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Versions: (draft-gandhi-spring-rfc6374-srpm-mpls) 00 01 02 03

MPLS Working Group                                        R. Gandhi, Ed.
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: December 13, 2020                                      D. Voyer
                                                             Bell Canada
                                                              S. Salsano
                                        Universita di Roma "Tor Vergata"
                                                                 M. Chen
                                                                  Huawei
                                                           June 11, 2020


Performance Measurement Using RFC 6374 for Segment Routing Networks with
                            MPLS Data Plane
                    draft-gandhi-mpls-rfc6374-sr-03

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 December 13, 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 Links  . . . . . . . . . . . . . . .   6
     4.2.  Probe Message for SR Policies . . . . . . . . . . . . . .   6
     4.3.  Probe Response Message for SR 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.  Delay Measurement . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Delay Measurement Message Format  . . . . . . . . . . . .  10
     5.2.  Timestamps  . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Loss Measurement  . . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  Loss Measurement Message Format . . . . . . . . . . . . .  11
     6.2.  Block Number TLV  . . . . . . . . . . . . . . . . . . . .  11
     6.3.  Combined Loss/Delay Measurement Message Format  . . . . .  12
   7.  Performance Measurement for P2MP SR Policies  . . . . . . . .  12
   8.  ECMP for SR Policies  . . . . . . . . . . . . . . . . . . . .  13
   9.  SR Link Extended TE Metrics Advertisements  . . . . . . . . .  13
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  14
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     12.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  18
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  18



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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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
   Links and Policies 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 querier 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 querier node R1.  The
   nodes R1 and R5 may be directly connected via a Link enabled with SR
   or there exists a Point-to-Point (P2P) SR Path e.g.  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].  In all cases,
   the data plane has MPLS enabled on the nodes.


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

                       Figure 1: Reference Topology

3.  Overview

   For one-way, two-way and round-trip delay measurements, the
   procedures defined in Section 2.4 and Section 2.6 of [RFC6374] are
   used.  For transmit and receive packet loss measurements, the
   procedures defined in Section 2.2 and Section 2.6 of [RFC6374] are
   used.  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 querier 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 querier 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 Links

   As described in Section 2.9.1 of [RFC6374], probe query and response
   messages flow over the MPLS Generic Associated Channel (G-ACh).  A
   probe message for SR 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 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 Link

4.2.  Probe Message for SR Policies

   As described in Section 2.9.1 of [RFC6374], probe query and response
   messages flow over the MPLS Generic Associated Channel (G-ACh).  A
   probe message for an end-to-end SR Policy measurement 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 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: Example Probe Message Header for an End-to-end SR 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 [RFC6374].

4.3.  Probe Response Message for SR Links and Policies

4.3.1.  One-way Measurement Mode

   In one-way performance measurement mode [RFC7679], the querier 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 querier node sets its own
   IP address in the URO TLV, the probe response is sent back by the
   responder node to the querier 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 to measure
   one-way delay.  The one-way mode is applicable to both SR Links and
   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
   querier 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 Links and 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 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 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 probe messages and generate
   response messages, and hence Loopback Request object (Type 3) is not
   required for SR.  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 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 querier
   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 querier node address may not
   be reachable via IP route from the responder node.  The querier 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 carried in a probe query
   message.  If responder does not support this TLV, it MUST return
   Error 0x17: Unsupported Mandatory TLV Object.  The querier 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.  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
   SR Policies measurements, 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.  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 Policy as shown in Figure 6.
   Different values of PSID can be used to measure packet loss per SR
   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: Example With Path Segment Identifier for SR 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 SR Policies measurements, 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
   correlate 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.  The
   probe query and response messages currently specified in [RFC6374]
   for Loss Measurement do not identify the Block Number of the
   counters.  The Block Number can also be used to aggregate performance
   metrics collected.

   [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 carried in a probe query
   message.  If responder does not support this TLV, it MUST return
   Error 0x17: Unsupported Mandatory TLV Object.  The querier 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.

6.3.  Combined Loss/Delay Measurement Message Format

   As defined in [RFC6374], Combined DM+LM probe query and response
   messages use Associated Channel Header (ACH) (value 0x000D for direct
   loss and delay measurement or value 0x000E for inferred loss and
   delay measurement), which identifies the message type, and the
   message payload following the ACH.  For both SR Links and end-to-end
   SR Policies measurements, the same MPLS ACH value is used.

   The message payload as defined in Section 3.3 of [RFC6374] is used
   for SR-MPLS combined delay and loss measurement, for both SR Links
   and end-to-end SR Policies.

7.  Performance Measurement for P2MP SR Policies

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

   o  The querier 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.



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      This TLV allows the querier root node to identify the responder
      leaf nodes of the P2MP SR Policy.

   o  The P2MP root node measures the delay and loss performance for
      each P2MP leaf node of the end-to-end P2MP 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              Replication SID          | TC  |S|      TTL      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              GAL (value 13)           | TC  |S|      TTL      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 1|Version| Reserved      | GAL Channel Type              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 8: Example Probe Query with Replication Segment for SR Policy

   The considerations for two-way and loopback modes for performance
   measurement for P2MP SR Policy are outside the scope of this
   document.

8.  ECMP for SR 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 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 Policy, sweeping of entropy
   label [RFC6790] values can be used in 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:





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   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.

   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 [RFC6374] to be carried in probe query
   message from the "MPLS Loss/Delay Measurement TLV Object" registry
   contained within the "Generic Associated Channel (G-ACh) Parameters"
   registry set:

   o  Type TBA1: Return Path TLV

   IANA is requested to create a sub-registry for "Return Path Sub-TLV
   Type" for the Return Path TLV.  All code points in the range 1
   through 32759 in this registry shall be allocated according to the
   "IETF Review" procedure as specified in [RFC8126].  Code points in
   the range 32760 through 65279 in this registry shall be allocated
   according to the "First Come First Served" procedure as specified in
   [RFC8126].  Remaining code points are allocated according to Table 1:









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   +---------------+-------------------------+-------------------------+
   | Value         |       Description       | Reference               |
   +---------------+-------------------------+-------------------------+
   | 0- 32767      |      Mandatory TLV,     | IETF Review             |
   |               |        unassigned       |                         |
   | 32768 - 65279 |      Optional TLV,      | First Come First Served |
   |               |        unassigned       |                         |
   | 65280 - 65519 |       Experimental      | This document           |
   | 65520 - 65534 |       Private Use       | This document           |
   | 65535         |         Reserved        | This document           |
   +---------------+-------------------------+-------------------------+

                Table 1: Return Path Sub-TLV Type Registry

   IANA is requested to allocate the values for the following Sub-TLV
   Types from this registry.

   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
   probe query and response messages for loss measurement from the "MPLS
   Loss/Delay Measurement TLV Object" registry contained within the
   "Generic Associated Channel (G-ACh) Parameters" registry set:

   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>.

   [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>.



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   [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>.

   [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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.







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   [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-07 (work in progress),
              May 2020.

   [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-03
              (work in progress), June 2020.

   [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-02 (work in progress), March 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-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, "MPLS Data Plane Encapsulation for In-situ
              OAM Data", draft-gandhi-mpls-ioam-sr-02 (work in
              progress), March 2020.

   [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-02 (work
              in progress), March 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.  Thanks to Huaimo Chen for MPLS-RT expert review.

Contributors

   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



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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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