I work on performance and architectural issues of computer networks. My
current work is on a range of topics spanning from overlay networks and
multimedia distribution to transport and link layer protocols to be
robust against loss. I am currently working on the following issues
with
several collaborators and students.
Current
Work:
Large scale XML-based information dissemination: Our proposal, XTreeNet, integrates both the publish/subscribe
and query/response models over a single overlay network.
Improving the performance of transport protocols over lossy
wireless networks: We have developed Loss-Tolerant TCP (LT-TCP) that uses a combination of
packet-level FEC, adaptive MSS and exploits Explicit Congestion
Notification (ECN) to make TCP robust against losses of even 50% over
multi-hop wireless networks.
Robust protection and restoration mechanisms of IP backbones to
be capable of supporting multimedia distribution.
I have also been working on Multiservice Network
Architectures and performance, emphasizing evolution of metro and
access networks to a packet-based infrastructure.
Professional activities:
I am an IEEE Fellow (2005)
“for
contributions to congestion control and traffic management in
communication networks”.
I am an AT&T
Fellow (2006), recognized for "fundamental contributions to
communications networks with lasting impact on AT&T and the
industry, including congestion control, trafic management and VPN
services". More information on the AT&T Fellows can be found here.
I have published over 100 papers, and have been awarded 79 patents.
In October 2003, I received the AT&T
Strategic Patent Award, in recognition of the patent
significantly contributing to AT&T's business, for Patent No.
6,324,279 — Method for Exchanging Signaling Message in Two Phases. (http://www.research.att.com/innovators.cfm?portal=17
). This was part of our work on IP Telephony
and the DOSA architecture, one of the first to integrate resource
management with call signaling.
My paper “A Binary Feedback Scheme for Congestion Avoidance in Computer
Networks with a Connectionless Network Layer” published in the
Proceedings of the ACM Sigcomm 1988 received the ACM Sigcomm Test of Time Paper Award in 2006
(http://www.sigcomm.org/tot/index.html).
This paper also received recognition in 1995 from ACM Sigcomm for being
one of the top innovations in networking in the last 25 years and was
republished by ACM SIGCOMM Computer Communications Review in its 25th
Anniversary Issue (Jan. 1995). The journal version ( "DECbit"
journal paper) appeared in ACM Transactions on Computer Systems in
1990. I continue to work on congestion control issues, and towards the
widespread deployment of Explicit Congestion Notification (ECN)-based
mechanisms in the Internet.
I am the General Co-Chair for the IEEE
LANMAN 2007 Workshop (http://www.ieee-lanman.org/),
to be held in June 2007. I was the Technical Program Committee Chair
for the LANMAN 2005 Workshop.
XTreeNet
Information is increasingly being created and exchanged in the
eXtensible Markup Language (XML). XTreeNet is an overlay network
that efficiently supports
large-scale XML content access and distribution XTreeNet unifies
two major information access paradigms, publish-subscribe and
query-response, based on the conceptual ideas of :
content descriptors (CDs), which are analogous to database index
entries and describe the information producers generate and consumers
desire at a fine granularity, and
CD distribution trees connecting producers and consumers
interested in the same CD.
An extended abstract of the XTreeNet concepts appeared in the 2005
Workshop on Web Content Caching and Distribution (XTreeNet-paper)
__________________________________________________________________________________________________________________ Loss
Tolerant TCP
TCP performance degrades rapidly as packet error rates increase.
LT-TCP is a highly loss-tolerant TCP enhancement that uses an
adaptive end-to-end hybrid ARQ/FEC reliability strategy exploiting ECN
for congestion detection. LT-TCP improves the performance of TCP
substantially, compared to TCP-SACK, for both uniform and bursty loss
scenarios, and for end-end error rates of even up to 50%. What is
attractive about LT-TCP is that the achieved goodput shows a relatively
smooth and linear decrease with increasing error rates, even with
substantial end-end round trip times over a path that may comprise
multiple wireless hops, each with significant bursty loss.
The first published paper on LT-TCP
appeared in IWQoS 2005. We are actively working on LT-TCP and seek to
find the right balance between link layer and transport layer support
for error-protection.
__________________________________________________________________________________________________________________ Virtual
Private Networks Virtual Private Networks (VPNs) are experiencing dramatic
growth. The number of endpoints per VPN is growing and the
communication
pattern between endpoints can be complex and difficult to predict.
Consequently, users are demanding dependable, dynamic connectivity
between endpoints, with the network expected to accommodate any traffic
matrix, as long as the traffic to the endpoints does not overwhelm the
capacity of the respective ingress and egress links. I proposed a new
service interface, termed a hose, to provide
the appropriate performance abstraction. A hose is characterized by the
aggregate traffic to and from one endpoint in the VPN to a set of other
endpoints in the VPN, and by an associated performance guarantee. Hoses
provide important advantages to a VPN customer: (i) flexibility to send
traffic to a set of endpoints without having to specify the detailed
traffic matrix, and (ii) reduction in the size of access links through
multiplexing gains obtained from the natural aggregation of the flows
between endpoints. Our paper introducing the Hose Model and related
resource management issues appeared in ACM Sigcomm 1999. A complete
version of the paper appeared in IEEE/ACM Transactions on Networking in
2002 (VPN-Hose_Model)
I continue to work with others on understanding the nature of
communication in VPNs. Based on measurements, we have analyzed the
behavior of a large number VPN customers over a few years. A paper on
the characterization based on the estimation of VPN traffic matrices
appeared in IMC 2004 (VPN-characterization).
We have also quantified the benefit of such characterization on
resource allocation in another paper that appeared in Infocom 2005 (VPN-Resource-Management).
We have also examined the characteristics of change in VPN customers
and their behavior over several years in a paper presented at the
Informs 8th Telecommunications Conference in 2006.
__________________________________________________________________________________________________________________ Multiservice Network Architectures
Our work on Multiservice access has resulted in defining the overall
strategy of AT&T for packet-based access networks. Metro
networks initially evolved from the need to support traditional voice
and private line services. However, the tremendous growth in access to
Frame Relay, ATM, IP and Ethernet services, coupled with the desire of
enterprise customers to interconnect via Ethernet interfaces, suggests
the need for a new approach. We proposed a new architecture for
Packet-Aware
Transport Networks (PATN) which supports both packet and traditional
TDM services and which leverages an assemblage of emerging technologies
to provide efficient aggregation and switching of packet traffic in
metro networks. The PATN has the potential to provide significant cost
savings to carriers by reducing the number of network elements,
reducing
transport costs through statistical multiplexing, and eliminating the
need for redundant multiplexing operations.
A paper on our proposed architecture for Packet Aware Transport
Networks (IEEE
Communications Magazine Paper on PATN) appeared in the IEEE
Communications Magazine in March 2004. A detailed paper on the
architecture and our experimental work to evaluate the benefits
appeared
in Journal of Optical Networking in 2006 (JON paper).
__________________________________________________________________________________________________________________
Remote Storage Replication
Our work on storage focuses on a remote replication service for
disaster recovery and business continuance. We have examined the
performance of a commercial replication sysetm and its feasibility for
remote replication. The network latency due to synchronous replication
is difficult to tolerate in scenarios where businesses
are required by regulation to separate their secondary sites from the
primary by hundreds of miles. We propose a semantic-aware remote
replication system to meet the contrasting needs of both system
efficiency and safe remote replication with tight recovery-point and
recovery-time objectives. A position paper on this subject is set to
appear in Oct. 2006 at the 2nd International Workshop on Storage
Security and Survivability (storagess_paper).
__________________________________________________________________________________________________________________
ECN
I have been working on congestion control and avoidance for a long
time, with particular emphasis on explicit feedback of congestion
information from the network. I have been involved in the congestion
control efforts of most packet networking technologies, especially
TCP/IP, Frame Relay, ATM, and IEEE 802.17 (Resilient Packet
Rings.) Sally Floyd and I have proposed the use of Explicit
Congestion Notification (ECN) for TCP/IP networks. A web page
(including
a pointer to RFC 3168, which is a Proposed Standard) is: ECN (Explicit Congestion
Notification) in TCP/IP .
__________________________________________________________________________________________________________________
IP
Telephony
Our work on the
Distributed Open Signaling Architecture (DOSA) architecturewas one of
the first to integrate Quality of Service with Call Signaling for IP
Telephony. A paper on DOSA is: (DOSA-Infocom
2000 Paper). Some of these ideas were patented, and in
October 2003, we received the AT&T Strategic Patent Award, in recognition of a patent that significantly
contributes to AT&T's business (see AT&T
Strategic Patent Award ). Our architectural work on IP
Telephony, "Telephony Over Packet networkS (TOPS)". TOPS allows users
to
move between terminals or to use mobile terminals while being reachable
by the same name. TOPS users can have multiple terminals and control
how
calls are routed to them. TOPS allows for terminals with a range of
capabilities such as support for video, whiteboard and other media with
a variety of coding formats. TOPS retains the necessary information on
terminal capabilities to determine the appropriate type of
communication
to be established with the remote terminal. The architecture assumes
that the underlying network supports the establishment of end-to-end
connectivity between terminals, with an appropriate quality of service.
A paper on this appeared in IEEE JSAC (TOPS-paper).
Previous Work: At AT&T, in the past I have worked on Multicast, Signaling
(Lightweight signaling for ATM), IP over ATM. IP Telephony and
Networked-multimedia support. Before coming to AT&T, I worked on
congestion management and network I/O while at Digital Equipment
Corporation for 11 years.
1.
Congestion Avoidance and Control
a. Connectionless Networks:
I have worked on congestion control for connectionless packet oriented
networks with window flow control such as DECnet, OSI and TCP/IP for a
long time. The most significant work on this is the so-called DECbit
work, which uses a single bit feedback for congestion avoidance.
Its mechanisms for increase/decrease (Additive Increase Multiplicative
Decrease) and a binary feedback for explicit congestion notification
have been widely adopted in many different network architectures
including TCP/IP, OSI, ATM and Frame Relay. A paper on this was
published in ACM Sigcomm 1988 and the full paper was published in the
ACM Transactions on Computer Systems in 1990 (DECbit-TOCS). Numerous
other papers were published on this topic. An early version of a paper
on the high-level principles behind DECbit is DEC-TR-506.
One follow-on work to achieve Max-Min fairness with binary feedback
used "Selective Feedback" (DEC-TR-510).
b. ATM Networks
Between 1995 and 1997, I worked on issues related to ATM. I worked on
algorithms for achieving max-min fairness for the ABR service in ATM.
Issues of scale were of concern. Scalability is aided by efficient
techniques to compute the max-min fair allocation, (described in HPN_95, brdcom96,)
as well as minimizing the amount of state information to be maintained
(using discrete rates ). We
have
published several papers on a rate-allocation algorithm for the ABR
service. I also looked at the interaction of TCP's window-flow
controlled mechanisms with a rate-controlled environment.
We have also analyzed in detail the effect of bi-directional traffic
on window flow control protocols such as TCP. The effect of
ack-compression is a degradation in throughput and unfairness. We
quantify it. We have also looked at its effect when the channels are
asymmetric - when the bandwidth in one direction is substantially
smaller than the other.
A formal specification of the source/destination behavior that is
specified in the ATM Forum's Traffic Management specification is
another
activity I have been involved in efsm
).
Other work related to ATM congestion management is in understanding
the issues of time-scale, (described in infocom_96 ) and exploring ways of
integrating different mechanisms for ATM congestion management ( rate_credit integration ).
__________________________________________________________________________________________________________________
2. Support
for Compressed Video over Networks
I also worked on supporting compressed video (source-adaptable
semi-real-time) traffic on ATM networks, using the Explicit Rate
algorithms of the type recommended for the ABR service explicit_rate_video ), and on
multipoint-to-multipoint communication over ATM seam ). A more detailed paper on
the
support of compressed video over rate based control schemes appeard in
Infocom 1997, and a journal version in IEEE/ACM Transactions on
Networking in 1999 (explicit-rate-video-TON).
I also worked on smoothing of compressed video. Supporting compressed
video efficiently on networks is a challenge because of its burstiness.
Although a large number of applications using compressed video allow
adaptive rates, it is also important to preserve quality as much as
possible. We propose a smoothing and rate
adaptation algorithm for compressed video, called SAVE, that is used in
conjunction with explicit rate based control in the network. SAVE
smooths the demand from the source to the network, thus helping achieve
good multiplexing gains. SAVE maintains the quality of the video and
ensures that the delay at the source buffer does not exceed a bound. We
show that SAVE is effective by demonstrating its performance across 28
different traces (entertainment and teleconferencing videos)
that use different compression algorithms. Two journal papers related
to SAVE are: IEEE/ACM Transactions on Networking, 1998 (SAVE-TON-1998) and IEEE Transactions on
Multimedia, 1999 (SAVE-Multimedia).
We proposed techniques to carry real-time data efficiently over ATM,
using an encapsulation that we call Real-Time AAL5. This was discussed
at the ATM Forum, and is described in Forum_SAA_0139.
__________________________________________________________________________________________________________________
3. Network I/O
I have worked on issues related to high performance Network I/O. In
particular, at Digitial Equipment Corporation, I worked on architecture
and scheduling for network adapters and the protocol implementation in
end-systems and host operating systems. A paper on Performance
Considerations for Designing Network Interfaces appeared in IEEE JSAC
in
1993 (Designing-NICS-JSAC-1993).
Another paper on scheduling considerations in the host operating system
for network I/O, particularly to mitigate "receive livelocks" appeared
in ACM Transactions on Computer Systems in 1997 (kk-mogul-1997).
__________________________________________________________________________________________________________________
4 Other
Work of Interest.
The first A few of other areas I have worked on, while at Digital
Equipment Corporation were in Operating System support for ATM,
particularly IP over ATM,
Operating System support for a Video-on-Demand Server ( VOD_paper ), and on the issues of
network overload in Interrupt-Driven kernels (Networking Implementation
). An interesting effect of the CSMA/CD protocol used on Ethernet is
called the "Ethernet Capture Effect". We came up with an interesting
solution for the problem which is in widespread use (Capture Effect.) My past work at
DEC included work on DECbit, on network I/O (adapters and protocol
implementation in end-systems) and file system performance.
