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SPRING Working Group                                      R. Gandhi, Ed.
Internet-Draft                                               C. Filsfils
Intended status: Informational                       Cisco Systems, Inc.
Expires: September 24, 2020                                     D. Voyer
                                                             Bell Canada
                                                                 M. Chen
                                                                  Huawei
                                                             B. Janssens
                                                                    Colt
                                                          March 23, 2020


 Performance Measurement Using TWAMP Light for Segment Routing Networks
                   draft-gandhi-spring-twamp-srpm-08

Abstract

   Segment Routing (SR) leverages the source routing paradigm.  SR is
   applicable to both Multiprotocol Label Switching (SR-MPLS) and IPv6
   (SRv6) data planes.  This document specifies procedure for sending
   and processing probe query and response messages for Performance
   Measurement (PM) in Segment Routing networks.  The procedure uses the
   messages defined in RFC 5357 (Two-Way Active Measurement Protocol
   (TWAMP) Light) for Delay Measurement, and uses the messages defined
   in this document for Loss Measurement.  The procedure specified is
   applicable to SR-MPLS and SRv6 data planes and is used for both Links
   and end-to-end SR Policies.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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

   This Internet-Draft will expire on September 24, 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 . . . . . . . . . . . . . .   3
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     2.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Reference Topology  . . . . . . . . . . . . . . . . . . .   5
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Example Provisioning Model  . . . . . . . . . . . . . . .   6
   4.  Probe Messages  . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Probe Query Message . . . . . . . . . . . . . . . . . . .   7
       4.1.1.  Delay Measurement Query Message . . . . . . . . . . .   7
       4.1.2.  Loss Measurement Query Message  . . . . . . . . . . .   8
       4.1.3.  Probe Query for Links . . . . . . . . . . . . . . . .   9
       4.1.4.  Probe Query for End-to-end Measurement for SR Policy    9
       4.1.5.  Control Code Field for TWAMP Light Messages . . . . .  10
       4.1.6.  Loss Measurement Query Message Formats for TWAMP
               Light . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.2.  Probe Response Message  . . . . . . . . . . . . . . . . .  14
       4.2.1.  One-way Measurement Mode  . . . . . . . . . . . . . .  15
       4.2.2.  Two-way Measurement Mode  . . . . . . . . . . . . . .  15
       4.2.3.  Loopback Measurement Mode . . . . . . . . . . . . . .  17
       4.2.4.  Loss Measurement Response Message Formats for TWAMP
               Light . . . . . . . . . . . . . . . . . . . . . . . .  17
   5.  Performance Measurement for P2MP SR Policies  . . . . . . . .  19
   6.  ECMP Support for SR Policies  . . . . . . . . . . . . . . . .  20
   7.  Additional Message Processing Rules . . . . . . . . . . . . .  20
     7.1.  TTL and Hop Limit . . . . . . . . . . . . . . . . . . . .  21
     7.2.  Router Alert Option . . . . . . . . . . . . . . . . . . .  21
     7.3.  UDP Checksum  . . . . . . . . . . . . . . . . . . . . . .  21
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22



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     10.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     10.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   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).

   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 using probe messages.  These protocols rely on
   control-channel signaling to establish a test-channel over an UDP
   path.  The TWAMP Light [Appendix I in RFC5357] [BBF.TR-390] provides
   simplified mechanisms for active performance measurement in Customer
   IP networks by provisioning UDP paths and eliminates the control-
   channel signaling.  As described in Appendix A of [RFC8545], TWAMP
   Light mechanism is informative only.  These protocols lack support
   for direct-mode Loss Measurement (LM) to detect actual Customer data
   traffic loss which is required in SR networks.

   This document specifies procedures for sending and processing probe
   query and response messages for Performance Measurement in SR
   networks.  The procedure uses the messages defined in [RFC5357]
   (TWAMP Light) for Delay Measurement (DM), and uses the messages
   defined in this document for Loss Measurement.  The procedure
   specified is applicable to SR-MPLS and SRv6 data planes and are used
   for both Links and end-to-end SR Policies.  This document also
   defines mechanisms for handling ECMPs of SR Policies for performance
   delay measurement.  Unless otherwise specified, the messages defined
   in [RFC5357] are not modified by this document.

2.  Conventions Used in This Document








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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

   BSID: Binding Segment ID.

   DM: Delay Measurement.

   ECMP: Equal Cost Multi-Path.

   HMAC: Hashed Message Authentication Code.

   LM: Loss Measurement.

   MPLS: Multiprotocol Label Switching.

   NTP: Network Time Protocol.

   OWAMP: One-Way Active Measurement Protocol.

   PM: Performance Measurement.

   PSID: Path Segment Identifier.

   PTP: Precision Time Protocol.

   SID: Segment ID.

   SL: Segment List.

   SR: Segment Routing.

   SRH: Segment Routing Header.

   SR-MPLS: Segment Routing with MPLS data plane.

   SRv6: Segment Routing with IPv6 data plane.

   TC: Traffic Class.

   TWAMP: Two-Way Active Measurement Protocol.





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2.3.  Reference Topology

   In the reference topology shown below, the sender node R1 initiates a
   probe query for performance measurement and the reflector node R5
   sends a probe response for the query message received.  The probe
   response is sent to the sender node R1.  The nodes R1 and R5 may be
   directly connected via a Link 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                       Reflector

                         Reference Topology

3.  Overview

   For one-way, two-way and round-trip delay measurements in Segment
   Routing networks, the probe messages defined in [RFC5357] are used.
   For direct-mode and inferred-mode loss measurements in Segment
   Routing networks, the messages defined in this document are used.
   Separate UDP destination port numbers are user-configured for delay
   and loss measurements.  As specified in [RFC8545], the reflector
   supports the destination UDP port 862 for delay measurement probe
   messages by default.  This UDP port however, is not used for loss
   measurement probe messages defined in this document.  The sender uses
   the UDP port number following the guidelines specified in Section 6
   in [RFC6335].  For both Links and end-to-end SR Policies, no PM
   session for delay or loss measurement is created on the reflector
   node R5 [RFC5357].

   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 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



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      collect the receive traffic counters for the incoming link or
      incoming SID where the probe query messages are received at the
      reflector node (incoming link or incoming SID needed since the
      reflector node does not have PM session state present).

   The In-Situ Operations, Administration, and Maintenance (IOAM)
   mechanisms for SR-MPLS defined in [I-D.gandhi-mpls-ioam-sr] and for
   SRv6 defined in [I-D.ali-spring-ioam-srv6] are used to carry PM
   information such as timestamp in-band as part of the data packets,
   and are outside the scope of this document.

3.1.  Example Provisioning Model

   An example of a provisioning model and typical measurement parameters
   for each user-configured destination UDP port for performance delay
   and loss measurements is shown in the following Figure 1:


                             +------------+
                             | Controller |
                             +------------+
   Destination UDP Port           /  \         Destination UDP port
   Measurement Protocol          /    \        Measurement Protocol
   Measurement Type             /      \       Measurement Type
     Delay/Loss                /        \        Delay/Loss
   Authentication Mode & Key  /          \     Authentication Mode & Key
   Timestamp Format          /            \    Loss Measurement Mode
   Delay Measurement Mode   /              \
   Loss Measurement Mode   /                \
                          v                  v
                     +-------+            +-------+
                     |       |            |       |
                     |   R1  |------------|   R5  |
                     |       |            |       |
                     +-------+            +-------+
                      Sender              Reflector

                   Figure 1: Example Provisioning Model

   Example of Measurement Protocol is TWAMP Light, the Timestamp Format
   is PTPv2 [IEEE1588] or NTP and the Loss Measurement mode is inferred-
   mode or direct-mode.  The mechanisms to provision the sender and
   reflector nodes are outside the scope of this document.

   The reflector node R5 uses the parameters for the timestamp format
   and delay measurement mode (i.e. one-way, two-way or loopback mode)
   from the received probe query message.




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4.  Probe Messages

4.1.  Probe Query Message

   The probe messages defined in [RFC5357] are used for Delay
   Measurement for Links and end-to-end SR Policies.  For Loss
   Measurement, the probe messages defined in this document are used.

   The Sender IPv4 or IPv6 address is used as the source address.  When
   known, the reflector IPv4 or IPv6 address is used as the destination
   address.  If not known, the address in the range of 127/8 for IPv4 or
   0:0:0:0:0:FFFF:7F00/104 for IPv6 is used as destination address.
   This is the case for example, when using SR Policy with IPv4 endpoint
   of 0.0.0.0 or IPv6 endpoint of ::0
   [I-D.ietf-spring-segment-routing-policy].

4.1.1.  Delay Measurement Query Message

   The message content for Delay Measurement probe query message using
   UDP header [RFC0768] is shown in Figure 2.  The DM probe query
   message is sent with user-configured Destination UDP port number for
   DM.  The Destination UDP port cannot be used as Source port, since
   the message does not have any indication to distinguish between the
   query and response message.  The payload of the DM probe query
   message contains the delay measurement message defined in
   Section 4.1.2 of [RFC5357].  For symmetrical size query and response
   messages as defined in [RFC6038], the DM probe query message contains
   the payload format defined in Section 4.2.1 of [RFC5357].

    +---------------------------------------------------------------+
    | IP Header                                                     |
    .  Source IP Address = Sender IPv4 or IPv6 Address              .
    .  Destination IP Address = Reflector IPv4 or IPv6 Address      .
    .  Protocol = UDP                                               .
    .                                                               .
    +---------------------------------------------------------------+
    | UDP Header                                                    |
    .  Source Port = As chosen by Sender                            .
    .  Destination Port = User-configured Port for Delay Measurement.
    .                                                               .
    +---------------------------------------------------------------+
    | Payload = Message as specified in Section 4.2.1 of RFC 5357 | |
    . Payload = Message as specified in Section 4.1.2 of RFC 5357 | .
    .                                                               .
    +---------------------------------------------------------------+

                     Figure 2: DM Probe Query Message




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   Timestamp field is eight bytes and use the format defined in
   Section 4.2.1 of [RFC5357].  It is recommended to use the IEEE 1588v2
   Precision Time Protocol (PTP) truncated 64-bit timestamp format
   [IEEE1588] as specified in [RFC8186], with hardware support in
   Segment Routing networks.

4.1.1.1.  Delay Measurement Authentication Mode

   When using the authenticated mode for delay measurement, the matching
   authentication type (e.g.  HMAC-SHA-256) and key are user-configured
   on both the sender and reflector nodes.  A separate user-configured
   destination UDP port is used for the delay measurement in
   authentication mode due to the different probe message format.

4.1.2.  Loss Measurement Query Message

   The message content for Loss Measurement probe query message using
   UDP header [RFC0768] is shown in Figure 3.  The LM probe query
   message is sent with user-configured Destination UDP port number for
   LM, which is a different Destination UDP port number than DM.
   Separate Destination UDP ports are used for direct-mode and inferred-
   mode loss measurements.  The Destination UDP port cannot be used as
   Source port, since the message does not have any indication to
   distinguish between the query and response message.  The LM probe
   query message contains the payload for loss measurement as defined in
   Figure 7 and Figure 8.

    +---------------------------------------------------------------+
    | IP Header                                                     |
    .  Source IP Address = Sender IPv4 or IPv6 Address              .
    .  Destination IP Address = Reflector IPv4 or IPv6 Address      .
    .  Protocol = UDP                                               .
    .                                                               .
    +---------------------------------------------------------------+
    | UDP Header                                                    |
    .  Source Port = As chosen by Sender                            .
    .  Destination Port = User-configured Port for Loss Measurement .
    .                                                               .
    +---------------------------------------------------------------+
    | Payload = Message as specified in Figure 7 or 8               |
    .                                                               .
    +---------------------------------------------------------------+

                     Figure 3: LM Probe Query Message







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4.1.2.1.  Loss Measurement Authentication Mode

   When using the authenticated mode for loss measurement, the matching
   authentication type (e.g.  HMAC-SHA-256) and key are user-configured
   on both the sender and reflector nodes.  A separate user-configured
   destination UDP port is used for the loss measurement in
   authentication mode due to the different message format.

4.1.3.  Probe Query for Links

   The probe query message as defined in Figure 2 for delay measurement
   and Figure 3 for loss measurement is sent on the congruent path of
   the data traffic.  The probe messages are routed over the Link for
   both delay and loss measurement.

4.1.4.  Probe Query for End-to-end Measurement for SR Policy

   The performance delay and loss measurement for segment routing is
   applicable to both SR-MPLS and SRv6 Policies.

4.1.4.1.  Probe Query Message for SR-MPLS Policy

   The probe query messages for end-to-end performance measurement of an
   SR-MPLS Policy is sent using its SR-MPLS header containing the MPLS
   segment list as shown in Figure 4.

    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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Segment(1)             | TC  |S|      TTL      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Segment(n)             | TC  |S|      TTL      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                PSID                   | TC  |S|      TTL      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Message as shown in Figure 2 for DM or Figure 3 for LM      |
    .                                                               .
    +---------------------------------------------------------------+

             Figure 4: Probe Query Message for SR-MPLS Policy

   The Segment List (SL) can be empty to indicate Implicit NULL label
   case for a single-hop SR Policy.




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   The Path Segment Identifier (PSID)
   [I-D.ietf-spring-mpls-path-segment] of the SR-MPLS Policy is used for
   accounting received traffic on the egress node for loss measurement.

4.1.4.2.  Probe Query Message for SRv6 Policy

   An SRv6 Policy setup using the SRv6 Segment Routing Header (SRH) and
   a Segment List as defined in [RFC8754].  For SRv6, network
   programming is defined in [I-D.ietf-spring-srv6-network-programming].
   The probe query messages for end-to-end performance measurement of an
   SRv6 Policy is sent using its SRH with Segment List as shown in
   Figure 5.

    +---------------------------------------------------------------+
    |                           SRH                                 |
    .                                                               .
    +---------------------------------------------------------------+
    |   Message as shown in Figure 2 for DM or Figure 3 for LM      |
    .   (Using IPv6 Source and Destination Addresses)               .
    .                                                               .
    +---------------------------------------------------------------+

               Figure 5: Probe Query Message for SRv6 Policy

   For delay measurement of SRv6 Policy using SRH, END function END.OTP
   [I-D.ietf-6man-spring-srv6-oam] is used with the target SRv6 SID to
   punt probe messages on the target node, as shown in Figure 5.
   Similarly, for loss measurement of SRv6 Policy, END function END.OP
   [I-D.ietf-6man-spring-srv6-oam] is used with target SRv6 SID to punt
   probe messages on the target node.

4.1.5.  Control Code Field for TWAMP Light Messages

   The Control Code field is defined for delay and loss measurement
   probe query and response messages for TWAMP Light in unauthenticated
   and authenticated modes.  The modified delay measurement probe query
   and response message format is shown in Figure 6.  This message
   format is backwards compatible with the message format defined in
   [RFC5357] as its reflectors ignore the received field (previously
   identified as MBZ).  The usage of the Control Code is not limited to
   the SR networks and can be used for various bidirectional paths in a
   network.









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    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Timestamp                            |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Error Estimate        |  MBZ                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         MBZ                                   |Se Control Code|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    .                                                               .



    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Session-Sender Error Estimate | MBZ           |Re Control Code|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Ses-Sender TTL |                 MBZ                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    .                                                               .

               Figure 6: Control Code in TWAMP Light Message

   Sender Control Code: Set as follows in TWAMP Light probe query
   message.

   For a Query:

       0x0: Out-of-band Response Requested.  Indicates that the probe
       response is not required over the same path in the reverse
       direction.  This is also the default behavior.

       0x1: In-band Response Requested.  Indicates that this query has
       been sent over a bidirectional path and the probe response is
       required over the same path in the reverse direction.  The
       bidirectional path does not have to be an SR path.

   Reflector Control Code: Set as follows in TWAMP Light probe response
   message.

   For a Response:

       0x1: Error - Invalid Message.  Indicates that the operation
       failed because the received query message could not be processed.



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       Additional Error Codes to be defined in future.

4.1.6.  Loss Measurement Query Message Formats for TWAMP Light

   In this document, TWAMP Light probe query message formats are defined
   for loss measurement as shown in Figure 7 and Figure 8.  The message
   formats are hardware efficient due to the well-known locations of the
   counters.  They are similar to the delay measurement message formats
   (e.g. location of the Counter and Timestamp) and do not require any
   backwards compatibility or support for the existing DM message
   formats from [RFC5357] as different user-configured destination UDP
   port is used for loss measurement.

    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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Sequence Number                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Transmit Counter                       |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |X|B| Reserved  | Block Number  | MBZ                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         MBZ                                   |Se Control Code|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                        Packet Padding                         .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 7: TWAMP Light LM Probe Query Message - Unauthenticated Mode




















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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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Sequence Number                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        MBZ (12 octets)                        |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Transmit Counter                       |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |X|B| Reserved  | Block Number  | MBZ                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         MBZ                                   |Se Control Code|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        HMAC (16 octets)                       |
    |                                                               |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                        Packet Padding                         .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 8: TWAMP Light LM Probe Query Message - Authenticated Mode

   Sequence Number (32-bit): As defined in [RFC5357].

   Transmit Counter (64-bit): The number of packets or octets sent by
   the sender node in the query message and by the reflector node in the
   response message.  The counter is always written at the well-known
   location in the probe query and response messages.

   Receive Counter (64-bit): The number of packets or octets received at
   the reflector node.  It is written by the reflector node in the probe
   response message.

   Sender Counter (64-bit): This is the exact copy of the transmit
   counter from the received query message.  It is written by the
   reflector node in the probe response message.

   Sender Sequence Number (32-bit): As defined in [RFC5357].

   Sender TTL: As defined in Section 7.1.

   LM Flags: The meanings of the Flag bits are:



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      X: Extended counter format indicator.  Indicates the use of
      extended (64-bit) counter values.  Initialized to 1 upon creation
      (and prior to transmission) of an LM Query and copied from an LM
      Query to an LM response.  Set to 0 when the LM message is
      transmitted or received over an interface that writes 32-bit
      counter values.

      B: Octet (byte) count.  When set to 1, indicates that the Counter
      1-4 fields represent octet counts.  The octet count applies to all
      packets within the LM scope, and the octet count of a packet sent
      or received includes the total length of that packet (but excludes
      headers, labels, or framing of the channel itself).  When set to
      0, indicates that the Counter fields represent packet counts.

   Block Number (8-bit): 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.

   HMAC: The PM probe message in authenticated mode includes a key
   Hashed Message Authentication Code (HMAC) ([RFC2104]) hash.  Each
   probe query and response messages are authenticated by adding
   Sequence Number with Hashed Message Authentication Code (HMAC) TLV.
   It can use HMAC-SHA-256 truncated to 128 bits (similarly to the use
   of it in IPSec defined in [RFC4868]); hence the length of the HMAC
   field is 16 octets.

   HMAC uses its own key and the mechanism to distribute the HMAC key is
   outside the scope of this document.

   In authenticated mode, only the sequence number is encrypted, and the
   other payload fields are sent in clear text.  The probe message may
   include Comp.MBZ (Must Be Zero) variable length field to align the
   packet on 16 octets boundary.

4.2.  Probe Response Message

   The probe response message is sent using the IP/UDP information from
   the received probe query message.  The content of the probe response
   message is shown in Figure 9.









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    +---------------------------------------------------------------+
    | IP Header                                                     |
    .  Source IP Address = Reflector IPv4 or IPv6 Address           .
    .  Destination IP Address = Source IP Address from Query        .
    .  Protocol = UDP                                               .
    .                                                               .
    +---------------------------------------------------------------+
    | UDP Header                                                    |
    .  Source Port = As chosen by Reflector                         .
    .  Destination Port = Source Port from Query                    .
    .                                                               .
    +---------------------------------------------------------------+
    | DM Payload as specified in Section 4.2.1 of RFC 5357 |        |
    . LM Payload as specified in Figure 12 or 13                    .
    .                                                               .
    +---------------------------------------------------------------+

                     Figure 9: Probe Response Message

4.2.1.  One-way Measurement Mode

   In one-way performance measurement mode, the probe response message
   as defined in Figure 9 is sent back out-of-band to the sender node,
   for both Links and SR Policies.  The Sender Control Code is set to
   "Out-of-band Response Requested".  In this delay measurement mode, as
   per Reference Topology, all timestamps t1, t2, t3, and t4 are
   collected by the probes.  However, only timestamps t1 and t2 are
   needed to measure one-way delay.

4.2.2.  Two-way Measurement Mode

   In two-way performance measurement mode, when using a bidirectional
   path, the probe response message as defined in Figure 9 is sent back
   to the sender node on the congruent path of the data traffic on the
   same reverse direction Link or associated reverse SR Policy
   [I-D.ietf-pce-sr-bidir-path].  The Sender Control Code 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 needed to measure two-way
   delay.

   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.






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4.2.2.1.  Probe Response Message for SR-MPLS Policy

   The message content for sending probe response message for two-way
   end-to-end performance measurement of an SR-MPLS Policy is shown in
   Figure 10.

    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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Segment(1)             | TC  |S|      TTL      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                               .
    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Segment(n)             | TC  |S|      TTL      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Message as shown in Figure 9                   |
    .                                                               .
    +---------------------------------------------------------------+

           Figure 10: Probe Response Message for SR-MPLS Policy

   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.

4.2.2.2.  Probe Response Message for SRv6 Policy

   The message content for sending probe response message on the
   congruent path of the data traffic for two-way end-to-end performance
   measurement of an SRv6 Policy with SRH is shown in Figure 11.

    +---------------------------------------------------------------+
    |                           SRH                                 |
    .                                                               .
    +---------------------------------------------------------------+
    |   Message as shown in Figure 9                                |
    .   (Using IPv6 Source and Destination Addresses)               .
    .                                                               .
    +---------------------------------------------------------------+

             Figure 11: Probe Response Message for SRv6 Policy






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4.2.3.  Loopback Measurement Mode

   The Loopback measurement mode can be used to measure round-trip delay
   for a bidirectional SR Path.  The IP header of the probe query
   message contains the destination address equals to the sender address
   and the source address equals to the reflector address.  Optionally,
   the probe query message can carry the reverse path information (e.g.
   reverse path label stack for SR-MPLS) as part of the SR header.  The
   probe messages are not punted at the reflector node and it does not
   process them and generate response messages.  The Sender Control Code
   is set to the default value of 0.  In this mode, as the probe packet
   is not punted on the reflector node for processing, the querier
   copies the 'Sequence Number' in 'Session-Sender Sequence Number'
   directly.  In this delay measurement mode, as per Reference Topology,
   the timestamps t1 and t4 are collected by the probes.  Both these
   timestamps are needed to measure round-trip delay.

4.2.4.  Loss Measurement Response Message Formats for TWAMP Light

   In this document, TWAMP Light probe response message formats are
   defined for loss measurement as shown in Figure 12 and Figure 13.






























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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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Sequence Number                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Transmit Counter                       |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |X|B| Reserved  | Block Number  | MBZ                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Receive Counter                        |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Sender Sequence Number                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Sender Counter                         |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |X|B| Reserved  |Sender Block Nu| MBZ           |Re Control Code|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Sender TTL   |                                               |
    +-+-+-+-+-+-+-+-+                                               +
    |                        Packet Padding                         |
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 12: TWAMP Light LM Probe Response Message - Unauthenticated
                                   Mode

    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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Sequence Number                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        MBZ (12 octets)                        |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Transmit Counter                       |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |X|B| Reserved  | Block Number  | MBZ                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        MBZ (4 octets)                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Receive Counter                        |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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    |                        MBZ (8 octets)                         |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Sender Sequence Number                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        MBZ (12 octets)                        |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Sender Counter                         |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |X|B| Reserved  |Sender Block Nu| MBZ           |Re Control Code|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        MBZ (4 octets)                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Sender TTL   |                                               |
    +-+-+-+-+-+-+-+-+                                               |
    |                        MBZ (15 octets)                        |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        HMAC (16 octets)                       |
    |                                                               |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                        Packet Padding                         .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 13: TWAMP Light LM Probe Response Message - Authenticated Mode

5.  Performance Measurement for P2MP SR Policies

   The procedures for 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 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 14.

   o  Each reflector leaf node sends its IP address in the Source
      Address of the probe response messages as shown in Figure 9.  This



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      allows the sender root node to identify the reflector 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.

     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      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Message as shown in Figure 2 for DM or Figure 3 for LM      |
    .                                                               .
    +---------------------------------------------------------------+

       Figure 14: Query with Replication Segment for SR-MPLS Policy

6.  ECMP Support 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 PM probe
   messages need to be sent to traverse different ECMP paths to measure
   performance delay of an SR Policy.

   Forwarding plane has various hashing functions available to forward
   packets on specific ECMP paths.  The mechanisms described in
   [RFC8029] and [RFC5884] for handling ECMPs are also applicable to the
   performance measurement.  In the IP header of the PM probe messages,
   sweeping of Destination Addresses in 127/8 range for IPv4 or
   0:0:0:0:0:FFFF:7F00/104 range for IPv6 can be used to exercise
   particular ECMP paths.  As specified in [RFC6437], Flow Label field
   in the outer IPv6 header can also be used for sweeping.

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

7.  Additional Message Processing Rules

   The processing rules defined in this section are applicable to TWAMP
   Light messages for delay and loss measurement for Links and end-to-
   end SR Policies.








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7.1.  TTL and Hop Limit

   The TTL field in the IPv4 and MPLS headers of the probe query
   messages is set to 255 [RFC5357].  Similarly, the Hop Limit field in
   the IPv6 and SRH headers of the probe query messages is set to 255
   [RFC5357].

   When using the Destination IPv4 Address from the 127/8 range, the TTL
   in the IPv4 header is set to 1 [RFC8029].  Similarly, when using the
   Destination IPv6 Address from the 0:0:0:0:0:FFFF:7F00/104 range, the
   Hop Limit field in the inner IPv6 header is set to 1 whereas in the
   outer IPv6 header is set to 255.

   For Link performance delay and loss measurements, the TTL and Hop
   Limit field in the probe message is set to 1 in both one-way and two-
   way measurement modes.

7.2.  Router Alert Option

   The Router Alert IP option is not set when using the routable
   Destination IP Address in the probe messages.

   When using the Destination IPv4 Address from the 127/8 range, to be
   able to punt probe packets on the reflector node, the Router Alert IP
   Option of value 0x0 [RFC2113] for IPv4 may be added [RFC8029].
   Similarly, when using the Destination IPv6 Address from the
   0:0:0:0:0:FFFF:7F00/104 range, the Router Alert IP Option of value 69
   [RFC7506] for IPv6 may be added in the destination option header,
   Section 4.6 of [RFC8200].  For SRv6 Policy using SRH, it is added in
   the inner IPv6 header.

7.3.  UDP Checksum

   The UDP Checksum Complement for delay and loss measurement messages
   follows the procedure defined in [RFC7820] and can be optionally used
   with the procedures defined in this document.

   For IPv4 and IPv6 probe messages, where the hardware is not capable
   of re-computing the UDP checksum or adding checksum complement
   [RFC7820], the sender node sets the UDP checksum to 0 [RFC6936]
   [RFC8085].  The receiving node bypasses the checksum validation and
   accepts the packets with UDP checksum value 0 for the UDP port being
   used for PM delay and loss measurements.








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8.  Security Considerations

   The performance measurement is intended for deployment in well-
   managed private and service provider networks.  As such, it assumes
   that a node involved in a measurement operation has previously
   verified the integrity of the path and the identity of the far-end
   reflector node.

   If desired, attacks can be mitigated by performing basic validation
   and sanity checks, at the sender, of the counter or timestamp fields
   in received measurement response messages.  The minimal state
   associated with these protocols also limits the extent of measurement
   disruption that can be caused by a corrupt or invalid message to a
   single query/response cycle.

   Use of HMAC-SHA-256 in the authenticated mode protects the data
   integrity of the probe messages.  SRv6 has HMAC protection
   authentication defined for SRH [RFC8754].  Hence, PM probe messages
   for SRv6 may not need authentication mode.  Cryptographic measures
   may be enhanced by the correct configuration of access-control lists
   and firewalls.

9.  IANA Considerations

   This document does not require any IANA action.

10.  References

10.1.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.

   [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>.

   [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>.



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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>.

   [I-D.ietf-6man-spring-srv6-oam]
              Ali, Z., Filsfils, C., Matsushima, S., Voyer, D., and M.
              Chen, "Operations, Administration, and Maintenance (OAM)
              in Segment Routing Networks with IPv6 Data plane (SRv6)",
              draft-ietf-6man-spring-srv6-oam-03 (work in progress),
              December 2019.

10.2.  Informative References

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

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [RFC2113]  Katz, D., "IP Router Alert Option", RFC 2113,
              DOI 10.17487/RFC2113, February 1997,
              <https://www.rfc-editor.org/info/rfc2113>.

   [RFC4868]  Kelly, S. and S. Frankel, "Using HMAC-SHA-256, HMAC-SHA-
              384, and HMAC-SHA-512 with IPsec", RFC 4868,
              DOI 10.17487/RFC4868, May 2007,
              <https://www.rfc-editor.org/info/rfc4868>.

   [RFC5884]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
              "Bidirectional Forwarding Detection (BFD) for MPLS Label
              Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884,
              June 2010, <https://www.rfc-editor.org/info/rfc5884>.

   [RFC6038]  Morton, A. and L. Ciavattone, "Two-Way Active Measurement
              Protocol (TWAMP) Reflect Octets and Symmetrical Size
              Features", RFC 6038, DOI 10.17487/RFC6038, October 2010,
              <https://www.rfc-editor.org/info/rfc6038>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <https://www.rfc-editor.org/info/rfc6335>.



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   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437,
              DOI 10.17487/RFC6437, November 2011,
              <https://www.rfc-editor.org/info/rfc6437>.

   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, DOI 10.17487/RFC6936, April 2013,
              <https://www.rfc-editor.org/info/rfc6936>.

   [RFC7506]  Raza, K., Akiya, N., and C. Pignataro, "IPv6 Router Alert
              Option for MPLS Operations, Administration, and
              Maintenance (OAM)", RFC 7506, DOI 10.17487/RFC7506, April
              2015, <https://www.rfc-editor.org/info/rfc7506>.

   [RFC7820]  Mizrahi, T., "UDP Checksum Complement in the One-Way
              Active Measurement Protocol (OWAMP) and Two-Way Active
              Measurement Protocol (TWAMP)", RFC 7820,
              DOI 10.17487/RFC7820, March 2016,
              <https://www.rfc-editor.org/info/rfc7820>.

   [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
              Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
              Switched (MPLS) Data-Plane Failures", RFC 8029,
              DOI 10.17487/RFC8029, March 2017,
              <https://www.rfc-editor.org/info/rfc8029>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

   [RFC8186]  Mirsky, G. and I. Meilik, "Support of the IEEE 1588
              Timestamp Format in a Two-Way Active Measurement Protocol
              (TWAMP)", RFC 8186, DOI 10.17487/RFC8186, June 2017,
              <https://www.rfc-editor.org/info/rfc8186>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [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>.





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   [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>.

   [RFC8545]  Morton, A., Ed. and G. Mirsky, Ed., "Well-Known Port
              Assignments for the One-Way Active Measurement Protocol
              (OWAMP) and the Two-Way Active Measurement Protocol
              (TWAMP)", RFC 8545, DOI 10.17487/RFC8545, March 2019,
              <https://www.rfc-editor.org/info/rfc8545>.

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

   [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.

   [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.ietf-spring-srv6-network-programming]
              Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
              Matsushima, S., and Z. Li, "SRv6 Network Programming",
              draft-ietf-spring-srv6-network-programming-14 (work in
              progress), March 2020.

   [BBF.TR-390]
              "Performance Measurement from IP Edge to Customer
              Equipment using TWAMP Light", BBF TR-390, May 2017.








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   [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.ali-spring-ioam-srv6]
              Ali, Z., Gandhi, R., Filsfils, C., Brockners, F., Kumar,
              N., Pignataro, C., Li, C., Chen, M., and G. Dawra,
              "Segment Routing Header encapsulation for In-situ OAM
              Data", draft-ali-spring-ioam-srv6-02 (work in progress),
              November 2019.

   [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 Performance Measurement in Segment Routing.  The
   authors would also like to thank Greg Mirsky for reviewing this
   document and providing useful comments and suggestions.  Patrick
   Khordoc and Radu Valceanu, both from Cisco Systems have helped
   significantly improve the mechanisms defined in this document.  The
   authors would like to acknowledge the earlier work on the loss
   measurement using TWAMP described in draft-xiao-ippm-twamp-ext-
   direct-loss.

Authors' Addresses

   Rakesh Gandhi (editor)
   Cisco Systems, Inc.
   Canada

   Email: rgandhi@cisco.com


   Clarence Filsfils
   Cisco Systems, Inc.

   Email: cfilsfil@cisco.com







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   Daniel Voyer
   Bell Canada

   Email: daniel.voyer@bell.ca


   Mach(Guoyi) Chen
   Huawei

   Email: mach.chen@huawei.com


   Bart Janssens
   Colt

   Email: Bart.Janssens@colt.net



































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