One document matched: draft-morton-ippm-composition-00.txt
Network Working Group A.Morton,Editor
Internet Draft AT&T Labs
Document: <draft-morton-ippm-composition-00.txt>
Category: Individual
Composition of Metrics
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This memo intends to define metrics that are applicable to both
complete paths and sub-paths, where a corresponding relationship can
be specified to compose the complete path metric from the sub-path
metrics with sufficient accuracy. The current memo gives some
background and proposes wording for a Scope and Application section
to define this new work. The description of an example metric and
statistic follows.
Morton, et al. Individual exp. January 2006 Page 1
Composition of Metrics July 2005
Contents
Status of this Memo................................................1
Copyright Notice...................................................1
Abstract...........................................................1
Authors/Contributors...............................................2
1. Conventions used in this document...............................3
2. Introduction....................................................3
3. Proposed Scope and Application..................................4
3.1 Scope of Work.................................................4
3.2 Application...................................................4
3.3 Measurement Points............................................5
4. One-way Delay Composition Metrics and Statistics................5
4.1 Name: Type-P-Finite-One-way-Delay-Poisson/Periodic-Stream......5
4.1.1 Metric Parameters:...........................................5
4.1.2 Definition:..................................................5
4.1.3 Discussion and other details.................................6
4.1.4 Mean Statistic...............................................6
4.1.5 Composition Relationship: Sum of Mean Delays.................6
4.1.6 Statement of Conjecture......................................6
4.1.7 Justification for the composite relationship.................6
4.1.8 Sources of Error.............................................6
4.1.9 Specific cases where the conjecture might fail...............7
4.1.10 Application of Measurement Methodology......................7
5. Loss Metrics/Statistics.........................................7
6. Delay Variation Metrics/Statistics..............................7
7. Other Metrics/Statistics........................................7
8. Security Considerations.........................................7
8.1 Denial of Service Attacks......................................7
8.2 User data confidentiality......................................8
8.3 Interference with the metric...................................8
9. IANA Considerations.............................................8
10. Normative References...........................................8
11. Informative References.........................................9
12. Acknowledgments................................................9
13. Author's Addresses.............................................9
Full Copyright Statement..........................................10
Intellectual Property.............................................10
Acknowledgement...................................................10
Authors/Contributors
Thus far, the following people have contributed useful ideas or
suggestions that have been incorporated into this memo:
- Phil Chimento <vze275m9@verizon.net>
- Reza Fardid <RFardid@Covad.COM>
- Roman Krzanowski <roman.krzanowski@verizon.com>
- Maurizio Molina <maurizio.molina@dante.org.uk>
- Emile Stephan <emile.stephan@francetelecom.com>
Morton, et al. Individual exp. January 2006 Page 2
Composition of Metrics July 2005
1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Although RFC 2119 was written with protocols in mind, the key words
are used in this document for similar reasons. They are used to
ensure the results of measurements from two different
implementations are comparable, and to note instances when an
implementation could perturb the network.
In this memo, the characters "<=" should be read as "less than or
equal to" and ">=" as "greater than or equal to".
2. Introduction
The IPPM framework RFC 2330 [RFC2330] describes two forms of metric
composition, spatial and temporal. Spatial composition encompasses
the definitions of performance metrics that are applicable to the
complete path, and to various sub-paths. Also, the text suggests
that the concepts of the analytical framework (or A-frame) would
help to define useful relationships between the complete path
metrics and the sub-path metrics. The effectiveness of such metrics
is dependent on their usefulness in analysis and applicability with
practical measurement methods.
The relationships may involve conjecture, and [RFC2330] lists four
points that the metric definitions should include:
+ the specific conjecture applied to the metric,
+ a justification of the practical utility of the composition in
terms of making accurate measurements of the metric on the path,
+ a justification of the usefulness of the composition in terms of
making analysis of the path using A-frame concepts more
effective, and
+ an analysis of how the conjecture could be incorrect.
RFC 2330 also gives an example where a conjecture that the delay of
a path is very nearly the sum of the delays of the exchanges and
clouds of the corresponding path digest. This example is
particularly relevant to those who wish to assess the performance of
an Inter-domain path without direct measurement, and the performance
estimate of the complete path is related to the measured results for
various sub-paths instead.
Approximate relationships between the sub-path and complete path
metrics are useful, with knowledge of the circumstances where the
relationships are/are not applicable. For example, we would not
expect that delay singletons from each sub-path would sum to produce
an accurate estimate of a delay singleton for the complete path
Morton, et al. Individual exp. January 2006 Page 3
Composition of Metrics July 2005
(unless all the delays were essentially constant - very unlikely).
However, other delay statistics (based on a reasonable sample size)
may have a sufficiently large set of circumstances where they are
applicable.
3. Proposed Scope and Application
3.1 Scope of Work
For the primary IPPM metrics (currently Loss, Delay, and Delay
Variation), this memo gives a set of complete path metrics that can
be composed from the same or similar sub-path metrics. This means
that the complete path metric may be composed from:
+ the same metric for each sub-path
+ multiple metrics for each sub-path (possibly one that is the same
as the complete path metric)
+ a single sub-path metrics that is different from the complete
path metric
Each metric will clearly state:
- the definition (and statistic, where appropriate)
- the composition relationship
- the specific conjecture on which the relationship is based
- a justification of practical utility or usefulness for analysis
using the A-frame concepts
- one or more examples of how the conjecture could be incorrect and
lead to inaccuracy
3.2 Application
For each metric, the applicable circumstances are defined, in terms
of whether the composition:
Requires the same test packets to traverse all sub-paths, or may use
similar packets sent and collected separately in each sub-path.
Requires homogeneity of measurement methodologies, or can allow a
degree of flexibility (e.g., active or passive methods produce the
"same" metric).
Needs information or access that will only be available within an
operator's domain, or is applicable to Inter-domain composition.
Morton, et al. Individual exp. January 2006 Page 4
Composition of Metrics July 2005
Requires synchronized measurement time intervals in all sub-paths,
or largely overlapping, or no timing requirements.
Requires assumption of sub-path independence w.r.t. the metric being
defined/composed, or other assumptions.
Has known sources of inaccuracy/error, and identifies the sources.
3.3 Measurement Points
This section will define the terminology applicable to both complete
path and sub-path metrics.
4. One-way Delay Composition Metrics and Statistics
4.1 Name: Type-P-Finite-One-way-Delay-Poisson/Periodic-Stream
4.1.1 Metric Parameters:
+ Src, the IP address of a host
+ Dst, the IP address of a host
+ T, a time (start of test interval)
+ Tf, a time (end of test interval)
+ lambda, a rate in reciprocal seconds (for Poisson Streams)
+ incT, the nominal duration of inter-packet interval, first bit to
first bit (for Periodic Streams)
+ T0, a time that MUST be selected at random from the interval
[T, T+dT] to start generating packets and taking measurements
(for Periodic Streams)
+ TstampSrc, the wire time of the packet as measured at MP(Src)
+ TstampDst, the wire time of the packet as measured at MP(Dst),
assigned to packets that arrive within a "reasonable" time.
4.1.2 Definition:
Using the parameters above, we obtain the value of Type-P-One-way-
Delay singleton as per RFC 2679 [RFC2679]. For each packet [i] that
has a finite One-way Delay (in other words, excluding packets which
have undefined, or infinite one-way delay):
Type-P-Finite-One-way-Delay-Poisson/Periodic-Stream[i] =
FiniteDelay[i] = TstampDst - TstampSrc
Morton, et al. Individual exp. January 2006 Page 5
Composition of Metrics July 2005
4.1.3 Discussion and other details...
4.1.4 Mean Statistic
+ L, the total number of packets received at Dst (sent between T0
and Tf)
The
Type-P-Finite-One-way-Delay-Mean =
MeanDelay = (1/L)Sum(from i=1 to L, FiniteDelay[i])
where all packets i= 1 through L have finite singleton delays.
4.1.5 Composition Relationship: Sum of Mean Delays
The Type-P-Finite-One-way-Delay-Mean, or MeanDelay for the complete
Source to Destination path can be calculated from sum of the Mean
Delays of all its constituent sub-paths.
4.1.6 Statement of Conjecture
The mean of a sufficiently large stream of packets measured on each
sub-path during the interval [T, Tf] will be representative of the
true mean of the delay distribution (and the distributions
themselves are sufficiently independent), such that the means may be
added to produce an estimate of the complete path mean delay.
4.1.7 Justification for the composite relationship
It is sometimes impractical to conduct active measurements between
every Src-Dst pair. For example, it may not be possible to collect
the desired sample size in each test interval when access link speed
is limited, because of the potential for measurement traffic to
degrade the user traffic performance. The conditions on a low-speed
access link may be understood well-enough to permit use of a small
sample size/rate, while a larger sample size/rate may be used on
other sub-paths.
Also, since measurement operations have a real monetary cost, there
is value in re-using measurements where they are applicable, rather
than launching new measurements for every possible source-
destination pair.
4.1.8 Sources of Error
The measurement packets, each having source and destination
addresses intended for collection at edges of the sub-path, may take
a different specific path through the network equipment and parallel
exchanges than packets with the source and destination addresses of
Morton, et al. Individual exp. January 2006 Page 6
Composition of Metrics July 2005
the complete path. Therefore, the sub-path measurements may differ
from the performance experienced by packets on the complete path.
Measurements employing sufficient sub-path address pairs might
produce bounds on the extent of this error.
others...
4.1.9 Specific cases where the conjecture might fail
If any of the sub-path distributions are bimodal, then the measured
means may not be stable, and in this case the mean will not be a
particularly useful statistic when describing the delay distribution
of the complete path.
The mean may not be sufficiently robust statistic to produce a
reliable estimate, or to be useful even if it can be measured.
others...
4.1.10 Application of Measurement Methodology
SHOULD use similar packets sent and collected separately in each
sub-path.
Allows a degree of flexibility (e.g., active or passive methods can
produce the "same" metric, but timing and correlation of passive
measurements is much more challenging).
Applicable to both Inter-domain and Intra-domain composition.
SHOULD have synchronized measurement time intervals in all sub-
paths, but largely overlapping intervals MAY suffice.
REQUIRES assumption of sub-path independence w.r.t. the metric being
defined/composed.
5. Loss Metrics/Statistics
6. Delay Variation Metrics/Statistics
7. Other Metrics/Statistics
8. Security Considerations
8.1 Denial of Service Attacks
This metric requires a stream of packets sent from one host (source)
to another host (destination) through intervening networks. This
method could be abused for denial of service attacks directed at
destination and/or the intervening network(s).
Morton, et al. Individual exp. January 2006 Page 7
Composition of Metrics July 2005
Administrators of source, destination, and the intervening
network(s) should establish bilateral or multi-lateral agreements
regarding the timing, size, and frequency of collection of sample
metrics. Use of this method in excess of the terms agreed between
the participants may be cause for immediate rejection or discard of
packets or other escalation procedures defined between the affected
parties.
8.2 User data confidentiality
Active use of this method generates packets for a sample, rather
than taking samples based on user data, and does not threaten user
data confidentiality. Passive measurement must restrict attention to
the headers of interest. Since user payloads may be temporarily
stored for length analysis, suitable precautions MUST be taken to
keep this information safe and confidential. In most cases, a
hashing function will produce a value suitable for payload
comparisons.
8.3 Interference with the metric
It may be possible to identify that a certain packet or stream of
packets is part of a sample. With that knowledge at the destination
and/or the intervening networks, it is possible to change the
processing of the packets (e.g. increasing or decreasing delay) that
may distort the measured performance. It may also be possible to
generate additional packets that appear to be part of the sample
metric. These additional packets are likely to perturb the results
of the sample measurement.
To discourage the kind of interference mentioned above, packet
interference checks, such as cryptographic hash, may be used.
9. IANA Considerations
Since this metric does not define a protocol or well-known values,
there are no IANA considerations in this memo.
10. Normative References
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
Obtain via: http://www.rfc-editor.org/rfc/rfc791.txt
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
Obtain via: http://www.rfc-editor.org/rfc/rfc2119.txt
[RFC2330] Paxson, V., Almes, G., Mahdavi, J., and Mathis, M.,
"Framework for IP Performance Metrics", RFC 2330, May
1998.
Obtain via: http://www.rfc-editor.org/rfc/rfc2330.txt
Morton, et al. Individual exp. January 2006 Page 8
Composition of Metrics July 2005
[RFC2679] Almes, G., Kalidindi, S. and M. Zekauskas, "A one-way
delay metric for IPPM", RFC 2679, September 1999.
Obtain via: http://www.rfc-editor.org/rfc/rfc2679.txt
[RFC3148] Mathis, M. and Allman, M., "A Framework for Defining
Empirical Bulk Transfer Capacity Metrics", RFC 3148, July
2001.
Obtain via: http://www.rfc-editor.org/rfc/rfc3148.txt
[RFC3432] Raisanen, V., Grotefeld, G., and Morton, A., "Network
performance measurement with periodic streams", RFC 3432,
November 2002.
11. Informative References
[I.356] ITU-T Recommendation I.356, "B-ISDN ATM layer cell
transfer performance", March 2000.
[Pax98] V.Paxson, "Measurements and Analysis of End-to-End
Internet Dynamics," Ph.D. dissertation, U.C. Berkeley,
1997, ftp://ftp.ee.lbl.gov/papers/vp-thesis/dis.ps.gz.
[RFC3393] Demichelis, C., and Chimento, P., "IP Packet Delay
Variation Metric for IP Performance Metrics (IPPM)", RFC
3393, November 2002.
[Y.1540] ITU-T Recommendation Y.1540, "Internet protocol data
communication service - IP packet transfer and
availability performance parameters", December 2002.
12. Acknowledgments
The authors would like to acknowledge many helpful discussions with
. . . (lots of people, eventually).
13. Author's Addresses
Al Morton
AT&T Labs
Room D3 - 3C06
200 Laurel Ave. South
Middletown, NJ 07748 USA
Phone +1 732 420 1571
EMail: <acmorton@att.com>
Morton, et al. Individual exp. January 2006 Page 9
Composition of Metrics July 2005
Need addresses for:
- Phil Chimento <vze275m9@verizon.net>
- Reza Fardid <RFardid@Covad.COM>
- Roman Krzanowski <roman.krzanowski@verizon.com>
- Maurizio Molina <maurizio.molina@dante.org.uk>
- Emile Stephan <emile.stephan@francetelecom.com>
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Acknowledgement
Morton, et al. Individual exp. January 2006 Page 10
Composition of Metrics July 2005
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Morton, et al. Individual exp. January 2006 Page 11
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