| PCE Working Group | C. Li |
| Internet-Draft | M. Chen |
| Intended status: Standards Track | Huawei Technologies |
| Expires: February 20, 2020 | W. Cheng |
| China Mobile | |
| Z. Li | |
| J. Dong | |
| Huawei Technologies | |
| R. Gandhi | |
| Cisco Systems, Inc. | |
| Q. Xiong | |
| ZTE Corporation | |
| August 19, 2019 |
PCEP Extensions for Associated Bidirectional Segment Routing (SR) Paths
draft-li-pce-sr-bidir-path-06
The Path Computation Element Communication Protocol (PCEP) provides mechanisms for Path Computation Elements (PCEs) to perform path computations in response to Path Computation Clients (PCCs) requests. The Stateful PCE extensions allow stateful control of Multiprotocol Label Switching (MPLS) Traffic Engineering (TE) Label Switched Paths (LSPs) using PCEP. Furthermore, PCEP can be used for computing paths in Segment Routing (SR) TE networks.
This document defines PCEP extensions for grouping two reverse unidirectional SR Paths into an Associated Bidirectional SR Path when using a Stateful PCE for both PCE-Initiated and PCC-Initiated LSPs as well as when using a Stateless PCE.
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Segment routing (SR) [RFC8402] leverages the source routing and tunneling paradigms. SR supports to steer packets into an explicit forwarding path at the ingress node.
[RFC5440] describes the Path Computation Element (PCE) Communication Protocol (PCEP). PCEP enables the communication between a Path Computation Client (PCC) and a PCE, or between PCE and PCE, for the purpose of computation of Multiprotocol Label Switching (MPLS) as well as Generalized MPLS (GMPLS) Traffic Engineering Label Switched Path (TE LSP) characteristics.
[RFC8231] specifies a set of extensions to PCEP to enable stateful control of TE LSPs within and across PCEP sessions in compliance with [RFC4657]. It includes mechanisms to effect LSP State Synchronization between PCCs and PCEs, delegation of control over LSPs to PCEs, and PCE control of timing and sequence of path computations within and across PCEP sessions. The model of operation where LSPs are initiated from the PCE is described in [RFC8281].
[I-D.ietf-pce-segment-routing] specifies extensions to the Path Computation Element Protocol (PCEP) [RFC5440] for SR networks, that allow a stateful PCE to compute and initiate SR-TE paths, as well as a PCC to request, report or delegate SR Paths.
[I-D.ietf-pce-association-group] introduces a generic mechanism to create a grouping of LSPs which can then be used to define associations between a set of LSPs and/or a set of attributes, for example primary and secondary LSP associations, and is equally applicable to the active and passive modes of a Stateful PCE [RFC8231] or a stateless PCE [RFC5440].
Currently, SR networks only support unidirectional paths. However, bidirectional SR Paths are required in some networks, for example, in mobile backhaul transport networks. The requirement of bidirectional SR Path is specified in [I-D.ietf-spring-mpls-path-segment].
[I-D.ietf-pce-association-bidir] defines PCEP extensions for grouping two reverse unidirectional MPLS TE LSPs into an Associated Bidirectional LSP when using a Stateful PCE for both PCE-Initiated and PCC-Initiated LSPs as well as when using a Stateless PCE.
This document extends the bidirectional association to segment routing by specifying PCEP extensions for grouping two reverse unidirectional SR Paths into a bidirectional SR Path.
[I-D.ietf-pce-association-bidir] specifies the Double-sided Bidirectional LSP Association procedure, where the PCE creates the association and provisions at both endpoints, the RSVP-TE does the signaling to the egress the status of the forward LSP and the ingress about the reverse LSP. Thus, the both endpoints learn the reverse LSPs forming the bidirectional LSP association. In case of SR, to support the bidirectional path use-case, this is done using the PCEP protocol. This is done so that both endpoints are aware of the the unidirectional SR Path, as well as the status and other SR path related information.
[I-D.li-pce-sr-path-segment] defines a procedure for Path Segment Identifier (PSID) in PCEP for SR using PATH-SEGMENT TLV. The PSID can be a Path Segment Identifier in SR-MPLS [I-D.ietf-spring-mpls-path-segment]. The PSID can be used for an associated bidirectional SR Path for identifying the SR Path.
This document makes use of the terms defined in [I-D.ietf-pce-segment-routing]. The reader is assumed to be familiar with the terminology defined in [RFC5440], [RFC8231], [RFC8281], [I-D.ietf-pce-association-group] and [I-D.ietf-pce-association-bidir].
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.
As per [I-D.ietf-pce-association-group], LSPs are associated by adding them to a common association group. [I-D.ietf-pce-association-bidir] specifies PCEP extensions for grouping two reverse unidirectional MPLS-TE LSPs into an Associated Bidirectional LSP for both single-sided and double-sided initiation cases by defining two new Bidirectional LSP Association Groups.
This document extends the procedure for associated bidirectional SR Paths by defining a new bidirectional association group (Double-sided Bidirectional SR Path Association Group). The document further describes the mechanism for associating two unidirectional SR Paths into a bidirectional SR Path. [I-D.li-pce-sr-path-segment] defines a procedure for communicating Path Segment in PCEP for SR using PATH-SEGMENT TLV. The bidirectional SR Path can also use the PATH-SEGMENT TLV.
Note that an association group is defined in this document to define procedures specific to SR Paths (and the procedures are different than the RSVP-TE bidirectional association groups defined in [I-D.ietf-pce-association-bidir]).
As defined in [I-D.ietf-pce-association-bidir], two LSPs are associated as a bidirectional MPLS-TE LSP by a common bidirectional LSP association group. For associating two SR paths, this document defines a new association group called 'Double-sided Bidirectional SR Path Association Group' as follows:
Similar to other bidirectional associations, this Association Type is operator-configured in nature and statically created by the operator on the PCEP peers. The paths belonging to this association is conveyed via PCEP messages to the PCEP peer. Operator-configured Association Range TLV [I-D.ietf-pce-association-group] MUST NOT be sent for these Association Types, and MUST be ignored, so that the entire range of association ID can be used for them. The handling of the Association ID, Association Source, optional Global Association Source and optional Extended Association ID in this association are set in the same way as [I-D.ietf-pce-association-bidir].
A member of the 'Double-sided Bidirectional SR Path Association Group' can take the role of a forward or reverse SR Path and follow the similar rules defined in [I-D.ietf-pce-association-bidir] for LSPs.
For describing the SR Paths in this association group, such as direction and co-routed information, this association group reuses the Bidirectional LSP Association Group TLV defined in [I-D.ietf-pce-association-bidir]. All fields and processing rules are as per [I-D.ietf-pce-association-bidir].
As defined in [RFC5440], the B-flag in RP object MUST be set when the PCC specifies that the path computation request relates to a bidirectional TE LSP. In this document, the B-flag also MUST be set when the PCC specifies that the path computation request relates to a bidirectional SR Path. When a stateful PCE initiates or updates a bidirectional SR Paths including LSPs and SR paths, the B-flag in SRP object [I-D.ietf-pce-pcep-stateful-pce-gmpls] MAY be set as well.
Two unidirectional SR Paths can be associated by the association group object as specified in [I-D.ietf-pce-association-group]. A bidirectional LSP association group object is defined in [I-D.ietf-pce-association-bidir] (for MPLS-TE). This document extends these association mechanisms for bidirectional SR Paths. Two SR Paths can be associated together by using the Bidirectional SR Path Association Group defined in this document for PCEP messages. The PATH-SEGMENT TLV [I-D.li-pce-sr-path-segment] SHOULD also be included in the LSP object for these SR Paths to support required use-cases.
For bidirectional SR Paths, there is a need to know the reverse direction SR paths. The PCE SHOULD inform the reverse SR Paths to the ingress PCCs and vice versa. To achieve this, a PCInitiate message for the reverse SR Path is sent to the ingress PCC and a PCInitiate message for the forward SR Path is sent to the egress PCC (with the same association group). These PCInitiate message MUST NOT trigger initiation of SR Paths. The reverse direction SR Path can be used for several use-cases, such as directed BFD [I-D.ietf-mpls-bfd-directed].
For a bidirectional LSP computation when using both direction LSPs on a node, the same LSP would need to be identified using 2 different PLSP-IDs based on the PCEP session to the ingress or the egress. In other words, the LSP will have a PLSP-ID A at the ingress node while it will have the PLSP-ID B at the egress node. The PCE will maintain the two PLSP-IDs for the same LSP. For instance, an ingress PCC requests a bidirectional SR Path computation, and the PCE computes a forward LSP1 with PLSP-ID say 100. The reverse LSP2 from the egress to the ingress with PLSP-ID say 200 is allocated by the egress PCC. Since the PLSP-ID space is independent at each PCC, the PLSP-ID allocated by the egress PCC can not be used for the LSP at the ingress PCC (PLSP-ID conflict may occur). Hence, the PCE needs to allocate a PLSP-ID for LSP2 from the ingress PCC's PLSP-ID space , say 101. Similarly for LSP1, it has PLSP-ID 100 at the ingress, and may have say PLSP-ID 201 at the egress node.
As specified in [I-D.ietf-pce-association-group], Bidirectional SR Path Association Group can be created by a Stateful PCE.
+-----+
| PCE |
+-----+
PCInitiate/PCUpd: / \ PCInitiate/PCUpd:
Tunnel 1 (F) / \ Tunnel 2 (F)
(LSP1 (F), LSP2 (R)) / \ (LSP2 (F), LSP1 (R))
Association #1 / \ Association #1
/ \
v v
+-----+ LSP1 +-----+
| S |------------>| D |
| |<------------| |
+-----+ LSP2 +-----+
<no signaling>
Figure 1: PCE-Initiated Double-sided Bidirectional SR Path
with Forward and Reverse Direction SR Paths
As specified in [I-D.ietf-pce-association-group], Bidirectional SR Path Association Group can also be created by a PCC.
+-----+
| PCE |
+-----+
Report/Delegate: ^ ^ Report/Delegate:
Tunnel 1 (F) / \ Tunnel 2 (F)
(LSP1 (F)) / \ (LSP2 (F))
Association #2 / \ Association #2
/ \
/ \
+-----+ LSP1 +-----+
| S |------------>| D |
| |<------------| |
+-----+ LSP2 +-----+
<no signaling>
Figure 2a: Step 1: PCC-Initiated Double-sided Bidirectional SR Path
with Forward Direction SR Paths
+-----+
| PCE |
+-----+
PCUpd/PCInitiate: / \ PCUpd/PCInitiate:
Tunnel 1 (F) / \ Tunnel 2 (F)
(LSP1 (F), LSP2 (R)) / \ (LSP2 (F), LSP1 (R))
Association #2 / \ Association #2
/ \
v v
+-----+ LSP1 +-----+
| S |------------>| D |
| |<------------| |
+-----+ LSP2 +-----+
<no signaling>
Figure 2b: Step 2: PCE-Upd/Initiated Double-sided Bidirectional Path
Along with Reverse Direction SR Paths
The error handling as described in section 5.5 of [I-D.ietf-pce-association-bidir] continue to apply.
The PCEP Path Setup Type (PST) MUST be set to 'TE Path is Setup using Segment Routing' [I-D.ietf-pce-segment-routing] for the LSP belonging to the 'Double-sided Bidirectional SR Path Association Group'. In case a PCEP speaker receives a different PST value for this association group, it MUST send a PCErr message with Error-Type = 29 (Early allocation by IANA) (Association Error) and Error-Value = TBD2 (Bidirectional LSP Association - Path Setup Type Mismatch).
[Note to the RFC Editor - remove this section before publication, as well as remove the reference to [RFC7942].
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".
The feature is developing based on Huawei VRP8.
This document defines a new Association Type for the Association Object defined [I-D.ietf-pce-association-group]. IANA is requested to make the assignment of a value for the sub-registry "ASSOCIATION Type Field" (to be created in [I-D.ietf-pce-association-group]), as follows:
Value Name Reference
-------------------------------------------------------------------
TBD1 Double-sided Bidirectional This document
SR Path Association Group
This document defines new Error value for Error Type 29 (Association Error). IANA is requested to allocate new Error value within the "PCEP-ERROR Object Error Types and Values" sub-registry of the PCEP Numbers registry, as follows:
Error Type Description Reference
-------------------------------------------------------------------
29 Association Error
Error value: TBD2 This document
Bidirectional LSP Association -
Path Setup Type Mismatch
The security considerations described in [RFC5440], [RFC8231], [RFC8281], and [I-D.ietf-pce-segment-routing] apply to the extensions defined in this document as well.
A new Association Type for the Association Object, 'Double-sided Associated Bidirectional SR Path Association Group' is introduced in this document. Additional security considerations related to LSP associations due to a malicious PCEP speaker is described in [I-D.ietf-pce-association-group] and apply to this Association Type. Hence, securing the PCEP session using Transport Layer Security (TLS) [RFC8253] is recommended.
All manageability requirements and considerations listed in [RFC5440], [RFC8231], and [RFC8281] apply to PCEP protocol extensions defined in this document. In addition, requirements and considerations listed in this section apply.
The mechanisms defined in this document do not imply any control or policy requirements in addition to those already listed in [RFC5440], [RFC8231], and [RFC8281].
[RFC7420] describes the PCEP MIB, there are no new MIB Objects defined for Bidirectional SR Path associations. The PCEP YANG module [I-D.ietf-pce-pcep-yang] defines data model for Bidirectional SR Path associations.
Mechanisms defined in this document do not imply any new liveness detection and monitoring requirements in addition to those already listed in [RFC5440], [RFC8231], and [RFC8281].
Mechanisms defined in this document do not imply any new operation verification requirements in addition to those already listed in [RFC5440], [RFC8231], and [I-D.ietf-pce-segment-routing] .
Mechanisms defined in this document do not imply any new requirements on other protocols.
Mechanisms defined in [RFC5440], [RFC8231], and [I-D.ietf-pce-segment-routing] also apply to PCEP extensions defined in this document.
The following people have substantially contributed to this document:
Dhruv Dhody
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore, Karnataka 560066
India
Email: dhruv.ietf@gmail.com
Many thanks to Marina Fizgeer for detailed review and comments.