Local Protection Enforcement in PCEP
draft-ietf-pce-local-protection-enforcement-09

Network Working Group                                           A. Stone
Internet-Draft                                               M. Aissaoui
Updates: 5440 (if approved)                                        Nokia
Intended status: Standards Track                                S. Sidor
Expires: 9 November 2023                             Cisco Systems, Inc.
                                                            S. Sivabalan
                                                      Ciena Coroporation
                                                              8 May 2023

                  Local Protection Enforcement in PCEP
             draft-ietf-pce-local-protection-enforcement-09

Abstract

   This document extends the base specification to clarify usage of the
   local protection desired bit signalled in the Path Computation
   Element Protocol (PCEP).  This document also introduces a new flag
   for signalling protection strictness in PCEP.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 9 November 2023.

Copyright Notice

   Copyright (c) 2023 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
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Implementation differences  . . . . . . . . . . . . . . .   4
     4.2.  SLA Enforcement . . . . . . . . . . . . . . . . . . . . .   4
   5.  Protection Enforcement Flag (E flag)  . . . . . . . . . . . .   5
     5.1.  Backwards Compatibility . . . . . . . . . . . . . . . . .   7
   6.  Implementation Status . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Nokia Implementation  . . . . . . . . . . . . . . . . . .   8
     6.2.  Cisco Implementation  . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The Path Computation Element (PCE) Communication Protocol (PCEP)
   [RFC5440] enables the communication between a Path Computation Client
   (PCC) and a PCE, or between two PCEs based on the PCE architecture
   [RFC4655].

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   PCEP [RFC5440] utilizes flags, values and concepts previously defined
   in RSVP-TE Extensions [RFC3209] and Fast Reroute Extensions to RSVP-
   TE [RFC4090].  One such concept in PCEP is the 'Local Protection
   Desired' (L flag in the LSPA Object in [RFC5440]), which was
   originally defined in the SESSION-ATTRIBUTE Object in RFC3209.  In
   RSVP, this flag signals to downstream routers that they may use a
   local repair mechanism.  The headend router calculating the path does
   not know whether a downstream router will or will not protect a hop
   during its calculation.  Therefore, a local protection desired does
   not require the transit router to satisfy protection in order to
   establish the RSVP signalled path.  This flag is signalled in PCEP as
   an attribute of the LSP via the LSP Attributes object.

   PCEP Extensions for Segment Routing ([RFC8664]) extends support in
   PCEP for Segment Routed paths.  The path list is encoded with Segment
   Identifiers, each of which offer local protection.  The PCE may
   discover the protection eligibility for a Segment Identifier (SID)
   via BGP-LS [RFC9085] and take the protection into consideration as a
   path constraint.

   It is desirable for an operator to be able to define the enforcement,
   or strictness of the protection requirement.

   This document updates [RFC5440] by further describing the behaviour
   with the Local Protection Desired Flag (L flag) and extends on it
   with the introduction of the Enforcement Flag (E flag).

   The document contains reference notes for Segment Routing, however
   the content described is path setup type and data plane technology
   agnostic.

2.  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.

3.  Terminology

   This document uses the following terminology:

   PROTECTION MANDATORY: The Path MUST have protection eligibility on
   all links.

   UNPROTECTED MANDATORY: The Path MUST NOT have protection eligibility
   on all links.

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   PROTECTION PREFERRED: The Path SHOULD have protection eligibility on
   all links but MAY contain links which do not have protection
   eligibility.

   UNPROTECTED PREFERRED: The Path SHOULD NOT have protection
   eligibility on all links but MAY contain links which have protection
   eligibility.

   PCC: Path Computation Client.  Any client application requesting a
   path computation to be performed by a Path Computation Element.

   PCE: Path Computation Element.  An entity (component, application, or
   network node) that is capable of computing a network path or route
   based on a network graph and applying computational constraints.

   PCEP: Path Computation Element Protocol.

   LSPA: LSP Attributes Object in PCEP, defined in RFC5440

4.  Motivation

4.1.  Implementation differences

   As defined in [RFC5440] the mechanism to signal protection
   enforcement in PCEP is the previously mentioned L flag defined in the
   LSPA Object.  The name of the flag uses the term "Desired", which by
   definition means "strongly wished for or intended" and the use case
   originated from the RSVP.  For RSVP signalled paths, local protection
   is not within control of the PCE.  However, [RFC5440] does state
   "When set, this means that the computed path must include links
   protected with Fast Reroute as defined in [RFC4090]."
   Implementations of [RFC5440] have either interpreted the L flag as
   PROTECTION MANDATORY or PROTECTION PREFERRED, leading to operational
   differences.

4.2.  SLA Enforcement

   The boolean bit L flag is unable to distinguish between the different
   options of PROTECTION MANDATORY, UNPROTECTED MANDATORY, PROTECTION
   PREFERRED and UNPROTECTED PREFERRED.  Selecting one of the options is
   typically dependent on the service level agreement the operator
   wishes to impose on the LSP.  A network may be providing transit to
   multiple service agreement definitions against the same base topology
   network, whose behavior could vary, such as wanting local protection
   to be invoked on some LSPs and not wanting local protection on
   others.  When enforcement is used, the resulting shortest path
   calculation is impacted.

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   For example, PROTECTION MANDATORY is for use cases where an operator
   may need the LSP to follow a path which has local protection provided
   along the full path, ensuring that if there is a failure anywhere
   along the path that traffic will be fast re-routed at the point.

   For example, UNPROTECTED MANDATORY is when an operator may
   intentionally prefer an LSP to not be locally protected, and thus
   would rather local failures cause the LSP to go down.  An example
   scenario is one where an LSP is protected with path protection via a
   secondary diverse LSP.  Each LSP is traffic engineered to follow
   specific traffic engineered criteria computed by the PCE to satisfy
   SLA.  Upon a failure, if local protection is invoked on the active
   LSP traffic, the traffic may temporarily traverse links which violate
   the TE requirements and could negatively impact the resources being
   traversed (e.g., insufficient bandwidth).  In addition, depending on
   the network topological scenario, it may be not feasible for the PCE
   to reroute the LSP while respecting the TE requirements which include
   path diversity, resulting in the LSP being torn down and switched to
   the protected path anyways.  In such scenarios its desirable for the
   LSP to be simply torn down immediately and not re-routed through
   local protection, so that traffic may be forwarded through an already
   established traffic-engineered secondary path.

   Both UNPROTECTED PREFERRED and PROTECTED PREFERRED options provide a
   relaxation of the protection constraint.  These options can be used
   when an operator does not require protection enforcement.  Regardless
   of the option selected, the protection status of a resource does not
   influence whether the link must be pruned during a path calculation.
   Furthermore, the selection of either option indicates a priority
   selection to PCE when there is an option to choose a protected or
   unprotected instruction associated with a resource, ensuring
   consistent PCE behavior across different implementations.

   When used with Segment Routing, an adjacency may have both a
   protected SID and an unprotected SID.  If the UNPROTECTED PREFERRED
   option is selected, PCE chooses the unprotected SID.  Alternatively,
   if the PROTECTED PREFERRED option is selected, PCE chooses the
   protected SID

5.  Protection Enforcement Flag (E flag)

   Section 7.11 in Path Computation Element Protocol [RFC5440] describes
   the encoding of the Local Protection Desired (L flag).  A Protection
   Enforcement flag "E" is specified below, extending the L flag.

   [RFC Editor Note: The text below assumes the E bit remains the early
   allocation value 6.  Please adjust if this changes and remove this
   note before publication.]

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   Codespace of the Flag field (LSPA Object)

        Bit      Description                      Reference

         7    Local Protection Desired             RFC5440

         6    Local Protection Enforcement        This document

   The format of the LSPA Object as defined in [RFC5440] is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Exclude-any                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Include-any                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Include-all                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Setup Prio   |  Holding Prio |     Flags |E|L|   Reserved    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      //                     Optional TLVs                           //
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Flags (8 bits)

   *  L Flag: As defined in [RFC5440] and further updated by this
      document.  When set to 1, protection is desired.  When set to 0,
      protection is not desired.  The enforcement of the protection is
      identified via the E flag.

   *  E Flag (Protection Enforcement): This flag controls the strictness
      in which the PCE must apply the L flag.  When set to 1, the value
      of the L flag MUST be respected during resource selection by the
      PCE.  When E flag is set to 0, the value of the L flag SHOULD be
      respected as selection criteria; however, the PCE is permitted to
      relax or ignore the L flag when computing a path.  The statements
      below indicate preference when E flag is set to 0 in combination
      with the L flag value.

   When both the L flag and E flag are set to 1, then the PCE MUST
   consider the protection eligibility as a PROTECTION MANDATORY
   constraint.

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   When the L flag is set to 1 and the E flag is set to 0, then the PCE
   MUST consider the protection eligibility as a PROTECTION PREFERRED
   constraint.

   When both L flag and E flag are set to 0, then the PCE SHOULD
   consider the protection eligibility as an UNPROTECTED PREFERRED
   constraint but MAY consider protection eligibility as an UNPROTECTED
   MANDATORY constraint.

   When L flag is set to 0 and E flag is set to 1, then the PCE MUST
   consider the protection eligibility as an UNPROTECTED MANDATORY
   constraint.

   If a PCE is unable to infer protection status of a resource, PCE MAY
   use local policy to define protected status assumptions.  When
   computing a Segment Routed path, It is RECOMMENDED that a PCE assume
   a Node SID is protected.  It is also RECOMMENDED that a PCE assume an
   Adjacency SID is protected if the backup flag advertised with the
   Adjacency SID is set.

5.1.  Backwards Compatibility

   Considerations in the message passing between the PCC and the PCE for
   the E flag bit which are not supported by the entity are outlined in
   this section, with requirements for the PCE and the PCC implementing
   this document described at the end.

   For a PCC or PCE which does not yet support this document, the E flag
   is ignored and set to zero in PCRpt and/or PCUpd as per [RFC5440] for
   PCC-initiated or as per [RFC8281] for PCE-initiated LSPs.  It is
   important to note that [RFC8231] and [RFC8281] permit the LSP
   Attribute Object to be included in PCUpd messages for PCC-initiated
   and PCE-initiated LSPs.

   For PCC-initiated LSPs, PCUpd E flag (and L flag) is an echo from the
   previous PCRpt however the bit value is ignored on the PCE from the
   previous PCRpt, therefore the E flag value set in the PCUpd is zero.
   A PCE which does not support this document sends PCUpd messages with
   the E flag set to 0 for PCC-initated LSPs even if set to 1 in the
   prior PCReq or PCRpt.

   A PCC which does not support this document sends PCRpt messages with
   the E flag set to 0 for PCE-initiated LSPs even if set to 1 in the
   prior PCInitiate or PCUpd.

   For a PCC which does support this document, it MAY set the E flag to
   1 depending on local configuration.  If communicating with a PCE
   which does not yet support this document, the PCE follows the

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   behaviour specified in [RFC5440] and will ignore the E flag.  Thus,
   it may compute a path which may not respect the enforcement
   constraint.

   For PCC-initiated LSPs, the PCC SHOULD ignore the E flag value
   received from the PCE in a PCUpd message as it may be communicating
   with a PCE which does not support this document.

   For PCE-initiated LSPs, the PCC MAY process the E flag value received
   from the PCE in a PCUpd message.  The PCE SHOULD ignore the E flag
   value received from the PCC in a PCRpt message as it may be
   communicating with a PCC which does not support this document.

6.  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 catalogue 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".

6.1.  Nokia Implementation

   *  Organization: Nokia

   *  Implementation: NSP PCE and SROS PCC.

   *  Description: Implementation for calculation and conveying
      intention described in this document

   *  Maturity Level: Demo

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   *  Coverage: Full

   *  Contact: andrew.stone@nokia.com

6.2.  Cisco Implementation

   *  Organization: Cisco Systems, Inc.

   *  Implementation: IOS-XR PCE and PCC.

   *  Description: Implementation for calculation and conveying
      intention described in this document

   *  Maturity Level: Demo

   *  Coverage: Full

   *  Contact: ssidor@cisco.com

7.  Security Considerations

   This document clarifies the behaviour of an existing flag and
   introduces a new flag to provide further control of that existing
   behaviour.  The introduction of this new flag and behaviour
   clarification does not create any new sensitive information.  No
   additional security measure is required.

   Securing the PCEP session using Transport Layer Security (TLS)
   [RFC8253], as per the recommendations and best current practices in
   [RFC7525] is RECOMMENDED.

8.  IANA Considerations

   [RFC Editor Note: The text below assumes the E bit remains the early
   allocation value 6.  Please adjust if this changes and remove this
   note before publication.]

   This document defines a new bit value in the sub-registry "LSPA
   Object Flag Field" in the "Path Computation Element Protocol (PCEP)
   Numbers" registry.  IANA has made the following codepoint allocation.

               Bit    Name                         Reference

                6     Protection Enforcement       This document

9.  References

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9.1.  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>.

   [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>.

   [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>.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.

   [RFC4090]  Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
              Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
              DOI 10.17487/RFC4090, May 2005,
              <https://www.rfc-editor.org/info/rfc4090>.

   [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>.

   [RFC8231]  Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for Stateful PCE", RFC 8231,
              DOI 10.17487/RFC8231, September 2017,
              <https://www.rfc-editor.org/info/rfc8231>.

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

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

9.2.  Informative References

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

   [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>.

   [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>.

   [RFC9085]  Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
              H., and M. Chen, "Border Gateway Protocol - Link State
              (BGP-LS) Extensions for Segment Routing", RFC 9085,
              DOI 10.17487/RFC9085, August 2021,
              <https://www.rfc-editor.org/info/rfc9085>.

Acknowledgements

   Thanks to Dhruv Dhody and Mike Koldychev for reviewing and providing
   very valuable feedback and discussions on this document.

   Thanks to Julien Meuric for shepherding this document.

Authors' Addresses

   Andrew Stone
   Nokia
   600 March Road
   Kanata Ontario K2K 2T6
   Canada
   Email: andrew.stone@nokia.com

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   Mustapha Aissaoui
   Nokia
   600 March Road
   Kanata Ontario K2K 2T6
   Canada
   Email: mustapha.aissaoui@nokia.com

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

   Siva Sivabalan
   Ciena Coroporation
   385 Terry Fox Drive
   Kanata Ontario K2K 0L1
   Canada
   Email: ssivabal@ciena.com

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