LSP-DB Synchronization between Stateful PCEs
draft-palle-pce-stateful-pce-lspdb-sync-02

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.

[STATEFUL-PCE] specifies a set of extensions to PCEP to enable stateful control of MPLS-TE and GMPLS Label Switched Paths (LSPs) via PCEP and maintaining of these LSPs at the stateful PCE. This document describes the mechanisms of LSP Database (LSP-DB) synchronization between stateful PCEs.

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Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 2. Terminology
• 3. Motivation and Use
• 4. Functions to Support LSP-DB Synchronization
• 5. Architectural Overview
• 5.1. LSP-DB Synchronization between Primary and Backup Stateful PCEs
• 5.2. LSP-DB Synchronization between Load-Balanced Stateful PCEs
• 5.3. Other Considerations
• 6. PCEP Messages
• 6.1. The PCRpt Message
• 6.2. The PCUpd Message
• 7. TLVs
• 7.1. Stateful PCE Capability TLV
• 7.2. PCE Redundancy Group Identifier TLV
• 7.3. PCE-CAP-FLAGS sub-TLV
• 8. Security Considerations
• 9. Manageability Considerations
• 9.1. Control of Function and Policy
• 9.2. Information and Data Models
• 9.3. Liveness Detection and Monitoring
• 9.4. Verify Correct Operations
• 9.5. Requirements On Other Protocols
• 9.6. Impact On Network Operations
• 10. IANA Considerations
• 10.1. STATEFUL-PCE-CAPABILITY TLV
• 10.2. PCE-CAP-FLAGS sub-TLV
• 11. Acknowledgments
• 12. References
• 12.1. Normative References
• 12.2. Informative References
• Appendix A. Contributor Addresses
• Authors' Addresses

1. Introduction

[RFC5440] describes the Path Computation Element Protocol (PCEP) as the communication between a Path Computation Client (PCC) and a Path Computation Element (PCE), or between PCEs, enabling computation of Multiprotocol Label Switching (MPLS) for Traffic Engineering Label Switched Paths (TE LSPs).

[STATEFUL-PCE] specifies a set of extensions to PCEP to enable stateful control of LSPs in compliance with [RFC4655]. It includes mechanisms for LSP state synchronization between a PCC and a PCE, i.e., all stateful PCEs synchronize their LSP states from the network.

When multiple stateful PCEs are operating in the network, they could be either Primary/Backup or Loadbalanced. In a scenario where the network operator has deployed backup stateful PCE(s) with only purpose to be used in the event of failure of the primary PCE, it makes sense that in such a deployment, PCE should have as latest LSP states as possible. Further as stateful PCE make changes to the delegated LSPs, these changes (pending LSPs and sticky resources) needs to be synchronized to other PCEs as soon as possible. [PCE-QUESTIONS] describes the synchronization issues when PCEs are synchronized from the network (PCC) only.

This document specifies the mechanisms of LSP-DB synchronization between stateful PCEs (in the same domain) to be able to get the latest LSP state.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

2. Terminology

The terminology is as per [RFC5440] and [STATEFUL-PCE].

LSP-DB:

A database of LSPs that are active in the network as maintained by a stateful PCE.

Sticky Resources:

The temporarily assigned resources that are allocated to a pending LSP and are provisionally blocked.

3. Motivation and Use

Distributed computation model ([RFC4655]) refers to a domain or network that may include multiple PCEs where computation of paths is shared among the PCEs, this is further clarified in [PCE-QUESTIONS].

When multiple stateful PCEs are operating in the network, they could be either -

Primary or Backup PCE:

A backup PCE exists to perform functions in the network, only in the event of a failure of the primary PCE. In this case, all LSPs to be delegated are under primary stateful PCE control while other PCEs in the domain act as backup. The backup PCE should have the same view of LSP-DB as primary stateful PCE. The LSP-DB of a backup PCE can be synchronized via the primary stateful PCE or collected from multiple network nodes (PCC). In case of latter only, the backup PCE may face synchronization issues as described in [PCE-QUESTIONS]. Thus it is suggested that backup PCE can be synchronized via the primary stateful PCE, this mechanism is described in Section 5.1. Note that backup PCE MAY use synchronization from network as a mechanism to cross-check the LSP-DB.

Load-Balanced 'Backup' PCE:

Load-Balanced PCEs share the computation load all the time as well as act backup to each other. One PCE MAY serve a set of PCCs as the primary computation server, and only addresses requests from other PCCs in the event of the failure of some other PCE. Delegated LSPs are thus distributed among stateful PCEs. It is suggested that in this case each load-balanced stateful PCE should build their LSP-DB independently from the network (PCCs) (via mechanism described in [STATEFUL-PCE]) during initial LSP state synchronization and not from other stateful PCEs. But it is important that these load-balanced stateful PCEs needs to be synchronized to have a similar view of pending LSPs and sticky resources, this mechanism is described in Section 5.2.

4. Functions to Support LSP-DB Synchronization

[STATEFUL-PCE] specifies new functions to support a stateful PCE. It also specifies that a function can be initiated either from a PCC towards a PCE (C-E) or from a PCE towards a PCC (E-C).

• Capability negotiation (E-C,C-E)
• LSP state synchronization (C-E)
• LSP update request (E-C)
• LSP state report (C-E)
• LSP control delegation (C-E,E-C)
• Stateful PCE discovery via [STATEFUL-PCE-DISC]

This document extends some of these functions to support LSP-DB synchronization. Some are initiated either from a PCE towards another PCE (E-E) or specifically from primary to backup PCE (PE-BE).

Capability negotiation (E-E):

both the PCEs must announce during PCEP session establishment that they support PCEP Stateful PCE extensions defined in [STATEFUL-PCE]. It should also declare whether it has primary or backup stateful PCE capability. This is done via Open message.

LSP state synchronization (PE-BE):

after the session between the stateful PCEs is initialized, the backup PCE must learn the state of LSPs from the primary PCE. This is done via PCRpt message.

LSP update request (E-E):

When a PCE requests modification of attributes of a delegated LSP, this information should also be sent to other PCEs. This is done via PCUpd message. This is needed to synchronize the pending LSPs and sticky resources.

Stateful PCE discovery:

PCE can advertise its primary or backup capability via IGP.

5. Architectural Overview

LSP-DB synchronization function is defined in section 5.4 of [STATEFUL-PCE] between PCC and PCEs. This document extends the LSP state synchronization between stateful PCEs.

5.1. LSP-DB Synchronization between Primary and Backup Stateful PCEs

As shown in Figure 1, PCE1 is the primary stateful PCE and PCE2 is the backup stateful PCE. PCC1 and PCC2 synchronize the LSP-DB with the primary stateful PCE1 after session initialization phase. And primary stateful PCE1 synchronizes LSP-DB with its backup stateful PCE2 after session initialization phase. This is LSP state synchronization as described in Section 4 and uses PCRpt message.

PCC1 and PCC2 delegates LSP1 and LSP2 to the primary PCE1. Whenever there is an update in LSP, PCE1 sends a PCUpd message to corresponding PCC and also to backup PCE2. This is LSP update request as described in Section 4 and uses PCUpd message. This makes sure that the pending LSP changes and sticky resources are backed up. The PCC sends a PCRpt message to the primary PCE, indicating the LSP's status, the primary PCE further synchronizes the state with backup PCEs via PCRpt message.

+----+            +----+             +----+              +----+ 
|PCC1|            |PCC2|             |PCE1|              |PCE2| 
+-+--+            +-+--+             +-+--+              +-+--+ 
  |                 |                  |                   |
  |---- LSP SYNC ---+----------------->|                   |
  |                 |---- LSP SYNC --->|                   |
  |                 |                  |---- LSP SYNC----->|
  |                 |---- LSP SYNC ----+------------------>| 
  |---- LSP SYNC ---+------------------+------------------>|
  |                 |                  |                   |
  |-- PCRpt,lsp1,D -+----------------->|                   |
  |<----------------+----PCUpd,lsp1 ---|                   |
  |                 |                  |--- PCUpd,lsp1 --->|
  |-- PCRpt,lsp1,up-+----------------->|                   |
  |-- PCRpt,lsp1,up-+------------------+------------------>| 
  |                 |                  |----PCRpt,lsp1,up->|
  |                 |                  |                   |
  |                 |-- PCRpt,lsp2,D ->|                   |
  |                 |<---PCUpd,lsp2 ---|                   |
  |                 |                  |--- PCUpd,lsp2---->|
  |                 |-- PCRpt,lsp2,up->|                   |
  |                 |-- PCRpt,lsp2,up--+------------------>|
  |                 |                  |----PCRpt,lsp2,up->|
  |                 |                  |                   |
      
	

+----+            +----+             +----+              +----+ 
|PCC1|            |PCC2|             |PCE1|              |PCE2| 
+-+--+            +-+--+             +-+--+              +-+--+ 
  |                 |                  |                   |
  |---- LSP SYNC ---+----------------->|                   |
  |---- LSP SYNC ---+------------------+------------------>|
  |                 |---- LSP SYNC --->|                   |
  |                 |---- LSP SYNC ----+------------------>| 
  |                 |                  |                   |
  |-- PCRpt,lsp1,D -+----------------->|                   |
  |<----------------+----PCUpd, lsp1---|                   |
  |                 |                  |--- PCUpd, lsp1--->|
  |-- PCRpt,lsp1,up-+----------------->|                   |
  |-- PCRpt,lsp1,up-+------------------+------------------>|
  |                 |                  |----PCRpt,lsp1,up->| 
  |                 |                  |                   |
  |                 |                  |                   |
  |                 |-- PCRpt,lsp2,D --+------------------>|
  |                 |<---PCUpd, lsp2---|-------------------|
  |                 |                  |<--- PCUpd, lsp2 --|
  |                 |-- PCRpt,lsp2,up->|                   |
  |                 |-- PCRpt,lsp2,up--+------------------>|
  |                 |                  |<---PCRpt,lsp1,up--|
  |                 |                  |                   |
   
	

      
      Bit       Capabilities

      TBD       Backup Stateful PCE
      

      
      Bit    Description                               Reference

      TBD    BACKUP                                    [This I.D.]
      

      
      Bit    Description                               Reference

      TBD    BACKUP                                    [This I.D.]
      

Young Lee
Huawei
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