Network Working Group                                        G. Fioccola
Internet-Draft                                                   T. Zhou
Intended status: Informational                                    Huawei
Expires: 11 August 2024                                      M. Cociglio
                                                          Telecom Italia
                                                               G. Mishra
                                                            Verizon Inc.
                                                                 X. Wang
                                                                  Ruijie
                                                                G. Zhang
                                                            China Mobile
                                                         8 February 2024


   Application of the Alternate Marking Method to the Segment Routing
                                 Header
                    draft-fz-spring-srv6-alt-mark-08

Abstract

   The Alternate Marking Method is a passive performance measurement
   method based on marking consecutive batches of packets, which can be
   used to measure packet loss, latency, and jitter of live traffic.
   This method requires a packet marking method so that packet flows can
   be distinguished and identified.

   A mechanism to carry suitable packet marking in the Hop-by-Hop Header
   and the Destination Options Header of an IPv6 packet is described in
   RFC 9343 and is also applicable to Segment Routing for IPv6 (SRv6).

   This document describes an alternative approach that uses a new TLV
   in the Segment Routing Header (SRH) of an SRv6 packet.  This approach
   has been implemented and has potential scaling and simplification
   benefits over the technique described in RFC 9343.

   This protocol extension has been developed outside the IETF and is
   published here to guide implementation, ensure interoperability among
   implementations, and enable wide-scale deployment to determine the
   potential benefits of this approach.

Status of This Memo

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







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Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Application of the Alternate Marking to SRv6  . . . . . . . .   4
     2.1.  Controlled Domain . . . . . . . . . . . . . . . . . . . .   4
   3.  Definition of the SRH AltMark TLV . . . . . . . . . . . . . .   5
     3.1.  The Flow Monitoring Identification (FlowMonID)  . . . . .   6
     3.2.  Optional Extended Data Fields for Enhanced Alternate
           Marking . . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Use of the SRH AltMark TLV  . . . . . . . . . . . . . . . . .  10
   5.  Observations on RFC 9343  . . . . . . . . . . . . . . . . . .  11
   6.  SRH AltMark TLV Compatibility . . . . . . . . . . . . . . . .  13
   7.  Implementation Overview . . . . . . . . . . . . . . . . . . .  14
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  15
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     12.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18






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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 as the Alternate
   Marking Method.

   The Alternate Marking Method requires a marking field so that packet
   flows can be distinguished and identified.  An IETF standards track
   solution is described in [RFC9343] which analyzes the possible
   implementation options for the application of the Alternate Marking
   Method in an IPv6 domain and defines how the marking field can be
   encoded in a new TLV that is carried in the Option Headers (both Hop-
   by-hop or Destination) of IPv6 packets for to achieve Alternate
   Marking in an IPv6 domain.  That solution is equally applicable to
   Segment Routing for IPv6 (SRv6) networks [RFC8402].

   This document describes an alternative approach that encodes the
   marking field in a new TLV carried in the Segment Routing Header
   (SRH) [RFC8754] of an SRv6 packet.  This approach is specific to SRv6
   networks and does not apply in native IPv6 networks, but it has
   potential scaling and simplification benefits over the technique
   described in [RFC9343].  Indeed, the rationale is to place an
   information related to an SRv6 path directly inside the SRH.  It has
   been implemented taking into account that SR nodes are supposed to
   support fast parsing and processing of the SRH, while the SR nodes
   may not handle properly Destination Options, as described in
   [RFC9098] and [I-D.ietf-6man-eh-limits].

   This protocol extension has been developed outside the IETF and is
   published here to guide implementation, ensure interoperability among
   implementations, and enable wide-scale deployment.  See Section 7 for
   more details .

1.1.  Requirements Language

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









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2.  Application of the Alternate Marking to SRv6

   [RFC9341] and [RFC9342] defines the Alternate Marking Method which
   employs one or two marking bits inside the packet header to perform
   network measurements.  The Flow Monitoring Identification field
   (FlowMonID), as introduced in Section 3, is used to identify
   monitored flows and aids the optimization of implementation and
   scaling of the Alternate Marking Method.

   Note that the Flow Label field of the IPv6 Header [RFC8200] is also
   used to identify packet flows, but its usage is somewhat different
   from the FlowMonID.  While the FlowMonID is used to identify the
   monitored flow, the Flow Label is used for application services, such
   as load-balancing, equal cost multi-path (ECMP), and QoS.  Reusing
   the Flow Label field for identifying monitored flows is rules out
   because setting it for flow monitoring purposes might cause changes
   to the application intent and the forwarding behaviour.  Conversely,
   the Flow Label might be changed by normal processing along the
   packet's path, and this would break the measurement/monitoring task.

   An important point that will also be discussed in this document is
   the the uniqueness of the FlowMonID and how to allow disambiguation
   of the FlowMonID in case of collision.

   Section 2.1 highlights an important requirement for the application
   of the Alternate Marking to IPv6 and SRv6.  The concept of the
   Controlled Domain is explained as an essential precondition.

2.1.  Controlled Domain

   [RFC8799] introduces the concept of specific limited domain solutions
   and notes application of the Alternate Marking Method as an example.

   Despite the flexibility of IPv6, when innovative applications are
   proposed they are often applied within controlled domains to help
   constrain the domain-wide policies, options supported, the style of
   network management, and security requirements.  This is also the case
   for the application of the Alternate Marking Method to SRv6.

   Therefore, the application of the Alternate Marking Method to SRv6
   MUST be deployed only within a controlled domain.  Implementations
   MUST reject or discard packets that carry Alternate Marking data
   (using the new SRH TLV) that attempt to enter the controlled domain,
   and SHOULD prevent packets carrying Alternate Marking data from
   leaving the controlled domains.






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   For SRv6, the controlled domain corresponds to an SR domain, as
   defined in [RFC8402].  [RFC9343] introduces the Alternate-Marking
   measurement domain that can overlap with the controlled domain or may
   be a subset of the controlled domain.  Therefore, it is also possible
   to enter the controlled domain with an SRH already in place and add
   the Alternate Marking data to the SRH.

3.  Definition of the SRH AltMark TLV

   The AltMark SRH TLV is defined to carry the data fields associated
   with the Alternate Marking Method.  The TLV has some initial fields
   that are always present, and further extension fields that are
   present when Enhanced Alternate Marking is in use.

   Figure 1 shows the format of the AltMark TLV.


      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | SRH TLV Type  |  SRH TLV Len  |         Reserved              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              FlowMonID                |L|D|  Reserved |  NH   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~              Optional extended data fields (variable)         ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


              Figure 1: AltMark: SRH TLV for alternate marking

   The fields of this TLV are as follows:

   *  SRH TLV Type: 8 bit identifier of the Alternate Marking SRH TLV.
      The value for this field is taken from the range 124-126.  That
      is, it is an Experimental code point that indicates a TLV that
      does not change en route.  Deployments of implementations of this
      document must coordinate the value used by all implementations
      participating in the deployment.  Thus, implementations MUST make
      this value configurable.  Further, deployments must carefully
      consider any other implementations running in the network to avoid
      clashes with other SRH TLVs.

   *  SRH TLV Len: The length of the Data Fields of this TLV in bytes.
      This is set to 6 when Enhanced Alternate Marking is not in use.

   *  FlowMonID: 20 bits unsigned integer.  The FlowMon identifier is
      described in Section 3.1.




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

   *  L: Loss flag as defined in [RFC9343].

   *  D: Delay flag as defined in [RFC9343].

   *  NH: The NH (NextHeader) field is used to indicate extended data
      fields are present to support Enhanced Alternate Marking as
      follows:

      -  NextHeader value of 0x0 means that there is no extended data
         field attached.

      -  NextHeader values of 0x1-0x8 are reserved for further usage.

      -  NextHeader value of 0x9 indicates the extended data fields are
         present as described in Section 3.2.

      -  NextHeader values of 0xA-0xF are reserved for further usage.

   *  Optional extended data fields may be present according to the
      setting of the NH field and as described in Section 3.2.

3.1.  The Flow Monitoring Identification (FlowMonID)

   The Flow Monitoring Identification (FlowMonID) is required for three
   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.

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

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










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   The FlowMonID field is used to uniquely identify a monitored flow
   within the controlled measurement domain.  The field is set at the
   entry node to the domain.  The FlowMonID can be assigned by a central
   controller or algorithmically generated by the domain entry node.
   The latter approach cannot guarantee the uniqueness of FlowMonID, but
   it may be preferred for a network where the conflict probability is
   small due to the large FlowMonID space.

   It is important to note that if the 20 bit FlowMonID is set by the
   domain entry nodes, there is a chance of collision even when the
   values are chosen using a pseudo-random algorithm.  In these cases a
   value may be not be sufficient to uniquely identify a monitored flow.
   In such cases the packets need to be tagged with additional flow
   information to allow disambiguation.  Such additional tagging is
   carried in the extended data fields described in Section 3.2.

3.2.  Optional Extended Data Fields for Enhanced Alternate Marking

   The optional extended data fields to support Enhanced Alternate
   Marking are illustrated in Figure 2.  They are present when the NH
   field of the AltMark TLV is set to 0x9.


      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           FlowMonID Ext               |M|F|W|R|  Len  | Rsvd  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           MetaInfo            |      Optional MetaData        ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~               Optional MetaData (variable)                    ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


       Figure 2: Optional Extended Data Fields for Enhanced Alternate
                                  Marking

   The extended data fields are as follows:

   *  FlowMonID Ext - 20 bits unsigned integer.  This is used to extend
      the FlowMonID in order to reduce the conflict when random
      allocation is applied.  The disambiguation of the FlowMonID field
      is discussed in IPv6 AltMark Option [RFC9343].

   *  Four bit-flags indicate special-purpose usage.

      M bit:  Measurement mode.  If M=0, it indicates that it is for




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         hop-by-hop monitoring.  If M=1, it indicates that it is for
         end-to-end monitoring.

      F bit:  Fragmentation.  If F=1, it indicates that the original
         packet is fragmented, therefore it is necessary to only count a
         single packet, ignoring all the following fragments with F set
         to 1.

      W bit:  Flow direction identification.  This flag is used if
         backward direction flow monitoring is requested to be set up
         automatically.  If W=1, it indicates that the flow direction is
         forward.  If W=0, it indicates that the flow direction is
         backward.

      R bit:  Reserved.  This bit MUST be set to zero and ignored on
         receipt.

   *  Len - Length.  Indicates the length of the extended data fields
      for enhanced alternate marking.  It includes all of the fields
      shown in Figure 2 including any meta data that is present.

   *  Rsvd - Reserved for further use.  These bits MUST be set to zero
      on transmission and ignored on receipt.

   *  MetaInfo - A 16-bit Bitmap to indicate more meta data attached in
      the Optional MetaData field for enhanced functions.  More than one
      bit may be set, in which case the additional meta data is present
      in the order that the bits are set.  MetaInfo bits are numbered
      from 0 as the most significant bit.  Three bits and associated
      meta data are defined as follows:

      bit 0:  If set to 1, it indicates that a 6 byte Timestamp is
       present as shown in Figure 3.


        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
                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                       |    Timestamp(s)               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Timestamp(ns)                                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                 Figure 3: The Timestamp Extended Data Field






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       This Timestamp can be filled by the encapsulation node, and is
       taken all the way to the decapsulation node so that all the
       intermediate nodes can compare it against their local time, and
       measure the one way delay.  The timestamp consists of two
       fields:

          Timestamp(s) is a 16 bit integer that carries the number of
          seconds.

          Timestamp(ns) is a 32 bit integer that carries the number of
          nanoseconds.

      bit 1:  If set to 1, it indicates that control information to set
       up the backward direction flow monitoring based on the trigger
       packet is present as shown in Figure 4.


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  DIP Mask     |  SIP Mask     |P|I|O|V|S|T|    Period         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


          Figure 4: Control Information for Backward Direction Flow
                                  Monitoring

       The control information includes several fields and flags to
       match in order to set up the backward direction:

          DIP Mask: The length of the destination IP prefix used to
          match the flow.

          SIP Mask: The length of the source IP prefix used to match
          the flow.

          P bit: If set to 1, it indicates to match the flow using the
          protocol identifier in the trigger packet.

          I bit: If set to 1, it indicates to match the source port.

          O bit: If set to 1, it indicates to match the destination
          port.

          V bit: If set to 1, the node will automatically set up
          reverse direction monitoring, and allocate a FlowMonID.

          S bit: If set to 1, it indicates to match the DSCP.



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          T bit: Used to control the scope of tunnel measurement.  T=1
          means meausre between Network-to-Network Interfaces (i.e.,
          NNI to NNI).  T=0 means measure between User-to-Network
          Interfaces (i.e., UNI to UNI).

          Period: Indicates the alternate marking period counted in
          seconds.

      bit 2:  If set to 1, it indicates a 4 byte sequence number is
       present as shown in Figure 5.


        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                    Sequence Number                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                     Figure 5: Sequence Number Data Field

       The unique Sequence Number can be used to detect the out-of-
       order packets, in addition to enabling packet loss measurement.
       Moreover, the Sequence Number can be used together with the
       latency measurement, to access per packet timestamps.

4.  Use of the SRH AltMark TLV

   SRv6 leverages the IPv6 Segment Routing Header (SRH).  The SRH can
   carry TLVs as described in [RFC8754].  This document defines the SRH
   AltMark TLV (see Section 3) to carry Alternative Marking data fields
   for use in SRv6 networks.

   Assuming that the measurement domain overlaps with the SR controlled
   domain, the procedure for AltMark data encapsulation in the SRv6 SRH
   is summarized as follows:

   *  Ingress SR Node: As part of the SRH encapsulation, the Ingress SR
      Node of an SR domain or an SR Policy [RFC9256] that supports the
      mechanisms defined in this document and that wishes to perform the
      Alternate Marking Method adds the AltMark TLV in the SRH of the
      data packets.

   *  Intermediate SR Node: The Intermediate SR Node is any node
      receiving an IPv6 packet where the destination address of that
      packet is a local Segment Identifier (SID).  If an Intermediate SR
      Node is not capable of processing AltMark TLV, it simply ignores
      it according to the processing rules of [RFC8754].  If an



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      Intermediate SR Node is capable of processing AltMark TLV, it
      checks if SRH AltMark TLV is present in the packet and processes
      it.

   *  Egress SR Node: The Egress SR Node is the last node in the segment
      list of the SRH.  The processing of AltMark TLV at the Egress SR
      Node is similar to the processing of AltMark TLV at the
      Intermediate SR Nodes.

   The use of the AltMark TLV may be combined with the network
   programming capability of SRv6 ([RFC8986]).  Specifically, the
   ability for an SRv6 endpoint to determine whether to process or
   ignore some specific SRH TLVs (such as the AltMark TLV) may be based
   on the SID function associated with the SID advertised by an
   Intermediate or Egress SR Node and used in the Destination Address
   field of the SRv6 packet.  When a packet is addressed to a SID which
   does not support the Alternate Marking functionality, the receiving
   node does not have to look for or process the SRH AltMark TLV and can
   simply ignore it.  This also enables collection of Alternate Marking
   data only from the supporting segment endpoints.

5.  Observations on RFC 9343

   Like any other IPv6 use case, Hop-by-Hop and Destination Options can
   also be used when the SRH is present.  As specified in [RFC8200], the
   Hop-by-Hop Options Header is used to carry optional information that
   needs be examined at every hop along the path, while the Destination
   Options Header is used to carry optional information that needs be
   examined only by the packet's destination node(s).

   When a Routing Header exists, the Destination Options before the
   Routing Header is "for options to be processed by the first
   destination that appears in the IPv6 Destination Address field plus
   subsequent destinations listed in the Routing header", while the
   Destination Options after the Routing Header is "for options to be
   processed only by the final destination of the packet".  Because the
   SRH is a Routing Header, Destination Options present in the IPv6
   packet before the SRH header are processed by destination indicated
   in the SRH's route list.  As specified in [RFC8754], SR segment
   endpoint nodes process the local SID corresponding to the packet
   destination address.  Then, the destination address is updated
   according to the segment list.  The SRH TLV provides metadata for
   segment processing, while processing the SID, if the node is locally
   configured to do so.  From the aspect of processing function, both
   the Destination Options Header before SRH and the SRH TLV are
   processed at the node being indicated in the destination address
   field of the IPv6 header.




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   The distinction between the approaches is most notable for SRv6
   packets that traverse a network where the paths between sequential
   segment end points include multiple hops.  If the Hop-by-Hop Option
   is used, then every hop along the path will process the AltMark data.
   If the Destination Option positioned before the SRH is used, or the
   SRH AltMark TLV is used, then only the segment end points will
   process the AltMark data.

   Thus, the Alternate Marking Method can be achieved in two ways in an
   SRv6 network: either using the mechanism defined in this document, or
   using Destination Option preceding the SRH to carry AltMark data
   fields as described in [RFC9343].  These solutions can co-exist
   according to the current specifications, which raises an issue in
   deployments.

   But, it is to be noted that the approach with the Destination Option
   requires two IPv6 extension headers and this can have operational
   implications, as described in [RFC9098] and
   [I-D.ietf-6man-eh-limits].  It may, therefore, be desirable to choose
   the use of the SRH AltMark TLV, as described in this document, in
   order to limit the number of extension headers present in the
   packets.  [I-D.peng-v6ops-eh-deployment-considerations] also analyzes
   the issues with the extension headers, and aims to provide deployment
   guidance when IPv6 options are used.

   Further, there is a simplification in placing all information related
   to an SRv6 path inside the SRH, and that includes the Alternate
   Marking Method information associated with that path.  Additionally,
   it is likely that SR nodes support fast parsing and processing of the
   SRH, while it is possible that SR nodes may be explicitly configured
   to not handle Destination Options for security and legacy reasons.

   From a device prospective, SRH TLV and Destination Options are
   generally two functional modules in the forwarding plane.  The
   difference is that SRH and SRH TLV are integrated modules, while
   Destination Option is a general IPv6 functional module.  Supporting
   two modules (SRH and Destination Options) at the same time may not be
   optimal and may consume resources.













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   Moreover, this document also introduces in Section 3.2 extended data
   fields, which are not defined in [RFC9343], to support additional
   telemetry requirements.  In particular,
   [I-D.ietf-opsawg-ipfix-on-path-telemetry] introduces new IP Flow
   Information Export (IPFIX) information elements to expose the On-Path
   Telemetry measured delay.  It defines how the timestamp can be
   encoded in the encapsulation node and be read at the intermediate and
   decapsulation node to calculate the on-path delay.  Therefore, the
   implementation of the SRH ALtMark TLV, as defined in this document,
   is also correlated with the implementation of
   [I-D.ietf-opsawg-ipfix-on-path-telemetry].

   For all these reasons, the preferred solution for Alternate Marking
   Method in SR networks can be the SRH AltMark TLV as defined in this
   document, while the solution for other IPv6 networks is as described
   in [RFC9343].  This document does not change or invalidate any
   procedures defined in [RFC9343].

   As noted in Section 3, implementations of this document must use a
   code point chosen from the Experimental range.  Such implementations
   should make it possible for the operator to configure the value used
   in a deployment such that it is possible to conduct multiple
   implementations within the same network.

6.  SRH AltMark TLV Compatibility

   As highlighted in the previous section, the use of the Destination
   Option to carry the AltMark data preceding the SRH is equivalent to
   the SRH AltMark TLV.  Therefore, it is important to analyze what
   happens when both the SRH AltMArk TLV and the Destination Option are
   used, and how that would impact processing and complexity.  There are
   significant benefits to having only one solution for any problem.  It
   simplifies implementation and makes deployment less complicated,
   reducing configuration and interoperability issues.

   It is worth mentioning that the SRH AltMark TLV and the the
   Destination Option carrying AltMark data can coexist without
   problems.  If both are present, the only issue could be the
   duplication of information but this will not affect in any way the
   device and the network services.  The security requirement of
   controlled domain applies to both this document and [RFC9343], and it
   also confines this duplication to a single service provider networks.
   However, duplication of the same information in different places
   should be avoided and this document recommends the use of SRH TLV to
   carry SRv6 related information.






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   How the Alternate Marking Method is applied in a specific controlled
   domain also involves the specific capabilities of the devices in the
   network.  The use of the SRH AltMark TLV should be evaluated before
   supporting the Alternate Marking Method capability.  It is
   recommended to employ capability advertisement mechanisms which can
   be utilized for this purpose.  The choice between SRH TLV and
   Destination Option can be up to the network operator, depending on
   the service requirements and network device characteristics.

7.  Implementation Overview

   This document describes a protocol extension built on existing
   technology and using an Experimental code point.  The purpose is to
   determine the practicality and optimality of the protocol extension,
   in particular in consideration of implementations that cannot support
   multiple IPv6 extension headers in the same packet, or which do not
   support Destination Option Header processing, or which process the
   Destination Option Header on the slow path.

   The deployment should determine whether the protocol extensions
   defined achieve the desired function and can be supported in the
   presence of normal SRv6 processing especially in regard to concerns
   about SRH size and the potential complexity of SRH TLV processing.
   In particular, it needs to verify the ability to support SR network
   programming support of SID function control of the support or non-
   support of the AltMark TLV.

   It is anticipated that the implementation with the AltMark TLV will
   be contained within single service provider networks in keeping with
   the normal constraints of an SR Domain, and also in keeping with the
   normal limits in sharing performance and monitoring data collected on
   the path of packets in the network.  The scope of the deployment may
   depend on the availability of implementations and the willingness of
   operators to deploy it on live networks.  Other implementers and
   depolyers are invited to share their experiences with the authors of
   this document.

   The results of this implementation will be collected and shared with
   the IETF SPRING working group (possibly as an Internet-Draft) to help
   forward the discussions that will determine the correct development
   of Alternate marking Method solutions in SRv6 networks.  It is
   expected that a first set of results will be made available within
   two years of the publication of this document as an RFC.








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8.  Security Considerations

   The security considerations of SRv6 are discussed in [RFC8754] and
   [RFC8986], and the security considerations of Alternate Marking in
   general and its application to IPv6 are discussed in [RFC9341] and
   [RFC9343].

   [RFC9343] analyzes different security concerns and related solutions.
   These aspects are valid and applicable also to this document.  In
   particular the fundamental security requirement is that Alternate
   Marking MUST only be applied in a limited domain, as also mentioned
   in [RFC8799] and Section 2.1.

   Alternate Marking is a feature applied to a trusted domain, where one
   or several operators decide on leveraging and configuring Alternate
   Marking according to their needs.  Additionally, operators need to
   properly secure the Alternate Marking domain to avoid malicious
   configuration and attacks, which could include injecting malicious
   packets into a domain.  So the implementation of Alternate Marking is
   applied within a controlled domain where the network nodes are
   locally administered and where packets containing the AltMark TLV are
   prevented from entering or leaving the domain.  A limited
   administrative domain provides the network administrator with the
   means to select, monitor and control the access to the network.

9.  IANA Considerations

   This document makes no requests for IANA actions.  The code point
   used is taken from an Experimental range, must be agreed between
   implementations within any individual deployment, and is not to be
   reported in any published specification.  [RFC8754] allows for SRH
   TLV code points for experimentation and testing.  It could be
   possible to reserve some code point values for specific behaviors,
   such as the implementation described in this document, but this is
   out of scope for this document.

10.  Acknowledgements

   The authors would like to thank Adrian Farrel and Haoyu Song for the
   precious comments and suggestions.

11.  Contributors

   The following people provided relevant contributions to this
   document:






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   Massimo Nilo
   Telecom Italia
   Email: massimo.nilo@telecomitalia.it

   Fabrizio Milan
   Telecom Italia
   Email: fabrizio.milan@telecomitalia.it

   Fabio Bulgarella
   Telecom Italia
   Email: fabio.bulgarella@guest.telecomitalia.it

12.  References

12.1.  Normative References

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

   [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>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC9341]  Fioccola, G., Ed., Cociglio, M., Mirsky, G., Mizrahi, T.,
              and T. Zhou, "Alternate-Marking Method", RFC 9341,
              DOI 10.17487/RFC9341, December 2022,
              <https://www.rfc-editor.org/info/rfc9341>.

   [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,
              <https://www.rfc-editor.org/info/rfc9342>.

   [RFC9343]  Fioccola, G., Zhou, T., Cociglio, M., Qin, F., and R.
              Pang, "IPv6 Application of the Alternate-Marking Method",
              RFC 9343, DOI 10.17487/RFC9343, December 2022,
              <https://www.rfc-editor.org/info/rfc9343>.

12.2.  Informative References





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   [I-D.ietf-6man-eh-limits]
              Herbert, T., "Limits on Sending and Processing IPv6
              Extension Headers", Work in Progress, Internet-Draft,
              draft-ietf-6man-eh-limits-12, 18 December 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-6man-eh-
              limits-12>.

   [I-D.ietf-opsawg-ipfix-on-path-telemetry]
              Graf, T., Claise, B., and A. H. Feng, "Export of On-Path
              Delay in IPFIX", Work in Progress, Internet-Draft, draft-
              ietf-opsawg-ipfix-on-path-telemetry-06, 14 January 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-opsawg-
              ipfix-on-path-telemetry-06>.

   [I-D.peng-v6ops-eh-deployment-considerations]
              Peng, S., Fioccola, G., and J. Dong, "Deployment
              considerations of IPv6 packets with options", Work in
              Progress, Internet-Draft, draft-peng-v6ops-eh-deployment-
              considerations-00, 13 March 2023,
              <https://datatracker.ietf.org/doc/html/draft-peng-v6ops-
              eh-deployment-considerations-00>.

   [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>.

   [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>.

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/info/rfc8986>.

   [RFC9098]  Gont, F., Hilliard, N., Doering, G., Kumari, W., Huston,
              G., and W. Liu, "Operational Implications of IPv6 Packets
              with Extension Headers", RFC 9098, DOI 10.17487/RFC9098,
              September 2021, <https://www.rfc-editor.org/info/rfc9098>.





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   [RFC9256]  Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
              A., and P. Mattes, "Segment Routing Policy Architecture",
              RFC 9256, DOI 10.17487/RFC9256, July 2022,
              <https://www.rfc-editor.org/info/rfc9256>.

Authors' Addresses

   Giuseppe Fioccola
   Huawei
   Palazzo Verrocchio, Centro Direzionale Milano 2
   20054 Segrate (Milan)
   Italy
   Email: giuseppe.fioccola@huawei.com


   Tianran Zhou
   Huawei
   156 Beiqing Rd.
   Beijing
   100095
   China
   Email: zhoutianran@huawei.com


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


   Gyan S. Mishra
   Verizon Inc.
   Email: gyan.s.mishra@verizon.com


   Xuewei Wang
   Ruijie
   Email: wangxuewei1@ruijie.com.cn


   Geng Zhang
   China Mobile
   Email: zhanggeng@chinamobile.com









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