| RFC 9504 | Stateful PCEP for GMPLS | December 2023 |
| Lee, et al. | Standards Track | [Page] |
The Path Computation Element Communication Protocol (PCEP) has been extended to support stateful PCE functions where the stateful PCE maintains information about paths and resource usage within a network; however, these extensions do not cover all requirements for GMPLS networks.¶
This document provides the extensions required for PCEP so as to enable the usage of a stateful PCE capability in GMPLS-controlled networks.¶
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/rfc9504.¶
Copyright (c) 2023 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.¶
[RFC4655] presents the architecture of a PCE-based model for computing Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) Traffic Engineering Label Switched Paths (TE LSPs). To perform such a constrained computation, a PCE stores the network topology (i.e., TE links and nodes) and resource information (i.e., TE attributes) in its TE Database (TED). A PCE that only maintains a TED is referred to as a "stateless PCE". [RFC5440] describes the Path Computation Element Communication Protocol (PCEP) for interaction between a Path Computation Client (PCC) and a PCE or between two PCEs, enabling computation of TE LSPs. PCEP is further extended to support GMPLS-controlled networks as per [RFC8779].¶
Stateful PCEs are shown to be helpful in many application scenarios, in both MPLS and GMPLS networks, as illustrated in [RFC8051]. Further discussion of the concept of a stateful PCE can be found in [RFC7399]. In order for these applications to be able to exploit the capability of stateful PCEs, extensions to stateful PCEP for GMPLS are required.¶
[RFC8051] describes how a stateful PCE can be applied to solve various problems for MPLS-TE and GMPLS networks and the benefits it brings to such deployments.¶
[RFC8231] specifies a set of extensions to PCEP to enable stateful control of TE LSPs where they are configured on the PCC and control over them could be delegated to the PCE. Furthermore, [RFC8281] describes the setup and teardown of PCE-initiated LSPs under the active stateful PCE model, without the need for local configuration on the PCC. However, both documents omit the specification for technology-specific objects and TLVs, and they do not cover GMPLS-controlled networks (e.g., Wavelength Switched Optical Network (WSON), Optical Transport Network (OTN), Synchronous Optical Network (SONET) / Synchronous Digital Hierarchy (SDH)).¶
This document focuses on the extensions that are necessary in order for the deployment of stateful PCEs and the requirements for PCE-initiated LSPs in GMPLS-controlled networks. Section 3 provides a general context of the usage of stateful PCEs and PCEP for GMPLS. The various requirements for stateful GMPLS, including PCE initiation for GMPLS LSPs, are provided in Section 4. An overview of the PCEP extensions is specified in Section 5. A solution to address such requirements with PCEP object extensions is specified in Section 6.¶
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.¶
Terminology used in this document is the same as terminology used in [RFC5440], [RFC8231], [RFC8281], and [RFC8779].¶
This section is built on the basis of stateful PCEs specified in [RFC8231] and PCEP for GMPLS specified in [RFC8779].¶
The operation of a stateful PCE on LSPs can be divided into two types: active stateful PCE and passive stateful PCE (as described in [RFC8051]).¶
Furthermore, the LSP Initiation function of PCEP is defined in [RFC8281] to allow the PCE to initiate LSP establishment after the path is computed. An LSP Initiate Request (PCInitiate) message is used to trigger the end node to set up the LSP. Any modifications to the objects and TLVs that are identified in this document to support GMPLS-specific attributes will be carried in the PCInitiate messages.¶
[RFC8779] defines GMPLS-specific objects and TLVs in stateless PCEP; this document makes use of these objects and TLVs without modifications where applicable. Where these objects and TLVs require modifications to incorporate stateful PCEs, they are described in this document. PCE-initiated LSPs follow the principle specified in [RFC8281], and the GMPLS-specific extensions are also included in this document.¶
This section notes the main functional requirements for PCEP extensions to support stateful PCEs for use in GMPLS-controlled networks, based on the description in [RFC8051]. Many requirements are common across a variety of network types (e.g., MPLS-TE networks and GMPLS networks) and the protocol extensions to meet the requirements are already described in [RFC8231] (such as LSP update, delegation, and state synchronization/report). Protection context information that describes the GMPLS requirement can also follow the description in [RFC8745]. This document does not repeat the description of those protocol extensions. This document presents protocol extensions for a set of requirements that are specific to the use of a stateful PCE in a GMPLS-controlled network.¶
The requirements for GMPLS-specific stateful PCEs are as follows:¶
Capability advertisement is specified in [RFC8231]; it can be achieved by using the STATEFUL-PCE-CAPABILITY TLV in the Open message. Another GMPLS-CAPABILITY TLV is defined in [RFC8779]. A subregistry to manage the Flag field of the GMPLS-CAPABILITY TLV has been created by IANA as requested by [RFC8779]. The following bits are introduced by this document in the GMPLS-CAPABILITY TLV as flags to indicate the capability for LSP report, update, and initiation in GMPLS networks: LSP-REPORT-CAPABILITY (31), LSP-UPDATE-CAPABILITY (30), and LSP-INSTANTIATION-CAPABILITY (29).¶
After the session between the PCC and a stateful PCE is initialized, the PCE must learn the state of a PCC's LSPs (including its attributes) before it can perform path computations or update LSP attributes in a PCC. This process is known as "LSP state synchronization". The LSP attributes, including bandwidth, associated route, and protection information etc., are stored by the PCE in the LSP database (LSP-DB). Note that, as described in [RFC8231], the LSP state synchronization covers both the bulk reporting of LSPs at initialization as well as the reporting of new or modified LSPs during normal operation. Incremental LSP-DB synchronization may be desired in a GMPLS-controlled network; it is specified in [RFC8232].¶
The format of the PCRpt message is specified in [RFC8231] and extended in [RFC8623] to include the END-POINTS object. The END-POINTS object is extended for GMPLS in [RFC8779]. The END-POINTS object can be carried in the PCRpt message as specified in [RFC8623]. The END-POINTS object type for GMPLS is included in the PCRpt message as per the same.¶
The following objects are extended for GMPLS in [RFC8779] and are also used in the PCRpt in the same manner: BANDWIDTH, LSP Attributes (LSPA), Include Route Object (IRO), and Exclude Route Object (XRO). These objects are carried in the PCRpt message as specified in [RFC8231] (as the attribute-list defined in Section 6.5 of [RFC5440] and extended by many other documents that define PCEP extensions for specific scenarios).¶
The SWITCH-LAYER object is defined in [RFC8282]. This object is carried in the PCRpt message as specified in Section 3.2 of [RFC8282].¶
The LSP delegation and cleanup procedure specified in [RFC8281] are equally applicable to GMPLS LSPs and this document does not modify the associated usage.¶
Both passive and active stateful PCE mechanisms in [RFC8231] are applicable in GMPLS-controlled networks. Remote LSP Initiation in [RFC8281] is also applicable in GMPLS-controlled networks.¶
Existing extensions defined in [RFC8779] can be used in stateful PCEP with no or slight changes for GMPLS network control, including the following:¶
The END-POINTS object was specified in [RFC8779] to include GMPLS capabilities. All stateful PCEP messages MUST include the END-POINTS object with Generalized Endpoint object type, containing the LABEL-REQUEST TLV. Further note that:¶
In [RFC8779], IANA allocates value 45 (GMPLS-CAPABILITY) from the "PCEP TLV Type Indicators" subregistry. This specification adds three flags to the Flag field of this TLV to indicate the Report, Update, and Initiation capabilities.¶
[RFC5521] defines a mechanism for a PCC to request or demand that specific nodes, links, or other network resources be excluded from paths computed by a PCE. A PCC may wish to request the computation of a path that avoids all links and nodes traversed by some other LSP.¶
To this end, this document defines a new subobject for use with route exclusion defined in [RFC5521]. The LSP Exclusion subobject 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |X|Type (11) | Length | Reserved | Flags | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | // Symbolic Path Name // | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This field is the same as the X-bit defined in the XRO subobjects in Section 2.1.1 of [RFC5521] where it says:¶
The X-bit indicates whether the exclusion is mandatory or desired. 0 indicates that the resource specified MUST be excluded from the path computed by the PCE. 1 indicates that the resource specified SHOULD be excluded from the path computed by the PCE, but MAY be included subject to PCE policy and the absence of a viable path that meets the other constraints and excludes the resource.¶
This is the identifier given to an LSP. Its syntax and semantics are identical to those of the Symbolic Path Name field defined in Section 7.3.2 of [RFC8231] where it says: "symbolic name for the LSP, unique in the PCC. It SHOULD be a string of printable ASCII characters, without a NULL terminator." The symbolic path name in the LSP Exclusion subobject MUST only vary from being a string of printable ASCII characters without a NULL terminator when it is matching the value contained in another subobject. It is worth noting that given that the symbolic path name is unique in the context of the headnode, only LSPs that share the same headnode or PCC could be excluded.¶
This subobject MAY be present multiple times in the XRO to exclude resources from multiple LSPs. When a stateful PCE receives a PCReq message carrying this subobject, it MUST search for the identified LSP in its LSP-DB and then exclude from the new path computation all resources used by the identified LSP.¶
Note that this XRO subobject could also be used by non-GMPLS LSPs. The usage of the XRO subobject for any non-GMPLS LSPs is not in the scope of this document.¶
The LSP object is defined in Section 7.3 of [RFC8231], and the new extended flags TLV is defined in [RFC9357]. This TLV is used in PCUpd, PCRpt and PCInitiate messages for GMPLS, with the following flags defined in this document:¶
The RG flag for GMPLS is also defined in the LSP-EXTENDED-FLAG TLV. The values are defined as per [RFC8779]:¶
A PCEP-ERROR object is used to report a PCEP error and is characterized by an Error-Type that specifies the type of error and an Error-value that provides additional information about the error. This section adds additional error handling procedures to those specified in Section 3 of [RFC8779]. Please note that all error handling specified in Section 3 of [RFC8779] is applicable and MUST be supported for a stateful PCE in GMPLS networks.¶
The PCEP extensions described in this document for stateful PCEs with GMPLS capabilities MUST NOT be used if the PCE has not advertised its capabilities with GMPLS as per Section 6.2.1.¶
If the PCC understands the U flag that indicates the stateful LSP-UPDATE-CAPABILITY, but did not advertise this capability, then upon receipt of a PCUpd message for GMPLS LSP from the PCE, it SHOULD generate a PCErr with Error-Type 19 ("Invalid Operation") Error-value 25 ("Attempted LSP update request for GMPLS if stateful PCE capability not advertised") and terminate the PCEP session. Such a PCC MAY decide to utilize the capability even though it did not advertise support for it.¶
If the PCE understands the R flag that indicates the stateful LSP-REPORT-CAPABILITY, but did not advertise this capability, then upon receipt of a PCRpt message for GMPLS LSP from the PCC, it SHOULD generate a PCErr with Error-Type 19 ("Invalid Operation") Error-value 26 ("Attempted LSP State Report for GMPLS if stateful PCE capability not advertised") and terminate the PCEP session. Such a PCE MAY decide to utilize the capability even though it did not advertise support for it.¶
If the PCC understands the I flag that indicates LSP-INSTANTIATION-CAPABILITY, but did not advertise this capability, then upon receipt of a PCInitiate message for GMPLS LSP from the PCE, it SHOULD generate a PCErr with Error-Type 19 ("Invalid Operation") Error-value 27 ("Attempted LSP instantiation request for GMPLS if stateful PCE instantiation capability for not advertised") and terminate the PCEP session. Such a PCC MAY decide to utilize the capability even though it did not advertise support for it.¶
A stateful PCE is expected to perform an LSP reoptimization when receiving a message with the R bit set in the RP object. If no LSP state information is available to carry out reoptimization, the stateful PCE SHOULD report Error-Type 19 ("Invalid Operation") Error-value 23 ("LSP state info unavailable for reoptimization"), although such a PCE MAY consider the reoptimization to have successfully completed. Note that this error message could also be used by non-GMPLS LSPs.¶
The LSP Exclusion subobject in XRO, as defined in Section 6.2.2 of this document, MAY be present multiple times. When a stateful PCE receives a PCEP message carrying this subobject, it searches for the identified LSP in its LSP-DB. It then excludes from the new path computation all the resources used by the identified LSP. If the stateful PCE cannot recognize the symbolic path name of the identified LSP, it SHOULD send an error message PCErr reporting Error-Type 19 ("Invalid Operation") Error-value 24 ("LSP state info for route exclusion not found"). Along with the unrecognized symbolic path name, it MAY also provide information to the requesting PCC using the error-reporting techniques described in [RFC5440]. An implementation MAY choose to ignore the requested exclusion when the LSP cannot be found because it could claim that it has avoided using all resources associated with an LSP that doesn't exist.¶
Note that the END-POINTS object in stateful PCEP messages was introduced for Point-to-Multipoint (P2MP) [RFC8623]. Similarly, the END-POINTS object MUST be carried for the GMPLS LSP. If the END-POINTS object is missing and the GMPLS flag in LSP-EXTENDED-FLAG is set, the receiving PCE or PCC MUST send a PCErr message with Error-Type 6 ("Mandatory Object missing") and Error-value 3 ("END-POINTS object missing") (defined in [RFC5440]). Similarly, if the END-POINTS object with the Generalized Endpoint object type is received but the LSP-EXTENDED-FLAG TLV is missing in the LSP object or the G flag in the LSP-EXTENDED-FLAG TLV is not set, the receiving PCE or PCC MUST send a PCErr message with Error-Type 19 ("Invalid Operation") Error-value 28 ("Use of the Generalized Endpoint object type for non-GMPLS LSPs").¶
If the END-POINTS object with Generalized Endpoint object type is missing the LABEL-REQUEST TLV, the receiving PCE or PCC MUST send a PCErr message with Error-Type 6 ("Mandatory Object missing") Error-value 20 ("LABEL-REQUEST TLV missing").¶
[RFC8779] defines the GMPLS-CAPABILITY TLV; per that RFC, IANA created the "GMPLS-CAPABILITY TLV Flag Field" registry to manage the values of the GMPLS-CAPABILITY TLV's Flag field. This document registers new bits in this registry as follows:¶
| Bit | Capability Description | Reference |
|---|---|---|
| 31 | LSP-REPORT-CAPABILITY (R) | RFC 9504 |
| 30 | LSP-UPDATE-CAPABILITY (U) | RFC 9504 |
| 29 | LSP-INSTANTIATION-CAPABILITY (I) | RFC 9504 |
IANA maintains the various XRO subobject types within the "XRO Subobjects" subregistry of the "Path Computation Element Protocol (PCEP) Numbers" registry. IANA has allocated a codepoint for another XRO subobject as follows:¶
| Value | Description | Reference |
|---|---|---|
| 11 | LSP | RFC 9504 |
IANA has created a registry named "LSP Exclusion Subobject Flag Field", within the "Path Computation Element Protocol (PCEP) Numbers" group, to manage the Flag field of the LSP Exclusion subobject in the XRO. No flag is currently defined for this Flag field in this document.¶
Codespace of the Flag field (LSP Exclusion Subobject)¶
| Bit | Capability Description | Reference |
|---|---|---|
| 0-7 | Unassigned | RFC 9504 |
New values are to be assigned by Standards Action [RFC8126]. Each bit should be registered with the following entries:¶
[RFC9357] requested IANA to create a subregistry, named the "LSP-EXTENDED-FLAG TLV Flag Field", within the "Path Computation Element Protocol (PCEP) Numbers" registry, to manage the Flag field of the LSP-EXTENDED-FLAG TLV.¶
IANA has made assignments from this registry as follows:¶
| Bit | Capability Description | Reference |
|---|---|---|
| 0 | GMPLS LSP (G) | RFC 9504 |
| 1 | Bidirectional Co-routed LSP (B) | RFC 9504 |
| 2-3 | Routing Granularity (RG) | RFC 9504 |
IANA has made the following allocations in the "PCEP-ERROR Object Error Types and Values" registry.¶
| Error-Type | Meaning | Error-value | Reference |
|---|---|---|---|
| 6 | Mandatory Object missing | 20: LABEL-REQUEST TLV missing | RFC 9504 |
| 19 | Invalid Operation | 23: LSP state info unavailable for reoptimization | RFC 9504 |
| 24: LSP state info for route exclusion not found | RFC 9504 | ||
| 25: Attempted LSP update request for GMPLS if stateful PCE capability not advertised | RFC 9504 | ||
| 26: Attempted LSP State Report for GMPLS if stateful PCE capability not advertised | RFC 9504 | ||
| 27: Attempted LSP instantiation request for GMPLS if stateful PCE instantiation capability not advertised | RFC 9504 | ||
| 28: Use of the Generalized Endpoint object type for non-GMPLS LSPs | RFC 9504 |
General PCE management considerations are discussed in [RFC4655] and [RFC5440], and GMPLS-specific PCEP management considerations are described in [RFC8779]. In this document, the management considerations for stateful PCEP extension in GMPLS are described.¶
This section follows the guidance of [RFC6123].¶
In addition to the parameters already listed in Section 8.1 of [RFC5440], a PCEP implementation SHOULD allow configuration of the following PCEP session parameters on a PCC. However, an implementation MAY choose to make these features available on all PCEP sessions:¶
In addition to the parameters already listed in Section 8.1 of [RFC5440], a PCEP implementation SHOULD allow configuration of 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 they may apply to a specific session with a given PCEP peer or a specific group of sessions with a specific group of PCEP peers.¶
The YANG module in [PCE-PCEP-YANG] can be used to configure and monitor PCEP states and messages. To make sure that the YANG module is useful for the extensions as described in this document, it would need to include advertised GMPLS stateful capabilities etc. A future version of [PCE-PCEP-YANG] will include this.¶
As described in [YANG-PATH-COMPUTATION], a YANG-based interface can be used in some cases to request GMPLS path computations, instead of PCEP. Refer to [YANG-PATH-COMPUTATION] for details.¶
This document makes no change to the basic operation of PCEP, so there are no changes to the requirements for liveness detection and monitoring in [RFC4657] and Section 8.3 of [RFC5440].¶
This document makes no change to the basic operations of PCEP and the considerations described in Section 8.4 of [RFC5440]. New errors defined by this document should satisfy the requirement to log error events.¶
When the detailed route information is included for LSP state synchronization (either at the initial stage or during the LSP State Report process), this requires the ingress node of an LSP to carry the Record Route Object (RRO) object in order to enable the collection of such information.¶
The management considerations concerning the impact on network operations described in Section 4.6 of [RFC8779] apply here.¶
The security considerations elaborated in [RFC5440] apply to this document. The PCEP extensions to support GMPLS-controlled networks should be considered under the same security as for MPLS networks, as noted in [RFC7025]. Therefore, the PCEP extension to support GMPLS specified in [RFC8779] is used as the foundation of this document; the security considerations in [RFC8779] should also be applicable to this document. The secure transport of PCEP specified in [RFC8253] allows the usage of Transport Layer Security (TLS). The same can also be used by the PCEP extension defined in this document.¶
This document provides additional extensions to PCEP so as to facilitate stateful PCE usage in GMPLS-controlled networks, on top of [RFC8231] and [RFC8281]. Security issues caused by the extension in [RFC8231] and [RFC8281] are not altered by the additions in this document. The security considerations in [RFC8231] and [RFC8281], including both issues and solutions, apply to this document as well.¶
This section uses the Routing Backus-Naur Form (RBNF) [RFC5511] to illustrate the PCEP messages. The RBNF in this section is reproduced for informative purposes. It is also expanded to show the GMPLS-specific objects.¶
According to [RFC8231], the PCRpt message is used to report the current state of an LSP. This document extends the message in reporting the status of LSPs with GMPLS characteristics.¶
The format of the PCRpt message is as follows:¶
<PCRpt Message> ::= <Common Header>
<state-report-list>
¶
Where:¶
<state-report-list> ::= <state-report>[<state-report-list>]
<state-report> ::= [<SRP>]
<LSP>
[<END-POINTS>]
<path>
¶
Where:¶
<path> ::= <intended-path>
[<actual-attribute-list><actual-path>]
<intended-attribute-list>
<actual-attribute-list> ::=[<BANDWIDTH>]
[<metric-list>]
¶
Where:¶
<attribute-list> ::= [<of-list>]
[<LSPA>]
[<BANDWIDTH>]
[<metric-list>]
[<IRO>][<XRO>]
[<INTER-LAYER>]
[<SWITCH-LAYER>]
[<REQ-ADAP-CAP>]
[<SERVER-INDICATION>]
¶
The format of a PCUpd message is as follows:¶
<PCUpd Message> ::= <Common Header>
<update-request-list>
¶
Where:¶
<update-request-list> ::= <update-request>[<update-request-list>]
<update-request> ::= <SRP>
<LSP>
[<END-POINTS>]
<path>
¶
Where:¶
<path> ::= <intended-path><intended-attribute-list>¶
Where:¶
According to [RFC8281], the PCInitiate message is used allow LSP Initiation. This document extends the message in initiating LSPs with GMPLS characteristics. The format of a PCInitiate message is as follows:¶
<PCInitiate Message> ::= <Common Header>
<PCE-initiated-lsp-list>
¶
Where:¶
<Common Header> is defined in <xref target="RFC5440" />.
<PCE-initiated-lsp-list> ::= <PCE-initiated-lsp-request>
[<PCE-initiated-lsp-list>]
<PCE-initiated-lsp-request> ::= (<PCE-initiated-lsp-instantiation>|
<PCE-initiated-lsp-deletion>)
<PCE-initiated-lsp-instantiation> ::= <SRP>
<LSP>
[<END-POINTS>]
<ERO>
[<attribute-list>]
<PCE-initiated-lsp-deletion> ::= <SRP>
<LSP>
¶
The format of the PCInitiate message is unchanged from Section 5.1 of [RFC8281]. All fields are similar to the PCRpt and the PCUpd messages.¶
We would like to thank Adrian Farrel, Cyril Margaria, George Swallow, Jan Medved, Sue Hares, and John Scudder for the useful comments and discussions.¶
Thanks to Dhruv Dhody for Shepherding this document and providing useful comments.¶