Extensions to Path Computation Element Communication Protocol (PCEP) for Hierarchical Path Computation Elements (PCE)
draft-zhang-pce-hierarchy-extensions-03

Abstract

The Hierarchical Path Computation Element (H-PCE) architecture, defined in the companion framework document [RFC6805], provides a mechanism to allow the optimum sequence of domains to be selected, and the optimum end-to-end path to be derived through the use of a hierarchical relationship between domains.

This document defines the Path Computation Element Protocol (PCEP) extensions for the purpose of implementing Hierarchical PCE procedures which are described in the aforementioned document.

Status of This Memo

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This Internet-Draft will expire on October 13, 2013.

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Table of Contents

• 1. Introduction
• 1.1. Scope
• 1.2. Terminology
• 1.3. Requirements Language
• 2. Requirements for H-PCE
• 2.1. PCEP Requests
• 2.1.1. Qualification of PCEP Requests
• 2.1.2. Multi-domain Objective Functions
• 2.1.3. Multi-domain Metrics
• 2.2. Parent PCE Capability Discovery
• 2.3. PCE Domain and PCE ID Discovery
• 3. PCEP Extensions (Encoding)
• 3.1. OPEN object
• 3.1.1. OF Codes
• 3.1.2. OPEN Object Flags
• 3.1.3. Domain-ID TLV
• 3.1.4. PCE-ID TLV
• 3.2. RP object
• 3.2.1. RP Object Flags
• 3.2.2. Domain-ID TLV
• 3.3. Metric Object
• 3.4. PCEP-ERROR object
• 3.4.1. Hierarchy PCE Error-Type
• 3.5. NO-PATH Object
• 4. H-PCE Procedures
• 4.1. OPEN Procedure between Child PCE and Parent PCE
• 4.2. Procedure to obtain Domain Sequence
• 5. Error Handling
• 6. Manageability Considerations
• 7. IANA Considerations
• 7.1. Objective Function (OF) codes
• 7.2. OPEN Object Flags
• 7.3. RP Object Flags
• 7.4. PCEP TLVs
• 7.5. PCEP PCEP-ERROR types
• 7.6. New No-Path Reasons
• 8. Security Considerations
• 9. Contributing Authors
• 10. Acknowledgments
• 11. Normative References
• Authors' Addresses

1. Introduction

[RFC6805] describes a Hierarchical PCE (H-PCE) architecture which can be used for computing end-to-end paths for inter-domain MPLS Traffic Engineering (TE) and GMPLS Label Switched Paths (LSPs).

Within the hierarchical PCE architecture, the parent PCE is used to compute a multi-domain path based on the domain connectivity information. A child PCE may be responsible for a single domain or multiple domains, it is used to compute the intra-domain path based on its domain topology information.

The H-PCE end-to-end domain path computation procedure is described below:

• A path computation client (PCC) sends the inter-domain path computation requests to the child PCE responsible for its domain;
• The child PCE forwards the request to the parent PCE;
• The parent PCE computes the likely domain paths from the ingress domain to the egress domain;
• The parent PCE sends the intra-domain path computation requests (between the domain border nodes) to the child PCEs which are responsible for the domains along the domain path;
• The child PCEs return the intra-domain paths to the parent PCE;
• The parent PCE constructs the end-to-end inter-domain path based on the intra-domain paths;
• The parent PCE returns the inter-domain path to the child PCE;
• The child PCE forwards the inter-domain path to the PCC;

In addition, the parent PCE may be requested to provide only the sequence of domains to a child PCE so that alternative inter-domain path computation procedures, including Per Domain (PD) [RFC5152] and Backwards Recursive Path Computation (BRPC) [RFC5441] may be used.

This document defines the PCEP extensions for the purpose of implementing Hierarchical PCE procedures, which are described in [RFC6805].

1.1. Scope

The following functions are out of scope of this document.

• Finding end point addresses;
• Parent Traffic Engineering Dabase (TED) methods;
• Domain connectivity;

The document also uses a number of [editor notes] to describe options and alternative solutions. These options and notes will be removed before publication once agreement is reached.

1.2. Terminology

This document uses the terminology defined in [RFC4655], [RFC5440] and the additional terms defined in section 1.4 of [RFC6805].

1.3. 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. Requirements for H-PCE

This section compiles the set of requirements of the PCEP protocol to support the H-PCE architecture and procedures.

[RFC6805] identifies high-level requirements of PCEP extensions required to support the hierarchical PCE model.

2.1. PCEP Requests

The PCReq messages are used by a PCC or PCE to make a path computation request to a PCE. In order to achieve the full functionality of the H-PCE procedures, the PCReq message needs to include:

• Qualification of PCE Requests.
• Multi-domain Objective Functions (OF).
• Multi-domain Metrics.

2.1.1. Qualification of PCEP Requests

As described in section 4.8.1 of [RFC6805], the H-PCE architecture introduces new request qualifications, which are:

• It MUST be possible for a child PCE to indicate that a request it sends to a parent PCE should be satisfied by a domain sequence only, that is, not by a full end-to-end path. This allows the child PCE to initiate a per-domain (PD) [RFC5152] or a backward recursive path computation (BRPC) [RFC5441].
• As stated in [RFC6805], section 4.5, if a PCC knows the egress domain, it can supply this information as the path computation request. It SHOULD be possible to specify the destination domain information in a PCEP request, if it is known.

2.1.2. Multi-domain Objective Functions

For inter-domain path computation, there are two new objective functions which are defined in section 1.3.1 and 4.1 of [RFC6805]:

• Minimize the number of domains crossed. A domain can be either an Autonomous System (AS) or an Internal Gateway Protocol (IGP) area depending on the type of multi-domain network hierarchical PCE is applied to.
• Disallow domain re-entry.[Editor's note: Disallow domain re-entry may not be an objective function, but an option in the request].

During the PCEP session establishment procedure, the parent PCE needs to be capable of indicating the Objective Functions (OF) capability in the Open message. This capability information may then be announced by child PCEs, and used for selecting the PCE when a PCC wants a path that satisfies one or multiple inter-domain objective functions.

When a PCC requests a PCE to compute an inter-domain path, the PCC needs also to be capable of indicating the new objective functions for inter-domain path. Note that a given child PCE may also act as a parent PCE.

For the reasons described previously, new OF codes need to be defined for the new inter-domain objective functions. Then the PCE can notify its new inter-domain objective functions to the PCC by carrying them in the OF-list TLV which is carried in the OPEN object. The PCC can specify which objective function code to use, which is carried in the OF object when requesting a PCE to compute an inter-domain path.

The proposed solution may need to differentiate between the OF code that is requested at the parent level, and the OF code that is requested at the intra-domain (child domain).

A parent PCE MUST be capable of ensuring homogeneity, across domains, when applying OF codes for strict OF intra-domain requests.

2.1.3. Multi-domain Metrics

For inter-domain path computation, there are several path metrics of interest [Editor's note: Current framework only mentions metric objectives. The metric itself should be also defined]:

• Domain count (number of domains crossed).
• Border Node count.

A PCC may be able to limit the number of domains crossed by applying a limit on these metrics.

2.2. Parent PCE Capability Discovery

Parent and child PCE relationships are likely to be configured. However, as mentioned in [RFC6805], it would assist network operators if the child and parent PCE could indicate their H-PCE capabilities.

During the PCEP session establishment procedure, the child PCE needs to be capable of indicating to the parent PCE whether it requests the parent PCE capability or not. Also, during the PCEP session establishment procedure, the parent PCE needs to be capable of indicating whether its parent capability can be provided or not.

2.3. PCE Domain and PCE ID Discovery

A PCE domain is a single domain with an associated PCE. Although it is possible for a PCE to manage multiple domains. The PCE domain may be an IGP area or AS.

The PCE ID is an IPv4 and/or IPv6 address that is used to reach the parent/child PCE. It is RECOMMENDED to use an address that is always reachable if there is any connectivity to the PCE.

The PCE ID information and PCE domain identifiers may be provided during the PCEP session establishment procedure or the domain connectivity information collection procedure.

3. PCEP Extensions (Encoding)

3.1. OPEN object

3.1.1. OF Codes

There are two new OF codes defined here for H-PCE:

• MTD
  • Name: Minimize the number of Transit Domains.
  • Objective Function Code: (to be assigned by IANA, recommended 12).
  • Description: Find a path P such that it passes through the lnumber of transit domains.
• MBN
  • Name: Minimize the number of border nodes.
  • Objective Function Code: (to be assigned by IANA, recommended 13).
  • Description: Find a path P such that it passes through the least number of border nodes.

• DDR
  • Name: Disallow Domain Re-entry (DDR)
  • Objective Function Code: (to be assigned by IANA, recommended 14)
  • Description: Find a path P such that does not entry a domain more than once.

3.1.2. OPEN Object Flags

There are two OPEN object flags defined here for H-PCE:

• Parent PCE Request bit (to be assigned by IANA, recommended bit 0): if set, it would signal that the child PCE wishes to use the peer PCE as a parent PCE.
• Parent PCE Indication bit (to be assigned by IANA, recommended bit 1): if set, it would signal that the PCE can be used as a parent PCE by the peer PCE.

3.1.3. Domain-ID TLV

The type of Domain-ID TLV is to be assigned by IANA (recommended 7). The length is variable. The format of this TLV is defined below:

 
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Domain Type         |            Reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Domain ID                               |
   //                                                             //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0|    Type     |     Length    |         Reserved              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          AS Id (4 bytes)                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0|    Type     |     Length    |  AS Id (2 bytes)              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Address Type        |            Reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //                     PCE IP Address                          //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 
   +------------+------------------------------------------------------+
   | Error-Type | Meaning                                              |
   +------------+------------------------------------------------------+
   | 19         | H-PCE error Error-value=1: parent PCE capability     |
   |            | cannot be provided                                   |
   +------------+------------------------------------------------------+