Path Segment for SRv6 (Segment Routing in IPv6)
draft-ietf-spring-srv6-path-segment-00

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Document Type Active Internet-Draft (spring WG)
Authors Cheng Li  , Weiqiang Cheng  , Mach Chen  , Dhruv Dhody  , Rakesh Gandhi 
Last updated 2020-11-24
Replaces draft-li-spring-srv6-path-segment
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SPRING Working Group                                               C. Li
Internet-Draft                                       Huawei Technologies
Intended status: Standards Track                                W. Cheng
Expires: May 29, 2021                                       China Mobile
                                                                 M. Chen
                                                                D. Dhody
                                                     Huawei Technologies
                                                               R. Gandhi
                                                     Cisco Systems, Inc.
                                                       November 25, 2020

            Path Segment for SRv6 (Segment Routing in IPv6)
                 draft-ietf-spring-srv6-path-segment-00

Abstract

   Segment Routing (SR) allows for a flexible definition of end-to-end
   paths by encoding an ordered list of instructions, called "segments".
   The SR architecture can be implemented over an MPLS data plane as
   well as an IPv6 data plane.

   Currently, Path Segment has been defined to identify an SR path in
   SR-MPLS networks, and is used for various use-cases such as end-to-
   end SR Path Protection and Performance Measurement (PM) of an SR
   path.  This document defines the Path Segment to identify an SRv6
   path in an IPv6 network.

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 29, 2021.

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Copyright Notice

   Copyright (c) 2020 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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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Use Cases for SRv6 Path Segment . . . . . . . . . . . . . . .   4
   3.  SRv6 Path Segment . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Format of an SRv6 Path Segment  . . . . . . . . . . . . .   5
       3.1.1.  SRv6 Path Segment: Locator and Local ID . . . . . . .   5
       3.1.2.  SRv6 Path Segment: Global ID  . . . . . . . . . . . .   6
   4.  SRv6 Path Segment Allocation  . . . . . . . . . . . . . . . .   6
   5.  Processing of SRv6 Path Segment . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Segment routing (SR) [RFC8402] is a source routing paradigm that
   explicitly indicates the forwarding path for packets at the ingress
   node by inserting an ordered list of instructions, called segments.

   When segment routing is deployed on an MPLS data plane, called SR-
   MPLS [I-D.ietf-spring-segment-routing-mpls], a segment identifier
   (SID) is present as an MPLS label.  When segment routing is deployed
   on an IPv6 data plane, a SID is presented as a 128-bit value, and it
   can be an IPv6 address of a local interface but it does not have to

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   be.  To support SR in an IPv6 network, a Segment Routing Header (SRH)
   [RFC8754] is used.

   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 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 can not
   determine from which ingress node or SR path the packet came from.
   Therefore, to identify an SR-MPLS path, a Path Segment is defined in
   [I-D.ietf-spring-mpls-path-segment].

   Likewise, a path needs to be identified in an SRv6 network for
   several use cases such as binding bidirectional paths
   [I-D.ietf-pce-sr-bidir-path] and end-to-end performance measurement
   [I-D.gandhi-spring-udp-pm].

   An SRv6 path MAY be identified by the content of a segment list.
   However, the segment list may not be a good key, since the length of
   a segment list is flexible according to the number of required SIDs.
   Also, the length of a segment list may be too long to be a key when
   it contains many SIDs.  For instance, if packet A uses an SRH with 3
   SIDs while Packet B uses an SRH with 10 SIDs, the key to identify
   these two paths will be a 384-bits value and a 1280-bits value,
   respectively.  Further, an SRv6 path cannot be identified by the
   information carried by the SRH in reduced mode [RFC8754] as the first
   SID is not present.

   Furthermore, different SRv6 policies may use the same segment list
   for different candidate paths, so the traffic of different SRv6
   policies are merged, resulting in the inability to measure the
   performance of the specific path.

   To solve the above issues, this document defines a new SRv6 segment
   called "SRv6 Path Segment", which is a 128-bits value, to identify an
   SRv6 path.

   When the SRv6 Path Segment is used in reduced mode SRH [RFC8754], the
   entire path information is indicated by the Path Segment, and the
   performance will be better than using the entire segment list as the
   path identifier, while the overhead is equivalent to the SRH in
   normal mode.  Furthermore, with SRv6 Path Segment, each SRv6
   candidate path can be identified and measured, even when they use the
   same segment list.

   An SRv6 Path Segment MUST NOT be copied to the IPv6 destination
   address, so it is not routable.

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1.1.  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.

1.2.  Terminology

   MPLS: Multiprotocol Label Switching.

   PM: Performance Measurement.

   SID: Segment ID.

   SR: Segment Routing.

   SR-MPLS: Segment Routing with MPLS data plane.

   SRH: Segment Routing Header.

   PSID: Path Segment Identifier.

   PSP: Penultimate Segment Popping.

   Further, this document makes use of the terms defined in [RFC8402]
   and [I-D.ietf-spring-srv6-network-programming].

2.  Use Cases for SRv6 Path Segment

   Similar to SR-MPLS Path Segment [I-D.ietf-spring-mpls-path-segment],
   SRv6 Path Segment may also be used to identify an SRv6 Path in some
   use cases:

   o  Performance Measurement: For Passive measurement [RFC7799], path
      identification at the measuring points is the pre-requisite
      [I-D.ietf-spring-mpls-path-segment].  SRv6 Path segment can be
      used by the measuring points (e.g., the ingress/egress nodes of an
      SRv6 path) or a centralized controller to correlate the packets
      counts/timestamps, then packet loss/delay can be calculated.

   o  Bi-directional SRv6 Path Association: In some scenarios, such as
      mobile backhaul transport networks, there are requirements to
      support bidirectional paths.  Like SR-MPLS
      [I-D.ietf-spring-mpls-path-segment], to support bidirectional SRv6
      paths, a straightforward way is to bind two unidirectional SRv6
      paths to a single bidirectional path.  SRv6 Path segments can be

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      used to correlate the two unidirectional SRv6 paths at both ends
      of the path.  [I-D.ietf-pce-sr-bidir-path] defines how to use PCEP
      and Path Segment to initiate a bidirectional SR path.

   o  End-to-end Path Protection: For end-to-end 1+1 path protection
      (i.e., Live-Live case), the egress node of an SRv6 path needs to
      know the set of paths that constitute the primary and the
      secondary(s), to select the primary packet for onward
      transmission, and to discard the packets from the secondary(s), so
      each SRv6 path needs a unique path identifier at the egress node,
      which can be an SRv6 Path Segment.

3.  SRv6 Path Segment

   As defined in [I-D.ietf-spring-srv6-network-programming], an SRv6
   segment is a 128-bit value.

   To identify an SRv6 path, this document defines a new segment called
   SRv6 Path Segment.

   The SRv6 Path Segment MUST appear only once in a segment list, and it
   MUST appear as the last entry in the segment list.  To indicate the
   SRv6 Path Segment, an SRH.P-flag is defined in
   [I-D.li-6man-srv6-path-segment-encap].

   Depending on the use case, an SRv6 Path Segment identifies:

   o  an SRv6 path within an SRv6 domain

   o  an SRv6 Policy

   o  a Candidate-path or a SID-List in a SRv6 Policy
      [I-D.ietf-spring-segment-routing-policy]

   Note that, based on the use-case, a SRv6 Path Segment can be used for
   different SID-Lists within an SR Policy.

3.1.  Format of an SRv6 Path Segment

   This document proposes two types of SRv6 Path Segment format.

3.1.1.  SRv6 Path Segment: Locator and Local ID

   As per [I-D.ietf-spring-srv6-network-programming], an SRv6 segment is
   a 128-bit value, which can be represented as LOC:FUNCT, where LOC is
   the L most significant bits and FUNCT is the 128-L least significant
   bits.  L is called the locator length and is flexible.  Each network
   operator is free to use the locator length it chooses.  Most often

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   the LOC part of the SID is routable and leads to the node which
   instantiates that SID.  The FUNCT part of the SID is an opaque
   identification of a local function bound to the SID.  The FUNCT value
   zero is invalid.

   SRv6 Path Segment can follow the format, where the LOC part
   identifies the egress node that allocates the Path Segment, and the
   FUNCT part is a unique local ID to identify an SRv6 Path and its
   endpoint behavior.

   The Function Type of an SRv6 Path Segment is END.PSID (End Function
   with Path Segment Identifier).

    +--------------------------------------------------------------+
    |  Locator              |        Function ID                   |
    +--------------------------------------------------------------+

    |<-------------------------128 bits--------------------------->|

                 Figure 2. PSID in Format LOC:FUNCT

3.1.2.  SRv6 Path Segment: Global ID

   An SRv6 Path Segment ID can be a Global ID, and its format depends on
   the use case.

   The SRv6 Path Segment will not be copied to the IPv6 Destination
   Address, so the SRv6 Path Segment ID can be allocated from an
   independent 128-bits ID Space.  In this case, a new table should be
   maintained at the node for SRv6 Path Segment.

    +--------------------------------------------------------------+
    |                         Global ID/PSID                       |
    +--------------------------------------------------------------+

    |<-------------------------128 bits--------------------------->|

                Figure 3. A Global ID as an PSID

4.  SRv6 Path Segment Allocation

   A Path Segment is a local segment allocated by an egress node.  A
   Path Segment can be allocated through several ways, such as CLI, BGP
   [I-D.ietf-idr-sr-policy-path-segment], PCEP
   [I-D.ietf-pce-sr-path-segment] or other ways.  The mechanisms through
   which a Path Segment is allocated are out of scope of this document.

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   When a Path Segment is allocated by the egress, it MUST be
   distributed to the ingress node of the path that identified by the
   path segment.  In this case, only the egress will process the Path
   Segment, and other nodes specified by SIDs in the segment list do not
   know how to process the Path Segment.

   Depending on the use case, a Path Segment may be distributed to the
   SRv6 nodes along the SRv6 path.  In this case, the SRv6 nodes that
   learned the Path Segment may process the Path Segment depending on
   the use case.

5.  Processing of SRv6 Path Segment

   When the SRv6 Path Segment is used, the following rules apply:

   o  The SRv6 Path Segment MUST appear only once in a segment list, and
      it MUST appear as the last entry.  Only the one that appears as
      the last entry in the SID list will be processed.  An SRv6 Path
      Segment that appears at any other location in the SID list will be
      treated as an error.

   o  When an SRv6 Path Segment is inserted, the SL MUST be initiated to
      be less than the value of Last Entry, and will not point to SRv6
      Path Segment.  For instance, when the Last entry is 4, the SID
      List[4] is the SRv6 Path Segment, so the SL MUST be set to 3 or
      other numbers less than Last entry.

   o  The SRv6 Path Segment MUST NOT be copied to the IPv6 destination
      address.

   o  Penultimate Segment Popping (PSP, as defined in
      [I-D.ietf-spring-srv6-network-programming]) MUST be disabled.

   o  The ingress needs to set the P-bit when an SRv6 Path Segment is
      inserted in the SID List.  Nodes that support SRv6 Path Segment
      processing will inspect the last entry to process SRv6 Path
      Segment when the P-bit is set.  When the P-bit is unset, the nodes
      will not inspect the last entry.

   o  The specific SRv6 Path Segment processing depends on use cases,
      and it is out of scope of this document.

6.  IANA Considerations

   This I-D requests the IANA to allocate, within the "SRv6 Endpoint
   Behaviors" sub-registry belonging to the top-level "Segment-routing
   with IPv6 data plane (SRv6) Parameters" registry, the following
   allocations:

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      Value      Description                               Reference
      --------------------------------------------------------------
      TBA1       End.PSID - SRv6 Path Segment                [This.ID]

7.  Security Considerations

   This document does not introduce additional security requirements and
   mechanisms other than the ones described in [RFC8402].

8.  Contributors

      Zhenbin Li
      Huawei Technologies
      Huawei Campus, No. 156 Beiqing Rd.
      Beijing  100095
      China

      Email: lizhenbin@huawei.com

      Jie Dong
      Huawei Technologies
      Huawei Campus, No. 156 Beiqing Rd.
      Beijing  100095
      China

      Email: jie.dong@huawei.com

9.  Acknowledgements

   The authors would like to thank Stefano Previdi and Zafar Ali for
   their valuable comments and suggestions.

10.  References

10.1.  Normative References

   [I-D.ietf-spring-srv6-network-programming]
              Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
              Matsushima, S., and Z. Li, "SRv6 Network Programming",
              draft-ietf-spring-srv6-network-programming-24 (work in
              progress), October 2020.

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   [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>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

10.2.  Informative References

   [I-D.gandhi-spring-udp-pm]
              Gandhi, R., Filsfils, C., daniel.voyer@bell.ca, d.,
              Salsano, S., Ventre, P., and M. Chen, "UDP Path for In-
              band Performance Measurement for Segment Routing
              Networks", draft-gandhi-spring-udp-pm-02 (work in
              progress), September 2018.

   [I-D.ietf-idr-sr-policy-path-segment]
              Li, C., Li, Z., Chen, H., Cheng, W., and K. Talaulikar,
              "SR Policy Extensions for Path Segment and Bidirectional
              Path", draft-ietf-idr-sr-policy-path-segment-02 (work in
              progress), November 2020.

   [I-D.ietf-pce-sr-bidir-path]
              Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong,
              "PCEP Extensions for Associated Bidirectional Segment
              Routing (SR) Paths", draft-ietf-pce-sr-bidir-path-03 (work
              in progress), September 2020.

   [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-02 (work in progress),
              November 2020.

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   [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-03 (work in progress),
              September 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.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-09 (work in progress),
              November 2020.

   [I-D.li-6man-srv6-path-segment-encap]
              Li, C., Cheng, W., Li, Z., and D. Dhody, "Encapsulation of
              Path Segment in SRv6", draft-li-6man-srv6-path-segment-
              encap-04 (work in progress), October 2020.

   [RFC7799]  Morton, A., "Active and Passive Metrics and Methods (with
              Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
              May 2016, <https://www.rfc-editor.org/info/rfc7799>.

Authors' Addresses

   Cheng Li
   Huawei Technologies

   Email: c.l@huawei.com

   Weiqiang Cheng
   China Mobile

   Email: chengweiqiang@chinamobile.com

   Mach(Guoyi) Chen
   Huawei Technologies

   Email: mach.chen@huawei.com

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   Dhruv Dhody
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore, Karnataka  560066
   India

   Email: dhruv.ietf@gmail.com

   Rakesh Gandhi
   Cisco Systems, Inc.
   Canada

   Email: rgandhi@cisco.com

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