Extension for Stateful PCE to allow Optional Processing of PCEP Objects
draft-ietf-pce-stateful-pce-optional-02

Document Type Active Internet-Draft (pce WG)
Authors Cheng Li  , Haomian Zheng  , Stephane Litkowski 
Last updated 2021-10-23
Replaces draft-dhody-pce-stateful-pce-optional
Stream Internet Engineering Task Force (IETF)
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PCE Working Group                                                  C. Li
Internet-Draft                                                  H. Zheng
Updates: 8231 (if approved)                          Huawei Technologies
Intended status: Standards Track                            S. Litkowski
Expires: 26 April 2022                                             Cisco
                                                         23 October 2021

Extension for Stateful PCE to allow Optional Processing of PCEP Objects
                draft-ietf-pce-stateful-pce-optional-02

Abstract

   This document introduces a mechanism to mark some of the Path
   Computation Element (PCE) Communication Protocol (PCEP) objects as
   optional during PCEP messages exchange for the Stateful PCE model to
   allow relaxing some constraints.  This document introduces this
   relaxation and updates RFC 8231.

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
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   This Internet-Draft will expire on 26 April 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Usage Example . . . . . . . . . . . . . . . . . . . . . .   4
   3.  PCEP Extension  . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  STATEFUL-PCE-CAPABILITY TLV . . . . . . . . . . . . . . .   5
     3.2.  Handling of P flag  . . . . . . . . . . . . . . . . . . .   5
       3.2.1.  The PCRpt Message . . . . . . . . . . . . . . . . . .   5
       3.2.2.  The PCUpd Message and the PCInitiate Message  . . . .   6
     3.3.  Handling of I flag  . . . . . . . . . . . . . . . . . . .   6
       3.3.1.  The PCUpd Message . . . . . . . . . . . . . . . . . .   6
       3.3.2.  The PCRpt Message . . . . . . . . . . . . . . . . . .   6
       3.3.3.  The PCInitiate Message  . . . . . . . . . . . . . . .   7
     3.4.  Delegation  . . . . . . . . . . . . . . . . . . . . . . .   7
     3.5.  Unknown Object Handling . . . . . . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  STATEFUL-PCE-CAPABILITY TLV . . . . . . . . . . . . . . .   8
   6.  Manageability Considerations  . . . . . . . . . . . . . . . .   8
     6.1.  Control of Function and Policy  . . . . . . . . . . . . .   8
     6.2.  Information and Data Models . . . . . . . . . . . . . . .   8
     6.3.  Liveness Detection and Monitoring . . . . . . . . . . . .   9
     6.4.  Verify Correct Operations . . . . . . . . . . . . . . . .   9
     6.5.  Requirements On Other Protocols . . . . . . . . . . . . .   9
     6.6.  Impact On Network Operations  . . . . . . . . . . . . . .   9
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Contributors . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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1.  Introduction

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

   [RFC5440] defined the P flag (Processing-Rule) in the Common Object
   Header to allow a PCC to specify in a Path Computation Request
   (PCReq) message (sent to a PCE) whether the object must be taken into
   account by the PCE during path computation or is optional.  The I
   flag (Ignore) is used by the PCE in a Path Computation Reply (PCRep)
   message to indicate to a PCC whether or not an optional object was
   considered by the PCE during path computation.  Stateful PCE
   [RFC8231] specified that the P and I flags of the PCEP objects
   defined in [RFC8231] is to be set to zero on transmission and ignored
   on receipt, since they are exclusively related to path computation
   requests.  The behavior for P and I flag in other messages defined in
   [RFC5440] and other extension was not specified.  This document
   clarifies how the P and I flag could be used in the stateful PCE
   model to identify optional objects in the Path Computation State
   Report (PCRpt) [RFC8231], the Path Computation Update Request (PCUpd)
   [RFC8231], and the LSP Initiate Request (PCInitiate) [RFC8281]
   message.

   This document updates [RFC8231] with respect to usage of the P and I
   flag as well as the handling of unknown objects in the stateful PCEP
   message exchange.

1.1.  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.

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2.  Overview

   [RFC5440] describes the handling of unknown objects as per the
   setting of the P flag for the PCReq message.  Further, [RFC8231]
   defined the usage of the LSP Error Code TLV in the PCRpt message in
   response to failed LSP Update Request via the PCUpd message (for
   example, due to an unsupported object/TLV).

   This document clarifies the procedure of marking some objects as
   'optional to be processed' by the PCEP peer in the stateful PCEP
   messages.  Furthermore, this document updates the procedure for
   handling unknown objects in the stateful PCEP messages based on the P
   flag.

2.1.  Usage Example

   The PCRpt message is used to report the current state of an LSP.  As
   part of the message both the <intended-attribute-list> and <actual-
   attribute-list> is encoded (see [RFC8231]).  For example, the
   <intended-attribute-list> could include the METRIC object to indicate
   a limiting constraint (B flag set) for the Path Delay Variation
   metric [RFC8233].  In some scenarios, it would be useful to state
   that this limiting constraint can be relaxed by the PCE in case it
   cannot find a path.  Similarly in the case of an association group
   [RFC8697] such as Disjoint Association [RFC8800], the PCE may need to
   completely relax the disjointness constraint in order to provide a
   path to all the LSPs that are part of the association.  In these case
   it would be useful to mark the objects as 'optional' and it could be
   ignored by the PCEP peer.  Also, it would be useful for the PCEP
   speaker to learn if the PCEP peer has relaxed the constraint and
   ignored the processing of the PCEP object.

   Thus, this document simply clarifies, how the already existing P and
   I flag in the PCEP common object header could be used during the
   stateful PCEP message exchange.  Further it should be noted that
   similar to handling of P and I flag in [RFC5440], the flag is
   applicable to full PCEP Object and could not be applied to the
   granularity of an optional TLV encoded in the PCEP Object.

3.  PCEP Extension

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3.1.  STATEFUL-PCE-CAPABILITY TLV

   A PCEP speaker indicates its ability to support the handling of the P
   and I flag in the stateful PCEP message exchange during the PCEP
   initialization phase, as follows.  When the PCEP session is
   established, a PCC sends an Open message with an OPEN object that
   contains the STATEFUL-PCE-CAPABILITY TLV, as defined in [RFC8231].  A
   new flag, the R (RELAX) flag, is added in this TLV to indicate the
   support for relaxing the processing of some objects via the use of
   the P and I flag in the PCEP common object header.

   R (RELAX bit - TBD1): If set to 1 by a PCEP Speaker, the R flag
   indicates that the PCEP Speaker is willing to send and receive PCEP
   objects with the P and I flags in the PCEP common object header for
   the stateful PCE messages.  In case the bit is unset, it indicates
   that the PCEP Speaker would not handle the P and I flags in the PCEP
   common object header for stateful PCE messages.

   The R flag MUST be set by both a PCC and a PCE to indicate support
   for the handling of the P and I flag in the PCEP common object header
   to allow relaxing some constraints by marking objects as optional to
   process.  If the PCEP speaker did not set the R flag but receives
   PCEP objects with P or I bit set, it MUST behave as per the
   processing rule in [RFC8231] i.e., the bits are simply ignored.

3.2.  Handling of P flag

3.2.1.  The PCRpt Message

   The P flag in the PCRpt message [RFC8231] allows a PCC to specify to
   a PCE whether the object must be taken into account by the PCE
   (during path computation, re-optimization, or state maintenance) or
   is optional o process.  When the P flag is set in the PCRpt message
   received on a PCEP session on which R bit was set by both peers, the
   object MUST be taken into account by the PCE.  Conversely, when the P
   flag is cleared, the object is optional and the PCE is free to ignore
   it.  The P flag for the mandatory objects such as the LSP and the ERO
   object (intended path) MUST be set in the PCRpt message.  If a
   mandatory object is received with the P flag set incorrectly
   according to the rules stated above, the receiving peer MUST send a
   PCErr message with Error-Type=10 (Reception of an invalid object) and
   Error-value=1 (reception of an object with P flag not set).  On a
   PCEP session on which R bit was set by both peers, the PCC SHOULD set
   the P flag by default, unless a local configuration or local policy
   indicates that some constraints (corresponding PCEP objects) can be
   marked as optional and could be ignored by the PCE.

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3.2.2.  The PCUpd Message and the PCInitiate Message

   The P flag in the PCUpd message [RFC8231] and the PCInitiate message
   [RFC8281] allows a PCE to specify to a PCC whether the object must be
   taken into account by the PCC (during path setup) or is optional to
   process.  When the P flag is set in the PCUpd/PCInitiate message
   received on a PCEP session on which R bit was set by both peers, the
   object MUST be taken into account by the PCC.  Conversely, when the P
   flag is cleared, the object is optional and the PCC is free to ignore
   it.  The P flag for the mandatory objects such as the SRP, the LSP
   and the ERO MUST be set in the PCUpd/PCInitiate message.  If a
   mandatory object is received with the P flag set incorrectly
   according to the rules stated above, the receiving peer MUST send a
   PCErr message with Error-Type=10 (Reception of an invalid object) and
   Error-value=1 (reception of an object with P flag not set).  By
   default, the PCE SHOULD set the P flag, unless a local configuration
   or local policy indicates that some constraints (corresponding PCEP
   objects) can be marked as optional and could be ignored by the PCC.

3.3.  Handling of I flag

3.3.1.  The PCUpd Message

   The I flag in the PCUpd message [RFC8231] allows a PCE to indicate to
   a PCC whether or not an optional object was processed.  The PCE MAY
   include the ignored optional object in its update request and set the
   I flag to indicate that the optional object was ignored.  When the I
   flag is cleared, the PCE indicates that the optional object was
   processed.

   Note that when a PCE is unable to find the path that meets all the
   constraints as per the PCEP Object that cannot be ignored (i.e.  P
   flag is set), the PCUpd message MAY optionally include the PCEP
   Objects that caused the path computation to fail along with the with
   the empty ERO.

3.3.2.  The PCRpt Message

   The I flag in the PCRpt message [RFC8231] allows a PCC to indicate to
   a PCE whether or not an optional object was processed in response to
   an LSP Update Request (PCUpd) or LSP Initiate Request (PCInitiate).
   The PCC MAY include the ignored optional object in its report and set
   the I flag to indicate that the optional object was ignored at PCC.
   When the I flag is cleared, the PCC indicates that the optional
   object was processed.  The I flag has no meaning if the PCRpt message
   is not in response to a PCUpd or PCInitiate message (i.e. without the
   SRP object in the PCRpt message).

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   Note that when a PCC is unable to setup the path that meets all the
   parameters as per the PCEP Object that cannot be ignored (i.e.  P
   flag is set), the PCRpt message MAY optionally include the PCEP
   Objects that caused the path setup to fail along with the LSP-ERROR-
   CODE TLV [RFC8231] indicating the reason for the failure.

3.3.3.  The PCInitiate Message

   The I flag has no meaning in the PCinitiate message [RFC8281] and is
   ignored.

3.4.  Delegation

   Delegation is an operation to grant a PCE temporary rights to modify
   a subset of parameters on one or more LSPs by a PCC as described in
   [RFC8051].  Note that for the delegated LSPs, the PCE can update and
   mark some objects as ignored even when the PCC had set the P flag
   during delegation.  Similarly, the PCE can update and mark some
   object as a must to process even when the PCC had not set the P flag
   during delegation.

   The PCC MUST acknowledge this by sending the PCRpt message with the P
   flag set as per the PCE expectation for the corresponding object.  In
   case PCC cannot accept this, it would react as per the processing
   rules of unacceptable update in [RFC8231].

3.5.  Unknown Object Handling

   This document updates the handling of unknown objects in the stateful
   PCEP messages as per the setting of the P flag in the common object
   header in a similar way as [RFC5440], i.e. if a PCEP speaker does not
   understand an object with the P flag set or understands the object
   but decides to ignore the object, the entire stateful PCEP message
   MUST be rejected and the PCE MUST send a PCErr message with Error-
   Type="Unknown Object" or "Not supported Object" [RFC5440].  In case
   the P flag is not set, the PCEP speaker is free to ignore the object
   and continue with message processing as defined.

   [RFC8231] defined LSP Error Code TLV to be carried in PCRpt message
   in the LSP object to convey error information.  This document does
   not change that procedure.

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4.  Security Considerations

   This document clarifies how the already existing P and I flag in PCEP
   common object header could be used during stateful PCEP exchanges.
   It updates the unknown object error handling in stateful PCEP message
   exchange.  These changes on their own do not add any new security
   concerns.  The security considerations identified in [RFC5440],
   [RFC8231], and [RFC8281] continue to apply.

   As per [RFC8231], it is RECOMMENDED that these PCEP extensions only
   be activated on authenticated and encrypted sessions across PCEs and
   PCCs belonging to the same administrative authority, using Transport
   Layer Security (TLS) [RFC8253] as per the recommendations and best
   current practices in [RFC7525] (unless explicitly set aside in
   [RFC8253]).

5.  IANA Considerations

5.1.  STATEFUL-PCE-CAPABILITY TLV

   [RFC8231] defines the STATEFUL-PCE-CAPABILITY TLV; per that RFC, IANA
   created a "STATEFUL-PCE-CAPABILITY TLV Flag Field" subregistry to
   manage the value of the STATEFUL-PCE-CAPABILITY TLV's Flag field.
   IANA is requested to allocate a new bit in the subregistry, as
   follows:

   Bit       Description                 Reference
   -------------------------------------------------
   TBD1      RELAX bit                   [This-I.D.]

6.  Manageability Considerations

6.1.  Control of Function and Policy

   An operator MUST be allowed to configure the capability to support
   relaxation of constraints in the stateful PCEP message exchange.
   They SHOULD also allow configuration of related LSP constraints (or
   parameters) that are optional to process.

6.2.  Information and Data Models

   An implementation SHOULD allow the operator to view the capability
   defined in this document.  To serve this purpose, the PCEP YANG
   module [I-D.ietf-pce-pcep-yang] could be extended in the future.

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6.3.  Liveness Detection and Monitoring

   Mechanisms defined in this document do not imply any new liveness
   detection and monitoring requirements in addition to those already
   listed in [RFC5440].

6.4.  Verify Correct Operations

   Mechanisms defined in this document do not imply any new operation
   verification requirements in addition to those already listed in
   [RFC5440].

6.5.  Requirements On Other Protocols

   Mechanisms defined in this document do not imply any new requirements
   on other protocols.

6.6.  Impact On Network Operations

   Mechanisms defined in this document do not have any impact on network
   operations in addition to those already listed in [RFC5440].

7.  Acknowledgments

   Thanks to Jonathan Hardwick for discussion and suggestions around
   this draft.

   Thanks to Oscar Gonzalez de Dios and Mike Koldychev for the review
   comments.

8.  References

8.1.  Normative References

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

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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   [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>.

8.2.  Informative References

   [I-D.ietf-pce-pcep-yang]
              Dhody, D., Hardwick, J., Beeram, V. P., and J. Tantsura,
              "A YANG Data Model for Path Computation Element
              Communications Protocol (PCEP)", Work in Progress,
              Internet-Draft, draft-ietf-pce-pcep-yang-16, 22 February
              2021, <https://www.ietf.org/archive/id/draft-ietf-pce-
              pcep-yang-16.txt>.

   [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>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC8051]  Zhang, X., Ed. and I. Minei, Ed., "Applicability of a
              Stateful Path Computation Element (PCE)", RFC 8051,
              DOI 10.17487/RFC8051, January 2017,
              <https://www.rfc-editor.org/info/rfc8051>.

   [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>.

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   [RFC8281]  Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for PCE-Initiated LSP Setup in a Stateful PCE
              Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
              <https://www.rfc-editor.org/info/rfc8281>.

   [RFC8697]  Minei, I., Crabbe, E., Sivabalan, S., Ananthakrishnan, H.,
              Dhody, D., and Y. Tanaka, "Path Computation Element
              Communication Protocol (PCEP) Extensions for Establishing
              Relationships between Sets of Label Switched Paths
              (LSPs)", RFC 8697, DOI 10.17487/RFC8697, January 2020,
              <https://www.rfc-editor.org/info/rfc8697>.

   [RFC8800]  Litkowski, S., Sivabalan, S., Barth, C., and M. Negi,
              "Path Computation Element Communication Protocol (PCEP)
              Extension for Label Switched Path (LSP) Diversity
              Constraint Signaling", RFC 8800, DOI 10.17487/RFC8800,
              July 2020, <https://www.rfc-editor.org/info/rfc8800>.

Appendix A.  Contributors

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

   Email: dhruv.ietf@gmail.com

Authors' Addresses

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

   Email: c.l@huawei.com

   Haomian Zheng
   Huawei Technologies
   H1, Huawei Xiliu Beipo Village, Songshan Lake
   Dongguan
   Guangdong, 523808
   China

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   Email: zhenghaomian@huawei.com

   Stephane Litkowski
   Cisco

   Email: slitkows.ietf@gmail.com

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