Network Working Group A. Morton
Internet-Draft G. Ramachandran
Expires: December 20, 2006 AT&T Labs
June 18, 2006
Reporting Metrics: Different Points of View
draft-morton-ippm-reporting-metrics-00
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Abstract
Consumers of IP network performance metrics have many different uses
in mind. This memo categorizes the different audience points of
view. It describes how the categories affect the selection of metric
parameters and options when seeking info that serves their needs.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . 3
3. Effect of POV on the Loss Metric . . . . . . . . . . . . . . . 4
3.1. Loss Threshold . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Errored Packet Designation . . . . . . . . . . . . . . . . 5
3.3. Causes of Lost Packets . . . . . . . . . . . . . . . . . . 6
4. Effect of POV on the Delay Metric . . . . . . . . . . . . . . 6
4.1. Treatment of Lost Packets . . . . . . . . . . . . . . . . 6
4.1.1. Application Performance . . . . . . . . . . . . . . . 7
4.1.2. Network Characterization . . . . . . . . . . . . . . . 7
4.1.3. Delay Variation . . . . . . . . . . . . . . . . . . . 8
4.1.4. Reordering . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Preferred Statistics . . . . . . . . . . . . . . . . . . . 8
4.3. Summary for Delay . . . . . . . . . . . . . . . . . . . . 9
5. Sampling: Test Stream Characteristics . . . . . . . . . . . . 9
6. Reporting Results . . . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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1. Introduction
When designing measurements of IP networks and presenting the
results, knowledge of the audience is a key consideration. To
present a useful and relevant portrait of network conditions, one
must answer the following question:
"How will the results be used?"
There are two main audience categories:
1. Network Characterization - describes conditions in an IP network
for quality assurance, troubleshooting, modeling, etc. The
point-of-view looks inward, toward the network, and the consumer
intends their actions there.
2. Application Performance Estimation - describes the network
conditions in a way that facilitates determining affects on user
applications, and ultimately the users themselves. This point-
of-view looks outward, toward the user(s), accepting the network
as-is. This consumer intends to estimate a network-dependent
aspect of performance, or design some aspect of an application's
accommodation of the network. (These are *not* application
metrics, they are defined at the IP layer.)
This memo considers how these different points-of-view affect both
the measurement design (parameters and options of the metrics) and
statistics reported when serving their needs.
The IPPM framework [RFC2330] and other RFCs describing IPPM metrics
provide a background for this memo.
2. Purpose and Scope
The purpose of this memo is to clearly delineate two points-of-view
(POV) for using measurements, and describe their effect on the test
and measurement design, including the selection of metric parameters
and reporting the results.
The current scope of this memo is primarily limited to design and
reporting of the loss and delay metrics [add refs] , but will also
discuss the delay variation and reordering metrics where applicable.
Sampling, or the design of the active packet stream that is the basis
for the measurements, is also discussed.
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3. Effect of POV on the Loss Metric
This section describes the ways in which the Loss metric can be tuned
to reflect the preferences of the two audience categories, or
different POV.
3.1. Loss Threshold
RFC 2680 [RFC2680] defines the concept of a waiting time for packets
to arrive, beyond which they are declared lost. The text of the RFC
declines to recommend a value, instead saying that "good engineering,
including an understanding of packet lifetimes, will be needed in
practice." Later, in the methodology, they give reasons for waiting
"a reasonable period of time", and leaving the definition of
"reasonable" intentionally vague.
Practical measurement experience has shown that unusual network
circumstances can cause long delays. One such circumstance is when
routing loops form during IGP re-convergence following a failure or
drastic link cost change. Packets will loop between two routers
until new routes are installed, or until the Time-to-Live (TTL) field
decrements to zero. Very long delays on the order of several seconds
have been measured [Casner] [Cia03].
Therefore, network characterization activities prefer a long waiting
time in order to distinguish these events from other causes of loss
(such as packet discard at a full queue, or tail drop). This way,
the metric design helps to distinguish more reliably between packets
that might yet arrive, and those that are no longer traversing the
network.
It is possible to calculate a worst-case waiting time, assuming that
a routing loop is the cause. We model the path between Source and
Destination as a series of delays in links (t) and queues (q), as
these two are the dominant contributors to delay. The normal path
delay across n hops without encountering a loop, D, is
n
---
\
D = t + > t + q
0 / i i
---
i = 1
and the time spent in the loop with L hops, is
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i + L
---
\ (TTL - n)
R = C > t + q where C = ---------
/ i i max L
---
i
and where C is the number of times a packet circles the loop.
If we take the delays of all links and queues as 100ms each, the
TTL=255, the number of hops n=5 and the hops in the loop L=4, then
D = 1.1 sec and R ~= 51.2 sec, and D + R ~= 52.3 seconds
We note that the link delays of 100ms would span most continents, and
a constant queue length of 100ms is also very generous. When a loop
occurs, it is almost certain to be resolved in 10 seconds or less.
The value calculated above is an upper limit for almost any realistic
circumstance.
A waiting time threshold parameter, dT, set consistent with this
calculation would not truncate the delay distribution (possibly
causing a change in its mathematical properties), because the packets
that might arrive have been given sufficient time to traverse the
network.
It is worth noting that packets that are stored and deliberately
forwarded at a much later time constitute a replay attack on the
measurement system, and are beyond the scope of normal performance
reporting.
Fortunately, application performance estimation activities are not
adversely affected by the estimated worst-case transfer time.
Although the designer's tendency might be to set the Loss Threshold
at a value equivalent to a particular application's threshold, this
specific threshold can be applied when post-processing the
measurements. A shorter waiting time can be enforced by locating
packets with delays longer than the application's threshold, and re-
designating such packets as lost.
3.2. Errored Packet Designation
RFC 2680 designates packets that arrive containing errors as lost
packets. Many packets that are corrupted by bit errors are discarded
within the network and do not reach their intended destination.
This is consistent with applications that would check the payload
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integrity at higher layers, and discard the packet. However, some
applications prefer to deal with errored payloads on their own, and
even a corrupted payload is better than no packet at all.
To address this possibility, and to make network characterization
more complete, it is recommended to distinguish between packets that
do not arrive (lost) and errored packets that arrive (conditionally
lost).
3.3. Causes of Lost Packets
Although many measurement systems use a waiting time to determine if
a packet is lost or not, most of the waiting is in vain. The packets
are no-longer traversing the network, and have not reached their
destination.
There are many causes of packet loss, including:
1. Queue drop, or discard
2. Corruption of the IP header, or other essential header info
3. TTL expiration (or use of a TTL value that is too small)
4. Link or router failure
After waiting sufficient time, packet loss can probably be attributed
to one of these causes.
4. Effect of POV on the Delay Metric
This section describes the ways in which the Delay metric can be
tuned to reflect the preferences of the two consumer categories, or
different POV.
4.1. Treatment of Lost Packets
The Delay Metric RFC 2679[RFC2679] specifies the treatment of packets
that do not successfully traverse the network: their delay is
undefined.
" >>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."
It is an accepted, but informal practice to assign infinite delay to
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lost packets. We next look at how these two different treatments
align with the needs of measurement consumers who wish to
characterize networks or estimate application performance. Also, we
look at the way that lost packets have been treated in other metrics:
delay variation and reordering.
4.1.1. Application Performance
Applications need to perform different functions, dependent on
whether or not each packet arrives within some finite tolerance. In
other words, a receivers' packet processing forks on packet arrival:
o Packets that arrive within expected tolerance are handled by
processes that remove headers, restore smooth delivery timing (as
in a de-jitter buffer), restore sending order, check for errors in
payloads, and many other operations.
o Packets that do not arrive when expected spawn other processes
that attempt recovery from the apparent loss, such as
retransmission requests, loss concealment, or forward error
correction to replace the missing packet.
So, it is important to maintain a distinction between packets that
actually arrive, and those that do not. Therefore, it is preferable
to leave the delay of lost packets undefined, and to characterize the
delay distribution as a conditional distribution (conditioned on
arrival).
4.1.2. Network Characterization
In this discussion, we assume that both loss and delay metrics will
be reported for network characterization (at least).
Assume packets that do not arrive are reported as Lost, usually as a
percentage of all sent packets. If these lost packets are assigned
undefined delay, then network's inability to deliver them in a timely
way is captured only in the loss metric.
However, if we assign infinite delay to all lost packets, then:
o The delay metric results are influenced by packets that arrive and
those that do not.
o The delay singleton and the loss singleton do not appear to be
orthogonal.
o The network is penalized in both the loss and delay metrics,
effectively double-counting the lost packets.
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Although infinity is a familiar mathematical concept, it is somewhat
disconcerting to see any time-related metric reported as infinity, in
the opinion of the authors. Questions are bound to arise, and tend
to detract from the goal of informing the consumer with a performance
report.
4.1.3. Delay Variation
[RFC3393] excludes lost packets from samples, effectively assigning
an undefined delay to packets that do not arrive in a reasonable
time. Section 4.1 describes this specification and its rationale.
"The treatment of lost packets as having "infinite" or "undefined"
delay complicates the derivation of statistics for ipdv.
Specifically, when packets in the measurement sequence are lost,
simple statistics such as sample mean cannot be computed. One
possible approach to handling this problem is to reduce the event
space by conditioning. That is, we consider conditional statistics;
namely we estimate the mean ipdv (or other derivative statistic)
conditioned on the event that selected packet pairs arrive at the
destination (within the given timeout). While this itself is not
without problems (what happens, for example, when every other packet
is lost), it offers a way to make some (valid) statements about ipdv,
at the same time avoiding events with undefined outcomes."
4.1.4. Reordering
draft-ietf-ippm-reordering [I-D.ietf-ippm-reordering]defines metrics
that are based on evaluation of packet arrival order, and include a
waiting time to declare a packet lost (to exclude them from further
processing).
If packets are assigned a delay value, then the reordering metric
would declare any packets with infinite delay to be reordered,
because their sequence numbers will surely be less than the "Next
Expected" threshold when (or if) they arrive. But this practice
would fail to maintain orthogonality between the reordering metric
and the loss metric. Confusion can be avoided by designating the
delay of non-arriving packets as undefined, and reserving delay
values only for packets that arrive within a sufficiently long
waiting time.
4.2. Preferred Statistics
Today in network characterization, the sample mean is one statistic
that is almost ubiquitously reported. It is easily computed and
understood by virtually everyone in this audience category. Also,
the sample is usually filtered on packet arrival, so that the mean is
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based a conditional distribution.
The median is another statistic that summarizes a distribution,
having somewhat different properties from the sample mean.
When both the sample mean and median are available, a comparison will
sometimes be informative, because these two statistics are equal only
when the delay distribution is perfectly symmetrical.
4.3. Summary for Delay
From the perspectives of:
1. application/receiver analysis, where processing forks on packet
arrival or time out,
2. straightforward network characterization without double-counting
defects, and
3. consistency with Delay variation and Reordering metric
definitions,
the most efficient practice is to distinguish between truly lost and
delayed packets with a sufficiently long waiting time, and to
designate the delay of non-arriving packets as undefined.
5. Sampling: Test Stream Characteristics
Network Characterization has traditionally used Poisson-distributed
interpacket spacing, as this provides an unbiased sample. The
average inter-packet spacing may be selected to allow observation of
specific network phenomena. Other test streams are designed to
sample some property of the network, such as the presence of
congestion, link bandwidth, or packet reordering.
If measuring a network in order to make inferences about applications
or receiver performance, then there are usually efficiencies derived
from a test stream that has similar characteristics to the sender.
In some cases, it is essential to synthesize the sender stream, as
with Bulk Transfer Capacity estimates. In other cases, it may be
sufficient to sample with a "known bias", e.g., a Periodic stream to
estimate real-time application performance.
6. Reporting Results
>>>>>>>>Note: this section will have sub-sections that address the
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different audience categories, for now it gives an overview for the
loss and delay metrics discussed above.
For Packet Loss, the loss ratio defined in RFC 2680 is a sufficient
starting point. We note that a loss ratio calculated according to
[Y.1540] would exclude errored packets form the numerator. In
practice, the difference between these two loss metrics is small if
any, depending on whether the last link prior to the destination
contributes errored packets.
For Packet Delay, we currently recommend providing both the mean
delay and the median delay with lost packets designated as undefined.
These would both be statistics on a conditional distribution, and the
condition is arrival prior to a waiting time dT, where dT has been
set to take maximum packet lifetimes into account.
7. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
8. Security Considerations
9. Acknowledgements
The authors would like to thank
10. References
10.1. Normative References
[I-D.ietf-ippm-reordering]
Morton, A., "Packet Reordering Metric for IPPM",
draft-ietf-ippm-reordering-13 (work in progress),
May 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
"Framework for IP Performance Metrics", RFC 2330,
May 1998.
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[RFC2679] Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
Delay Metric for IPPM", RFC 2679, September 1999.
[RFC2680] Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
Packet Loss Metric for IPPM", RFC 2680, September 1999.
[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation
Metric for IP Performance Metrics (IPPM)", RFC 3393,
November 2002.
10.2. Informative References
[Casner] "A Fine-Grained View of High Performance Networking, NANOG
22 Conf.; http://www.nanog.org/mtg-0105/agenda.html", May
20-22 2001.
[Cia03] "Standardized Active Measurements on a Tier 1 IP Backbone,
IEEE Communications Mag., pp 90-97.", June 2003.
[Y.1540] ITU-T Recommendation Y.1540, "Internet protocol data
communication service - IP packet transfer and availability
performance parameters", December 2002.
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Authors' Addresses
Al Morton
AT&T Labs
200 Laurel Avenue South
Middletown,, NJ 07748
USA
Phone: +1 732 420 1571
Fax: +1 732 368 1192
Email: acmorton@att.com
URI: http://home.comcast.net/~acmacm/
Gomathi Ramachandran
AT&T Labs
200 Laurel Avenue South
Middletown,, NJ 07748
USA
Phone: +1 732 420 2353
Fax: +1 732 368 1192
Email: gomathi@att.com
URI:
Phone:
Fax:
Email:
URI:
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