| Internet-Draft | Protection Enforcement | June 2023 |
| Stone, et al. | Expires 25 December 2023 | [Page] |
This document updates RFC5440 to clarify usage of the local protection desired bit signalled in the Path Computation Element Protocol (PCEP). This document also introduces a new flag for signalling protection strictness in PCEP.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶
This Internet-Draft will expire on 25 December 2023.¶
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.¶
The Path Computation Element (PCE) 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 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 whether a downstream router will or will not protect a hop during its calculation. Therefore, a local protection desired does not require the transit router to satisfy protection in order to establish the RSVP signalled path. This flag is signalled in PCEP as an attribute of the LSP via the LSP Attributes object.¶
PCEP Extensions for Segment Routing ([RFC8664]) extends support in PCEP for Segment Routed paths. The path list is encoded with Segment Identifiers, each of which might offer local protection. The PCE may discover the protection eligibility for a Segment Identifier (SID) via 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, or strictness of the protection requirement.¶
This document updates [RFC5440] by further describing the behaviour with the Local Protection Desired Flag (L flag) and extends on it with the introduction of the Enforcement Flag (E flag).¶
The document contains reference notes for Segment Routing, however the content described is path setup type and data plane technology agnostic.¶
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.¶
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 which do not have protection eligibility.¶
UNPROTECTED PREFERRED: The Path should not have protection eligibility on all links but might contain links which 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 Protocol.¶
LSPA: LSP Attributes Object in PCEP, defined in RFC5440¶
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" and the use case originated from the RSVP. For RSVP signalled paths, local protection is not within control of the PCE. However, [RFC5440] does state "When set, this means that the computed path must include links protected with Fast Reroute as defined in [RFC4090]." Implementations of [RFC5440] have either interpreted the L flag as PROTECTION MANDATORY or PROTECTION PREFERRED, leading to operational differences.¶
The boolean bit L flag is unable to distinguish between the different options of PROTECTION MANDATORY, UNPROTECTED MANDATORY, PROTECTION PREFERRED and UNPROTECTED PREFERRED. Selecting one of the options is typically dependent on the service level agreement the operator wishes to impose on the LSP. A network may be providing transit to multiple service agreement 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 where an operator may need the LSP to follow a path which has local protection provided along the full path, ensuring that if there is a failure anywhere along the path that traffic will be fast re-routed at the point.¶
For example, UNPROTECTED MANDATORY is when 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 with path protection via a secondary diverse LSP. Each LSP is traffic engineered to follow specific traffic engineered criteria computed by the PCE to satisfy SLA. Upon a failure, if local protection is invoked on the active LSP traffic, the traffic may temporarily traverse links which 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 be not feasible for the PCE to reroute the LSP while respecting the TE requirements which include path diversity, resulting in the LSP being torn down and switched to the protected path anyways. In such scenarios its desirable for the LSP to be simply torn down immediately and not re-routed through local protection, so that traffic may be forwarded through an already established traffic-engineered secondary path.¶
Both 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 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, PCE chooses the unprotected SID. Alternatively, if the PROTECTED PREFERRED option is selected, PCE chooses the protected SID¶
Section 7.11 in Path Computation Element Protocol [RFC5440] describes the encoding of the Local Protection Desired (L flag). A Protection Enforcement flag "E" is specified below, extending the L flag.¶
[RFC Editor Note: The text below assumes the E bit remains the early allocation value 6. Please adjust if this changes and remove this note before publication.]¶
Codespace of the Flag field (LSPA Object)
Bit Description Reference
7 Local Protection Desired RFC5440
6 Local Protection Enforcement This document¶
The format of the LSPA Object as defined in [RFC5440] is:¶
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)¶
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 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 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 L flag is set to 0 and 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 Routed 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.¶
Considerations in the message passing between the PCC and the PCE for the E flag bit which are not supported by the entity are outlined in this section, with requirements for the PCE and the PCC implementing this document described at the end.¶
For a PCC or PCE which does not yet support this document, the E flag is ignored and set to zero in PCRpt and/or PCUpd as per [RFC5440] for PCC-initiated or as per [RFC8281] for PCE-initiated LSPs. It is important to note that [RFC8231] and [RFC8281] permit the LSP Attribute Object to be included in PCUpd messages for PCC-initiated and PCE-initiated LSPs.¶
For PCC-initiated LSPs, PCUpd E flag (and L flag) is an echo from the previous PCRpt however the bit value is ignored on the PCE from the previous PCRpt, therefore the E flag value set in the PCUpd is zero. A PCE which does not support this document sends PCUpd messages with the E flag set to 0 for PCC-initated LSPs even if set to 1 in the prior PCReq or PCRpt.¶
A PCC which 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.¶
For a PCC which does support this document, it MAY set the E flag to 1 depending on local configuration. If communicating with a PCE which does not yet support this document, the PCE follows the behaviour specified in [RFC5440] and will ignore 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 which 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 which does not support this document.¶
[Note to the RFC Editor - remove this section before publication, as well as remove the reference to RFC 7942.]¶
This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalogue of available implementations or their features. Readers are advised to note that other implementations may exist.¶
According to [RFC7942], "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".¶
This document clarifies the behaviour of an existing flag and introduces a new flag to provide further control of that existing behaviour. The introduction of this new flag and behaviour clarification does 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.¶
[RFC Editor Note: The text below assumes the E bit remains the early allocation value 6. Please adjust if this changes and remove this note before publication.]¶
This document defines a new bit value in the sub-registry "LSPA Object Flag Field" in the "Path Computation Element Protocol (PCEP) Numbers" registry. IANA has made the following codepoint allocation.¶
Bit Name Reference
6 Protection Enforcement This document
¶
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.¶