Stateful PCE for SR-MPLS Inter-domain
draft-xiong-pce-stateful-pce-sr-inter-domain-02

Versions: 00 01 02                                                      
PCE WG                                                        Quan Xiong
Internet-Draft                                               Greg Mirsky
Intended status: Standards Track                         ZTE Corporation
Expires: April 24, 2020                                       Fangwei Hu
                                                              Individual
                                                          Weiqiang Cheng
                                                            China Mobile
                                                        October 22, 2019


                 Stateful PCE for SR-MPLS Inter-domain
            draft-xiong-pce-stateful-pce-sr-inter-domain-02

Abstract

   This document proposes a solution to perform the Segment Routing with
   MPLS data plane (SR-MPLS) inter-domain path computation and
   initiation with stateful PCEs and the use of Path Segments.

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  The SR-MPLS Inter-domain with Path Segments . . . . . . . . .   3
   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.ietf-spring-mpls-path-segment], a path segment is
   used to identify a SR path and support bidirectional SR paths
   correlation.  [I-D.ietf-pce-sr-path-segment] proposed the extension
   for PCEP to operate with Path Segment.  [I-D.li-pce-sr-bidir-path]
   proposed the extension for PCEP to group two unidirectional SR Paths
   into an Associated Bidirectional SR Path.
   [I-D.xiong-spring-path-segment-sr-inter-domain] proposes the use of
   Path Segment in inter-domain scenarios for SR-MPLS network.  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].
   Inter-domain path computation can be performed by a single stateful



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   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.xiong-spring-path-segment-sr-inter-domain], the
   SR-MPLS inter-domain models includes stitching and nesting inter-
   domain models between inter-Area or inter-AS domains.  This document
   proposes a solution to perform the SR-MPLS inter-domain path
   computation and initiation with stateful PCEs and the use of Path
   Segments.

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.ietf-spring-mpls-path-segment].

2.  The SR-MPLS Inter-domain with Path Segments

   The SR-MPLS inter-domain scenario is described in
   [I-D.xiong-spring-path-segment-sr-inter-domain].  The domains of the
   networks may be IGP Areas or ASes and the inter- domain scenario may
   be inter-Area or inter-AS.  The multiple SR-MPLS domains may be
   interconnect with a ABR within areas or inter-link between ASes.  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, H-PCE is parent PCE and PCE-1, PCE-2 and PCE-3
   are child PCEs which is responsible for each own domain.  SR-AS1, SR-
   AS2 and SR-AS3 interconnect with logical links and SR-Area1, SR-Area2



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   and SR-Area3 interconnect within border nodes.  The SR end-to-end
   bidirectional LSP needs to be provided along the multi-domain paths.
   The Path 1~5 are forward path segments and Path 1'~5' are the related
   reverse path segments and these are all inter-domain path segments.

   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].






































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                               +-------+
            +------------------+ H-PCE +-------------------+
            |                  +---+---+                   |
            |                      |                       |
            v                      v                       v
         +--+--+                +--+--+                 +--+--+
         |PCE-1|                |PCE-2|                 |PCE-3|
         +--+--+                +--+--+                 +--+--+
            |                      |                       |
            v                      v                       v
SR Inter-Area:
      ..................   .................   ....................
      .                .   .               .   .                  .
  +-----+             +-----+             +-----+              +-----+
  |  A  |             |  X  |             |  Y  |              |  Z  |
  +-----+             +-----+             +-----+              +-----+
      .    SR-Area1      .   .   SR-Area2      .   .    SR-Area3        .
      ..................   .................   ....................

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

SR Inter-AS:
   ....................   ....................    .....................
   .                  .   .                  .    .                   .
   . +---+      +---+ .   . +---+     +---+  .    . +---+     +----+  .
   . | A |------| B |-------| C |-----| X |---------| Y |-----| Z  |  .
   . +---+      +---+ .   . +---+     +---+  .    . +---+     +----+  .
   .    SR-AS1        .   .      SR-AS2      .    .     SR-AS3        .
   ....................   ....................    .....................

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

             Figure 1 The SR Inter-Domain with H-PCE


   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 forward 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



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   PCE.  The LSP 1~3 could be stitched to a forward end-to-end LSP and
   the LSP 1'~3' could be stitched to a reverse end-to-end LSP.
   Furthermore, the two unidirectional end-to-end LSPs MAY be bound to a
   bidirectional end-to-end LSP as decribed in
   [I-D.li-pce-sr-bidir-path].


                                  +-------+
               +------------------+ 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'  +-----+
   |----------------------Forward End-to-end LSP--------------------->|
   |<---------------------Reverse End-to-end LSP----------------------|

                   Figure 2 The SR inter-domain Stitching LSP Association


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 operation of path
   segment request and reply may be the same with that in single domain
   as defined in [I-D.ietf-pce-sr-path-segment].

3.1.  PCC Allocated

   As defined in [I-D.xiong-spring-path-segment-sr-inter-domain], 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
   domains.  The ingress and egress PCC need to exchange messages which
   carrying path segment information between the two PCEs.



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   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.  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.

   The stateful H-PCE sends PCInitiate message to every PCEs which the
   end-to-end path traversed, carrying inter-domain path segments




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   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.ietf-pce-sr-path-segment].

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.hu-pce-stitching-lsp-association]
              Xiong, Q., Mirsky, G., hu, f., and W. Cheng, "Stitching
              LSP Association", draft-hu-pce-stitching-lsp-
              association-01 (work in progress), July 2019.

   [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-16 (work in progress),
              March 2019.

   [I-D.ietf-pce-sr-path-segment]
              Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong,
              "Path Computation Element Communication Protocol (PCEP)
              Extension for Path Segment in Segment Routing (SR)",
              draft-ietf-pce-sr-path-segment-00 (work in progress),
              October 2019.

   [I-D.ietf-pce-stateful-hpce]
              Dhody, D., Lee, Y., Ceccarelli, D., Shin, J., and D. King,
              "Hierarchical Stateful Path Computation Element (PCE)",
              draft-ietf-pce-stateful-hpce-15 (work in progress),
              October 2019.

   [I-D.ietf-spring-mpls-path-segment]
              Cheng, W., Li, H., Chen, M., Gandhi, R., and R. Zigler,
              "Path Segment in MPLS Based Segment Routing Network",
              draft-ietf-spring-mpls-path-segment-01 (work in progress),
              September 2019.






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   [I-D.li-pce-sr-bidir-path]
              Li, C., Chen, M., Cheng, W., Li, Z., Dong, J., Gandhi, R.,
              and Q. Xiong, "PCEP Extensions for Associated
              Bidirectional Segment Routing (SR) Paths", draft-li-pce-
              sr-bidir-path-06 (work in progress), August 2019.

   [I-D.xiong-spring-path-segment-sr-inter-domain]
              Xiong, Q., Mirsky, G., and W. Cheng, "The Use of Path
              Segment in SR Inter-domain Scenarios", draft-xiong-spring-
              path-segment-sr-inter-domain-00 (work in progress), July
              2019.

   [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>.









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   [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>.

   [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


   Greg Mirsky
   ZTE Corporation
   USA

   Email: gregimirsky@gmail.com


   Fangwei Hu
   Individual
   China

   Email: hufwei@gmail.com








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   Weiqiang Cheng
   China Mobile
   Beijing
   China

   Email: chengweiqiang@chinamobile.com













































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