| Network Working Group | A. Stone |
| Internet-Draft | M. Aissaoui |
| Intended status: Standards Track | Nokia |
| Expires: December 18, 2020 | S. Sidor |
| Cisco Systems, Inc. | |
| S. Sivabalan | |
| Ciena Coroporation | |
| June 16, 2020 |
Local Protection Enforcement in PCEP
draft-stone-pce-local-protection-enforcement-01
This document aims to clarify existing usage of the local protection desired bit signalled in 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.
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This Internet-Draft will expire on December 18, 2020.
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Path Computation Element (PCE) Communication Protocol (PCEP) [RFC5440] enables the communication between a Path Computation Client (PCC) and a Path Control Element (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 local protection is desired, which indicates to transit 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 it's 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 (draft-ietf-pce-segment-routing) extends support in PCEP for Segment Routed LSPs (SR-LSPs) as defined in the Segment Routing Architecture [RFC8402]. As per the Segment Routing Architecture, Adjacency Segment Identifiers(Adj-SID) may be eligible for protection (using IPFRR or MPLS-FRR). The protection eligibility is advertised into IGP (draft-ietf-ospf-segment-routing-extensions and draft-ietf-isis-segment-routing-extensions) as the B-Flag part of the Adjacency SID sub-tlv and can be discovered by a PCE via BGP-LS [RFC7752] using the BGP-LS Segment Routing Extensions (draft-ietf-idr-bgp-ls-segment-routing-ext). An Adjacency SID may or may not have protection eligibility and for a given adjacency between two routers there may be multiple Adjacency SIDs, some of which are protected and some which are not.
A Segment Routed path calculated by PCE may contain various types of segments, as defined in [RFC8402] such as Adjacency, Node or Binding. The protection eligibility for Adjacency SIDs can be discovered by PCE, so therefore the PCE can take the protection eligibility into consideration as a path constraint. If a path is calculated to include other segment identifiers which are not applicable to having their protection state advertised, as they may only be locally significant for each router processing the SID such as Node SIDs, it may not be possible for PCE to include the protection constraint as part of the path calculation.
It is desirable for an operator to define the enforcement, or strictness of the protection requirement when it can be applied.
In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in BCP 14, [RFC2119].
This document uses the following terminology:
PROTECTION MANDATORY: path MUST have protection eligibility on all links.
UNPROTECTED MANDATORY: path MUST NOT have protection eligibility on all links.
PROTECTION PREFERRED: path SHOULD have protection eligibility on all links but MAY contain links which do not have protection eligibility.
UNPROTECTED PREFERRED: path SHOULD NOT have protection eligibility on all links but MAY 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.
As defined in [RFC5440] the mechanism to signal protection enforcement in PCEP is with 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 is rooted in the RSVP use case. For RSVP, this 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 flag is unable to distinguish between the different options of PROTECTION MANDATORY, UNPROTECTED MANDATORY, PROTECTION PREFERRED and UNPROTECTED PREFERRED. The selection of the options are typically dependent on the service level agreement the operator wishes to impose on the LSP. 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 anywhere along the path that traffic will be fast re-routed at the point of failure.
For another example, UNPROTECTED MANDATORY is when an operator may intentionally prefer an LSP to not be locally protected, and thus would rather local failures to cause the LSP to go down and/or rely on other protection mechanisms such as a secondary diverse path.
There are also use cases where there is simply no requirement to enforce protection or no protection along a path. This can be considered as "do not care to enforce". This is a relaxation of the protection constraint. The path calculation is permitted the use of any SID which is available along the calculated path. The SID backup availability does not impact the shortest path computation. Since links may have both protected and unprotected SIDs available, the option PROTECTION PREFERRED or UNPROTECTED PREFERRED is used to instruction PCE a preference on which SID to select, as the behaviour of the LSP would differ during a local failure depending on which SID is selected.
Section 7.11 in Path Computation Element Protocol [RFC5440] describes the encoding of the Local Protection Desired (L-Flag). A new flag is proposed in this document in the LSP Attributes Object which extends the L-Flag to identify the protection enforcement.
The flag bit is to be allocated by IANA following IETF Consensus.
This draft version proposes using bit 6.
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 L-flag is set and E-flag is set then PCE MUST consider the protection eligibility as PROTECTION MANDATORY constraint.
When L-flag is set and E-flag is not set then PCE MUST consider the protection eligibility as PROTECTION PREFERRED constraint.
When L-flag is not set and E-flag is not set then PCE SHOULD consider the protection eligibility as UNPROTECTED PREFERRED but MAY consider protection eligibility as UNPROTECTED MANDATORY constraint.
When L-flag is not set and E-flag is set then PCE MUST consider the protection eligibility as UNPROTECTED MANDATORY constraint.
For a PCC which does not yet support this draft, the E-flag bit is always set to zero as per [RFC5440]. Therefore, a PCE communicating with a PCC which does not support this draft would treat the L-Flag set as being PROTECTION PREFERRED.
The protection constraint can only be applied to resource selection in which the protection state is known to PCE. A PCE calculating a path that includes resources which does not support the protection state being known to PCE (such as Node SID), then the protection state MAY ignore the protection enforcement constraint.
UNPROTECTED PREFERRED and PROTECTED PREFERRED may seem similar but they indicate the preference of selection if PCE has an option of either protected or unprotected available for a link. When presented with either option, PCE SHOULD select the SID which has a protection state matching the state of the L-Flag.
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 [RFC7525],, is RECOMMENDED.
This document defines a new LSP Attribute Flag; IANA is requested to make the following bit allocation from the "LSPA Object" sub registry of the PCEP Numbers registry, as follows:
Value Name Reference
6 PROTECTION-ENFORCEMENT This document