Network Working Group                                   G. Fioccola, Ed.
Internet-Draft                                                A. Capello, Ed. Capello
Intended status: Experimental                                M. Cociglio
Expires: March 11, 15, 2018                                   L. Castaldelli
                                                          Telecom Italia
                                                                 M. Chen, Ed. Chen
                                                                L. Zheng, Ed. Zheng
                                                     Huawei Technologies
                                                               G. Mirsky, Ed. Mirsky
                                                              T. Mizrahi, Ed. Mizrahi
                                                      September 7, 11, 2017

 Alternate Marking method for passive and hybrid performance monitoring


   This document describes a method to perform packet loss, delay and
   jitter measurements on live traffic.  This method is based on
   Alternate Marking (Coloring) technique.  A report is provided in
   order to explain an example and show the method applicability.  This
   technique can be applied in various situations as detailed in this
   document and could be considered passive or hybrid depending on the

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

Status of This Memo

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

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   This Internet-Draft will expire on March 11, 15, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview of the method  . . . . . . . . . . . . . . . . . . .   4
   3.  Detailed description of the method  . . . . . . . . . . . . .   6
     3.1.  Packet loss measurement . . . . . . . . . . . . . . . . .   6
       3.1.1.  Coloring the packets  . . . . . . . . . . . . . . . .  11
       3.1.2.  Counting the packets  . . . . . . . . . . . . . . . .  11
       3.1.3.  Collecting data and calculating packet loss . . . . .  12
     3.2.  Timing aspects  . . . . . . . . . . . . . . . . . . . . .  12
     3.3.  One-way delay measurement . . . . . . . . . . . . . . . .  14
       3.3.1.  Single marking methodology  . . . . . . . . . . . . .  14
       3.3.2.  Double marking methodology  . . . . . . . . . . . . .  16
     3.4.  Delay variation measurement . . . . . . . . . . . . . . .  17
   4.  Considerations  . . . . . . . . . . . . . . . . . . . . . . .  18
     4.1.  Synchronization . . . . . . . . . . . . . . . . . . . . .  18
     4.2.  Data Correlation  . . . . . . . . . . . . . . . . . . . .  18
     4.3.  Packet Re-ordering  . . . . . . . . . . . . . . . . . . .  19
   5.  Implementation and deployment . . . . . . . . . . . . . . . .  20
     5.1.  Report on the operational experiment at Telecom Italia  .  20
       5.1.1.  Metric transparency . . . . . . . . . . . . . . . . .  22
     5.2.  IP flow performance measurement (IPFPM) . . . . . . . . .  22
     5.3.  OAM Passive Performance Measurement . . . . . . . . . . .  22
     5.4.  RFC6374 Use Case  . . . . . . . . . . . . . . . . . . . .  23
     5.5.  Application to active performance measurement . . . . . .  23
   6.  Hybrid measurement  . . . . . . . . . . . . . . . . . . . . .  23
   7.  Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .  23
   8.  Compliance with RFC6390 guidelines  . . . . . . . . . . . . .  24
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  27
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  27
     12.2.  Informative References . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   Nowadays, most Service Providers' networks carry traffic with
   contents that are highly sensitive to packet loss [RFC7680], delay
   [RFC7679], and jitter [RFC3393].

   In view of this scenario, Service Providers need methodologies and
   tools to monitor and measure network performances with an adequate
   accuracy, in order to constantly control the quality of experience
   perceived by their customers.  On the other hand, performance
   monitoring provides useful information for improving network
   management (e.g.  isolation of network problems, troubleshooting,

   A lot of work related to OAM, that includes also performance
   monitoring techniques, has been done by Standards Developing
   Organizations(SDOs): [RFC7276] provides a good overview of existing
   OAM mechanisms defined in IETF, ITU-T and IEEE.  Considering IETF, a
   lot of work has been done on fault detection and connectivity
   verification, while a minor effort has been dedicated so far to
   performance monitoring.  The IPPM WG has defined standard metrics to
   measure network performance; however, the methods developed in this
   WG mainly refer to focus on active measurement techniques.  More
   recently, the MPLS WG has defined mechanisms for measuring packet
   loss, one-way and two-way delay, and delay variation in MPLS
   networks[RFC6374], but their applicability to passive measurements
   has some limitations, especially for pure connection-less networks.

   The lack of adequate tools to measure packet loss with the desired
   accuracy drove an effort to design a new method for the performance
   monitoring of live traffic, possibly easy to implement and deploy.
   The effort led to the method described in this document: basically,
   it is a passive performance monitoring technique, potentially
   applicable to any kind of packet based traffic, including Ethernet,
   IP, and MPLS, both unicast and multicast.  The method addresses
   primarily packet loss measurement, but it can be easily extended to
   one-way delay and delay variation measurements as well.

   The method has been explicitly designed for passive measurements but
   it can also be used with active probes.  Passive measurements are
   usually more easily understood by customers and provide a much better
   accuracy, especially for packet loss measurements.

   RFC 7799 [RFC7799] defines passive and hybrid methods of measurement.
   In particular, Passive Methods of Measurement are based solely on
   observations of an undisturbed and unmodified packet stream of
   interest; Hybrid Methods are Methods of Measurement that use a
   combination of Active Methods and Passive Methods.

   Taking into consideration these definitions, Alternate Marking Method
   could be considered Hybrid or Passive depending on the case.  In case
   the marking field is obtained by changing existing field values of
   the packets (e.g.  DSCP field), the technique is Hybrid.  In case the
   marking field is dedicated, reserved and is included in the protocol
   specification Alternate Marking technique can be considered as
   Passive (e.g.  RFC6374 Synonymous Flow Label or OAM Marking Bits in
   BIER Header).

   This document is organized as follows:

   o  Section 2 gives an overview of the method, including a comparison
      with different measurement strategies;

   o  Section 3 describes the method in detail;

   o  Section 4 reports considerations about synchronization, data
      correlation and packet re-ordering;

   o  Section 5 reports examples of implementation and deployment of the
      method.  Furthermore the operational experiment done at Telecom
      Italia is described;

   o  Section 6 introduces Hybrid measurement aspects;

   o  Section 7 is about the Compliance with RFC6390 guidelines;

   o  Section 8 includes some security aspects;

   o  Section 9 finally summarizes some concluding remarks.

2.  Overview of the method

   In order to perform packet loss measurements on a live traffic flow,
   different approaches exist.  The most intuitive one consists in
   numbering the packets, so that each router that receives the flow can
   immediately detect a packet missing.  This approach, though very
   simple in theory, is not simple to achieve: it requires the insertion
   of a sequence number into each packet and the devices must be able to
   extract the number and check it in real time.  Such a task can be
   difficult to implement on live traffic: if UDP is used as the
   transport protocol, the sequence number is not available; on the
   other hand, if a higher layer sequence number (e.g. in the RTP
   header) is used, extracting that information from each packet and
   process it in real time could overload the device.

   An alternate approach is to count the number of packets sent on one
   end, the number of packets received on the other end, and to compare
   the two values.  This operation is much simpler to implement, but
   requires that the devices performing the measurement are in sync: in
   order to compare two counters it is required that they refer exactly
   to the same set of packets.  Since a flow is continuous and cannot be
   stopped when a counter has to be read, it could be difficult to
   determine exactly when to read the counter.  A possible solution to
   overcome this problem is to virtually split the flow in consecutive
   blocks by inserting periodically a delimiter so that each counter
   refers exactly to the same block of packets.  The delimiter could be
   for example a special packet inserted artificially into the flow.
   However, delimiting the flow using specific packets has some
   limitations.  First, it requires generating additional packets within
   the flow and requires the equipment to be able to process those
   packets.  In addition, the method is vulnerable to out of order
   reception of delimiting packets and, to a lesser extent, to their

   The method proposed in this document follows the second approach, but
   it doesn't use additional packets to virtually split the flow in
   blocks.  Instead, it "marks" the packets so that the packets
   belonging to the same block will have the same color, whilst
   consecutive blocks will have different colors.  Each change of color
   represents a sort of auto-synchronization signal that guarantees the
   consistency of measurements taken by different devices along the path
   (see also [I-D.cociglio-mboned-multicast-pm] and
   [I-D.tempia-opsawg-p3m], where this technique was introduced).

   Figure 1 represents a very simple network and shows how the method
   can be used to measure packet loss on different network segments: by
   enabling the measurement on several interfaces along the path, it is
   possible to perform link monitoring, node monitoring or end-to-end
   monitoring.  The method is flexible enough to measure packet loss on
   any segment of the network and can be used to isolate the faulty

                            Traffic flow
          +------+       +------+       +------+       +------+
      ---<>  R1  <>-----<>  R2  <>-----<>  R3  <>-----<>  R4  <>---
          +------+       +------+       +------+       +------+
          .              .      .              .       .      .
          .              .      .              .       .      .
          .              <------>              <------->      .
          .          Node Packet Loss      Link Packet Loss   .
          .                                                   .
                           End-to-End Packet loss

                     Figure 1: Available measurements

3.  Detailed description of the method

   This section describes in detail how the method operate.  A special
   emphasis is given to the measurement of packet loss, that represents
   the core application of the method, but applicability to delay and
   jitter measurements is also considered.

3.1.  Packet loss measurement

   The basic idea is to virtually split traffic flows into consecutive
   blocks: each block represents a measurable entity unambiguously
   recognizable by all network devices along the path.  By counting the
   number of packets in each block and comparing the values measured by
   different network devices along the path, it is possible to measure
   packet loss occurred in any single block between any two points.

   As discussed in the previous section, a simple way to create the
   blocks is to "color" the traffic (two colors are sufficient) so that
   packets belonging to different consecutive blocks will have different
   colors.  Whenever the color changes, the previous block terminates
   and the new one begins.  Hence, all the packets belonging to the same
   block will have the same color and packets of different consecutive
   blocks will have different colors.  The number of packets in each
   block depends on the criterion used to create the blocks:

   o  if the color is switched after a fixed number of packets, then
      each block will contain the same number of packets (except for any

   o  if the color is switched according to a fixed timer, then the
      number of packets may be different in each block depending on the
      packet rate.

   The following figure shows how a flow looks like when it is split in
   traffic blocks with colored packets.

   A: packet with A coloring
   B: packet with B coloring

            |           |           |           |           |
            |           |    Traffic flow       |           |
       ...  |  Block 5  |  Block 4  |  Block 3  |  Block 2  |  Block 1
            |           |           |           |           |

                        Figure 2: Traffic coloring

   Figure 3 shows how the method can be used to measure link packet loss
   between two adjacent nodes.

   Referring to the figure, let's assume we want to monitor the packet
   loss on the link between two routers: router R1 and router R2.
   According to the method, the traffic is colored alternatively with
   two different colors, A and B.  Whenever the color changes, the
   transition generates a sort of square-wave signal, as depicted in the
   following figure.

   Color A   ----------+           +-----------+           +----------
                       |           |           |           |
   Color B             +-----------+           +-----------+
              Block n        ...      Block 3     Block 2     Block 1
            <---------> <---------> <---------> <---------> <--------->

                                Traffic flow

                 Figure 3: Computation of link packet loss

   Traffic coloring could be done by R1 itself or by an upward router.
   R1 needs two counters, C(A)R1 and C(B)R1, on its egress interface:
   C(A)R1 counts the packets with color A and C(B)R1 counts those with
   color B.  As long as traffic is colored A, only counter C(A)R1 will
   be incremented, while C(B)R1 is not incremented; vice versa, when the
   traffic is colored as B, only C(B)R1 is incremented.  C(A)R1 and
   C(B)R1 can be used as reference values to determine the packet loss
   from R1 to any other measurement point down the path.  Router R2,
   similarly, will need two counters on its ingress interface, C(A)R2
   and C(B)R2, to count the packets received on that interface and
   colored with color A and B respectively.  When an A block ends, it is
   possible to compare C(A)R1 and C(A)R2 and calculate the packet loss
   within the block; similarly, when the successive B block terminates,
   it is possible to compare C(B)R1 with C(B)R2, and so on for every
   successive block.

   Likewise, by using two counters on R2 egress interface it is possible
   to count the packets sent out of R2 interface and use them as
   reference values to calculate the packet loss from R2 to any
   measurement point down R2.

   Using a fixed timer for color switching offers a better control over
   the method: the (time) length of the blocks can be chosen large
   enough to simplify the collection and the comparison of measures
   taken by different network devices.  It's preferable to read the
   value of the counters not immediately after the color switch: some
   packets could arrive out of order and increment the counter
   associated to the previous block (color), so it is worth waiting for
   some time.  A safe choice is to wait L/2 time units (where L is the
   duration for each block) after the color switch, to read the still
   counter of the previous color, so the possibility to read a running
   counter instead of a still one is minimized.  The drawback is that
   the longer the duration of the block, the less frequent the
   measurement can be taken.

   The following table shows how the counters can be used to calculate
   the packet loss between R1 and R2.  The first column lists the
   sequence of traffic blocks while the other columns contain the
   counters of A-colored packets and B-colored packets for R1 and R2.
   In this example, we assume that the values of the counters are reset
   to zero whenever a block ends and its associated counter has been
   read: with this assumption, the table shows only relative values,
   that is the exact number of packets of each color within each block.
   If the values of the counters were not reset, the table would contain
   cumulative values, but the relative values could be determined simply
   by difference from the value of the previous block of the same color.

   The color is switched on the basis of a fixed timer (not shown in the
   table), so the number of packets in each block is different.

           | Block | C(A)R1 | C(B)R1 | C(A)R2 | C(B)R2 | Loss |
           | 1     | 375    | 0      | 375    | 0      | 0    |
           |       |        |        |        |        |      |
           | 2     | 0      | 388    | 0      | 388    | 0    |
           |       |        |        |        |        |      |
           | 3     | 382    | 0      | 381    | 0      | 1    |
           |       |        |        |        |        |      |
           | 4     | 0      | 377    | 0      | 374    | 3    |
           |       |        |        |        |        |      |
           | ...   | ...    | ...    | ...    | ...    | ...  |
           |       |        |        |        |        |      |
           | 2n    | 0      | 387    | 0      | 387    | 0    |
           |       |        |        |        |        |      |
           | 2n+1  | 379    | 0      | 377    | 0      | 2    |

       Table 1: Evaluation of counters for packet loss measurements

   During an A block (blocks 1, 3 and 2n+1), all the packets are
   A-colored, therefore the C(A) counters are incremented to the number
   seen on the interface, while C(B) counters are zero.  Vice versa,
   during a B block (blocks 2, 4 and 2n), all the packets are B-colored:
   C(A) counters are zero, while C(B) counters are incremented.

   When a block ends (because of color switching) the relative counters
   stop incrementing and it is possible to read them, compare the values
   measured on router R1 and R2 and calculate the packet loss within
   that block.

   For example, looking at the table above, during the first block
   (A-colored), C(A)R1 and C(A)R2 have the same value (375), which
   corresponds to the exact number of packets of the first block (no
   loss).  Also during the second block (B-colored) R1 and R2 counters
   have the same value (388), which corresponds to the number of packets
   of the second block (no loss).  During blocks three and four, R1 and
   R2 counters are different, meaning that some packets have been lost:
   in the example, one single packet (382-381) was lost during block
   three and three packets (377-374) were lost during block four.

   The method applied to R1 and R2 can be extended to any other router
   and applied to more complex networks, as far as the measurement is
   enabled on the path followed by the traffic flow(s) being observed.

   It's worth mentioning two different strategies that can be used when
   implementing the method:

   o  flow-based: the flow-based strategy is used when only a limited
      number of traffic flows need to be monitored.  According to this
      strategy, only a subset of the flows is colored.  Counters for
      packet loss measurements can be instantiated for each single flow,
      or for the set as a whole, depending on the desired granularity.
      A relevant problem with this approach is the necessity to know in
      advance the path followed by flows that are subject to
      measurement.  Path rerouting and traffic load-balancing increase
      the issue complexity, especially for unicast traffic.  The problem
      is easier to solve for multicast traffic where load balancing is
      seldom used and static joins are frequently used to force traffic
      forwarding and replication.

   o  link-based: measurements are performed on all the traffic on a
      link by link basis.  The link could be a physical link or a
      logical link.  Counters could be instantiated for the traffic as a
      whole or for each traffic class (in case it is desired to monitor
      each class separately), but in the second case a couple of
      counters is needed for each class.

   As mentioned, the flow-based measurement requires the identification
   of the flow to be monitored and the discovery of the path followed by
   the selected flow.  It is possible to monitor a single flow or
   multiple flows grouped together, but in this case measurement is
   consistent only if all the flows in the group follow the same path.
   Moreover if a measurement is performed by grouping many flows, it is
   not possible to determine exactly which flow was affected by packets
   loss.  In order to have measures per single flow it is necessary to
   configure counters for each specific flow.  Once the flow(s) to be
   monitored have been identified, it is necessary to configure the
   monitoring on the proper nodes.  Configuring the monitoring means
   configuring the rule to intercept the traffic and configuring the
   counters to count the packets.  To have just an end-to-end
   monitoring, it is sufficient to enable the monitoring on the first
   and the last hop routers of the path: the mechanism is completely
   transparent to intermediate nodes and independent from the path
   followed by traffic flows.  On the contrary, to monitor the flow on a
   hop-by-hop basis along its whole path it is necessary to enable the
   monitoring on every node from the source to the destination.  In case
   the exact path followed by the flow is not known a priori (i.e. the
   flow has multiple paths to reach the destination) it is necessary to
   enable the monitoring system on every path: counters on interfaces
   traversed by the flow will report packet count, counters on other
   interfaces will be null.

3.1.1.  Coloring the packets

   The coloring operation is fundamental in order to create packet
   blocks.  This implies choosing where to activate the coloring and how
   to color the packets.

   In case of flow-based measurements, it is desirable, in general, to
   have a single coloring node because it is easier to manage and
   doesn't rise any risk of conflict (consider the case where two nodes
   color the same flow).  Thus it is advantageous to color the flow as
   close as possible to the source.  In addition, coloring a flow close
   to the source allows an end-to-end measure if a measurement point is
   enabled on the last-hop router as well.  The only requirement is that
   the coloring must change periodically and every node along the path
   must be able to identify unambiguously the colored packets.  For
   link-based measurements, all traffic needs to be colored when
   transmitted on the link.  If the traffic had already been colored,
   then it has to be re-colored because the color must be consistent on
   the link.  This means that each hop along the path must (re-)color
   the traffic; the color is not required to be consistent along
   different links.

   Traffic coloring can be implemented by setting a specific bit in the
   packet header and changing the value of that bit periodically.  How
   to choose the marking field depends on the application and is out of
   scope here.

3.1.2.  Counting the packets

   Assuming that the coloring of the packets is performed only by the
   source node, the nodes between source and destination (included) have
   to count the colored packets that they receive and forward: this
   operation can be enabled on every router along the path or only on a
   subset, depending on which network segment is being monitored (a
   single link, a particular metro area, the backbone, the whole path).

   Since the color switches periodically between two values, two
   counters (one for each value) are needed: one counter for packets
   with color A and one counter for packets with color B.  For each flow
   (or group of flows) being monitored and for every interface where the
   monitoring is active, a couple of counters is needed.  For example,
   in order to monitor separately 3 flows on a router with 4 interfaces
   involved, 24 counters are needed (2 counters for each of the 3 flows
   on each of the 4 interfaces).

   In case of link-based measurements the behaviour is similar except
   that coloring and counting operations are performed on a link by link
   basis at each endpoint of the link.

   Another important aspect to take into consideration is when to read
   the counters: in order to count the exact number of packets of a
   block the routers must perform this operation when that block has
   ended: in other words, the counter for color A must be read when the
   current block has color B, in order to be sure that the value of the
   counter is stable.  This task can be accomplished in two ways.  The
   general approach suggests to read the counters periodically, many
   times during a block duration, and to compare these successive
   readings: when the counter stops incrementing means that the current
   block has ended and its value can be elaborated safely.
   Alternatively, if the coloring operation is performed on the basis of
   a fixed timer, it is possible to configure the reading of the
   counters according to that timer: for example, reading the counter
   for color A every period in the middle of the subsequent block with
   color B is a safe choice.  A sufficient margin should be considered
   between the end of a block and the reading of the counter, in order
   to take into account any out-of-order packets.

3.1.3.  Collecting data and calculating packet loss

   The nodes enabled to perform performance monitoring collect the value
   of the counters, but they are not able to directly use this
   information to measure packet loss, because they only have their own
   samples.  For this reason, an external Network Management System
   (NMS) can be used to collect and elaborate data and to perform packet
   loss calculation.  The NMS compares the values of counters from
   different nodes and can calculate if some packets were lost (even a
   single packet) and also where packets were lost.

   The value of the counters needs to be transmitted to the NMS as soon
   as it has been read.  This can be accomplished by using SNMP or FTP
   and can be done in Push Mode or Polling Mode.  In the first case,
   each router periodically sends the information to the NMS, in the
   latter case it is the NMS that periodically polls routers to collect
   information.  In any case, the NMS has to collect all the relevant
   values from all the routers within one cycle of the timer.

   If link-based measurement is used, it would be possible to use a
   protocol to exchange values of counters between the two endpoints in
   order to let them perform the packet loss calculation for each
   traffic direction.  A similar approach could be also applied to a
   flow-based measurement.

3.2.  Timing aspects

   This document introduces two color switching method: one is based on
   fixed number of packet, the other is based on fixed timer.  But the
   method based on fixed timer is preferable because is more
   deterministic, and will be considered in the rest of the dcoument.

   By considering the clock error between network devices R1 and R2,
   they must be synchronized to the same clock reference with an
   accuracy of +/- L/2 time units, where L is the time duration of the
   block.  So each colored packet can be assigned to the right batch by
   each router.  This is because the minimum time distance between two
   packets of the same color but belonging to different batches is L
   time units.

   In practice, there are also out of order at batch boundaries,
   strictly related to the delay between measurement points.  This means
   that, without considering clock error, we wait L/2 after color
   switching to be sure to take a still counter.

   In summary we need to take into account two contributions: clock
   error between network devices and the interval we need to wait to
   avoid out of order because of network delay.

   The following figure explains both issues.

                |                   L                    |
                |       L/2                   L/2        |
                |<===>|                            |<===>|
                   d  |                            |   d
                       available counting interval

                         Figure 4: Timing aspects

   It is assumed that all network devices are synchronized to a common
   reference time with an accuracy of +/- A/2.  Thus, the difference
   between the clock values of any two network devices is bounded by A.

   The guardband d is given by:

   d = A + D_max - D_min,

   where A is the clock accuracy, D_max is an upper bound on the network
   delay between the network devices, and D_min is a lower bound on the

   The available counting interval is L - 2d that must be > 0.

   The condition that must be satisfied and is a requirement on the
   synchronization accuracy is:

   d < L/2.

3.3.  One-way delay measurement

   The same principle used to measure packet loss can be applied also to
   one-way delay measurement.  There are three alternatives, as
   described hereinafter.

3.3.1.  Single marking methodology

   The alternation of colors can be used as a time reference to
   calculate the delay.  Whenever the color changes (that means that a
   new block has started) a network device can store the timestamp of
   the first packet of the new block; that timestamp can be compared
   with the timestamp of the same packet on a second router to compute
   packet delay.  Considering Figure 2, R1 stores a timestamp TS(A1)R1
   when it sends the first packet of block 1 (A-colored), a timestamp
   TS(B2)R1 when it sends the first packet of block 2 (B-colored) and so
   on for every other block.  R2 performs the same operation on the
   receiving side, recording TS(A1)R2, TS(B2)R2 and so on.  Since the
   timestamps refer to specific packets (the first packet of each block)
   we are sure that timestamps compared to compute delay refer to the
   same packets.  By comparing TS(A1)R1 with TS(A1)R2 (and similarly
   TS(B2)R1 with TS(B2)R2 and so on) it is possible to measure the delay
   between R1 and R2.  In order to have more measurements, it is
   possible to take and store more timestamps, referring to other
   packets within each block.

   In order to coherently compare timestamps collected on different
   routers, the clocks on the network nodes must be in sync.
   Furthermore, a measurement is valid only if no packet loss occurs and
   if packet misordering can be avoided, otherwise the first packet of a
   block on R1 could be different from the first packet of the same
   block on R2 (f.i. if that packet is lost between R1 and R2 or it
   arrives after the next one).

   The following table shows how timestamps can be used to calculate the
   delay between R1 and R2.  The first column lists the sequence of
   blocks while other columns contain the timestamp referring to the
   first packet of each block on R1 and R2.  The delay is computed as a
   difference between timestamps.  For the sake of simplicity, all the
   values are expressed in milliseconds.

      | Block | TS(A)R1 | TS(B)R1 | TS(A)R2 | TS(B)R2 | Delay R1-R2 |
      | 1     | 12.483  | -       | 15.591  | -       | 3.108       |
      |       |         |         |         |         |             |
      | 2     | -       | 6.263   | -       | 9.288   | 3.025       |
      |       |         |         |         |         |             |
      | 3     | 27.556  | -       | 30.512  | -       | 2.956       |
      |       |         |         |         |         |             |
      |       | -       | 18.113  | -       | 21.269  | 3.156       |
      |       |         |         |         |         |             |
      | ...   | ...     | ...     | ...     | ...     | ...         |
      |       |         |         |         |         |             |
      | 2n    | 77.463  | -       | 80.501  | -       | 3.038       |
      |       |         |         |         |         |             |
      | 2n+1  | -       | 24.333  | -       | 27.433  | 3.100       |

         Table 2: Evaluation of timestamps for delay measurements

   The first row shows timestamps taken on R1 and R2 respectively and
   referring to the first packet of block 1 (which is A-colored).  Delay
   can be computed as a difference between the timestamp on R2 and the
   timestamp on R1.  Similarly, the second row shows timestamps (in
   milliseconds) taken on R1 and R2 and referring to the first packet of
   block 2 (which is B-colored).  Comparing timestamps taken on
   different nodes in the network and referring to the same packets
   (identified using the alternation of colors) it is possible to
   measure delay on different network segments.

   For the sake of simplicity, in the above example a single measurement
   is provided within a block, taking into account only the first packet
   of each block.  The number of measurements can be easily increased by
   considering multiple packets in the block: for instance, a timestamp
   could be taken every N packets, thus generating multiple delay
   measurements.  Taking this to the limit, in principle the delay could
   be measured for each packet, by taking and comparing the
   corresponding timestamps (possible but impractical from an
   implementation point of view).  Mean delay

   As mentioned before, the method previously exposed for measuring the
   delay is sensitive to out of order reception of packets.  In order to
   overcome this problem, a different approach has been considered: it
   is based on the concept of mean delay.  The mean delay is calculated
   by considering the average arrival time of the packets within a
   single block.  The network device locally stores a timestamp for each
   packet received within a single block: summing all the timestamps and
   dividing by the total number of packets received, the average arrival
   time for that block of packets can be calculated.  By subtracting the
   average arrival times of two adjacent devices it is possible to
   calculate the mean delay between those nodes.  When computing the
   mean delay, measurement error could be augmented by accumulating
   measurement error of a lot of packets.  This method is robust to out
   of order packets and also to packet loss (only a small error is
   introduced).  Moreover, it greatly reduces the number of timestamps
   (only one per block for each network device) that have to be
   collected by the management system.  On the other hand, it only gives
   one measure for the duration of the block (f.i. 5 minutes), and it
   doesn't give the minimum, maximum and median delay values (RFC 6703
   [RFC6703]).  This limitation could be overcome by reducing the
   duration of the block (f.i. from 5 minutes to a few seconds), that
   implicates an highly optimized implementation of the method.

   By summing the mean delays of the two directions of a path, it is
   also possible to measure the two-way mean delay (round-trip delay).

3.3.2.  Double marking methodology

   The Single marking methodology for one-way delay measurement is
   sensitive to out of order reception of packets.  The first approach
   to overcome this problem is described before and is based on the
   concept of mean delay.  But the limitation of mean delay is that it
   doesn't give information about the delay values distribution for the
   duration of the block.  Additionally it may be useful to have not
   only the mean delay but also the minimum, maximum and median delay
   values and, in wider terms, to know more about the statistic
   distribution of delay values.  So in order to have more information
   about the delay and to overcome out of order issues, a different
   approach can be introduced: it is based on double marking

   Basically, the idea is to use the first marking to create the
   alternate flow and, within this colored flow, a second marking to
   select the packets for measuring delay/jitter.  The first marking is
   needed for packet loss and mean delay measurement.  The second
   marking creates a new set of marked packets that are fully identified
   over the network, so that a network device can store the timestamps
   of these packets; these timestamps can be compared with the
   timestamps of the same packets on a second router to compute packet
   delay values for each packet.  The number of measurements can be
   easily increased by changing the frequency of the second marking.
   But the frequency of the second marking must be not too high in order
   to avoid out of order issues.  Between packets with the second
   marking there should be a security time gap (e.g. this gap could be,
   at the minimum, the mean network delay calculated with the previous
   methodology) to avoid out of order issues and also to have a number
   of measurement packets that is rate independent.  If a second marking
   packet is lost, the delay measurement for the considered block is
   corrupted and should be discarded.

   Mean delay is calculated on all the packets of a sample and is a
   simple computation to be performed for single marking method.  In
   some cases the mean delay measure is not sufficient to characterize
   the sample, and more statistics of delay extent data are needed, e.g.
   percentiles, variance and median delay values.  The conventional
   range (maximum-minimum) should be avoided for several reasons,
   including stability of the maximum delay due to the influence by
   outliers.  RFC 5481 [RFC5481] section 6.5 highlights how the 99.9th
   percentile of delay and delay variation is more helpful to
   performance planners.  To overcome this drawback the idea is to
   couple the mean delay measure for the entire batch with double
   marking method, where a subset of batch packets are selected for
   extensive delay calculation by using a second marking.  In this way
   it is possible to perform a detailed analysis on these double marked
   packets.  Please note that there are classic algorithms for median
   and variance calculation, but are out of the scope of this document.
   The comparison between the mean delay for the entire batch and the
   mean delay on these double marked packets gives an useful information
   since it is possible to understand if the double marking measurements
   are actually representative of the delay trends.

3.4.  Delay variation measurement

   Similarly to one-way delay measurement (both for single marking and
   double marking), the method can also be used to measure the inter-
   arrival jitter.  We refer to the definition in RFC 3393 [RFC3393].
   The alternation of colors, for single marking method, can be used as
   a time reference to measure delay variations.  In case of double
   marking, the time reference is given by the second marked packets.
   Considering the example depicted in Figure 2, R1 stores a timestamp
   TS(A)R1 whenever it sends the first packet of a block and R2 stores a
   timestamp TS(B)R2 whenever it receives the first packet of a block.
   The inter-arrival jitter can be easily derived from one-way delay
   measurement, by evaluating the delay variation of consecutive

   The concept of mean delay can also be applied to delay variation, by
   evaluating the average variation of the interval between consecutive
   packets of the flow from R1 to R2.

4.  Considerations

   This section highlights some considerations about the methodology.

4.1.  Synchronization

   The Alternate Marking technique does not require a strong
   synchronization, especially for packet loss and two-way delay
   measurement.  Only one-way delay measurement requires network devices
   to have synchronized clocks.

   The color switching is the reference for all the network devices, and
   the only requirement to be achieved is that all network devices have
   to recognize the right batch along the path.

   If the length of the measurement period is L time units, then all
   network devices must be synchronized to the same clock reference with
   an accuracy of +/- L/2 time units (without considering network
   delay).  This level of accuracy guarantees that all network devices
   consistently match the color bit to the correct block.  For example,
   if the color is toggeled every second (L = 1 second), then clocks
   must be synchronized with an accuracy of +/- 0.5 second to a common
   time reference.

   This synchronization requirement can be satisfied even with a
   relatively inaccurate synchronization method.  This is true for
   packet loss and two-way delay measurement, instead, for one-way delay
   measurement clock synchronization must be accurate.

   Therefore, a system that uses only packet loss and two-way delay
   measurement does not require synchronization.  This is because the
   value of the clocks of network devices does not affect the
   computation of the two-way delay measurement.

4.2.  Data Correlation

   Data Correlation is the mechanism to compare counters and timestamps
   for packet loss, delay and delay variation calculation.  It could be
   performed in several ways depending on the alternate marking
   application and use case.

   o  A possibility is to use a centralized solution using Network
      Management System (NMS) to correlate data;

   o  Another possibility is to define a protocol based distributed
      solution, by defining a new protocol or by extending the existing
      protocols (e.g.  RFC6374, TWAMP, OWAMP) in order to communicate
      the counters and timestamps between nodes.

   In the following paragraphs an example data correlation mechanism is
   explained and could be use independently of the adopted solutions.

   When data is collected on the upstream and downstream node, e.g.,
   packet counts for packet loss measurement or timestamps for packet
   delay measurement, and periodically reported to or pulled by other
   nodes or NMS, a certain data correlation mechanism SHOULD be in use
   to help the nodes or NMS to tell whether any two or more packet
   counts are related to the same block of markers, or any two
   timestamps are related to the same marked packet.

   The alternate marking method described in this document literally
   split the packets of the measured flow into different measurement
   blocks, in addition a Block Number could be assigned to each of such
   measurement block.  The BN is generated each time a node reads the
   data (packet counts or timestamps), and is associated with each
   packet count and timestamp reported to or pulled by other nodes or
   NMS.  The value of BN could be calculated as the modulo of the local
   time (when the data are read) and the interval of the marking time

   When the nodes or NMS see, for example, same BNs associated with two
   packet counts from an upstream and a downstream node respectively, it
   considers that these two packet counts corresponding to the same
   block, i.e. that these two packet counts belong to the same block of
   markers from the upstream and downstream node.  The assumption of
   this BN mechanism is that the measurement nodes are time
   synchronized.  This requires the measurement nodes to have a certain
   time synchronization capability (e.g., the Network Time Protocol
   (NTP) RFC 5905 [RFC5905], or the IEEE 1588 Precision Time Protocol
   (PTP) [IEEE-1588]).  Synchronization aspects are further discussed in
   Section 4.

4.3.  Packet Re-ordering

   Due to ECMP, packet re-ordering is very common in IP network.  The
   accuracy of marking based PM, especially packet loss measurement, may
   be affected by packet re-ordering.  Take a look at the following

   Block   :    1    |    2    |    3    |    4    |    5    |...

                        Figure 5: Packet Reordering

   In Figure 5 the packet stream for Node R1 isn't being reordered, and
   can be safely assigned to interval blocks, but the packet stream for
   Node R2 is being reordered, so, looking at the packet with the marker
   of "B" in block 3, there is no safe way to tell whether the packet
   belongs to block 2 or block 4.

   In general there is the need to assign packets with the marker of "B"
   or "A" to the right interval blocks.  Most of the packet re-ordering
   occur at the edge of adjacent blocks, and they are easy to handle if
   the interval of each block is sufficient large.  Then, it can assume
   that the packets with different marker belong to the block that they
   are more close to.  If the interval is small, it is difficult and
   sometime impossible to determine to which block a packet belongs.

   To choose a proper interval is important and how to choose a proper
   interval is out of the scope of this document.  But an implementation
   SHOULD provide a way to configure the interval and allow a certain
   degree of packet re-ordering.

5.  Implementation and deployment

   The methodology described in the previous sections can be applied in
   various situations.  Basically Alternate Marking technique could be
   used in many cases for performance measurement.  The only requirement
   is to select and mark the flow to be monitored; in this way packets
   are batched by the sender and each batch is alternately marked such
   that can be easily recognized by the receiver.

   An example of implementation and deployment is explained in the next
   section, just to clarify how the method can work.

5.1.  Report on the operational experiment at Telecom Italia

   The method described in this document, also called PNPM (Packet
   Network Performance Monitoring), has been invented and engineered in
   Telecom Italia and it's currently being used in Telecom Italia's
   network.  The methodology has been applied by leveraging functions
   and tools available on IP routers and it's currently being used to
   monitor packet loss in some portions of Telecom Italia's network.
   The application of the method to delay measurement is currently being
   evaluated in Telecom Italia's labs.  This section describes how the
   features currently available on existing routing platforms can be
   used to apply the method, in order to give an example of
   implementation and deployment.

   The current implementation in Telecom Italia uses the flow-based
   strategy, as defined in section 3.  The link-based strategy could be
   applied to physical link or a logical link (e.g.  Ethernet VLAN or a
   MPLS PW).

   The method is applied in Telecom Italia's network to multicast IPTV
   channels or other specific traffic flows with high QoS requirements
   (i.e.  Mobile Backhauling traffic implemented with a VPN MPLS).

   The implementation of the method by a Service Provider needs to use
   the router features.  With current router implementations, only QoS
   related fields and features offer the required flexibility to set
   bits in the packet header.  In case a Service Provider only uses the
   three most significant bits of the DSCP field (corresponding to IP
   Precedence) for QoS classification and queuing, it is possible to use
   the two less significant bits of the DSCP field (bit 0 and bit 1) to
   implement the method without affecting QoS policies.  One of the two
   bits (bit 0) could be used to identify flows subject to traffic
   monitoring (set to 1 if the flow is under monitoring, otherwise it is
   set to 0), while the second (bit 1) can be used for coloring the
   traffic (switching between values 0 and 1, corresponding to color A
   and B) and creating the blocks.

   In practice, coloring the traffic using the DSCP field can be
   implemented by configuring on the router output interface an access
   list that intercepts the flow(s) to be monitored and applies to them
   a policy that sets the DSCP field accordingly.  Since traffic
   coloring has to be switched between the two values over time, the
   policy needs to be modified periodically: an automatic script is used
   to perform this task on the basis of a fixed timer.

   In Telecom Italia's implementation the timer is set to 5 minutes:
   this value showed to be a good compromise between measurement
   frequency and stability of the measurement (i.e. possibility to
   collect all the measures referring to the same block).

   If traffic is colored using the DSCP field an access-list that
   matches specific DSCP values can be used to count the packets of the
   flow(s) being monitored.  The access-list is installed on all the
   routers of the path.  Also, a 5 minutes timer for color switching is
   a safe choice for reading the counters.

   The counters are collected by using an automatic script that sends
   out these to a Network Management System (NMS).  The NMS is
   responsible for packet loss calculation, performed by comparing the
   values of counters from the routers along the flow(s) path.

5.1.1.  Metric transparency

   Since a Service Provider application is described here, the method
   can be applied to end-to-end services supplied to Customers.  So it
   is important to highlight that the method SHOULD be transparent
   outside the Service Provider domain.

   In Telecom Italia's implementation the source node colors the packets
   with a policy that is modified periodically via an automatic script
   in order to alternate the DSCP field of the packets.  The nodes
   between source and destination (included) have to count with an
   access-list the colored packets that they receive and forward.

   Moreover the destination node has an important role: the colored
   packets are intercepted and a policy restores and sets the DSCP field
   of all the packets to the initial value.  In this way the metric is
   transparent because outside the section of the network under
   monitoring the traffic flow is unchanged.

   In such a case, thanks to this restoring technique, network elements
   outside the Alternate Marking monitoring domain (e.g. the two
   Provider Edge nodes of the Mobile Backhauling VPN MPLS) are totally
   anaware that packets were marked.  So this restoring technique makes
   Alternate Marking completely transparent outside its monitoring

5.2.  IP flow performance measurement (IPFPM)

   This application of marking method is described in

5.3.  OAM Passive Performance Measurement

   In [I-D.ietf-bier-mpls-encapsulation] two OAM bits from Bit Index
   Explicit Replication (BIER) Header are reserved for the passive
   performance measurement marking method.  [I-D.ietf-bier-pmmm-oam]
   details the measurement for multicast service over BIER domain.

   In addition, the alternate marking method could also be used in a
   Service Function Chaining (SFC) domain.

   The application of the marking method to Network Virtualization
   Overlays (NVO3) protocols is a work in progress (see

5.4.  RFC6374 Use Case

   RFC6374 [RFC6374] uses the LM packet as the packet accounting
   demarcation point.  Unfortunately this gives rise to a number of
   problems that may lead to significant packet accounting errors in
   certain situations.  [I-D.ietf-mpls-flow-ident] discusses the desired
   capabilities for MPLS flow identification in order to perform a
   better in-band performance monitoring of user data packets.  A method
   of accomplishing identification is Synonymous Flow Labels (SFL)
   introduced in [I-D.bryant-mpls-sfl-framework], while
   [I-D.ietf-mpls-rfc6374-sfl] describes RFC6374 performance
   measurements with SFL.

5.5.  Application to active performance measurement

   [I-D.fioccola-ippm-alt-mark-active] describes how to extend the
   existing Active Measurement Protocol, in order to implement alternate
   marking methodology.  [I-D.fioccola-ippm-rfc6812-alt-mark-ext]
   describes an extension to the Cisco SLA Protocol Measurement-Type

6.  Hybrid measurement

   The method has been explicitly designed for passive measurements but
   it can also be used with active measurements.  In order to have both
   end to end measurements and intermediate measurements (hybrid
   measurements) two end points can exchanges artificial traffic flows
   and apply alternate marking over these flows.  In the intermediate
   points artificial traffic is managed in the same way as real traffic
   and measured as specified before.  So the application of marking
   method can simplify also the active measurement, as explained in

7.  Summary

   The advantages of the method described in this document are:

   o  easy implementation: it can be implemented using features already
      available on major routing platforms;

   o  low computational effort: the additional load on processing is

   o  accurate packet loss measurement: single packet loss granularity
      is achieved with a passive measurement;

   o  potential applicability to any kind of packet/frame -based
      traffic: Ethernet, IP, MPLS, etc., both unicast and multicast;

   o  robustness: the method can tolerate out of order packets and it's
      not based on "special" packets whose loss could have a negative

   o  no interoperability issues: the features required to implement the
      method are available on all current routing platforms.

   The method doesn't raise any specific need for protocol extension,
   but it could be further improved by means of some extension to
   existing protocols.  Specifically, the use of DiffServ bits for
   coloring the packets could not be a viable solution in some cases: a
   standard method to color the packets for this specific application
   could be beneficial.

8.  Compliance with RFC6390 guidelines

   RFC6390 [RFC6390] defines a framework and a process for developing
   Performance Metrics for protocols above and below the IP layer (such
   as IP-based applications that operate over reliable or datagram
   transport protocols).

   This document doesn't aim to propose a new Performance Metric but a
   new method of measurement for a few Performance Metrics that have
   already been standardized.  Nevertheless, it's worth applying
   [RFC6390] guidelines to the present document, in order to provide a
   more complete and coherent description of the proposed method.  We
   used a subset of the Performance Metric Definition template defined
   by [RFC6390].

   o  Metric name and description: as already stated, this document
      doesn't propose any new Performance Metric.  On the contrary, it
      describes a novel method for measuring packet loss [RFC7680].  The
      same concept, with small differences, can also be used to measure
      delay [RFC7679], and jitter [RFC3393].  The document mainly
      describes the applicability to packet loss measurement.

   o  Method of Measurement or Calculation: according to the method
      described in the previous sections, the number of packets lost is
      calculated by subtracting the value of the counter on the source
      node from the value of the counter on the destination node.  Both
      counters must refer to the same color.  The calculation is
      performed when the value of the counters is in a steady state.

   o  Units of Measurement: the method calculates and reports the exact
      number of packets sent by the source node and not received by the
      destination node.

   o  Measurement Points: the measurement can be performed between
      adjacent nodes, on a per-link basis, or along a multi-hop path,
      provided that the traffic under measurement follows that path.  In
      case of a multi-hop path, the measurements can be performed both
      end-to-end and hop-by-hop.

   o  Measurement Timing: the method have a constraint on the frequency
      of measurements.  In order to perform a measure, the counter must
      be in a steady state: this happens when the traffic is being
      colored with the alternate color; for example in the Telecom
      Italia application of the method the time interval is set to 5

   o  Implementation: the Telecom Italia application of the method uses
      two encodings of the DSCP field to color the packets; this enables
      the use of policy configurations on the router to color the
      packets and accordingly configure the counter for each color.  The
      path followed by traffic being measured should be known in advance
      in order to configure the counters along the path and be able to
      compare the correct values.

   o  Use and Applications: the method can be used to measure packet
      loss with high precision on live traffic; moreover, by combining
      end-to-end and per-link measurements, the method is useful to
      pinpoint the single link that is experiencing loss events.

   o  Reporting Model: the value of the counters has to be sent to a
      centralized management system that perform the calculations; such
      samples must contain a reference to the time interval they refer
      to, so that the management system can perform the correct
      correlation; the samples have to be sent while the corresponding
      counter is in a steady state (within a time interval), otherwise
      the value of the sample should be stored locally.

   o  Dependencies: the values of the counters have to be correlated to
      the time interval they refer to; moreover, as far the Telecom
      Italia application of the method is based on DSCP values, there
      are significant dependencies on the usage of the DSCP field: it
      must be possible to rely on unused DSCP values without affecting
      QoS-related configuration and behavior; moreover, the intermediate
      nodes must not change the value of the DSCP field not to alter the

   o  Organization of Results: the method of measurement produces

   o  Parameters: currently, the main parameter of the method is the
      time interval used to alternate the colors and read the counters.

9.  Security Considerations

   This document specifies a method to perform measurements in the
   context of a Service Provider's network and has not been developed to
   conduct Internet measurements, so it does not directly affect
   Internet security nor applications which run on the Internet.
   However, implementation of this method must be mindful of security
   and privacy concerns.

   There are two types of security concerns: potential harm caused by
   the measurements and potential harm to the measurements.

   o  Harm caused by the measurement: the measurements described in this
      document are passive, so there are no new packets injected into
      the network causing potential harm to the network itself and to
      data traffic.  Nevertheless, the method implies modifications on
      the fly to the IP header of data packets: this must be performed
      in a way that doesn't alter the quality of service experienced by
      packets subject to measurements and that preserve stability and
      performance of routers doing the measurements.  One of the main
      security threats in OAM protocols is network reconnaissance; an
      attacker can gather information about the network performance by
      passively eavesdropping to OAM messages.  The advantage of the
      methods described in this document is that the marking bits are
      the only information that is exchanged between the network
      devices.  Therefore, passive eavesdropping to data plane traffic
      does not allow attackers to gain information about the network

   o  Harm to the measurement: the measurements could be harmed by
      routers altering the marking of the packets, or by an attacker
      injecting artificial traffic.  Authentication techniques, such as
      digital signatures, may be used where appropriate to guard against
      injected traffic attacks.  Since the measurement itself may be
      affected by routers (or other network devices) along the path of
      IP packets intentionally altering the value of marking bits of
      packets, as mentioned above, the mechanism specified in this
      document can be applied just in the context of a controlled
      domain, and thus the routers (or other network devices) are
      locally administered and this type of attack can be avoided.  In
      addition, an attacker can't gain information about network
      performance from a single monitoring point, and must use
      synchronized monitoring points at multiple points on the path,
      because they have to do the same kind of measurement and
      aggregation that Service Providers using Alternate Marking must

   The privacy concerns of network measurement are limited because the
   method only relies on information contained in the IP header without
   any release of user data.

   Delay attacks are another potential threat in the context of this
   document.  Delay measurement is performed using a specific packet in
   each block, marked by a dedicated color bit.  Therefore, a man-in-
   the-middle attacker can selectively induce synthetic delay only to
   delay-colored packets, causing systematic error in the delay
   measurements.  As discussed in previous sections, the methods
   described in this document rely on an underlying time synchronization
   protocol.  Thus, by attacking the time protocol an attacker can
   potentially compromise the integrity of the measurement.  A detailed
   discussion about the threats against time protocols and how to
   mitigate them is presented in RFC 7384 [RFC7384].

10.  IANA Considerations

   There are no IANA actions required.

11.  Acknowledgements

   The previous IETF drafts about this technique were:
   [I-D.cociglio-mboned-multicast-pm] and [I-D.tempia-opsawg-p3m].

   The authors would like to thank Alberto Tempia Bonda, Domenico
   Laforgia, Daniele Accetta and Mario Bianchetti for their contribution
   to the definition and the implementation of the method.

12.  References

12.1.  Normative References

              IEEE 1588-2008, "IEEE Standard for a Precision Clock
              Synchronization Protocol for Networked Measurement and
              Control Systems", July 2008.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3393]  Demichelis, C. and P. Chimento, "IP Packet Delay Variation
              Metric for IP Performance Metrics (IPPM)", RFC 3393,
              DOI 10.17487/RFC3393, November 2002,

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,

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

   [RFC7680]  Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
              Ed., "A One-Way Loss Metric for IP Performance Metrics
              (IPPM)", STD 82, RFC 7680, DOI 10.17487/RFC7680, January
              2016, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

12.2.  Informative References

              Bryant, S., Chen, M., Li, Z., Swallow, G., Sivabalan, S.,
              and G. Mirsky, "Synonymous Flow Label Framework", draft-
              bryant-mpls-sfl-framework-05 (work in progress), June

              Chen, M., Zheng, L., Mirsky, G., Fioccola, G., and T.
              Mizrahi, "IP Flow Performance Measurement Framework",
              draft-chen-ippm-coloring-based-ipfpm-framework-06 (work in
              progress), March 2016.

              Cociglio, M., Capello, A., Bonda, A., and L. Castaldelli,
              "A method for IP multicast performance monitoring", draft-
              cociglio-mboned-multicast-pm-01 (work in progress),
              October 2010.

              Fioccola, G., Clemm, A., Bryant, S., Cociglio, M.,
              Chandramouli, M., and A. Capello, "Alternate Marking
              Extension to Active Measurement Protocol", draft-fioccola-
              ippm-alt-mark-active-01 (work in progress), March 2017.

              Fioccola, G., Clemm, A., Cociglio, M., Chandramouli, M.,
              and A. Capello, "Alternate Marking Extension to Cisco SLA
              Protocol RFC6812", draft-fioccola-ippm-rfc6812-alt-mark-
              ext-01 (work in progress), March 2016.

              Wijnands, I., Rosen, E., Dolganow, A., Tantsura, J.,
              Aldrin, S., and I. Meilik, "Encapsulation for Bit Index
              Explicit Replication in MPLS and non-MPLS Networks",
              draft-ietf-bier-mpls-encapsulation-07 (work in progress),
              June 2017.

              Mirsky, G., Zheng, L., Chen, M., and G. Fioccola,
              "Performance Measurement (PM) with Marking Method in Bit
              Index Explicit Replication (BIER) Layer", draft-ietf-bier-
              pmmm-oam-02 (work in progress), July 2017.

              Bryant, S., Pignataro, C., Chen, M., Li, Z., and G.
              Mirsky, "MPLS Flow Identification Considerations", draft-
              ietf-mpls-flow-ident-05 (work in progress), July 2017.

              Bryant, S., Chen, M., Li, Z., Swallow, G., Sivabalan, S.,
              Mirsky, G., and G. Fioccola, "RFC6374 Synonymous Flow
              Labels", draft-ietf-mpls-rfc6374-sfl-00 (work in
              progress), June 2017.

              Boutros, S., Ganga, I., Garg, P., Manur, R., Mizrahi, T.,
              Mozes, D., and E. Nordmark, "NVO3 Encapsulation
              Considerations", draft-ietf-nvo3-encap-00 (work in
              progress), June 2017.

              Capello, A., Cociglio, M., Castaldelli, L., and A. Bonda,
              "A packet based method for passive performance
              monitoring", draft-tempia-opsawg-p3m-04 (work in
              progress), February 2014.

   [RFC5481]  Morton, A. and B. Claise, "Packet Delay Variation
              Applicability Statement", RFC 5481, DOI 10.17487/RFC5481,
              March 2009, <>.

   [RFC6374]  Frost, D. and S. Bryant, "Packet Loss and Delay
              Measurement for MPLS Networks", RFC 6374,
              DOI 10.17487/RFC6374, September 2011,

   [RFC6390]  Clark, A. and B. Claise, "Guidelines for Considering New
              Performance Metric Development", BCP 170, RFC 6390,
              DOI 10.17487/RFC6390, October 2011,

   [RFC6703]  Morton, A., Ramachandran, G., and G. Maguluri, "Reporting
              IP Network Performance Metrics: Different Points of View",
              RFC 6703, DOI 10.17487/RFC6703, August 2012,

   [RFC7276]  Mizrahi, T., Sprecher, N., Bellagamba, E., and Y.
              Weingarten, "An Overview of Operations, Administration,
              and Maintenance (OAM) Tools", RFC 7276,
              DOI 10.17487/RFC7276, June 2014,

   [RFC7384]  Mizrahi, T., "Security Requirements of Time Protocols in
              Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
              October 2014, <>.

   [RFC7799]  Morton, A., "Active and Passive Metrics and Methods (with
              Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
              May 2016, <>.

Authors' Addresses

   Giuseppe Fioccola (editor)
   Telecom Italia
   Via Reiss Romoli, 274
   Torino  10148


   Alessandro Capello (editor)
   Telecom Italia
   Via Reiss Romoli, 274
   Torino  10148

   Mauro Cociglio
   Telecom Italia
   Via Reiss Romoli, 274
   Torino  10148


   Luca Castaldelli
   Telecom Italia
   Via Reiss Romoli, 274
   Torino  10148


   Mach(Guoyi) Chen (editor)
   Huawei Technologies


   Lianshu Zheng (editor)
   Huawei Technologies


   Greg Mirsky  (editor)


   Tal Mizrahi (editor)
   6 Hamada st.