My main areas of research revolve around
the design and evaluation of control mechanisms for adaptive applications,
the measurement and analysis of Internet traffic.
and the design of ``lightweight'' resource allocation and pricing schemes
for the Internet,
All three areas above are related and (I believe)
form a coherent whole, as I try to explain briefly.
From a connection's point of view, the current single class
best effort service in the Internet amounts in practice to offering
a channel with time varying characteristics such as delay and
loss distributions. These characteristics
are not known in advance since they depend on the (a priori
unknown) behavior of other connections on the network.
This makes it hard to provide performance guarantees such as
minimum delay or maximum loss rate.
One approach to this problem is to adapt applications to the
channel characteristics -i.e. to the service
provided by the network-, the goal being to maximise the
quality of the data received at the destination(s).
This is done in practice using a panoply of control mechanisms,
each one tailored for a particular channel characteristics.
For example, rate control mechanisms control bandwidth
requirements of an application to match available bandwidth;
FEC-based control mechanisms control the amount of redundant
data to minimze the impact of lost packets, etc.
This approach is attractive because it can be implemented
relatively easily (it does not require any change in routers)
in the current Internet, and it provides immediate
benefits. For example, rate control mechanisms make sure
that no application would hog Internet bandwidth unfairly
(which is not what is done by many applications such as
commercial telephony software which keep sending constant bit rate
data independently of the level of congestion in the Internet).
We (with Andres Vega-Garcia
and Sacha
Fosse-Parisis)
have developed rate, jitter, and FEC-based error control
schemes for multicast multimedia applications.
Early mechanisms were developed for and experimented
with the INRIA Videoconference System
IVS
.
Better mechanisms are included (and new ones
experimented with) in
Free Phone.
Work on resource allocation and control in general, and adaptive
applications in particular, require good characterization
and estimation of Internet connection parameters such as
end-to-end delay or loss. My approach to this problem
combines measurements with analytic models. The results have
proved important to design efficient
control mechanisms for audio and video applications. For example,
measurements show (and analytic models confirm) that the number of
consecutive losses in a CBR stream sent into the Internet is generally small.
This has led us to
develop efficient FEC-based error control schemes for audio tools,
which
have been shown to provide clear improvements in
subjective audio quality.
The measurements have also been used to dimension playout buffers,
to support the case for feedback control of video applications, etc.
Current work examines end to end inference issues,
wavelet-based estimation of Internet
traffic (with Darryl Veitch and Patrice Abry), and the design
of efficient mechanisms using such estimates.
In related work, with
Matthias
Grossglauser, we
have examined under which conditions the salient long range dependence
feature of network traffic must be taken into account in network
performance evaluation. Our results confirm that "it is all a
matter of time scales". Specifically, when studying the performance
of a networking system or an application, many time scales must
be taken into account -- the time scales in the input
traffic, but also the time scales of the system (they show
up for example because of finite buffer queues) and the time
scales of the performance metric of interest.
The "adaptive application" approach above is attractive, but it
still does
not provide performance guarantees in the current single class
FIFO Internet. Another approach to providing users with good
quality applications is to augment the service model of the
Internet and include services which provide (deterministic
or statistical) performance
garantees. This approach requires that specific mechanisms
be implemented at the edge and inside the network. Many
such mechanisms have been examined. However, it is important
to keep such mechanisms (including their interface(s) to
applications) simple.
With
Martin May,
we have examined simple schemes in which service discrimination
in routers is done on the basis of one (or a few) bits
in packet headers. These bits indicate preferential treatment
expressed in terms of throughput, delay, or a combination of these.
Our current work aims at quantifying the service provided by
such schemes, designing appropriate tariffing policies for them,
and experimenting with them. We started this work a year and a
half ago, so this fits in well with the general current interest
in differentiated services in the IETF.
With
Matthias
Grossglauser, we are extending the RCBR (Renegociable
CBR) service Matthias examined a couple of years ago with Keshav and
David Tse to the multicast case. In the process of doing this,
we have had to tackle a couple of neat research issues, which
we will report on (soon....)
I have also worked on caching schemes for distributed systems
such as the DNS or the Web. The main idea is that cache
replacement algorithms should take document size and document retrieval
time into account, in addition to usual parameters such
as locality of reference.
The work on adaptive real-time audio applications described
above translates (in part) into "IP telephony".
Papers:
What's New?
Directions: Hwy 101, exit Shoreline Blvd South,
make a right
at first light (Terra Bella Ave), Ensim is in the first building on
the left ("Valicert" building). We are 24 miles from San Francsico
Airport (SFO), and 9 miles from San Jose Airport (SJC).
Research
Control Mechanisms for Adaptive Applications
Measurement and Analysis of Network Traffic
Leightweight Service Discrimination in the Internet
Other stuff
Advising and Teaching
Recent Papers and Talks
Talks (viewgraphs):
The complete bibliography is here.
Other interests/commitments
How to reach me