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Versions: 00 01 02 03 04                                 Standards Track
          draft-ietf-ippm-rfc8321bis                                    
Network Working Group                                   G. Fioccola, Ed.
Internet-Draft                                       Huawei Technologies
Obsoletes: 8321 (if approved)                                M. Cociglio
Intended status: Standards Track                          Telecom Italia
Expires: October 7, 2022                                       G. Mirsky
                                                                Ericsson
                                                              T. Mizrahi
                                                                 T. Zhou
                                                     Huawei Technologies
                                                           April 5, 2022


                        Alternate-Marking Method
                      draft-fioccola-rfc8321bis-04

Abstract

   This document describes the Alternate-Marking technique to perform
   packet loss, delay, and jitter measurements on live traffic.  This
   technology can be applied in various situations and for different
   protocols.  It could be considered Passive or Hybrid depending on the
   application.  This document obsoletes [RFC8321].

Status of This Memo

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

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

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

   This Internet-Draft will expire on October 7, 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Overview of the Method  . . . . . . . . . . . . . . . . . . .   4
   3.  Detailed Description of the Method  . . . . . . . . . . . . .   5
     3.1.  Packet Loss Measurement . . . . . . . . . . . . . . . . .   5
     3.2.  One-Way Delay Measurement . . . . . . . . . . . . . . . .   9
       3.2.1.  Single-Marking Methodology  . . . . . . . . . . . . .   9
       3.2.2.  Double-Marking Methodology  . . . . . . . . . . . . .  10
     3.3.  Delay Variation Measurement . . . . . . . . . . . . . . .  11
   4.  Alternate Marking Functions . . . . . . . . . . . . . . . . .  12
     4.1.  Marking the Packets . . . . . . . . . . . . . . . . . . .  12
     4.2.  Counting and Timestamping Packets . . . . . . . . . . . .  12
     4.3.  Data Collection and Correlation . . . . . . . . . . . . .  13
   5.  Synchronization and Timing  . . . . . . . . . . . . . . . . .  14
   6.  Packet Fragmentation  . . . . . . . . . . . . . . . . . . . .  16
   7.  Results of the Alternate Marking Experiment . . . . . . . . .  16
     7.1.  Controlled Domain requirement . . . . . . . . . . . . . .  18
   8.  Compliance with Guidelines from RFC 6390  . . . . . . . . . .  18
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  20
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  22
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     13.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Appendix A.  Changes Log  . . . . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

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

   Service Providers need methodologies and tools to monitor and
   accurately measure network performance, in order to constantly
   control the quality of experience perceived by their customers.
   Performance monitoring also provides useful information for improving




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   network management (e.g., isolation of network problems,
   troubleshooting, etc.).

   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.

   [RFC7276] provides a good overview of existing Operations,
   Administration, and Maintenance (OAM) mechanisms defined in the IETF,
   ITU-T, and IEEE.  In the IETF, a lot of work has been done on fault
   detection and connectivity verification, while little has been thus
   far dedicated to performance monitoring.  The IETF has defined
   standard metrics to measure network performance; however, its methods
   mainly focus on Active measurement techniques.For example, [RFC6374]
   defines mechanisms for measuring packet loss, one-way and two-way
   delay, and delay variation in MPLS networks, but its applicability to
   Passive measurements has some limitations, especially for connection-
   less networks.

   This document proposes 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 or two-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 much better
   accuracy, especially for packet loss measurements.

   Therefore, the Alternate-Marking Method could be considered Hybrid or
   Passive, depending on the case.  In the case where the marking method
   is obtained by changing existing field values of the packets the
   technique is Hybrid.  In the case where the marking field is
   dedicated, reserved, and included in the protocol specification, the
   Alternate-Marking technique can be considered as Passive.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.




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2.  Overview of the Method

   In order to perform packet loss measurements on a production 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 that is 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
   processing it in real time could overload the device.

   An alternate approach is to count the number of packets sent on one
   end, count the number of packets received on the other end, and
   compare the two values.  This operation is much simpler to implement,
   but it requires the devices performing the measurement to be 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 can 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 periodically inserting 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
   loss.

   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.

   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



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   any segment of the network and can be used to isolate the faulty
   element.

                               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 operates.  A
   special emphasis is given to the measurement of packet loss, which
   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
   if 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
      losses); and





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   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 rest of the document assumes that the blocks are created
   according to a fixed timer.  The switching after a fixed number of
   packets is an additional possibility but its detailed specification
   is out of scope.

   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       |           |
    ------------------------------------------------------------------->
     BBBBBBB AAAAAAAAAAA BBBBBBBBBBB AAAAAAAAAAA BBBBBBBBBBB AAAAAAA
    ------------------------------------------------------------------->
       ...  |  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.
















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

                                Traffic Flow
            ===========================================================>
   Color   ...AAAAAAAAAAA BBBBBBBBBBB AAAAAAAAAAA BBBBBBBBBBB AAAAAAA...
            ===========================================================>


                 Figure 3: Computation of Link Packet Loss

   Traffic coloring can be done by R1 itself if the traffic is not
   already colored.  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 as A, only
   counter C(A)R1 will be incremented, while C(B)R1 is not incremented;
   conversely, 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 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 the R2 egress interface, it is
   possible to count the packets sent out of the R2 interface and use
   them as reference values to calculate the packet loss from R2 to any
   measurement point downstream from R2.

   Using a fixed timer for color switching offers 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 with 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
   counter of the previous color.  The drawback is that the longer the
   duration of the block, the less frequently the measurement can be
   taken.




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   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, two counters are
      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 packet
   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 has 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 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, whereas counters on
   other interfaces will be null.




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3.2.  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.

   Note that, for all the one-way delay alternatives described in the
   next sections, by summing the one-way delays of the two directions of
   a path, it is always possible to measure the two-way delay (round-
   trip "virtual" delay).

3.2.1.  Single-Marking Methodology

   The alternation of colors can be used as a time reference to
   calculate the delay.  Whenever the color changes (which 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.  When looking at Figure 2, R1 stores the timestamp
   TS(A1)R1 when it sends the first packet of block 1 (A-colored), the
   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.  The number of
   measurements could be 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).

   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 (for instance, if that packet is lost between R1 and R2
   or it arrives after the next one).  Since packet misordering is
   generally undetectable it is not possible to check whether the first
   packet on R1 is the same on R2 and this is part of the intrinsic
   error in this measurement.




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3.2.1.1.  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, the measurement error could be augmented by accumulating
   the 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, and it doesn't give the
   minimum, maximum, and median delay values [RFC6703].  This limitation
   could be overcome by reducing the duration of the block (for
   instance, from 5 minutes to a few seconds), which implies a highly
   optimized implementation of the method.

3.2.2.  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 has been 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 value's 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 a Double-Marking
   methodology.

   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



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   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 not be 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 are 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 a 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 a
   Double-Marking Method, where a subset of batch packets is 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 they 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 useful
   information since it is possible to understand if the Double-Marking
   measurements are actually representative of the delay trends.

3.3.  Delay Variation Measurement

   Similar 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 a 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 the timestamp
   TS(A)R1 whenever it sends the first packet of a block, and R2 stores
   the 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
   samples.





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   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.  Alternate Marking Functions

4.1.  Marking the Packets

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

   In case of flow-based measurements, the flow to monitor can be
   defined by a set of selection rules (e.g., header fields) used to
   match a subset of the packets; in this way, it is possible to control
   the number of involved nodes, the path followed by the packets, and
   the size of the flows.  It is possible, in general, to have multiple
   coloring nodes or a single coloring node that is easier to manage and
   doesn't raise any risk of conflict.  Coloring in multiple nodes can
   be done, and the requirement is that the coloring must change
   periodically between the nodes according to the timing considerations
   in Section 5; so every node that is designated as a measurement point
   along the path should be able to identify unambiguously the colored
   packets.  Furthermore, [I-D.fioccola-rfc8889bis] generalizes the
   coloring for multipoint-to-multipoint flow.  In addition, it can be
   advantageous to color the flow as close as possible to the source
   because it allows an end-to-end measure if a measurement point is
   enabled on the last-hop router as well.

   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 specific flags 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.

4.2.  Counting and Timestamping Packets

   For flow-based measurements, assuming that the coloring of the
   packets is performed only by the source nodes, the nodes between
   source and destination (included) have to count and timestamp 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



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   on which network segment is being monitored (a single link, a
   particular metro area, the backbone, or the whole path).  Since the
   color switches periodically between two values, two counters (one for
   each value) are needed for each flow and for every interface being
   monitored.  The number of timestamps to be stored depends on the
   method for delay measurement that is applied.  Furthermore,
   [I-D.fioccola-rfc8889bis] generalizes the counting for multipoint-to-
   multipoint flow.

   In case of link-based measurements, the behavior is similar except
   that coloring, counting and timestamping 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 or when to select the packets to be double-marked for
   delay measurement.  It involves timing aspects to consider that are
   further described in Section 5.

4.3.  Data Collection and Correlation

   The nodes enabled to perform performance monitoring collect the value
   of the counters and timestamps, but they are not able to directly use
   this information to measure packet loss and delay, because they only
   have their own samples.

   Data collection enables the transmission of the counters and
   timestamps as soon as it has been read.  While, data correlation is
   the mechanism to compare counters and timestamps for packet loss,
   delay, and delay variation calculation.

   There are two main possibilities to perform both data collection and
   correlation depending on the Alternate-Marking application and use
   case:

   o  Use of a centralized solution using Network Management System
      (NMS) to correlate data.  This 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.

   o  Definition of a protocol-based distributed solution to exchange
      values of counters and timestamps between the endpoints.  This can
      be done by introducing a new protocol or by extending the existing
      protocols (e.g., the Two-Way Active Measurement Protocol (TWAMP)
      as defined in RFC 5357 [RFC5357] or the One-Way Active Measurement




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      Protocol (OWAMP) as defined in RFC 4656 [RFC4656]) in order to
      communicate the counters and timestamps between nodes.

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

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

   The Alternate-Marking Method described in this document literally
   splits the packets of the measured flow into different measurement
   blocks.  An implementation MAY use a Block Number (BN) for data
   correlation.  The BN MAY be assigned to each measurement block and
   associated with each packet count and timestamp reported to or pulled
   by other nodes or NMSs.  When the nodes or NMS see, for example, the
   same BNs associated with two packet counts from an upstream and a
   downstream node, respectively, it considers that these two packet
   counts correspond to the same block.  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) [RFC5905] or the
   IEEE 1588 Precision Time Protocol (PTP) [IEEE-1588]).

5.  Synchronization and Timing

   This document introduces two color-switching methods: one is based on
   a fixed number of packets, and the other is based on a fixed timer.
   But the method based on a fixed timer is preferable because it is
   more deterministic, and it is considered in the document.

   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.

   In general, clocks in network devices are not accurate and for this
   reason, there is a clock error between the measurement points R1 and
   R2.  And, to implement the methodology, they must be synchronized to
   the same clock reference with an adequate accuracy in order to
   guarantee that all network devices consistently match the marking bit
   to the correct block.  Additionally, in practice, besides clock
   errors, packet reordering is also very common in a packet network due
   to equal-cost multipath (ECMP).  In particular, the delay between
   measurement points is the main cause of out of order because each



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   packet can be delayed differently.  If the block is sufficiently
   large, packet reordering occurs only at the edge of adjacent blocks
   and it can be easy to assign reordered packets to the right interval
   blocks.

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

   The following figure explains both issues.

   ...BBBBBBBBB | AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA | BBBBBBBBB...
                |<======================================>|
                |                   L                    |
   ...=========>|<==================><==================>|<==========...
                |       L/2                   L/2        |
                |<===>|                            |<===>|
                   d  |                            |   d
                      |<==========================>|
                       available counting interval

                         Figure 4: Timing Aspects

   Where L is the time duration of each block.

   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 network delay between the network devices can be represented as a
   data set and 99.7% of the samples are within 3 standard deviation of
   the average.

   The guard band d is given by:

   d = A + D_avg + 3*D_stddev,

   where A is the clock accuracy, D_avg is the average value of the
   network delay between the network devices, and D_stddev is the
   standard deviation of the delay.

   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.




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   It is worth mentioning that, if the time duration L of each block is
   not so small, the synchronization requirement could be satisfied even
   with a relatively inaccurate synchronization method.  This is true
   for packet loss and two-way delay measurement, but not for one-way
   delay measurement, where clock synchronization must be accurate.
   Therefore, a system that uses only packet loss and two-way delay
   measurement may not require a very precise synchronization.  This is
   because the value of the clocks of network devices does not affect
   the computation of the two-way delay measurement.

6.  Packet Fragmentation

   Fragmentation can be managed with the Alternate-Marking Method and in
   particular it is possible to give the following guidance:

      Marking nodes MUST mark all fragments if there are flag bits to
      use (i.e. it is in the specific encapsulation), as if they were
      separate packets.

      Nodes that fragment packets within the measurement domain SHOULD,
      if they have the capability to do so, ensure that only one
      resulting fragment carries the marking bit(s) of the original
      packet.  Failure to do so can introduce errors into the
      measurement.

      Measurement points MAY simply ignore unmarked fragments and count
      marked fragments as full packets.  However, if resources allow,
      measurement points MAY make note of both marked and unmarked
      initial fragments and only increment the corresponding counter if
      (a) other fragments are also marked, or (b) it observes all other
      fragments and they are unmarked.

   The proposed approach allows the marking node to mark all the
   fragments except in the case of fragmentation within the network
   domain, in that event it is suggested to mark only the first
   fragment.

7.  Results of the Alternate Marking Experiment

   The methodology described in the previous sections can be applied to
   various performance measurement problems, as explained in [RFC8321].
   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 it can be easily recognized by the
   receiver.

   Either one or two flag bits might be available for marking in
   different deployments:



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      One flag: packet loss measurement SHOULD be done as described in
      Section 3.1, while delay measurement MAY be done according to the
      single-marking method described in Section 3.2.1.  Mean delay
      (Section 3.2.1.1) is NOT RECOMMENDED since it implies more
      computational load.

      Two flags: packet loss measurement SHOULD be done as described in
      Section 3.1, while delay measurement SHOULD be done according to
      double-marking method Section 3.2.2.  In this case single-marking
      MAY also be used in combination with double-marking and the two
      approaches provide slightly different pieces of information that
      can be combined to have a more robust data set.

   The experiment with Alternate Marking methodologies confirmed the
   following benefits:

   o  easy implementation: it can be implemented by using features
      already available on major routing platforms, or by applying an
      optimized implementation of the method for both legacy and newest
      technologies;

   o  low computational effort: the additional load on processing is
      negligible;

   o  accurate loss and delay measurements: single packet loss
      granularity is achieved with a Passive measurement;

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

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

   o  flexibility: all the timestamp formats are allowed, because they
      are managed out of band.  The format (the Network Time Protocol
      (NTP) [RFC5905] or the IEEE 1588 Precision Time Protocol (PTP)
      [IEEE-1588]) depends on the precision you want; and

   o  no interoperability issues: the features required are available on
      all current routing platforms.  Both a centralized or distributed
      solution can be used to harvest data from the routers.

   A deployment of the Alternate-Marking Method SHOULD also take into
   account how to handle and recognize marked and unmarked traffic
   depending on whether the technique is applied as Hybrid or Passive.
   In the case where the marking method is applied by changing existing
   fields of the packets, it is RECOMMENDED to use an additional flag or



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   some out-of-band signaling to indicate if the measurement is
   activated or not in order to inform the measurement points.  While,
   in the case where the marking field is dedicated, reserved, and
   included in a protocol extension, the measurement points can learn
   whether the measurement is activated or not by checking if the
   specific extension is included or not within the packets.

   It is worth mentioning some related work: in particular
   [IEEE-Network-PNPM] explains the Alternate-Marking method together
   with new mechanisms based on hashing techniques as also further
   described in [I-D.mizrahi-ippm-marking]; while
   [I-D.zhou-ippm-enhanced-alternate-marking] extends the Alternate-
   Marking Data Fields, to provide enhanced capabilities and allow
   advanced functionalities.

7.1.  Controlled Domain requirement

   The Alternate Marking Method is an example of a solution limited to a
   controlled domain [RFC8799].

   A controlled domain is a managed network that selects, monitors, and
   controls access by enforcing policies at the domain boundaries, in
   order to discard undesired external packets entering the domain and
   check internal packets leaving the domain.  It does not necessarily
   mean that a controlled domain is a single administrative domain or a
   single organization.  A controlled domain can correspond to a single
   administrative domain or multiple administrative domains under a
   defined network management.  It must be possible to control the
   domain boundaries, and use specific precautions if traffic traverses
   the Internet.

   For security reasons, the Alternate Marking Method is RECOMMENDED
   only for controlled domains.

8.  Compliance with Guidelines from RFC 6390

   RFC 6390 [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
   rather a new Method of Measurement for a few Performance Metrics that
   have already been standardized.  Nevertheless, it's worth applying
   guidelines from [RFC6390] to the present document, in order to
   provide a more complete and coherent description of the proposed
   method.  We used a combination of the Performance Metric Definition




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   template defined in Section 5.4 of [RFC6390] and the Dependencies
   laid out in Section 5.5 of that document.

   o  Metric Name / Metric Description: as already stated, this document
      doesn't propose any new Performance Metrics.  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.
      The steady state is an intrinsic characteristic of the marking
      method counters because the alternation of color makes the counter
      associated with a color inactive for the duration of a marking
      period.

   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 Point(s) with Potential Measurement Domain: 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 has a constraint on the frequency
      of measurements.  This is detailed in Section 5, where it is
      specified that the marking period and the guard band interval are
      strictly related each other to avoid out-of-order issues.  That is
      because, in order to perform a measurement, the counter must be in
      a steady state, and this happens when the traffic is being colored
      with the alternate color.

   o  Implementation: the method uses one or two marking bits 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  Verification: both in the lab and in the operational network, the
      methodology has been tested and experimented for packet loss and



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      delay measurements by using traffic generators together with
      precision test instruments and network emulators.

   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 performs 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.

   o  Organization of Results: the Method of Measurement produces
      singletons.

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

9.  IANA Considerations

   This document has no IANA actions.

10.  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 that 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 a header or encapsulation of the 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



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      preserves 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 on 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 on data-plane traffic does not allow attackers to
      gain information about the network performance.

   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; thus, the routers (or other network devices) are locally
      administered and this type of attack can be avoided.

   It is worth highlighting that an attacker can't gain information
   about network performance from a single monitoring point; it 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 do.

   Attacks on the data collection and reporting of the statistics
   between the monitoring points and the network management system can
   interfere with the proper functioning of the system.  Hence, the
   channels used to report back flow statistics MUST be secured.

   The privacy concerns of network measurement are limited because the
   method only relies on information contained in the header or
   encapsulation without any release of user data.  Although information
   in the header or encapsulation is metadata that can be used to
   compromise the privacy of users, the limited marking technique in
   this document seems unlikely to substantially increase the existing
   privacy risks from header or encapsulation metadata.  It might be
   theoretically possible to modulate the marking to serve as a covert
   channel, but it would have a very low data rate if it is to avoid
   adversely affecting the measurement systems that monitor the marking.

   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



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   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].

11.  Contributors

   Xiao Min
   ZTE Corp.
   Email: xiao.min2@zte.com.cn

   Mach(Guoyi) Chen
   Huawei Technologies
   Email: mach.chen@huawei.com

   Alessandro Capello
   Telecom Italia
   Email: alessandro.capello@telecomitalia.it

12.  Acknowledgements

   The authors would like to thank Alberto Tempia Bonda, Luca
   Castaldelli and Lianshu Zheng for their contribution to the
   experimentation of the method.

   The authors would also thank Martin Duke and Tommy Pauly for their
   assistance and their detailed and precious reviews.

13.  References

13.1.  Normative References

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.







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   [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,
              <https://www.rfc-editor.org/info/rfc5905>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

13.2.  Informative References

   [I-D.fioccola-rfc8889bis]
              Fioccola, G., Cociglio, M., Sapio, A., Sisto, R., and T.
              Zhou, "Multipoint Alternate-Marking Method", draft-
              fioccola-rfc8889bis-03 (work in progress), February 2022.

   [I-D.mizrahi-ippm-marking]
              Mizrahi, T., Fioccola, G., Cociglio, M., Chen, M., and G.
              Mirsky, "Marking Methods for Performance Measurement",
              draft-mizrahi-ippm-marking-00 (work in progress), October
              2021.

   [I-D.zhou-ippm-enhanced-alternate-marking]
              Zhou, T., Fioccola, G., Liu, Y., Cociglio, M., Lee, S.,
              and W. Li, "Enhanced Alternate Marking Method", draft-
              zhou-ippm-enhanced-alternate-marking-09 (work in
              progress), February 2022.

   [IEEE-Network-PNPM]
              IEEE Network, "AM-PM: Efficient Network Telemetry using
              Alternate Marking", DOI 10.1109/MNET.2019.1800152, 2019.

   [RFC3393]  Demichelis, C. and P. Chimento, "IP Packet Delay Variation
              Metric for IP Performance Metrics (IPPM)", RFC 3393,
              DOI 10.17487/RFC3393, November 2002,
              <https://www.rfc-editor.org/info/rfc3393>.

   [RFC4656]  Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
              Zekauskas, "A One-way Active Measurement Protocol
              (OWAMP)", RFC 4656, DOI 10.17487/RFC4656, September 2006,
              <https://www.rfc-editor.org/info/rfc4656>.

   [RFC5357]  Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
              Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
              RFC 5357, DOI 10.17487/RFC5357, October 2008,
              <https://www.rfc-editor.org/info/rfc5357>.





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   [RFC5481]  Morton, A. and B. Claise, "Packet Delay Variation
              Applicability Statement", RFC 5481, DOI 10.17487/RFC5481,
              March 2009, <https://www.rfc-editor.org/info/rfc5481>.

   [RFC6374]  Frost, D. and S. Bryant, "Packet Loss and Delay
              Measurement for MPLS Networks", RFC 6374,
              DOI 10.17487/RFC6374, September 2011,
              <https://www.rfc-editor.org/info/rfc6374>.

   [RFC6390]  Clark, A. and B. Claise, "Guidelines for Considering New
              Performance Metric Development", BCP 170, RFC 6390,
              DOI 10.17487/RFC6390, October 2011,
              <https://www.rfc-editor.org/info/rfc6390>.

   [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,
              <https://www.rfc-editor.org/info/rfc6703>.

   [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,
              <https://www.rfc-editor.org/info/rfc7276>.

   [RFC7384]  Mizrahi, T., "Security Requirements of Time Protocols in
              Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
              October 2014, <https://www.rfc-editor.org/info/rfc7384>.

   [RFC7679]  Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
              Ed., "A One-Way Delay Metric for IP Performance Metrics
              (IPPM)", STD 81, RFC 7679, DOI 10.17487/RFC7679, January
              2016, <https://www.rfc-editor.org/info/rfc7679>.

   [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, <https://www.rfc-editor.org/info/rfc7680>.

   [RFC7799]  Morton, A., "Active and Passive Metrics and Methods (with
              Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
              May 2016, <https://www.rfc-editor.org/info/rfc7799>.

   [RFC8321]  Fioccola, G., Ed., Capello, A., Cociglio, M., Castaldelli,
              L., Chen, M., Zheng, L., Mirsky, G., and T. Mizrahi,
              "Alternate-Marking Method for Passive and Hybrid
              Performance Monitoring", RFC 8321, DOI 10.17487/RFC8321,
              January 2018, <https://www.rfc-editor.org/info/rfc8321>.



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   [RFC8799]  Carpenter, B. and B. Liu, "Limited Domains and Internet
              Protocols", RFC 8799, DOI 10.17487/RFC8799, July 2020,
              <https://www.rfc-editor.org/info/rfc8799>.

Appendix A.  Changes Log

   Changes from RFC 8321 include:

   o  Minor editorial changes

   o  Replacement of the section on "Applications, Implementation, and
      Deployment" with "Finding of the Alternate Marking Implementations
      and Deployments"

   o  Moved advantages and benefits of the method from "Introduction" to
      the new section on "Finding of the Alternate Marking
      Implementations and Deployments"

   o  Removed section on "Hybrid Measurement"

   Changes in v-(01) include:

   o  Considerations on the reference: [IEEE-Network-PNPM]

   o  Clarified that the method based on a fixed timer is specified in
      this document while the method based on a fixed number of packets
      is only mentioned but not detailed.

   o  Explanation of the the intrinsic error in section 3.3.1 on
      "Single-Marking Methodology"

   o  Deleted some parts in section 4 "Considerations" that no longer
      apply

   o  New section on "Packet Fragmentation"

   Changes in v-(02) include:

   o  Considerations on how to handle unmarked traffic in section 5 on
      "Results of the Alternate Marking Experiment"

   o  Minor rewording in section 4.4 on "Packet Fragmentation"

   Changes in v-(03) include:

   o  Deleted numeric examples in sections on "Packet Loss Measurement"
      and on "Single-Marking Methodology"




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   o  New section on "Alternate Marking Functions"

   o  Moved sections 3.1.1 on "Coloring the Packets", 3.1.2 on "Counting
      the Packets" and 3.1.3 on "Collecting Data and Calculating Packet
      Loss" into the new section on "Alternate Marking Functions"

   o  Renamed sections 4.1 as "Marking the Packets", 4.2 as "Counting
      and Timestamping Packets" and 4.3 as "Data Collection and
      Correlation"

   o  Merged old section on "Data Correlation" with section 4.3 on "Data
      Collection and Correlation"

   o  Moved and renamed section on "Timing Aspects" as "Synchronization
      and Timing"

   o  Merged old section on "Synchronization" with section on
      "Synchronization and Timing"

   o  Merged old section on "Packet Reordering" with section on
      "Synchronization and Timing"

   Changes in v-(04) include:

   o  Revised "Introduction" section

   o  Revised sections 4.2 "Counting and Timestamping Packets" and 4.3
      on "Data Collection and Correlation"

   o  Revised section 5 on "Synchronization and Timing"

Authors' Addresses

   Giuseppe Fioccola (editor)
   Huawei Technologies
   Riesstrasse, 25
   Munich  80992
   Germany

   Email: giuseppe.fioccola@huawei.com











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   Mauro Cociglio
   Telecom Italia
   Via Reiss Romoli, 274
   Torino  10148
   Italy

   Email: mauro.cociglio@telecomitalia.it


   Greg Mirsky
   Ericsson

   Email: gregimirsky@gmail.com


   Tal Mizrahi
   Huawei Technologies

   Email: tal.mizrahi.phd@gmail.com


   Tianran Zhou
   Huawei Technologies
   156 Beiqing Rd.
   Beijing  100095
   China

   Email: zhoutianran@huawei.com























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