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
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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