Internet Engineering Task Force (IETF) L. Ciavattone
Request for Comments: 6808 AT&T Labs
Category: Informational R. Geib
ISSN: 2070-1721 Deutsche Telekom
A. Morton
AT&T Labs
M. Wieser
Technical University Darmstadt
November
December 2012
Test Plan and Results Supporting Advancement of
RFC 2679 on the Standards Track
Abstract
This memo provides the supporting test plan and results to advance
RFC 2679 on one-way delay metrics along the Standards Track,
following the process in RFC 6576. Observing that the metric
definitions themselves should be the primary focus rather than the
implementations of metrics, this memo describes the test procedures
to evaluate specific metric requirement clauses to determine if the
requirement has been interpreted and implemented as intended. Two
completely independent implementations have been tested against the
key specifications of RFC 2679. This memo also provides direct input
for development of a revision of RFC 2679.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see 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/rfc6808.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 ....................................................3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5 ......................................5
2. A Definition-Centric Metric Advancement Process . . . . . . . 5 .................5
3. Test Configuration . . . . . . . . . . . . . . . . . . . . . . 5 ..............................................5
4. Error Calibration, RFC 2679 . . . . . . . . . . . . . . . . . 9 .....................................9
4.1. NetProbe Error and Type-P . . . . . . . . . . . . . . . . 10 .................................10
4.2. Perfas+ Error and Type-P . . . . . . . . . . . . . . . . . 12 ..................................12
5. Pre-Determined Predetermined Limits on Equivalence . . . . . . . . . . . . . 13 ............................12
6. Tests to Evaluate RFC 2679 Specifications . . . . . . . . . . 13 ......................13
6.1. One-Way Delay, ADK Sample Comparison: Same- and
Cross-Implementation . . . . . . . . . . . . . . . . . . . 14 Cross- ....13
6.1.1. NetProbe Same-Implementation Results . . . . . . . . . 15 ...............15
6.1.2. Perfas+ Same-Implementation Results . . . . . . . . . 16 ................16
6.1.3. One-Way Delay, Cross-Implementation ADK
Comparison . . 17 .........................................16
6.1.4. Conclusions on the ADK Results for One-Way Delay . . . 17 ...17
6.1.5. Additional Investigations . . . . . . . . . . . . . . 18 ..........................17
6.2. One-Way Delay, Loss Threshold, RFC 2679 . . . . . . . . . 21 ...................20
6.2.1. NetProbe Results for Loss Threshold . . . . . . . . . 22 ................21
6.2.2. Perfas+ Results for Loss Threshold . . . . . . . . . . 22 .................21
6.2.3. Conclusions for Loss Threshold . . . . . . . . . . . . 22 .....................21
6.3. One-Way Delay, First Bit to Last Bit, RFC 2679 . . . . . . 22 ............21
6.3.1. NetProbe and Perfas+ Results for Serialization . . . . 23 .....22
6.3.2. Conclusions for Serialization . . . . . . . . . . . . 24 ......................23
6.4. One-Way Delay, Difference Sample Metric . . . . . . . . . 25 ...................24
6.4.1. NetProbe Results for Differential Delay . . . . . . . 25 ............24
6.4.2. Perfas+ Results for Differential Delay . . . . . . . . 26 .............25
6.4.3. Conclusions for Differential Delay . . . . . . . . . . 26 .................25
6.5. Implementation of Statistics for One-Way Delay . . . . . . 26 ............25
7. Conclusions and RFC 2679 Errata . . . . . . . . . . . . . . . 27 ................................26
8. Security Considerations . . . . . . . . . . . . . . . . . . . 27 ........................................26
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27 ...............................................27
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28 ....................................................27
10.1. Normative References . . . . . . . . . . . . . . . . . . . 28 .....................................27
10.2. Informative References . . . . . . . . . . . . . . . . . . 28 ...................................28
1. Introduction
The IETF IP Performance Metrics (IPPM) working group has considered
how to advance their metrics along the Standards Track since 2001,
with the initial publication of Bradner/Paxson/Mankin's memo
[METRICS-TEST]. The original proposal was to compare the performance
of metric implementations. This was similar to the usual procedures
for advancing protocols, which did not directly apply. It was found
to be difficult to achieve consensus on exactly how to compare
implementations, since there were many legitimate sources of
variation that would emerge in the results despite the best attempts
to keep the network paths equal, and because considerable variation
was allowed in the parameters (and therefore implementation) of each
metric. Flexibility in metric definitions, essential for
customization and broad appeal, made the comparison task quite
difficult.
A renewed work effort investigated ways in which the measurement
variability could be reduced and thereby simplify the problem of
comparison for equivalence.
The consensus process documented in [RFC6576] is that metric
definitions rather than the implementations of metrics should be the
primary focus of evaluation. Equivalent test results are deemed to
be evidence that the metric specifications are clear and unambiguous.
This is now the metric specification equivalent of protocol
interoperability. The [RFC6576] advancement process either produces
confidence that the metric definitions and supporting material are
clearly worded and unambiguous, or it identifies ways in which the
metric definitions should be revised to achieve clarity.
The metric RFC advancement process requires documentation of the
testing and results. [RFC6576] retains the testing requirement of
the original Standards Track advancement process described in
[RFC2026] and [RFC5657], because widespread deployment is
insufficient to determine whether RFCs that define performance
metrics result in consistent implementations.
The process also permits identification of options that were not
implemented, so that they can be removed from the advancing
specification (this is a similar aspect to protocol advancement along
the Standards Track). All errata must also be considered.
This memo's purpose is to implement the advancement process of
[RFC6576] for [RFC2679]. It supplies the documentation that
accompanies the protocol action request submitted to the Area
Director, including description of the test setup, results for each
implementation, evaluation of each metric specification, and
conclusions.
In particular, this memo documents the consensus on the extent of
tolerable errors when assessing equivalence in the results. The IPPM
working group agreed that the test plan and procedures should include
the threshold for determining equivalence, and that this aspect
should be decided in advance of cross-implementation comparisons.
This memo includes procedures for same-implementation comparisons
that may influence the equivalence threshold.
Although the conclusion reached through testing is that [RFC2679]
should be advanced on the Standards Track with modifications, the
revised text of RFC 2679 is not yet ready for review. Therefore,
this memo documents the information to support [RFC2679] advancement,
and the approval of a revision of RFC 2769 is left for future action.
1.1. 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. A Definition-Centric Metric Advancement Process
As a first principle, the process described in Section 3.5 of
[RFC6576] takes the fact that the metric definitions (embodied in the
text of the RFCs) are the objects that require evaluation and
possible revision in order to advance to the next step on the
Standards Track. This memo follows that process.
3. Test Configuration
One metric implementation used was NetProbe version 5.8.5 (an earlier
version is used in AT&T's IP network performance measurement system
and deployed worldwide [WIPM]). NetProbe uses UDP packets of
variable size, and it can produce test streams with Periodic
[RFC3432] or Poisson [RFC2330] sample distributions.
The other metric implementation used was Perfas+ version 3.1,
developed by Deutsche Telekom [Perfas]. Perfas+ uses UDP unicast
packets of variable size (but also supports TCP and multicast). Test
streams with Periodic, Poisson, or uniform sample distributions may
be used.
Figure 1 shows a view of the test path as each implementation's test
flows pass through the Internet and the Layer 2 Tunneling Protocol,
version 3 (L2TPv3) tunnel IDs (1 and 2), based on Figures 2 and 3 of
[RFC6576].
+----+ +----+ +----+ +----+
|Imp1| |Imp1| ,---. |Imp2| |Imp2|
+----+ +----+ / \ +-------+ +----+ +----+
| V100 | V200 / \ | Tunnel| | V300 | V400
| | ( ) | Head | | |
+--------+ +------+ | |__| Router| +----------+
|Ethernet| |Tunnel| |Internet | +---B---+ |Ethernet |
|Switch |--|Head |-| | | |Switch |
+-+--+---+ |Router| | | +---+---+--+--+--+----+
|__| +--A---+ ( ) |Network| |__|
\ / |Emulat.|
U-turn \ / |"netem"| U-turn
V300 to V400 `-+-' +-------+ V100 to V200
Implementations ,---. +--------+
+~~~~~~~~~~~/ \~~~~~~| Remote |
+------->-----F2->-| / \ |->---. |
| +---------+ | Tunnel ( ) | | |
| | transmit|-F1->-| ID 1 ( ) |->. | |
| | Imp 1 | +~~~~~~~~~| |~~~~| | | |
| | receive |-<--+ ( ) | F1 F2 |
| +---------+ | |Internet | | | | |
*-------<-----+ F1 | | | | | |
+---------+ | | +~~~~~~~~~| |~~~~| | | |
| transmit|-* *-| | | |<-* | |
| Imp 2 | | Tunnel ( ) | | |
| receive |-<-F2-| ID 2 \ / |<----* |
+---------+ +~~~~~~~~~~~\ /~~~~~~| Switch |
`-+-' +--------+
Illustrations of a test setup with a bidirectional tunnel. The upper
diagram emphasizes the VLAN connectivity and geographical location.
The lower diagram shows example flows traveling between two
measurement implementations (for simplicity, only two flows are
shown).
Figure 1
The testing employs the Layer 2 Tunneling Protocol, version 3
(L2TPv3) [RFC3931] tunnel between test sites on the Internet. The
tunnel IP and L2TPv3 headers are intended to conceal the test
equipment addresses and ports from hash functions that would tend to
spread different test streams across parallel network resources, with
likely variation in performance as a result.
At each end of the tunnel, one pair of VLANs encapsulated in the
tunnel are looped back so that test traffic is returned to each test
site. Thus, test streams traverse the L2TP tunnel twice, but appear
to be one-way tests from the test equipment point of view.
The network emulator is a host running Fedora 14 Linux [Fedora14]
with IP forwarding enabled and the "netem" Network emulator [netem]
loaded and operating as part of the Fedora Kernel 2.6.35.11.
Connectivity across the netem/Fedora host was accomplished by
bridging Ethernet VLAN interfaces together with "brctl" commands
(e.g., eth1.100 <-> eth2.100). The netem emulator was activated on
one interface (eth1) and only operates on test streams traveling in
one direction. In some tests, independent netem instances operated
separately on each VLAN.
The links between the netem emulator host and router and switch were
found to be 100baseTx-HD (100 Mbps half duplex) when the testing was
complete. Use of half duplex was not intended, but probably added a
small amount of delay variation that could have been avoided in full
duplex mode.
Each individual test was run with common packet rates (1 pps, 10 pps)
Poisson/Periodic distributions, and IP packet sizes of 64, 340, and
500 Bytes. These sizes cover a reasonable range while avoiding
fragmentation and the complexities it causes, thus complying with the
notion of "standard formed packets" described in Section 15 of
[RFC2330].
For these tests, a stream of at least 300 packets were sent from
Source to Destination in each implementation. Periodic streams (as
per [RFC3432]) with 1 second spacing were used, except as noted.
With the L2TPv3 tunnel in use, the metric name for the testing
configured here (with respect to the IP header exposed to Internet
processing) is:
Type-IP-protocol-115-One-way-Delay-<StreamType>-Stream
With (Section 4.2 of [RFC2679]) Metric Parameters:
+ Src, the IP address of a host (12.3.167.16 or 193.159.144.8)
+ Dst, the IP address of a host (193.159.144.8 or 12.3.167.16)
+ T0, a time
+ Tf, a time
+ lambda, a rate in reciprocal seconds
+ Thresh, a maximum waiting time in seconds (see Section 3.8.2 of
[RFC2679] and Section 4.3 of [RFC2679])
Metric Units: A sequence of pairs; the elements of each pair are:
+ T, a time, and
+ dT, either a real number or an undefined number of seconds.
The values of T in the sequence are monotonic increasing. Note that
T would be a valid parameter to Type-P-One-way-Delay and that dT
would be a valid value of Type-P-One-way-Delay.
Also, Section 3.8.4 of [RFC2679] recommends that the path SHOULD be
reported. In this test setup, most of the path details will be
concealed from the implementations by the L2TPv3 tunnels; thus, a
more informative path trace route can be conducted by the routers at
each location.
When NetProbe is used in production, a traceroute is conducted in
parallel with, and at the outset of, measurements.
Perfas+ does not support traceroute.
IPLGW#traceroute 193.159.144.8
Type escape sequence to abort.
Tracing the route to 193.159.144.8
1 12.126.218.245 [AS 7018] 0 msec 0 msec 4 msec
2 cr84.n54ny.ip.att.net (12.123.2.158) [AS 7018] 4 msec 4 msec
cr83.n54ny.ip.att.net (12.123.2.26) [AS 7018] 4 msec
3 cr1.n54ny.ip.att.net (12.122.105.49) [AS 7018] 4 msec
cr2.n54ny.ip.att.net (12.122.115.93) [AS 7018] 0 msec
cr1.n54ny.ip.att.net (12.122.105.49) [AS 7018] 0 msec
4 n54ny02jt.ip.att.net (12.122.80.225) [AS 7018] 4 msec 0 msec
n54ny02jt.ip.att.net (12.122.80.237) [AS 7018] 4 msec
5 192.205.34.182 [AS 7018] 0 msec
192.205.34.150 [AS 7018] 0 msec
192.205.34.182 [AS 7018] 4 msec
6 da-rg12-i.DA.DE.NET.DTAG.DE (62.154.1.30) [AS 3320] 88 msec 88 msec
88 msec
7 217.89.29.62 [AS 3320] 88 msec 88 msec 88 msec
8 217.89.29.55 [AS 3320] 88 msec 88 msec 88 msec
9 * * *
It was only possible to conduct the traceroute for the measured path
on one of the tunnel-head routers (the normal trace facilities of the
measurement systems are confounded by the L2TPv3 tunnel
encapsulation).
4. Error Calibration, RFC 2679
An implementation is required to report on its error calibration in
Section 3.8 of [RFC2679] (also required in Section 4.8 for sample
metrics). Sections 3.6, 3.7, and 3.8 of [RFC2679] give the detailed
formulation of the errors and uncertainties for calibration. In
summary, Section 3.7.1 of [RFC2679] describes the total time-varying
uncertainty as:
Esynch(t)+ Rsource + Rdest
where:
Esynch(t) denotes an upper bound on the magnitude of clock
synchronization uncertainty.
Rsource and Rdest denote the resolution of the source clock and the
destination clock, respectively.
Further, Section 3.7.2 of [RFC2679] describes the total wire-time
uncertainty as:
Hsource + Hdest
referring to the upper bounds on host-time to wire-time for source
and destination, respectively.
Section 3.7.3 of [RFC2679] describes a test with small packets over
an isolated minimal network where the results can be used to estimate
systematic and random components of the sum of the above errors or
uncertainties. In a test with hundreds of singletons, the median is
the systematic error and when the median is subtracted from all
singletons, the remaining variability is the random error.
The test context, or Type-P of the test packets, must also be
reported, as required in Section 3.8 of [RFC2679] and all metrics
defined there. Type-P is defined in Section 13 of [RFC2330] (as are
many terms used below).
4.1. NetProbe Error and Type-P
Type-P for this test was IP-UDP with Best Effort Differentiated
Services Code Point (DSCP). These headers were encapsulated
according to the L2TPv3 specifications [RFC3931]; thus, they may not
influence the treatment received as the packets traversed the
Internet.
In general, NetProbe error is dependent on the specific version and
installation details.
NetProbe operates using host-time above the UDP layer, which is
different from the wire-time preferred in [RFC2330], but it can be
identified as a source of error according to Section 3.7.2 of
[RFC2679].
Accuracy of NetProbe measurements is usually limited by NTP
synchronization performance (which is typically taken as ~+/-1 ms
error or greater), although the installation used in this testing
often exhibits errors much less than typical for NTP. The primary
stratum 1 NTP server is closely located on a sparsely utilized
network management LAN; thus, it avoids many concerns raised in
Section 10 of [RFC2330] (in fact, smooth adjustment, long-term drift
analysis and compensation, and infrequent adjustment all lead to
stability during measurement intervals, the main concern).
The resolution of the reported results is 1 us (us = microsecond) in
the version of NetProbe tested here, which contributes to at least
+/-1 us error.
NetProbe implements a timekeeping sanity check on sending and
receiving time-stamping processes. When a significant process
interruption takes place, individual test packets are flagged as
possibly containing unusual time errors, and they are excluded from
the sample used for all "time" metrics.
We performed a NetProbe calibration of the type described in Section
3.7.3 of [RFC2679], using 64-Byte packets over a cross-connect cable.
The results estimate systematic and random components of the sum of
the Hsource + Hdest errors or uncertainties. In a test with 300
singletons conducted over 30 seconds (periodic sample with 100 ms
spacing), the median is the systematic error and the remaining
variability is the random error. One set of results is tabulated
below:
(Results from the "R" software environment for statistical computing
and graphics - http://www.r-project.org/ )
> summary(XD4CAL)
CAL1 CAL2 CAL3
Min. : 89.0 Min. : 68.00 Min. : 54.00
1st Qu.: 99.0 1st Qu.: 77.00 1st Qu.: 63.00
Median :110.0 Median : 79.00 Median : 65.00
Mean :116.8 Mean : 83.74 Mean : 69.65
3rd Qu.:127.0 3rd Qu.: 88.00 3rd Qu.: 74.00
Max. :205.0 Max. :177.00 Max. :163.00
>
NetProbe Calibration with Cross-Connect Cable, one-way delay values
in microseconds (us)
The median or systematic error can be as high as 110 us, and the
range of the random error is also on the order of 116 us for all
streams.
Also, anticipating the Anderson-Darling K-sample (ADK) [ADK]
comparisons to follow, we corrected the CAL2 values for the
difference between the means of CAL2 and CAL3 (as permitted in
Section 3.2 of [RFC6576]), and found strong support (for the Null
Hypothesis) that the samples are from the same distribution
(resolution of 1 us and alpha equal 0.05 and 0.01)
> XD4CVCAL2 <- XD4CAL$CAL2 - (mean(XD4CAL$CAL2)-mean(XD4CAL$CAL3))
> boxplot(XD4CVCAL2,XD4CAL$CAL3)
> XD4CV2_ADK <- adk.test(XD4CVCAL2, XD4CAL$CAL3)
> XD4CV2_ADK
Anderson-Darling k-sample test.
Number of samples: 2
Sample sizes: 300 300
Total number of values: 600
Number of unique values: 97
Mean of Anderson Darling Criterion: 1
Standard deviation of Anderson Darling Criterion: 0.75896
T = (Anderson-Darling Criterion - mean)/sigma
Null Hypothesis: All samples come from a common population.
t.obs P-value extrapolation
not adj. for ties 0.71734 0.17042 0
adj. for ties -0.39553 0.44589 1
>
using [Rtool] and [Radk].
4.2. Perfas+ Error and Type-P
Perfas+ is configured to use GPS synchronization and uses NTP
synchronization as a fall-back or default. GPS synchronization
worked throughout this test with the exception of the calibration
stated here (one implementation was NTP synchronized only). The time
stamp accuracy typically is 0.1 ms.
The resolution of the results reported by Perfas+ is 1 us (us =
microsecond) in the version tested here, which contributes to at
least +/-1 us error.
Port 5001 5002 5003
Min. -227 -226 294
Median -169 -167 323
Mean -159 -157 335
Max. 6 -52 376
s 102 102 93
Perfas+ Calibration with Cross-Connect Cable, one-way delay values in
microseconds (us)
The median or systematic error can be as high as 323 us, and the
range of the random error is also less than 232 us for all streams.
5. Pre-Determined Predetermined Limits on Equivalence
This section provides the numerical limits on comparisons between
implementations, in order to declare that the results are equivalent
and therefore, the tested specification is clear. These limits have
their basis in Section 3.1 of [RFC6576] and the Appendix of
[RFC2330], with additional limits representing IP Performance Metrics
(IPPM) consensus prior to publication of results.
A key point is that the allowable errors, corrections, and confidence
levels only need to be sufficient to detect misinterpretation of the
tested specification resulting in diverging implementations.
Also, the allowable error must be sufficient to compensate for
measured path differences. It was simply not possible to measure
fully identical paths in the VLAN-loopback test configuration used,
and this practical compromise must be taken into account.
For Anderson-Darling K-sample (ADK) comparisons, the required
confidence factor for the cross-implementation comparisons SHALL be
the smallest of:
o 0.95 confidence factor at 1 ms resolution, or
o the smallest confidence factor (in combination with resolution) of
the two same-implementation comparisons for the same test
conditions.
A constant time accuracy error of as much as +/-0.5 ms MAY be removed
from one implementation's distributions (all singletons) before the
ADK comparison is conducted.
A constant propagation delay error (due to use of different sub-nets
between the switch and measurement devices at each location) of as
much as +2 ms MAY be removed from one implementation's distributions
(all singletons) before the ADK comparison is conducted.
For comparisons involving the mean of a sample or other central
statistics, the limits on both the time accuracy error and the
propagation delay error constants given above also apply.
6. Tests to Evaluate RFC 2679 Specifications
This section describes some results from real-world (cross-Internet)
tests with measurement devices implementing IPPM metrics and a
network emulator to create relevant conditions, to determine whether
the metric definitions were interpreted consistently by implementors.
The procedures are slightly modified from the original procedures
contained in Appendix A.1 of [RFC6576]. The modifications include
the use of the mean statistic for comparisons.
Note that there are only five instances of the requirement term
"MUST" in [RFC2679] outside of the boilerplate and [RFC2119]
reference.
6.1. One-Way Delay, ADK Sample Comparison: Same- and Cross-
Implementation
This test determines if implementations produce results that appear
to come from a common delay distribution, as an overall evaluation of
Section 4 of [RFC2679], "A Definition for Samples of One-way Delay".
Same-implementation comparison results help to set the threshold of
equivalence that will be applied to cross-implementation comparisons.
This test is intended to evaluate measurements in Sections 3 and 4 of
[RFC2679].
By testing the extent to which the distributions of one-way delay
singletons from two implementations of [RFC2679] appear to be from
the same distribution, we economize on comparisons, because comparing
a set of individual summary statistics (as defined in Section 5 of
[RFC2679]) would require another set of individual evaluations of
equivalence. Instead, we can simply check which statistics were
implemented, and report on those facts.
1. Configure an L2TPv3 path between test sites, and each pair of
measurement devices to operate tests in their designated pair of
VLANs.
2. Measure a sample of one-way delay singletons with two or more
implementations, using identical options and network emulator
settings (if used).
3. Measure a sample of one-way delay singletons with *four*
instances of the *same* implementations, using identical options,
noting that connectivity differences SHOULD be the same as for
the cross-implementation testing.
4. Apply the ADK comparison procedures (see Appendices A and B of
[RFC6576]) and determine the resolution and confidence factor for
distribution equivalence of each same-implementation comparison
and each cross-implementation comparison.
5. Take the coarsest resolution and confidence factor for
distribution equivalence from the same-implementation pairs, or
the limit defined in Section 5 above, as a limit on the
equivalence threshold for these experimental conditions.
6. Apply constant correction factors to all singletons of the sample
distributions, as described and limited in Section 5 above.
7. Compare the cross-implementation ADK performance with the
equivalence threshold determined in step 5 to determine if
equivalence can be declared.
The common parameters used for tests in this section are:
o IP header + payload = 64 octets
o Periodic sampling at 1 packet per second
o Test duration = 300 seconds (March 29, 2011)
The netem emulator was set for 100 ms average delay, with uniform
delay variation of +/-50 ms. In this experiment, the netem emulator
was configured to operate independently on each VLAN; thus, the
emulator itself is a potential source of error when comparing streams
that traverse the test path in different directions.
In the result analysis of this section:
o All comparisons used 1 microsecond resolution.
o No correction factors were applied.
o The 0.95 confidence factor (1.960 for paired stream comparison)
was used.
6.1.1. NetProbe Same-Implementation Results
A single same-implementation comparison fails the ADK criterion (s1
<-> sB). We note that these streams traversed the test path in
opposite directions, making the live network factors a possibility to
explain the difference.
All other pair comparisons pass the ADK criterion.
+------------------------------------------------------+
| | | | |
| ti.obs (P) | s1 | s2 | sA |
| | | | |
.............|.............|.............|.............|
| | | | |
| s2 | 0.25 (0.28) | | |
| | | | |
...........................|.............|.............|
| | | | |
| sA | 0.60 (0.19) |-0.80 (0.57) | |
| | | | |
...........................|.............|.............|
| | | | |
| sB | 2.64 (0.03) | 0.07 (0.31) |-0.52 (0.48) |
| | | | |
+------------+-------------+-------------+-------------+
NetProbe ADK results for same-implementation
6.1.2. Perfas+ Same-Implementation Results
All pair comparisons pass the ADK criterion.
+------------------------------------------------------+
| | | | |
| ti.obs (P) | p1 | p2 | p3 |
| | | | |
.............|.............|.............|.............|
| | | | |
| p2 | 0.06 (0.32) | | |
| | | | |
.........................................|.............|
| | | | |
| p3 | 1.09 (0.12) | 0.37 (0.24) | |
| | | | |
...........................|.............|.............|
| | | | |
| p4 |-0.81 (0.57) |-0.13 (0.37) | 1.36 (0.09) |
| | | | |
+------------+-------------+-------------+-------------+
Perfas+ ADK results for same-implementation
6.1.3. One-Way Delay, Cross-Implementation ADK Comparison
The cross-implementation results are compared using a combined ADK
analysis [Radk], where all NetProbe results are compared with all
Perfas+ results after testing that the combined same-implementation
results pass the ADK criterion.
When 4 (same) samples are compared, the ADK criterion for 0.95
confidence is 1.915, and when all 8 (cross) samples are compared it
is 1.85.
Combination of Anderson-Darling K-Sample Tests.
Sample sizes within each data set:
Data set 1 : 299 297 298 300 (NetProbe)
Data set 2 : 300 300 298 300 (Perfas+)
Total sample size per data set: 1194 1198
Number of unique values per data set: 1188 1192
...
Null Hypothesis:
All samples within a data set come from a common distribution.
The common distribution may change between data sets.
NetProbe ti.obs P-value extrapolation
not adj. for ties 0.64999 0.21355 0
adj. for ties 0.64833 0.21392 0
Perfas+
not adj. for ties 0.55968 0.23442 0
adj. for ties 0.55840 0.23473 0
Combined Anderson-Darling Criterion:
tc.obs P-value extrapolation
not adj. for ties 0.85537 0.17967 0
adj. for ties 0.85329 0.18010 0
The combined same-implementation samples and the combined cross-
implementation comparison all pass the ADK criterion at P>=0.18 and
support the Null Hypothesis (both data sets come from a common
distribution).
We also see that the paired ADK comparisons are rather critical.
Although the NetProbe s1-sB comparison failed, the combined data set
from four streams passed the ADK criterion easily.
6.1.4. Conclusions on the ADK Results for One-Way Delay
Similar testing was repeated many times in the months of March and
April 2011. There were many experiments where a single test stream
from NetProbe or Perfas+ proved to be different from the others in
paired comparisons (even same-implementation comparisons). When the
outlier stream was removed from the comparison, the remaining streams
passed combined ADK criterion. Also, the application of correction
factors resulted in higher comparison success.
We conclude that the two implementations are capable of producing
equivalent one-way delay distributions based on their interpretation
of [RFC2679].
6.1.5. Additional Investigations
On the final day of testing, we performed a series of measurements to
evaluate the amount of emulated delay variation necessary to achieve
successful ADK comparisons. The need for correction factors (as
permitted by Section 5) and the size of the measurement sample
(obtained as sub-sets of the complete measurement sample) were also
evaluated.
The common parameters used for tests in this section are:
o IP header + payload = 64 octets
o Periodic sampling at 1 packet per second
o Test duration = 300 seconds at each delay variation setting, for a
total of 1200 seconds (May 2, 2011 at 1720 UTC)
The netem emulator was set for 100 ms average delay, with (emulated)
uniform delay variation of:
o +/-7.5 ms
o +/-5.0 ms
o +/-2.5 ms
o 0 ms
In this experiment, the netem emulator was configured to operate
independently on each VLAN; thus, the emulator itself is a potential
source of error when comparing streams that traverse the test path in
different directions.
In the result analysis of this section:
o All comparisons used 1 microsecond resolution.
o Correction factors *were* applied as noted (under column heading
"mean adj"). The difference between each sample mean and the
lowest mean of the NetProbe or Perfas+ stream samples was
subtracted from all values in the sample. ("raw" indicates no
correction factors were used.) All correction factors applied met
the limits described in Section 5.
o The 0.95 confidence factor (1.960 for paired stream comparison)
was used.
When 8 (cross) samples are compared, the ADK criterion for 0.95
confidence is 1.85. The Combined ADK test statistic ("TC observed")
must be less than 1.85 to accept the Null Hypothesis (all samples in
the data set are from a common distribution).
Emulated Delay Sub-Sample size
Variation 0ms
adk.combined (all) 300 values 75 values
Adj. for ties raw mean adj raw mean adj
TC observed 226.6563 67.51559 54.01359 21.56513
P-value 0 0 0 0
Mean std dev (all),us 719 635
Mean diff of means,us 649 0 606 0
Variation +/- 2.5ms
adk.combined (all) 300 values 75 values
Adj. for ties raw mean adj raw mean adj
TC observed 14.50436 -1.60196 3.15935 -1.72104
P-value 0 0.873 0.00799 0.89038
Mean std dev (all),us 1655 1702
Mean diff of means,us 471 0 513 0
Variation +/- 5ms
adk.combined (all) 300 values 75 values
Adj. for ties raw mean adj raw mean adj
TC observed 8.29921 -1.28927 0.37878 -1.81881
P-value 0 0.81601 0.29984 0.90305
Mean std dev (all),us 3023 2991
Mean diff of means,us 582 0 513 0
Variation +/- 7.5ms
adk.combined (all) 300 values 75 values
Adj. for ties raw mean adj raw mean adj
TC observed 2.53759 -0.72985 0.29241 -1.15840
P-value 0.01950 0.66942 0.32585 0.78686
Mean std dev (all),us 4449 4506
Mean diff of means,us 426 0 856 0
From the table above, we conclude the following:
1. None of the raw or mean adjusted results pass the ADK criterion
with 0 ms emulated delay variation. Use of the 75 value sub-
sample yielded the same conclusion. (We note the same results
when comparing same-implementation samples for both NetProbe and
Perfas+.)
2. When the smallest emulated delay variation was inserted (+/-2.5
ms), the mean adjusted samples pass the ADK criterion and the
high P-value supports the result. The raw results do not pass.
3. At higher values of emulated delay variation (+/-5.0 ms and
+/-7.5 ms), again the mean adjusted values pass ADK. We also see
that the 75-value sub-sample passed the ADK in both raw and mean
adjusted cases. This indicates that sample size may have played
a role in our results, as noted in the Appendix of [RFC2330] for
Goodness-of-Fit testing.
We note that 150 value sub-samples were also evaluated, with ADK
conclusions that followed the results for 300 values. Also, same-
implementation analysis was conducted with results similar to the
above, except that more of the "raw" or uncorrected samples passed
the ADK criterion.
6.2. One-Way Delay, Loss Threshold, RFC 2679
This test determines if implementations use the same configured
maximum waiting time delay from one measurement to another under
different delay conditions, and correctly declare packets arriving in
excess of the waiting time threshold as lost.
See the requirements of Section 3.5 of [RFC2679], third bullet point,
and also Section 3.8.2 of [RFC2679].
1. configure an L2TPv3 path between test sites, and each pair of
measurement devices to operate tests in their designated pair of
VLANs.
2. configure the network emulator to add 1.0 sec. one-way constant
delay in one direction of transmission.
3. measure (average) one-way delay with two or more implementations,
using identical waiting time thresholds (Thresh) for loss set at
3 seconds.
4. configure the network emulator to add 3 sec. one-way constant
delay in one direction of transmission equivalent to 2 seconds of
additional one-way delay (or change the path delay while test is
in progress, when there are sufficient packets at the first delay
setting).
5. repeat/continue measurements.
6. observe that the increase measured in step 5 caused all packets
with 2 sec. additional delay to be declared lost, and that all
packets that arrive successfully in step 3 are assigned a valid
one-way delay.
The common parameters used for tests in this section are:
o IP header + payload = 64 octets
o Poisson sampling at lambda = 1 packet per second
o Test duration = 900 seconds total (March 21, 2011)
The netem emulator was set to add constant delays as specified in the
procedure above.
6.2.1. NetProbe Results for Loss Threshold
In NetProbe, the Loss Threshold is implemented uniformly over all
packets as a post-processing routine. With the Loss Threshold set at
3 seconds, all packets with one-way delay >3 seconds are marked
"Lost" and included in the Lost Packet list with their transmission
time (as required in Section 3.3 of [RFC2680]). This resulted in 342
packets designated as lost in one of the test streams (with average
delay = 3.091 sec.).
6.2.2. Perfas+ Results for Loss Threshold
Perfas+ uses a fixed Loss Threshold that was not adjustable during
this study. The Loss Threshold is approximately one minute, and
emulation of a delay of this size was not attempted. However, it is
possible to implement any delay threshold desired with a post-
processing routine and subsequent analysis. Using this method, 195
packets would be declared lost (with average delay = 3.091 sec.).
6.2.3. Conclusions for Loss Threshold
Both implementations assume that any constant delay value desired can
be used as the Loss Threshold, since all delays are stored as a pair
<Time, Delay> as required in [RFC2679]. This is a simple way to
enforce the constant loss threshold envisioned in [RFC2679] (see
specific section references above). We take the position that the
assumption of post-processing is compliant and that the text of the
RFC should be revised slightly to include this point.
6.3. One-Way Delay, First Bit to Last Bit, RFC 2679
This test determines if implementations register the same relative
change in delay from one packet size to another, indicating that the
first-to-last time-stamping convention has been followed. This test
tends to cancel the sources of error that may be present in an
implementation.
See the requirements of Section 3.7.2 of [RFC2679], and Section 10.2
of [RFC2330].
1. configure an L2TPv3 path between test sites, and each pair of
measurement devices to operate tests in their designated pair of
VLANs, and ideally including a low-speed link (it was not
possible to change the link configuration during testing, so the
lowest speed link present was the basis for serialization time
comparisons).
2. measure (average) one-way delay with two or more implementations,
using identical options and equal size small packets (64-octet IP
header and payload).
3. maintain the same path with additional emulated 100 ms one-way
delay.
4. measure (average) one-way delay with two or more implementations,
using identical options and equal size large packets (500 octet
IP header and payload).
5. observe that the increase measured between steps 2 and 4 is
equivalent to the increase in ms expected due to the larger
serialization time for each implementation. Most of the
measurement errors in each system should cancel, if they are
stationary.
The common parameters used for tests in this section are:
o IP header + payload = 64 octets
o Periodic sampling at l packet per second
o Test duration = 300 seconds total (April 12)
The netem emulator was set to add constant 100 ms delay.
6.3.1. NetProbe and Perfas+ Results for Serialization
When the IP header + payload size was increased from 64 octets to 500
octets, there was a delay increase observed.
Mean Delays in us
NetProbe
Payload s1 s2 sA sB
500 190893 191179 190892 190971
64 189642 189785 189747 189467
Diff 1251 1394 1145 1505
Perfas
Payload p1 p2 p3 p4
500 190908 190911 191126 190709
64 189706 189752 189763 190220
Diff 1202 1159 1363 489
Serialization tests, all values in microseconds
The typical delay increase when the larger packets were used was 1.1
to 1.5 ms (with one outlier). The typical measurements indicate that
a link with approximately 3 Mbit/s capacity is present on the path.
Through investigation of the facilities involved, it was determined
that the lowest speed link was approximately 45 Mbit/s, and therefore
the estimated difference should be about 0.077 ms. The observed
differences are much higher.
The unexpected large delay difference was also the outcome when
testing serialization times in a lab environment, using the NIST Net
Emulator and NetProbe [ADV-METRICS].
6.3.2. Conclusions for Serialization
Since it was not possible to confirm the estimated serialization time
increases in field tests, we resort to examination of the
implementations to determine compliance.
NetProbe performs all time stamping above the IP layer, accepting
that some compromises must be made to achieve extreme portability and
measurement scale. Therefore, the first-to-last bit convention is
supported because the serialization time is included in the one-way
delay measurement, enabling comparison with other implementations.
Perfas+ is optimized for its purpose and performs all time stamping
close to the interface hardware. The first-to-last bit convention is
supported because the serialization time is included in the one-way
delay measurement, enabling comparison with other implementations.
6.4. One-Way Delay, Difference Sample Metric
This test determines if implementations register the same relative
increase in delay from one measurement to another under different
delay conditions. This test tends to cancel the sources of error
that may be present in an implementation.
This test is intended to evaluate measurements in Sections 3 and 4 of
[RFC2679].
1. configure an L2TPv3 path between test sites, and each pair of
measurement devices to operate tests in their designated pair of
VLANs.
2. measure (average) one-way delay with two or more implementations,
using identical options.
3. configure the path with X+Y ms one-way delay.
4. repeat measurements.
5. observe that the (average) increase measured in steps 2 and 4 is
~Y ms for each implementation. Most of the measurement errors in
each system should cancel, if they are stationary.
In this test, X = 1000 ms and Y = 1000 ms.
The common parameters used for tests in this section are:
o IP header + payload = 64 octets
o Poisson sampling at lambda = 1 packet per second
o Test duration = 900 seconds total (March 21, 2011)
The netem emulator was set to add constant delays as specified in the
procedure above.
6.4.1. NetProbe Results for Differential Delay
Average pre-increase delay, microseconds 1089868.0
Average post 1 s additional, microseconds 2089686.0
Difference (should be ~= Y = 1 s) 999818.0
Average delays before/after 1 second increase
The NetProbe implementation observed a 1 second increase with a 182
microsecond error (assuming that the netem emulated delay difference
is exact).
We note that this differential delay test has been run under lab
conditions and published in prior work [ADV-METRICS]. The error was
6 microseconds.
6.4.2. Perfas+ Results for Differential Delay
Average pre-increase delay, microseconds 1089794.0
Average post 1 s additional, microseconds 2089801.0
Difference (should be ~= Y = 1 s) 1000007.0
Average delays before/after 1 second increase
The Perfas+ implementation observed a 1 second increase with a 7
microsecond error.
6.4.3. Conclusions for Differential Delay
Again, the live network conditions appear to have influenced the
results, but both implementations measured the same delay increase
within their calibration accuracy.
6.5. Implementation of Statistics for One-Way Delay
The ADK tests the extent to which the sample distributions of one-way
delay singletons from two implementations of [RFC2679] appear to be
from the same overall distribution. By testing this way, we
economize on the number of comparisons, because comparing a set of
individual summary statistics (as defined in Section 5 of [RFC2679])
would require another set of individual evaluations of equivalence.
Instead, we can simply check which statistics were implemented, and
report on those facts, noting that Section 5 of [RFC2679] does not
specify the calculations exactly, and gives only some illustrative
examples.
NetProbe Perfas+
5.1. Type-P-One-way-Delay-Percentile yes no
5.2. Type-P-One-way-Delay-Median yes no
5.3. Type-P-One-way-Delay-Minimum yes yes
5.4. Type-P-One-way-Delay-Inverse-Percentile no no
Implementation of Section 5 Statistics
Only the Type-P-One-way-Delay-Inverse-Percentile has been ignored in
both implementations, so it is a candidate for removal or deprecation
in a revision of RFC 2679 (this small discrepancy does not affect
candidacy for advancement).
7. Conclusions and RFC 2679 Errata
The conclusions throughout Section 6 support the advancement of
[RFC2679] to the next step of the Standards Track, because its
requirements are deemed to be clear and unambiguous based on
evaluation of the test results for two implementations. The results
indicate that these implementations produced statistically equivalent
results under network conditions that were configured to be as close
to identical as possible.
Sections 6.2.3 and 6.5 indicate areas where minor revisions are
warranted in RFC 2679. The IETF has reached consensus on guidance
for reporting metrics in [RFC6703], and this memo should be
referenced in the revision to RFC 2679 to incorporate recent
experience where appropriate.
We note that there is currently one erratum with status "Held for
Document Update" for [RFC2679], and it appears this minor revision
and additional text should be incorporated in RFC2679bis. a revision of RFC 2679.
The authors that revise [RFC2679] should review all errata filed at
the time the document is being written. They should not rely upon
this document to indicate all relevant errata updates.
8. Security Considerations
The security considerations that apply to any active measurement of
live networks are relevant here as well. See [RFC4656] and
[RFC5357].
9. Acknowledgements
The authors thank Lars Eggert for his continued encouragement to
advance the IPPM metrics during his tenure as AD Advisor.
Nicole Kowalski supplied the needed CPE router for the NetProbe side
of the test setup, and graciously managed her testing in spite of
issues caused by dual-use of the router. Thanks Nicole!
The "NetProbe Team" also acknowledges many useful discussions with
Ganga Maguluri.
10. References
10.1. Normative References
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[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.
[RFC3432] Raisanen, V., Grotefeld, G., and A. Morton, "Network
performance measurement with periodic streams", RFC 3432,
November 2002.
[RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
Zekauskas, "A One-way Active Measurement Protocol
(OWAMP)", RFC 4656, September 2006.
[RFC5357] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
RFC 5357, October 2008.
[RFC5657] Dusseault, L. and R. Sparks, "Guidance on Interoperation
and Implementation Reports for Advancement to Draft
Standard", BCP 9, RFC 5657, September 2009.
[RFC6576] Geib, R., Morton, A., Fardid, R., and A. Steinmitz, "IP
Performance Metrics (IPPM) Standard Advancement Testing",
BCP 176, RFC 6576, March 2012.
[RFC6703] Morton, A., Ramachandran, G., and G. Maguluri, "Reporting
IP Network Performance Metrics: Different Points of View",
RFC 6703, August 2012.
10.2. Informative References
[ADK] Scholz, F. and M. Stephens, "K-sample Anderson-Darling
Tests of fit, for continuous and discrete cases",
University of Washington, Technical Report No. 81,
May 1986.
[ADV-METRICS]
Morton, A., "Lab Test Results for Advancing Metrics on the
Standards Track", Work in Progress, October 2010.
[Fedora14]
Fedora Project, "Fedora Project Home Page", 2012,
<http://fedoraproject.org/>.
[METRICS-TEST]
Bradner, S. and V. Paxson, "Advancement of metrics
specifications on the IETF Standards Track", Work
in Progress, August 2007.
[Perfas] Heidemann, C., "Qualitaet in IP-Netzen Messverfahren",
published by ITG Fachgruppe, 2nd meeting 5.2.3 (NGN),
November 2001, <http://www.itg523.de/oeffentlich/01nov/
Heidemann_QOS_Messverfahren.pdf>.
[RFC3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.
[Radk] Scholz, F., "adk: Anderson-Darling K-Sample Test and
Combinations of Such Tests. R package version 1.0.", 2008.
[Rtool] R Development Core Team, "R: A language and environment
for statistical computing. R Foundation for Statistical
Computing, Vienna, Austria. ISBN 3-900051-07-0", 2011,
<http://www.R-project.org/>.
[WIPM] AT&T, "AT&T Global IP Network", 2012,
<http://ipnetwork.bgtmo.ip.att.net/pws/index.html>.
[netem] The Linux Foundation, "netem", 2009, <http://
www.linuxfoundation.org/collaborate/workgroups/networking/
netem>.
Authors' Addresses
Len Ciavattone
AT&T Labs
200 Laurel Avenue South
Middletown, NJ 07748
USA
Phone: +1 732 420 1239
EMail: lencia@att.com
Ruediger Geib
Deutsche Telekom
Heinrich Hertz Str. 3-7
Darmstadt, 64295
Germany
Phone: +49 6151 58 12747
EMail: Ruediger.Geib@telekom.de
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/
Matthias Wieser
Technical University Darmstadt
Darmstadt,
Germany
EMail: matthias_michael.wieser@stud.tu-darmstadt.de