As per Relevance of the word standards, we have this rfc below:
Network Working Group G.
Request for Comments: 2680 S.
Category: Standards Track M.
Advanced Network &
September 1999
A One-way Packet Loss Metric for
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (1999). All Rights Reserved
1.
This memo defines a metric for one-way packet loss across
paths. It builds on notions introduced and discussed in the
Framework document, RFC 2330 [1]; the reader is assumed to
familiar with that document
This memo is intended to be parallel in structure to a
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
document are to be interpreted as described in RFC 2119 [5].
Although RFC 2119 was written with protocols in mind, the key
are used in this document for similar reasons. They are used
ensure the results of measurements from two different
are comparable, and to note instances when an implementation
perturb the network
The structure of the memo is as follows
+ A 'singleton' analytic metric, called Type-P-One-way-Loss,
introduced to measure a single observation of packet
or loss
Almes, et al. Standards Track [Page 1]
RFC 2680 One Way Packet Loss Metric for IPPM September 1999
+ Using this singleton metric, a 'sample', called Type-P-One-way
Loss-Poisson-Stream, is introduced to measure a sequence
singleton transmissions and/or losses measured at times taken
a Poisson process
+ Using this sample, several 'statistics' of the sample are
and discussed
This progression from singleton to sample to statistics, with
separation among them, is important
Whenever a technical term from the IPPM Framework document is
used in this memo, it will be tagged with a trailing asterisk.
example, "term*" indicates that "term" is defined in the Framework
1.1. Motivation
Understanding one-way packet loss of Type-P* packets from a
host* to a destination host is useful for several reasons
+ Some applications do not perform well (or at all) if end-to-
loss between hosts is large relative to some threshold value
+ Excessive packet loss may make it difficult to support
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
transport-layer protocols to sustain high bandwidths
+ The sensitivity of real-time applications and of transport-
protocols to loss become especially important when very
delay-bandwidth products must be supported
The measurement of one-way loss instead of round-trip loss
motivated by the following factors
+ In today's Internet, the path from a source to a destination
be different than the path from the destination back to the
("asymmetric paths"), such that different sequences of routers
used for the forward and reverse paths. Therefore round-
measurements actually measure the performance of two
paths together. Measuring each path independently highlights
performance difference between the two paths which may
different Internet service providers, and even radically
types of networks (for example, research versus
networks, or ATM versus packet-over-SONET).
Almes, et al. Standards Track [Page 2]
RFC 2680 One Way Packet Loss Metric for IPPM September 1999
+ Even when the two paths are symmetric, they may have
different performance characteristics due to asymmetric queueing
+ Performance of an application may depend mostly on the
in one direction. For example, a file transfer using TCP
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
direction may be radically different than provisioning in
reverse direction, and thus the QoS guarantees differ.
the paths independently allows the verification of
guarantees
It is outside the scope of this document to say precisely how
metrics would be applied to specific problems
1.2. General Issues Regarding
{Comment: the terminology below differs from that defined by ITU-
documents (e.g., G.810, "Definitions and terminology
synchronization networks" and I.356, "B-ISDN ATM layer cell
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
authors of this document (and the Framework) have a computer
background. Although the terms defined below have no
equivalent in the ITU-T definitions, after our definitions we
provide a rough mapping. However, note one potential confusion:
definition of "clock" is the computer operating systems
denoting a time-of-day clock, while the ITU-T definition of
denotes a frequency reference.}
Whenever a time (i.e., a moment in history) is mentioned here, it
understood to be measured in seconds (and fractions) relative to UTC
As described more fully in the Framework document, there are
distinct, but related notions of clock uncertainty
synchronization
Synchronization measures the extent to which two clocks agree
what time it is. For example, the clock on one host might
5.4 msec ahead of the clock on a second host. {Comment: A
ITU-T equivalent is "time error".}
Almes, et al. Standards Track [Page 3]
RFC 2680 One Way Packet Loss Metric for IPPM September 1999
accuracy
Accuracy measures the extent to which a given clock agrees
UTC. For example, the clock on a host might be 27.1 msec
UTC. {Comment: A rough ITU-T equivalent is "time error
UTC".}
resolution
Resolution measures the precision of a given clock.
example, the clock on an old Unix host might advance only
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
1.3 msec per hour and thus be 27.1 msec behind UTC at one
and only 25.8 msec an hour later. In this case, we say that
clock of the given host has a skew of 1.3 msec per hour
to UTC, which threatens accuracy. We might also speak of
skew of one clock relative to another clock, which
synchronization. {Comment: A rough ITU-T equivalent is "
drift".}
2. A Singleton Definition for One-way Packet
2.1. Metric Name
Type-P-One-way-Packet-
2.2. Metric Parameters
+ Src, the IP address of a
+ Dst, the IP address of a
+ T, a
2.3. Metric Units
The value of a Type-P-One-way-Packet-Loss is either a
(signifying successful transmission of the packet) or a
(signifying loss).
Almes, et al. Standards Track [Page 4]
RFC 2680 One Way Packet Loss Metric for IPPM September 1999
2.4. Definition
>>The *Type-P-One-way-Packet-Loss* from Src to Dst at T is 0<<
that Src sent the first bit of a Type-P packet to Dst at wire-time*
and that Dst received that packet
>>The *Type-P-One-way-Packet-Loss* from Src to Dst at T is 1<<
that Src sent the first bit of a type-P packet to Dst at wire-time
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
between a packet loss and a very large (but finite) delay.
noted by Mahdavi and Paxson [3], simple upper bounds (such as
255 seconds theoretical upper bound on the lifetimes of
packets [4]) could be used, but good engineering, including
understanding of packet lifetimes, will be needed in practice
{Comment: Note that, for many applications of these metrics,
may be no harm in treating a large delay as packet loss. An
playback packet, for example, that arrives only after the
point may as well have been lost.}
+ If the packet arrives, but is corrupted, then it is counted
lost. {Comment: one is tempted to count the packet as
since corruption and packet loss are related but
phenomena. If the IP header is corrupted, however, one cannot
sure about the source or destination IP addresses and is thus
shaky grounds about knowing that the corrupted received
corresponds to a given sent test packet. Similarly, if
parts of the packet needed by the methodology to know that
corrupted received packet corresponds to a given sent test packet
then such a packet would have to be counted as lost.
these packets as lost but packet with corruption in other parts
the packet as not lost would be inconsistent.}
+ If the packet is duplicated along the path (or paths) so
multiple non-corrupt copies arrive at the destination, then
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
Almes, et al. Standards Track [Page 5]
RFC 2680 One Way Packet Loss Metric for IPPM September 1999
2.6. Methodologies
As with other Type-P-* metrics, the detailed methodology will
on the Type-P (e.g., protocol number, UDP/TCP port number, size
precedence).
Generally, for a given Type-P, one possible methodology would
as follows
+ Arrange that Src and Dst have clocks that are synchronized
each other. The degree of synchronization is a parameter of
methodology, and depends on the threshold used to determine
(see below).
+ At the Src host, select Src and Dst IP addresses, and form a
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
threshold of "reasonable" here is a parameter of the methodology
{Comment: The definition of reasonable is intentionally vague,
is intended to indicate a value "Th" so large that any value
the closed interval [Th-delta, Th+delta] is an
threshold for loss. Here, delta encompasses all error in
synchronization along the measured path. If there is a
value after which the packet must be counted as lost, then
reintroduce the need for a degree of clock synchronization
to that needed for one-way delay. Therefore, if a measure
packet loss parameterized by a specific non-huge "reasonable
time-out value is needed, one can always measure one-way delay
see what percentage of packets from a given stream exceed a
time-out value.}
Issues such as the packet format, the means by which Dst knows
to expect the test packet, and the means by which Src and Dst
synchronized are outside the scope of this document. {Comment:
plan to document elsewhere our own work in describing such
detailed implementation techniques and we encourage others to
well.}
Almes, et al. Standards Track [Page 6]
RFC 2680 One Way Packet Loss Metric for IPPM September 1999
2.7. Errors and Uncertainties
The description of any specific measurement method should include
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
between clocks).
+ Resource limits in the network interface or software on
receiving instrument
The first two sources are interrelated and could result in a
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
arrive and the difference between Src timestamp and Dst timestamp
greater than the "reasonable period of time", or loss threshold.
the clocks are not sufficiently synchronized, the loss threshold
not be "reasonable" - the packet may take much less time to
than its Src timestamp indicates. Similarly, if the loss
is set too low, then many packets may be counted as lost. The
threshold must be high enough, and the clocks synchronized
enough so that a packet that arrives is rarely counted as lost. (
the discussions in the previous two sections.)
Since the sensitivity of packet loss measurement to lack of
synchronization is less than for delay, we refer the reader to
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
dropped by the measurement instrument, and counted as lost when
fact the network delivered the packet in reasonable time
The measurement instruments should be calibrated such that the
threshold is reasonable for application of the metrics and the
are synchronized enough so the loss threshold remains reasonable
In addition, the instruments should be checked to ensure the that
possibility a packet arrives at the network interface, but is
due to congestion on the interface or to other resource
(e.g., buffers) on the instrument is low
Almes, et al. Standards Track [Page 7]
RFC 2680 One Way Packet Loss Metric for IPPM September 1999
2.8. Reporting the metric
The calibration and context in which the metric is measured MUST
carefully considered, and SHOULD always be reported along with
results. We now present four items to consider: Type-P of the
packets, the loss threshold, instrument calibration, and the
traversed by the test packets. This list is not exhaustive;
additional information that could be useful in
applications of the metrics should also be reported
2.8.1. Type-
As noted in the Framework document [1], the value of the metric
depend on the type of IP packets used to make the measurement,
"Type-P". The value of Type-P-One-way-Delay could change if
protocol (UDP or TCP), port number, size, or arrangement for
treatment (e.g., IP precedence or RSVP) changes. The exact Type-
used to make the measurements MUST be accurately reported
2.8.2. Loss
The threshold (or methodology to distinguish) between a large
delay and loss MUST be reported
2.8.3. Calibration
The degree of synchronization between the Src and Dst clocks MUST
reported. If possible, possibility that a test packet that
at the Dst network interface is reported as lost due to
exhaustion on Dst SHOULD be reported
2.8.4.
Finally, the path traversed by the packet SHOULD be reported,
possible. In general it is impractical to know the precise path
given packet takes through the network. The precise path may
known for certain Type-P on short or stable paths. If Type-
includes the record route (or loose-source route) option in the
header, and the path is short enough, and all routers* on the
support record (or loose-source) route, then the path will
precisely recorded. This is impractical because the route must
short enough, many routers do not support (or are not configured for
record route, and use of this feature would often artificially
the performance observed by removing the packet from common-
processing. However, partial information is still valuable context
For example, if a host can choose between two links* (and hence
separate routes from Src to Dst), then the initial link used
valuable context. {Comment: For example, with Merit's NetNow setup
Almes, et al. Standards Track [Page 8]
RFC 2680 One Way Packet Loss Metric for IPPM September 1999
a Src on one NAP can reach a Dst on another NAP by either of
different backbone networks.}
3. A Definition for Samples of One-way Packet
Given the singleton metric Type-P-One-way-Packet-Loss, we now
one particular sample of such singletons. The idea of the sample
to select a particular binding of the parameters Src, Dst, and Type
P, then define a sample of values of parameter T. The means
defining the values of T is to select a beginning time T0, a
time Tf, and an average rate lambda, then define a pseudo-
Poisson process of rate lambda, whose values fall between T0 and Tf
The time interval between successive values of T will then
1/lambda
{Comment: Note that Poisson sampling is only one way of defining
sample. Poisson has the advantage of limiting bias, but
methods of sampling might be appropriate for different situations
We encourage others who find such appropriate cases to use
general framework and submit their sampling method
standardization.}
3.1. Metric Name
Type-P-One-way-Packet-Loss-Poisson-
3.2. Metric Parameters
+ Src, the IP address of a
+ Dst, the IP address of a
+ T0, a
+ Tf, a
+ lambda, a rate in reciprocal
3.3. Metric Units
A sequence of pairs; the elements of each pair are
+ T, a time,
+ L, either a zero or a
Almes, et al. Standards Track [Page 9]
RFC 2680 One Way Packet Loss Metric for IPPM September 1999
The values of T in the sequence are monotonic increasing. Note
T would be a valid parameter to Type-P-One-way-Packet-Loss, and
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
beginning at or before T0, with average arrival rate lambda,
ending at or after Tf. Those time values greater than or equal to T
and less than or equal to Tf are then selected. At each of the
in this process, we obtain the value of Type-P-One-way-Packet-Loss
this time. The value of the sample is the sequence made up of
resulting pairs. If there are no such pairs,
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
sampling in the Framework document [1], which includes methods
compute and verify the pseudo-random Poisson process
We specifically do not constrain the value of lambda, except to
the extremes. If the rate is too large, then the measurement
will perturb the network, and itself cause congestion. If the
is too small, then you might not capture interesting
behavior. {Comment: We expect to document our experiences with,
suggestions for, lambda elsewhere, culminating in a "best
practices" document.}
Since a pseudo-random number sequence is employed, the sequence
times, and hence the value of the sample, is not fully specified
Pseudo-random number generators of good quality will be needed
achieve the desired qualities
The sample is defined in terms of a Poisson process both to avoid
effects of self-synchronization and also capture a sample that
statistically as unbiased as possible. The Poisson process is
to schedule the delay measurements. The test packets will
not arrive at Dst according to a Poisson distribution, since they
influenced by the network
{Comment: there is, of course, no claim that real Internet
arrives according to a Poisson arrival process
It is important to note that, in contrast to this metric, loss
observed by transport connections do not reflect unbiased samples
For example, TCP transmissions both (1) occur in bursts, which
Almes, et al. Standards Track [Page 10]
RFC 2680 One Way Packet Loss Metric for IPPM September 1999
induce loss due to the burst volume that would not otherwise
been observed, and (2) adapt their transmission rate in an attempt
minimize the loss rate observed by the connection.}
All the singleton Type-P-One-way-Packet-Loss metrics in the
will have the same values of Src, Dst, and Type-P
Note also that, given one sample that runs from T0 to Tf, and
new time values T0' and Tf' such that T0 <= T0' <= Tf' <= Tf,
subsequence of the given sample whose time values fall between T0'
and Tf' are also a valid Type-P-One-way-Packet-Loss-Poisson-
sample
3.6. Methodologies
The methodologies follow directly from
+ the selection of specific times, using the specified
arrival process,
+ 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
packets; it is possible that the Src could send one test packet
TS[i], then send a second one (later) at TS[i+1], while the Dst
receive the second test packet at TR[i+1], and then receive the
one (later) at TR[i].
3.7. Errors and Uncertainties
In addition to sources of errors and uncertainties associated
methods employed to measure the singleton values that make up
sample, care must be given to analyze the accuracy of the
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
generate the Poisson arrival process. The Framework document
how to use the Anderson-Darling test verify the accuracy of
Poisson process over small time frames. {Comment: The goal is
ensure that the test packets are sent "close enough" to a
schedule, and avoid periodic behavior.}
3.8. Reporting the metric
The calibration and context for the underlying singletons MUST
reported along with the stream. (See "Reporting the metric"
Type-P-One-way-Packet-Loss.)
Almes, et al. Standards Track [Page 11]
RFC 2680 One Way Packet Loss Metric for IPPM September 1999
4. Some Statistics Definitions for One-way Packet
Given the sample metric Type-P-One-way-Packet-Loss-Poisson-Stream,
now offer several statistics of that sample. These statistics
offered mostly to be illustrative of what could be done
4.1. Type-P-One-way-Packet-Loss-
Given a Type-P-One-way-Packet-Loss-Poisson-Stream, the average of
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 = <
>
Then the average would be 0.2.
Note that, since healthy Internet paths should be operating at
rates below 1% (particularly if high delay-bandwidth products are
be sustained), the sample sizes needed might be larger than one
like. Thus, for example, if one wants to discriminate
various fractions of 1% over one-minute periods, then several
samples per minute might be needed. This would result in
values of lambda than one would ordinarily want
Note that although the loss threshold should be set such that
errors in loss are not significant, if the possibility that a
which arrived is counted as lost due to resource exhaustion
significant compared to the loss rate of interest, Type-P-One-way
Packet-Loss-Average will be meaningless
5. Security
Conducting Internet measurements raises both security and
concerns. This memo does not specify an implementation of
metrics, so it does not directly affect the security of the
nor of applications which run on the Internet. However
implementations of these metrics must be mindful of security
privacy concerns
Almes, et al. Standards Track [Page 12]
RFC 2680 One Way Packet Loss Metric for IPPM September 1999
There are two types of security concerns: potential harm caused
the measurements, and potential harm to the measurements.
measurements could cause harm because they are active, and
packets into the network. The measurement parameters MUST
carefully selected so that the measurements inject trivial amounts
additional traffic into the networks they measure. If they
"too much" traffic, they can skew the results of the measurement,
in extreme cases cause congestion and denial of service
The measurements themselves could be harmed by routers
measurement traffic a different priority than "normal" traffic, or
an attacker injecting artificial measurement traffic. If routers
recognize measurement traffic and treat it separately,
measurements will not reflect actual user traffic. If an
injects artificial traffic that is accepted as legitimate, the
rate will be artificially lowered. Therefore, the
methodologies SHOULD include appropriate techniques to reduce
probability measurement traffic can be distinguished from "normal
traffic. Authentication techniques, such as digital signatures,
be used where appropriate to guard against injected traffic attacks
The privacy concerns of network measurement are limited by the
measurements described in this memo. Unlike passive measurements
there can be no release of existing user data
6.
Thanks are due to Matt Mathis for encouraging this work and
calling attention on so many occasions to the significance of
loss
Thanks are due also to Vern Paxson for his valuable comments on
drafts, and to Garry Couch and Will Leland for several
suggestions
7.
[1] Paxson, V., Almes,G., Mahdavi, J. and M. Mathis, "Framework
IP Performance Metrics", RFC 2330, May 1998.
[2] Almes, G., Kalidindi, S. and M. Zekauskas, "A One-way
Metric for IPPM", RFC 2679, September 1999.
[3] Mahdavi, J. and V. Paxson, "IPPM Metrics for
Connectivity", RFC 2678, September 1999.
Almes, et al. Standards Track [Page 13]
RFC 2680 One Way Packet Loss Metric for IPPM September 1999
[4] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
[5] Bradner, S., "Key words for use in RFCs to Indicate
Levels", BCP 14, RFC 2119, March 1997.
[6] Bradner, S., "The Internet Standards Process -- Revision 3",
9, RFC 2026, October 1996.
8. Authors'
Guy
Advanced Network & Services, Inc
200 Business Park
Armonk, NY 10504
Phone: +1 914 765 1120
EMail: almes@advanced.
Sunil
Advanced Network & Services, Inc
200 Business Park
Armonk, NY 10504
Phone: +1 914 765 1128
EMail: kalidindi@advanced.
Matthew J.
Advanced Network & Services, Inc
200 Business Park
Armonk, NY 10504
Phone: +1 914 765 1112
EMail: matt@advanced.
Almes, et al. Standards Track [Page 14]
RFC 2680 One Way Packet Loss Metric for IPPM September 1999
9. Full Copyright
Copyright (C) The Internet Society (1999). All Rights Reserved
This document and translations of it may be copied and furnished
others, and derivative works that comment on or otherwise explain
or assist in its implementation may be prepared, copied,
and distributed, in whole or in part, without restriction of
kind, provided that the above copyright notice and this paragraph
included on all such copies and derivative works. However,
document itself may not be modified in any way, such as by
the copyright notice or references to the Internet Society or
Internet organizations, except as needed for the purpose
developing Internet standards in which case the procedures
copyrights defined in the Internet Standards process must
followed, or as required to translate it into languages other
English
The limited permissions granted above are perpetual and will not
revoked by the Internet Society or its successors or assigns
This document and the information contained herein is provided on
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE
Funding for the RFC Editor function is currently provided by
Internet Society
Almes, et al. Standards Track [Page 15]
if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
RFC documents can be found at I.E.T.F.
Relevance System Copyright © 2002 Spectrum WorldResearch
other technical nosh by ServerMasters Corporation
collaboration of BobX
