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Scheme to Measure One-way Delay Variation withDetection and Removal of Clock Skew
Makoto Aoki, Eiji Oki, and Roberto Rojas-Cessa
AbstractβOne-Way Delay Variation (πππ·π ) has becomeincreasingly of interest to evaluate network state and servicequality, especially for real-time and streaming services such asVoIP and video. Measurement of these parameters needs to beperformed with the layout infrastructure. Many schemes forOWD measurements require clock synchronization at the sourceand destination through Global-Positioning System (GPS) orthe Network Time Protocol (NTP). In clock-synchronized ap-proaches, the accuracy of the measurement ofπππ·π dependson the achieved accuracy of clock synchronization. GPS provideshigh-accuracy clock synchronization. However, the deployment ofGPS on legacy network equipment might be slow and costly. Thispaper proposes a method for measuringπππ·π without recur-ring to clock synchronization. However, clock skew may affectthe measurement ofπππ·π . The proposed approach is based onthe measurement of Inter-Packet Delay (πΌππ·) and Accumulatedπππ·π (AOWDV). This paper shows the performance of theproposed scheme via simulation and through experimentationin a VoIP network. The presented simulation and experimentalresults indicate that clock skew can be efficiently measured andremoved and that πππ·π can be measured without requiringclock synchronization.
Index Termsβdelay measurement, clock skew, clock rateadjustment, one-way delay, inter-packet delay.
I. INTRODUCTION
Real-time applications over the Internet, such as Voice-over-IP (VoIP) and video streaming services, use One-Way DelayVariation (πππ·π ) information to monitor and maintainQuality of Service (QoS) or quality perception by the user.OWD is define as the delay that a data packet suffers in thetrip from source to destination. If the source and destinationuse the same clock, this is equivalent to the difference betweenthe arrival time at the destination minus the departure time atthe source. πππ·π is define as the difference of OWDsexperienced by two packets of the same length [3]. Accordingto [3], the definitio of πππ·π between any two packets πand π, where π β= π, is the difference of their OWDs.
πππ·π (π, π) = πππ·(π)βπππ·(π). (1)
However, the source and destination use different clocks inthe general case. Therefore, a large effort has been made in therecent years to provide clock synchronization between remotehosts.
Makoto Aoki is with Cyber Creative Institute, Tokyo, Japan, Email:[email protected]. He is also a Visiting Researcher at The Universityof Electro-Communications, Tokyo, Japan, Email:[email protected]
Eiji Oki is with The University of Electro-Communications, Tokyo, Japan,Email:[email protected].
Roberto Rojas-Cessa is with the Dept. of Electrical and Computer Engineer-ing, New Jersey Institute of Technology, Newark, NJ 07102, USA.Email: [email protected].
To describe clock synchronization, the following definitionare used in this paper. Clock A, πΆπ΄ has time π‘π΄ at the sametime that Clock B, πΆπ΅ , indicates π‘π΅ , where π‘π΄ and π‘π΅ are theindicated times or time stamps at the same time. The offsettime is ππ΄,π΅ = π‘π΄ β π‘π΅ . The clocks A and B advance at rateππ΄ = π(π‘π΄)
ππ‘ and ππ΅ = π(π‘π΅)ππ‘ , respectively. The clock drift is
the rate of change of the clock rate. However, for simplicity,this clock drift is considered to be zero. Then the clock skewusing πΆπ΄ as reference is define as ππ΄ = ππ΅
ππ΄. In similar way
the clock skew referenced at πΆπ΅ is ππ΅ = ππ΄ππ΅
.Clock synchronization is the adjustment of time on the
local clock in relation to the other endβs clock to make themeasurements in reference to a single time scale, using as thereference the clock at the source, destination, or any other. Inother words, clock synchronization is the adjustment of theclocksβ offsets.
There are two broad groups of measurements approaches,active and passive. Active measurements inject probing pack-ets into the network, specificall between source and destina-tion hosts, with the purpose to measure the network parameterof interest [6], [7], [8]. In several of the existing active-measurement schemes, Global Positioning System (GPS), Net-work Time Protocol (NTP) [4], or Precision Time Protocol(PTP), also known as IEEE 1588 standard [5], are used withthe objective to perform clock synchronization. However, theintroduction of GPS increases equipment costs. Furthermore,it is difficul to be deployed on legacy equipment.
NTP and PTP require to measure Round-Trip Time (RTT)to perform clock synchronization, and even more restrictive,they require each way (from source to destination and fromdestination to source) has equal OWD to function. In general,each way has a different OWD unless a network has beenengineered with this feature in mind, and it is difficul to expectthat from a network such as the Internet.
An active measurement scheme to measure OWD that doesnot require clock synchronization was proposed [9]. Thisscheme needs to select cyclic-paths properly to calculate eachone-way delay. However, it is based on the measurement ofround-trip delays.
A passive measurement scheme uses the fl wing traffi inthe network, usually user traffic to perform the measurementprocess. Passive measurement is attractive because it does notneed to inject additional traffi to the network and becausemeasurement is directly performed on the user traffi (the ef-fect that the user actually experiences). However, the accuracyof this approach depends on the fl wing traffi at a giventime, which is beyond the control of the network operator. Apassive-measurement scheme to measure OWD was recentlyproposed [10]. This scheme, yet passive, it is based onGPS and NTP for clock synchronization. Another scheme for
measurement of OWDwas proposed [11]. However, it is basedon the assumption that PTP achieves clock synchronization.
Recently, a scheme to measure OWD clock skew removalwas proposed [15]. This scheme uses the real time protocol(RTP) [1] and the Real-Time Control Protocol (RTCP) [2]to create fi ed length and fi ed inter-packet delay, πΌππ·, toperform OWD measurements. πΌππ· is the time between twoconsecutive packet departures at the source and measured atthe destination. This approach is based on a simple modelto estimate clock skew, showing a different perspective fromthe existing complex approaches to the estimation of clockskew, where for example, linear programming is used [16].However, the efficien y of clock skew removal depends onexperiencing low queuing delays during the measurement.However, this depends on the user traffi rather than in themeasurement method. The case where the network remainswith long queuing delays is not considered in the scheme.
This paper proposes a scheme to measure πππ·π withoutrequiring to clock synchronization between the source anddestination hosts. The proposed approach, measured πΌππ·sand the combined and accumulated πππ·π , called AOWDV,which is the sum of continuing πΌππ·s. However, as clockskew is included in the πΌππ· measurements, this paper uses amechanism to estimate and remove clock skew using AOWDV.πππ·π is obtained from AOWDVs after clock skew is re-moved. In this process, the proposed method estimates theclock-rate difference between the source and the destination.Evaluation of the proposed approach is presented with bothsimulation and analysis of real network data. The evaluationresults show that the proposed method can effectively estimateπππ·π as derived from πΌππ·.
The remainder of this paper is organized as follows. SectionII introduces the proposed mechanism to measure πππ·π .This section also describes a methodology detection andadjustment of clock skew on the performed measurements.Section III presents a simulation study of the proposedπππ·π scheme. Section IV presents the experimental resultsof πππ·π evaluations on a VoIP network. Section V presentsour conclusions.
II. SCHEME TO MEASURE ONE-WAY DELAY VARIATION
The proposed mechanism is based on the measurementof the inter-packet delay πΌππ· at the destination host. Theadopted method, uses the smallest experienced πΌππ·, calledπ·πππ, to evaluate the contribution of every new πΌππ· mea-surement as packets are sent from source to destination.For the evaluation of the clock skew, the source explicitlysends packets, where the dispatching time between consecutivepackets at the source host is conveyed to the destination.The packets have fi ed length to comply with the πππ·πdefinitio [3]. Every packet has an identificatio number. Forthe description of the proposed mechanism, π is the firs packettransmitted from the source to perform measurement. Then,πΌππ·(π+ 1) is measured between packets π and π+ 1. Figure1 shows an example of the delay times between packets π andπ+1 in the transmission of multiple packets. In this example,packet π departs at time π (π) and packet π+1 at time π (π+1).The arrival times of packets π and π + 1 are then π (π) andπ (π+1). For simplicity, it assumes that packets are transmitted
in order of sequence. If out-of-order packets were received atthe destination, these packets would be discarded. In that case,ππ and πΌππ· between π+π and π+π (π > π+1) of correctlyreceived packets are measured.
1
TS(i+1)
OWD(i+1)
Source
Destination
T(i+1)
R(i) R(i+1)
Packet i Packet i+1
OWD(i)IPD(i+1)
T(i)
Fig. 1. Variables to measure πππ·π based on πΌππ·.
The inter-packet delay at the source, ππ(π+ 1), is defineas
ππ(π+ 1) = π (π+ 1)β π (π), (2)
This value is set by the source. The inter-packet delaymeasured at the destination, πΌππ·(π+ 1), is:
πΌππ·(π+ 1) = π (π+ 1)βπ (π) (3)
This value is ππ plus the variable OWD that a packetundergoes, and it is measured in reference to the clockat the destination. The value of OWD would have severalparameters, or πππ· = π‘π + π‘π‘ + π‘π, where π‘π, π‘π‘, and π‘πare the total propagation, transmission, and queuing delaysaccumulated in the path from source to destination hosts.These variables are susceptible to variations caused by paths ortraffi variations during the measurement period. This paper,rather than focusing on the measurement of these specifiparameters, is centered on the measurement of the variationsof OWD,πππ·π .
The measurement of πππ·π is based on the measurementof πΌππ·. As shown in Figure 1, πΌππ·(π + 1) is measuredbetween packets π and π+1, which in this case are consecutivepackets, and the minimum πΌππ· experienced among all thosemeasured, denoted as π·πππ, including that of packet π + 1.The consideration of consecutive packets is not required aslong as ππ between two packets is conveyed to the destinationhost (there are several options for the implementation of thisrequirement [11], however, they are left out of the scope ofthis paper).
Here, π·πππ uses as reference value the smallest OWDcollected during the measurement period. Figure 2 describesthe relationship of πππ·π and π·πππ.
The experienced πππ·π by packet π + 1 in reference toπ·πππ can be re-written as
πππ·π (π+ 1) = πππ·(π+ 1)βπ·πππ (4)
On the other hand, the inter-packet delay at the source canbe described in function of πΌππ·:
ππ(π+ 1) +πππ·(π+ 1) = πππ·(π) + πΌππ·(π+ 1) (5)
1
OWD(i+1)
Delay
Probability distributionof OWDV(i+1)
Dminof
OWDV(i+1)
Prob
abili
ty
Fig. 2. Relationship of π·πππ and πππ·π (π+ 1).
Using (5) and for a long measured period, πππ·(π+π) ofthe OWD for the last received packet π, can be expressed as
πππ·(π+π) =πβ
π=1
(πΌππ·(π+π)βππ(π+π))+πππ·(π) (6)
where the sum of collected (πΌππ·(π + π) β ππ(π + π) iscalled AOWDVor:
π΄πππ·π (π+ π) =
πβπ=1
(πΌππ·(π+ π)β ππ(π+ π)). (7)
Using (7) πππ·(π+ π) can be described as:
πππ·(π+ π) = π΄πππ·π (π+ π) +πππ·(π). (8)
π·πππ in (4) can be expressed in terms of (6) as
π·πππ = min{πππ·(π+ π)} π = 1, 2...π= min{π΄πππ·π (π+ 1), . . . ,
π΄πππ·π (π+ π)}+πππ·(π)(9)
where min{π₯π} π = 1, 2...π is define as a function ofselecting a minimum value in a set of {π₯1, π₯2, ...π₯π}.
Using (8) and (9), πππ·π for packet π can be expressedas:
πππ·π (π+ π) = π΄πππ·π (π+ π)βmin{π΄πππ·π (π+ 1), . . . , π΄πππ·π (π+ π)} (10)
This is, πππ·π uses the smallest π·πππ experienced duringthe measurement time, independently of the firs experienceOWD, orπππ·(π).
A. Removal of Clock Skew
The removal (or also called adjustment) of the clock skewfrom the πππ·π measurements are needed. The adjustmentneeds to be done in reference to the destination clock πΆπ.Measurements of ππ are made with the source clock, sothey include information about the clock rate of πΆπ . Since
1
Cs
Cd
Packet
i i+1 i+2 n-1 n
TS(i+1) TS(i+2) TS(i+n-1) TS(i+n)
IPD(i+1) IPD(i+2)
IPD(i+n-1)
IPD(i+n)
OWD(i) OWD(i+1) OWD(i+n)OWD(i+2)
β=
+β+=+n
j
jiTSjiIPDniAOWDV1
))()(()(
β=
+n
j
j)TS(i1
β=
+n
j
j)IPD(i1
β=
+n
j
j)IPD(i1
Fig. 3. Example of data collected for clock synchronization.
the information is the difference of the departure times, clocksynchronization is not needed.
For this, the accumulated πππ·π as in (7) per-packetbasis is considered. Figure 3 shows a general descriptionof how AOWDV is collected. AOWDV contains clock skewinformation (considering the rate of change of clock skewequal to zero) that can be observed when this parameter isplotted against time (πΆπ). However, no every AOWDVvalueprovides useful information. For this, only the smallest valuesof AOWDV, min{π΄πππ·π }, including negative values areused. The selection of min{π΄πππ·π } is independent ofthe difference of the smallest value in the complete set ofcollected measurement (i.e., π), which can make it dependableof user traffic Instead, information of the rate of small valuescollected is used to divide the measurement time in windowswhose size is determined by the rate of small values π(π)collected in π samples under normalized traffi load (π), whichuses the link or network capacity as reference value. This is
π(π) =ππ (π)
π(11)
where ππ (π) is the number of small AOWDVvalues collectedin π packets at load π. The window size is them given innumber of packets.
III. SIMULATION STUDY OF THE PROPOSED SCHEME
Simulation of a VoIP environment was performed usingthe network simulator ns2 [17]. The network traffi (CBR)simulated a VoIP traffic using 240-byte packets every 20ms, using 96 kbps of bandwidth. In addition, there are threeindependent fl ws of cross traffics modeled as variable bitrate (VBR) traffic of which both packet lengths and intervalshave exponential distributions. The traffi is assumed to bedata traffi using TCP/IP with an average packet length of700 bytes. The clock rate difference between source anddestination is set to 1000 parts per million (ppm), or 1 ms.Figure 4 shows the network model used in the simulation. Inthis model, a VoIP transmission is performed between a sourcehost and a destination host. The traffi for this transmission
is modeled as CBR traffic Data are captured as a networkoperator would collect them in an actual network. Only πΌππ·values are collected. Each packet carries a timestamp, using thelocal clock (i.e., πΆπ ). With these data, πππ·π and AOWDVare obtained, whoever, including clock skew information.
1
S S S
DDD
S D
CBR
VBRοΌ VBR2 VBR3
Node1 Node2 Node3 Node4
: Cross-traffic (VBR) : Observed-traffic (CBR)
R1 R2 R3
CBR: Constant Bit Rate VBR: Variable Bit Rate
Source Destination
Fig. 4. Network simulation model for πππ·π estimation.
The network load (define as the traffi load on the tightlink, which is the link with the smallest capacity) was set at80%. Figure 5 shows the collected AOWDV data, calculatedfrom the collected ππ and πΌππ· values. AOWDV is plottedvs. the arrival times as clocked by πΆπ. In addition to thevariability in the AOWDV observed created by the queuingdelays of the cross traffic the figur shows that there is asteady offset increase in the AOWDV measured. This offsetincrease is created by the clock skew.
1
20 40 60 80 100 120 140-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Time at Cd (second)
AOWDV(second)
Fig. 5. πΌππ· with offset produced by clock skew between πΆπ and πΆπ.
To remove the clock skew, the smaller values of AOWDVare collected, as define in Section II-A. Different from [15],where an acceptable error π from the minimum experienceddelay is used to determine the selection of delays for thecalculation of the clock skew, the approach adopted in thispaper segments the simulation time into windows, and fromeach window, the lower values are collected. The windowsselected here have a length of 100 packets, or 2 seconds (asCBR traffi is used).
Figure 6 shows the minimum values of AOWDVcollected.These values shows similar linear increasing offset to thatobserved in Figure 5. The advantage of this method is thateven if queuing delay remains constant, the collectible AOWDV
values can be used for the calculation of the clock skew. Inthe πππππ approach, if the queuing delay is constant but largerthan π, no values can be collected, requiring longer time andlower queuing delays, which are out of the control of networkoperators as the queuing delays are caused by the cross traffic
The clock skew was estimated using linear regression, andthe resulting is 1037.39e-06. This means that the calculationthe clock skew has an error of 3.7% (in reference to the 1000e-06 in the experiment setup). After the skew is calculated,it is removed from the measured πππ·π . This is the actualπππ·π
1
0 20 40 60 80 100 120 140 160 180 200
0
0.05
0.1
0.15
Time at Cd (second)
min
{AO
WD
V}
(sec
ond)
Fig. 6. Minimum AOWDVvalues used for clock skew estimation.
Figure 7 shows πππ·π of this experiment, where the offsetcreated by the clock skew is removed. As the figur shows,the image shows similar distribution to that shown in Figure5, so the information is kept.
1
OWDV(second)
Fig. 7. πππ·π with clock skew removed.
IV. EXPERIMENTAL TEST OF THE PROPOSED SCHEME
OWD measurements were experimented in a practical fielenvironment. The used system offers private VoIP servicesto around 20,000 IP phones over major cities in Japan.The networks consist of various network subsystems such asprivate core networks, public VoIP networks, Internet, ADSL,
Wireless LAN (IEEE 802.11). The network systems, includingIP phones, are run without clock synchronization.
1
Fig. 8. Network model for experimental measurement of πππ·π .
VoIP quality metrics such as packet loss and jitter exceptone-way delay (variation) are monitored at the Network Oper-ation Center (NOC). Figure 8 shows the experimental systemused to monitor the transmitted voice packets. The voicepackets on the monitoring paths are copied and transferredto packet capturing tools installed at the NOC. The packetcapturing tools record the information of proper RTP packetsincluding transmission (timestamp) and reception times. Infor-mation of VoIP calls is stored in the used packet capturingtools to estimate OWD. The source of this information isindicated with red dotted lines in the figure
Figure 9 shows the estimated AOWDV based on sampleddata at the NOC. The AOWD shows an apparent constantoffset, indicating that clocks skew is small.
1
0 20 40 60 80 100 120-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
Time at Cd (second)
AO
WD
V(s
econ
d)
Fig. 9. AOWDVof values collected in the experimental VoIP network.
The collected minimum values of AOWDV are shown inFigure 10. The values, due to the duration of the collectionprocess, shows a variation that could indicate the magnitudeof the clock skew, when in fact the largest values are the mostrepresentative, as πΆπ has slightly smaller rate than πΆπ .
The VoIP calls related to examples are characterized byhaving large jitters among the captured calls. The figur showsthat the clock skew between the source and the destination arealmost zero and the values of this clock skew varied between3 and 17 ppm at the collection time.
Figure 11 shows the πππ·π of the collected values afterclock skew removal. In this case, the small clock skew looksremovable by a simple process and most data shows similar
1
20 30 40 50 60 70 80 90 100 110-0.0012
-0.001
-0.0008
-0.0006
-0.0004
-0.0002
0
Time at Cd (second)
min {AOWDV} (second)
Fig. 10. Minimum AOWDV values in the the firs experiment in theexperimental VoIP network.
1
0 20 40 60 80 100 1200
0.05
0.1
0.15
0.2
0.25
0.3
Time at Cd (second)
OWDV(second)
Fig. 11. πππ·π of the firs experiment in the experimental VoIP network.
1
OWDV(second)
A
Fig. 12. AOWDVvalues in the second experiment in the experimental VoIPnetwork.
trend. However, linear regression is used for this purpose,applied on the minimum values.
In a second experiment, the new collected data shows asmaller variation of the offset values indicated the clock skew.Figure 12 shows the AOWDV values of the this experiment.
1
20 40 60 80 100 120-0.0045
-0.004
-0.0035
-0.003
-0.0025
-0.002
-0.0015
-0.001
-0.0005
0
Time at Cd (second)
min
{AO
WD
V}
(sec
ond)
Fig. 13. Minimum AOWDV values of the second experiment in theexperimental VoIP network.
1
OWDV(second)
Fig. 14. πππ·π of the second experiment in the experimental VoIPnetwork.
The offset values have a small slope, indicating a small clockskew in this practical network.
Figure 13 shows the minimum values of πππ·π used todetect the clock skew. Figure 14 shows the πππ·π of thisexperiment with the clock skew removed after using linearregression among the minimum π΄πππ·π values. The figurshows the πππ·π , which shows similarities to the AOWDVshown in Figure 12. As shown in the πππ·π , the clockskew is effectively removed without affecting the networkinformation.
V. CONCLUSIONS
One-way delay variation (πππ·π ) experienced by travel-ing packets between source and destination host can be easilymeasured if clock synchronization is performed between thesetwo hosts. Clock synchronization has been proved difficulwith means other than the use of GPS as time adjustment pro-tocols require symmetrical paths for round trips measurements.However, as legacy equipment may need total replacementfor the deployment of GPS, this replacement may be slow
or expensive. As an alternative to use additional resources,this paper presents a scheme for measuring one-way delayvariation (OWDV) that does not require clock synchronization.The method depends on the clock skew adjustment. This paperpresents the dependability of OWDV on the clock skew. Theproposed measurement scheme is based on the measurementof inter-packet delays (πΌππ·s), which is the difference of thearrival times between consecutive packets. The πΌππ· valuesare closely related to the πππ·π but however, they areaffected by the clock skew between the source and destinationclocks. This paper shows a method for the estimation and de-tection of clock skew to enable the measurement of πππ·π .Different from other existing methods, the method to estimatethe clock skew is based on the accumulated values of πππ·πor AOWDV. The use of AOWDV speeds up the collection ofsamples as it does not discard useful data as in error-basedmethods.
The proposed measurement scheme was tested with com-puter simulation and on an experimental VoIP network. Thesimulation and experimental results show that the clock skewcan be effectively removed and that the πππ·π values canbe efficientl obtained. The error for clock skew estimationachieved was equal to or smaller than 3.7%. The proposedmethod can by network operators to detect large jitters orOWDVand to make network adjustments.
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