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Versions: (draft-mizrahi-ippm-multiplexed-alternate-marking) 00 01 02 03

Network Working Group                                         T. Mizrahi
Internet-Draft                          Huawei Network.IO Innovation Lab
Intended status: Informational                                   C. Arad
Expires: April 13, 2019
                                                             G. Fioccola
                                                     Huawei Technologies
                                                             M. Cociglio
                                                          Telecom Italia
                                                                 M. Chen
                                                                L. Zheng
                                                     Huawei Technologies
                                                               G. Mirsky
                                                               ZTE Corp.
                                                        October 10, 2018


  Compact Alternate Marking Methods for Passive and Hybrid Performance
                               Monitoring
            draft-mizrahi-ippm-compact-alternate-marking-03

Abstract

   This memo introduces new alternate marking methods that require a
   compact overhead of either a single bit per packet, or zero bits per
   packet.  This memo also presents a summary of alternate marking
   methods, and discusses the tradeoffs among them.  The target audience
   of this document is network protocol designers; this document is
   intended to help protocol designers choose the best alternate marking
   method(s) based on the protocol's constraints and requirements.

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 April 13, 2019.





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Copyright Notice

   Copyright (c) 2018 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
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  The Scope of This Document  . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
     2.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Marking Abstractions  . . . . . . . . . . . . . . . . . . . .   5
   4.  Double Marking  . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Single-bit Marking  . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Single Marking Using the First Packet . . . . . . . . . .   8
     5.2.  Single Marking using the Mean Delay . . . . . . . . . . .   8
     5.3.  Single Marking using a Multiplexed Marking Bit  . . . . .   8
       5.3.1.  Overview  . . . . . . . . . . . . . . . . . . . . . .   8
       5.3.2.  Timing and Synchronization Aspects  . . . . . . . . .   9
     5.4.  Pulse Marking . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Zero Marking Hashed . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  Hash-based Sampling . . . . . . . . . . . . . . . . . . .  12
       6.1.1.  Hashed Pulse Marking  . . . . . . . . . . . . . . . .  13
       6.1.2.  Hashed Step Marking . . . . . . . . . . . . . . . . .  13
   7.  Single Marking Hashed . . . . . . . . . . . . . . . . . . . .  13
   8.  Summary of Marking Methods  . . . . . . . . . . . . . . . . .  14
   9.  Alternate Marking using Reserved Values . . . . . . . . . . .  19
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  20
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     12.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21






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1.  Introduction

1.1.  Background

   Alternate marking, defined in [RFC8321], is a method for measuring
   packet loss, packet delay, and packet delay variation.  Typical delay
   measurement protocols require the two measurement points (MPs) to
   exchange timestamped test packets.  In contrast, the alternate
   marking method does not require control packets to be exchanged.
   Instead, every data packet carries a color indicator, which divides
   the traffic into consecutive blocks of packets.

   The color value is toggled periodically, as illustrated in Figure 1.


   A: packet with color 0
   B: packet with color 1

   Packets      AAAAAAAAAA BBBBBBBBBB AAAAAAAAAA BBBBBBBBBB AAAAAAAAAA
      Time   ---------------------------------------------------------->
               |          |          |          |          |
               | Block 1  | Block 2  | Block 3  | Block 4  | Block 5 ...
               |          |          |          |          |
   Color        0000000000 1111111111 0000000000 1111111111 0000000000

     Figure 1: Alternate marking: packets are monitored on a per-color
                                  basis.

   Alternate marking is used between two MPs, the initiating MP, and the
   monitoring MP.  The initiating MP incorporates the marking field into
   en-route packets, allowing the monitoring MP to use the marking field
   in order to bind each packet to the corresponding block.

   Each of the MPs maintains two counters, one per color.  At the end of
   each block the counter values can be collected by a central
   management system, and analyzed; the packet loss can be computed by
   comparing the counter values of the two MPs.

   When using alternate marking delay measurement can be performed in
   one of three ways (as per [RFC8321]):

   o  Single marking using the first packet: in this method each packet
      uses a single marking bit, used as a color indicator.  The first
      packet of each block is used by both MPs as a reference for delay
      measurement.  The timestamp of this packet is measured by the two
      measurement points, and can be collected by the mangement system
      from each of the measurement points, which can compute the path
      delay by comparing the two timestamps.  The drawback of this



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      approach is that it is not accurate when packets arrive out-of-
      order, as the two MPs may have a different view of which packet
      was the first in the block.

   o  Single marking using the mean delay: as in the previous method,
      each packet uses a single marking method, indicating the color.
      Each of the MPs computes the average packet timestamp of each
      block.  The management system can then compute the delay by
      comparing the average times of the two MPs.  The drawback of this
      approach is that it may be computationally heavy, or difficult to
      implement at the data plane.

   o  Double marking: each packet uses two marking bits.  One bit is
      used as a color indicator, and one is used as a timestamping
      indicator.  This method resolves the drawbacks raised for the two
      previous methods, at the expense of an extra bit in the packet
      header.

   The double marking method is the most straightforward approach.  It
   allows for accurate measurement without incurring expensive
   computational load.  However, in some cases allocating two bits for
   passive measurement is not possible.  For example, if alternate
   marking is implemented over IPv4, allocating 2 marking bits in the
   IPv4 header is challenging, as every bit in the 20-octet header is
   costly; one of the possible approaches discussed in [RFC8321] is to
   reserve one or two bits from the DSCP field for remarking.  In this
   case every marking bit comes at the expense of reducing the DSCP
   range by a factor of two.

1.2.  The Scope of This Document

   This memo extends the marking methods of [RFC8321], and introduces
   methods that require a single marking bit, or zero marking bits.

   Two single-bit marking methods are proposed, multiplexed marking and
   pulse marking.  In multiplexed marking the color indicator and the
   timestamp indicator are multiplexed into a single bit, providing the
   advantages of the double marking method while using a single bit in
   the packet header.  In pulse marking both delay and loss measurement
   are triggered by a 'pulse' value in a single marking field.

   This document also discusses zero-bit marking methods that leverage
   well-known hash-based selection approaches ([RFC5474], [RFC5475]).

   Alternate marking is discussed in this memo as a single-bit or a two-
   bit marking method.  However, these methods can similarly be applied
   to larger fields, such as an IPv6 Flow Label or an MPLS Label;
   single-bit marking can be applied using two reserved values, and two-



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   bit marking can be applied using four reserved values.  Marking based
   on reserved values is further discussed in this document, including
   its application to MPLS and IPv6.

   Finally, this memo summarizes the alternate marking methods, and
   discusses the tradeoffs among them.  It is expected that different
   network protocols will have different constraints, and therefore may
   choose to use different alternate marking methods.  In some cases it
   may be preferable to support more than one marking method; in this
   case the particular marking method may be signaled through the
   control plane.

2.  Terminology

2.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.2.  Abbreviations

   The following abbreviations are used in this document:

   DSCP          Differentiated Services Code Point

   DM            Delay Measurement

   LM            Loss Measurement

   LSP           Label Switched Path

   MP            Measurement Point

   MPLS          Multiprotocol Label Switching

   SFL           Synonymous Flow Label [I-D.ietf-mpls-sfl-framework]

3.  Marking Abstractions

   The marking methods that were discussed in Section 1, as well as the
   methods introduced in this document, use two basic abstractions,
   pulse detection, and step detection.

   The common thread along the various marking methods is that one or
   two marking bits are used by the MPs to signal a measurement event.
   The value of the marking bit indicates when the event takes place, in
   one of two ways:



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   Pulse         An event is detected when the value of the marking bit
                 is toggled in a single packet.

   Step          An event is detected when the value of the marking bit
                 is toggled, and remains at the new value.

   The double marking method (Section 1) uses pulse-based detection for
   DM, and step-based detection for LM.

   Pulse-based detection affects the processing of a single packet; the
   packet that indicates the pulse is processed differently than the
   packets around it.  For example, in the double marking method, the
   marked packet is timestamped for DM, without affecting the packets
   before or after it.  Note that if the marked packet is lost, no pulse
   is detected, yielding a missing measurement (see Figure 2).


   P: indicates a packet

   Packets      PPPPPPPPPP PPPPPPPPPP PPPPPPPPPP PPPPPPPPPP PPPPPPPPPP
      Time   ---------------------------------------------------------->
   Marking bit  0000010000 0000010000 0000010000 0000010000 00000 0000
                     ^          ^          ^          ^          ^
     Pulse-based     |          |          |          |          |
     detection       |          |          |          |          |
                                                         Dropped packet:
                                                         no detection


                     Figure 2: Pulse-based Detection.

   In step-based detection the event is detected by observing a value
   change in stream of packets.  Specifically, when the step approach is
   used for LM (as in the double marking method), two counters are used
   per flow; each MP decides which counter to use based on the value of
   the marking bit.  Thus, the step-based approach allows accurate
   counting even when packets arrive out-of-order (see Figure 3).  When
   the step approach is used for DM (e.g., single marking using the
   first packet), out-of-order causes the delay measurement to be false,
   without any indication to the management system.











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   P: indicates a packet

   Packets      PPPPPPPPPP PPPPPPPPPP PPPPPPPPPP PPPPPPPPPP PPPPPPPPPP
      Time   ---------------------------------------------------------->
   Marking bit  0000000000 1111111111 000000000 10111111111 0000000000
                           ^          ^         ^          ^
     Step-based            |          |         |          |
     detection             |          |         |          |
                                           out-of-order


                      Figure 3: Step-based Detection.

4.  Double Marking

   The two-bit marking method of [RFC8321] uses two marking bits: a
   color indicator, and a delay measurement indicator.  The color bit is
   used for step-based LM, while the delay bit is used as a pulse-based
   DM trigger.  This double marking approach is the most straightforward
   of the approaches discussed in this memo, as it allows accurate
   measurement, it is resilient to out-of-order delivery, and is
   relatively simple to implement.  The main drawback is that it
   requires two bits, which are not always available.

   Figure 4 illustrates the double marking method: each block of packets
   includes a packet that is marked for timestamping, and therefore has
   its delay bit set.


   A: packet with color 0
   B: packet with color 1

   Packets      AAAAAAAAAA BBBBBBBBBB AAAAAAAAAA BBBBBBBBBB AAAAAAAAAA
      Time   ---------------------------------------------------------->
               |          |          |          |          |
               | Block 1  | Block 2  | Block 3  | Block 4  | Block 5 ...
               |          |          |          |          |
   Color bit    0000000000 1111111111 0000000000 1111111111 0000000000
   Delay bit    0000100000 0000100000 0000100000 0000100000 0001000000
                    ^          ^          ^          ^         ^
     Packets        |          |          |          |         |
     marked for     |          |          |          |         |
     timestamping   |          |          |          |         |


                   Figure 4: The double marking method.





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5.  Single-bit Marking

5.1.  Single Marking Using the First Packet

   This method uses a single marking bit that indicates the color, as
   described in [RFC8321].  Both LM and DM are implemented using a step-
   based approach; LM is implemented using two color-based counters per
   flow.  The first packet of every period is used by the two MPs as the
   reference for measuring the delay.  As denoted above, the delay
   computed in this method may be erroneous when packets are delivered
   out-of-order.


   A: packet with color 0
   B: packet with color 1

   Packets      AAAAAAAAAA BBBBBBBBBB AAAAAAAAAA BBBBBBBBBB AAAAAAAAAA
      Time   ---------------------------------------------------------->
               |          |          |          |          |
               | Block 1  | Block 2  | Block 3  | Block 4  | Block 5 ...
               |          |          |          |          |
   Color bit    0000000000 1111111111 0000000000 1111111111 0000000000
                ^          ^          ^          ^          ^
    Packets     |          |          |          |          |
    used for DM |          |          |          |          |


       Figure 5: Single marking using the first packet of the block.

5.2.  Single Marking using the Mean Delay

   As in the first-packet approach, in the mean delay approach
   ([RFC8321]) a single marking bit is used to indicate the color,
   enabling step-based loss measurement.  Delay is measured in each
   period by averaging the measured delay over all the packets in the
   period.  As discussed above, this approach is not sensitive to out-
   of-order delivery, but may be heavy from a computational perspective.

5.3.  Single Marking using a Multiplexed Marking Bit

5.3.1.  Overview

   This section introduces a method that uses a single marking bit that
   serves two purposes: a color indicator, and a timestamp indicator.
   The double marking method that was discussed in the previous section
   uses two 1-bit values: a color indicator C, and a timestamp indicator
   T.  The multiplexed marking bit, denoted by M, is an exclusive or
   between these two values: M = C XOR T.



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   An example of the use of the multiplexed marking bit is depicted in
   Figure 6.  The example considers two routers, R1 and R2, that use the
   multiplexed bit method to measure traffic from R1 to R2.  In each
   block R1 designates one of the packets for delay measurement.  In
   each of these designated packets the value of the multiplexed bit is
   reversed compared to the other packets in the same block, allowing R2
   to distinguish the designated packets from the other packets.


   A: packet with color 0
   B: packet with color 1

   Packets      AAAAAAAAAA BBBBBBBBBB AAAAAAAAAA BBBBBBBBBB AAAAAAAAAA
      Time   ---------------------------------------------------------->
               |          |          |          |          |
               | Block 1  | Block 2  | Block 3  | Block 4  | Block 5 ...
               |          |          |          |          |
   Color        0000000000 1111111111 0000000000 1111111111 0000000000
                    ^          ^          ^           ^        ^
     Packets        |          |          |           |        |
     marked for     |          |          |           |        |
     timestamping   |          |          |           |        |
                    v          v          v           v        v
   Muxed bit    0000100000 1111011111 0000100000 1111101111 0001000000


             Figure 6: Alternate marking with multiplexed bit.

5.3.2.  Timing and Synchronization Aspects

   It is assumed that all MPs are synchronized to a common reference
   time with an accuracy of +/- A/2.  Thus, the difference between the
   clock values of any two MPs is bounded by A.  Clocks can be
   synchronized for example using NTP [RFC5905], PTP [IEEE1588], or by
   other means.  The common reference time is used for dividing the time
   domain into equal-sized measurement periods, such that all packets
   forwarded during a measurement period have the same color, and
   consecutive periods have alternating colors.

   The single marking bit incorporates two multiplexed values.  From the
   monitoring MP's perspective, the two values are Time-Division
   Multiplexed (TDM), as depicted in Figure 7.  It is assumed that the
   start time of every measurement period is known to both the
   initiating MP and the monitoring MP.  If the measurement period is L,
   then during the first and the last L/4 time units of each block the
   marking bit is interpreted by the monitoring MP as a color indicator.
   During the middle part of the block, the marking bit is interpreted
   as a timestamp indicator; if the value of this bit is different than



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   the color value, the corresponding packet is used as a reference for
   delay measurement.


                 +--- Beginning of measurement period
                 |
                 v

    ...BBBBBBBBB | AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA | BBBBBBBBB...
                 |<======================================>|
                 |                   L                    |
       <========>|<========><==================><========>|<========>
           L/4       L/4            L/2             L/4       L/4

       <===================><==================><===================>
           Detect color     Detect timestamping      Detect color
             change              indication            change

    Figure 7: Multiplexed marking field interpretation at the receiving
                            measurement point.

   In order to prevent ambiguity in the receiver's interpretation of the
   marking field, the initiating MP is permitted to set the timestamp
   indication only during a specific interval, as depicted in Figure 8.
   Since the receiver is willing to receive the timestamp indication
   during the middle L/2 time units of the block, the sender refrains
   from sending the timestamp indication during a guardband interval of
   d time units at the beginning and end of the L/2-period.


                 +--- Beginning of measurement period
                 |
                 v

    ...BBBBBBBBB | AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA | BBBBBBBBB...
                 |<======================================>|
                 |                   L                    |
       <========>|<========>|<================>|<========>|
           L/4       L/4    |       L/2        |    L/4
                         <=>|<=>            <=>|<=>
                          d   d              d   d
                                <==========>
                                permissible
                                timestamping
                                indication
                                interval

                       Figure 8: A time domain view.



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   The guardband d is given by d = A + D_max - D_min, where A is the
   clock accuracy, D_max is an upper bound on the network delay between
   the MPs, and D_min is a lower bound on the delay.  It is
   straightforward from Figure 8 that d < L/4 must be satisfied.  The
   latter implies a minimal requirement on the synchronization accuracy.

   All MPs must be synchronized to the same reference time with an
   accuracy of +/- L/8.  Depending on the system topology, in some
   systems the accuracy requirement will be even more stringent, subject
   to d < L/4.  Note that the accuracy requirement of the conventional
   alternate marking method [RFC8321] is +/- L/2, while the multiplexed
   marking method requires an accuracy of +/- L/8.

   Note that we assume that the middle L/2-period is designated as the
   timestamp indication period, allowing a sufficiently long guardband
   between the transitions.  However, a system may be configured to use
   a longer timestamp indication period or a shorter one, if it is
   guaranteed that the synchronization accuracy meets the guardband
   requirements (i.e., the constraints on d).

5.4.  Pulse Marking

   Pulse marking uses a single marking bit that is used as a trigger for
   both LM and DM.  In this method the two MPs maintain a single per-
   flow counter for LM, in contrast to the color-based methods which
   require two counters per flow.  In each block one of the packets is
   marked.  The marked packet triggers two actions in each of MPs:

   o  The timestamp is captured for DM.

   o  The value of the counter is captured for LM.

   In each period, each of the MPs exports the timestamp and counter-
   stamp to the management system, which can then compute the loss and
   delay in that period.  It should be noted that as in [RFC8321], if
   the length of the measurement period is L time units, then all
   network devices must be synchronized to the same clock reference with
   an accuracy of +/- L/2 time units.

   The pulse marking approach is illustrated in Figure 9.  Since both LM
   and DM use a pulse-based trigger, if the marked packet is lost then
   no measurement is available in this period.  Moreover, the LM
   accuracy may be affected by out-of-order delivery.








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   P: packet - all packets have the same color

   Packets      PPPPPPPPPP PPPPPPPPP  PPPPPPPPPP PPPPPPPPPP PPPPPPPPPP
      Time   ---------------------------------------------------------->
               |          |          |          |          |
               | Block 1  | Block 2  | Block 3  | Block 4  | Block 5 ...
               |          |          |          |          |
                    ^          ^          ^           ^        ^
     Packets        |          |          |           |        |
     marked for     |          |          |           |        |
     DM and LM      |          |          |           |        |
                    v          v          v           v        v
   Marking bit  0000100000 0000100000 0000100000 0000010000 0001000000


                      Figure 9: Pulse marking method.

6.  Zero Marking Hashed

6.1.  Hash-based Sampling

   Hash based selection [RFC5475] is a well-known method for sampling a
   subset of packets.  As defined in [RFC5475]:

      A Hash Function h maps the Packet Content c, or some portion of
      it, onto a Hash Range R.  The packet is selected if h(c) is an
      element of S, which is a subset of R called the Hash Selection
      Range.

   Hash-based selection can be leveraged as a marking method, allowing a
   zero-bit marking approach.  Specifically, the pulse and step
   abstractions can be implemented using hashed selection:

   o  Hashed pulse-based trigger: in this approach, a packet is selected
      if h(c) is an element of S, which is a strict subset of the hash
      range R.  When |S|<<|R|, the average sampling period is long,
      reducing the probability of ambiguity between consecutive
      packets. |S| and |R| denote the number of elements in S and R,
      respectively.

   o  Hashed step-based trigger: the hash values of a given traffic flow
      are said to be monotonically increasing if for two packets p1 and
      p2, if p1 is sent before p2 then h(p1)<=h(p2).  If it is
      guaranteed that the hash values of a flow are monotonically
      increasing, then a step-based approach can be used on the range R.
      For example, in an IPv4 flow the Identification field can be used
      as the hash value of each packet.  Since the Identification field
      is monotonically increasing, the step-based trigger can be



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      implemented using consecutive ranges of the Identification value.
      For example, the fourth bit of the Identification field is toggled
      every 8 packets.  Thus, a possible hash function simply takes the
      fourth bit of the Identification field as the hash value.  This
      hash value is toggled every 8 packets, simulating the alternate
      marking behavior of Section 4.

   Note that as opposed to the double marking and single marking
   methods, hashed sampling is not based on fixed time intervals, as the
   duration between sampled packets depends only on the hash value.

   It is also important to note that all methods that use hash-based
   marking require the hash function and the set S to be configured
   consistently across the MPs.

6.1.1.  Hashed Pulse Marking

   In this approach a hash is computed over the packet content, and both
   LM and DM are triggered based on the pulse-based trigger
   (Section 6.1).  A pulse is detected when the hash value h(c) is equal
   to one of the values in S.  The hash function h and the set S
   determine the probability (or frequency) of the pulse event.

6.1.2.  Hashed Step Marking

   As in the previous approach, hashed step marking also uses a hash
   that is computed over the packet content.  In this approach DM is
   performed using a pulse-based trigger, whereas the LM trigger is
   step-based (Section 6.1).  The main drawback of this method is that
   the step-based trigger is possible only under the assumption that the
   hash function is monotonically increasing, which is not necessarily
   possible in all cases.  Specifically, a measured flow is not
   necessarily an IPv4 5-tuple.  For example, a measured flow may
   include multiple IPv4 5-tuple flows, and in this case the
   Identification field is not monotonically increasing.

7.  Single Marking Hashed

   Mixed hashed marking combines the single marking approach with hash-
   based sampling.  A single marking bit is used in the packet header as
   a color indicator, while a hash-based pulse is used to trigger DM.
   Although this method requires a single bit, it is described in this
   section as it is closely related to the other hash-based methods that
   require zero marking bits.

   The hash-based selection for DM can be applied in one of two possible
   approaches: the basic approach, and the dynamic approach.  In the
   basic approach, packets forwarded between two MPs, MP1 and MP2, are



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   selected using a hash function, as described above.  One of the
   challenges is that the frequency of the sampled packets may vary
   considerably, making it difficult for the management system to
   correlate samples from the two MPs.  Thus, the dynamic approach can
   be used.

   In the dynamic hash-based sampling, alternate marking is used to
   create divide time into periods, so that hash-based samples are
   divided into batches, allowing to anchor the selected samples to
   their period.  Moreover, by dynamically adapting the length of the
   hash value, the number of samples is bounded in each marking period.
   This can be realized by choosing first the maximum number of samples
   (NMAX) to be used with the initial hash length.  The algorithm starts
   with only few hash bits, that permit to select a greater percentage
   of packets (e.g. with 1 bit of hash half of the packets are sampled).
   When the number of selected packets reaches NMAX, a hashing bit is
   added.  As a consequence, the sampling proceeds at half of the
   original rate and the packets already selected that do not match the
   new hash are discarded.  This step can be repeated iteratively.  It
   is assumed that each sample includes the timestamp (used for DM) and
   the hash value, allowing the management system to match the samples
   received from the two MPs.

   The dynamic process statistically converges at the end of a marking
   period and the number of selected samples beyond the initial NMAX
   samples mentioned above is between NMAX/2 and NMAX.  Therefore, the
   dynamic approach paces the sampling rate, allowing to bound the
   number of sampled packets per sampling period.

8.  Summary of Marking Methods

   This section summarizes the marking methods described in this memo.
   Each row in the table of Figure 10 represents a marking method.  For
   each method the table specifies the number of bits required in the
   header, the number of counters per flow for LM, the methods used for
   LM and DM (pulse or step), and also the resilience to disturbances.















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   +--------------+----+----+------+------+-------------+-------------+
   | Method       |# of|# of|LM    |DM    |Resilience to|Resilience to|
   |              |bits|coun|Method|Method|Reordering   |Packet drops |
   |              |    |ters|      |      +------+------+------+------+
   |              |    |    |      |      |  LM  |  DM  |  LM  |  DM  |
   +--------------+----+----+------+------+------+------+------+------+
   |Single marking| 1  | 2  |Step  |Step  |  +   |  --  |  +   |  --  |
   |- 1st packet  |    |    |      |      |      |      |      |      |
   +--------------+----+----+------+------+------+------+------+------+
   |Single marking| 1  | 2  |Step  |Mean  |  +   |  +   |  +   |  -   |
   |- mean delay  |    |    |      |      |      |      |      |      |
   +--------------+----+----+------+------+------+------+------+------+
   |Double marking| 2  | 2  |Step  |Pulse |  +   |  +   |  +   |  =   |
   +--------------+----+----+------+------+------+------+------+------+
   |Single marking| 1  | 2  |Step  |Pulse |  +   |  +   |  +   |  =   |
   |multiplexed   |    |    |      |      |      |      |      |      |
   +--------------+----+----+------+------+------+------+------+------+
   |Pulse marking | 1  | 1  |Pulse |Pulse |  --  |  +   |  -   |  =   |
   +--------------+----+----+------+------+------+------+------+------+
   |Zero marking  | 0  | 1  |Hashed|Hashed|  --  |  +   |  -   |  +   |
   |hashed        |    |(2) |pulse |pulse | (-)  |      |      |      |
   |              |    |    |(step)|      |      |      |      |      |
   +--------------+----+----+------+------+------+------+------+------+
   |Single marking| 1  | 2  |Step  |Hashed|  +   |  +   |  +   |  +   |
   |hashed        |    |    |      |pulse |      |      |      |      |
   +--------------+----+----+------+------+------+------+------+------+

   +  Accurate measurement.
   =  Invalidate only if a measured packet is lost (detectable)
   -  No measurement in case of disturbance (detectable).
   -- False measurement in case of disturbance (not detectable).

              Figure 10: Detailed Summary of Marking Methods

   In the context of this comparison two possible disturbances are
   considered: out-of-order delivery, and packet drops.  Generally
   speaking, pulse based methods are sensitive to packet drops, since if
   the marked packet is dropped no measurement is recorded in the
   current period.  Notably, a missing measurement is detectable by the
   management system, and is not as severe as a false measurement.
   Step-based triggers are generally resilient to out-of-order delivery
   for LM, but are not resilient to out-of-order delivery for DM.
   Notably, a step-based trigger may yield a false delay measurement
   when packets are delivered out-of-order, and this inaccuracy is not
   detectable.

   As mentioned above, the double marking method is the most
   straightforward approach, and is resilient to most of the



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   disturbances that were analyzed.  Its obvious drawback is that it
   requires two marking bits.

   Several single marking methods are discussed in this memo.  In this
   case there is no clear verdict which method is the optimal one.  The
   first packet method may be simple to implement, but may present
   erroneous delay measurements in case of dropped or reordered packets.
   Arguably, the mean delay approach and the multiplexed approach may be
   more difficult to implement (depending on the underlying platform),
   but are more resilient to the disturbances that were considered here.
   Note that the computational complexity of the mean delay approach can
   be reduced by combining it with a hashed approach, i.e., by computing
   the mean delay over a hash-based subset of the packets.  The pulse
   marking method requires only a single counter per flow, while the
   other methods require two counters per flow.

   The hash-based sampling approaches reduce the overhead to zero bits,
   which is a significant advantage.  However, the sampling period in
   these approaches is not associated with a fixed time interval.
   Therefore, in some cases adjacent packets may be selected for the
   sampling, potentially causing measurement errors.  Furthermore, when
   the traffic rate is low, measurements may become signifcantly
   infrequent.

   It should be noted that most of the marking methods that were
   presented in this memo are intended for point-to-point measurements,
   e.g., from MP1 to MP2 in Figure 11.  In point-to-multipoint
   measurements, the mean delay method can be used to measure the loss
   and delay of the entire point-to-multipoint flow (which includes all
   the traffic from MP3 to either MP4 or MP5), while other methods such
   as double marking can be used to measure the point-to-point
   performance, for example from MP3 to MP5.  Alternate marking in
   multipoint scenarios is discussed in detail in
   [I-D.fioccola-ippm-multipoint-alt-mark].

















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       MP1            MP2               MP3                 MP4
      +--+           +--+              +--+      +--+      +--+
      |  |---------->|  |              |  |----->|  |----->|  |
      +--+           +--+              +--+      +--+      +--+
                                                   |
                                                   |        MP5
                                                   |       +--+
                                                   +------>|  |
                                                           +--+

   Point-to-point measurement        Point-to-multipoint measurement


      Figure 11: Point-to-point and point-to-multipoint measurements.

   It is clear from the previous table that packet loss measurement can
   be considered resilient to both reordering and packet drops if at
   least one bit is used with a step-based approach.  Thus, since the
   packet loss can be considered obvious, the previous table can be
   simplified into Figure 12, where only the characteristics of delay
   measurements are highlighted, along with multipoint-to-multipoint
   delay measurement compatibility (refer to
   [I-D.fioccola-ippm-multipoint-alt-mark] for more details).




























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   +--------------+----+--------+------------+------------+-----------+
   | Marking      |# of|LM      |DM          |DM          |DM         |
   | Method       |bits|on      |Resilience  |Resilience  |Multipoint |
   |              |    |All     |to          |to          |compatible |
   |              |    |Packets |Reordering  |Packet drops|           |
   +--------------+----+--------+------------+------------+-----------+
   |Single marking| 1  | Yes    |     --     |     -      | No        |
   |- 1st packet  |    |        |            |            |           |
   +--------------+----+--------+------------+------------+-----------+
   |Single marking| 1  | Yes    |     +      |     -      | Yes       |
   |- mean delay  |    |        |            |            |           |
   +--------------+----+--------+------------+------------+-----------+
   |Double marking| 2  | Yes    |     +      |     =      | No        |
   +--------------+----+--------+------------+------------+-----------+
   |Single marking| 1  | Yes    |     +      |     =      | No        |
   |multiplexed   |    |        |            |            |           |
   +--------------+----+--------+------------+------------+-----------+
   |Pulse marking | 1  | No     |     +      |     =      | No        |
   +--------------+----+--------+------------+------------+-----------+
   |Zero marking  | 0  | No     |     +      |     +      | Yes       |
   |hashed        |    |        |            |            |           |
   |              |    |        |            |            |           |
   +--------------+----+--------+------------+------------+-----------+
   |Single marking| 1  | Yes    |     +      |     +      | Yes       |
   |hashed        |    |        |            |            |           |
   +--------------+----+--------+------------+------------+-----------+

   +  Accurate measurement.
   =  Invalidate only if a measured packet is lost (detectable)
   -  No measurement in case of disturbance (detectable).
   -- False measurement in case of disturbance (not detectable).

     Figure 12: Summary of Marking Methods: focus on Delay Measurement

   In the context of delay measurement, both zero marking hashed and
   single marking hashed are resilient to packet drops.  Using double
   marking it could also be possible to perform an accurate measurement
   in case of packet drops, as long as the packet that is marked for DM
   is not dropped.

   The single marking hashed method seems the most complete approach,
   especially because it is also compatible with multipoint-to-
   multipoint measurements.








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9.  Alternate Marking using Reserved Values

   As mentioned in Section 1, a marking bit is not necessarily a single
   bit, but may be implemented by using two well-known values in one of
   the header fields.  Similarly, two-bit marking can be implemented
   using four reserved values.

   A notable example is MPLS Synonymous Flow Labels (SFL), as defined in
   [I-D.ietf-mpls-rfc6374-sfl].  Two MPLS Label values can be used to
   indicate the two colors of a given LSP: the original Label value, and
   an SFL value.  A similar approach can be applied to IPv6 using the
   Flow Label field.

   The following example illustrates how alternate marking can be
   implemented using reserved values.  The bit multiplexing approach of
   Section 5.3 is applicable not only to single-bit color indicators,
   but also to two-value indicators; instead of using a single bit that
   is toggled between '0' and '1', two values of the indicator field, U
   and W, can be used in the same manner, allowing both loss and delay
   measurement to be performed using only two reserved values.  Thus,
   the multiplexing approach of Figure 6 can be illustrated more
   generally with two values, U and W, as depicted in Figure 13.


   A: packet with color 0
   B: packet with color 1

   Packets      AAAAAAAAAA BBBBBBBBBB AAAAAAAAAA BBBBBBBBBB AAAAAAAAAA
      Time   ---------------------------------------------------------->
               |          |          |          |          |
               | Block 1  | Block 2  | Block 3  | Block 4  | Block 5 ...
               |          |          |          |          |
   Color        0000000000 1111111111 0000000000 1111111111 0000000000
                    ^          ^          ^           ^        ^
     Packets        |          |          |           |        |
     marked for     |          |          |           |        |
     timestamping   |          |          |           |        |
                    v          v          v           v        v
   Muxed        UUUUWUUUUU WWWWUWWWWW UUUUWUUUUU WWWWWUWWWW UUUWUUUUUU
   marking
   values

    Figure 13: Alternate marking with two multiplexed marking values, U
                                  and W.







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10.  IANA Considerations

   This memo includes no requests from IANA.

11.  Security Considerations

   The security considerations of the alternate marking method are
   discussed in [RFC8321].  The analysis of Section 8 emphasizes the
   sensitivity of some of the alternate marking methods to packet drops
   and to packet reordering.  Thus, a malicious attacker may attempt to
   tamper with the measurements by either selectively dropping packets,
   or by selectively reordering specific packets.  The multiplexed
   marking method Section 5.3 that is defined in this document requires
   slightly more stringent synchronization than the conventional marking
   method, potentially making the method more vulnerable to attacks on
   the time synchronization protocol.  A detailed discussion about the
   threats against time protocols and how to mitigate them is presented
   in [RFC7384].

12.  References

12.1.  Normative References

   [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>.

   [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>.

12.2.  Informative References

   [I-D.fioccola-ippm-multipoint-alt-mark]
              Fioccola, G., Cociglio, M., Sapio, A., and R. Sisto,
              "Multipoint Alternate Marking method for passive and
              hybrid performance monitoring", draft-fioccola-ippm-
              multipoint-alt-mark-04 (work in progress), June 2018.

   [I-D.ietf-mpls-rfc6374-sfl]
              Bryant, S., Chen, M., Li, Z., Swallow, G., Sivabalan, S.,
              Mirsky, G., and G. Fioccola, "RFC6374 Synonymous Flow
              Labels", draft-ietf-mpls-rfc6374-sfl-02 (work in
              progress), June 2018.




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   [I-D.ietf-mpls-sfl-framework]
              Bryant, S., Chen, M., Li, Z., Swallow, G., Sivabalan, S.,
              and G. Mirsky, "Synonymous Flow Label Framework", draft-
              ietf-mpls-sfl-framework-03 (work in progress), June 2018.

   [IEEE1588]
              IEEE, "IEEE 1588 Standard for a Precision Clock
              Synchronization Protocol for Networked Measurement and
              Control Systems Version 2", 2008.

   [RFC5474]  Duffield, N., Ed., Chiou, D., Claise, B., Greenberg, A.,
              Grossglauser, M., and J. Rexford, "A Framework for Packet
              Selection and Reporting", RFC 5474, DOI 10.17487/RFC5474,
              March 2009, <https://www.rfc-editor.org/info/rfc5474>.

   [RFC5475]  Zseby, T., Molina, M., Duffield, N., Niccolini, S., and F.
              Raspall, "Sampling and Filtering Techniques for IP Packet
              Selection", RFC 5475, DOI 10.17487/RFC5475, March 2009,
              <https://www.rfc-editor.org/info/rfc5475>.

   [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>.

   [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>.

Authors' Addresses

   Tal Mizrahi
   Huawei Network.IO Innovation Lab
   Israel

   Email: tal.mizrahi.phd@gmail.com


   Carmi Arad

   Email: carmi.arad@gmail.com


   Giuseppe Fioccola
   Huawei Technologies

   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


   Mach(Guoyi) Chen
   Huawei Technologies

   Email: mach.chen@huawei.com


   Lianshu Zheng
   Huawei Technologies

   Email: vero.zheng@huawei.com


   Greg Mirsky
   ZTE Corp.

   Email: gregimirsky@gmail.com


























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