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Versions: 00

PCE WG                                                        Quan Xiong
Internet-Draft                                                Fangwei Hu
Intended status: Standards Track                             Greg Mirsky
Expires: June 13, 2019                                   ZTE Corporation
                                                          Weiqiang Cheng
                                                            China Mobile
                                                            Dec 10, 2018


                Stateful PCE for SR-MPLS-TP Inter-domain
            draft-xiong-pce-stateful-pce-sr-inter-domain-00

Abstract

   This document proposes two solutions to perform the Segment routing
   transport Profile with MPLS data plane(SR-MPLS-TP) inter-domain path
   computation and initiation with stateful PCEs including stitching
   Association Group and stitching labels.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on June 13, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  The SR-MPLS-TP Inter-domain with PCE  . . . . . . . . . . . .   3
     2.1.  The Stitching LSP Association Solution  . . . . . . . . .   5
     2.2.  The Stitching Label Solution  . . . . . . . . . . . . . .   6
   3.  Inter-domain Path Segment Allocation  . . . . . . . . . . . .   6
     3.1.  PCC Allocated . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  PCE Allocated . . . . . . . . . . . . . . . . . . . . . .   7
   4.  PCEP Procedure  . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  HPCE-initiated LSP  . . . . . . . . . . . . . . . . . . .   7
     4.2.  PCC-initiated LSP . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Informative References  . . . . . . . . . . . . . . . . .   9
     8.2.  Normative References  . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The Path Computation Element (PCE) architecture is defined in
   [RFC4655] for MPLS Traffic Engineering (MPLS-TE) and Generalized MPLS
   (GMPLS) networks.  The Path Computation Element Communication
   Protocol (PCEP) defined in [RFC5440] provides mechanisms for PCEs to
   perform path computations in response to Path Computation Clients
   (PCCs) requests.

   [I-D.ietf-pce-segment-routing] proposes extensions to PCEP that allow
   a stateful PCE to compute TE paths in segment routing (SR) networks.
   As defined in [I-D.cheng-spring-mpls-path-segment], a path segment is
   used to identify a SR path and support bidirectional SR paths
   correlation.  [I-D.xiong-pce-pcep-extension-sr-tp] proposed a
   mechanism to create the bidirectional SR tunnel in a single SR
   domain.  [I-D.hu-mpls-sr-inter-domain-use-cases] defined a inter-
   domain path segment and related inter-domain use cases for Segment
   Routing in MPLS Transport Profile (SR-MPLS-TP) networks.  It is
   required to perform the SR inter-domain path computation and
   initiation with PCE deployment.

   The path computation requirments for Label Switched Paths (LSPs)
   across multiple domains are discussed in [RFC4105] and [RFC4216].



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   Inter-domain path computation can be performed by a single stateful
   PCE and multiple stateful PCEs.  The PCE may has no ability to
   collect the topologies all over the domains.  So the single PCE model
   is not applied in deployment.  Three multiple PCEs models can be uesd
   to perform PCE-based inter-domain path computation including Per-
   Domain Path Computation [RFC5152], Backward-Recursive PCE-Based
   Computation (BRPC) [RFC5441] and Hierarchical PCE (H-PCE) [RFC6805].
   Computing the optimum inter-domain path requires co-operation between
   multiple PCEs.  But the sequence of domains need to be known before
   the path computation in BRPC mechanism.  Stateful H-PCE architecture
   is appropriate to compute an optimal end-to-end path across multiple
   domains.

   As defined in [I-D.hu-mpls-sr-inter-domain-use-cases], two SR-MPLS-TP
   inter-domain models includes stitching and nesting inter-domain
   models.  This document proposes two solutions to perform the SR-MPLS-
   TP inter-domain path computation and initiation with stateful PCEs
   including stitching LSPs Association and stitching labels.

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].

1.2.  Terminology

   The terminology is defined as [RFC5440],
   [I-D.ietf-pce-segment-routing] ,
   [I-D.cheng-spring-mpls-path-segment].

2.  The SR-MPLS-TP Inter-domain with PCE

   The SR-MPLS-TP stitching inter-domain scenario is described in
   [I-D.hu-mpls-sr-inter-domain-use-cases].  The SR nodes belong to
   three Autonomous Systems (AS) including SR-AS1, SR-AS2 and SR-AS3
   which connecting with logical links or border nodes as the Figure 1
   shown.  The SR inter-domain LSP needs to be provided along the inter-
   AS paths.  The Path 1~5 are forwarding path segments and Path 1'~5'
   are the related reverse path segments and these are all inter-domain
   path segments.










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                                +-------+
             +------------------+ H-PCE +-------------------+
             |                  +---+---+                   |
             |                      |                       |
             v                      v                       v
          +--+--+                +--+--+                 +--+--+
          |PCE-1|                |PCE-2|                 |PCE-3|
          +--+--+                +--+--+                 +--+--+
             |                      |                       |
             v                      v                       v
 SR Border Node Network:
       ..................   .................   ....................
       .                .   .               .   .                  .
   +-----+             +-----+             +-----+              +-----+
   |  A  |             |  X  |             |  Y  |              |  Z  |
   +-----+             +-----+             +-----+              +-----+
       .    SR-AS1      .   .   SR-AS2      .   .    SR-AS3        .
       ..................   .................   ....................

    Forwarding Path Segments:
       |------Path1------->|-----Path2------->|--------Path3------>|
    Reverse Path Segments:
       |<-----Path1'-------|<----Path2'-------|<--------Path3'-----|

 SR Border Link Network:
    ....................   ....................    .....................
    .                  .   .                  .    .                   .
    . +---+      +---+ .   . +---+     +---+  .    . +---+     +----+  .
    . | A |------| B |-------| C |-----| X |---------| Y |-----| Z  |  .
    . +---+      +---+ .   . +---+     +---+  .    . +---+     +----+  .
    .    SR-AS1        .   .      SR-AS2      .    .     SR-AS3        .
    ....................   ....................    .....................

    Forwarding Path Segments:
    |----Path1---->|-Path2-->|----Path3--->|-Path4-->|-----Path5------>|
    Reverse Path Segments:
    |<---Path1'----|<-Path2'-|<---Path3'---|<-Path4'-|<----Path5'------|

              Figure 1 The SR Stitching Inter-Domain with H-PCE


   The hierarchical PCE architecture is described in [RFC6805], a parent
   PCE maintains a domain topology map that contains the child domains
   (seen as vertices in the topology) and their interconnections (links
   in the topology) but no information about the content of the child
   domains.  Each child domain has one PCE taking in charge of computing
   paths across its own domain.  These PCEs are known as child PCEs and
   have a relationship with the parent PCE.  As the Figure 1 shown,



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   H-PCE is parent PCE and PCE-1, PCE-2 and PCE-3 are child PCEs which
   is responsible for each own SR-AS.

   When an optimal inter-domain path is required, the ingress PCE sends
   a request to the parent PCE or the stateful parent PCE itself to
   initiate the path computation.  The parent PCE selects a set of
   candidate domain paths based on the domain topology and the state of
   the inter-domain links.  It then sends computation requests to the
   child PCEs responsible for each of the domains on the candidate
   domain paths.  The stateful child PCE in each domain performs active
   stateful procedure as defined [RFC8231].

2.1.  The Stitching LSP Association Solution

   The LSPs of multiple domains can be stitched together by adding them
   to a stitching LSP association group as defined in [I-D.hu-pce-
   stitching-lsp-association].  As the Figure 2 shown, the stateful
   H-PCE sends the PCInit message defined in [RFC8281] to initiate the
   inter-domain path computation adding the forwarding LSP 1~3 to
   Assoc#1 and reverse LSP 1'~3' to Assoc#2.  The child PCEs may
   initiate the intra-domain LSPs when receiving the message from parent
   PCE.


                                  +-------+
               +------------------+ H-PCE +-----------------+
      PCInit   |                  +---+---+                 |
(LSP1,Assoc#1) |  PCInit(LSP2,Assoc#1)| PCInit(LSP3,Assoc#1)|
      PCInit   | PCInit(LSP2',Assoc#2 |PCInit(LSP3',Assoc#2 |
(LSP1',Assoc#2)|                      |                     |
               v                      v                     v
            +-----+                +-----+               +-----+
            |PCE-1|                |PCE-2|               |PCE-3|
            +-----+                +-----+               +-----+
      PCInit/   \PCInit       PCInit/  \PCInit       PCInit/ \PCInit
       LSP1/     \LSP1'        LSP2/    \LSP2'        LSP3/   \LSP3'
   Assoc#1/       \Assoc#2 Assoc#1/      \Assoc#2 Assoc#1/     \Assoc#2
         v         v            v         v            v        v
   +-----+   LSP1  +-----------+   LSP2   +-----------+  LSP3   +-----+
   |  A  |-------->|     X     |--------->|     Y     |-------->|  Z  |
   |     |<--------|           |<---------|           |<--------|     |
   +-----+  LSP1'  +-----------|   LSP2'  +-----------+  LSP3'  +-----+

                   Figure 2 The SR inter-domain Stitching LSP Association







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2.2.  The Stitching Label Solution

   This section defined the inter-domain path segments as stitching
   Labels which used to stitch per-domain LSP tunnels in order to form
   inter-domain path that cross multiple domains.

   SR intra-domain path is setup as part of inter-domain SR path.  When
   PCC requests the PCE or the PCE itself to initiate The SR path, the
   inter-domain path segments should be carried as a stitching Label
   with the associated link.


                                   +-------+
                +------------------+ H-PCE +-----------------+
         PCInit |                  +---+---+                 |
    (LSP1,LSP1')|    PCInit(LSP2,LSP2')|   PCInit(LSP3,LSP3')|
       SL1,SL1' |    SL1,SL1',SL2,SL2' |            SL2,SL2' |
                v                      v                     v
    ....................   ....................    .....................
    .                  .   .                  .    .                   .
    . +---+ LSP1 +---+ .   . +---+ LSP2+---+  .    . +---+ LSP3+----+  .
    . | A |----->| B |--SL1->| C |---->| X |---SL2-->| Y |---->| Z  |  .
    . |   |<-----|   |<-SL1'-|   |<----|   |<--SL2'--|   |<----|    |  .
    . +---+ LSP1'+---+ .   . +---+LSP2'+---+  .    . +---+LSP3'+----+  .
    .    SR-AS1        .   .      SR-AS2      .    .     SR-AS3        .
    ....................   ....................    .....................

    SL:Stiching Label

              Figure 3 The SR Inter-Domain Stitching Label


3.  Inter-domain Path Segment Allocation

   The inter-domain path segment may be allocated by PCC or PCE.  The
   PCE may be the single domain PCE which taking in charge of the
   respective domain.  The inter-domain path segments is a unique value
   in the domain which PCC or PCE belongs to.  The mechanism of path
   segment request and reply may be the same with that in single domain
   as defined in [I-D.xiong-pce-pcep-extension-sr-tp].

3.1.  PCC Allocated

   As defined in [I-D.hu-mpls-sr-inter-domain-use-cases], an inter-
   domain path segment can be allocated by egress PCC and may be
   maintained on the PCC itself.  The inter-domain path segment connects
   two domains and the ingress and egress PCC are belong to different




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   domains.  The ingress and egress PCC need to exchange messages which
   carrying path segment information between the two PCEs.

   The Ingress PCC may request to allocate a path segment from egress
   PCC.  Once egress PCC allocated the inter-domain path segment, it
   need to inform the PCE in respective domain with the PCRpt message.
   The PCE need to communicate with the PCE which the ingress PCC
   belongs to inform the value allocated.

3.2.  PCE Allocated

   The ingress PCC may request the inter-domain path segment to be
   allocated by the PCE in PCC-Initiated LSP.  The PCE may allocate the
   inter-domain path segment on its own domain in PCEs-Initiated LSP.
   The allocated path segment needs to be informed to the ingress and
   egress PCC.

   The inter-domain path segments may be allocated separately by the
   PCEs which control the ingress and egress PCC along with the LSP
   initiation.

4.  PCEP Procedure

   [RFC8281] describes setup, maintenance and teardown of PCE-initiated
   LSPs under the stateful PCE model, without the need for local
   configuration on the PCC.  Similar to LSP updation, the inter-domain
   LSP can be initiated by the ingress PCE using the PCInitiate message
   to the ingress LSR.  The inter-domain path segment is viewed as
   stitching label.  Per-domain LSP may also be initiated by respective
   domain's PCE and stitched together.

4.1.  HPCE-initiated LSP

   In H-PCE [RFC6805] architecture, the parent PCE is used to compute a
   multi-domain path based on the domain connectivity information.  The
   stateful H-PCE in active model can be used to initiate the inter-
   domain bidirectional path for SR networks.  PCE sends PCInitiate
   message to its domain SR nodes with ERO={SID LIST} and carrying
   stitching association group TLV and path segments.  If the SR nodes
   is the border nodes of the SR domain, it correlates the two path
   segments and the related SID list if the related association ID is
   the same value.

   The PECP procedure for the HPCE-initiated LSP is following:

   The stateful H-PCE initiates the end-to-end path computation across
   multiple domains and selects a set of candidate domain paths based on
   the topology.



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   The stateful H-PCE sends PCInitiate message to every PCEs which the
   end-to-end path traversed, carrying inter-domain path segments
   allocated by H-PCE, stitching LSP association group and the SID list
   in the ERO object.

   The stateful child PCE in each domain perform active stateful
   procedure as defined in [I-D.xiong-pce-pcep-extension-sr-tp].

4.2.  PCC-initiated LSP

   In case of passive path computation request to the ingress PCE from
   the ingress LSR, the H-PCE path computation procedure is applied to
   compute sequence of domains or end-to-end path by using PCReq and
   PCRep messages among stateful PCEs in passive mode.

   In case of delegation to the ingress PCE (active stateful PCE), the
   ingress child PCE may further delegate to parent PCE as per
   [I-D.ietf-pce-stateful-hpce].  The parent PCE could update the path
   of the inter-domain LSP.

   The ingress nodes of the source AS sends the PCReq message to its
   PCE, then the PCE sends PCReq message to the H-PCE or stateful PCEs
   in other domains.  The PECP procedure for the PCC-initiated LSP in
   H-PCE model is as follow.

   The ingress PCC from the ingress domain sends a PCReq request to the
   PCE which is responsible for the domain containing the destination
   information.

   The ingress PCE sends the path computation request direct to the
   parent PCE.

   The parent PCE computes the optimal end-to-end path and initiates the
   inter-domain paths to the child PCEs which the path traversed.

   Each PCE sends PCInitiate message to ingress or egress nodes of its
   domain to initiate the LSPs.

5.  Security Considerations

   TBD.

6.  IANA Considerations

   TBD.






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7.  Acknowledgements

   TBD.

8.  References

8.1.  Informative References

   [RFC6805]  King, D., Ed. and A. Farrel, Ed., "The Application of the
              Path Computation Element Architecture to the Determination
              of a Sequence of Domains in MPLS and GMPLS", RFC 6805,
              DOI 10.17487/RFC6805, November 2012,
              <https://www.rfc-editor.org/info/rfc6805>.

8.2.  Normative References

   [I-D.cheng-spring-mpls-path-segment]
              Cheng, W., Wang, L., Li, H., Chen, M., Gandhi, R., Zigler,
              R., and S. Zhan, "Path Segment in MPLS Based Segment
              Routing Network", draft-cheng-spring-mpls-path-segment-03
              (work in progress), October 2018.

   [I-D.hu-mpls-sr-inter-domain-use-cases]
              hu, f., Xiong, Q., Mirsky, G., and W. Cheng, "Segment
              Routing in MPLS-TP Inter-domain Use Cases", draft-hu-mpls-
              sr-inter-domain-use-cases-00 (work in progress), December
              2018.

   [I-D.ietf-pce-association-bidir]
              Barth, C., Gandhi, R., and B. Wen, "PCEP Extensions for
              Associated Bidirectional Label Switched Paths (LSPs)",
              draft-ietf-pce-association-bidir-02 (work in progress),
              November 2018.

   [I-D.ietf-pce-segment-routing]
              Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "PCEP Extensions for Segment Routing",
              draft-ietf-pce-segment-routing-14 (work in progress),
              October 2018.

   [I-D.ietf-pce-stateful-hpce]
              Dhody, D., Lee, Y., Ceccarelli, D., Shin, J., King, D.,
              and O. Dios, "Hierarchical Stateful Path Computation
              Element (PCE).", draft-ietf-pce-stateful-hpce-06 (work in
              progress), October 2018.






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   [I-D.xiong-pce-pcep-extension-sr-tp]
              Xiong, Q., hu, f., and S. Zhan, "PCEP extensions for SR
              MPLS-TP", draft-xiong-pce-pcep-extension-sr-tp-02 (work in
              progress), October 2018.

   [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>.

   [RFC4105]  Le Roux, J., Ed., Vasseur, J., Ed., and J. Boyle, Ed.,
              "Requirements for Inter-Area MPLS Traffic Engineering",
              RFC 4105, DOI 10.17487/RFC4105, June 2005,
              <https://www.rfc-editor.org/info/rfc4105>.

   [RFC4216]  Zhang, R., Ed. and J. Vasseur, Ed., "MPLS Inter-Autonomous
              System (AS) Traffic Engineering (TE) Requirements",
              RFC 4216, DOI 10.17487/RFC4216, November 2005,
              <https://www.rfc-editor.org/info/rfc4216>.

   [RFC4655]  Farrel, A., Vasseur, J., 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>.

   [RFC5152]  Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang, "A
              Per-Domain Path Computation Method for Establishing Inter-
              Domain Traffic Engineering (TE) Label Switched Paths
              (LSPs)", RFC 5152, DOI 10.17487/RFC5152, February 2008,
              <https://www.rfc-editor.org/info/rfc5152>.

   [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>.

   [RFC5441]  Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux,
              "A Backward-Recursive PCE-Based Computation (BRPC)
              Procedure to Compute Shortest Constrained Inter-Domain
              Traffic Engineering Label Switched Paths", RFC 5441,
              DOI 10.17487/RFC5441, April 2009,
              <https://www.rfc-editor.org/info/rfc5441>.

   [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>.



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   [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>.

Authors' Addresses

   Quan Xiong
   ZTE Corporation
   No.6 Huashi Park Rd
   Wuhan, Hubei  430223
   China

   Phone: +86 27 83531060
   Email: xiong.quan@zte.com.cn


   Fangwei Hu
   ZTE Corporation
   No.889 Bibo Rd
   Shanghai  201203
   China

   Phone: +86 21 68896273
   Email: hu.fangwei@zte.com.cn


   Greg Mirsky
   ZTE Corporation
   USA

   Email: gregimirsky@gmail.com


   Weiqiang Cheng
   China Mobile
   Beijing
   China

   Email: chengweiqiang@chinamobile.com










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