Network Working Group G. Almes
Internet Draft S. Kalidindi
Expiration Date: December 1998 M. Zekauskas
Advanced Network & Services
June 1998
A One-way Delay Metric for IPPM
<draft-ietf-ippm-delay-03.txt>
1. Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months, and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet- Drafts as
reference material or to cite them other than as "work in progress."
To view the entire list of current Internet-Drafts, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts shadow
directories on ftp.is.co.za (Africa), nic.nordu.net (Northern
Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific
Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
2. Introduction
This memo defines a metric for one-way delay of packets across
Internet paths. It builds on notions introduced and discussed in the
IPPM Framework document, RFC 2223 [1]; the reader is assumed to be
familiar with that document.
This memo is intended to be parallel in structure to a companion
document for Packet Loss ("A Packet Loss Metric for IPPM"
<draft-ietf-ippm-loss-02.txt>) [2].
The structure of the memo is as follows:
Almes et al. [Page 1]
INTERNET-DRAFT One-way Delay Metric June 1998
+ A 'singleton' analytic metric, called Type-P-One-way-Delay, will
be introduced to measure a single observation of one-way delay.
+ Using this singleton metric, a 'sample', called Type-P-One-way-
Delay-Poisson-Stream, will be introduced to measure a sequence of
singleton delays measured at times taken from a Poisson process.
+ Using this sample, several 'statistics' of the sample will be
defined and discussed.
This progression from singleton to sample to statistics, with clear
separation among them, is important.
Whenever a technical term from the IPPM Framework document is first
used in this memo, it will be tagged with a trailing asterisk. For
example, "term*" indicates that "term" is defined in the Framework.
2.1. Motivation:
One-way delay of a type-P packet from a source host* to a destination
host is useful for several reasons:
+ Some applications do not perform well (or at all) if end-to-end
delay between hosts is large relative to some threshold value.
+ Erratic variation in delay makes it difficult (or impossible) to
support many real-time applications.
+ The larger the value of delay, the more difficult it is for
transport-layer protocols to sustain high bandwidths.
+ The minimum value of this metric provides an indication of the
delay due only to propagation and transmission delay.
+ The minimum value of this metric provides an indication of the
delay that will likely be experienced when the path* traversed is
lightly loaded.
+ Values of this metric above the minimum provide an indication of
the congestion present in the path.
It is outside the scope of this document to say precisely how delay
metrics would be applied to specific problems.
Almes et al. [Page 2]
INTERNET-DRAFT One-way Delay Metric June 1998
2.2. General Issues Regarding Time
Whenever a time (i.e., a moment in history) is mentioned here, it is
understood to be measured in seconds (and fractions) relative to UTC.
As described more fully in the Framework document, there are four
distinct, but related notions of clock uncertainty:
synchronization*
measures the extent to which two clocks agree on what time it
is. For example, the clock on one host might be 5.4 msec ahead
of the clock on a second host.
accuracy*
measures the extent to which a given clock agrees with UTC. For
example, the clock on a host might be 27.1 msec behind UTC.
resolution*
measures the precision of a given clock. For example, the clock
on an old Unix host might tick only once every 10 msec, and thus
have a resolution of only 10 msec.
skew*
measures the change of accuracy, or of synchronization, with
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 and
only 25.8 msec an hour later. In this case, we say that the
clock of the given host has a skew of 1.3 msec per hour relative
to UTC, and this threatens accuracy. We might also speak of the
skew of one clock relative to another clock, and this threatens
synchronization.
3. A Singleton Definition for One-way Delay
3.1. Metric Name:
Type-P-One-way-Delay
Almes et al. [Page 3]
INTERNET-DRAFT One-way Delay Metric June 1998
3.2. Metric Parameters:
+ Src, the IP address of a host
+ Dst, the IP address of a host
+ T, a time
3.3. Metric Units:
The value of a type-P-One-way-Delay is either a non-negative real
number or an undefined (informally, infinite) number of seconds.
3.4. Definition:
For a non-negative real number dT, >>the *Type-P-One-way-Delay* from
Src to Dst at T is dT<< means that Src sent the first bit of a type-P
packet to Dst at wire-time* T and that Dst received the last bit of
that packet at wire-time T+dT.
>>The *Type-P-One-way-Delay* from Src to Dst at T is undefined
(informally, infinite)<< means that Src sent the first bit of a type-
P packet to Dst at wire-time T and that Dst did not receive that
packet.
3.5. Discussion:
Type-P-One-way-Delay is a relatively simple analytic metric, and one
that we believe will afford effective methods of measurement.
The following issues are likely to come up in practice:
+ Since delay values will often be as low as the 100 usec to 10 msec
range, it will be important for Src and Dst to synchronize very
closely. GPS systems afford one way to achieve synchronization to
within several 10s of usec. Ordinary application of NTP may allow
synchronization to within several msec, but this depends on the
stability and symmetry of delay properties among those NTP agents
used, and this delay is what we are trying to measure. A
combination of some GPS-based NTP servers and a conservatively
designed and deployed set of other NTP servers should yield good
results, but this is yet to be tested.
Almes et al. [Page 4]
INTERNET-DRAFT One-way Delay Metric June 1998
+ A given methodology will have to include a way to determine
whether a delay value is infinite or whether it is merely very
large (and the packet is yet to arrive at Dst). As noted by
Mahdavi and Paxson [4], simple upper bounds (such as the 255
seconds theoretical upper bound on the lifetimes of IP
packets [5]) could be used, but good engineering, including an
understanding of packet lifetimes, will be needed in practice.
{Comment: Note that, for many applications of these metrics, the
harm in treating a large delay as infinite might be zero or very
small. A TCP data packet, for example, that arrives only after
several multiples of the RTT may as well have been lost.}
+ The context in which the metric is measured must be carefully
considered, and should always be reported along with metric
results.
As noted in the Framework document [1], the value of the metric
may depend on the type of IP packets used to make the measurement,
or "type-P". The value of Type-P-One-way-Delay could change if
the protocol (UDP or TCP), port number, size, or arrangement for
special treatment (e.g., IP precedence or RSVP) changes. The
exact Type-P used to make the measurements must be accurately
reported.
In addition, the threshold (or methodology to distinguish) between
a large finite delay and loss should be reported.
Finally, the path traversed by the packet should be reported, if
possible. In general it is impractical to know the precise path a
given packet takes through the network. The precise path may be
known for certain Type-P on short or stable paths. If Type-P
includes the record route (or loose-source route) option in the IP
header, and the path is short enough, and all routers* on the path
support record (or loose-source) route, then the path will be
precisely recorded. This is impractical because the route must be
short enough, many routers do not support (or are not configured
for) record route, and use of this feature would often
artificially worsen the performance observed by removing the
packet from common-case processing. However, partial information
is still valuable context. For example, if a host can choose
between two links* (and hence two separate routes from src to
dst), then the initial link used is valuable context. {Comment:
For example, with Merit's NetNow setup, a Src on one NAP can reach
a Dst on another NAP by either of several different backbone
networks.}
The above list is not exhaustive; any additional information that
could be useful in interpreting applications of the metrics should
Almes et al. [Page 5]
INTERNET-DRAFT One-way Delay Metric June 1998
be reported.
+ If the packet is duplicated along the path (or paths) so that
multiple non-corrupt copies arrive at the destination, then the
packet is counted as received, and the first copy to arrive
determines the packet's one-way delay.
+ If the packet is fragmented and if, for whatever reason,
reassembly does not occur, then the packet will be deemed lost.
3.6. Methodologies:
As with other Type-P-* metrics, the detailed methodology will depend
on the Type-P (e.g., protocol number, UDP/TCP port number, size,
precedence).
Generally, for a given Type-P, the methodology would proceed as
follows:
+ Arrange that Src and Dst are synchronized; that is, that they have
clocks that are very closely synchronized with each other and each
fairly close to the actual time.
+ At the Src host, select Src and Dst IP addresses, and form a test
packet of Type-P with these addresses. Any 'padding' portion of
the packet needed only to make the test packet a given size should
be filled with randomized bits to avoid a situation in which the
measured delay is lower than it would otherwise be due to
compression techniques along the path.
+ At the Dst host, arrange to receive the packet.
+ At the Src host, place a timestamp in the prepared Type-P packet,
and send it towards Dst.
+ If the packet arrives within a reasonable period of time, take a
timestamp as soon as possible upon the receipt of the packet. By
subtracting the two timestamps, an estimate of one-way delay can
be computed. Error analysis of a given implementation of the
method must take into account the closeness of synchronization
between Src and Dst. If the delay between Src's 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 error analysis. Similarly, if the
delay between the actual receipt of the packet and Dst's timestamp
is known, then the estimate could be adjusted by subtracting this
Almes et al. [Page 6]
INTERNET-DRAFT One-way Delay Metric June 1998
amount; uncertainty in this value must be taken into account in
error analysis.
+ If the packet fails to arrive within a reasonable period of time,
the one-way delay is taken to be undefined (informally, infinite).
Note that the threshold of 'reasonable' here is a parameter of the
methodology.
Issues such as the packet format, the means by which Dst knows when
to expect the test packet, and the means by which Src and Dst are
synchronized are outside the scope of this document. {Comment: We
plan to document elsewhere our own work in describing such more
detailed implementation techniques and we encourage others to as
well.}
3.7. Errors and Uncertainties:
The description of any specific measurement method should include an
accounting and analysis of various sources of error/uncertainty. The
Framework document provides general guidence on this point, but we
note here the following specifics related to delay metrics:
+ Errors/uncertainties due to uncertainties in the clocks of the Src
and Dst hosts.
+ Errors/uncertainties due to the difference between 'wire time' and
'host time'.
Each of these are discussed in more detail below.
3.7.1. Errors/uncertainties related to Clocks
The uncertainty in a measurement of one-way delay is related, in
part, to uncertainties in the clocks of the Src and Dst hosts. In
the following, we refer to the clock used to measure when the packet
was sent from Src as the source clock, we refer to the clock used to
measure when the packet was received by Dst as the dest clock, we
refer to the observed time when the packet was sent by the source
clock as Tsource, and the observed time when the packet was received
by the dest clock as Tdest. Alluding to the notions of
synchronization, accuracy, resolution, and skew mentioned in the
Introduction, we note the following:
+ Any error in the synchronization between the source clock and the
dest clock will contribute to error in the delay measurement. We
say that the source clock and the dest clock have a
Almes et al. [Page 7]
INTERNET-DRAFT One-way Delay Metric June 1998
synchronization error of Tsynch if the source clock is Tsynch
ahead of the dest clock. Thus, if we know the value of Tsynch
exactly, we could correct for clock synchronization by adding
Tsynch to the uncorrected value of Tdest-Tsource.
+ The accuracy of a clock is important only in identifying the time
at which a given delay was measured. Accuracy, per se, has no
importance to the accuracy of the measurement of delay. This is
because, when computing delays, we are interested only in the
differences between clock values.
+ The resolution of a clock adds to uncertainty about any time
measured with it. Thus, if the source clock has a resolution of
10 msec, then this adds 10 msec of uncertainty to any time value
measured with it. We will denote the resolution of the source
clock and the dest clock as Rsource and Rdest, respectively.
+ The skew of a clock is not so much an additional issue as it is a
realization of the fact that Tsynch is itself a function of time.
Thus, if we attempt to measure or to bound Tsynch, this needs to
be done periodically. Over some periods of time, this function
can be approximated as a linear function plus some higher order
terms; in these cases, one option is to use knowledge of the
linear component to correct the clock. Using this correction, the
residual Tsynch is made smaller, but remains a source of
uncertainty that must be accounted for. We use the function
Esynch(t) to denote an upper bound on the uncertainty in
synchronization. Thus, |Tsynch(t)| <= Esynch(t).
Taking these items together, we note that naive computation Tdest-
Tsource will be off by Tsynch(t) +/- (|Rsource|+|Rdest|). Using the
notion of Esynch(t), we note that these clock-related problems
introduce a total uncertainty of Esynch(t)+|Rsource|+|Rdest|. This
estimate of total clock-related uncertainty should be included in the
error/uncertainty analysis of any measurement implementation.
3.7.2. Errors/uncertainties related to Wire-time vs Host-time
As we've defined one-way delay, we'd like to measure the time between
when the test packet leaves the network interface of Src and when it
(completely) arrives at the network interface of Dst, and we refer to
this as 'wire time'. If the timings are themselves performed by
software on Src and Dst, however, then this software can only
directly measure the time between when Src grabs a timestamp just
prior to sending the test packet and when Dst grabs a timestamp just
after having received the test packet, and we refer to this as 'host
time'.
Almes et al. [Page 8]
INTERNET-DRAFT One-way Delay Metric June 1998
To the extent that the difference between wire time and host time is
accurately known, this knowledge can be used to correct for host time
measurements and the corrected value more accurately estimates the
desired (wire time) metric.
To the extent, however, that the difference between wire time and
host time is uncertain, this uncertainty must be accounted for in an
analysis of a given measurement method. We denote by Hsource an
upper bound on the uncertainty in the difference between wire time
and host time on the Src host, and similarly define Hdest for the Dst
host. We then note that these problems introduce a total uncertainty
of Hsource+Hdest. This estimate of total wire-vs-host uncertainty
should be included in the error/uncertainty analysis of any
measurement implementation.
4. A Definition for Samples of One-way Delay
Given the singleton metric Type-P-One-way-Delay, we now define one
particular sample of such singletons. The idea of the sample is to
select a particular binding of the parameters Src, Dst, and Type-P,
then define a sample of values of parameter T. The means for
defining the values of T is to select a beginning time T0, a final
time Tf, and an average rate lambda, then define a pseudo-random
Poisson arrival process of rate lambda, whose values fall between T0
and Tf. The time interval between successive values of T will then
average 1/lambda.
4.1. Metric Name:
Type-P-One-way-Delay-Poisson-Stream
4.2. Metric Parameters:
+ Src, the IP address of a host
+ Dst, the IP address of a host
+ T0, a time
+ Tf, a time
+ lambda, a rate in reciprocal seconds
Almes et al. [Page 9]
INTERNET-DRAFT One-way Delay Metric June 1998
4.3. Metric Units:
A sequence of pairs; the elements of each pair are:
+ T, a time, and
+ dT, either a non-negative real number or an undefined number of
seconds.
The values of T in the sequence are monotonic increasing. Note that
T would be a valid parameter to Type-P-One-way-Delay, and that dT
would be a valid value of Type-P-One-way-Delay.
4.4. Definition:
Given T0, Tf, and lambda, we compute a pseudo-random Poisson process
beginning at or before T0, with average arrival rate lambda, and
ending at or after Tf. Those time values greater than or equal to T0
and less than or equal to Tf are then selected. At each of the times
in this process, we obtain the value of Type-P-One-way-Delay at this
time. The value of the sample is the sequence made up of the
resulting <time, delay> pairs. If there are no such pairs, the
sequence is of length zero and the sample is said to be empty.
4.5. Discussion:
Note first that, since a pseudo-random number sequence is employed,
the sequence of times, and hence the value of the sample, is not
fully specified. Pseudo-random number generators of good quality
will be needed to achieve the desired qualities.
The sample is defined in terms of a Poisson process both to avoid the
effects of self-synchronization and also capture a sample that is
statistically as unbiased as possible. {Comment: there is, of
course, no claim that real Internet traffic arrives according to a
Poisson arrival process.}
All the singleton Type-P-One-way-Delay metrics in the sequence will
have the same values of Src, Dst, and Type-P.
Note also that, given one sample that runs from T0 to Tf, and given
new time values T0' and Tf' such that T0 <= T0' <= Tf' <= Tf, the
subsequence of the given sample whose time values fall between T0'
and Tf' are also a valid Type-P-One-way-Delay-Poisson-Stream sample.
Almes et al. [Page 10]
INTERNET-DRAFT One-way Delay Metric June 1998
4.6. Methodologies:
The methodologies follow directly from:
+ the selection of specific times, using the specified Poisson
arrival process, and
+ the methodologies discussion already given for the singleton Type-
P-One-way-Delay metric.
Care must, of course, be given to correctly handle out-of-order
arrival of test packets; it is possible that the Src could send one
test packet at TS[i], then send a second one (later) at TS[i+1],
while the Dst could receive the second test packet at TR[i+1], and
then receive the first one (later) at TR[i].
4.7. Errors and Uncertainties:
In addition to sources of errors and uncertainties associated with
methods employed to measure the singleton values that make up the
sample, care must be given to analyze the accuracy of the Poisson
arrival process of the wire-time of the sending of the test packets.
Problems with this process could be caused by either of several
things, including problems with the pseudo-random number techniques
used to generate the Poisson arrival process, or with jitter in the
value of Hsource (mentioned above as uncertainty in the singleton
delay metric). The Framework document shows how to use an Anderson-
Darling test for this.
5. Some Statistics Definitions for One-way Delay
Given the sample metric Type-P-One-way-Delay-Poisson-Stream, we now
offer several statistics of that sample. These statistics are
offered mostly to be illustrative of what could be done.
5.1. Type-P-One-way-Delay-Percentile
Given a Type-P-One-way-Delay-Poisson-Stream and a percent X between
0% and 100%, the Xth percentile of all the dT values in the Stream.
In computing this percentile, undefined values are treated as
infinitely large. Note that this means that the percentile could
thus be undefined (informally, infinite). In addition, the Type-P-
One-way-Delay-Percentile is undefined if the sample is empty.
Almes et al. [Page 11]
INTERNET-DRAFT One-way Delay Metric June 1998
Example: suppose we take a sample and the results are:
Stream1 = <
<T1, 100 msec>
<T2, 110 msec>
<T3, undefined>
<T4, 90 msec>
<T5, 500 msec>
>
Then the 50th percentile would be 110 msec, since 90 msec and 100
msec are smaller and 110 msec and 'undefined' are larger.
5.2. Type-P-One-way-Delay-Median
Given a Type-P-One-way-Delay-Poisson-Stream, the median of all the dT
values in the Stream. In computing the median, undefined values are
treated as infinitely large.
As noted in the Framework document, the median differs from the 50th
percentile only when the sample contains an even number of values, in
which case the mean of the two central values is used.
Example: suppose we take a sample and the results are:
Stream2 = <
<T1, 100 msec>
<T2, 110 msec>
<T3, undefined>
<T4, 90 msec>
>
Then the median would be 105 msec, the mean of 100 msec and 110 msec,
the two central values.
5.3. Type-P-One-way-Delay-Minumum
Given a Type-P-One-way-Delay-Poisson-Stream, the minimum of all the
dT values in the Stream. In computing this, undefined values are
treated as infinitely large. Note that this means that the minimum
could thus be undefined (informally, infinite) if all the dT values
are undefined. In addition, the Type-P-One-way-Delay-Minimum is
undefined if the sample is empty.
In the above example, the minimum would be 90 msec.
Almes et al. [Page 12]
INTERNET-DRAFT One-way Delay Metric June 1998
5.4. Type-P-One-way-Delay-Inverse-Percentile
Given a Type-P-One-way-Delay-Poisson-Stream and a non-negative time
duration threshold, the fraction of all the dT values in the Stream
less than or equal to the threshold. The result could be as low as
0% (if all the dT values exceed threshold) or as high as 100%.
In the above example, the Inverse-Percentile of 103 msec would be
50%.
6. Security Considerations
Conducting Internet measurements raises both security and privacy
concerns. This memo does not specify an implementation of the
metrics, so it does not directly affect the security of the Internet
nor of applications which run on the Internet. However,
implementations of these metrics must be mindful of security and
privacy concerns.
There are two types of security concerns: potential harm caused by
the measurements, and potential harm to the measurements. The
measurements could cause harm because they are active, and inject
packets into the network. The measurement parameters must be
carefully selected so that the measurements inject trivial amounts of
additional traffic into the networks they measure. If they inject
"too much" traffic, they can skew the results of the measurement, and
in extreme cases cause congestion and denial of service.
The measurements themselves could be harmed by routers giving
measurement traffic a different priority than "normal" traffic, or by
an attacker injecting artificial measurement traffic. If routers can
recognize measurement traffic and treat it separately, the
measurements will not reflect actual user traffic. If an attacker
injects artificial traffic that is accepted as legitimate, the loss
rate will be artificially lowered. Therefore, the measurement
methodologies should include appropriate techniques to reduce the
probability measurement traffic can be distinguished from "normal"
traffic. Authentication techniques, such as digital signatures, may
be used where appropriate to guard against injected traffic attacks.
The privacy concerns of network measurement are limited by the active
measurements described in this memo. Unlike passive measurements,
there can be no release of existing user data.
Almes et al. [Page 13]
INTERNET-DRAFT One-way Delay Metric June 1998
7. Acknowledgements
Special thanks are due to Vern Paxson of Lawrence Berkeley Labs for
his helpful comments on issues of clock uncertainty and statistics.
Thanks also to Sean Shapira and to Roland Wittig for several useful
suggestions.
8. References
[1] V. Paxson, G. Almes, J. Mahdavi, and M. Mathis, "Framework for
IP Performance Metrics", RFC 2330, May 1998.
[2] G. Almes, S. Kalidindi, and M. Zekauskas, "A One-way Delay
Metric for IPPM", Internet-Draft <draft-ietf-ippm-delay-02.txt>,
June 1998.
[3] D. Mills, "Network Time Protocol (v3)", RFC 1305, April 1992.
[4] J. Mahdavi and V. Paxson, "Connectivity", Work in Progress,
November 1997.
[5] J. Postel, "Internet Protocol", RFC 791, September 1981.
9. Authors' Addresses
Guy Almes
Advanced Network & Services, Inc.
200 Business Park Drive
Armonk, NY 10504
USA
Phone: +1 914 765 1120
EMail: almes@advanced.org
Sunil Kalidindi
Advanced Network & Services, Inc.
200 Business Park Drive
Armonk, NY 10504
USA
Phone: +1 914 765 1128
EMail: kalidindi@advanced.org
Almes et al. [Page 14]
INTERNET-DRAFT One-way Delay Metric June 1998
Matthew J. Zekauskas
Advanced Network & Services, Inc.
200 Buisiness Park Drive
Armonk, NY 10504
USA
Phone: +1 914 765 1112
EMail: matt@advanced.org
Expiration date: December, 1998
Almes et al. [Page 15]