The Use of Path Segment in SR Inter-domain Scenarios
draft-xiong-spring-path-segment-sr-inter-domain-02

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SPRING                                                          Q. Xiong
Internet-Draft                                                 G. Mirsky
Intended status: Informational                           ZTE Corporation
Expires: January 14, 2021                                       W. Cheng
                                                            China Mobile
                                                           July 13, 2020

          The Use of Path Segment in SR Inter-domain Scenarios
           draft-xiong-spring-path-segment-sr-inter-domain-02

Abstract

   This document illustrates the inter-domain scenarios for SR-MPLS
   networks to support end-to-end bidirectional tunnel across multiple
   domains with the use of Path Segments.

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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 . . . . . . . . . . . . . . . . . .   3
   3.  End-to-end Path Segment . . . . . . . . . . . . . . . . . . .   4
     3.1.  S-PSID  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  N-PSID  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  SR-MPLS Inter-domain Scenarios  . . . . . . . . . . . . . . .   4
     4.1.  Stitching of Path Segments  . . . . . . . . . . . . . . .   5
     4.2.  Nesting of Path Segments  . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   8
   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".  A segment can represent any
   instruction, topological or service based.  A segment can have a
   semantic local to an SR node or global within an SR domain.  SR
   supports 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.

   As defined in [RFC8402], the headend of an SR Policy binds a Binding
   Segment ID (B-SID) to its policy.  The B-SID could be bound to a SID
   List or selected path and used to stitch the SR list and the SR Label
   Switched Paths (LSP) across multiple domains.  In some scenarios, for
   example, a mobile backhaul transport network, it is required to
   provide end-to-end bidirectional path across SR networks.
   [I-D.ietf-spring-mpls-path-segment] defines a path segment identifier
   to support bidirectional path correlation for transport network.  In
   the multi-domain scenarios, the SR bidirectional end-to-end path MAY
   be established with the use of path segments.  Path segment MAY be
   used to indicate the end-to-end bidirectional path to achieve the
   path monitoring including nesting of Path Segments or Path SID
   (N-PSID) and stitching of Path Segments or Path SID (S-PSID).

   This document illustrates the inter-domain scenarios for SR-MPLS
   networks to support end-to-end bidirectional tunnel across multiple
   domains with the use of Path Segments.

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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).

   A->B SID list: The SID List from SR node A to SR node B.

   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.

   B-SID: Binding Segment ID.

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

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

   Inter-Area: Two IGP areas interconnects with an ABR in an AS.

   Inter-AS: Two ASes interconnects with an ASBR.

   IGP: Interior Gateway Protocol.

   N-PSID: Nesting of Path Segments.

   S-PSID: Stitching of Path Segments.

   SR: Segment Routing.

   SR-MPLS: Segment Routing with MPLS data plane.

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.

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3.  End-to-end Path Segment

3.1.  S-PSID

   As described in [I-D.ietf-spring-mpls-path-segment], an end-to-end
   Path Segment, also referred to as e-Path.  In the inter-domain
   scenario, the end-to-end SR path is split into multiple segments.
   And each segment can be identified by S-PSIDs in stitching model.
   The correlation of path segments can stitch the inter-domain paths
   and bind unidirectional paths.  The S-PSIDs are valid in the
   corresponding domain and the border nodes maintain the forwarding
   entries of that S-PSID.  At the headend node, the S-PSID can
   correlate the inter-domain path of reverse direction and bind the two
   unidirectional paths.

   The S-PSID can be a locally unique label and assigned from the
   Segment Routing Local Block (SRLB).  It is required that the
   controller(e.g., PCE) assigns the label to ensure the ingress and the
   egress node can recognize it and it also can be assigned from egress
   node of each domain.  PCEP based S-PSID allocation and procedure is
   defined in [I-D.xiong-pce-stateful-pce-sr-inter-domain].

3.2.  N-PSID

   As described in [I-D.ietf-spring-mpls-path-segment], an end-to-end
   Path Segment, also referred to as e-Path.  In nesting model, the
   e-Path is also referred to as N-PSID which 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 N-PSID.
   The use of the B-SID is also recommended to reduce t he size of label
   stack section 4.2 and stitch the SR list and the SR LSP.  The N-PSID
   can be used to indicate the end-to-end path and achieve the
   bidirectional path monitoring.

   The N-PSID can be a globally unique or local label.  If the N-PSID is
   globally unique, it MUST be assigned from the SRGB block of each
   domain.  If the N-PSID is a local label, it is required that the
   controller(e.g., PCE) or a super controller (e.g., hierarchical PCE)
   assigns the label to ensure the ingress(A) and the egress node(Z) can
   recognize it and there is no SID collision in the ingress and egress
   domains.

4.  SR-MPLS Inter-domain Scenarios

   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 interconnected with a ABR within areas or inter-link
   between ASes.  This document takes IGP Areas domains for example.

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   The border link scenarios are in future discussion.  SR-MPLS domains
   can be deployed as Figure 1 shown.

                  +                    +                    +
               +     +              +     +              +     +
            +           +         +          +         +          +
         +                 +    +              +    +                +
      A        SR-MPLS       X       SR-MPLS      Y       SR-MPLS      Z
         +      IGP 1      +    +     IGP 2    +    +      IGP 3     +
            +           +         +          +        +            +
               +     +              +     +              +      +
                  +                    +                     +

                  Figure 1: SR-MPLS inter-domain Scenario

   Two SR-MPLS inter-domain models are discussed in this document
   including using the stitching and nesting of Path Segments which are
   described in Section 4.1 and Section 4.2 respectively.

4.1.  Stitching of Path Segments

   The Figure 1 displays the border node inter-domain scenario.  SR node
   X and SR node Y are the border nodes of two different domains.  The
   S-PSIDs from A->X, X->Y, and Y->Z are used for the inter-domain path
   segment.  The ingress SR node A encapsulates the data packet with
   S-PSID (A->X), B-SID(Y->Z), B-SID(X->Y) and A->X SID list.  The data
   packet is forwarded to SR node X according to the A->X SID list.
   Node X pushes the S-PSID (X->Y), B-SID(Y->Z) and X->Y SID list based
   on the above mentioned forwarding entry.  The data packet is
   forwarded to node Y and then to the SR node Z b ased on the same
   forwarding procedure.  In node Z, the S-PSID (Y->Z) can be mapped to
   the path from Z to Y of reverse direction and correlates the two
   unidirectional paths.  The packet transmission of the reverse
   direction is the same with the forwarding direction with different
   S-PSID.  The stitching of path segments can achieve the inter-domain
   path monitoring.

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      ..................   .................   ....................
      .                .   .               .   .                  .
  +-----+             +-----+             +-----+              +-----+
  |  A  |             |  X  |             |  Y  |              |  Z  |
  +-----+             +-----+             +-----+              +-----+
      .  SR Domain 1   .   .  SR Domain 2  .   .   SR Domain 3    .
      ..................   .................   ....................

Service Layer:
    |<----------------------End-to-end Service--------------->|
Path Segment:
    |<-----S-PSID----->o<------S-PSID----->o<-----S-PSID----->|
LSP/Tunnel:
    |<------SR-LSP---->|<-----SR-LSP------>|<-----SR-LSP----->|
Node:
    |<----SID List---->|<-----SID List---->|<----SID List---->|

    Node A               Node X             Node Y             Node Z
+-------------+
|A->X SID list|
+-------------+     +-------------+
| B-SID(X->Y) | --->|X->Y SID list|
+-------------+     +-------------+     +-------------+
| B-SID(Y->Z) |     | B-SID(Y->Z) | --->|Y->Z SID list|
+-------------+     +-------------+     +-------------+    +--------------+
|S-PSID(A->X) |     |S-PSID(X->Y) |     |S-PSID(Y->Z) | -->|   Payload    |
+-------------+     +-------------+     +-------------+    +--------------+
|  Payload    |     |   Payload   |     |  Payload    |
+-------------+     +-------------+     +-------------+

       Figure 2: Stitching of Path Segments in Inter-Domain Scenario

4.2.  Nesting of Path Segments

   Figure 3 shows the SR-MPLS nesting inter-domain scenario.  The
   e-Path(A->Z) is used to indicate the end-to-end path.  The N-PSID,
   B-SID and SR list are pushed by the ingress node.  The N-PSID is used
   to correlate the two unidirectional SR paths to an SR bidirectional
   path.

   The use of the B-SID is also recommended to replace the SR list of
   each domain.  As shown in Figure 3, the B-SID(X->Y) is used to
   replace the X->Y SID list.  Ingress node A pushes N-PSID(A->Z),
   B-SID(Y->Z), B-SID(X->Y), and A->X SID list in turn.  When the packet
   is received at node X, the X->Y SID list are popped.  Also, X->Y SID
   list replaces B-SID(X->Y) to indicate that packet to be forwarded
   from node X to node Y.  The data packet reaches the SR node Z

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   according to the same forwarding procedure.  In SR node Z, the N-PSID
   (A->Z) is used to correlate the two unidirectional end-to-end paths.

      ..................   .................   ....................
      .                .   .               .   .                  .
  +-----+             +-----+             +-----+              +-----+
  |  A  |             |  X  |             |  Y  |              |  Z  |
  +-----+             +-----+             +-----+              +-----+
      .  SR Domain 1   .   .  SR Domain 2  .   .   SR Domain 3    .
      ..................   .................   ....................

Service Layer:
    |<----------------------End-to-end Service--------------->|
Path Segment:
    |<------------------------N-PSID------------------------->|
LSP/Tunnel:
    |<------SR-LSP---->o<-------SR-LSP----->o<-----SR-LSP---->|
Node:
    |<----SID List---->|<-----SID List----->|<----SID List--->|

     Node A            Node X                Node Y            Node Z
+-------------+
|A->X SID list|
+-------------+     +-------------+
|B-SID(X->Y)  | --> |X->Y SID list|
+-------------+     +-------------+     +-------------+
|B-SID(Y->Z)  |     |B-SID(Y->Z)  | --> |Y->Z SID list|
+-------------+     +-------------+     +-------------+     +-------------+
|N-PSID(A->Z) |     |N-PSID(A->Z) |     |N-PSID(A->Z) | --> |  Payload    |
+-------------+     +-------------+     +-------------+     +-------------+
|  Payload    |     |   Payload   |     |  Payload    |
+-------------+     +-------------+     +-------------+

        Figure 3: Nesting of Path Segments in Inter-Domain Scenario

5.  Security Considerations

   TBA

6.  Acknowledgements

   TBA

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7.  IANA Considerations

   TBA

8.  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-02 (work in progress),
              February 2020.

   [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.

   [I-D.xiong-pce-stateful-pce-sr-inter-domain]
              Xiong, Q., Mirsky, G., hu, f., and W. Cheng, "Stateful PCE
              for SR-MPLS Inter-domain", draft-xiong-pce-stateful-pce-
              sr-inter-domain-02 (work in progress), October 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

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   Greg Mirsky
   ZTE Corporation
   USA

   Email: gregimirsky@gmail.com

   Weiqiang Cheng
   China Mobile
   Beijing
   China

   Email: chengweiqiang@chinamobile.com

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