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Versions: 00 draft-ietf-mpls-loss-delay

MPLS                                                       D. Frost, Ed.
Internet-Draft                                            S. Bryant, Ed.
Intended status: Standards Track                           Cisco Systems
Expires: June 11, 2011                                  December 8, 2010


          Packet Loss and Delay Measurement for MPLS Networks
                     draft-frost-mpls-loss-delay-00

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

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].

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 11, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  6
   2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Packet Loss Measurement  . . . . . . . . . . . . . . . . .  7
     2.2.  Throughput Measurement . . . . . . . . . . . . . . . . . .  9
     2.3.  Delay Measurement  . . . . . . . . . . . . . . . . . . . .  9
     2.4.  Delay Variation Measurement  . . . . . . . . . . . . . . . 11
     2.5.  Unidirectional Measurement . . . . . . . . . . . . . . . . 11
     2.6.  Loopback Measurement . . . . . . . . . . . . . . . . . . . 12
     2.7.  Measurement Considerations . . . . . . . . . . . . . . . . 12
       2.7.1.  Types of Channels  . . . . . . . . . . . . . . . . . . 12
       2.7.2.  Quality of Service . . . . . . . . . . . . . . . . . . 12
       2.7.3.  Equal Cost Multipath . . . . . . . . . . . . . . . . . 13
       2.7.4.  Intermediate Nodes . . . . . . . . . . . . . . . . . . 13
       2.7.5.  Distributed Systems  . . . . . . . . . . . . . . . . . 14
       2.7.6.  Loss Measurement Modes . . . . . . . . . . . . . . . . 14
       2.7.7.  Loss Measurement Scope . . . . . . . . . . . . . . . . 16
       2.7.8.  Delay Measurement Accuracy . . . . . . . . . . . . . . 16
       2.7.9.  Delay Measurement Timestamp Format . . . . . . . . . . 16
   3.  Message Formats  . . . . . . . . . . . . . . . . . . . . . . . 17
     3.1.  Loss Measurement Message Format  . . . . . . . . . . . . . 17
     3.2.  Delay Measurement Message Format . . . . . . . . . . . . . 22
     3.3.  Combined Loss/Delay Measurement Message Format . . . . . . 24
     3.4.  Timestamp Field Formats  . . . . . . . . . . . . . . . . . 25
     3.5.  TLV Objects  . . . . . . . . . . . . . . . . . . . . . . . 26
       3.5.1.  Padding  . . . . . . . . . . . . . . . . . . . . . . . 27
       3.5.2.  Addressing . . . . . . . . . . . . . . . . . . . . . . 27
   4.  Operation  . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     4.1.  Loss Measurement Procedures  . . . . . . . . . . . . . . . 28
       4.1.1.  Initiating a Loss Measurement Operation  . . . . . . . 28
       4.1.2.  Transmitting a Loss Measurement Query  . . . . . . . . 29
       4.1.3.  Receiving a Loss Measurement Query . . . . . . . . . . 29
       4.1.4.  Transmitting a Loss Measurement Response . . . . . . . 30
       4.1.5.  Receiving a Loss Measurement Response  . . . . . . . . 30
       4.1.6.  Loss Calculation . . . . . . . . . . . . . . . . . . . 31



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       4.1.7.  Quality of Service . . . . . . . . . . . . . . . . . . 31
       4.1.8.  G-ACh Packets  . . . . . . . . . . . . . . . . . . . . 31
       4.1.9.  Test Messages  . . . . . . . . . . . . . . . . . . . . 31
       4.1.10. Message Loss and Packet Misorder Conditions  . . . . . 32
     4.2.  Delay Measurement Procedures . . . . . . . . . . . . . . . 33
       4.2.1.  Transmitting a Delay Measurement Query . . . . . . . . 33
       4.2.2.  Receiving a Delay Measurement Query  . . . . . . . . . 33
       4.2.3.  Transmitting a Delay Measurement Response  . . . . . . 34
       4.2.4.  Receiving a Delay Measurement Response . . . . . . . . 34
       4.2.5.  Timestamp Format Negotiation . . . . . . . . . . . . . 35
       4.2.6.  Quality of Service . . . . . . . . . . . . . . . . . . 36
     4.3.  Combined Loss/Delay Measurement Procedures . . . . . . . . 36
   5.  Congestion Considerations  . . . . . . . . . . . . . . . . . . 36
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 37
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 37
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 38
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 38
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 38
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 39































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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 capability, in
   addition, provides operators with greater visibility into the
   performance characteristics of their networks, thereby facilitating
   planning, troubleshooting, and 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.

   o  The LM and DM protocols operate over the MPLS Generic Associated
      Channel (G-ACh) [RFC5586] and support measurement of loss and
      delay 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].

   o  The LM and DM protocols can be used for both 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 either be carried out
      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.





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   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
      packets 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.

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

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

   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.



















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1.1.  Terminology

   Term    Definition
   ------- -------------------------------------------
   ACH     Associated Channel Header
   DM      Delay Measurement
   G-ACh   Generic Associated Channel
   LM      Loss Measurement
   LSE     Label Stack Entry
   LSP     Label Switched Path
   LSR     Label Switching Router
   MPLS-TP MPLS Transport Profile
   NTP     Network Time Protocol
   OAM     Operations, Administration, and Maintenance
   PTP     Precision Time Protocol
   PW      Pseudowire
   TC      Traffic Class


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 are discussed that may come into play to
   inform or modify these simple procedures.

   The following figure shows the reference scenario.

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

                                 Figure 1

   The 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.
   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



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   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.

2.1.  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.

   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.



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   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 inserts an appropriate response code into the
   message and transmits it 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.

   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 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.  Consider for
   example 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



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   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
   proportionately.

2.2.  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 a measurement of the throughput sustained over the
   channel during the interval.  This metric can be called the delivered
   throughput.  As for loss measurement, the interval counts can be
   accumulated to arrive at the delivered throughput of the channel
   since the start of the measurement operation.  This procedure also
   enables out-of-service throughput testing when combined with a simple
   packet generator.

2.3.  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.




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   In the case of two-way delay, there are actually two possible metrics
   of interest.  The "strict" two-way delay is the sum of the one-way
   delays in each direction and reflects the two-way delay of the
   channel itself, irrespective of processing delays within the remote
   endpoint B. The "loose" two-way delay 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 can be measured
   directly even when this is not the case (provided A and B have stable
   clocks).

   The measurement is accomplished by sending a Delay Measurement (DM)
   query message over the channel to B which 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 inserts an appropriate response code into the message
   and transmits it 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.

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

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

   and the loose two-way delay as

      loose two-way delay = T4 - T1.

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





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      forward one-way delay = T2 - T1

      reverse one-way delay = T4 - T3

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

2.4.  Delay Variation Measurement

   Packet Delay Variation (PDV) [RFC3393] is another performance metric
   important in some applications.  The PDV of a pair of packets within
   a stream of packets is defined for a selected pair of packets in the
   stream going from measurement point 1 to measurement point 2.  The
   PDV is the difference between the one-way delay of the selected
   packets.

   A PDV measurement can therefore be derived from successive delay
   measurements obtained through the procedures in Section 2.3.  An
   important point regarding PDV 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.

2.5.  Unidirectional Measurement

   In the case that the channel from A to (B1, ..., Bk) 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 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, i.e.

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

   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.







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2.6.  Loopback Measurement

   Some bidirectional channels may be placed into a loopback state such
   that query messages are looped back to the querier without
   modification.  In this situation, LM and DM procedures can be used to
   carry out measurements associated with the circular path.

   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])

   For DM, the loose two-way 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.

   Query messages must include some form of source identifier in order
   for looped-back queries to be differentiated from queries initiated
   by the far end.

2.7.  Measurement Considerations

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

2.7.1.  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.

2.7.2.  Quality of Service

   Quality of Service (QoS) capabilities, in the form of the
   Differentiated Services architecture, apply to MPLS as specified in



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   [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 therefore 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.

   Another aspect of packet processing which often arises in the context
   of QoS concerns the location of the measurement points for loss and
   delay within the sending and receiving nodes, which is
   implementation-dependent.  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-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 impacts what, precisely, is being
   measured.  The principal consideration here is that the behavior of
   an implementation in these respects SHOULD be made clear to the user.

2.7.3.  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 which 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.

2.7.4.  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



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   (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.  Often the motivation for conducting
   measurements to intermediate nodes is 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.

2.7.5.  Distributed Systems

   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.

   [Editor's note: The last two restrictions could be relaxed if
   desired, at the expense of some additional protocol complexity.]

2.7.6.  Loss Measurement Modes

   The summary of loss measurement at the beginning of Section 2 above
   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".

   Direct LM has the advantage of being able to provide perfect loss



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   accounting when it is available.  There are, however, several
   limitations 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.

   Another 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.

   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.

   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.



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2.7.7.  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
   SHOULD make their supported LM scopes clear to the user, and care
   must be taken to ensure that the scopes of the channel endpoints
   agree.

2.7.8.  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
   main difference in the two cases is one of measurement accuracy.
   Implementations SHOULD make their delay measurement accuracy levels
   clear to the user.

2.7.9.  Delay Measurement Timestamp Format

   There are two significant timestamp formats in common use: the
   timestamp format of the Internet standard 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 was specifically designed to make the computation
   of timestamp differences as simple and efficient as possible.  On the
   other hand, there is also now a significant deployment of equipment
   designed to support the PTP format.

   The approach taken in this document is therefore to include in DM
   messages fields which 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.  Support for multiple timestamp formats is OPTIONAL.  An
   implementation SHOULD, however, make clear which timestamp formats it
   supports and the extent of its support for computation with and
   reconciliation of different formats for purposes of delay
   measurement.

   In recognition of the wide deployment, particularly in hardware-based



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   timing implementations, of IEEE 1588 PTP, the PTP timestamp format is
   the default format used in DM messages.  This format MUST be
   supported.


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

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

   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.

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                | TC  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Origin Timestamp                       |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Counter 1                           |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Counter 4                           |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           TLV Block                           ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 2: 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.

   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   Format of the Origin Timestamp field
   (OTF)
   Reserved                  Reserved for future specification
   Session Identifier        Set arbitrarily by the querier
   Traffic Class (TC)        TC being measured
   Origin Timestamp          Query message transmission timestamp
   Counter 1-4               Packet counter values in network byte order
   TLV Block                 Optional block of Type-Length-Value fields

   The possible values for these fields are as follows.



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   Version: Currently set to 0.

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

                               +-+-+-+-+
                               | Flags |
                               +-+-+-+-+

                               +-+-+-+-+
                               |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, and 0 otherwise.  When set to 1, the TC 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.

      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.

      For a Response:

         Codes 0x0-0xF are reserved for non-error responses.

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





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         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 which may
         render the data in this response inconsistent relative to
         earlier responses.

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

         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.




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         0x18: Error - Query Rate Exceeded.  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 - Temporary Resource Exhaustion.  Indicates that
         the operation failed because node resources were not available.

   Message Length: Set to the total length of this message in bytes.

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

                               +-+-+-+-+
                               | DFlags|
                               +-+-+-+-+

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

                      Loss Measurement Message 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.  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.

   TC: When the T flag is set to 1, this field is set to the TC being
   measured.  When the T flag is set to 0, the value of this field is



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   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 2.1, 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 fields 1-4
   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 is important for hardware processing.

   All counter values MUST be in network byte order.  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                | TC  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Timestamp 1                         |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Timestamp 4                         |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           TLV Block                           ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 3: 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
   Traffic Class (TC)    TC being 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.

   Flags: As specified in Section 3.1, except for the X flag, which is
   set to 0, and the T flag, which is set to 1.



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   Control Code: As specified in Section 3.1.

   Message Length: Set to the total length of this message in bytes.

   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.

   TC: Set to the TC being measured.

   Timestamp 1-4: Referring to Section 2.3, 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 fields 1-4 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                | TC  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Timestamp 1                         |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Timestamp 4                         |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Counter 1                           |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Counter 4                           |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           TLV Block                           ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 4: Loss/Delay Measurement Message Format

   The LM/DM message fields have the same meanings as the corresponding
   fields in the LM and DM message formats.

3.4.  Timestamp Field Formats

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

      0x0: Null timestamp format.  This value is a placeholder
      indicating that the timestamp field does not contain a meaningful
      timestamp.

      0x1: Sequence number.  This value indicates that the timestamp
      field is to be viewed as a simple 64-bit sequence number.



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      0x2: 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.

      0x3: IEEE 1588-2002 (1588v1) Precision Time Protocol timestamp
      format [IEEE1588].  This format consists of a 32-bit seconds field
      followed by a 32-bit nanoseconds field.

   In recognition of the wide deployment, particularly in hardware-based
   timing implementations, of IEEE 1588 PTP, the PTP timestamp format is
   the default format used in Delay Measurement messages.  This format
   MUST be supported.  Support for other timestamp formats is OPTIONAL.

   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.

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

   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 discard the message or respond with an
   appropriate error code.





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   The types defined are as follows:

   Type           Definition
   -------------- ---------------------------------
   Mandatory
   0              Padding - copy in response
   1              Return Address
   2-119          Reserved
   120-127        Vendor-specific usage

   Optional
   128            Padding - do not copy in response
   129            Destination Address
   130            Source Address
   131-247        Reserved
   248-255        Vendor-specific usage

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.  More than one padding object MAY
   be present, in which case they SHOULD be continguous.  Padding
   objects SHOULD occur at the end of the TLV Block.  The Value field of
   a padding object is arbitrary.

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     | AType |      Reserved         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           Address                             ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Addressing Object Format

   The AType (Address Type) field indicates the type of the address.
   Address types defined are:








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   Type    Definition
   ------- --------------------
   0       IP version 4 address
   1       IP version 6 address

   The Source and Destination address objects indicate the addresses of
   the sender and the intended recipient of the message, respectively.
   The Source Address SHOULD be used as the destination for out-of-band
   responses 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.


4.  Operation

4.1.  Loss Measurement Procedures

4.1.1.  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 2.1, 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.

   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 2.1 cannot arise.  The
   implementation SHOULD assume, in evaluating this rate, 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.

   When initiating an LM operation, the far end may require a period of
   time to become ready for the requested measurement operation.  During
   this period, LM queries MAY simply be discarded, and the querier
   expecting a response SHOULD be prepared for this situation, for
   example by setting a timer to differentiate between an acceptable
   initialization delay and a permanent unavailability condition at the
   far end.  Alternatively, the receiver MAY respond, possibly in a
   rate-limited manner, to queries received during this period with an
   appropriate notification code.




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4.1.2.  Transmitting a Loss Measurement Query

   When transmitting an LM Query over a channel, 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 TC 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.

   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 SHOULD 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.  The remaining Counter fields MUST be
   set to 0.

4.1.3.  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.






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4.1.4.  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, and if the value of the T flag is
   1, the value of the TC field MUST also be copied.

   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.

4.1.5.  Receiving a Loss Measurement Response

   Upon in-band receipt of an LM Response message, the Counter 2 field
   SHOULD be 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 MUST be set to 0.

   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.  When the Control Code indicates an
   error condition, the LM operation SHOULD be suspended and an
   appropriate notification to the user generated.  If a temporary error
   condition is indicated, the LM operation MAY be restarted
   automatically.







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4.1.6.  Loss Calculation

   Calculation of packet loss is carried out according to the procedures
   in Section 2.1.  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
   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.

4.1.7.  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.

4.1.8.  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.

4.1.9.  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.

   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 TC fields SHOULD be used for this
   purpose when possible.  That is, test messages in this case will
   include a 32-bit field which can carry the value of the combined
   Session Identifier + TC field present in LM messages.  When TC-
   specific LM is conducted, the TC field of the LSE in the label stack
   of a test message corresponding to the channel (e.g.  LSP) over which



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   the message is sent MUST equal the TC value in the associated LM
   messages.

4.1.10.  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.

   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 2.1.  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, which is usually what is desired.

   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 2.1)



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   indicates the maximum amount of time that can elapse before the LM
   state is discarded.  If some messages are lost, but a message is
   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.

4.2.  Delay Measurement Procedures

4.2.1.  Transmitting a Delay Measurement Query

   When transmitting a DM Query over a channel, 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 TC 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 other timestamp fields MUST be set to 0.

4.2.2.  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 is received.

   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



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   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.

4.2.3.  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, TC, 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 either to the QTF or the
   RPTF field.  See Section 4.2.5 for guidelines on 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.

   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.

4.2.4.  Receiving a Delay Measurement Response

   Upon in-band receipt of a DM Response message, the Timestamp 2 field
   SHOULD be set to the time at which this DM Response is received.




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   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.  When the Control Code indicates an
   error condition, an appropriate notification to the user SHOULD be
   generated.

4.2.5.  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,
   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.

4.2.5.1.  Single-Format Procedures

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




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   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.

4.2.6.  Quality of Service

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

4.3.  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 which happens to carry additional
   timestamp data, with the timestamp fields processed as per delay
   measurement procedures.


5.  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 2.1.

   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



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   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 pro-active 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.  Authentication
   procedures can also be used to ensure that only queries from
   authorized devices are processed.


6.  Security Considerations

   There are three main types of security considerations associated with
   the exchange of performance monitoring messages such as those
   described in this document: the possibility of a malicious or
   misconfigured device generating an excessive quantity of messages,
   causing service impairment; the possibility of unauthorized
   alteration of messages in transit; and the possibility of an
   unauthorized device learning the data contained in or implied by such
   messages.

   The first consideration is discussed in Section 5.  If reception or
   alteration of performance-related data by unauthorized devices is an
   operational concern, authentication and/or encryption procedures
   should be used to ensure message integrity and confidentiality.  Such
   procedures are outside the scope of this document, but have general
   applicability to OAM protocols in MPLS-TP networks.


7.  IANA Considerations

   A future version of this document will detail IANA considerations
   for:

   o  ACH Channel Types for LM and DM messages




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   o  Timestamp format registry

   o  LM and DM Control Codes

   o  TLV Objects


8.  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, and to Linda Dunbar for the idea of using LM messages
   for throughput measurement.


9.  References

9.1.  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.

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

   [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.

9.2.  Informative References

   [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.

   [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



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              Services", RFC 3270, May 2002.

   [RFC3393]  Demichelis, C. and P. Chimento, "IP Packet Delay Variation
              Metric for IP Performance Metrics (IPPM)", RFC 3393,
              November 2002.

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

   [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.

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

   [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.


Authors' Addresses

   Dan Frost (editor)
   Cisco Systems

   Email: danfrost@cisco.com


   Stewart Bryant (editor)
   Cisco Systems

   Email: stbryant@cisco.com







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