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Versions: (draft-gandhi-spring-rfc6374-srpm-mpls) 00 01 02

MPLS Working Group                                        R. Gandhi, Ed.
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: September 7, 2020                                      D. Voyer
                                                             Bell Canada
                                                              S. Salsano
                                        Universita di Roma "Tor Vergata"
                                                                 M. Chen
                                                                  Huawei
                                                           March 6, 2020


  Performance Measurement for Segment Routing Networks with MPLS Data
                                 Plane
                    draft-gandhi-mpls-rfc6374-sr-02

Abstract

   Segment Routing (SR) leverages the source routing paradigm.  RFC 6374
   specifies protocol mechanisms to enable the efficient and accurate
   measurement of packet loss, one-way and two-way delay, as well as
   related metrics such as delay variation in MPLS networks using probe
   messages.  This document utilizes these mechanisms for Performance
   Delay and Loss Measurements in Segment Routing networks with MPLS
   data plane (SR-MPLS), for both SR Links and end-to-end SR Policies.
   In addition, this document defines Return Path TLV for two-way
   performance measurement and Block Number TLV for loss measurement.

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
   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 September 7, 2020.







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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
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   4
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     2.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Reference Topology  . . . . . . . . . . . . . . . . . . .   5
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Probe Query and Response Messages . . . . . . . . . . . . . .   6
     4.1.  Probe Message for SR-MPLS Links . . . . . . . . . . . . .   6
     4.2.  Probe Message for SR-MPLS Policies  . . . . . . . . . . .   6
     4.3.  Probe Response Message for SR-MPLS Links and Policies . .   7
       4.3.1.  One-way Measurement Mode  . . . . . . . . . . . . . .   7
       4.3.2.  Two-way Measurement Mode  . . . . . . . . . . . . . .   8
       4.3.3.  Loopback Measurement Mode . . . . . . . . . . . . . .   8
     4.4.  Return Path TLV . . . . . . . . . . . . . . . . . . . . .   8
   5.  Performance Delay Measurement . . . . . . . . . . . . . . . .  10
     5.1.  Delay Measurement Message Format  . . . . . . . . . . . .  10
     5.2.  Timestamps  . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Performance Loss Measurement  . . . . . . . . . . . . . . . .  10
     6.1.  Loss Measurement Message Format . . . . . . . . . . . . .  11
     6.2.  Block Number TLV  . . . . . . . . . . . . . . . . . . . .  11
   7.  Performance Measurement for P2MP SR Policies  . . . . . . . .  12
   8.  ECMP for SR-MPLS Policies . . . . . . . . . . . . . . . . . .  13
   9.  SR Link Extended TE Metrics Advertisements  . . . . . . . . .  13
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  14
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     12.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  17
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18



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1.  Introduction

   Service provider's ability to satisfy Service Level Agreements (SLAs)
   depend on the ability to measure and monitor performance metrics for
   packet loss and one-way and two-way delay, as well as related metrics
   such as delay variation.  The ability to monitor these performance
   metrics also provides operators with greater visibility into the
   performance characteristics of their networks, thereby facilitating
   planning, troubleshooting, and network performance evaluation.

   Segment Routing (SR) leverages the source routing paradigm and
   greatly simplifies network operations for Software Defined Networks
   (SDNs).  SR is applicable to both Multiprotocol Label Switching (SR-
   MPLS) and IPv6 (SRv6) data planes.  SR takes advantage of the Equal-
   Cost Multipaths (ECMPs) between source and transit nodes, between
   transit nodes and between transit and destination nodes.  SR Policies
   as defined in [I-D.ietf-spring-segment-routing-policy] are used to
   steer traffic through a specific, user-defined paths using a stack of
   Segments.  Built-in SR Performance Measurement (PM) is one of the
   essential requirements to provide Service Level Agreements (SLAs).

   [RFC6374] specifies protocol mechanisms to enable the efficient and
   accurate measurement of performance metrics in MPLS networks using
   probe messages.  The One-Way Active Measurement Protocol (OWAMP)
   defined in [RFC4656] and Two-Way Active Measurement Protocol (TWAMP)
   defined in [RFC5357] provide capabilities for the measurement of
   various performance metrics in IP networks.  However, mechanisms
   defined in [RFC6374] are more suitable for Segment Routing when using
   MPLS data plane (SR-MPLS).  [RFC6374] also supports "direct mode"
   Loss Measurement (LM), which is required in SR networks.

   [RFC7876] specifies the procedures to be used when sending and
   processing out-of-band performance measurement probe replies over an
   UDP return path when receiving RFC 6374 based probe queries.  These
   procedures can be used to send out-of-band PM replies for both SR-
   MPLS Links and Policies [I-D.ietf-spring-segment-routing-policy] for
   one-way measurement.

   This document utilizes the probe-based mechanisms defined in
   [RFC6374] for Performance Delay and Loss Measurements in SR networks
   with MPLS data plane, for both SR Links and end-to-end SR Policies.
   In addition, this document defines Return Path TLV for two-way
   performance measurement and Block Number TLV for loss measurement.
   The Performance Measurements (PM) for SR Links are used to compute
   extended Traffic Engineering (TE) metrics for delay and loss and can
   be advertised in the network using the routing protocol extensions.





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2.  Conventions Used in This Document

2.1.  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 [RFC2119] [RFC8174]
   when, and only when, they appear in all capitals, as shown here.

2.2.  Abbreviations

   ACH: Associated Channel Header.

   DM: Delay Measurement.

   ECMP: Equal Cost Multi-Path.

   G-ACh: Generic Associated Channel (G-ACh).

   GAL: Generic Associated Channel (G-ACh) Label.

   LM: Loss Measurement.

   MPLS: Multiprotocol Label Switching.

   NTP: Network Time Protocol.

   PM: Performance Measurement.

   PSID: Path Segment Identifier.

   PTP: Precision Time Protocol.

   SID: Segment ID.

   SL: Segment List.

   SR: Segment Routing.

   SR-MPLS: Segment Routing with MPLS data plane.

   TC: Traffic Class.

   TE: Traffic Engineering.

   URO: UDP Return Object.





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2.3.  Reference Topology

   In the reference topology shown in Figure 1, the sender node R1
   initiates a performance measurement probe query and the responder
   node R5 sends a probe response for the query message received.  The
   probe response is typically sent back to the sender node R1.  The
   nodes R1 and R5 may be directly connected via a Link enabled with
   Segment Routing or there exists a Point-to-Point (P2P) SR Policy
   [I-D.ietf-spring-segment-routing-policy] on node R1 with destination
   to node R5.  In case of Point-to-Multipoint (P2MP), SR Policy
   originating from source node R1 may terminate on multiple destination
   leaf nodes [I-D.voyer-spring-sr-replication-segment].


                +-------+ t1     Query     t2 +-------+
                |       | - - - - - - - - - ->|       |
                |   R1  |---------------------|   R5  |
                |       |<- - - - - - - - - - |       |
                +-------+ t4     Response  t3 +-------+
                 Sender                       Responder

                       Figure 1: Reference Topology

3.  Overview

   One-way delay and two-way delay measurement procedure defined in
   Section 2.4 of [RFC6374] are used.  Transmit and Receive packet loss
   measurement procedures defined in Section 2.2 and Section 2.6 of
   [RFC6374] are used.  One-way loss measurement provides receive packet
   loss whereas two-way loss measurement provides both transmit and
   receive packet loss.  For both SR Links and end-to-end SR Policies,
   no PM session for delay or loss measurement is created on the
   responder node R5 [RFC6374].

   For Performance Measurement, probe query and response messages are
   sent as following:

   o  For Delay Measurement, the probe messages are sent on the
      congruent path of the data traffic by the sender node, and are
      used to measure the delay experienced by the actual data traffic
      flowing on the SR Links and SR Policies.

   o  For Loss Measurement, the probe messages are sent on the congruent
      path of the data traffic by the sender node, and are used to
      collect the receive traffic counters for the incoming link or
      incoming SID where the probe query messages are received at the
      responder node (incoming link or incoming SID needed since the
      responder node does not have PM session state present).



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   The In-Situ Operations, Administration, and Maintenance (IOAM)
   mechanisms for SR-MPLS defined in [I-D.gandhi-mpls-ioam-sr] are used
   to carry PM information in-band as part of the data traffic packets,
   and are outside the scope of this document.

4.  Probe Query and Response Messages

4.1.  Probe Message for SR-MPLS Links

   As described in Section 2.9.1 of [RFC6374], MPLS PM probe query and
   response messages flow over the MPLS Generic Associated Channel
   (G-ACh).  A probe message for SR-MPLS Links contains G-ACh Label
   (GAL) (with S=1).  The GAL is followed by an Associated Channel
   Header (ACH), which identifies the message type, and the message
   payload following the ACH as shown in Figure 2.  The probe messages
   are routed over the SR Links for both delay and loss measurement.
   For SR-MPLS Links, the TTL value is set to 1 in the SR-MPLS header
   for one-way and two-way measurement modes.


    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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             GAL (value 13)            | TC  |S|      TTL      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 1|Version| Reserved      | GAL Channel Type              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 2: Probe Message Header for an SR-MPLS Link

4.2.  Probe Message for SR-MPLS Policies

   As described in Section 2.9.1 of [RFC6374], MPLS PM probe query and
   response messages flow over the MPLS Generic Associated Channel
   (G-ACh).  A probe message for an end-to-end measurement for SR Policy
   contains SR-MPLS label stack
   [I-D.ietf-spring-segment-routing-policy], with the G-ACh Label (GAL)
   at the bottom of the stack (with S=1).  The GAL is followed by an
   Associated Channel Header (ACH), which identifies the message type,
   and the message payload following the ACH as shown in Figure 3.  For
   SR-MPLS Policies, the TTL value is set to 255 in the SR-MPLS header.










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    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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Label(1)             | TC  |S|      TTL      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Label(n)             | TC  |S|      TTL      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  GAL (value 13)       | TC  |S|      TTL      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 1|Version| Reserved      | GAL Channel Type              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 3: Probe Message Header for an End-to-end SR-MPLS Policy

   The SR-MPLS label stack can be empty (as shown in Figure 2) to
   indicate Implicit NULL label case.

   For SR Policy performance measurement, in order to ensure that the
   probe query message is processed by the intended responder node,
   Destination Address TLV (Type 129) [RFC6374] can be sent in the probe
   query message.  The responder node only replies with Success in
   Control Code if it is the intended destination for the probe query.
   Otherwise, it MUST return 0x15: Error - Invalid Destination Node
   Identifier.

4.3.  Probe Response Message for SR-MPLS Links and Policies

4.3.1.  One-way Measurement Mode

   In one-way performance measurement mode [RFC7679], the PM sender node
   can receive "out-of-band" probe replies by properly setting the UDP
   Return Object (URO) TLV in the probe query message.  The URO TLV
   (Type=131) is defined in [RFC7876] and includes the UDP-Destination-
   Port and IP Address.  In particular, if the sender sets its own IP
   address in the URO TLV, the probe response is sent back by the
   responder node to the sender node.  In addition, the "control code"
   in the probe query message is set to "out-of-band response
   requested".  In this delay measurement mode, as per Reference
   Topology, timestamps t1 and t2 are collected by the probes.  Only
   timestamps t1 and t2 are used to measure one-way delay.  The one-way
   mode is applicable to both SR-MPLS Links and SR-MPLS Policies.






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4.3.2.  Two-way Measurement Mode

   In two-way performance measurement mode [RFC6374], when using a
   bidirectional path, the probe response message is sent back to the
   sender node on the congruent path of the data traffic on the reverse
   direction SR Link or associated SR Policy
   [I-D.ietf-pce-sr-bidir-path] using a message with format similar to
   their probe query message.  In this case, the "control code" in the
   probe query message is set to "in-band response requested".  In this
   delay measurement mode, as per Reference Topology, all timestamps t1,
   t2, t3, and t4 are collected by the probes.  All four timestamps are
   used to measure two-way delay.  The two-way mode is applicable to
   both SR-MPLS Links and SR-MPLS Policies.

   Specifically, the probe response message is sent back on the incoming
   physical interface where the probe query message is received.  This
   is useful for example, in case of two-way measurement mode for Link
   delay.

   The Path Segment Identifier (PSID)
   [I-D.ietf-spring-mpls-path-segment] of the forward SR-MPLS Policy in
   the probe query can be used to find the associated reverse SR Policy
   [I-D.ietf-pce-sr-bidir-path] to send the probe response message for
   two-way measurement of SR-MPLS Policy unless when using the Return
   Path TLV.

4.3.3.  Loopback Measurement Mode

   The Loopback measurement mode defined in Section 2.8 of [RFC6374] can
   be used to measure round-trip delay for a bidirectional SR Path
   [I-D.ietf-pce-sr-bidir-path].  The probe query messages in this case
   carries the reverse SR Path label stack as part of the MPLS header.
   The GAL is still carried at the bottom of the label stack (with S=1).
   The responder node does not process the PM probe messages and
   generate response messages.  In this delay measurement mode, as per
   Reference Topology, the timestamps t1 and t4 are collected by the
   probes.  Both these timestamps are used to measure round-trip delay.
   The loopback mode for SR-MPLS Links is outside the scope of this
   document.

4.4.  Return Path TLV

   For two-way performance measurement, the responder node needs to send
   the probe response message on a specific reverse path.  The sender
   node can request in the probe query message to the responder node to
   send a response message back on a given reverse path (e.g. co-routed
   path for two-way measurement).  This way the destination node does
   not require any additional SR Policy state.



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   For one-way performance measurement, the sender node address may not
   be reachable via IP route from the responder node.  The sender node
   in this case needs to send its reachability path information to the
   responder node.

   [RFC6374] defines DM and LM probe query messages that can include one
   or more optional TLVs.  New TLV Type (TBA1) is defined in this
   document for Return Path to carry reverse path for probe response
   messages (in the payload of the message).  The format of the Return
   Path TLV is shown in Figure 4 and Figure 5:


    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 = TBA1  |    Length     |      Reserved                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Return Path Sub-TLVs                       |
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 4: Return Path TLV


    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     |      Reserved                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Label(1)                                   |
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Label(n)                                   |
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 5: Segment List Sub-TLV in Return Path TLV

   The Segment List Sub-TLV in the Return Path TLV can be one of the
   following Types:

   o  Type (value 1): SR-MPLS Label Stack of the Reverse SR Path





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   o  Type (value 2): SR-MPLS Binding SID
      [I-D.ietf-pce-binding-label-sid] of the Reverse SR Policy

   The Return Path TLV is Mandatory when used.  If responder does not
   support this TLV, it MUST return Error 0x17: Unsupported Mandatory
   TLV Object.  The PM sender node MUST only insert one Return Path TLV
   in the probe query message and the responder node MUST only process
   the first Return Path TLV in the probe query message and ignore other
   Return Path TLVs if present.  The responder node MUST send probe
   response message back on the reverse path specified in the Return
   Path TLV and MUST NOT add Return Path TLV in the probe response
   message.

5.  Performance Delay Measurement

5.1.  Delay Measurement Message Format

   As defined in [RFC6374], MPLS DM probe query and response messages
   use Associated Channel Header (ACH) (value 0x000C for delay
   measurement) [RFC6374], which identifies the message type, and the
   message payload following the ACH.  For both SR Links and end-to-end
   measurement for SR-MPLS Policies, the same MPLS DM ACH value is used.

   The DM message payload as defined in Section 3.2 of [RFC6374] is used
   for SR-MPLS delay measurement, for both SR Links and end-to-end SR
   Policies.

5.2.  Timestamps

   The Section 3.4 of [RFC6374] defines timestamp format that can be
   used for delay measurement.  The IEEE 1588 Precision Time Protocol
   (PTP) timestamp format [IEEE1588] is used by default as described in
   Appendix A of [RFC6374], with hardware support in Segment Routing
   networks.

6.  Performance Loss Measurement

   The LM protocol can perform two distinct kinds of loss measurement as
   described in Section 2.9.8 of [RFC6374].

   o  In inferred mode, LM will measure the loss of specially generated
      test messages in order to infer the approximate data plane loss
      level.  Inferred mode LM provides only approximate loss
      accounting.

   o  In direct mode, LM will directly measure data plane packet loss.
      Direct mode LM provides perfect loss accounting, but may require
      hardware support.



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   For both of these modes of LM, Path Segment Identifier (PSID)
   [I-D.ietf-spring-mpls-path-segment] is used for accounting received
   traffic on the egress node of the SR-MPLS Policy as shown in
   Figure 6.  Different values of PSID can be used to measure packet
   loss per SR-MPLS Policy, per Candidate Path or per Segment List of
   the SR Policy.


    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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  PSID                 | TC  |S|      TTL      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  GAL (value 13)       | TC  |S|      TTL      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 1|Version| Reserved      | GAL Channel Type              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 6: With Path Segment Identifier for SR-MPLS Policy

6.1.  Loss Measurement Message Format

   As defined in [RFC6374], MPLS LM probe query and response messages
   use Associated Channel Header (ACH) (value 0x000A for direct loss
   measurement or value 0x000B for inferred loss measurement), which
   identifies the message type, and the message payload following the
   ACH.  For both SR Links and end-to-end measurement for SR-MPLS
   Policies, the same MPLS LM ACH value is used.

   The LM message payload as defined in Section 3.1 of [RFC6374] is used
   for SR-MPLS loss measurement, for both SR Links and end-to-end SR
   Policies.

6.2.  Block Number TLV

   The Loss Measurement using Alternate-Marking method defined in
   [RFC8321] requires to color the data traffic.  To be able to compare
   the transmit and receive traffic counters of the matching color, the
   Block Number (or color) of the traffic counters is carried by the
   probe query and response messages for loss measurement.  Probe query
   and response messages specified in [RFC6374] for Loss Measurement do
   not identify the Block Number of the counters.

   [RFC6374] defines probe query and response messages that can include
   one or more optional TLVs.  New TLV Type (value TBA2) is defined in
   this document to carry the Block Number (8-bit) of the traffic
   counters in the probe query and response messages for loss




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   measurement.  The format of the Block Number TLV is shown in
   Figure 7:


    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 TBA2   |    Length     | Reserved      | Block Number  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 7: Block Number TLV

   The Block Number TLV is Mandatory when used.  If responder does not
   support this TLV, it MUST return Error 0x17: Unsupported Mandatory
   TLV Object.  The PM sender node SHOULD only insert one Block Number
   TLV in the probe query message and the responder node in the probe
   response message SHOULD return the first Block Number TLV from the
   probe query messages and ignore other Block Number TLVs if present.
   In probe messages, the counters MUST belong to the same Block Number.

7.  Performance Measurement for P2MP SR Policies

   The procedures for delay and loss measurement described in this
   document for Point-to-Point (P2P) SR-MPLS Policies
   [I-D.ietf-spring-segment-routing-policy] are also equally applicable
   to the Point-to-Multipoint (P2MP) SR-MPLS Policies as following:

   o  The sender root node sends probe query messages using the
      Replication Segment defined in
      [I-D.voyer-spring-sr-replication-segment] for the P2MP SR Policy
      as shown in Figure 8.

   o  Each responder leaf node adds the "Source Address" TLV (Type 130)
      [RFC6374] with its IP address in the probe response messages.
      This TLV allows the sender root node to identify the responder
      leaf nodes of the P2MP SR Policy.

   o  The P2MP root node measures the end-to-end delay and loss
      performance for each P2MP leaf node of the P2MP SR Policy.












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    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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              Replication SID          | TC  |S|      TTL      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              GAL (value 13)           | TC  |S|      TTL      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 1|Version| Reserved      | GAL Channel Type              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 8: Query with Replication Segment for SR-MPLS Policy

8.  ECMP for SR-MPLS Policies

   An SR Policy can have ECMPs between the source and transit nodes,
   between transit nodes and between transit and destination nodes.
   Usage of Anycast SID [RFC8402] by an SR Policy can result in ECMP
   paths via transit nodes part of that Anycast group.  The PM probe
   messages need to be sent to traverse different ECMP paths to measure
   performance delay of each of the ECMP path of an SR Policy.

   Forwarding plane has various hashing functions available to forward
   packets on specific ECMP paths.  For SR-MPLS Policy, sweeping of
   entropy label [RFC6790] values can be used in PM probe messages to
   take advantage of the hashing function in forwarding plane to
   influence the ECMP path taken by them.

   The considerations for performance loss measurement for different
   ECMP paths of an SR Policy are outside the scope of this document.

9.  SR Link Extended TE Metrics Advertisements

   The extended TE metrics for SR Link delay and loss computed using the
   performance measurement procedures described in this document can be
   advertised in the routing domain as follows:

   o  For OSPF, ISIS, and BGP-LS, protocol extensions defined in
      [RFC7471], [RFC8570], and [RFC8571] are used, respectively for
      advertising the extended TE link metrics in the network.

   o  The advertised delay-variance metric is computed as specified in
      Section 4.2 of [RFC5481].

   o  The extended TE link one-way delay metrics can also be computed
      using two-way delay measurement or round-trip delay measurement
      from loopback mode by dividing the measured delay values by 2.





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   o  The extended TE link delay and loss metrics are advertised for
      Layer 2 bundle members in OSPF [I-D.ketant-lsr-ospf-l2bundles] and
      ISIS [RFC8668] using the same mechanisms defined in [RFC7471] and
      [RFC8570], respectively.

10.  Security Considerations

   This document describes the procedures for performance delay and loss
   measurement for SR-MPLS networks, for both SR Links and end-to-end SR
   Policies using the mechanisms defined in [RFC6374] and [RFC7876].
   This document does not introduce any additional security
   considerations other than those covered in [RFC6374], [RFC7471],
   [RFC8570], [RFC8571], and [RFC7876].

11.  IANA Considerations

   IANA is requested to allocate a value for the following mandatory
   Return Path TLV Type for RFC 6374 to be carried in PM probe query
   messages:

   o  Type TBA1: Return Path TLV

   IANA is requested to allocate the values for the following Sub-TLV
   Types for the Return Path TLV for RFC 6374.

   o  Type (value 1): SR-MPLS Label Stack of the Reverse SR Path

   o  Type (value 2): SR-MPLS Binding SID
      [I-D.ietf-pce-binding-label-sid] of the Reverse SR Policy

   IANA is also requested to allocate a value for the following
   mandatory Block Number TLV Type for RFC 6374 to be carried in the PM
   probe query and response messages for loss measurement:

   o  Type TBA2: Block Number TLV

12.  References

12.1.  Normative References

   [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>.







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   [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>.

   [RFC7876]  Bryant, S., Sivabalan, S., and S. Soni, "UDP Return Path
              for Packet Loss and Delay Measurement for MPLS Networks",
              RFC 7876, DOI 10.17487/RFC7876, July 2016,
              <https://www.rfc-editor.org/info/rfc7876>.

   [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>.

12.2.  Informative References

   [IEEE1588]
              IEEE, "1588-2008 IEEE Standard for a Precision Clock
              Synchronization Protocol for Networked Measurement and
              Control Systems", March 2008.

   [RFC4656]  Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
              Zekauskas, "A One-way Active Measurement Protocol
              (OWAMP)", RFC 4656, DOI 10.17487/RFC4656, September 2006,
              <https://www.rfc-editor.org/info/rfc4656>.

   [RFC5357]  Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
              Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
              RFC 5357, DOI 10.17487/RFC5357, October 2008,
              <https://www.rfc-editor.org/info/rfc5357>.

   [RFC5481]  Morton, A. and B. Claise, "Packet Delay Variation
              Applicability Statement", RFC 5481, DOI 10.17487/RFC5481,
              March 2009, <https://www.rfc-editor.org/info/rfc5481>.

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, DOI 10.17487/RFC6790, November 2012,
              <https://www.rfc-editor.org/info/rfc6790>.

   [RFC7679]  Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
              Ed., "A One-Way Delay Metric for IP Performance Metrics
              (IPPM)", STD 81, RFC 7679, DOI 10.17487/RFC7679, January
              2016, <https://www.rfc-editor.org/info/rfc7679>.







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   [RFC7471]  Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
              Previdi, "OSPF Traffic Engineering (TE) Metric
              Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,
              <https://www.rfc-editor.org/info/rfc7471>.

   [RFC8321]  Fioccola, G., Ed., Capello, A., Cociglio, M., Castaldelli,
              L., Chen, M., Zheng, L., Mirsky, G., and T. Mizrahi,
              "Alternate-Marking Method for Passive and Hybrid
              Performance Monitoring", RFC 8321, DOI 10.17487/RFC8321,
              January 2018, <https://www.rfc-editor.org/info/rfc8321>.

   [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>.

   [RFC8570]  Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward,
              D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE)
              Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March
              2019, <https://www.rfc-editor.org/info/rfc8570>.

   [RFC8571]  Ginsberg, L., Ed., Previdi, S., Wu, Q., Tantsura, J., and
              C. Filsfils, "BGP - Link State (BGP-LS) Advertisement of
              IGP Traffic Engineering Performance Metric Extensions",
              RFC 8571, DOI 10.17487/RFC8571, March 2019,
              <https://www.rfc-editor.org/info/rfc8571>.

   [RFC8668]  Ginsberg, L., Ed., Bashandy, A., Filsfils, C., Nanduri,
              M., and E. Aries, "Advertising Layer 2 Bundle Member Link
              Attributes in IS-IS", RFC 8668, DOI 10.17487/RFC8668,
              December 2019, <https://www.rfc-editor.org/info/rfc8668>.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Sivabalan, S., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-06 (work in progress),
              December 2019.

   [I-D.voyer-spring-sr-replication-segment]
              Voyer, D., Filsfils, C., Parekh, R., Bidgoli, H., and Z.
              Zhang, "SR Replication Segment for Multi-point Service
              Delivery", draft-voyer-spring-sr-replication-segment-02
              (work in progress), November 2019.








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   [I-D.ietf-pce-binding-label-sid]
              Sivabalan, S., Filsfils, C., Tantsura, J., Hardwick, J.,
              Previdi, S., and C. Li, "Carrying Binding Label/Segment-ID
              in PCE-based Networks.", draft-ietf-pce-binding-label-
              sid-01 (work in progress), November 2019.

   [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.gandhi-mpls-ioam-sr]
              Gandhi, R., Ali, Z., Filsfils, C., Brockners, F., Wen, B.,
              and V. Kozak, "Segment Routing with MPLS Data Plane
              Encapsulation for In-situ OAM Data", draft-gandhi-mpls-
              ioam-sr-01 (work in progress), December 2019.

   [I-D.ketant-lsr-ospf-l2bundles]
              Talaulikar, K. and P. Psenak, "Advertising L2 Bundle
              Member Link Attributes in OSPF", draft-ketant-lsr-ospf-
              l2bundles-01 (work in progress), January 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-01 (work
              in progress), February 2020.

Acknowledgments

   The authors would like to thank Thierry Couture for the discussions
   on the use-cases for the performance measurement in segment routing
   networks.  Authors would like to thank Patrick Khordoc for
   implementing the mechanisms defined in this document.  The authors
   would like to thank Greg Mirsky for providing many useful comments
   and suggestions.  The authors would also like to thank Stewart
   Bryant, Sam Aldrin, Tarek Saad, and Rajiv Asati for their review
   comments.

Contributors










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   Sagar Soni
   Cisco Systems, Inc.
   Email: sagsoni@cisco.com

   Zafar Ali
   Cisco Systems, Inc.
   Email: zali@cisco.com

   Pier Luigi Ventre
   CNIT
   Italy
   Email: pierluigi.ventre@cnit.it

Authors' Addresses

   Rakesh Gandhi (editor)
   Cisco Systems, Inc.
   Canada

   Email: rgandhi@cisco.com


   Clarence Filsfils
   Cisco Systems, Inc.

   Email: cfilsfil@cisco.com


   Daniel Voyer
   Bell Canada

   Email: daniel.voyer@bell.ca


   Stefano Salsano
   Universita di Roma "Tor Vergata"
   Italy

   Email: stefano.salsano@uniroma2.it


   Mach(Guoyi) Chen
   Huawei

   Email: mach.chen@huawei.com






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