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
draft-stone-pce-local-protection-enforcement-00
Abstract
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.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 3, 2020.
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1. Introduction
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.
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It is desirable for an operator to define the enforcement, or
strictness of the protection requirement when it can be applied.
2. Requirements Language
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].
3. Terminology
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.
4. Motivation
4.1. Implementation differences
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
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interpreted the L-Flag as PROTECTION MANDATORY or PROTECTION
PREFERRED, leading to operational differences.
4.2. SLA Enforcement
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.
5. Protection Enforcement Flag (E-Flag)
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.
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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)
o L flag: As defined in [RFC5440] and further updated by this
document. When set, protection is desired. When not set,
protection is not desired. The enforcement of the protection is
identified via the E-Flag.
o E flag (Protection Enforcement): When set, the value of the L-Flag
MUST be treated as a MUST constraint where applicable, when
protection state of a SID is known. When E flag is not set, the
value of the L-Flag MUST be treated as a MAY constraint.
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.
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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.
6. Security Considerations
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.
7. IANA Considerations
8. LSP Attributes Protection Enforcement Flag
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
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9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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>.
[RFC4655] Farrel, A., Vasseur, J., 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>.
[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>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.
[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>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
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Authors' Addresses
Andrew Stone
Nokia
Email: andrew.stone@nokia.com
Mustapha Aissaoui
Nokia
Email: mustapha.aissaoui@nokia.com
Siva Sivabalan
Cisco Systems, Inc.
Email: msiva@cisco.com
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