Path Computation Element communication Protocol extension for signaling LSP diversity constraint
draft-ietf-pce-association-diversity-00

Abstract

This document introduces a simple mechanism to signal path diversity for a group of Label Switched Paths (LSPs) via an extension to the Path Computation Element Communication Protocol (PCEP).

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Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 2. Terminology
• 3. Motivation
• 4. Protocol extension
• 4.1. Association group
• 4.2. Optional TLVs
• 5. Security Considerations
• 6. IANA Considerations
• 6.1. Association object Type Indicators
• 7. Manageability Considerations
• 7.1. Control of Function and Policy
• 7.2. Information and Data Models
• 7.3. Liveness Detection and Monitoring
• 7.4. Verify Correct Operations
• 7.5. Requirements On Other Protocols
• 7.6. Impact On Network Operations
• 8. Acknowledgments
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Authors' Addresses

1. Introduction

[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 a set of attributes (such as configuration parameters or behaviours) and is equally applicable to the active and passive modes of a stateful PCE [I-D.ietf-pce-stateful-pce] or a stateless PCE [RFC5440].

This document specifies a PCEP extension to signal that a particular group of LSPs should use diverse paths including the requested type of diversity.

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 following terminology is used in this document.

LSR:

Label Switch Router.

MPLS:

Multiprotocol Label Switching.

PCC:

Path Computation Client. Any client application requesting a path computation to be performed by a Path Computation Element.

PCE:

Path Computation Element. An entity (component, application, or network node) that is capable of computing a network path or route based on a network graph and applying computational constraints.

PCEP:

Path Computation Element Communication Protocol.

SRLG:

Shared Risk Link Group.

3. Motivation

Path diversity is a very common use case today in IP/MPLS networks especially for layer 2 transport over MPLS. A customer may request that the operator provide two end-to-end disjoint paths across the IP/MPLS core. The customer may use those paths as primary/backup or active/active.

Different level of disjointness may be offered:

• Link disjointness: the paths of the associated LSPs should transit different links (but may use common nodes or different links that may have some shared fate).
• Node disjointness: the paths of the associated LSPs should transit different nodes (but may use different links that may have some shared fate).
• SRLG disjointness: the paths of the associated LSPs should transit different links that do not share fate (but may use common transit nodes).
• Node+SRLG disjointness: the paths of the associated LSPs should transit different links that do not have any common shared fate and should transit different nodes.

The associated LSPs may originate from the same or from different head-end(s) and may terminate at the same or different tail-end(s).

         _________________________________________
        /                                         \
       /        +------+                           \
      |         | PCE  |                            |
      |         +------+                            |
      |                                             |
      |          ***********************>           |
      | +------+           10             +------+  |
CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2
      | +------+       |        |         +------+  |
      |                |        |                   |
      |                |        |                   |
      | +------+       |        |         +------+  |
CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4
      | +------+ ***********************> +------+  |
      |                                             |
       \                                           /
        \_________________________________________/
	
	

Figure 1 - Disjoint paths with different head-ends

In the figure above, the customer wants to have two disjoint paths between CE1/CE2 and CE3/CE4. From an IP/MPLS network point view, in this example, the CEs are connected to different PEs to maximize their disjointness. When LSPs originate from different head-ends, distributed computation of diverse paths can be difficult. Whereas, computation via a centralized PCE ensures path disjointness correctness and simplicity.

Using PCEP, the PCC MUST indicate that disjoint path computation is required, such indication SHOULD include disjointness parameters such as the type of disjointness, the disjoint group-id, and any customization parameters according to local policy.

The PCC can use the generic mechanism as per [I-D.ietf-pce-association-group] to associate a set of LSPs with a particular disjoint-group.

The management of the disjoint group-ids will be a key point for the operator as the Association ID field is limited to 65535. The local configuration of IPv4/IPv6 association source, or Global Association Source/Extended Association ID should allow to overcome this limitation. For example, when a PCC or PCE initiates all the LSPs in a particular disjoint-group, it can set the IPv4/IPv6 association source can be set to one of its IP address. When disjoint LSPs are initiated from different head-ends, a unique association identifier SHOULD be used for those LSPs: this can be achieved by setting the IPv4/IPv6 source address to a common value (zero value can be used) as well as the Association ID.

                                                         
          Initiate & Monitor LSP                       
                   |                                          
                   |                          PCReq           
                   V                  {Disjoint-group Y}      
                +-----+                   ----------------> +-----+
     _ _ _ _ _ _| PCE |                  |                  | PCE |
    |           +-----+                  |      ----------> +-----+
    | PCEInitiate                        |     |    PCReq
    |{Disjoint-group X}                  |     | {Disjoint-group Y}
    |                                    |     |
    |              .-----.               |     |         .-----.
    |             (       )              |  +----+      (       )
    |         .--(         )--.          |  |PE 1|--.--(         )--.
    V        (                 )         |  +----+ (                 )
  +---+     (                   )        |        (                   )
  |PCC|----(   (G)MPLS network    )   +----+     ( (G)MPLS network     )
  +---+     (                   )     |PE 3|------(                   )
Disjoint-group X               )      +----+       (                 )
{Monitor LSP} '--(         )--'                     '--(         )--'
                  (       )                             (       )
                   '-----'                               '-----'

 Case 1: Disjointness initiated by    Case 2: Disjointness initiated by
         PCE and enforced by PCC             PCC and enforced by PCE

Figure 2 - Sample use-cases for carrying disjoint-group over PCEP session

Using the disjoint-group within a PCUpdate or PCInit may have two purposes:

• Information: in case the PCE is performing the path computation, it may communicate to the PCC the locally used configured parameters in the attribute-list of the LSP.
• Configuration: in case the PCC is performing the path computation but the PCE (without computation engine) is managing the LSP parameters, the PCE should add the disjoint-group within the PCUpdate message to communicate to the PCC the disjointness constraint.

4. Protocol extension

4.1. Association group

As per [I-D.ietf-pce-association-group], LSPs are associated with other LSPs with which they interact by adding them to a common association group. The Association ID will be used to identify the disjoint group a set of LSPs belongs to. This document defines four new Association types, based on the generic Association object -

• Association type = TBD1 ("Disjointness Association Type") for link disjoint group.
• Association type = TBD2 ("Disjointness Association Type") for node disjoint group.
• Association type = TBD3 ("Disjointness Association Type") for srlg disjoint group.
• Association type = TBD4 ("Disjointness Association Type") for node+srlg disjoint group.

A disjoint group can have two or more LSPs. But a PCE may be limited in how many LSPs it can take into account when computing disjointness: usually PCEs are able to compute a pair of disjoint paths. If a PCE receives more LSPs in the group than it can handle in its computation algorithm, it SHOULD apply disjointness computation to only a subset of LSPs in the group. The subset of disjoint LSPs will be decided by the implementation.

Local polices on the PCC or PCE MAY define the computational behavior for the other LSPs in the group. For example, the PCE may provide no path, a shortest path, or a constrained path based on relaxing disjointness, etc.

Associating a particular LSP to multiple disjoint groups is authorized from a protocol perspective, however there is no insurance that the PCE will be able to compute properly the multi-disjointness constraint.

4.2. Optional TLVs

The disjoint group MAY carry some optional TLVs including but not limited to:

• VENDOR-INFORMATION-TLV: Used to communicate arbitrary vendor specific behavioral information, described in [RFC7150].
• DISJOINTNESS-INFORMATION-TLV: Used to communicate some disjointness specific parameters.

       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 = [TBD5]         |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Flags                                           |P|S|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	

The DISJOINTNESS-INFORMATION-TLV is shown in the following figure:

• Flags:
  • P: shortest path, this particular LSP in the group SHOULD use the shortest constrained path and others MAY use a non shortest constrained path. The shortest constrained path is the shortest path (from the requested metric point of view) that fills all the LSP constraints without taking into account the disjointness constraint. This means that an LSP with P flag set should be placed as if the disjointness constraint has not been configured, while the other LSP in the association with P flag unset should be placed by taking into account the disjointness constraint. Setting P flag changes the relationship between LSPs to a unidirectional relationship (LSP 1 with P=0 depends of LSP 2 with P=1, but LSP 2 with P=1 does not depend of LSP 1 with P=0).
  • S: strict disjointness, when set if disjoint paths cannot be found, PCE should return no path for LSPs that could not be be disjoint. When unset, PCE is allowed to relax disjointness by using either using a lower disjoint type (link instead of node) or relaxing disjointness constraint at all.

If a PCEP speaker receives a disjoint-group without DISJOINTNESS-INFORMATION-TLV, it SHOULD use its locally configured parameters or use the following default parameters:

• Strict disjointness is assumed.
• LSP can use a non shortest-path.

If a PCEP speaker receives two LSPs with the same disjoint-group but with a different S flag value, it SHOULD apply a strict disjointness path computation for this disjoint-group (it considers S flag set for all LSPs).

5. Security Considerations

This document defines one new type for association, which do not add any new security concerns beyond those discussed in [RFC5440], [I-D.ietf-pce-stateful-pce] and [I-D.ietf-pce-association-group] in itself.

6. IANA Considerations

6.1. Association object Type Indicators

This document defines the following new association type originally defined in [I-D.ietf-pce-association-group].

Value                     Name                             Reference

TBD1                      Link Disjoint-group 
                          Association Type                 [This I.D.]
TBD2                      Node Disjoint-group 
                          Association Type                 [This I.D.]
TBD3                      SRLG Disjoint-group 
                          Association Type                 [This I.D.]
TBD4                      Node+SRLG Disjoint-group
                          Association Type                 [This I.D.]

This document defines the following new PCEP TLV:

Value     Name                                   Reference

TBD5      DISJOINTNESS-INFORMATION-TLV           [This I.D.]

IANA is requested to manage the space of flags carried in the DISJOINTNESS-INFORMATION TLV defined in this document, numbering them from 0 as the least significant bit.

New bit numbers may be allocated in future.

IANA is requested to allocate the following bit numbers in the DISJOINTNESS-INFORMATION-TLV flag space:

Bit Number     Name                                   Reference
    0          Strict disjointness                    [This I.D.]
    1          Shortest-path                          [This I.D.]

7. Manageability Considerations

7.1. Control of Function and Policy

An operator MUST be allowed to configure the disjointness associations and parameters at PCEP peers and associate it with the LSPs.

7.2. Information and Data Models

[RFC7420] describes the PCEP MIB, there are no new MIB Objects for this document.

7.3. 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 [RFC5440].

7.4. Verify Correct Operations

Mechanisms defined in this document do not imply any new operation verification requirements in addition to those already listed in [RFC5440].

7.5. Requirements On Other Protocols

Mechanisms defined in this document do not imply any new requirements on other protocols.

7.6. Impact On Network Operations

Mechanisms defined in this document do not have any impact on network operations in addition to those already listed in [RFC5440].

8. Acknowledgments

A special thanks to author of [I-D.ietf-pce-association-group], this document borrow some of the text from it.

9. References

9.1. Normative References

9.2. Informative References

Authors' Addresses