在時脈偏移的測量上，當接收端時間戳記的時間精度低於發送端的時間精度時，
即可能生成一特別的偏移量分布圖，稱之為虛線(Dotted Line) 現象。當虛線產生，
量測時脈偏移時必須花費較長的時間才能收集到足夠的數量，以求得較精準的時
脈偏移。過往虛線現象產生時，若離群值大量出現在主群體上方，會使用線性規
劃方法計算時脈偏移；但若離群值出現在主群體下方時則要改用霍式轉換方法來
計算。以過去大多數的研究方法測量，會發現當虛線圖形出現離群值時，影響測
量結果程度遠大於其他圖形。為了改善虛線時脈偏移的精準度，本研究利用觀察
虛線圖形的變化規律，與偏移量的變化進行實驗，提出一新方法降低離群值的影
響，求得一較精準的時脈偏移，其誤差值1 ppm 以內。

Upon measuring the clock skew of network devices, when the time resolution of measurer
lower than time resolution of sender, there is a special pattern in the scatter diagram of
time offsets, or the dotted line phenomenon. In order to get precise clock skew measurement
of dotted line, the period of time to collect enough offsets has be lengthened. In the
past, if the outliers of dotted line appear above the main group, the Linear Programming
Method would be used to calculate the clock skew ; however, if the outliers appear below
the main group, the Hough Transform-base Method should be used instead. In the past
research methods, it is found that when the outliers appear in the dotted line, the influence
of accuracy of clock skew of is much greater than other patterns. To improve the accuracy
of clock skew of the dotted line, this research utilizes the distribution of the change rule
of offsets in the dotted line pattern, proposes a new method to reduce the influence of the
outliers, and get a more accurate clock skew.

[1] T. Kohno, A. Broido, and K. C. Claffy, “Remote physical device fingerprinting,” in
IEEE Transactions on Dependable and Secure Computing, pp. 93–108, 2005.
[2] D. J. Huang, K. T. Yang, C. C. Ni, W. C. Teng, T. R. Hsiang, and Y. J. Lee, “Clock
skew based client device identification in cloud environments,” in Proceedings -
International Conference on Advanced Information Networking and Applications,
AINA, pp. 526–533, 2012.
[3] 鄭理介, “基於時脈偏移的可攜式裝置識別技術.” 國立臺灣科技大學, 2013.
[4] D.-J. Huang and W.-C. Teng, “A defense against clock skew replication attacks in
wireless sensor networks,” Journal of Network and Computer Applications, pp. 26–
37, 2014.
[5] M. Cristea and B. Groza, “Fingerprinting smartphones remotely via ICMP timestamps,”
IEEE Communications Letters, 2013.
[6] S. Sharma, A. Hussain, and H. Saran, “Experience with heterogenous clock-skew
based device fingerprinting,” Proceedings of the 2012 Workshop on Learning from
Authoritative Security Experiment Results, pp. 9–18, 2012.
[7] M. Uddin and C. Castelluccia, “Toward Clock Skew Based Wireless Sensor Node
Services,” in Proceedings of the 5th Annual ICST Wireless Internet Conference
(WICON 2010), pp. 1–9, 2010.
[8] 周品言, “藍芽通訊傳輸延遲之研究.” 國立臺灣科技大學, 2014.
[9] 孫國仁, “網路時鐘偏斜測量中虛線現象之研究.” 國立臺灣科技大學, 2016.
[10] 陳致佑, “利用虛線特性測量時脈偏移之研究.” 國立臺灣科技大學, 2017.
[11] D.-J. Huang, W.-C. Teng, C.-Y. Wang, H.-Y. Huang, and J. M. Hellerstein, “Clock
Skew Based Node Identification in Wireless Sensor Networks,” in IEEE GLOBECOM
2008 - 2008 IEEE Global Telecommunications Conference, pp. 1–5, 2008.
[12] S. B. Moon, P. Skelly, and D. Towsley, “Estimation and removal of clock skew from
network delay measurements,” in Proceedings - IEEE International Conference on
Computer Communications, 1999.
[13] V. Paxson, “On calibrating measurements of packet transit times,” ACM SIGMETRICS
Performance Evaluation Review, pp. 11–21, 1998.
[14] C. H. Hsu and W. C. Feng, “A power-aware run-time system for high-performance
computing,” in Proceedings of the ACM/IEEE 2005 Supercomputing Conference,
SC’05, p. 1, 2005.
[15] K. Oka Saputra, W. C. Teng, and T. H. Chen, “Hough Transform-Based Clock Skew
Measurement over Network,” IEEE Transactions on Instrumentation and Measurement,
pp. 3209–3216, 2015.
[16] 楊祐齊, “一個改良的基於霍氏轉換時脈偏移測量方法.” 國立臺灣科技大學,
2015.
[17] C. E. Shannon, “A mathematical theory of communication,” The Bell System Technical
Journal, pp. 3–55, 1948.
[18] 何建德, “一個利用熵的時脈偏移測量方法.” 國立臺灣科技大學, 2016.