP2
   are signaled as separate state-report elements and each has a unique
   PLSP-ID, assigned by the PCC.  Let us assign PLSP-ID 100 to CP1 and
   PLSP-ID 200 to CP2.

   The state-report for CP1 can be encoded as:

   <state-report> =
           <LSP PLSP_ID=100>
           <ASSOCIATION>
           <END-POINT>
           <PATH-ATTRIB Path_ID=1 <WEIGHT-TLV Weight=W1>>
           <ERO SID-List1>
           <PATH-ATTRIB Path_ID=2 <WEIGHT-TLV Weight=W2>>
           <ERO SID-List2>

   The state-report for CP2 can be encoded as:

   <state-report> =
           <LSP PLSP_ID=200>
           <ASSOCIATION>
           <END-POINT>
           <PATH-ATTRIB Path_ID=1 <WEIGHT-TLV Weight=W3>>
           <ERO SID-List3>
           <PATH-ATTRIB Path_ID=2 <WEIGHT-TLV Weight=W4>>
           <ERO SID-List4>

   The above sample state-report elements only specify the minimum
   mandatory objects, of course other objects like SRP, LSPA, METRIC,
   etc., are allowed to be inserted.

   Note that the syntax

   <PATH-ATTRIB Path_ID=1 <WEIGHT-TLV Weight=W1>>

   , simply means that this is PATH-ATTRIB object with Path ID field set
   to "1" and with a MULTIPATH-WEIGHT TLV carrying weight of "W1".

7.2.  Two Primary Paths Protected by One Backup Path

   Suppose there are 3 paths: A, B, C.  Where A,B are primary and C is
   to be used only when A or B fail.  Suppose the Path IDs for A, B, C
   are respectively 1, 2, 3.  This would be encoded in a state-report
   as:







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   <state-report> =
           <LSP>
           <ASSOCIATION>
           <END-POINT>
           <PATH-ATTRIB Path_ID=1 <BACKUP-TLV B=0, Backup_Paths=[3]>>
           <ERO A>
           <PATH-ATTRIB Path_ID=2 <BACKUP-TLV B=0, Backup_Paths=[3]>>
           <ERO B>
           <PATH-ATTRIB Path_ID=3 <BACKUP-TLV B=1, Backup_Paths=[]>>
           <ERO C>

   Note that the syntax

   <PATH-ATTRIB Path_ID=1 <BACKUP-TLV B=0, Backup_Paths=[3]>>

   , simply means that this is PATH-ATTRIB object with Path ID field set
   to "1" and with a MULTIPATH-BACKUP TLV that has B-flag cleared and
   contains a single backup path with Backup Path ID of 3.

7.3.  Composite Candidate Path

   Consider the following Composite Candidate Path, taken from
   [I-D.ietf-spring-segment-routing-policy].

      SR policy POL100 <headend = H1, color = 100, endpoint = E1>
           Candidate-path CP1 <protocol-origin = 20, originator =
      100:1.1.1.1, discriminator = 1>
               Preference 200
               Weight W1, SR policy <color = 1>
               Weight W2, SR policy <color = 2>

   This is signaled in PCEP as:

       <LSP PLSP_ID=100>
           <ASSOCIATION>
           <END-POINT>
           <PATH-ATTRIB Path_ID=1
               <WEIGHT-TLV Weight=W1>
               <COLOR-TLV Color=1>>
           <SR-ERO (empty)>
           <PATH-ATTRIB Path_ID=2
               <WEIGHT-TLV Weight=W2>
               <COLOR-TLV Color=2>>
           <SR-ERO (empty)>







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7.4.  Opposite Direction Tunnels

   Consider the two opposite-direction SR Policies between end-points H1
   and E1.

       SR policy POL1 <headend = H1, color, endpoint = E1>
           Candidate-path CP1
               Preference 200
               Bidirectional Association = A1
               SID-List = <H1,M1,M2,E1>
               SID-List = <H1,M3,M4,E1>
           Candidate-path CP2
               Preference 100
               Bidirectional Association = A2
               SID-List = <H1,M5,M6,E1>
               SID-List = <H1,M7,M8,E1>

       SR policy POL2 <headend = E1, color, endpoint = H1>
           Candidate-path CP1
               Preference 200
               Bidirectional Association = A1
               SID-List = <E1,M2,M1,H1>
               SID-List = <E1,M4,M3,H1>
           Candidate-path CP2
               Preference 100
               Bidirectional Association = A2
               SID-List = <E1,M6,M5,H1>

   The state-report for POL1, CP1 can be encoded as:

       <state-report> =
           <LSP PLSP_ID=100>
           <BIDIRECTIONAL ASSOCIATION = A1>
           <PATH-ATTRIB PathID=1
               <OPPDIR-PATH-TLV R-flag=0 OppositePathID=3>>
           <ERO <H1,M1,M2,E1>>
           <PATH-ATTRIB PathID=2
               <OPPDIR-PATH-TLV R-flag=0 OppositePathID=4>>
           <ERO <H1,M3,M4,E1>>
           <PATH-ATTRIB PathID=3
               <OPPDIR-PATH-TLV R-flag=1 OppositePathID=1>>
           <ERO <E1,M2,M1,H1>>
           <PATH-ATTRIB PathID=4
               <OPPDIR-PATH-TLV R-flag=1 OppositePathID=2>>
           <ERO <E1,M4,M3,H1>>

   The state-report for POL1, CP2 can be encoded as:




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       <state-report> =
           <LSP PLSP_ID=200>
           <BIDIRECTIONAL ASSOCIATION = A2>
           <PATH-ATTRIB PathID=1
               <OPPDIR-PATH-TLV R-flag=0 OppositePathID=3>>
           <ERO <H1,M5,N6,E1>>
           <PATH-ATTRIB PathID=2
               <OPPDIR-PATH-TLV R-flag=0 OppositePathID=0>>
           <ERO <H1,M7,M8,E1>>
           <PATH-ATTRIB PathID=3
               <OPPDIR-PATH-TLV R-flag=1 OppositePathID=1>>
           <ERO <E1,M6,M5,H1>>

   The state-report for POL2, CP1 can be encoded as:

       <state-report> =
           <LSP PLSP_ID=100>
           <BIDIRECTIONAL ASSOCIATION = A1>
           <PATH-ATTRIB PathID=1
               <OPPDIR-PATH-TLV R-flag=0 OppositePathID=3>>
           <ERO <E1,M2,M1,H1>>
           <PATH-ATTRIB PathID=2
               <OPPDIR-PATH-TLV R-flag=0 OppositePathID=4>>
           <ERO <E1,M4,M3,H1>>
           <PATH-ATTRIB PathID=3
               <OPPDIR-PATH-TLV R-flag=1 OppositePathID=1>>
           <ERO <H1,M1,M2,E1>>
           <PATH-ATTRIB PathID=4
               <OPPDIR-PATH-TLV R-flag=1 OppositePathID=2>>
           <ERO <H1,M3,M4,E1>>

   The state-report for POL2, CP2 can be encoded as:

       <state-report> =
           <LSP PLSP_ID=200>
           <BIDIRECTIONAL ASSOCIATION = A2>
           <PATH-ATTRIB PathID=1
               <OPPDIR-PATH-TLV R-flag=0 OppositePathID=3>>
           <ERO <E1,M6,M5,H1>>
           <PATH-ATTRIB PathID=2
               <OPPDIR-PATH-TLV R-flag=1 OppositePathID=0>>
           <ERO <H1,M7,M8,E1>>
           <PATH-ATTRIB PathID=3
               <OPPDIR-PATH-TLV R-flag=1 OppositePathID=1>>
           <ERO <H1,M5,N6,E1>>

8.  IANA Considerations




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8.1.  PCEP Object

   IANA is requested to make the assignment of a new value for the
   existing "PCEP Objects" registry as follows:

    +--------------+-------------+-------------------+-----------------+
    | Object-Class | Name        | Object-Type       | Reference       |
    | Value        |             | Value             |                 |
    +--------------+-------------+-------------------+-----------------+
    | TBD2         | PATH-ATTRIB | 1                 | This document   |
    +--------------+-------------+-------------------+-----------------+

8.2.  PCEP TLV

   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      |                                   |                 |
    +------------+-----------------------------------+-----------------+
    | TBD1       | MULTIPATH-CAP                     | This document   |
    +------------+-----------------------------------+-----------------+
    | TBD3       | MULTIPATH-WEIGHT                  | This document   |
    +------------+-----------------------------------+-----------------+
    | TBD4       | MULTIPATH-BACKUP                  | This document   |
    +------------+-----------------------------------+-----------------+
    | TBD9       | MULTIPATH-OPPDIR-PATH             | This document   |
    +------------+-----------------------------------+-----------------+

8.3.  PCEP-Error Object

   IANA is requested to make the assignment of a new value for the
   existing "PCEP-ERROR Object Error Types and Values" sub-registry of
   the PCEP Numbers registry for the following errors:

    +------------+-----------------------------------+-----------------+
    | Error-Type | Error-Value                       | Reference       |
    +------------+-----------------------------------+-----------------+
    | 10         | TBD5 - Conflicting Path ID        | This document   |
    +------------+-----------------------------------+-----------------+
    | 10         | TBD6 - No primary path for pure   | This document   |
    |            |        backup                     |                 |
    +------------+-----------------------------------+-----------------+
    | 19         | TBD7 - Not supported path backup  | This document   |
    +------------+-----------------------------------+-----------------+
    | 19         | TBD8 - Non-empty path             | This document   |
    +------------+-----------------------------------+-----------------+



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8.4.  Flags in the Multipath Capability TLV

   IANA is requested to create a new sub-registry to manage the Flag
   field of the MULTIPATH-CAP TLV, called "Flags in MULTIPATH-CAP TLV".

    +------------+-----------------------------------+-----------------+
    | Bit        | Description                       | Reference       |
    +------------+-----------------------------------+-----------------+
    | 0-12       | Unassigned                        | This document   |
    +------------+-----------------------------------+-----------------+
    | 13         | 0-flag: support for processing    | This document   |
    |            | MULTIPATH-OPPDIR-PATH TLV         |                 |
    +------------+-----------------------------------+-----------------+
    | 14         | B-flag: support for processing    | This document   |
    |            | MULTIPATH-BACKUP TLV              |                 |
    +------------+-----------------------------------+-----------------+
    | 15         | W-flag: support for processing    | This document   |
    |            | MULTIPATH-WEIGHT TLV              |                 |
    +------------+-----------------------------------+-----------------+

8.5.  Flags in the Path Attribute Object

   IANA is requested to create a new sub-registry to manage the Flag
   field of the PATH-ATTRIBUTE object, called "Flags in PATH-ATTRIBUTE
   Object".

    +------------+-----------------------------------+-----------------+
    | Bit        | Description                       | Reference       |
    +------------+-----------------------------------+-----------------+
    | 0-12       | Unassigned                        | This document   |
    +------------+-----------------------------------+-----------------+
    | 13-15      | O-flag: Operational state         | This document   |
    +------------+-----------------------------------+-----------------+

8.6.  Flags in the Multipath Backup TLV

   IANA is requested to create a new sub-registry to manage the Flag
   field of the MULTIPATH-BACKUP TLV, called "Flags in MULTIPATH-BACKUP
   TLV".

    +------------+-----------------------------------+-----------------+
    | Bit        | Description                       | Reference       |
    +------------+-----------------------------------+-----------------+
    | 0-14       | Unassigned                        | This document   |
    +------------+-----------------------------------+-----------------+
    | 15         | B-flag: Pure backup               | This document   |
    +------------+-----------------------------------+-----------------+




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8.7.  Flags in the Multipath Opposite Direction Path TLV

   IANA is requested to create a new sub-registry to manage the flag
   fields of the MULTIPATH-OPPDIR-PATH TLV, called "Flags in the
   MULTIPATH-OPPDIR-PATH TLV".

    +------------+-----------------------------------+-----------------+
    | Bit        | Description                       | Reference       |
    +------------+-----------------------------------+-----------------+
    | 0-12       | Unassigned                        | This document   |
    +------------+-----------------------------------+-----------------+
    | 13         | L-flag: Link co-routed            | This document   |
    +------------+-----------------------------------+-----------------+
    | 14         | N-flag: Node co-routed            | This document   |
    +------------+-----------------------------------+-----------------+
    | 15         | R-flag: Reverse path              | This document   |
    +------------+-----------------------------------+-----------------+

9.  Security Considerations

   None at this time.

10.  Acknowledgement

   Thanks to Dhruv Dhody for ideas and discussion.

11.  Contributors

      Andrew Stone
      Nokia

      Email: andrew.stone@nokia.com

12.  References

12.1.  Normative References

   [I-D.ietf-pce-segment-routing-policy-cp]
              Koldychev, M., Sivabalan, S., Barth, C., Peng, S., and H.
              Bidgoli, "PCEP extension to support Segment Routing Policy
              Candidate Paths", Work in Progress, Internet-Draft, draft-
              ietf-pce-segment-routing-policy-cp-05, 23 May 2021,
              <https://www.ietf.org/archive/id/draft-ietf-pce-segment-
              routing-policy-cp-05.txt>.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", Work in



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              Progress, Internet-Draft, draft-ietf-spring-segment-
              routing-policy-13, 28 May 2021,
              <https://www.ietf.org/archive/id/draft-ietf-spring-
              segment-routing-policy-13.txt>.

   [I-D.koldychev-pce-operational]
              Koldychev, M., Sivabalan, S., Peng, S., Achaval, D., and
              H. Kotni, "PCEP Operational Clarification", Work in
              Progress, Internet-Draft, draft-koldychev-pce-operational-
              03, 19 February 2021, <https://www.ietf.org/archive/id/
              draft-koldychev-pce-operational-03.txt>.

   [I-D.peng-pce-te-constraints]
              Peng, S., Xiong, Q., Qin, F., Koldychev, M., and S.
              Sivabalan, "PCE TE Constraints", Work in Progress,
              Internet-Draft, draft-peng-pce-te-constraints-06, 11 July
              2021, <https://www.ietf.org/archive/id/draft-peng-pce-te-
              constraints-06.txt>.

   [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>.







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   [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>.

12.2.  Informative References

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

   [RFC8745]  Ananthakrishnan, H., Sivabalan, S., Barth, C., Minei, I.,
              and M. Negi, "Path Computation Element Communication
              Protocol (PCEP) Extensions for Associating Working and
              Protection Label Switched Paths (LSPs) with Stateful PCE",
              RFC 8745, DOI 10.17487/RFC8745, March 2020,
              <https://www.rfc-editor.org/info/rfc8745>.

Authors' Addresses

   Mike Koldychev
   Cisco Systems, Inc.

   Email: mkoldych@cisco.com


   Siva Sivabalan
   Ciena Corporation

   Email: ssivabal@ciena.com


   Tarek Saad
   Juniper Networks, Inc.

   Email: tsaad@juniper.net


   Vishnu Pavan Beeram
   Juniper Networks, Inc.

   Email: vbeeram@juniper.net







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   Hooman Bidgoli
   Nokia

   Email: hooman.bidgoli@nokia.com


   Bhupendra Yadav
   Ciena

   Email: byadav@ciena.com


   Shuping Peng
   Huawei Technologies

   Email: pengshuping@huawei.com



































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