Active networks allow their users to inject customized programs into the nodes of the network. An extreme case, in which we are most interested, replaces packets with "capsules" -- program fragments that are executed at each network router/switch they traverse. Active architectures permit a massive increase in the sophistication of the computation that is performed within the network. They will enable new applications, especially those based on application-specific multicast, information fusion, and other services that leverage network-based computation and storage. Furthermore, they will accelerate the pace of innovation by decoupling network services from the underlying hardware and allowing new services to be loaded into the infrastructure on demand. In this paper, we describe our vision of an active network architecture, outline our approach to its design, and survey the technologies that can be brought to bear on its implementation. We propose that the research community mount a j...

The deployment of IP Multicast has fostered the development of a suite of applications, collectively known as the MBone tools, for real-time multimedia conferencingover the Internet. Two of these tools --- nv from Xerox PARC and ivs from INRIA --- provide video transmission using softwarebased codecs. We describe a new video tool, vic, that extends the groundbreaking work of nv and ivs with a more flexible system architecture. This flexibility is characterized by network layer independence, support for hardware-based codecs, a conference coordination model, an extensible user interface, and support for diverse compression algorithms. We also propose a novel compression scheme called "IntraH. 261". Created as a hybrid of the nv and ivs codecs, IntraH. 261 provides a factor of 2-3 improvement in compression gain over the nv encoder (6 dB of PSNR) as well as a substantial improvement in run-time performance over the ivs H.261 coder. KEYWORDS Conferencing protocols; digital video; image ...

This paper describes the INRIA Videoconferencing System (IVS), a low bandwidth tool for real-time video between workstations on the Internet using UDP datagrams and the IP multicast extension. The video coder-decoder (codec) is a software implementation of the UIT-T recommendation H.261 originally developed for the Integrated Services Digital Network (ISDN). Our focus in this paper is on adapting this codec for the Internet environment. We propose a packetization scheme, an error control scheme and an output rate control scheme that adapts the image coding process based on network conditions. This work shows that it is possible to maintain videoconferences with reasonable quality across packet-switched networks without requiring special support from the network such as resource reservation or admission control.

We investigate the performance of TCP connections over ATM networks without ATM-level congestion control, and compare it to the performance of TCP over packet-based networks. For simulations of congested networks, the effective throughput of TCP over ATM can be quite low when cells are dropped at the congested ATM switch. The low throughput is due to wasted bandwidth as the congested link transmits cells from `corrupted' packets, i.e., packets in which at least one cell is dropped by the switch. We investigate two packet discard strategies which alleviate the effects of fragmentation. Partial Packet Discard, in which remaining cells are discarded after one cell has been dropped from a packet, somewhat improves throughput. We introduce Early Packet Discard, a strategy in which the switch drops whole packets prior to buffer overflow. This mechanism prevents fragmentation and restores throughput to maximal levels.

This paper presents a novel framework for managing network congestion from an end-to-end perspective. Our work is motivated by several trends in traffic patterns that threaten the long-term stability of the Internet. These trends include the use of multiple independent concurrent flows by Web applications and the increasing use of transport protocols and applications that do not adapt to congestion. We present an end-system architecture centered around a Congestion Manager (CM) that ensures proper congestion behavior and allows applications to easily adapt to network congestion. Our framework integrates congestion management across all applications and transport protocols. The CM maintains congestion parameters and exposes an API to enable applications to learn about network characteristics, pass information to the CM, and schedule data transmissions. Internally, it uses a stable rate-based control algorithm, a scheduler to regulate transmissions, and a lightweight loss-resilient protocol to elicit feedback from receivers. Its ratebased scheme uses additive increase/multiplicative decrease, combined with a novel exponential aging scheme when receiver feedback is infrequent, to obtain both stable network behavior and good application performance.

ly, the remote procedure call problem, which an RPC protocol undertakes to solve, consists of emulating LPC using message passing. LPC has a number of "properties" -- a single procedure invocation results in exactly one execution of the procedure body, the result returned is reliably delivered to the invoker, and exceptions are raised if (and only if) an error occurs. Given a completely reliable communication environment, which never loses, duplicates, or reorders messages, and given client and server processes that never fail, RPC would be trivial to solve. The sender would merely package the invocation into one or more messages, and transmit these to the server. The server would unpack the data into local variables, perform the desired operation, and send back the result (or an indication of any exception that occurred) in a reply message. The challenge, then, is created by failures. Were it not for the possibility of process and machine crashes, an RPC protocol capable of overcomi...

This paper makes a case for paths as an explicit abstraction in operating system design. Paths provide a unifying infrastructure for several OS mechanisms that have been introduced in the last several years, including fbufs, integrated layer processing, packet classifiers, code specialization, and migrating threads. This paper articulates the potential advantages of a path-based OS structure, describes the specific path architecture implemented in the Scout OS, and demonstrates the advantages in a particular application domain---receiving, decoding, and displaying MPEG-compressed video. 1 Introduction Layering is a fundamental structuring technique with a long history in system design. From early work on layered operating systems and network architectures [12, 32], to more recent advances in stackable systems [27, 15, 14, 26], layering has played a central role in managing complexity, isolating failure, and enhancing configurability. This paper describes a complementary, but equally f...

The World Wide Web has succeeded in large part because its software architecture has been designed to meet the needs of an Internet-scale distributed hypermedia application. The modern Web architecture emphasizes scalability of component interactions, generality of interfaces, independent deployment of components, and intermediary components to reduce interaction latency, enforce security, and encapsulate legacy systems. In this article we introduce the Representational State Transfer (REST) architectural style, developed as an abstract model of the Web architecture and used to guide our redesign and definition of the Hypertext Transfer Protocol and Uniform Resource Identifiers. We describe the software engineering principles guiding REST and the interaction constraints chosen to retain those principles, contrasting them to the constraints of other architectural styles. We then compare the abstract model to the currently deployed Web architecture in order to elicit mismatches between the existing protocols and the applications they are intended to support.

Over the past several years there has been a considerable amount of research within the field of quality of service (QoS) support for distributed multimedia systems. To date, most of the work has been within the context of individual architectural layers such as the distributed system platform, operating system, transport subsystem and network. Much less progress has been made in addressing the issue of overall end-to-end support for multimedia communications. In recognition of this, a number of research teams have proposed the development of QoS architectures which incorporate quality of service configurable interfaces and quality of service driven control and management mechanisms across all architectural layers. This paper examines the state-of-the-art in the development of QoS architectures. The approach taken is to present QoS terminology and a generalised QoS framework for understanding and discussing quality of service in the context of distributed multimedia systems. Following this, we evaluate a number of QoS architectures that have emerged in the literature. 1.

Network software is a critical component of any distributed system. Because of its complexity, network software is commonly layered into a hierarchy of protocols, or more generally, into a protocol graph Typical protocol graphs-including those standardized in the IS0 and TCP/IP network architectures-share three important properties: the protocol graph is simple, the nodes of the graph (protocols) encapsulate complex functionality, and the topology of the graph is relatively static. This paper describes a new way to organize network software that differs from conventional architectures in all three of these properties In our approach, the protocol graph is complex, individual protocols encapsulate a single function. and the topology of the graph is dynamic. The main contribution of this paper is to describe the ideas behind our new architec-ture, illustrate the advantages of using the architecture, and demonstrate that the architecture results in efficient network software.