| 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: September 10, 2019 | J. Fabini |
| TU Wien | |
| C. Pignataro | |
| Cisco Systems, Inc. | |
| R. Geib | |
| Deutsche Telekom | |
| March 9, 2019 |
Advanced Unidirectional Route Assessment (AURA)
draft-ietf-ippm-route-04
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 one of j Discoverable Hosts (or Cooperating Hosts) that are i hops away from the host with address = Src during the measurement interval, T0 to Tf. As defined above, a Hop singleton measurement MUST contain a Host Identity, hid(i,j), and MAY contain one or more of the following attributes:
Now that Host Identities and related information can be positioned according to their distance from the host with address Src in hops, we introduce two forms of Routes:
A Route Ensemble is defined as the combination of all routes traversed by different flows from the host at Src address to the host at Dst address. The route traversed by each flow (with addresses Src and Dst, and other fields which constitute flow criteria) is a member of the 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 by 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, turs each list item into a rich structure. There may be information on the links between Hops, possibly information on the routing (arrival int. to departure int.), an estimate of distance between Hops based on Round Trip Delay measurements and calculations, and a time stamp indicating when all the additional detail was 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 hosts with Src and Dst addresses. More formally, with the host 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 various routes may overlap).
RTD(i,j,T) is defined as a singleton of the [RFC2681] Round-trip Delay between the host with address = Src and the host 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 host with address = Src and the host at Hop h(i,j) at time T.
Depending on the way that Host Identity is revealed, it may be difficult to determine parallel subpaths between the same pair of hosts (i.e. multiple parallel links). It is easier to detect parallel subpaths involving different hosts.
@@@@ This paragraph on Temporal Composition moved to support a more complete section on Methodology (section 4).
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).
@@@@ The Temporal Measurement and Route Class C (unrelated to address classes of the past) is now partly addressed in Section 4.
@@@@ now partly addressed, based on feedback at IETF-101:
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, host symbolic address, host 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 host 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.
@@@@ can we leave updates to RFC 5388 for further work? Or, do we need to take-on this topic in an Appendix here?
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 hosts on a path, and Hybrid active-passive methods that involve direct interrogation of cooperating hosts (usually within a single domain). Description of these methods follow.
@@@@ Editor's Note: We need to incorporate description of Type-P packets (with the flow parameters) used in each method below (done for Active).
We have chosen to describe the method based on that employed in current open source tools, thereby providing a practical framework for further advanced techniques to be included as method variants. This method is applicable to use across multiple administrative domains.
Paris-traceroute [PT] provides some measure of protection from path variation generated by ECMP load balancing, and it ensures traceroute packets will follow the same path in 98% of cases according to [SCAMPER]. If it is necessary to find every path possible between two hosts, Paris-traceroute provides “exhaustive” mode while scamper provides “tracelb” (stands for traceroute load balance).
The Type-P of packets used could be ICMP (as in the original traceroute), UDP or TCP. The later are used when a particular characteristic needs to be to verified, such as filtering or traffic shaping on specific ports (i.e., services). [SCAMPER] supports IPv6 traceroute measurements, keeping the FlowLable constant in all packets.
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. Since route assessment can be conducted using TCP, UDP or ICMP packets, this method REQUIRES the Diffserv field, the protocol number, IP source and destination addresses, and the port settings for TCP or UDP kept constant. For ICMP probes, the method additionally REQUIRES keeping the type, code, and ICMP checksum constant; which occupy the corresponding positions in the header of an IP packet, e.g., bytes 20 to 23 when the header IP has no options.
Maintaining a constant checksum in ICMP is most challenging because the ICMP Sequence Number is part of the calculation. The advanced traceroute method requires calculations using the IP Sequence Number Field and the Identifier Field, yielding a constant ICMP checksum in successive packets. For an example of calculations to maintain a constant checksum, see Appendix A of [RFC7820], where revision of a timestamp field is complemented by modifying the 2 octet checksum complement field (these fields take the roles of the ICMP Sequence Number and Identifier Fields, respectively).
For TCP and UDP packets, the checksum must also be kept constant. Therefore, the first four bytes of UDP (or TCP) data field are modified to compensate for fields that change from packet to packet.
@@@@ Note: other variants of advanced traceroute are planned be described.
Finally, the return path is also important to check. Taking into account that it is an ICMP time exceeded (during transit) packet, the source and destination IP are constant for every reply. Then, we should consider the fields in the first 32 bits of the protocol on the top of IP: the type and code of ICMP packet, and its checksum. Again, to maintain the ICMP checksum constant for the returning packets, we need to consider the whole ICMP message. It contains the IP header of the discarded packet plus the first 8 bytes of the IP payload; that is some of the fields of TCP header, the UDP header plus four data bytes, the ICMP header plus four bytes. Therefore, for UDP case the data field is used to maintain the ICMP checksum constant in the returning packet. For the ICMP case, the identifier and sequence fields of the sent ICMP probe are manipulated to be constant. The TCP case presents no problem because its first eight bytes will be the same for every packet probe.
Formally, to maintain the same flow in the measurements to a certain 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:
@@@@ Comments on this material are very welcome!
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. 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 Routing 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 Routing 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 Routing Class C characteristics benefit from the following measurement capabilities:
@@@@ Comments on this material are very welcome! Especially suggestions for tools that might lend themselves to support these measurements.
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 anomolous dely 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 mesurement of the route where anomolous 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.
@@@@ Comments on this new material are very welcome!
@@@@ This is a topic for investigation at the Hackfest-103 Measurements and Standards table.
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]).
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 hosts involved in the path that qualify as Cooperating Hosts, but not as Discoverable Hosts, 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 hosts that are part of a domain supporting an interrogation protocol have the following attributes:
When Hosts 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-hosts 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) while the last one binds RTD with queueing delay on routers. 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 greater 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]).
Internet routing is complex because it depends on the policies of thousands 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 keeps flow state in the router to ensure 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 well-known tool traceroute was developed to gather the IP addresses of each hop along a path using the ICMP protocol [RFC0792]. Besides, traceroute adds the measured RTD from each hop. However, the growing complexity of the Internet makes it more challenging to develop accurate traceroute implementation. For instance, the early traceroute tools would be inaccurate in the current network, mainly because they were not designed to retain 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.
Paris-traceroute-like tools operate in the following way: every packet should follow the same path because the sensitive fields of the header are controlled to appear as the same flow. This means that source and destination IP addresses, source and destination port numbers are the same in every packet. Additionally, Differentiated Services Code Point (DSCP), checksum and ICMP code should remain constant since they may affect the path selection.
Today's traceroute tools can send either UDP, TCP or ICMP packet probes. Since ICMP header does not include transport layer information, there are no fields for source and destination port numbers. For this reason, these tools keep constant ICMP type, code, and checksum fields to generate a kind of flow. However, the checksum may vary in every packet, therefore when probes use ICMP packets, ICMP Identifier and Sequence Number are manipulated to maintain constant checksum in every packet. On the other hand, when UDP probes are generated, the expected variation in the checksum of each packet is again compensated by manipulating the payload.
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. 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.
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 generate 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 allow 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 in performing traceroutes several times to obtain samples of the RTD in every hop from a path, during a time window (W) and compute the quantiles for every hop. This could be done for a single path flow or for every detected path flow.
Even though a particular hop may be understood as the amount of hops away from the source, a more detailed classification could be used. For example, a possible classification may be identify ICMP Time Exceeded packets coming from the same routers to those who have the same hop distance, IP address of the router which is replying and TTL value of the received ICMP packet.
Thus, the proposed methodology is based on this algorithm:
================================================================ 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 in the path(s) to Dst) ---------------------------------------------------------------- 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) ================================================================
In line 9 the advance-traceroute could be either Paris-traceroute or Scamper, which will use “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 in the path towards the Dst. Additionally, it could also return path variations using “alt” variable.
Combining the method proposed in Section 4 and statistics in Section 7, we can measure the performance of paths interconnecting two endpoints in Internet, and attempt the categorization of link types and congestion presence based on RTD.
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).
@@@@ add reference to security considerations from [I-D.ietf-ippm-ioam-data].
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. We thank them all!
A host 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 hosts. 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.
RFC to be 8403 (draft-ietf-spring-oam-usecase-10) 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.