James Griffioen

Concast: design and implementation of an active network service

Concast is a network layer service that provides many-to-one channels: multiple sources send messages toward one destination, and the network delivers a single merged copy to that destination. As we have defined it, the service is generic but the relationship between the sent and received messages can be customized for particular applications. We describe the concast service and show how it can be implemented in a back ward-compatible manner in the Internet. We describe its use to solve a problem that has eluded scalable end-system-only solutions: collecting feedback in multicast applications. Our preliminary analysis of concasting effectiveness shows that it provides significant benefits, even with partial deployment. We argue that concast has the characteristics needed for a programmable service to be widely accepted and deployed in the Internet.

Lightweight network support for scalable end-to-end services

Some end-to-end network services benefit greatly from network support in terms of utility and scalability. However, when such support is provided through service-specific mechanisms, the proliferation of one-off solutions tend to decrease the robustness of the network over time. Programmable routers, on the other hand, offer generic support for a variety of end-to-end services, but face a different set of challenges with respect to performance, scalability, security, and robustness. Ideally, router-based support for end-to-end services should exhibit the kind of generality, simplicity, scalability, and performance that made the Internet Protocol (IP) so successful. In this paper we present a router-based building block called ephemeral state processing (ESP), which is designed to have IP-like characteristics. ESP allows packets to create and manipulate small amounts of temporary state at routers via short, predefined computations. We discuss the issues involved in the design of such a service and describe three broad classes of problems for which ESP enables robust solutions. We also present performance measurements from a network-processor-based implementation.

A simple loss differentiation approach to layered multicast

Layered multicast is a promising technique for broadcasting adaptive-quality TV video to heterogeneous receivers. While several-layered multicast approaches have been proposed, prior work has identified several problems including significant and persistent instability in video quality, arbitrary unfairness with other sessions, low access link utilization due to conservative bandwidth allocation, and problems with receiver synchronization. In this paper we propose a new layered multicast scheme, where we exploit a simple, coarse-grained, two-tier loss differentiation architecture to achieve stable and fair bandwidth allocation for viewers. Despite the simplicity of our loss differentiation model, we show that it achieves most of the benefits of complex and costly priority dropping schemes. In addition, our protocol is receiver-driven and thus retains the incentives to limit bandwidth usage that are not present in existing priority dropping schemes.

Building multicast services from unicast forwarding and ephemeral state

We present an approach to building multicast services at the network layer using unicast forwarding and two additional building blocks: ephemeral state probes, i.e. extremely lightweight distributed computations based on a time-bounded associative memory; and the ability to inject or enable packet processing functions that modify router behavior in a very limited way. In our approach, senders and receivers use ephemeral state probes to determine where to inject functionality. A special function that duplicates packets matching a particular pattern and forwards them to a specific destination is then instantiated at the desired network location. Our approach eliminates the need for sophisticated multicast routing protocols and gives the end-systems control over the multicast service, allowing the application to tailor the service to its needs. At the same time, our approach creates efficient forwarding paths by using ephemeral state probes to determine (only) the relevant aspects of the network and group topology. We present two multicast implementations: one builds a multicast tree with centralized control, another provides the traditional IP multicast abstraction. Both implementations can be done in a simple and scalable manner with minimal added functionality in the routers beyond unicast forwarding.

Concast: design and implementation of a new network service

This paper introduces concast, a new network service. Concast is the inverse of multicast: multiple sources send messages toward the same destination, which results in a single message being delivered to the destination. The received message appears to come from the concast group rather than any particular receiver. Different forms of concast service can be defined by varying the mapping from the set of sent messages to the received message. The service is useful for preventing implosion and reducing bandwidth consumption in cases where many senders transmit to the same receiver-for example in aggregating (or suppressing) positive (or negative) acknowledgements. We define the semantics of a simple concast service that is the inverse of multicast, as well as a more general custom concast, which allows users to define certain aspects of the services semantics. We describe how to implement the service so that it scales approximately as well as IP multicast. We also present results from a simulation study showing that concast provides significant benefits in a layered-video application.

CALM: congestion-aware layered multicast

This paper demonstrates the flexibility and utility of two lightweight, general-purpose network services (Emphemeral State Processing and LightWeight Processing modules) by showing how end-systems can use these services to obtain timely and accurate information about the location of congested links and their level of congestion. Although accurate congestion information is useful to a wide range of network services, here we illustrate its benefits to layered multicast systems. In particular, we show how these services can be used to overcome well-known problems with layered multicast, including the desire to reduce router state, the need to drop layers quickly (i.e., within one RTT), the problem of coordinating receivers, and the desire to support fine-grained layering without thrashing between layers-even in the face of join experiments.

Passive inference of path correlation

Overlays have been proposed as a means to improve application performance in many areas, including multimedia streaming and content distribution. Some overlays use parallel transmission to increase aggregate throughput or use backup paths to improve reliability. For such applications, an important consideration is whether the virtual links at the overlay level (i.e. paths between overlay nodes) share links in the underlying network. In particular, choosing parallel or backup paths without any information about path correlation can reduce the effectiveness of the overlay.In this paper we show how to use passive measurement of TCP throughput to provide information about path correlation, for use in overlay routing decisions. Our methods have the advantage that they send no probe traffic to collect path information. We present results of experimental evaluation in both controlled testbed (Emulab) and real wide area network (Planetlab). Our results demonstrate that the methods together work well across a wide range of operating conditions.

A multi-path routing service for immersive environments

The Metaverse project aims to develop technology for low-cost, high-resolution networked immersive display environments that can be used for distributed collaboration, exploration of 3D data models, scientific visualization, and other Grid-related applications. Such applications often deal with massive data sets and need a high-capacity, low-latency transport service to effectively connect distant locations across the wide-area (best-effort) Internet. This work presents the initial design of such an end-to-end transport service for Metaverse applications, along with results of a simulation study evaluating its effectiveness. The transport service features an application programming interface providing enhanced control over the way resources are allocated to data objects, and uses multiple overlay-based end-to-end paths to increase bandwidth delivered to the application.

Building a programmable multiplexing service using concast

Concast is a scalable inverse-multicast network service: messages sent from multiple sources toward the same destination are merged into a single message that is delivered to the destination. The mapping from sent messages to received messages is programmable, so the service can be tailored to the needs of specific applications. However the service can also be used as a building block for other generic network services, such as a packet multiplexing service that encapsulates multiple small packets into a single larger packet and then unencapsulates the small packets at their (common) destination. Such a service offers several potential benefits, including reduced packet processing overhead and increased rate-sharing, but must also be carefully designed to avoid problems caused by added packet delays. We show how concast can serve as the basis for a multiplexing service that can be tailored to the needs of the application. We present simulation results showing that the benefits of our multiplexing service vary with delay. We also show that given certain queue-manipulation capabilities, benefits can be achieved with zero added delay.

Building layered active services

The NodeOS and execution environment (EE) layers of the active network architectural framework are designed to offer a general purpose programming environment to active applications (AAs). However, the architecture suffers from the lack of higher-level APIs and services that would simplify AA code and could be shared by multiple AAs. We propose a modified architecture that introduces an Application Environment (AE) layer above the EE layer to fill the gap. The AE layer offers higher-level services to simplified User-define Processing Modules (UPMs). Together they replace the conventional AA and offer a higher-level API to code to. This paper discusses the issues that arise when breaking the existing AA layer into two layers, and presents our experiences implementing a concast service using the new architecture. We also present performance numbers for our concast service (running on the ASP EE) that show the additional overhead incurred is acceptable.