| Network Working Group | J. Alvarez-Hamelin |
| Internet-Draft | Universidad de Buenos Aires |
| Updates: 2330 (if approved) | A. Morton |
| Intended status: Standards Track | AT&T Labs |
| Expires: December 20, 2020 | J. Fabini |
| TU Wien | |
| C. Pignataro | |
| Cisco Systems, Inc. | |
| R. Geib | |
| Deutsche Telekom | |
| June 18, 2020 |
Advanced Unidirectional Route Assessment (AURA)
draft-ietf-ippm-route-08
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 multi-path technologies.
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.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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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 [RFC2330] framework motivated the development of "performance and reliability metrics for paths through the Internet," and Section 5 of [RFC2330] defines terms that support description of a path under test. However, metrics for assessment of path components and related performance aspects had not been attempted in IPPM when the [RFC2330] framework 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).
Section 7 of [RFC2330] presented 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:
The refined definition of Route metrics begins in the sections that follow.
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 with IP, 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.
Also, to specify a framework for active methods of measurement which use the techniques described in [PT] at a minimum, and a framework for hybrid active-passive methods of measurement, such as the Hybrid Type I method [RFC7799] described in [I-D.ietf-ippm-ioam-data](intended only for single administrative domains), which do not rely on ICMP and 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 Equal Cost Multi-Path (ECMP) and Unequal Cost Multi-Path (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.
This section sets requirements for the following components to support the Route Metric:
Type-P-Route-Ensemble-Method-Variant, abbreviated as Route Ensemble.
Note that Type-P depends heavily on the chosen method and variant.
This section lists the REQUIRED input factors to specify a Route metric.
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* definition for a Hop on the path, with sufficient indexes to identify all Hops identified in a measurement interval.
A Hop, 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:
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 1 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, possibly 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 time stamp indicating when all these additional details were valid.
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}
}
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).
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.
Depending on the way that 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.
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).
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 3.3 above. 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.
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.
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 (AS). While most of the routers perform load balancing on flows using Equal Cost Multiple Path (ECMP), a few still divide the workload through packet-based techniques. The former scenario is defined according to [RFC2991], while the latter generates a round-robin scheme to deliver every new outgoing packet. ECMP uses a hashing function to ensure that every packet of a flow is delivered by the same path, and this avoids increasing the packet delay variation and possibly producing overwhelming packet reordering in TCP flows.
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 the ICMP protocol [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, 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). [SCAMPER] supports IPv6 traceroute measurements, keeping the FlowLabel constant in all packets.
Paris-traceroute allows its users to measure RTD in every hop 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 this method). This task is accomplished by repeating complete traceroute measurements with different flow parameters for each measurement; Paris-traceroute provides “exhaustive” mode while scamper provides “tracelb” (stands for traceroute load balance). The Framework for IP Performance Metrics (IPPM) ([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. In IPv6, it is sufficient to be routed identically if the IP source and destination addresses and the FlowLabel are constant, see [RFC6437]. In IPv4, certain fields of the IP header and the first four 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 four 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 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 eight bytes of the IP payload, which affects its ICMP checksum. 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 be[PT]:
Then, the way to identify different hops and attempts of the same flow is:
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 which 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 which are flow-invariant.
Section 9.2 of [RFC2330] describes 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:
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:
There is an opportunity to apply the [RFC2330] notion 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:
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 which includes the flow of interest is as described in the section above, aided by computation of the relevant hash function output as the target.
The Hybrid Type I methods provide an alternative method for Route Member assessment. As mentioned in the Scope section, [I-D.ietf-ippm-ioam-data] provides a possible set of data fields that would support route identification.
In general, nodes in the measured domain would be equipped with specific abilities:
In addition to node identity, nodes may also identify the ingress and egress interfaces utilized by the tracing packet, the time of day when the packet was processed, and other generic data (as described in section 4 of [I-D.ietf-ippm-ioam-data]). Interface identification isn't necessarily limited to IP, i.e. different links in a bundle (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(s) added by the Hybrid method can certainly be specified in Type-P, in unpopulated form.
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:
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 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.
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:
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 Round-Trip Delay (RTD), the next one binds RTD with queueing 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 100ms on transatlantic hops, depending on the geolocation of the vantage points. Moreover, RTD is typically higher than 480ms when two hops are connected using geostationary satellite technology (i.e., their orbit is at 36000km). 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, after sending several traceroute probes, it is possible to detect congestion observing different statistics parameters (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 RIR delegations among other (for more detail, please consult [bdrmap]).
Several articles have shown that network traffic presents a self-similar nature [SSNT] [MLRM] which 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 non-parametric 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, and consequently, the Inter-Quartile Range (IQR), the distance between Q3 and Q1, increases its value.
This procedure requires to compute quartile values “on the fly” using the algorithm presented in [P2].
This procedure allows us to update the quartiles value 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, with parameter E set as False, or for every detected Route Ensemble flow (E=True).
The identification of a specific Hop in 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 field inserted by traceroute. As this specific Hop can be reached by different paths, also the IP source and destination addresses of the traceroute packet need to be recorded. Finally, different return paths are distinguished by evaluating the ICMP Time Exceeded TTL (of the reply message): if this TTL 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 value is normally initialized with values 64, 128, or 255. The 5-tuple (origin IP, destination IP, reply IP, distance, response TTL) univocally 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, response TTL), i.e., one for each possible combination.
================================================================ 1 input: W (window time of the measurement) 2 i_t (time between two measurements) 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. Line 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 advance-traceroute could be either Paris-traceroute or Scamper, which will use the “exhaustive” mode or “tracelb” option if E is set True, respectively. The procedure returns a list of tuples (m,Q1,Q2,Q3,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 stores each measurement into the real-time quartiles computation.
Notice there are cases where the even having a unique hop at distance h from the Src to Dst, the returning path could have several possibilities, yielding in different total paths. In this situation, the algorithm will return more "Alt" for this particular hop.
This document introduces a method to perform statistical RTD measurements in a path, according to the actual state of the art regarding the traffic nature and the flow balance method in ECMP cases, which can help to tackle different performance situations in the network. Some of these cases are enumerated in Section 5, while our method is proposed in Section 4, and the algorithm in Section 6. The importance of this algorithm is that it deals with the different topological aspects and the self-similar (i.e., not Poisson-distributed) nature of the traffic.
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).
For applicable Hybrid methods, the security considerations in[I-D.ietf-ippm-ioam-data] apply.
When considering 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 which 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.
This memo makes no requests of IANA. We thank the good folks at IANA for having checked this section anyway.
The original 3 authors acknowledge Ruediger Geib, for his penetrating comments on the initial draft, 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, so did Footer Foote. We thank them all!
A Node assessing an MPLS path must be part of the MPLS domain where the path is implemented. When this condition is met, RFC 8029 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” [RFC8029].
MPLS routing is based on the presence of a Forwarding Equivalence Class (FEC) Stack in all visited Nodes. Selecting one of several Equal Cost Multi Path (ECMP) is however based on information hidden deeper in the stack. Early deployments may support a so called “Entropy label” for this purpose. State of the art deployments base their choice of an ECMP member based on the IP addresses (see Section 2.4 of [RFC7325]). Both methods allow load sharing information to be decoupled from routing information. Thus, an MPLS traceroute is able to check how packets with a contiguous number of ECMP relevant addresses (and the same destination) are routed by a particular router. The minimum number of 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.