



Internet Engineering Task Force (IETF)                       A. Raghuram
Request for Comments: 8741                                    A. Goddard
Category: Standards Track                                           AT&T
ISSN: 2070-1721                                               J. Karthik
                                                            S. Sivabalan
                                                     Cisco Systems, Inc.
                                                                 M. Negi
                                                     Huawei Technologies
                                                              March 2020


  Ability for a Stateful Path Computation Element (PCE) to Request and
             Obtain Control of a Label Switched Path (LSP)

Abstract

   A stateful Path Computation Element (PCE) retains information about
   the placement of Multiprotocol Label Switching (MPLS) Traffic
   Engineering Label Switched Paths (TE LSPs).  When a PCE has stateful
   control over LSPs, it may send indications to LSP head-ends to modify
   the attributes (especially the paths) of the LSPs.  A Path
   Computation Client (PCC) that has set up LSPs under local
   configuration may delegate control of those LSPs to a stateful PCE.

   There are use cases in which a stateful PCE may wish to obtain
   control of locally configured LSPs that it is aware of but have not
   been delegated to the PCE.

   This document describes an extension to the Path Computation Element
   Communication Protocol (PCEP) to enable a PCE to make requests for
   such control.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8741.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
     2.1.  Requirements Language
   3.  LSP Control Request Flag
   4.  Operation
   5.  Security Considerations
   6.  IANA Considerations
   7.  Manageability Considerations
     7.1.  Control of Function and Policy
     7.2.  Information and Data Models
     7.3.  Liveness Detection and Monitoring
     7.4.  Verify Correct Operations
     7.5.  Requirements on Other Protocols
     7.6.  Impact on Network Operations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   "Path Computation Element Communication Protocol (PCEP) Extensions
   for Stateful PCE" [RFC8231] specifies a set of extensions to PCEP
   [RFC5440] to enable stateful control of Traffic Engineering Label
   Switched Paths (TE LSPs) between and across PCEP sessions in
   compliance with [RFC4657].  It includes mechanisms to synchronize LSP
   state between Path Computation Clients (PCCs) and PCEs, delegate
   control of LSPs to PCEs, and allow PCEs to control the timing and
   sequence of path computations within and across PCEP sessions.  The
   stateful PCEP defines the following two useful network operations:

   Delegation:  As per [RFC8051], an operation to grant a PCE temporary
                rights to modify a subset of LSP parameters on one or
                more LSPs of a PCC.  LSPs are delegated from a PCC to a
                PCE and are referred to as "delegated" LSPs.

   Revocation:  As per [RFC8231], an operation performed by a PCC on a
                previously delegated LSP.  Revocation revokes the rights
                granted to the PCE in the delegation operation.

   For redundant stateful PCEs (Section 5.7.4 of [RFC8231]), during a
   PCE failure, one of the redundant PCEs might want to request to take
   control over an LSP.  The redundant PCEs may use a local policy or a
   proprietary election mechanism to decide which PCE would take
   control.  In this case, a mechanism is needed for a stateful PCE to
   request control of one or more LSPs from a PCC so that a newly
   elected primary PCE can request to take over control.

   In case of virtualized PCEs (vPCEs) running in virtual network
   function (VNF) mode, as the computation load in the network
   increases, a new instance of vPCE could be instantiated to balance
   the current load.  The PCEs could use a proprietary algorithm to
   decide which LSPs can be assigned to the new vPCE.  Thus, having a
   mechanism for the PCE to request control of some LSPs is needed.

   In some deployments, the operator would like to use stateful PCE for
   global optimization algorithms but would still like to keep the
   control of the LSP at the PCC.  In such cases, a stateful PCE could
   request to take control during the global optimization and return the
   delegation once done.

   Note that [RFC8231] specifies a mechanism for a PCC to delegate an
   orphaned LSP to another PCE.  The mechanism defined in this document
   can be used in conjunction with [RFC8231].  Ultimately, it is the PCC
   that decides which PCE to delegate the orphaned LSP to.

   This specification provides a simple extension that allows a PCE to
   request control of one or more LSPs from any PCC over the stateful
   PCEP session.  The procedures for granting and relinquishing control
   of the LSPs are specified in accordance with [RFC8231] unless
   explicitly set aside in this document.

2.  Terminology

   This document uses the following terms defined in [RFC5440]:

   PCC:  Path Computation Client

   PCE:  Path Computation Element

   PCEP:  Path Computation Element communication Protocol

   This document uses the following terms defined in [RFC8231]:

   PCRpt:  Path Computation State Report message

   PCUpd:  Path Computation Update Request message

   PLSP-ID:  A PCEP-specific identifier for the LSP

   SRP:  Stateful PCE Request Parameters

   Readers of this document are expected to have some familiarity with
   [RFC8231].

2.1.  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.

3.  LSP Control Request Flag

   The Stateful PCE Request Parameters (SRP) object is defined in
   Section 7.2 of [RFC8231] and includes a Flags field.

   A new "LSP Control Request" flag (30), also called the C flag, is
   introduced in the SRP object.  In a PCUpd message, a PCE sets the C
   flag to 1 to indicate that it wishes to gain control of LSPs.  The
   LSPs are identified by the PLSP-ID in the LSP object following the
   SRP object.  A PLSP-ID value other than 0 and 0xFFFFF is used to
   identify the LSP for which the PCE requests control.  A PLSP-ID value
   of 0 indicates that the PCE is requesting control of all LSPs
   originating from the PCC that it wishes to delegate.  The C flag has
   no meaning in other PCEP messages that carry SRP objects and for
   which the C flag MUST be set to 0 on transmission and MUST be ignored
   on receipt.

   The C flag is ignored in case the R flag [RFC8281] in the SRP object
   is set.

4.  Operation

   During normal operation, a PCC that wishes to delegate the control of
   an LSP sets the Delegate (D) flag (Section 7.3 of [RFC8231]) to 1 in
   all PCRpt messages pertaining to the LSP.  The PCE confirms the
   delegation by setting the D flag to 1 in all PCUpd messages
   pertaining to the LSP.  The PCC revokes the control of the LSP from
   the PCE by setting the D flag to 0 in PCRpt messages pertaining to
   the LSP.  If the PCE wishes to relinquish the control of the LSP, it
   sets the D flag to 0 in all PCUpd messages pertaining to the LSP.

   If a PCE wishes to gain control over an LSP, it sends a PCUpd message
   with the C flag set to 1 in the SRP object.  The LSP for which the
   PCE requests control is identified by the PLSP-ID in the associated
   LSP object.  A PLSP-ID value of 0 indicates that the PCE wants
   control over all LSPs originating from the PCC.  An implementation of
   this feature needs to make sure to check for the LSP control feature
   (C flag set to 1) before any check for PLSP-ID (as per [RFC8231]).
   The D flag and C flag are mutually exclusive in a PCUpd message.  The
   PCE MUST NOT send a control request for the LSP that is already
   delegated to the PCE, i.e., if the D flag is set in the PCUpd
   message, then the C flag MUST NOT be set.  If a PCC receives a PCUpd
   message with the D flag set in the LSP object (i.e., LSP is already
   delegated) and the C flag is also set (i.e., PCE is making a control
   request), the PCC MUST ignore the C flag.  A PCC can decide to
   delegate the control of the LSP at its own discretion.  If the PCC
   grants or denies the control, it sends a PCRpt message with the D
   flag set to 1 and 0, respectively, in accordance with stateful PCEP
   [RFC8231].  If the PCC does not grant the control, it MAY choose to
   not respond, and the PCE MAY choose to retry requesting the control,
   preferably using an exponentially increasing timer.  Note that, if
   the PCUpd message with the C flag set is received for a currently
   non-delegated LSP (for which the PCE is requesting delegation), this
   MUST NOT trigger the error handling as specified in [RFC8231] (a
   PCErr with Error-type=19 (Invalid Operation) and error-value 1
   (Attempted LSP Update Request for a non-delegated LSP)).

   As per [RFC8231], a PCC cannot delegate an LSP to more than one PCE
   at any time.  If a PCE requests control of an LSP that has already
   been delegated by the PCC to another PCE, the PCC MAY ignore the
   request or MAY revoke the delegation to the first PCE before
   delegating it to the second.  This choice is a matter of local
   policy.

   It should be noted that a legacy implementation of PCC that does not
   support this extension may receive an LSP control request: a PCUpd
   message with the C flag set and the D flag unset.  The legacy
   implementation would ignore the C flag and trigger the error
   condition for the D flag, as specified in [RFC8231] (i.e., a PCErr
   with Error-type=19 (Invalid Operation) and error-value 1 (Attempted
   LSP Update Request for a non-delegated LSP)).  Further, in case of a
   PLSP-ID value of 0, the error condition, as specified in [RFC8231],
   (i.e., a PCErr with Error-type=19 (Invalid Operation) and error-value
   3 (Attempted LSP Update Request for an LSP identified by an unknown
   PLSP-ID)) would be triggered.

   [RFC8281] describes the setup, maintenance, and teardown of PCE-
   initiated LSPs under the stateful PCE model.  It also specifies how a
   PCE may obtain control over an orphaned LSP that was PCE-initiated.
   A PCE implementation can apply the mechanism described in this
   document in conjunction with those in [RFC8281].

5.  Security Considerations

   The security considerations listed in [RFC8231] and [RFC8281] apply
   to this document as well.  However, this document also introduces a
   new attack vector.  An attacker may flood the PCC with requests to
   delegate all of its LSPs at a rate that exceeds the PCC's ability to
   process them, either by spoofing messages or by compromising the PCE
   itself.  The PCC SHOULD be configured with a threshold rate for the
   delegation requests received from the PCE.  If the threshold is
   reached, it is RECOMMENDED to log the issue.

   A PCC is the ultimate arbiter of delegation.  As per [RFC8231], a
   local policy at the PCC is used to influence the delegation.  A PCC
   can also revoke the delegation at any time.  A PCC need not blindly
   trust the control requests and SHOULD take local policy and other
   factors into consideration before honoring the request.

   Note that a PCE may not be sure if a PCC supports this feature.  A
   PCE would try sending a control request to a 'legacy' PCC that would
   in turn respond with an error, as described in Section 4.  So, a PCE
   would learn this fact only when it wants to take control over an LSP.
   A PCE might also be susceptible to downgrade attacks by falsifying
   the error condition.

   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 BCP 195 [RFC7525] (unless explicitly excluded in
   [RFC8253]).

6.  IANA Considerations

   IANA has allocated the following code point in the "SRP Object Flag
   Field" subregistry in the "Path Computation Element Protocol (PCEP)
   Numbers" registry.

                 +-----+---------------------+-----------+
                 | Bit | Description         | Reference |
                 +=====+=====================+===========+
                 | 30  | LSP Control Request | RFC 8741  |
                 +-----+---------------------+-----------+

                                  Table 1

7.  Manageability Considerations

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

7.1.  Control of Function and Policy

   A PCC implementation SHOULD allow the operator to configure the
   policy rules that specify the conditions under which it honors the
   request to control the LSPs.  This includes the handling of the case
   where an LSP control request is received for an LSP that is currently
   delegated to some other PCE.  A PCC implementation SHOULD also allow
   the operator to configure the threshold rate for the delegation
   requests received from the PCE.  Further, the operator MAY be allowed
   to trigger the LSP control request for a particular LSP at the PCE.
   A PCE implementation SHOULD also allow the operator to configure an
   exponentially increasing timer to retry the control requests for
   which the PCE did not get a response.

7.2.  Information and Data Models

   The PCEP YANG module [PCEP-YANG] could be extended to include a
   mechanism to trigger the LSP control request.

7.3.  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].

7.4.  Verify Correct Operations

   Mechanisms defined in this document do not imply any new operation
   verification requirements in addition to those already listed in
   [RFC5440] and [RFC8231].

7.5.  Requirements on Other Protocols

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

7.6.  Impact on Network Operations

   Mechanisms defined in [RFC5440] and [RFC8231] also apply to PCEP
   extensions defined in this document.  Further, the mechanism
   described in this document can help the operator to request control
   of the LSPs at a particular PCE.

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>.

   [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>.

8.2.  Informative References

   [PCEP-YANG]
              Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A
              YANG Data Model for Path Computation Element
              Communications Protocol (PCEP)", Work in Progress,
              Internet-Draft, draft-ietf-pce-pcep-yang-13, 31 October
              2019,
              <https://tools.ietf.org/html/draft-ietf-pce-pcep-yang-13>.

   [RFC4657]  Ash, J., Ed. and J.L. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol Generic
              Requirements", RFC 4657, DOI 10.17487/RFC4657, September
              2006, <https://www.rfc-editor.org/info/rfc4657>.

   [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>.

   [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>.

Acknowledgements

   Thanks to Jonathan Hardwick for reminding the authors to not use
   suggested values in IANA section.

   Thanks to Adrian Farrel, Haomian Zheng, and Tomonori Takeda for their
   valuable comments.

   Thanks to Shawn M. Emery for his Security Directorate review.

   Thanks to Francesca Palombini for GENART review.

   Thanks to Benjamin Kaduk, Martin Vigoureux, Alvaro Retana, and Barry
   Leiba for IESG reviews.

Contributors

   The following people contributed substantially to the content of this
   document and should be considered coauthors:

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

   Email: dhruv.ietf@gmail.com


   Jon Parker
   Cisco Systems, Inc.
   2000 Innovation Drive
   Kanata Ontario K2K 3E8
   Canada

   Email: jdparker@cisco.com


   Chaitanya Yadlapalli
   AT&T
   200 S Laurel Avenue
   Middletown, NJ 07748
   United States of America

   Email: cy098@att.com


Authors' Addresses

   Aswatnarayan Raghuram
   AT&T
   200 S Laurel Avenue
   Middletown, NJ 07748
   United States of America

   Email: ar2521@att.com


   Al Goddard
   AT&T
   200 S Laurel Avenue
   Middletown, NJ 07748
   United States of America

   Email: ag6941@att.com


   Jay Karthik
   Cisco Systems, Inc.
   125 High Street
   Boston, Massachusetts 02110
   United States of America

   Email: jakarthi@cisco.com


   Siva Sivabalan
   Cisco Systems, Inc.
   2000 Innovation Drive
   Kanata Ontario K2K 3E8
   Canada

   Email: msiva@cisco.com


   Mahendra Singh Negi
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore 560066
   Karnataka
   India

   Email: mahend.ietf@gmail.com
