A One-way Packet Loss Metric for IPPM

Status of this Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (1999). All Rights Reserved.

1. Introduction

This memo defines a metric for one-way packet loss across Internet paths. It builds on notions introduced and discussed in the IPPM Framework document, RFC 2330 [1]; the reader is assumed to be familiar with that document.

This memo is intended to be parallel in structure to a companion document for One-way Delay ("A One-way Delay Metric for IPPM") [2]; the reader is assumed to be familiar with that document.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [5]. Although RFC 2119 was written with protocols in mind, the key words are used in this document for similar reasons. They are used to ensure the results of measurements from two different implementations are comparable, and to note instances when an implementation could perturb the network.

The structure of the memo is as follows:

   +  A 'singleton' analytic metric, called Type-P-One-way-Loss, is
   
      introduced to measure a single observation of packet transmission
      or loss.
   
   +  Using this singleton metric, a 'sample', called Type-P-One-way-
      Loss-Poisson-Stream, is introduced to measure a sequence of
      singleton transmissions and/or losses measured at times taken from
      a Poisson process.

• Using this sample, several 'statistics' of the sample are defined and discussed.

This progression from singleton to sample to statistics, with clear separation among them, is important.

Whenever a technical term from the IPPM Framework document is first used in this memo, it will be tagged with a trailing asterisk. For example, "term*" indicates that "term" is defined in the Framework.

1.1. Motivation:

Understanding one-way packet loss of Type-P* packets from a source host* to a destination host is useful for several reasons:

• Some applications do not perform well (or at all) if end-to-end loss between hosts is large relative to some threshold value.

• Excessive packet loss may make it difficult to support certain real-time applications (where the precise threshold of "excessive" depends on the application).

• The larger the value of packet loss, the more difficult it is for transport-layer protocols to sustain high bandwidths.

• The sensitivity of real-time applications and of transport-layer protocols to loss become especially important when very large delay-bandwidth products must be supported.

The measurement of one-way loss instead of round-trip loss is motivated by the following factors:

• In today's Internet, the path from a source to a destination may be different than the path from the destination back to the source ("asymmetric paths"), such that different sequences of routers are used for the forward and reverse paths. Therefore round-trip measurements actually measure the performance of two distinct paths together. Measuring each path independently highlights the performance difference between the two paths which may traverse different Internet service providers, and even radically different types of networks (for example, research versus commodity networks, or ATM versus packet-over-SONET).

• Even when the two paths are symmetric, they may have radically different performance characteristics due to asymmetric queueing.

• Performance of an application may depend mostly on the performance in one direction. For example, a file transfer using TCP may depend more on the performance in the direction that data flows, rather than the direction in which acknowledgements travel.

   +  In quality-of-service (QoS) enabled networks, provisioning in one
      direction may be radically different than provisioning in the
      reverse direction, and thus the QoS guarantees differ.  Measuring
      the paths independently allows the verification of both
      guarantees.

It is outside the scope of this document to say precisely how loss metrics would be applied to specific problems.

1.2. General Issues Regarding Time

{Comment: the terminology below differs from that defined by ITU-T documents (e.g., G.810, "Definitions and terminology for synchronization networks" and I.356, "B-ISDN ATM layer cell transfer performance"), but is consistent with the IPPM Framework document. In general, these differences derive from the different backgrounds; the ITU-T documents historically have a telephony origin, while the authors of this document (and the Framework) have a computer systems background. Although the terms defined below have no direct equivalent in the ITU-T definitions, after our definitions we will provide a rough mapping. However, note one potential confusion: our definition of "clock" is the computer operating systems definition denoting a time-of-day clock, while the ITU-T definition of clock denotes a frequency reference.}

Whenever a time (i.e., a moment in history) is mentioned here, it is understood to be measured in seconds (and fractions) relative to UTC.

As described more fully in the Framework document, there are four distinct, but related notions of clock uncertainty:

   synchronization*

Synchronization measures the extent to which two clocks agree on what time it is. For example, the clock on one host might be 5.4 msec ahead of the clock on a second host. {Comment: A rough ITU-T equivalent is "time error".}

   accuracy*

Accuracy measures the extent to which a given clock agrees with UTC. For example, the clock on a host might be 27.1 msec behind UTC. {Comment: A rough ITU-T equivalent is "time error from UTC".}

   resolution*

Resolution measures the precision of a given clock. For example, the clock on an old Unix host might advance only once every 10 msec, and thus have a resolution of only 10 msec. {Comment: A very rough ITU-T equivalent is "sampling period".}

   skew*

Skew measures the change of accuracy, or of synchronization, with time. For example, the clock on a given host might gain 1.3 msec per hour and thus be 27.1 msec behind UTC at one time and only 25.8 msec an hour later. In this case, we say that the clock of the given host has a skew of 1.3 msec per hour relative to UTC, which threatens accuracy. We might also speak of the skew of one clock relative to another clock, which threatens synchronization. {Comment: A rough ITU-T equivalent is "time drift".}

2. A Singleton Definition for One-way Packet Loss

2.1. Metric Name:

      Type-P-One-way-Packet-Loss

2.2. Metric Parameters:

• Src, the IP address of a host

• Dst, the IP address of a host

• T, a time

2.3. Metric Units:

The value of a Type-P-One-way-Packet-Loss is either a zero (signifying successful transmission of the packet) or a one (signifying loss).

2.4. Definition:

>>The *Type-P-One-way-Packet-Loss* from Src to Dst at T is 0<< means that Src sent the first bit of a Type-P packet to Dst at wire-time* T and that Dst received that packet.

>>The *Type-P-One-way-Packet-Loss* from Src to Dst at T is 1<< means that Src sent the first bit of a type-P packet to Dst at wire-time T and that Dst did not receive that packet.

2.5. Discussion:

   Thus, Type-P-One-way-Packet-Loss is 0 exactly when Type-P-One-way-
   Delay is a finite value, and it is 1 exactly when Type-P-One-way-
   Delay is undefined.

The following issues are likely to come up in practice:

• A given methodology will have to include a way to distinguish between a packet loss and a very large (but finite) delay. As noted by Mahdavi and Paxson [3], simple upper bounds (such as the 255 seconds theoretical upper bound on the lifetimes of IP packets [4]) could be used, but good engineering, including an understanding of packet lifetimes, will be needed in practice. {Comment: Note that, for many applications of these metrics, there may be no harm in treating a large delay as packet loss. An audio playback packet, for example, that arrives only after the playback point may as well have been lost.}

• If the packet arrives, but is corrupted, then it is counted as lost. {Comment: one is tempted to count the packet as received since corruption and packet loss are related but distinct phenomena. If the IP header is corrupted, however, one cannot be sure about the source or destination IP addresses and is thus on shaky grounds about knowing that the corrupted received packet corresponds to a given sent test packet. Similarly, if other parts of the packet needed by the methodology to know that the corrupted received packet corresponds to a given sent test packet, then such a packet would have to be counted as lost. Counting these packets as lost but packet with corruption in other parts of the packet as not lost would be inconsistent.}

• If the packet is duplicated along the path (or paths) so that multiple non-corrupt copies arrive at the destination, then the packet is counted as received.

• If the packet is fragmented and if, for whatever reason, reassembly does not occur, then the packet will be deemed lost.

2.6. Methodologies:

   As with other Type-P-* metrics, the detailed methodology will depend
   on the Type-P (e.g., protocol number, UDP/TCP port number, size,
   precedence).

Generally, for a given Type-P, one possible methodology would proceed as follows:

• Arrange that Src and Dst have clocks that are synchronized with each other. The degree of synchronization is a parameter of the methodology, and depends on the threshold used to determine loss (see below).

• At the Src host, select Src and Dst IP addresses, and form a test packet of Type-P with these addresses.

• At the Dst host, arrange to receive the packet.

• At the Src host, place a timestamp in the prepared Type-P packet, and send it towards Dst.

• If the packet arrives within a reasonable period of time, the one- way packet-loss is taken to be zero.

• If the packet fails to arrive within a reasonable period of time, the one-way packet-loss is taken to be one. Note that the threshold of "reasonable" here is a parameter of the methodology.

{Comment: The definition of reasonable is intentionally vague, and is intended to indicate a value "Th" so large that any value in the closed interval [Th-delta, Th+delta] is an equivalent threshold for loss. Here, delta encompasses all error in clock synchronization along the measured path. If there is a single value after which the packet must be counted as lost, then we reintroduce the need for a degree of clock synchronization similar to that needed for one-way delay. Therefore, if a measure of packet loss parameterized by a specific non-huge "reasonable" time-out value is needed, one can always measure one-way delay and see what percentage of packets from a given stream exceed a given time-out value.}

Issues such as the packet format, the means by which Dst knows when to expect the test packet, and the means by which Src and Dst are synchronized are outside the scope of this document. {Comment: We plan to document elsewhere our own work in describing such more detailed implementation techniques and we encourage others to as well.}

2.7. Errors and Uncertainties:

The description of any specific measurement method should include an accounting and analysis of various sources of error or uncertainty. The Framework document provides general guidance on this point.

For loss, there are three sources of error:

• Synchronization between clocks on Src and Dst.

• The packet-loss threshold (which is related to the synchronization between clocks).

• Resource limits in the network interface or software on the receiving instrument.

The first two sources are interrelated and could result in a test packet with finite delay being reported as lost. Type-P-One-way- Packet-Loss is 0 if the test packet does not arrive, or if it does arrive and the difference between Src timestamp and Dst timestamp is greater than the "reasonable period of time", or loss threshold. If the clocks are not sufficiently synchronized, the loss threshold may not be "reasonable" - the packet may take much less time to arrive than its Src timestamp indicates. Similarly, if the loss threshold is set too low, then many packets may be counted as lost. The loss threshold must be high enough, and the clocks synchronized well enough so that a packet that arrives is rarely counted as lost. (See the discussions in the previous two sections.)

Since the sensitivity of packet loss measurement to lack of clock synchronization is less than for delay, we refer the reader to the treatment of synchronization errors in the One-way Delay metric [2] for more details.

The last source of error, resource limits, cause the packet to be dropped by the measurement instrument, and counted as lost when in fact the network delivered the packet in reasonable time.

The measurement instruments should be calibrated such that the loss threshold is reasonable for application of the metrics and the clocks are synchronized enough so the loss threshold remains reasonable.

In addition, the instruments should be checked to ensure the that the possibility a packet arrives at the network interface, but is lost due to congestion on the interface or to other resource exhaustion (e.g., buffers) on the instrument is low.

2.8. Reporting the metric:

The calibration and context in which the metric is measured MUST be carefully considered, and SHOULD always be reported along with metric results. We now present four items to consider: Type-P of the test packets, the loss threshold, instrument calibration, and the path traversed by the test packets. This list is not exhaustive; any additional information that could be useful in interpreting applications of the metrics should also be reported.

2.8.1. Type-P

As noted in the Framework document [1], the value of the metric may depend on the type of IP packets used to make the measurement, or "Type-P". The value of Type-P-One-way-Delay could change if the protocol (UDP or TCP), port number, size, or arrangement for special treatment (e.g., IP precedence or RSVP) changes. The exact Type-P used to make the measurements MUST be accurately reported.

2.8.2. Loss threshold

The threshold (or methodology to distinguish) between a large finite delay and loss MUST be reported.

2.8.3. Calibration results

The degree of synchronization between the Src and Dst clocks MUST be reported. If possible, possibility that a test packet that arrives at the Dst network interface is reported as lost due to resource exhaustion on Dst SHOULD be reported.

2.8.4. Path

Finally, the path traversed by the packet SHOULD be reported, if possible. In general it is impractical to know the precise path a given packet takes through the network. The precise path may be known for certain Type-P on short or stable paths. If Type-P includes the record route (or loose-source route) option in the IP header, and the path is short enough, and all routers* on the path support record (or loose-source) route, then the path will be precisely recorded. This is impractical because the route must be short enough, many routers do not support (or are not configured for) record route, and use of this feature would often artificially worsen the performance observed by removing the packet from common-case processing. However, partial information is still valuable context. For example, if a host can choose between two links* (and hence two separate routes from Src to Dst), then the initial link used is valuable context. {Comment: For example, with Merit's NetNow setup, a Src on one NAP can reach a Dst on another NAP by either of several different backbone networks.}

3. A Definition for Samples of One-way Packet Loss

Given the singleton metric Type-P-One-way-Packet-Loss, we now define one particular sample of such singletons. The idea of the sample is to select a particular binding of the parameters Src, Dst, and Type- P, then define a sample of values of parameter T. The means for defining the values of T is to select a beginning time T0, a final time Tf, and an average rate lambda, then define a pseudo-random Poisson process of rate lambda, whose values fall between T0 and Tf. The time interval between successive values of T will then average 1/lambda.

{Comment: Note that Poisson sampling is only one way of defining a sample. Poisson has the advantage of limiting bias, but other methods of sampling might be appropriate for different situations. We encourage others who find such appropriate cases to use this general framework and submit their sampling method for standardization.}

3.1. Metric Name:

   Type-P-One-way-Packet-Loss-Poisson-Stream

3.2. Metric Parameters:

• Src, the IP address of a host

• Dst, the IP address of a host

• T0, a time

• Tf, a time

• lambda, a rate in reciprocal seconds

3.3. Metric Units:

A sequence of pairs; the elements of each pair are:

   +  T, a time, and
   
   +  L, either a zero or a one

The values of T in the sequence are monotonic increasing. Note that T would be a valid parameter to Type-P-One-way-Packet-Loss, and that L would be a valid value of Type-P-One-way-Packet-Loss.

3.4. Definition:

Given T0, Tf, and lambda, we compute a pseudo-random Poisson process beginning at or before T0, with average arrival rate lambda, and ending at or after Tf. Those time values greater than or equal to T0 and less than or equal to Tf are then selected. At each of the times in this process, we obtain the value of Type-P-One-way-Packet-Loss at this time. The value of the sample is the sequence made up of the resulting <time, loss> pairs. If there are no such pairs, the sequence is of length zero and the sample is said to be empty.

3.5. Discussion:

The reader should be familiar with the in-depth discussion of Poisson sampling in the Framework document [1], which includes methods to compute and verify the pseudo-random Poisson process.

We specifically do not constrain the value of lambda, except to note the extremes. If the rate is too large, then the measurement traffic will perturb the network, and itself cause congestion. If the rate is too small, then you might not capture interesting network behavior. {Comment: We expect to document our experiences with, and suggestions for, lambda elsewhere, culminating in a "best current practices" document.}

Since a pseudo-random number sequence is employed, the sequence of times, and hence the value of the sample, is not fully specified. Pseudo-random number generators of good quality will be needed to achieve the desired qualities.

The sample is defined in terms of a Poisson process both to avoid the effects of self-synchronization and also capture a sample that is statistically as unbiased as possible. The Poisson process is used to schedule the delay measurements. The test packets will generally not arrive at Dst according to a Poisson distribution, since they are influenced by the network.

{Comment: there is, of course, no claim that real Internet traffic arrives according to a Poisson arrival process.

It is important to note that, in contrast to this metric, loss rates observed by transport connections do not reflect unbiased samples. For example, TCP transmissions both (1) occur in bursts, which can induce loss due to the burst volume that would not otherwise have been observed, and (2) adapt their transmission rate in an attempt to minimize the loss rate observed by the connection.}

All the singleton Type-P-One-way-Packet-Loss metrics in the sequence will have the same values of Src, Dst, and Type-P.

Note also that, given one sample that runs from T0 to Tf, and given new time values T0' and Tf' such that T0 <= T0' <= Tf' <= Tf, the subsequence of the given sample whose time values fall between T0' and Tf' are also a valid Type-P-One-way-Packet-Loss-Poisson-Stream sample.

3.6. Methodologies:

The methodologies follow directly from:

• the selection of specific times, using the specified Poisson arrival process, and

• the methodologies discussion already given for the singleton Type- P-One-way-Packet-Loss metric.

Care must be given to correctly handle out-of-order arrival of test packets; it is possible that the Src could send one test packet at TS[i], then send a second one (later) at TS[i+1], while the Dst could receive the second test packet at TR[i+1], and then receive the first one (later) at TR[i].

3.7. Errors and Uncertainties:

In addition to sources of errors and uncertainties associated with methods employed to measure the singleton values that make up the sample, care must be given to analyze the accuracy of the Poisson arrival process of the wire-times of the sending of the test packets. Problems with this process could be caused by several things, including problems with the pseudo-random number techniques used to generate the Poisson arrival process. The Framework document shows how to use the Anderson-Darling test verify the accuracy of the Poisson process over small time frames. {Comment: The goal is to ensure that the test packets are sent "close enough" to a Poisson schedule, and avoid periodic behavior.}

3.8. Reporting the metric:

The calibration and context for the underlying singletons MUST be reported along with the stream. (See "Reporting the metric" for Type-P-One-way-Packet-Loss.)

4. Some Statistics Definitions for One-way Packet Loss

Given the sample metric Type-P-One-way-Packet-Loss-Poisson-Stream, we now offer several statistics of that sample. These statistics are offered mostly to be illustrative of what could be done.

4.1. Type-P-One-way-Packet-Loss-Average

   Given a Type-P-One-way-Packet-Loss-Poisson-Stream, the average of all
   the L values in the Stream.  In addition, the Type-P-One-way-Packet-
   Loss-Average is undefined if the sample is empty.

Example:

suppose we take a sample and the results are:

      Stream1 = <
      <T1, 0>
      <T2, 0>
      <T3, 1>
      <T4, 0>
      <T5, 0>
      >

Then the average would be 0.2.

Note that, since healthy Internet paths should be operating at loss rates below 1% (particularly if high delay-bandwidth products are to be sustained), the sample sizes needed might be larger than one would like. Thus, for example, if one wants to discriminate between various fractions of 1% over one-minute periods, then several hundred samples per minute might be needed. This would result in larger values of lambda than one would ordinarily want.

Note that although the loss threshold should be set such that any errors in loss are not significant, if the possibility that a packet which arrived is counted as lost due to resource exhaustion is significant compared to the loss rate of interest, Type-P-One-way- Packet-Loss-Average will be meaningless.

5. 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 nor of applications which 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 will 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 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. Unlike passive measurements, there can be no release of existing user data.

6. Acknowledgements

Thanks are due to Matt Mathis for encouraging this work and for calling attention on so many occasions to the significance of packet loss.

Thanks are due also to Vern Paxson for his valuable comments on early drafts, and to Garry Couch and Will Leland for several useful suggestions.

7. References

   [1]  Paxson, V.,  Almes,G., Mahdavi, J. and M. Mathis, "Framework for
        IP Performance Metrics", RFC 2330, May 1998.
   
   [2]  Almes, G.,  Kalidindi, S.  and M. Zekauskas, "A One-way Delay
        Metric for IPPM", RFC 2679, September 1999.
   
   [3]  Mahdavi, J. and V. Paxson, "IPPM Metrics for Measuring
        Connectivity", RFC 2678, September 1999.
   
   [4]  Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
   
   [5]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.
   
   [6]  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
        9, RFC 2026, October 1996.

8. Authors' Addresses

   Guy Almes
   Advanced Network & Services, Inc.
   200 Business Park Drive
   Armonk, NY  10504
   USA
   
   Phone: +1 914 765 1120
   EMail: almes@advanced.org
   
   Sunil Kalidindi
   Advanced Network & Services, Inc.
   200 Business Park Drive
   Armonk, NY  10504
   USA
   
   Phone: +1 914 765 1128
   EMail: kalidindi@advanced.org

Matthew J. Zekauskas
Advanced Network & Services, Inc.
200 Business Park Drive
Armonk, NY 10504
USA

   Phone: +1 914 765 1112
   EMail: matt@advanced.org

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