draft-ietf-ippm-loss-03.txt   draft-ietf-ippm-loss-04.txt 
Network Working Group G. Almes Network Working Group G. Almes
INTERNET-DRAFT S. Kalidindi INTERNET-DRAFT S. Kalidindi
Expiration Date: December 1998 M. Zekauskas Expiration Date: March 1999 M. Zekauskas
Advanced Network & Services Advanced Network & Services
June 1998 August 1998
A Packet Loss Metric for IPPM A Packet Loss Metric for IPPM
<draft-ietf-ippm-loss-03.txt> <draft-ietf-ippm-loss-04.txt>
1. Status of this Memo 1. Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute and its working groups. Note that other groups may also distribute
working documents as Internet Drafts. working documents as Internet Drafts.
Internet-Drafts are draft documents valid for a maximum of six Internet-Drafts are draft documents valid for a maximum of six
months, and may be updated, replaced, or obsoleted by other documents months, and may be updated, replaced, or obsoleted by other documents
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2. Introduction 2. Introduction
This memo defines a metric for packet loss across Internet paths. It This memo defines a metric for packet loss across Internet paths. It
builds on notions introduced and discussed in the IPPM Framework builds on notions introduced and discussed in the IPPM Framework
document, RFC 2330 [1]; the reader is assumed to be familiar with document, RFC 2330 [1]; the reader is assumed to be familiar with
that document. that document.
This memo is intended to be parallel in structure to a companion This memo is intended to be parallel in structure to a companion
document for One-way Delay (currently "A One-way Delay Metric for document for One-way Delay (currently "A One-way Delay Metric for
IPPM" <draft-ietf-ippm-delay-02.txt>) [2]; the reader is assumed to IPPM" <draft-ietf-ippm-delay-04.txt>) [2]; the reader is assumed to
be familiar with that document. be familiar with that document.
The structure of the memo is as follows: The structure of the memo is as follows:
+ A 'singleton' analytic metric, called Type-P-One-way-Loss, is + A 'singleton' analytic metric, called Type-P-One-way-Loss, is
introduced to measure a single observation of packet transmission introduced to measure a single observation of packet transmission
or loss. or loss.
+ Using this singleton metric, a 'sample', called Type-P-One-way- + Using this singleton metric, a 'sample', called Type-P-One-way-
Loss-Poisson-Stream, is introduced to measure a sequence of Loss-Poisson-Stream, is introduced to measure a sequence of
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This progression from singleton to sample to statistics, with clear This progression from singleton to sample to statistics, with clear
separation among them, is important. separation among them, is important.
Whenever a technical term from the IPPM Framework document is first Whenever a technical term from the IPPM Framework document is first
used in this memo, it will be tagged with a trailing asterisk. For used in this memo, it will be tagged with a trailing asterisk. For
example, "term*" indicates that "term" is defined in the Framework. example, "term*" indicates that "term" is defined in the Framework.
2.1. Motivation: 2.1. Motivation:
Understanding one-way packet loss of type-P* packets from a source Understanding one-way packet loss of Type-P* packets from a source
host* to a destination host is useful for several reasons: host* to a destination host is useful for several reasons:
+ Some applications do not perform well (or at all) if end-to-end + Some applications do not perform well (or at all) if end-to-end
loss between hosts is large relative to some threshold value. loss between hosts is large relative to some threshold value.
+ Excessive packet loss may make it difficult to support certain + Excessive packet loss may make it difficult to support certain
real-time applications (where the precise threshold of "excessive" real-time applications (where the precise threshold of "excessive"
depends on the application). depends on the application).
+ The larger the value of packet loss, the more difficult it is for + The larger the value of packet loss, the more difficult it is for
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3.3. Metric Units: 3.3. Metric Units:
The value of a Type-P-One-way-Packet-Loss is either a zero The value of a Type-P-One-way-Packet-Loss is either a zero
(signifying successful transmission of the packet) or a one (signifying successful transmission of the packet) or a one
(signifying loss). (signifying loss).
3.4. Definition: 3.4. Definition:
>>The *Type-P-One-way-Packet-Loss* from Src to Dst at T is 0<< means >>The *Type-P-One-way-Packet-Loss* from Src to Dst at T is 0<< means
that Src sent the first bit of a type-P packet to Dst at wire-time* T that Src sent the first bit of a Type-P packet to Dst at wire-time* T
and that Dst received that packet. and that Dst received that packet.
>>The *Type-P-One-way-Packet-Loss* from Src to Dst at T is 1<< means >>The *Type-P-One-way-Packet-Loss* from Src to Dst at T is 1<< means
that Src sent the first bit of a type-P packet to Dst at wire-time T 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. and that Dst did not receive that packet.
3.5. Discussion: 3.5. Discussion:
Thus, Type-P-One-way-Packet-Loss is 0 exactly when Type-P-One-way- Thus, Type-P-One-way-Packet-Loss is 0 exactly when Type-P-One-way-
Delay is a finite positive value, and it is 1 exactly when Type-P- Delay is a finite positive value, and it is 1 exactly when Type-P-
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+ A given methodology will have to include a way to distinguish + A given methodology will have to include a way to distinguish
between a packet loss and a very large (but finite) delay. As between a packet loss and a very large (but finite) delay. As
noted by Mahdavi and Paxson [3], simple upper bounds (such as the noted by Mahdavi and Paxson [3], simple upper bounds (such as the
255 seconds theoretical upper bound on the lifetimes of IP 255 seconds theoretical upper bound on the lifetimes of IP
packets [4]) could be used, but good engineering, including an packets [4]) could be used, but good engineering, including an
understanding of packet lifetimes, will be needed in practice. understanding of packet lifetimes, will be needed in practice.
{Comment: Note that, for many applications of these metrics, there {Comment: Note that, for many applications of these metrics, there
may be no harm in treating a large delay as packet loss. An audio may be no harm in treating a large delay as packet loss. An audio
playback packet, for example, that arrives only after the playback playback packet, for example, that arrives only after the playback
point may as well have been lost.} point 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
be reported.
+ If the packet arrives, but is corrupted, then it is counted as + If the packet arrives, but is corrupted, then it is counted as
lost. {Comment: one is tempted to count the packet as received lost. {Comment: one is tempted to count the packet as received
since corruption and packet loss are related but distinct since corruption and packet loss are related but distinct
phenomena. If the IP header is corrupted, however, one cannot be phenomena. If the IP header is corrupted, however, one cannot be
sure about the source or destination IP addresses and is thus on sure about the source or destination IP addresses and is thus on
shaky grounds about knowing that the corrupted received packet shaky grounds about knowing that the corrupted received packet
corresponds to a given sent test packet. Similarly, if other corresponds to a given sent test packet. Similarly, if other
parts of the packet needed by the methodology to know that the parts of the packet needed by the methodology to know that the
corrupted received packet corresponds to a given sent test packet, corrupted received packet corresponds to a given sent test packet,
then such a packet would have to be counted as lost. Counting then such a packet would have to be counted as lost. Counting
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+ If the packet arrives within a reasonable period of time, the one- + If the packet arrives within a reasonable period of time, the one-
way packet-loss is taken to be zero. way packet-loss is taken to be zero.
+ If the packet fails to arrive within a reasonable period of time, + If the packet fails to arrive within a reasonable period of time,
the one-way packet-loss is taken to be one. Note that the the one-way packet-loss is taken to be one. Note that the
threshold of "reasonable" here is a parameter of the methodology. threshold of "reasonable" here is a parameter of the methodology.
{Comment: The definition of reasonable is intentionally vague, and {Comment: The definition of reasonable is intentionally vague, and
is intended to indicate a value "Th" so large that any value in is intended to indicate a value "Th" so large that any value in
(Th-delta,Th+delta) is an equivalent threshold for loss. Here, the closed interval [Th-delta, Th+delta] is an equivalent
delta encompasses all error in clock synchronization along the threshold for loss. Here, delta encompasses all error in clock
measured path. If there is a single value after which the packet synchronization along the measured path. If there is a single
must be counted as lost, then we reintroduce the need for a degree value after which the packet must be counted as lost, then we
of clock synchronization similar to that needed for one-way delay. reintroduce the need for a degree of clock synchronization similar
Therefore, if a measure of packet loss parameterized by a specific to that needed for one-way delay. Therefore, if a measure of
non-huge "reasonable" time-out value is needed, one can always packet loss parameterized by a specific non-huge "reasonable"
measure one-way delay and see what percentage of packets from a time-out value is needed, one can always measure one-way delay and
given stream exceed a given time-out value.} see what percentage of packets from a given stream exceed a given
time-out value.}
Issues such as the packet format, the means by which Dst knows when 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 to expect the test packet, and the means by which Src and Dst are
synchronized are outside the scope of this document. {Comment: We synchronized are outside the scope of this document. {Comment: We
plan to document elsewhere our own work in describing such more plan to document elsewhere our own work in describing such more
detailed implementation techniques and we encourage others to as detailed implementation techniques and we encourage others to as
well.} well.}
3.7. Errors and Uncertainties: 3.7. Errors and Uncertainties:
The description of any specific measurement method should include an The description of any specific measurement method should include an
accounting and analysis of various sources of error/uncertainty. The accounting and analysis of various sources of error or uncertainty.
Framework document provides general guidance on this point. The Framework document provides general guidance on this point.
Errors due to gross lack of synchronization between the Src and Dst For loss, there are three sources of error:
hosts should be dealt with. Since the sensitivity of packet loss
measurement to lack of synchronization is much less than for delay, + Synchronization between clocks on Src and Dst.
we refer the reader to the treatment of synchronization errors in the
One-way Delay metric. + The packet-loss threshold (which is related to the synchronization
between clocks).
+ Resource limits in the network interface or software on the
receiving instrument.
The first two sources are interrelated and could result in a test
packet with finite delay being reported as lost. Type-P-One-way-
Packet-Loss is 0 if the test packet does not arrive, or if it does
arrive and the difference between Src timestamp and Dst timestamp is
greater than the "reasonable period of time", or loss threshold. If
the clocks are not sufficiently synchronized, the loss threshold may
not be "reasonable" - the packet may take much less time to arrive
than its Src timestamp indicates. Similarly, if the loss threshold
is set too low, then many packets may be counted as lost. The loss
threshold must be high enough, and the clocks synchronized well
enough so that a packet that arrives is rarely counted as lost. (See
the discussions in the previous two sections.)
Since the sensitivity of packet loss measurement to lack of clock
synchronization is less than for delay, we refer the reader to the
treatment of synchronization errors in the One-way Delay metric [2]
for more details.
The last source of error, resource limits, cause the packet to be
dropped by the measurement instrument, and counted as lost when in
fact the network delivered the packet in reasonable time.
The measurement instruments should be calibrated such that the loss
threshold is reasonable for application of the metrics and the clocks
are synchronized enough so the loss threshold remains reasonable.
In addition, the instruments should be checked to ensure the
probability is low that a packet arrives at the network interface,
but is lost due to congestion on the interface or to other resource
exhaustion (e.g., buffers) on the instrument.
3.8. Reporting the metric:
The calibration and context in which the metric is measured must be
carefully considered, and should always be reported along with metric
results. We now present four items to consider: Type-P of the test
packets, the loss threshold, instrument calibration, and the path
traversed by the test packets. This list is not exhaustive; any
additional information that could be useful in interpreting
applications of the metrics should also be reported.
3.8.1. Type-P
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.
3.8.2. Loss threshold
The threshold (or methodology to distinguish) between a large finite
delay and loss should be reported.
3.8.3. Calibration results
The degree of synchronization between the Src and Dst clocks should
be reported. If possible, report the probability that a test packet
that arrives at the Dst network interface is reported as lost due to
resource exhaustion on Dst.
3.8.4. Path
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.}
4. A Definition for Samples of One-way Packet Loss 4. A Definition for Samples of One-way Packet Loss
Given the singleton metric Type-P-One-way-Packet-Loss, we now define Given the singleton metric Type-P-One-way-Packet-Loss, we now define
one particular sample of such singletons. The idea of the sample is one particular sample of such singletons. The idea of the sample is
to select a particular binding of the parameters Src, Dst, and Type- 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 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 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 time Tf, and an average rate lambda, then define a pseudo-random
Poisson arrival process of rate lambda, whose values fall between T0 Poisson arrival process of rate lambda, whose values fall between T0
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TS[i], then send a second one (later) at TS[i+1], while the Dst could 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 receive the second test packet at TR[i+1], and then receive the first
one (later) at TR[i]. one (later) at TR[i].
4.7. Errors and Uncertainties: 4.7. Errors and Uncertainties:
In addition to sources of errors and uncertainties associated with In addition to sources of errors and uncertainties associated with
methods employed to measure the singleton values that make up the methods employed to measure the singleton values that make up the
sample, care must be given to analyze the accuracy of the Poisson 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. arrival process of the wire-time of the sending of the test packets.
Problems with this process could be caused by either of several Problems with this process could be caused by several things,
things, including problems with the pseudo-random number techniques including problems with the pseudo-random number techniques used to
used to generate the Poisson arrival process. The Framework document generate the Poisson arrival process. The Framework document shows
shows how to use an Anderson-Darling test for this. how to use the Anderson-Darling test to verify the Poisson process.
4.8. Reporting the metric:
The calibration and context for the underlying singletons should be
reported along with the stream. (See "Reporting the metric" for
Type-P-One-way-Packet-Loss.)
5. Some Statistics Definitions for One-way Packet Loss 5. Some Statistics Definitions for One-way Packet Loss
Given the sample metric Type-P-One-way-Packet-Loss-Poisson-Stream, we Given the sample metric Type-P-One-way-Packet-Loss-Poisson-Stream, we
now offer several statistics of that sample. These statistics are now offer several statistics of that sample. These statistics are
offered mostly to be illustrative of what could be done. offered mostly to be illustrative of what could be done.
5.1. Type-P-One-way-Packet-Loss-Average 5.1. Type-P-One-way-Packet-Loss-Average
Given a Type-P-One-way-Packet-Loss-Poisson-Stream, the average of all Given a Type-P-One-way-Packet-Loss-Poisson-Stream, the average of all
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Then the average would be 0.2. Then the average would be 0.2.
Note that, since healthy Internet paths should be operating at loss Note that, since healthy Internet paths should be operating at loss
rates below 1% (particularly if high delay-bandwidth products are to rates below 1% (particularly if high delay-bandwidth products are to
be sustained), the sample sizes needed might be larger than one would be sustained), the sample sizes needed might be larger than one would
like. Thus, for example, if one wants to discriminate between like. Thus, for example, if one wants to discriminate between
various fractions of 1% over one-minute periods, then several hundred various fractions of 1% over one-minute periods, then several hundred
samples per minute might be needed. This would result in larger samples per minute might be needed. This would result in larger
values of lambda than one would ordinarily want. values of lambda than one would ordinarily want.
Note that although the loss threshold should be set such that any
errors in loss are not significant, if the probability that a packet
which arrived is counted as lost due to resource exhaustion is
significant compared to the loss rate of interest, Type-P-One-way-
Packet-Loss-Average will be meaningless.
6. Security Considerations 6. Security Considerations
Conducting Internet measurements raises both security and privacy Conducting Internet measurements raises both security and privacy
concerns. This memo does not specify an implementation of the concerns. This memo does not specify an implementation of the
metrics, so it does not directly affect the security of the Internet metrics, so it does not directly affect the security of the Internet
nor of applications which run on the Internet. However, nor of applications which run on the Internet. However,
implementations of these metrics must be mindful of security and implementations of these metrics must be mindful of security and
privacy concerns. privacy concerns.
There are two types of security concerns: potential harm caused by There are two types of security concerns: potential harm caused by
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Thanks are due also to Vern Paxson for his valuable comments on early Thanks are due also to Vern Paxson for his valuable comments on early
drafts. drafts.
8. References 8. References
[1] V. Paxson, G. Almes, J. Mahdavi, and M. Mathis, "Framework for [1] V. Paxson, G. Almes, J. Mahdavi, and M. Mathis, "Framework for
IP Performance Metrics", RFC 2330, May 1998. IP Performance Metrics", RFC 2330, May 1998.
[2] G. Almes, S. Kalidindi, and M. Zekauskas, "A One-way Delay [2] G. Almes, S. Kalidindi, and M. Zekauskas, "A One-way Delay
Metric for IPPM", Internet-Draft <draft-ietf-ippm-delay-02.txt>, Metric for IPPM", Internet-Draft <draft-ietf-ippm-delay-04.txt>,
June 1998. August 1998.
[3] J. Mahdavi and V. Paxson, "Connectivity", Work in Progress, [3] J. Mahdavi and V. Paxson, "IPPM Metrics for Measuring
November 1997. Connectivity", Internet-Draft <draft-ietf-ippm-
connectivity-02.txt>, August 1998.
[4] J. Postel, "Internet Protocol", RFC 791, September 1981. [4] J. Postel, "Internet Protocol", RFC 791, September 1981.
9. Authors' Addresses 9. Authors' Addresses
Guy Almes Guy Almes
Advanced Network & Services, Inc. Advanced Network & Services, Inc.
200 Business Park Drive 200 Business Park Drive
Armonk, NY 10504 Armonk, NY 10504
USA USA
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EMail: almes@advanced.org EMail: almes@advanced.org
Sunil Kalidindi Sunil Kalidindi
Advanced Network & Services, Inc. Advanced Network & Services, Inc.
200 Business Park Drive 200 Business Park Drive
Armonk, NY 10504 Armonk, NY 10504
USA USA
Phone: +1 914 765 1128 Phone: +1 914 765 1128
EMail: kalidindi@advanced.org EMail: kalidindi@advanced.org
Matthew J. Zekauskas Matthew J. Zekauskas
Advanced Network & Services, Inc. Advanced Network & Services, Inc.
200 Buisiness Park Drive 200 Buisiness Park Drive
Armonk, NY 10504 Armonk, NY 10504
USA USA
Phone: +1 914 765 1112 Phone: +1 914 765 1112
EMail: matt@advanced.org EMail: matt@advanced.org
Expiration date: December, 1998 Expiration date: March, 1999
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