AT&T Labs - Research

K. K. Ramakrishnan

K. K. Ramakrishnan's photo

Email: kkrama AT research dot att dot com
Click here for a list of my publications (with links to some of the papers).


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:
  1. Large scale XML-based information dissemination: Our proposal, XTreeNet, integrates both the publish/subscribe and query/response models over a single overlay network.
  2. 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.
  3. Robust protection and restoration mechanisms of IP backbones to be capable of supporting multimedia distribution.
  4. Understanding the nature of Virtual Private Network communications.
  5. The value of Quality of Service support in IP networks.
  6. Remote Storage Replication for disaster recovery and business continuance.
  7. 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. ( ). 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 ( 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 (, to be held in June 2007. I was the Technical Program Committee Chair for the LANMAN 2005 Workshop.

A section on my early work is here.


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 :
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).

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) architecture was 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.  

I am affiliated with the Internet and  Network Systems Research Center.