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MPLS                                                          Quan Xiong
Internet-Draft                                               Greg Mirsky
Intended status: Standards Track                         ZTE Corporation
Expires: April 19, 2020                                   Weiqiang Cheng
                                                            China Mobile
                                                        October 17, 2019


        The Use of Path Segment in SR-MPLS and MPLS Interworking
         draft-xiong-mpls-path-segment-sr-mpls-interworking-01

Abstract

   This document discusses the SR-MPLS and MPLS interworking scenarios
   and proposes the solution with the use of path segments.

Status of This Memo

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   This Internet-Draft will expire on April 19, 2020.

Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   3.  SR-MPLS Interworking with MPLS  . . . . . . . . . . . . . . .   4
     3.1.  Stitching Model with Path Segment . . . . . . . . . . . .   5
     3.2.  Nesting Model with Path Segment . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Segment Routing (SR) leverages the source routing paradigm.  A node
   steers a packet through an SR Policy instantiated as an ordered list
   of instructions called "segments".  SR supports a per-flow explicit
   routing while maintaining per-flow state only at the ingress nodes of
   the SR domain.  Segment Routing can be instantiated on MPLS data
   plane which is referred to as SR-MPLS
   [I-D.ietf-spring-segment-routing-mpls].  SR-MPLS leverages the MPLS
   label stack to construct the SR path.

   In some scenarios, for example, a mobile backhaul transport network,
   it is required to provide end-to-end bidirectional tunnel across
   multiple domains.  IP/MPLS technology can be deployed in these
   domains, which may serve as an access, aggregation, or core network.
   Further, using SR architecture, the IP/MPLS network may be upgraded
   to support the SR-MPLS technology.  As such transformation is
   performed incrementally, by one domain at the time, operators are
   faced with a requirement to support the interworking between MPLS and
   SR-MPLS networks at the boundaries to provide the end-to-end
   bidirectional service.  [I-D.ietf-spring-mpls-path-segment] defined a
   path segment identifier to support SR path PM, end-to-end 1+1 SR path
   protection and bidirectional SR paths correlation.

   As defined in [RFC8402], the headend of an SR Policy binds a Bingding
   Segment ID (BSID) to a policy.  The BSID could be bound to a SID List
   or selected path and used to stitch the service across multiple
   domains.  For example, as discussed in Section 3
   [I-D.ietf-spring-mpls-path-segment], the BSID can be used to identify
   a sub-path and stitched them into an end-to-end SR path in the
   nesting model.  The BSID and path segment can be combined to achieve
   the inter-domain path monitoring.  But the solution is not
   appropriate for the stitching model.  The policy MUST be instantiated



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   before the end-to-end service and it can not deploy domains
   incrementally.  Moreover, all of the BSIDs MUST be pushed onto the
   label stack at the headend but not all of them are popped at an edge
   node.  The edge node pops one BSID that is bound to a SID List.  That
   cannot meet the independence requirement in the stitching model
   especially when the domains belong to different operators.

   This document discusses the interworking scenarios between SR-MPLS
   and MPLS networks and proposes the solution with the use of path
   segments.  The stitching and correlation of Path Segments are
   proposed to realize the inter-domain stitching and path monitoring.

2.  Conventions used in this document

2.1.  Terminology

   ABR: Area Border Routers.  Routers used to connect two IGP areas
   (areas in OSPF or levels in IS-IS).

   AS: Autonomous System.  An Autonomous System is composed by one or
   more IGP areas.

   ASBR: Autonomous System Border Router.  A router used to connect
   together ASes of the same or different service providers via one or
   more inter-AS links.

   Border Node: An ABR that interconnects two or more IGP areas.

   Border Link: Two ASes are interconnected with ASBRs.

   BSID: Binding Segment ID.

   Domains: Autonomous System (AS) or IGP Area.  An Autonomous System is
   composed of one or more IGP areas.

   e-Path: End-to-end Path segment.

   IGP: Interior Gateway Protocol.

   i-Path/i-PSID: Inter-domain Path Segment.

   PM: Performance Measurement.

   SR: Segment Routing.

   SR-MPLS: Segment Routing with MPLS data plane.

   s-Path: Sub-path Path Segment.



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   VPN: Virtual Private Network.

2.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  SR-MPLS Interworking with MPLS

   It is required to establish the end-to-end VPN service across the
   access network, aggregation network, and core network.  For example,
   SR-MPLS may be deployed in access and core network, and MPLS may be
   deployed in the aggregation network.  The network interworking should
   be taken into account in deployment are the following:

   o  Border Node or Border Link

   o  Stitching Model or Nesting Model

   o  End-to-End OAM or Per-domain OAM

   The domains of the networks may be IGP Areas or ASes.  The SR-MPLS
   and MPLS networks can be interconnected with a border node between
   IGP areas or border links between ASes.  This document takes IGP
   Areas domains for example.  MPLS domain can be deployed between two
   SR-MPLS domains, as Figure 1 shows.  The packets being transmitted
   along the SR path in SR-MPLS domains by using the SID list at the
   ingress node.  And the path in MPLS domains can be pre-configuration
   either via NMS or via the MPLS control plane signaling.



















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                  B                     E                      X
               +     +              .       .               +      +
            +           +         .           .          +            +
         +                 +    .               .     +                  +
      A        SR-MPLS       C        MPLS         G       SR-MPLS          Z
         +      IGP 1      +    .     IGP 2     .     +      IGP 3       +
            +           +         .           .          +            +
               +     +              .      .                +      +
                  D                     F                       Y

         |<---Access Network--->|<-Aggregation Network->|<----Core Network---->|



             Figure 1: SR-MPLS and MPLS interworking Scenario

   The VPN service across the SR-MPLS and MPLS domains is an end-to-end
   bidirectional path.  In the SR-MPLS network, a Path Segment uniquely
   identifies an SR path and can be used for the bidirectional path.
   This document proposed the solution with path segment used in the
   interworking scenario including the stitching and nesting models.

3.1.  Stitching Model with Path Segment

   It is a common requirement that SR-MPLS needs to interwork with MPLS
   when SR is incrementally deployed in the MPLS domain.  Figure 2 shows
   the stitching model of SR-MPLS inter-working with MPLS.

   The end-to-end bidirectional path across the SR-MPLS and MPLS network
   is split into multiple segments.  And each segment can be identified
   by an inter-domain path segment (i-Path or i-PSID) as defined in
   [I-D.xiong-spring-path-segment-sr-inter-domain].  The correlation of
   path segments can stitch the inter-domain paths and bind
   unidirectional paths.  The i-Paths are valid in the corresponding
   domain and the border nodes maintain the forwarding entries of that
   i-Path segment that maps to the next i-Path and the related labels,
   e.g, SID list or MPLS labels.  In the headend node, the i-Path can
   correlate the inter-domain path of reverse direction and bind the two
   unidirectional paths.  The interworking with path segments can
   support the inter-domain stitching and path monitoring.











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         +-----------------+  ................  +-----------------+
         |        +---+    |  .    +---+     .  |    +---+        |
         |        | B |    |  .    | E |     .  |    | X |        |
         |        +---+    |  .    +---+     .  |    +---+        |
         |      /       \  |  .  /       \   .  |  /       \      |
         | +---+ SR-MPLS +-----+    MPLS   +-----+  SR-MPLS +---+ |
         | | A | domain1 |  C  |  domain2  |  G  |  domain3 | Z | |
         | +---+         +-----+           +-----+          +---+ |
         |      \      /   |  .  \      /    .  |  \       /      |
         |       +---+     |  .   +---+      .  |    +---+        |
         |       | D |     |  .   | F |      .  |    | Y |        |
         |       +---+     |  .   +---+      .  |    +---+        |
         +-----------------+  ................  +-----------------+

        |<----SID List---->|<--- MPLS Label--->|<----SID List---->|
        |<-----i-Path----->|<------i-Path----->|<-----i-Path----->|
        |<----------------------VPN Service---------------------->|


        Figure 2: Stitching Model of SR-MPLS and MPLS interworking

3.2.  Nesting Model with Path Segment

   Figure 3 displays the nesting model of SR-MPLS and MPLS interworking.
   Comparing with the stitching model, the path segment presents end-to-
   end encapsulation in the packet from an SR-MPLS domain to an MPLS
   domain.  As described in [I-D.ietf-spring-mpls-path-segment], an end-
   to-end path segment, also referred to as e-Path, is used to indicate
   the end-to-end path, and an s-Path is used to indicate the intra-
   domain path.  The e-Path is encapsulated at the ingress nodes and
   decapsulated at the egress nodes.  The transit nodes, even the border
   nodes of domains, are not aware of the e-Path segment.  The s-Path
   can be used as a stitching label to correlate the two domains.  The
   use of the binding SID [RFC8402] is also recommended to reduce the
   size of the label stack and stitch the inter-domain paths.
















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         +-----------------+  ................  +-----------------+
         |        +---+    |  .    +---+     .  |    +---+        |
         |        | B |    |  .    | E |     .  |    | X |        |
         |        +---+    |  .    +---+     .  |    +---+        |
         |      /       \  |  .  /       \   .  |  /      \       |
         | +---+ SR-MPLS +-----+  MPLS     +-----+  SR-MPLS +---+ |
         | | A | domain1 |  C  |  domain2  |  G  |  domain3 | Z | |
         | +---+         +-----+           +-----+          +---+ |
         |      \      /   |  .  \      /    .  |  \       /      |
         |       +---+     |  .   +---+      .  |    +---+        |
         |       | D |     |  .   | F |      .  |    | Y |        |
         |       +---+     |  .   +---+      .  |    +---+        |
         +-----------------+  ................  +-----------------+

        |<----SID List---->|<--   MPLS Label-->|<----SID List---->|
        |<-----s-Path----->|<------s-Path----->|<-----s-Path----->|
        |<------------------------e-Path------------------------->|
        |<----------------------VPN Service---------------------->|



         Figure 3: Nesting Model of SR-MPLS and MPLS interworking

4.  Security Considerations

   TBA

5.  Acknowledgements

   TBA

6.  IANA Considerations

   TBA

7.  Normative References

   [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.

   [I-D.ietf-spring-segment-routing-mpls]
              Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing with MPLS
              data plane", draft-ietf-spring-segment-routing-mpls-22
              (work in progress), May 2019.



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   [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>.

   [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>.

   [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>.

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


   Weiqiang Cheng
   China Mobile
   Beijing
   China

   Email: chengweiqiang@chinamobile.com






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