Round-Trip Packet Loss Metrics
RFC 6673

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
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   publication of this document.  Please review these documents
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   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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