| Internet-Draft | PCEP Ext for Flexi-grid | July 2024 |
| Lee, et al. | Expires 6 January 2025 | [Page] |
This document provides the Path Computation Element Communication Protocol (PCEP) extensions for the support of Routing and Spectrum Assignment (RSA) in Flexible Grid networks.¶
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Copyright (c) 2024 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 carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
This document uses the terminology defined in [RFC4655], [RFC5440] and [RFC7698].¶
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.¶
[RFC4655] defines a Path Computation Element (PCE) based path computation architecture and explains how a Path Computation Element (PCE) may compute Label Switched Paths (LSP) in Multiprotocol Label Switching Traffic Engineering (MPLS-TE) and Generalized MPLS (GMPLS) networks at the request of Path Computation Clients (PCCs). A PCC is said to be any network component that makes such a request and may be, for instance, an Optical Switching Element within a Wavelength Division Multiplexing (WDM) network. The PCE, itself, can be located anywhere within the network, and may be within an optical switching element, a Network Management System (NMS) or Operational Support System (OSS), or may be an independent network server.¶
The PCE communications Protocol (PCEP) is the communication protocol used between a PCC and a PCE, and can also be used between cooperating PCEs. [RFC4657] sets out the common protocol requirements for PCEP. Additional application-specific requirements for PCEP are deferred to separate documents.¶
[RFC8780] provides the PCEP extensions for the support of Routing and Wavelength Assignment (RWA) in Wavelength Switched Optical Networks (WSON) based on the requirements specified in [RFC6163] and [RFC7449].¶
To allow efficient allocation of optical spectral bandwidth for systems that have high bit-rates, the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) has extended its Recommendations [ITU-T_G.694.1] to include a new Dense Wavelength Division Multiplexing (DWDM) grid by defining a set of nominal central frequencies, channel spacings, and the concept of the "frequency slot". In such an environment, a data-plane connection is switched based on allocated, variable-sized frequency ranges within the optical spectrum, creating what is known as a flexible grid (flexi-grid). [RFC7698] provides Framework and Requirements for GMPLS-Based Control of Flexi-Grid Dense Wavelength Division Multiplexing (DWDM) Networks.¶
The terms "Routing and Spectrum Assignment" (RSA) is introduced in [RFC7698] to refer to the process determines a route and frequency slot for an LSP. Hence, when a route is computed, the spectrum assignment process determines the central frequency and slot width. The term "Spectrum Switched Optical Networks" is also introduced in [RFC7698] to refer to a flexi-grid enabled DWDM network, which can be controlled by a GMPLS or PCE control plane.¶
This document provides PCEP extensions to support RSA in Flexi-grid networks.¶
Figure 1 shows one typical PCE based implementation, which is referred to as the Combined Routing and Spectrum Assignment (RSA) [RFC7698]. With this architecture, the two processes of routing and spectrum assignment are accessed via a single PCE. This architecture is the base architecture from which the PCEP extensions are specified in this document.¶
+----------------------------+
+-----+ | +-------+ +--+ |
| | | |Routing| |SA| |
| PCC |<----->| +-------+ +--+ |
| | | |
+-----+ | PCE |
+----------------------------+
This document aligns with GMPLS extensions for PCEP [RFC8779] for generic property such as label, label-set and label assignment noting that frequency is a type of label. Frequency restrictions and constraints are also formulated in terms of labels per [RFC7579].¶
Spectrum allocation can be performed by the PCE by different means:¶
Option (b) allows distributed spectrum allocation (performed during signaling) to complete spectrum assignment. Additionally, given a range of potential spectrums to allocate, a PC Request SHOULD convey the heuristic / mechanism to the allocation.¶
The format Routing Backus-Naur Form (RBNF) [RFC5511] of a PCReq message per [RFC5440] after incorporating the Spectrum Assignment (SA) object is as follows:¶
<PCReq Message> ::= <Common Header>
[<svec-list>]
<request-list>
Where:
<request-list>::=<request>[<request-list>]
<request>::= <RP>
<GENERALIZED ENDPOINTS>
[ <SA> ]
[other optional objects...]
¶
If the SA object is present in the request, it MUST be encoded after the GENERALIZED ENDPOINTS object.¶
SA Object-Class is (TBD1) (To be assigned by IANA). SA Object-Type is 1.¶
The format of the Spectrum Assignment (SA) object body is as follows:¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Flags |M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Frequency-Slot Selection TLV | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Frequency-Slot Restriction Constraint TLV | . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Optional TLVs // | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved (16 bits)¶
Flags (16 bits)¶
One Flag bit is allocated as follows:¶
M (Mode - 1 bit): M bit is used to indicate the mode of spectrum assignment. When M bit is set to 1, this indicates that the spectrum assigned by the PCE must be explicit. That is, the selected way to convey the allocated spectrum is by means of Explicit Label Control (ELC) [RFC4003] for each hop of a computed LSP. Otherwise, the spectrum assigned by the PCE needs not be explicit (i.e., it can be suggested in the form of label set objects in the corresponding response, to allow distributed SA. In such case, the PCE MUST return a Label Set Field as described in Section 2.6 of [RFC7579] in the response. See Section 5 of this document for the encoding discussion of a Label Set Field in a PCRep message.¶
The Frequency-Slot Selection TLV is used to indicate the frequency-slot selection constraint in regard to the order of frequency-slot assignment to be returned by the PCE. This TLV is only applied when M bit is set in the SA Object specified in Section 4. This TLV SHOULD NOT be present and MUST be ignored when the M bit is cleared.¶
The Frequency-Slot Selection sub-TLV value field is defined as:¶
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| FSA Method | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
¶
Frequency-Slot Assignment (FSA) Method (7 bits):¶
S (Symmetry, 1 bit): This flag is only meaningful when the request is for a bidirectional LSP (see [RFC5440]).0 denotes requiring the same frequency-slot in both directions; 1 denotes that different spectrums on both directions are allowed.¶
IANA is to allocate a new PCEP TLV type, Frequency-Slot Selection TLV (TBD2) in the "PCEP TLV Type Indicators" subregistry (http://www.iana.org/assignments/pcep/pcep.xhtml#pcep-tlv-type-indicators).¶
If a PCE does not support the attribute(s), its behavior is specified below:¶
For any request that contains a Frequency-slot assignment, the requester (PCC) must be able to specify a restriction on the frequency-slots to be used. This restriction is to be interpreted by the PCE as a constraint on the tuning ability of the origination laser transmitter or on any other maintenance related constraints.¶
The format of the Frequency-Slot Restriction Constraint TLV is as follows:¶
<Frequency-lot Restriction Constraint> ::=
(<Action>
<Link Identifiers> <Freq-slot Restriction>)...
Where
<Link Identifiers> ::= <Link Identifier> [<Link Identifiers>]
¶
See Section 4.3.1 in [RFC8780] for the encoding of the Link Identifiers Field.¶
IANA is to allocate a new PCEP TLV, the Frequency slot Restriction Constraint TLV type (TBD5). This TLV MAY appear more than once to be able to specify multiple restrictions. The TLV data is defined as follows:¶
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Action | Count | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Identifiers |
| . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Frequency Slot Restriction Field |
// . . . . //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Action: 8 bits.¶
Note that a PCC MAY add a frequency slot restriction that applies to all links by setting the Count field to zero and specifying just a set of frequency slots.¶
Note that all link identifiers in the same list must be of the same type.¶
The Frequency-Slot Restriction Field of the Frequency slot restriction TLV is encoded as defined in section 4.2 of [RFC8363].¶
This section provides the encoding of a RSA Path Reply, in the PCRep/PCUpd message, for frequency slot allocation as discussed in Section 4. Spectrum Allocation TLV IANA is to allocate a new PCEP TLV type, the Spectrum Allocation TLV type (TBD6). The TLV data is defined as follows:¶
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 | Length |M|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Identifier |
| . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Allocated Spectrum(s) |
// . . . . //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
M (Mode): 1 bit¶
Note that all link identifiers in the same list must be of the same type.¶
This TLV is encoded as an attributes TLV, per [RFC5420], which is carried in the ERO LSP Attribute Subobjects per [RFC7570].¶
To indicate errors associated with the RSA request, a new Error Type (TDB) and subsequent error-values are defined as follows for inclusion in the PCEP-ERROR Object:¶
A new Error-Type (TBD7) and subsequent error-values are defined as follows:¶
To communicate the reason(s) for not being able to find RSA for the path request, the NO-PATH object can be used in the corresponding response. The format of the NO-PATH object body is defined in [RFC5440]. The object may contain a NO-PATH-VECTOR TLV to provide additional information about why a path computation has failed.¶
One new bit flag is defined to be carried in the Flags field in the NO-PATH-VECTOR TLV carried in the NO-PATH Object.¶
Manageability of flexi-grid Routing and Spectrum Assignment (RSA) with PCE must address the following considerations:¶
In addition to the parameters already listed in Section 8.1 of [RFC5440], a PCEP implementation SHOULD allow configuring the following PCEP session parameters on a PCC:¶
In addition to the parameters already listed in Section 8.1 of [RFC5440], a PCEP implementation SHOULD allow configuring the following PCEP session parameters on a PCE:¶
These parameters may be configured as default parameters for any PCEP session the PCEP speaker participates in, or may apply to a specific session with a given PCEP peer or a specific group of sessions with a specific group of PCEP peers.¶
Extensions to the PCEP YANG module may include to cover the Flexi-Grid RSA information introduced in this document. Liveness Detection and Monitoring Mechanisms defined in this document do not imply any new liveness detection and monitoring requirements in addition to those already listed in section 8.3 of [RFC5440].¶
Mechanisms defined in this document do not imply any new verification requirements in addition to those already listed in section 8.4 of [RFC5440].¶
The PCE Discovery mechanisms ([RFC5088] and [RFC5089]) may be used to advertise Flexi-Grid RSA path computation capabilities to PCCs. This draft has requirements on other protocols (ERO objects, etc. which are under TEAS or CCAMP.).¶
Mechanisms defined in this document do not imply any new network operation requirements in addition to those already listed in section 8.6 of [RFC5440].¶
[NOTE TO RFC EDITOR: This whole section and the reference to [RFC7942] is to be removed before publication as an RFC]¶
This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942].¶
The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.¶
According to [RFC7942]], "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".¶
At the time of posting the -10 version of this document, Huawei has implemented some of the features specified in this document, on the WDM network. Details could be as follow:¶
Organization: Huawei Technologies Co.,Ltd.
Implementation: Huawei's WDM systems
Description: supporting PCE Protocol with WDM extensions
Maturity Level: supported features
Coverage: Partial
Contact: zhenghaomian@huawei.com
¶
This document has no requirement for a change to the security models within PCEP. However, the additional information distributed in order to address the RSA problem represents a disclosure of network capabilities that an operator may wish to keep private. Consideration should be given to securing this information.¶
This document requests IANA actions to allocate code points for the objects and sub-registries defined in this document.¶
As described in Section 4.1, a new PCEP Object is defined to carry frequency-slot assignment related constraints. IANA is to allocate the following from 'PCEP Objects' sub-registry (http://www.iana.org/assignments/pcep/pcep.xhtml#pcep-objects):¶
Object Class Name Object Reference
Value Type
-----------------------------------------------------------
TBD1 SA 1: Spectrum Assignment [This.I-D]
¶
As described in Sections 4.2, a new PCEP TLV is defined to indicate spectrum selection constraints. IANA is to allocate this new TLV from the 'PCEP TLV Type Indicators' subregistry (http://www.iana.org/assignments/pcep/pcep.xhtml#pcep-tlv-type-indicators).¶
Value Description Reference
---------------------------------------------------------
TBD2 Spectrum Selection [This.I-D]
¶
As described in Section 4.3, a new PCEP TLV is defined to indicate wavelength restriction constraints. IANA is to allocate this new TLV from the 'PCEP TLV Type Indicators' subregistry (http://www.iana.org/assignments/pcep/pcep.xhtml#pcep-tlv-type-indicators).¶
Value Description Reference
---------------------------------------------------------
TBD5 Frequency Slot Restriction [This.I-D]
Constraint
¶
As described in Section 5, a new PCEP TLV is defined to indicate the allocation of freq-slots(s) by the PCE in response to a request by the PCC. IANA is to allocate this new TLV from the "PCEP TLV Type Indicators" subregistry (http://www.iana.org/assignments/pcep/pcep.xhtml#pcep-tlv-type-indicators).¶
Value Description Reference
---------------------------------------------------------
TBD6 Spectrum Allocation [This.I-D]
¶
As described in Section 4.3, a new bit flag are defined to be carried in the Flags field in the NO-PATH-VECTOR TLV carried in the NO-PATH Object. This flag, when set, indicates that no feasible route was found that meets all the RSA constraints (e.g., spectrum restriction, signal compatibility, etc.) associated with a RSA path computation request.¶
IANA is to allocate this new bit flag from the "PCEP NO-PATH-VECTOR TLV Flag Field" subregistry (http://www.iana.org/assignments/pcep/pcep.xhtml#no-path-vector-tlv).¶
Bit Description Reference
---------------------------------------------------------
TBD8 No RSA constraints met [This.I-D]
¶
As described in Section 5.1, new PCEP error codes are defined for WSON RWA errors. IANA is to allocate from the 'PCEP-ERROR Object Error Types and Values' sub-registry (http://www.iana.org/assignments/pcep/pcep.xhtml#pcep-error-object)¶
Error- Meaning Error-Value Reference
Type
---------------------------------------------------------------
TBD7 Flexi-Grid RSA Error 1: Insufficient [This.I-D]
Memory
2: RSA computation [This.I-D]
Not supported
¶
As discussed in Section 4.1, two new PathErr values for the Existing Error Type (24) are to be allocated:¶
Meaning Error-Value Reference
---------------------------------------------------------------
Unsupported Frequency slot TBD3 [This.I-D]
Selection Symmetry value
Unsupported Frequency Slot TBD4 [This.I-D]
Assignment value
¶
TBD¶