PCEP Extensions for Associated Bidirectional Segment Routing (SR) Paths
draft-li-pce-sr-bidir-path-05

Abstract

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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Table of Contents

• 1. Introduction
• 2. Terminology
• 2.1. Requirements Language
• 3. PCEP Extension for Bidirectional SR Path
• 3.1. Double-sided Bidirectional SR Path Association Group Object
• 4. Bidirectional Flag
• 5. Procedures for Associated Bidirectional SR Path Computation
• 5.1. PCE Initiated Associated Bidirectional SR Paths
• 5.2. PCC Initiated Associated Bidirectional SR Paths
• 5.3. Error Handling
• 6. IANA Considerations
• 6.1. Association Type
• 6.2. PCEP Errors
• 7. Security Considerations
• 8. Contributors
• 9. Acknowledgments
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Authors' Addresses

1. Introduction

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-segment-routing-ipv6] extend PCEP to support SR for IPv6 data plane.

[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], or a Path Segment Identifier in SRv6 [I-D.li-spring-srv6-path-segment]. The PSID can be used for an associated bidirectional SR Path for identifying the SR Path.

2. Terminology

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].

2.1. 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.

3. PCEP Extension for Bidirectional SR Path

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]).

3.1. Double-sided Bidirectional SR Path Association Group Object

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:

• Association Type (TBD1 to be assigned by IANA) = Double-sided Bidirectional SR Path Association Group

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.

• An SR Path (forward or reverse) can not be part of more than one 'Double-sided Bidirectional SR Path Association Group'.
• The endpoints of the SR Paths in this associations cannot be different.

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].

4. Bidirectional Flag

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.

5. Procedures for Associated Bidirectional SR Path Computation

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.

5.1. PCE Initiated Associated Bidirectional SR Paths

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
           

• Stateful PCE can create and update the forward and reverse SR Paths independently for a 'Double-sided Bidirectional SR Path Association Group'.
• Stateful PCE can establish and remove the association relationship on a per SR Path basis.
• Stateful PCE can create and update the SR Path and the association on a PCC via PCInitiate and PCUpd messages, respectively, using the procedures described in [I-D.ietf-pce-association-group].
• The PATH-SEGMENT TLV SHOULD be included for each SR Path in the LSP object.
• The reverse direction SR Path (LSP2(R) at node S, LSP1(R) at node D) SHOULD be informed by PCE via PCInitiate message with the matching association group.

5.2. PCC Initiated Associated Bidirectional 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

• PCC can create and update the forward and reverse SR Paths independently for a 'Double-sided Bidirectional SR Path Association Group'.
• PCC can establish and remove the association relationship on a per SR Path basis.
• PCC MUST report the change in the association group of an SR Path to PCE(s) via PCRpt message.
• PCC can report the forward and reverse SR Paths independently to PCE(s) via PCRpt message.
• PCC can delegate the forward and reverse SR Paths independently to a Stateful PCE, where PCE would control the SR Paths.
• Stateful PCE can update the SR Paths in the 'Double-sided Bidirectional SR Path Association Group' via PCUpd message, using the procedures described in [I-D.ietf-pce-association-group].
• The PATH-SEGMENT TLV MUST be handled as defined in [I-D.li-pce-sr-path-segment].
• The reverse direction SR Path (LSP2(R) at node S, LSP1(R) at node D) SHOULD be informed by PCE via PCInitiate message with the matching association group.

5.3. Error Handling

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).

6. IANA Considerations

6.1. Association Type

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 

6.2. PCEP Errors

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

7. Security Considerations

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.

8. Contributors

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

9. Acknowledgments

Many thanks to Marina Fizgeer for detailed review and comments.

10. References

10.1. Normative References

10.2. Informative References

Authors' Addresses