Carrying SR-Algorithm in Path Computation Element Communication Protocol (PCEP).
draft-ietf-pce-sid-algo-17

PCE Working Group                                               S. Sidor
Internet-Draft                                                   Z. Rose
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: 17 July 2025                                            S. Peng
                                                         ZTE Corporation
                                                                 S. Peng
                                                     Huawei Technologies
                                                                A. Stone
                                                                   Nokia
                                                         13 January 2025

Carrying SR-Algorithm in Path Computation Element Communication Protocol
                                (PCEP).
                       draft-ietf-pce-sid-algo-17

Abstract

   This document specifies extensions to the Path Computation Element
   Communication Protocol (PCEP) to support Segment Routing (SR) with a
   focus on the use of Segment Identifiers (SIDs) and SR-Algorithms in
   Traffic Engineering (TE).  The SR-Algorithm associated with a SID
   defines the path computation algorithm used by Interior Gateway
   Protocols (IGPs).  This document proposes an approach for informing
   PCEP peers about the SR-Algorithm associated with each SID used, as
   well as signaling a specific SR-Algorithm as a constraint to the PCE.
   The mechanisms for specifying SR-Algorithm constraint is allowing
   refined path computations that meet specific operational needs, such
   as low-latency or high-bandwidth paths mostly based on operator-
   defined criteria using Flexible Algorithms.

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/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 17 July 2025.

Copyright Notice

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Object Formats  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  OPEN Object . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  SR PCE Capability Sub-TLV . . . . . . . . . . . . . .   6
       3.1.2.  SRv6 PCE Capability sub-TLV . . . . . . . . . . . . .   6
     3.2.  SR-ERO Subobject  . . . . . . . . . . . . . . . . . . . .   6
     3.3.  SRv6-ERO Subobject  . . . . . . . . . . . . . . . . . . .   7
     3.4.  LSPA Object . . . . . . . . . . . . . . . . . . . . . . .   7
     3.5.  Extensions to METRIC Object . . . . . . . . . . . . . . .   8
       3.5.1.  Path Min Delay Metric . . . . . . . . . . . . . . . .   9
       3.5.2.  P2MP Path Min Delay Metric  . . . . . . . . . . . . .   9
       3.5.3.  Path Min Delay Metric value . . . . . . . . . . . . .   9
       3.5.4.  Path Bandwidth Metric . . . . . . . . . . . . . . . .  10
       3.5.5.  P2MP Path Bandwidth Metric  . . . . . . . . . . . . .  10
       3.5.6.  Path Bandwidth Metric value . . . . . . . . . . . . .  10
       3.5.7.  User Defined Metric . . . . . . . . . . . . . . . . .  11
   4.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  SR-ERO and SRv6-ERO . . . . . . . . . . . . . . . . . . .  12
     4.2.  SR-Algorithm Constraint . . . . . . . . . . . . . . . . .  12
       4.2.1.  Flexible Algorithm Path computation . . . . . . . . .  14
       4.2.2.  Path computation with SID filtering . . . . . . . . .  15
       4.2.3.  New Metric types  . . . . . . . . . . . . . . . . . .  15
   5.  Manageability Considerations  . . . . . . . . . . . . . . . .  15
     5.1.  Control of Function and Policy  . . . . . . . . . . . . .  15

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     5.2.  Information and Data Models . . . . . . . . . . . . . . .  15
     5.3.  Verify Correct Operations . . . . . . . . . . . . . . . .  15
     5.4.  Impact On Network Operations  . . . . . . . . . . . . . .  16
   6.  Operational Considerations  . . . . . . . . . . . . . . . . .  16
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .  16
     7.1.  Cisco . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     7.2.  Huawei  . . . . . . . . . . . . . . . . . . . . . . . . .  17
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  SR Capability Flag  . . . . . . . . . . . . . . . . . . .  18
     9.2.  SRv6 PCE Capability Flag  . . . . . . . . . . . . . . . .  18
     9.3.  SR-ERO Flag . . . . . . . . . . . . . . . . . . . . . . .  18
     9.4.  SRv6-ERO Flag . . . . . . . . . . . . . . . . . . . . . .  19
     9.5.  PCEP TLV Types  . . . . . . . . . . . . . . . . . . . . .  19
     9.6.  Metric Types  . . . . . . . . . . . . . . . . . . . . . .  19
     9.7.  PCEP-Error Object . . . . . . . . . . . . . . . . . . . .  20
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     10.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Appendix A.  Acknowledgement  . . . . . . . . . . . . . . . . . .  23
   Appendix B.  Contributors . . . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

   Segment Routing (SR) enables the use of Segment Identifiers (SIDs) to
   define paths across a network, offering a flexible approach to
   Traffic Engineering (TE).  The Path Computation Element (PCE) plays a
   crucial role in computing SR-TE paths, which can utilize various SR-
   Algorithms based on specific use cases, constraints, and
   requirements.

   Both the PCE and the headend router may independently compute SR-TE
   paths with different SR-Algorithms.  This information needs to be
   exchanged between PCEP peers for purposes such as data collection and
   troubleshooting.  In scenarios involving multiple (redundant) PCEs,
   when a headend receives a path from the primary PCE, it needs to be
   able to report the complete path information, including the SR-
   Algorithm, to a backup PCE.  This is essential for high availability
   (HA) scenarios, ensuring that the backup PCE can correctly verify
   Prefix SIDs.

   The introduction of an SR-Algorithm TLV within the Label Switched
   Path Attributes (LSPA) object allows operators to specify SR-
   algorithm constraints directly, thereby refining path computations to
   meet specific needs, such as low-latency paths.

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   Building on concepts introduced in [RFC9350], this document extends
   the applicability of Flexible Algorithms for SR-TE paths in PCEP.
   Flexible Algorithms allow the customization of routing behavior by
   defining link attributes specific to SR-Algorithms, but also
   specifying constraints as part of Flexible Algorithm Definitions
   (FADs).  The PCE leverages these FADs to compute paths that adhere to
   operator-defined criteria, such as minimizing delay or optimizing
   bandwidth utilization.

   In the context of SR-TE, the PCE must ensure that paths computed
   using Flexible Algorithms are congruent with the desired routing
   policies and constraints.  This involves using the same ordered rules
   to select FADs when multiple options are available, and considering
   node participation in the specified SR-Algorithm during path
   computation.  The PCE must also optimize paths based on metrics
   defined within the FAD, ensuring alignment with the operator's
   objectives.  The introduction of new metric types, such as Path Min
   Delay Metric and Path Bandwidth Metric, further enhances the ability
   of PCE to compute paths that meet these criteria.

   The PCEP extensions for the SR-Algorithm based path computation
   include following components:

   *  Extends OPEN Object to indicate support for SR-Algorithm

   *  Extends SR-ERO, SRv6-ERO, SR-RRO and SRv6-RRO Subobjects to
      include Algorithm field

   *  Extends LSPA Object to include SR-Algorithm constraint

   *  Defines several new types for METRIC Object required to support
      SR-Algorithm based path computation

   The ability to specify an SR-Algorithm per SID in ERO and RRO is
   crucial for multiple reasons for example:

   *  SID types without algorithm specified - Certain SID types, such as
      Binding SIDs (BSIDs), may not have an SR-algorithm specified.  It
      may be inaccurate to state that an entire end-to-end path adheres
      to a specific algorithm if it includes a BSID from another policy.

   *  Topologies with two Interior Gateway Protocol (IGP) domains, each
      using the same Flexible Algorithm Definition (FAD) but with
      differing algorithm numbers.

   The mechanisms described in this document are equally applicable to
   both SR-MPLS and SRv6.

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2.  Terminology

   The following terminologies are used in this document:

   ASLA:  Application-Specific Link Attribute.

   BSID:  Binding Segment Identifier.

   ERO:  Explicit Route Object.

   FAD:  Flexible Algorithm Definition.

   IGP:  Interior Gateway Protocol.

   NAI:  Node or Adjacency Identifier.

   P2P:  Point-to-Point.

   P2MP:  Point-to-Multipoint.

   PCE:  Path Computation Element.

   PCEP:  Path Computation Element Protocol.

   SID:  Segment Identifier.

   SR:  Segment Routing.

   SR-TE:  Segment Routing Traffic Engineering.

   LSP:  Label Switched Path.

   LSPA:  Label Switched Path Attributes.

   Winning FAD:  The FAD selected according to rules described in
      Section 5.3 of [RFC9350].

3.  Object Formats

3.1.  OPEN Object

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3.1.1.  SR PCE Capability Sub-TLV

   A new flag S is proposed in the SR PCE Capability Sub-TLV introduced
   in Section 4.1.2 of [RFC8664] to indicate support for SR-Algorithm.
   If S flag is set, PCEP peer indicates support for Algorithm field in
   SR-ERO Subject and SR-Algorithm constraint only for Traffic-
   engineering paths with Segment Routing Path Setup Type.  It is not
   indicating support for these extensions for other Path Setup Types.

      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=26               |            Length=4           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Reserved              |   Flags |S|N|X|      MSD      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.1.2.  SRv6 PCE Capability sub-TLV

   A new flag S is proposed in the SRv6 PCE Capability sub-TLV
   introduced in 4.1.1 of [RFC9603] to indicate support for SR-
   Algorithm.  If S flag is set, PCEP peer indicates support for
   Algorithm field in SRv6-ERO Subobject and SR-Algorithm constraint
   only for Traffic-engineering paths with SRv6 Path Setup Type.  It is
   not indicating support for these extensions for other Path Setup
   Types.

    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=27            |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Reserved           |             Flags       |S|N| |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   MSD-Type    | MSD-Value     |   MSD-Type    |   MSD-Value   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                             ...                             //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   MSD-Type    | MSD-Value     |            Padding            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.  SR-ERO Subobject

   The SR-ERO subobject encoding is extended with new flag "A" to
   indicate if the Algorithm field is included after other optional
   fields.

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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |L|   Type=36   |     Length    |  NT   |     Flags   |A|F|S|C|M|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         SID (optional)                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                   NAI (variable, optional)                  //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Reserved                     |  Algorithm    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.3.  SRv6-ERO Subobject

   The SRv6-ERO subobject encoding is extended with new flag "A" to
   indicate if the Algorithm field is set.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |L|  Type=40    |     Length    |   NT  |    Flags    |A|V|T|F|S|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Reserved   |   Algorithm   |        Endpoint Behavior      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                      SRv6 SID (optional)                      |
     |                           (128-bit)                           |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                    NAI (variable, optional)                 //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     SID Structure (optional)                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.4.  LSPA Object

   A new TLV for the LSPA Object with TLV type=66 is introduced to carry
   the SR-Algorithm constraint.  This TLV SHOULD only be used when PST
   (Path Setup type) = SR or SRv6.  Only the first instance of this TLV
   SHOULD be processed, subsequent instances SHOULD be ignored

   The format of the SR-Algorithm TLV is as follows:

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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=66               |            Length=4           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Reserved              |   Flags   |F|S|   Algorithm   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 1: SR-Algorithm TLV Format

   The code point for the TLV type is 66.  The TLV length is 4 octets.

   The 32-bit value is formatted as follows.

   Reserved:  MUST be set to zero by the sender and MUST be ignored by
      the receiver.

   Flags:  This document defines the following flag bits.  The other
      bits MUST be set to zero by the sender and MUST be ignored by the
      receiver.

      *  S (Strict): If set, the PCE MUST fail the path computation if
         specified SR-Algorithm constraint cannot be satisfied.  If
         unset, the PCE SHOULD try to compute path with SR-algorithm
         constraint specified.  If such computation is not successful,
         then a path that that does not satisfy the specified SR-
         algorithm constraint can be computed.

      *  F (Flexible Algorithm Path Computation): If set, the PCE
         follows procedures defined in Section 4.2.1.  If unset, the PCE
         follows procedures defined in Section 4.2.2.  The flag SHOULD
         be ignored if Algorithm field is set to value in range 0 to
         127.

   Algorithm:  SR-Algorithm to be used during path computation.

3.5.  Extensions to METRIC Object

   The METRIC object is defined in Section 7.8 of [RFC5440].  This
   document defines the following types for the METRIC object.

   *  T=22: Path Min Delay metric (Section 3.5.2)

   *  T=23: P2MP Path Min Delay metric (Section 3.5.3)

   *  T=24: Path Bandwidth Metric (Section 3.5.5)

   *  T=25: P2MP Path Bandwidth Metric (Section 3.5.6)

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   *  T=128-255: User-defined metric (Section 3.5.7)

   Metric type values for "Path Bandwidth Metric", "P2MP Path Bandwidth
   Metric" and "User Defined metric" are suggested values only for IANA
   to allocate.

3.5.1.  Path Min Delay Metric

   The minimum Link Delay metric is defined in [RFC7471] and [RFC8570]
   as "Min Unidirectional Link Delay".  The Path Min Link Delay metric
   represents measured minimum link delay value over a configurable
   interval.

   The Path Min Delay metric type of the METRIC object in PCEP
   represents the sum of the Min Link Delay metric of all links along a
   P2P path.

   *  A Min Link Delay metric of link L is denoted D(L).

   *  A path P of a P2P LSP is a list of K links {Lpi,(i=1...K)}.

   *  A Path Min Delay metric for the P2P path P = Sum {D(Lpi),
      (i=1...K)}.

3.5.2.  P2MP Path Min Delay Metric

   The P2MP Path Min Delay metric type of the METRIC object in PCEP
   encodes the Path Min Delay metric for the destination that observes
   the worst delay metric among all destinations of the P2MP tree.

   *  A P2MP tree T comprises a set of M destinations {Dest_j,
      (j=1...M)}.

   *  The P2P Path Min Delay metric of the path to destination Dest_j is
      denoted by PMDM(Dest_j).

   *  The P2MP Path Min Delay metric for the P2MP tree T =
      Maximum{PMDM(Dest_j), (j=1...M)}.

3.5.3.  Path Min Delay Metric value

   [RFC7471] and [RFC8570] define "Min/Max Unidirectional Link Delay
   Sub-TLV" to advertise the link minimum and maximum delay in
   microseconds in a 24-bit field.

   [RFC5440] defines the METRIC object with a 32-bit metric value
   encoded in IEEE floating point format.

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   The encoding for the Path Min Delay metric value is quantified in
   units of microseconds and encoded in IEEE floating point format.

   The conversion from 24-bit integer to 32-bit IEEE floating point
   could introduce some loss of precision.

3.5.4.  Path Bandwidth Metric

   The Path Bandwidth metric type of the METRIC object in PCEP
   represents the sum of the Bandwidth Metric of all links along a P2P
   path.  Note: the link Bandwidth Metric utilized in the formula may be
   the original metric advertised on the link, which may have a value
   inversely proportional to the link capacity.

   *  A Bandwidth Metric of link L is denoted B(L).

   *  A path P of a P2P LSP is a list of K links {Lpi,(i=1...K)}.

   *  A Path Bandwidth metric for the P2P path P = Sum {B(Lpi),
      (i=1...K)}.

3.5.5.  P2MP Path Bandwidth Metric

   The Bandwidth metric type of the METRIC object in PCEP encodes the
   Path Bandwidth metric for the destination that observes the worst
   bandwidth metric among all destinations of the P2MP tree.

   *  A P2MP tree T comprises a set of M destinations {Dest_j,
      (j=1...M)}.

   *  The P2P Bandwidth metric of the path to destination Dest_j is
      denoted by BM(Dest_j).

   *  The P2MP Path Bandwidth metric for the P2MP tree T =
      Maximum{BM(Dest_j), (j=1...M)}.

3.5.6.  Path Bandwidth Metric value

   The section 4 of [I-D.ietf-lsr-flex-algo-bw-con] defines new metric
   type "Bandwidth Metric", which MAY be advertised in their link metric
   advertisements.

   When performing Flexible Algorithm path computation as described in
   section 4.2.1, procedures described in section 4.1 and 5 from
   [I-D.ietf-lsr-flex-algo-bw-con] MUST be followed with automatic
   metric calculation attempted.

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   When performing path computation for other algorithms and Generic
   Metric sub-TLV with Bandwidth metric type is not advertised for the
   link then PCE implementation MAY have local policy to specify
   attributes similar to section 4.1.3 and 4.1.4 in
   [I-D.ietf-lsr-flex-algo-bw-con] and compute metric value
   automatically or the link SHOULD be treated as if the metric value is
   not available for other metric types (e.g. use default value
   instead).  If Bandwidth metric value is advertised for the link, then
   PCE MUST use value advertised and compute path metric as described in
   Section 3.5.5 and 3.5.6.

   The Path Bandwidth metric value is encoded in IEEE floating point
   format.

   The conversion from 24-bit integer to 32-bit IEEE floating point
   could introduce some loss of precision.

3.5.7.  User Defined Metric

   The section 2 of [I-D.ietf-lsr-flex-algo-bw-con] defined new metric
   type range for "User defined metric", which MAY be advertised in
   their link metric advertisements.  These are user defined and can be
   assigned by an operator for local use.

   The encoding for the User Defined metric values is encoded in IEEE
   floating point format.

   The conversion from 24-bit integer to 32-bit IEEE floating point
   could introduce some loss of precision.

   Proposed metric type range was chosen to allow mapping with values
   assigned in "IGP Metric-Type Registry".  For example, the User
   Defined metric type 130 of the METRIC object in PCEP can represent
   the sum of the User Defined Metric 130 of all links along a P2P or
   P2MP path.

   User Defined Metric are equally applicable to P2P and P2MP paths.

4.  Operation

   The PCEP protocol extensions defined in Sections 3.2, 3.3 and 3.4 of
   this draft MUST NOT be used if one or both PCEP speakers have not
   indicated the support using S flag in Path Setup Type specific Sub-
   TLVs in their respective OPEN messages.

   SR-Algorithm used in this document refers to complete range of SR-
   Algorithm values (0-255) if specific section does not specify
   otherwise.  Valid SR-Algorithm values are defined in registry "IGP

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   Algorithm Types" of "Interior Gateway Protocol (IGP) Parameters" IANA
   registry.  Refer to Section 3.1.1 of [RFC8402] and [RFC9256] for
   definition of SR-Algorithm in Segment Routing.  [RFC8665] and
   [RFC8667] are describing use of SR-Algorithm in IGP.  Note that some
   RFCs are referring to SR-Algorithm with different names, for example
   "Prefix-SID Algorithm" and "SR Algorithm".

4.1.  SR-ERO and SRv6-ERO

   PCEP speaker MAY set the A flag and include the Algorithm field in
   SR-ERO or SRv6-ERO subobject if the S flag was advertised by both
   PCEP speakers.

   If PCC received the Algorithm field in subobjects of ERO in
   PCInitiate, PCUpd or PCRep and path received from those messages is
   being included in ERO of PCRpt, then PCC MUST include the Algorithm
   field in encoded subobjects with received SR-Algorithm value.

   If PCEP peer receives SR-ERO subobject with the A flag set or with
   the SR-Algorithm included, but the S flag was not advertised, then it
   MUST consider entire ERO as invalid as described in Section 5.2.1 of
   [RFC8664]

   In case of SR-ERO subobject, the Algorithm field MUST be included
   after optional SID, NAI or SID structure and length of SR-ERO
   subobject MUST be increased with additional 4 bytes for Reserved and
   Algorithm field.

   In case of SRv6-ERO subobject, the Algorithm field MUST be included
   in position specified in Section 3.3, length of SRv6-ERO subobject is
   not impacted by inclusion of Algorithm field.

   If the length and the A flag are not consistent, PCEP peer MUST
   consider the entire ERO invalid and MUST send a PCErr message with
   Error-Type = 10 ("Reception of an invalid object") and Error-value =
   11 ("Malformed object").

   As per [RFC9603] and [RFC8664], the format of the SR-RRO subobject is
   the same as that of the SR-ERO subobject, but without the L-Flag,
   therefore SR-RRO subobject may also carry the A flag and Algorithm
   field.

4.2.  SR-Algorithm Constraint

   In order to signal a specific SR-Algorithm constraint to the PCE, the
   headend MUST encode the SR-Algorithm TLV inside the LSPA object.

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   If PCC received LSPA object with SR-Algorithm TLV as part of
   PCInitiate, PCUpd messages, then it MUST include LSPA object with SR-
   Algorithm TLV in PCRpt message as part of intended-attribute-list.

   If PCE received LSPA object with SR-Algorithm TLV in PCRpt or PCReq,
   then it MUST include LSPA object with SR-Algorithm TLV in PCUpd
   message, or PCRep message in case of unsuccessful path computation
   based on rules described in Section 7.11 of [RFC5440].

   If PCEP peer receives LSPA object with SR-Algorithm TLV in it, but
   the S flag was not advertised, then PCEP peer MUST ignore it as per
   Section 7.1 of [RFC5440].

   Path computation MUST occur on the topology associated with specified
   SR-Algorithm.  The PCE MUST NOT use Prefix SIDs of SR-Algorithm other
   than specified in SR-Algorithm constraint.  It is allowed to use
   other SID types (e.g., Adjacency or BSID), but only from nodes
   participating in specified SR-Algorithm.  Furthermore, the inclusion
   of a path BSID from another policy is allowed only if the path
   associated with such a policy fully satisfies all the constraints of
   the current path computation.  This ensures that while leveraging
   BSIDs from different policies, requirements of the SR-Algorithm
   constraints are maintained.

   Specified SR-Algorithm constraint is applied to end-to-end SR policy
   path.  Using different SR-Algorithm constraint in each domain or part
   of the topology in single path computation is out of scope of this
   document.  One possible solution is to determine FAD mapping using
   PCE local policy.

   If the PCE is unable to find a path with the given SR-Algorithm
   constraint or it does not support combination of specified
   constraints, it MUST use empty ERO in PCInitiate for LSP
   instantiation or PCUpdate message if update is required or NO-PATH
   object in PCRep to indicate that it was not able to find valid path.

   If headend is part of multiple IGP domains and winning FAD for
   specified SR-Algorithm in each of them has different constraints, the
   PCE implementation MAY have local policy with defined behavior for
   selecting FAD for such path computation or even completely not
   supporting it.  If such path computation is not supported, PCE MUST
   send a PCErr message with Error-Type = 6 (Path computation failure)
   and Error-value = TBD3 (Unsupported combination of constraints).

   If NO-PATH object is included in PCRep, then PCE MAY include SR-
   Algorithm TLV to indicate constraint, which cannot be satisfied as
   described in section 7.5 of [RFC5440].

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   SR-Algorithm does not replace the Objective Function defined in
   [RFC5541]

4.2.1.  Flexible Algorithm Path computation

   This section is applicable only to Flexible Algorithms range of SR-
   Algorithm values.

   The PCE MUST follow IGP Flexible Algorithm path computation logic as
   described in [RFC9350].  That includes using same ordered rules to
   select FAD if multiple FADs are available, considering node
   participation of specified SR-Algorithm in the path computation,
   using ASLA specific link attributes and other rules for Flexible
   Algorithm path computation described in that document.

   The PCE MUST optimize computed path based on metric type specified in
   the FAD, metric type included in PCEP messages from PCC MUST be
   ignored.  The PCE SHOULD use metric type from FAD in messages sent to
   the PCC.  If corresponding metric type is not defined in PCEP, PCE
   SHOULD skip encoding of metric object for optimization metric.

   There are corresponding metric types in PCEP for IGP and TE metric
   from FAD introduced in [RFC9350], but there were no corresponding
   metric types defined for "Min Unidirectional Link Delay" from
   [RFC9350] and "Bandwidth Metric", "User Defined Metric" from
   [I-D.ietf-lsr-flex-algo-bw-con].  Section 3.5 of this document is
   introducing them.  Note that the defined "Path Bandwidth Metric" is
   accumulative and is different from the Bandwidth Object defined in
   [RFC5440]

   The PCE MUST use constraints specified in the FAD and also
   constraints (except optimization metric type) directly included in
   PCEP messages from PCC.  The PCE implementation MAY decide to ignore
   specific constraints received from PCC based on existing processing
   rules for PCEP Objects and TLVs, e.g.  P flag described in
   Section 7.2 of [RFC5440] and processing rules described in
   [I-D.ietf-pce-stateful-pce-optional].  If the PCE does not support
   specified combination of constraints, it MUST fail path computation
   and respond with PCEP message with PCInitiate or PCUpdate message
   with empty ERO or PCRep with NO-PATH object.  PCC MUST NOT include
   constraints from FAD in PCEP message sent to PCE as it can result in
   undesired behavior in various cases.  PCE SHOULD NOT include
   constraints from FAD in PCEP messages sent to PCC.

   The combinations of constraints specified in the FAD and constraints
   directly included in PCEP messages from PCC may decrease a chance
   that Flex-algo specific Prefix SIDs represents optimal path while
   satisfying all specified constraints, as a result a longer SID list

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   may be required for the computed path.  Adding more constraints on
   top of FAD requires complex path computation, and may reduce the
   benefit of this scheme.

4.2.2.  Path computation with SID filtering

   The SR-Algorithm constraint acts as a filter, restricting which SIDs
   may be used as a result of the path computation function.  Path
   computation is done based on optimization metric type and constraints
   specified in PCEP message received from PCC.

   If the specified SR-Algorithm is Flexible Algorithm, the PCE MUST
   ensure that IGP path of Flexible Algorithm SIDs is congruent with
   computed path.

4.2.3.  New Metric types

   All the rules of processing the METRIC object as explained in
   [RFC5440] and [RFC8233] are applicable to new metric types defined in
   this document.

5.  Manageability Considerations

   All manageability requirements and considerations listed in
   [RFC5440], [RFC8231], [RFC8281], [RFC8664] and [RFC9603] apply to
   PCEP protocol extensions defined in this document.  In addition,
   requirements and considerations listed in this section apply.

5.1.  Control of Function and Policy

   A PCE or PCC implementation MAY allow the capability of supporting
   PCEP extensions introduced in this document to be enabled/disabled as
   part of the global configuration.

5.2.  Information and Data Models

   An implementation SHOULD allow the operator to view the capability
   defined in this document.  Section 4.1 and 4.1.1 of
   [I-D.ietf-pce-pcep-yang] should be extended to include that
   capabilities introduced in Section 3.1.1 and 3.1.2 for PCEP peer.

5.3.  Verify Correct Operations

   Operation verification requirements already listed in [RFC5440],
   [RFC8231], [RFC8281] and [RFC8664] are applicable to mechanisms
   defined in this document.

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   An implementation SHOULD also allow the operator to view FADs, which
   MAY be used in Flexible Algorithm path computation defined in
   Section 4.2.1.

   An implementation SHOULD allow the operator to view nodes
   participating in specified SR-Algorithm.

5.4.  Impact On Network Operations

   The mechanisms defined in [RFC5440], [RFC8231], and [RFC8281] also
   apply to the PCEP extensions defined in this document.

   This document inherits considerations from documents describing IGP
   Flexible Algorithm - for example [RFC9350] and
   [I-D.ietf-lsr-flex-algo-bw-con].

6.  Operational Considerations

   This document inherits considerations from documents describing IGP
   Flexible Algorithm - for example [RFC9350] and
   [I-D.ietf-lsr-flex-algo-bw-con].

7.  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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7.1.  Cisco

   *  Organization: Cisco Systems

   *  Implementation: IOS-XR PCC and PCE.

   *  Description: SR-MPLS part with experimental codepoints.

   *  Maturity Level: Production.

   *  Coverage: Partial.

   *  Contact: ssidor@cisco.com

7.2.  Huawei

   *  Organization: Huawei

   *  Implementation: NE Series Routers

   *  Description: SR Policy with SR Algorithm.

   *  Maturity Level: Production.

   *  Coverage: Partial.

   *  Contact: pengshuping@huawei.com

8.  Security Considerations

   The security considerations described in [RFC5440], [RFC8231],
   [RFC8253], [RFC8281], [RFC8664], [RFC9603] and [RFC9350] in itself.

   Note that this specification introduces possibility to compute paths
   by PCE based on Flexible Algorithm related topology attributes and
   based on metric type and constraints from FAD.  This creates
   additional vulnerabilities, which are already described for path
   computation done by IGP like those described in Security
   Considerations section of [RFC9350], but which are also applicable to
   path computation done by PCE.

9.  IANA Considerations

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9.1.  SR Capability Flag

   IANA maintains a registry, named "SR Capability Flag Field", within
   the "Path Computation Element Protocol (PCEP) Numbers" registry group
   to manage the Flags field of the SR-PCE-CAPABILITY TLV.  IANA is
   requested to confirm the following early allocation:

             +=====+=========================+===============+
             | Bit | Description             | Reference     |
             +=====+=========================+===============+
             +-----+-------------------------+---------------+
             |  5  | SR-Algorithm Capability | This document |
             +-----+-------------------------+---------------+

                                  Table 1

9.2.  SRv6 PCE Capability Flag

   IANA maintains a registry, named "SRv6 PCE Capability Flags", within
   the "Path Computation Element Protocol (PCEP) Numbers" registry group
   to manage the Flags field of SRv6-PCE-CAPABILITY sub-TLV.  IANA is
   requested to make the following assignment:

            +======+=========================+===============+
            | Bit  | Description             | Reference     |
            +======+=========================+===============+
            +------+-------------------------+---------------+
            | TBD1 | SR-Algorithm Capability | This document |
            +------+-------------------------+---------------+

                                 Table 2

9.3.  SR-ERO Flag

   IANA maintains a registry, named "SR-ERO Flag Field", within the
   "Path Computation Element Protocol (PCEP) Numbers" registry group to
   manage the Flags field of the SR-ERO Subobject.  IANA is requested to
   confirm the following early allocation:

                +=====+===================+===============+
                | Bit | Description       | Reference     |
                +=====+===================+===============+
                +-----+-------------------+---------------+
                |  7  | SR-Algorithm Flag | This document |
                +-----+-------------------+---------------+

                                  Table 3

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9.4.  SRv6-ERO Flag

   IANA maintains a registry, named "SRv6-ERO Flag Field", within the
   "Path Computation Element Protocol (PCEP) Numbers" registry group to
   manage the Flags field of the SRv6-ERO subobject.  IANA is requested
   to make the following assignment:

               +======+===================+===============+
               | Bit  | Description       | Reference     |
               +======+===================+===============+
               +------+-------------------+---------------+
               | TBD2 | SR-Algorithm Flag | This document |
               +------+-------------------+---------------+

                                 Table 4

9.5.  PCEP TLV Types

   IANA maintains a registry, named "PCEP TLV Type Indicators", within
   the "Path Computation Element Protocol (PCEP) Numbers" registry
   group.  IANA is requested to confirm the early allocation of a new
   TLV type for the new LSPA TLV specified in this document.

                  +======+==============+===============+
                  | Type | Description  | Reference     |
                  +======+==============+===============+
                  +------+--------------+---------------+
                  |  66  | SR-Algorithm | This document |
                  +------+--------------+---------------+

                                  Table 5

9.6.  Metric Types

   IANA maintains a registry for "METRIC Object T Field" within the
   "Path Computation Element Protocol (PCEP) Numbers" registry group.
   IANA is requested to confirm the early allocated codepoints as
   follows:

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         +=========+============================+===============+
         |   Type  | Description                | Reference     |
         +=========+============================+===============+
         +---------+----------------------------+---------------+
         |    22   | Path Min Delay Metric      | This document |
         +---------+----------------------------+---------------+
         |    23   | P2MP Path Min Delay Metric | This document |
         +---------+----------------------------+---------------+
         |    24   | Path Bandwidth Metric      | This document |
         +---------+----------------------------+---------------+
         |    25   | P2MP Path Bandwidth Metric | This document |
         +---------+----------------------------+---------------+
         | 128-255 | User Defined Metric        | This document |
         +---------+----------------------------+---------------+

                                 Table 6

9.7.  PCEP-Error Object

   IANA is requested to allocate new error types and error values within
   the "PCEP-ERROR Object Error Types and Values" sub-registry of the
   PCEP Numbers registry for the following errors.

      +============+==================+============================+
      | Error-Type | Meaning          | Error-Value                |
      +============+==================+============================+
      +------------+------------------+----------------------------+
      |     29     | Path computation | TBD3:Unsupported           |
      |            | failure          | combination of constraints |
      +------------+------------------+----------------------------+

                                 Table 7

10.  References

10.1.  Normative References

   [I-D.ietf-lsr-flex-algo-bw-con]
              Hegde, S., Britto, W., Shetty, R., Decraene, B., Psenak,
              P., and T. Li, "Flexible Algorithms: Bandwidth, Delay,
              Metrics and Constraints", Work in Progress, Internet-
              Draft, draft-ietf-lsr-flex-algo-bw-con-17, 4 December
              2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
              lsr-flex-algo-bw-con-17>.

   [I-D.ietf-pce-pcep-yang]
              Dhody, D., Beeram, V. P., Hardwick, J., and J. Tantsura,
              "A YANG Data Model for Path Computation Element

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              Communications Protocol (PCEP)", Work in Progress,
              Internet-Draft, draft-ietf-pce-pcep-yang-28, 18 December
              2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
              pce-pcep-yang-28>.

   [I-D.ietf-pce-stateful-pce-optional]
              Li, C., Zheng, H., and S. Litkowski, "Extension for
              Stateful PCE to allow Optional Processing of PCE
              Communication Protocol (PCEP) Objects", Work in Progress,
              Internet-Draft, draft-ietf-pce-stateful-pce-optional-13,
              27 November 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-pce-stateful-pce-optional-13>.

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

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [RFC5541]  Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of
              Objective Functions in the Path Computation Element
              Communication Protocol (PCEP)", RFC 5541,
              DOI 10.17487/RFC5541, June 2009,
              <https://www.rfc-editor.org/info/rfc5541>.

   [RFC7471]  Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
              Previdi, "OSPF Traffic Engineering (TE) Metric
              Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,
              <https://www.rfc-editor.org/info/rfc7471>.

   [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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   [RFC8233]  Dhody, D., Wu, Q., Manral, V., Ali, Z., and K. Kumaki,
              "Extensions to the Path Computation Element Communication
              Protocol (PCEP) to Compute Service-Aware Label Switched
              Paths (LSPs)", RFC 8233, DOI 10.17487/RFC8233, September
              2017, <https://www.rfc-editor.org/info/rfc8233>.

   [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>.

   [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>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8570]  Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward,
              D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE)
              Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March
              2019, <https://www.rfc-editor.org/info/rfc8570>.

   [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>.

   [RFC8665]  Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
              H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", RFC 8665,
              DOI 10.17487/RFC8665, December 2019,
              <https://www.rfc-editor.org/info/rfc8665>.

   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

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   [RFC9256]  Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
              A., and P. Mattes, "Segment Routing Policy Architecture",
              RFC 9256, DOI 10.17487/RFC9256, July 2022,
              <https://www.rfc-editor.org/info/rfc9256>.

   [RFC9350]  Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
              and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
              DOI 10.17487/RFC9350, February 2023,
              <https://www.rfc-editor.org/info/rfc9350>.

   [RFC9603]  Li, C., Ed., Kaladharan, P., Sivabalan, S., Koldychev, M.,
              and Y. Zhu, "Path Computation Element Communication
              Protocol (PCEP) Extensions for IPv6 Segment Routing",
              RFC 9603, DOI 10.17487/RFC9603, July 2024,
              <https://www.rfc-editor.org/info/rfc9603>.

10.2.  Informative References

   [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>.

Appendix A.  Acknowledgement

   Would like to thank Adrian Farrel, Aijun Wang, Boris Khasanov, Dhruv
   Dhody, Jie Dong, Marina Fizgeer, Nagendra Nainar, Rakesh Gandhi, Russ
   White, Tom Petch for review and suggestions.

Appendix B.  Contributors

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   Mike Koldychev
   Cisco Systems, Inc.
   Email: mkoldych@cisco.com

   Zafar Ali
   Cisco Systems, Inc.
   Email: zali@cisco.com

   Stephane Litkowski
   Cisco Systems, Inc.
   Email: slitkows.ietf@gmail.com

   Siva Sivabalan
   Ciena
   Email: msiva282@gmail.com

   Tarek Saad
   Cisco Systems, Inc.
   Email: tsaad.net@gmail.com

   Mahendra Singh Negi
   RtBrick Inc
   Email: mahend.ietf@gmail.com

   Tom Petch
   Email: ietfc@btconnect.com

Authors' Addresses

   Samuel Sidor
   Cisco Systems, Inc.
   Eurovea Central 3.
   Pribinova 10
   811 09 Bratislava
   Slovakia
   Email: ssidor@cisco.com

   Zoey Rose
   Cisco Systems, Inc.
   2300 East President George
   Richardson,  TX 75082
   United States of America
   Email: atokar@cisco.com

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   Shaofu Peng
   ZTE Corporation
   No.50 Software Avenue
   Nanjing
   Jiangsu, 210012
   China
   Email: peng.shaofu@zte.com.cn

   Shuping Peng
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing
   100095
   China
   Email: pengshuping@huawei.com

   Andrew Stone
   Nokia
   Email: andrew.stone@nokia.com

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