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Versions: 00 01 02 03 draft-ietf-ippm-alt-mark

Network Working Group                                         A. Capello
Internet-Draft                                               M. Cociglio
Intended status: Experimental                                G. Fioccola
Expires: September 22, 2016                               L. Castaldelli
                                                          Telecom Italia
                                                         A. Tempia Bonda
                                                          March 21, 2016


        A packet based method for passive performance monitoring
                        draft-tempia-ippm-p3m-03

Abstract

   This document describes a passive method to perform packet loss,
   delay and jitter measurements on live traffic.  This method is based
   on Alternate Marking (Coloring) technique.  A report on the
   operational experiment done at Telecom Italia is explained in order
   to give an example and show the method applicability.  This technique
   can be applied in various situations as detailed in this document.
   The previous IETF drafts about this technique were:
   [I-D.cociglio-mboned-multicast-pm] and [I-D.tempia-opsawg-p3m].

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on September 22, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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



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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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   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.  Average delay . . . . . . . . . . . . . . . . . . . .  11
       3.2.3.  Double marking methodology  . . . . . . . . . . . . .  11
     3.3.  Delay variation measurement . . . . . . . . . . . . . . .  12
   4.  Implementation and deployment . . . . . . . . . . . . . . . .  12
     4.1.  Report on the operational experiment at Telecom Italia  .  13
       4.1.1.  Coloring the packets  . . . . . . . . . . . . . . . .  14
       4.1.2.  Counting the packets  . . . . . . . . . . . . . . . .  15
       4.1.3.  Collecting data and calculating packet loss . . . . .  16
       4.1.4.  Metric transparency . . . . . . . . . . . . . . . . .  17
     4.2.  IP flow performance measurement (IPFPM) . . . . . . . . .  17
     4.3.  Performance Measurement Marking Method in BIER Domain . .  17
     4.4.  RFC6374 Use Case  . . . . . . . . . . . . . . . . . . . .  17
     4.5.  Application to active performance measurement . . . . . .  18
   5.  Hybrid measurement  . . . . . . . . . . . . . . . . . . . . .  18
   6.  Compliance with RFC6390 guidelines  . . . . . . . . . . . . .  18
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   8.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  20
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     11.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   Nowadays, most of the traffic in Service Providers' networks carries
   real time content.  These contents are highly sensitive to packet
   loss [RFC2680], while interactive contents are sensitive to delay
   [RFC2679], 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



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

   A lot of work related to OAM, that includes also performance
   monitoring techniques, has been done by Standards Developing
   Organizations: [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 the WG
   mainly refer to 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.  It doesn't
   require any protocol extension or interaction with existing
   protocols, thus avoiding any interoperability issue.  Even if the
   method doesn't raise any specific need for standardization, it could
   be further improved by means of some extension to existing protocols,
   but this aspect is left for further study and it is out of the scope
   of this document.

   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.

   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 previous IETF drafts about this technique were:
   [I-D.cociglio-mboned-multicast-pm] and [I-D.tempia-opsawg-p3m].
   There are some references to this methodology in other IETF works
   (e.g.  [I-D.ietf-mpls-flow-ident], [I-D.bryant-mpls-sfl-framework]
   [I-D.bryant-mpls-rfc6374-sfl], [I-D.ietf-bier-mpls-encapsulation],
   [I-D.mirsky-bier-pmmm-oam]
   [I-D.chen-ippm-coloring-based-ipfpm-framework]).



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

   o  Section 5 includes some considerations about security aspects;

   o  Section 6 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
   loss.



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   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 "colors" 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
   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 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.




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   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: 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); but
   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       |           |
    ------------------------------------------------------------------->
     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: Application of the method to compute 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.



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   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    |
           |       |        |        |        |        |      |
           | ...   | ...    | ...    | ...    | ...    | ...  |
           |       |        |        |        |        |      |
           | n     | 0      | 387    | 0      | 387    | 0    |
           |       |        |        |        |        |      |
           | n+1   | 379    | 0      | 377    | 0      | 2    |
           +-------+--------+--------+--------+--------+------+

       Table 1: Evaluation of counters for packet loss measurements

   During an A block (blocks 1, 3 and n+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 n), 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



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   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.

   R1 and R2 require a clock error less than +/-L/2 time units, where L
   is the time duration of the block.  In this way each colored packet
   can be assigned to the right block by each router.  This is because
   the minimum time distance between two packets of the same color but
   belonging to different blocks is L time units.

   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.

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.

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 (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 4, 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 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



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   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       |
      |       |         |         |         |         |             |
      | ...   | ...     | ...     | ...     | ...     | ...         |
      |       |         |         |         |         |             |
      | n     | 77.463  | -       | 80.501  | -       | 3.038       |
      |       |         |         |         |         |             |
      | n+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



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   corresponding timestamps (possible but impractical from an
   implementation point of view).

3.2.2.  Average 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 average delay.  The average 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 average delay between those nodes.  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) by means of an highly optimized implementation of the
   method.

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

3.2.3.  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 average delay.  But the limitation of average 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 average delay but also the minimum and maximum 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
   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



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   needed for packet loss and average 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 average 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.

3.3.  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.  The alternation of colors can be used as a time
   reference to measure delay variations.  Considering the example
   depicted in Figure 4, 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 samples.

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

4.  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.








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4.1.  Report on the operational experiment at Telecom Italia

   The methodology has been applied in Telecom Italia 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 fundamental steps for this implementation of the method can be
   summarized in the following items:

   o  coloring the packets;

   o  counting the packets;

   o  collecting data and calculating the packet loss.

   o  metric transparency.

   Before going deeper into the implementation details, 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.  This could be the
      case, for example, of IPTV channels or other specific applications
      traffic with high QoS requirements (i.e.  Mobile Backhauling
      traffic).  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,
      especially for IPTV traffic where static joins are frequently used
      to force traffic forwarding and replication.  Another application
      is on Mobile Backhauling, implemented with a VPN MPLS in Telecom
      Italia's network; in this case the problem with unicast traffic is
      overcome by monitoring just the two Provider Edge nodes of the VPN
      MPLS.

   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



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      logical link (for instance an Ethernet VLAN or a MPLS PW).
      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.

   The current implementation in Telecom Italia uses the first strategy.
   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, a Service Provider should be aware that, 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 policy 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.

4.1.1.  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 necessary 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



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   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.  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 ca be
   used perform this task on the basis of a fixed timer.  In Telecom
   Italia's implementation this 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).

4.1.2.  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 od counters is needed.  For example,
   in order to monitor separately 3 flows on a router with 4 interfaces



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   involved, 24 counters are needed (2 counters for each of the 3 flows
   on each of the 4 interfaces).  If traffic is colored using the DSCP
   field, as in Telecom Italia's implementation, an access-list that
   matches specific DSCP values can be used to count the packets of the
   flow(s) being monitored.

   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, if each block is 5
   minutes long, reading the counter for color A every 5 minute 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.  The choice of a 5 minutes timer for colore switching
   was also inspired by these considerations.

4.1.3.  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) is required 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




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   values from all the routers within one cycle of the timer (5
   minutes).

   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 complicated if
   applied to a flow-based measurement.

4.1.4.  Metric transparency

   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
   domain.

4.2.  IP flow performance measurement (IPFPM)

   This application of marking method is described in
   [I-D.chen-ippm-coloring-based-ipfpm-framework].

4.3.  Performance Measurement Marking Method in BIER Domain

   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.mirsky-bier-pmmm-oam]
   details the measurement for multicast service over BIER domain.

4.4.  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



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   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.bryant-mpls-rfc6374-sfl] describes RFC6374 performance
   measurements with SFL.

4.5.  Application to active performance measurement

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

5.  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.

6.  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 [RFC2680].  The
      same concept, with small differences, can also be used to measure
      delay [RFC2679], 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



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      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
      minutes.

   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



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      nodes must not change the value of the DSCP field not to alter the
      measurement.

   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.

7.  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.  For what
   concerns the first point, the measurements described in this document
   are passive, so there are no 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.  The measurements themselves could be harmed
   by routers altering the coloring 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.

   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.

8.  Conclusions

   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
      negligible;





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   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
      impact;

   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 standardization, 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.

9.  IANA Considerations

   There are no IANA actions required.

10.  Acknowledgements

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

11.  References

11.1.  Normative References

   [RFC2679]  Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
              Delay Metric for IPPM", RFC 2679, DOI 10.17487/RFC2679,
              September 1999, <http://www.rfc-editor.org/info/rfc2679>.

   [RFC2680]  Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
              Packet Loss Metric for IPPM", RFC 2680,
              DOI 10.17487/RFC2680, September 1999,
              <http://www.rfc-editor.org/info/rfc2680>.

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




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11.2.  Informative References

   [I-D.bryant-mpls-rfc6374-sfl]
              Bryant, S., Swallow, G., Sivabalan, S., Mirsky, G., Chen,
              M., and Z. Li, "RFC6374 Synonymous Flow Labels", draft-
              bryant-mpls-rfc6374-sfl-00 (work in progress), October
              2015.

   [I-D.bryant-mpls-sfl-framework]
              Bryant, S., Swallow, G., Sivabalan, S., Mirsky, G., Chen,
              M., and Z. Li, "Synonymous Flow Label Framework", draft-
              bryant-mpls-sfl-framework-00 (work in progress), October
              2015.

   [I-D.bryant-mpls-synonymous-flow-labels]
              Bryant, S., Swallow, G., Sivabalan, S., Mirsky, G., Chen,
              M., and Z. Li, "RFC6374 Synonymous Flow Labels", draft-
              bryant-mpls-synonymous-flow-labels-01 (work in progress),
              July 2015.

   [I-D.chen-ippm-coloring-based-ipfpm-framework]
              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.

   [I-D.cociglio-mboned-multicast-pm]
              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.

   [I-D.fioccola-ippm-rfc6812-alt-mark-ext]
              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-00 (work in progress), October 2015.

   [I-D.ietf-bier-mpls-encapsulation]
              Wijnands, I., Rosen, E., Dolganow, A., Tantsura, J., and
              S. Aldrin, "Encapsulation for Bit Index Explicit
              Replication in MPLS Networks", draft-ietf-bier-mpls-
              encapsulation-03 (work in progress), February 2016.

   [I-D.ietf-mpls-flow-ident]
              Bryant, S., Pignataro, C., Chen, M., Li, Z., and G.
              Mirsky, "MPLS Flow Identification Considerations", draft-
              ietf-mpls-flow-ident-00 (work in progress), December 2015.



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   [I-D.mirsky-bier-pmmm-oam]
              Mirsky, G., Zheng, L., Chen, M., and G. Fioccola,
              "Performance Measurement (PM) with Marking Method in Bit
              Index Explicit Replication (BIER) Layer", draft-mirsky-
              bier-pmmm-oam-01 (work in progress), March 2016.

   [I-D.tempia-opsawg-p3m]
              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.

   [RFC6374]  Frost, D. and S. Bryant, "Packet Loss and Delay
              Measurement for MPLS Networks", RFC 6374,
              DOI 10.17487/RFC6374, September 2011,
              <http://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,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc7276>.

Authors' Addresses

   Alessandro Capello
   Telecom Italia
   Via Reiss Romoli, 274
   Torino  10148
   Italy

   Email: alessandro.capello@telecomitalia.it









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

   Email: mauro.cociglio@telecomitalia.it


   Giuseppe Fioccola
   Telecom Italia
   Via Reiss Romoli, 274
   Torino  10148
   Italy

   Email: giuseppe.fioccola@telecomitalia.it


   Luca Castaldelli
   Telecom Italia
   Via Reiss Romoli, 274
   Torino  10148
   Italy

   Email: luca.castaldelli@telecomitalia.it


   Alberto Tempia Bonda

   Email: alberto.tempia@gmail.com





















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