As per Relevance of the word september, we have this rfc below:
Network Working Group G.
Request for Comments: 2681 S.
Category: Standards Track M.
Advanced Network &
September 1999
A Round-trip Delay Metric for
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (1999). All Rights Reserved
1.
This memo defines a metric for round-trip delay of packets
Internet paths. It builds on notions introduced and discussed in
IPPM Framework document, RFC 2330 [1], and follows closely
corresponding metric for One-way Delay ("A One-way Delay Metric
IPPM") [2]; the reader is assumed to be familiar with
documents
The memo was largely written by copying material from the One-
Delay metric. The intention is that, where the two metrics
similar, they will be described with similar or identical text,
that where the two metrics differ, new or modified text will be used
This memo is intended to be parallel in structure to a
companion document for Packet Loss
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
document are to be interpreted as described in RFC 2119 [6].
Although RFC 2119 was written with protocols in mind, the key
are used in this document for similar reasons. They are used
ensure the results of measurements from two different
are comparable, and to note instances when an implementation
perturb the network
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The structure of the memo is as follows
+ A 'singleton' analytic metric, called Type-P-Round-trip-Delay
will be introduced to measure a single observation of round-
delay
+ Using this singleton metric, a 'sample', called Type-P-Round-trip
Delay-Poisson-Stream, will be introduced to measure a sequence
singleton delays measured at times taken from a Poisson process
+ Using this sample, several 'statistics' of the sample will
defined and discussed
This progression from singleton to sample to statistics, with
separation among them, is important
Whenever a technical term from the IPPM Framework document is
used in this memo, it will be tagged with a trailing asterisk.
example, "term*" indicates that "term" is defined in the Framework
1.1.
Round-trip delay of a Type-P* packet from a source host* to
destination host is useful for several reasons
+ Some applications do not perform well (or at all) if end-to-
delay between hosts is large relative to some threshold value
+ Erratic variation in delay makes it difficult (or impossible)
support many interactive real-time applications
+ The larger the value of delay, the more difficult it is
transport-layer protocols to sustain high bandwidths
+ The minimum value of this metric provides an indication of
delay due only to propagation and transmission delay
+ The minimum value of this metric provides an indication of
delay that will likely be experienced when the path* traversed
lightly loaded
+ Values of this metric above the minimum provide an indication
the congestion present in the path
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The measurement of round-trip delay instead of one-way delay
several weaknesses, summarized here
+ The Internet path from a source to a destination may differ
the path from the destination back to the source ("
paths"), such that different sequences of routers are used for
forward and reverse paths. Therefore round-trip
actually measure the performance of two distinct paths together
+ Even when the two paths are symmetric, they may have
different performance characteristics due to asymmetric queueing
+ Performance of an application may depend mostly on the
in one direction
+ In quality-of-service (QoS) enabled networks, provisioning in
direction may be radically different than provisioning in
reverse direction, and thus the QoS guarantees differ
On the other hand, the measurement of round-trip delay has
specific advantages
+ Ease of deployment: unlike in one-way measurement, it is
possible to perform some form of round-trip delay
without installing measurement-specific software at the
destination. A variety of approaches are well-known,
use of ICMP Echo or of TCP-based methodologies (similar to
outlined in "IPPM Metrics for Measuring Connectivity" [4]).
However, some approaches may introduce greater uncertainty in
time for the destination to produce a response (
Section 2.7.3).
+ Ease of interpretation: in some circumstances, the round-trip
is in fact the quantity of interest. Deducing the round-trip
from matching one-way measurements and an assumption of
destination processing time is less direct and potentially
accurate
1.2. General Issues Regarding
Whenever a time (i.e., a moment in history) is mentioned here, it
understood to be measured in seconds (and fractions) relative to UTC
As described more fully in the Framework document, there are
distinct, but related notions of clock uncertainty
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synchronization
measures the extent to which two clocks agree on what time
is. For example, the clock on one host might be 5.4 msec
of the clock on a second host
accuracy
measures the extent to which a given clock agrees with UTC.
example, the clock on a host might be 27.1 msec behind UTC
resolution
measures the precision of a given clock. For example, the
on an old Unix host might tick only once every 10 msec, and
have a resolution of only 10 msec
skew
measures the change of accuracy, or of synchronization,
time. For example, the clock on a given host might gain 1.3
msec per hour and thus be 27.1 msec behind UTC at one time
only 25.8 msec an hour later. In this case, we say that
clock of the given host has a skew of 1.3 msec per hour
to UTC, which threatens accuracy. We might also speak of
skew of one clock relative to another clock, which
synchronization
2. A Singleton Definition for Round-trip
2.1. Metric Name
Type-P-Round-trip-
2.2. Metric Parameters
+ Src, the IP address of a
+ Dst, the IP address of a
+ T, a
2.3. Metric Units
The value of a Type-P-Round-trip-Delay is either a real number, or
undefined (informally, infinite) number of seconds
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2.4. Definition
For a real number dT, >>the *Type-P-Round-trip-Delay* from Src to
at T is dT<< means that Src sent the first bit of a Type-P packet
Dst at wire-time* T, that Dst received that packet, then
sent a Type-P packet back to Src, and that Src received the last
of that packet at wire-time T+dT
>>The *Type-P-Round-trip-Delay* from Src to Dst at T is
(informally, infinite)<< means that Src sent the first bit of
Type-P packet to Dst at wire-time T and that (either Dst did
receive the packet, Dst did not send a Type-P packet in response, or
Src did not receive that response packet
>>The *Type-P-Round-trip-Delay between Src and Dst at T<<
either the *Type-P-Round-trip-Delay from Src to Dst at T or
*Type-P-Round-trip-Delay from Dst to Src at T. When this notion
used, it is understood to be specifically ambiguous which host
as Src and which as Dst. {Comment: This ambiguity will usually be
small price to pay for being able to have one measurement,
from either Src or Dst, rather than having two measurements.}
Suggestions for what to report along with metric values appear
Section 3.8 after a discussion of the metric, methodologies
measuring the metric, and error analysis
2.5. Discussion
Type-P-Round-trip-Delay is a relatively simple analytic metric,
one that we believe will afford effective methods of measurement
The following issues are likely to come up in practice
+ The timestamp values (T) for the time at which delays are
should be fairly accurate in order to draw meaningful
about the state of the network at a given T. Therefore,
should have an accurate knowledge of time-of-day. NTP [3]
one way to achieve time accuracy to within several milliseconds
Depending on the NTP server, higher accuracy may be achieved,
example when NTP servers make use of GPS systems as a time source
Note that NTP will adjust the instrument's clock. If
adjustment is made between the time the initial timestamp is
and the time the final timestamp is taken the adjustment
affect the uncertainty in the measured delay. This
must be accounted for in the instrument's calibration
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+ A given methodology will have to include a way to
whether a delay value is infinite or whether it is merely
large (and the packet is yet to arrive at Dst). As noted
Mahdavi and Paxson [4], simple upper bounds (such as the 255
seconds theoretical upper bound on the lifetimes of
packets [5]) could be used, but good engineering, including
understanding of packet lifetimes, will be needed in practice
{Comment: Note that, for many applications of these metrics,
harm in treating a large delay as infinite might be zero or
small. A TCP data packet, for example, that arrives only
several multiples of the RTT may as well have been lost.}
+ If the packet is duplicated so that multiple non-corrupt
of the response arrive back at the source, then the packet
counted as received, and the first instance to arrive back at
source determines the packet's round-trip delay
+ If the packet is fragmented and if, for whatever reason
reassembly does not occur, then the packet will be deemed lost
2.6. Methodologies
As with other Type-P-* metrics, the detailed methodology will
on the Type-P (e.g., protocol number, UDP/TCP port number, size
precedence).
Generally, for a given Type-P, the methodology would proceed
follows
+ At the Src host, select Src and Dst IP addresses, and form a
packet of Type-P with these addresses. Any 'padding' portion
the packet needed only to make the test packet a given size
be filled with randomized bits to avoid a situation in which
measured delay is lower than it would otherwise be due
compression techniques along the path. The test packet must
some identifying information so that the response to it can
identified by Src when Src receives the response; one means to
this is by placing the timestamp generated just before sending
test packet in the packet itself
+ At the Dst host, arrange to receive and respond to the
packet. At the Src host, arrange to receive the
response packet
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+ At the Src host, take the initial timestamp and then send
prepared Type-P packet towards Dst. Note that the timestamp
be placed inside the packet, or kept separately as long as
packet contains a suitable identifier so the received
can be compared with the send timestamp
+ If the packet arrives at Dst, send a corresponding response
back from Dst to Src as soon as possible
+ If the response packet arrives within a reasonable period of time
take the final timestamp as soon as possible upon the receipt
the packet. By subtracting the two timestamps, an estimate
round-trip delay can be computed. If the delay between
initial timestamp and the actual sending of the packet is known
then the estimate could be adjusted by subtracting this amount
uncertainty in this value must be taken into account in
analysis. Similarly, if the delay between the actual receipt
the response packet and final timestamp is known, then
estimate could be adjusted by subtracting this amount;
in this value must be taken into account in error analysis.
the next section, "Errors and Uncertainties", for a more
discussion
+ If the packet fails to arrive within a reasonable period of time
the round-trip delay is taken to be undefined (informally
infinite). Note that the threshold of 'reasonable' is a
of the methodology
Issues such as the packet format and the means by which Dst
when to expect the test packet are outside the scope of
document
{Comment: Note that you cannot in general add two Type-P-One-way
Delay values (see [2]) to form a Type-P-Round-trip-Delay value.
order to form a Type-P-Round-trip-Delay value, the return packet
be triggered by the reception of a packet from Src.}
{Comment: "ping" would qualify as a round-trip measure under
definition, with a Type-P of ICMP echo request/reply with 60-
packets. However, the uncertainties associated with a typical
program must be analyzed as in the next section, including the
of reflecting point (a router may not handle an ICMP request in
fast path) and effects of load on the reflecting point.}
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2.7. Errors and Uncertainties
The description of any specific measurement method should include
accounting and analysis of various sources of error or uncertainty
The Framework document provides general guidance on this point,
we note here the following specifics related to delay metrics
+ Errors or uncertainties due to uncertainty in the clock of the
host
+ Errors or uncertainties due to the difference between 'wire time
and 'host time'.
+ Errors or uncertainties due to time required by the Dst to
the packet from the Src and send the corresponding response
In addition, the loss threshold may affect the results. Each
these are discussed in more detail below, along with a
("Calibration") on accounting for these errors and uncertainties
2.7.1. Errors or Uncertainties Related to
The uncertainty in a measurement of round-trip delay is related,
part, to uncertainty in the clock of the Src host. In the following
we refer to the clock used to measure when the packet was sent
Src as the source clock, and we refer to the observed time when
packet was sent by the source as Tinitial, and the observed time
the packet was received by the source as Tfinal. Alluding to
notions of synchronization, accuracy, resolution, and skew
in the Introduction, we note the following
+ While in one-way delay there is an issue of the synchronization
the source clock and the destination clock, in round-trip
there is an (easier) issue of self-synchronization, as it were
between the source clock at the time the test packet is sent
the (same) source clock at the time the response packet
received. Theoretically a very severe case of skew could
this. In practice, the greater threat is anything that
cause a discontinuity in the source clock during the time
the taking of the initial and final timestamp. This might happen
for example, with certain implementations of NTP
+ The accuracy of a clock is important only in identifying the
at which a given delay was measured. Accuracy, per se, has
importance to the accuracy of the measurement of delay
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+ The resolution of a clock adds to uncertainty about any
measured with it. Thus, if the source clock has a resolution
10 msec, then this adds 10 msec of uncertainty to any time
measured with it. We will denote the resolution of the
clock as Rsource
Taking these items together, we note that naive computation Tfinal
Tinitial will be off by 2*Rsource
2.7.2. Errors or Uncertainties Related to Wire-time vs Host-
As we have defined round-trip delay, we would like to measure
time between when the test packet leaves the network interface of
and when the corresponding response packet (completely) arrives
the network interface of Src, and we refer to these as "wire times".
If the timings are themselves performed by software on Src, however
then this software can only directly measure the time between
Src grabs a timestamp just prior to sending the test packet and
it grabs a timestamp just after having received the response packet
and we refer to these two points as "host times".
Another contributor to this problem is time spent at Dst between
receipt there of the test packet and the sending of the
packet. Ideally, this time is zero; it is explored further in
next section
To the extent that the difference between wire time and host time
accurately known, this knowledge can be used to correct for host
measurements and the corrected value more accurately estimates
desired (wire time) metric
To the extent, however, that the difference between wire time
host time is uncertain, this uncertainty must be accounted for in
analysis of a given measurement method. We denote by Hinitial
upper bound on the uncertainty in the difference between wire
and host time on the Src host in sending the test packet,
similarly define Hfinal for the difference on the Src host
receiving the response packet. We then note that these
introduce a total uncertainty of Hinitial + Hfinal. This estimate
total wire-vs-host uncertainty should be included in
error/uncertainty analysis of any measurement implementation
2.7.3. Errors or Uncertainties Related to Dst Producing a
Any time spent by the destination host in receiving and
the packet from Src, and then producing and sending the
response adds additional error and uncertainty to the round-
delay measurement. The error equals the difference between the
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time the first bit of the packet is received by Dst and the wire
the first bit of the response is sent by Dst. To the extent
this difference is accurately known, this knowledge can be used
correct the desired metric. To the extent, however, that
difference is uncertain, this uncertainty must be accounted for
the error analysis of a measurement implementation. We denote
uncertainty by Hrefl. This estimate of uncertainty should
included in the error/uncertainty analysis of any
implementation
2.7.4.
Generally, the measured values can be decomposed as follows
measured value = true value + systematic error + random
If the systematic error (the constant bias in measured values) can
determined, it can be compensated for in the reported results
reported value = measured value - systematic
reported value = true value + random
The goal of calibration is to determine the systematic and
error generated by the instruments themselves in as much detail
possible. At a minimum, a bound ("e") should be found such that
reported value is in the range (true value - e) to (true value + e
at least 95 percent of the time. We call "e" the calibration
for the measurements. It represents the degree to which the
produced by the measurement instrument are repeatable; that is,
closely an actual delay of 30 ms is reported as 30 ms. {Comment: 95
percent was chosen because (1) some confidence level is desirable
be able to remove outliers, which will be found in measuring
physical property; and (2) a particular confidence level should
specified so that the results of independent implementations can
compared.}
From the discussion in the previous three sections, the error
measurements could be bounded by determining all the
uncertainties, and adding them together to
2*Rsource + Hinitial + Hfinal + Hrefl
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However, reasonable bounds on both the clock-related
captured by the first term and the host-related uncertainty
by the last three terms should be possible by careful
techniques and calibrating the instruments using a known, isolated
network in a lab
The host-related uncertainties, Hinitial + Hfinal + Hrefl, could
bounded by connecting two instruments back-to-back with a high-
serial link or isolated LAN segment. In this case,
measurements are measuring the same round-trip delay
If the test packets are small, such a network connection has
minimal delay that may be approximated by zero. The measured
therefore contains only systematic and random error in
instrumentation. The "average value" of repeated measurements is
systematic error, and the variation is the random error
One way to compute the systematic error, and the random error to
95% confidence is to repeat the experiment many times - at
hundreds of tests. The systematic error would then be the median
The random error could then be found by removing the systematic
from the measured values. The 95% confidence interval would be
range from the 2.5th percentile to the 97.5th percentile of
deviations from the true value. The calibration error "e" could
be taken to be the largest absolute value of these two numbers,
the clock-related uncertainty. {Comment: as described, this bound
relatively loose since the uncertainties are added, and the
value of the largest deviation is used. As long as the
value is not a significant fraction of the measured values, it is
reasonable bound. If the resulting value is a significant
of the measured values, then more exact methods will be needed
compute the calibration error.}
Note that random error is a function of measurement load.
example, if many paths will be measured by one instrument, this
increase interrupts, process scheduling, and disk I/O (for example
recording the measurements), all of which may increase the
error in measured singletons. Therefore, in addition to minimal
measurements to find the systematic error, calibration
should be performed with the same measurement load that
instruments will see in the field
We wish to reiterate that this statistical treatment refers to
calibration of the instrument; it is used to "calibrate the
stick" and say how well the meter stick reflects reality
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In addition to calibrating the instruments for finite delay,
checks should be made to ensure that packets reported as losses
really lost. First, the threshold for loss should be verified.
particular, ensure the "reasonable" threshold is reasonable: that
is very unlikely a packet will arrive after the threshold value,
therefore the number of packets lost over an interval is
sensitive to the error bound on measurements. Second, consider
possibility that a packet arrives at the network interface, but
lost due to congestion on that interface or to other
exhaustion (e.g. buffers) in the instrument
2.8. Reporting the Metric
The calibration and context in which the metric is measured MUST
carefully considered, and SHOULD always be reported along with
results. We now present four items to consider: the Type-P of
packets, the threshold of infinite delay (if any), error calibration
and the path traversed by the test packets. This list is
exhaustive; any additional information that could be useful
interpreting applications of the metrics should also be reported
2.8.1. Type-
As noted in the Framework document [1], the value of the metric
depend on the type of IP packets used to make the measurement,
"type-P". The value of Type-P-Round-trip-Delay could change if
protocol (UDP or TCP), port number, size, or arrangement for
treatment (e.g., IP precedence or RSVP) changes. The exact Type-
used to make the measurements MUST be accurately reported
2.8.2. Loss
In addition, the threshold (or methodology to distinguish) between
large finite delay and loss MUST be reported
2.8.3. Calibration
+ If the systematic error can be determined, it SHOULD be
from the measured values
+ You SHOULD also report the calibration error, e, such that
true value is the reported value plus or minus e, with 95%
confidence (see the last section.)
+ If possible, the conditions under which a test packet with
delay is reported as lost due to resource exhaustion on
measurement instrument SHOULD be reported
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2.8.4.
Finally, the path traversed by the packet SHOULD be reported,
possible. In general it is impractical to know the precise path
given packet takes through the network. The precise path may
known for certain Type-P on short or stable paths. For example,
Type-P includes the record route (or loose-source route) option
the IP header, and the path is short enough, and all routers* on
path support record (or loose-source) route, and the Dst host
the path from Src to Dst into the corresponding reply packet,
the path will be precisely recorded. This is impractical because
route must be short enough, many routers do not support (or are
configured for) record route, and use of this feature would
artificially worsen the performance observed by removing the
from common-case processing. However, partial information is
valuable context. For example, if a host can choose between
links* (and hence two separate routes from Src to Dst), then
initial link used is valuable context. {Comment: For example,
Merit's NetNow setup, a Src on one NAP can reach a Dst on another
by either of several different backbone networks.}
3. A Definition for Samples of Round-trip
Given the singleton metric Type-P-Round-trip-Delay, we now define
particular sample of such singletons. The idea of the sample is
select a particular binding of the parameters Src, Dst, and Type-P
then define a sample of values of parameter T. The means
defining the values of T is to select a beginning time T0, a
time Tf, and an average rate lambda, then define a pseudo-
Poisson process of rate lambda, whose values fall between T0 and Tf
The time interval between successive values of T will then
1/lambda
{Comment: Note that Poisson sampling is only one way of defining
sample. Poisson has the advantage of limiting bias, but
methods of sampling might be appropriate for different situations
We encourage others who find such appropriate cases to use
general framework and submit their sampling method
standardization.}
3.1. Metric Name
Type-P-Round-trip-Delay-Poisson-
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3.2. Metric Parameters
+ Src, the IP address of a
+ Dst, the IP address of a
+ T0, a
+ Tf, a
+ lambda, a rate in reciprocal
3.3. Metric Units
A sequence of pairs; the elements of each pair are
+ T, a time,
+ dT, either a real number or an undefined number of seconds
The values of T in the sequence are monotonic increasing. Note
T would be a valid parameter to Type-P-Round-trip-Delay, and that
would be a valid value of Type-P-Round-trip-Delay
3.4. Definition
Given T0, Tf, and lambda, we compute a pseudo-random Poisson
beginning at or before T0, with average arrival rate lambda,
ending at or after Tf. Those time values greater than or equal to T
and less than or equal to Tf are then selected. At each of the
in this process, we obtain the value of Type-P-Round-trip-Delay
this time. The value of the sample is the sequence made up of
resulting pairs. If there are no such pairs,
sequence is of length zero and the sample is said to be empty
3.5. Discussion
The reader should be familiar with the in-depth discussion of
sampling in the Framework document [1], which includes methods
compute and verify the pseudo-random Poisson process
We specifically do not constrain the value of lambda, except to
the extremes. If the rate is too large, then the measurement
will perturb the network, and itself cause congestion. If the
is too small, then you might not capture interesting
behavior. {Comment: We expect to document our experiences with,
suggestions for, lambda elsewhere, culminating in a "best
practices" document.}
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Since a pseudo-random number sequence is employed, the sequence
times, and hence the value of the sample, is not fully specified
Pseudo-random number generators of good quality will be needed
achieve the desired qualities
The sample is defined in terms of a Poisson process both to avoid
effects of self-synchronization and also capture a sample that
statistically as unbiased as possible. {Comment: there is,
course, no claim that real Internet traffic arrives according to
Poisson arrival process.} The Poisson process is used to
the delay measurements. The test packets will generally not
at Dst according to a Poisson distribution, nor will response
arrive at Src according to a Poisson distribution, since they
influenced by the network
All the singleton Type-P-Round-trip-Delay metrics in the
will have the same values of Src, Dst, and Type-P
Note also that, given one sample that runs from T0 to Tf, and
new time values T0' and Tf' such that T0 <= T0' <= Tf' <= Tf,
subsequence of the given sample whose time values fall between T0'
and Tf' are also a valid Type-P-Round-trip-Delay-Poisson-
sample
3.6. Methodologies
The methodologies follow directly from
+ the selection of specific times, using the specified
arrival process,
+ the methodologies discussion already given for the singleton Type
P-Round-trip-Delay metric
Care must, of course, be given to correctly handle out-of-
arrival of test or response packets; it is possible that the
could send one test packet at TS[i], then send a second test
(later) at TS[i+1], and it could receive the second response
at TR[i+1], and then receive the first response packet (later)
TR[i].
3.7. Errors and Uncertainties
In addition to sources of errors and uncertainties associated
methods employed to measure the singleton values that make up
sample, care must be given to analyze the accuracy of the
process with respect to the wire-times of the sending of the
packets. Problems with this process could be caused by
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things, including problems with the pseudo-random number
used to generate the Poisson arrival process, or with jitter in
value of Hinitial (mentioned above as uncertainty in the
delay metric). The Framework document shows how to use
Anderson-Darling test to verify the accuracy of a Poisson
over small time frames. {Comment: The goal is to ensure that
packets are sent "close enough" to a Poisson schedule, and
periodic behavior.}
3.8. Reporting the Metric
You MUST report the calibration and context for the
singletons along with the stream. (See "Reporting the metric"
Type-P-Round-trip-Delay.)
4. Some Statistics Definitions for Round-trip
Given the sample metric Type-P-Round-trip-Delay-Poisson-Stream,
now offer several statistics of that sample. These statistics
offered mostly to be illustrative of what could be done
4.1. Type-P-Round-trip-Delay-
Given a Type-P-Round-trip-Delay-Poisson-Stream and a percent
between 0% and 100%, the Xth percentile of all the dT values in
Stream. In computing this percentile, undefined values are
as infinitely large. Note that this means that the percentile
thus be undefined (informally, infinite). In addition, the Type-P
Round-trip-Delay-Percentile is undefined if the sample is empty
Example: suppose we take a sample and the results are
Stream1 = <
>
Then the 50th percentile would be 110 msec, since 90 msec and 100
msec are smaller and 110 msec and 'undefined' are larger
Note that if the possibility that a packet with finite delay
reported as lost is significant, then a high percentile (90th
95th) might be reported as infinite instead of finite
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4.2. Type-P-Round-trip-Delay-
Given a Type-P-Round-trip-Delay-Poisson-Stream, the median of all
dT values in the Stream. In computing the median, undefined
are treated as infinitely large. As with Type-P-Round-trip-Delay
Percentile, Type-P-Round-trip-Delay-Median is undefined if the
is empty
As noted in the Framework document, the median differs from the 50
percentile only when the sample contains an even number of values,
which case the mean of the two central values is used
Example: suppose we take a sample and the results are
Stream2 = <
>
Then the median would be 105 msec, the mean of 100 msec and 110 msec
the two central values
4.3. Type-P-Round-trip-Delay-
Given a Type-P-Round-trip-Delay-Poisson-Stream, the minimum of
the dT values in the Stream. In computing this, undefined values
treated as infinitely large. Note that this means that the
could thus be undefined (informally, infinite) if all the dT
are undefined. In addition, the Type-P-Round-trip-Delay-Minimum
undefined if the sample is empty
In the above example, the minimum would be 90 msec
4.4. Type-P-Round-trip-Delay-Inverse-
Given a Type-P-Round-trip-Delay-Poisson-Stream and a time
threshold, the fraction of all the dT values in the Stream less
or equal to the threshold. The result could be as low as 0% (if
the dT values exceed threshold) or as high as 100%. Type-P-Round
trip-Delay-Inverse-Percentile is undefined if the sample is empty
In the above example, the Inverse-Percentile of 103 msec would
50%.
Almes, et al. Standards Track [Page 17]
RFC 2681 Round-trip for Delay Metric for IPPM September 1999
5. Security
Conducting Internet measurements raises both security and
concerns. This memo does not specify an implementation of
metrics, so it does not directly affect the security of the
nor of applications which run on the Internet. However
implementations of these metrics must be mindful of security
privacy concerns
There are two types of security concerns: potential harm caused
the measurements, and potential harm to the measurements.
measurements could cause harm because they are active, and
packets into the network. The measurement parameters MUST
carefully selected so that the measurements inject trivial amounts
additional traffic into the networks they measure. If they
"too much" traffic, they can skew the results of the measurement,
in extreme cases cause congestion and denial of service
The measurements themselves could be harmed by routers
measurement traffic a different priority than "normal" traffic, or
an attacker injecting artificial measurement traffic. If routers
recognize measurement traffic and treat it separately,
measurements will not reflect actual user traffic. If an
injects artificial traffic that is accepted as legitimate, the
rate will be artificially lowered. Therefore, the
methodologies SHOULD include appropriate techniques to reduce
probability measurement traffic can be distinguished from "normal
traffic. Authentication techniques, such as digital signatures,
be used where appropriate to guard against injected traffic attacks
The privacy concerns of network measurement are limited by the
measurements described in this memo. Unlike passive measurements
there can be no release of existing user data
6.
Special thanks are due to Vern Paxson and to Will Leland for
useful suggestions
7.
[1] Paxson, D., Almes, G., Mahdavi, J. and M. Mathis, "Framework
IP Performance Metrics", RFC 2330, May 1998.
[2] Almes, G., Kalidindi,S. and M. Zekauskas, "A One-way
Metric for IPPM", RFC 2679, September 1999.
[3] Mills, D., "Network Time Protocol (v3)", RFC 1305, April 1992.
Almes, et al. Standards Track [Page 18]
RFC 2681 Round-trip for Delay Metric for IPPM September 1999
[4] Mahdavi, J. and V. Paxson, "IPPM Metrics for
Connectivity", RFC 2678, September 1999.
[5] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
[6] Bradner, S., "Key words for use in RFCs to Indicate
Levels", BCP 14, RFC 2119, March 1997.
8. Authors'
Guy
Advanced Network & Services, Inc
200 Business Park
Armonk, NY 10504
Phone: +1 914 765 1120
EMail: almes@advanced.
Sunil
Advanced Network & Services, Inc
200 Business Park
Armonk, NY 10504
Phone: +1 914 765 1128
EMail: kalidindi@advanced.
Matthew J.
Advanced Network & Services, Inc
200 Business Park
Armonk, NY 10504
Phone: +1 914 765 1112
EMail: matt@advanced.
Almes, et al. Standards Track [Page 19]
RFC 2681 Round-trip for Delay Metric for IPPM September 1999
9. Full Copyright
Copyright (C) The Internet Society (1999). All Rights Reserved
This document and translations of it may be copied and furnished
others, and derivative works that comment on or otherwise explain
or assist in its implementation may be prepared, copied,
and distributed, in whole or in part, without restriction of
kind, provided that the above copyright notice and this paragraph
included on all such copies and derivative works. However,
document itself may not be modified in any way, such as by
the copyright notice or references to the Internet Society or
Internet organizations, except as needed for the purpose
developing Internet standards in which case the procedures
copyrights defined in the Internet Standards process must
followed, or as required to translate it into languages other
English
The limited permissions granted above are perpetual and will not
revoked by the Internet Society or its successors or assigns
This document and the information contained herein is provided on
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE
Funding for the RFC Editor function is currently provided by
Internet Society
Almes, et al. Standards Track [Page 20]
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