path: root/doc/rfc/rfc6374.txt

Internet Engineering Task Force (IETF)                          D. Frost
Request for Comments: 6374                                     S. Bryant
Category: Standards Track                                  Cisco Systems
ISSN: 2070-1721                                           September 2011


          Packet Loss and Delay Measurement for MPLS Networks

Abstract

   Many service provider 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 and channel throughput.  This measurement capability
   also provides operators with greater visibility into the performance
   characteristics of their networks, thereby facilitating planning,
   troubleshooting, and network performance evaluation.  This document
   specifies protocol mechanisms to enable the efficient and accurate
   measurement of these performance metrics in MPLS networks.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6374.

Copyright Notice

   Copyright (c) 2011 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
   (http://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.



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Table of Contents

   1. Introduction ....................................................3
      1.1. Applicability and Scope ....................................5
      1.2. Terminology ................................................6
      1.3. Requirements Language ......................................6
   2. Overview ........................................................6
      2.1. Basic Bidirectional Measurement ............................6
      2.2. Packet Loss Measurement ....................................7
      2.3. Throughput Measurement ....................................10
      2.4. Delay Measurement .........................................10
      2.5. Delay Variation Measurement ...............................12
      2.6. Unidirectional Measurement ................................12
      2.7. Dyadic Measurement ........................................13
      2.8. Loopback Measurement ......................................13
      2.9. Measurement Considerations ................................14
           2.9.1. Types of Channels ..................................14
           2.9.2. Quality of Service .................................14
           2.9.3. Measurement Point Location .........................14
           2.9.4. Equal Cost Multipath ...............................15
           2.9.5. Intermediate Nodes .................................15
           2.9.6. Different Transmit and Receive Interfaces ..........16
           2.9.7. External Post-Processing ...........................16
           2.9.8. Loss Measurement Modes .............................16
           2.9.9. Loss Measurement Scope .............................18
           2.9.10. Delay Measurement Accuracy ........................18
           2.9.11. Delay Measurement Timestamp Format ................18
   3. Message Formats ................................................19
      3.1. Loss Measurement Message Format ...........................19
      3.2. Delay Measurement Message Format ..........................25
      3.3. Combined Loss/Delay Measurement Message Format ............27
      3.4. Timestamp Field Formats ...................................28
      3.5. TLV Objects ...............................................29
           3.5.1. Padding ............................................30
           3.5.2. Addressing .........................................31
           3.5.3. Loopback Request ...................................31
           3.5.4. Session Query Interval .............................32
   4. Operation ......................................................33
      4.1. Operational Overview ......................................33
      4.2. Loss Measurement Procedures ...............................34
           4.2.1. Initiating a Loss Measurement Operation ............34
           4.2.2. Transmitting a Loss Measurement Query ..............34
           4.2.3. Receiving a Loss Measurement Query .................35
           4.2.4. Transmitting a Loss Measurement Response ...........35
           4.2.5. Receiving a Loss Measurement Response ..............36
           4.2.6. Loss Calculation ...................................36
           4.2.7. Quality of Service .................................37
           4.2.8. G-ACh Packets ......................................37



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           4.2.9. Test Messages ......................................37
           4.2.10. Message Loss and Packet Misorder Conditions .......38
      4.3. Delay Measurement Procedures ..............................39
           4.3.1. Transmitting a Delay Measurement Query .............39
           4.3.2. Receiving a Delay Measurement Query ................39
           4.3.3. Transmitting a Delay Measurement Response ..........40
           4.3.4. Receiving a Delay Measurement Response .............41
           4.3.5. Timestamp Format Negotiation .......................41
                  4.3.5.1. Single-Format Procedures ..................42
           4.3.6. Quality of Service .................................42
      4.4. Combined Loss/Delay Measurement Procedures ................42
   5. Implementation Disclosure Requirements .........................42
   6. Congestion Considerations ......................................44
   7. Manageability Considerations ...................................44
   8. Security Considerations ........................................45
   9. IANA Considerations ............................................46
      9.1. Allocation of PW Associated Channel Types .................47
      9.2. Creation of Measurement Timestamp Type Registry ...........47
      9.3. Creation of MPLS Loss/Delay Measurement Control
           Code Registry .............................................47
      9.4. Creation of MPLS Loss/Delay Measurement TLV Object
           Registry ..................................................49
   10. Acknowledgments ...............................................49
   11. References ....................................................49
      11.1. Normative References .....................................49
      11.2. Informative References ...................................50
   Appendix A. Default Timestamp Format Rationale ....................52

1.  Introduction

   Many service provider 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 and channel throughput.  This measurement capability
   also provides operators with greater visibility into the performance
   characteristics of their networks, thereby facilitating planning,
   troubleshooting, and network performance evaluation.  This document
   specifies protocol mechanisms to enable the efficient and accurate
   measurement of these performance metrics in MPLS networks.

   This document specifies two closely related protocols, one for packet
   loss measurement (LM) and one for packet delay measurement (DM).
   These protocols have the following characteristics and capabilities:

   o  The LM and DM protocols are intended to be simple and to support
      efficient hardware processing.





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   o  The LM and DM protocols operate over the MPLS Generic Associated
      Channel (G-ACh) [RFC5586] and support measurement of loss, delay,
      and related metrics over Label Switched Paths (LSPs), pseudowires,
      and MPLS sections (links).

   o  The LM and DM protocols are applicable to the LSPs, pseudowires,
      and sections of networks based on the MPLS Transport Profile
      (MPLS-TP), because the MPLS-TP is based on a standard MPLS data
      plane.  The MPLS-TP is defined and described in [RFC5921], and
      MPLS-TP LSPs, pseudowires, and sections are discussed in detail in
      [RFC5960].  A profile describing the minimal functional subset of
      the LM and DM protocols in the MPLS-TP context is provided in
      [RFC6375].

   o  The LM and DM protocols can be used both for continuous/proactive
      and selective/on-demand measurement.

   o  The LM and DM protocols use a simple query/response model for
      bidirectional measurement that allows a single node -- the querier
      -- to measure the loss or delay in both directions.

   o  The LM and DM protocols use query messages for unidirectional loss
      and delay measurement.  The measurement can be carried out either
      at the downstream node(s) or at the querier if an out-of-band
      return path is available.

   o  The LM and DM protocols do not require that the transmit and
      receive interfaces be the same when performing bidirectional
      measurement.

   o  The DM protocol is stateless.

   o  The LM protocol is "almost" stateless: loss is computed as a delta
      between successive messages, and thus the data associated with the
      last message received must be retained.

   o  The LM protocol can perform two distinct kinds of loss
      measurement: it can measure the loss of specially generated test
      messages in order to infer the approximate data-plane loss level
      (inferred measurement) or it can directly measure data-plane
      packet loss (direct measurement).  Direct measurement provides
      perfect loss accounting, but may require specialized hardware
      support and is only applicable to some LSP types.  Inferred
      measurement provides only approximate loss accounting but is
      generally applicable.






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      The direct LM method is also known as "frame-based" in the context
      of Ethernet transport networks [Y.1731].  Inferred LM is a
      generalization of the "synthetic" measurement approach currently
      in development for Ethernet networks, in the sense that it allows
      test messages to be decoupled from measurement messages.

   o  The LM protocol supports measurement in terms of both packet
      counts and octet counts.

   o  The LM protocol supports both 32-bit and 64-bit counters.

   o  The LM protocol can be used to measure channel throughput as well
      as packet loss.

   o  The DM protocol supports multiple timestamp formats, and provides
      a simple means for the two endpoints of a bidirectional connection
      to agree on a preferred format.  This procedure reduces to a
      triviality for implementations supporting only a single timestamp
      format.

   o  The DM protocol supports varying the measurement message size in
      order to measure delays associated with different packet sizes.

   The One-Way Active Measurement Protocol (OWAMP) [RFC4656] and Two-Way
   Active Measurement Protocol (TWAMP) [RFC5357] provide capabilities
   for the measurement of various performance metrics in IP networks.
   These protocols are not streamlined for hardware processing and rely
   on IP and TCP, as well as elements of the Network Time Protocol
   (NTP), which may not be available or optimized in some network
   environments; they also lack support for IEEE 1588 timestamps and
   direct-mode LM, which may be required in some environments.  The
   protocols defined in this document thus are similar in some respects
   to, but also differ from, these IP-based protocols.

1.1.  Applicability and Scope

   This document specifies measurement procedures and protocol messages
   that are intended to be applicable in a wide variety of circumstances
   and amenable to implementation by a wide range of hardware- and
   software-based measurement systems.  As such, it does not attempt to
   mandate measurement quality levels or analyze specific end-user
   applications.









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1.2.  Terminology

   Term  Definition
   ----- -------------------------------------------
   ACH   Associated Channel Header
   DM    Delay Measurement
   ECMP  Equal Cost Multipath
   G-ACh Generic Associated Channel
   LM    Loss Measurement
   LSE   Label Stack Entry
   LSP   Label Switched Path
   NTP   Network Time Protocol
   OAM   Operations, Administration, and Maintenance
   PTP   Precision Time Protocol
   TC    Traffic Class

1.3.  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 RFC 2119 [RFC2119].

2.  Overview

   This section begins with a summary of the basic methods used for the
   bidirectional measurement of packet loss and delay.  These
   measurement methods are then described in detail.  Finally, a list of
   practical considerations is discussed that may come into play to
   inform or modify these simple procedures.  This section is limited to
   theoretical discussion; for protocol specifics, the reader is
   referred to Sections 3 and 4.

2.1.  Basic Bidirectional Measurement

   The following figure shows the reference scenario.

                             T1              T2
                   +-------+/     Query       \+-------+
                   |       | - - - - - - - - ->|       |
                   |   A   |===================|   B   |
                   |       |<- - - - - - - - - |       |
                   +-------+\     Response    /+-------+
                             T4              T3

   This figure shows a bidirectional channel between two nodes, A and B,
   and illustrates the temporal reference points T1-T4 associated with a
   measurement operation that takes place at A.  The operation consists
   of A sending a query message to B, and B sending back a response.



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   Each reference point indicates the point in time at which either the
   query or the response message is transmitted or received over the
   channel.

   In this situation, A can arrange to measure the packet loss over the
   channel in the forward and reverse directions by sending Loss
   Measurement (LM) query messages to B, each of which contains the
   count of packets transmitted prior to time T1 over the channel to B
   (A_TxP).  When the message reaches B, it appends two values and
   reflects the message back to A: the count of packets received prior
   to time T2 over the channel from A (B_RxP) and the count of packets
   transmitted prior to time T3 over the channel to A (B_TxP).  When the
   response reaches A, it appends a fourth value: the count of packets
   received prior to time T4 over the channel from B (A_RxP).

   These four counter values enable A to compute the desired loss
   statistics.  Because the transmit count at A and the receive count at
   B (and vice versa) may not be synchronized at the time of the first
   message, and to limit the effects of counter wrap, the loss is
   computed in the form of a delta between messages.

   To measure at A the delay over the channel to B, a Delay Measurement
   (DM) query message is sent from A to B containing a timestamp
   recording the instant at which it is transmitted, i.e., T1.  When the
   message reaches B, a timestamp is added recording the instant at
   which it is received (T2).  The message can now be reflected from B
   to A, with B adding its transmit timestamp (T3) and A adding its
   receive timestamp (T4).  These four timestamps enable A to compute
   the one-way delay in each direction, as well as the two-way delay for
   the channel.  The one-way delay computations require that the clocks
   of A and B be synchronized; mechanisms for clock synchronization are
   outside the scope of this document.

2.2.  Packet Loss Measurement

   Suppose a bidirectional channel exists between the nodes A and B.
   The objective is to measure at A the following two quantities
   associated with the channel:

      A_TxLoss (transmit loss): the number of packets transmitted by A
      over the channel but not received at B;

      A_RxLoss (receive loss): the number of packets transmitted by B
      over the channel but not received at A.







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   This is accomplished by initiating a Loss Measurement (LM) operation
   at A, which consists of transmission of a sequence of LM query
   messages (LM[1], LM[2], ...) over the channel at a specified rate,
   such as one every 100 milliseconds.  Each message LM[n] contains the
   following value:

      A_TxP[n]: the total count of packets transmitted by A over the
      channel prior to the time this message is transmitted.

   When such a message is received at B, the following value is recorded
   in the message:

      B_RxP[n]: the total count of packets received by B over the
      channel at the time this message is received (excluding the
      message itself).

   At this point, B transmits the message back to A, recording within it
   the following value:

      B_TxP[n]: the total count of packets transmitted by B over the
      channel prior to the time this response is transmitted.

   When the message response is received back at A, the following value
   is recorded in the message:

      A_RxP[n]: the total count of packets received by A over the
      channel at the time this response is received (excluding the
      message itself).

   The transmit loss A_TxLoss[n-1,n] and receive loss A_RxLoss[n-1,n]
   within the measurement interval marked by the messages LM[n-1] and
   LM[n] are computed by A as follows:

   A_TxLoss[n-1,n] = (A_TxP[n] - A_TxP[n-1]) - (B_RxP[n] - B_RxP[n-1])
   A_RxLoss[n-1,n] = (B_TxP[n] - B_TxP[n-1]) - (A_RxP[n] - A_RxP[n-1])

   where the arithmetic is modulo the counter size.

   (Strictly speaking, it is not necessary that the fourth count,
   A_RxP[n], actually be written in the message, but this is convenient
   for some implementations and useful if the message is to be forwarded
   on to an external measurement system.)









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   The derived values

      A_TxLoss = A_TxLoss[1,2] + A_TxLoss[2,3] + ...

      A_RxLoss = A_RxLoss[1,2] + A_RxLoss[2,3] + ...

   are updated each time a response to an LM message is received and
   processed, and they represent the total transmit and receive loss
   over the channel since the LM operation was initiated.

   When computing the values A_TxLoss[n-1,n] and A_RxLoss[n-1,n], the
   possibility of counter wrap must be taken into account.  For example,
   consider the values of the A_TxP counter at sequence numbers n-1 and
   n.  Clearly if A_TxP[n] is allowed to wrap to 0 and then beyond to a
   value equal to or greater than A_TxP[n-1], the computation of an
   unambiguous A_TxLoss[n-1,n] value will be impossible.  Therefore, the
   LM message rate MUST be sufficiently high, given the counter size and
   the speed and minimum packet size of the underlying channel, that
   this condition cannot arise.  For example, a 32-bit counter for a
   100-Gbps link with a minimum packet size of 64 bytes can wrap in 2^32
   / (10^11/(64*8)) = ~22 seconds, which is therefore an upper bound on
   the LM message interval under such conditions.  This bound will be
   referred to as the MaxLMInterval of the channel.  It is clear that
   the MaxLMInterval will be a more restrictive constraint in the case
   of direct LM and for smaller counter sizes.

   The loss measurement approach described in this section has the
   characteristic of being stateless at B and "almost" stateless at A.
   Specifically, A must retain the data associated with the last LM
   response received, in order to use it to compute loss when the next
   response arrives.  This data MAY be discarded, and MUST NOT be used
   as a basis for measurement, if MaxLMInterval elapses before the next
   response arrives, because in this case an unambiguous measurement
   cannot be made.

   The foregoing discussion has assumed the counted objects are packets,
   but this need not be the case.  In particular, octets may be counted
   instead.  This will, of course, reduce the MaxLMInterval accordingly.

   In addition to absolute aggregate loss counts, the individual loss
   counts yield other metrics, such as the average loss rate over any
   multiple of the measurement interval.  An accurate loss rate can be
   determined over time even in the presence of anomalies affecting
   individual measurements, such as those due to packet misordering
   (Section 4.2.10).






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   Note that an approach for conducting packet loss measurement in IP
   networks is documented in [RFC2680].  This approach differs from the
   one described here, for example by requiring clock synchronization
   between the measurement points and lacking support for direct-mode
   LM.

2.3.  Throughput Measurement

   If LM query messages contain a timestamp recording their time of
   transmission, this data can be combined with the packet or octet
   counts to yield measurements of the throughput offered and delivered
   over the channel during the interval in terms of the counted units.

   For a bidirectional channel, for example, given any two LM response
   messages (separated in time by not more than the MaxLMInterval), the
   difference between the counter values tells the querier the number of
   units successfully transmitted and received in the interval between
   the timestamps.  Absolute offered throughput is the number of data
   units transmitted and absolute delivered throughput is the number of
   data units received.  Throughput rate is the number of data units
   sent or received per unit time.

   Just as for loss measurement, the interval counts can be accumulated
   to arrive at the absolute throughput of the channel since the start
   of the measurement operation or be used to derive related metrics
   such as the throughput rate.  This procedure also enables out-of-
   service throughput testing when combined with a simple packet
   generator.

2.4.  Delay Measurement

   Suppose a bidirectional channel exists between the nodes A and B.
   The objective is to measure at A one or more of the following
   quantities associated with the channel:

   o  The one-way delay associated with the forward (A to B) direction
      of the channel;

   o  The one-way delay associated with the reverse (B to A) direction
      of the channel;

   o  The two-way delay (A to B to A) associated with the channel.

   The one-way delay metric for packet networks is described in
   [RFC2679].  In the case of two-way delay, there are actually two
   possible metrics of interest.  The "two-way channel delay" is the sum
   of the one-way delays in each direction and reflects the delay of the
   channel itself, irrespective of processing delays within the remote



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   endpoint B.  The "round-trip delay" is described in [RFC2681] and
   includes in addition any delay associated with remote endpoint
   processing.

   Measurement of the one-way delay quantities requires that the clocks
   of A and B be synchronized, whereas the two-way delay metrics can be
   measured directly even when this is not the case (provided A and B
   have stable clocks).

   A measurement is accomplished by sending a Delay Measurement (DM)
   query message over the channel to B that contains the following
   timestamp:

      T1: the time the DM query message is transmitted from A.

   When the message arrives at B, the following timestamp is recorded in
   the message:

      T2: the time the DM query message is received at B.

   At this point, B transmits the message back to A, recording within it
   the following timestamp:

      T3: the time the DM response message is transmitted from B.

   When the message arrives back at A, the following timestamp is
   recorded in the message:

      T4: the time the DM response message is received back at A.

   (Strictly speaking, it is not necessary that the fourth timestamp,
   T4, actually be written in the message, but this is convenient for
   some implementations and useful if the message is to be forwarded on
   to an external measurement system.)

   At this point, A can compute the two-way channel delay associated
   with the channel as

      two-way channel delay = (T4 - T1) - (T3 - T2)

   and the round-trip delay as

      round-trip delay = T4 - T1.








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   If the clocks of A and B are known at A to be synchronized, then both
   one-way delay values, as well as the two-way channel delay, can be
   computed at A as

      forward one-way delay = T2 - T1

      reverse one-way delay = T4 - T3

      two-way channel delay = forward delay + reverse delay.

   Note that this formula for the two-way channel delay reduces to the
   one previously given, and clock synchronization is not required to
   compute this metric.

2.5.  Delay Variation Measurement

   Inter-Packet Delay Variation (IPDV) and Packet Delay Variation (PDV)
   [RFC5481] are performance metrics derived from one-way delay
   measurement and are important in some applications.  IPDV represents
   the difference between the one-way delays of successive packets in a
   stream.  PDV, given a measurement test interval, represents the
   difference between the one-way delay of a packet in the interval and
   that of the packet in the interval with the minimum delay.

   IPDV and PDV measurements can therefore be derived from delay
   measurements obtained through the procedures in Section 2.4.  An
   important point regarding delay variation measurement, however, is
   that it can be carried out based on one-way delay measurements even
   when the clocks of the two systems involved in those measurements are
   not synchronized with one another.

2.6.  Unidirectional Measurement

   In the case that the channel from A to (B1, ..., Bk) (where B2, ...,
   Bk refers to the point-to-multipoint case) is unidirectional, i.e.,
   is a unidirectional LSP, LM and DM measurements can be carried out at
   B1, ..., Bk instead of at A.

   For LM, this is accomplished by initiating an LM operation at A and
   carrying out the same procedures as used for bidirectional channels,
   except that no responses from B1, ..., Bk to A are generated.
   Instead, each terminal node B uses the A_TxP and B_RxP values in the
   LM messages it receives to compute the receive loss associated with
   the channel in essentially the same way as described previously, that
   is:

   B_RxLoss[n-1,n] = (A_TxP[n] - A_TxP[n-1]) - (B_RxP[n] - B_RxP[n-1])




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   For DM, of course, only the forward one-way delay can be measured and
   the clock synchronization requirement applies.

   Alternatively, if an out-of-band channel from a terminal node B back
   to A is available, the LM and DM message responses can be
   communicated to A via this channel so that the measurements can be
   carried out at A.

2.7.  Dyadic Measurement

   The basic procedures for bidirectional measurement assume that the
   measurement process is conducted by and for the querier node A.
   Instead, it is possible, with only minor variation of these
   procedures, to conduct a dyadic or "dual-ended" measurement process
   in which both nodes A and B perform loss or delay measurement based
   on the same message flow.  This is achieved by stipulating that A
   copy the third and fourth counter or timestamp values from a response
   message into the third and fourth slots of the next query, which are
   otherwise unused, thereby providing B with equivalent information to
   that learned by A.

   The dyadic procedure has the advantage of halving the number of
   messages required for both A and B to perform a given kind of
   measurement, but comes at the expense of each node's ability to
   control its own measurement process independently, and introduces
   additional operational complexity into the measurement protocols.
   The quantity of measurement traffic is also expected to be low
   relative to that of user traffic, particularly when 64-bit counters
   are used for LM.  Consequently, this document does not specify a
   dyadic operational mode.  However, it is still possible, and may be
   useful, for A to perform the extra copy, thereby providing additional
   information to B even when its participation in the measurement
   process is passive.

2.8.  Loopback Measurement

   Some bidirectional channels may be placed into a loopback state such
   that messages are looped back to the sender without modification.  In
   this situation, LM and DM procedures can be used to carry out
   measurements associated with the circular path.  This is done by
   generating "queries" with the Response flag set to 1.

   For LM, the loss computation in this case is:

   A_Loss[n-1,n] = (A_TxP[n] - A_TxP[n-1]) - (A_RxP[n] - A_RxP[n-1])






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   For DM, the round-trip delay is computed.  In this case, however, the
   remote endpoint processing time component reflects only the time
   required to loop the message from channel input to channel output.

2.9.  Measurement Considerations

   A number of additional considerations apply in practice to the
   measurement methods summarized above.

2.9.1.  Types of Channels

   There are several types of channels in MPLS networks over which loss
   and delay measurement may be conducted.  The channel type may
   restrict the kinds of measurement that can be performed.  In all
   cases, LM and DM messages flow over the MPLS Generic Associated
   Channel (G-ACh), which is described in detail in [RFC5586].

   Broadly, a channel in an MPLS network may be either a link, a Label
   Switched Path (LSP) [RFC3031], or a pseudowire [RFC3985].  Links are
   bidirectional and are also referred to as MPLS sections; see
   [RFC5586] and [RFC5960].  Pseudowires are bidirectional.  Label
   Switched Paths may be either unidirectional or bidirectional.

   The LM and DM protocols discussed in this document are initiated from
   a single node: the querier.  A query message may be received either
   by a single node or by multiple nodes, depending on the nature of the
   channel.  In the latter case, these protocols provide point-to-
   multipoint measurement capabilities.

2.9.2.  Quality of Service

   Quality of Service (QoS) capabilities, in the form of the
   Differentiated Services architecture, apply to MPLS as specified in
   [RFC3270] and [RFC5462].  Different classes of traffic are
   distinguished by the three-bit Traffic Class (TC) field of an MPLS
   Label Stack Entry (LSE).  Delay measurement applies on a per-traffic-
   class basis, and the TC values of LSEs above the G-ACh Label (GAL)
   that precedes a DM message are significant.  Packet loss can be
   measured with respect either to the channel as a whole or to a
   specific traffic class.

2.9.3.  Measurement Point Location

   The location of the measurement points for loss and delay within the
   sending and receiving nodes is implementation dependent but directly
   affects the nature of the measurements.  For example, a sending
   implementation may or may not consider a packet to be "lost", for LM
   purposes, that was discarded prior to transmission for queuing-



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   related reasons; conversely, a receiving implementation may or may
   not consider a packet to be "lost", for LM purposes, if it was
   physically received but discarded during receive-path processing.
   The location of delay measurement points similarly determines what,
   precisely, is being measured.  The principal consideration here is
   that the behavior of an implementation in these respects MUST be made
   clear to the user.

2.9.4.  Equal Cost Multipath

   Equal Cost Multipath (ECMP) is the behavior of distributing packets
   across multiple alternate paths toward a destination.  The use of
   ECMP in MPLS networks is described in BCP 128 [RFC4928].  The typical
   result of ECMP being performed on an LSP that is subject to delay
   measurement will be that only the delay of one of the available paths
   is, and can be, measured.

   The effects of ECMP on loss measurement will depend on the LM mode.
   In the case of direct LM, the measurement will account for any
   packets lost between the sender and the receiver, regardless of how
   many paths exist between them.  However, the presence of ECMP
   increases the likelihood of misordering both of LM messages relative
   to data packets and of the LM messages themselves.  Such misorderings
   tend to create unmeasurable intervals and thus degrade the accuracy
   of loss measurement.  The effects of ECMP are similar for inferred
   LM, with the additional caveat that, unless the test packets are
   specially constructed so as to probe all available paths, the loss
   characteristics of one or more of the alternate paths cannot be
   accounted for.

2.9.5.  Intermediate Nodes

   In the case of an LSP, it may be desirable to measure the loss or
   delay to or from an intermediate node as well as between LSP
   endpoints.  This can be done in principle by setting the Time to Live
   (TTL) field in the outer LSE appropriately when targeting a
   measurement message to an intermediate node.  This procedure may
   fail, however, if hardware-assisted measurement is in use, because
   the processing of the packet by the intermediate node occurs only as
   the result of TTL expiry, and the handling of TTL expiry may occur at
   a later processing stage in the implementation than the hardware-
   assisted measurement function.  The motivation for conducting
   measurements to intermediate nodes is often an attempt to localize a
   problem that has been detected on the LSP.  In this case, if
   intermediate nodes are not capable of performing hardware-assisted
   measurement, a less accurate -- but usually sufficient -- software-
   based measurement can be conducted instead.




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2.9.6.  Different Transmit and Receive Interfaces

   The overview of the bidirectional measurement process presented in
   Section 2 is also applicable when the transmit and receive interfaces
   at A or B differ from one another.  Some additional considerations,
   however, do apply in this case:

   o  If different clocks are associated with transmit and receive
      processing, these clocks must be synchronized in order to compute
      the two-way delay.

   o  The DM protocol specified in this document requires that the
      timestamp formats used by the interfaces that receive a DM query
      and transmit a DM response agree.

   o  The LM protocol specified in this document supports both 32-bit
      and 64-bit counter sizes, but the use of 32-bit counters at any of
      the up to four interfaces involved in an LM operation will result
      in 32-bit LM calculations for both directions of the channel.

2.9.7.  External Post-Processing

   In some circumstances, it may be desirable to carry out the final
   measurement computation at an external post-processing device
   dedicated to the purpose.  This can be achieved in supporting
   implementations by, for example, configuring the querier, in the case
   of a bidirectional measurement session, to forward each response it
   receives to the post-processor via any convenient protocol.  The
   unidirectional case can be handled similarly through configuration of
   the receiver or by including an instruction in query messages for the
   receiver to respond out-of-band to the appropriate return address.

   Post-processing devices may have the ability to store measurement
   data for an extended period and to generate a variety of useful
   statistics from them.  External post-processing also allows the
   measurement process to be completely stateless at the querier and
   responder.

2.9.8.  Loss Measurement Modes

   The summary of loss measurement at the beginning of Section 2 made
   reference to the "count of packets" transmitted and received over a
   channel.  If the counted packets are the packets flowing over the
   channel in the data plane, the loss measurement is said to operate in
   "direct mode".  If, on the other hand, the counted packets are
   selected control packets from which the approximate loss
   characteristics of the channel are being inferred, the loss
   measurement is said to operate in "inferred mode".



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   Direct LM has the advantage of being able to provide perfect loss
   accounting when it is available.  There are, however, several
   constraints associated with direct LM.

   For accurate direct LM to occur, packets must not be sent between the
   time the transmit count for an outbound LM message is determined and
   the time the message is actually transmitted.  Similarly, packets
   must not be received and processed between the time an LM message is
   received and the time the receive count for the message is
   determined.  If these "synchronization conditions" do not hold, the
   LM message counters will not reflect the true state of the data
   plane, with the result that, for example, the receive count of B may
   be greater than the transmit count of A, and attempts to compute loss
   by taking the difference will yield an invalid result.  This
   requirement for synchronization between LM message counters and the
   data plane may require special support from hardware-based forwarding
   implementations.

   A limitation of direct LM is that it may be difficult or impossible
   to apply in cases where the channel is an LSP and the LSP label at
   the receiver is either nonexistent or fails to identify a unique
   sending node.  The first case happens when Penultimate Hop Popping
   (PHP) is used on the LSP, and the second case generally holds for
   LSPs based on the Label Distribution Protocol (LDP) [RFC5036] as
   opposed to, for example, those based on Traffic Engineering
   extensions to the Resource Reservation Protocol (RSVP-TE) [RFC3209].
   These conditions may make it infeasible for the receiver to identify
   the data-plane packets associated with a particular source and LSP in
   order to count them, or to infer the source and LSP context
   associated with an LM message.  Direct LM is also vulnerable to
   disruption in the event that the ingress or egress interface
   associated with an LSP changes during the LSP's lifetime.

   Inferred LM works in the same manner as direct LM except that the
   counted packets are special control packets, called test messages,
   generated by the sender.  Test messages may be either packets
   explicitly constructed and used for LM or packets with a different
   primary purpose, such as those associated with a Bidirectional
   Forwarding Detection (BFD) [RFC5884] session.

   The synchronization conditions discussed above for direct LM also
   apply to inferred LM, the only difference being that the required
   synchronization is now between the LM counters and the test message
   generation process.  Protocol and application designers MUST take
   these synchronization requirements into account when developing tools
   for inferred LM, and make their behavior in this regard clear to the
   user.




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   Inferred LM provides only an approximate view of the loss level
   associated with a channel, but is typically applicable even in cases
   where direct LM is not.

2.9.9.  Loss Measurement Scope

   In the case of direct LM, where data-plane packets are counted, there
   are different possibilities for which kinds of packets are included
   in the count and which are excluded.  The set of packets counted for
   LM is called the "loss measurement scope".  As noted above, one
   factor affecting the LM scope is whether all data packets are counted
   or only those belonging to a particular traffic class.  Another is
   whether various "auxiliary" flows associated with a data channel are
   counted, such as packets flowing over the G-ACh.  Implementations
   MUST make their supported LM scopes clear to the user, and care must
   be taken to ensure that the scopes of the channel endpoints agree.

2.9.10.  Delay Measurement Accuracy

   The delay measurement procedures described in this document are
   designed to facilitate hardware-assisted measurement and to function
   in the same way whether or not such hardware assistance is used.  The
   measurement accuracy will be determined by how closely the transmit
   and receive timestamps correspond to actual packet departure and
   arrival times.

   As noted in Section 2.4, measurement of one-way delay requires clock
   synchronization between the devices involved, while two-way delay
   measurement does not involve direct comparison between non-local
   timestamps and thus has no synchronization requirement.  The
   measurement accuracy will be limited by the quality of the local
   clock and, in the case of one-way delay measurement, by the quality
   of the synchronization.

2.9.11.  Delay Measurement Timestamp Format

   There are two significant timestamp formats in common use: the
   timestamp format of the Network Time Protocol (NTP), described in
   [RFC5905], and the timestamp format used in the IEEE 1588 Precision
   Time Protocol (PTP) [IEEE1588].

   The NTP format has the advantages of wide use and long deployment in
   the Internet, and it was specifically designed to make the
   computation of timestamp differences as simple and efficient as
   possible.  On the other hand, there is now also a significant
   deployment of equipment designed to support the PTP format.





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   The approach taken in this document is therefore to include in DM
   messages fields that identify the timestamp formats used by the two
   devices involved in a DM operation.  This implies that a node
   attempting to carry out a DM operation may be faced with the problem
   of computing with and possibly reconciling different timestamp
   formats.  To ensure interoperability, it is necessary that support of
   at least one timestamp format is mandatory.  This specification
   requires the support of the IEEE 1588 PTP format.  Timestamp format
   support requirements are discussed in detail in Section 3.4.

3.  Message Formats

   Loss Measurement and Delay Measurement messages flow over the MPLS
   Generic Associated Channel (G-ACh) [RFC5586].  Thus, a packet
   containing an LM or DM message contains an MPLS label stack, with the
   G-ACh Label (GAL) at the bottom of the stack.  The GAL is followed by
   an Associated Channel Header (ACH), which identifies the message
   type, and the message body follows the ACH.

   This document defines the following ACH Channel Types:

      MPLS Direct Loss Measurement (DLM)
      MPLS Inferred Loss Measurement (ILM)
      MPLS Delay Measurement (DM)
      MPLS Direct Loss and Delay Measurement (DLM+DM)
      MPLS Inferred Loss and Delay Measurement (ILM+DM)

   The message formats for direct and inferred LM are identical.  The
   formats of the DLM+DM and ILM+DM messages are also identical.

   For these channel types, the ACH SHALL NOT be followed by the ACH TLV
   Header defined in [RFC5586].

   The fixed-format portion of a message MAY be followed by a block of
   Type-Length-Value (TLV) fields.  The TLV block provides an extensible
   way of attaching subsidiary information to LM and DM messages.
   Several such TLV fields are defined below.

   All integer values for fields defined in this document SHALL be
   encoded in network byte order.

3.1.  Loss Measurement Message Format

   The format of a Loss Measurement message, which follows the
   Associated Channel Header (ACH), is as follows:






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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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Version| Flags |  Control Code |        Message Length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | DFlags|  OTF  |                   Reserved                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Session Identifier          |    DS     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Origin Timestamp                       |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Counter 1                           |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Counter 4                           |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           TLV Block                           ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Loss Measurement Message Format

   Reserved fields MUST be set to 0 and ignored upon receipt.  The
   possible values for the remaining fields are as follows.






















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   Field                 Meaning
   --------------------- -----------------------------------------------
   Version               Protocol version
   Flags                 Message control flags
   Control Code          Code identifying the query or response type
   Message Length        Total length of this message in bytes
   Data Format Flags     Flags specifying the format of message data
   (DFlags)
   Origin Timestamp      Format of the Origin Timestamp field
   Format (OTF)
   Reserved              Reserved for future specification
   Session Identifier    Set arbitrarily by the querier
   Differentiated        Differentiated Services Code Point (DSCP) being
   Services (DS) Field   measured
   Origin Timestamp      64-bit field for query message transmission
                         timestamp
   Counter 1-4           64-bit fields for LM counter values
   TLV Block             Optional block of Type-Length-Value fields

   The possible values for these fields are as follows.

   Version: Currently set to 0.

   Flags: The format of the Flags field is shown below.

                               +-+-+-+-+
                               |R|T|0|0|
                               +-+-+-+-+

                      Loss Measurement Message Flags

   The meanings of the flag bits are:

      R: Query/Response indicator.  Set to 0 for a Query and 1 for a
      Response.

      T: Traffic-class-specific measurement indicator.  Set to 1 when
      the measurement operation is scoped to packets of a particular
      traffic class (DSCP value), and 0 otherwise.  When set to 1, the
      DS field of the message indicates the measured traffic class.

      0: Set to 0.

   Control Code: Set as follows according to whether the message is a
   Query or a Response as identified by the R flag.






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      For a Query:

         0x0: In-band Response Requested.  Indicates that this query has
         been sent over a bidirectional channel and the response is
         expected over the same channel.

         0x1: Out-of-band Response Requested.  Indicates that the
         response should be sent via an out-of-band channel.

         0x2: No Response Requested.  Indicates that no response to the
         query should be sent.  This mode can be used, for example, if
         all nodes involved are being controlled by a Network Management
         System.

      For a Response:

         Codes 0x0-0xF are reserved for non-error responses.  Error
         response codes imply that the response does not contain valid
         measurement data.

         0x1: Success.  Indicates that the operation was successful.

         0x2: Notification - Data Format Invalid.  Indicates that the
         query was processed, but the format of the data fields in this
         response may be inconsistent.  Consequently, these data fields
         MUST NOT be used for measurement.

         0x3: Notification - Initialization in Progress.  Indicates that
         the query was processed but this response does not contain
         valid measurement data because the responder's initialization
         process has not completed.

         0x4: Notification - Data Reset Occurred.  Indicates that the
         query was processed, but a reset has recently occurred that may
         render the data in this response inconsistent relative to
         earlier responses.

         0x5: Notification - Resource Temporarily Unavailable.
         Indicates that the query was processed, but resources were
         unavailable to complete the requested measurement and that,
         consequently, this response does not contain valid measurement
         data.

         0x10: Error - Unspecified Error.  Indicates that the operation
         failed for an unspecified reason.






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         0x11: Error - Unsupported Version.  Indicates that the
         operation failed because the protocol version supplied in the
         query message is not supported.

         0x12: Error - Unsupported Control Code.  Indicates that the
         operation failed because the Control Code requested an
         operation that is not available for this channel.

         0x13: Error - Unsupported Data Format.  Indicates that the
         operation failed because the data format specified in the query
         is not supported.

         0x14: Error - Authentication Failure.  Indicates that the
         operation failed because the authentication data supplied in
         the query was missing or incorrect.

         0x15: Error - Invalid Destination Node Identifier.  Indicates
         that the operation failed because the Destination Node
         Identifier supplied in the query is not an identifier of this
         node.

         0x16: Error - Connection Mismatch.  Indicates that the
         operation failed because the channel identifier supplied in the
         query did not match the channel over which the query was
         received.

         0x17: Error - Unsupported Mandatory TLV Object.  Indicates that
         the operation failed because a TLV Object received in the query
         and marked as mandatory is not supported.

         0x18: Error - Unsupported Query Interval.  Indicates that the
         operation failed because the query message rate exceeded the
         configured threshold.

         0x19: Error - Administrative Block.  Indicates that the
         operation failed because it has been administratively
         disallowed.

         0x1A: Error - Resource Unavailable.  Indicates that the
         operation failed because node resources were not available.

         0x1B: Error - Resource Released.  Indicates that the operation
         failed because node resources for this measurement session were
         administratively released.

         0x1C: Error - Invalid Message.  Indicates that the operation
         failed because the received query message was malformed.




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         0x1D: Error - Protocol Error.  Indicates that the operation
         failed because a protocol error was found in the received query
         message.

   Message Length: Set to the total length of this message in bytes,
   including the Version, Flags, Control Code, and Message Length fields
   as well as the TLV Block, if any.

   DFlags: The format of the DFlags field is shown below.

                               +-+-+-+-+
                               |X|B|0|0|
                               +-+-+-+-+

                             Data Format Flags

   The meanings of the DFlags bits are:

      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 (Section 2.9.9), and the octet count
      of a packet sent or received over a channel 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
      1-4 fields represent packet counts.

      0: Set to 0.

   Origin Timestamp Format: The format of the Origin Timestamp field, as
   specified in Section 3.4.

   Session Identifier: Set arbitrarily in a query and copied in the
   response, if any.  This field uniquely identifies a measurement
   operation (also called a session) that consists of a sequence of
   messages.  All messages in the sequence have the same Session
   Identifier.

   DS: When the T flag is set to 1, this field is set to the DSCP value
   [RFC3260] that corresponds to the traffic class being measured.  For
   MPLS, where the traffic class of a channel is identified by the
   three-bit Traffic Class in the channel's LSE [RFC5462], this field



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   SHOULD be set to the Class Selector Codepoint [RFC2474] that
   corresponds to that Traffic Class.  When the T flag is set to 0, the
   value of this field is arbitrary, and the field can be considered
   part of the Session Identifier.

   Origin Timestamp: Timestamp recording the transmit time of the query
   message.

   Counter 1-4: Referring to Section 2.2, when a query is sent from A,
   Counter 1 is set to A_TxP and the other counter fields are set to 0.
   When the query is received at B, Counter 2 is set to B_RxP.  At this
   point, B copies Counter 1 to Counter 3 and Counter 2 to Counter 4,
   and re-initializes Counter 1 and Counter 2 to 0.  When B transmits
   the response, Counter 1 is set to B_TxP.  When the response is
   received at A, Counter 2 is set to A_RxP.

   The mapping of counter types such as A_TxP to the Counter 1-4 fields
   is designed to ensure that transmit counter values are always written
   at the same fixed offset in the packet, and likewise for receive
   counters.  This property may be important for hardware processing.

   When a 32-bit counter value is written to one of the counter fields,
   that value SHALL be written to the low-order 32 bits of the field;
   the high-order 32 bits of the field MUST, in this case, be set to 0.

   TLV Block: Zero or more TLV fields.

3.2.  Delay Measurement Message Format

   The format of a Delay Measurement message, which follows the
   Associated Channel Header (ACH), is as follows:




















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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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Version| Flags |  Control Code |        Message Length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  QTF  |  RTF  | RPTF  |              Reserved                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Session Identifier          |    DS     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Timestamp 1                         |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Timestamp 4                         |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           TLV Block                           ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Delay Measurement Message Format

   The meanings of the fields are summarized in the following table.

   Field                 Meaning
   --------------------- -----------------------------------------------
   Version               Protocol version
   Flags                 Message control flags
   Control Code          Code identifying the query or response type
   Message Length        Total length of this message in bytes
   QTF                   Querier timestamp format
   RTF                   Responder timestamp format
   RPTF                  Responder's preferred timestamp format
   Reserved              Reserved for future specification
   Session Identifier    Set arbitrarily by the querier
   Differentiated        Differentiated Services Code Point (DSCP) being
   Services (DS) Field   measured

   Timestamp 1-4         64-bit timestamp values
   TLV Block             Optional block of Type-Length-Value fields

   Reserved fields MUST be set to 0 and ignored upon receipt.  The
   possible values for the remaining fields are as follows.

   Version: Currently set to 0.




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   Flags: As specified in Section 3.1.  The T flag in a DM message is
   set to 1.

   Control Code: As specified in Section 3.1.

   Message Length: Set to the total length of this message in bytes,
   including the Version, Flags, Control Code, and Message Length fields
   as well as the TLV Block, if any.

   Querier Timestamp Format: The format of the timestamp values written
   by the querier, as specified in Section 3.4.

   Responder Timestamp Format: The format of the timestamp values
   written by the responder, as specified in Section 3.4.

   Responder's Preferred Timestamp Format: The timestamp format
   preferred by the responder, as specified in Section 3.4.

   Session Identifier: As specified in Section 3.1.

   DS: As specified in Section 3.1.

   Timestamp 1-4: Referring to Section 2.4, when a query is sent from A,
   Timestamp 1 is set to T1 and the other timestamp fields are set to 0.
   When the query is received at B, Timestamp 2 is set to T2.  At this
   point, B copies Timestamp 1 to Timestamp 3 and Timestamp 2 to
   Timestamp 4, and re-initializes Timestamp 1 and Timestamp 2 to 0.
   When B transmits the response, Timestamp 1 is set to T3.  When the
   response is received at A, Timestamp 2 is set to T4.  The actual
   formats of the timestamp fields written by A and B are indicated by
   the Querier Timestamp Format and Responder Timestamp Format fields
   respectively.

   The mapping of timestamps to the Timestamp 1-4 fields is designed to
   ensure that transmit timestamps are always written at the same fixed
   offset in the packet, and likewise for receive timestamps.  This
   property is important for hardware processing.

   TLV Block: Zero or more TLV fields.

3.3.  Combined Loss/Delay Measurement Message Format

   The format of a combined Loss and Delay Measurement message, which
   follows the Associated Channel Header (ACH), is as follows:







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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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Version| Flags |  Control Code |        Message Length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | DFlags|  QTF  |  RTF  | RPTF  |           Reserved            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Session Identifier          |    DS     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Timestamp 1                         |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Timestamp 4                         |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Counter 1                           |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Counter 4                           |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           TLV Block                           ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Loss/Delay Measurement Message Format

   The fields of this message have the same meanings as the
   corresponding fields in the LM and DM message formats, except that
   the roles of the OTF and Origin Timestamp fields for LM are here
   played by the QTF and Timestamp 1 fields, respectively.

3.4.  Timestamp Field Formats

   The following timestamp format field values are specified in this
   document:

      0: Null timestamp format.  This value is a placeholder indicating
      that the timestamp field does not contain a meaningful timestamp.





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      1: Sequence number.  This value indicates that the timestamp field
      is to be viewed as a simple 64-bit sequence number.  This provides
      a simple solution for applications that do not require a real
      absolute timestamp, but only an indication of message ordering; an
      example is LM exception detection.

      2: Network Time Protocol version 4 64-bit timestamp format
      [RFC5905].  This format consists of a 32-bit seconds field
      followed by a 32-bit fractional seconds field, so that it can be
      regarded as a fixed-point 64-bit quantity.

      3: Low-order 64 bits of the IEEE 1588-2008 (1588v2) Precision Time
      Protocol timestamp format [IEEE1588].  This truncated format
      consists of a 32-bit seconds field followed by a 32-bit
      nanoseconds field, and is the same as the IEEE 1588v1 timestamp
      format.

   Timestamp formats of n < 64 bits in size SHALL be encoded in the
   64-bit timestamp fields specified in this document using the n high-
   order bits of the field.  The remaining 64 - n low-order bits in the
   field SHOULD be set to 0 and MUST be ignored when reading the field.

   To ensure that it is possible to find an interoperable mode between
   implementations, it is necessary to select one timestamp format as
   the default.  The timestamp format chosen as the default is the
   truncated IEEE 1588 PTP format (format code 3 in the list above);
   this format MUST be supported.  The rationale for this choice is
   discussed in Appendix A.  Implementations SHOULD also be capable of
   reading timestamps written in NTPv4 64-bit format and reconciling
   them internally with PTP timestamps for measurement purposes.
   Support for other timestamp formats is OPTIONAL.

   The implementation MUST make clear which timestamp formats it
   supports and the extent of its support for computation with and
   reconciliation of different formats for measurement purposes.

3.5.  TLV Objects

   The TLV Block in LM and DM messages consists of zero or more objects
   with the following format:

        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     |        Value                  ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                TLV Format



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   The Type and Length fields are each 8 bits long, and the Length field
   indicates the size in bytes of the Value field, which can therefore
   be up to 255 bytes long.

   The Type space is divided into Mandatory and Optional subspaces:

   Type Range     Semantics
   -------------- ---------
   0-127          Mandatory
   128-255        Optional

   Upon receipt of a query message including an unrecognized mandatory
   TLV object, the recipient MUST respond with an Unsupported Mandatory
   TLV Object error code.

   The types defined are as follows:

   Type           Definition
   -------------- ---------------------------------
   Mandatory
   0              Padding - copy in response
   1              Return Address
   2              Session Query Interval
   3              Loopback Request
   4-126          Unallocated
   127            Experimental use

   Optional
   128            Padding - do not copy in response
   129            Destination Address
   130            Source Address
   131-254        Unallocated
   255            Experimental use

3.5.1.  Padding

   The two padding objects permit the augmentation of packet size; this
   is mainly useful for delay measurement.  The type of padding
   indicates whether the padding supplied by the querier is to be copied
   to, or omitted from, the response.  Asymmetrical padding may be
   useful when responses are delivered out-of-band or when different
   maximum transmission unit sizes apply to the two components of a
   bidirectional channel.

   More than one padding object MAY be present, in which case they MUST
   be contiguous.  The Value field of a padding object is arbitrary.





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3.5.2.  Addressing

   The addressing objects have the following format:

        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     |        Address Family         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           Address                             ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Addressing Object Format

   The Address Family field indicates the type of the address, and it
   SHALL be set to one of the assigned values in the "IANA Address
   Family Numbers" registry.

   The Source and Destination Address objects indicate the addresses of
   the sender and the intended recipient of the message, respectively.
   The Source Address of a query message SHOULD be used as the
   destination for an out-of-band response unless some other out-of-band
   response mechanism has been configured, and unless a Return Address
   object is present, in which case the Return Address specifies the
   target of the response.  The Return Address object MUST NOT appear in
   a response.

3.5.3.  Loopback Request

   The Loopback Request object, when included in a query, indicates a
   request that the query message be returned to the sender unmodified.
   This object has a Length of 0.

   Upon receiving the reflected query message back from the responder,
   the querier MUST NOT retransmit the message.  Information that
   uniquely identifies the original query source, such as a Source
   Address object, can be included to enable the querier to
   differentiate one of its own loopback queries from a loopback query
   initiated by the far end.

   This object may be useful, for example, when the querier is
   interested only in the round-trip delay metric.  In this case, no
   support for delay measurement is required at the responder at all,
   other than the ability to recognize a DM query that includes this
   object and return it unmodified.






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3.5.4.  Session Query Interval

   The Value field of the Session Query Interval object is a 32-bit
   unsigned integer that specifies a time interval in milliseconds.

        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     |            Session Query      >
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       <        Interval (ms)          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Session Query Interval Object Format

   This time interval indicates the interval between successive query
   messages in a specific measurement session.  The purpose of the
   Session Query Interval (SQI) object is to enable the querier and
   responder of a measurement session to agree on a query rate.  The
   procedures for handling this object SHALL be as follows:

   1.  The querier notifies the responder that it wishes to be informed
       of the responder's minimum query interval for this session by
       including the SQI object in its query messages, with a Value of
       0.

   2.  When the responder receives a query that includes an SQI object
       with a Value of 0, the responder includes an SQI object in the
       response with the Value set to the minimum query interval it
       supports for this session.

   3.  When the querier receives a response that includes an SQI object,
       it selects a query interval for the session that is greater than
       or equal to the Value specified in the SQI object and adjusts its
       query transmission rate accordingly, including in each subsequent
       query an SQI object with a Value equal to the selected query
       interval.  Once a response to one of these subsequent queries has
       been received, the querier infers that the responder has been
       apprised of the selected query interval and MAY then stop
       including the SQI object in queries associated with this session.

   Similar procedures allow the query rate to be changed during the
   course of the session by either the querier or the responder.  For
   example, to inform the querier of a change in the minimum supported
   query interval, the responder begins including a corresponding SQI
   object in its responses, and the querier adjusts its query rate if
   necessary and includes a corresponding SQI object in its queries
   until a response is received.



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   Shorter query intervals (i.e., higher query rates) provide finer
   measurement granularity at the expense of additional load on
   measurement endpoints and the network; see Section 6 for further
   discussion.

4.  Operation

4.1.  Operational Overview

   A loss or delay measurement operation, also called a session, is
   controlled by the querier and consists of a sequence of query
   messages associated with a particular channel and a common set of
   measurement parameters.  If the session parameters include a response
   request, then the receiving node or nodes will (under normal
   conditions) generate a response message for each query message
   received, and these responses are also considered part of the
   session.  All query and response messages in a session carry a common
   session identifier.

   Measurement sessions are initiated at the discretion of the network
   operator and are terminated either at the operator's request or as
   the result of an error condition.  A session may be as brief as a
   single message exchange, for example when a DM query is used by the
   operator to "ping" a remote node, or it may extend throughout the
   lifetime of the channel.

   When a session is initiated for which responses are requested, the
   querier SHOULD initialize a timer, called the SessionResponseTimeout,
   that indicates how long the querier will wait for a response before
   abandoning the session and notifying the user that a timeout has
   occurred.  This timer persists for the lifetime of the session and is
   reset each time a response message for the session is received.

   When a query message is received that requests a response, a variety
   of exceptional conditions may arise that prevent the responder from
   generating a response that contains valid measurement data.  Such
   conditions fall broadly into two classes: transient exceptions from
   which recovery is possible and fatal exceptions that require
   termination of the session.  When an exception arises, the responder
   SHOULD generate a response with an appropriate Notification or Error
   control code according to whether the exception is, respectively,
   transient or fatal.  When the querier receives an Error response, the
   session MUST be terminated and the user informed.

   A common example of a transient exception occurs when a new session
   is initiated and the responder requires a period of time to become
   ready before it can begin providing useful responses.  The response
   control code corresponding to this situation is Notification -



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   Initialization in Progress.  Typical examples of fatal exceptions are
   cases where the querier has requested a type of measurement that the
   responder does not support or where a query message is malformed.

   When initiating a session, the querier SHOULD employ the Session
   Query Interval mechanism (Section 3.5.4) to establish a mutually
   agreeable query rate with the responder.  Responders SHOULD employ
   rate-limiting mechanisms to guard against the possibility of
   receiving an excessive quantity of query messages.

4.2.  Loss Measurement Procedures

4.2.1.  Initiating a Loss Measurement Operation

   An LM operation for a particular channel consists of sending a
   sequence (LM[1], LM[2], ...) of LM query messages over the channel at
   a specific rate and processing the responses received, if any.  As
   described in Section 2.2, the packet loss associated with the channel
   during the operation is computed as a delta between successive
   messages; these deltas can be accumulated to obtain a running total
   of the packet loss for the channel or be used to derive related
   metrics such as the average loss rate.

   The query message transmission rate MUST be sufficiently high, given
   the LM message counter size (which can be either 32 or 64 bits) and
   the speed and minimum packet size of the underlying channel, that the
   ambiguity condition noted in Section 2.2 cannot arise.  In evaluating
   this rate, the implementation SHOULD assume that the counter size is
   32 bits unless explicitly configured otherwise or unless (in the case
   of a bidirectional channel) all local and remote interfaces involved
   in the LM operation are known to be 64-bit-capable, which can be
   inferred from the value of the X flag in an LM response.

4.2.2.  Transmitting a Loss Measurement Query

   When transmitting an LM Query, the Version field MUST be set to 0.
   The R flag MUST be set to 0.  The T flag SHALL be set to 1 if, and
   only if, the measurement is specific to a particular traffic class,
   in which case the DS field SHALL identify that traffic class.

   The X flag MUST be set to 1 if the transmitting interface writes
   64-bit LM counters and otherwise MUST be set to 0 to indicate that
   32-bit counters are written.  The B flag SHALL be set to 1 to
   indicate that the counter fields contain octet counts or to 0 to
   indicate packet counts.






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   The Control Code field MUST be set to one of the values for Query
   messages listed in Section 3.1; if the channel is unidirectional,
   this field MUST NOT be set to 0x0 (Query: In-band Response
   Requested).

   The Session Identifier field can be set arbitrarily.

   The Origin Timestamp field SHALL be set to the time at which this
   message is transmitted, and the Origin Timestamp Format field MUST be
   set to indicate its format, according to Section 3.4.

   The Counter 1 field SHOULD be set to the total count of units
   (packets or octets, according to the B flag) transmitted over the
   channel prior to this LM Query, or to 0 if this is the beginning of a
   measurement session for which counter data is not yet available.  The
   Counter 2 field MUST be set to 0.  If a response was previously
   received in this measurement session, the Counter 1 and Counter 2
   fields of the most recent such response MAY be copied to the Counter
   3 and Counter 4 fields, respectively, of this query; otherwise, the
   Counter 3 and Counter 4 fields MUST be set to 0.

4.2.3.  Receiving a Loss Measurement Query

   Upon receipt of an LM Query message, the Counter 2 field SHOULD be
   set to the total count of units (packets or octets, according to the
   B flag) received over the channel prior to this LM Query.  If the
   receiving interface writes 32-bit LM counters, the X flag MUST be set
   to 0.

   At this point, the LM Query message must be inspected.  If the
   Control Code field is set to 0x2 (No Response Requested), an LM
   Response message MUST NOT be transmitted.  If the Control Code field
   is set to 0x0 (In-band Response Requested) or 0x1 (Out-of-band
   Response Requested), then an in-band or out-of-band response,
   respectively, SHOULD be transmitted unless this has been prevented by
   an administrative, security, or congestion control mechanism.

   In the case of a fatal exception that prevents the requested
   measurement from being made, the error SHOULD be reported, via either
   a response, if one was requested, or else as a notification to the
   user.

4.2.4.  Transmitting a Loss Measurement Response

   When constructing a Response to an LM Query, the Version field MUST
   be set to 0.  The R flag MUST be set to 1.  The value of the T flag
   MUST be copied from the LM Query.




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   The X flag MUST be set to 0 if the transmitting interface writes
   32-bit LM counters; otherwise, its value MUST be copied from the LM
   Query.  The B flag MUST be copied from the LM Query.

   The Session Identifier, Origin Timestamp, and Origin Timestamp Format
   fields MUST be copied from the LM Query.  The Counter 1 and Counter 2
   fields from the LM Query MUST be copied to the Counter 3 and Counter
   4 fields, respectively, of the LM Response.

   The Control Code field MUST be set to one of the values for Response
   messages listed in Section 3.1.  The value 0x10 (Unspecified Error)
   SHOULD NOT be used if one of the other more specific error codes is
   applicable.

   If the response is transmitted in-band, the Counter 1 field SHOULD be
   set to the total count of units transmitted over the channel prior to
   this LM Response.  If the response is transmitted out-of-band, the
   Counter 1 field MUST be set to 0.  In either case, the Counter 2
   field MUST be set to 0.

4.2.5.  Receiving a Loss Measurement Response

   Upon in-band receipt of an LM Response message, the Counter 2 field
   is set to the total count of units received over the channel prior to
   this LM Response.  If the receiving interface writes 32-bit LM
   counters, the X flag is set to 0.  (Since the life of the LM message
   in the network has ended at this point, it is up to the receiver
   whether these final modifications are made to the packet.  If the
   message is to be forwarded on for external post-processing
   (Section 2.9.7), then these modifications MUST be made.)

   Upon out-of-band receipt of an LM Response message, the Counter 1 and
   Counter 2 fields MUST NOT be used for purposes of loss measurement.

   If the Control Code in an LM Response is anything other than 0x1
   (Success), the counter values in the response MUST NOT be used for
   purposes of loss measurement.  If the Control Code indicates an error
   condition, or if the response message is invalid, the LM operation
   MUST be terminated and an appropriate notification to the user
   generated.

4.2.6.  Loss Calculation

   Calculation of packet loss is carried out according to the procedures
   in Section 2.2.  The X flag in an LM message informs the device
   performing the calculation whether to perform 32-bit or 64-bit
   arithmetic.  If the flag value is equal to 1, all interfaces involved
   in the LM operation have written 64-bit counter values, and 64-bit



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   arithmetic can be used.  If the flag value is equal to 0, at least
   one interface involved in the operation has written a 32-bit counter
   value, and 32-bit arithmetic is carried out using the low-order 32
   bits of each counter value.

   Note that the semantics of the X flag allow all devices to
   interoperate regardless of their counter size support.  Thus, an
   implementation MUST NOT generate an error response based on the value
   of this flag.

4.2.7.  Quality of Service

   The TC field of the LSE corresponding to the channel (e.g., LSP)
   being measured SHOULD be set to a traffic class equal to or better
   than the best TC within the measurement scope to minimize the chance
   of out-of-order conditions.

4.2.8.  G-ACh Packets

   By default, direct LM MUST exclude packets transmitted and received
   over the Generic Associated Channel (G-ACh).  An implementation MAY
   provide the means to alter the direct LM scope to include some or all
   G-ACh messages.  Care must be taken when altering the LM scope to
   ensure that both endpoints are in agreement.

4.2.9.  Test Messages

   In the case of inferred LM, the packets counted for LM consist of
   test messages generated for this purpose, or of some other class of
   packets deemed to provide a good proxy for data packets flowing over
   the channel.  The specification of test protocols and proxy packets
   is outside the scope of this document, but some guidelines are
   discussed below.

   An identifier common to both the test or proxy messages and the LM
   messages may be required to make correlation possible.  The combined
   value of the Session Identifier and DS fields SHOULD be used for this
   purpose when possible.  That is, test messages in this case will
   include a 32-bit field that can carry the value of the combined
   Session Identifier + DS field present in LM messages.  When TC-
   specific LM is conducted, the DS field of the LSE in the label stack
   of a test message corresponding to the channel (e.g., LSP) over which
   the message is sent MUST correspond to the DS value in the associated
   LM messages.

   A separate test message protocol SHOULD include a timeout value in
   its messages that informs the responder when to discard any state
   associated with a specific test.



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4.2.10.  Message Loss and Packet Misorder Conditions

   Because an LM operation consists of a message sequence with state
   maintained from one message to the next, LM is subject to the effects
   of lost messages and misordered packets in a way that DM is not.
   Because this state exists only on the querier, the handling of these
   conditions is, strictly speaking, a local matter.  This section,
   however, presents recommended procedures for handling such
   conditions.  Note that in the absence of ECMP, packet misordering
   within a traffic class is a relatively rare event.

   The first kind of anomaly that may occur is that one or more LM
   messages may be lost in transit.  The effect of such loss is that
   when an LM Response is next received at the querier, an unambiguous
   interpretation of the counter values it contains may be impossible,
   for the reasons described at the end of Section 2.2.  Whether this is
   so depends on the number of messages lost and the other variables
   mentioned in that section, such as the LM message rate and the
   channel parameters.

   Another possibility is that LM messages are misordered in transit, so
   that, for instance, the response to LM[n] is received prior to the
   response to LM[n-1].  A typical implementation will discard the late
   response to LM[n-1], so that the effect is the same as the case of a
   lost message.

   Finally, LM is subject to the possibility that data packets are
   misordered relative to LM messages.  This condition can result, for
   example, in a transmit count of 100 and a corresponding receive count
   of 101.  The effect here is that the A_TxLoss[n-1,n] value (for
   example) for a given measurement interval will appear to be extremely
   (if not impossibly) large.  The other case, where an LM message
   arrives earlier than some of the packets, simply results in those
   packets being counted as lost.

   An implementation SHOULD identify a threshold value that indicates
   the upper bound of lost packets measured in a single computation
   beyond which the interval is considered unmeasurable.  This is called
   the "MaxLMIntervalLoss threshold".  It is clear that this threshold
   should be no higher than the maximum number of packets (or bytes) the
   channel is capable of transmitting over the interval, but it may be
   lower.  Upon encountering an unmeasurable interval, the LM state
   (i.e., data values from the last LM message received) SHOULD be
   discarded.

   With regard to lost LM messages, the MaxLMInterval (see Section 2.2)
   indicates the maximum amount of time that can elapse before the LM
   state is discarded.  If some messages are lost, but a message is



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   subsequently received within MaxLMInterval, its timestamp or sequence
   number will quantify the loss, and it MAY still be used for
   measurement, although the measurement interval will in this case be
   longer than usual.

   If an LM message is received that has a timestamp less than or equal
   to the timestamp of the last LM message received, this indicates that
   an exception has occurred, and the current interval SHOULD be
   considered unmeasurable unless the implementation has some other way
   of handling this condition.

4.3.  Delay Measurement Procedures

4.3.1.  Transmitting a Delay Measurement Query

   When transmitting a DM Query, the Version and Reserved fields MUST be
   set to 0.  The R flag MUST be set to 0, the T flag MUST be set to 1,
   and the remaining flag bits MUST be set to 0.

   The Control Code field MUST be set to one of the values for Query
   messages listed in Section 3.1; if the channel is unidirectional,
   this field MUST NOT be set to 0x0 (Query: In-band Response
   Requested).

   The Querier Timestamp Format field MUST be set to the timestamp
   format used by the querier when writing timestamp fields in this
   message; the possible values for this field are listed in
   Section 3.4.  The Responder Timestamp Format and Responder's
   Preferred Timestamp Format fields MUST be set to 0.

   The Session Identifier field can be set arbitrarily.  The DS field
   MUST be set to the traffic class being measured.

   The Timestamp 1 field SHOULD be set to the time at which this DM
   Query is transmitted, in the format indicated by the Querier
   Timestamp Format field.  The Timestamp 2 field MUST be set to 0.  If
   a response was previously received in this measurement session, the
   Timestamp 1 and Timestamp 2 fields of the most recent such response
   MAY be copied to the Timestamp 3 and Timestamp 4 fields,
   respectively, of this query; otherwise, the Timestamp 3 and Timestamp
   4 fields MUST be set to 0.

4.3.2.  Receiving a Delay Measurement Query

   Upon receipt of a DM Query message, the Timestamp 2 field SHOULD be
   set to the time at which this DM Query was received.





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   At this point, the DM Query message must be inspected.  If the
   Control Code field is set to 0x2 (No Response Requested), a DM
   Response message MUST NOT be transmitted.  If the Control Code field
   is set to 0x0 (In-band Response Requested) or 0x1 (Out-of-band
   Response Requested), then an in-band or out-of-band response,
   respectively, SHOULD be transmitted unless this has been prevented by
   an administrative, security, or congestion control mechanism.

   In the case of a fatal exception that prevents the requested
   measurement from being made, the error SHOULD be reported, via either
   a response, if one was requested, or else as a notification to the
   user.

4.3.3.  Transmitting a Delay Measurement Response

   When constructing a Response to a DM Query, the Version and Reserved
   fields MUST be set to 0.  The R flag MUST be set to 1, the T flag
   MUST be set to 1, and the remaining flag bits MUST be set to 0.

   The Session Identifier and Querier Timestamp Format (QTF) fields MUST
   be copied from the DM Query.  The Timestamp 1 and Timestamp 2 fields
   from the DM Query MUST be copied to the Timestamp 3 and Timestamp 4
   fields, respectively, of the DM Response.

   The Responder Timestamp Format (RTF) field MUST be set to the
   timestamp format used by the responder when writing timestamp fields
   in this message, i.e., Timestamp 4 and (if applicable) Timestamp 1;
   the possible values for this field are listed in Section 3.4.
   Furthermore, the RTF field MUST be set equal to either the QTF or the
   RPTF field.  See Section 4.3.5 for guidelines on the selection of the
   value for this field.

   The Responder's Preferred Timestamp Format (RPTF) field MUST be set
   to one of the values listed in Section 3.4 and SHOULD be set to
   indicate the timestamp format with which the responder can provide
   the best accuracy for purposes of delay measurement.

   The Control Code field MUST be set to one of the values for Response
   messages listed in Section 3.1.  The value 0x10 (Unspecified Error)
   SHOULD NOT be used if one of the other more specific error codes is
   applicable.

   If the response is transmitted in-band, the Timestamp 1 field SHOULD
   be set to the time at which this DM Response is transmitted.  If the
   response is transmitted out-of-band, the Timestamp 1 field MUST be
   set to 0.  In either case, the Timestamp 2 field MUST be set to 0.





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   If the response is transmitted in-band and the Control Code in the
   message is 0x1 (Success), then the Timestamp 1 and Timestamp 4 fields
   MUST have the same format, which will be the format indicated in the
   Responder Timestamp Format field.

4.3.4.  Receiving a Delay Measurement Response

   Upon in-band receipt of a DM Response message, the Timestamp 2 field
   is set to the time at which this DM Response was received.  (Since
   the life of the DM message in the network has ended at this point, it
   is up to the receiver whether this final modification is made to the
   packet.  If the message is to be forwarded on for external post-
   processing (Section 2.9.7), then these modifications MUST be made.)

   Upon out-of-band receipt of a DM Response message, the Timestamp 1
   and Timestamp 2 fields MUST NOT be used for purposes of delay
   measurement.

   If the Control Code in a DM Response is anything other than 0x1
   (Success), the timestamp values in the response MUST NOT be used for
   purposes of delay measurement.  If the Control Code indicates an
   error condition, or if the response message is invalid, the DM
   operation MUST be terminated and an appropriate notification to the
   user generated.

4.3.5.  Timestamp Format Negotiation

   In case either the querier or the responder in a DM transaction is
   capable of supporting multiple timestamp formats, it is desirable to
   determine the optimal format for purposes of delay measurement on a
   particular channel.  The procedures for making this determination
   SHALL be as follows.

   Upon sending an initial DM Query over a channel, the querier sets the
   Querier Timestamp Format (QTF) field to its preferred timestamp
   format.

   Upon receiving any DM Query message, the responder determines whether
   it is capable of writing timestamps in the format specified by the
   QTF field.  If so, the Responder Timestamp Format (RTF) field is set
   equal to the QTF field.  If not, the RTF field is set equal to the
   Responder's Preferred Timestamp Format (RPTF) field.

   The process of changing from one timestamp format to another at the
   responder may result in the Timestamp 1 and Timestamp 4 fields in an
   in-band DM Response having different formats.  If this is the case,





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   the Control Code in the response MUST NOT be set to 0x1 (Success).
   Unless an error condition has occurred, the Control Code MUST be set
   to 0x2 (Notification - Data Format Invalid).

   Upon receiving a DM Response, the querier knows from the RTF field in
   the message whether the responder is capable of supporting its
   preferred timestamp format: if it is, the RTF will be equal to the
   QTF.  The querier also knows the responder's preferred timestamp
   format from the RPTF field.  The querier can then decide whether to
   retain its current QTF or to change it and repeat the negotiation
   procedures.

4.3.5.1.  Single-Format Procedures

   When an implementation supports only one timestamp format, the
   procedures above reduce to the following simple behavior:

   o  All DM Queries are transmitted with the same QTF;

   o  All DM Responses are transmitted with the same RTF, and the RPTF
      is always set equal to the RTF;

   o  All DM Responses received with RTF not equal to QTF are discarded;

   o  On a unidirectional channel, all DM Queries received with QTF not
      equal to the supported format are discarded.

4.3.6.  Quality of Service

   The TC field of the LSE corresponding to the channel (e.g., LSP)
   being measured MUST be set to the value that corresponds to the DS
   field in the DM message.

4.4.  Combined Loss/Delay Measurement Procedures

   The combined LM/DM message defined in Section 3.3 allows loss and
   delay measurement to be carried out simultaneously.  This message
   SHOULD be treated as an LM message that happens to carry additional
   timestamp data, with the timestamp fields processed as per delay
   measurement procedures.

5.  Implementation Disclosure Requirements

   This section summarizes the requirements placed on implementations
   for capabilities disclosure.  The purpose of these requirements is to
   ensure that end users have a clear understanding of implementation





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   capabilities and characteristics that have a direct impact on how
   loss and delay measurement mechanisms function in specific
   situations.  Implementations are REQUIRED to state:

   o  METRICS: Which of the following metrics are supported: packet
      loss, packet throughput, octet loss, octet throughput, average
      loss rate, one-way delay, round-trip delay, two-way channel delay,
      packet delay variation.

   o  MP-LOCATION: The location of loss and delay measurement points
      with respect to other stages of packet processing, such as
      queuing.

   o  CHANNEL-TYPES: The types of channels for which LM and DM are
      supported, including LSP types, pseudowires, and sections (links).

   o  QUERY-RATE: The minimum supported query intervals for LM and DM
      sessions, both in the querier and responder roles.

   o  LOOP: Whether loopback measurement (Section 2.8) is supported.

   o  LM-TYPES: Whether direct or inferred LM is supported, and for the
      latter, which test protocols or proxy message types are supported.

   o  LM-COUNTERS: Whether 64-bit counters are supported.

   o  LM-ACCURACY: The expected measurement accuracy levels for the
      supported forms of LM, and the expected impact of exception
      conditions such as lost and misordered messages.

   o  LM-SYNC: The implementation's behavior in regard to the
      synchronization conditions discussed in Section 2.9.8.

   o  LM-SCOPE: The supported LM scopes (Sections 2.9.9 and 4.2.8).

   o  DM-ACCURACY: The expected measurement accuracy levels for the
      supported forms of DM.

   o  DM-TS-FORMATS: The supported timestamp formats and the extent of
      support for computation with and reconciliation of different
      formats.










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6.  Congestion Considerations

   An MPLS network may be traffic-engineered in such a way that the
   bandwidth required both for client traffic and for control,
   management, and OAM traffic is always available.  The following
   congestion considerations therefore apply only when this is not the
   case.

   The proactive generation of Loss Measurement and Delay Measurement
   messages for purposes of monitoring the performance of an MPLS
   channel naturally results in a degree of additional load placed on
   both the network and the terminal nodes of the channel.  When
   configuring such monitoring, operators should be mindful of the
   overhead involved and should choose transmit rates that do not stress
   network resources unduly; such choices must be informed by the
   deployment context.  In case of slower links or lower-speed devices,
   for example, lower Loss Measurement message rates can be chosen, up
   to the limits noted at the end of Section 2.2.

   In general, lower measurement message rates place less load on the
   network at the expense of reduced granularity.  For delay
   measurement, this reduced granularity translates to a greater
   possibility that the delay associated with a channel temporarily
   exceeds the expected threshold without detection.  For loss
   measurement, it translates to a larger gap in loss information in
   case of exceptional circumstances such as lost LM messages or
   misordered packets.

   When carrying out a sustained measurement operation such as an LM
   operation or continuous proactive DM operation, the querier SHOULD
   take note of the number of lost measurement messages (queries for
   which a response is never received) and set a corresponding
   Measurement Message Loss Threshold.  If this threshold is exceeded,
   the measurement operation SHOULD be suspended so as not to exacerbate
   the possible congestion condition.  This suspension SHOULD be
   accompanied by an appropriate notification to the user so that the
   condition can be investigated and corrected.

   From the receiver perspective, the main consideration is the
   possibility of receiving an excessive quantity of measurement
   messages.  An implementation SHOULD employ a mechanism such as rate-
   limiting to guard against the effects of this case.

7.  Manageability Considerations

   The measurement protocols described in this document are intended to
   serve as infrastructure to support a wide range of higher-level
   monitoring and diagnostic applications, from simple command-line



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   diagnostic tools to comprehensive network performance monitoring and
   analysis packages.  The specific mechanisms and considerations for
   protocol configuration, initialization, and reporting thus depend on
   the nature of the application.

   In the case of on-demand diagnostics, the diagnostic application may
   provide parameters such as the measurement type, the channel, the
   query rate, and the test duration when initiating the diagnostic;
   results and exception conditions are then reported directly to the
   application.  The system may discard the statistics accumulated
   during the test after the results have been reported or retain them
   to provide a historical measurement record.

   Alternatively, measurement configuration may be supplied as part of
   the channel configuration itself in order to support continuous
   monitoring of the channel's performance characteristics.  In this
   case, the configuration will typically include quality thresholds
   depending on the service level agreement, the crossing of which will
   trigger warnings or alarms, and result reporting and exception
   notification will be integrated into the system-wide network
   management and reporting framework.

8.  Security Considerations

   This document describes procedures for the measurement of performance
   metrics over a pre-existing MPLS path (a pseudowire, LSP, or
   section).  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 using existing MPLS mechanisms such as
   Bidirectional Forwarding Detection (BFD) [RFC5884]; tools,
   techniques, and considerations for securing MPLS paths are discussed
   in detail in [RFC5920].

   When such mechanisms are not available, and where security of the
   measurement operation is a concern, reception of Generic Associated
   Channel messages with the Channel Types specified in this document
   SHOULD be disabled.  Implementations MUST provide the ability to
   disable these protocols on a per-Channel-Type basis.

   Even when the identity of the far end has been verified, the
   measurement protocols remain vulnerable to injection and man-in-the-
   middle attacks.  The impact of such an attack would be to compromise
   the quality of performance measurements on the affected path.  An
   attacker positioned to disrupt these measurements is, however,
   capable of causing much greater damage by disrupting far more
   critical elements of the network such as the network control plane or
   user traffic flows.  At worst, a disruption of the measurement
   protocols would interfere with the monitoring of the performance



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   aspects of the service level agreement associated with the path; the
   existence of such a disruption would imply that a serious breach of
   basic path integrity had already occurred.

   If desired, such attacks can be mitigated by performing basic
   validation and sanity checks, at the querier, 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.

   Cryptographic mechanisms capable of signing or encrypting the
   contents of the measurement packets without degrading the measurement
   performance are not currently available.  In light of the preceding
   discussion, the absence of such cryptographic mechanisms does not
   raise significant security issues.

   Users concerned with the security of out-of-band responses over IP
   networks SHOULD employ suitable security mechanisms such as IPsec
   [RFC4301] to protect the integrity of the return path.

9.  IANA Considerations

   Per this document, IANA has completed the following actions:

   o  Allocation of Channel Types in the "PW Associated Channel Type"
      registry

   o  Creation of a "Measurement Timestamp Type" registry

   o  Creation of an "MPLS Loss/Delay Measurement Control Code" registry

   o  Creation of an "MPLS Loss/Delay Measurement Type-Length-Value
      (TLV) Object" registry

















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9.1.  Allocation of PW Associated Channel Types

   As per the IANA considerations in [RFC5586], IANA has allocated the
   following Channel Types in the "PW Associated Channel Type" registry:

   Value  Description                              TLV Follows Reference
   ------ ---------------------------------------- ----------- ---------
   0x000A MPLS Direct Loss Measurement (DLM)       No          RFC 6374
   0x000B MPLS Inferred Loss Measurement (ILM)     No          RFC 6374
   0x000C MPLS Delay Measurement (DM)              No          RFC 6374
   0x000D MPLS Direct Loss and Delay Measurement   No          RFC 6374
          (DLM+DM)
   0x000E MPLS Inferred Loss and Delay Measurement No          RFC 6374
          (ILM+DM)

9.2.  Creation of Measurement Timestamp Type Registry

   IANA has created a new "Measurement Timestamp Type" registry, with
   format and initial allocations as follows:

   Type Description                               Size in Bits Reference
   ---- ----------------------------------------- ------------ ---------
   0    Null Timestamp                            64           RFC 6374
   1    Sequence Number                           64           RFC 6374
   2    Network Time Protocol version 4 64-bit    64           RFC 6374
        Timestamp
   3    Truncated IEEE 1588v2 PTP Timestamp       64           RFC 6374

   The range of the Type field is 0-15.

   The allocation policy for this registry is IETF Review.

9.3.  Creation of MPLS Loss/Delay Measurement Control Code Registry

   IANA has created a new "MPLS Loss/Delay Measurement Control Code"
   registry.  This registry is divided into two separate parts, one for
   Query Codes and the other for Response Codes, with formats and
   initial allocations as follows:

   Query Codes

   Code Description                    Reference
   ---- ------------------------------ ---------
   0x0  In-band Response Requested     RFC 6374
   0x1  Out-of-band Response Requested RFC 6374
   0x2  No Response Requested          RFC 6374





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   Response Codes

   Code Description                         Reference
   ---- ----------------------------------- ---------
   0x0  Reserved                            RFC 6374
   0x1  Success                             RFC 6374
   0x2  Data Format Invalid                 RFC 6374
   0x3  Initialization in Progress          RFC 6374
   0x4  Data Reset Occurred                 RFC 6374
   0x5  Resource Temporarily Unavailable    RFC 6374
   0x10 Unspecified Error                   RFC 6374
   0x11 Unsupported Version                 RFC 6374
   0x12 Unsupported Control Code            RFC 6374
   0x13 Unsupported Data Format             RFC 6374
   0x14 Authentication Failure              RFC 6374
   0x15 Invalid Destination Node Identifier RFC 6374
   0x16 Connection Mismatch                 RFC 6374
   0x17 Unsupported Mandatory TLV Object    RFC 6374
   0x18 Unsupported Query Interval          RFC 6374
   0x19 Administrative Block                RFC 6374
   0x1A Resource Unavailable                RFC 6374
   0x1B Resource Released                   RFC 6374
   0x1C Invalid Message                     RFC 6374
   0x1D Protocol Error                      RFC 6374

   IANA has indicated that the values 0x0 - 0xF in the Response Code
   section are reserved for non-error response codes.

   The range of the Code field is 0 - 255.

   The allocation policy for this registry is IETF Review.




















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9.4.  Creation of MPLS Loss/Delay Measurement TLV Object Registry

   IANA has created a new "MPLS Loss/Delay Measurement TLV Object"
   registry, with format and initial allocations as follows:

   Type Description                       Reference
   ---- --------------------------------- ---------
   0    Padding - copy in response        RFC 6374
   1    Return Address                    RFC 6374
   2    Session Query Interval            RFC 6374
   3    Loopback Request                  RFC 6374
   127  Experimental use                  RFC 6374
   128  Padding - do not copy in response RFC 6374
   129  Destination Address               RFC 6374
   130  Source Address                    RFC 6374
   255  Experimental use                  RFC 6374

   IANA has indicated that Types 0-127 are classified as Mandatory, and
   that Types 128-255 are classified as Optional.

   The range of the Type field is 0 - 255.

   The allocation policy for this registry is IETF Review.

10.  Acknowledgments

   The authors wish to thank the many participants of the MPLS working
   group who provided detailed review and feedback on this document.
   The authors offer special thanks to Alexander Vainshtein, Loa
   Andersson, and Hiroyuki Takagi for many helpful thoughts and
   discussions, to Linda Dunbar for the idea of using LM messages for
   throughput measurement, and to Ben Niven-Jenkins, Marc Lasserre, and
   Ben Mack-Crane for their valuable comments.

11.  References

11.1.  Normative References

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

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.







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   [RFC2474]   Nichols, K., Blake, S., Baker, F., and D. Black,
               "Definition of the Differentiated Services Field (DS
               Field) in the IPv4 and IPv6 Headers", RFC 2474,
               December 1998.

   [RFC3031]   Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
               Label Switching Architecture", RFC 3031, January 2001.

   [RFC3270]   Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
               P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
               Protocol Label Switching (MPLS) Support of Differentiated
               Services", RFC 3270, May 2002.

   [RFC5462]   Andersson, L. and R. Asati, "Multiprotocol Label
               Switching (MPLS) Label Stack Entry: "EXP" Field Renamed
               to "Traffic Class" Field", RFC 5462, February 2009.

   [RFC5586]   Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
               Associated Channel", RFC 5586, June 2009.

   [RFC5905]   Mills, D., Martin, J., Burbank, J., and W. Kasch,
               "Network Time Protocol Version 4: Protocol and Algorithms
               Specification", RFC 5905, June 2010.

11.2.  Informative References

   [RFC2679]   Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
               Delay Metric for IPPM", RFC 2679, September 1999.

   [RFC2680]   Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
               Packet Loss Metric for IPPM", RFC 2680, September 1999.

   [RFC2681]   Almes, G., Kalidindi, S., and M. Zekauskas, "A Round-trip
               Delay Metric for IPPM", RFC 2681, September 1999.

   [RFC3209]   Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
               and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
               Tunnels", RFC 3209, December 2001.

   [RFC3260]   Grossman, D., "New Terminology and Clarifications for
               Diffserv", RFC 3260, April 2002.

   [RFC3985]   Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
               Edge (PWE3) Architecture", RFC 3985, March 2005.

   [RFC4301]   Kent, S. and K. Seo, "Security Architecture for the
               Internet Protocol", RFC 4301, December 2005.




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   [RFC4656]   Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
               Zekauskas, "A One-way Active Measurement Protocol
               (OWAMP)", RFC 4656, September 2006.

   [RFC4928]   Swallow, G., Bryant, S., and L. Andersson, "Avoiding
               Equal Cost Multipath Treatment in MPLS Networks",
               BCP 128, RFC 4928, June 2007.

   [RFC5036]   Andersson, L., Minei, I., and B. Thomas, "LDP
               Specification", RFC 5036, October 2007.

   [RFC5357]   Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
               Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
               RFC 5357, October 2008.

   [RFC5481]   Morton, A. and B. Claise, "Packet Delay Variation
               Applicability Statement", RFC 5481, March 2009.

   [RFC5884]   Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
               "Bidirectional Forwarding Detection (BFD) for MPLS Label
               Switched Paths (LSPs)", RFC 5884, June 2010.

   [RFC5920]   Fang, L., "Security Framework for MPLS and GMPLS
               Networks", RFC 5920, July 2010.

   [RFC5921]   Bocci, M., Bryant, S., Frost, D., Levrau, L., and L.
               Berger, "A Framework for MPLS in Transport Networks",
               RFC 5921, July 2010.

   [RFC5960]   Frost, D., Bryant, S., and M. Bocci, "MPLS Transport
               Profile Data Plane Architecture", RFC 5960, August 2010.

   [RFC6375]   Frost, D., Ed. and S. Bryant, Ed., "A Packet Loss and
               Delay Measurement Profile for MPLS-Based Transport
               Networks", RFC 6375, September 2011.

   [Y.1731]    ITU-T Recommendation Y.1731, "OAM Functions and
               Mechanisms for Ethernet based Networks", February 2008.













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Appendix A.  Default Timestamp Format Rationale

   This document initially proposed the Network Time Protocol (NTP)
   timestamp format as the mandatory default, as this is the normal
   default timestamp in IETF protocols and thus would seem the "natural"
   choice.  However, a number of considerations have led instead to the
   specification of the truncated IEEE 1588 Precision Time Protocol
   (PTP) timestamp as the default.  NTP has not gained traction in
   industry as the protocol of choice for high-quality timing
   infrastructure, whilst IEEE 1588 PTP has become the de facto time
   transfer protocol in networks that are specially engineered to
   provide high-accuracy time distribution service.  The PTP timestamp
   format is also the ITU-T format of choice for packet transport
   networks, which may rely on MPLS protocols.  Applications such as
   one-way delay measurement need the best time service available, and
   converting between the NTP and PTP timestamp formats is not a trivial
   transformation, particularly when it is required that this be done in
   real time without loss of accuracy.

   The truncated IEEE 1588 PTP format specified in this document is
   considered to provide a more than adequate wrap time and greater time
   resolution than it is expected will be needed for the operational
   lifetime of this protocol.  By truncating the timestamp at both the
   high and low order bits, the protocol achieves a worthwhile reduction
   in system resources.

Authors' Addresses

   Dan Frost
   Cisco Systems

   EMail: danfrost@cisco.com


   Stewart Bryant
   Cisco Systems

   EMail: stbryant@cisco.com













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