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Versions: 00

Network Working Group                                           W. Cheng
Internet-Draft                                                   L. Wang
Intended status: Standards Track                                   H. Li
Expires: May 3, 2018                                        China Mobile
                                                                 M. Chen
                                                                  Huawei
                                                               R. Zigler
                                                                Broadcom
                                                                 S. Zhan
                                                                     ZTE
                                                        October 30, 2017


           Path Segment in MPLS Based Sement Routing Network
                draft-cheng-spring-mpls-path-segment-00

Abstract

   An SR path is identified by an SR segment list, one or partial
   segments of the list cannot uniquely identify the SR path.

   This document introduces the concept of Path Segment that is used to
   identify an SR path.  When used, it is inserted at the ingress node
   of the SR path and immediately follows the last segment of the SR
   path.  The Path Segment will not be popped off until it reaches the
   egress of the SR path, it can be used by the egress node to implement
   end-2-end SR path protection or performance measurement (PM) of an SR
   path.

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 RFC 2119 [RFC2119].

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any




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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 3, 2018.

Copyright Notice

   Copyright (c) 2017 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
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Path Segment  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  One Label Solution  . . . . . . . . . . . . . . . . . . .   3
       2.1.1.  Path Segment Assignment . . . . . . . . . . . . . . .   4
     2.2.  Two Labels Solution . . . . . . . . . . . . . . . . . . .   5
   3.  Path Segment Application  . . . . . . . . . . . . . . . . . .   7
     3.1.  Performance Measurement . . . . . . . . . . . . . . . . .   7
     3.2.  End-2-end Path Protection . . . . . . . . . . . . . . . .   7
     3.3.  Bi-directional SR Tunnel  . . . . . . . . . . . . . . . .   8
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Segment Routing (SR) [I-D.ietf-spring-segment-routing] is a source
   routed forwarding method that allows to directly encode forwarding
   instructions (called segments) in each packet, hence it enables to
   steer traffic through a network without the per-flow states
   maintained in the transit nodes.  Segment Routing can be instantiated
   on MPLS data plane or IPv6 data plane.  The former is called SR-MPLS



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   [I-D.ietf-spring-segment-routing-mpls], the latter is called SRv6
   [I-D.ietf-6man-segment-routing-header].  SR-MPLS leverages the MPLS
   label stack to construct SR path, and SRv6 uses the Segment Routing
   Header to construct SR path.

   In an SR-MPLS network, when a packet is transmitted along an SR path,
   the labels in the MPLS label stack will be swapped or popped.  So
   that no label or only the last label may be left in the MPLS label
   stack when the packet reaches the egress node.  Thus, the egress node
   cannot determine from which ingress node or SR path the packet comes.

   However, to support use cases like end-2-end 1+1 path protection,
   bidirectional path correlation or performance measurement(PM), the
   ability to implement path identification is the pre-condition.

   Therefore, this document introduces a new segment that is referred to
   as Path Segment.  A Path Segment is defined to unique identify an SR
   path in a specific context. (e.g., in the context of the egress node
   or ingress node of an SR path, or within an SR domain).  It is
   normally used by egress nodes for path identification or correlation.
   Path Segment can only apply to SR-MPLS.

2.  Path Segment

   This document introduces two options for SR path identification: one
   label solution and two labels solution.

   [Editor notes: it is supposed that the WG will discuss and decide
   which one is the better solution.]

2.1.  One Label Solution

   The Path Segment is a single label that is assigned from the Segment
   Routing Local Block (SRLB) or Segment Routing Global Block (SRGB) of
   the egress node of an SR path.  It means that the Path Segment is
   unique in the context of the egress node of SR paths.  When Path
   Segment is used, a Path label MUST be inserted at the ingress node
   and MUST immediately follow the last label of the SR path.

   If the Path label is the bottom label, the S bit MUST be set.  The
   value of the TC field MUST be set to the same value as the last
   segment label of the SR path.  The value of the TTL field MUST be set
   to the same value of the last segment label of the SR path.

   Normally, the intermediates node will not see the Path Segment label
   and do not know how to process it even if they see it.  A Path
   Segment label presenting to an intermediate node is error situation.




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   The egress node MUST pop the Path label and deliver it to relevant
   components for further processing.

   The label stack with Path Segment is as below (Figure1):

               +--------------------+
               |       ...          |
               +--------------------+
               |      Label 1       |
               +--------------------+
               |      Label 2       |
               +--------------------+
               |       ...          |
               +--------------------+
               |      Label n       |
               +--------------------+
               |     Path Label     |
               +--------------------+
               |       ...          |
               +--------------------+

      Figure 1: Label Stack with Path Segment


   Where:

   o  The Label 1-n are the segment labels that are used to direct how
      to steer the packets along the SR path.

   o  The Path Label identifies the SR path in the context of the egress
      node of the SR path.

2.1.1.  Path Segment Assignment

   Several ways can be used to assign the Path Segment.  One way is that
   the Path Segment label is directly assigned by the egress node of an
   SR path.  Where the ingress node of the SR path can directly send a
   request to the egress node to ask for a Path label.  With this way,
   it needs to set up a communication channel between the ingress node
   and the egress node.  New protocols or extensions to existing
   protocol may be required.

   Another candidate way is to leverage a centralized controller (e.g.,
   PCE) to assign the Path label.  The ingress node sends a request to
   the PCE to compute a SR path and indicate that a Path label is
   desired.  The PCE will compute the path as required.  Once the path
   computed, the PCE will send a request (with computed path and
   relevant information) to the egress node to ask for a Path label for



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   the SR path.  The egress node will allocate a label to the SR path
   and build mapping relationship between the label and the path.  A
   reply will be sent back to the PCE, the PCE will send a reply to the
   ingress node about the path information and the corresponding Path
   Segment label.

   With either way or the variations, the final purpose is to assign a
   label from the egress node's label space, hence a single label is
   enough for path identification.  Then the ingress node can put the
   Path Segment label into the label stack when needed, and the egress
   node can use that Path Segment to implement relevant functionalities.

2.2.  Two Labels Solution

   Two segments (Source segment and Path segment) are used to identify
   an SR path.  The Source segment is a global node segment, it can
   uniquely identify a node within an SR domain.  It MUST NOT be used
   for forwarding and indicates that a Path segment immediately follows.
   The Path segment is a local segment generated at the ingress node to
   identify an SR path.  The combination of Source segment and Path
   segment can uniquely identify an SR Path with an SR domain.

   A node that enables Path segment function will be assigned two node
   segments.  One is for forwarding just as defined in
   [I-D.ietf-spring-segment-routing], the other is for source
   identification.  The corresponding label of the Source Segment is
   indexed in the SRGB (or in a of the node to which the Source Segment
   will be presented.

   The Path segment label is a local label that is assigned to an SR
   path at the ingress node.

   The label stack with Source and Path segments is as below (Figure 2):


















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               +--------------------+
               |       ...          |
               +--------------------+
               |      Label 1       |
               +--------------------+
               |      Label 2       |
               +--------------------+
               |       ...          |
               +--------------------+
               |      Label n       |
               +--------------------+
               |     Source Label   |
               +--------------------+
               |     Path Label     |
               +--------------------+
               |       ...          |
               +--------------------+

      Figure 2: Label Stack with Source and Path Segments


   Where:

   o  The Label 1-n are the segment labels that are used to direct how
      to steer the packets along an SR path, and the "label n" is the
      last label of the SR path or the label that directs forwarding
      packets to the node to which the Source Segment will be presented.

   o  The Source Label identifies the source of the SR path.  The value
      of the TC and TTL fields of the Source Label MUST be set to the
      same values as the label (e.g., the Label n) it follows.

   o  The Path Label identifies the SR path in the context of source
      node.  If the Path label is the bottom label, the S bit MUST be
      set.  The value of the TC and TTL fields SHOULD be set to the same
      values as the Source label.

   The Source and Path label MUST be inserted at the ingress node of an
   SR path.  And they MUST immediately follow the label that directs
   forwarding packets to the node (e.g., the egress or an intermediate
   node) to which the Source Segment (as the stack top label) and Path
   Segment are presented.

   If a node receives a packet with an unknown Source Label, the packet
   MUST be discarded and an error SHOULD be reported.

   The Source label and Path label MUST be popped at the node who
   receives a packet with the Source label as the stack top label.



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3.  Path Segment Application

3.1.  Performance Measurement

   To measure the packet loss and delay of the real traffic of an SR
   path, one fundamental condition is path identification at the
   measuring points.  For an SR path, the ingress node have the complete
   information of the path, it can use those information for packet
   counting and/or timestamping.  At the egress node, since the Path
   Segment label (or combination with Source label) can be used to
   identify the path, path based packet counting and/or timestamping can
   be implemented as well.  Then combined with the mechanisms defined
   [RFC6374], end-2-end packet loss and/or delay measurement of an SR
   path can be achieved.

   Measuring at intermediate nodes needs more consideration, it will be
   added in the next version.

3.2.  End-2-end Path Protection

   For end-2-end 1+1 path protection, the egress node of an path needs
   to know the set of paths that constitute the primary and the
   backup(s), in order to select the primary packet for onward
   transmission, and to discard the packets from the backups.

   To do this each path needs a path identifier that is unique at the
   egress node.  Depending on the design, this single unique label
   chosen by the egress PE or the combination of the source node
   identifier and a unique path identifier chosen by the source.

   There then needs to be a method of binding this path identifiers into
   equivalence groups such that the egress PE can determine the set of
   packets that represent a single path and its backup.

   It is obvious that this group can be instantiated in the network by
   an SDN controller.

   In a network that is using a distributed control plane the approach
   will depend on the control protocol used, but the essence of the
   solution is that which ever PE is responsible for creating the group
   advertises then as a group of equivalent paths.  Whether one of these
   is advertised as primary and the others as secondary will or all are
   advertised as of equal status will depend on the details of the
   underlying protection mechanism.







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3.3.  Bi-directional SR Tunnel

   With the current SR architecture, an SR path is an unidirectional
   path.  In some scenarios, for example, mobile backhaul transport
   network, there are requirements to support bi-directional path, and
   the path is normally treated as a single entity and both directions
   of the path have same fate, for example, failure in one direction
   will result in switching at both directions.

   MPLS supports this by introducing the concepts of co-routed
   bidirectional LSP and associated bi-directional LSP.  With SR, to
   support bidirectional path, a straightforward way is to bind two
   unidirectional SR paths to a single bi-directional path.  Path
   segments can be used to correlate the two unidirectional SR paths at
   both ends of the paths.

4.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.

5.  Security Considerations

6.  Contributors

   The following individuals also contribute to this document.

   o  Shuangping Zhan, ZTE

   o  Cheng Li, Huawei

7.  Acknowledgements

   The authors would like to thank Stewart Bryant for his review,
   suggestion and comments to this document.

8.  References

8.1.  Normative References










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   [I-D.ietf-6man-segment-routing-header]
              Previdi, S., Filsfils, C., Raza, K., Leddy, J., Field, B.,
              daniel.voyer@bell.ca, d., daniel.bernier@bell.ca, d.,
              Matsushima, S., Leung, I., Linkova, J., Aries, E., Kosugi,
              T., Vyncke, E., Lebrun, D., Steinberg, D., and R. Raszuk,
              "IPv6 Segment Routing Header (SRH)", draft-ietf-6man-
              segment-routing-header-07 (work in progress), July 2017.

   [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>.

8.2.  Informative References

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-13 (work
              in progress), October 2017.

   [I-D.ietf-spring-segment-routing-mpls]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing with MPLS
              data plane", draft-ietf-spring-segment-routing-mpls-10
              (work in progress), June 2017.

   [RFC6374]  Frost, D. and S. Bryant, "Packet Loss and Delay
              Measurement for MPLS Networks", RFC 6374,
              DOI 10.17487/RFC6374, September 2011,
              <https://www.rfc-editor.org/info/rfc6374>.

Authors' Addresses

   Weiqiang Cheng
   China Mobile

   Email: chengweiqiang@chinamobile.com


   Lei Wang
   China Mobile

   Email: wangleiyj@chinamobile.com







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   Han Li
   China Mobile

   Email: lihan@chinamobile.com


   Mach(Guoyi) Chen
   Huawei

   Email: mach.chen@huawei.com


   Royi Zigler
   Broadcom

   Email: royi.zigler@broadcom.com


   Shuangping Zhan
   ZTE

   Email: zhan.shuangping@zte.com.cn





























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