RFC 9005: Path Computation Element Communication Protocol (PCEP) Extension for Associating Policies and Label Switched Paths (LSPs)

Abstract

This document introduces a simple mechanism to associate policies with a group of Label Switched Paths (LSPs) via an extension to the Path Computation Element Communication Protocol (PCEP). The extension allows a PCEP speaker to advertise to a PCEP peer that a particular LSP belongs to a particular Policy Association Group (PAG).¶

1. Introduction

[RFC5440] describes the Path Computation Element Communication Protocol (PCEP), which enables the communication between a Path Computation Client (PCC) and a Path Control Element (PCE) or between two PCEs based on the PCE architecture [RFC4655]. [RFC5394] provides additional details on policy within the PCE architecture and also provides context for the support of PCE policy.¶

"Path Computation Element Communication Protocol (PCEP) Extensions for Stateful PCE" ([RFC8231]) describes a set of extensions to PCEP to enable active control of Multiprotocol Label Switching Traffic Engineering (MPLS-TE) and Generalized MPLS (GMPLS) tunnels. [RFC8281] describes the setup and teardown of PCE-initiated LSPs under the active stateful PCE model without the need for local configuration on the PCC, thus allowing for a dynamic network. Currently, the LSPs can either be signaled via Resource Reservation Protocol Traffic Engineering (RSVP-TE) or segment routed as specified in [RFC8664].¶

[RFC8697] introduces a generic mechanism to create a grouping of LSPs that can then be used to define associations between a set of LSPs and a set of attributes (such as configuration parameters or behaviors) and is equally applicable to stateful PCE (active and passive modes) and stateless PCE.¶

This document specifies a PCEP extension to associate one or more LSPs with policies using the generic association mechanism.¶

A PCEP speaker may want to influence the PCEP peer with respect to path selection and other policies. This document describes a PCEP extension to associate policies by creating a Policy Association Group (PAG) and encoding this association in PCEP messages. The specification is applicable to both stateful and stateless PCEP sessions.¶

Note that the actual policy definition and the associated parameters are out of scope of this document.¶

2. Terminology

The following terminology is used in this document.¶

Association parameters:

As described in [RFC8697], the combination of the mandatory fields Association Type, Association ID, and Association Source in the ASSOCIATION object uniquely identifies the association group. If the optional TLVs -- Global Association Source or Extended Association ID -- are included, then they are included in combination with mandatory fields to uniquely identify the association group.¶

Association information:

As described in [RFC8697], the ASSOCIATION object could include other optional TLVs based on the Association Types that provide "information" related to the association.¶

LSR:

Label Switching Router¶

MPLS:

Multiprotocol Label Switching¶

PAG:

Policy Association Group¶

PAT:

Policy Association Type¶

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¶

3. Motivation

Paths computed using PCE can be subjected to various policies at both the PCE and the PCC. For example, in a centralized TE scenario, network operators may instantiate LSPs and specify policies for traffic accounting, path monitoring, telemetry, etc., for some LSPs via the stateful PCE. Similarly, a PCC could request a user-specific or service-specific policy to be applied at the PCE, such as a constraints relaxation policy, to meet optimal QoS and resiliency levels.¶

PCEP speakers can use the generic mechanism of [RFC8697] to associate a set of LSPs with a policy, without the need to know the details of such a policy. This simplifies network operations, avoids frequent software upgrades, and provides the ability to introduce new policies more quickly.¶

                                                         PAG Y
                                          {Service-Specific Policy
                                                    for constraint
            Monitor LSP                                relaxation}
                 |                                          |
                 | PAG X                    PCReq/PCRpt     |
                 V {Monitor LSP}            {PAG Y}         V
              +-----+                   ----------------> +-----+
   _ _ _ _ _ _| PCE |                  |                  | PCE |
  |           +-----+                  |      ----------> +-----+
  | PCInitiate/PCUpd                   |     |    PCReq/PCRpt
  |{PAG X}                             |     |    {PAG Y}
  |                                    |     |
  |              .-----.               |     |         .-----.
  |             (       )              |  +----+      (       )
  |         .--(         )--.          |  |PCC1|--.--(         )--.
  V        (                 )         |  +----+ (                 )
+---+     (                   )        |        (                   )
|PCC|----(   (G)MPLS network    )   +----+     ( (G)MPLS network   )
+---+     (                   )     |PCC2|------(                   )
PAG X      (                 )      +----+       (                 )
{Monitor    '--(         )--'                     '--(         )--'
LSP}            (       )                             (       )
                 '-----'                               '-----'

Case 1: Policy requested by PCE        Case 2: Policy requested by
        and enforced by PCC                    PCC and enforced by
                                               PCE

4. Overview

As per [RFC8697], LSPs are associated with other LSPs with which they interact by adding them to a common association group. Grouping can also be used to define the association between LSPs and the policies associated with them. As described in [RFC8697], the association group is uniquely identified by the combination of the following fields in the ASSOCIATION object: Association Type, Association ID, Association Source, and (if present) Global Association Source or Extended Association ID. This document defines a new Association Type called "Policy Association" with value 3 based on the generic ASSOCIATION object. This new Association Type is called "Policy Association Type" (PAT).¶

[RFC8697] specifies the mechanism for the capability advertisement of the Association Types supported by a PCEP speaker by defining an ASSOC-Type-List TLV to be carried within an OPEN object. This capability exchange for the PAT MUST be done before using the Policy Association. Thus, the PCEP speaker MUST include the PAT in the ASSOC-Type-List TLV and MUST receive the same from the PCEP peer before using the PAG in PCEP messages.¶

The Policy Association Type (3) is operator configured (as specified in [RFC8697]), i.e., the association is created by the operator manually on the PCEP peers, and an LSP belonging to this association is conveyed via PCEP messages to the PCEP peer. There is no need to convey an explicit Operator-configured Association Range, which could only serve to artificially limit the available Association IDs. Thus, for the Policy Association Type, the Operator-configured Association Range MUST NOT be set and MUST be ignored if received.¶

A PAG can have one or more LSPs. The association parameters including Association Identifier, Policy Association Type (PAT), as well as the Association Source IP address are manually configured by the operator and are used to identify the PAG as described in [RFC8697]. The Global Association Source and Extended Association ID MAY also be included.¶

As per the processing rules specified in Section 6.4 of [RFC8697], if a PCEP speaker does not support this Policy Association Type, it would return a PCEP error (PCErr) message with Error-Type 26 "Association Error" and Error-value 1 "Association type is not supported". The PAG and the policy MUST be configured on the PCEP peers as per the operator-configured association procedures. All further processing is as per Section 6.4 of [RFC8697]. If a PCE speaker receives a PAG in a PCEP message and the Policy Association information is not configured, it MUST return a PCErr message with Error-Type 26 "Association Error" and Error-value 4 "Association unknown".¶

Associating a particular LSP with multiple policy groups is allowed from a protocol perspective; however, there is no assurance that the PCEP speaker will be able to apply multiple policies. If a PCEP speaker does not support handling of multiple policies for an LSP, it MUST NOT add the LSP into the association group and MUST return a PCErr with Error-Type 26 "Association Error" and Error-value 7 "Cannot join the association group".¶

5. Policy Association Group

Association groups and their memberships are defined using the ASSOCIATION object defined in [RFC8697]. Two object types for IPv4 and IPv6 are defined. The ASSOCIATION object includes "Association type" indicating the type of the association group. This document adds a new Association Type, Policy Association Type (PAT).¶

PAG may carry optional TLVs including but not limited to:¶

POLICY-PARAMETERS-TLV:

Used to communicate opaque information useful to applying the policy, described in Section 5.1.¶

VENDOR-INFORMATION-TLV:

Used to communicate arbitrary vendor-specific behavioral information, described in [RFC7470].¶

    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=48               |          Length               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //                     Policy Parameters                       //
   |                                                               |
   +---------------------------------------------------------------+

6. Security Considerations

The security considerations described in [RFC8697], [RFC8231], [RFC5394], and [RFC5440] apply to the extensions described in this document as well. In particular, a malicious PCEP speaker could be spoofed and used as an attack vector by creating spurious Policy Associations as described in [RFC8697]. Further, as described in [RFC8697], a spurious LSP can have policies that are inconsistent with those of the legitimate LSPs of the group and, thus, cause problems in the handling of the policy for the legitimate LSPs. It should be noted that Policy Association could provide an adversary with the opportunity to eavesdrop on the relationship between the LSPs. [RFC8697] suggests that the implementations and operators use indirect values as a way to hide any sensitive business relationships. Thus, securing the PCEP session using Transport Layer Security (TLS) [RFC8253], as per the recommendations and best current practices in BCP 195 [RFC7525], is RECOMMENDED.¶

Further, extra care needs to be taken by the implementation with respect to the POLICY-PARAMETERS-TLV while decoding, verifying, and applying these policy variables. This TLV parsing could be exploited by an attacker; thus, extra care must be taken while configuring a Policy Association that uses the POLICY-PARAMETERS-TLV and making sure that the data is easy to parse and verify before use. Ensuring agreement among all relevant PCEP peers as to the format and layout of the policy parameters information is key for correct operations. Note that the parser for POLICY-PARAMETERS-TLV is particularly sensitive since it is opaque to PCEP and can be used to convey data with many different internal structures/formats. The choice of decoder is dependent on the additional metadata associated with the policy; thus, additional risk of using a wrong decoder and getting garbage results is incurred. Using standard and well-known policy formats could help alleviate those risks.¶

7. IANA Considerations

8. Manageability Considerations

9. References

Acknowledgments

We would like to acknowledge and thank Santiago Alvarez, Zafar Ali, Luis Tomotaki, Victor Lopez, Rob Shakir, and Clarence Filsfils for working on earlier draft versions with similar motivation.¶

Special thanks to the authors of [RFC8697]. This document borrowed some of its text. The authors would like to thank Aijun Wang, Peng Shuping, and Gyan Mishra for their useful comments.¶

Thanks to Hariharan Ananthakrishnan for shepherding this document. Thanks to Deborah Brungard for providing comments and being the responsible AD for this document.¶

Thanks to Nic Leymann for the RTGDIR review.¶

Thanks to Benjamin Kaduk and Murray Kucherawy for their comments during the IESG review.¶

Contributors

The following individuals have contributed extensively:¶

The following individuals have contributed text that was incorporated:¶