Internet Engineering Task Force (IETF) A. Morton
Request for Comments: 9097 AT&T Labs
Category: Standards Track R. Geib
ISSN: 2070-1721 Deutsche Telekom
L. Ciavattone
AT&T Labs
November 2021
Metrics and Methods for One-Way IP Capacity
Abstract
This memo revisits the problem of Network Capacity Metrics first
examined in RFC 5136. This memo specifies a more practical Maximum
IP-Layer Capacity Metric definition catering to measurement and
outlines the corresponding Methods of Measurement.
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/rfc9097.
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Table of Contents
1. Introduction
1.1. Requirements Language
2. Scope, Goals, and Applicability
3. Motivation
4. General Parameters and Definitions
5. IP-Layer Capacity Singleton Metric Definitions
5.1. Formal Name
5.2. Parameters
5.3. Metric Definitions
5.4. Related Round-Trip Delay and One-Way Loss Definitions
5.5. Discussion
5.6. Reporting the Metric
6. Maximum IP-Layer Capacity Metric Definitions (Statistics)
6.1. Formal Name
6.2. Parameters
6.3. Metric Definitions
6.4. Related Round-Trip Delay and One-Way Loss Definitions
6.5. Discussion
6.6. Reporting the Metric
7. IP-Layer Sender Bit Rate Singleton Metric Definitions
7.1. Formal Name
7.2. Parameters
7.3. Metric Definition
7.4. Discussion
7.5. Reporting the Metric
8. Method of Measurement
8.1. Load Rate Adjustment Algorithm
8.2. Measurement Qualification or Verification
8.3. Measurement Considerations
9. Reporting Formats
9.1. Configuration and Reporting Data Formats
10. Security Considerations
11. IANA Considerations
12. References
12.1. Normative References
12.2. Informative References
Appendix A. Load Rate Adjustment Pseudocode
Appendix B. RFC 8085 UDP Guidelines Check
B.1. Assessment of Mandatory Requirements
B.2. Assessment of Recommendations
Acknowledgments
Authors' Addresses
1. Introduction
The IETF's efforts to define Network Capacity and Bulk Transport
Capacity (BTC) have been chartered and progressed for over twenty
years. Over that time, the performance community has seen the
development of Informative definitions in [RFC3148] for the Framework
for Bulk Transport Capacity, [RFC5136] for Network Capacity and
Maximum IP-Layer Capacity, and the Experimental metric definitions
and methods in "Model-Based Metrics for Bulk Transport Capacity"
[RFC8337].
This memo revisits the problem of Network Capacity Metrics examined
first in [RFC3148] and later in [RFC5136]. Maximum IP-Layer Capacity
and Bulk Transfer Capacity [RFC3148] (goodput) are different metrics.
Maximum IP-Layer Capacity is like the theoretical goal for goodput.
There are many metrics in [RFC5136], such as Available Capacity.
Measurements depend on the network path under test and the use case.
Here, the main use case is to assess the Maximum Capacity of one or
more networks where the subscriber receives specific performance
assurances, sometimes referred to as Internet access, or where a
limit of the technology used on a path is being tested. For example,
when a user subscribes to a 1 Gbps service, then the user, the
Service Provider, and possibly other parties want to assure that the
specified performance level is delivered. When a test confirms the
subscribed performance level, a tester can seek the location of a
bottleneck elsewhere.
This memo recognizes the importance of a definition of a Maximum IP-
Layer Capacity Metric at a time when Internet subscription speeds
have increased dramatically -- a definition that is both practical
and effective for the performance community's needs, including
Internet users. The metric definitions are intended to use Active
Methods of Measurement [RFC7799], and a Method of Measurement is
included for each metric.
The most direct Active Measurement of IP-Layer Capacity would use IP
packets, but in practice a transport header is needed to traverse
address and port translators. UDP offers the most direct assessment
possibility, and in the measurement study to investigate whether UDP
is viable as a general Internet transport protocol [copycat], the
authors found that a high percentage of paths tested support UDP
transport. A number of liaison statements have been exchanged on
this topic [LS-SG12-A] [LS-SG12-B], discussing the laboratory and
field tests that support the UDP-based approach to IP-Layer Capacity
measurement.
This memo also recognizes the updates to the IP Performance Metrics
(IPPM) Framework [RFC2330] that have been published since 1998. In
particular, it makes use of [RFC7312] for the Advanced Stream and
Sampling Framework and [RFC8468] for its IPv4, IPv6, and IPv4-IPv6
Coexistence Updates.
Appendix A describes the load rate adjustment algorithm, using
pseudocode. Appendix B discusses the algorithm's compliance with
[RFC8085].
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Scope, Goals, and Applicability
The scope of this memo is to define Active Measurement metrics and
corresponding methods to unambiguously determine Maximum IP-Layer
Capacity and useful secondary metrics.
Another goal is to harmonize the specified Metric and Method across
the industry, and this memo is the vehicle that captures IETF
consensus, possibly resulting in changes to the specifications of
other Standards Development Organizations (SDOs) (through each SDO's
normal contribution process or through liaison exchange).
Secondary goals are to add considerations for test procedures and to
provide interpretation of the Maximum IP-Layer Capacity results (to
identify cases where more testing is warranted, possibly with
alternate configurations). Fostering the development of protocol
support for this Metric and Method of Measurement is also a goal of
this memo (all active testing protocols currently defined by the IPPM
WG are UDP based, meeting a key requirement of these methods). The
supporting protocol development to measure this metric according to
the specified method is a key future contribution to Internet
measurement.
The load rate adjustment algorithm's scope is limited to helping
determine the Maximum IP-Layer Capacity in the context of an
infrequent, diagnostic, short-term measurement. It is RECOMMENDED to
discontinue non-measurement traffic that shares a subscriber's
dedicated resources while testing: measurements may not be accurate,
and throughput of competing elastic traffic may be greatly reduced.
The primary application of the Metrics and Methods of Measurement
described here is the same as what is described in Section 2 of
[RFC7497], where:
| The access portion of the network is the focus of this problem
| statement. The user typically subscribes to a service with
| bidirectional [Internet] access partly described by rates in bits
| per second.
In addition, the use of the load rate adjustment algorithm described
in Section 8.1 has the following additional applicability
limitations:
* It MUST only be used in the application of diagnostic and
operations measurements as described in this memo.
* It MUST only be used in circumstances consistent with Section 10
("Security Considerations").
* If a network operator is certain of the IP-Layer Capacity to be
validated, then testing MAY start with a fixed-rate test at the
IP-Layer Capacity and avoid activating the load adjustment
algorithm. However, the stimulus for a diagnostic test (such as a
subscriber request) strongly implies that there is no certainty,
and the load adjustment algorithm is RECOMMENDED.
Further, the Metrics and Methods of Measurement are intended for use
where specific exact path information is unknown within a range of
possible values:
* The subscriber's exact Maximum IP-Layer Capacity is unknown (which
is sometimes the case; service rates can be increased due to
upgrades without a subscriber's request or increased to provide a
surplus to compensate for possible underestimates of TCP-based
testing).
* The size of the bottleneck buffer is unknown.
Finally, the measurement system's load rate adjustment algorithm
SHALL NOT be provided with the exact capacity value to be validated
a priori. This restriction fosters a fair result and removes an
opportunity for nefarious operation enabled by knowledge of the
correct answer.
3. Motivation
As with any problem that has been worked on for many years in various
SDOs without any special attempts at coordination, various solutions
for Metrics and Methods have emerged.
There are five factors that have changed (or began to change) in the
2013-2019 time frame, and the presence of any one of them on the path
requires features in the measurement design to account for the
changes:
1. Internet access is no longer the bottleneck for many users (but
subscribers expect network providers to honor contracted
performance).
2. Both transfer rate and latency are important to a user's
satisfaction.
3. UDP's role in transport is growing in areas where TCP once
dominated.
4. Content and applications are moving physically closer to users.
5. There is less emphasis on ISP gateway measurements, possibly due
to less traffic crossing ISP gateways in the future.
4. General Parameters and Definitions
This section lists the REQUIRED input factors to specify a Sender or
Receiver metric.
Src: One of the addresses of a host (such as a globally routable IP
address).
Dst: One of the addresses of a host (such as a globally routable IP
address).
MaxHops: The limit on the number of Hops a specific packet may visit
as it traverses from the host at Src to the host at Dst
(implemented in the TTL or Hop Limit).
T0: The time at the start of a measurement interval, when packets
are first transmitted from the Source.
I: The nominal duration of a measurement interval at the Destination
(default 10 sec).
dt: The nominal duration of m equal sub-intervals in I at the
Destination (default 1 sec).
dtn: The beginning boundary of a specific sub-interval, n, one of m
sub-intervals in I.
FT: The feedback time interval between status feedback messages
communicating measurement results, sent from the Receiver to
control the Sender. The results are evaluated throughout the test
to determine how to adjust the current offered load rate at the
Sender (default 50 msec).
Tmax: A maximum waiting time for test packets to arrive at the
Destination, set sufficiently long to disambiguate packets with
long delays from packets that are discarded (lost), such that the
distribution of one-way delay is not truncated.
F: The number of different flows synthesized by the method (default
one flow).
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 SHOULD be included in
the flow definition when routers have complied with the guidelines
provided in [RFC6438].
Type-P: The complete description of the test packets for which this
assessment applies (including the flow-defining fields). Note
that the UDP transport layer is one requirement for test packets
specified below. Type-P is a concept parallel to "population of
interest" as defined in Clause 6.1.1 of [Y.1540].
Payload Content: An aspect of the Type-P Parameter that can help to
improve measurement determinism. Specifying packet payload
content helps to ensure IPPM Framework-conforming Metrics and
Methods. If there is payload compression in the path and tests
intend to characterize a possible advantage due to compression,
then payload content SHOULD be supplied by a pseudorandom sequence
generator, by using part of a compressed file, or by other means.
See Section 3.1.2 of [RFC7312].
PM: A list of fundamental metrics, such as loss, delay, and
reordering, and corresponding target performance threshold(s). At
least one fundamental metric and target performance threshold MUST
be supplied (such as one-way IP packet loss [RFC7680] equal to
zero).
A non-Parameter that is required for several metrics is defined
below:
T: The host time of the *first* test packet's *arrival* as measured
at the Destination Measurement Point, or MP(Dst). There may be
other packets sent between Source and Destination hosts that are
excluded, so this is the time of arrival of the first packet used
for measurement of the metric.
Note that timestamp format and resolution, sequence numbers, etc.
will be established by the chosen test protocol standard or
implementation.
5. IP-Layer Capacity Singleton Metric Definitions
This section sets requirements for the Singleton metric that supports
the Maximum IP-Layer Capacity Metric definitions in Section 6.
5.1. Formal Name
"Type-P-One-way-IP-Capacity" is the formal name; it is informally
called "IP-Layer Capacity".
Note that Type-P depends on the chosen method.
5.2. Parameters
This section lists the REQUIRED input factors to specify the metric,
beyond those listed in Section 4.
No additional Parameters are needed.
5.3. Metric Definitions
This section defines the REQUIRED aspects of the measurable IP-Layer
Capacity Metric (unless otherwise indicated) for measurements between
specified Source and Destination hosts:
Define the IP-Layer Capacity, C(T,dt,PM), to be the number of IP-
Layer bits (including header and data fields) in packets that can be
transmitted from the Src host and correctly received by the Dst host
during one contiguous sub-interval, dt in length. The IP-Layer
Capacity depends on the Src and Dst hosts, the host addresses, and
the path between the hosts.
The number of these IP-Layer bits is designated n0[dtn,dtn+1] for a
specific dt.
When the packet size is known and of fixed size, the packet count
during a single sub-interval dt multiplied by the total bits in IP
header and data fields is equal to n0[dtn,dtn+1].
Anticipating a Sample of Singletons, the number of sub-intervals with
duration dt MUST be set to a natural number m, so that T+I = T + m*dt
with dtn+1 - dtn = dt for 1 <= n <= m.
Parameter PM represents other performance metrics (see Section 5.4
below); their measurement results SHALL be collected during
measurement of IP-Layer Capacity and associated with the
corresponding dtn for further evaluation and reporting. Users SHALL
specify the Parameter Tmax as required by each metric's reference
definition.
Mathematically, this definition is represented as (for each n):
( n0[dtn,dtn+1] )
C(T,dt,PM) = -------------------------
dt
Figure 1: Equation for IP-Layer Capacity
and:
* n0 is the total number of IP-Layer header and payload bits that
can be transmitted in standard-formed packets [RFC8468] from the
Src host and correctly received by the Dst host during one
contiguous sub-interval, dt in length, during the interval
[T,T+I].
* C(T,dt,PM), the IP-Layer Capacity, corresponds to the value of n0
measured in any sub-interval beginning at dtn, divided by the
length of the sub-interval, dt.
* PM represents other performance metrics (see Section 5.4 below);
their measurement results SHALL be collected during measurement of
IP-Layer Capacity and associated with the corresponding dtn for
further evaluation and reporting.
* All sub-intervals MUST be of equal duration. Choosing dt as non-
overlapping consecutive time intervals allows for a simple
implementation.
* The bit rate of the physical interface of the measurement devices
MUST be higher than the smallest of the links on the path whose
C(T,I,PM) is to be measured (the bottleneck link).
Measurements according to this definition SHALL use the UDP transport
layer. Standard-formed packets are specified in Section 5 of
[RFC8468]. The measurement SHOULD use a randomized Source port or
equivalent technique, and SHOULD send responses from the Source
address matching the test packet Destination address.
Some effects of compression on measurement are discussed in Section 6
of [RFC8468].
5.4. Related Round-Trip Delay and One-Way Loss Definitions
RTD[dtn,dtn+1] is defined as a Sample of the Round-Trip Delay
[RFC2681] between the Src host and the Dst host during the interval
[T,T+I] (that contains equal non-overlapping intervals of dt). The
"reasonable period of time" mentioned in [RFC2681] is the Parameter
Tmax in this memo. The statistics used to summarize RTD[dtn,dtn+1]
MAY include the minimum, maximum, median, mean, and the range =
(maximum - minimum). Some of these statistics are needed for load
adjustment purposes (Section 8.1), measurement qualification
(Section 8.2), and reporting (Section 9).
OWL[dtn,dtn+1] is defined as a Sample of the One-Way Loss [RFC7680]
between the Src host and the Dst host during the interval [T,T+I]
(that contains equal non-overlapping intervals of dt). The
statistics used to summarize OWL[dtn,dtn+1] MAY include the count of
lost packets and the ratio of lost packets.
Other metrics MAY be measured: one-way reordering, duplication, and
delay variation.
5.5. Discussion
See the corresponding section for Maximum IP-Layer Capacity
(Section 6.5).
5.6. Reporting the Metric
The IP-Layer Capacity SHOULD be reported with at least single-Megabit
resolution, in units of Megabits per second (Mbps) (which, to avoid
any confusion, is 1,000,000 bits per second).
The related One-Way Loss metric and Round-Trip Delay measurements for
the same Singleton SHALL be reported, also with meaningful resolution
for the values measured.
Individual Capacity measurements MAY be reported in a manner
consistent with the Maximum IP-Layer Capacity; see Section 9.
6. Maximum IP-Layer Capacity Metric Definitions (Statistics)
This section sets requirements for the following components to
support the Maximum IP-Layer Capacity Metric.
6.1. Formal Name
"Type-P-One-way-Max-IP-Capacity" is the formal name; it is informally
called "Maximum IP-Layer Capacity".
Note that Type-P depends on the chosen method.
6.2. Parameters
This section lists the REQUIRED input factors to specify the metric,
beyond those listed in Section 4.
No additional Parameters or definitions are needed.
6.3. Metric Definitions
This section defines the REQUIRED aspects of the Maximum IP-Layer
Capacity Metric (unless otherwise indicated) for measurements between
specified Source and Destination hosts:
Define the Maximum IP-Layer Capacity, Maximum_C(T,I,PM), to be the
maximum number of IP-Layer bits n0[dtn,dtn+1] divided by dt that can
be transmitted in packets from the Src host and correctly received by
the Dst host, over all dt-length intervals in [T,T+I] and meeting the
PM criteria. An equivalent definition would be the maximum of a
Sample of size m of Singletons C(T,I,PM) collected during the
interval [T,T+I] and meeting the PM criteria.
The number of sub-intervals with duration dt MUST be set to a natural
number m, so that T+I = T + m*dt with dtn+1 - dtn = dt for 1 <= n <=
m.
Parameter PM represents the other performance metrics (see
Section 6.4 below) and their measurement results for the Maximum IP-
Layer Capacity. At least one target performance threshold (PM
criterion) MUST be defined. If more than one metric and target
performance threshold is defined, then the sub-interval with the
maximum number of bits transmitted MUST meet all the target
performance thresholds. Users SHALL specify the Parameter Tmax as
required by each metric's reference definition.
Mathematically, this definition can be represented as:
max ( n0[dtn,dtn+1] )
[T,T+I]
Maximum_C(T,I,PM) = -------------------------
dt
where:
T T+I
_________________________________________
| | | | | | | | | | |
dtn=1 2 3 4 5 6 7 8 9 10 n+1
n=m
Figure 2: Equation for Maximum Capacity
and:
* n0 is the total number of IP-Layer header and payload bits that
can be transmitted in standard-formed packets from the Src host
and correctly received by the Dst host during one contiguous sub-
interval, dt in length, during the interval [T,T+I].
* Maximum_C(T,I,PM), the Maximum IP-Layer Capacity, corresponds to
the maximum value of n0 measured in any sub-interval beginning at
dtn, divided by the constant length of all sub-intervals, dt.
* PM represents the other performance metrics (see Section 6.4) and
their measurement results for the Maximum IP-Layer Capacity. At
least one target performance threshold (PM criterion) MUST be
defined.
* All sub-intervals MUST be of equal duration. Choosing dt as non-
overlapping consecutive time intervals allows for a simple
implementation.
* The bit rate of the physical interface of the measurement systems
MUST be higher than the smallest of the links on the path whose
Maximum_C(T,I,PM) is to be measured (the bottleneck link).
In this definition, the m sub-intervals can be viewed as trials when
the Src host varies the transmitted packet rate, searching for the
maximum n0 that meets the PM criteria measured at the Dst host in a
test of duration I. When the transmitted packet rate is held
constant at the Src host, the m sub-intervals may also be viewed as
trials to evaluate the stability of n0 and metric(s) in the PM list
over all dt-length intervals in I.
Measurements according to these definitions SHALL use the UDP
transport layer.
6.4. Related Round-Trip Delay and One-Way Loss Definitions
RTD[dtn,dtn+1] and OWL[dtn,dtn+1] are defined in Section 5.4. Here,
the test intervals are increased to match the capacity Samples,
RTD[T,I] and OWL[T,I].
The interval dtn,dtn+1 where Maximum_C(T,I,PM) occurs is the
reporting sub-interval for RTD[dtn,dtn+1] and OWL[dtn,dtn+1] within
RTD[T,I] and OWL[T,I].
Other metrics MAY be measured: one-way reordering, duplication, and
delay variation.
6.5. Discussion
If traffic conditioning (e.g., shaping, policing) applies along a
path for which Maximum_C(T,I,PM) is to be determined, different
values for dt SHOULD be picked and measurements executed during
multiple intervals [T,T+I]. Each duration dt SHOULD be chosen so
that it is an integer multiple of increasing values k times
serialization delay of a Path MTU (PMTU) at the physical interface
speed where traffic conditioning is expected. This should avoid
taking configured burst tolerance Singletons as a valid
Maximum_C(T,I,PM) result.
A Maximum_C(T,I,PM) without any indication of bottleneck congestion,
be that increased latency, packet loss, or Explicit Congestion
Notification (ECN) marks during a measurement interval, I, is likely
an underestimate of Maximum_C(T,I,PM).
6.6. Reporting the Metric
The IP-Layer Capacity SHOULD be reported with at least single-Megabit
resolution, in units of Megabits per second (Mbps) (which, to avoid
any confusion, is 1,000,000 bits per second).
The related One-Way Loss metric and Round-Trip Delay measurements for
the same Singleton SHALL be reported, also with meaningful resolution
for the values measured.
When there are demonstrated and repeatable Capacity modes in the
Sample, the Maximum IP-Layer Capacity SHALL be reported for each
mode, along with the relative time from the beginning of the stream
that the mode was observed to be present. Bimodal Maximum IP-Layer
Capacities have been observed with some services, sometimes called a
"turbo mode" intending to deliver short transfers more quickly or
reduce the initial buffering time for some video streams. Note that
modes lasting less than duration dt will not be detected.
Some transmission technologies have multiple methods of operation
that may be activated when channel conditions degrade or improve, and
these transmission methods may determine the Maximum IP-Layer
Capacity. Examples include line-of-sight microwave modulator
constellations, or cellular modem technologies where the changes may
be initiated by a user moving from one coverage area to another.
Operation in the different transmission methods may be observed over
time, but the modes of Maximum IP-Layer Capacity will not be
activated deterministically as with the "turbo mode" described in the
paragraph above.
7. IP-Layer Sender Bit Rate Singleton Metric Definitions
This section sets requirements for the following components to
support the IP-Layer Sender Bit Rate Metric. This metric helps to
check that the Sender actually generated the desired rates during a
test, and measurement takes place at the interface between the Src
host and the network path (or as close as practical within the Src
host). It is not a metric for path performance.
7.1. Formal Name
"Type-P-IP-Sender-Bit-Rate" is the formal name; it is informally
called the "IP-Layer Sender Bit Rate".
Note that Type-P depends on the chosen method.
7.2. Parameters
This section lists the REQUIRED input factors to specify the metric,
beyond those listed in Section 4.
S: The duration of the measurement interval at the Source.
st: The nominal duration of N sub-intervals in S (default st = 0.05
seconds).
stn: The beginning boundary of a specific sub-interval, n, one of N
sub-intervals in S.
S SHALL be longer than I, primarily to account for on-demand
activation of the path, or any preamble to testing required, and the
delay of the path.
st SHOULD be much smaller than the sub-interval dt and on the same
order as FT; otherwise, the rate measurement will include many rate
adjustments and include more time smoothing, possibly smoothing the
interval that contains the Maximum IP-Layer Capacity (and therefore
losing relevance). The st Parameter does not have relevance when the
Source is transmitting at a fixed rate throughout S.
7.3. Metric Definition
This section defines the REQUIRED aspects of the IP-Layer Sender Bit
Rate Metric (unless otherwise indicated) for measurements at the
specified Source on packets addressed for the intended Destination
host and matching the required Type-P:
Define the IP-Layer Sender Bit Rate, B(S,st), to be the number of IP-
Layer bits (including header and data fields) that are transmitted
from the Source with address pair Src and Dst during one contiguous
sub-interval, st, during the test interval S (where S SHALL be longer
than I) and where the fixed-size packet count during that single sub-
interval st also provides the number of IP-Layer bits in any
interval, [stn,stn+1].
Measurements according to this definition SHALL use the UDP transport
layer. Any feedback from the Dst host to the Src host received by
the Src host during an interval [stn,stn+1] SHOULD NOT result in an
adaptation of the Src host traffic conditioning during this interval
(rate adjustment occurs on st interval boundaries).
7.4. Discussion
Both the Sender and Receiver (or Source and Destination) bit rates
SHOULD be assessed as part of an IP-Layer Capacity measurement.
Otherwise, an unexpected sending rate limitation could produce an
erroneous Maximum IP-Layer Capacity measurement.
7.5. Reporting the Metric
The IP-Layer Sender Bit Rate SHALL be reported with meaningful
resolution, in units of Megabits per second (which, to avoid any
confusion, is 1,000,000 bits per second).
Individual IP-Layer Sender Bit Rate measurements are discussed
further in Section 9.
8. Method of Measurement
It is REQUIRED per the architecture of the method that two
cooperating hosts operate in the roles of Src (test packet Sender)
and Dst (Receiver) with a measured path and return path between them.
The duration of a test, Parameter I, MUST be constrained in a
production network, since this is an active test method and it will
likely cause congestion on the path from the Src host to the Dst host
during a test.
8.1. Load Rate Adjustment Algorithm
The algorithm described in this section MUST NOT be used as a general
Congestion Control Algorithm (CCA). As stated in Section 2 ("Scope,
Goals, and Applicability"), the load rate adjustment algorithm's goal
is to help determine the Maximum IP-Layer Capacity in the context of
an infrequent, diagnostic, short-term measurement. There is a trade-
off between test duration (also the test data volume) and algorithm
aggressiveness (speed of ramp-up and ramp-down to the Maximum IP-
Layer Capacity). The Parameter values chosen below strike a well-
tested balance among these factors.
A table SHALL be pre-built (by the test administrator), defining all
the offered load rates that will be supported (R1 through Rn, in
ascending order, corresponding to indexed rows in the table). It is
RECOMMENDED that rates begin with 0.5 Mbps at index zero, use 1 Mbps
at index one, and then continue in 1 Mbps increments to 1 Gbps.
Above 1 Gbps, and up to 10 Gbps, it is RECOMMENDED that 100 Mbps
increments be used. Above 10 Gbps, increments of 1 Gbps are
RECOMMENDED. A higher initial IP-Layer Sender Bit Rate might be
configured when the test operator is certain that the Maximum IP-
Layer Capacity is well above the initial IP-Layer Sender Bit Rate and
factors such as test duration and total test traffic play an
important role. The sending rate table SHOULD bracket the Maximum
Capacity where it will make measurements, including constrained rates
less than 500 kbps if applicable.
Each rate is defined as datagrams of size ss, sent as a burst of
count cc, each time interval tt (the default for tt is 100 microsec,
a likely system tick interval). While it is advantageous to use
datagrams of as large a size as possible, it may be prudent to use a
slightly smaller maximum that allows for secondary protocol headers
and/or tunneling without resulting in IP-Layer fragmentation.
Selection of a new rate is indicated by a calculation on the current
row, Rx. For example:
"Rx+1": The Sender uses the next-higher rate in the table.
"Rx-10": The Sender uses the rate 10 rows lower in the table.
At the beginning of a test, the Sender begins sending at rate R1 and
the Receiver starts a feedback timer of duration FT (while awaiting
inbound datagrams). As datagrams are received, they are checked for
sequence number anomalies (loss, out-of-order, duplication, etc.) and
the delay range is measured (one-way or round-trip). This
information is accumulated until the feedback timer FT expires and a
status feedback message is sent from the Receiver back to the Sender,
to communicate this information. The accumulated statistics are then
reset by the Receiver for the next feedback interval. As feedback
messages are received back at the Sender, they are evaluated to
determine how to adjust the current offered load rate (Rx).
If the feedback indicates that no sequence number anomalies were
detected AND the delay range was below the lower threshold, the
offered load rate is increased. If congestion has not been confirmed
up to this point (see below for the method for declaring congestion),
the offered load rate is increased by more than one rate setting
(e.g., Rx+10). This allows the offered load to quickly reach a near-
maximum rate. Conversely, if congestion has been previously
confirmed, the offered load rate is only increased by one (Rx+1).
However, if a rate threshold above a high sending rate (such as 1
Gbps) is exceeded, the offered load rate is only increased by one
(Rx+1) in any congestion state.
If the feedback indicates that sequence number anomalies were
detected OR the delay range was above the upper threshold, the
offered load rate is decreased. The RECOMMENDED threshold values are
10 for sequence number gaps and 30 msec for lower and 90 msec for
upper delay thresholds, respectively. Also, if congestion is now
confirmed for the first time by the current feedback message being
processed, then the offered load rate is decreased by more than one
rate setting (e.g., Rx-30). This one-time reduction is intended to
compensate for the fast initial ramp-up. In all other cases, the
offered load rate is only decreased by one (Rx-1).
If the feedback indicates that there were no sequence number
anomalies AND the delay range was above the lower threshold but below
the upper threshold, the offered load rate is not changed. This
allows time for recent changes in the offered load rate to stabilize
and for the feedback to represent current conditions more accurately.
Lastly, the method for inferring congestion is that there were
sequence number anomalies AND/OR the delay range was above the upper
threshold for three consecutive feedback intervals. The algorithm
described above is also illustrated in Annex B of ITU-T
Recommendation Y.1540, 2020 version [Y.1540] and is implemented in
Appendix A ("Load Rate Adjustment Pseudocode") in this memo.
The load rate adjustment algorithm MUST include timers that stop the
test when received packet streams cease unexpectedly. The timeout
thresholds are provided in Table 1, along with values for all other
Parameters and variables described in this section. Operations of
non-obvious Parameters appear below:
load packet timeout:
The load packet timeout SHALL be reset to the configured value
each time a load packet is received. If the timeout expires, the
Receiver SHALL be closed and no further feedback sent.
feedback message timeout:
The feedback message timeout SHALL be reset to the configured
value each time a feedback message is received. If the timeout
expires, the Sender SHALL be closed and no further load packets
sent.
+=============+==========+===========+=========================+
| Parameter | Default | Tested | Expected Safe Range |
| | | Range or | (not entirely tested, |
| | | Values | other values NOT |
| | | | RECOMMENDED) |
+=============+==========+===========+=========================+
| FT, | 50 msec | 20 msec, | 20 msec <= FT <= 250 |
| feedback | | 50 msec, | msec; larger values may |
| time | | 100 msec | slow the rate increase |
| interval | | | and fail to find the |
| | | | max |
+-------------+----------+-----------+-------------------------+
| Feedback | L*FT, | L=100 | 0.5 sec <= L*FT <= 30 |
| message | L=20 (1 | with | sec; upper limit for |
| timeout | sec with | FT=50 | very unreliable test |
| (stop test) | FT=50 | msec (5 | paths only |
| | msec) | sec) | |
+-------------+----------+-----------+-------------------------+
| Load packet | 1 sec | 5 sec | 0.250-30 sec; upper |
| timeout | | | limit for very |
| (stop test) | | | unreliable test paths |
| | | | only |
+-------------+----------+-----------+-------------------------+
| Table index | 0.5 Mbps | 0.5 Mbps | When testing <= 10 Gbps |
| 0 | | | |
+-------------+----------+-----------+-------------------------+
| Table index | 1 Mbps | 1 Mbps | When testing <= 10 Gbps |
| 1 | | | |
+-------------+----------+-----------+-------------------------+
| Table index | 1 Mbps | 1 Mbps <= | Same as tested |
| (step) size | | rate <= 1 | |
| | | Gbps | |
+-------------+----------+-----------+-------------------------+
| Table index | 100 Mbps | 1 Gbps <= | Same as tested |
| (step) | | rate <= | |
| size, rate | | 10 Gbps | |
| > 1 Gbps | | | |
+-------------+----------+-----------+-------------------------+
| Table index | 1 Gbps | Untested | >10 Gbps |
| (step) | | | |
| size, rate | | | |
| > 10 Gbps | | | |
+-------------+----------+-----------+-------------------------+
| ss, UDP | None | <=1222 | Recommend max at |
| payload | | | largest value that |
| size, bytes | | | avoids fragmentation; |
| | | | using a payload size |
| | | | that is too small might |
| | | | result in unexpected |
| | | | Sender limitations |
+-------------+----------+-----------+-------------------------+
| cc, burst | None | 1 <= cc | Same as tested. Vary |
| count | | <= 100 | cc as needed to create |
| | | | the desired maximum |
| | | | sending rate. Sender |
| | | | buffer size may limit |
| | | | cc in the |
| | | | implementation |
+-------------+----------+-----------+-------------------------+
| tt, burst | 100 | 100 | Available range of |
| interval | microsec | microsec, | "tick" values (HZ |
| | | 1 msec | param) |
+-------------+----------+-----------+-------------------------+
| Low delay | 30 msec | 5 msec, | Same as tested |
| range | | 30 msec | |
| threshold | | | |
+-------------+----------+-----------+-------------------------+
| High delay | 90 msec | 10 msec, | Same as tested |
| range | | 90 msec | |
| threshold | | | |
+-------------+----------+-----------+-------------------------+
| Sequence | 10 | 0, 1, 5, | Same as tested |
| error | | 10, 100 | |
| threshold | | | |
+-------------+----------+-----------+-------------------------+
| Consecutive | 3 | 2, 3, 4, | Use values >1 to avoid |
| errored | | 5 | misinterpreting |
| status | | | transient loss |
| report | | | |
| threshold | | | |
+-------------+----------+-----------+-------------------------+
| Fast mode | 10 | 10 | 2 <= steps <= 30 |
| increase, | | | |
| in table | | | |
| index steps | | | |
+-------------+----------+-----------+-------------------------+
| Fast mode | 3 * Fast | 3 * Fast | Same as tested |
| decrease, | mode | mode | |
| in table | increase | increase | |
| index steps | | | |
+-------------+----------+-----------+-------------------------+
Table 1: Parameters for Load Rate Adjustment Algorithm
As a consequence of default parameterization, the Number of table
steps in total for rates less than 10 Gbps is 1090 (excluding index
0).
A related Sender backoff response to network conditions occurs when
one or more status feedback messages fail to arrive at the Sender.
If no status feedback messages arrive at the Sender for the interval
greater than the Lost Status Backoff timeout:
UDRT + (2+w)*FT = Lost Status Backoff timeout
where:
UDRT = upper delay range threshold (default 90 msec)
FT = feedback time interval (default 50 msec)
w = number of repeated timeouts (w=0 initially, w++ on each
timeout, and reset to 0 when a message is received)
Beginning when the last message (of any type) was successfully
received at the Sender:
The offered load SHALL then be decreased, following the same process
as when the feedback indicates the presence of one or more sequence
number anomalies OR the delay range was above the upper threshold (as
described above), with the same load rate adjustment algorithm
variables in their current state. This means that lost status
feedback messages OR sequence errors OR delay variation can result in
rate reduction and congestion confirmation.
The RECOMMENDED initial value for w is 0, taking a Round-Trip Time
(RTT) of less than FT into account. A test with an RTT longer than
FT is a valid reason to increase the initial value of w
appropriately. Variable w SHALL be incremented by one whenever the
Lost Status Backoff timeout is exceeded. So, with FT = 50 msec and
UDRT = 90 msec, a status feedback message loss would be declared at
190 msec following a successful message, again at 50 msec after that
(240 msec total), and so on.
Also, if congestion is now confirmed for the first time by a Lost
Status Backoff timeout, then the offered load rate is decreased by
more than one rate setting (e.g., Rx-30). This one-time reduction is
intended to compensate for the fast initial ramp-up. In all other
cases, the offered load rate is only decreased by one (Rx-1).
Appendix B discusses compliance with the applicable mandatory
requirements of [RFC8085], consistent with the goals of the IP-Layer
Capacity Metric and Method, including the load rate adjustment
algorithm described in this section.
8.2. Measurement Qualification or Verification
It is of course necessary to calibrate the equipment performing the
IP-Layer Capacity measurement, to ensure that the expected capacity
can be measured accurately and that equipment choices (processing
speed, interface bandwidth, etc.) are suitably matched to the
measurement range.
When assessing a maximum rate as the metric specifies, artificially
high (optimistic) values might be measured until some buffer on the
path is filled. Other causes include bursts of back-to-back packets
with idle intervals delivered by a path, while the measurement
interval (dt) is small and aligned with the bursts. The artificial
values might result in an unsustainable Maximum Capacity observed
when the Method of Measurement is searching for the maximum, and that
would not do. This situation is different from the bimodal service
rates (discussed in "Reporting the Metric", Section 6.6), which are
characterized by a multi-second duration (much longer than the
measured RTT) and repeatable behavior.
There are many ways that the Method of Measurement could handle this
false-max issue. The default value for measurement of Singletons (dt
= 1 second) has proven to be of practical value during tests of this
method, allows the bimodal service rates to be characterized, and has
an obvious alignment with the reporting units (Mbps).
Another approach comes from Section 24 of [RFC2544] and its
discussion of trial duration, where relatively short trials conducted
as part of the search are followed by longer trials to make the final
determination. In the production network, measurements of Singletons
and Samples (the terms for trials and tests of Lab Benchmarking) must
be limited in duration because they may affect service. But there is
sufficient value in repeating a Sample with a fixed sending rate
determined by the previous search for the Maximum IP-Layer Capacity,
to qualify the result in terms of the other performance metrics
measured at the same time.
A Qualification measurement for the search result is a subsequent
measurement, sending at a fixed 99.x percent of the Maximum IP-Layer
Capacity for I, or an indefinite period. The same Maximum Capacity
Metric is applied, and the Qualification for the result is a Sample
without supra-threshold packet losses or a growing minimum delay
trend in subsequent Singletons (or each dt of the measurement
interval, I). Samples exhibiting supra-threshold packet losses or
increasing queue occupation require a repeated search and/or test at
a reduced fixed Sender rate for Qualification.
Here, as with any Active Capacity test, the test duration must be
kept short. Ten-second tests for each direction of transmission are
common today. The default measurement interval specified here is I =
10 seconds. The combination of a fast and congestion-aware search
method and user-network coordination makes a unique contribution to
production testing. The Maximum IP Capacity Metric and Method for
assessing performance is very different from the classic Throughput
Metric and Methods provided in [RFC2544]: it uses near-real-time load
adjustments that are sensitive to loss and delay, similar to other
congestion control algorithms used on the Internet every day, along
with limited duration. On the other hand, Throughput measurements
[RFC2544] can produce sustained overload conditions for extended
periods of time. Individual trials in a test governed by a binary
search can last 60 seconds for each step, and the final confirmation
trial may be even longer. This is very different from "normal"
traffic levels, but overload conditions are not a concern in the
isolated test environment. The concerns raised in [RFC6815] were
that the methods discussed in [RFC2544] would be let loose on
production networks, and instead the authors challenged the standards
community to develop Metrics and Methods like those described in this
memo.
8.3. Measurement Considerations
In general, the widespread measurements that this memo encourages
will encounter widespread behaviors. The bimodal IP Capacity
behaviors already discussed in Section 6.6 are good examples.
In general, it is RECOMMENDED to locate test endpoints as close to
the intended measured link(s) as practical (for reasons of scale,
this is not always possible; there is a limit on the number of test
endpoints coming from many perspectives -- for example, management
and measurement traffic). The testing operator MUST set a value for
the MaxHops Parameter, based on the expected path length. This
Parameter can keep measurement traffic from straying too far beyond
the intended path.
The measured path may be stateful based on many factors, and the
Parameter "Time of day" when a test starts may not be enough
information. Repeatable testing may require knowledge of the time
from the beginning of a measured flow -- and how the flow is
constructed, including how much traffic has already been sent on that
flow when a state change is observed -- because the state change may
be based on time, bytes sent, or both. Both load packets and status
feedback messages MUST contain sequence numbers; this helps with
measurements based on those packets.
Many different types of traffic shapers and on-demand communications
access technologies may be encountered, as anticipated in [RFC7312],
and play a key role in measurement results. Methods MUST be prepared
to provide a short preamble transmission to activate on-demand
communications access and to discard the preamble from subsequent
test results.
The following conditions might be encountered during measurement,
where packet losses may occur independently of the measurement
sending rate:
1. Congestion of an interconnection or backbone interface may appear
as packet losses distributed over time in the test stream, due to
much-higher-rate interfaces in the backbone.
2. Packet loss due to the use of Random Early Detection (RED) or
other active queue management may or may not affect the
measurement flow if competing background traffic (other flows) is
simultaneously present.
3. There may be only a small delay variation independent of the
sending rate under these conditions as well.
4. Persistent competing traffic on measurement paths that include
shared transmission media may cause random packet losses in the
test stream.
It is possible to mitigate these conditions using the flexibility of
the load rate adjustment algorithm described in Section 8.1 above
(tuning specific Parameters).
If the measurement flow burst duration happens to be on the order of
or smaller than the burst size of a shaper or a policer in the path,
then the line rate might be measured rather than the bandwidth limit
imposed by the shaper or policer. If this condition is suspected,
alternate configurations SHOULD be used.
In general, results depend on the sending stream's characteristics;
the measurement community has known this for a long time and needs to
keep it foremost in mind. Although the default is a single flow
(F=1) for testing, the use of multiple flows may be advantageous for
the following reasons:
1. The test hosts may be able to create a higher load than with a
single flow, or parallel test hosts may be used to generate one
flow each.
2. Link aggregation may be present (flow-based load balancing), and
multiple flows are needed to occupy each member of the aggregate.
3. Internet access policies may limit the IP-Layer Capacity
depending on the Type-P of the packets, possibly reserving
capacity for various stream types.
Each flow would be controlled using its own implementation of the
load rate adjustment (search) algorithm.
It is obviously counterproductive to run more than one independent
and concurrent test (regardless of the number of flows in the test
stream) attempting to measure the *maximum* capacity on a single
path. The number of concurrent, independent tests of a path SHALL be
limited to one.
Tests of a v4-v6 transition mechanism might well be the intended
subject of a capacity test. As long as both IPv4 packets and IPv6
packets sent/received are standard-formed, this should be allowed
(and the change in header size easily accounted for on a per-packet
basis).
As testing continues, implementers should expect the methods to
evolve. The ITU-T has published a supplement (Supplement 60) to the
Y-series of ITU-T Recommendations, "Interpreting ITU-T Y.1540 maximum
IP-layer capacity measurements" [Y.Sup60], which is the result of
continued testing with the metric. Those results have improved the
methods described here.
9. Reporting Formats
The Singleton IP-Layer Capacity results SHOULD be accompanied by the
context under which they were measured.
* Timestamp (especially the time when the maximum was observed in
dtn).
* Source and Destination (by IP or other meaningful ID).
* Other inner Parameters of the test case (Section 4).
* Outer Parameters, such as "test conducted in motion" or other
factors belonging to the context of the measurement.
* Result validity (indicating cases where the process was somehow
interrupted or the attempt failed).
* A field where unusual circumstances could be documented, and
another one for "ignore / mask out" purposes in further
processing.
The Maximum IP-Layer Capacity results SHOULD be reported in tabular
format. There SHOULD be a column that identifies the test Phase.
There SHOULD be a column listing the number of flows used in that
Phase. The remaining columns SHOULD report the following results for
the aggregate of all flows, including the Maximum IP-Layer Capacity,
the Loss Ratio, the RTT minimum, RTT maximum, and other metrics
tested having similar relevance.
As mentioned in Section 6.6, bimodal (or multi-modal) maxima SHALL be
reported for each mode separately.
+========+==========+==================+========+=========+=========+
| Phase | Number | Maximum IP-Layer | Loss | RTT min | RTT |
| | of Flows | Capacity (Mbps) | Ratio | (msec) | max |
| | | | | | (msec) |
+========+==========+==================+========+=========+=========+
| Search | 1 | 967.31 | 0.0002 | 30 | 58 |
+--------+----------+------------------+--------+---------+---------+
| Verify | 1 | 966.00 | 0.0000 | 30 | 38 |
+--------+----------+------------------+--------+---------+---------+
Table 2: Maximum IP-Layer Capacity Results
Static and configuration Parameters:
The sub-interval time, dt, MUST accompany a report of Maximum IP-
Layer Capacity results, as well as the remaining Parameters from
Section 4 ("General Parameters and Definitions").
The PM list metrics corresponding to the sub-interval where the
Maximum Capacity occurred MUST accompany a report of Maximum IP-Layer
Capacity results, for each test Phase.
The IP-Layer Sender Bit Rate results SHOULD be reported in tabular
format. There SHOULD be a column that identifies the test Phase.
There SHOULD be a column listing each individual (numbered) flow used
in that Phase, or the aggregate of flows in that Phase. A
corresponding column SHOULD identify the specific sending rate sub-
interval, stn, for each flow and aggregate. A final column SHOULD
report the IP-Layer Sender Bit Rate results for each flow used, or
the aggregate of all flows.
+========+==========================+===========+=============+
| Phase | Flow Number or Aggregate | stn (sec) | Sender Bit |
| | | | Rate (Mbps) |
+========+==========================+===========+=============+
| Search | 1 | 0.00 | 345 |
+--------+--------------------------+-----------+-------------+
| Search | 2 | 0.00 | 289 |
+--------+--------------------------+-----------+-------------+
| Search | Agg | 0.00 | 634 |
+--------+--------------------------+-----------+-------------+
| Search | 1 | 0.05 | 499 |
+--------+--------------------------+-----------+-------------+
| Search | ... | 0.05 | ... |
+--------+--------------------------+-----------+-------------+
Table 3: IP-Layer Sender Bit Rate Results (Example with Two
Flows and st = 0.05 (sec))
Static and configuration Parameters:
The sub-interval duration, st, MUST accompany a report of Sender IP-
Layer Bit Rate results.
Also, the values of the remaining Parameters from Section 4 ("General
Parameters and Definitions") MUST be reported.
9.1. Configuration and Reporting Data Formats
As a part of the multi-Standards Development Organization (SDO)
harmonization of this Metric and Method of Measurement, one of the
areas where the Broadband Forum (BBF) contributed its expertise was
in the definition of an information model and data model for
configuration and reporting. These models are consistent with the
metric Parameters and default values specified as lists in this memo.
[TR-471] provides the information model that was used to prepare a
full data model in related BBF work. The BBF has also carefully
considered topics within its purview, such as the placement of
measurement systems within the Internet access architecture. For
example, timestamp resolution requirements that influence the choice
of the test protocol are provided in Table 2 of [TR-471].
10. Security Considerations
Active Metrics and Active Measurements have a long history of
security considerations. The security considerations that apply to
any Active Measurement of live paths are relevant here. See
[RFC4656] and [RFC5357].
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.
There are some new considerations for Capacity measurement as
described in this memo.
1. Cooperating Source and Destination hosts and agreements to test
the path between the hosts are REQUIRED. Hosts perform in either
the Src role or the Dst role.
2. It is REQUIRED to have a user client-initiated setup handshake
between cooperating hosts that allows firewalls to control
inbound unsolicited UDP traffic that goes to either a control
port (expected and with authentication) or ephemeral ports that
are only created as needed. Firewalls protecting each host can
both continue to do their job normally.
3. Client-server authentication and integrity protection for
feedback messages conveying measurements are RECOMMENDED.
4. Hosts MUST limit the number of simultaneous tests to avoid
resource exhaustion and inaccurate results.
5. Senders MUST be rate limited. This can be accomplished using a
pre-built table defining all the offered load rates that will be
supported (Section 8.1). The recommended load control search
algorithm results in "ramp-up" from the lowest rate in the table.
6. Service subscribers with limited data volumes who conduct
extensive capacity testing might experience the effects of
Service Provider controls on their service. Testing with the
Service Provider's measurement hosts SHOULD be limited in
frequency and/or overall volume of test traffic (for example, the
range of duration values, I, SHOULD be limited).
The exact specification of these features is left for future protocol
development.
11. IANA Considerations
This document has no IANA actions.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
[RFC4737] Morton, A., Ciavattone, L., Ramachandran, G., Shalunov,
S., and J. Perser, "Packet Reordering Metrics", RFC 4737,
DOI 10.17487/RFC4737, November 2006,
<https://www.rfc-editor.org/info/rfc4737>.
[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>.
[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>.
[RFC7497] Morton, A., "Rate Measurement Test Protocol Problem
Statement and Requirements", RFC 7497,
DOI 10.17487/RFC7497, April 2015,
<https://www.rfc-editor.org/info/rfc7497>.
[RFC7680] Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
Ed., "A One-Way Loss Metric for IP Performance Metrics
(IPPM)", STD 82, RFC 7680, DOI 10.17487/RFC7680, January
2016, <https://www.rfc-editor.org/info/rfc7680>.
[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>.
12.2. Informative References
[copycat] Edeline, K., Kühlewind, M., Trammell, B., and B. Donnet,
"copycat: Testing Differential Treatment of New Transport
Protocols in the Wild", ANRW '17,
DOI 10.1145/3106328.3106330, July 2017,
<https://irtf.org/anrw/2017/anrw17-final5.pdf>.
[LS-SG12-A]
"Liaison statement: LS - Harmonization of IP Capacity and
Latency Parameters: Revision of Draft Rec. Y.1540 on IP
packet transfer performance parameters and New Annex A
with Lab Evaluation Plan", From ITU-T SG 12, March 2019,
<https://datatracker.ietf.org/liaison/1632/>.
[LS-SG12-B]
"Liaison statement: LS on harmonization of IP Capacity and
Latency Parameters: Consent of Draft Rec. Y.1540 on IP
packet transfer performance parameters and New Annex A
with Lab & Field Evaluation Plans", From ITU-T-SG-12, May
2019, <https://datatracker.ietf.org/liaison/1645/>.
[RFC2544] Bradner, S. and J. McQuaid, "Benchmarking Methodology for
Network Interconnect Devices", RFC 2544,
DOI 10.17487/RFC2544, March 1999,
<https://www.rfc-editor.org/info/rfc2544>.
[RFC3148] Mathis, M. and M. Allman, "A Framework for Defining
Empirical Bulk Transfer Capacity Metrics", RFC 3148,
DOI 10.17487/RFC3148, July 2001,
<https://www.rfc-editor.org/info/rfc3148>.
[RFC5136] Chimento, P. and J. Ishac, "Defining Network Capacity",
RFC 5136, DOI 10.17487/RFC5136, February 2008,
<https://www.rfc-editor.org/info/rfc5136>.
[RFC6815] Bradner, S., Dubray, K., McQuaid, J., and A. Morton,
"Applicability Statement for RFC 2544: Use on Production
Networks Considered Harmful", RFC 6815,
DOI 10.17487/RFC6815, November 2012,
<https://www.rfc-editor.org/info/rfc6815>.
[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>.
[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>.
[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>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8337] Mathis, M. and A. Morton, "Model-Based Metrics for Bulk
Transport Capacity", RFC 8337, DOI 10.17487/RFC8337, March
2018, <https://www.rfc-editor.org/info/rfc8337>.
[TR-471] Morton, A., "Maximum IP-Layer Capacity Metric, Related
Metrics, and Measurements", Broadband Forum TR-471, July
2020, <https://www.broadband-forum.org/technical/download/
TR-471.pdf>.
[Y.1540] ITU-T, "Internet protocol data communication service - IP
packet transfer and availability performance parameters",
ITU-T Recommendation Y.1540, December 2019,
<https://www.itu.int/rec/T-REC-Y.1540-201912-I/en>.
[Y.Sup60] ITU-T, "Interpreting ITU-T Y.1540 maximum IP-layer
capacity measurements", ITU-T Recommendation Y.Sup60,
October 2021, <https://www.itu.int/rec/T-REC-Y.Sup60/en>.
Appendix A. Load Rate Adjustment Pseudocode
This appendix provides a pseudocode implementation of the algorithm
described in Section 8.1.
Rx = 0 # The current sending rate (equivalent to a row
# of the table)
seqErr = 0 # Measured count that includes Loss or Reordering
# or Duplication impairments (all appear
# initially as errors in the packet sequence
# numbering)
seqErrThresh = 10 # Threshold on seqErr count that includes Loss or
# Reordering or Duplication impairments (all
# appear initially as errors in the packet
# sequence numbering)
delay = 0 # Measured Range of Round Trip Delay (RTD), msec
lowThresh = 30 # Low threshold on the Range of RTD, msec
upperThresh = 90 # Upper threshold on the Range of RTD, msec
hSpeedThresh = 1 # Threshold for transition between sending rate
# step sizes (such as 1 Mbps and 100 Mbps), Gbps
slowAdjCount = 0 # Measured Number of consecutive status reports
# indicating loss and/or delay variation above
# upperThresh
slowAdjThresh = 3 # Threshold on slowAdjCount used to infer
# congestion. Use values > 1 to avoid
# misinterpreting transient loss.
highSpeedDelta = 10 # The number of rows to move in a single
# adjustment when initially increasing offered
# load (to ramp up quickly)
maxLoadRates = 2000 # Maximum table index (rows)
if ( seqErr <= seqErrThresh && delay < lowThresh ) {
if ( Rx < hSpeedThresh && slowAdjCount < slowAdjThresh ) {
Rx += highSpeedDelta;
slowAdjCount = 0;
} else {
if ( Rx < maxLoadRates - 1 )
Rx++;
}
} else if ( seqErr > seqErrThresh || delay > upperThresh ) {
slowAdjCount++;
if ( Rx < hSpeedThresh && slowAdjCount == slowAdjThresh ) {
if ( Rx > highSpeedDelta * 3 )
Rx -= highSpeedDelta * 3;
else
Rx = 0;
} else {
if ( Rx > 0 )
Rx--;
}
}
Appendix B. RFC 8085 UDP Guidelines Check
Section 3.1 of [RFC8085] (BCP 145), which provides UDP usage
guidelines, focuses primarily on congestion control. The guidelines
appear in mandatory (MUST) and recommendation (SHOULD) categories.
B.1. Assessment of Mandatory Requirements
The mandatory requirements in Section 3 of [RFC8085] include the
following:
| Internet paths can have widely varying characteristics, ...
| Consequently, applications that may be used on the Internet MUST
| NOT make assumptions about specific path characteristics. They
| MUST instead use mechanisms that let them operate safely under
| very different path conditions. Typically, this requires
| conservatively probing the current conditions of the Internet path
| they communicate over to establish a transmission behavior that it
| can sustain and that is reasonably fair to other traffic sharing
| the path.
The purpose of the load rate adjustment algorithm described in
Section 8.1 is to probe the network and enable Maximum IP-Layer
Capacity measurements with as few assumptions about the measured path
as possible and within the range of applications described in
Section 2. There is tension between the goals of probing
conservatism and minimization of both the traffic dedicated to
testing (especially with Gigabit rate measurements) and the duration
of the test (which is one contributing factor to the overall
algorithm fairness).
The text of Section 3 of [RFC8085] goes on to recommend alternatives
to UDP to meet the mandatory requirements, but none are suitable for
the scope and purpose of the Metrics and Methods in this memo. In
fact, ad hoc TCP-based methods fail to achieve the measurement
accuracy repeatedly proven in comparison measurements with the
running code [LS-SG12-A] [LS-SG12-B] [Y.Sup60]. Also, the UDP aspect
of these methods is present primarily to support modern Internet
transmission where a transport protocol is required [copycat]; the
metric is based on the IP Layer, and UDP allows simple correlation to
the IP Layer.
Section 3.1.1 of [RFC8085] discusses protocol timer guidelines:
| Latency samples MUST NOT be derived from ambiguous transactions.
| The canonical example is in a protocol that retransmits data, but
| subsequently cannot determine which copy is being acknowledged.
Both load packets and status feedback messages MUST contain sequence
numbers; this helps with measurements based on those packets, and
there are no retransmissions needed.
| When a latency estimate is used to arm a timer that provides loss
| detection -- with or without retransmission -- expiry of the timer
| MUST be interpreted as an indication of congestion in the network,
| causing the sending rate to be adapted to a safe conservative rate
| ...
The methods described in this memo use timers for sending rate
backoff when status feedback messages are lost (Lost Status Backoff
timeout) and for stopping a test when connectivity is lost for a
longer interval (feedback message or load packet timeouts).
This memo does not foresee any specific benefit of using Explicit
Congestion Notification (ECN).
Section 3.2 of [RFC8085] discusses message size guidelines:
| To determine an appropriate UDP payload size, applications MUST
| subtract the size of the IP header (which includes any IPv4
| optional headers or IPv6 extension headers) as well as the length
| of the UDP header (8 bytes) from the PMTU size.
The method uses a sending rate table with a maximum UDP payload size
that anticipates significant header overhead and avoids
fragmentation.
Section 3.3 of [RFC8085] provides reliability guidelines:
| Applications that do require reliable message delivery MUST
| implement an appropriate mechanism themselves.
The IP-Layer Capacity Metrics and Methods do not require reliable
delivery.
| Applications that require ordered delivery MUST reestablish
| datagram ordering themselves.
The IP-Layer Capacity Metrics and Methods do not need to reestablish
packet order; it is preferable to measure packet reordering if it
occurs [RFC4737].
B.2. Assessment of Recommendations
The load rate adjustment algorithm's goal is to determine the Maximum
IP-Layer Capacity in the context of an infrequent, diagnostic, short-
term measurement. This goal is a global exception to many SHOULD-
level requirements as listed in [RFC8085], of which many are intended
for long-lived flows that must coexist with other traffic in a more
or less fair way. However, the algorithm (as specified in
Section 8.1 and Appendix A above) reacts to indications of congestion
in clearly defined ways.
A specific recommendation is provided as an example. Section 3.1.5
of [RFC8085] (regarding the implications of RTT and loss measurements
on congestion control) says:
| A congestion control [algorithm] designed for UDP SHOULD respond
| as quickly as possible when it experiences congestion, and it
| SHOULD take into account both the loss rate and the response time
| when choosing a new rate.
The load rate adjustment algorithm responds to loss and RTT
measurements with a clear and concise rate reduction when warranted,
and the response makes use of direct measurements (more exact than
can be inferred from TCP ACKs).
Section 3.1.5 of [RFC8085] goes on to specify the following:
| The implemented congestion control scheme SHOULD result in
| bandwidth (capacity) use that is comparable to that of TCP within
| an order of magnitude, so that it does not starve other flows
| sharing a common bottleneck.
This is a requirement for coexistent streams, and not for diagnostic
and infrequent measurements using short durations. The rate
oscillations during short tests allow other packets to pass and don't
starve other flows.
Ironically, ad hoc TCP-based measurements of "Internet Speed" are
also designed to work around this SHOULD-level requirement, by
launching many flows (9, for example) to increase the outstanding
data dedicated to testing.
The load rate adjustment algorithm cannot become a TCP-like
congestion control, or it will have the same weaknesses of TCP when
trying to make a Maximum IP-Layer Capacity measurement and will not
achieve the goal. The results of the referenced testing [LS-SG12-A]
[LS-SG12-B] [Y.Sup60] supported this statement hundreds of times,
with comparisons to multi-connection TCP-based measurements.
A brief review of requirements from [RFC8085] follows (marked "Yes"
when this memo is compliant, or "NA" (Not Applicable)):
+======+============================================+=========+
| Yes? | Recommendation in RFC 8085 | Section |
+======+============================================+=========+
| Yes | MUST tolerate a wide range of Internet | 3 |
| | path conditions | |
+------+--------------------------------------------+---------+
| NA | SHOULD use a full-featured transport | |
| | (e.g., TCP) | |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| Yes | SHOULD control rate of transmission | 3.1 |
+------+--------------------------------------------+---------+
| NA | SHOULD perform congestion control over all | |
| | traffic | |
+------+--------------------------------------------+---------+
+======+============================================+=========+
| | For bulk transfers, | 3.1.2 |
+======+============================================+=========+
| NA | SHOULD consider implementing TFRC | |
+------+--------------------------------------------+---------+
| NA | else, SHOULD in other ways use bandwidth | |
| | similar to TCP | |
+------+--------------------------------------------+---------+
+======+============================================+=========+
| | For non-bulk transfers, | 3.1.3 |
+======+============================================+=========+
| NA | SHOULD measure RTT and transmit max. 1 | 3.1.1 |
| | datagram/RTT | |
+------+--------------------------------------------+---------+
| NA | else, SHOULD send at most 1 datagram every | |
| | 3 seconds | |
+------+--------------------------------------------+---------+
| NA | SHOULD back-off retransmission timers | |
| | following loss | |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| Yes | SHOULD provide mechanisms to regulate the | 3.1.6 |
| | bursts of transmission | |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| NA | MAY implement ECN; a specific set of | 3.1.7 |
| | application mechanisms are REQUIRED if ECN | |
| | is used | |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| Yes | For DiffServ, SHOULD NOT rely on | 3.1.8 |
| | implementation of PHBs | |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| Yes | For QoS-enabled paths, MAY choose not to | 3.1.9 |
| | use CC | |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| Yes | SHOULD NOT rely solely on QoS for their | 3.1.10 |
| | capacity | |
+------+--------------------------------------------+---------+
| NA | non-CC controlled flows SHOULD implement a | |
| | transport circuit breaker | |
+------+--------------------------------------------+---------+
| Yes | MAY implement a circuit breaker for other | |
| | applications | |
+------+--------------------------------------------+---------+
+======+============================================+=========+
| | For tunnels carrying IP traffic, | 3.1.11 |
+======+============================================+=========+
| NA | SHOULD NOT perform congestion control | |
+------+--------------------------------------------+---------+
| NA | MUST correctly process the IP ECN field | |
+------+--------------------------------------------+---------+
+======+============================================+=========+
| | For non-IP tunnels or rate not determined | 3.1.11 |
| | by traffic, | |
+======+============================================+=========+
| NA | SHOULD perform CC or use circuit breaker | |
+------+--------------------------------------------+---------+
| NA | SHOULD restrict types of traffic | |
| | transported by the tunnel | |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| Yes | SHOULD NOT send datagrams that exceed the | 3.2 |
| | PMTU, i.e., | |
+------+--------------------------------------------+---------+
| Yes | SHOULD discover PMTU or send datagrams < | |
| | minimum PMTU | |
+------+--------------------------------------------+---------+
| NA | Specific application mechanisms are | |
| | REQUIRED if PLPMTUD is used | |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| Yes | SHOULD handle datagram loss, duplication, | 3.3 |
| | reordering | |
+------+--------------------------------------------+---------+
| NA | SHOULD be robust to delivery delays up to | |
| | 2 minutes | |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| Yes | SHOULD enable IPv4 UDP checksum | 3.4 |
+------+--------------------------------------------+---------+
| Yes | SHOULD enable IPv6 UDP checksum; specific | 3.4.1 |
| | application mechanisms are REQUIRED if a | |
| | zero IPv6 UDP checksum is used | |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| NA | SHOULD provide protection from off-path | 5.1 |
| | attacks | |
+------+--------------------------------------------+---------+
| | else, MAY use UDP-Lite with suitable | 3.4.2 |
| | checksum coverage | |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| NA | SHOULD NOT always send middlebox keep- | 3.5 |
| | alive messages | |
+------+--------------------------------------------+---------+
| NA | MAY use keep-alives when needed (min. | |
| | interval 15 sec) | |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| Yes | Applications specified for use in limited | 3.6 |
| | use (or controlled environments) SHOULD | |
| | identify equivalent mechanisms and | |
| | describe their use case | |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| NA | Bulk-multicast apps SHOULD implement | 4.1.1 |
| | congestion control | |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| NA | Low volume multicast apps SHOULD implement | 4.1.2 |
| | congestion control | |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| NA | Multicast apps SHOULD use a safe PMTU | 4.2 |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| Yes | SHOULD avoid using multiple ports | 5.1.2 |
+------+--------------------------------------------+---------+
| Yes | MUST check received IP source address | |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| NA | SHOULD validate payload in ICMP messages | 5.2 |
+------+--------------------------------------------+---------+
+------+--------------------------------------------+---------+
| Yes | SHOULD use a randomized Source port or | 6 |
| | equivalent technique, and, for client/ | |
| | server applications, SHOULD send responses | |
| | from source address matching request | |
+------+--------------------------------------------+---------+
| NA | SHOULD use standard IETF security | 6 |
| | protocols when needed | |
+------+--------------------------------------------+---------+
Table 4: Summary of Key Guidelines from RFC 8085
Acknowledgments
Thanks to Joachim Fabini, Matt Mathis, J. Ignacio Alvarez-Hamelin,
Wolfgang Balzer, Frank Brockners, Greg Mirsky, Martin Duke, Murray
Kucherawy, and Benjamin Kaduk for their extensive comments on this
memo and related topics. In a second round of reviews, we
acknowledge Magnus Westerlund, Lars Eggert, and Zaheduzzaman Sarker.
Authors' Addresses
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
Rüdiger Geib
Deutsche Telekom
Heinrich Hertz Str. 3-7
64295 Darmstadt
Germany
Phone: +49 6151 5812747
Email: Ruediger.Geib@telekom.de
Len Ciavattone
AT&T Labs
200 Laurel Avenue South
Middletown, NJ 07748
United States of America
Phone: +1 732 420 1239
Email: lencia@att.com