Flexible Candidate Path Selection of SR Policy
draft-liu-spring-sr-policy-flexible-path-selection-00

SPRING Working Group                                             Y. Liu
Internet Draft                                             China Mobile
Intended status: Standards Track                                 C. Lin
Expires: September 5, 2023                                       Y. Qiu
                                                   New H3C Technologies
                                                          March 4, 2023

              Flexible Candidate Path Selection of SR Policy
           draft-liu-spring-sr-policy-flexible-path-selection-00

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Abstract

   This document proposes a flexible SR policy candidate path selection
   method. Based on the real-time resource usage and forwarding quality
   of candidate paths, the head node can perform dynamic path switching
   among multiple candidate paths in the SR policy.

Table of Contents

   1. Introduction ................................................ 2
   2. Terminology ................................................. 3
   3. Background of requirements .................................. 3
   4. Flexible Candidate Path Selection Method .................... 4
      4.1. Threshold Parameters of Candidate Paths ................ 5
      4.2. Rules for Selecting the Best Path ...................... 6
      4.3. Flexible Candidate Path Selection Process .............. 7
   5. Examples of Flexible Candidate Path Selection ............... 8
   6. IANA Considerations ......................................... 9
   7. Security Considerations ..................................... 9
   8. References .................................................. 9
      8.1. Normative References ................................... 9
      8.2. Informative References ................................. 9
   9. Acknowledgments ............................................ 10
   Authors' Addresses ............................................ 11

  1. Introduction

   Segment routing (SR) [RFC8402] is a source routing paradigm that
   explicitly indicates the forwarding path for packets at the ingress
   node. The ingress node steers packets into a specific path according
   to the Segment Routing Policy (SR Policy) as defined in [RFC9256].

   An SR Policy may have multiple candidate paths that are provisioned
   or signaled [I-D.ietf-idr-segment-routing-te-policy] [RFC8664] from
   one of more sources. The tie-breaker rules defined in [RFC9256]
   result in determination of a single "active path" in a formal
   definition.

   Refer to [RFC9256] only the active candidate path MUST be used for
   forwarding traffic that is being steered onto that policy except for
   certain scenarios such as fast reroute where a backup candidate path
   may be used. A candidate path can be represented as a segment list
   or a set of segment lists. If a set of segment lists is associated
   with the active path of the policy, then the steering is per flow

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   and weighted-ECMP (W-ECMP) based according to the relative weight of
   each valid segment list.

   According to the criteria for the validity of candidate paths
   described in Section 5 of [RFC9256], as long as there is a valid
   segment list in the active candidate path, the active candidate path
   is still valid.  When some segment lists of the active candidate
   path are invalid, the active candidate path may still be valid, but
   it may not continue to meet the actual forwarding requirements.

   This document proposes a flexible SR policy candidate path selection
   method. Based on the real-time resource usage and forwarding quality
   of candidate paths, the head node can perform dynamic path switching
   among multiple candidate paths in the SR policy.

  2. Terminology

   The definitions of the basic terms are identical to those found in
   Segment Routing Policy Architecture [RFC9256].

  3. Background of requirements

   When some segment lists of the active candidate path are invalid,
   according to [RFC9256], as long as there is a valid segment list in
   the active candidate path, the active candidate path is still valid.
   But the paths of remaining segment lists may not meet the SR policy
   forwarding performance requirements, such as insufficient path
   bandwidth. Even if there are other candidate paths with lower
   preference that can meet the forwarding performance requirements in
   the SR policy, the traffic will continue to be forwarded along the
   original active candidate path.

   Take the following SR Policy as an example to explain in detail the
   problems existing in the current candidate path selection process.

      SR Policy POL1
         Candidate Path CP1
            Preference 200
            Segment List 1 <SID11...SID1i>, Weight 1
            Segment List 2 <SID21...SID2j>, Weight 1
            Segment List 3 <SID31...SID3k>, Weight 1
         Candidate Path CP2
          Preference 100
            Segment List 4 <SID41...SID4i>, Weight 1
            Segment List 5 <SID51...SID5j>, Weight 1
            Segment List 6 <SID61...SID6k>, Weight 1

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   There are two candidate paths CP1 and CP2 in SR policy POL1. CP1 has
   a higher preference. Both candidate paths are composed of three
   segment lists with the same weight. The path indicated by each
   segment list can carry traffic of 100M bandwidth. When the Segment
   Lists are valid, the candidate path can carry traffic with bandwidth
   less than 300M.

   The bandwidth of the actual traffic forwarded by the SR policy is
   between 100M and 150M. Because the traffic forwarded on the
   candidate path will share the load on the three segment list paths
   according to the weight value. Therefore, normally, the candidate
   path can meet the forwarding requirements. The traffic is forwarded
   on the three segment lists of the high preference candidate paths of
   the SR policy.

   When the segment list 1 and 2 in the high-preference candidate path
   CP1 are invalid, according to the candidate path validity criteria
   described in [RFC9256] Section 5, because the segment list 3 in CP1
   is still valid, the active candidate path CP1 is still valid. All
   traffic of SR policy POL1 will continue to be forwarded based on the
   path of CP1. However, because segment list 3 can only forward 100M
   traffic, over-bandwidth traffic will be discarded.

   Of course, when the Segment List path fault is detected, the network
   device can report the detected fault information to the controller.
   The controller optimizes the forwarding path after receiving the
   message. However, this interaction process is relatively long, and
   it is difficult to meet the requirement for fast switching.

   To solve this problem, this document proposes a new candidate path
   selection rule, which sets resource thresholds and forwarding
   quality requirements for candidate path. This candidate path can
   only be selected if the current path can meet the preset
   requirements.

  4. Flexible Candidate Path Selection Method

   As described in [RFC9256], the candidate path selection process
   operates primarily on the candidate path Preference.  A candidate
   path is selected when it is valid and it has the highest Preference
   value among all the valid candidate paths of the SR Policy.

   In the case of multiple valid candidate paths of the same Preference,
   the tie-breaking rules are evaluated on the identification tuple in
   the following order until only one valid best path is selected:

      1. The higher value of Protocol-Origin is selected.

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      2. If specified by configuration, prefer the existing installed
   path.

      3. The lower value of the Originator is selected.

      4. Finally, the higher value of the Discriminator is selected.

   This document proposes to take the forwarding quality requirements
   and resource requirements of candidate paths as the selection
   criteria of candidate paths.

   Set the threshold parameters of forwarding quality and resources for
   candidate paths. First, find the paths that meet the threshold from
   the candidate paths of SR policy, and then select the best path as
   the active path according to the rules in the above standards.

  4.1. Threshold Parameters of Candidate Paths

   The threshold parameters of candidate paths can include but are not
   limited to the following:

      Jitter

      Latency

      Packet loss

      If there are multiple segment lists in the candidate path, as long
      as the delay, jitter or packet loss rate parameters of any valid
      segment list in the candidate path fail to meet the specified
      threshold requirements, it is considered that the candidate path
      does not meet the threshold requirements.

      Available bandwidth

      If there are multiple segment lists in the candidate path, the
      available bandwidth is the sum of all valid segment lists in the
      candidate path, or the cumulative value calculated based on the
      weight and bandwidth of each segment list.

      Bandwidth utilization

      Current flow rate

      Ratio of valid segment lists in candidate path

      This parameter reflects the failure ratio of the segment list in
      the candidate path.

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      The higher ratio of valid segment lists, the candidate path is
      more robust.

   If the weight of the segment list is different, a threshold for each
   segment list separately can be specified. The threshold of the
   candidate path is the sum of the thresholds of the segment list
   calculated based on the weight.

   When multiple threshold parameters are specified on the candidate
   path at the same time, the candidate path is considered to meet the
   threshold requirements only if all the threshold requirements are
   met.

   If the candidate path does not specify any threshold parameters,
   select the primary candidate path according to the selection method
   defined in RFC9256.

   By default, there is no threshold parameter specified on the
   candidate path.

  4.2. Rules for Selecting the Best Path

   When the current forwarding quality and hardware resources of a
   candidate path meet the specified threshold requirements, it only
   means that this candidate path has the ability to forward traffic.

   If there are multiple candidate paths in the SR policy that meet the
   forwarding requirements at the same time, the candidate paths need
   to be sorted to select the best one.

   Under the condition that multiple valid candidate paths meet the
   threshold requirements, evaluate the tie break rule in the following
   order until only one valid best path is selected:

     1. If the quality requirements of the candidate path are specified,
        it is necessary to check whether the path meets the quality
        requirements. Only the valid path that meets the quality
        requirements can be selected as the active path.

        If only one path in the SR policy meets the quality requirements,
        the path is selected.

        If multiple candidate paths meet the quality requirements at the
        same time, or if all candidate paths fail to meet the
        requirements, then select the following second step according to
        the Preference.

     2. The higher value of the Preference is selected.

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     3. The higher value of Protocol-Origin is selected.

     4. If specified by configuration, prefer the existing installed
        path.

     5. The lower value of the Originator is selected.

     6. Finally, the higher value of the Discriminator is selected.

  4.3. Flexible Candidate Path Selection Process

   The process of selecting the best path for SR policy through the
   threshold parameter of the path is as follows.

   1. Configure the threshold parameters on the candidate path of the
      head node through static manual configuration or controlled
      distribution.

   2. The head node monitors whether the available resources and
      forwarding quality of the SR policy candidate path exceed the
      thresholds.

      The forwarding quality of path can be obtained through active or
      passive performance measurement methods, such as iOAM, STAMP,
      TWAMP, etc. The real-time quality data can be calculated by the
      controller and distributed to the head node, or calculated by the
      head node according to the network measurement data. The
      measurement method and quality data acquisition method are beyond
      the scope of this document.

   3. According to the rules described in Section 4.2, when the
      available resources are less than the threshold, or the
      forwarding quality cannot meet the threshold requirements, select
      a new active candidate path.

   4. After the old active candidate path eliminates the fault or
      improves the forwarding quality, whether to recover can be
      specified by the configuration. If fault recovery is required,
      start a wait timer for delay recovery. If the timer expires and
      the old candidate path still meets the threshold requirements,
      the traffic will be switched to the old higher preference
      candidate path.

   For avoiding path switching frequently, both over-threshold
   switching and fault recovery should be delayed. The interval of
   delay waiting can be adjusted by configuration.

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   In order to distribute the threshold parameters of SR Policy to the
   head node, it may be necessary to extend the control plane, such as
   NetConf, PCEP and BGP. This document does not limit the specific
   distribution method. The specific control plane extension will be
   described in other documents.

  5. Examples of Flexible Candidate Path Selection

   The SR policy in Section 3 is still used to illustrate how the
   flexible candidate path selection method switches candidate paths.

   SR policy POL1 has two candidate paths CP1 and CP2. The Preference
   of CP1 is 200, and the Preference of CP2 is 100. Both candidate
   paths are composed of three segment lists with the same weight.

   The path indicated by each segment list can carry traffic of 100M
   bandwidth. When the Segment Lists are valid, the candidate path can
   carry traffic with bandwidth less than 300M. The bandwidth of the
   actual traffic forwarded by the SR policy is between 100M and 150M.

      SR Policy POL1
         Candidate Path CP1
            Preference 200
            Available bandwidth ratio 50
            Segment List 1 <SID11...SID1i>, Weight 1
            Segment List 2 <SID21...SID2j>, Weight 1
            Segment List 3 <SID31...SID3k>, Weight 1
         Candidate Path CP2
          Preference 100
          Available bandwidth ratio 50
            Segment List 4 <SID41...SID4i>, Weight 1
            Segment List 5 <SID51...SID5j>, Weight 1
            Segment List 6 <SID61...SID6k>, Weight 1

   First, take the available bandwidth as the threshold parameter of
   POL1. The threshold for configuring the ratio of available bandwidth
   is 50%. When the available bandwidth of the candidate path is less
   than 50%, path switching is performed.

   Normally, the three segment lists of CP1 and CP2 are valid. The
   available bandwidth of CP1 is 300M, and the ratio of available
   bandwidth is 100%, which can meet the threshold requirements of the
   path. So CP1 is selected as the active candidate path according to
   the Preference.

   If the paths indicated by Segment 1 and 2 fail, Segment List 1 and 2
   become invalid, and the available bandwidth of CP1 becomes 100M. The
   ratio of available bandwidth becomes 33.3% (i.e. 100/300). Because

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   the ratio of available bandwidth of CP1 is lower than the specified
   threshold, CP1 has failed to meet the forwarding quality
   requirements. Need to reselect the active candidate path for POL1.

   The three segment lists of the low-preference candidate path CP2 of
   POL1 are valid, and the available bandwidth can meet the threshold
   requirements. CP2 is selected as the new active candidate path of
   POL1. The traffic forwarded by POL1 is switched to the path of CP2
   for forwarding.

  6. IANA Considerations

   This document has no IANA actions.

  7. Security Considerations

   This document does not introduce any security considerations.

8. References

           8.1. Normative References

   [I-D.ietf-idr-segment-routing-te-policy] Previdi, S., Filsfils, C.,
             Talaulikar, K., Mattes, P., Jain, D., and S. Lin,
             "Advertising Segment Routing Policies in BGP", draft-ietf-
             idr-segment-routing-te-policy-20 (work in progress), July
             2022.

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

   [RFC9256] Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
             P. Mattes, "Segment Routing Policy Architecture", RFC 9256,
             DOI 10.17487/RFC9256, July 2022, <https://www.rfc-
             editor.org/info/rfc9256>.

           8.2. Informative References

   TBD

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  9. Acknowledgments

   The authors would like to thank the following for their valuable
   contributions of this document:

   TBD

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Authors' Addresses

   Yisong Liu
   China Mobile

   Email: liuyisong@chinamobile.com

   Changwang Lin
   New H3C Technologies

   Email: linchangwang.04414@h3c.com

   Yuanxiang Qiu
   New H3C Technologies

   Email: qiuyuanxiang@h3c.com

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