Internet Engineering Task Force (IETF) Q. Xiong
Request for Comments: 9357
Category: Standards Track February 2023
Label Switched Path (LSP) Object Flag Extension for Stateful PCE
Abstract RFC 8231
describes a set of extensions to the Path Computation
Element Communication Protocol (PCEP) to enable stateful control of
MPLS-TE and GMPLS Label Switched Paths (LSPs) via PCEP. One of the
extensions is the LSP object, which includes a Flag field with a
length of 12 bits. However, all bits of the Flag field have already
This document defines a new LSP-EXTENDED-FLAG TLV for the LSP object
for an extended Flag field.
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/rfc9357
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Table of Contents 1.
Conventions Used in this Document 2.1.
Requirements Language 3.
PCEP Extension 3.1.
The LSP-EXTENDED-FLAG TLV 3.2.
Advice for Specification of New Flags 5.
Backward Compatibility 6.
IANA Considerations 6.1.
LSP Object 6.1.1.
PCEP TLV Type Indicators 6.1.2.
LSP Extended Flags Field 7.
Management Considerations 8.
Security Considerations 9.
Normative References 9.2.
Informative References Appendix A
. Working Group Discussion
] describes the Path Computation Element Communication
Protocol (PCEP), which is used between a PCE and a Path Computation
Client (PCC) (or other PCE) to enable computation of Multi-protocol
Label Switching for Traffic Engineering (MPLS-TE) Label Switched
PCEP Extensions for the Stateful PCE Model [RFC8231
] describes a set
of extensions to PCEP to enable active control of MPLS-TE and
Generalized MPLS (GMPLS) tunnels. One of the extensions is the LSP
object, which contains a Flag field; bits in the Flag field are used
to indicate delegation, synchronization, removal, etc.
As defined in [RFC8231
], the length of the Flag field is 12 bits, and
all of the bits have already been defined as shown in Table 1. This
document extends the Flag field of the LSP object for other use by
defining a new LSP-EXTENDED-FLAG TLV for an extended Flag field in
the LSP object (see Section 3.1
| Bit | Description | Reference |
| 0 | PCE-allocation | [BIND-LABEL-SID] |
| 1 | ERO-compression | [RFC8623
| 2 | Fragmentation | [RFC8623
| 3 | P2MP | [RFC8623
| 4 | Create | [RFC8281
| 5-7 | Operational (3 bits) | [RFC8281
| 8 | Administrative | [RFC8281
| 9 | Remove | [RFC8281
| 10 | SYNC | [RFC8281
| 11 | Delegate | [RFC8281
Table 1: LSP Object Flag Field
2. Conventions Used in this Document
The terminology is defined in [RFC5440
] and [RFC8231
2.2. Requirements Language
The key words "MUST
", "MUST NOT
", "SHALL NOT
", "SHOULD NOT
", "NOT RECOMMENDED
" in this document are to be interpreted as described in
BCP 14 [RFC2119
] when, and only when, they appear in all
capitals, as shown here.
3. PCEP Extension
The LSP object is defined in Section 7.3 of [RFC8231
]. This document
defines a new LSP-EXTENDED-FLAG TLV for an extended Flag field in the
3.1. The LSP-EXTENDED-FLAG TLV
The format of the LSP-EXTENDED-FLAG TLV shown in Figure 1 follows the
format of all PCEP TLVs, as defined in [RFC5440
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
| Type=64 | Length |
// LSP Extended Flags //
Figure 1: LSP-EXTENDED-FLAG TLV Format
Type (16 bits): 64
Length (16 bits): This indicates the length of the value portion in
bytes. It MUST
be in multiples of 4 and greater than 0.
LSP Extended Flags: This contains an array of units of 32-bit flags
numbered from the most significant as bit zero, where each bit
represents one LSP flag (for operation, feature, or state). The
LSP Extended Flags field SHOULD
use the minimal amount of space
needed to encode the flag bits. Currently, no bits are assigned.
Unassigned bits MUST
be set to zero on transmission and MUST
ignored on receipt.
As an example of usage of the LSP-EXTENDED-FLAG TLV, the E-flag is
requested for entropy label configuration, as proposed in
The LSP Extended Flags field is an array of units of 32 flags that
are allocated starting from the most significant bit. The bits of
the LSP Extended Flags field will be assigned by future documents.
This document does not define any flags. Flags that an
implementation is not supporting MUST
be set to zero on transmission.
Implementations that do not understand any particular flag MUST
ignore the flag.
Note that PCEP peers MUST
handle varying lengths of the LSP-EXTENDED-
If a PCEP speaker receives the LSP-EXTENDED-FLAG TLV of a length more
than it currently supports or understands, it MUST
ignore the bits
beyond that length.
If a PCEP speaker receives the LSP-EXTENDED-FLAG TLV of a length less
than the one supported by the implementation, it MUST
act as if the
bits beyond the length were not set.
4. Advice for Specification of New Flags
Following the model provided in Section 3.1
], we provide
the following advice for new specifications that define additional
flags. Each such specification is expected to describe the
interaction between these new flags and any existing flags. In
particular, new specifications are expected to explain how to handle
the cases when both new and preexisting flags are set. They are also
expected to discuss any security implications of the additional flags
(if any) and their interactions with existing flags.
5. Backward Compatibility
The LSP-EXTENDED-FLAG TLV defined in this document does not introduce
any backward compatibility issues. An implementation that does not
understand or support the LSP-EXTENDED-FLAG TLV MUST
ignore the TLV,
as per [RFC5440
]. Future documents that define bits in the LSP-
EXTENDED-FLAG TLV are expected to also define the error handling
required for cases in which the LSP-EXTENDED-FLAG TLV is missing when
Further, any additional bits in the LSP-EXTENDED-FLAG TLV that are
not understood by an implementation MUST
be ignored. It is expected
that future documents that define bits in the LSP-EXTENDED-FLAG TLV
will take that into consideration.
6. IANA Considerations
6.1. LSP Object
6.1.1. PCEP TLV Type Indicators
IANA has allocated the following TLV Type Indicator value within the
"PCEP TLV Type Indicators" registry of the "Path Computation Element
Protocol (PCEP) Numbers" registry:
| Value | Description | Reference |
| 64 | LSP-EXTENDED-FLAG | RFC 9357
6.1.2. LSP Extended Flags Field
IANA has created the "LSP-EXTENDED-FLAG TLV Flag Field" registry
within the "Path Computation Element Protocol (PCEP) Numbers"
registry to manage the LSP Extended Flags field of the LSP-EXTENDED-
FLAG TLV. New values are assigned by Standards Action [RFC8126
Each bit should be tracked with the following qualities:
* Bit number (counting from bit 0 as the most significant bit)
* Capability Description
No values are currently defined. Bits 0-31 are initially marked as
"Unassigned". Bits with a higher ordinal than 31 will be added to
the registry in future documents if necessary.
7. Management Considerations
Implementations receiving set LSP Extended Flags that they do not
log this. That could be helpful for diagnosing
backward compatibility issues with future features that utilize those
8. Security Considerations
] sets out security considerations for PCEP when used for
communication with a stateful PCE. This document does not change
those considerations. For LSP object processing, see [RFC8231
The flags for the LSP object and their associated security
considerations are specified in [RFC8231
This document provides for the future addition of flags in the LSP
object. Any future document that specifies new flags must also
discuss any associated security implications. No additional security
issues are raised in this document beyond those that exist in the
referenced documents. Note that [RFC8231
] recommends that the
stateful PCEP extension be authenticated and encrypted using
Transport Layer Security (TLS) [RFC8253
] [PCEPS-TLS1.3], as per the
recommendations and best current practices in [RFC9325
that the recommendation is followed, then the flags will be protected
9.1. Normative References
] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
, March 1997,
] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440
, March 2009,
] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126
, DOI 10.17487/RFC8126
, June 2017,
] 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
] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for Stateful PCE", RFC 8231
, September 2017,
9.2. Informative References
Sivabalan, S., Filsfils, C., Tantsura, J., Previdi, S.,
and C. Li, Ed., "Carrying Binding Label/Segment Identifier
(SID) in PCE-based Networks.", Work in Progress, Internet-
Draft, draft-ietf-pce-binding-label-sid-15, 20 March 2022,
Xiong, Q., Peng, S., and F. Qin, "PCEP Extension for SR-
MPLS Entropy Label Position", Work in Progress, Internet-
Draft, draft-peng-pce-entropy-label-position-08, 29 August
Dhody, D., Turner, S., and R. Housley, "PCEPS with TLS
1.3", Work in Progress, Internet-Draft, draft-dhody-pce-
pceps-tls13-01, 20 October 2022,
] Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and R.
Zhang, "OSPF Protocol Extensions for Path Computation
Element (PCE) Discovery", RFC 5088
, DOI 10.17487/RFC5088
January 2008, <https://www.rfc-editor.org/info/rfc5088
] Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and R.
Zhang, "IS-IS Protocol Extensions for Path Computation
Element (PCE) Discovery", RFC 5089
, DOI 10.17487/RFC5089
January 2008, <https://www.rfc-editor.org/info/rfc5089
] 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,
] 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,
] Palle, U., Dhody, D., Tanaka, Y., and V. Beeram, "Stateful
Path Computation Element (PCE) Protocol Extensions for
Usage with Point-to-Multipoint TE Label Switched Paths
(LSPs)", RFC 8623
, DOI 10.17487/RFC8623
, June 2019,
] Farrel, A., "Updated Rules for Processing Stateful PCE
Request Parameters Flags", RFC 8786
, DOI 10.17487/RFC8786
May 2020, <https://www.rfc-editor.org/info/rfc8786
] Sheffer, Y., Saint-Andre, P., and T. Fossati,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 9325
, DOI 10.17487/RFC9325
Appendix A. Working Group Discussion
The working group discussed the idea of a fixed length (with 32 bits)
for the LSP-EXTENDED-FLAG TLV. Though 32 bits would be sufficient
for quite a while, the use of variable length with a multiple of 32
bits allows for future extensibility where we would never run out of
flags and there would not be a need to define yet another TLV in the
future. Further, note that [RFC5088
] and [RFC5089
] use the same
approach for the PCE-CAP-FLAGS sub-TLV and are found to be useful.
The authors would like to thank Loa Andersson, Adrian Farrel, Aijun
Wang, and Gyan Mishra for their reviews, suggestions, and comments
for this document.
The following people have substantially contributed to this document:
No.6 Huashi Park Rd