PCEP LSP Extended Flags: Label Switched Path (LSP) Object Flag Extension for Stateful PCE
A second, variable-length flag field for the PCEP LSP object, added after its 12 original flag bits were all taken.
Xiong, Q., "Label Switched Path (LSP) Object Flag Extension for Stateful PCE", RFC 9357, DOI 10.17487/RFC9357, February 2023, <https://www.rfc-editor.org/info/rfc9357>.
@misc{rfc9357,
series = {Request for Comments},
number = 9357,
howpublished = {RFC 9357},
publisher = {RFC Editor},
doi = {10.17487/RFC9357},
url = {https://www.rfc-editor.org/info/rfc9357},
author = {Q. Xiong},
title = {{Label Switched Path (LSP) Object Flag Extension for Stateful PCE}},
pagetotal = 9,
year = 2023,
month = feb,
} RFC text
Contents
Internet Engineering Task Force (IETF) Q. Xiong
Request for Comments: 9357 ZTE Corporation
Category: Standards Track February 2023
ISSN: 2070-1721
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
been assigned.
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.
Copyright Notice
Copyright (c) 2023 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 Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Conventions Used in this Document
2.1. Terminology
2.2. Requirements Language
3. PCEP Extension
3.1. The LSP-EXTENDED-FLAG TLV
3.2. Processing
4. 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. References
9.1. Normative References
9.2. Informative References
Appendix A. Working Group Discussion
Acknowledgements
Contributors
Author's Address
1. Introduction
[RFC5440] 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
Paths (LSPs).
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
2.1. Terminology
The terminology is defined in [RFC5440] and [RFC8231].
2.2. 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. 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
LSP object.
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 be
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
[PCEP-ENTROPY-LABEL].
3.2. Processing
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-
FLAG TLV.
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 of [RFC8786], 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
it MUST be present.
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 |
+-------+-------------------+-----------+
Table 2
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
* Reference
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
recognize MAY log this. That could be helpful for diagnosing
backward compatibility issues with future features that utilize those
flags.
8. Security Considerations
[RFC8231] 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], [RFC8281], [RFC8623], and
[BIND-LABEL-SID].
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]. Assuming
that the recommendation is followed, then the flags will be protected
by TLS.
9. References
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>.
[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>.
[RFC8126] 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,
<https://www.rfc-editor.org/info/rfc8126>.
[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>.
9.2. Informative References
[BIND-LABEL-SID]
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,
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-
binding-label-sid-15>.
[PCEP-ENTROPY-LABEL]
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
2022, <https://datatracker.ietf.org/doc/html/draft-peng-
pce-entropy-label-position-08>.
[PCEPS-TLS1.3]
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,
<https://datatracker.ietf.org/doc/html/draft-dhody-pce-
pceps-tls13-01>.
[RFC5088] 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>.
[RFC5089] 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>.
[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>.
[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>.
[RFC8623] 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,
<https://www.rfc-editor.org/info/rfc8623>.
[RFC8786] 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>.
[RFC9325] 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, November
2022, <https://www.rfc-editor.org/info/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.
Acknowledgements
The authors would like to thank Loa Andersson, Adrian Farrel, Aijun
Wang, and Gyan Mishra for their reviews, suggestions, and comments
for this document.
Contributors
The following people have substantially contributed to this document:
Dhruv Dhody
Huawei Technologies
Email: dhruv.ietf@gmail.com
Greg Mirsky
Ericsson
Email: gregimirsky@gmail.com
Author's Address
Quan Xiong
ZTE Corporation
No.6 Huashi Park Rd
Wuhan
Hubei, 430223
China
Email: xiong.quan@zte.com.cn
ISO Plain Language
On this page
§1Why the Flag Field Ran Out
The Path Computation Element Communication Protocol (PCEP) runs between a Path Computation Element (PCE) and a Path Computation Client (PCC), or between two PCEs, so that paths for Multi-protocol Label Switching for Traffic Engineering (MPLS-TE) can be computed. RFC 5440 defines the protocol. RFC 8231 adds extensions for a stateful PCE, where the PCE keeps and updates state for the paths it manages.
One of those extensions is the LSP object, which carries a Flag field whose bits signal things such as delegation, synchronization and removal. As defined in RFC 8231, that field is 12 bits long, and Table 1 of this RFC shows that all 12 bits already have a meaning: PCE-allocation, ERO-compression, Fragmentation, P2MP, Create, a three-bit Operational value, Administrative, Remove, SYNC and Delegate.
Because every bit is taken, no new flag can be added to that field. This RFC leaves the original field alone and instead defines a separate TLV, the LSP-EXTENDED-FLAG TLV, that carries an extended Flag field inside the same LSP object.
PCEP is the protocol a network controller uses to ask for paths. The LSP object it carries has 12 flag bits, and every one of them is already in use. This RFC adds a second field of flags so that new flags can be defined later.
- 0PCE-allocation — From the work in progress on carrying the binding label in PCE-based networks.
- 1ERO-compression — Defined in RFC 8623.
- 2Fragmentation — Defined in RFC 8623.
- 3P2MP — Defined in RFC 8623.
- 4Create — Defined in RFC 8281.
- 5-7Operational — Three bits defined in RFC 8281.
- 8Administrative — Defined in RFC 8281.
- 9Remove — Defined in RFC 8281.
- 10SYNC — Defined in RFC 8281.
- 11Delegate — Defined in RFC 8281.
§3.1The LSP-EXTENDED-FLAG TLV
The new TLV follows the format of every other PCEP TLV, as defined in RFC 5440. It carries a 16-bit Type field set to 64, a 16-bit Length field, and a value that holds the flags.
Length counts the bytes of the value. It MUST be a multiple of 4 and greater than 0. The value is an array of units of 32-bit flags, numbered from the most significant bit as bit zero, so each bit in a unit carries one LSP flag. A bit can stand for an operation, a feature or a state.
The field SHOULD use the minimum amount of space needed to encode the flag bits. This document assigns no bits at all. Unassigned bits MUST be set to zero on transmission and MUST be ignored on receipt. As an example of intended use, the RFC notes that an E-flag for entropy label configuration was requested in a work in progress on the PCEP extension for the SR-MPLS entropy label position.
The new TLV carries a list of flag bits, 32 bits at a time. Its length must be a multiple of 4 bytes. This RFC reserves the space but assigns no flags to it.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 2 bytes | Always 64 for this TLV. | |
LENGTH | Length | 2 bytes | Length of the value in bytes. Must be a multiple of 4 and greater than 0. | |
FLAGS | LSP Extended Flags | 4 bytes | One unit of 32 flags, numbered from the most significant bit as bit zero. More units may follow. |
§3.2How Receivers Process the TLV
Flags are allocated starting from the most significant bit of each 32-bit unit. An implementation that does not support a particular flag MUST set that flag to zero when it transmits the object, and an implementation that does not understand a particular flag MUST ignore it. PCEP peers MUST handle TLVs of varying lengths.
When a speaker receives a TLV longer than it currently supports, it MUST ignore the bits beyond the length it understands. When it receives a TLV shorter than the one its implementation supports, it MUST act as if the bits beyond that length were not set.
A sender sets any flag it does not support to zero, and a receiver ignores any flag it does not understand. If the flag list is longer or shorter than the receiver expects, the receiver uses only the part it knows and treats the rest as unset.
§4Advice for Defining New Flags
Following the model given in Section 3.1 of RFC 8786, this RFC gives guidance for later documents that define additional flags. Each such document is expected to describe the interaction between the new flags and the existing ones. In particular, a new specification is expected to explain how to handle the case where both a new flag and a pre-existing flag are set. It is also expected to discuss any security implications of the additional flags and of their interaction with the existing flags.
A document that adds a new flag has to explain how that flag combines with the flags already in use, and what security effects it has.
§5Backward Compatibility
The RFC states that the LSP-EXTENDED-FLAG TLV does not raise any backward compatibility issues. An implementation that does not understand or support the TLV MUST ignore it, as RFC 5440 requires for TLVs it does not recognise.
Any additional bits in the TLV that an implementation does not understand MUST be ignored. Future documents that define bits in the TLV are expected to define the error handling required for the case where the TLV is missing although it must be present.
Equipment that does not know the new TLV skips it, so older and newer implementations can talk to each other unchanged. Later documents that use the TLV must say what to do when it is missing.
§6IANA Registries
IANA allocated the value 64 in the 'PCEP TLV Type Indicators' registry, which sits inside the 'Path Computation Element Protocol (PCEP) Numbers' registry. The same body created a new registry, the 'LSP-EXTENDED-FLAG TLV Flag Field', to manage the bits of the new field. New values are assigned by Standards Action, as described in RFC 8126.
Each bit in the registry is tracked with three pieces of information: the bit number, counting from bit 0 as the most significant bit, a capability description, and a reference to the document that defines it. No values are currently defined. Bits 0 to 31 are initially marked 'Unassigned', and bits with a higher ordinal than 31 will be added to the registry in future documents if they are needed.
IANA keeps a public list of the flag bits in the new field. Each entry records the bit number, what the bit does, and the document that defines it. All 32 bits are unassigned so far.
§7Management Considerations
An implementation that receives LSP Extended Flags it does not recognise MAY log them. The RFC suggests this because it helps diagnose backward compatibility problems with future features that make use of those flags.
An implementation may write a log entry when it sees a flag it does not recognise. That makes it easier to spot problems with a later feature.
§8Security Considerations
RFC 8231 sets out the security considerations for PCEP used with a stateful PCE, and this document does not change them. The security considerations for the LSP object flags are given in RFC 8231, RFC 8281, RFC 8623 and the work in progress on carrying the binding label and segment identifier in PCE-based networks.
Because the RFC opens the door to future flags, any document that specifies new flags must also discuss any associated security implications. No additional security issues are raised here beyond those in the referenced documents. RFC 8231 recommends that the stateful PCEP extension be authenticated and encrypted using Transport Layer Security (TLS), as set out in RFC 8253 and the current best practice in RFC 9325. When that recommendation is followed, the flags are protected by TLS.
This document adds no security rules of its own. The flags travel inside PCEP, and RFC 8231 says that PCEP with a stateful PCE should be authenticated and encrypted with TLS, so the flags are protected along with the rest of the session.