Clockscalpel: Understanding Root Causes of Internet Clock Synchronization Inaccuracy

  • Chi-Yao Hong
  • Chia-Chi Lin
  • Matthew Caesar
Part of the Lecture Notes in Computer Science book series (LNCS, volume 6579)


Synchronizing clocks is an integral part of modern network and security architectures. However, the ability to synchronize clocks in modern networks is not well-understood. In this work, we use testbeds equipped with a high-accuracy GPS receiver to acquire ground truth, to study the accuracy of probe-based synchronization techniques to over 1861 public time servers. We find that existing synchronization protocols provide a median error of 2 – 5 ms, but suffer from a long-tail. We analyze sources of inaccuracy by decoupling and quantifying different network factors. We found that most inaccuracies stem from asymmetry of propagation delay and queueing delay. We discuss possible schemes to compensate these errors to improve synchronization accuracy.


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. 1.
    Minar, N.: A survey of the NTP network (December 1999)Google Scholar
  2. 2.
    Mills, D.L.: On the accuracy and stablility of clocks synchronized by the network time protocol in the Internet system. SIGCOMM Comp. Comm. Rev. (1990)Google Scholar
  3. 3.
    Veitch, D., Ridoux, J., Korada, S.B.: Robust synchronization of absolute and difference clocks over networks. IEEE/ACM Trans. Netw. (2009)Google Scholar
  4. 4.
    Ridoux, J., Veitch, D.: Principles of robust timing over the Internet. Commun. ACM (2010)Google Scholar
  5. 5.
    Kompella, R.R., Levchenko, K., Snoeren, A.C., Varghese, G.: Every microsecond counts: tracking fine-grain latencies with a lossy difference aggregator. In: SIGCOMM (2009)Google Scholar
  6. 6.
    Pathak, A., Pucha, H., Zhang, Y., Hu, Y.C., Mao, Z.M.: A measurement study of internet delay asymmetry. In: Claypool, M., Uhlig, S. (eds.) PAM 2008. LNCS, vol. 4979, pp. 182–191. Springer, Heidelberg (2008)CrossRefGoogle Scholar
  7. 7.
    The NTP Public Services Project,
  8. 8.
    NIST Automated Computer Time Service (ACTS),
  9. 9.
    Paxson, V.: End-to-end routing behavior in the internet. SIGCOMM Comput. Commun. Rev. (2006)Google Scholar
  10. 10.
    He, Y., Faloutsos, M., Krishnamurthy, S., Huffaker, B.: On routing asymmetry in the internet. In: GLOBECOM (2005)Google Scholar
  11. 11.
    Rosen, E., Viswanathan, A., Callon, R.: Multiprotocol Label Switching Architecture. RFC 3031 (January 2001)Google Scholar
  12. 12.
    Hanks, S., Li, T., Farinacci, D., Traina, P.: Generic Routing Encapsulation over IPv4 networks. RFC 1702 (October 1994)Google Scholar
  13. 13.
    Chang, C.-C., Lin, C.-J.: LIBSVM: a library for support vector machines (2001)Google Scholar
  14. 14.
    Katz-Bassett, E., Madhyastha, H., Adhikari, V., Scott, C., Sherry, J., van Wesep, P., Krishnamurthy, A., Anderson, T.: Reverse traceroute. In: NSDI (2010)Google Scholar
  15. 15.
  16. 16.
    Katz-Bassett, E., John, J.P., Krishnamurthy, A., Wetherall, D., Anderson, T., Chawathe, Y.: Towards IP geolocation using delay and topology measurements. In: IMC (2006)Google Scholar

Copyright information

© Springer-Verlag Berlin Heidelberg 2011

Authors and Affiliations

  • Chi-Yao Hong
    • 1
  • Chia-Chi Lin
    • 1
  • Matthew Caesar
    • 1
  1. 1.University of IllinoisUrbana-ChampaignUSA

Personalised recommendations