Path Computation Element Communication Protocol (PCEP) Procedures and Extensions for Using PCE as a Central Controller (PCECC) for Segment Routing (SR) MPLS Segment Identifier (SID) Allocation and Distribution.

Abstract

The Path Computation Element (PCE) is a core component of Software-Defined Networking (SDN) systems.¶

A PCE-based Central Controller (PCECC) can simplify the processing of a distributed control plane by blending it with elements of SDN and without necessarily completely replacing it. Thus, the LSP can be calculated/set up/initiated and the label forwarding entries can also be downloaded through a centralized PCE server to each network device along the path while leveraging the existing PCE technologies as much as possible.¶

This document specifies the procedures and PCEP extensions when a PCE-based controller is also responsible for configuring the forwarding actions on the routers, in addition to computing the paths for packet flows in a segment routing (SR) network and telling the edge routers what instructions to attach to packets as they enter the network. PCECC as defined in RFC 9050 is further enhanced for SR-MPLS SID (Segment Identifier) allocation and distribution.¶

1. Introduction

The Path Computation Element (PCE) [RFC4655] was developed to offload the path computation function from routers in an MPLS traffic-engineered network. It can compute optimal paths for traffic across a network and can also update the paths to reflect changes in the network or traffic demands. Since then, the role and function of the PCE has grown to cover a number of other uses (such as GMPLS [RFC7025]) and to allow delegated control [RFC8231] and PCE-initiated use of network resources [RFC8281].¶

According to [RFC7399], Software-Defined Networking (SDN) refers to a separation between the control elements and the forwarding components so that software running in a centralized system, called a controller, can act to program the devices in the network to behave in specific ways. A required element in an SDN architecture is a component that plans how the network resources will be used and how the devices will be programmed. It is possible to view this component as performing specific computations to place traffic flows within the network given knowledge of the availability of network resources, how other forwarding devices are programmed, and the way that other flows are routed. This is the function and purpose of a PCE, and the way that a PCE integrates into a wider network control system (including an SDN system) is presented in [RFC7491].¶

In early PCE implementations, where the PCE was used to derive paths for MPLS Label Switched Paths (LSPs), paths were requested by network elements (known as Path Computation Clients (PCCs)), and the results of the path computations were supplied to network elements using the Path Computation Element Communication Protocol (PCEP) [RFC5440]. This protocol was later extended to allow a PCE to send unsolicited requests to the network for LSP establishment [RFC8281].¶

PCE was developed to derive paths for MPLS Label Switched Paths (LSPs), which are supplied to the head end of the LSP using the Path Computation Element Communication Protocol (PCEP). But SDN has a broader applicability than signaled (G)MPLS traffic-engineered (TE) networks, and the PCE may be used to determine paths in a range of use cases. PCEP has been proposed as a control protocol for use in these environments to allow the PCE to be fully enabled as a central controller.¶

[RFC8283] introduces the architecture for PCE as a central controller as an extension of the architecture described in [RFC4655] and assumes the continued use of PCEP as the protocol used between PCE and PCC. [RFC8283] further examines the motivations and applicability for PCEP as a Southbound Interface (SBI), and introduces the implications for the protocol. [I-D.ietf-teas-pcecc-use-cases] describes the use cases for the PCE-based Central Controller (PCECC) architecture. As described in [RFC8283], PCECC simplifies the processing of a distributed IGP based control plane by blending it with elements of SDN, without replacing it.¶

[RFC9050] specify the procedures and PCEP extensions for using the PCE as the central controller for static LSPs, where LSPs can be provisioned as explicit label instructions at each hop on the end-to-end path.¶

Segment Routing (SR) technology leverages the source routing and tunneling paradigms. A source node can choose a path without relying on hop-by-hop signaling protocols such as LDP or RSVP-TE. Each path is specified as a set of "segments" advertised by link-state routing protocols (IS-IS or OSPF). [RFC8402] provides an introduction to SR architecture. The corresponding IS-IS and OSPF extensions are specified in [RFC8667] and [RFC8665] , respectively. It relies on a series of forwarding instructions being placed in the header of a packet. The segment routing architecture supports operations that can be used to steer packet flows in a network, thus providing a form of traffic engineering. [RFC8664] specify the SR specific PCEP extensions.¶

Segment Routing Policy for Traffic Engineering [I-D.ietf-spring-segment-routing-policy] details the concepts of SR Policy and approaches to steering traffic into an SR Policy. An SR Policy contains one or more SR Policy Candidate Paths where one or more such paths can be computed via PCE. [I-D.ietf-pce-segment-routing-policy-cp] specifies PCEP extensions to signal additional information to map candidate paths to their SR policies.¶

PCECC may further use PCEP for SR SID (Segment Identifier) allocation and distribution to all the SR nodes with some benefits. The SR nodes continue to rely on IGP for distributed computation (nexthop selection, protection etc) where PCE (and PCEP) does only the allocation and distribution of SIDs in the network. Note that the topology at PCE is still learned via existing mechanisms.¶

This document specifies the procedures and PCEP extensions when a PCE-based controller is also responsible for configuring the forwarding actions on the routers (i.e. the SR SID allocation and distribution in this case), in addition to computing the SR paths for packet flows in a segment routing network and telling the edge routers what instructions to attach to packets as they enter the network as described in [RFC8283].¶

Only SR using MPLS dataplane (SR-MPLS) is in the scope of this document. Refer [I-D.dhody-pce-pcep-extension-pce-controller-srv6] for use of PCECC technique for SR in IPv6 (SRv6) dataplane.¶

2. Terminology

Terminologies used in this document is the same as described in the [RFC8283] and [I-D.ietf-teas-pcecc-use-cases].¶

6. PCEP Messages

As defined in [RFC5440], a PCEP message consists of a common header followed by a variable-length body made of a set of objects that can be either mandatory or optional. An object is said to be mandatory in a PCEP message when the object must be included for the message to be considered valid. For each PCEP message type, a set of rules is defined that specify the set of objects that the message can carry. An implementation MUST form the PCEP messages using the object ordering specified in this document.¶

Message formats in this section are presented using Routing Backus-Naur Format (RBNF) as specified in [RFC5511].¶

     <PCInitiate Message> ::= <Common Header>
                              <PCE-initiated-lsp-list>
  Where:
     <Common Header> is defined in [RFC5440]

     <PCE-initiated-lsp-list> ::= <PCE-initiated-lsp-request>
                                  [<PCE-initiated-lsp-list>]

     <PCE-initiated-lsp-request> ::=
                          (<PCE-initiated-lsp-instantiation>|
                           <PCE-initiated-lsp-deletion>|
                           <PCE-initiated-lsp-central-control>)

     <PCE-initiated-lsp-central-control> ::= <SRP>
                                             <LSP>
                                             (<cci-list>|
                                             (<FEC>
                                             <CCI>))

     <cci-list> ::=  <CCI>
                     [<cci-list>]

  Where:
      <PCE-initiated-lsp-instantiation> and
      <PCE-initiated-lsp-deletion> are as per
      [RFC8281].

     The LSP and SRP object is defined in [RFC8231].

      <PCRpt Message> ::= <Common Header>
                          <state-report-list>
   Where:

      <state-report-list> ::= <state-report>[<state-report-list>]

      <state-report> ::= (<lsp-state-report>|
                          <central-control-report>)

      <lsp-state-report> ::= [<SRP>]
                             <LSP>
                             <path>

      <central-control-report> ::= [<SRP>]
                                   <LSP>
                                   (<cci-list>|
                                   (<FEC>
                                   <CCI>))

      <cci-list> ::=  <CCI>
                      [<cci-list>]


    Where:
      <path> is as per [RFC8231] and the LSP and SRP object are
      also defined in [RFC8231].

7. PCEP Objects

    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=TBD        |            Length=4           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Flags                         |S|L|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            CC-ID                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      MT-ID    |    Algorithm  |    Flags      |B|P|G|C|N|E|V|L|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       SID/Label/Index                         |
+---------------------------------------------------------------+
|                                                               |
//                        Optional TLV                         //
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

9. Security Considerations

As per [RFC8283], the security considerations for a PCE-based controller is a little different from those for any other PCE system. That is, the operation relies heavily on the use and security of PCEP, so consideration should be given to the security features discussed in [RFC5440] and the additional mechanisms described in [RFC8253]. It further lists the vulnerability of a central controller architecture, such as a central point of failure, denial-of-service, and a focus for interception and modification of messages sent to individual NEs.¶

The PCECC extension builds on the existing PCEP messages and thus the security considerations described in [RFC5440], [RFC8231], [RFC8281], and [RFC9050] continue to apply.¶

As per [RFC8231], it is RECOMMENDED that these PCEP extensions only be activated on mutually-authenticated and encrypted sessions across PCEs and PCCs belonging to the same administrative authority, using Transport Layer Security (TLS) [RFC8253] as per the recommendations and best current practices in [RFC7525] (unless explicitly set aside in [RFC8253]).¶

10. Manageability Considerations

11. IANA Considerations

12. Acknowledgments

We would like to thank Robert Tao, Changjing Yan, Tieying Huang, Avantika, and Aijun Wang for their useful comments and suggestions.¶

Further thanks to Stephane Litkowski, Robert Sawaya, Zafar Ali, and Mike Koldychev for useful discussion and ideas to improve the document.¶

13. Normative References

[I-D.dhodylee-pce-pcep-ls]

Dhody, D., Peng, S., Lee, Y., Ceccarelli, D., Wang, A., Mishra, G., and S. Sivabalan, "PCEP extensions for Distribution of Link-State and TE Information", Work in Progress, Internet-Draft, draft-dhodylee-pce-pcep-ls-23, 5 March 2022, <https://datatracker.ietf.org/doc/html/draft-dhodylee-pce-pcep-ls-23>.

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.

[RFC3630]

Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering (TE) Extensions to OSPF Version 2", RFC 3630, DOI 10.17487/RFC3630, September 2003, <https://www.rfc-editor.org/info/rfc3630>.

[RFC4203]

Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005, <https://www.rfc-editor.org/info/rfc4203>.

[RFC4915]

Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P. Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF", RFC 4915, DOI 10.17487/RFC4915, June 2007, <https://www.rfc-editor.org/info/rfc4915>.

[RFC5305]

Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305, DOI 10.17487/RFC5305, October 2008, <https://www.rfc-editor.org/info/rfc5305>.

[RFC5307]

Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008, <https://www.rfc-editor.org/info/rfc5307>.

[RFC5440]

Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation Element (PCE) Communication Protocol (PCEP)", RFC 5440, DOI 10.17487/RFC5440, March 2009, <https://www.rfc-editor.org/info/rfc5440>.

[RFC7752]

Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and S. Ray, "North-Bound Distribution of Link-State and Traffic Engineering (TE) Information Using BGP", RFC 7752, DOI 10.17487/RFC7752, March 2016, <https://www.rfc-editor.org/info/rfc7752>.

[RFC7942]

Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", BCP 205, RFC 7942, DOI 10.17487/RFC7942, July 2016, <https://www.rfc-editor.org/info/rfc7942>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8231]

Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path Computation Element Communication Protocol (PCEP) Extensions for Stateful PCE", RFC 8231, DOI 10.17487/RFC8231, September 2017, <https://www.rfc-editor.org/info/rfc8231>.

[RFC8253]

Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody, "PCEPS: Usage of TLS to Provide a Secure Transport for the Path Computation Element Communication Protocol (PCEP)", RFC 8253, DOI 10.17487/RFC8253, October 2017, <https://www.rfc-editor.org/info/rfc8253>.

[RFC8281]

Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path Computation Element Communication Protocol (PCEP) Extensions for PCE-Initiated LSP Setup in a Stateful PCE Model", RFC 8281, DOI 10.17487/RFC8281, December 2017, <https://www.rfc-editor.org/info/rfc8281>.

[RFC8408]

Sivabalan, S., Tantsura, J., Minei, I., Varga, R., and J. Hardwick, "Conveying Path Setup Type in PCE Communication Protocol (PCEP) Messages", RFC 8408, DOI 10.17487/RFC8408, July 2018, <https://www.rfc-editor.org/info/rfc8408>.

[RFC8664]

Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., and J. Hardwick, "Path Computation Element Communication Protocol (PCEP) Extensions for Segment Routing", RFC 8664, DOI 10.17487/RFC8664, December 2019, <https://www.rfc-editor.org/info/rfc8664>.

[RFC9050]

Li, Z., Peng, S., Negi, M., Zhao, Q., and C. Zhou, "Path Computation Element Communication Protocol (PCEP) Procedures and Extensions for Using the PCE as a Central Controller (PCECC) of LSPs", RFC 9050, DOI 10.17487/RFC9050, July 2021, <https://www.rfc-editor.org/info/rfc9050>.

14. Informative References

[I-D.dhody-pce-pcep-extension-pce-controller-srv6]

Li, Z., Peng, S., Geng, X., and M. Negi, "Path Computation Element Communication Protocol (PCEP) Procedures and Extensions for Using the PCE as a Central Controller (PCECC) for SRv6 SID Allocation and Distribution.", Work in Progress, Internet-Draft, draft-dhody-pce-pcep-extension-pce-controller-srv6-07, 27 August 2021, <https://datatracker.ietf.org/doc/html/draft-dhody-pce-pcep-extension-pce-controller-srv6-07>.

[I-D.ietf-pce-binding-label-sid]

Sivabalan, S., Filsfils, C., Tantsura, J., Previdi, S., and C. L. (editor), "Carrying Binding Label/Segment Identifier (SID) in PCE-based Networks.", Work in Progress, Internet-Draft, draft-ietf-pce-binding-label-sid-14, 3 March 2022, <https://datatracker.ietf.org/doc/html/draft-ietf-pce-binding-label-sid-14>.

[I-D.ietf-pce-pcep-yang]

Dhody, D., Hardwick, J., Beeram, V. P., and J. Tantsura, "A YANG Data Model for Path Computation Element Communications Protocol (PCEP)", Work in Progress, Internet-Draft, draft-ietf-pce-pcep-yang-18, 25 January 2022, <https://datatracker.ietf.org/doc/html/draft-ietf-pce-pcep-yang-18>.

[I-D.ietf-pce-segment-routing-policy-cp]

Koldychev, M., Sivabalan, S., Barth, C., Peng, S., and H. Bidgoli, "PCEP extension to support Segment Routing Policy Candidate Paths", Work in Progress, Internet-Draft, draft-ietf-pce-segment-routing-policy-cp-06, 22 October 2021, <https://datatracker.ietf.org/doc/html/draft-ietf-pce-segment-routing-policy-cp-06>.

[I-D.ietf-pce-state-sync]

Litkowski, S., Sivabalan, S., Li, C., and H. Zheng, "Inter Stateful Path Computation Element (PCE) Communication Procedures.", Work in Progress, Internet-Draft, draft-ietf-pce-state-sync-01, 20 October 2021, <https://datatracker.ietf.org/doc/html/draft-ietf-pce-state-sync-01>.

[I-D.ietf-spring-segment-routing-policy]

Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and P. Mattes, "Segment Routing Policy Architecture", Work in Progress, Internet-Draft, draft-ietf-spring-segment-routing-policy-18, 17 February 2022, <https://datatracker.ietf.org/doc/html/draft-ietf-spring-segment-routing-policy-18>.

[I-D.ietf-teas-pcecc-use-cases]

Li, Z. (., Dhody, D., Zhao, Q., Ke, K., Khasanov, B., Fang, L., Zhou, C., Zhang, B., Rachitskiy, A., and A. Gulida, "The Use Cases for Path Computation Element (PCE) as a Central Controller (PCECC).", Work in Progress, Internet-Draft, draft-ietf-teas-pcecc-use-cases-08, 25 October 2021, <https://datatracker.ietf.org/doc/html/draft-ietf-teas-pcecc-use-cases-08>.

[I-D.li-pce-controlled-id-space]

Li, C., Chen, M., Wang, A., Cheng, W., and C. Zhou, "PCE Controlled ID Space", Work in Progress, Internet-Draft, draft-li-pce-controlled-id-space-10, 24 February 2022, <https://datatracker.ietf.org/doc/html/draft-li-pce-controlled-id-space-10>.

[RFC3031]

Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label Switching Architecture", RFC 3031, DOI 10.17487/RFC3031, January 2001, <https://www.rfc-editor.org/info/rfc3031>.

[RFC4655]

Farrel, A., Vasseur, J.-P., and J. Ash, "A Path Computation Element (PCE)-Based Architecture", RFC 4655, DOI 10.17487/RFC4655, August 2006, <https://www.rfc-editor.org/info/rfc4655>.

[RFC5511]

Farrel, A., "Routing Backus-Naur Form (RBNF): A Syntax Used to Form Encoding Rules in Various Routing Protocol Specifications", RFC 5511, DOI 10.17487/RFC5511, April 2009, <https://www.rfc-editor.org/info/rfc5511>.

[RFC7025]

Otani, T., Ogaki, K., Caviglia, D., Zhang, F., and C. Margaria, "Requirements for GMPLS Applications of PCE", RFC 7025, DOI 10.17487/RFC7025, September 2013, <https://www.rfc-editor.org/info/rfc7025>.

[RFC7399]

Farrel, A. and D. King, "Unanswered Questions in the Path Computation Element Architecture", RFC 7399, DOI 10.17487/RFC7399, October 2014, <https://www.rfc-editor.org/info/rfc7399>.

[RFC7420]

Koushik, A., Stephan, E., Zhao, Q., King, D., and J. Hardwick, "Path Computation Element Communication Protocol (PCEP) Management Information Base (MIB) Module", RFC 7420, DOI 10.17487/RFC7420, December 2014, <https://www.rfc-editor.org/info/rfc7420>.

[RFC7491]

King, D. and A. Farrel, "A PCE-Based Architecture for Application-Based Network Operations", RFC 7491, DOI 10.17487/RFC7491, March 2015, <https://www.rfc-editor.org/info/rfc7491>.

[RFC7525]

Sheffer, Y., Holz, R., and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015, <https://www.rfc-editor.org/info/rfc7525>.

[RFC8126]

Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, <https://www.rfc-editor.org/info/rfc8126>.

[RFC8232]

Crabbe, E., Minei, I., Medved, J., Varga, R., Zhang, X., and D. Dhody, "Optimizations of Label Switched Path State Synchronization Procedures for a Stateful PCE", RFC 8232, DOI 10.17487/RFC8232, September 2017, <https://www.rfc-editor.org/info/rfc8232>.

[RFC8283]

Farrel, A., Ed., Zhao, Q., Ed., Li, Z., and C. Zhou, "An Architecture for Use of PCE and the PCE Communication Protocol (PCEP) in a Network with Central Control", RFC 8283, DOI 10.17487/RFC8283, December 2017, <https://www.rfc-editor.org/info/rfc8283>.

[RFC8402]

Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, July 2018, <https://www.rfc-editor.org/info/rfc8402>.

[RFC8660]

Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing with the MPLS Data Plane", RFC 8660, DOI 10.17487/RFC8660, December 2019, <https://www.rfc-editor.org/info/rfc8660>.

[RFC8665]

Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler, H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF Extensions for Segment Routing", RFC 8665, DOI 10.17487/RFC8665, December 2019, <https://www.rfc-editor.org/info/rfc8665>.

[RFC8667]

Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C., Bashandy, A., Gredler, H., and B. Decraene, "IS-IS Extensions for Segment Routing", RFC 8667, DOI 10.17487/RFC8667, December 2019, <https://www.rfc-editor.org/info/rfc8667>.

Appendix A. Contributor Addresses

Dhruv Dhody
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore, Karnataka  560066
India

EMail: dhruv.ietf@gmail.com

Satish Karunanithi
Huawei Technologies
Divyashree Techno Park, Whitefield
Bangalore, Karnataka  560066
India

EMail: satishk@huawei.com

Adrian Farrel
Old Dog Consulting
UK

EMail: adrian@olddog.co.uk

Xuesong Geng
Huawei Technologies
China

Email: gengxuesong@huawei.com

Udayasree Palle

EMail: udayasreereddy@gmail.com

Katherine Zhao
Huawei Technologies
2330 Central Expressway
Santa Clara, CA  95050
USA

EMail: katherine.zhao@huawei.com

Boris Zhang
Telus Ltd.
Toronto
Canada

EMail: boris.zhang@telus.com

Alex Tokar
Cisco Systems
Slovak Republic

EMail: atokar@cisco.com