Internet Engineering Task Force (IETF) A. Stone

Internet Engineering Task Force (IETF) A. Stone
Request for Comments: 9488 M. Aissaoui
Updates: 5440 Nokia
Category: Standards Track S. Sidor
ISSN: 2070-1721 Cisco Systems, Inc.
S. Sivabalan
Ciena Corporation
October 2023

Local Protection Enforcement in the Path Computation Element
Communication Protocol (PCEP)

Abstract

This document updates RFC 5440 to clarify usage of the Local
Protection Desired bit signaled in the Path Computation Element
Communication Protocol (PCEP). This document also introduces a new
flag for signaling protection enforcement in PCEP.

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/rfc9488.

Copyright Notice

Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.

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in the Revised BSD License.

Table of Contents

1. Introduction
2. Requirements Language
3. Terminology
4. Motivation
4.1. Implementation Differences
4.2. SLA Enforcement
5. Protection Enforcement Flag (E Flag)
5.1. Backwards Compatibility
6. Security Considerations
7. IANA Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Authors' Addresses

1. Introduction

The Path Computation Element 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].

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 LSP Attributes (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 if a downstream router will
or will not protect a hop during its calculation. Therefore, the L
flag does not require the transit router to satisfy protection in
order to establish the RSVP-signaled path. This flag is signaled in
PCEP as an attribute of the Label Switched Path (LSP) via the LSPA
object.

PCEP Extensions for Segment Routing [RFC8664] extends support in PCEP
for Segment Routing paths. The path list is encoded with Segment
Identifiers (SIDs), each of which might offer local protection. The
PCE may discover the protection eligibility for a SID via the Border
Gateway Protocol - Link State (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
of the protection requirement.

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

The document contains descriptions in the context of Segment Routing;
however, the content described is agnostic in regard to path setup
type and data plane technology.

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.

PROTECTION PREFERRED: The path should have protection eligibility on
all links but might contain links that do not have protection
eligibility.

UNPROTECTED PREFERRED: The path should not have protection
eligibility on all links but might contain links that 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 Communication Protocol

LSPA: LSP Attributes object [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". The use case for
this flag originated in RSVP. For RSVP-signaled paths, local
protection is not within control of the PCE. However, [RFC5440] does
state that "[w]hen set, this means that the computed path must
include links protected with Fast Reroute as defined in [RFC4090]."
Implementations that use PCEP [RFC5440] have interpreted the L flag
as either PROTECTION MANDATORY or PROTECTION PREFERRED, leading to
operational differences.

4.2. SLA Enforcement

The L flag is a boolean bit and thus unable to distinguish between
the different options of PROTECTION MANDATORY, UNPROTECTED MANDATORY,
PROTECTION PREFERRED, and UNPROTECTED PREFERRED. Selecting one of
these options is typically dependent on the Service Level Agreement
(SLA) the operator wishes to impose on the LSP. A network may be
providing transit to multiple SLA 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.

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

As another example, UNPROTECTED MANDATORY is for use cases in which
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 via a
secondary diverse LSP. Each LSP is traffic engineered to follow
specific traffic-engineered criteria computed by the PCE to satisfy
the SLA. Upon a failure, if local protection is invoked on the
active LSP traffic, the traffic may temporarily traverse links that
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 not be feasible
for the PCE to reroute the LSP while respecting the TE requirements,
which include path diversity; this results in the LSP being torn down
and switched to the protected path anyways. In such scenarios, it is
desirable for the LSP to be simply torn down immediately and not
rerouted through local protection, so that traffic may be forwarded
through an already-established traffic-engineered secondary path.

Both the 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 the 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, the PCE chooses the unprotected SID.
Alternatively, if the PROTECTED PREFERRED option is selected, the PCE
chooses the protected SID.

5. Protection Enforcement Flag (E Flag)

Section 7.11 of [RFC5440] describes the encoding of the Local
Protection Desired (L) flag. The Protection Enforcement (E) flag,
which extends the L flag, is specified below.

+=====+==========================+===========+
| Bit | Description | Reference |
+=====+==========================+===========+
| 6 | Protection Enforcement | RFC 9488 |
+-----+--------------------------+-----------+
| 7 | Local Protection Desired | RFC 5440 |
+-----+--------------------------+-----------+

Table 1: Codespace of the Flag Field (LSPA
Object)

The following shows the format of the LSPA object as defined in
[RFC5440] with the addition of the E flag defined in this document:

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 (Local Protection Desired): This flag is 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 (Protection Enforcement): This flag controls the strictness
with which the PCE must apply the L flag. When set to 1, the
value of the L flag needs to be respected during resource
selection by the PCE. When the E flag is set to 0, an attempt
to respect the value of the L flag is made; however, the PCE
could relax or ignore the L flag when computing a path. The
statements below indicate preference when the 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.

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 the 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 the protection eligibility as an
UNPROTECTED MANDATORY constraint. An example of when the latter
behavior might be chosen is if the PCE has some means (outside the
scope of this document) to detect that it is interacting with a
legacy PCC that expects the legacy behavior.

When the L flag is set to 0 and the 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 the protection status of a resource, the
PCE MAY use local policy to define protected status assumptions.
When computing a Segment Routing 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

This section outlines considerations for the E flag bit in the
message passing between the PCC and the PCE that are not supported by
the entity. The requirements for the PCE and the PCC implementing
this document are described at the end.

For a PCC or PCE that does not yet support this document, the E flag
is ignored and set to 0 in PCRpt and/or PCUpd messages as per
[RFC5440] for PCC-initiated LSPs or as per [RFC8281] for PCE-
initiated LSPs. It is important to note that [RFC8231] and [RFC8281]
permit the LSPA object [RFC5440] to be included in PCUpd messages for
PCC-initiated and PCE-initiated LSPs.

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

A PCC that 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 message.

For a PCC that does support this document, the E flag MAY be set to 1
depending on local configuration. If communicating with a PCE that
does not yet support this document, the PCE follows the behavior
specified in [RFC5440] and ignores the E flag. Thus, a computed path
might 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 that 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 that does not support this document.

6. Security Considerations

This document clarifies the behavior of an existing flag and
introduces a new flag to provide further control of that existing
behavior. The introduction of this new flag and the behavior
clarification do 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
[RFC9325], is RECOMMENDED.

7. IANA Considerations

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

+=====+========================+===========+
| Bit | Description | Reference |
+=====+========================+===========+
| 6 | Protection Enforcement | RFC 9488 |
+-----+------------------------+-----------+

Table 2: Addition to LSPA Object Flag
Field Registry

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>.

[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>.

[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>.

[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>.

[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>.

8.2. 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>.

[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, Mike Koldychev, and John Scudder 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: [email protected]

Mustapha Aissaoui
Nokia
600 March Road
Kanata Ontario K2K 2T6
Canada
Email: [email protected]

Samuel Sidor
Cisco Systems, Inc.
Eurovea Central 3
Pribinova 10
811 09 Bratislava
Slovakia
Email: [email protected]

Siva Sivabalan
Ciena Corporation
385 Terry Fox Drive
Kanata Ontario K2K 0L1
Canada
Email: [email protected]