IPv6 Application of the Alternate Marking Method
draft-ietf-6man-ipv6-alt-mark-14

6MAN Working Group                                           G. Fioccola
Internet-Draft                                                   T. Zhou
Intended status: Standards Track                                  Huawei
Expires: October 30, 2022                                    M. Cociglio
                                                          Telecom Italia
                                                                  F. Qin
                                                            China Mobile
                                                                 R. Pang
                                                            China Unicom
                                                          April 28, 2022

            IPv6 Application of the Alternate Marking Method
                    draft-ietf-6man-ipv6-alt-mark-14

Abstract

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

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on October 30, 2022.

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Alternate Marking application to IPv6 . . . . . . . . . . . .   3
     2.1.  Controlled Domain . . . . . . . . . . . . . . . . . . . .   5
       2.1.1.  Alternate Marking Measurement Domain  . . . . . . . .   6
   3.  Definition of the AltMark Option  . . . . . . . . . . . . . .   7
     3.1.  Data Fields Format  . . . . . . . . . . . . . . . . . . .   7
   4.  Use of the AltMark Option . . . . . . . . . . . . . . . . . .   8
   5.  Alternate Marking Method Operation  . . . . . . . . . . . . .  10
     5.1.  Packet Loss Measurement . . . . . . . . . . . . . . . . .  10
     5.2.  Packet Delay Measurement  . . . . . . . . . . . . . . . .  12
     5.3.  Flow Monitoring Identification  . . . . . . . . . . . . .  13
     5.4.  Multipoint and Clustered Alternate Marking  . . . . . . .  15
     5.5.  Data Collection and Calculation . . . . . . . . . . . . .  16
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   [I-D.ietf-ippm-rfc8321bis] and [I-D.ietf-ippm-rfc8889bis] 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.

   The Alternate Marking is an on-path telemetry technique and consists
   of synchronizing the measurements in different points of a network by

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   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.

   The format of IPv6 addresses is defined in [RFC4291] while [RFC8200]
   defines the IPv6 Header, including a 20-bit Flow Label and the IPv6
   Extension Headers.

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

   The threat model for the application of the Alternate Marking Method
   in an IPv6 domain is reported in Section 6.  As with all on-path
   telemetry techniques, the only definitive solution is that this
   methodology MUST be applied in a controlled domain.

1.1.  Terminology

   This document uses the terms related to the Alternate Marking Method
   as defined in [I-D.ietf-ippm-rfc8321bis] and
   [I-D.ietf-ippm-rfc8889bis].

1.2.  Requirements Language

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

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

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      Using the IPv6 destination address to encode the Alternate Marking
      processing is very expensive;

      Using the IPv6 Flow Label for Alternate Marking conflicts with the
      utilization of the Flow Label for load distribution purpose
      ([RFC6438]).

   In the end, a new Hop-by-Hop or a new 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
   operations:

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

   o  In case of Hop-by-Hop Option Header carrying Alternate Marking
      bits, it is not inserted or deleted, but 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 can happen that
      packets with hop-by-hop option could also 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
      ([I-D.peng-v6ops-hbh], [I-D.hinden-6man-hbh-processing]), these
      aspects are out of the scope for this document.

   o  In case of Destination Option 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 Option Header.

   Hop-by-Hop Option 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 scaling of the Alternate
   Marking Method is also considered and a way to identify flows is

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   required.  The Flow Monitoring Identification field (FlowMonID), 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/equal cost multi-path (LB/ECMP).  The reuse
   of 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.  Furthermore, since the
   Flow Label is pseudo-random, there is always a finite probability of
   collision.  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 scope, are used to
   identify flows based on different criteria, and are intended for
   different use cases.

   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 traditional
   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 it is considered an essential
   precondition, as also highlighted in Section 6.

2.1.  Controlled Domain

   [RFC8799] introduces the concept of specific limited domain solutions
   and, in this regard, it is reported the IPv6 Application of the
   Alternate Marking Method as an example.

   IPv6 has much more flexibility than IPv4 and innovative applications
   have been proposed, but for a number of reasons, such as the
   policies, options supported, the style of network management and
   security requirements, it is suggested to limit some of these
   applications to a controlled domain.  This is also the case of the
   Alternate Marking application to IPv6 as assumed hereinafter.

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   Therefore, the IPv6 application of the Alternate Marking Method MUST
   be deployed in a controlled domain.  It is RECOMMENDED that an
   implementation filters packets that carry Alternate Marking data and
   are entering or leaving the controlled domains.

   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 traverse the
   Internet.

   The security considerations reported in Section 6 also highlight this
   requirement.

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 in
      case it is fully managed and if it 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 CPE (Customer Premises Equipment) is most likely to be the
      starting or ending node since it connects the user's premises with
      the service provider's network and therefore belongs to the
      operator's controlled domain.  Typically the CPE encapsulates a
      received packet in an outer IPv6 header which contains the
      Alternate Marking data.  The CPE can also be able to filter and
      drop packets from outside of the domain with inconsistent fields
      to make effective the relevant security rules at the domain

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      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 new TLV for the Options Extension Headers,
   carrying the data fields dedicated to the Alternate Marking method,
   is reported below.

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
   Option).

    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      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Option Type: 8-bit identifier of the type of Option that needs to
      be allocated.  Unrecognized Types MUST be ignored on processing.
      For 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 and that the data of this
      Option do not change en route, indeed the source is the only one
      that can write it.

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

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   o  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.

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

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

   o  Reserved: is 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 and the destination node processes it.  While, in case of
   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 Destination Option and as 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 is not enough and it is required the hop-by-hop
   measurement, so the most complete choice can be 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 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 that can be mentioned is the presence of a Routing
   Header, in particular it is possible to consider SRv6.  A new type of
   Routing Header, referred as Segment Routing Header (SRH), has been
   defined in [RFC8754] for SRv6.  Like any other use case of IPv6, Hop-

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   by-Hop and Destination Options are usable when SRv6 header is
   present.  Because SRv6 is implemented through a Segment Routing
   Header (SRH), Destination Options before the Routing Header are
   processed by each destination in the route list, that means, in case
   of SRH, by every SR node that is identified by the SR path.  More
   details about the SRv6 application are described in
   [I-D.fz-spring-srv6-alt-mark].

   In summary, it is possible to list the alternative possibilities:

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

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

   o  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 new 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, 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 new Hop-by-Hop Option is that it is needed for OAM (Operations,
   Administration, and Maintenance).  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 mark
   alternately the flow, so, 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 SHOULD ignore them.  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 node
   that support it.  Indeed, the three high-order bits of the Options
   Header defined in this draft are 000 and, in theory, as per [RFC8200]
   and [I-D.hinden-6man-hbh-processing], this means "skip if do not
   recognize and data do not change en route".  [RFC8200] also mentions
   that the nodes only examine and process the Hop-by-Hop Options header

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   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 the 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 [I-D.hinden-6man-hbh-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.
   [I-D.ietf-ippm-rfc8321bis] introduces several applicable methods
   which are reported below, and a new 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
   [I-D.ietf-ippm-rfc8321bis].  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
   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 fixed timer can be used by the
   source node to create packet batches.  But, as also explained in
   [I-D.ietf-ippm-rfc8321bis], the timer-based batches are preferable
   because they are more deterministic than the counter-based batches.
   There is no definitive rule for counter-based batches, differently
   from timer-based batches.  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.

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   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 of the packets.  The mathematical formula on timing aspects,
   explained in section 5 of [I-D.ietf-ippm-rfc8321bis], must be
   satisfied and it takes into considerations the different causes of
   reordering such as clock error and network delay.  The assumption is
   to define the available counting interval where 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, which refers to the original
   marking interval at the source node considering that this interval
   could fluctuate along the path.  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 take
   still 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.

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5.2.  Packet Delay Measurement

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

   1.  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.

   2.  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 apply also 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

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       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 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 could also be 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).

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      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 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:

   o  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.

   o  If the 20 bits 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 can be 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 into the measurement SHOULD
   use the same mode for identification.  As mentioned, it is
   RECOMMENDED to use the FlowMonID for identification purpose in
   combination with source and destination addresses to identify a flow.
   By considering source and destination addresses together with the
   FlowMonID it can be possible to monitor 145 concurrent flows per host
   pairs with a 1% chance of collision in case of pseudo randomly

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   generated FlowMonID, or 1048576 concurrent flows per host pairs in
   case of 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:

   a.  It can be algorithmically generated by the source node, that 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).

   b.  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.  While, 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.

5.4.  Multipoint and Clustered Alternate Marking

   The Alternate Marking method can also be extended to any kind of
   multipoint to multipoint paths, and the network clustering approach
   allows a flexible and optimized performance measurement, as described
   in [I-D.ietf-ippm-rfc8889bis].

   The Cluster is the smallest identifiable subnetwork of the entire
   Network graph that still satisfies the condition that the number of
   packets that goes in is the same that goes out.  With network
   clustering, it is possible to use the partition of the network into
   clusters at different levels in order to perform the needed degree of
   detail.  So, for Multipoint Alternate Marking, FlowMonID can identify

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   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.  [I-D.ietf-idr-sr-policy-ifit],
   [I-D.chen-pce-pcep-ifit].

6.  Security Considerations

   This document aims to apply a method to perform 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
   modifications on the fly to an Option Header of 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, it is RECOMMENDED that the AltMark
   Option does not affect the throughput and therefore 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, FlowMonID) or by a malicious attacker adding
   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 Option Header while the
   intermediate nodes and destination node only read it without
   modifying the Option Header.  But, for example, an on-path attacker
   can modify the flags, whether intentionally or accidentally, or
   deliberately insert a new 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 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

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   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 HbH 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 HbH Option, and it can
   be easily recognized.

   An attacker that does not belong to the controlled domain can
   maliciously send packets with 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 MTU
   (by 48 bits).  While if Alternate Marking is supported in the
   controlled domain, it is also necessary to avoid that the
   measurements are affected and external packets with 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) composes the AltMark Option together
   with the two marking bits (L and D).  As explained in Section 5.3,
   there is a chance of collision if the FlowMonID is set pseudo
   randomly and a solution exists.  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 Option Header without any
   release of user data.  Indeed, from a confidentiality perspective,
   although 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.  AltMark Option contains two kinds of
   metadata: the marking bits (L and D bits) and the flow identifier
   (FlowMonID).

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      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 a 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 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 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.  [RFC8799] analyzes and discusses the trend towards
   network behaviors that can be applied only within a limited domain.
   This is due to the specific set of requirements especially related to
   security, network management, policies and options supported which
   may vary between such limited domains.  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 AltMark Option
   entering the domain and internal packets with AltMark Option leaving
   the domain.  Therefore, the trusted domain is unlikely subject to
   hijacking of packets since packets with AltMark Option are processed
   and used only within the controlled domain.

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   As stated, the application to a controlled domain ensures the 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 MUST simply ignore
   packets with AltMark Option since they are not configured to handle
   it and should not process it.

   Additionally, it is to be noted that the AltMark Option is carried by
   the Options Header and it may 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 bit in total)
   of these Option 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 Hop-
   by-Hop or Destination Option and, in case of 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, 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
   domains.  To this end, the inter-domain links need to be secured
   (e.g., by IPsec, 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 can be 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 domain.

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7.  IANA Considerations

   The Option Type should be assigned in IANA's "Destination Options and
   Hop-by-Hop Options" registry.

   This draft requests the following IPv6 Option Type assignment from
   the Destination Options and Hop-by-Hop Options sub-registry of
   Internet Protocol Version 6 (IPv6) Parameters
   (https://www.iana.org/assignments/ipv6-parameters/).

      Hex Value    Binary Value      Description           Reference
                   act chg rest
      ----------------------------------------------------------------
      TBD          00   0  tbd       AltMark               [This draft]

8.  Acknowledgements

   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, Christopher Wood, Yoshifumi
   Nishida, Tom Herbert, Stefano Previdi, Brian Carpenter, Greg Mirsky,
   Ron Bonica for the precious comments and suggestions.

9.  References

9.1.  Normative References

   [I-D.ietf-ippm-rfc8321bis]
              Fioccola, G., Cociglio, M., Mirsky, G., Mizrahi, T., and
              T. Zhou, "Alternate-Marking Method", draft-ietf-ippm-
              rfc8321bis-01 (work in progress), April 2022.

   [I-D.ietf-ippm-rfc8889bis]
              Fioccola, G., Cociglio, M., Sapio, A., Sisto, R., and T.
              Zhou, "Multipoint Alternate-Marking Clustered Method",
              draft-ietf-ippm-rfc8889bis-01 (work in progress), April
              2022.

   [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>.

   [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>.

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   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

9.2.  Informative References

   [I-D.chen-pce-pcep-ifit]
              Yuan, H., Zhou, T., Li, W., Fioccola, G., and Y. Wang,
              "Path Computation Element Communication Protocol (PCEP)
              Extensions to Enable IFIT", draft-chen-pce-pcep-ifit-06
              (work in progress), February 2022.

   [I-D.fz-spring-srv6-alt-mark]
              Fioccola, G., Zhou, T., and M. Cociglio, "Segment Routing
              Header encapsulation for Alternate Marking Method", draft-
              fz-spring-srv6-alt-mark-02 (work in progress), February
              2022.

   [I-D.hinden-6man-hbh-processing]
              Hinden, R. M. and G. Fairhurst, "IPv6 Hop-by-Hop Options
              Processing Procedures", draft-hinden-6man-hbh-
              processing-01 (work in progress), June 2021.

   [I-D.ietf-idr-sr-policy-ifit]
              Qin, F., Yuan, H., Zhou, T., Fioccola, G., and Y. Wang,
              "BGP SR Policy Extensions to Enable IFIT", draft-ietf-idr-
              sr-policy-ifit-03 (work in progress), January 2022.

   [I-D.peng-v6ops-hbh]
              Peng, S., Li, Z., Xie, C., Qin, Z., and G. Mishra,
              "Processing of the Hop-by-Hop Options Header", draft-peng-
              v6ops-hbh-06 (work in progress), August 2021.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [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,
              <https://www.rfc-editor.org/info/rfc6437>.

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   [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,
              <https://www.rfc-editor.org/info/rfc6438>.

   [RFC7045]  Carpenter, B. and S. Jiang, "Transmission and Processing
              of IPv6 Extension Headers", RFC 7045,
              DOI 10.17487/RFC7045, December 2013,
              <https://www.rfc-editor.org/info/rfc7045>.

   [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,
              <https://www.rfc-editor.org/info/rfc8754>.

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

   [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,
              <https://www.rfc-editor.org/info/rfc8915>.

Authors' Addresses

   Giuseppe Fioccola
   Huawei
   Riesstrasse, 25
   Munich  80992
   Germany

   Email: giuseppe.fioccola@huawei.com

   Tianran Zhou
   Huawei
   156 Beiqing Rd.
   Beijing  100095
   China

   Email: zhoutianran@huawei.com

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

   Email: mauro.cociglio@telecomitalia.it

   Fengwei Qin
   China Mobile
   32 Xuanwumenxi Ave.
   Beijing  100032
   China

   Email: qinfengwei@chinamobile.com

   Ran Pang
   China Unicom
   9 Shouti South Rd.
   Beijing  100089
   China

   Email: pangran@chinaunicom.cn

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