A One-Way Packet Duplication Metric
A way to measure how often one packet arrives more than once, and how much of a stream that affects.
Uijterwaal, H., "A One-Way Packet Duplication Metric", RFC 5560, DOI 10.17487/RFC5560, May 2009, <https://www.rfc-editor.org/info/rfc5560>.
@misc{rfc5560,
series = {Request for Comments},
number = 5560,
howpublished = {RFC 5560},
publisher = {RFC Editor},
doi = {10.17487/RFC5560},
url = {https://www.rfc-editor.org/info/rfc5560},
author = {H. Uijterwaal},
title = {{A One-Way Packet Duplication Metric}},
pagetotal = 14,
year = 2009,
month = may,
} RFC text
Contents
Network Working Group H. Uijterwaal
Request for Comments: 5560 RIPE NCC
Category: Standards Track May 2009
A One-Way Packet Duplication Metric
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 (c) 2009 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 in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
When a packet is sent from one host to the other, one normally
expects that exactly one copy of the packet that was sent arrives at
the destination. It is, however, possible that a packet is either
lost or that multiple copies arrive.
In earlier work, a metric for packet loss was defined. This metric
quantifies the case where a packet that is sent does not arrive at
its destination within a reasonable time. In this memo, a metric for
another case is defined: a packet is sent, but multiple copies
arrive. The document also discusses streams and methods to summarize
the results of streams.
Table of Contents
1. Introduction ....................................................3
1.1. Requirements Notation ......................................3
1.2. Motivation .................................................4
2. A Singleton Definition for One-Way Packet Arrival Count .........4
2.1. Metric Name ................................................4
2.2. Metrics Parameters .........................................4
2.3. Metric Units ...............................................4
2.4. Definition .................................................4
2.5. Discussion .................................................5
2.6. Methodology ................................................6
2.7. Errors and Uncertainties ...................................6
2.8. Reporting the Metric .......................................6
3. A Singleton Definition for One-Way Packet Duplication ...........6
3.1. Metric Name ................................................6
3.2. Metrics Parameters .........................................7
3.3. Metric Units ...............................................7
3.4. Definition .................................................7
3.5. Discussion .................................................7
4. Definition for Samples for One-Way Packet Duplication ...........7
4.1. Poisson Streams ............................................7
4.1.1. Metric Name .........................................7
4.1.2. Metric Parameters ...................................8
4.1.3. Metric Units ........................................8
4.1.4. Definition ..........................................8
4.1.5. Methodology .........................................8
4.1.6. Errors and Uncertainties ............................8
4.1.7. Reporting the Metric ................................8
4.2. Periodic Streams ...........................................9
4.2.1. Metric Name .........................................9
4.2.2. Metric Parameters ...................................9
4.2.3. Metric Units ........................................9
4.2.4. Definition ..........................................9
4.2.5. Methodology .........................................9
4.2.6. Errors and uncertainties ............................9
4.2.7. Reporting the metric ...............................10
5. Some Statistics Definitions for One-Way Duplication ............10
5.1. Type-P-one-way-packet-duplication-fraction ................10
5.2. Type-P-one-way-replicated-packet-rate .....................10
5.3. Examples ..................................................11
6. Security Considerations ........................................12
7. IANA Considerations ............................................12
8. Acknowledgements ...............................................13
9. References .....................................................13
9.1. Normative References ......................................13
9.2. Informative References ....................................13
1. Introduction
This document defines a metric for one-way packet duplication across
Internet paths. It builds on the IP Performance Metrics (IPPM)
Framework document [RFC2330]; the reader is assumed to be familiar
with that document.
This document follows the same structure as the document for one-way
packet loss [RFC2680]; the reader is assumed to be familiar with that
document as well.
The structure of this memo is as follows:
o First, a singleton metric, called Type-P-one-way-packet-arrival-
count, is introduced to measure the number of arriving packets for
each packet sent.
o Then, a singleton metric, called Type-P-one-way-packet-
duplication, is defined to describe a single instance of packet
duplication.
o Next, this singleton metric is used to define samples, Type-P-one-
way-Packet-Duplication-Poisson-Stream and Type-P-one-way-Packet-
Duplication-Periodic-Stream. These are introduced to measure
duplication in a series of packets sent with either Poisson-
distributed [RFC2680] or periodic [RFC3432] intervals between the
packets.
o Finally, statistics that summarize the properties of these samples
are introduced.
1.1. Requirements Notation
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 [RFC2119].
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.
1.2. Motivation
When a packet is sent from one host to the other, one normally
expects that exactly one copy of the packet that was sent arrives at
the destination. It is, however, possible that a packet is either
lost or that multiple copies arrive.
In earlier work, a metric for packet loss was defined [RFC2680].
This metric distinguishes between cases where the packet arrives and
where the packet does not arrive within a reasonable time. In this
memo, a metric for a third outcome is defined: a single packet is
sent, but multiple copies arrive.
As this document describes a case similar to the one discussed in
[RFC2680], all considerations from that document on timing and
accuracy apply.
2. A Singleton Definition for One-Way Packet Arrival Count
2.1. Metric Name
Type-P-one-way-packet-arrival-count
2.2. Metrics Parameters
o src, the IP address of a host
o dst, the IP address of a host
o T, the wire time of a packet at the source
o T0, the maximum waiting time for a packet to arrive at the
destination.
2.3. Metric Units
An integer number.
2.4. Definition
Two packets are considered identical if and only if:
o Both contain identical information fields (see Section 2.5). The
recipient thus could take either packet and use the data in an
application. The other packet does not contain any additional
information.
o Both packets appear to have been sent by one and the same host, to
one and the same destination. Hosts are identified by their IP
addresses.
The value of a Type-P-one-way-packet-arrival-count is a positive
integer number indicating the number of (uncorrupted and identical)
copies received by dst in the interval [T, T+T0] for a packet sent by
src at time T.
If a packet is sent, but it is lost or does not arrive in the
interval [T, T+T0], then the metric is undefined. Applications MAY
report an "impossible" value (for example, -1) to indicate this
condition instead of undefined.
If a packet is fragmented during transport and if, for whatever
reason, reassembly does not occur, then the packet will be deemed
lost. It is thus not included in the Type-P-one-way-packet-arrival-
count.
2.5. Discussion
This metric counts the number of packets arriving for each packet
sent. The time-out value T0 SHOULD be set to a value when the
application could potentially still use the packet and would not
discard it automatically.
If this metric is used in parallel with the Packet Loss Metric
[RFC2680], the value of T0 MUST be the same for both cases in order
to keep the results comparable.
The metric only counts packets that are not corrupted during
transmission and may have been resent automatically by lower layers
or intermediate devices. Packets that were corrupted during
transmission but, nevertheless, still arrived at dst are not counted.
Clocks do have to be synchronized between src and dst such that it is
possible to uniquely and accurately determine the interval [T, T+T0]
at both sides.
If this metric is used in an active measurement system, the system
MUST NOT send multiple packets with identical information fields in
order to avoid that all packets will be declared duplicates. This
metric can be used inside a passive measurement system as well, using
packets generated by another source. However, if the source can send
two identical packets within the interval [T, T+T0], this will be
incorrectly labeled as a duplicate, resulting in a false positive.
It is up to the implementor to estimate if this scenario is likely to
happen and the rate of false positives that is acceptable.
The definition of identical information fields is such that two
packets are considered to be identical if they are sent from the same
source and contain the same information. This does not necessarily
mean that all bits in the packet are the same. For example, when a
packet is replicated and the copies are transferred along different
paths, the Time to Live (TTL) may be different. The implementation
MUST specify which fields are compared when deciding whether or not
two packets are identical.
In the case of IPv4, these will usually be: version, ihl,
identification, src, dst, protocol, some or all upper-layer protocol
data.
In IPv6, these will usually be: version, next header, source,
destination, some or all upper-layer protocol data
Note that the use of the identification field is not present in non-
fragmented IPv6 packets and may not be sufficient to distinguish
packets from each even in IPv4, particularly at higher transmission
speeds
2.6. Methodology
The basic technique to measure this metric follows the methodology
described in Section 2.6 of [RFC2680] with one exception.
[RFC2680] does not specify that the receiving host should be able to
receive multiple copies of a single packet, as it only needs one copy
to determine the metrics. Implementations for this metric should
obviously be capable of receiving multiple copies.
2.7. Errors and Uncertainties
Refer to Section 2.7 of [RFC2680].
2.8. Reporting the Metric
Refer to Section 2.8 of [RFC2680].
3. A Singleton Definition for One-Way Packet Duplication
3.1. Metric Name
Type-P-one-way-packet-duplication
3.2. Metrics Parameters
o src, the IP address of a host
o dst, the IP address of a host
o T, the wire time of a packet at the source
o T0, the maximum waiting time for a packet to arrive at the
destination.
3.3. Metric Units
An integer number.
3.4. Definition
The value of a Type-P-one-way-packet-duplication is a positive
integer number indicating the number of (uncorrupted and identical)
additional copies of an individual packet received by dst in the
interval [T, T+T0] as sent by src at time T.
If a packet is sent and only one copy arrives in the interval [T,
T+T0], then the metric is 0. If no copy arrives in this interval,
then the metric is undefined. Applications MAY report an
"impossible" value (for example, -1) to indicate this condition.
3.5. Discussion
This metric is equal to:
Type-P-one-way-packet-arrival-count - 1
This metric is expected to be used for applications that need to know
duplication for an individual packet. All considerations regarding
methodology, errors, and reporting from the previous section apply.
4. Definition for Samples for One-Way Packet Duplication
4.1. Poisson Streams
4.1.1. Metric Name
Type-P-one-way-Packet-Duplication-Poisson-Stream
4.1.2. Metric Parameters
o src, the IP address of a host.
o dst, the IP address of a host.
o Ts, a time.
o Tf, a time. Ts and Tf specify the time interval when packets can
be sent for this stream.
o T0, the maximum waiting time for a packet to arrive at the
destination.
o lambda, a rate in reciprocal seconds.
4.1.3. Metric Units
A sequence of pairs; the elements of each pair are:
o T, a time
o Type-P-one-way-packet-arrival-count for the packet sent at T.
4.1.4. Definition
Given Ts, Tf, and lambda, we compute a pseudo-random Poisson process
beginning at or before Ts, with average-rate lambda, and ending at or
after Tf. Those time values greater than or equal to Ts, 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-arrival-count.
The value of the sample is the sequence made up of the resulting
{time, duplication} pairs. If there are no such pairs, the sequence
is of length zero, and the sample is said to be empty.
4.1.5. Methodology
Refer to Section 3.6 of [RFC2680].
4.1.6. Errors and Uncertainties
Refer to Section 3.7 of [RFC2680].
4.1.7. Reporting the Metric
Refer to Section 3.8 of [RFC2680].
4.2. Periodic Streams
4.2.1. Metric Name
Type-P-one-way-Packet-Duplication-Periodic-Stream
4.2.2. Metric Parameters
o src, the IP address of a host.
o dst, the IP address of a host.
o Ts, a time.
o Tf, a time. Ts and Tf specify the time interval when packets can
be sent for this stream.
o T0, the maximum waiting time for a packet to arrive at the
destination.
o lambda, a rate in reciprocal seconds.
4.2.3. Metric Units
A sequence of pairs; the elements of each pair are:
o T, a time
o Type-P-one-way-packet-arrival-count for the packet sent at T.
4.2.4. Definition
At time Ts, we start sending packets with a constant-rate lambda,
until time Tf. For each packet sent, we obtain the value of Type-P-
one-way-packet-arrival-count. The value of the sample is the
sequence made up of the resulting {time, duplication} pairs. If
there are no such pairs, the sequence is of length zero and the
sample is said to be empty.
4.2.5. Methodology
Refer to Section 4.5 of [RFC3432].
4.2.6. Errors and uncertainties
Refer to Section 4.6 of [RFC3432].
4.2.7. Reporting the metric
Refer to Section 4.7 of [RFC3432].
5. Some Statistics Definitions for One-Way Duplication
Note: the statistics described in this section can be used for both
Type-P-one-way-Packet-Duplication-Poisson-Stream and Type-P-one-way-
Packet-Duplication-Periodic-Stream. The application SHOULD report
which sample was used as input.
5.1. Type-P-one-way-packet-duplication-fraction
This statistic gives the fraction of additional packets that arrived
in a stream.
Given a Type-P-one-way-Packet-Duplication-Poisson-Stream, one first
removes all values of Type-P-one-way-Packet-Duplication that are
undefined. For the remaining pairs in the stream, one calculates:
(Sum Type-P-one-way-packet-arrival-count/Number of pairs left) - 1
(In other words, (number of packets received)/(number of packets sent
and not lost).)
The number can be expressed as a percentage.
Note: this statistic is the equivalent to the Y.1540 IPDR [Y1540].
5.2. Type-P-one-way-replicated-packet-rate
This statistic gives the fraction of packets that was duplicated (one
or more times) in a stream.
Given a Type-P-one-way-Packet-Duplication-Poisson-Stream, one first
removes all values of Type-P-one-way-packet-arrival-count that are
undefined. For the remaining pairs in the stream, one counts the
number of pairs with Type-P-one-way-packet-arrival-count greater than
1. Then, one calculates the fraction of packets that meet this
criterion as a fraction of the total. (In other words: (number of
duplicated packets)/(number of packets sent and not lost).)
The number can be expressed as a percentage.
Note: this statistic is the equivalent of the Y.1540 RIPR [Y1540].
5.3. Examples
Consider a stream of 4 packets, sent as:
(1, 2, 3, 4)
and arriving as:
o Case 1: (1, 2, 3, 4)
o Case 2: (1, 1, 2, 2, 3, 3, 4, 4)
o Case 3: (1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4)
o Case 4: (1, 1, 1, 2, 3, 3, 3, 4)
Case 1: No packets are duplicated in a stream, and both the Type-P-
one-way-packet-duplication-fraction and the Type-P-one-way-packet-
replicated-packet-rate are 0.
Case 2: Every packet is duplicated once, and the Type-P-one-way-
packet-duplication-fraction is 100%. The Type-P-one-way-replicated-
packet-rate is 100%, too.
Case 3: Every packet is duplicated twice, so the Type-P-one-way-
packet-duplication-fraction is 200%. The Type-P-one-way-replicated-
packet-rate is still 100%.
Case 4: Half the packets are duplicated twice and the other half are
not duplicated. The Type-P-one-way-packet-duplication-fraction is
again 100%, and this number does not show the difference with case 2.
However, the Type-P-one-way-packet-replicated-packet-rate is 50% in
this case and 100% in case 2.
However, the Type-P-one-way-packet-duplication-rate will not show the
difference between cases 2 and 3. For this, one has to look at the
Type-P-one-way-packet-duplication-fraction.
Finally, note that the order in which the packets arrived does not
affect the results. For example, these variations of case 2:
o Case 2a: (1, 1, 2, 2, 3, 3, 4, 4)
o Case 2b: (1, 2, 3, 4, 1, 2, 3, 4)
o Case 2c: (1, 2, 3, 4, 4, 3, 2, 1)
(as well as any other permutation) all yield the same results for
Type-P-one-way-packet-duplication-fraction and the Type-P-one-way-
replicated-packet-rate.
6. 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 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 they
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 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. Unlike passive measurements,
there can be no release of existing user data.
7. IANA Considerations
IANA has registered the metrics defined in this document in the IP
Performance Metrics (IPPM) Metrics Registry, see [RFC4148].
8. Acknowledgements
The idea to write this document came up in a meeting with Al Morton,
Stanislav Shalunov, Emile Stephan, and the author on the IPPM
reporting document.
This document relies heavily on [RFC2680], and the author would like
to thank the authors of that document for writing it.
Finally, thanks are due to Lars Eggert, Al Morton, Martin Swany, and
Matt Zekauskas for their comments.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2680] Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
Packet Loss Metric for IPPM", RFC 2680, September 1999.
[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.
[RFC4148] Stephan, E., "IP Performance Metrics (IPPM) Metrics
Registry", BCP 108, RFC 4148, August 2005.
[Y1540] "Y.1540 ITU-T Recommendation Y.1540 (2007), Internet
protocol data communication service IP packet transfer and
availability performance parameters.", 2007.
Author's Address
Henk Uijterwaal
RIPE NCC
Singel 258
1016 AB Amsterdam
The Netherlands
Phone: +31 20 535 4444
EMail: henk@ripe.net
ISO Plain Language
On this page
§1Why Duplication Needs a Metric
The document defines a metric for one-way packet duplication across Internet paths. It builds on the IP Performance Metrics framework of RFC 2330, and it follows the same structure as the document for one-way packet loss, RFC 2680.
The structure of the memo is layered. First, a singleton metric called Type-P-one-way-packet-arrival-count is introduced to measure the number of arriving packets for each packet sent. Then a second singleton metric, Type-P-one-way-packet-duplication, is defined to describe a single instance of packet duplication. That singleton metric is then used to define two samples, Type-P-one-way-Packet-Duplication-Poisson-Stream and Type-P-one-way-Packet-Duplication-Periodic-Stream, which measure duplication in a series of packets sent with either Poisson-distributed or periodic intervals between the packets. Finally, statistics that summarise the properties of these samples are introduced.
When a packet is sent from one host to another, one normally expects that exactly one copy arrives at the destination. It is, however, possible that a packet is either lost or that multiple copies arrive. The earlier packet loss metric distinguishes between cases where the packet arrives and where it does not arrive within a reasonable time. This memo defines a metric for a third outcome: a single packet is sent but multiple copies arrive. Since the document describes a case similar to the one in RFC 2680, all considerations from that document on timing and accuracy apply.
The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY and OPTIONAL are to be interpreted as described in RFC 2119. Although RFC 2119 was written with protocols in mind, the key words are used here for similar reasons: to ensure the results of measurements from two different implementations are comparable, and to note instances when an implementation could perturb the network.
The RFC builds a measurement in layers, from one packet up to a whole stream, and then adds two summary percentages. The requirement words are used here to make sure two measurement systems produce results that can be compared.
§2Type-P-one-way-packet-arrival-count
The metric takes the parameters src, the IP address of a host, dst, the IP address of a host, T, the wire time of a packet at the source, and T0, the maximum waiting time for a packet to arrive at the destination. Its unit is an integer number.
Two packets are considered identical if and only if both contain identical information fields, so the recipient could take either packet and use the data in an application while the other adds nothing, and both appear to have been sent by the same host to the same destination, hosts being identified by their IP addresses.
The value of a Type-P-one-way-packet-arrival-count is a positive integer giving the number of uncorrupted and identical copies received by dst in the interval [T, T+T0] for a packet sent by src at time T. If a packet is sent but is lost or does not arrive in that interval, the metric is undefined, and applications MAY report an impossible value such as -1 instead. If a packet is fragmented during transport and reassembly does not occur, the packet is deemed lost and is not included in the count.
The time-out value T0 SHOULD be set to a value at which the application could potentially still use the packet and would not discard it automatically. If this metric is used in parallel with the packet loss metric, the value of T0 MUST be the same in both cases so the results stay comparable. The metric counts only packets that were not corrupted during transmission, including packets that may have been resent automatically by lower layers or intermediate devices; packets that were corrupted but still arrived are not counted.
Clocks have to be synchronised between src and dst so that the interval [T, T+T0] can be determined accurately at both sides. If the metric is used in an active measurement system, the system MUST NOT send multiple packets with identical information fields, or all of them would be declared duplicates. The metric can also be used inside a passive measurement system with packets generated by another source, but if that source can send two identical packets within the interval, this will be incorrectly labelled as a duplicate, giving a false positive. It is up to the implementor to estimate whether that is likely and what rate of false positives is acceptable.
Identical does not necessarily mean that all bits in the packet are the same. When a packet is replicated and the copies travel along different paths, the Time to Live may differ. The implementation MUST specify which fields are compared. In IPv4 these will usually be version, ihl, identification, src, dst, protocol and some or all upper-layer protocol data; in IPv6, version, next header, source, destination and some or all upper-layer protocol data. The use of the identification field is not present in non-fragmented IPv6 packets and may not be sufficient to distinguish packets from each other even in IPv4, particularly at higher transmission speeds.
The measurement counts the copies of one packet that arrive within a set waiting time, treating copies as the same when they carry the same data and came from the same place. Copies damaged in transit do not count, and a packet that never gets through is reported as undefined rather than as zero.
- Same dataBoth copies carry identical information fields, so either one would do for the application.
- Same path endpointsBoth appear to come from the same source host and go to the same destination, judged by IP address.
- Damaged copies do not countA packet corrupted in transit is not counted, even if it still arrived.
- A lost packet is not a duplicateIf a fragmented packet is never reassembled it counts as lost, not as a duplicate.
- Clocks must agreeBoth ends need synchronised clocks to fix the waiting window [T, T+T0].
§3Type-P-one-way-packet-duplication
The second singleton metric, Type-P-one-way-packet-duplication, takes the same parameters, src, dst, T and T0, and its unit is also an integer number. Its value is a positive integer giving the number of uncorrupted and identical additional copies of an individual packet received by dst in the interval [T, T+T0], as sent by src at time T.
If a packet is sent and only one copy arrives in the interval, the metric is 0. If no copy arrives in the interval, the metric is undefined, and applications MAY report an impossible value such as -1 to indicate this condition.
The metric is equal to Type-P-one-way-packet-arrival-count minus 1. It is expected to be used by applications that need to know about duplication for an individual packet. All considerations regarding methodology, errors and reporting from the previous section apply.
This is the arrival count minus one. One copy means no duplication, three copies means two extra copies, and no copy at all means the figure is undefined.
§4Samples for One-Way Duplication
Two samples are defined on top of the singleton metric, one for each way of sending packets.
Type-P-one-way-Packet-Duplication-Poisson-Stream takes the parameters src, dst, Ts and Tf, which specify the time interval when packets can be sent for this stream, T0, the maximum waiting time, and lambda, a rate in reciprocal seconds. Its unit is a sequence of pairs, each pair being a time T and the Type-P-one-way-packet-arrival-count for the packet sent at T. Given Ts, Tf and lambda, a pseudo-random Poisson process is computed beginning at or before Ts, with average rate lambda, and ending at or after Tf. The time values greater than or equal to Ts and less than or equal to Tf are selected, and at each of those times the value of the arrival count is obtained. The value of the sample is the sequence of the resulting time and duplication pairs. If there are no such pairs, the sequence is of length zero and the sample is said to be empty. Methodology, errors and uncertainties and reporting are as specified in the corresponding sections of RFC 2680.
Type-P-one-way-Packet-Duplication-Periodic-Stream takes the same parameters and has the same unit. At time Ts, packets are sent with a constant rate lambda until time Tf. For each packet sent the value of the arrival count is obtained, and the value of the sample is the sequence of the resulting time and duplication pairs. If there are no such pairs, the sequence is of length zero and the sample is empty. Methodology, errors and uncertainties and reporting are as specified in the corresponding sections of RFC 3432.
Both samples are just a list of pairs: the time a packet was sent, and how many copies turned up. One sample sends packets at random intervals, the other sends them at a steady rate.
§5The Two Summary Statistics
The statistics in this section can be used for both the Poisson stream and the periodic stream, and the application SHOULD report which sample was used as input.
Type-P-one-way-packet-duplication-fraction gives the fraction of additional packets that arrived in a stream. Given a Poisson stream, one first removes all values of the duplication metric that are undefined. For the remaining pairs, one calculates the sum of the arrival counts divided by the number of pairs left, minus one. In other words, the number of packets received divided by the number of packets sent and not lost. The number can be expressed as a percentage. This statistic is the equivalent to the Y.1540 IPDR.
Type-P-one-way-replicated-packet-rate gives the fraction of packets that were duplicated, one or more times, in a stream. Again, undefined arrival counts are removed first. One then counts the number of pairs with an arrival count greater than 1, and calculates that count as a fraction of the total. In other words, the number of duplicated packets divided by the number of packets sent and not lost. The number can be expressed as a percentage, and the statistic is the equivalent of the Y.1540 RIPR.
The document works through four cases with a stream of four packets sent as 1, 2, 3, 4. In case 1 the packets arrive as 1, 2, 3, 4, so no packets are duplicated and both statistics are 0. In case 2 they arrive as 1, 1, 2, 2, 3, 3, 4, 4, so every packet is duplicated once, the duplication fraction is 100% and the replicated packet rate is 100% too. In case 3 they arrive three times each, so the duplication fraction is 200% while the replicated packet rate is still 100%. In case 4 they arrive as 1, 1, 1, 2, 3, 3, 3, 4, so half the packets are duplicated twice and the other half are not duplicated: the duplication fraction is again 100%, which does not show the difference with case 2, but the replicated packet rate is 50% here against 100% in case 2. The replicated packet rate will not show the difference between cases 2 and 3, so for that one has to look at the duplication fraction. Finally, the order in which the packets arrived does not affect the results, so permutations of case 2 yield the same values for both statistics.
Two percentages summarise a run of measurements. The duplication fraction says how many extra copies arrived in total. The replicated packet rate says how many packets were duplicated at all. A stream where every packet arrived three times and a stream where every packet arrived twice look the same on the second measure but not on the first.
| Case | Packets arriving | Duplication fraction | Replicated packet rate |
|---|---|---|---|
| 1 | (1, 2, 3, 4) | 0 | 0 |
| 2 | (1, 1, 2, 2, 3, 3, 4, 4) | 100% | 100% |
| 3 | (1, 1, 1, 2, 2, 2, 3, 3, 3, 4, 4, 4) | 200% | 100% |
| 4 | (1, 1, 1, 2, 3, 3, 3, 4) | 100% | 50% |
Type-P-one-way-packet-duplication-fraction
= (Sum Type-P-one-way-packet-arrival-count / Number of pairs left) - 1
= (number of packets received) / (number of packets sent and not lost)
Type-P-one-way-replicated-packet-rate
= (number of duplicated packets) / (number of packets sent and not lost)§6Security Considerations
Conducting Internet measurements raises both security and privacy concerns. The memo does not specify an implementation of the metrics, so it does not directly affect the security of the Internet nor of applications that run on it, but implementations of these metrics must be mindful of security and privacy concerns.
There are two types of security concern: 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 recognise 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. The measurement methodologies SHOULD therefore 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 the memo: unlike passive measurements, there can be no release of existing user data.
These measurements add traffic to the network, so they must add very little, or they distort what they measure. They can also be attacked: injected fake packets would make the numbers look better than they are.
§7IANA Considerations
IANA has registered the metrics defined in the document in the IP Performance Metrics Metrics Registry, described in RFC 4148.
The metric names were entered in the IETF registry of IP performance metrics, so that measurement results can be labelled in a standard way.