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Path Computation Element (PCE) Protocol Extensions for Stateful PCE Usage in GMPLS-controlled Networks

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Xian Zhang , Young Lee , Fatai Zhang , Ramon Casellas , Oscar Gonzalez de Dios , Zafar Ali
Last updated 2018-02-13
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PCE Working Group                                            Xian Zhang
Internet-Draft                                       Young Lee (Editor)
Intended status: Standards Track                            Fatai Zhang
                                                         Ramon Casellas
                                                 Oscar Gonzalez de Dios
                                                         Telefonica I+D
                                                              Zafar Ali
                                                          Cisco Systems

Expires: August 13, 2018                              February 13, 2018

  Path Computation Element (PCE) Protocol Extensions for Stateful PCE
                   Usage in GMPLS-controlled Networks



   The Path Computation Element (PCE) facilitates Traffic Engineering
   (TE) based path calculation in large, multi-domain, multi-region, or
   multi-layer networks. The PCE communication Protocol (PCEP) has been
   extended to support stateful PCE functions where the PCE retains
   information about the paths already present in the network, but
   those extensions are technology-agnostic. This memo provides
   extensions required for PCEP so as to enable the usage of a stateful
   PCE capability in GMPLS-controlled networks.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with
   the provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   documents at any   time.  It is inappropriate to use Internet-Drafts

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   as reference   material or to cite them other than as "work in

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on August 13, 2018.

Copyright Notice

   Copyright (c) 2018 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
   ( 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.

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [RFC2119].

Table of Contents

   Table of Contents.................................................2
   1. Introduction...................................................3
   2. Context of Stateful PCE and PCE for GMPLS......................3
   3. Main Requirements..............................................4
   4. PCEP Extensions................................................5
      4.1. LSP Update in GMPLS-controlled Networks...................5
      4.2. LSP Synchronization in GMPLS-controlled Networks..........5
      4.3. Modification of Existing PCEP Messages and Procedures.....7
         4.3.1. Modification for LSP Re-optimization.................7
         4.3.2. Modification for Route Exclusion.....................7
      4.4. Object Encoding...........................................9

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   5. IANA Considerations............................................9
      5.1. New PCEP Error Codes......................................9
      5.2. New Subobject for the Exclude Route Object................9
   6. Manageability Considerations..................................10
      6.1. Requirements on Other Protocols and Functional Components10
   7. Security Considerations.......................................10
   8. Acknowledgement...............................................10
   9. References....................................................10
      9.1. Normative References.....................................10
      9.2. Informative References...................................11
   10. Contributors' Address........................................11
   Authors' Addresses...............................................12

1. Introduction

   [RFC4655] presents the architecture of a Path Computation Element
   (PCE)-based model for computing Multiprotocol Label Switching (MPLS)
   and Generalized MPLS (GMPLS) Traffic Engineering Label Switched
   Paths (TE LSPs).  To perform such a constrained computation, a PCE
   stores the network topology (i.e., TE links and nodes) and resource
   information (i.e., TE attributes) in its TE Database (TED).  Such a
   PCE is usually referred as a stateless PCE. To request path
   computation services to a PCE, [RFC5440] defines the PCE
   communication Protocol (PCEP) for interaction between a Path
   Computation Client (PCC) and a PCE, or between two PCEs.  PCEP as
   specified in [RFC 5440] mainly focuses on MPLS networks and the PCEP
   extensions needed for GMPLS-controlled networks are provided in

   Stateful PCEs are shown to be helpful in many application scenarios,
   in both MPLS and GMPLS networks, as illustrated in [RFC8051].
   Further discussion of concept of a stateful PCE can be found in
   [RFC7399].  In order for these applications to able to exploit the
   capability of stateful PCEs, extensions to PCEP are required.

   [RFC8231] provides the fundamental extensions needed for stateful
   PCE to support general functionality, but leaves out the
   specification for technology-specific objects/TLVs. This document
   focuses on the extensions that are necessary in order for the
   deployment of stateful PCEs in GMPLS-controlled networks.

2. Context of Stateful PCE and PCEP for GMPLS

   This document is built on the basis of Stateful PCE [RFC8231] and

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   There are two types of LSP operation for Stateful PCE.

   For Active Stateful PCE, PCUpd message is sent from PCE to PCC to
   update the LSP state for the LSP delegated to PCE. Any changes to
   the delegated LSPs generate a PCRpt message by the PCC to PCE to
   convey the changes of the LSP. Any modifications to the Objects/TLVs
   that are identified in this document to support GMPLS technology-
   specific attributes will be carried in the PCRpt and PCUpd messages.

   For Passive Stateful PCE where PCReq/PCRep messages are used to
   convey path computation instruction. As GMPLS-technology specific
   Objects/TLVs are defined in [PCEP-GMPLS], this document just points
   to the work in [PCEP-GMPLS] and add only the stateful PCE aspect
   only if applicable. Passive Stateful PCE makes use of PCRep messages
   when reporting LSP State changes sent by PCC to PCEs. Any
   modifications to the Objects/TLVs that are identified in this
   document to support GMPLS technology-specific attributes will be
   carried in the PCRpt message.

   [PCEP-GMPLS] defines GMPLS-technology specific Objects/TLVs and this
   document makes use of these Objects/TLVs without modifications where
   applicable. Some of these Objects/TLVs may require modifications to
   incorporate stateful PCE element where applicable.

3. Main Requirements

   This section notes the main functional requirements for PCEP
   extensions to support stateful PCE for use in GMPLS-controlled
   networks, based on the description in [RFC8051].  Many
   requirements are common across a variety of network types (e.g.,
   MPLS-TE networks and GMPLS networks) and the protocol extensions to
   meet the requirements are already described in [RFC8231].  This
   document does not repeat the description of those protocol
   extensions.  This document presents protocol extensions for a set of
   requirements which are specific to the use of a stateful PCE in a
   GMPLS-controlled network.

   The basic requirements are as follows:

  o  Advertisement of the stateful PCE capability.  This generic
     requirement is covered in Section 5.7. of [RFC8231].  This
     document does not provide any further extensions.

  o  LSP delegation is already covered in Section 5.7. of [RFC8231].
     Section 2.2. of this document does not provide any further

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   o  Active LSP update is covered in Section 6.2 of [RFC8231]. Section
      4.1. of this document provides extension for its application in
      GMPLS-controlled networks.

  o  LSP state synchronization and LSP state report. This is a generic
     requirement already covered in Section 5.6. of [RFC8231].  However,
     there are further extensions required specifically for GMPLS-
     controlled networks and discussed in Section 4.2.

4. PCEP Extensions

4.1. LSP Update in GMPLS-controlled Networks

   [RFC8231] defines the Path Computation LSP Update Request (PCUpd)
   message to enable to update the attributes of an LSP. However, that
   document does not define technology-specific parameters.

   A key element of the PCUpd message is the attribute-list construct
   defined in [RFC5440] and extended by many other PCEP specifications.

   For GMPLS purposes we note that the BANDWIDTH object used in the
   attribute-list is defined in [PCEP-GMPLS].  Furthermore, additional
   TLVs are defined for the LSPA object in [PCEP-GMPLS] and MAY be
   included to indicate technology-specific attributes. There are other
   technology-specific attributes that need to be conveyed in the
   <intended-attribute-list> of the <path> object of the PCUpd message.
   Note that these path details in the PCUpd message are the same as
   the <attribute-list> of the PCRep message. See Section 4.2 for the

   LSP parameter update controlled by a stateful PCE in a multi-domain
   network is complex and requires well-defined operational procedures
   as well as protocol design and is out of scope of this document and
   left for further study.

4.2. LSP Synchronization in GMPLS-controlled Networks

   PCCs need to report the attributes of LSPs to the PCE to enable
   stateful operation of a GMPLS network.  This process is known as
   LSP state synchronization.  The LSP attributes include bandwidth,
   associated route, and protection information etc., are stored by the
   PCE in the LSP database (LSP-DB).  Note that, as described in
   [RFC8231], the LSP state synchronization covers both the bulk
   reporting of LSPs at initialization as well the reporting of new or
   modified LSP during normal operation. Incremental LSP-DB
   synchronization may be desired in a GMPLS-controlled network and it
   is specified in [RFC8232].

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   [RFC8231] describes mechanisms for LSP synchronization using the
   Path Computation State Report (PCRpt) message, but does not cover
   reporting of technology-specific attributes. As stated in [RFC8231],
   the <path> construct is further composed of a compulsory ERO object
   and a compulsory attribute-list and an optional RRO object. In order
   to report LSP states in GMPLS networks, this specification allows
   the use within a PCRpt message both of technology- and GMPLS-
   specific attribute objects and TLVs defined in [PCEP-GMPLS] as

     o  IRO/XRO Extensions to support the inclusion/exclusion of labels
        and label sub-objects for GMPLS. (See Section 2.6 and 2.7 in

     o  END-POINTS (Generalized END-POINTS Object Type. See Section 2.5
        in [PCEP-GMPLS])

     o  BANDWIDTH (Generalized BANDWIDTH Object Type. See Section 2.3
        in [PCEP-GMPLS])


      o IF_ID_ERROR_SPEC (extending [RFC8231] section 7.3.4 that only
        considers the use of RSVP ERROR_SPEC)

   The END-POINTS object SHOULD be carried within the attribute-list to
   specify the endpoints pertaining to the reported LSP. The XRO object
   MAY be carried to specify the network resources that the reported
   LSP avoids and a PCE SHOULD consider avoid these network resources
   during the process of re-optimizing after this LSP is delegated to
   the PCE.  To be more specific, the <attribute-list> is updated as

   <attribute-list> ::= [<END-POINTS>]

   <metric-list>::= <METRIC>[<metric-list>]

   If the LSP being reported protects another LSP, the PROTECTION-
   ATTRIBUTE TLV [PCEP-GMPLS] MUST be included in the LSPA object to
   describe its attributes and restrictions.  Moreover, if the status

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   of the protecting LSP changes from non-operational to operational,
   this SHOULD to be synchronized to the stateful PCE using a PCRpt

4.3.  Modification of Existing PCEP Messages and Procedures

   One of the advantages mentioned in [RFC8051] is that the stateful
   nature of a PCE simplifies the information conveyed in PCEP messages,
   notably between PCC and PCE, since it is possible to refer to PCE
   managed state for active LSPs. To be more specific, with a stateful
   PCE, it is possible to refer to an LSP with a unique identifier in
   the scope of the PCC-PCE session and thus use such identifier to
   refer to that LSP. Note this MAY also be applicable to packet

4.3.1. Modification for LSP Re-optimization

   The Request Parameters (RP) object on a Path Computation Request
   (PCReq) message carries the R bit.  When set, this indicates that
   the PCC is requesting re-optimization of an existing LSP. Upon
   receiving such a PCReq, a stateful PCE SHOULD perform the    re-
   optimization in the following cases:

     o  The existing bandwidth and route information of the LSP to be
         re-optimized is provided in the PCReq message using the
         BANDWIDTH object and the ERO.

     o  The existing bandwidth and route information is not supplied
         in the PCReq message, but can be found in the PCE's LSP-DB.
         In this case, the LSP MUST be identified using an LSP
         identifier carried in the PCReq message, and that fact
         requires that the LSP identifier was previously supplied
         either by the PCC in a PCRpt message or by the PCE in a PCRep
         message.  [RFC8231] defines how this is achieved using a
         combination of the per-node LSP identifier (PLSP-ID) and the
         PCC's address.

   If no LSP state information is available to carry out re-
   optimization, the stateful PCE should report the error "LSP state
   information unavailable for the LSP re-optimization" (Error Type =
   TBD1, Error value= TBD2).

4.3.2. Modification for Route Exclusion

   [RFC5521] defines a mechanism for a PCC to request or demand that
   specific nodes, links, or other network resources are excluded from

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   paths computed by a PCE.  A PCC may wish to request the computation
   of a path that avoids all link and nodes traversed by some other LSP.

   To this end this document defines a new sub-object for use with
   route exclusion defined in [RFC5521].  The LSP exclusion sub-object
   is as follows:

      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
     |X|Type (TBD3) |     Length    |   Attributes  |    Flag        |
     |               PLSP-ID                   |      Reserved       |

     X bit and Attribute fields are defined in [RFC5521].

     X bit: indicates whether the exclusion is mandatory (X=1) and MUST
     be accommodated, or desired (X=0) and SHOULD be accommodated.

     Type: Subobject Type for an LSP exclusion sub-object. Value of
     TBD3. To be assigned by IANA.

     Length: The Length contains the total length of the subobject in
     bytes, including the Type and Length fields.

     Attributes: indicates how the exclusion object is to be
     interpreted. Currently, Interface (Attributes = 0), Node
     (Attributes =1) and SRLG (Attributes =2) are defined in [RFC5521]
     and this document does not define new values.

     Flags: This field may be used to further specify the exclusion
     constraint with regard to the LSP. Currently, no values are

     PLSP-ID: This is the identifier given to a LSP and is unique in
     the context of the PCC address as defined in [RFC8231].

     Reserved: MUST be transmitted as zero and SHOULD be ignored on

   This sub-object is OPTIONAL in the exclude route object (XRO) and
   can be present multiple times.  When a stateful PCE receives a PCReq
   message carrying this sub-object, it SHOULD search for the
   identified LSP in its LSP-DB and then exclude it from the new path
   computation all resources used by the identified LSP.  If the
   stateful PCE cannot recognize one or more of the received LSP

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   identifiers, it should send an error message PCErr reporting "The
   LSP state information for route exclusion purpose cannot be found"
   (Error-type = TBD1, Error-value = TBD4).  Optionally, it may provide
   with the unrecognized identifier information to the requesting PCC
   using the error reporting techniques described in [RFC5440].

4.4. Object Encoding

   Note that, as is stated in Section 7 of [RFC8231], the P flag   and
   the I flag of the PCEP objects used on PCUpd and PCRpt messages
   SHOULD be set to 0 on transmission and SHOULD be ignored on receipt
   since these flags are exclusively related to path computation

5. IANA Considerations

   IANA is requested to allocate new Types for the TLV/Object defined
   in this document.

5.1. New PCEP Error Codes

   IANA is requested to make the following allocation in the "PCEP-
   ERROR Object Error Types and Values" registry.

   Error Type        Meaning                                Reference

   TBD1         LSP state information missing              [This.I-D]

   Error-value TBD2:    LSP state information unavailable  [This.I-D]

                    for the LSP re-optimization

   Error-value TBD4:   LSP state information for route

                   exclusion purpose cannot be found       [This.I-D]

5.2. New Subobject for the Exclude Route Object

   IANA maintains the "PCEP Parameters" registry containing a
   subregistry called "PCEP Objects".  This registry has a subregistry
   for the XRO (Exclude Route Object) listing the sub-objects that can
   be carried in the XRO.  IANA is requested to assign a further sub-
   object that can be carried in the XRO as follows:

      Value       Description                    Reference

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      TBD3        LSP identifier sub-object     [This.I-D]

6. Manageability Considerations

   The description and functionality specifications presented related
   to stateful PCEs should also comply with the manageability
   specifications covered in Section 8 of [RFC4655]. Furthermore, a
   further list of manageability issues presented in [RFC8231] should
   also be considered.

   Additional considerations are presented in the next sections.

6.1. Requirements on Other Protocols and Functional Components

   When the detailed route information is included for LSP state
   synchronization (either at the initial stage or during LSP state
   report process), this requires the ingress node of an LSP carry the
   RRO object in order to enable the collection of such information.

7. Security Considerations

   This draft provides additional extensions to PCEP so as to
   facilitate stateful PCE usage in GMPLS-controlled networks, on top
   of [RFC8231].  The PCEP extensions to support GMPLS-controlled
   networks should be considered under the same security as for MPLS
   networks, as noted in [RFC7025]. Therefore, the security
   considerations elaborated in [RFC5440] still apply to this draft.
   Furthermore, [RFC8231] provides a detailed analysis of the
   additional security issues incurred due to the new extensions and
   possible solutions needed to support for the new stateful PCE
   capabilities and they apply to this document as well.

8. Acknowledgement

   We would like to thank Adrian Farrel and Cyril Margaria for the
   useful comments and discussions.

9. References

9.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to indicate
             requirements levels", RFC 2119, March 1997.

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   [RFC4655] Farrel, A., Vasseur, J.-P., and Ash, J., "A Path
             Computation Element (PCE)-Based Architecture", RFC 4655,
             August 2006.

   [RFC5440] Vasseur, J.-P., and Le Roux, JL., "Path Computation
             Element (PCE) Communication Protocol (PCEP)", RFC 5440,
             March 2009.

   [RFC8231] Crabbe, E., Medved, J., Varga, R., Minei, I., "Path
             Computation Element Communication Protocol (PCEP)
             Extensions for Stateful PCE", RFC 8231, September 2017.

   [PCEP-GMPLS] Margaria, C., Gonzalez de Dios, O., Zhang, F., "PCEP
             extensions for GMPLS", draft-ietf-pce-gmpls-pcep-
             extensions, work in progress.

9.2. Informative References

   [RFC8051] Zhang, X., Minei, I., et al, "Applicability of Stateful
             Path Computation Element (PCE) ", RFC 8051, January 2017.

   [RFC8232] Crabbe, E., Minei, I., Medved, J., Varga, R., Zhang, X.,
             and D. Dhody, "Optimizations of Label Switched Path State
             Synchronization Procedures for a Stateful PCE", RFC 8232,
             September 2017.

10. Contributors' Address

   Dhruv Dhody
   Huawei Technology
   Leela Palace
   Bangalore, Karnataka 560008


   Yi Lin
   Huawei Technologies
   F3-5-B R&D Center, Huawei Base
   Bantian, Longgang District
   Shenzhen 518129 P.R.China

   Phone: +86-755-28972914

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Authors' Addresses

   Xian Zhang
   Huawei Technologies
   F3-5-B R&D Center, Huawei Base
   Bantian, Longgang District
   Shenzhen 518129 P.R.China

   Phone: +86-755-28972645

   Young Lee (Editor)
   5340 Legacy Drive, Suite 170
   Plano, TX  75023

   Phone: +1 469 278 5838

   Fatai Zhang
   F3-5-B R&D Center, Huawei Base
   Bantian, Longgang District
   P.R. China

   Phone: +86-755-28972912

   Ramon Casellas
   Av. Carl Friedrich Gauss n7
   Castelldefels, Barcelona 08860


   Oscar Gonzalez de Dios
   Telefonica Investigacion y Desarrollo
   Emilio Vargas 6
   Madrid,   28045

   Phone: +34 913374013

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   Zafar Ali
   Cisco Systems

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