Internet Engineering Task Force (IETF) F. Zhang
Request for Comments: 8685 Q. Zhao
Category: Standards Track Huawei
ISSN: 2070-1721 O. Gonzalez de Dios
Telefonica I+D
R. Casellas
CTTC
D. King
Old Dog Consulting
December 2019
Path Computation Element Communication Protocol (PCEP) Extensions
for the Hierarchical Path Computation Element (H-PCE) Architecture
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
Communication Protocol (PCEP) to support H-PCE procedures.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8685.
Copyright Notice
Copyright (c) 2019 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
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
1.1. Scope
1.2. Terminology
1.3. Requirements Language
2. Requirements for the H-PCE Architecture
2.1. Path Computation Requests
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. Applicability to PCC-PCE Communications
3.2. OPEN Object
3.2.1. H-PCE-CAPABILITY TLV
3.2.1.1. Backwards Compatibility
3.2.2. Domain-ID TLV
3.3. RP Object
3.3.1. H-PCE-FLAG TLV
3.3.2. Domain-ID TLV
3.4. Objective Functions
3.4.1. OF Codes
3.4.2. OF Object
3.5. METRIC Object
3.6. SVEC Object
3.7. PCEP-ERROR Object
3.7.1. Hierarchical PCE Error-Type
3.8. NO-PATH Object
4. H-PCE Procedures
4.1. OPEN Procedure between Child PCE and Parent PCE
4.2. Procedure for Obtaining the 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
6.3. Liveness Detection and Monitoring
6.4. Verifying 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 Object Types
7.7. New PCEP Error-Types and Values
7.8. New NO-PATH-VECTOR TLV Bit Flag
7.9. SVEC Flag
8. Security Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgements
Contributors
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 Path Computation Element (H-PCE)
architecture that can be used for computing end-to-end paths for
inter-domain MPLS-TE and GMPLS LSPs.
In the H-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 single or multiple domains and 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:
* A PCC sends the inter-domain Path Computation Request (PCReq)
messages [RFC5440] to the child PCE responsible for its domain.
* The child PCE forwards the request to the parent PCE.
* The parent PCE computes the likely domain paths from the ingress
domain to the egress domain.
* The parent PCE sends the intra-domain PCReq messages (between the
domain border nodes) to the child PCEs that are responsible for
the domains along the domain path.
* The child PCEs return the intra-domain paths to the parent PCE.
* The parent PCE constructs the end-to-end inter-domain path based
on the intra-domain paths.
* The parent PCE returns the inter-domain path to the child PCE.
* The child PCE forwards the inter-domain path to the PCC.
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) path computation
[RFC5152] and Backward-Recursive PCE-Based Computation (BRPC)
[RFC5441], may be used.
This document defines the PCEP extensions for the purpose of
implementing H-PCE procedures, which are described in [RFC6805].
1.1. Scope
The following functions are out of scope for this document:
* Determination of the destination domain (Section 4.5 of
[RFC6805]):
- via a collection of reachability information from child
domains,
- via requests to the child PCEs to discover if they contain the
destination node, or
- via any other methods.
* Parent Traffic Engineering Database (TED) methods (Section 4.4 of
[RFC6805]), although suitable mechanisms include:
- YANG-based management interfaces.
- BGP - Link State (BGP-LS) [RFC7752].
- Future extensions to PCEP (for example, see [PCEP-LS]).
* Learning of domain connectivity and border node addresses.
Methods to achieve this function include:
- YANG-based management interfaces.
- BGP-LS [RFC7752].
- Future extensions to PCEP (for example, see [PCEP-LS]).
* Stateful PCE operations. (Refer to [STATEFUL-HPCE].)
* Applicability of the H-PCE model to large multi-domain
environments.
- The hierarchical relationship model is described in [RFC6805].
It is applicable to environments with small groups of domains
where visibility from the ingress Label Switching Routers
(LSRs) is limited. As highlighted in [RFC7399], applying the
H-PCE model to very 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] and
[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 the H-PCE Architecture
This section compiles the set of requirements for the PCEP extensions
to support the H-PCE architecture and procedures. [RFC6805]
identifies high-level requirements for PCEP extensions that are
required for supporting the H-PCE model.
2.1. Path Computation Requests
The PCReq messages [RFC5440] 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:
* Qualification of PCE requests (Section 4.8.1 of [RFC6805]).
* Multi-domain Objective Functions (OFs).
* 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 as follows:
* The ability for a child PCE to indicate that a PCReq message 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 PD path computation per [RFC5152] or a BRPC
procedure [RFC5441].
* As stated in [RFC6805], Section 4.5, if a PCC knows the egress
domain, it can supply this information as part of the PCReq
message. The PCC may also want to specify the destination domain
information in a PCEP request, if it is known.
* An inter-domain path computed by a parent PCE should be capable of
disallowing re-entry into a specified domain.
2.1.2. Multi-domain Objective Functions
For H-PCE inter-domain path computation, there are three new OFs
defined in this document:
* Minimize the number of Transit Domains (MTD)
* Minimize the number of Border Nodes (MBN)
* Minimize the number of Common Transit Domains (MCTD)
The PCC may specify the multi-domain OF code to use when requesting
inter-domain path computation. It may also include intra-domain OFs,
such as Minimum Cost Path (MCP) [RFC5541], which must be considered
by participating child PCEs.
2.1.3. Multi-domain Metrics
For inter-domain path computation, there are two path metrics of
interest.
* Domain Count (number of domains crossed).
* Border Node Count.
A PCC may be able to limit the number of domains crossed by applying
a limit on these metrics. See Section 3.4 for details.
2.2. Parent PCE Capability Advertisement
A PCEP speaker (parent PCE or child PCE) that supports and wishes to
use the procedures described in this document must advertise this
fact and negotiate its role with its PCEP peers. It does this using
the "H-PCE Capability" TLV, as described in Section 3.2.1, in the
OPEN object [RFC5440] to advertise its support for PCEP extensions
for the H-PCE capability.
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 Autonomous System (AS).
The PCE domain identifiers MAY be provided during the PCEP session
establishment procedure.
2.4. Domain Diversity
"Domain diversity" in the context of a multi-domain environment is
defined in [RFC6805] and described as follows:
| A pair of paths are domain-diverse if they do not transit any of
| the same domains. A pair of paths that share a common ingress and
| egress are domain-diverse if they only share the same domains at
| the ingress and egress (the ingress and egress domains). Domain
| diversity may be maximized for a pair of paths by selecting paths
| that have the smallest number of shared domains.
The main motivation behind domain diversity is to avoid fate-sharing.
However, domain diversity may also be requested to avoid specific
transit domains due to security, geopolitical, and commercial
reasons. For example, a pair of paths should choose different
transit ASes because of certain 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 domain-diversity techniques (node,
link, Shared Risk Link Group (SRLG), etc.) can still be applied
inside the commonly shared domains.
3. PCEP Extensions
This section defines extensions to PCEP [RFC5440] to support the
H-PCE procedures.
3.1. Applicability to PCC-PCE Communications
Although the extensions defined in this document are intended
primarily for use between a child PCE and a parent PCE, they are also
applicable for communications between a PCC and its PCE.
Thus, the information that may be encoded in a PCReq can be sent from
a PCC towards the child PCE. This includes the Request Parameters
(RP) object ([RFC5440] and Section 3.3), the OF codes
(Section 3.4.1), and the OF object (Section 3.4.2). A PCC and a
child PCE could also exchange the H-PCE capability (Section 3.2.1)
during its session.
This allows a PCC to request paths that transit multiple domains
utilizing the capabilities defined in this document.
3.2. OPEN Object
This document defines two new TLVs to be carried in an OPEN object.
This way, during the PCEP session establishment, the H-PCE capability
and domain information can be advertised.
3.2.1. H-PCE-CAPABILITY TLV
The H-PCE-CAPABILITY TLV is an optional TLV associated with the OPEN
object [RFC5440] to exchange the 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=13 | Length=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |P|
+---------------------------------------------------------------+
Figure 1: H-PCE-CAPABILITY TLV Format
The type of the TLV is 13, 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 setting of the P-flag (Parent PCE Request bit) would mean that
the PCEP speaker wants the peer to be a parent PCE, so in the case of
a PCC-to-child-PCE relationship, neither entity would set the P-flag.
If both peers attempt to set the P-flag, then the session
establishment MUST fail, and the PCEP speaker MUST respond with a
PCErr message using Error-Type 1 (PCEP session establishment failure)
as per [RFC5440].
If the PCE understands the H-PCE PCReq message but did not advertise
its H-PCE capability, it MUST send a PCErr message with Error-Type=28
(H-PCE Error) and Error-Value=1 (H-PCE Capability not advertised).
3.2.1.1. Backwards Compatibility
Section 7.1 of [RFC5440] specifies the following requirement:
"Unrecognized TLVs MUST be ignored."
The OPEN object [RFC5440] contains the necessary PCEP information
between the PCE entities, including session information and PCE
capabilities via TLVs (including if H-PCE is supported). If the PCE
does not support this document but receives an Open message
containing an OPEN object that includes an H-PCE-CAPABILITY TLV, it
will ignore that TLV and continue to attempt to establish a PCEP
session. However, it will not include the TLV in the Open message
that it sends, so the H-PCE relationship will not be created.
If a PCE does not support the extensions defined in this document but
receives them in a PCEP message (notwithstanding the fact that the
session was not established as supporting an H-PCE relationship), the
receiving PCE will ignore the H-PCE related parameters because they
are all encoded in TLVs in standard PCEP objects.
3.2.2. Domain-ID TLV
The Domain-ID TLV, when used in the OPEN object, identifies the
domains served by the PCE. The child PCE uses this mechanism to
provide 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=14 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Domain Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// Domain ID //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Domain-ID TLV Format
The type of the TLV is 14, and it has a variable Length of the value
portion. The value part comprises the following:
Domain Type (8 bits): Indicates the domain type. Four types of
domains 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 a 2-byte Area-Len and a
variable-length IS-IS area ID. Padded with trailing
zeros to a 4-byte boundary.
Reserved: Zero at transmission; ignored on 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 the
case of IS-IS, it includes the Area-Len as well.
In the case where a PCE serves more than one domain, multiple Domain-
ID TLVs are included for each domain it serves.
3.3. RP Object
3.3.1. H-PCE-FLAG TLV
The H-PCE-FLAG TLV is an optional TLV associated with the RP object
[RFC5440] to indicate the H-PCE PCReq message 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=15 | Length=4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |D|S|
+---------------------------------------------------------------+
Figure 3: H-PCE-FLAG TLV Format
The type of the TLV is 15, and it has a fixed length of 4 octets.
The value comprises a single field -- Flags (32 bits):
D (Disallow Domain Re-entry bit):
If set, will signal that the computed path does not enter a
domain more than once.
S (Domain Sequence bit):
If set, will signal that the child PCE wishes to get only the
domain sequence in the Path Computation Reply (PCRep) message
[RFC5440]. Refer to Section 3.7 of [RFC7897] for details.
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.
3.3.2. Domain-ID TLV
The Domain-ID TLV, carried in an OPEN object, is used to indicate a
managed domain (or a list of managed domains) and is described in
Section 3.2.2. 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. Section 3.2.2 also
defines the format for this TLV and the procedure for using it.
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 the NO-PATH-VECTOR TLV should be
used. A new bit number is assigned to indicate "Destination is not
found in the indicated domain" (see Section 3.8).
3.4. Objective Functions
3.4.1. OF Codes
[RFC5541] defines a mechanism to specify an OF that is used by a PCE
when it computes a path. Three new OFs are defined for the H-PCE
model; these are:
* MTD
Name: Minimize the number of Transit Domains (MTD)
OF code: 12
Description: Find a path P such that it passes through the least
number of transit domains.
- OFs are formulated using the following terminology:
o A network comprises a set of N domains {Di, (i=1...N)}.
o A path P passes through K unique domains {Dpi, (i=1...K)}.
o Find a path P such that the value of K is minimized.
* MBN
Name: Minimize the number of Border Nodes (MBN)
OF code: 13
Description: Find a path P such that it passes through the least
number of border nodes.
- OFs are formulated using the following terminology:
o A network comprises a set of N links {Li, (i=1...N)}.
o A path P is a list of K links {Lpi, (i=1...K)}.
o D(Lpi) is 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.
o The number of border nodes in a path P is denoted by B(P),
where B(P) = sum{D(Lpi), (i=1...K-1)}.
o Find a path P such that B(P) is minimized.
There is one OF that applies to a set of synchronized PCReq messages
to increase the domain diversity:
* MCTD
Name: Minimize the number of Common Transit Domains (MCTD)
OF code: 14
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} has 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.
3.4.2. OF Object
The OF object [RFC5541] is carried in a PCReq message so as to
indicate the desired/required OF 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 PCReq message.
The new OF codes described in Section 3.4.1 are applicable to the
inter-domain path computation performed by the parent PCE. It is
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 the 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 PCReq message 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 OFs defined
in this document. The OF code inside the OF-List TLV MUST NOT
include an H-PCE OF. 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=23 (Incompatible OF codes in H-PCE).
If the OFs defined in this document are unknown or unsupported by a
PCE, then the procedure as defined in [RFC5440] is followed.
3.5. METRIC Object
The METRIC object is defined in Section 7.8 of [RFC5440] and is
comprised of the metric-value field, the metric type (the T field),
and flags (the Flags field). This document defines the following
types for the METRIC object for the H-PCE model:
T=20: Domain Count metric (number of domains crossed).
T=21: Border Node Count metric (number of border nodes crossed).
The Domain Count metric type of the METRIC object encodes the number
of domains 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 the domain should be double counted.
A PCC or child PCE MAY use the metric in a PCReq message for an
inter-domain path computation, meeting the requirement for the number
of domains or border nodes being crossed. 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
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 the metric in a PCRep message along with a NO-
PATH object in the case where the PCE cannot compute a path that
meets this constraint. A PCE MAY also use this metric to send the
computed end-to-end metric value in a reply message.
3.6. SVEC Object
[RFC5440] defines the Synchronization Vector (SVEC) object, which
includes flags for the potential dependency between the set of PCReq
messages (Link, Node, and SRLG diverse). This document defines a new
flag (the O-bit) for domain diversity.
The following new bit is added to the Flags field:
Domain Diverse O-bit - 18:
When set, this indicates that the computed paths corresponding
to the requests specified by any RP objects that might be
provided MUST NOT have any transit domains in common.
The Domain Diverse O-bit can be used in H-PCE path computation to
compute synchronized domain-diverse end-to-end paths or diverse
domain sequences.
When the Domain Diverse O-bit is set, it is applied to the transit
domains. The other bit in SVEC object L (Link diverse), N (Node
diverse), S (SRLG diverse), etc. MAY be set and MUST still be applied
in the ingress and egress shared domain.
3.7. PCEP-ERROR Object
3.7.1. Hierarchical PCE Error-Type
A new PCEP Error-Type [RFC5440] is used for the H-PCE extension as
defined below:
+------------+------------------------------------------------------+
| Error-Type | Meaning |
+============+======================================================+
| 28 | H-PCE Error |
| | |
| | Error-Value=1: H-PCE Capability not |
| | advertised |
| | |
| | Error-Value=2: Parent PCE Capability cannot |
| | be provided |
+------------+------------------------------------------------------+
Table 1: H-PCE Error
3.8. NO-PATH Object
To communicate the reason(s) for not being able to find a multi-
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.
This document defines four new bit flags in the "NO-PATH-VECTOR TLV
Flag Field" subregistry. These flags are to be carried in the Flags
field in the NO-PATH-VECTOR TLV carried in the NO-PATH object.
Bit number 22: When set, the parent PCE indicates that the
destination domain is unknown.
Bit number 21: When set, the parent PCE indicates that one or
more child PCEs are unresponsive.
Bit number 20: When set, the parent PCE indicates that no
resources are available in one or more domains.
Bit number 19: 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-flag (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.
This object is 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 OFs.
This information can be used for selecting a proper parent PCE when a
child PCE wants to get a path that satisfies a certain OF.
When a child PCE sends a PCReq to a peer PCE that requires parental
activity and the H-PCE-CAPABILITY TLV but these items were not taken
into account in the session establishment procedure described above,
the peer PCE SHOULD send a PCErr message to the child PCE and MUST
specify Error-Type=28 (H-PCE Error) and Error-Value=1 (H-PCE
Capability 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
MUST specify Error-Type=28 (H-PCE Error) and Error-Value=2 (Parent
PCE Capability cannot be provided) in the PCEP-ERROR object.
4.2. Procedure for Obtaining the 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 that receives the PCReq message tries to
compute a domain sequence for it (instead of the end-to-end path).
If the domain path computation succeeds, the parent PCE sends a PCRep
message that carries the domain sequence in the Explicit Route Object
(ERO) to the child PCE. Refer to [RFC7897] for more details about
domain subobjects in the ERO. Otherwise, it sends a PCReq message
that 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.
When the child PCE sends a PCReq that requires parental activity, a
negative response in the form of a PCEP Error (PCErr) message that
includes H-PCE Error-Type=28 (H-PCE Error) and an applicable Error-
Value (Section 3.7) might result.
Additionally, the parent PCE may fail to find the multi-domain path
or domain sequence for one or more of the following reasons:
* A child PCE cannot find a suitable path to the egress.
* The parent PCE does not hear from a child PCE for a specified
time.
* The OFs specified in the path request cannot be met.
In this case, the parent PCE MAY need to send a negative PCRep
message specifying the reason for the failure. This can be achieved
by including the NO-PATH object in the PCRep message. An extension
to the NO-PATH object is needed in order to include the reasons
defined in Section 3.8.
6. Manageability Considerations
General PCE and PCEP management/manageability considerations are
discussed in [RFC4655] and [RFC5440]. There are additional
management considerations for the H-PCE model; these 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:
* Multiple domains (e.g., IGP areas or multiple ASes) in a single
service provider network. The management responsibility for the
parent PCE would most likely be handled by the service provider.
* Multiple ASes in 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 of H-PCE function will need to be carefully managed via
configuration and interaction policies, which may be controlled via a
policy module on the H-PCE. 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
does not form part of the H-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 behavior of the child and
parent PCEs is described in the following subsections.
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 PCReq messages from authorized child
PCEs. If a parent PCE receives requests from an unauthorized child
PCE, the request SHOULD be dropped. This means that a parent PCE
MUST be able to cryptographically authenticate requests from child
PCEs.
Multi-party shared key authentication schemes are not recommended for
inter-domain relationships because of (1) the potential for
impersonation and repudiation and (2) operational difficulties should
revocation be required.
The choice of authentication schemes to employ may be left to
implementers of the H-PCE architecture and are not discussed further
in this document.
6.1.3. Policy Control
It may be necessary to maintain H-PCE policy [RFC5394] via a policy
control module on the parent PCE. 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
[RFC7420] provides a MIB module for PCEP and describes managed
objects for the modeling of PCEP communication. A YANG module for
PCEP has also been proposed [PCEP-YANG].
An H-PCE MIB module or an additional data model will also be required
for reporting parent PCE and child PCE information, including:
* parent PCE configuration and status,
* child PCE configuration and information,
* notifications to indicate session changes between parent PCEs and
child PCEs, and
* 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. Verifying 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:
* number of child PCE requests,
* number of successful H-PCE procedure completions on a per-PCE-peer
basis,
* number of H-PCE procedure-completion failures on a per-PCE-peer
basis, and
* number of H-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 H-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. IANA has allocated code points for the protocol elements
defined in this document.
7.1. PCEP TLV Type Indicators
IANA maintains the "PCEP TLV Type Indicators" subregistry (see
[RFC5440]) within the "Path Computation Element Protocol (PCEP)
Numbers" registry.
IANA has allocated the following three new PCEP TLVs:
+------+------------------+-----------+
| Type | TLV Name | Reference |
+======+==================+===========+
| 13 | H-PCE-CAPABILITY | RFC 8685 |
+------+------------------+-----------+
| 14 | Domain-ID | RFC 8685 |
+------+------------------+-----------+
| 15 | H-PCE-FLAG | RFC 8685 |
+------+------------------+-----------+
Table 2: New PCEP TLVs
7.2. H-PCE-CAPABILITY TLV Flags
IANA has created the "H-PCE-CAPABILITY TLV Flag Field" subregistry
within the "Path Computation Element Protocol (PCEP) Numbers"
registry to manage the Flag field in the H-PCE-CAPABILITY TLV of the
PCEP OPEN object.
New values are assigned by Standards Action [RFC8126]. Each
registered bit should include the following information:
* Bit number (counting from bit 0 as the most significant bit)
* Capability description
* Defining RFC
The following value is defined in this document:
+-----+----------------------------+-----------+
| Bit | Description | Reference |
+=====+============================+===========+
| 31 | P (Parent PCE Request bit) | RFC 8685 |
+-----+----------------------------+-----------+
Table 3: Parent PCE Request Bit
7.3. Domain-ID TLV Domain Type
IANA has created the "Domain-ID TLV Domain Type" subregistry 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 subregistry is IETF Review [RFC8126].
The following values are defined in this document:
+-------+-------------------------------+
| Value | Meaning |
+=======+===============================+
| 0 | Reserved |
+-------+-------------------------------+
| 1 | 2-byte AS number |
+-------+-------------------------------+
| 2 | 4-byte AS number |
+-------+-------------------------------+
| 3 | 4-byte OSPF area ID |
+-------+-------------------------------+
| 4 | Variable-length IS-IS area ID |
+-------+-------------------------------+
| 5-255 | Unassigned |
+-------+-------------------------------+
Table 4: Parameters for Domain-ID TLV
Domain Type
7.4. H-PCE-FLAG TLV Flags
IANA has created the "H-PCE-FLAG TLV Flag Field" subregistry within
the "Path Computation Element Protocol (PCEP) Numbers" registry to
manage the Flag field in the H-PCE-FLAG TLV of the PCEP RP object.
New values are to be assigned by Standards Action [RFC8126]. Each
registered bit should include the following information:
* Bit number (counting from bit 0 as the most significant bit)
* Capability description
* Defining RFC
The following values are defined in this document:
+-----+----------------------------------+-----------+
| Bit | Description | Reference |
+=====+==================================+===========+
| 30 | D (Disallow Domain Re-entry bit) | RFC 8685 |
+-----+----------------------------------+-----------+
| 31 | S (Domain Sequence bit) | RFC 8685 |
+-----+----------------------------------+-----------+
Table 5: New H-PCE-FLAG TLV Flag Field Entries
7.5. OF Codes
IANA maintains a list of OFs (described in [RFC5541]) in the
"Objective Function" subregistry within the "Path Computation Element
Protocol (PCEP) Numbers" registry.
IANA has allocated the following OFs:
+------------+-------------------------------+-----------+
| Code Point | Name | Reference |
+============+===============================+===========+
| 12 | Minimize the number of | RFC 8685 |
| | Transit Domains (MTD) | |
+------------+-------------------------------+-----------+
| 13 | Minimize the number of Border | RFC 8685 |
| | Nodes (MBN) | |
+------------+-------------------------------+-----------+
| 14 | Minimize the number of Common | RFC 8685 |
| | Transit Domains (MCTD) | |
+------------+-------------------------------+-----------+
Table 6: New OF Codes
7.6. METRIC Object Types
IANA maintains the "METRIC Object T Field" subregistry [RFC5440]
within the "Path Computation Element Protocol (PCEP) Numbers"
registry.
The following two new metric types for the METRIC object are defined
in this document:
+-------+--------------------------+-----------+
| Value | Description | Reference |
+=======+==========================+===========+
| 20 | Domain Count metric | RFC 8685 |
+-------+--------------------------+-----------+
| 21 | Border Node Count metric | RFC 8685 |
+-------+--------------------------+-----------+
Table 7: New METRIC Object Types
7.7. New PCEP Error-Types and Values
IANA maintains a list of Error-Types and Error-Values for use in PCEP
messages. This list is maintained in the "PCEP-ERROR Object Error
Types and Values" subregistry within the "Path Computation Element
Protocol (PCEP) Numbers" registry.
IANA has allocated the following:
+------------+------------------------------------------+-----------+
| Error-Type | Meaning and Error Values | Reference |
+============+==========================================+===========+
| 28 | H-PCE Error | RFC 8685 |
| | | |
| | Error-Value=1: H-PCE Capability | |
| | not advertised | |
| | | |
| | Error-Value=2: Parent PCE | |
| | Capability cannot be provided | |
+------------+------------------------------------------+-----------+
| 10 | Reception of an invalid object | RFC 5440 |
| | | |
| | Error-Value=23: Incompatible OF | RFC 8685 |
| | codes in H-PCE | |
+------------+------------------------------------------+-----------+
Table 8: New PCEP Error-Types and Values
7.8. New NO-PATH-VECTOR TLV Bit Flag
IANA maintains the "NO-PATH-VECTOR TLV Flag Field" subregistry, which
contains a list of bit flags carried in the PCEP NO-PATH-VECTOR TLV
in the PCEP NO-PATH object as defined in [RFC5440].
IANA has allocated the following four new bit flags:
+------------+----------------------------+-----------+
| Bit Number | Description | Reference |
+============+============================+===========+
| 22 | Destination domain unknown | RFC 8685 |
+------------+----------------------------+-----------+
| 21 | Unresponsive child PCE(s) | RFC 8685 |
+------------+----------------------------+-----------+
| 20 | No available resource in | RFC 8685 |
| | one or more domains | |
+------------+----------------------------+-----------+
| 19 | Destination is not found | RFC 8685 |
| | in the indicated domain | |
+------------+----------------------------+-----------+
Table 9: PCEP NO-PATH Object Flags
7.9. SVEC Flag
IANA maintains the "SVEC Object Flag Field" subregistry, which
contains a list of bit flags carried in the PCEP SVEC object as
defined in [RFC5440].
IANA has allocated the following new bit flag:
+------------+----------------------+-----------+
| Bit Number | Description | Reference |
+============+======================+===========+
| 18 | Domain Diverse O-bit | RFC 8685 |
+------------+----------------------+-----------+
Table 10: Domain Diverse O-Bit
8. Security Considerations
The H-PCE procedure relies on PCEP and inherits the security
considerations defined in [RFC5440]. As PCEP operates over TCP, it
may also make use of TCP security mechanisms, such as the TCP
Authentication Option (TCP-AO) [RFC5925] or Transport Layer Security
(TLS) [RFC8253] [RFC8446].
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
information related to domain topology and capabilities as possible.
For further considerations regarding the security issues related to
inter-AS path computation, see [RFC5376].
9. References
9.1. 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
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>.
9.2. Informative References
[RFC4105] Le Roux, J.-L., Ed., Vasseur, J.-P., 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.-P. 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.-P., 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.-P., 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,
"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,
<https://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>.
[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>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[PCEP-YANG]
Dhody, D., Ed., Hardwick, J., Beeram, V., and J. Tantsura,
"A YANG Data Model for Path Computation Element
Communications Protocol (PCEP)", Work in Progress,
Internet-Draft, draft-ietf-pce-pcep-yang-13, 31 October
2019,
<https://tools.ietf.org/html/draft-ietf-pce-pcep-yang-13>.
[STATEFUL-HPCE]
Dhody, D., Lee, Y., Ceccarelli, D., Shin, J., and D. King,
"Hierarchical Stateful Path Computation Element (PCE)",
Work in Progress, Internet-Draft, draft-ietf-pce-stateful-
hpce-15, 20 October 2019, <https://tools.ietf.org/html/
draft-ietf-pce-stateful-hpce-15>.
[PCEP-LS] Dhody, D., Lee, Y., and D. Ceccarelli, "PCEP Extension for
Distribution of Link-State and TE Information.", Work in
Progress, Internet-Draft, draft-dhodylee-pce-pcep-ls-14,
21 October 2019, <https://tools.ietf.org/html/draft-
dhodylee-pce-pcep-ls-14>.
Acknowledgements
The authors would like to thank Mike McBride, Kyle Rose, and Roni
Even for their detailed review, comments, and suggestions, which
helped improve this document.
Contributors
The following people contributed substantially to the content of this
document and should be considered coauthors:
Xian Zhang
Huawei
Email: zhang.xian@huawei.com
Dhruv Dhody
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore 560066
Karnataka
India
Email: dhruv.ietf@gmail.com
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
United States of America
Email: quintinzhao@gmail.com
Oscar Gonzalez de Dios
Telefonica I+D
Don Ramon de la Cruz 82-84
28045 Madrid
Spain
Email: oscar.gonzalezdedios@telefonica.com
Ramon Casellas
CTTC
Av. Carl Friedrich Gauss n.7
Castelldefels Barcelona
Spain
Email: ramon.casellas@cttc.es
Daniel King
Old Dog Consulting
United Kingdom