Local Protection Enforcement in PCEP
Network Working Group A. Stone
Internet-Draft M. Aissaoui
Intended status: Standards Track Nokia
Expires: September 3, 2020 S. Sivabalan
Cisco Systems, Inc.
March 2, 2020
Local Protection Enforcement in PCEP
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.
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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
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