Update to Automatic Bandwidth Adjustment procedure of Stateful PCE

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Table of Contents

1. Introduction

[RFC5440] describes the Path Computation Element Communication Protocol (PCEP). PCEP defines the communication between a Path Computation Client (PCC) and a Path Computation Element (PCE), or between PCEs, enabling computation of Multiprotocol Label Switching (MPLS) for Traffic Engineering Label Switched Path (TE LSP) characteristics.¶

[RFC8231] specifies extensions to PCEP to enable stateful control of MPLS TE LSPs. It describes two modes of operations - Passive stateful PCE and Active stateful PCE. Further, [RFC8281] describes the setup, maintenance and teardown of PCE-Initiated LSPs for the stateful PCE model.¶

[RFC8733] describes the auto-bandwidth feature that allows automatic and dynamic adjustment of the TE LSP bandwidth reservation based on the volume of traffic flowing through the LSP. It describes PCEP extensions for auto-bandwidth adjustment when employing an active stateful PCE for both PCE-initiated [RFC8281] and PCC-initiated LSPs. It defines the AUTO-BANDWIDTH-ATTRIBUTES TLV that provides the 'configurable knobs' of the feature, and it can be included as an optional TLV in the LSPA object. The TLV is encoded in all PCEP messages for the LSP while the auto-bandwidth adjustment feature is enabled. The absence of the TLV indicates the PCEP speaker wishes to disable the feature. The TLV includes multiple AUTO-BANDWIDTH-ATTRIBUTES sub-TLVs defined in [RFC8733]. The AUTO-BANDWIDTH-ATTRIBUTES sub-TLVs are included if there is a change since the last information sent in the PCEP message. It also states that in the case of a missing sub-TLV, as per the local policy, either the default value or some other operator-configured value is used.¶

Since the missing sub-TLV in a subsequent PCEP message is considered as no change, there is no mechanism to remove a particular attribute encoded in the sub-TLV. This document updates [RFC8733] to define such a procedure.¶

Note that for the attributes that have an associated default value, they could simply encode the default value in the sub-TLV but this cannot be used for the attributes that do not have a default value.¶

This document proposes to use a special value of all zeros to indicate "restore to default", which could mean going back to the default values or removal of the attribute itself.¶

The following table includes the sub-TLVs and the default values as per [RFC8733].¶

Thus, the use of the special value of all zeros in the value portion of the sub-TLV can be used to indicate "restore to default", which could mean :¶

• if an explicit default value is set for the sub-TLV:¶

  • Restore to the default values¶

• if the default value is set to another sub-TLV value:¶

  • Remove the associated attribute¶

• if there is no default value for the sub-TLV:¶

  • Remove the associated attribute¶

The value portion of the sub-TLV consists of encoded data (of the specified length and type), which is set to zero. In cases where the value portion of the sub-TLV contains multiple fields, all fields are set to zero.¶

2. Conventions

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. Updated Procedures

Section 5.2 of [RFC8733] defines the AUTO-BANDWIDTH-ATTRIBUTES TLV and its associated sub-TLVs.¶

This document updates [RFC8733] by adding this text at the end of paragraph 3 in section 5.2:¶

A special value of all zeros in the value portion of the sub-TLV
indicates the attribute identified by the sub-TLV is restored to
the default value. The value of all zeros is not considered an
invalid value and MUST be checked before individual fields.

For the attributes that have an associated default value, on
receiving such a sub-TLV, the PCEP speaker MUST consider it as
an instruction to restore to the default values. Note that, the
PCEP speaker could also set the default value in the sub-TLV
itself.

For the attributes that do not have an associated default value,
on receiving such a sub-TLV, the PCEP speaker MUST consider it
as a removal of the specific auto-bandwidth attribute.

4. Extensions

Section 5.1.1 of [RFC8733] defines the AUTO-BANDWIDTH-CAPABILITY TLV as an optional TLV for use in the OPEN Object for auto-bandwidth adjustment. This document adds a new flag -¶

Z (TBD): The flag indicates that a PCEP speaker supports the use of the special value of all zeros in the value field as specified in this document.¶

The presence of the Z flag can give a clear indication to the PCEP peer if they can use the updated procedures defined in this document.¶

5. Examples

6. Backward Compatibility

Note that to achieve the same objective, an [RFC8733] compliant implementation could send a PCEP message without AUTO-BANDWIDTH-ATTRIBUTES TLV first and then include the AUTO-BANDWIDTH-ATTRIBUTES TLV with the updated sub-TLV. This is the same as "turning it off and on again" but would cause unnecessary path computation churn (compared to targeted removal of the attribute).¶

An existing implementation of [RFC8733] that does not support this update (where the Z flag is not set) will not recognize or use the special value of all zeros in the sub-TLV. If such a sub-TLV is received, as per [RFC8733], implementations may treat the sub-TLV as malformed and ignore it.¶

7. Security Considerations

This document does not add any substantial new security concerns beyond those already discussed in [RFC8733].¶

8. IANA Considerations

9. 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/rfc/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/rfc/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/rfc/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/rfc/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/rfc/rfc8281>.

[RFC8733]

Dhody, D., Ed., Gandhi, R., Ed., Palle, U., Singh, R., and L. Fang, "Path Computation Element Communication Protocol (PCEP) Extensions for MPLS-TE Label Switched Path (LSP) Auto-Bandwidth Adjustment with Stateful PCE", RFC 8733, DOI 10.17487/RFC8733, February 2020, <https://www.rfc-editor.org/rfc/rfc8733>.

Appendix A. Acknowledgments

Thanks to Aijun Wang, Andrew Stone, and Luis Miguel Contreras Murillo for their review comments.¶

Authors' Addresses