Internet Engineering Task Force (IETF) A. Morton
Request for Comments: 6673 AT&T Labs
Category: Standards Track August 2012
ISSN: 2070-1721
Round-Trip Packet Loss Metrics
Abstract
Many user applications (and the transport protocols that make them
possible) require two-way communications. To assess this capability,
and to achieve test system simplicity, round-trip loss measurements
are frequently conducted in practice. The Two-Way Active Measurement
Protocol specified in RFC 5357 establishes a round-trip loss
measurement capability for the Internet. However, there is currently
no round-trip packet loss metric specified according to the RFC 2330
framework.
This memo adds round-trip loss to the set of IP Performance Metrics
(IPPM).
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/rfc6673.
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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
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to this document. Code Components extracted from this document must
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.
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Table of Contents
1. Introduction ....................................................3
1.1. Motivation .................................................4
1.2. Requirements Language ......................................5
2. Scope ...........................................................5
3. Common Specifications for Round-Trip Metrics ....................5
3.1. Name: Type-P-* .............................................5
3.2. Metric Parameters ..........................................5
3.3. Metric Definition ..........................................6
3.4. Metric Units ...............................................6
4. A Singleton Round-Trip Loss Metric ..............................7
4.1. Name: Type-P-Round-trip-Loss ...............................7
4.2. Metric Parameters ..........................................7
4.3. Definition and Metric Units ................................7
4.4. Discussion and Other Details ...............................8
5. A Sample Round-Trip Loss Metric .................................9
5.1. Name: Type-P-Round-trip-Loss-<Sample>-Stream ...............9
5.2. Metric Parameters ..........................................9
5.3. Definition and Metric Units ................................9
5.4. Discussion and Other Details ..............................10
6. Round-Trip Loss Statistic ......................................10
6.1. Type-P-Round-trip-Loss-<Sample>-Ratio .....................10
7. Round-Trip Testing and One-Way Reporting .......................11
8. Measurement Considerations and Calibration .....................11
9. Security Considerations ........................................12
9.1. Denial-of-Service Attacks .................................12
9.2. User Data Confidentiality .................................12
9.3. Interference with the Metrics .............................12
10. IANA Considerations ...........................................13
11. Acknowledgements ..............................................13
12. References ....................................................13
12.1. Normative References .....................................13
12.2. Informative References ...................................14
1. Introduction
This memo defines a metric to quantify an IP network's ability to
transfer packets in both directions from one host to another host.
Two-way communication is almost always needed; thus, failure to
transfer a packet in either direction constitutes a round-trip packet
loss.
This memo defines a metric for round-trip packet loss on Internet
paths. It builds on the notions and conventions introduced in the IP
Performance Metrics (IPPM) framework [RFC2330]. Also, the
specifications of the one-way packet loss metric for IPPM [RFC2680]
and the round-trip delay metric for IPPM [RFC2681] are frequently
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referenced and modified to match the round-trip circumstances
addressed here. However, this memo assumes that the reader is
familiar with the references; thus, it does not repeat material as
was done in [RFC2681].
This memo uses the terms "two-way" and "round-trip" synonymously.
1.1. Motivation
Many user applications and the transport protocols that make them
possible require two-way communications. For example, the TCP SYN->,
<-SYN-ACK, ACK-> three-way handshake attempted billions of times each
day cannot be completed without two-way connectivity in a near-
simultaneous time interval. Thus, measurements of Internet round-
trip packet loss performance provide a basis to infer application
performance more easily.
Measurement system designers have also recognized advantages of
system simplicity when one host simply echoes or reflects test
packets to the sender. Round-trip packet loss measurements are
frequently conducted and reported in practice. The ubiquitous "ping"
tools allow the measurement of round-trip packet loss and delay but
usually require ICMP Echo-Request/Reply support, and ICMP packets may
encounter exceptional treatment on the measurement path (see
Section 2.6 of [RFC2681]). The Two-Way Active Measurement Protocol
(TWAMP) specified in [RFC5357] establishes a round-trip packet loss
measurement capability for the Internet. However, there is currently
no round-trip packet loss metric specified according to the [RFC2330]
framework.
[RFC2681] indicates that round-trip measurements may sometimes
encounter "asymmetric" paths. When loss is observed using a round-
trip measurement, there is often a desire to ascertain which of the
two directional paths "lost" the packet. Under some circumstances,
it is possible to make this inference. The round-trip measurement
method raises a few complications when interpreting the embedded one-
way results, and the user should be aware of them.
[RFC2681] also points out that loss measurement conducted
sequentially in both directions of a path and reported as a round-
trip result may be exactly the desired metric. On the other hand, it
may be difficult to derive the state of round-trip packet loss from
one-way measurements conducted in each direction unless a method to
match the appropriate one-way measurements has been pre-arranged.
Finally, many measurement systems report statistics on a conditional
delay distribution, where the condition is packet arrival at the
destination. This condition is encouraged in [RFC3393], [RFC5481],
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and [RFC6703]. As a result, lost packets need to be reported
separately, according to a standardized metric. This memo defines
such a metric.
See Section 1.1 of [RFC2680] for additional motivation of the packet
loss metric.
1.2. 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].
2. Scope
This memo defines a round-trip packet loss metric using the
conventions of the IPPM framework [RFC2330].
The memo defines a singleton metric, a sample metric, and a
statistic, as per [RFC2330]. The [RFC2330] framework is for active
measurement methods. Although this metric MAY be applicable in
passive measurement as well, discussion of additional considerations
for the passive scenario are beyond the normative scope of this memo.
The memo also investigates the topic of one-way loss inference from a
two-way measurement and lists some key considerations.
3. Common Specifications for Round-Trip Metrics
To reduce the redundant information presented in the detailed metrics
sections that follow, this section presents the specifications that
are common to two or more metrics. The section is organized using
the same subsections as the individual metrics, to simplify
comparisons.
3.1. Name: Type-P-*
All metrics use the Type-P convention as described in [RFC2330]. The
rest of the name is unique to each metric.
3.2. Metric Parameters
o Src, the IP address of a host
o Dst, the IP address of a host
o T, a time (start of test interval)
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o Tf, a time (end of test interval)
o lambda, a rate in reciprocal seconds (for Poisson Streams)
o incT, the nominal duration of inter-packet interval, first bit to
first bit (for Periodic Streams)
o T0, a time that MUST be selected at random from the interval
[T, T+dT] to start generating packets and taking measurements (for
Periodic Streams)
o TstampSrc, the wire time of the packet as measured at MP(Src) as
it leaves for Dst.
o TstampDst, the wire time of the packet as measured at MP(Dst),
assigned to packets that arrive within a "reasonable" time (less
than Tmax).
o Tmax, a maximum waiting time for packets to arrive at Src, set
sufficiently long to disambiguate packets with long delays from
packets that are discarded (lost).
o M, the total number of packets sent between T0 and Tf
o N, the total number of packets received at Dst (sent between T0
and Tf)
o Type-P, as defined in [RFC2330], which includes any field that may
affect a packet's treatment as it traverses the network
3.3. Metric Definition
This section is specific to each metric.
3.4. Metric Units
The metric units are logical (1 or 0) when describing a single
packet's loss performance, where a 0 indicates successful packet
transmission and a 1 indicates packet loss.
Units of time are as specified in [RFC2330].
Other units used are defined in the associated section where
needed (e.g., Section 6.1 in the case of
Type-P-Round-trip-Loss-<Sample>-Ratio).
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4. A Singleton Round-Trip Loss Metric
4.1. Name: Type-P-Round-trip-Loss
4.2. Metric Parameters
See Section 3.2.
4.3. Definition and Metric Units
Type-P-Round-trip-Loss SHALL be represented by the binary logical
values (or their equivalents) when the following conditions are met:
Type-P-Round-trip-Loss = 0:
o Src sent the first bit of a Type-P packet to Dst at wire-time
TstampSrc,
o that Dst received that packet,
o the Dst sent a Type-P packet back to the Src as quickly as
possible (certainly less than Tmax, and fast enough for the
intended purpose), and
o that Src received the last bit of the reflected packet prior to
wire-time TstampSrc + Tmax.
Type-P-Round-trip-Loss = 1:
o Src sent the first bit of a Type-P packet to Dst at wire-time
TstampSrc,
o that Src did not receive the last bit of the reflected packet
before the waiting time lapsed at TstampSrc + Tmax.
Possible causes for the Loss = 1 outcome are as follows:
o the Dst did not receive that packet,
o the Dst did not send a Type-P packet back to the Src, or
o the Src did not receive a reflected Type-P packet sent from
the Dst.
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Following the precedent of Section 2.4 of [RFC2681], we make the
simplifying assertion that round-trip loss measured between two hosts
is equal regardless of the host that originates the test:
Type-P-Round-trip-Loss(Src->Dst->Src) =
Type-P-Round-trip-Loss(Dst->Src->Dst)
(and agree with the rationale presented there -- that the ambiguity
introduced is a small price to pay for measurement efficiency).
Therefore, each singleton can be represented by pairs of elements as
follows:
o TstampSrc, the wire time of the packet at the Src (beginning the
round-trip journey).
o L, either zero or one (or some logical equivalent), where L=1
indicates loss and L=0 indicates successful round-trip arrival
prior to TstampSrc + Tmax.
4.4. Discussion and Other Details
See [RFC2680] and [RFC2681] for extensive discussion, methods of
measurement, errors and uncertainties, and other fundamental
considerations that need not be repeated here.
We add the following guidance regarding the responder process to
"send a Type-P packet back to the Src as quickly as possible".
A response that was not generated within Tmax is inadequate for any
realistic test, and the Src will discard such responses. A responder
that serves typical round-trip packet loss testing (which is relevant
to higher-layer application performance) SHOULD produce a response in
1 second or less. A responder that is unable to satisfy this
requirement SHOULD log the fact so that an operator can adjust the
load and priorities as necessary. Analysis of responder timestamps
[RFC5357] that finds responses are not generated in a timely fashion
SHOULD result in operator notification, and the operator SHOULD
suspend tests to the responder, since it may be overloaded.
Additional measurement considerations are described in Section 8
below.
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5. A Sample Round-Trip Loss Metric
Given the singleton metric Type-P-Round-trip-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 TstampSrc. This can be
done in several ways, including the following:
1. Poisson: a pseudo-random Poisson process of rate lambda, whose
values fall between T and Tf. The time interval between
successive values of TstampSrc will then average 1/lambda, as per
Section 11.1.1 of [RFC2330].
2. Periodic: a periodic stream process with pseudo-random start time
T0 between T and dT, and nominal inter-packet interval incT, as
per [RFC3432].
In the metric name, the variable <Sample> SHALL be replaced with the
process used to define the sample, using one of the above processes
(or another sample process meeting the criteria in Section 11.1 of
[RFC2330], the details of which MUST be reported with the results if
used).
5.1. Name: Type-P-Round-trip-Loss-<Sample>-Stream
5.2. Metric Parameters
See Section 3.2.
5.3. Definition and Metric Units
Given one of the methods for defining the test interval -- the sample
of times (TstampSrc) and other metric parameters -- we obtain a
sequence of Type-P-Round-trip-Loss singletons as defined in
Section 4.3.
Type-P-Round-trip-Loss-<Sample>-Stream SHALL be a sequence of pairs
with elements as follows:
o TstampSrc, as above
o L, either zero or one (or some logical equivalent), where L=1
indicates loss and L=0 indicates successful round-trip arrival
prior to TstampSrc + Tmax
and where <Sample> SHALL be replaced with "Poisson", "Periodic", or
an appropriate term to designate another sample method as described
in Section 5 above.
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5.4. Discussion and Other Details
See [RFC2680] and [RFC2681] for extensive discussion, methods of
measurement, errors and uncertainties, and other fundamental
considerations that need not be repeated here. However, when these
references were approved, the packet reordering metrics in [RFC4737]
had not yet been defined, nor had reordering been addressed in IPPM
methodologies.
[RFC4737] defines packets that arrive "late" with respect to their
sending order as reordered -- for example, when packets arrive with
sequence numbers 4, 7, 5, 6, then packets 5 and 6 are reordered, and
they are obviously not lost because they have arrived within some
reasonable waiting time threshold. The presence of reordering on a
round-trip path has several likely effects on the measurement.
1. Methods of measurement should continue to wait the specified time
for packets and avoid prematurely declaring round-trip packet
loss when a sequence gap or error is observed.
2. The time distribution of the singletons in the sample has been
significantly changed.
3. Either the original packet stream or the reflected packet stream
experienced path instability, and the original conditions may no
longer be present.
Measurement implementations MUST address the possibility of packet
reordering and avoid related errors in their processes.
6. Round-Trip Loss Statistic
This section gives the primary and overall statistic for loss
performance. Additional statistics and metrics originally prepared
for one-way loss MAY also be applicable.
6.1. Type-P-Round-trip-Loss-<Sample>-Ratio
Given a Type-P-Round-trip-Loss-<Sample>-Stream, the average of
all the logical values, L, in the stream is the
Type-P-Round-trip-Loss-<Sample>-Ratio. This ratio is in units of
lost packets per round-trip transmissions actually attempted.
In addition, the Type-P-Round-trip-Loss-<Sample>-Ratio is undefined
if the sample is empty.
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7. Round-Trip Testing and One-Way Reporting
This section raises considerations for results collected using a
round-trip measurement architecture, such as in TWAMP [RFC5357].
The sampling process for the reverse path (Dst->Src) is a conditional
process that depends on successful packet arrival at the Dst and
correct operation at the Dst to generate the reflected packet.
Therefore, the sampling process for the reverse path will be
significantly affected when appreciable loss occurs on the Src->Dst
path, making an attempt to assess the reverse path performance
invalid (for loss or possibly any metric).
Further, the sampling times for the reverse path (Dst->Src) are a
random process that depends on the original sample times (TstampSrc),
the one-way delay for successful packet arrival at the Dst, and time
taken at the Dst to generate the reflected packet. Therefore, the
sampling process for the reverse path will be significantly affected
when appreciable delay variation occurs on the Src->Dst path, making
an attempt to assess the reverse path performance invalid (for loss
or possibly any metric).
As discussed above in Section 5.4, packet reordering is always a
possibility. In addition to the severe delay variation that usually
accompanies it, reordering on the Src->Dst path will cause a
misalignment of sequence numbers applied at the Dst when compared to
the sender numbers. Measurement implementations MUST address this
possible outcome.
8. Measurement Considerations and Calibration
Prior to conducting this measurement, the participating hosts MUST be
configured to send and receive test packets of the chosen Type-P.
Standard measurement protocols are capable of this task [RFC5357],
but any reliable method is sufficient (e.g., if the issues with ICMP
discussed in Section 2.6 of [RFC2681] can be alleviated, and the
requirements of Sections 4.3 and 4.4 above are met, then ICMP could
be used).
Two key features of the host that receives test packets and returns
them to the originating host are described in Section 4.2 of
[RFC5357]. Every received test packet MUST result in a responding
packet, and the response MUST be generated as quickly as possible.
This implies that interface buffers will be serviced promptly and
that buffer discards will be extremely rare. These features of the
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measurement equipment MUST be calibrated according to Section 3.7.3
of [RFC2679] when operating under a representative measurement load
(as defined by the user). Both unexpected test packet discards, and
the systematic and random errors and uncertainties, MUST be recorded.
We note that Section 4.2.1 of [RFC5357] specifies a method to collect
all four significant timestamps needed to describe a packet's round-
trip delay [RFC2681] and remove the processing time incurred at the
responding host. This information supports the measurement of the
corresponding one-way delays encountered on the round-trip path,
which can identify path asymmetry or unexpected processing time at
the responding host.
9. Security Considerations
9.1. Denial-of-Service Attacks
This metric requires a stream of packets sent from one host (source)
to another host (destination) through intervening networks, and back.
This method could be abused for denial-of-service attacks directed at
the destination and/or the intervening network(s).
Administrators of source, destination, and intervening network(s)
should establish bilateral or multilateral agreements regarding the
timing, size, and frequency of collection of sample metrics. Use of
this method in excess of the terms agreed upon by the participants
may be cause for immediate rejection or discard of packets, or other
escalation procedures as defined between the affected parties.
9.2. User Data Confidentiality
Active use of this method generates packets for a sample, rather than
taking samples based on user data, and does not threaten user data
confidentiality. Passive measurement must restrict attention to the
headers of interest. Since user payloads may be temporarily stored
for length analysis, suitable precautions MUST be taken to keep this
information safe and confidential. In most cases, a hashing function
will produce a value suitable for payload comparisons.
9.3. Interference with the Metrics
It may be possible to identify that a certain packet or stream of
packets is part of a sample. With that knowledge at the destination
and/or the intervening networks, it is possible to change the
processing of the packets (e.g., increasing or decreasing delay) in a
way that may distort the measured performance. It may also be
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possible to generate additional packets that appear to be part of the
sample metric. These additional packets are likely to perturb the
results of the sample measurement.
Authentication or encryption techniques, such as digital signatures,
MAY be used where appropriate to guard against injected traffic
attacks. [RFC5357] includes both authentication and encryption
features.
10. IANA Considerations
Metrics previously defined in the IETF were registered in the IANA
IPPM Metrics Registry; however, this process was discontinued when
the registry structure was found to be inadequate, and the registry
was declared obsolete [RFC6248].
Although the metrics in this document may be considered for some form
of registration in the future, no IANA action is requested at this
time.
11. Acknowledgements
The author thanks Tiziano Ionta for his careful review of this memo,
primarily resulting in the development of measurement considerations
using TWAMP [RFC5357] as an example method. The reviews of Adrian
Farrel and Benoit Claise also contributed to the clarity of the memo.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
"Framework for IP Performance Metrics", RFC 2330,
May 1998.
[RFC2679] Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
Delay Metric for IPPM", RFC 2679, September 1999.
[RFC2680] Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
Packet Loss Metric for IPPM", RFC 2680, September 1999.
[RFC2681] Almes, G., Kalidindi, S., and M. Zekauskas, "A Round-trip
Delay Metric for IPPM", RFC 2681, September 1999.
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[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation
Metric for IP Performance Metrics (IPPM)", RFC 3393,
November 2002.
[RFC3432] Raisanen, V., Grotefeld, G., and A. Morton, "Network
performance measurement with periodic streams", RFC 3432,
November 2002.
[RFC4737] Morton, A., Ciavattone, L., Ramachandran, G., Shalunov,
S., and J. Perser, "Packet Reordering Metrics", RFC 4737,
November 2006.
[RFC5357] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
RFC 5357, October 2008.
12.2. Informative References
[RFC5481] Morton, A. and B. Claise, "Packet Delay Variation
Applicability Statement", RFC 5481, March 2009.
[RFC6248] Morton, A., "RFC 4148 and the IP Performance Metrics
(IPPM) Registry of Metrics Are Obsolete", RFC 6248,
April 2011.
[RFC6703] Morton, A., Ramachandran, G., and G. Maguluri, "Reporting
IP Network Performance Metrics: Different Points of View",
RFC 6703, August 2012.
Author's Address
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/
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