A Scheme to Estimate One-way Delay Variations for Diagnosing Network Traffic Conditions

Transcription

1 Cyber Journals: Multiisciplinary Journals in Science an Technology Journal of Selecte Areas in Telecommunications (JSAT) August Eition 2011 A Scheme to Estimate One-way Delay Variations for Diagnosing Network Traffic Conitions Makoto Aoki an Eiji Oki Abstract Real-time applications over the Internet such as Voice-over-IP an vieo streaming services are increasingly being applie to practical communication services so how to evaluate service qualities an how to iagnosis the cause of service egraation has become one of the key issue for QoS management Various kins of measurement metrics over IP networks incluing one-way elay variation (OWDV) inter-packet elay variation (IPDV) an packet losses have been stanarize In particular OWDV is useful for analyzing the network traffic conitions since this metric irectly reflects the queuing buffer elays generate in the network noes Unfortunately OWDV is ifficult to measure because it requires the use of time information at source an estination hosts This paper proposes an OWDV estimation scheme that solves this problem; it estimates OWDV values using IPDV values measure at the estination host The clock ifference between the source an the estination mainly cause by clock skew can be estimate by using IPDV values Base on this scheme we evelop a tool for iagnosis of network traffic conitions an apply the tool to analyze network traffic conitions in actual networks The results show that the problems with network traffic conitions can be accurately analyze using the estimate OWDV values an the cause of the conitions can be clarifie Inex Terms One-way elay One-way elay variation Inter-packet elay Packet elay variation OWDV PDV Packet loss Quality of Service Diagnosis of network traffic conitions R I INTRODUCTION eal-time applications over the Internet such as Voice-over-IP (VoIP) an vieo streaming services are increasingly being applie to practical communication services so that how to evaluate service performance an how to iagnose the cause of service egraation has become a key issue for QoS management [1] Measurement metrics are categorize by how many information types must be measure into one-point an two-point types ``One-point" means that measurement nee to be conucte only at one point for instance the estination ``Two-point" means that measurements must be performe at two ifferent points ie source an estination The latter raises the issue of a clock reference to measure the elays between the two points M Aoki is with Cyber Creative Institute Tokyo Japan aoki@cybersokencom He is also with The University of Electro-Communications Tokyo Japan E Oki is with The University of Electro-Communications Tokyo Japan oki@uecacjp 1 Measurements are also categorize by the measurement metho use into active an passive measurements Active measurements inject probing packets into the observe path an measure network traffic conitions that these packets experience Passive measurements involve only the actual packets being use for communication services Since our goal is to analyze the behavior of actual packet flows we focus on passive measurements The various kins of measurement metrics known for streaming services on IP networks inclue one-way elay variation (OWDV) Inter-Packet Delay Variation (IPDV) an packet loss all of which have been internationally stanarize by IETF (Internet Engineering Task Force) an ITU-T (International Telecommunication Union Telecommunication Stanarization Sector) [2]-[4] OWDV a two-point type is efine as the variation in packet elay between the source an the estination; it reflects the change in queuing elays in the IP networks Since the queuing elays are irectly affecte by the network traffic conitions OWDV is useful for analyzing the network traffic conitions However OWDV is ifficult to measure ue to its requirement for clock synchronization between the source an the estination Most of OWDV schemes assume the use of an external clock synchronization mechanism base on GPS (Global Positioning System) Therefore OWDV has not been wiely use to measure elay variations in actual network paths IPDV a two-point type is efine as the ifference between the elays of the current an the previous packets Therefore IPDV has no nee for clock synchronization because the clock ifference is negligible ue to the short inter-packet spacing This metric however can not express the network traffic conitions in etail because it means the ifference between the elays of ajacent packets Packet loss a one-point type is efine as the loss of one or more packets ue to transmission errors in the network links or buffer overflow in the network noes This metric is also efine as the packet loss rate (PLR) which means the ratio of the numbers of lost packets to the numbers of transmitte packets The packet loss metric can be easily measure However it offers poor sensitivity in measuring the network traffic conitions since packet losses occur only uring buffer overflow OWDV measurement schemes without external clock synchronization have been propose by [5]-[7] [5] an [6] escribe a way to estimate OWDV using IPDV values

2 measure by packet capturing tools [8][9] [5] an [6] give an estimation metho that inclues transformation mechanism from IPDV values to OWDV ones uner the requirement of the synchronize clocks The OWDV values which inclue the effect of the clock ifference between the source an the estination are compensate to estimate the actual OWDV values to offset clock skew However since a truly practical scheme must operate in general conitions incluing those in which the source an estination clocks are not synchronize it is require to give the mechanism in more etail uner the general conitions The scheme in [7] measures the time-interval values between a pair of packets at the sener an the receiver using eicate user atagram protocol (UDP) packets an calculates the ifference between the values These values are use to euce the Fourier-transforme magnitue of queuing elay PDF (probability istribution function); clock synchronization is not neee This scheme however is classifie as an active measurement approach an so is not applicable to our OWDV estimation scheme Requirements place on a practical OWDV measurement scheme are summarize below: The measurement must be sensitive to network traffic conitions so that we are able to investigate fine changes in elay precisely The measurement must be conucte on a packet-by-packet basis so that we are able to trace the elays of packets which will change wiely with each packet reception The measurement must be simple an not require any external clock synchronization mechanism This will yiel wie eployment in actual networks One question arises: Is there any measurement scheme that satisfies the above requirements? This paper proposes a measurement scheme base on OWDV to iagnose network traffic conitions in cooperation with IPDV an packet loss To estimate OWDV without recourse to a clock synchronization mechanism the propose scheme etects an ajusts the clock ifference which is mainly cause by clock skew In aition the scheme calculates an removes the offset time which is etermine by the time of the first packet reception The reminer of this paper is organize as follows Section Ⅱ escribes the basic efinitions for elay measurements an the propose scheme Section Ⅲ epicts a evelope iagnosis tool for analyzing the egraation in service quality Section Ⅳ presents our results on the iagnosis of traffic conitions base on OWDV as experience in actual VoIP networks Section Ⅳ escribes the relate works to our work Section Ⅴ presents finally our conclusion II DEFINITIONS FOR MEASURING DELAY VARIATIONS For comprehensive unerstaning the packet transmission scheme at the source is efine as shown in Fig 1 Measurements are conucte uring communication on a 2 packet-by-packet basis The n packets each of which is enote as a sequence number i where 0 i n-1 are transmitte at a fixe interval an with a fixe packet length uring communication When a packet is transmitte the transmission time (measure at the source) is set in the packet an carrie to the estination The carrie transmission time calle "time-stamp" is use to measure the transmission elay from the source to the estination as escribe below Fig 1 Measurement is active uring communication an is terminate at the en of communication The n packets are transmitte at fixe intervals Fig 2 shows an example of packet transmission sequences from the source an the estination to ai the consieration of the elay measurement by using ifferent clocks C s an C are enote as clocks at the source an the estination respectively an the times of the clocks are not equal in general if these clocks are not synchronize using an external synchronization scheme T x ( R x ( TD x ( an RD x ( represent the time values at which the measurements of packet i are conucte Fig 2 Delay moel for packet transmission from the source to the estination Polygonal lines in the figure show the transmission times measure by the estination clock (C ) which have the ifferences of E(i-1) an E( at packet sequence numbers i-1 an imeasurement is active uring communication an is terminate at the en of communication The n packets are transmitte at fixe intervals where suffix x shows that time measurements are performe using the source clock (C s ) or the estination clock (C ) respectively TD s ( is efine as Eq (1) This means that TD s ( is calculate using the interval between transmission times T s (i-1) an T s ( For simplicity but without loosing generality the reference clock is taken to be C If time is measure by C s the error enote by E ue to the ifference

3 between C s an C shoul be consiere Accoringly T s ( is epicte as Eq (2) TD s( T s ( T s ( i 1) (1) T ( T s ( T s ( i 1) (2) Eq (2) means that T ( measure by C can be obtaine from T s ( measure by C s an E( generate by the ifference between C s an C D ( the transmission elay of packet i is epicte by Eq (3) in which Eq (2) is substitute for T ( D ( R ( T ( R ( Ts ( E( (3) A Definition of IPDV base on the elay moel IPDV ( is efine as Eq (4) in accorance with [10] IPDV ( D ( D ( i 1) ( R ( T ( ) ( R ( i 1) T ( i 1)) RD ( TD ( ( E( E( i 1)) Eq (4) means that IPDV ( can be obtaine by measuring two elay parameters: The time ifference between packet i an packet i-1 of reception interval time base on C The time ifference between packet i an packet i-1 of transmission interval time base on C s (This information is transmitte to the estination as time-stamps) E( - E(i-1) will be close to zero because E( is almost the same as E(i-1) given the short interval time perio (eg 10 or 20 ms) Therefore Eq (5) remains vali in practical measurements: IPDV ( RD ( TD ( (5) B Definition of OWDV base on the elay moel OWDV ( in the same way as IPDV ( is efine as Eq (6) in accorance with [11] OWDV ( is base on the probability istribution of one-way packet transmission elay as shown in Fig 3 OWDV ( D ( min { D ( j)} (6) s 0 j n 1 where min 0 j n-1 {D (j)} is the packet transmission elay with the lowest value over the observation time (consecutive n packets) an is taken to be the elay without queuing elay in the networks It shoul be note that min 0 j n-1 {D (j) is etermine to be a constant value when the communication is terminate Referring to Fig 2 D ( is efine as Eq (7) in the recurrence formula consiering the clock ifference between the source an the estination: (4) Fig 3 Probability istribution of OWDV an the efinitions of variables for OWDV measurement: D (0) an δ min mean the elay of packet 0 an the ifference between D (0) an min{d (} P' is efine as the temporary reference point for OWDV measurement which is etermine at the time of receiving the first packet 0 P stans for the "true" reference point to OWDV ( or OWDV (=0 The change of the reference point from P' to P is processe at the en of measurement D ( ( RD ( TD ( ) D ( j) ( RD ( TDs ( ) ( E( j) E( i 1)) D ( i 1) This equation is transforme into Eq (8) within the range of zero to n-1: D ( (7) ( D (0) (8) where let δ( enote the equation as shown in j i ( ( RD ( j) TDs ( j)) ( E( E(0)) (9) j 1 In Eq (9) E(-E(0) is the time ifference from packet 0 to packet i ue to the ifference between the source an the estination clocks D (0) is efine as the one-way elay of packet 0 However its value is not able to be measure because reference point P is not etermine at the reception of this packet Therefore P' which correspons to the reception time of packet 0 is efine as a pseuo reference point δ( correspons to OWDV ( being measure from the reference point of P' so that its value will be negative when D ( is greater than D (0) Using Eq (8) min 0 j n-1 {D (j)} is transforme into Eq (10): min min { D ( j)} { ( j) D ( j)} 0 j n 1 0 j n 1 min D (0) (10) where δ min is efine as min 1 j n-1 {δ(j)}; its value correspons to the minimum value of OWDV ( being measure from the reference point of D (0) Since D (0) is greater than or equal to 3

4 min 0 j n-1 {D (j)} hols as shown in Fig 3 δ min remains less than or equal to zero Eq (6) is transforme into Eq (11) by substituting Eqs (8) an (10) for Eq (6) Note that the term of D (0) is remove by the substitution OWDV ( ( min (11) Eq (11) also means that OWDV ( can be evaluate if E(-E(0) or the ifference between C s an C can be estimate in some way We will iscuss about how to estimate E(-E(0) in the next subsection C Removal of the clock ifference The clock ifferences inclue clock offset clock skew an clock rift Clock offset is the ifference between C s an C referre to as θ as measure against the same reference clock efine as θ= t - t s Note thatθ correspons to E(0) when i=0 Clock skew referre to as φ is the ifference between the clock rates (r) of the source an the estination hosts also efine as φ = r s/ r Clock rift is the rate of change (ν) in the clock rates Since clock rift will be assume to be nearly zero E( is efine as Eq (12) by introucing a iscrete expression in time: ψ will be estimate using linear regression or linear programing to reuce the estimation errors ue to misleaing outlier (smallest) ata Using linear regression the error of clock skew estimation is given by less than 37% in simulation stuies [6] Fig 4 summarizes the estimation process of Eq (11): the ata shown as circles on the long otte lines represent j i ( RD ( j) TD ( j)) j 1 s which are measure using ifferent C s an C in the same way as in IPDV measurement The ata shown as triangles on the short otte lines represent δ(-d (0) These ata correspon to the values that woul be measure with synchronize clocks However the time errors cause by the time of the first receive packet remain The ata shown as the rectangles on soli lines represent OWDV ( in which the time error or δ min is cancele III IMPLEMENTED MEASUREMENT TOOL FOR DIAGNOSIS OF IP NETWORKS As mentione in the previous section the IPDV-OWDV E ( ir E(0) (12) where the time-unit is efine as τ which correspons to packet transmission interval time measure by C Therefore the estimation of E(-E(0) results in etermining the slope or the value of φ among the values of j i ( RD ( j ) TDs( j )) j 1 in Eq (9) These values consist of propagation plus transmission elays an queuing elays: The former hols a constant value when packets with the same length are transmitte the latter epens on the traffic conitions in the networks OWDV ( will be close to zero (ie not incluing queuing elays) at higher probability when packets are transmitte uner less than extremely high traffic loas (for instance link utilization 08) Assuming that we can select OWDV ( an OWDV (i+k) (k is natural number) which are nearly close to zero from among all OWDV (j) (0 j n-1) Eq (11) can be transforme into Eq (13): j i k E ( i k) E( ( RD ( j) TD s ( j)) (13) j i 1 Consiering E(i+k)-E(= φkτ Eq (14) is given by: j i k ( j j j i RD ( ) 1 TDs ( )) k (14) Eq (14) means that the clock skew (ψ) can be etermine by measurable ata between packet i an packet i+k In practice 4 Fig 4 The slope of E(-E(0) an δ min are remove from δ( to obtain OWDV ( transformation scheme yiels OWDV information at the same time as conucting IPDV measurements We can use this scheme to create a metho that observes the traffic streams on a packet-by-packet basis in aition to packet loss We have evelope a measurement tool base on this scheme Fig 5 shows its block iagram With this tool the information of packet streams incluing also packet reception time measure by C are recore on a per communication session basis The packets to be analyze are selecte from among these collecte packets The selecte packets are checke to confirm if they were correctly receive If correctly receive the information of the time-stamps an packet sequence numbers inclue in the packets is extracte an analyze Whereas if incorrectly receive ue to violation of the communications protocol such as FCS (Frame Check Sequence) errors the packets are iscare an their sequence numbers remain lost

5 These information are transmitte to the functional blocks epicte as an OWDV an packet loss measurement blocks for analyzing the elays an packet loss characteristics The two blocks output their analyze information to the isplay function block in which isplays the two metrics on the same clock an time scale Voice signals are moulate base on ITU-T G711 [16] (moulation rate of 64 Kb/s) an their packets are transmitte at fixe interval (20 ms) from the source to the estination hosts using RTP [10] The estination host transforms the packets into voice signals an the voice signal's elay is mae constant by using a jitter buffer an a partial packet loss can be helpe hie by packet loss concealment (PLC) function [17] IV EXPERIMENTS AND RESULTS This section escribes the experimental configurations an methos use to assess the propose scheme an the results obtaine from practical voice communications sessions in actual IP networks Specifically the relationships between voice quality an the three measurement metrics were analyze A System configuration for measurements Fig 6 epicts the communications paths over several subnetworks We focuse on the network traffic conitions on two communication paths: one is a path the short-otte line (a) between two IP-phones over a wireless LAN (IEEE 80211g) [12] ADSL (ITU-T G9923) [13] Internet an VoIP B Measurement analysis for the case of fairly goo voice quality Fig 7 shows an example of the measurement results of the IPDV OWDV an lost packets for VoIP communication on path (a) The voice quality of that call is assesse overall by the users as ``fairly goo quality" which means ``no appreciable effort require for telephone conversation" [15] The voice quality is evaluate by user-a an/or user-b using subjective assessment mainly from the view point of the presence of elay echo noise etc The primary reason why the voice quality is ``fairly goo" is assume to be that the VoIP packets experience no packet losses an long elay variations ue to the network congestion transmission errors an the like as shown in the figure It however shows that OWDV excees 100 ms with bursty traffic when the communication times Fig 5 A block iagram of measurement tool base on OWDV together with Packet loss In this measurement scheme OWDV an packet loss metrics can be shown simultaneously on a packet-by-packet basis This scheme allows us to analyze network traffic conitions in etail (QoS-manage) networks The other is a path the long-otte line (b) between analog telephones over PSTN an IP phones connecte to private VoIP networks We measure transmission qualities of the voice packets at a network operation center (NOC) The packets being measure are copie an transmitte from probing points (routers) to the NOC These packets incluing packet heaers are recore onto storage meia at the NOC We focuse on the voice packets which transmitte from the left sie to the right sie because they experience many of the factors yieling voice quality egraation such as elay jitter an packet loss The recore ata of VoIP calls are selectively analyze if there are any complaints from users about voice qualities on their conversations The ata are extracte among all of the call ata using the information of phone numbers communication time incluing the beginning an the en times The networks were assume to support enterprise-level communications an inclue aroun IP telephones Fig 6 System configuration for measurement of QoS of voice communications The packets on observe paths are copie an transferre to the network operation center (NOC) to be measure The observe paths inclue two routes; path(a) consists of IP-telephone (wireless) Wireless LAN (IEEE 80211g) ADSL (maximum upwar rate: 12 Mb/s Internet an private VoIP networks Path (b) consists of PSTN public VoIP networks an the private VoIP networks (calle CT in the following) are equal to aroun an 100 sec In these cases elay variations beyon 50 ms will lea to packet iscar at the jitter buffer of IP-telephone receivers The total packet loss rate ue to the elay variations beyon 50 ms is estimate to be 39% so that the voice quality of this call is assume to be slightly egrae The IPDV in Fig 7 appears to match the elay variations of OWDV However as shown in Fig 8 in which the time perio between CTs values of 25 an 35 sec is graphically expane the number of the packets iscare at the jitter buffer is estimate lower value by IPDV metrics to be lower than that estimate by OWDV metrics The packets that follow a packet with long elay ten to have long elays therefore these packets will be iscare with higher probability 5

6 C Measurement analysis for the case of poor performance Here the voice packets experience longer-elays an higher packet loss rates: the OWDV values were almost flat at aroun 300 ms an packet loss rate were high at aroun 4% between CTs values of 53 an 99 sec as shown in Fig 9 The loss inclue instances of consecutive packet loss However between CTs values of 99 an 136 sec there was no packet loss an longer elays In this measurement CTs values of 53 an 136 sec correspon to the beginning an the en points in the VoIP connection These voice packet ata are observe in an actual communication session on the same path (path (a)) as in the previous example User-A an -B evaluate the voice quality of this call: the user-b felt iscomfort in communicating with user-a because of click noises an/or istorte voice However the user-a i not feel any great egraation in voice quality From the analytical point of view the primary egraation in OWDV metric is presume to be overloaing of the ADSL upwar link in other wors insufficient ADSL upwar link capacity The reasoning is as follows First the long an constant elays mean that any queue in the observe path became full when the VoIP connection starte using the insufficient resources on path (a) so that packets input into the queue were iscare with higher loss rate Secon the minimum link capacity over the path is ADSL upwar link of Fig 7 An example of measurement results of IPDV OWDV an Packet loss when the voice quality is evaluate as "fairly goo quality" on the observe path (a) Fig 8 IPDV an OWDV values between CT s values of 25 an 35 sec are enlarge from Fig 7 Focusing on the number of the packets with elay variations greater than 50 ms it is shown that the OWDV measurements inicate more packets are iscare at the jitter buffer greater than the IPDV measurements which link rate is estimate aroun 250 kb/s by the metho escribe in [20] Thir since user-a i not feel any egraation in voice quality the path with ba voice quality inclues the ADSL upwar link mentione above Fig 9 Another example of measurement results of OWDV an Packet loss These figures show that quality is estimate to be low over the observe path The high OWDV values an packet loss rates are cause mainly by the assignment of voice channels to the ADSL link We estimate that the cause is the insufficient of ADSL upwar link capacity The bursty OWDV values between CTs values of 99 an 136 sec are assume to reflect the bursty traffic on the ADSL links incluing that on the wireless LAN because the available link banwith is estimate to be aroun 250 Kb/s These elays trigger iscar of the voice packets at the jitter buffer of the IP-telephone receiver as mentione in Section Ⅳ -B Since the packet loss cause by the jitter correspon to packet loss rate of aroun 3% the total packet loss rate is estimate to be 7% (= 4 + 3) between CTs of 53 an 99 sec an 3% between CTs values of 99 an 136 sec We shoul note that the change of OWDV values shows the events generate in the queuing buffer extremely ifferent from that of IPDV values as shown in Fig 9 So OWDV metric can play an important role as a measurement metric in cooperation with packet loss metrics D Measurement in another case of poor performance This case examines ynamic elay experience by voice packets ue to processor overloa The VoIP communication session was establishe call on path (b) which consists of public an private VoIP networks Path (b) requires the conjunction of two manage QoS networks After the call was complete the users complaine about two problems; both of the users sometimes notice a response elay uring the conversation which isrupte communication; User-B often hear an echoe voice The voice quality of that call was evaluate as ``ba quality" because of recognizing the long response elay an the echoe voice On path (b) the echoe voice signals coul be echoe at the two an four wire transmission converter which is locate at the interface between PSTN (Public Switche Telephone Networks) an the Public VoIP network 1 1 Generally echoe voice signals are cancele by using an echo canceller However when transmission elays on the path are long there is a higher probability of experiencing echoing 6

7 As shown in Fig 10 the peak OWDV value change is 150 ms up to CT value of 100 sec however beyon CT value of 100 sec it graually increases to 600 ms at CT value of 135 sec After that it ecreases to aroun 300 ms until call completion There was no packet loss uring the call These phenomena were observe a few times per ay especially uring the perios of heavy VoIP traffic for instance in the morning The primary issue to solving the problem is to fin the cause of the elays starting at CT value of 100 sec in this case The big ifference from previous cases shows that OWDV between CT values of 100 an 155 sec increase an then ecrease at a small rate (several secons orer) which o not change with bursty traffic This shows that the queue length at some noes on the path (b) graually increase between CT values of 100 an 135 sec The change of OWDV values mainly reflects the events with the change of processing loas in the networks It is assume that the loas o not consist of the events generate stochastically on a packet-by-packet basis such as the packet transmission but the events which involve call processing per packet protocol processing an the like Therefore we pai attention to the processing loas of noe equipment in the VoIP networks which are connecte on the path (b) As a result we foun out the noe with the interface of public VoIP network which is assume to generate longer elays by investigating a correlation between OWDV values an the numbers of connecting VoIP calls The noe was erroneously assigne to process VoIP protocol conversion approaches clock synchronization approach measurement principles an applicable areas OWD incluing OWDV measurement has two major challenges to solve: clock synchronization an OWD measurement scheme There are two approaches to using external mechanisms for clock synchronization: active an passive methos Several existing active measurement schemes use the global-positioning system (GPS) network time protocol (NTP) [29] or precision time protocol (PTP) also known as IEEE 1588 stanar [30] The introuction of GPS allows us to realize high clock accuracy [1] for elay measurement but increases equipment costs Furthermore GPS is ifficult to eploy on legacy equipment NTP an PTP require measurement of roun-trip time (RTT) to perform clock synchronization an are even more restrictive since they require that both paths (from source to estination an from estination to source) have the same OWD to function In general each path has a ifferent OWD unless the network was engineere with this feature an it is ifficult to expect a network such as the Internet woul be able to meet this restriction Active methos that can measure OWD without requiring clock synchronization have been propose [25][26][7] In [25] cyclic-paths are selecte properly to calculate each one-way elay while a new relationship between OWD an RTT is introuce in [26] However both are base on the measurement of roun-trip elays The scheme in [7] as escribe in Section Ⅰ is use to euce the Fourier-transforme magnitue of the queuing elay probability istribution function an so avoi the clock synchronization problem Passive methos use the traffic flowing through the network most often user traffic to implement the measurement process Fig 10 An example of measurement results of IPDV OWDV an Packet loss The measurement was conucte to iagnose malfunction of communication equipment The result shows that OWDV values change significantly an increase continuously in the later half of the communication session whereas IPDV showe no such change beyon its performance limit As in Section Ⅳ-C OWDV is shown to play an important role as a measurement metric in cooperation with packet loss metrics V RELATED WORKS Table Ⅰ compares existing one-way elay measurement techniques to our work The categories are measurement [19] 7

8 Measurement approach Active metho Passive metho TABLE I ONE-WAY DELAY MEASUREMENT METHODS Clock External Measurement Applicable Reference synchronization mechanism principle area GPS [21][22] Wie area NTP Require OWD=R - T s [21] PTP Small subnetworks [21] OWD estimation [24] using pairs of packets Deicate Not require OWD estimation Wie area base on cyclic-path [25] elay measurement GPS [21][22][23][27] Wie area NTP Require OWD=R - T s [21] PTP Small subnetworks [21][31] OWD estimation [28] using pairs of packets Deicate Not require OWD estimation Wie area base on cyclic-path [5][6] this work elay measurement R an T s represent the reception time at the estination an the transmission time at the source respectively A passive measurement metho was recently propose in [27] This metho uses GPS an NTP for clock synchronization Another passive OWD measurement scheme is base on the assumption that PTP achieves clock synchronization [31] A scheme to measure clock skew was recently propose in [28] This scheme uses the real-time protocol (RTP) [10] an the real-time control protocol (RTCP) [32] to create fixe packet length an fixe inter-eparture gap This approach is base on a simple moel for estimating clock skew an emonstrates a ifferent perspective from the existing complex approaches to the estimation of clock skew which for example use linear programming The accuracy of clock skew removal epens on queuing elays being short uring the measurement The contra case long queuing elays is not consiere in the scheme From the perspective of measurement accuracy only GPS-base schemes are able to lower the error rate of clock frequency etermination to uner 10-8 [27] Almost none of the methos base on clock skew estimation aresse the issue of measurement accuracy [25][31][33] From the user point of view it must be possible to measure the OWD of the user ata anywhere with no limitation For example this means that the tools offer passive measurement an interoperability with general purpose ata capture tools such as Wireshark [9] an that the nee for clock synchronization is avoie OWDV is an important factor in jitter buffer esign an the analysis of network conitions This is because OWDV involves only queuing elays which strongly epen on network traffic conitions Our research targets a passive OWDV estimation scheme that oes not nee an external mechanism for clock synchronization between the source an estination hosts Our approach measures IPDV an the combine an accumulate OWDV which is the sum of IPDV However as clock skew is inclue in the IPDV measurements we introuce an OWDV-base mechanism to estimate an remove clock skew In orer to clarify its mechanism more precisely than is one in [5][6] this work analyzes the mechanism incluing the error 8 ue to the clock ifference between the source an estination hosts It also shows that OWDV measurements are useful in analyzing the network traffic conitions in actual VoIP networks VI CONCLUSION Real-time applications over the Internet such as Voice-over-IP an vieo streaming services are increasingly being applie to practical communication services so how to evaluate service qualities has become one of the key issue for QoS management This paper propose a scheme that can estimate the one-way elay variations (OWDV) which is one of important evaluation metrics This scheme allows us to estimate OWDV values from inter-packet elays (IPDV) values which are easily measure The paper shows stuy results from two points of view: theoretical analysis of estimating process in the scheme an experimental results for iagnoses of traffic events in actual IP networks First we analyze this scheme in etail uner the moel of ifferent clocks in the source an estination hosts which correspons to actual IP networks The result shows that the scheme is able to be applie to estimate OWDV values an the accuracy of the OWDV estimation is etermine by estimating the clock skew between the source an the estination clocks Next we applie this scheme to iagnose traffic conitions in actual IP networks such as Voice-over-IP (VoIP) networks Our results shows that the scheme is able to estimate OWDV values in actual networks an be applie to iagnose traffic conitions with istinctive characteristics in comparison with IPDV values The further work is necessary to stuy to apply the scheme to high-spee networks more then ten Gb/s with high-accurate clock skew estimation ACKNOWLEDGMENT The authors woul like to thank Roberto Rojas-Cessa for his

9 instructive suggestions on performance measurement schemes in IP networks REFERENCE [1] J Wang M Zhou an Y Li ``Survey on the En-to-En Internet Delay Measurements" High Spee Networks Multimeia Communications Springer 2004 [2] C Demichelis an P Chimento ``IP Packet Delay Variation Metric for IP Performance Metrics (IPPM)" RFC 3393 Nov 2002 [3] ``Internet protocol ata communication service? IP packet transfer an availability performance parameters" ITU-T Recommenation Y1540 Nov 2007 [4] ``Network performance objectives for IP-base services" ITU-T Recommenation Y1541 Feb 2006 [5] M Aoki E Oki an R Rojas-Cessa ~``Scheme to Measure One-way Delay Variation with Detection an Removal of Clock Skew" IEEE HPSR 2010 Dallas TX Jun 2010 [6] M Aoki E Oki an R Rojas-Cessa ~``Measurement Scheme for One-way Delay Variation with Detection an Removal of Clock Skew" ETRI-Journal Vol 32 No 6 Dec 2010 pp [7] W Lu W Gu an S Yu ``One-way queuing elay measurement an its application on etecting DDoS attack" Journal of Network an Computer Applications Vol 32 Issue 2 Mar 2009 pp [8] [9] [10] H Schulzrinne S Casner R Freerick an V Jacobson ``RTP: A Transport Protocol for Real-Time Applications" RFC 3550 Jul 2003 [11] A Morton an B Claise ``Packet Delay Variation Applicability Statement" RFC 5481 Mar 2009 [12] [13] Asymmetric igital subscriber line (ADSL) transceivers ITU-T Recommenation G9923 Jan 2005 [14] J Postel ~``Internet Control Message Protocol'' RFC 792 Sep 1981 [15] ``Methos for subjective etermination of transmission quality" ITU-T Recommenation P800 Aug 1996 [16] ``Pulse coe moulation (PCM) of voice frequencies" ITU-T Recommenation G [17] ``A high quality low-complexity algorithm for packet loss concealment with G711" ITU-T Recommenation G711 Appenix I Sep 1999 [18] K Lai an M Baker ~``Nettimer: A Tool for Measuring Bottleneck Link Banwith" Proceeings of the USENIX Symposium on Internet Technologies an Systems Mar 2001 pp [19] ``Talker echo an its control" ITU-T Recommenation G131 Nov 2003 [20] M Aoki an E Oki ``Estimating ADSL Link Capacity by Measuring RTT of Different Length Packets" Globecom 2010 Dec pages [21] L D Vito S Rapuano an L Tomaciello ~``One-Way Delay Measurement: State of the Art" Instrumentation an Measurement IEEE Transactions on Vol 57 Issue pp [22] A Hernanez an E Magana ~``One-way Delay Measurement an Characterization" ICNS 2007 Jun pages [23] S Niccolini M Molina F Raspall an S Tartarelli ~``Design an implementation of a one way elay passive measurement system" NOMS 2004 IEEE/IFIP Apr 2007 pp [24] J Wang j Yang G Xie Z Li an M Zhou ~``On-line Estimating Skew in One-way Delay Measurement" PDCAT 2003 Design an implementation of a one way elay passive measurement system" NOMS 2004 IEEE/IFIP Aug 2003 pp [25] O Gurewitz an M Sii ~``Estimating one-way elays from cyclic-path elay measurements" INFOCOM 2001 Apr 2001 pp [26] D Kim an J Lee ~``En-to-En One-way Delay Estimation Using One-Way Delay Variation an Roun-Trip Time" Qshine 2007 ACM Aug pages [27] T Zseby L Mark C Schmoll an G Pohl ~``Passive One-Way-Delay Measurement an Data Export" Proc Int Workshop Inter-omain Performance Simulation Feb pages [28] B Ngamwongwattana an R Thompson ~``Measuring One-Way Delay of VoIP Packets Without Clock Synchronization" I2MTC 2009 May pages [29] DL Mills ~``Network Time Protocol (Version 3) Specification Implementation an Analysis" RFC [30] C Goron~``Introuction to IEEE 1588 an Transparent Clocks" White Paper Tekron 2009 [31] M Cola G De Lucia D Mazza M Patrignani an M Rimonini ~``Covert Channel for One-Way Delay Measurements" ICCCN 2009 Aug pages [32] C Huitema ~ ``Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)" RFC 3605 Oct 2003 [33] SB Moon P Skelly an D Towsley ~``Estimation an Removal of Clock Skew from Network Delay Measurements" INFOCOM 1999 Mar pages Makoto Aoki receive the BS in Electrical Engineering from Tokyo University of Science Tokyo Japan in 1971 an joine NTT laboratory Since 1971 he worke in the Switching System Department to conuct research an evelopment of remote control switching systems ata communication systems communications protocol control LSI (X25 an high-spee packet communications protocol) an so on From 1998 to 2007 he worke with Infonex an NTT-ata Customer Service where he was responsible for multimeia packet network/system esign for the company s networks He joine Cyber Creative Institute in 2007 an mainly conucts research/consultation about VoIP networks next generation network IPTV an so on His research interests are quality-of-service networks network measurements an high-spee communications protocol control for future Internet He is an IEEE Member an IEICE Member Eiji Oki is an associate professor of The University of Electro-Communications Tokyo Japan He receive his BE an ME in instrumentation engineering an a PhD in electrical engineering from Keio University Yokohama Japan in an 1999 respectively In 1993 he joine Nippon Telegraph an Telephone Corporation's Communication Switching Laboratories Tokyo Japan He researche IP an optical network architectures traffic-control methos high-spee switching systems an communications protocols From 2000 to 2001 he was a visiting scholar at Polytechnic University Brooklyn New York where he was involve in esigning tera-bit switch/router systems He joine The University of Electro-Communications Tokyo Japan in July 2008 Dr Oki was the recipient of the 1998 Switching System Research Awar the 1999 Excellent Paper Awar presente by IEICE an the 2001 Asia-Pacific Outstaning Young Researcher Awar presente by IEEE Communications Society for his contribution to broaban network ATM an optical IP technologies He has co-authore two books Broaban Packet Switching Technologies publishe by John Wiley New York in 2001 an GMPLS Technologies publishe by RC Press Boca Raton in 2005 He is an IEEE Senior Member an an IEICE Senior Member 9