draft-ietf-ippm-npmps-01.txt   draft-ietf-ippm-npmps-02.txt 
Network Working Group V. Raisanen Network Working Group V. Raisanen
INTERNET-DRAFT Nokia INTERNET-DRAFT Nokia
Expiration Date: November 2000 G. Grotefeld Expiration Date: January 2001 G. Grotefeld
Motorola Motorola
May 2000 July 2000
Network performance measurement for periodic streams Network performance measurement for periodic streams
<draft-ietf-ippm-npmps-01.txt> <draft-ietf-ippm-npmps-02.txt>
1. Status of this Memo 1. Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
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for use as bearer for multimedia streams. In this document, for use as bearer for multimedia streams. In this document,
the reader is assumed to be familiar with the terminology of the the reader is assumed to be familiar with the terminology of the
Framework for IP Performance Metrics RFC 2330 [1]. This document is Framework for IP Performance Metrics RFC 2330 [1]. This document is
parallel to A One-way Delay Metric for IPPM RFC 2679[2]. A sample parallel to A One-way Delay Metric for IPPM RFC 2679[2]. A sample
metric is described that is suitable for application-level measurement metric is described that is suitable for application-level measurement
for streaming multimedia over IP. Using such a measurement, for streaming multimedia over IP. Using such a measurement,
transmission service of a network is probed with a traffic stream transmission service of a network is probed with a traffic stream
similar to that of the application of interest, which is likely to be similar to that of the application of interest, which is likely to be
very dissimilar to the Poisson inter-arrival interval described in [2]. very dissimilar to the Poisson inter-arrival interval described in [2].
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3. Introduction 3. Introduction
This document discusses concepts relevant to application-level This document discusses concepts relevant to application-level
performance measurements of an IP network. The original driver for performance measurements of an IP network. The original driver for
this work is Quality of Service of interactive periodic streams such this work is Quality of Service of interactive periodic streams such
as multimedia conference over IP, but the idea of application-level as multimedia conference over IP, but the idea of application-level
measurement may have a wider scope. In the following, interactive measurement may have a wider scope. In the following, interactive
multimedia traffic is used as an example to illustrate the concept. multimedia traffic is used as an example to illustrate the concept.
A streaming (hereinafter called periodic) multimedia bit stream may A constant bit-rate (CBR), or nearly CBR, streaming (hereinafter
be simulated by transmitting uniformly sized packets (or mostly called periodic) multimedia bit stream may be simulated by
uniformly sized packets) at regular intervals through the network to transmitting uniformly sized packets (or mostly uniformly sized
be evaluated. The "mostly uniformly sized packets" may be found in packets) at regular intervals through the network to be evaluated.
applications that may use smaller packets during a portion of the The "mostly uniformly sized packets" may be found in applications
stream (e.g. digitally coded voice during silence periods). As that may use smaller packets during a portion of the stream (e.g.
noted in the framework document [1], a sample metric using digitally coded voice during silence periods). As noted in the
regularly spaced singleton tests has some limitations when framework document [1], a sample metric using regularly spaced
considered from a general measurement point of view: only part of singleton tests has some limitations when considered from a
the network performance spectrum is sampled. However, from the point general measurement point of view: only part of the network
of view of application-level performance, this is actually good news performance spectrum is sampled. However, from the point of view of
as explained below. application-level performance, this is actually good news as
explained below.
IP delivery service measurements have been discussed within the IP delivery service measurements have been discussed within the
International Telecommunications Union (ITU). A framework for IP International Telecommunications Union (ITU). A framework for IP
service level measurements (with references to the framework for IP service level measurements (with references to the framework for IP
performance [1]) that is intended to be suitable for service planning performance [1]) that is intended to be suitable for service planning
has been approved as I.380 [3]. The emphasis in the ITU has been approved as I.380 [3]. The emphasis in the ITU
recommendation is on passive measurements, though not explicitly recommendation is on passive measurements, though not explicitly
forbidding active measurements. The present contribution proposes a forbidding active measurements. The present contribution proposes a
method that is usable both for service planning and end-user testing method that is usable both for service planning and end-user testing
purposes, and is based on active measurements. purposes, and is based on active measurements.
3.1 Terminology 3.1 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [5]. document are to be interpreted as described in RFC 2119 [4].
Although RFC 2119 was written with protocols in mind, the key words Although RFC 2119 was written with protocols in mind, the key words
are used in this document for similar reasons. They are used to are used in this document for similar reasons. They are used to
ensure the results of measurements from two different implementations ensure the results of measurements from two different implementations
are comparable, and to note instances when an implementation could are comparable, and to note instances when an implementation could
perturb the network. perturb the network.
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3.2 Considerations related to delay 3.2 Considerations related to delay
For interactive multimedia sessions, end-to-end delay is an For interactive multimedia sessions, end-to-end delay is an
important factor. Too large a delay reduces the quality of the important factor. Too large a delay reduces the quality of the
multimedia session as perceived by the participants. End-to-end multimedia session as perceived by the participants. One approach for
delay may be best managed in the general case by assigning maximum managing end-to-end delays on an Internet path involving
per-domain quotas (e.g. 50 ms for a particular IP domain). The heterogeneous link layer technologies is to use per-domain delay
50 ms would then be included into a calculation of an end-to-end quotas (e.g. 50 ms for a particular IP domain). The 50 ms would
delay bound. then be included into a calculation of an end-to-end delay bound. A
practical implementation of such as scheme ought to address issues
like possibility of asymmetric delays in a route in different
directions, and sensitivity of an application to delay variations in
a given domain. There are several alternatives as to which kind of
derivative delay metric one ought to use in managing end-to-end QoS.
This question, although very interesting, is not within the scope of
this draft and is not discussed further here.
For example, in estimating the delay bound that can be guaranteed for In the following, a methodology and metric are presented for
connections, these measurements can provide a useful tool. This is measuring media stream transport QoS in an IP domain. The
probably true irrespective of the possible QoS mechanism utilized in measurement results may be used in derivative metrics such as
the core network. As an example, for a QoS mechanism without hard average and maximum delays. A metric is presented that is a standard
guarantees, measurements may be used to ascertain that the "best" way for performing a measurement irrespective of the possible QoS
class gets the service that has been promised for the traffic class mechanism utilized in the core network. As an example, for a QoS
in question. Moreover, an operator could study the quality of a mechanism without hard guarantees, measurements may be used to
cheap, low-guarantee service implemented using possible slack ascertain that the "best" class gets the service that has been
bandwidth in other classes. Such measurements could be made either promised for the traffic class in question. Moreover, an operator
in studying the feasibility of a new service, or on a regular could study the quality of a cheap, low-guarantee service
basis. implemented using possible slack bandwidth in other classes. Such
measurements could be made either in studying the feasibility of a
new service, or on a regular basis.
The present draft seeks to formalize the measurements in such a way
that interoperable results are achieved.
3.3 Protocol level issues 3.3 Protocol level issues
Fig.1 illustrates measurements on multiple protocol levels that
are relevant to this draft. The major focus of the present draft
is on transport quality evaluation from application point of
view. However, to properly account for quality effects of, e.g.,
operating system and codec on packet voice, it is beneficial to be
able to measure quality at IP level [5]. Link layer monitoring
provides a way of accounting for link layer characteristics such
as bit error rates.
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---------------
| application |
---------------
| transport | <--
---------------
| network | <--
---------------
| link | <--
---------------
| physical |
---------------
Fig. 1: Different possibilities for performing measurements: a
protocol view. Above, "application" refers to all layers above
L4 and is not used in the OSI sense.
In general, the results of measurements may be influenced by In general, the results of measurements may be influenced by
individual application requirements/responses related to the individual application requirements/responses related to the
following issues: following issues:
+ Lost packets: Applications may have varying tolerance to lost + Lost packets: Applications may have varying tolerance to lost
packets. Another consideration is the distribution of lost packets. Another consideration is the distribution of lost
packets (i.e. random or bursty). packets (i.e. random or bursty).
+ Long delays: Many applications will consider packets delayed + Long delays: Many applications will consider packets delayed
longer than a certain value to be equivalent to lost packets longer than a certain value to be equivalent to lost packets
(i.e. real time applications). (i.e. real time applications).
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corruption. corruption.
+ Spurious packet: Dst may receive spurious packets (i.e. packets + Spurious packet: Dst may receive spurious packets (i.e. packets
that are not part of the metric). Many applications may be that are not part of the metric). Many applications may be
perturbed by spurious packets. perturbed by spurious packets.
Depending, e.g., on the observed protocol level, some issues listed Depending, e.g., on the observed protocol level, some issues listed
above may be indistinguishable from others by the application, it above may be indistinguishable from others by the application, it
may be important to preserve the distinction for the operators of may be important to preserve the distinction for the operators of
Src, Dst, and/or the intermediate network(s). Src, Dst, and/or the intermediate network(s).
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Because of the possible errors listed above, in most cases it is Because of the possible errors listed above, in most cases it is
recommended to use a packet identifier for each packet generated at recommended to use a packet identifier for each packet generated at
Src. Identifiers for the metric sample may be those used by the Src. Identifiers for the metric sample may be those used by the
underlying transport layer (e.g. RTP sequence number) or the same underlying transport layer (e.g. RTP sequence number) or the same
identifiers used by an application if the application to be modeled identifiers used by an application if the application to be modeled
by the metric uses an identifier. The possibility of identifier by the metric uses an identifier. The possibility of identifier
roll-over (reuse if intentional) during a metric collected over roll-over (reuse if intentional) during a metric collected over
a "long" (application dependent) time should be observed. a "long" (application dependent) time should be observed.
If the application does not use an identifier, it may still be If the application does not use an identifier, it may still be
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estimation of performance variation at timescale that is estimation of performance variation at timescale that is
important to the multimedia application (packetization important to the multimedia application (packetization
interval). interval).
+ Effects of elastic traffic (TCP) on measurement packets are + Effects of elastic traffic (TCP) on measurement packets are
different for a sustained stream than for single packets during different for a sustained stream than for single packets during
overloading situations as discussed in [3]. overloading situations as discussed in [3].
3.5 Measurement types 3.5 Measurement types
Delay measurements can be one-way [2,3], paired one-way, or Delay measurements can be one-way [2,3], paired one-way, or
round-trip [4]. Accordingly, the measurements may be performed round-trip [6]. Accordingly, the measurements may be performed
either with synchronized or unsynchronized Src/Dst host clocks. either with synchronized or unsynchronized Src/Dst host clocks.
Different possibilities are listed below. Different possibilities are listed below.
The reference measurement setup for all measurement types is The reference measurement setup for all measurement types is
shown in Fig. 1. shown in Fig. 2.
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----------------< IP >-------------------- ----------------< IP >--------------------
| | | | | | | |
------- ------- -------- -------- ------- ------- -------- --------
| Src | | MP | | MP | | Dst | | Src | | MP | | MP | | Dst |
------- |(Src)| |(Dst) | -------- ------- |(Src)| |(Dst) | --------
------- -------- ------- --------
Fig. 1: Example setup for the metric usage. Fig. 2: Example setup for the metric usage.
An example of the use of the metric is a setup with a source host An example of the use of the metric is a setup with a source host
(Src), a destination host (Dst), and corresponding measurement (Src), a destination host (Dst), and corresponding measurement
points (MP(Src) and MP(Dst)) as shown in Figure 1. Separate equipment points (MP(Src) and MP(Dst)) as shown in Figure 2. Separate equipment
for measurement points may be used if having Src and/or Dst conduct for measurement points may be used if having Src and/or Dst conduct
the measurement may significantly affect the delay performance to be the measurement may significantly affect the delay performance to be
measured. MP(Src)should be placed/measured close to the egress point measured. MP(Src)should be placed/measured close to the egress point
of packets from Src. MP(Dst) should be placed/measure close to of packets from Src. MP(Dst) should be placed/measure close to
the ingress point of packets for Dst. "Close" is defined as a the ingress point of packets for Dst. "Close" is defined as a
distance sufficiently small so that application-level performance distance sufficiently small so that application-level performance
characteristics measured (such as delay) can be expected to follow characteristics measured (such as delay) can be expected to follow
the corresponding performance characteristic between Src and Dst to the corresponding performance characteristic between Src and Dst to
an adequate accuracy. an adequate accuracy.
The test setup just described fulfills two important criteria: The test setup just described fulfills two important criteria:
1) Test is made with realistic stream metrics, emulating - for example - 1) Test is made with realistic stream metrics, emulating - for example -
a full-duplex VoIP call. a full-duplex Voice over IP (VoIP) call.
2) Either one-way or round-trip characteristics may be obtained. 2) Either one-way or round-trip characteristics may be obtained.
It is also possible to have intermediate measurement points between It is also possible to have intermediate measurement points between
MP(Src) and MP(Dst), but that is beyond the scope of this document. MP(Src) and MP(Dst), but that is beyond the scope of this document.
3.5.1 One way measurement 3.5.1 One way measurement
In the interests of specifying metrics that are as generally usable In the interests of specifying metrics that are as generally usable
as possible, application-level measurements based on one-way delays as possible, application-level measurements based on one-way delays
are used in the example metrics. The implication of application-level are used in the example metrics. The implication of application-level
measurement for bi-directional applications such as interactive measurement for bi-directional applications such as interactive
multimedia conferencing is discussed below. multimedia conferencing is discussed below.
Performing a single one-way measurement only yields information on Performing a single one-way measurement only yields information on
network behavior in one direction. Moreover, the stream at the network behavior in one direction. Moreover, the stream at the
network transport level does not emulate accurately a full-duplex network transport level does not emulate accurately a full-duplex
multimedia connection. multimedia connection.
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3.5.2 Paired one way measurement 3.5.2 Paired one way measurement
Paired one way delay refers to two multimedia streams: Src to Dst Paired one way delay refers to two multimedia streams: Src to Dst
and Dst to Src for the same Src and Dst. By way of example, for and Dst to Src for the same Src and Dst. By way of example, for
some applications, the delay performance of each one way path is some applications, the delay performance of each one way path is
more important than the round trip delay. This is the case for more important than the round trip delay. This is the case for
delay-limited signals such as voice over IP (VoIP). Possible reasons delay-limited signals such as VoIP. Possible reasons for the
for the difference between one-way delays is different routing of difference between one-way delays is different routing of streams
streams from Src to Dst vs. Dst to Src. from Src to Dst vs. Dst to Src.
For example, a paired one way measurement may show that Src to Dst
has an average delay of 30ms while Dst to Src has an average delay
of 120ms. To a round trip delay measurement, this example would
look like an average of 150ms delay. Without the knowledge of the
asymmetry, we might miss a problem that the application at either
end may have with delays averaging more than 100ms.
Moreover, paired one way delay measurement emulates a full-duplex Moreover, paired one way delay measurement emulates a full-duplex
VoIP call more accurately than a single one-way measurement only. VoIP call more accurately than a single one-way measurement only.
3.5.3 Round trip measurement 3.5.3 Round trip measurement
From the point of view of periodic multimedia streams, From the point of view of periodic multimedia streams,
round-trip measurements have two advantages: they avoid the need of round-trip measurements have two advantages: they avoid the need of
host clock synchronization and they allow for a simulation of host clock synchronization and they allow for a simulation of
full-duplex connections. The former aspect means that a measurement full-duplex connections. The former aspect means that a measurement
is easily performed, since no NTP setup is needed. The latter is easily performed, since no special equipment or NTP setup is
property means that measurement streams are transmitted in both needed. The latter property means that measurement streams are
directions. Thus, the measurement provides information on quality transmitted in both directions. Thus, the measurement provides
of service as experienced by appropriate application. information on quality of service as experienced by appropriate
application.
The downsides of round-trip measurement are the need for more The downsides of round-trip measurement are the need for more
bandwidth than an one-way test and more complex accounting of bandwidth than an one-way test and more complex accounting of
packet loss. Moreover, the stream that is returning towards the packet loss. Moreover, the stream that is returning towards the
original sender may be more bursty than the one on the first "leg" of original sender may be more bursty than the one on the first "leg" of
the round-trip journey. The last issue, however, means in practice the round-trip journey. The last issue, however, means in practice
that returning stream experiences worse QoS than the other one, and that returning stream experiences worse QoS than the other one, and
the performance estimates thus obtained are pessimistic ones. The the performance estimates thus obtained are pessimistic ones. The
possibility of asymmetric routing and queuing must be taken into possibility of asymmetric routing and queuing must be taken into
account during analysis of the results. account during analysis of the results.
Please note that with suitable arrangements, round-trip measurements Please note that with suitable arrangements, round-trip measurements
may be performed using paired one way measurements. may be performed using paired one way measurements.
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4 Sample metric for multimedia stream simulation 4 Sample metric for multimedia stream simulation
The sample metric presented here is similar to the sample metric The sample metric presented here is similar to the sample metric
Type-P-One-way-Delay-Poisson-Stream presented in [2]. "Singletons", as Type-P-One-way-Delay-Poisson-Stream presented in [2]. "Singletons", as
defined in [1] and [2] are not directly used in this document because defined in [1] and [2] are not directly used in this document because
certain key results (such as duplicate or out of sequence packets) certain key results (such as duplicate or out of sequence packets)
cannot be identified in the context of a singleton, but only as part cannot be identified in the context of a singleton, but only as part
of a sample. of a sample.
4.1 Metric name 4.1 Metric name
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Especially in cases of congestion, it may be useful to have Especially in cases of congestion, it may be useful to have
packets smaller than the maximum or predominant size of packets packets smaller than the maximum or predominant size of packets
in the periodic stream. in the periodic stream.
4.2.2 Metrics collected at MP(Src) 4.2.2 Metrics collected at MP(Src)
+ Tstamp(Src)[i], for each packet [i], the time of the packet as + Tstamp(Src)[i], for each packet [i], the time of the packet as
measured at MP(Src) measured at MP(Src)
+ PktID [i], for each packet [i], an identification number for the + PktID [i], for each packet [i], an identification number for the
the packet sent from Src to Dst the packet sent from Src to Dst
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+ PktSiTy [i], for each packet [i], the packet size and/or type. + PktSiTy [i], for each packet [i], the packet size and/or type.
Some applications may use packets of different size, either Some applications may use packets of different size, either
because of application requirements or in response to IP because of application requirements or in response to IP
performance experienced. performance experienced.
4.2.3 Metrics collected at MP (Dst) 4.2.3 Metrics collected at MP (Dst)
+ dTstop, a time interval, used to add to time Tf to determine when to + dTstop, a time interval, used to add to time Tf to determine when to
stop collecting metrics for a sample stop collecting metrics for a sample
+ Tstamp(Dst)[i], for each packet [i], the time of the packet as + Tstamp(Dst)[i], for each packet [i], the time of the packet as
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parameters from the preceding sections (4.2.1, 4.2.2, and 4.2.3 are parameters from the preceding sections (4.2.1, 4.2.2, and 4.2.3 are
combined. combined.
+ Tstamp(Src)[i], for each packet [i], the time of the packet as + Tstamp(Src)[i], for each packet [i], the time of the packet as
measured at MP(Src). This entry may be blank or noted as N/A measured at MP(Src). This entry may be blank or noted as N/A
for spurious packets received at MP(Dst) for spurious packets received at MP(Dst)
+ Tstamp(Dst)[i], for each packet [i], the time of the packet as + Tstamp(Dst)[i], for each packet [i], the time of the packet as
measured at MP(Dst). This entry may be blank or noted as N/A measured at MP(Dst). This entry may be blank or noted as N/A
for packets not received at MP(Dst), received with corrupt for packets not received at MP(Dst), received with corrupt
packet headers, or for duplicate packets received at MP(Dst). packet headers, or for duplicate packets received at MP(Dst).
+ PktID [i], for each packet [i], an identification number for the + PktID [i], for each packet [i], an identification number for the
the packet received. This identification number may be corrupted the packet received. This identification number may be corrupted
for certain packets received at MP (Dst). for certain packets received at MP (Dst).
+ PktSiTy [i], for each packet [i], the packet size and/or type. + PktSiTy [i], for each packet [i], the packet size and/or type.
+ PktStatus [i], for each packet [i], the status of the packet + PktStatus [i], for each packet [i], the status of the packet
received. Possible status includes: OK, packet header corrupt, received. Possible status includes: OK, packet header corrupt,
packet payload corrupt, spurious, duplicate, out of sequence. packet payload corrupt, spurious, duplicate, out of sequence.
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+ Delay [i], for each packet [i], the time interval Tstamp(Dst)[i] - + Delay [i], for each packet [i], the time interval Tstamp(Dst)[i] -
Tstamp(Src)[i]. For the following conditions, it will not be Tstamp(Src)[i]. For the following conditions, it will not be
possible to be able to compute delay: possible to be able to compute delay:
Spurious: There will be no Tstamp(Src)[i] time Spurious: There will be no Tstamp(Src)[i] time
Not received: There will be no Tstamp (Dst) [i] Not received: There will be no Tstamp (Dst) [i]
Corrupt packet header: There will be no Tstamp (Dst) [i] Corrupt packet header: There will be no Tstamp (Dst) [i]
Duplicate: Only the first non-corrupt copy of the packet Duplicate: Only the first non-corrupt copy of the packet
received at Dst should have Delay [i] computed. received at Dst should have Delay [i] computed.
+ SDV[i] [optional] , for each packet [i] except the first one: + SDV[i] [optional] , for each packet [i] except the first one:
momentary delay variation between successive packets, i.e., the momentary delay variation between successive packets, i.e., the
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zero, or positive. Delay for both packets i and i+1 must be zero, or positive. Delay for both packets i and i+1 must be
calculable according to the definition above or SDV[i] is calculable according to the definition above or SDV[i] is
undefined. undefined.
4.3 High level description of the procedure to collect a sample 4.3 High level description of the procedure to collect a sample
Beginning on or after time T0, Type-P packets are generated Beginning on or after time T0, Type-P packets are generated
by Src and sent to Dst until time Tf is reached with a nominal by Src and sent to Dst until time Tf is reached with a nominal
interval between the first bit of successive packets of incT as interval between the first bit of successive packets of incT as
measured at MP(Src). incT may be nominal due to a number of reasons: measured at MP(Src). incT may be nominal due to a number of reasons:
variation in packet generation at Src, clock issues (see section 4.7), variation in packet generation at Src, clock issues (see section 4.6),
etc. etc.
MP(Src) records the following information only for packets with MP(Src) records the following information only for packets with
timestamps between and including T0 and Tf: timestamp, timestamps between and including T0 and Tf: timestamp,
packet identifier, and packet size/type of each packet sent from Src packet identifier, and packet size/type of each packet sent from Src
to Dst that is part of the sample. to Dst that is part of the sample.
MP (Dst) records the following information only for packets with MP (Dst) records the following information only for packets with
time stamps between T0 and (Tf+ dTstop): timestamp, packet identifier, time stamps between T0 and (Tf+ dTstop): timestamp, packet identifier,
packet size/type, and received status of each packet received from packet size/type, and received status of each packet received from
Src at Dst that is part of the sample. Optionally, at a time Tf + Src at Dst that is part of the sample. Optionally, at a time Tf +
Tcons, the data from MP(Src) and MP(Dst) are consolidated to derive Tcons, the data from MP(Src) and MP(Dst) are consolidated to derive
the results of the sample metric. the results of the sample metric.
To prevent stopping data collection too soon, dTcons should be greater To prevent stopping data collection too soon, dTcons should be greater
than or equal to dTstop. Conversely, to keep data collection than or equal to dTstop. Conversely, to keep data collection
reasonably efficient, dTstop should be some reasonable time interval reasonably efficient, dTstop should be some reasonable time interval
(seconds/minutes/hours), even if dTloss is infinite or extremely long. (seconds/minutes/hours), even if dTloss is infinite or extremely long.
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4.4 Discussion 4.4 Discussion
The sample metric thus defined is intended to probe the delays and The sample metric thus defined is intended to probe the delays and
the delay variation as experienced by multimedia streams of the delay variation as experienced by multimedia streams of
an application. Subsequently, the delay is assumed to be measured at an application. Subsequently, the delay is assumed to be measured at
transport layer level. Since a range of packet sizes and nominal transport layer level. Since a range of packet sizes and nominal
interval between packets is used, the method probes only a specific interval between packets is used, the method probes only a specific
time scale of network QoS variations. time scale of network QoS variations.
There are a number of factors that should be taken into account when There are a number of factors that should be taken into account when
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100 usec to 10 msec, whereby it may be important for Src and Dst to 100 usec to 10 msec, whereby it may be important for Src and Dst to
synchronize very closely. GPS systems afford one way to achieve synchronize very closely. GPS systems afford one way to achieve
synchronization to within several 10s of usec. Ordinary application synchronization to within several 10s of usec. Ordinary application
of NTP may allow synchronization to within several msec, but this of NTP may allow synchronization to within several msec, but this
depends on the stability and symmetry of delay properties among those 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 NTP agents used, and this delay is what we are trying to measure. A
combination of some GPS-based NTP servers and a conservatively combination of some GPS-based NTP servers and a conservatively
designed and deployed set of other NTP servers should yield good designed and deployed set of other NTP servers should yield good
results, but this is yet to be tested. results, but this is yet to be tested.
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+ A given methodology will have to include a way to determine + A given methodology will have to include a way to determine
whether a delay value is infinite or whether it is merely very whether packet was lost or whether delay is merely very large (and
large (and the packet is yet to arrive at Dst). The global metric the packet is yet to arrive at Dst). The global metric parameter
parameter dTloss defines a time interval such that delays larger dTloss defines a time interval such that delays larger than dTloss
than dTloss are interpreted as losses. are interpreted as losses.
{Comment: Note that, for many applications of these metrics, the {Comment: Note that, for many applications of these metrics, the
harm in treating a large delay as infinite might be zero or very harm in treating a large delay as infinite might be zero or very
small. A TCP data packet, for example, that arrives only after small. A TCP data packet, for example, that arrives only after
several multiples of the RTT may as well have been lost.} several multiples of the RTT may as well have been lost.}
4.5 Additional Methodology Aspects 4.5 Additional Methodology Aspects
As with other Type-P-* metrics, the detailed methodology will depend As with other Type-P-* metrics, the detailed methodology will depend
on the Type-P (e.g., protocol number, UDP/TCP port number, size, on the Type-P (e.g., protocol number, UDP/TCP port number, size,
precedence). precedence).
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+ Any error in the synchronization between the MP(Src) clock and + Any error in the synchronization between the MP(Src) clock and
the MP(Dst) clock will contribute to error in the delay the MP(Dst) clock will contribute to error in the delay
measurement. We say that the MP(Src) clock and the MP(Dst) measurement. We say that the MP(Src) clock and the MP(Dst)
clock have a synchronization error of Tsynch if the MP(Src) clock clock have a synchronization error of Tsynch if the MP(Src) clock
is Tsynch ahead of the MP(Dst) clock. Thus, if we know the is Tsynch ahead of the MP(Dst) clock. Thus, if we know the
value of Tsynch exactly, we could correct for clock value of Tsynch exactly, we could correct for clock
synchronization by adding Tsynch to the uncorrected value of synchronization by adding Tsynch to the uncorrected value of
Tstamp(Dst)[i] - Tstamp(Src) [i]. Tstamp(Dst)[i] - Tstamp(Src) [i].
Raisanen, Grotefeld expires January 2001 [Page 12]
+ The accuracy of a clock is important only in identifying the time + The accuracy of a clock is important only in identifying the time
at which a given delay was measured. Accuracy, per se, has no at which a given delay was measured. Accuracy, per se, has no
importance to the accuracy of the measurement of delay. When importance to the accuracy of the measurement of delay. When
computing delays, we are interested only in the differences computing delays, we are interested only in the differences
between clock values, not the values themselves. between clock values, not the values themselves.
+ The resolution of a clock adds to uncertainty about any time + The resolution of a clock adds to uncertainty about any time
measured with it. Thus, if the MP(Src) clock has a resolution of measured with it. Thus, if the MP(Src) clock has a resolution of
10 msec, then this adds 10 msec of uncertainty to any time value 10 msec, then this adds 10 msec of uncertainty to any time value
measured with it. We will denote the resolution of the source measured with it. We will denote the resolution of the source
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only directly measure the time between when Src generates the packet only directly measure the time between when Src generates the packet
just prior to sending the test packet and when Dst has started to just prior to sending the test packet and when Dst has started to
process the packet after having received the test packet, and we refer process the packet after having received the test packet, and we refer
to these two points as "host times". to these two points as "host times".
To the extent that the difference between wire time and host time is To the extent that the difference between wire time and host time is
accurately known, this knowledge can be used to correct for wire time accurately known, this knowledge can be used to correct for wire time
measurements and the corrected value more accurately estimates the measurements and the corrected value more accurately estimates the
desired (host time) metric. desired (host time) metric.
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To the extent, however, that the difference between wire time and To the extent, however, that the difference between wire time and
host time is uncertain, this uncertainty must be accounted for in an host time is uncertain, this uncertainty must be accounted for in an
analysis of a given measurement method. We denote by Hsource an analysis of a given measurement method. We denote by Hsource an
upper bound on the uncertainty in the difference between wire time upper bound on the uncertainty in the difference between wire time
of MP(Src) and host time on the Src host, and similarly define Hdest of MP(Src) and host time on the Src host, and similarly define Hdest
for the difference between the host time on the Dst host and the wire for the difference between the host time on the Dst host and the wire
time of MP(Dst). We then note that these problems introduce a total time of MP(Dst). We then note that these problems introduce a total
uncertainty of Hsource+Hdest. This estimate of total wire-vs-host uncertainty of Hsource+Hdest. This estimate of total wire-vs-host
uncertainty should be included in the error/uncertainty analysis of uncertainty should be included in the error/uncertainty analysis of
any measurement implementation. any measurement implementation.
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reported value = true value + random error reported value = true value + random error
The goal of calibration is to determine the systematic and random The goal of calibration is to determine the systematic and random
error generated by the instruments themselves in as much detail as error generated by the instruments themselves in as much detail as
possible. At a minimum, a bound ("e") should be found such that the possible. At a minimum, a bound ("e") should be found such that the
reported value is in the range (true value - e) to (true value + e) 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 error at least 95 percent of the time. We call "e" the calibration error
for the measurements. It represents the degree to which the values for the measurements. It represents the degree to which the values
produced by the measurement instrument are repeatable; that is, how produced by the measurement instrument are repeatable; that is, how
closely an actual delay of 30 ms is reported as 30 ms. {Comment: 95 closely an actual delay of 30 ms is reported as 30 ms. {Comment: 95
percent was chosen because (1) some confidence level is desirable to percent was chosen due to reasons discussed in [2], briefly
be able to remove outliers, which will be found in measuring any summarized as (1) some confidence level is desirable to be able to
physical property; (2) a particular confidence level should be remove outliers, which will be found in measuring any physical
specified so that the results of independent implementations can be property; (2) a particular confidence level should be specified so
compared; and (3) even with a prototype user-level implementation, that the results of independent implementations can be compared.}
95% was loose enough to exclude outliers.}
From the discussion in the previous two sections, the error in From the discussion in the previous two sections, the error in
measurements could be bounded by determining all the individual measurements could be bounded by determining all the individual
uncertainties, and adding them together to form uncertainties, and adding them together to form
Esynch(t) + ResMP(Src) + ResMP(Dst) + Hsource + Hdest. Esynch(t) + ResMP(Src) + ResMP(Dst) + Hsource + Hdest.
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However, reasonable bounds on both the clock-related uncertainty However, reasonable bounds on both the clock-related uncertainty
captured by the first three terms and the host-related uncertainty captured by the first three terms and the host-related uncertainty
captured by the last two terms should be possible by careful design captured by the last two terms should be possible by careful design
techniques and calibrating the instruments using a known, isolated, techniques and calibrating the instruments using a known, isolated,
network in a lab. network in a lab.
For example, the clock-related uncertainties are greatly reduced For example, the clock-related uncertainties are greatly reduced
through the use of a GPS time source. The sum of Esynch(t) + through the use of a GPS time source. The sum of Esynch(t) +
ResMP(Src) + ResMP(Dst) is small, and is also bounded for the ResMP(Src) + ResMP(Dst) is small, and is also bounded for the
duration of the measurement because of the global time source. duration of the measurement because of the global time source.
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Note that random error is a function of measurement load. For Note that random error is a function of measurement load. For
example, if many paths will be measured by one instrument, this might example, if many paths will be measured by one instrument, this might
increase interrupts, process scheduling, and disk I/O (for example, increase interrupts, process scheduling, and disk I/O (for example,
recording the measurements), all of which may increase the random recording the measurements), all of which may increase the random
error in measured singletons. Therefore, in addition to minimal load error in measured singletons. Therefore, in addition to minimal load
measurements to find the systematic error, calibration measurements measurements to find the systematic error, calibration measurements
should be performed with the same measurement load that the should be performed with the same measurement load that the
instruments will see in the field. instruments will see in the field.
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We wish to reiterate that this statistical treatment refers to the We wish to reiterate that this statistical treatment refers to the
calibration of the instrument; it is used to "calibrate the meter calibration of the instrument; it is used to "calibrate the meter
stick" and say how well the meter stick reflects reality. stick" and say how well the meter stick reflects reality.
4.7 Reporting the metric 4.7 Reporting the metric
The calibration and context in which the metric is measured MUST be The calibration and context in which the metric is measured MUST be
carefully considered, and SHOULD always be reported along with metric carefully considered, and SHOULD always be reported along with metric
results. We now present five items to consider: the Type-P of test results. We now present five items to consider: the Type-P of test
packets, the threshold of delay equivalent to loss, error packets, the threshold of delay equivalent to loss, error
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from the measured values. from the measured values.
+ You SHOULD also report the calibration error, e, such that the + You SHOULD also report the calibration error, e, such that the
true value is the reported value plus or minus e, with 95% true value is the reported value plus or minus e, with 95%
confidence (see the last section.) confidence (see the last section.)
+ If possible, the conditions under which a test packet with finite + If possible, the conditions under which a test packet with finite
delay is reported as lost due to resource exhaustion on the delay is reported as lost due to resource exhaustion on the
measurement instrument SHOULD be reported. measurement instrument SHOULD be reported.
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4.7.4. Path 4.7.4. Path
Finally, the path traversed by the packets SHOULD be reported, if The path traversed by the packets SHOULD be reported, if possible.
possible. In general it is impractical to know the precise path a In general it is impractical to know the precise path a given packet
given packet takes through the network. The precise path may be takes through the network. The precise path may be known for
known for certain Type-P packets on short or stable paths. If certain Type-P packets on short or stable paths. If Type-P includes
Type-P includes the record route (or loose-source route) option in the record route (or loose-source route) option in the IP header,
the IP header, and the path is short enough, and all routers* on the and the path is short enough, and all routers* on the path support
path support record (or loose-source) route, then the path will be record (or loose-source) route, then the path will be precisely
precisely recorded. recorded.
This may be impractical because the route must be short enough, This may be impractical because the route must be short enough,
many routers do not support (or are not configured for) record route, many routers do not support (or are not configured for) record route,
and use of this feature would often artificially worsen the and use of this feature would often artificially worsen the
performance observed by removing the packet from common-case performance observed by removing the packet from common-case
processing. However, partial information is still valuable context. processing. However, partial information is still valuable context.
For example, if a host can choose between two links* (and hence two 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 separate routes from Src to Dst), then the initial link used is
valuable context. {Comment: For example, with Merit's NetNow setup, 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 a Src on one NAP can reach a Dst on another NAP by either of several
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test 1,000 two-minute VoIP calls rather than a single 2,000 minute test 1,000 two-minute VoIP calls rather than a single 2,000 minute
VoIP call. When considering collection of a sample of samples, issues VoIP call. When considering collection of a sample of samples, issues
like the interval between samples (e.g. Poisson vs. periodic, time of like the interval between samples (e.g. Poisson vs. periodic, time of
day/day of week), composition of samples (e.g. equal (Tf-T0 duration, day/day of week), composition of samples (e.g. equal (Tf-T0 duration,
different packet sizes), and network considerations (e.g. run different different packet sizes), and network considerations (e.g. run different
samples over different intervening link-host combinations) should be samples over different intervening link-host combinations) should be
taken into account. For items like the interval between samples, taken into account. For items like the interval between samples,
the pattern of use of the application being measured should be the pattern of use of the application being measured should be
considered. considered.
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4.9 Statistics based on Type-P-One-way-Delay-Periodic-Stream 4.9 Statistics based on Type-P-One-way-Delay-Periodic-Stream
4.9.1 Statistics calculable from one sample 4.9.1 Statistics calculable from one sample
As a metric based on a sample representative of certain As a metric based on a sample representative of certain
applications, some general purpose statistics (e.g. median and applications, some general purpose statistics (e.g. median and
percentile) may be less applicable than ways to characterize the percentile) may be less applicable than ways to characterize the
range of delay values recorded during the sample metrics. range of delay values recorded during the sample metrics.
Example, a sample metric generates 100 packets as measured at MP(Src) Example, a sample metric generates 100 packets as measured at MP(Src)
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5. Security Considerations 5. Security Considerations
5.1 Denial of Service Attacks 5.1 Denial of Service Attacks
This metric generates a periodic stream of packets from one host (Src) This metric generates a periodic stream of packets from one host (Src)
to another host (Dst) through intervening networks. This metric to another host (Dst) through intervening networks. This metric
could be abused for denial of service attacks directed at Dst and/or could be abused for denial of service attacks directed at Dst and/or
the intervening network(s). the intervening network(s).
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Administrators of Src, Dst, and the intervening network(s) should Administrators of Src, Dst, and the intervening network(s) should
establish bilateral or multi-lateral agreements regarding the timing, establish bilateral or multi-lateral agreements regarding the timing,
size, and frequency of collection of sample metrics. Use of this size, and frequency of collection of sample metrics. Use of this
metric in excess the terms agreed between the participants MAY BE metric in excess the terms agreed between the participants MAY BE
cause for immediate rejection or discard of packets or other cause for immediate rejection or discard of packets or other
escalation procedures defined between the affected parties. escalation procedures defined between the affected parties.
5.2 User data confidentiality 5.2 User data confidentiality
This metric generates packets for a sample metric, rather than This metric generates packets for a sample metric, rather than
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packets are part of a sample metric. With that knowledge at Dst packets are part of a sample metric. With that knowledge at Dst
and/or the intervening networks, it is possible to change the and/or the intervening networks, it is possible to change the
processing of the packets (e.g. increasing or decreasing delay) processing of the packets (e.g. increasing or decreasing delay)
that may distort the measured performance. It may also be that may distort the measured performance. It may also be
possible to generate additional packets that appear to be part of possible to generate additional packets that appear to be part of
the sample metric. These additional packets are likely to perturb the sample metric. These additional packets are likely to perturb
the results of the sample measurement. the results of the sample measurement.
6. Acknowledgements 6. Acknowledgements
The authors wish to thank the chairs of the IPPM WG, Howard The authors wish to thank the chairs of the IPPM WG for comments
Stanislevic and Al Morton for comments that have made the present that have made the present draft clearer and more focused. Howard
draft clearer and more focused. The authors have also built on the Stanislevic and Al Morton ahave presented useful comments and
substantial foundations laid by the authors of the framework for questions. The authors have also built on the substantial
IP performance [1]. foundations laid by the authors of the framework for IP
performance [1].
7. References 7. References
[1] V.Paxson, G.Almes, J.Mahdavi, and M.Mathis: Framework for IP [1] V.Paxson, G.Almes, J.Mahdavi, and M.Mathis: Framework for IP
Performance Metrics, IETF RFC 2330, May 1998. Performance Metrics, IETF RFC 2330, May 1998.
[2] G.Almes, S.Kalidindi, and M.Zekauskas: A one-way delay metric [2] G.Almes, S.Kalidindi, and M.Zekauskas: A one-way delay metric
for IPPM, IETF RFC 2679, September 1999. for IPPM, IETF RFC 2679, September 1999.
[3] International Telecommunications Union recommendation I.380, [3] International Telecommunications Union recommendation I.380,
February 1999. February 1999.
[4] G.Almes, S.Kalidindi, and M.Zekauskas: A round-trip delay [4] S. Bradner: Key words for use in RFCs to Indicate Requirement
metric for IPPM, IETF RFC 2681.
[5] S. Bradner: Key words for use in RFCs to Indicate Requirement
Levels, RFC 2119, March 1997. Levels, RFC 2119, March 1997.
[5] ETSI TIPHON document TS-101329-5 (to be published in July).
[6] G.Almes, S.Kalidindi, and M.Zekauskas: A round-trip delay
metric for IPPM, IETF RFC 2681.
Raisanen, Grotefeld expires January 2001 [Page 19]
8. Authors' contact information 8. Authors' contact information
Vilho Raisanen <Vilho.Raisanen@nokia.com> Vilho Raisanen <Vilho.Raisanen@nokia.com>
P.O. Box 407 P.O. Box 407
Communication Systems Laboratory Communication Systems Laboratory
Nokia Research Center Nokia Research Center
FIN-00045 Nokia Group FIN-00045 Nokia Group
Finland Finland
Phone +358 9 4376 1 Phone +358 9 4376 1
skipping to change at line 841 skipping to change at line 916
Glenn Grotefeld <g.grotefeld@motorola.com> Glenn Grotefeld <g.grotefeld@motorola.com>
Motorola, Inc. Motorola, Inc.
1303 E. Algonquin Road 1303 E. Algonquin Road
4th Floor 4th Floor
Schaumburg, IL 60196 Schaumburg, IL 60196
USA USA
Phone +1 847 576-5992 Phone +1 847 576-5992
Fax +1 847 538-7455 Fax +1 847 538-7455
EXPIRES NOVEMBER 2000 EXPIRES JANUARY 2001
 End of changes. 

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