Research Interests

I work in computer networking, with a particular interest in the modelling and measurement of tele-traffic, in particular the TCP/IP packet  data flowing over the Internet. In general terms the aim is to understand in greater detail how the traffic sources and network structure and protocols interact, with a view to making the network, and end applications, more efficient.  This has lead to work in a number of seemingly different areas including statistical estimation and clock synchronisation.

Scaling Properties of Traffic

Packet traffic has scale invariance features, in particular long range dependent (LRD), which impacts on network performance, performance analysis, accuracy of simulation, and parameter estimation.  My main interest has been in traffic modelling, but in the analysis of real data, the need for more powerful estimation tools naturally arises. Much of my work on this 'side interest' involves wavelets and is in collaboration with Patrice Abry from the Signal Analysis group of the Ecole Normale Supérieure de Lyon. Associated Matlab code can be downloaded.

With my former student Nicolas Hohn we developed models of packet arrivals based on cluster point processes which seem to describe backbone traffic rather well. One of the conclusions is that TCP flows can be treated as independent in the Internet core!

Clock Synchronisation

Software clocks in computers are based on local hardware synchronising to more accurate remote clocks. Currently the NTP system is used to synchronise hosts to remote servers across the Internet.  The stability of modern PC hardware however actually supports higher accuracy and robustness that NTP currently delivers.  We are developing replacement for the NTP clients and servers based on new principles, in particular the need to distinguish between difference clocks and absolute clocks, and the associated primacy of rate stability over absolute clock error.  The RAD difference clock, for example, can measure RTTs to under a microsecond, even if connectively to the time server is lost for periods of over a week!

The RADclock Project has as its aim to provide a complete new system for network timing within two years. Currently client software is available for Linux and BSD Unix which can connect to existing NTP servers. Download details, documentation and several publications can be found on the project page above. This work is in collaboration with Dr. Julien Ridoux.

This project has been made possible in part by a grant from the Cisco University Research Program Fund at Silicon Valley Community Foundation, and by a Google Research Award.

Active Probing

Another key interest is in active probing techniques for traffic and network measurement.  This is where test packets or `probes' are injected into the network, collected on the other side, and inferences made on the end-to-end path based on measured end-to-end delays and/or losses. My interests in this area range from the underlying measurement infrastructure, the `heuristic' design of effective probe streams and their analysis, and the rigorous application of queueing theory to active probing problems. My colleagues in this area include François Baccelli, and  Sridhar Machiraju, and Jean Bolot from Sprint ATL.


Wavelet based tools for estimation with scaling processes

  The second order estimation code includes a number of related capabilities:
  The multifractal estimation code goes beyond second order:

Clock Synchronisation

  Details of the RADclock project (formally known as the TSCclock) and the current release can be found on the RADclock page.