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+Internet Engineering Task Force (IETF) N. Duffield
+Request for Comments: 6534 AT&T Labs-Research
+Category: Standards Track A. Morton
+ISSN: 2070-1721 AT&T Labs
+ J. Sommers
+ Colgate University
+ May 2012
+
+
+ Loss Episode Metrics for IP Performance Metrics (IPPM)
+
+Abstract
+
+ The IETF has developed a one-way packet loss metric that measures the
+ loss rate on a Poisson and Periodic probe streams between two hosts.
+ However, the impact of packet loss on applications is, in general,
+ sensitive not just to the average loss rate but also to the way in
+ which packet losses are distributed in loss episodes (i.e., maximal
+ sets of consecutively lost probe packets). This document defines
+ one-way packet loss episode metrics, specifically, the frequency and
+ average duration of loss episodes and a probing methodology under
+ which the loss episode metrics are to be measured.
+
+Status of This Memo
+
+ This is an Internet Standards Track document.
+
+ This document is a product of the Internet Engineering Task Force
+ (IETF). It represents the consensus of the IETF community. It has
+ received public review and has been approved for publication by the
+ Internet Engineering Steering Group (IESG). Further information on
+ Internet Standards is available in Section 2 of RFC 5741.
+
+ Information about the current status of this document, any errata,
+ and how to provide feedback on it may be obtained at
+ http://www.rfc-editor.org/info/rfc6534.
+
+Copyright Notice
+
+ Copyright (c) 2012 IETF Trust and the persons identified as the
+ document authors. All rights reserved.
+
+ This document is subject to BCP 78 and the IETF Trust's Legal
+ Provisions Relating to IETF Documents
+ (http://trustee.ietf.org/license-info) in effect on the date of
+ publication of this document. Please review these documents
+ carefully, as they describe your rights and restrictions with respect
+ to this document. Code Components extracted from this document must
+
+
+
+Duffield, et al. Standards Track [Page 1]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+ include Simplified BSD License text as described in Section 4.e of
+ the Trust Legal Provisions and are provided without warranty as
+ described in the Simplified BSD License.
+
+ This document may contain material from IETF Documents or IETF
+ Contributions published or made publicly available before November
+ 10, 2008. The person(s) controlling the copyright in some of this
+ material may not have granted the IETF Trust the right to allow
+ modifications of such material outside the IETF Standards Process.
+ Without obtaining an adequate license from the person(s) controlling
+ the copyright in such materials, this document may not be modified
+ outside the IETF Standards Process, and derivative works of it may
+ not be created outside the IETF Standards Process, except to format
+ it for publication as an RFC or to translate it into languages other
+ than English.
+
+Table of Contents
+
+ 1. Introduction ....................................................4
+ 1.1. Background and Motivation ..................................4
+ 1.1.1. Requirements Language ...............................5
+ 1.2. Loss Episode Metrics and Bi-Packet Probes ..................5
+ 1.3. Outline and Contents .......................................6
+ 2. Singleton Definition for Type-P-One-way Bi-Packet Loss ..........7
+ 2.1. Metric Name ................................................7
+ 2.2. Metric Parameters ..........................................7
+ 2.3. Metric Units ...............................................7
+ 2.4. Metric Definition ..........................................7
+ 2.5. Discussion .................................................8
+ 2.6. Methodologies ..............................................8
+ 2.7. Errors and Uncertainties ...................................8
+ 2.8. Reporting the Metric .......................................8
+ 3. General Definition of Samples for
+ Type-P-One-way-Bi-Packet-Loss ...................................8
+ 3.1. Metric Name ................................................9
+ 3.2. Metric Parameters ..........................................9
+ 3.3. Metric Units ...............................................9
+ 3.4. Metric Definition ..........................................9
+ 3.5. Discussion .................................................9
+ 3.6. Methodologies .............................................10
+ 3.7. Errors and Uncertainties ..................................10
+ 3.8. Reporting the Metric ......................................10
+ 4. An Active Probing Methodology for Bi-Packet Loss ...............10
+ 4.1. Metric Name ...............................................10
+ 4.2. Metric Parameters .........................................10
+ 4.3. Metric Units ..............................................11
+ 4.4. Metric Definition .........................................11
+ 4.5. Discussion ................................................11
+
+
+
+Duffield, et al. Standards Track [Page 2]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+ 4.6. Methodologies .............................................11
+ 4.7. Errors and Uncertainties ..................................12
+ 4.8. Reporting the Metric ......................................12
+ 5. Loss Episode Proto-Metrics .....................................12
+ 5.1. Loss-Pair-Counts ..........................................13
+ 5.2. Bi-Packet-Loss-Ratio ......................................13
+ 5.3. Bi-Packet-Loss-Episode-Duration-Number ....................13
+ 5.4. Bi-Packet-Loss-Episode-Frequency-Number ...................13
+ 6. Loss Episode Metrics Derived from Bi-Packet Loss Probing .......14
+ 6.1. Geometric Stream: Loss Ratio ..............................14
+ 6.1.1. Metric Name ........................................14
+ 6.1.2. Metric Parameters ..................................14
+ 6.1.3. Metric Units .......................................15
+ 6.1.4. Metric Definition ..................................15
+ 6.1.5. Discussion .........................................15
+ 6.1.6. Methodologies ......................................15
+ 6.1.7. Errors and Uncertainties ...........................15
+ 6.1.8. Reporting the Metric ...............................15
+ 6.2. Geometric Stream: Loss Episode Duration ...................16
+ 6.2.1. Metric Name ........................................16
+ 6.2.2. Metric Parameters ..................................16
+ 6.2.3. Metric Units .......................................16
+ 6.2.4. Metric Definition ..................................16
+ 6.2.5. Discussion .........................................16
+ 6.2.6. Methodologies ......................................16
+ 6.2.7. Errors and Uncertainties ...........................17
+ 6.2.8. Reporting the Metric ...............................17
+ 6.3. Geometric Stream: Loss Episode Frequency ..................17
+ 6.3.1. Metric Name ........................................17
+ 6.3.2. Metric Parameters ..................................17
+ 6.3.3. Metric Units .......................................17
+ 6.3.4. Metric Definition ..................................18
+ 6.3.5. Discussion .........................................18
+ 6.3.6. Methodologies ......................................18
+ 6.3.7. Errors and Uncertainties ...........................18
+ 6.3.8. Reporting the Metric ...............................18
+ 7. Applicability of Loss Episode Metrics ..........................18
+ 7.1. Relation to Gilbert Model .................................18
+ 8. Security Considerations ........................................19
+ 9. References .....................................................20
+ 9.1. Normative References ......................................20
+ 9.2. Informative References ....................................20
+
+
+
+
+
+
+
+
+
+Duffield, et al. Standards Track [Page 3]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+1. Introduction
+
+1.1. Background and Motivation
+
+ Packet loss in the Internet is a complex phenomenon due to the bursty
+ nature of traffic and congestion processes, influenced by both end-
+ users and applications and the operation of transport protocols such
+ as TCP. For these reasons, the simplest model of packet loss -- the
+ single parameter Bernoulli (independent) loss model -- does not
+ represent the complexity of packet loss over periods of time.
+ Correspondingly, a single loss metric -- the average packet loss
+ ratio over some period of time -- arising, e.g., from a stream of
+ Poisson probes as in [RFC2680] is not sufficient to determine the
+ effect of packet loss on traffic in general.
+
+ Moving beyond single parameter loss models, Markovian and Markov-
+ modulated loss models involving transitions between a good and bad
+ state, each with an associated loss rate, have been proposed by
+ Gilbert [Gilbert] and more generally by Elliot [Elliot]. In
+ principle, Markovian models can be formulated over state spaces
+ involving patterns of loss of any desired number of packets.
+ However, further increase in the size of the state space makes such
+ models cumbersome both for parameter estimation (accuracy decreases)
+ and prediction in practice (due to computational complexity and
+ sensitivity to parameter inaccuracy). In general, the relevance and
+ importance of particular models can change in time, e.g., in response
+ to the advent of new applications and services. For this reason, we
+ are drawn to empirical metrics that do not depend on a particular
+ model for their interpretation.
+
+ An empirical measure of packet loss complexity, the index of
+ dispersion of counts (IDC), comprise, for each t >0, the ratio v(t) /
+ a(t) of the variance v(t) and average a(t) of the number of losses
+ over successive measurement windows of a duration t. However, a full
+ characterization of packet loss over time requires specification of
+ the IDC for each window size t>0.
+
+ In the standards arena, loss pattern sample metrics are defined in
+ [RFC3357]. Following the Gilbert-Elliot model, burst metrics
+ specific for Voice over IP (VoIP) that characterize complete episodes
+ of lost, transmitted, and discarded packets are defined in [RFC3611].
+
+ The above considerations motivate the formulation of empirical
+ metrics of one-way packet loss that provide the simplest
+ generalization of [RFC2680] (which is widely adopted but only defines
+ a single loss-to-total ratio metric). The metrics defined here
+
+
+
+
+
+Duffield, et al. Standards Track [Page 4]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+ capture deviations from independent packet loss in a robust model-
+ independent manner. The document also defines efficient measurement
+ methodologies for these metrics.
+
+1.1.1. Requirements Language
+
+ 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].
+
+1.2. Loss Episode Metrics and Bi-Packet Probes
+
+ The losses experienced by the packet stream can be viewed as
+ occurring in loss episodes, i.e., a maximal set of consecutively lost
+ packets. This memo describes one-way loss episode metrics: their
+ frequency and average duration. Although the average loss ratio can
+ be expressed in terms of these quantities, they go further in
+ characterizing the statistics of the patterns of packet loss within
+ the stream of probes. This is useful information in understanding
+ the effect of packet losses on application performance, since
+ different applications can have different sensitivities to patterns
+ of loss, being sensitive not only to the long-term average loss rate,
+ but how losses are distributed in time. As an example, MPEG video
+ traffic may be sensitive to loss involving the I-frame in a group of
+ pictures, but further losses within an episode of sufficiently short
+ duration have no further impact; the damage is already done.
+
+ The loss episode metrics presented here have the following useful
+ properties:
+
+ 1. the metrics are empirical and do not depend on an underlying
+ model; e.g., the loss process is not assumed to be Markovian. On
+ the other hand, it turns out that the metrics of this memo can be
+ related to the special case of the Gilbert Model parameters; see
+ Section 7.
+
+ 2. the metric units can be directly compared with applications or
+ user requirements or tolerance for network loss performance, in
+ the frequency and duration of loss episodes, as well as the usual
+ packet loss ratio, which can be recovered from the loss episode
+ metrics upon dividing the average loss episode duration by the
+ loss episode frequency.
+
+ 3. the metrics provide the smallest possible increment in complexity
+ beyond, but in the spirit of, the IP Performance Metrics (IPPM)
+ average packet loss ratio metrics [RFC2680], i.e., moving from a
+ single metric (average packet loss ratio) to a pair of metrics
+ (loss episode frequency and average loss episode duration).
+
+
+
+Duffield, et al. Standards Track [Page 5]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+ The document also describes a probing methodology under which loss
+ episode metrics are to be measured. The methodology comprises
+ sending probe packets in pairs, where packets within each probe pair
+ have a fixed separation, and the time between pairs takes the form of
+ a geometric distributed number multiplied by the same separation.
+ This can be regarded a generalization of Poisson probing where the
+ probes are pairs rather than single packets as in [RFC2680], and also
+ of geometric probing described in [RFC2330]. However, it should be
+ distinguished from back-to-back packet pairs whose change in
+ separation on traversing a link is used to probe bandwidth. In this
+ document, the separation between the packets in a pair is the
+ temporal resolution at which different loss episodes are to be
+ distinguished. The methodology does not measure episodes of loss of
+ consecutive background packets on the measured path. One key feature
+ of this methodology is its efficiency: it estimates the average
+ length of loss episodes without directly measuring the complete
+ episodes themselves. Instead, this information is encoded in the
+ observed relative frequencies of the four possible outcomes arising
+ from the loss or successful transmission of each of the two packets
+ of the probe pairs. This is distinct from the approach of [RFC3611],
+ which reports on directly measured episodes.
+
+ The metrics defined in this memo are "derived metrics", according to
+ Section 6.1 of [RFC2330] (the IPPM framework). They are based on the
+ singleton loss metric defined in Section 2 of [RFC2680] .
+
+1.3. Outline and Contents
+
+ o Section 2 defines the fundamental singleton metric for the
+ possible outcomes of a probe pair: Type-P-One-way-Bi-Packet-Loss.
+
+ o Section 3 defines sample sets of this metric derived from a
+ general probe stream: Type-P-One-way-Bi-Packet-Loss-Stream.
+
+ o Section 4 defines the prime example of the Bi-Packet-Loss-Stream
+ metrics, specifically Type-P-One-way-Bi-Packet-Loss-Geometric-
+ Stream arising from the geometric stream of packet-pair probes
+ that was described informally in Section 1.
+
+ o Section 5 defines loss episode proto-metrics that summarize the
+ outcomes from a stream metrics as an intermediate step to forming
+ the loss episode metrics; they need not be reported in general.
+
+ o Section 6 defines the final loss episode metrics that are the
+ focus of this memo, the new metrics:
+
+ * Type-P-One-way-Bi-Packet-Loss-Geometric-Stream-Episode-
+ Duration, the average duration, in seconds, of a loss episode.
+
+
+
+Duffield, et al. Standards Track [Page 6]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+ * Type-P-One-way-Bi-Packet-Loss-Geometric-Stream-Episode-
+ Frequency, the average frequency, per second, at which loss
+ episodes start.
+
+ * Type-P-One-way-Bi-Packet-Loss-Geometric-Stream-Ratio, which is
+ the average packet loss ratio metric arising from the geometric
+ stream probing methodology
+
+ o Section 7 details applications and relations to existing loss
+ models.
+
+2. Singleton Definition for Type-P-One-way Bi-Packet Loss
+
+2.1. Metric Name
+
+ Type-P-One-way-Bi-Packet-Loss
+
+2.2. Metric Parameters
+
+ o Src, the IP address of a source host
+
+ o Dst, the IP address of a destination host
+
+ o T1, a sending time of the first packet
+
+ o T2, a sending time of the second packet, with T2>T1
+
+ o F, a selection function defining unambiguously the two packets
+ from the stream selected for the metric
+
+ o P, the specification of the packet type, over and above the source
+ and destination addresses
+
+2.3. Metric Units
+
+ A Loss Pair is pair (l1, l2) where each of l1 and l2 is a binary
+ value 0 or 1, where 0 signifies successful transmission of a packet
+ and 1 signifies loss.
+
+ The metric unit of Type-P-One-way-Bi-Packet-Loss is a Loss Pair.
+
+2.4. Metric Definition
+
+ 1. "The Type-P-One-way-Bi-Packet-Loss with parameters (Src, Dst, T1,
+ T2, F, P) is (1,1)" means that Src sent the first bit of a Type-P
+ packet to Dst at wire-time T1 and the first bit of a Type-P
+ packet to Dst at wire-time T2>T1 and that neither packet was
+ received at Dst.
+
+
+
+Duffield, et al. Standards Track [Page 7]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+ 2. "The Type-P-One-way-Bi-Packet-Loss with parameters (Src, Dst, T1,
+ T2, F, P) is (1,0)" means that Src sent the first bit of a Type-P
+ packet to Dst at wire-time T1 and the first bit of a Type-P
+ packet to Dst at wire-time T2>T1 and that the first packet was
+ not received at Dst, and the second packet was received at Dst
+
+ 3. "The Type-P-One-way-Bi-Packet-Loss with parameters (Src, Dst, T1,
+ T2, F, P) is (0,1)" means that Src sent the first bit of a Type-P
+ packet to Dst at wire-time T1 and the first bit of a Type-P
+ packet to Dst at wire-time T2>T1 and that the first packet was
+ received at Dst, and the second packet was not received at Dst
+
+ 4. "The Type-P-One-way-Bi-Packet-Loss with parameters (Src, Dst, T1,
+ T2, F, P) is (0,0)" means that Src sent the first bit of a Type-P
+ packet to Dst at wire-time T1 and the first bit of a Type-P
+ packet to Dst at wire-time T2>T1 and that both packets were
+ received at Dst.
+
+2.5. Discussion
+
+ The purpose of the selection function is to specify exactly which
+ packets are to be used for measurement. The notion is taken from
+ Section 2.5 of [RFC3393], where examples are discussed.
+
+2.6. Methodologies
+
+ The methodologies related to the Type-P-One-way-Packet-Loss metric in
+ Section 2.6 of [RFC2680] are similar for the Type-P-One-way-Bi-
+ Packet-Loss metric described above. In particular, the methodologies
+ described in RFC 2680 apply to both packets of the pair.
+
+2.7. Errors and Uncertainties
+
+ Sources of error for the Type-P-One-way-Packet-Loss metric in Section
+ 2.7 of [RFC2680] apply to each packet of the pair for the Type-P-One-
+ way-Bi-Packet-Loss metric.
+
+2.8. Reporting the Metric
+
+ Refer to Section 2.8 of [RFC2680].
+
+3. General Definition of Samples for Type-P-One-way-Bi-Packet-Loss
+
+ Given the singleton metric for Type-P-One-way-Bi-Packet-Loss, we now
+ define examples of samples of singletons. The basic idea is as
+ follows. We first specify a set of times T1 < T2 <...<Tn, each of
+
+
+
+
+
+Duffield, et al. Standards Track [Page 8]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+ which acts as the first time of a packet pair for a single Type-P-
+ One-way-Bi-Packet-Loss measurement. This results is a set of n
+ metric values of Type-P-One-way-Bi-Packet-Loss.
+
+3.1. Metric Name
+
+ Type-P-One-way-Bi-Packet-Loss-Stream
+
+3.2. Metric Parameters
+
+ o Src, the IP address of a source host
+
+ o Dst, the IP address of a destination host
+
+ o (T11,T12), (T21,T22)....,(Tn1,Tn2) a set of n times of sending
+ times for packet pairs, with T11 < T12 <= T21 < T22 <=...<= Tn1 <
+ Tn2
+
+ o F, a selection function defining unambiguously the two packets
+ from the stream selected for the metric
+
+ o P, the specification of the packet type, over and above the source
+ and destination address
+
+3.3. Metric Units
+
+ A set L1,L2,...,Ln of Loss Pairs
+
+3.4. Metric Definition
+
+ Each Loss Pair Li for i = 1,....n is the Type-P-One-way-Bi-Packet-
+ Loss with parameters (Src, Dst, Ti1, Ti2, Fi, P) where Fi is the
+ restriction of the selection function F to the packet pair at time
+ Ti1, Ti2.
+
+3.5. Discussion
+
+ The metric definition of Type-P-One-way-Bi-Packet-Loss-Stream is
+ sufficiently general to describe the case where packets are sampled
+ from a preexisting stream. This is useful in the case in which there
+ is a general purpose measurement stream set up between two hosts, and
+ we wish to select a substream from it for the purposes of loss
+ episode measurement. Packet pairs selected as bi-packet loss probes
+ need not be consecutive within such a stream. In the next section,
+ we specialize this somewhat to more concretely describe a purpose
+ built packet stream for loss episode measurement.
+
+
+
+
+
+Duffield, et al. Standards Track [Page 9]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+3.6. Methodologies
+
+ The methodologies related to the Type-P-One-way-Packet-Loss metric in
+ Section 2.6 of [RFC2680] are similar for the Type-P-One-way-Bi-
+ Packet-Loss-Stream metric described above. In particular, the
+ methodologies described in RFC 2680 apply to both packets of each
+ pair.
+
+3.7. Errors and Uncertainties
+
+ Sources of error for the Type-P-One-way-Packet-Loss metric in Section
+ 2.7 of [RFC2680] apply to each packet of each pair for the Type-P-
+ One-way-Bi-Packet-Loss-Stream metric.
+
+3.8. Reporting the Metric
+
+ Refer to Section 2.8 of [RFC2680].
+
+4. An Active Probing Methodology for Bi-Packet Loss
+
+ This section specializes the preceding section for an active probing
+ methodology. The basic idea is a follows. We set up a sequence of
+ evenly spaced times T1 < T2 < ... < Tn. Each time Ti is potentially
+ the first packet time for a packet pair measurement. We make an
+ independent random decision at each time, whether to initiate such a
+ measurement. Hence, the interval count between successive times at
+ which a pair is initiated follows a geometric distribution. We also
+ specify that the spacing between successive times Ti is the same as
+ the spacing between packets in a given pair. Thus, if pairs happen
+ to be launched at the successive times Ti and T(i+1), the second
+ packet of the first pair is actually used as the first packet of the
+ second pair.
+
+4.1. Metric Name
+
+ Type-P-One-way-Bi-Packet-Loss-Geometric-Stream
+
+4.2. Metric Parameters
+
+ o Src, the IP address of a source host
+
+ o Dst, the IP address of a destination host
+
+ o T0, the randomly selected starting time [RFC3432] for periodic
+ launch opportunities
+
+ o d, the time spacing between potential launch times, Ti and T(i+1)
+
+
+
+
+Duffield, et al. Standards Track [Page 10]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+ o n, a count of potential measurement instants
+
+ o q, a launch probability
+
+ o F, a selection function defining unambiguously the two packets
+ from the stream selected for the metric
+
+ o P, the specification of the packet type, over and above the source
+ and destination address
+
+4.3. Metric Units
+
+ A set of Loss Pairs L1, L2, ..., Lm for some m <= n
+
+4.4. Metric Definition
+
+ For each i = 0, 1, ..., n-1 we form the potential measurement time Ti
+ = T0 + i*d. With probability q, a packet pair measurement is
+ launched at Ti, resulting in a Type-P-One-way-Bi-Packet-Loss with
+ parameters (Src, Dst, Ti, T(i+1), Fi, P) where Fi is the restriction
+ of the selection function F to the packet pair at times Ti, T(i+1).
+ L1, L2,...Lm are the resulting Loss Pairs; m can be less than n since
+ not all times Ti have an associated measurement.
+
+4.5. Discussion
+
+ The above definition of Type-P-One-way-Bi-Packet-Loss-Geometric-
+ Stream is equivalent to using Type-P-One-way-Bi-Packet-Loss-Stream
+ with an appropriate statistical definition of the selection function
+ F.
+
+ The number m of Loss Pairs in the metric can be less than the number
+ of potential measurement instants because not all instants may
+ generate a probe when the launch probability q is strictly less than
+ 1.
+
+4.6. Methodologies
+
+ The methodologies follow from:
+
+ o the specific time T0, from which all successive Ti follow, and
+
+ o the specific time spacing, and
+
+ o the methodologies discussion given above for the singleton Type-P-
+ One-way-Bi-Packet-Loss metric.
+
+
+
+
+
+Duffield, et al. Standards Track [Page 11]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+ The issue of choosing an appropriate time spacing (e.g., one that is
+ matched to expected characteristics of loss episodes) is outside the
+ scope of this document.
+
+ Note that as with any active measurement methodology, consideration
+ must be made to handle out-of-order arrival of packets; see also
+ Section 3.6. of [RFC2680].
+
+4.7. Errors and Uncertainties
+
+ In addition to sources of errors and uncertainties related to
+ methodologies for measuring the singleton Type-P-One-way-Bi-Packet-
+ Loss metric, a key source of error when emitting packets for Bi-
+ Packet Loss relates to resource limits on the host used to send the
+ packets. In particular, the choice of T0, the choice of the time
+ spacing, and the choice of the launch probability results in a
+ schedule for sending packets. Insufficient CPU resources on the
+ sending host may result in an inability to send packets according to
+ schedule. Note that the choice of time spacing directly affects the
+ ability of the host CPU to meet the required schedule (e.g., consider
+ a 100 microsecond spacing versus a 100 millisecond spacing).
+
+ For other considerations, refer to Section 3.7 of [RFC2680].
+
+4.8. Reporting the Metric
+
+ Refer to Section 3.8. of [RFC2680].
+
+5. Loss Episode Proto-Metrics
+
+ This section describes four generic proto-metric quantities
+ associated with an arbitrary set of Loss Pairs. These are the Loss-
+ Pair-Counts, Bi-Packet-Loss-Ratio, Bi-Packet-Loss-Episode-Duration-
+ Number, Bi-Packet-Loss-Episode-Frequency-Number. Specific loss
+ episode metrics can then be constructed when these proto-metrics
+ take, as their input, sets of Loss Pairs samples generated by the
+ Type-P-One-way-Bi-Packet-Loss-Stream and Type-P-One-way-Bi-Packet-
+ Loss-Geometric-Stream. The second of these is described in
+ Section 4. It is not expected that these proto-metrics would be
+ reported themselves. Rather, they are intermediate quantities in the
+ production of the final metrics of Section 6 below, and could be
+ rolled up into metrics in implementations. The metrics report loss
+ episode durations and frequencies in terms of packet counts, since
+ they do not depend on the actual time between probe packets. The
+ final metrics of Section 6 incorporate timescales and yield durations
+ in seconds and frequencies as per second.
+
+
+
+
+
+Duffield, et al. Standards Track [Page 12]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+5.1. Loss-Pair-Counts
+
+ Loss-Pair-Counts are the absolute frequencies of the four types of
+ Loss Pair outcome in a sample. More precisely, the Loss-Pair-Counts
+ associated with a set of Loss Pairs L1,,,,Ln are the numbers N(i,j)
+ of such Loss Pairs that take each possible value (i,j) in the set (
+ (0,0), (0,1), (1,0), (1,1)).
+
+5.2. Bi-Packet-Loss-Ratio
+
+ The Bi-Packet-Loss-Ratio associated with a set of n Loss Pairs
+ L1,,,,Ln is defined in terms of their Loss-Pair-Counts by the
+ quantity (N(1,0) + N(1,1))/n.
+
+ Note this is formally equivalent to the loss metric Type-P-One-way-
+ Packet-Loss-Average from [RFC2680], since it averages single packet
+ losses.
+
+5.3. Bi-Packet-Loss-Episode-Duration-Number
+
+ The Bi-Packet-Loss-Episode-Duration-Number associated with a set of n
+ Loss Pairs L1,,,,Ln is defined in terms of their Loss-Pair-Counts in
+ the following cases:
+
+ o (2*N(1,1) + N(0,1) + N(1,0)) / (N(0,1) + N(1,0)) if N(0,1) +
+ N(1,0) > 0
+
+ o 0 if N(0,1) + N(1,0) + N(1,1) = 0 (no probe packets lost)
+
+ o Undefined if N(0,1) + N(1,0) + N(0,0) = 0 (all probe packets lost)
+
+ Note N(0,1) + N(1,0) is zero if there are no transitions between loss
+ and no-loss outcomes.
+
+5.4. Bi-Packet-Loss-Episode-Frequency-Number
+
+ The Bi-Packet-Loss-Episode-Frequency-Number associated with a set of
+ n Loss Pairs L1,,,,Ln is defined in terms of their Loss-Pair-Counts
+ as Bi-Packet-Loss-Ratio / Bi-Packet-Loss-Episode-Duration-Number,
+ when this can be defined, specifically, it is as follows:
+
+ o (N(1,0) + N(1,1)) * (N(0,1) + N(1,0)) / (2*N(1,1) + N(0,1) +
+ N(1,0) ) / n if N(0,1) + N(1,0) > 0
+
+ o 0 if N(0,1) + N(1,0) + N(1,1) = 0 (no probe packets lost)
+
+ o 1 if N(0,1) + N(1,0) + N(0,0) = 0 (all probe packets lost)
+
+
+
+
+Duffield, et al. Standards Track [Page 13]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+6. Loss Episode Metrics Derived from Bi-Packet Loss Probing
+
+ Metrics for the time frequency and time duration of loss episodes are
+ now defined as functions of the set of n Loss Pairs L1,....,Ln.
+ Although a loss episode is defined as a maximal set of successive
+ lost packets, the loss episode metrics are not defined directly in
+ terms of the sequential patterns of packet loss exhibited by Loss
+ Pairs. This is because samples, including Type-P-One-way-Bi-Packet-
+ Loss-Geometric-Stream, generally do not report all lost packets in
+ each episode. Instead, the metrics are defined as functions of the
+ Loss-Pair-Counts of the sample, for reasons that are now described.
+
+ Consider an idealized Type-P-One-way-Bi-Packet-Loss-Geometric-Stream
+ sample in which the launch probability q =1. It is shown in [SBDR08]
+ that the average number of packets in a loss episode of this ideal
+ sample is exactly the Bi-Packet-Loss-Episode-Duration derived from
+ its set of Loss Pairs. Note this computation makes no reference to
+ the position of lost packet in the sequence of probes.
+
+ A general Type-P-One-way-Bi-Packet-Loss-Geometric-Stream sample with
+ launch probability q < 1, independently samples, with probability q,
+ each Loss Pair of an idealized sample. On average, the Loss-Pair-
+ Counts (if normalized by the total number of pairs) will be the same
+ as in the idealized sample. The loss episode metrics in the general
+ case are thus estimators of those for the idealized case; the
+ statistical properties of this estimation, including a derivation of
+ the estimation variance, is provided in [SBDR08].
+
+6.1. Geometric Stream: Loss Ratio
+
+6.1.1. Metric Name
+
+ Type-P-One-way-Bi-Packet-Loss-Geometric-Stream-Ratio
+
+6.1.2. Metric Parameters
+
+ o Src, the IP address of a source host
+
+ o Dst, the IP address of a destination host
+
+ o T0, the randomly selected starting time [RFC3432] for periodic
+ launch opportunities
+
+ o d, the time spacing between potential launch times, Ti and T(i+1)
+
+ o n, a count of potential measurement instants
+
+
+
+
+
+Duffield, et al. Standards Track [Page 14]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+ o q, a launch probability
+
+ o F, a selection function defining unambiguously the two packets
+ from the stream selected for the metric
+
+ o P, the specification of the packet type, over and above the source
+ and destination address
+
+6.1.3. Metric Units
+
+ A decimal number in the interval [0,1]
+
+6.1.4. Metric Definition
+
+ The result obtained by computing the Bi-Packet-Loss-Ratio over a
+ Type-P-One-way-Bi-Packet-Loss-Geometric-Stream sample with the metric
+ parameters.
+
+6.1.5. Discussion
+
+ Type-P-One-way-Bi-Packet-Loss-Geometric-Stream-Ratio estimates the
+ fraction of packets lost from the geometric stream of Bi-Packet
+ probes.
+
+6.1.6. Methodologies
+
+ Refer to Section 4.6.
+
+6.1.7. Errors and Uncertainties
+
+ Because Type-P-One-way-Bi-Packet-Loss-Geometric-Stream is sampled in
+ general (when the launch probability q <1), the metrics described in
+ this section can be regarded as statistical estimators of the
+ corresponding idealized version corresponding to q = 1. Estimation
+ variance as it applies to Type-P-One-way-Bi-Packet-Loss-Geometric-
+ Stream-Loss-Ratio is described in [SBDR08].
+
+ For other issues, refer to Section 4.7
+
+6.1.8. Reporting the Metric
+
+ Refer to Section 4.8.
+
+
+
+
+
+
+
+
+
+Duffield, et al. Standards Track [Page 15]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+6.2. Geometric Stream: Loss Episode Duration
+
+6.2.1. Metric Name
+
+ Type-P-One-way-Bi-Packet-Loss-Geometric-Stream-Episode-Duration
+
+6.2.2. Metric Parameters
+
+ o Src, the IP address of a source host
+
+ o Dst, the IP address of a destination host
+
+ o T0, the randomly selected starting time [RFC3432] for periodic
+ launch opportunities
+
+ o d, the time spacing between potential launch times, Ti and T(i+1)
+
+ o n, a count of potential measurement instants
+
+ o q, a launch probability
+
+ o F, a selection function defining unambiguously the two packets
+ from the stream selected for the metric
+
+ o P, the specification of the packet type, over and above the source
+ and destination address
+
+6.2.3. Metric Units
+
+ A non-negative number of seconds
+
+6.2.4. Metric Definition
+
+ The result obtained by computing the Bi-Packet-Loss-Episode-Duration-
+ Number over a Type-P-One-way-Bi-Packet-Loss-Geometric-Stream sample
+ with the metric parameters, then multiplying the result by the launch
+ spacing parameter d.
+
+6.2.5. Discussion
+
+ Type-P-One-way-Bi-Packet-Loss-Geometric-Stream-Episode-Duration
+ estimates the average duration of a loss episode, measured in
+ seconds. The duration measured in packets is obtained by dividing
+ the metric value by the packet launch spacing parameter d.
+
+6.2.6. Methodologies
+
+ Refer to Section 4.6.
+
+
+
+Duffield, et al. Standards Track [Page 16]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+6.2.7. Errors and Uncertainties
+
+ Because Type-P-One-way-Bi-Packet-Loss-Geometric-Stream is sampled in
+ general (when the launch probability q <1), the metrics described in
+ this section can be regarded as statistical estimators of the
+ corresponding idealized version corresponding to q = 1. Estimation
+ variance as it applies to Type-P-One-way-Bi-Packet-Loss-Geometric-
+ Stream-Episode-Duration is described in [SBDR08].
+
+ For other issues, refer to Section 4.7
+
+6.2.8. Reporting the Metric
+
+ Refer to Section 4.8.
+
+6.3. Geometric Stream: Loss Episode Frequency
+
+6.3.1. Metric Name
+
+ Type-P-One-way-Bi-Packet-Loss-Geometric-Stream-Episode-Frequency
+
+6.3.2. Metric Parameters
+
+ o Src, the IP address of a source host
+
+ o Dst, the IP address of a destination host
+
+ o T0, the randomly selected starting time [RFC3432] for periodic
+ launch opportunities
+
+ o d, the time spacing between potential launch times, Ti and T(i+1)
+
+ o n, a count of potential measurement instants
+
+ o q, a launch probability
+
+ o F, a selection function defining unambiguously the two packets
+ from the stream selected for the metric
+
+ o P, the specification of the packet type, over and above the source
+ and destination address
+
+6.3.3. Metric Units
+
+ A positive number
+
+
+
+
+
+
+Duffield, et al. Standards Track [Page 17]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+6.3.4. Metric Definition
+
+ The result obtained by computing the Bi-Packet-Loss-Episode-
+ Frequency-Number over a Type-P-One-way-Bi-Packet-Loss-Geometric-
+ Stream sample with the metric parameters, then dividing the result by
+ the launch spacing parameter d.
+
+6.3.5. Discussion
+
+ Type-P-One-way-Bi-Packet-Loss-Geometric-Stream-Episode-Frequency
+ estimates the average frequency per unit time with which loss
+ episodes start (or finish). The frequency relative to the count of
+ potential probe launches is obtained by multiplying the metric value
+ by the packet launch spacing parameter d.
+
+6.3.6. Methodologies
+
+ Refer to Section 4.6.
+
+6.3.7. Errors and Uncertainties
+
+ Because Type-P-One-way-Bi-Packet-Loss-Geometric-Stream is sampled in
+ general (when the launch probability q <1), the metrics described in
+ this section can be regarded as statistical estimators of the
+ corresponding idealized version corresponding to q = 1. Estimation
+ variance as it applies to Type-P-One-way-Bi-Packet-Loss-Geometric-
+ Stream-Episode-Frequency is described in [SBDR08].
+
+ For other issues, refer to Section 4.7
+
+6.3.8. Reporting the Metric
+
+ Refer to Section 4.8.
+
+7. Applicability of Loss Episode Metrics
+
+7.1. Relation to Gilbert Model
+
+ The general Gilbert-Elliot model is a discrete time Markov chain over
+ two states, Good (g) and Bad (b), each with its own independent
+ packet loss ratio. In the simplest case, the Good loss ratio is 0,
+ while the Bad loss ratio is 1. Correspondingly, there are two
+ independent parameters, the Markov transition probabilities P(g|b) =
+ 1- P(b|b) and P(b|g) = 1- P(g|g), where P(i|j) is the probability to
+ transition from state j and step n to state i at step n+1. With
+ these parameters, the fraction of steps spent in the bad state is
+ P(b|g)/(P(b|g) + P(g|b)), while the average duration of a sojourn in
+ the bad state is 1/P(g|b) steps.
+
+
+
+Duffield, et al. Standards Track [Page 18]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+ Now identify the steps of the Markov chain with the possible sending
+ times of packets for a Type-P-One-way-Bi-Packet-Loss-Geometric-Stream
+ with launch spacing d. Suppose the loss episode metrics Type-P-One-
+ way-Bi-Packet-Loss-Geometric-Stream-Ratio and Type-P-One-way-Bi-
+ Packet-Loss-Geometric-Stream-Episode-Duration take the values r and
+ m, respectively. Then, from the discussion in Section 6.1.5, the
+ following can be equated:
+
+ r = P(b|g)/(P(b|g) + P(g|b)) and m/d = 1/P(g|b).
+
+ These relationships can be inverted in order to recover the Gilbert
+ model parameters:
+
+ P(g|b) = d/m and P(b|g)=d/m/(1/r - 1)
+
+8. Security Considerations
+
+ Conducting Internet measurements raises both security and privacy
+ concerns. This memo does not specify an implementation of the
+ metrics, so it does not directly affect the security of the Internet
+ or of applications that run on the Internet. However,implementations
+ of these metrics must be mindful of security and privacy concerns.
+
+ There are two types of security concerns: potential harm caused by
+ the measurements and potential harm to the measurements. The
+ measurements could cause harm because they are active and inject
+ packets into the network. The measurement parameters MUST be
+ carefully selected so that the measurements inject trivial amounts of
+ additional traffic into the networks they measure. If they inject
+ "too much" traffic, they can skew the results of the measurement and,
+ in extreme cases, cause congestion and denial of service. The
+ measurements themselves could be harmed by routers giving measurement
+ traffic a different priority than "normal" traffic, or by an attacker
+ injecting artificial measurement traffic. If routers can recognize
+ measurement traffic and treat it separately, the measurements may not
+ reflect actual user traffic. If an attacker injects artificial
+ traffic that is accepted as legitimate, the loss rate will be
+ artificially lowered. Therefore, the measurement methodologies
+ SHOULD include appropriate techniques to reduce the probability that
+ measurement traffic can be distinguished from "normal" traffic.
+ Authentication techniques, such as digital signatures, may be used
+ where appropriate to guard against injected traffic attacks. The
+ privacy concerns of network measurement are limited by the active
+ measurements described in this memo: they involve no release of user
+ data.
+
+
+
+
+
+
+Duffield, et al. Standards Track [Page 19]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+9. References
+
+9.1. Normative References
+
+ [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.
+
+ [RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control
+ Protocol Extended Reports (RTCP XR)", RFC 3611,
+ November 2003.
+
+ [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
+ Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+ [RFC3432] Raisanen, V., Grotefeld, G., and A. Morton, "Network
+ performance measurement with periodic streams", RFC 3432,
+ November 2002.
+
+9.2. Informative References
+
+ [RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
+ "Framework for IP Performance Metrics", RFC 2330,
+ May 1998.
+
+ [RFC3357] Koodli, R. and R. Ravikanth, "One-way Loss Pattern Sample
+ Metrics", RFC 3357, August 2002.
+
+ [SBDR08] IEEE/ACM Transactions on Networking, 16(2): 307-320, "A
+ Geometric Approach to Improving Active Packet Loss
+ Measurement", 2008.
+
+ [Gilbert] Gilbert, E.N., "Capacity of a Burst-Noise Channel. Bell
+ System Technical Journal 39 pp 1253-1265", 1960.
+
+ [Elliot] Elliott, E.O., "Estimates of Error Rates for Codes on
+ Burst-Noise Channels. Bell System Technical Journal 42 pp
+ 1977-1997", 1963.
+
+
+
+
+
+
+
+
+
+
+Duffield, et al. Standards Track [Page 20]
+
+RFC 6534 Loss Episode Metrics for IPPM May 2012
+
+
+Authors' Addresses
+
+ Nick Duffield
+ AT&T Labs-Research
+ 180 Park Avenue
+ Florham Park, NJ 07932
+ USA
+
+ Phone: +1 973 360 8726
+ Fax: +1 973 360 8871
+ EMail: duffield@research.att.com
+ URI: http://www.research.att.com/people/Duffield_Nicholas_G
+
+
+ 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/
+
+
+ Joel Sommers
+ Colgate University
+ 304 McGregory Hall
+ Hamilton, NY 13346
+ USA
+
+ Phone: +1 315 228 7587
+ Fax:
+ EMail: jsommers@colgate.edu
+ URI: http://cs.colgate.edu/faculty/jsommers
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Duffield, et al. Standards Track [Page 21]
+