PCE                                                        L. Zhang, Ed.
Internet-Draft                                                   J. Dong
Intended status: Standards Track                                 T. Zhou
Expires: 4 September 2025                                         Huawei
                                                            3 March 2025


              PCE SR Policy Extensions for Path Scheduling
                 draft-zzd-pce-sr-policy-scheduling-00

Abstract

   Segment Routing (SR) policy enables instantiation of an ordered list
   of segments with a specific intent for traffic steering.  When using
   SR policy in a time-variant network, delivering the time-variant
   information associated with paths is necessary in some scenarios.

   This document proposes extensions to PCE SR Policy to deliver the
   schedule information of candidate path (segment list) and its
   associated attributes.

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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   This Internet-Draft will expire on 4 September 2025.

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components



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   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Network with Discontinuous Links  . . . . . . . . . . . .   3
     2.2.  Network with Frequent Topology Changes  . . . . . . . . .   4
   3.  Schedule Information in PCE SR Policy . . . . . . . . . . . .   4
   4.  Schedule Information TLV  . . . . . . . . . . . . . . . . . .   4
     4.1.  Candidate Paths with Schedule . . . . . . . . . . . . . .   6
     4.2.  Segment Lists with Schedule . . . . . . . . . . . . . . .   7
   5.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   [RFC9657] introduces a set of time-variant network use cases where
   the topology of the network changes predictably.  When the networks
   uses traditional routing protocols, it takes these topology changes
   as unexpected events and may cause and packet loss.  However, the
   topology changes of these networks can be predicted in advance,
   therefore some measures can be taken in advance to prevent the packet
   loss.  With this idea, [I-D.ietf-tvr-requirements] describes the
   requirements of using the time-variant information in a network.  In
   Section 3.4.1 of [I-D.ietf-tvr-requirements], it describes the
   centralized routing scenarios with time-variant information, in which
   the network entities receive the time variable information and
   traffic forwarding rules directly from a logically centralized
   source(an Orchestrator or network controller).  The time-variant
   information is especially essential when there is a risk that a
   logically centralized source may loses connectivity with the network
   entities.

   [RFC8664]specifies extensions to the Path Computation Element
   Communication Protocol (PCEP) that allow a stateful PCE to compute
   and initiate Traffic-Engineering (TE) paths, as well as a Path
   Computation Client (PCC) to request a path subject to certain
   constraints and optimization criteria in SR networks.
   [I-D.ietf-pce-segment-routing-policy-cp] extends [RFC8664] to support



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   signaling SR Policy Candidate Paths as PCEP LSPs and to signal
   candidate paths of the SR Policy.  It signals SR Policy Candidate
   Paths as PCEP LSPs and signal Candidate Path membership in an SR
   Policy by means of the Association mechanism.  The segment lists of
   each candidate path and their associated attributes are signaled by
   the Path Attribute Object defined in [I-D.ietf-pce-multipath].
   However, when using SR Policy in a time-variant network, it can't
   advertise the schedule information associated with paths.

   This document proposes extensions to PCE SR Policy to carry the
   schedule information of candidate paths/segment lists.

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.

2.  Motivation

   Most of the time-variant network use cases using PCE SR Policy could
   benefit from this work.  In some cases, carrying the time-variant
   information with SR Policy is essential.

   This section describes the cases that requires extending SR Policy
   with schedule information.

2.1.  Network with Discontinuous Links

   In some time-variant network cases, the links between the network
   entities and network controller may very weak or intermittent, this
   is very typical in Resource Preservation and Dynamic Reachability
   network[RFC9657].  In these cases, Real-time SR policy advertising
   (before changes occur) may not be timely.  For example, when a link
   of an old path is about to be disconnected, the network controller is
   going to advertise a new path to the headend.  However, the link
   between the headend and the network controller is not available.  As
   a result, the new path cannot be advertised in time, causing packet
   loss.

   Therefore, in these cases, once the links between the headend and
   network controller are available, the controller need to advertise
   the paths with schedule information for a period in the future to the
   headend.  Then the headend could determine valid paths in the future
   based on the schedule information of SR policy.




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2.2.  Network with Frequent Topology Changes

   There are also some time-variant network cases that topology changes
   frequently.  This is very typical when the number of network entities
   is very large (For example, a Dynamic Reachability network with
   hundreds or thousands of nodes).  In this kind of time-variant
   network, a path form one network entity to another changes
   frequently, sometimes it can only be maintained for a few minutes or
   seconds.

   Considering that there are multiple paths in a network that computed
   by the controller, the SR Policies with candidate paths may be
   advertised to the headend every few seconds.  It poses great
   changeling to the network controller.  However, using schedule
   information could advertise several paths at a time, which greatly
   mitigate the pressure of network controllers.

3.  Schedule Information in PCE SR Policy

4.  Schedule Information TLV

   [RFC8934] defines the SCHED-LSP-ATTRIBUTE and SCHED-PD-LSP-ATTRIBUTE
   TLV to indicate the LSP is a non-periodical scheduling LSP or a
   periodical scheduling LSP.  However, it can't express a LSP with
   complex schedules.  On the one hand, the format of these TLVs are
   very simple, each TLV can only descripts one duration or a periodical
   duration, on the other hand, it requires that only one SCHED-LSP-
   ATTRIBUTE TLV SHOULD be present in the LSP object, which means each
   scheduling LSP can only have one duration or periodical duration.

   Therefore, the extensions of [RFC8934] could be applicable in some
   cases with simple schedules, but it is not flexible enough to be
   applied in the cases with complex schedules(such as the cases listed
   in Section 2).  A more general format of Schedule Information TLV is
   defined in this draft to cover different kind of cases.

   The schedule information TLV indicates one or more valid durations.
   The format of Schedule Information TLV is shown as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Type             |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   /                        Schedules                              /
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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                     Figure 1: Schedule Information TLV

   Type: TBD1

   Length: the size of the value field in octets.

   Schedules: one or more schedules, each schedule indicates the
   duration when the candidate path (segment list) is active.  The
   format of each schedule is shown as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Schedule-id                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Flags  |S|P|R|    Length     |          Reserved             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Start Time                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Start Time(Continue)                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       End Time/Duration                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   End Time/Duration(Continue)                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Recurrence count/Bound(Optional)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Recurrence count/Bound(Optional)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Frequency (Optional)                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2: Format of Schedules

   Schedule-id: 32-bit value, the unique identifier to distinguish each
   schedule within a SR Policy, this value is allocated by the SR Policy
   generator.

   Flags: 8 bits, currently only 3 bits are used, the other bits are
   reserved.

   Length: 8 bits, indicates the length of this schedule in octets.









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   S (Schedule type): one-bit flag to indicate the type of a schedule.
   If S=0, it indicates the schedule only has one instance, the
   Recurrence Count/Bound, Frequency and Interval field should not be
   included in the sub-TLV; If S=1, it indicates the schedule has
   multiple instances, the Recurrence Count/Bound, Frequency and
   Interval field should be included.

   P (Period type): one-bit flag to indicate the description type of a
   period. if P=1, then the period is described by a start time filed
   and an end time field; If P =0, then the period is described by a
   start time field and a duration time field.

   R (Recurrence bound type): one-bit flag to indicate the how to
   determine whether the recurrence is end. if R=1, then the end of
   recurrence is determined by a detail timepoint; If R = 0, then the
   end of the recurrence is determined by the number of occurrences.

   Start Time: 64-bit value, the number of seconds since the epoch, it
   indicates when the candidate path (segment list) and its associated
   attributes start to take effect.

   End Time/Duration: 64-bit value, if the flag P=1, then it is the
   number of seconds since the epoch, it indicates when the candidate
   path (segment list) and its associated attributes becomes
   ineffective.  If the flag P=0, then it is the number of seconds since
   the Start Time, it indicates how long the candidate path (segment
   list) and its associated attributes are effective.

   Recurrence Count/Bound(optional): 64-bit value, this field SHOULD be
   included when the flag P is set to 1.  When the flag R=0, then this
   field indicates the max number of occurrences.  For example, if it is
   set to 2, then the schedule will repeat twice with the specified
   Frequency and Interval.  When the flag R=1, then tis field indicates
   the bounded timepoint of recurrence, it is descripted by the number
   of seconds since the epoch.

   Frequency(optional): 32-bit value, this field should be included when
   the flag S is set to 1.  It is the numbers of seconds since the Start
   Time of an instance to the Start Time of next instance.  This field
   indicates the recurrence frequency for all the instance of this
   schedule.

4.1.  Candidate Paths with Schedule

   As described in [I-D.ietf-pce-segment-routing-policy-cp], SR Policy
   is represented by a new type of PCEP Association, called the SR
   Policy Association.  The SR Candidate Paths of an SR Policy are
   represented by the PCEP LSPs within the same SRPA.



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   When applying schedules to a Candidate Path of an SR Policy, the LSP
   Object[RFC8231] is required to be extended to support the Schedule
   Information TLV.  The Schedule Information TLV could be an optional
   TLV present in the LSP Object.

4.2.  Segment Lists with Schedule

   When there are multiple segment lists within an SR Policy Candidate
   Paht, [I-D.ietf-pce-multipath] defines the Path Attribute
   object(PATH-ATTRIB) to carry per-path information.  When applying
   schedules to a Segment List, the PATH-ATTRIB object is required to be
   extended to support the Schedule Information TLV.  The Schedule
   Information TLV could be an optional TLV present in the LSP Object.

5.  Procedures

   TBD

6.  Security Considerations

   TBD

7.  IANA Considerations

   IANA maintains a sub-registry "PCEP TLV Type Indicators" in the "Path
   Computation Element Protocol (PCEP) Numbers" registry.  IANA is
   requested to make the following allocations from this sub-registry.

         +=======+===============================+===============+
         | Value | Description                   | Reference     |
         +=======+===============================+===============+
         | TBD1  | Schedule Information (SI) TLV | This document |
         +-------+-------------------------------+---------------+

                                  Table 1

8.  References

8.1.  Normative References

   [I-D.ietf-tvr-requirements]
              King, D., Contreras, L. M., Sipos, B., and L. Zhang, "TVR
              (Time-Variant Routing) Requirements", Work in Progress,
              Internet-Draft, draft-ietf-tvr-requirements-04, 13
              September 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-tvr-requirements-04>.





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   [RFC8664]  Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "Path Computation Element Communication
              Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
              DOI 10.17487/RFC8664, December 2019,
              <https://www.rfc-editor.org/info/rfc8664>.

   [I-D.ietf-pce-segment-routing-policy-cp]
              Koldychev, M., Sivabalan, S., Sidor, S., Barth, C., Peng,
              S., and H. Bidgoli, "Path Computation Element
              Communication Protocol (PCEP) Extensions for Segment
              Routing (SR) Policy Candidate Paths", Work in Progress,
              Internet-Draft, draft-ietf-pce-segment-routing-policy-cp-
              22, 25 February 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-pce-
              segment-routing-policy-cp-22>.

   [I-D.ietf-pce-multipath]
              Koldychev, M., Sivabalan, S., Saad, T., Beeram, V. P.,
              Bidgoli, H., Yadav, B., Peng, S., and G. S. Mishra, "PCEP
              Extensions for Signaling Multipath Information", Work in
              Progress, Internet-Draft, draft-ietf-pce-multipath-12, 8
              October 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-pce-multipath-12>.

   [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>.

8.2.  Informative References

   [RFC9657]  Birrane, III, E., Kuhn, N., Qu, Y., Taylor, R., and L.
              Zhang, "Time-Variant Routing (TVR) Use Cases", RFC 9657,
              DOI 10.17487/RFC9657, October 2024,
              <https://www.rfc-editor.org/info/rfc9657>.

   [RFC8934]  Chen, H., Ed., Zhuang, Y., Ed., Wu, Q., and D. Ceccarelli,
              "PCE Communication Protocol (PCEP) Extensions for Label
              Switched Path (LSP) Scheduling with Stateful PCE",
              RFC 8934, DOI 10.17487/RFC8934, October 2020,
              <https://www.rfc-editor.org/info/rfc8934>.






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   [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>.

Authors' Addresses

   Li Zhang (editor)
   Huawei
   Beiqing Road
   Beijing
   China
   Email: zhangli344@huawei.com


   Jie Dong
   Huawei
   Email: jie.dong@huawei.com


   Tianran Zhou
   Huawei
   Email: zhoutianran@huawei.com



























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