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From: draft-ietf-ippm-route-10                         Proposed Standard
Internet Engineering Task Force (IETF)                J. Alvarez-Hamelin
Request for Comments: 9198                   Universidad de Buenos Aires
Updates: 2330                                                  A. Morton
Category: Standards Track                                      AT&T Labs
ISSN: 2070-1721                                                J. Fabini
                                                                 TU Wien
                                                            C. Pignataro
                                                     Cisco Systems, Inc.
                                                                 R. Geib
                                                        Deutsche Telekom
                                                                May 2022


            Advanced Unidirectional Route Assessment (AURA)

Abstract

   This memo introduces an advanced unidirectional route assessment
   (AURA) metric and associated measurement methodology based on the IP
   Performance Metrics (IPPM) framework (RFC 2330).  This memo updates
   RFC 2330 in the areas of path-related terminology and path
   description, primarily to include the possibility of parallel
   subpaths between a given Source and Destination pair, owing to the
   presence of multipath technologies.

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9198.

Copyright Notice

   Copyright (c) 2022 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
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Issues with Earlier Work to Define a Route Metric
     1.2.  Requirements Language
   2.  Scope
   3.  Route Metric Specifications
     3.1.  Terms and Definitions
     3.2.  Formal Name
     3.3.  Parameters
     3.4.  Metric Definitions
     3.5.  Related Round-Trip Delay and Loss Definitions
     3.6.  Discussion
     3.7.  Reporting the Metric
   4.  Route Assessment Methodologies
     4.1.  Active Methodologies
       4.1.1.  Temporal Composition for Route Metrics
       4.1.2.  Routing Class Identification
       4.1.3.  Intermediate Observation Point Route Measurement
     4.2.  Hybrid Methodologies
     4.3.  Combining Different Methods
   5.  Background on Round-Trip Delay Measurement Goals
   6.  RTD Measurements Statistics
   7.  Security Considerations
   8.  IANA Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  MPLS Methods for Route Assessment
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The IETF IP Performance Metrics (IPPM) Working Group first created a
   framework for metric development in [RFC2330].  This framework has
   stood the test of time and enabled development of many fundamental
   metrics.  It has been updated in the area of metric composition
   [RFC5835] and in several areas related to active stream measurement
   of modern networks with reactive properties [RFC7312].

   The framework in [RFC2330] motivated the development of "performance
   and reliability metrics for paths through the Internet"; Section 5 of
   [RFC2330] defines terms that support description of a path under
   test.  However, metrics for assessment of paths and related
   performance aspects had not been attempted in IPPM when the framework
   in [RFC2330] was written.

   This memo takes up the Route measurement challenge and specifies a
   new Route metric, two practical frameworks for methods of measurement
   (using either active or hybrid active-passive methods [RFC7799]), and
   Round-Trip Delay and link information discovery using the results of
   measurements.  All Route measurements are limited by the willingness
   of Hosts along the path to be discovered, to cooperate with the
   methods used, or to recognize that the measurement operation is
   taking place (such as when tunnels are present).

1.1.  Issues with Earlier Work to Define a Route Metric

   Section 7 of [RFC2330] presents a simple example of a "Route" metric
   along with several other examples.  The example is reproduced below
   (where the reference is to Section 5 of [RFC2330]):

   |  route:  The path, as defined in Section 5, from A to B at a given
   |     time.

   This example provides a starting point to develop a more complete
   definition of Route.  Areas needing clarification include:

   Time:  In practice, the Route will be assessed over a time interval
      because active path detection methods like Paris-traceroute [PT]
      rely on Hop Limits for their operation and cannot accomplish
      discovery of all Hosts using a single packet.

   Type-P:  The legacy Route definition lacks the option to cater for
      packet-dependent routing.  In this memo, we assess the Route for a
      specific packet of Type-P and reflect this in the metric
      definition.  The methods of measurement determine the specific
      Type-P used.

   Parallel Paths:  Parallel paths are a reality of the Internet and a
      strength of advanced Route assessment methods, so the metric must
      acknowledge this possibility.  Use of Equal-Cost Multipath (ECMP)
      and Unequal-Cost Multipath (UCMP) technologies are common sources
      of parallel subpaths.

   Cloud Subpath:  Cloud subpaths may contain Hosts that do not
      decrement the Hop Limit but may have two or more exchange links
      connecting "discoverable" Hosts or routers.  Parallel subpaths
      contained within clouds cannot be discovered.  The assessment
      methods only discover Hosts or routers on the path that decrement
      Hop Limit or cooperate with interrogation protocols.  The presence
      of tunnels and nested tunnels further complicate assessment by
      hiding Hops.

   Hop:  The definition of Hop in [RFC2330] was a link-Host pair.
      However, only Hosts that were discoverable and cooperated with
      interrogation protocols (where link information may be exposed)
      provided both link and Host information.

   Note that the actual definitions appear in Section 3.1.

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.  Scope

   The purpose of this memo is to add new Route metrics and methods of
   measurement to the existing set of IPPM metrics.

   The scope is to define Route metrics that can identify the path taken
   by a packet or a flow traversing the Internet between two Hosts.
   Although primarily intended for Hosts communicating on the Internet,
   the definitions and metrics are constructed to be applicable to other
   network domains, if desired.  The methods of measurement to assess
   the path may not be able to discover all Hosts comprising the path,
   but such omissions are often deterministic and explainable sources of
   error.

   This memo also specifies a framework for active methods of
   measurement that uses the techniques described in [PT] as well as a
   framework for hybrid active-passive methods of measurement, such as
   the Hybrid Type I method [RFC7799] described in [RFC9197].  Methods
   using [RFC9197] are intended only for single administrative domains
   that provide a protocol for explicit interrogation of Nodes on a
   path.  Combinations of active methods and hybrid active-passive
   methods are also in scope.

   Further, this memo provides additional analysis of the Round-Trip
   Delay measurements made possible by the methods in an effort to
   discover more details about the path, such as the link technology in
   use.

   This memo updates Section 5 of [RFC2330] in the areas of path-related
   terminology and path description, primarily to include the
   possibility of parallel subpaths between a given Source and
   Destination address pair (possibly resulting from ECMP and UCMP
   technologies).

   There are several simple non-goals of this memo.  There is no attempt
   to assess the reverse path from any Host on the path to the Host
   attempting the path measurement.  The reverse path contribution to
   delay will be that experienced by ICMP packets (in active methods)
   and may be different from delays experienced by UDP or TCP packets.
   Also, the Round-Trip Delay will include an unknown contribution of
   processing time at the Host that generates the ICMP response.
   Therefore, the ICMP-based active methods are not supposed to yield
   accurate, reproducible estimations of the Round-Trip Delay that UDP
   or TCP packets will experience.

3.  Route Metric Specifications

   This section sets requirements for the components of the route
   metric.

3.1.  Terms and Definitions


   Host
      A Host (as defined in [RFC2330]) is a computer capable of IP
      communication, including routers (aka an RFC 2330 Host).

   Node
      A Node is any network function on the path capable of IP-layer
      Communication, including RFC 2330 Hosts.

   Node Identity
      The Node identity is the unique address for Nodes communicating
      within the network domain.  For Nodes communicating on the
      Internet with IP, it is the globally routable IP address that the
      Node uses when communicating with other Nodes under normal or
      error conditions.  The Node identity revealed (and its connection
      to a Node name through reverse DNS) determines whether interfaces
      to parallel links can be associated with a single Node or appear
      to identify unique Nodes.

   Discoverable Node
      Discoverable Nodes are Nodes that convey their Node identity
      according to the requirements of their network domain, such as
      when error conditions are detected by that Node.  For Nodes
      communicating with IP packets, compliance with Section 3.2.2.4 of
      [RFC1122], when discarding a packet due to TTL or Hop Limit
      Exceeded condition, MUST result in sending the corresponding Time
      Exceeded message (containing a form of Node identity) to the
      source.  This requirement is also consistent with Section 5.3.1 of
      [RFC1812] for routers.

   Cooperating Node
      Cooperating Nodes are Nodes that respond to direct queries for
      their Node identity as part of a previously established and agreed
      upon interrogation protocol.  Nodes SHOULD also provide
      information such as arrival/departure interface identification,
      arrival timestamp, and any relevant information about the Node or
      specific link that delivered the query to the Node.

   Hop specification
      A Hop specification MUST contain a Node identity and MAY contain
      arrival and/or departure interface identification, Round-Trip
      Delay, and an arrival timestamp.

   Routing Class
      Routing Class is a Route that treats a class of different types of
      packets, designated "C" (unrelated to address classes of the past)
      equally ([RFC2330] [RFC8468]).  Knowledge of such a class allows
      any one of the types of packets within that class to be used for
      subsequent measurement of the Route.  The designator "class C" is
      used for historical reasons; see [RFC2330].

3.2.  Formal Name

   The formal name of the metric is:

   Type-P-Route-Ensemble-Method-Variant

   abbreviated as Route Ensemble.

   Note that Type-P depends heavily on the chosen method and variant.

3.3.  Parameters

   This section lists the REQUIRED input factors to define and measure a
   Route metric, as specified in this memo.

   Src:  the address of a Node (such as the globally routable IP
      address).

   Dst:  the address of a Node (such as the globally routable IP
      address).

   i:  the limit on the number of Hops a specific packet may visit as it
      traverses from the Node at Src to the Node at Dst (such as the TTL
      or Hop Limit).

   MaxHops:  the maximum value of i used (i=1,2,3,...MaxHops).

   T0:  a time (start of measurement interval).

   Tf:  a time (end of measurement interval).

   MP(address):  the Measurement Point at address, such as Src or Dst,
      usually at the same Node stack layer as "address".

   T:  the Node time of a packet as measured at MP(Src), meaning
      Measurement Point at the Source.

   Ta:  the Node time of a reply packet's *arrival* as measured at
      MP(Src), assigned to packets that arrive within a "reasonable"
      time (see parameter below).

   Tmax:  a maximum waiting time for reply packets to return to the
      source, set sufficiently long to disambiguate packets with long
      delays from packets that are discarded (lost), such that the
      distribution of Round-Trip Delay is not truncated.

   F:  the number of different flows simulated by the method and
      variant.

   flow:  the stream of packets with the same n-tuple of designated
      header fields that (when held constant) result in identical
      treatment in a multipath decision (such as the decision taken in
      load balancing).  Note: The IPv6 flow label MAY be included in the
      flow definition if the MP(Src) is a Tunnel Endpoint (TEP)
      complying with the guidelines in [RFC6438].

   Type-P:  the complete description of the packets for which this
      assessment applies (including the flow-defining fields).

3.4.  Metric Definitions

   This section defines the REQUIRED measurement components of the Route
   metrics (unless otherwise indicated):

   M:  the total number of packets sent between T0 and Tf.

   N:  the smallest value of i needed for a packet to be received at Dst
      (sent between T0 and Tf).

   Nmax:  the largest value of i needed for a packet to be received at
      Dst (sent between T0 and Tf).  Nmax may be equal to N.

   Next, define a *singleton* for a Node on the path with sufficient
   indexes to identify all Nodes identified in a measurement interval
   (where *singleton* is part of the IPPM Framework [RFC2330]).

   singleton:  A Hop specification, designated h(i,j), the IP address
      and/or identity of Discoverable Nodes (or Cooperating Nodes) that
      are i Hops away from the Node with address = Src and part of Route
      j during the measurement interval T0 to Tf.  As defined here, a
      Hop singleton measurement MUST contain a Node identity, hid(i,j),
      and MAY contain one or more of the following attributes:

   *  a(i,j) Arrival Interface ID (e.g., when [RFC5837] is supported)

   *  d(i,j) Departure Interface ID (e.g., when [RFC5837] is supported)

   *  t(i,j) arrival timestamp, where t(i,j) is ideally supplied by the
      Hop (note that t(i,j) might be approximated from the sending time
      of the packet that revealed the Hop, e.g., when the round-trip
      response time is available and divided by 2)

   *  Measurements of Round-Trip Delay (for each packet that reveals the
      same Node identity and flow attributes, then this attribute is
      computed; see next section)

   Node identities and related information can be ordered by their
   distance from the Node with address Src in Hops h(i,j).  Based on
   this, two forms of Routes are distinguished:

   A Route Ensemble is defined as the combination of all Routes
   traversed by different flows from the Node at Src address to the Node
   at Dst address.  A single Route traversed by a single flow
   (determined by an unambiguous tuple of addresses Src and Dst and
   other identical flow criteria) is a member of the Route Ensemble and
   called a Member Route.

   Using h(i,j) and components and parameters further define:

   When considering the set of Hops in the context of a single flow, a
   Member Route j is an ordered list {h(1,j), ... h(Nj, j)} where h(i-1,
   j) and h(i, j) are one Hop away from each other and Nj satisfying
   h(Nj,j)=Dst is the minimum count of Hops needed by the packet on
   member Route j to reach Dst. Member Routes must be unique.  The
   uniqueness property requires that any two Member Routes, j and k,
   that are part of the same Route Ensemble differ either in terms of
   minimum Hop count Nj and Nk to reach the destination Dst or, in the
   case of identical Hop count Nj=Nk, they have at least one distinct
   Hop: h(i,j) != h(i,k) for at least one i (i=1..Nj).

   All the optional information collected to describe a Member Route,
   such as the arrival interface, departure interface, and Round-Trip
   Delay at each Hop, turns each list item into a rich structure.  There
   may be information on the links between Hops, possible information on
   the routing (arrival interface and departure interface), an estimate
   of distance between Hops based on Round-Trip Delay measurements and
   calculations, and a timestamp indicating when all these additional
   details were measured.

   The Route Ensemble from Src to Dst, during the measurement interval
   T0 to Tf, is the aggregate of all m distinct Member Routes discovered
   between the two Nodes with Src and Dst addresses.  More formally,
   with the Node having address Src omitted:

   Route Ensemble = {
   {h(1,1), h(2,1), h(3,1), ... h(N1,1)=Dst},
   {h(1,2), h(2,2), h(3,2),..., h(N2,2)=Dst},
   ...
   {h(1,m), h(2,m), h(3,m), ....h(Nm,m)=Dst}
   }

   where the following conditions apply: i <= Nj <= Nmax (j=1..m)

   Note that some h(i,j) may be empty (null) in the case that systems do
   not reply (not discoverable or not cooperating).

   h(i-1,j) and h(i,j) are the Hops on the same Member Route one Hop
   away from each other.

   Hop h(i,j) may be identical with h(k,l) for i!=k and j!=l, which
   means there may be portions shared among different Member Routes
   (parts of Member Routes may overlap).

3.5.  Related Round-Trip Delay and Loss Definitions

   RTD(i,j,T) is defined as a singleton of the [RFC2681] Round-Trip
   Delay between the Node with address = Src and the Node at Hop h(i,j)
   at time T.

   RTL(i,j,T) is defined as a singleton of the [RFC6673] Round-Trip Loss
   between the Node with address = Src and the Node at Hop h(i,j) at
   time T.

3.6.  Discussion

   Depending on the way that the Node identity is revealed, it may be
   difficult to determine parallel subpaths between the same pair of
   Nodes (i.e., multiple parallel links).  It is easier to detect
   parallel subpaths involving different Nodes.

   *  If a pair of discovered Nodes identify two different addresses (IP
      or not), then they will appear to be different Nodes.  See item
      below.

   *  If a pair of discovered Nodes identify two different IP addresses
      and the IP addresses resolve to the same Node name (in the DNS),
      then they will appear to be the same Node.

   *  If a discovered Node always replies using the same network
      address, regardless of the interface a packet arrives on, then
      multiple parallel links cannot be detected in that network domain.
      This condition may apply to traceroute-style methods but may not
      apply to other hybrid methods based on In situ Operations,
      Administration, and Maintenance (IOAM).  For example, if the ICMP
      extension mechanism described in [RFC5837] is implemented, then
      parallel links can be detected with the discovery traceroute-style
      methods.

   *  If parallel links between routers are aggregated below the IP
      layer, then, from the Node's point of view, all these links share
      the same pair of IP addresses.  The existence of these parallel
      links can't be detected at the IP layer.  This applies to other
      network domains with layers below them as well.  This condition
      may apply to traceroute-style methods but may not apply to other
      hybrid methods based on IOAM.

   When a Route assessment employs IP packets (for example), the reality
   of flow assignment to parallel subpaths involves layers above IP.
   Thus, the measured Route Ensemble is applicable to IP and higher
   layers (as described in the methodology's packet of Type-P and flow
   parameters).

3.7.  Reporting the Metric

   An Information Model and an XML Data Model for Storing Traceroute
   Measurements is available in [RFC5388].  The measured information at
   each Hop includes four pieces of information: a one-dimensional Hop
   index, Node symbolic address, Node IP address, and RTD for each
   response.

   The description of Hop information that may be collected according to
   this memo covers more dimensions, as defined in Section 3.4.  For
   example, the Hop index is two-dimensional to capture the complexity
   of a Route Ensemble, and it contains corresponding Node identities at
   a minimum.  The models need to be expanded to include these features
   as well as Arrival Interface ID, Departure Interface ID, and arrival
   timestamp, when available.  The original sending Timestamp from the
   Src Node anchors a particular measurement in time.

4.  Route Assessment Methodologies

   There are two classes of methods described in this section, active
   methods relying on the reaction to TTL or Hop Limit Exceeded
   condition to discover Nodes on a path and hybrid active-passive
   methods that involve direct interrogation of Cooperating Nodes
   (usually within a single domain).  Description of these methods
   follow.

4.1.  Active Methodologies

   This section describes the method employed by current open-source
   tools, thereby providing a practical framework for further advanced
   techniques to be included as method variants.  This method is
   applicable for use across multiple administrative domains.

   Internet routing is complex because it depends on the policies of
   thousands of Autonomous Systems (ASes).  Most routers perform load
   balancing on flows using a form of ECMP.  [RFC2991] describes a
   number of flow-based or hashed approaches (e.g., Modulo-N Hash, Hash-
   Threshold, and Highest Random Weight (HRW)) and makes some good
   suggestions.  Flow-based ECMP avoids increased packet Delay Variation
   and possibly overwhelming levels of packet reordering in flows.

   A few routers still divide the workload through packet-based
   techniques, such as a round-robin scheme to distribute every new
   outgoing packet to multiple links, as explained in [RFC2991].  The
   methods described in this section assume flow-based ECMP.

   Taking into account that Internet protocol was designed under the
   "end-to-end" principle, the IP payload and its header do not provide
   any information about the Routes or path necessary to reach some
   destination.  For this reason, the popular tool, traceroute, was
   developed to gather the IP addresses of each Hop along a path using
   ICMP [RFC0792].  Traceroute also measures RTD from each Hop. However,
   the growing complexity of the Internet makes it more challenging to
   develop an accurate traceroute implementation.  For instance, the
   early traceroute tools would be inaccurate in the current network,
   mainly because they were not designed to retain a flow state.
   However, evolved traceroute tools, such as Paris-traceroute ([PT]
   [MLB]) and Scamper ([SCAMPER]), expect to encounter ECMP and achieve
   more accurate results when they do, where Scamper ensures traceroute
   packets will follow the same path in 98% of cases ([SCAMPER]).

   Today's traceroute tools send Type-P of packets, which are either
   ICMP, UDP, or TCP.  UDP and TCP are used when a particular
   characteristic needs to be verified, such as filtering or traffic
   shaping on specific ports (i.e., services).  UDP and TCP traceroute
   are also used when ICMP responses are not received.  [SCAMPER]
   supports IPv6 traceroute measurements, keeping the Flow Label
   constant in all packets.

   Paris-traceroute allows its users to measure the RTD to every Node of
   the path for a particular flow.  Furthermore, either Paris-traceroute
   or Scamper is capable of unveiling the many available paths between a
   source and destination (which are visible to active methods).  This
   task is accomplished by repeating complete traceroute measurements
   with different flow parameters for each measurement; Paris-traceroute
   provides an "exhaustive" mode, while Scamper provides "tracelb"
   (which stands for "traceroute load balance").  "Framework for IP
   Performance Metrics" [RFC2330], updated by [RFC7312], has the
   flexibility to require that the Round-Trip Delay measurement
   [RFC2681] uses packets with the constraints to assure that all
   packets in a single measurement appear as the same flow.  This
   flexibility covers ICMP, UDP, and TCP.  The accompanying methodology
   of [RFC2681] needs to be expanded to report the sequential Hop
   identifiers along with RTD measurements, but no new metric definition
   is needed.

   The advanced Route assessment methods used in Paris-traceroute [PT]
   keep the critical fields constant for every packet to maintain the
   appearance of the same flow.  When considering IPv6 headers, it is
   necessary to ensure that the IP Source and Destination addresses and
   Flow Label are constant (but note that many routers ignore the Flow
   Label field at this time); see [RFC6437].  Use of IPv6 Extension
   Headers may add critical fields and SHOULD be avoided.  In IPv4,
   certain fields of the IP header and the first 4 bytes of the IP
   payload should remain constant in a flow.  In the IPv4 header, the IP
   Source and Destination addresses, protocol number, and Diffserv
   fields identify flows.  The first 4 payload bytes include the UDP and
   TCP ports and the ICMP type, code, and checksum fields.

   Maintaining a constant ICMP checksum in IPv4 is most challenging, as
   the ICMP sequence number or identifier fields will usually change for
   different probes of the same path.  Probes should use arbitrary bytes
   in the ICMP data field to offset changes to the sequence number and
   identifier, thus keeping the checksum constant.

   Finally, it is also essential to Route the resulting ICMP Time
   Exceeded messages along a consistent path.  In IPv6, the fields above
   are sufficient.  In IPv4, the ICMP Time Exceeded message will contain
   the IP header and the first 8 bytes of the IP payload, both of which
   affect its ICMP checksum calculation.  The TCP sequence number, UDP
   length, and UDP checksum will affect this value and should remain
   constant.

   Formally, to maintain the same flow in the measurements to a
   particular Hop, the Type-P-Route-Ensemble-Method-Variant packets
   should have the following attributes (see [PT]):

   TCP case:  For IPv4, the fields Src, Dst, port-Src, port_Dst,
      sequence number, and Diffserv SHOULD be the same.  For IPv6, the
      fields Flow Label, Src, and Dst SHOULD be the same.

   UDP case:  For IPv4, the fields Src, Dst, port-Src, port-Dst, and
      Diffserv should be the same, and the UDP checksum SHOULD change to
      keep the IP checksum of the ICMP Time Exceeded reply constant.
      Then, the data length should be fixed, and the data field is used
      to make it so (consider that ICMP checksum uses its data field,
      which contains the original IP header plus 8 bytes of UDP, where
      TTL, IP identification, IP checksum, and UDP checksum changes).
      For IPv6, the field Flow Label and Source and Destination
      addresses SHOULD be the same.

   ICMP case:  For IPv4, the data field SHOULD compensate variations on
      TTL or Hop Limit, IP identification, and IP checksum for every
      packet.  There is no need to consider ICMPv6 because only Flow
      Label of IPv6 and Source and Destination addresses are used, and
      all of them SHOULD be constant.

   Then, the way to identify different Hops and attempts of the same
   IPv4 flow is:

   TCP case:  The IP identification field.

   UDP case:  The IP identification field.

   ICMP case:  The IP identification field and ICMP sequence number.

4.1.1.  Temporal Composition for Route Metrics

   The active Route assessment methods described above have the ability
   to discover portions of a path where ECMP load balancing is present,
   observed as two or more unique Member Routes having one or more
   distinct Hops that are part of the Route Ensemble.  Likewise,
   attempts to deliberately vary the flow characteristics to discover
   all Member Routes will reveal portions of the path that are flow
   invariant.

   Section 9.2 of [RFC2330] describes the Temporal Composition of
   metrics and introduces the possibility of a relationship between
   earlier measurement results and the results for measurement at the
   current time (for a given metric).  There is value in establishing a
   Temporal Composition relationship for Route metrics; however, this
   relationship does not represent a forecast of future Route conditions
   in any way.

   For Route-metric measurements, the value of Temporal Composition is
   to reduce the measurement iterations required with repeated
   measurements.  Reduced iterations are possible by inferring that
   current measurements using fixed and previously measured flow
   characteristics:

   *  will have many common Hops with previous measurements.

   *  will have relatively time-stable results at the ingress and egress
      portions of the path when measured from user locations, as opposed
      to measurements of backbone networks and across inter-domain
      gateways.

   *  may have greater potential for time variation in path portions
      where ECMP load balancing is observed (because increasing or
      decreasing the pool of links changes the hash calculations).

   Optionally, measurement systems may take advantage of the inferences
   above when seeking to reduce measurement iterations after exhaustive
   measurements indicate that the time-stable properties are present.
   Repetitive active Route measurement systems:

   1.  SHOULD occasionally check path portions that have exhibited
       stable results over time, particularly ingress and egress
       portions of the path (e.g., daily checks if measuring many times
       during a day).

   2.  SHOULD continue testing portions of the path that have previously
       exhibited ECMP load balancing.

   3.  SHALL trigger reassessment of the complete path and Route
       Ensemble if any change in Hops is observed for a specific (and
       previously tested) flow.


4.1.2.  Routing Class Identification

   There is an opportunity to apply the notion from [RFC2330] of equal
   treatment for a class of packets, "...very useful to know if a given
   Internet component treats equally a class C of different types of
   packets", as it applies to Route measurements.  The notion of class C
   was examined further in [RFC8468] as it applied to load-balancing
   flows over parallel paths, which is the case we develop here.
   Knowledge of class C parameters (unrelated to address classes of the
   past) on a path potentially reduces the number of flows required for
   a given method to assess a Route Ensemble over time.

   First, recognize that each Member Route of a Route Ensemble will have
   a corresponding class C.  Class C can be discovered by testing with
   multiple flows, all of which traverse the unique set of Hops that
   comprise a specific Member Route.

   Second, recognize that the different classes depend primarily on the
   hash functions used at each instance of ECMP load balancing on the
   path.

   Third, recognize the synergy with Temporal Composition methods
   (described above), where evaluation intends to discover time-stable
   portions of each Member Route so that more emphasis can be placed on
   ECMP portions that also determine class C.

   The methods to assess the various class C characteristics benefit
   from the following measurement capabilities:

   *  flows designed to determine which n-tuple header fields are
      considered by a given hash function and ECMP Hop on the path and
      which are not.  This operation immediately narrows the search
      space, where possible, and partially defines a class C.

   *  a priori knowledge of the possible types of hash functions in use
      also helps to design the flows for testing (major router vendors
      publish information about these hash functions; examples are in
      [LOAD_BALANCE]).

   *  ability to direct the emphasis of current measurements on ECMP
      portions of the path, based on recent past measurement results
      (the Routing Class of some portions of the path is essentially
      "all packets").


4.1.3.  Intermediate Observation Point Route Measurement

   There are many examples where passive monitoring of a flow at an
   Observation Point within the network can detect unexpected Round-Trip
   Delay or Delay Variation.  But how can the cause of the anomalous
   delay be investigated further *from the Observation Point* possibly
   located at an intermediate point on the path?

   In this case, knowledge that the flow of interest belongs to a
   specific Routing Class C will enable measurement of the Route where
   anomalous delay has been observed.  Specifically, Round-Trip Delay
   assessment to each Hop on the path between the Observation Point and
   the Destination for the flow of interest may discover high or
   variable delay on a specific link and Hop combination.

   The determination of a Routing Class C that includes the flow of
   interest is as described in the section above, aided by computation
   of the relevant hash function output as the target.


4.2.  Hybrid Methodologies

   The Hybrid Type I methods provide an alternative for Route
   assessment.  The "Scope, Applicability, and Assumptions" section of
   [RFC9197] provides one possible set of data fields that would support
   Route identification.

   In general, Nodes in the measured domain would be equipped with
   specific abilities:

   *  Store the identity of Nodes that a packet has visited in header
      data fields in the order the packet visited the Nodes.

   *  Support of a "Loopback" capability where a copy of the packet is
      returned to the encapsulating Node and the packet is processed
      like any other IOAM packet on the return transfer.

   In addition to Node identity, Nodes may also identify the ingress and
   egress interfaces utilized by the tracing packet, the absolute time
   when the packet was processed, and other generic data (as described
   in Section 3 of [RFC9197]).  Interface identification isn't
   necessarily limited to IP, i.e., different links in a bundle (Link
   Aggregation Control Protocol (LACP)) could be identified.  Equally
   well, links without explicit IP addresses can be identified (like
   with unnumbered interfaces in an IGP deployment).

   Note that the Type-P packet specification for this method will likely
   be a partial specification because most of the packet fields are
   determined by the user traffic.  The packet encapsulation header or
   headers added by the hybrid method can certainly be specified in
   Type-P, in unpopulated form.

4.3.  Combining Different Methods

   In principle, there are advantages if the entity conducting Route
   measurements can utilize both forms of advanced methods (active and
   hybrid) and combine the results.  For example, if there are Nodes
   involved in the path that qualify as Cooperating Nodes but not as
   Discoverable Nodes, then a more complete view of Hops on the path is
   possible when a hybrid method (or interrogation protocol) is applied
   and the results are combined with the active method results collected
   across all other domains.

   In order to combine the results of active and hybrid/interrogation
   methods, the network Nodes that are part of a domain supporting an
   interrogation protocol have the following attributes:

   1.  Nodes at the ingress to the domain SHOULD be both Discoverable
       and Cooperating.

   2.  Any Nodes within the domain that are both Discoverable and
       Cooperating SHOULD reveal the same Node identity in response to
       both active and hybrid methods.

   3.  Nodes at the egress to the domain SHOULD be both Discoverable and
       Cooperating and SHOULD reveal the same Node identity in response
       to both active and hybrid methods.

   When Nodes follow these requirements, it becomes a simple matter to
   match single-domain measurements with the overlapping results from a
   multidomain measurement.

   In practice, Internet users do not typically have the ability to
   utilize the Operations, Administrations, and Maintenance (OAM)
   capabilities of networks that their packets traverse, so the results
   from a remote domain supporting an interrogation protocol would not
   normally be accessible.  However, a network operator could combine
   interrogation results from their access domain with other
   measurements revealing the path outside their domain.

5.  Background on Round-Trip Delay Measurement Goals

   The aim of this method is to use packet probes to unveil the paths
   between any two End-Nodes of the network.  Moreover, information
   derived from RTD measurements might be meaningful to identify:

   1.  Intercontinental submarine links

   2.  Satellite communications

   3.  Congestion

   4.  Inter-domain paths

   This categorization is widely accepted in the literature and among
   operators alike, and it can be trusted with empirical data and
   several sources as ground of truth (e.g., [RTTSub]), but it is an
   inference measurement nonetheless [bdrmap] [IDCong].

   The first two categories correspond to the physical distance
   dependency on RTD, the next one binds RTD with queuing delay on
   routers, and the last one helps to identify different ASes using
   traceroutes.  Due to the significant contribution of propagation
   delay in long-distance Hops, RTD will be on the order of 100 ms on
   transatlantic Hops, depending on the geolocation of the vantage
   points.  Moreover, RTD is typically higher than 480 ms when two Hops
   are connected using geostationary satellite technology (i.e., their
   orbit is at 36000 km).  Detecting congestion with latency implies
   deeper mathematical understanding, since network traffic load is not
   stationary.  Nonetheless, as the first approach, a link seems to be
   congested if observing different/varying statistical results after
   sending several traceroute probes (e.g., see [IDCong]).  Finally, to
   recognize distinctive ASes in the same traceroute path is challenging
   because more data is needed, like AS relationships and Regional
   Internet Registry (RIR) delegations among others (for more details,
   please consult [bdrmap]).

6.  RTD Measurements Statistics

   Several articles have shown that network traffic presents a self-
   similar nature [SSNT] [MLRM] that is accountable for filling the
   queues of the routers.  Moreover, router queues are designed to
   handle traffic bursts, which is one of the most remarkable features
   of self-similarity.  Naturally, while queue length increases, the
   delay to traverse the queue increases as well and leads to an
   increase on RTD.  Due to traffic bursts generating short-term
   overflow on buffers (spiky patterns), every RTD only depicts the
   queueing status on the instant when that packet probe was in transit.
   For this reason, several RTD measurements during a time window could
   begin to describe the random behavior of latency.  Loss must also be
   accounted for in the methodology.

   To understand the ongoing process, examining the quartiles provides a
   nonparametric way of analysis.  Quartiles are defined by five values:
   minimum RTD (m), RTD value of the 25% of the Empirical Cumulative
   Distribution Function (ECDF) (Q1), the median value (Q2), the RTD
   value of the 75% of the ECDF (Q3), and the maximum RTD (M).
   Congestion can be inferred when RTD measurements are spread apart;
   consequently, the Interquartile Range (IQR), i.e., the distance
   between Q3 and Q1, increases its value.

   This procedure requires the algorithm presented in [P2] to compute
   quartile values "on the fly".

   This procedure allows us to update the quartile values whenever a new
   measurement arrives, which is radically different from classic
   methods of computing quartiles, because they need to use the whole
   dataset to compute the values.  This way of calculus provides savings
   in memory and computing time.

   To sum up, the proposed measurement procedure consists of performing
   traceroutes several times to obtain samples of the RTD in every Hop
   from a path during a time window (W) and compute the quartiles for
   every Hop. This procedure could be done for a single Member Route
   flow, for a non-exhaustive search with parameter E (defined below)
   set to False, or for every detected Route Ensemble flow (E=True).

   The identification of a specific Hop in a traceroute is based on the
   IP origin address of the returned ICMP Time Exceeded packet and on
   the distance identified by the value set in the TTL (or Hop Limit)
   field inserted by traceroute.  As this specific Hop can be reached by
   different paths, the IP Source and Destination addresses of the
   traceroute packet also need to be recorded.  Finally, different
   return paths are distinguished by evaluating the ICMP Time Exceeded
   TTL (or Hop Limit) of the reply message; if this TTL (or Hop Limit)
   is constant for different paths containing the same Hop, the return
   paths have the same distance.  Moreover, this distance can be
   estimated considering that the TTL (or Hop Limit) value is normally
   initialized with values 64, 128, or 255.  The 5-tuple (origin IP,
   destination IP, reply IP, distance, and response TTL or Hop Limit)
   unequivocally identifies every measurement.

   This algorithm below runs in the origin of the traceroute.  It
   returns the Qs quartiles for every Hop and Alt (alternative paths
   because of balancing).  Notice that the "Alt" parameter condenses the
   parameters of the 5-tuple (origin IP, destination IP, reply IP,
   distance, and response TTL), i.e., one for each possible combination.

   ================================================================
   0  input:   W (window time of the measurement)
   1           i_t (time between two measurements, set the i_t time
   2                long enough to avoid incomplete results)
   3           E (True: exhaustive, False: a single path)
   4           Dst (destination IP address)
   5  output:  Qs (quartiles for every Hop and Alt)
   ----------------------------------------------------------------
   6  T := start_timer(W)
   7  while T is not finished do:
   8  |       start_timer(i_t)
   9  |       RTD(Hop,Alt) = advanced-traceroute(Dst,E)
   10 |       for each Hop and Alt in RTD do:
   11 |       |     Qs[Dst,Hop,Alt] := ComputeQs(RTD(Hop,Alt))
   12 |       done
   13 |       wait until i_t timer is expired
   14 done
   15  return (Qs)
   ================================================================

   During the time W, lines 6 and 7 assure that the measurement loop is
   made.  Lines 8 and 13 set a timer for each cycle of measurements.  A
   cycle comprises the traceroutes packets, considering every possible
   Hop and the alternatives paths in the Alt variable (ensured in lines
   9-12).  In line 9, the advanced-traceroute could be either Paris-
   traceroute or Scamper, which will use the "exhaustive" mode or
   "tracelb" option if E is set to True, respectively.  The procedure
   returns a list of tuples (m, Q1, Q2, Q3, and M) for each intermediate
   Hop, or "Alt" in as a function of the 5-tuple, in the path towards
   the Dst. Finally, lines 10 through 12 store each measurement into the
   real-time quartiles computation.

   Notice there are cases where even having a unique Hop at distance h
   from the Src to Dst, the returning path could have several
   possibilities, yielding different total paths.  In this situation,
   the algorithm will return another "Alt" for this particular Hop.

7.  Security Considerations

   The security considerations that apply to any active measurement of
   live paths are relevant here as well.  See [RFC4656] and [RFC5357].

   The active measurement process of changing several fields to keep the
   checksum of different packets identical does not require special
   security considerations because it is part of synthetic traffic
   generation and is designed to have minimal to zero impact on network
   processing (to process the packets for ECMP).

   Some of the protocols used (e.g., ICMP) do not provide cryptographic
   protection for the requested/returned data, and there are risks of
   processing untrusted data in general, but these are limitations of
   the existing protocols where we are applying new methods.

   For applicable hybrid methods, the security considerations in
   [RFC9197] apply.

   When considering the privacy of those involved in measurement or
   those whose traffic is measured, the sensitive information available
   to potential observers is greatly reduced when using active
   techniques that are within this scope of work.  Passive observations
   of user traffic for measurement purposes raise many privacy issues.
   We refer the reader to the privacy considerations described in the
   Large-scale Measurement of Broadband Performance (LMAP) Framework
   [RFC7594], which covers active and passive techniques.

8.  IANA Considerations

   This document has no IANA actions.

9.  References

9.1.  Normative References

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <https://www.rfc-editor.org/info/rfc792>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,
              <https://www.rfc-editor.org/info/rfc1812>.

   [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>.

   [RFC2330]  Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
              "Framework for IP Performance Metrics", RFC 2330,
              DOI 10.17487/RFC2330, May 1998,
              <https://www.rfc-editor.org/info/rfc2330>.

   [RFC2681]  Almes, G., Kalidindi, S., and M. Zekauskas, "A Round-trip
              Delay Metric for IPPM", RFC 2681, DOI 10.17487/RFC2681,
              September 1999, <https://www.rfc-editor.org/info/rfc2681>.

   [RFC4656]  Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
              Zekauskas, "A One-way Active Measurement Protocol
              (OWAMP)", RFC 4656, DOI 10.17487/RFC4656, September 2006,
              <https://www.rfc-editor.org/info/rfc4656>.

   [RFC5388]  Niccolini, S., Tartarelli, S., Quittek, J., Dietz, T., and
              M. Swany, "Information Model and XML Data Model for
              Traceroute Measurements", RFC 5388, DOI 10.17487/RFC5388,
              December 2008, <https://www.rfc-editor.org/info/rfc5388>.

   [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>.

   [RFC6673]  Morton, A., "Round-Trip Packet Loss Metrics", RFC 6673,
              DOI 10.17487/RFC6673, August 2012,
              <https://www.rfc-editor.org/info/rfc6673>.

   [RFC7799]  Morton, A., "Active and Passive Metrics and Methods (with
              Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
              May 2016, <https://www.rfc-editor.org/info/rfc7799>.

   [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
              Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
              Switched (MPLS) Data-Plane Failures", RFC 8029,
              DOI 10.17487/RFC8029, March 2017,
              <https://www.rfc-editor.org/info/rfc8029>.

   [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>.

   [RFC8468]  Morton, A., Fabini, J., Elkins, N., Ackermann, M., and V.
              Hegde, "IPv4, IPv6, and IPv4-IPv6 Coexistence: Updates for
              the IP Performance Metrics (IPPM) Framework", RFC 8468,
              DOI 10.17487/RFC8468, November 2018,
              <https://www.rfc-editor.org/info/rfc8468>.

   [RFC9197]  Brockners, F., Ed., Bhandari, S., Ed., and T. Mizrahi,
              Ed., "Data Fields for In Situ Operations, Administration,
              and Maintenance (IOAM)", RFC 9197, DOI 10.17487/RFC9197,
              May 2022, <https://www.rfc-editor.org/info/rfc9197>.

9.2.  Informative References

   [bdrmap]   Luckie, M., Dhamdhere, A., Huffaker, B., Clark, D., and
              KC. Claffy, "bdrmap: Inference of Borders Between IP
              Networks", Proceedings of the 2016 ACM on Internet
              Measurement Conference, pp. 381-396,
              DOI 10.1145/2987443.2987467, November 2016,
              <https://doi.org/10.1145/2987443.2987467>.

   [IDCong]   Luckie, M., Dhamdhere, A., Clark, D., and B. Huffaker,
              "Challenges in Inferring Internet Interdomain Congestion",
              Proceedings of the 2014 Conference on Internet Measurement
              Conference, pp. 15-22, DOI 10.1145/2663716.2663741,
              November 2014, <https://doi.org/10.1145/2663716.2663741>.

   [LOAD_BALANCE]
              Sanguanpong, S., Pittayapitak, W., and K. Kasom Koht-Arsa,
              "COMPARISON OF HASH STRATEGIES FOR FLOW-BASED LOAD
              BALANCING", International Journal of Electronic Commerce
              Studies, Vol.6, No.2, pp.259-268, DOI 10.7903/ijecs.1346,
              December 2015, <https://doi.org/10.7903/ijecs.1346>.

   [MLB]      Augustin, B., Friedman, T., and R. Teixeira, "Measuring
              load-balanced paths in the internet", Proceedings of the
              7th ACM SIGCOMM conference on Internet measurement, pp.
              149-160, DOI 10.1145/1298306.1298329, October 2007,
              <https://doi.org/10.1145/1298306.1298329>.

   [MLRM]     Fontugne, R., Mazel, J., and K. Fukuda, "An empirical
              mixture model for large-scale RTT measurements", 2015 IEEE
              Conference on Computer Communications (INFOCOM), pp.
              2470-2478, DOI 10.1109/INFOCOM.2015.7218636, April 2015,
              <https://doi.org/10.1109/INFOCOM.2015.7218636>.

   [P2]       Jain, R. and I. Chlamtac, "The P 2 algorithm for dynamic
              calculation of quartiles and histograms without storing
              observations", Communications of the ACM 28.10 (1985):
              1076-1085, DOI 10.1145/4372.4378, October 1985,
              <https://doi.org/10.1145/4372.4378>.

   [PT]       Augustin, B., Cuvellier, X., Orgogozo, B., Viger, F.,
              Friedman, T., Latapy, M., Magnien, C., and R. Teixeira,
              "Avoiding traceroute anomalies with Paris traceroute",
              Proceedings of the 6th ACM SIGCOMM conference on Internet
              measurement, pp. 153-158, DOI 10.1145/1177080.1177100,
              October 2006, <https://doi.org/10.1145/1177080.1177100>.

   [RFC2991]  Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
              Multicast Next-Hop Selection", RFC 2991,
              DOI 10.17487/RFC2991, November 2000,
              <https://www.rfc-editor.org/info/rfc2991>.

   [RFC5357]  Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
              Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
              RFC 5357, DOI 10.17487/RFC5357, October 2008,
              <https://www.rfc-editor.org/info/rfc5357>.

   [RFC5835]  Morton, A., Ed. and S. Van den Berghe, Ed., "Framework for
              Metric Composition", RFC 5835, DOI 10.17487/RFC5835, April
              2010, <https://www.rfc-editor.org/info/rfc5835>.

   [RFC5837]  Atlas, A., Ed., Bonica, R., Ed., Pignataro, C., Ed., Shen,
              N., and JR. Rivers, "Extending ICMP for Interface and
              Next-Hop Identification", RFC 5837, DOI 10.17487/RFC5837,
              April 2010, <https://www.rfc-editor.org/info/rfc5837>.

   [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>.

   [RFC7312]  Fabini, J. and A. Morton, "Advanced Stream and Sampling
              Framework for IP Performance Metrics (IPPM)", RFC 7312,
              DOI 10.17487/RFC7312, August 2014,
              <https://www.rfc-editor.org/info/rfc7312>.

   [RFC7325]  Villamizar, C., Ed., Kompella, K., Amante, S., Malis, A.,
              and C. Pignataro, "MPLS Forwarding Compliance and
              Performance Requirements", RFC 7325, DOI 10.17487/RFC7325,
              August 2014, <https://www.rfc-editor.org/info/rfc7325>.

   [RFC7594]  Eardley, P., Morton, A., Bagnulo, M., Burbridge, T.,
              Aitken, P., and A. Akhter, "A Framework for Large-Scale
              Measurement of Broadband Performance (LMAP)", RFC 7594,
              DOI 10.17487/RFC7594, September 2015,
              <https://www.rfc-editor.org/info/rfc7594>.

   [RFC8403]  Geib, R., Ed., Filsfils, C., Pignataro, C., Ed., and N.
              Kumar, "A Scalable and Topology-Aware MPLS Data-Plane
              Monitoring System", RFC 8403, DOI 10.17487/RFC8403, July
              2018, <https://www.rfc-editor.org/info/rfc8403>.

   [RTTSub]   Bischof, Z., Rula, J., and F. Bustamante, "In and out of
              Cuba: Characterizing Cuba's Connectivity", Proceedings of
              the 2015 ACM Conference on Internet Measurement
              Conference, pp. 487-493, DOI 10.1145/2815675.2815718,
              October 2015, <https://doi.org/10.1145/2815675.2815718>.

   [SCAMPER]  Matthew Luckie, M., "Scamper: a scalable and extensible
              packet prober for active measurement of the internet",
              Proceedings of the 10th ACM SIGCOMM conference on Internet
              measurement, pp. 239-245, DOI 10.1145/1879141.1879171,
              November 2010, <https://doi.org/10.1145/1879141.1879171>.

   [SSNT]     Park, K. and W. Willinger, "Self-Similar Network Traffic
              and Performance Evaluation (1st ed.)",
              DOI 10.1002/047120644X,  John Wiley & Sons, Inc., New
              York, NY, USA, August 2000,
              <https://doi.org/10.1002/047120644X>.

Appendix A.  MPLS Methods for Route Assessment

   A Node assessing an MPLS path must be part of the MPLS domain where
   the path is implemented.  When this condition is met, [RFC8029]
   provides a powerful set of mechanisms to detect "correct operation of
   the data plane, as well as a mechanism to verify the data plane
   against the control plane".

   MPLS routing is based on the presence of a Forwarding Equivalence
   Class (FEC) Stack in all visited Nodes.  Selecting one of several
   Equal-Cost Multipaths (ECMPs) is, however, based on information
   hidden deeper in the stack.  Late deployments may support a so-called
   "Entropy label" for this purpose.  State-of-the-art deployments base
   their choice of an ECMP member interface on the complete MPLS label
   stack and on IP addresses up to the complete 5-tuple IP header
   information (see Section 2.4 of [RFC7325]).  Load sharing based on IP
   information decouples this function from the actual MPLS routing
   information.  Thus, an MPLS traceroute is able to check how packets
   with a contiguous number of ECMP-relevant IP addresses (and an
   identical MPLS label stack) are forwarded by a particular router.
   The minimum number of equivalent MPLS paths traceable at a router
   should be 32.  Implementations supporting more paths are available.

   The MPLS echo request and reply messages offering this feature must
   support the Downstream Detailed Mapping TLV (was Downstream Mapping
   initially, but the latter has been deprecated).  The MPLS echo
   response includes the incoming interface where a router received the
   MPLS echo request.  The MPLS echo reply further informs which of the
   n addresses relevant for the load-sharing decision results in a
   particular next-hop interface and contains the next Hop's interface
   address (if available).  This ensures that the next Hop will receive
   a properly coded MPLS echo request in the next step Route of
   assessment.

   [RFC8403] explains how a central Path Monitoring System could be used
   to detect arbitrary MPLS paths between any routers within a single
   MPLS domain.  The combination of MPLS forwarding, Segment Routing,
   and MPLS traceroute offers a simple architecture and a powerful
   mechanism to detect and validate (segment-routed) MPLS paths.

Acknowledgements

   The original three authors (Ignacio, Al, Joachim) acknowledge
   Ruediger Geib for his penetrating comments on the initial document
   and his initial text for the appendix on MPLS.  Carlos Pignataro
   challenged the authors to consider a wider scope and applied his
   substantial expertise with many technologies and their measurement
   features in his extensive comments.  Frank Brockners also shared
   useful comments and so did Footer Foote.  We thank them all!

Authors' Addresses

   J. Ignacio Alvarez-Hamelin
   Universidad de Buenos Aires
   Av. Paseo Colón 850
   C1063ACV Buenos Aires
   Argentina
   Phone: +54 11 5285-0716
   Email: ihameli@cnet.fi.uba.ar
   URI:   http://cnet.fi.uba.ar/ignacio.alvarez-hamelin/


   Al Morton
   AT&T Labs
   200 Laurel Avenue South
   Middletown, NJ 07748
   United States of America
   Phone: +1 732 420 1571
   Email: acm@research.att.com


   Joachim Fabini
   TU Wien
   Gusshausstrasse 25/E389
   1040 Vienna
   Austria
   Phone: +43 1 58801 38813
   Email: Joachim.Fabini@tuwien.ac.at
   URI:   http://www.tc.tuwien.ac.at/about-us/staff/joachim-fabini/


   Carlos Pignataro
   Cisco Systems, Inc.
   7200-11 Kit Creek Road
   Research Triangle Park, NC 27709
   United States of America
   Email: cpignata@cisco.com


   Ruediger Geib
   Deutsche Telekom
   Heinrich Hertz Str. 3-7
   64295 Darmstadt
   Germany
   Phone: +49 6151 5812747
   Email: Ruediger.Geib@telekom.de