PCE Working Group F. Zhang
Internet-Draft Q. Zhao
Intended status: Standards Track Huawei
Expires: June 6, 2019 O. Gonzalez de Dios
Telefonica I+D
R. Casellas
CTTC
D. King
Old Dog Consulting
December 5, 2018
Extensions to Path Computation Element Communication Protocol (PCEP) for
Hierarchical Path Computation Elements (PCE)
draft-ietf-pce-hierarchy-extensions-07
Abstract
The Hierarchical Path Computation Element (H-PCE) architecture is
defined in RFC 6805. It provides a mechanism to derive an optimum
end-to-end path in a multi-domain environment by using a hierarchical
relationship between domains to select the optimum sequence of
domains and optimum paths across those domains.
This document defines extensions to the Path Computation Element
Protocol (PCEP) to support Hierarchical PCE procedures.
Status of This Memo
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 May 6, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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(https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . .
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . .
1.3. Requirements Language . . . . . . . . . . . . . . . . . .
2. Requirements for H-PCE . . . . . . . . . . . . . . . . . . .
2.1. Path Computation Request . . . . . . . . . . . . . . . .
2.1.1. Qualification of PCEP Requests . . . . . . . . . . .
2.1.2. Multi-domain Objective Functions . . . . . . . . . .
2.1.3. Multi-domain Metrics . . . . . . . . . . . . . . . .
2.2. Parent PCE Capability Advertisement . . . . . . . . . . .
2.3. PCE Domain Identification . . . . . . . . . . . . . . . .
2.4. Domain Diversity . . . . . . . . . . . . . . . . . . . .
3. PCEP Extensions . . . . . . . . . . . . . . . . . . . . . . .
3.1. OPEN Object . . . . . . . . . . . . . . . . . . . . . . .
3.1.1. H-PCE Capability TLV . . . . . . . . . . . . . . . .
3.1.2. Domain-ID TLV . . . . . . . . . . . . . . . . . . . .
3.2. RP Object . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1. H-PCE-FLAG TLV . . . . . . . . . . . . . . . . . . .
3.2.2. Domain-ID TLV . . . . . . . . . . . . . . . . . . . .
3.3. Objective Functions . . . . . . . . . . . . . . . . . . .
3.3.1. OF Codes . . . . . . . . . . . . . . . . . . . . . .
3.3.2. OF Object . . . . . . . . . . . . . . . . . . . . . .
3.4. Metric Object . . . . . . . . . . . . . . . . . . . . . .
3.5. SVEC Object . . . . . . . . . . . . . . . . . . . . . . .
3.6. PCEP-ERROR object . . . . . . . . . . . . . . . . . . . .
3.6.1. Hierarchy PCE Error-Type . . . . . . . . . . . . . .
3.7. NO-PATH Object . . . . . . . . . . . . . . . . . . . . .
4. H-PCE Procedures . . . . . . . . . . . . . . . . . . . . . .
4.1. OPEN Procedure between Child PCE and Parent PCE . . . . .
4.2. Procedure to Obtain Domain Sequence . . . . . . . . . . .
5. Error Handling . . . . . . . . . . . . . . . . . . . . . . .
6. Manageability Considerations . . . . . . . . . . . . . . . .
6.1. Control of Function and Policy . . . . . . . . . . . . .
6.1.1. Child PCE . . . . . . . . . . . . . . . . . . . . . .
6.1.2. Parent PCE . . . . . . . . . . . . . . . . . . . . .
6.1.3. Policy Control . . . . . . . . . . . . . . . . . . .
6.2. Information and Data Models . . . . . . . . . . . . . . .
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6.3. Liveness Detection and Monitoring . . . . . . . . . . . .
6.4. Verify Correct Operations . . . . . . . . . . . . . . . .
6.5. Requirements On Other Protocols . . . . . . . . . . . . .
6.6. Impact On Network Operations . . . . . . . . . . . . . .
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . .
7.1. PCEP TLV Type Indicators . . . . . . . . . . . . . . . .
7.2. H-PCE-CAPABILITY TLV Flags . . . . . . . . . . . . . . .
7.3. Domain-ID TLV Domain type . . . . . . . . . . . . . . . .
7.4. H-PCE-FLAG TLV Flags . . . . . . . . . . . . . . . . . .
7.5. OF Codes . . . . . . . . . . . . . . . . . . . . . . . .
7.6. METRIC Types . . . . . . . . . . . . . . . . . . . . . .
7.7. New PCEP Error-Types and Values . . . . . . . . . . . . .
7.8. New NO-PATH-VECTOR TLV Bit Flag . . . . . . . . . . . . .
7.9. SVEC Flag . . . . . . . . . . . . . . . . . . . . . . . .
7.10. NO-PATH VECTOR TLV Bit Flag. . . . . . . . . . . . . . .
8. Security Considerations . . . . . . . . . . . . . . . . . . .
9. Contributing Authors. . . . . . . . . . . . . . . . . . . . .
10. References . . . . . . . . . . . . . . . . . . . . . . . . .
10.1. Normative References . . . . . . . . . . . . . . . . . .
10.2. Informative References . . . . . . . . . . . . . . . . .
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
The Path Computation Element communication Protocol (PCEP) provides
a mechanism for Path Computation Elements (PCEs) and Path Computation
Clients (PCCs) to exchange requests for path computation and
responses that provide computed paths.
The capability to compute the routes of end-to-end inter-domain MPLS
Traffic Engineering (MPLS-TE) and GMPLS Label Switched Paths (LSPs)
is expressed as requirements in [RFC4105] and [RFC4216]. This
capability may be realized by a PCE [RFC4655]. The methods for
establishing and controlling inter-domain MPLS-TE and GMPLS LSPs are
documented in [RFC4726].
[RFC6805] describes a Hierarchical PCE (H-PCE) architecture which can
be used for computing end-to-end paths for inter-domain MPLS Traffic
Engineering (TE) and GMPLS Label Switched Paths (LSPs).
Within the hierarchical PCE architecture, the parent PCE is used to
compute a multi-domain path based on the domain connectivity
information. A child PCE may be responsible for a single domain or
multiple domains, it is used to compute the intra-domain path based
on its own domain topology information.
The H-PCE end-to-end domain path computation procedure is described
below:
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o A path computation client (PCC) sends the inter-domain path
computation requests to the child PCE responsible for its domain;
o The child PCE forwards the request to the parent PCE;
o The parent PCE computes the likely domain paths from the ingress
domain to the egress domain;
o The parent PCE sends the intra-domain path computation requests
(between the domain border nodes) to the child PCEs which are
responsible for the domains along the domain path;
o The child PCEs return the intra-domain paths to the parent PCE;
o The parent PCE constructs the end-to-end inter-domain path based
on the intra-domain paths;
o The parent PCE returns the inter-domain path to the child PCE;
o The child PCE forwards the inter-domain path to the PCC.
In addition, the parent PCE may be requested to provide only the
sequence of domains to a child PCE so that alternative inter-domain
path computation procedures, including Per Domain (PD) [RFC5152] and
Backwards Recursive Path Computation (BRPC) [RFC5441] may be used.
This document defines the PCEP extensions for the purpose of
implementing Hierarchical PCE procedures, which are described in
[RFC6805].
1.1. Scope
The following functions are out of scope of this document.
o Determination of Destination Domain (section 4.5 of [RFC6805]).
This could be done
* via a collection of reachability information from child domain;
* via requests to the child PCEs to discover if they contain the
destination node;
* or any other methods.
o Parent Traffic Engineering Database (TED) methods (section 4.4 of
[RFC6805]). This could be done via
* Yang based management interfaces
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* BGP-LS [RFC7752]
* Future extension to PCEP (such as PCEP-LS)
o Learning of Domain connectivity and boundary nodes (BN) addresses.
This could be done via
* Yang based management interfaces
* BGP-LS [RFC7752]
* Future extension to PCEP (such as PCEP-LS)
o Stateful PCE Operations. (Refer [I-D.ietf-pce-stateful-hpce])
The hierarchical relationship model is described in [RFC6805]. It is
applicable to environments with small groups of domains where
visibility from the ingress LSRs is limited. As highlighted in
[RFC7399] applying the hierarchical PCE model to large groups of
domains such as the Internet is not considered feasible or desirable.
1.2. Terminology
This document uses the terminology defined in [RFC4655], [RFC5440]
and the additional terms defined in section 1.4 of [RFC6805].
1.3. 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.
2. Requirements for H-PCE
This section compiles the set of requirements to the PCEP extension
to support the H-PCE architecture and procedures.
[RFC6805] identifies high-level requirements of PCEP extensions
required to support the hierarchical PCE model.
2.1. Path Computation Request
The Path Computation Request (PCReq) [RFC5440] messages are used by
a PCC or a PCE to make a path computation request to a PCE. In order
to achieve the full functionality of the H-PCE procedures, the PCReq
message needs to include:
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o Qualification of PCE Requests (Section 4.8.1. of [RFC6805]);
o Multi-domain Objective Functions (OF);
o Multi-domain Metrics.
2.1.1. Qualification of PCEP Requests
As described in section 4.8.1 of [RFC6805], the H-PCE architecture
introduces new request qualifications, which are:
o The ability for a child PCE to indicate that a path computation
request sent to a parent PCE should be satisfied by a domain
sequence only, that is, not by a full end-to-end path. This allows
the child PCE to initiate a per-domain (PD) [RFC5152] or a
backward recursive path computation (BRPC) [RFC5441].
o As stated in [RFC6805], section 4.5, if a PCC knows the egress
domain, it can supply this information as the path computation
request. The PCC may also want to specify the destination domain
information in a PCEP request, if it is known.
o An inter domain path computed by parent PCE should be capable of
disallowing specific domain re-entry.
2.1.2. Multi-domain Objective Functions
For H-PCE inter-domain path computation, there is three new Objective
Functions defined in this document:
o Minimize the number of Transit Domains (MTD)
o Minimize the number of border nodes (MBN)
o Minimize the number of Common Transit Domains (MCTD)
The PCC may specify the multi-domain Objective Function code to
use when requesting inter-domain path computation, it may also
include intra-domain OFs, such as Minimum Cost Path (MCP) [RFC5441],
which must be considered by participating child PCEs.
2.1.3. Multi-domain Metrics
For inter-domain path computation, there are several path metrics of
interest.
o Domain count (number of domains crossed);
o Border Node count.
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A PCC may be able to limit the number of domains crossed by applying
a limit on these metrics. Details in Section 3.4.
2.2. Parent PCE Capability Advertisement
A PCEP Speaker (Parent PCE or Child PCE or PCC) includes the "H-PCE
Capability" TLV, described in Section 3.1.1, in the OPEN Object to
advertise its support for PCEP extensions for H-PCE Capability.
Parent and child PCE relationships are likely to be configured.
However, as mentioned in [RFC6805], it would assist network operators
if the child and parent PCEs could indicate their H-PCE capabilities.
During the PCEP session establishment procedure, the child PCE needs
to be capable of indicating to the parent PCE whether it requests the
parent PCE capability or not.
2.3. PCE Domain Identification
A PCE domain is a single domain with an associated PCE. Although it
is possible for a PCE to manage multiple domains simultaneously. The
PCE domain could be an IGP area or AS.
The PCE domain identifiers MAY be provided during the PCEP session
establishment procedure.
2.4. Domain Diversity
In a multi-domain environment, Domain Diversity is defined in
[RFC6805]. A pair of paths is domain-diverse if they do not
traverse any of the same transit domains. Domain diversity may be
maximized for a pair of paths by selecting paths that have the
smallest number of shared domains. Path computation should
facilitate the selection of domain diverse paths as a way to reduce
the risk of shared failure and automatically helps to ensure path
diversity for a pair of LSPs.
The main motivation behind domain diversity is to avoid fate sharing,
but it can also be because of some geo-political reasons and
commercial relationships that would require domain diversity. For
example, a pair of paths should choose different transit Autonomous
System (AS) because of some policy considerations.
In the case when full domain diversity could not be achieved, it is
helpful to minimize the commonly shared domains. Also, it is
interesting to note that other scope of diversity (node, link, SRLG
etc.) can still be applied inside the commonly shared domains.
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3. PCEP Extensions
This section defines extensions to PCEP [RFC5440] to support the
H-PCE procedures.
3.1. OPEN object
Two new TLVs are defined in this document to be carried within an
OPEN object. This way, during the PCEP session establishment, the
H-PCE capability and Domain information can be advertised.
3.1.1. H-PCE Capability TLV
The H-PCE-CAPABILITY TLV is an optional TLV associated with the OPEN
Object [RFC5440] to exchange H-PCE capability of PCEP speakers.
Its format is shown in the following figure:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type= TBD1 | Length=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |P|
+---------------------------------------------------------------+
Figure 1: H-PCE-CAPABILITY TLV format
The type of the TLV is TBD1 (to be assigned by IANA), and it has a
fixed length of 4 octets.
The value comprises a single field - Flags (32 bits):
P (Parent PCE Request bit): if set, will signal that the child PCE
wishes to use the peer PCE as a parent PCE.
Unassigned bits MUST be set to 0 on transmission and MUST be ignored
on receipt.
The inclusion of this TLV in an OPEN object indicates that the H-PCE
extensions are supported by the PCEP speaker. The child PCE MUST
include this TLV and set the P flag. The parent PCE MUST include
this TLV and unset the P flag. The PCC MUST include this TLV to
indicate that it understands the H-PCE extensions with P flag unset.
If both peers attempt to set the P flag then the session
establishment MUST fail, and the PCEP speaker MUST respond with PCErr
message using Error-Type 1: "PCEP Session Establishment Failure" as
per [RFC5440].
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If the PCE understands the H-PCE path computation request but did not
advertise its H-PCE capability, it MUST send a PCErr message with
Error-Type=TBD8 (H-PCE error) and Error-Value=1 (Parent PCE
Capability not advertised).
3.1.1.1 Backwards Compatibility
If the PCE does not understand an H-PCE path computation request as
specified in this document, the PCE will ignore the H-PCE related
parameters, and behave as per [RFC5440].
3.1.2. Domain-ID TLV
The Domain-ID TLV when used in the OPEN object, identify the domains
served by the PCE. The child PCE uses this mechanism to inform the
domain information to the parent PCE.
The Domain-ID TLV is defined below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type= TBD2 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Domain Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Domain ID //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Domain-ID TLV format
The type of the TLV is TBD2 (to be assigned by IANA), and it has a
variable Length of the value portion. The value part comprises of -
Domain Type (8 bits): Indicates the domain type. Four types of
domain are currently defined:
* Type=1: the Domain ID field carries a 2-byte AS number. Padded
with trailing zeros to a 4-byte boundary.
* Type=2: the Domain ID field carries a 4-byte AS number.
* Type=3: the Domain ID field carries a 4-byte OSPF area ID.
* Type=4: the Domain ID field carries (2-byte Area-Len, variable
length IS-IS area ID). Padded with trailing zeros to a 4-byte
boundary.
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Reserved: Zero at transmission; ignored at the receipt.
Domain ID (variable): Indicates an IGP Area ID or AS number as
per the Domain Type field. It can be 2 bytes, 4 bytes or variable
length depending on the domain identifier used. It is padded with
trailing zeros to a 4-byte boundary. In case of IS-IS it includes
the Area-Len as well.
In the case a PCE serves more than one domain, multiple Domain-ID
TLVs are included for each domain it serves.
3.2. RP Object
3.2.1. H-PCE-FLAG TLV
The H-PCE-FLAG TLV is an optional TLV associated with the RP Object
[RFC5440] to indicate the H-PCE path computation request and options.
Its format is shown in the following figure:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type= TBD3 | Length=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |D|S|
+---------------------------------------------------------------+
Figure 3: H-PCE-FLAG TLV format
The type of the TLV is TBD3 (to be assigned by IANA), and it has a
fixed length of 4 octets.
The value comprises a single field - Flags (32 bits):
S (Domain Sequence bit): if set, will signal that the child PCE
wishes to get only the domain sequence in the path computation
reply. Refer section 3.7 of [RFC7897] for details.
D (Disallow Domain Re-entry bit): if set, will signal that the
computed path does not enter a domain more than once.
Unassigned bits MUST be set to 0 on transmission and MUST be ignored
on receipt.
The presence of the TLV indicates that the H-PCE based path
computation is requested as per this document.
3.2.2. Domain-ID TLV
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The usage of Domain-ID TLV carried in an OPEN object is used to
indicate a (list of) managed domains and is described in
Section 3.1.2. This TLV when carried in an RP object, indicates the
destination domain ID. If a PCC knows the egress domain, it can
supply this information in the PCReq message. The format and
procedure of this TLV are defined in Section 3.1.2.
If a Domain-id TLV is used in the RP object, and the destination is
not actually in the indicated domain, then the parent
PCE should respond with a NO-PATH object and NO-PATH VECTOR TLV
should be used, and a new bit number is assigned to indicate
"Destination not found in the indicated domain" (see section 3.7).
3.3. Objective Functions
3.3.1. OF Codes
[RFC5541] defines a mechanism to specify an Objective Function that
is used by a PCE when it computes a path. Three new Objective
Functions are defined for H-PCE, these are:
o MTD
* Name: Minimize the number of Transit Domains (MTD)
* Objective Function Code - TBD4 (to be assigned by IANA)
* Description: Find a path P such that it passes through the
least number of transit domains.
* Objective functions are formulated using the following
terminology:
+ A network comprises a set of N domains {Di, (i=1...N)}.
+ A path P passes through K unique domains {Dpi,(i=1...K)}.
+ Find a path P such that the value of K is minimized.
o MBN
* Name: Minimize the number of border nodes.
* Objective Function Code - TBD5 (to be assigned by IANA)
* Description: Find a path P such that it passes through the
least number of border nodes.
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* Objective functions are formulated using the following
terminology:
+ A network comprises a set of N links {Li, (i=1...N)}.
+ A path P is a list of K links {Lpi,(i=1...K)}.
+ D(Lpi) if a function that determines if the links Lpi
and Lpi+1 belong to different domains, D(Li) = 1 if link
Li and Li+1 belong to different domains, D(Lk) = 0 if
link Lk and Lk+1 belong to the same domain.
+ The number of border node in a path P is denoted by B(P),
where B(P) = sum{D(Lpi),(i=1...K-1)}.
+ Find a path P such that B(P) is minimized.
There is one objective function that applies to a set of synchronized
path computation requests to increase the domain diversity:
MCTD
o Name: Minimize the number of Common Transit Domains
o Objective Function Code - TBD13 (to be assigned by IANA)
o Description: Find a set of paths such that it passes through the
least number of common transit domains.
+ A network comprises a set of N domains {Di, (i=1...N)}.
+ A path P passes through K unique domains {Dpi,(i=1...K)}.
+ A set of paths {P1...Pm} have L transit domains that are
common to more than one path {Dpi,(i=1...L)}.
+ Find a set of paths such that the value of L is minimized.
3.3.2. OF Object
The OF (Objective Function) object [RFC5541] is carried within a
PCReq message so as to indicate the desired/required objective
function to be applied by the PCE during path computation. As per
section 3.2 of [RFC5541] a single OF object may be included in a path
computation request.
The new OF codes described in Section 3.3.1 are applicable at the
inter-domain path computation performed by the parent PCE, it is
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also necessary to specify the OF code that may be applied for the
intra-domain path computation performed by the child PCE. To
accommodate this, the OF-List TLV (described in section 2.1. of
[RFC5541]) is included in the OF object as an optional TLV.
The OF-List TLV allows encoding of multiple OF codes. When this TLV
is included inside the OF object, only the first OF-code in the
OF-LIST TLV is considered. The parent PCE MUST use this OF code in
the OF object when sending the intra domain path computation request
to the child PCE. If the OF list TLV is included in the OF Object,
the OF Code inside the OF Object MUST include one of the H-PCE
Objective Functions defined in this document, the OF Code inside the
OF List TLV MUST NOT include an H-PCE Objective Function. If this
condition is not met, the PCEP speaker MUST respond with a PCErr
message with Error-Type=10 (Reception of an invalid object) and
Error-Value=TBD15 (Incompatible OF codes in H-PCE).
If the Objective Functions defined in this document are unknown or
unsupported by a PCE, then the procedure as defined in [RFC5541]
is followed.
3.4. Metric Object
The METRIC object is defined in section 7.8 of [RFC5440], comprising
of metric-value, metric-type (T field) and flags. This document
defines the following types for the METRIC object for H-PCE:
o T=TBD6: Domain count metric (number of domains crossed);
o T=TBD7: Border Node count metric (number of border nodes crossed).
The domain count metric type of the METRIC object encodes the number
of domain crossed in the path. The border node count metric type of
the METRIC object encodes the number of border nodes in the path. If
a domain is re-entered, then domain should be double counted.
A PCC or child PCE MAY use these metric in PCReq message for an
inter-domain path computation meeting the number of domain or border
nodes crossing requirement. As per [RFC5440], in this case, the B bit
is set to suggest a bound (a maximum) for the metric that must not be
exceeded for the PCC to consider the computed path as acceptable.
A PCC or child PCE MAY also use this metric to ask the PCE to
optimize the metric during inter-domain path computation. In this
case, the B flag is cleared, and the C flag is set.
The Parent PCE MAY use these metric in a PCRep message along with a
NO-PATH object in the case where the PCE cannot compute a path
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meeting this constraint. A PCE MAY also use this metric to send the
computed end to end metric value in a reply message.
3.5. SVEC Object
[RFC5440] defines SVEC object which includes flags for the potential
dependency between the set of path computation requests (Link, Node
and SRLG diverse). This document defines a new flag O for domain
diversity.
The following new bit is added to the Flags field:
o O (Domain diverse) bit - TBD14 : when set, this indicates that the
computed paths corresponding to the requests specified by the
following RP objects MUST NOT have any transit domains in
common.
The Domain Diverse O-bit can be used in Hierarchical PCE path
computation to compute synchronized domain diverse end to end path or
diverse domain sequences.
When domain diverse O bit is set, it is applied to the transit
domains. The other bit in SVEC object (N, L, S etc.) MAY be set and
MUST still be applied in the ingress and egress shared domain.
3.6. PCEP-ERROR object
3.6.1. Hierarchy PCE Error-Type
A new PCEP Error-Type [RFC5440] is used for the H-PCE extension as
defined below:
+------------+-----------------------------------------+
| Error-Type | Meaning |
+------------+-----------------------------------------+
| TBD8 | H-PCE error |
| | Error-value=1: H-PCE capability |
| | was not advertised |
| | Error-value=2: parent PCE capability |
| | cannot be provided |
+------------+-----------------------------------------+
Figure 4: H-PCE error
3.7. NO-PATH Object
To communicate the reason(s) for not being able to find a multi-
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domain path or domain sequence, the NO-PATH object can be used in the
PCRep message. [RFC5440] defines the format of the NO-PATH object.
The object may contain a NO-PATH-VECTOR TLV to provide additional
information about why a path computation has failed.
Three new bit flags are defined to be carried in the Flags field in
the NO-PATH-VECTOR TLV carried in the NO-PATH Object.
o Bit number TBD9: When set, the parent PCE indicates that
destination domain unknown;
o Bit number TBD10: When set, the parent PCE indicates unresponsive
child PCE(s);
o Bit number TBD11: When set, the parent PCE indicates no available
resource available in one or more domains.
o Bit number TBD12: When set, the parent PCE indicates that
the destination is not found in the indicated domain.
4. H-PCE Procedures
The H-PCE path computation procedure is described in [RFC6805].
4.1. OPEN Procedure between Child PCE and Parent PCE
If a child PCE wants to use the peer PCE as a parent, it MUST set the
P (parent PCE request flag) in the H-PCE-CAPABILITY TLV inside the
OPEN object carried in the Open message during the PCEP session
initialization procedure.
The child PCE MAY also report its list of domain IDs to the parent
PCE by specifying them in the Domain-ID TLVs in the OPEN object
carried in the Open message during the PCEP session initialization
procedure.
The OF codes defined in this document can be carried in the OF-list
TLV of the OPEN object. If the OF-list TLV carries the OF codes, it
means that the PCE is capable of implementing the corresponding
objective functions. This information can be used for selecting a
proper parent PCE when a child PCE wants to get a path that satisfies
a certain Objective Function.
When a specific child PCE sends a PCReq to a peer PCE that requires
parental activity and H-PCE capability flags TLV was not included in
the session establishment procedure as described above, the peer PCE
should send a PCErr message to the child PCE and specify the error-
type=TBD8 (H-PCE error) and error-value=1 (H-PCE capability was
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not advertised) in the PCEP-ERROR object.
When a specific child PCE sends a PCReq to a peer PCE that requires
parental activity and the peer PCE does not want to act as the parent
for it, the peer PCE should send a PCErr message to the child PCE and
specify the error-type=TBD8 (H-PCE error) and error-value=2 (Parent
PCE capability cannot be provided) in the PCEP-ERROR object.
4.2. Procedure to Obtain Domain Sequence
If a child PCE only wants to get the domain sequence for a multi-
domain path computation from a parent PCE, it can set the Domain Path
Request bit in the H-PCE-FLAG TLV in the RP object carried in a PCReq
message. The parent PCE which receives the PCReq message tries to
compute a domain sequence for it (instead of the E2E path). If the
domain path computation succeeds the parent PCE sends a PCRep message
which carries the domain sequence in the ERO to the child PCE. Refer
[RFC7897] for more details about domain sub-objects in the ERO.
Otherwise, it sends a PCReq message which carries the NO-PATH object
to the child PCE.
5. Error Handling
A PCE that is capable of acting as a parent PCE might not be
configured or willing to act as the parent for a specific child PCE.
This fact could be determined when the child sends a PCReq that
requires parental activity, and could result in a negative response
in a PCEP Error (PCErr) message and indicate the hierarchy PCE error-
type=TBD8 (H-PCE error) and suitable error-value. (Section 3.6)
Additionally, the parent PCE may fail to find the multi-domain path
or domain sequence due to one or more of the following reasons:
o A child PCE cannot find a suitable path to the egress;
o The parent PCE do not hear from a child PCE for a specified time;
o The Objective Functions specified in the path request cannot be
met.
In this case, the parent PCE MAY need to send a negative path
computation reply specifying the reason. This can be achieved by
including NO-PATH object in the PCRep message. Extension to NO-PATH
object is needed to include the aforementioned reasons described in
Section 3.7.
6. Manageability Considerations
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General PCE and PCEP management considerations are discussed in
[RFC4655] and [RFC5440]. There are additional management
considerations for H-PCE which are described in [RFC6805], and
repeated in this section.
The administrative entity responsible for the management of the
parent PCEs must be determined for the following cases:
o multi-domains (e.g., IGP areas or multiple ASes) within a single
service provider network, the management responsibility for the
parent PCE would most likely be handled by the service provider,
o multiple ASes within different service provider networks, it may
be necessary for a third party to manage the parent PCEs according
to commercial and policy agreements from each of the participating
service providers.
6.1. Control of Function and Policy
Control and function will need to be carefully managed in an H-PCE
network. A child PCE will need to be configured with the address of
its parent PCE. It is expected that there will only be one or two
parents of any child.
The parent PCE also needs to be aware of the child PCEs for all child
domains that it can see. This information is most likely to be
configured (as part of the administrative definition of each domain).
Discovery of the relationships between parent PCEs and child PCEs
do not form part of the hierarchical PCE architecture. Mechanisms
that rely on advertising or querying PCE locations across domain or
provider boundaries are undesirable for security, scaling,
commercial, and confidentiality reasons. The specific behaviour of
the child and parent PCE are described in the following sub-sections.
6.1.1. Child PCE
Support of the hierarchical procedure will be controlled by the
management organization responsible for each child PCE. A child PCE
must be configured with the address of its parent PCE in order for it
to interact with its parent PCE. The child PCE must also be
authorized to peer with the parent PCE.
6.1.2. Parent PCE
The parent PCE must only accept path computation requests from
authorized child PCEs. If a parent PCE receives requests from an
unauthorized child PCE, the request should be dropped. This means
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that a parent PCE must be configured with the identities and security
credentials of all of its child PCEs, or there must be some form of
shared secret that allows an unknown child PCE to be authorized by
the parent PCE.
6.1.3. Policy Control
It may be necessary to maintain a policy module on the parent PCE
[RFC5394]. This would allow the parent PCE to apply commercially
relevant constraints such as SLAs, security, peering preferences, and
monetary costs.
It may also be necessary for the parent PCE to limit the
end-to-end path selection by including or excluding specific domains
based on commercial relationships, security implications, and
reliability.
6.2. Information and Data Models
A MIB module for PCEP was published as RFC 7420 [RFC7420] that
describes managed objects for modelling of PCEP communication. A
YANG module for PCEP has also been proposed [I-D.ietf-pce-pcep-yang].
Aditionally, H-PCE MIB module, or additional data model, will be
required to report parent PCE and child PCE information, including:
o parent PCE configuration and status,
o child PCE configuration and information,
o notifications to indicate session changes between parent PCEs and
child PCEs, and
o notification of parent PCE TED updates and changes.
6.3. Liveness Detection and Monitoring
The hierarchical procedure requires interaction with multiple PCEs.
Once a child PCE requests an end-to-end path, a sequence of events
occurs that requires interaction between the parent PCE and each
child PCE. If a child PCE is not operational, and an alternate
transit domain is not available, then the failure must be reported.
6.4. Verify Correct Operations
Verifying the correct operation of a parent PCE can be performed by
monitoring a set of parameters. The parent PCE implementation should
provide the following parameters monitored at the parent PCE:
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o number of child PCE requests,
o number of successful hierarchical PCE procedures completions on a
per-PCE-peer basis,
o number of hierarchical PCE procedure completion failures on a per-
PCE-peer basis, and
o number of hierarchical PCE procedure requests from unauthorized
child PCEs.
6.5. Requirements On Other Protocols
Mechanisms defined in this document do not imply any new requirements
on other protocols.
6.6. Impact On Network Operations
The hierarchical PCE procedure is a multiple-PCE path computation
scheme. Subsequent requests to and from the child and parent PCEs do
not differ from other path computation requests and should not have
any significant impact on network operations.
7. IANA Considerations
IANA maintains the "Path Computation Element Protocol (PCEP) Numbers"
registry. This document requests IANA actions to allocate code
points for the protocol elements defined in this document.
7.1. PCEP TLV Type Indicators
IANA Manages the PCEP TLV code point registry (see [RFC5440]). This
is maintained as the "PCEP TLV Type Indicators" sub-registry of the
"Path Computation Element Protocol (PCEP) Numbers" registry.
This document defines three new PCEP TLVs. IANA is requested to make
the following allocation:
Type TLV name References
-----------------------------------------------
TBD1 H-PCE-CAPABILITY TLV This I-D
TBD2 Domain-ID TLV This I-D
TBD3 H-PCE-FLAG TLV This I-D
7.2. H-PCE-CAPABILITY TLV Flags
This document requests that a new sub-registry, named "H-PCE-
CAPABILITY TLV Flag Field", is created within the "Path Computation
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Element Protocol (PCEP) Numbers" registry to manage the Flag field in
the H-PCE-CAPABILITY TLV of the PCEP OPEN object.
New values are to be assigned by Standards Action [RFC8126]. Each
bit should be tracked with the following qualities:
o Bit number (counting from bit 0 as the most significant bit)
o Capability description
o Defining RFC
The following values are defined in this document:
Bit Description Reference
--------------------------------------------------
31 P (Parent PCE Request bit) This I.D.
7.3. Domain-ID TLV Domain type
This document requests that a new sub-registry, named "Domain-ID TLV
Domain type", is created within the "Path Computation Element
Protocol (PCEP) Numbers" registry to manage the Domain-Type field of
the Domain-ID TLV. The allocation policy for this new sub-registry is
IETF Review [RFC8126].
Value Meaning
-----------------------------------------------
1 2-byte AS number
2 4-byte AS number
3 4-byte OSPF area ID
4 Variable length IS-IS area ID
7.4. H-PCE-FLAG TLV Flags
This document requests that a new sub-registry, named "H-PCE-FLAGS
TLV Flag Field", is created within the "Path Computation Element
Protocol (PCEP) Numbers" registry to manage the Flag field in the H-
PCE-FLAGS TLV of the PCEP RP object. New values are to be assigned
by Standards Action [RFC8126]. Each bit should be tracked with the
following qualities:
o Bit number (counting from bit 0 as the most significant bit)
o Capability description
o Defining RFC
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The following values are defined in this document:
Bit Description Reference
-----------------------------------------------
31 S (Domain This I.D.
Sequence bit)
30 D (Disallow Domain This I.D.
Re-entry bit)
7.5. OF Codes
IANA maintains a registry of Objective Function (described in
[RFC5541]) at the sub-registry "Objective Function". Three new
Objective Functions have been defined in this document.
IANA is requested to make the following allocations:
Code
Point Name Reference
------------------------------------------------------
TBD4 Minimum number of Transit This I.D.
Domains (MTD)
TBD5 Minimize number of Border This I.D.
Nodes (MBN)
TBD13 Minimize the number of This I.D.
Common Transit Domains
(MCTD)
7.6. METRIC Types
IANA maintains one sub-registry for "METRIC object T field". Two new
metric types are defined in this document for the METRIC object
(specified in [RFC5440]).
IANA is requested to make the following allocations:
Value Description Reference
----------------------------------------------------------
TBD6 Domain Count metric This I.D.
TBD7 Border Node Count metric This I.D.
7.7. New PCEP Error-Types and Values
IANA maintains a registry of Error-Types and Error-values for use in
PCEP messages. This is maintained as the "PCEP-ERROR Object Error
Types and Values" sub-registry of the "Path Computation Element
Protocol (PCEP) Numbers" registry.
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IANA is requested to make the following allocations:
Error-Type Meaning and error values Reference
------------------------------------------------------
TBD8 H-PCE Error This I.D.
Error-value=1 H-PCE
Capability not advertised
Error-value=2 Parent PCE
Capability cannot be provided
10 Reception of an invalid object [RFC5440]
Error-value=TBD15: Incompatible This I.D.
OF codes in H-PCE
7.8. New NO-PATH-VECTOR TLV Bit Flag
IANA maintains a sub-registry "NO-PATH-VECTOR TLV Flag Field" of
bit flags carried in the PCEP NO-PATH-VECTOR TLV in the PCEP NO-PATH
object as defined in [RFC5440]. IANA is requested to assign three
new bit flag as follows:
Bit Number Name Flag Reference
------------------------------------------------------
TBD9 Destination Domain unknown This I.D.
TBD10 Unresponsive child PCE(s) This I.D.
TBD11 No available resource in This I.D.
one or more domain
TBD12 Destination is not found This I.D.
in the indicated domain.
7.9. SVEC Flag
IANA maintains a sub-registry "SVEC Object Flag Field" of bit flags
carried in the PCEP SVEC object as defined in [RFC5440]. IANA is
requested to assign one new bit flag as follows:
Bit Number Name Flag Reference
------------------------------------------------------
TBD14 Domain Diverse This I.D.
8. Security Considerations
The hierarchical PCE procedure relies on PCEP and inherits the
security requirements defined in [RFC5440]. As PCEP operates over
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TCP, it may also make use of TCP security mechanisms, such as TCP
Authentication Option (TCP-AO) [RFC5925] or Transport Layer
Security (TLS) [RFC8253].
Any multi-domain operation necessarily involves the exchange of
information across domain boundaries. This may represent a
significant security and confidentiality risk especially when the
child domains are controlled by different commercial concerns. PCEP
allows individual PCEs to maintain the confidentiality of their
domain path information using path-keys [RFC5520], and the H-PCE
architecture is specifically designed to enable as much isolation of
domain topology and capabilities information as is possible.
For further considerations of the security issues related to inter-AS
path computation, see [RFC5376].
9. Contributing Authors
Xian Zhang
Huawei
EMail: zhang.xian@huawei.com
Dhruv Dhody
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore, Karnataka 560066
India
EMail: dhruv.ietf@gmail.com
10. References
10.1. 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>.
[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>.
[RFC5541] Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of
Objective Functions in the Path Computation Element
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Communication Protocol (PCEP)", RFC 5541,
DOI 10.17487/RFC5541, June 2009,
<https://www.rfc-editor.org/info/rfc5541>.
[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>.
10.2. Informative References
[RFC4105] Le Roux, J., Ed., Vasseur, J., Ed., and J. Boyle, Ed.,
"Requirements for Inter-Area MPLS Traffic Engineering",
RFC 4105, DOI 10.17487/RFC4105, June 2005,
<https://www.rfc-editor.org/info/rfc4105>.
[RFC4216] Zhang, R., Ed. and J. Vasseur, Ed., "MPLS Inter-Autonomous
System (AS) Traffic Engineering (TE) Requirements",
RFC 4216, DOI 10.17487/RFC4216, November 2005,
<https://www.rfc-editor.org/info/rfc4216>.
[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>.
[RFC4726] Farrel, A., Vasseur, J., and A. Ayyangar, "A Framework for
Inter-Domain Multiprotocol Label Switching Traffic
Engineering", RFC 4726, DOI 10.17487/RFC4726, November
2006, <https://www.rfc-editor.org/info/rfc4726>.
[RFC5152] Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang, "A
Per-Domain Path Computation Method for Establishing Inter-
Domain Traffic Engineering (TE) Label Switched Paths
(LSPs)", RFC 5152, DOI 10.17487/RFC5152, February 2008,
<https://www.rfc-editor.org/info/rfc5152>.
[RFC5376] Bitar, N., Zhang, R., and K. Kumaki, "Inter-AS
Requirements for the Path Computation Element
Communication Protocol (PCECP)", RFC 5376,
DOI 10.17487/RFC5376, November 2008,
<https://www.rfc-editor.org/info/rfc5376>.
[RFC5394] Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash,
"Policy-Enabled Path Computation Framework", RFC 5394,
DOI 10.17487/RFC5394, December 2008,
<https://www.rfc-editor.org/info/rfc5394>.
[RFC5520] Bradford, R., Ed., Vasseur, JP., and A. Farrel,
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"Preserving Topology Confidentiality in Inter-Domain Path
Computation Using a Path-Key-Based Mechanism", RFC 5520,
DOI 10.17487/RFC5520, April 2009,
<https://www.rfc-editor.org/info/rfc5520>.
[RFC5441] Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
"A Backward-Recursive PCE-Based Computation (BRPC)
Procedure to Compute Shortest Constrained Inter-Domain
Traffic Engineering Label Switched Paths", RFC 5441,
DOI 10.17487/RFC5441, April 2009,
<https://www.rfc-editor.org/info/rfc5441>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC6805] King, D., Ed. and A. Farrel, Ed., "The Application of the
Path Computation Element Architecture to the Determination
of a Sequence of Domains in MPLS and GMPLS", RFC 6805,
DOI 10.17487/RFC6805, November 2012,
<https://www.rfc-editor.org/info/rfc6805>.
[RFC7399] Farrel, A. and D. King, "Unanswered Questions in the Path
Computation Element Architecture", RFC 7399,
DOI 10.17487/RFC7399, October 2014,
<http://www.rfc-editor.org/info/rfc7399>.
[RFC7420] Koushik, A., Stephan, E., Zhao, Q., King, D., and J.
Hardwick, "Path Computation Element Communication Protocol
(PCEP) Management Information Base (MIB) Module",
RFC 7420, DOI 10.17487/RFC7420, December 2014,
<https://www.rfc-editor.org/info/rfc7420>.
[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>.
[RFC7897] Dhody, D., Palle, U., and R. Casellas, "Domain Subobjects
for the Path Computation Element Communication Protocol
(PCEP)", RFC 7897, DOI 10.17487/RFC7897, June 2016,
<https://www.rfc-editor.org/info/rfc7897>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
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[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>.
[I-D.ietf-pce-pcep-yang]
Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A
YANG Data Model for Path Computation Element
Communications Protocol (PCEP)", draft-ietf-pce-pcep-
yang-09 (work in progress), October 2018.
[I-D.ietf-pce-stateful-hpce]
Dhody, D., Lee, Y., Ceccarelli, D., Shin, J., King, D.,
and O. Dios, "Hierarchical Stateful Path Computation
Element (PCE).", draft-ietf-pce-stateful-hpce-06 (work in
progress), June 2018.
Authors' Addresses
Fatai Zhang
Huawei
Huawei Base, Bantian, Longgang District
Shenzhen 518129
China
EMail: zhangfatai@huawei.com
Quintin Zhao
Huawei
125 Nagog Technology Park
Acton, MA 01719
USA
EMail: quintin.zhao@huawei.com
Oscar Gonzalez de Dios
Telefonica I+D
Don Ramon de la Cruz 82-84
Madrid 28045
Spain
EMail: ogondio@tid.es
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Ramon Casellas
CTTC
Av. Carl Friedrich Gauss n.7
Barcelona, Castelldefels
Spain
EMail: ramon.casellas@cttc.es
Daniel King
Old Dog Consulting
UK
EMail: daniel@olddog.co.uk
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