RFC9343: IPv6 Application of the Alternate-Marking Method

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Internet Engineering Task Force (IETF)                       G. Fioccola
Request for Comments: 9343                                       T. Zhou
Category: Standards Track                                         Huawei
ISSN: 2070-1721                                              M. Cociglio
                                                          Telecom Italia
                                                                  F. Qin
                                                            China Mobile
                                                                 R. Pang
                                                            China Unicom
                                                           December 2022

            IPv6 Application of the Alternate-Marking Method


   This document describes how the Alternate-Marking Method can be used
   as a passive performance measurement tool in an IPv6 domain.  It
   defines an Extension Header Option to encode Alternate-Marking
   information in both the Hop-by-Hop Options Header and Destination
   Options Header.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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

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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology
     1.2.  Requirements Language
   2.  Alternate-Marking Application to IPv6
     2.1.  Controlled Domain
       2.1.1.  Alternate-Marking Measurement Domain
   3.  Definition of the AltMark Option
     3.1.  Data Fields Format
   4.  Use of the AltMark Option
   5.  Alternate-Marking Method Operation
     5.1.  Packet Loss Measurement
     5.2.  Packet Delay Measurement
     5.3.  Flow Monitoring Identification
     5.4.  Multipoint and Clustered Alternate Marking
     5.5.  Data Collection and Calculation
   6.  Security Considerations
   7.  IANA Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Authors' Addresses

1.  Introduction

   [RFC9341] and [RFC9342] describe a passive performance measurement
   method, which can be used to measure packet loss, latency, and jitter
   on live traffic.  Since this method is based on marking consecutive
   batches of packets, the method is often referred to as the Alternate-
   Marking Method.

   This document defines how the Alternate-Marking Method can be used to
   measure performance metrics in IPv6.  The rationale is to apply the
   Alternate-Marking methodology to IPv6 and therefore allow detailed
   packet loss, delay, and delay variation measurements both hop by hop
   and end to end to exactly locate the issues in an IPv6 network.

   Alternate Marking is an on-path telemetry technique and consists of
   synchronizing the measurements in different points of a network by
   switching the value of a marking bit and therefore dividing the
   packet flow into batches.  Each batch represents a measurable entity
   recognizable by all network nodes along the path.  By counting the
   number of packets in each batch and comparing the values measured by
   different nodes, it is possible to precisely measure the packet loss.
   Similarly, the alternation of the values of the marking bits can be
   used as a time reference to calculate the delay and delay variation.
   The Alternate-Marking operation is further described in Section 5.

   This document introduces a TLV (type-length-value) that can be
   encoded in the Options Headers (Hop-by-Hop or Destination), according
   to [RFC8200], for the purpose of the Alternate-Marking Method
   application in an IPv6 domain.

   The Alternate-Marking Method MUST be applied to IPv6 only in a
   controlled environment, as further described in Section 2.1.
   [RFC8799] provides further discussion of network behaviors that can
   be applied only within limited domains.

   The threat model for the application of the Alternate-Marking Method
   in an IPv6 domain is reported in Section 6.

1.1.  Terminology

   This document uses the terms related to the Alternate-Marking Method
   as defined in [RFC9341] and [RFC9342].

1.2.  Requirements Language

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

2.  Alternate-Marking Application to IPv6

   The Alternate-Marking Method requires a marking field.  Several
   alternatives could be considered such as IPv6 Extension Headers, IPv6
   Address, and Flow Label.  But, it is necessary to analyze the
   drawbacks for all the available possibilities, more specifically:

   *  reusing an existing Extension Header for Alternate Marking leads
      to a non-optimized implementation;

   *  using the IPv6 destination address to encode the Alternate-Marking
      processing is very expensive; and

   *  using the IPv6 Flow Label for Alternate Marking conflicts with the
      utilization of the Flow Label for load distribution purposes

   In the end, a Hop-by-Hop or a Destination Option is the best choice.

   The approach for the Alternate-Marking application to IPv6 specified
   in this memo is compliant with [RFC8200].  It involves the following

   *  The source node is the only one that writes the Options Header to
      mark alternately the flow (for both the Hop-by-Hop and Destination
      Option).  The intermediate nodes and destination node MUST only
      read the marking values of the Option without modifying the
      Options Header.

   *  In case of a Hop-by-Hop Options Header carrying Alternate-Marking
      bits, the Options Header is not inserted or deleted on the path,
      but it can be read by any node along the path.  The intermediate
      nodes may be configured to support this Option or not, and the
      measurement can be done only for the nodes configured to read the
      Option.  As further discussed in Section 4, the presence of the
      Hop-by-Hop Option should not affect the traffic throughput both on
      nodes that do not recognize this Option and on the nodes that
      support it.  However, it is worth mentioning that there is a
      difference between theory and practice.  Indeed, in a real
      implementation, it is possible for packets with a Hop-by-Hop
      Option to be skipped or processed in the slow path.  While some
      proposals are trying to address this problem and make Hop-by-Hop
      Options more practical (see [PROC-HBH-OPT-HEADER] and
      [HBH-OPTIONS-PROCESSING]), these aspects are out of the scope for
      this document.

   *  In case of a Destination Options Header carrying Alternate-Marking
      bits, it is not processed, inserted, or deleted by any node along
      the path until the packet reaches the destination node.  Note
      that, if there is also a Routing Header (RH), any visited
      destination in the route list can process the Options Header.

   A Hop-by-Hop Options Header is also useful to signal to routers on
   the path to process the Alternate Marking.  However, as said, routers
   will only examine this Option if properly configured.

   The optimization of both implementation and the scaling of the
   Alternate-Marking Method is also considered, and a way to identify
   flows is required.  The Flow Monitoring Identification (FlowMonID)
   field, as introduced in Section 5.3, goes in this direction, and it
   is used to identify a monitored flow.

   The FlowMonID is different from the Flow Label field of the IPv6
   header [RFC6437].  The Flow Label field in the IPv6 header is used by
   a source to label sequences of packets to be treated in the network
   as a single flow and, as reported in [RFC6438], it can be used for
   load balancing (LB) and equal-cost multipath (ECMP).  The reuse of
   the Flow Label field for identifying monitored flows is not
   considered because it may change the application intent and
   forwarding behavior.  Also, the Flow Label may be changed en route,
   and this may also invalidate the integrity of the measurement.  Those
   reasons make the definition of the FlowMonID necessary for IPv6.
   Indeed, the FlowMonID is designed and only used to identify the
   monitored flow.  Flow Label and FlowMonID within the same packet are
   totally disjoint, have different scopes, are used to identify flows
   based on different criteria, and are intended for different use

   The rationale for the FlowMonID is further discussed in Section 5.3.
   This 20-bit field allows easy and flexible identification of the
   monitored flow and enables improved measurement correlation and finer
   granularity since it can be used in combination with the conventional
   TCP/IP 5-tuple to identify a flow.  An important point that will be
   discussed in Section 5.3 is the uniqueness of the FlowMonID and how
   to allow disambiguation of the FlowMonID in case of collision.

   The following section highlights an important requirement for the
   application of the Alternate Marking to IPv6.  The concept of the
   controlled domain is explained and is considered an essential
   precondition, as also highlighted in Section 6.

2.1.  Controlled Domain

   IPv6 has much more flexibility than IPv4 and innovative applications
   have been proposed, but for security and compatibility reasons, some
   of these applications are limited to a controlled environment.  This
   is also the case of the Alternate-Marking application to IPv6 as
   assumed hereinafter.  In this regard, [RFC8799] reports further
   examples of specific limited domain solutions.

   The IPv6 application of the Alternate-Marking Method MUST be deployed
   in a controlled domain.  It is not common that the user traffic
   originates and terminates within the controlled domain, as also noted
   in Section 2.1.1.  For this reason, it will typically only be
   applicable in an overlay network, where user traffic is encapsulated
   at one domain border and decapsulated at the other domain border, and
   the encapsulation incorporates the relevant extension header for
   Alternate Marking.  This requirement also implies that an
   implementation MUST filter packets that carry Alternate-Marking data
   and are entering or leaving the controlled domain.

   A controlled domain is a managed network where it is required to
   select, monitor, and control the access to the network by enforcing
   policies at the domain boundaries in order to discard undesired
   external packets entering the domain and check the internal packets
   leaving the domain.  It does not necessarily mean that a controlled
   domain is a single administrative domain or a single organization.  A
   controlled domain can correspond to a single administrative domain or
   can be composed by multiple administrative domains under a defined
   network management.  Indeed, some scenarios may imply that the
   Alternate-Marking Method involves more than one domain, but in these
   cases, it is RECOMMENDED that the multiple domains create a whole
   controlled domain while traversing the external domain by employing
   IPsec [RFC4301] authentication and encryption or other VPN technology
   that provides full packet confidentiality and integrity protection.
   In a few words, it must be possible to control the domain boundaries
   and eventually use specific precautions if the traffic traverses the

   The security considerations reported in Section 6 also highlight this

2.1.1.  Alternate-Marking Measurement Domain

   The Alternate-Marking measurement domain can overlap with the
   controlled domain or may be a subset of the controlled domain.  The
   typical scenarios for the application of the Alternate-Marking Method
   depend on the controlled domain boundaries; in particular:

   *  The user equipment can be the starting or ending node only when/if
      it is fully managed and belongs to the controlled domain.  In this
      case, the user-generated IPv6 packets contain the Alternate-
      Marking data.  But, in practice, this is not common due to the
      fact that the user equipment cannot be totally secured in the
      majority of cases.

   *  The Customer Premises Equipment (CPE) or the Provider Edge (PE)
      routers are most likely to be the starting or ending nodes since
      they can be border routers of the controlled domain.  For
      instance, the CPE, which connects the user's premises with the
      service provider's network, belongs to a controlled domain only if
      it is managed by the service provider and if additional security
      measures are taken to keep it trustworthy.  Typically, the CPE or
      the PE can encapsulate a received packet in an outer IPv6 header,
      which contains the Alternate-Marking data.  They are also able to
      filter and drop packets from outside of the domain with
      inconsistent fields to make effective the relevant security rules
      at the domain boundaries; for example, a simple security check can
      be to insert the Alternate-Marking data if and only if the
      destination is within the controlled domain.

3.  Definition of the AltMark Option

   The definition of a TLV for the Extension Header Option, carrying the
   data fields dedicated to the Alternate-Marking Method, is reported

3.1.  Data Fields Format

   The following figure shows the data fields format for enhanced
   Alternate-Marking TLV (AltMark).  This AltMark data can be
   encapsulated in the IPv6 Options Headers (Hop-by-Hop or Destination

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                   |  Option Type  |  Opt Data Len |
   |              FlowMonID                |L|D|     Reserved      |


   Option Type:  8-bit identifier of the type of Option that needs to be
      allocated.  Unrecognized Types MUST be ignored on processing.  For
      the Hop-by-Hop Options Header or Destination Options Header,
      [RFC8200] defines how to encode the three high-order bits of the
      Option Type field.  The two high-order bits specify the action
      that must be taken if the processing IPv6 node does not recognize
      the Option Type; for AltMark, these two bits MUST be set to 00
      (skip over this Option and continue processing the header).  The
      third-highest-order bit specifies whether the Option Data can
      change en route to the packet's final destination; for AltMark,
      the value of this bit MUST be set to 0 (Option Data does not
      change en route).  In this way, since the three high-order bits of
      the AltMark Option are set to 000, it means that nodes can simply
      skip this Option if they do not recognize it and that the data of
      this Option does not change en route; indeed the source is the
      only one that can write it.

   Opt Data Len:  4.  It is the length of the Option Data Fields of this
      Option in bytes.

   FlowMonID:  20-bit unsigned integer.  The FlowMon identifier is
      described in Section 5.3.  As further discussed below, it has been
      picked as 20 bits since it is a reasonable value and a good
      compromise in relation to the chance of collision.  It MUST be set
      pseudo-randomly by the source node or by a centralized controller.

   L:  Loss flag for Packet Loss Measurement as described in
      Section 5.1.

   D:  Delay flag for Single Packet Delay Measurement as described in
      Section 5.2.

   Reserved:  Reserved for future use.  These bits MUST be set to zero
      on transmission and ignored on receipt.

4.  Use of the AltMark Option

   The AltMark Option is the best way to implement the Alternate-Marking
   Method, and it is carried by the Hop-by-Hop Options Header and the
   Destination Options Header.  In case of Destination Option, it is
   processed only by the source and destination nodes: the source node
   inserts it and the destination node processes it.  In case of the
   Hop-by-Hop Option, it may be examined by any node along the path if
   explicitly configured to do so.

   It is important to highlight that the Option Layout can be used both
   as the Destination Option and as the Hop-by-Hop Option depending on
   the use cases, and it is based on the chosen type of performance
   measurement.  In general, it is needed to perform both end-to-end and
   hop-by-hop measurements, and the Alternate-Marking methodology
   allows, by definition, both performance measurements.  In many cases,
   the end-to-end measurement may not be enough, and the hop-by-hop
   measurement is required.  To meet this need, the most complete choice
   is the Hop-by-Hop Options Header.

   IPv6, as specified in [RFC8200], allows nodes to optionally process
   Hop-by-Hop headers.  Specifically, the Hop-by-Hop Options Header is
   not inserted or deleted, but it may be examined or processed by any
   node along a packet's delivery path, until the packet reaches the
   node (or each of the set of nodes in the case of multicast)
   identified in the Destination Address field of the IPv6 header.
   Also, it is expected that nodes along a packet's delivery path only
   examine and process the Hop-by-Hop Options Header if explicitly
   configured to do so.

   Another scenario is the presence of a Routing Header.  Both Hop-by-
   Hop Options and Destination Options Headers can be used when a
   Routing Header is present.  Depending on where the Destination
   Options are situated in the header chain (before or after the Routing
   Header if any), Destination Options Headers can be processed by
   either intermediate routers specified in the Routing Header or the
   destination node.  As an example, a type of Routing Header, referred
   to as a Segment Routing Header (SRH), has been defined in [RFC8754]
   for the Segment Routing over IPv6 (SRv6) data place, and more details
   about the SRv6 application can be found in [SRv6-AMM].

   In summary, using these tools, it is possible to control on which
   nodes measurement occurs:

   *  Destination Option not preceding a Routing Header => measurement
      only by node in Destination Address

   *  Hop-by-Hop Option => every router on the path with feature enabled

   *  Destination Option preceding a Routing Header => every destination
      node in the route list

   In general, Hop-by-Hop and Destination Options are the most suitable
   ways to implement Alternate Marking.

   It is worth mentioning that Hop-by-Hop Options are not strongly
   recommended in [RFC7045] and [RFC8200], unless there is a clear
   justification to standardize it, because nodes may be configured to
   ignore the Options Header or drop or assign packets containing an
   Options Header to a slow processing path.  In case of the AltMark
   Data Fields described in this document, the motivation to standardize
   a Hop-by-Hop Option is that it is needed for Operations,
   Administration, and Maintenance (OAM).  An intermediate node can read
   it or not, but this does not affect the packet behavior.  The source
   node is the only one that writes the Hop-by-Hop Option to alternately
   mark the flow; therefore, the performance measurement can be done for
   those nodes configured to read this Option, while the others are
   simply not considered for the metrics.

   The Hop-by-Hop Option defined in this document is designed to take
   advantage of the property of how Hop-by-Hop Options are processed.
   Nodes that do not support this Option would be expected to ignore it
   if encountered, according to the procedures of [RFC8200].  This can
   mean that, in this case, the performance measurement does not account
   for all links and nodes along a path.  The definition of the Hop-by-
   Hop Options in this document is also designed to minimize throughput
   impact both on nodes that do not recognize the Option and on nodes
   that support it.  Indeed, the three high-order bits of the Options
   Header defined in this document are 000 and, in theory, as per
   [RFC8200] and [HBH-OPTIONS-PROCESSING], this means "skip if not
   recognized and data does not change en route".  [RFC8200] also
   mentions that the nodes only examine and process the Hop-by-Hop
   Options Header if explicitly configured to do so.  For these reasons,
   this Hop-by-Hop Option should not affect the throughput.  However, in
   practice, it is important to be aware that things may be different in
   the implementation, and it can happen that packets with Hop by Hop
   are forced onto the slow path, but this is a general issue, as also
   explained in [HBH-OPTIONS-PROCESSING].  It is also worth mentioning
   that the application to a controlled domain should avoid the risk of
   arbitrary nodes dropping packets with Hop-by-Hop Options.

5.  Alternate-Marking Method Operation

   This section describes how the method operates.  [RFC9341] introduces
   several applicable methods, which are reported below, and an
   additional field is introduced to facilitate the deployment and
   improve the scalability.

5.1.  Packet Loss Measurement

   The measurement of the packet loss is really straightforward in
   comparison to the existing mechanisms, as detailed in [RFC9341].  The
   packets of the flow are grouped into batches, and all the packets
   within a batch are marked by setting the L bit (Loss flag) to a same
   value.  The source node can switch the value of the L bit between 0
   and 1 after a fixed number of packets or according to a fixed timer,
   and this depends on the implementation.  The source node is the only
   one that marks the packets to create the batches, while the
   intermediate nodes only read the marking values and identify the
   packet batches.  By counting the number of packets in each batch and
   comparing the values measured by different network nodes along the
   path, it is possible to measure the packet loss that occurred in any
   single batch between any two nodes.  Each batch represents a
   measurable entity recognizable by all network nodes along the path.

   Both fixed number of packets and a fixed timer can be used by the
   source node to create packet batches.  But, as also explained in
   [RFC9341], the timer-based batches are preferable because they are
   more deterministic than the counter-based batches.  Unlike the timer-
   based batches, there is no definitive rule for counter-based batches,
   which are not considered in [RFC9341].  Using a fixed timer for the
   switching offers better control over the method; indeed, the length
   of the batches can be chosen large enough to simplify the collection
   and the comparison of the measures taken by different network nodes.
   In the implementation, the counters can be sent out by each node to
   the controller that is responsible for the calculation.  It is also
   possible to exchange this information by using other on-path
   techniques, but this is out of scope for this document.

   Packets with different L values may get swapped at batch boundaries,
   and in this case, it is required that each marked packet can be
   assigned to the right batch by each router.  It is important to
   mention that for the application of this method, there are two
   elements to consider: the clock error between network nodes and the
   network delay.  These can create offsets between the batches and out-
   of-order packets.  The mathematical formula on timing aspects,
   explained in Section 5 of [RFC9341], must be satisfied, and it takes
   into consideration the different causes of reordering such as clock
   error and network delay.  The assumption is to define the available
   counting interval to get stable counters and to avoid these issues.
   Specifically, if the effects of network delay are ignored, the
   condition to implement the methodology is that the clocks in
   different nodes MUST be synchronized to the same clock reference with
   an accuracy of +/- B/2 time units, where B is the fixed time duration
   of the batch.  In this way, each marked packet can be assigned to the
   right batch by each node.  Usually, the counters can be taken in the
   middle of the batch period to be sure to read quiescent counters.  In
   a few words, this implies that the length of the batches MUST be
   chosen large enough so that the method is not affected by those
   factors.  The length of the batches can be determined based on the
   specific deployment scenario.

   L bit=1   ----------+           +-----------+           +----------
                       |           |           |           |
   L bit=0             +-----------+           +-----------+
              Batch n        ...      Batch 3     Batch 2     Batch 1
            <---------> <---------> <---------> <---------> <--------->

                                Traffic Flow
   L bit   ...1111111111 0000000000 11111111111 00000000000 111111111...

      Figure 1: Packet Loss Measurement and Single-Marking Methodology
                                Using L Bit

   It is worth mentioning that the duration of the batches is considered
   stable over time in the previous figure.  In theory, it is possible
   to change the length of batches over time and among different flows
   for more flexibility.  But, in practice, it could complicate the
   correlation of the information.

5.2.  Packet Delay Measurement

   The same principle used to measure packet loss can also be applied to
   one-way delay measurement.  Delay metrics MAY be calculated using the
   following two possibilities:

   Single-Marking Methodology:  This approach uses only the L bit to
      calculate both packet loss and delay.  In this case, the D flag
      MUST be set to zero on transmit and ignored by the monitoring
      points.  The alternation of the values of the L bit can be used as
      a time reference to calculate the delay.  Whenever the L bit
      changes and a new batch starts, a network node can store the
      timestamp of the first packet of the new batch; that timestamp can
      be compared with the timestamp of the first packet of the same
      batch on a second node to compute packet delay.  But, this
      measurement is accurate only if no packet loss occurs and if there
      is no packet reordering at the edges of the batches.  A different
      approach can also be considered, and it is based on the concept of
      the mean delay.  The mean delay for each batch is calculated by
      considering the average arrival time of the packets for the
      relative batch.  There are limitations also in this case indeed;
      each node needs to collect all the timestamps and calculate the
      average timestamp for each batch.  In addition, the information is
      limited to a mean value.

   Double-Marking Methodology:  This approach is more complete and uses
      the L bit only to calculate packet loss, and the D bit (Delay
      flag) is fully dedicated to delay measurements.  The idea is to
      use the first marking with the L bit to create the alternate flow
      and, within the batches identified by the L bit, a second marking
      is used to select the packets for measuring delay.  The D bit
      creates a new set of marked packets that are fully identified over
      the network so that a network node can store the timestamps of
      these packets; these timestamps can be compared with the
      timestamps of the same packets on a second node to compute packet
      delay values for each packet.  The most efficient and robust mode
      is to select a single double-marked packet for each batch; in this
      way, there is no time gap to consider between the double-marked
      packets to avoid their reorder.  Regarding the rule for the
      selection of the packet to be double-marked, the same
      considerations in Section 5.1 also apply here, and the double-
      marked packet can be chosen within the available counting interval
      that is not affected by factors such as clock errors.  If a
      double-marked packet is lost, the delay measurement for the
      considered batch is simply discarded, but this is not a big
      problem because it is easy to recognize the problematic batch and
      skip the measurement just for that one.  So in order to have more
      information about the delay and to overcome out-of-order issues,
      this method is preferred.

   In summary, the approach with Double Marking is better than the
   approach with Single Marking.  Moreover, the two approaches provide
   slightly different pieces of information, and the data consumer can
   combine them to have a more robust data set.

   Similar to what is said in Section 5.1 for the packet counters, in
   the implementation, the timestamps can be sent out to the controller
   that is responsible for the calculation or exchanged using other on-
   path techniques.  But, this is out of scope for this document.

   L bit=1   ----------+           +-----------+           +----------
                       |           |           |           |
   L bit=0             +-----------+           +-----------+

   D bit=1         +          +          +          +            +
                   |          |          |          |            |
   D bit=0   ------+----------+----------+----------+------------+-----

                                Traffic Flow
   L bit   ...1111111111 0000000000 11111111111 00000000000 111111111...

   D bit   ...0000010000 0000010000 00000100000 00001000000 000001000...

         Figure 2: Double-Marking Methodology Using L Bit and D Bit

   Likewise, to packet delay measurement (both for Single Marking and
   Double Marking), the method can also be used to measure the inter-
   arrival jitter.

5.3.  Flow Monitoring Identification

   The Flow Monitoring Identification (FlowMonID) identifies the flow to
   be measured and is required for some general reasons:

   *  First, it helps to reduce the per-node configuration.  Otherwise,
      each node needs to configure an access control list (ACL) for each
      of the monitored flows.  Moreover, using a flow identifier allows
      a flexible granularity for the flow definition; indeed, it can be
      used together with other identifiers (e.g., 5-tuple).

   *  Second, it simplifies the counters handling.  Hardware processing
      of flow tuples (and ACL matching) is challenging and often incurs
      into performance issues, especially in tunnel interfaces.

   *  Third, it eases the data export encapsulation and correlation for
      the collectors.

   The FlowMonID MUST only be used as a monitored flow identifier in
   order to determine a monitored flow within the measurement domain.
   This entails not only an easy identification but improved correlation
   as well.

   The FlowMonID allocation procedure can be stateful or stateless.  In
   case of a stateful approach, it is required that the FlowMonID
   historic information can be stored and tracked in order to assign
   unique values within the domain.  This may imply a complex procedure,
   and it is considered out of scope for this document.  The stateless
   approach is described hereinafter where FlowMonID values are pseudo-
   randomly generated.

   The value of 20 bits has been selected for the FlowMonID since it is
   a good compromise and implies a low rate of ambiguous FlowMonIDs that
   can be considered acceptable in most of the applications.  The
   disambiguation issue can be solved by tagging the pseudo-randomly
   generated FlowMonID with additional flow information.  In particular,
   it is RECOMMENDED to consider the 3-tuple FlowMonID, source, and
   destination addresses:

   *  If the 20-bit FlowMonID is set independently and pseudo-randomly
      in a distributed way, there is a chance of collision.  Indeed, by
      using the well-known birthday problem in probability theory, if
      the 20-bit FlowMonID is set independently and pseudo-randomly
      without any additional input entropy, there is a 50% chance of
      collision for 1206 flows.  So, for more entropy, FlowMonID is
      combined with source and destination addresses.  Since there is a
      1% chance of collision for 145 flows, it is possible to monitor
      145 concurrent flows per host pairs with a 1% chance of collision.

   *  If the 20-bit FlowMonID is set pseudo-randomly but in a
      centralized way, the controller can instruct the nodes properly in
      order to guarantee the uniqueness of the FlowMonID.  With 20 bits,
      the number of combinations is 1048576, and the controller should
      ensure that all the FlowMonID values are used without any
      collision.  Therefore, by considering source and destination
      addresses together with the FlowMonID, it is possible to monitor
      1048576 concurrent flows per host pairs.

   A consistent approach MUST be used in the Alternate-Marking
   deployment to avoid the mixture of different ways of identifying.
   All the nodes along the path and involved in the measurement SHOULD
   use the same mode for identification.  As mentioned, it is
   RECOMMENDED to use the FlowMonID for identification purposes in
   combination with source and destination addresses to identify a flow.
   By considering source and destination addresses together with the
   FlowMonID, it is possible to monitor 145 concurrent flows per host
   pairs with a 1% chance of collision in case of pseudo-randomly
   generated FlowMonID, or 1048576 concurrent flows per host pairs in
   case of a centralized controller.  It is worth mentioning that the
   solution with the centralized control allows finer granularity and
   therefore adds even more flexibility to the flow identification.

   The FlowMonID field is set at the source node, which is the ingress
   point of the measurement domain, and can be set in two ways:

   *  It can be algorithmically generated by the source node, which can
      set it pseudo-randomly with some chance of collision.  This
      approach cannot guarantee the uniqueness of FlowMonID since
      conflicts and collisions are possible.  But, considering the
      recommendation to use FlowMonID with source and destination
      addresses, the conflict probability is reduced due to the
      FlowMonID space available for each endpoint pair (i.e., 145 flows
      with 1% chance of collision).

   *  It can be assigned by the central controller.  Since the
      controller knows the network topology, it can allocate the value
      properly to avoid or minimize ambiguity and guarantee the
      uniqueness.  In this regard, the controller can verify that there
      is no ambiguity between different pseudo-randomly generated
      FlowMonIDs on the same path.  The conflict probability is really
      small given that the FlowMonID is coupled with source and
      destination addresses, and up to 1048576 flows can be monitored
      for each endpoint pair.  When all values in the FlowMonID space
      are consumed, the centralized controller can keep track and
      reassign the values that are not used any more by old flows.

   If the FlowMonID is set by the source node, the intermediate nodes
   can read the FlowMonIDs from the packets in flight and act
   accordingly.  If the FlowMonID is set by the controller, both
   possibilities are feasible for the intermediate nodes, which can
   learn by reading the packets or can be instructed by the controller.

   The FlowMonID setting by the source node may seem faster and more
   scalable than the FlowMonID setting by the controller.  But, it is
   supposed that the controller does not slow the process since it can
   enable the Alternate-Marking Method and its parameters (like
   FlowMonID) together with the flow instantiation, as further described

5.4.  Multipoint and Clustered Alternate Marking

   The Alternate-Marking Method can be extended to any kind of
   multipoint-to-multipoint paths.  [RFC9341] only applies to point-to-
   point unicast flows, while the Clustered Alternate-Marking Method,
   introduced in [RFC9342], is valid for multipoint-to-multipoint
   unicast flows, anycast, and ECMP flows.

   [RFC9342] describes the network clustering approach, which allows a
   flexible and optimized performance measurement.  A cluster is the
   smallest identifiable non-trivial subnetwork of the entire network
   graph that still satisfies the condition that the number of packets
   that goes in is the same number that goes out.  With network
   clustering, it is possible to partition the network into clusters at
   different levels in order to perform the needed degree of detail.

   For Multipoint Alternate Marking, FlowMonID can identify in general a
   multipoint-to-multipoint flow and not only a point-to-point flow.

5.5.  Data Collection and Calculation

   The nodes enabled to perform performance monitoring collect the value
   of the packet counters and timestamps.  There are several
   alternatives to implement data collection and calculation, but this
   is not specified in this document.

   There are documents on the control plane mechanisms of Alternate
   Marking, e.g., [BGP-SR-POLICY-IFIT] and [PCEP-IFIT].

6.  Security Considerations

   This document aims to apply a method to the performance measurements
   that does not directly affect Internet security nor applications that
   run on the Internet.  However, implementation of this method must be
   mindful of security and privacy concerns.

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

   Harm caused by the measurement:  Alternate Marking implies the
      insertion of an Options Header to the IPv6 packets by the source
      node, but this must be performed in a way that does not alter the
      quality of service experienced by the packets and that preserves
      stability and performance of routers doing the measurements.  As
      already discussed in Section 4, the design of the AltMark Option
      has been chosen with throughput in mind, such that it can be
      implemented without affecting the user experience.

   Harm to the measurement:  Alternate-Marking measurements could be
      harmed by routers altering the fields of the AltMark Option (e.g.,
      marking of the packets or FlowMonID) or by a malicious attacker
      adding the AltMark Option to the packets in order to consume the
      resources of network devices and entities involved.  As described
      above, the source node is the only one that writes the Options
      Header while the intermediate nodes and destination node only read
      it without modifying the Options Header.  But, for example, an on-
      path attacker can modify the flags, whether intentionally or
      accidentally, or deliberately insert an Option to the packet flow
      or delete the Option from the packet flow.  The consequent effect
      could be to give the appearance of loss or delay or to invalidate
      the measurement by modifying Option identifiers, such as
      FlowMonID.  The malicious implication can be to cause actions from
      the network administrator where an intervention is not necessary
      or to hide real issues in the network.  Since the measurement
      itself may be affected by network nodes intentionally altering the
      bits of the AltMark Option or injecting Options Headers as a means
      for Denial of Service (DoS), the Alternate Marking MUST be applied
      in the context of a controlled domain, where the network nodes are
      locally administered and this type of attack can be avoided.  For
      this reason, the implementation of the method is not done on the
      end node if it is not fully managed and does not belong to the
      controlled domain.  Packets generated outside the controlled
      domain may consume router resources by maliciously using the Hop-
      by-Hop Option, but this can be mitigated by filtering these
      packets at the controlled domain boundary.  This can be done
      because if the end node does not belong to the controlled domain,
      it is not supposed to add the AltMark Hop-by-Hop Option, and it
      can be easily recognized.

   An attacker that does not belong to the controlled domain can
   maliciously send packets with the AltMark Option.  But, if Alternate
   Marking is not supported in the controlled domain, no problem happens
   because the AltMark Option is treated as any other unrecognized
   Option and will not be considered by the nodes since they are not
   configured to deal with it; so, the only effect is the increased
   packet size (by 48 bits).  If Alternate Marking is supported in the
   controlled domain, it is necessary to keep the measurements from
   being affected, and external packets with the AltMark Option MUST be
   filtered.  As any other Hop-by-Hop Options or Destination Options, it
   is possible to filter AltMark Options entering or leaving the domain,
   e.g., by using ACL extensions for filtering.

   The flow identifier (FlowMonID), together with the two marking bits
   (L and D), comprises the AltMark Option.  As explained in
   Section 5.3, there is a chance of collision if the FlowMonID is set
   pseudo-randomly, but there is a solution for this issue.  In general,
   this may not be a problem, and a low rate of ambiguous FlowMonIDs can
   be acceptable since this does not cause significant harm to the
   operators or their clients, and this harm may not justify the
   complications of avoiding it.  But, for large scale measurements, a
   big number of flows could be monitored and the probability of a
   collision is higher; thus, the disambiguation of the FlowMonID field
   can be considered.

   The privacy concerns also need to be analyzed even if the method only
   relies on information contained in the Options Header without any
   release of user data.  Indeed, from a confidentiality perspective,
   although the AltMark Option does not contain user data, the metadata
   can be used for network reconnaissance to compromise the privacy of
   users by allowing attackers to collect information about network
   performance and network paths.  The AltMark Option contains two kinds
   of metadata: the marking bits (L and D) and the flow identifier

   *  The marking bits are the small information that is exchanged
      between the network nodes.  Therefore, due to this intrinsic
      characteristic, network reconnaissance through passive
      eavesdropping on data plane traffic is difficult.  Indeed, an
      attacker cannot gain information about network performance from a
      single monitoring point.  The only way for an attacker can be to
      eavesdrop on multiple monitoring points at the same time, because
      they have to do the same kind of calculation and aggregation as
      Alternate Marking requires.

   *  The FlowMonID field is used in the AltMark Option as the
      identifier of the monitored flow.  It represents more sensitive
      information for network reconnaissance and may allow a flow
      tracking type of attack because an attacker could collect
      information about network paths.

   Furthermore, in a pervasive surveillance attack, the information that
   can be derived over time is more.  But, as further described
   hereinafter, the application of the Alternate Marking to a controlled
   domain helps to mitigate all the above aspects of privacy concerns.

   At the management plane, attacks can be set up by misconfiguring or
   by maliciously configuring the AltMark Option.  Thus, AltMark Option
   configuration MUST be secured in a way that authenticates authorized
   users and verifies the integrity of configuration procedures.
   Solutions to ensure the integrity of the AltMark Option are outside
   the scope of this document.  Also, attacks on the reporting of the
   statistics between the monitoring points and the network management
   system (e.g., centralized controller) can interfere with the proper
   functioning of the system.  Hence, the channels used to report back
   flow statistics MUST be secured.

   As stated above, the precondition for the application of the
   Alternate Marking is that it MUST be applied in specific controlled
   domains, thus confining the potential attack vectors within the
   network domain.  A limited administrative domain provides the network
   administrator with the means to select, monitor, and control the
   access to the network, making it a trusted domain.  In this regard,
   it is expected to enforce policies at the domain boundaries to filter
   both external packets with the AltMark Option entering the domain and
   internal packets with the AltMark Option leaving the domain.
   Therefore, the trusted domain is unlikely subject to the hijacking of
   packets since packets with AltMark Option are processed and used only
   within the controlled domain.

   As stated, the application to a controlled domain ensures control
   over the packets entering and leaving the domain, but despite that,
   leakages may happen for different reasons such as a failure or a
   fault.  In this case, nodes outside the domain are expected to ignore
   packets with the AltMark Option since they are not configured to
   handle it and should not process it.

   Additionally, note that the AltMark Option is carried by the Options
   Header and it will have some impact on the packet sizes for the
   monitored flow and on the path MTU since some packets might exceed
   the MTU.  However, the relative small size (48 bits in total) of
   these Options Headers and its application to a controlled domain help
   to mitigate the problem.

   It is worth mentioning that the security concerns may change based on
   the specific deployment scenario and related threat analysis, which
   can lead to specific security solutions that are beyond the scope of
   this document.  As an example, the AltMark Option can be used as a
   Hop-by-Hop or Destination Option and, in case of a Destination
   Option, multiple administrative domains may be traversed by the
   AltMark Option that is not confined to a single administrative
   domain.  In this case, the user, who is aware of the kind of risks,
   may still want to use Alternate Marking for telemetry and test
   purposes, but the controlled domain must be composed by more than one
   administrative domain.  To this end, the inter-domain links need to
   be secured (e.g., by IPsec or VPNs) in order to avoid external
   threats and realize the whole controlled domain.

   It might be theoretically possible to modulate the marking or the
   other fields of the AltMark Option to serve as a covert channel to be
   used by an on-path observer.  This may affect both the data and
   management plane, but, here too, the application to a controlled
   domain helps to reduce the effects.

   The Alternate-Marking application described in this document relies
   on a time synchronization protocol.  Thus, by attacking the time
   protocol, an attacker can potentially compromise the integrity of the
   measurement.  A detailed discussion about the threats against time
   protocols and how to mitigate them is presented in [RFC7384].
   Network Time Security (NTS), described in [RFC8915], is a mechanism
   that can be employed.  Also, the time, which is distributed to the
   network nodes through the time protocol, is centrally taken from an
   external accurate time source such as an atomic clock or a GPS clock.
   By attacking the time source, it is possible to compromise the
   integrity of the measurement as well.  There are security measures
   that can be taken to mitigate the GPS spoofing attacks, and a network
   administrator should certainly employ solutions to secure the network

7.  IANA Considerations

   IANA has allocated the Option Type in the "Destination Options and
   Hop-by-Hop Options" subregistry of the "Internet Protocol Version 6
   (IPv6) Parameters" registry (<https://www.iana.org/assignments/
   ipv6-parameters/>) as follows:

        | Hex Value | Binary Value      | Description | Reference |
        |           | act | chg | rest  |             |           |
        | 0x12      | 00  | 0   | 10010 | AltMark     | RFC 9343  |

            Table 1: Destination Options and Hop-by-Hop Options

8.  References

8.1.  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,

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

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

   [RFC9341]  Fioccola, G., Ed., Cociglio, M., Mirsky, G., Mizrahi, T.,
              and T. Zhou, "Alternate-Marking Method", RFC 9341,
              DOI 10.17487/RFC9341, December 2022,

   [RFC9342]  Fioccola, G., Ed., Cociglio, M., Sapio, A., Sisto, R., and
              T. Zhou, "Clustered Alternate-Marking Method", RFC 9342,
              DOI 10.17487/RFC9342, December 2022,

8.2.  Informative References

              Qin, F., Yuan, H., Yang, S., Zhou, T., and G. Fioccola,
              "BGP SR Policy Extensions to Enable IFIT", Work in
              Progress, Internet-Draft, draft-ietf-idr-sr-policy-ifit-
              05, 24 October 2022,

              Hinden, R. and G. Fairhurst, "IPv6 Hop-by-Hop Options
              Processing Procedures", Work in Progress, Internet-Draft,
              draft-ietf-6man-hbh-processing-04, 21 October 2022,

              Yuan, H., Wang, X., Yang, P., Li, W., and G. Fioccola,
              "Path Computation Element Communication Protocol (PCEP)
              Extensions to Enable IFIT", Work in Progress, Internet-
              Draft, draft-ietf-pce-pcep-ifit-01, 3 August 2022,

              Peng, S., Li, Z., Xie, C., Qin, Z., and G. Mishra,
              "Operational Issues with Processing of the Hop-by-Hop
              Options Header", Work in Progress, Internet-Draft, draft-
              ietf-v6ops-hbh-02, 21 October 2022,

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437,
              DOI 10.17487/RFC6437, November 2011,

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,

   [RFC7045]  Carpenter, B. and S. Jiang, "Transmission and Processing
              of IPv6 Extension Headers", RFC 7045,
              DOI 10.17487/RFC7045, December 2013,

   [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>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,

   [RFC8799]  Carpenter, B. and B. Liu, "Limited Domains and Internet
              Protocols", RFC 8799, DOI 10.17487/RFC8799, July 2020,

   [RFC8915]  Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
              Sundblad, "Network Time Security for the Network Time
              Protocol", RFC 8915, DOI 10.17487/RFC8915, September 2020,

   [SRv6-AMM] Fioccola, G., Zhou, T., and M. Cociglio, "Segment Routing
              Header encapsulation for Alternate Marking Method", Work
              in Progress, Internet-Draft, draft-fz-spring-srv6-alt-
              mark-03, 5 August 2022,


   The authors would like to thank Bob Hinden, Ole Troan, Martin Duke,
   Lars Eggert, Roman Danyliw, Alvaro Retana, Eric Vyncke, Warren
   Kumari, Benjamin Kaduk, Stewart Bryant, C. A. Wood, Yoshifumi
   Nishida, Tom Herbert, Stefano Previdi, Brian Carpenter, Greg Mirsky,
   and Ron Bonica for their valuable comments and suggestions.

Authors' Addresses

   Giuseppe Fioccola
   Riesstrasse, 25
   80992 Munich
   Email: giuseppe.fioccola@huawei.com

   Tianran Zhou
   156 Beiqing Rd.
   Email: zhoutianran@huawei.com

   Mauro Cociglio
   Telecom Italia
   Email: mauro.cociglio@outlook.com

   Fengwei Qin
   China Mobile
   32 Xuanwumenxi Ave.
   Email: qinfengwei@chinamobile.com

   Ran Pang
   China Unicom
   9 Shouti South Rd.
   Email: pangran@chinaunicom.cn