Scientific computing is becoming more data-centric and more collaborative, requiring increasingly large datasets to be transferred across the Internet. Transferring these datasets efficiently and making them accessible to scientific workflows is an increasingly difficult task. In addition, the data transfer time can be a significant portion of the overall workflow running time. This paper presents SDM (Syndicate Dataset Manager), a scientific dataset delivery platform. Unlike general-purpose data transfer tools, SDM offers on-demand access to remote scientific datasets. On-demand access doesn't require staging datasets to local file systems prior to computing on them, and it also enables overlapping computation and I/O. In addition, SDM offers a simple interface for users to locate and access datasets. To validate the usefulness of SDM, we performed realistic metagenomic sequence analysis workflows on remote genomic datasets. In general, SDM configured with a CDN outperforms existing data access methods. With warm CDN caches, SDM completes the workflow 17-20% faster than staging methods. Its performance is even comparable to local storage. SDM is only 9% slower than local HDD storage and 18% slower than local SSD storage. Together, its performance and its ease-of-use make SDM an attractive platform for performing scientific workflows on remote datasets.

The cloud is changing the way we share data. We can keep data on local workstations and file servers for quick access, but face the challenge of sharing it with a large number of people. Alternatively, we can put our data into one or more cloud storage systems to share it with many other users, but then we cannot access it as quickly or as cheaply. Moreover, local copies of data can get out of sync with cloud copies, causing remote users to see old versions. Our solution is Syndicate, a virtual cloud storage system that composes local storage, cloud storage, and commodity CDNs and network caches to transparently give users the best of both worlds.

Recent efforts to federate computation and communication resources across organizational boundaries face a challenge in establishing the policies by which one organization's users can access resources in other organizations. This paper describes an approach to defining, communicating, analyzing, and enforcing resource allocation policies in this new setting. Our approach was designed to address the needs of Planet Lab, but we demonstrate through a range of examples that it is general enough to accommodate a diverse collection of computing facilities. Our policy engine is implemented in a specific tool chain, called sfatables, that is patterned after the iptables mechanism used to define packet processing policies for network traffic. The interface to our policy engine thus uses the familiar paradigm of a firewall and provides a flexible interface for resource owners to specify access policies for their resources. Our implementation makes it possible to precisely document policies, query, and analyze them.

We describe Trellis, a platform for hosting virtual networks on shared commodity hardware. Trellis allows each virtual network to define its own topology, control protocols, and forwarding tables, while amortizing costs by sharing the physical infrastructure. Trellis synthesizes two container-based virtualization technologies, VServer and NetNS, as well as a new tunneling mechanism, EGRE, into a coherent platform that enables high-speed virtual networks. We describe the design and implementation of Trellis and evaluate its packet-forwarding rates relative to other virtualization technologies and native kernel forwarding performance.

Hypervisors, popularized by Xen and VMware, are quickly becoming commodity. They are appropriate for many usage scenarios, but there are scenarios that require system virtualization with high degrees of both isolation and efficiency. Examples include HPC clusters, the Grid, hosting centers, and PlanetLab. We present an alternative to hypervisors that is better suited to such scenarios. The approach is a synthesis of prior work on resource containers and security containers applied to general-purpose, time-shared operating systems. Examples of such container-based systems include Solaris 10, Virtuozzo for Linux, and Linux-VServer. As a representative instance of container-based systems, this paper describes the design and implementation of Linux-VServer. In addition, it contrasts the architecture of Linux-VServer with current generations of Xen, and shows how Linux-VServer provides comparable support for isolation and superior system efficiency.

A key step in the semantic analysis of network traffic is to parse the traffic stream according to the high-level protocols it contains. This process transforms raw bytes into structured, typed, and semantically meaningful data fields that provide a high-level representation of the traffic. However, constructing protocol parsers by hand is a tedious and error-prone affair due to the complexity and sheer number of application protocols.This paper presents binpac, a declarative language and compiler designed to simplify the task of constructing robust and efficient semantic analyzers for complex network protocols. We discuss the design of the binpac language and a range of issues in generating efficient parsers from high-level specifications. We have used binpac to build several protocol parsers for the "Bro" network intrusion detection system, replacing some of its existing analyzers (handcrafted in C++), and supplementing its operation with analyzers for new protocols. We can then use Bro's powerful scripting language to express application-level analysis of network traffic in high-level terms that are both concise and expressive. binpac is now part of the open-source Bro distribution.

This paper describes VINI, a virtual network infrastructure that allows network researchers to evaluate their protocols and services in a realistic environment that also provides a high degree of control over network conditions. VINI allows researchers to deploy and evaluate their ideas with real routing software, traffic loads, and network events. To provide researchers flexibility in designing their experiments, VINI supports simultaneous experiments with arbitrary network topologies on a shared physical infrastructure. This paper tackles the following important design question: What set of concepts and techniques facilitate flexible, realistic, and controlled experimentation (e.g., multiple topologies and the ability to tweak routing algorithms) on a fixed physical infrastructure? We first present VINI's high-level design and the challenges of virtualizing a single network. We then present PL-VINI, an implementation of VINI on PlanetLab, running the "Internet In a Slice". Our evaluation of PL-VINI shows that it provides a realistic and controlled environment for evaluating new protocols and services.

In mobile devices, the wireless network interface card (WNIC) consumes a significant portion of overall system energy. One way to reduce energy consumed by a device is to transition its WNIC to a lower-power sleep mode when data is not being received or transmitted. In this paper, we investigate client-centered techniques for energy efficient communication, using IEEE 802.11b, within the network layer. The basic idea is to conserve energy by keeping the WNIC in high-power mode only when necessary. We track each connection, which allows us to determine inactive intervals during which to transition the WNIC to sleep mode. Whenever necessary, we also shape the traffic from the client side to maximize sleep intervals—convincing the server to send data in bursts. This trades lower WNIC energy consumption for an increase in transmission time. Our techniques are compatible with standard TCP and do not rely on any assistance from the server or network infrastructure. Results show that during Web browsing, our client-centered technique saved 21 percent energy compared to PSM and incurred less than a 1 percent increase in transmission time compared to regular TCP. For a large file download, our scheme saved 27 percent energy on average with a transmission time increase of only 20 percent.

This paper describes our effort to build extensible routers using a combination of general-purpose and network processors. We emphasize five overriding challenges that dictate our design decisions: (1) optimal resource allocation; (2) efficient but flexible scheduling of the CPU; (3) maintaining overall router robustness; (4) maximizing router performance; and (5) providing sufficient extensibility to enable the injection of new functionality into the router. We adopt a hierarchical architecture, in which packet flows traverse a range of processing/forwarding paths, thereby partitioning hardware and software in concert. This paper both presents the architecture, and describes our experiences implementing the architecture and addressing the five design challenges in a prototype built from Intel IXP 1200 and a Pentium. Copyright © 2005 John Wiley & Sons, Ltd.

Monitoring any portion of the Internet address space reveals incessant activity. This holds even when monitoring traffic sent to unused addresses, which we term "background radiation. " Background radiation reflects fundamentally nonproductive traffic, either malicious (flooding backscatter, scans for vulnerabilities, worms) or benign (misconfigurations). While the general presence of background radiation is well known to the network operator community, its nature has yet to be broadly characterized. We develop such a characterization based on data collected from four unused networks in the Internet. Two key elements of our methodology are (i) the use of filtering to reduce load on the measurement system, and (ii) the use of active responders to elicit further activity from scanners in order to differentiate different types of background radiation. We break down the components of background radiation by protocol, application, and often specific exploit; analyze temporal patterns and correlated activity; and assess variations across different networks and over time. While we find a menagerie of activity, probes from worms and autorooters heavily dominate. We conclude with considerations of how to incorporate our characterizations into monitoring and detection activities.

In this paper we evaluate a method of using interleaved spanning trees to compose a resilient, high performance overlay mesh. Though spanning trees of arbitrary type could be used to construct an overlay mesh, we focus on a distributed algorithm that computes k minimum spanning trees on an arbitrary graph. The principal motivation behind this strategy is to provide applications with a k-redundant, high quality mesh suitable for demanding applications like A/V broadcast, video conferencing, data collection, multi-path routing, and file mirroring/transfer. We elaborate details of k-MST, pointing out advantages and potential problem points of the protocol, and then analyze its performance using a variety of metrics with simulation as well as a functional PlanetLab implementation.

In mobile devices, the wireless network interface card (WNIC) consumes a significant portion of overall system energy. One way to reduce energy consumed by a mobile device is to transition its WNIC to a lower-power sleep mode when data is not being received or transmitted. This paper investigates client-centered techniques for trading download time for energy savings during TCP downloads, in an attempt to reduce the energy' delay product. Effectively saving WNIC energy during a TCP download is difficult because TCP streams tend to be smooth, leaving little potential sleep time. The basic idea behind our technique is that the client increases the amount of time that can be spent in sleep mode by shaping the traffic. In particular, the client convinces the server to send data in predictable bursts, trading lower WNIC energy cost for increased transmission time. Our technique does not rely on any assistance from the server, a proxy, or IEEE 802.11b power-saving mode. Results show that in Internet experiments our scheme outperforms baseline TCP by 64% in the best case, with an average improvement of 19%.

TCP performs poorly on paths that reorder packets significantly, where it misinterprets out-of-order delivery as packet loss. The sender responds with a fast retransmit though no actual loss has occurred. These repeated false fast retransmits keep the sender's window small, and severely degrade the throughput it attains. Requiring nearly in-order delivery needlessly restricts and complicates Internet routing systems and routers. Such beneficial systems as multi-path routing and parallel packet switches are difficult to deploy in a way that preserves ordering. Toward a more reordering-tolerant Internet architecture, we present enhancements to TCP that improve the protocol's robustness to reordered and delayed packets. We extend the sender to detect and recover from false fast retransmits using DSACK information, and to avoid false fast retransmits proactively, by adaptively varying dupthresh. Our algorithm is the first that adaptively balances increasing dupthresh, to avoid false fast retransmits, and limiting the growth of dupthresh, to avoid unnecessary timeouts. Finally, we demonstrate that TCP's RTO estimator tolerates delayed packets poorly, and present enhancements to it that ensure it is sufficiently conservative, without using timestamps or additional TCP header hits. Our simulations show that these enhancements significantly improve TCP's performance over paths that reorder or delay packets.

We argue that designing overlay services to independently probe the Internet--with the goal of making informed application-specific routing decisions--is an untenable strategy. Instead, we propose a shared routing underlay that overlay services query. We posit that this underlay must adhere to two high-level principles. First, it must take cost (in terms of network probes) into account. Second, it must be layered so that specialized routing services can be built from a set of basic primitives. These principles lead to an underlay design where lower layers expose large-scale, coarse-grained static information already collected by the network, and upper layers perform more frequent probes over a narrow set of nodes. This paper proposes a set of primitive operations and three library routing services that can be built on top of them, and describes how such libraries could be useful to overlay services.

This paper describes and evaluates a probabilistic packet scheduling (PPS) algorithm for providing different levels of service to TCP flows. With our approach, each router defines a local currency in terms of tickets and assigns tickets to its inputs based on contractual agreements with its upstream routers. A flow is tagged with tickets to represent the relative share of bandwidth it should receive at each link. When multiple flows share the same bottleneck, the bandwidth that each flow obtains is proportional to the relative tickets assigned to that flow. Simulations show that PPS does a better job of proportionally allocating bandwidth than DiffServ and weighted CSFQ. In addition, PPS accommodates flows that cross multiple currency domains.

We view congestion control as a distributed primal--dual algorithm carried out by sources and links over a network to solve a global optimization problem. We describe a multilink multisource model of the TCP Vegas congestion control mechanism. The model provides a fundamental understanding of delay, fairness and loss properties of TCP Vegas. It implies that Vegas stabilizes around a weighted proportionally fair allocation of network capacity when there is sufficient buffering in the network. It clarifies the mechanism through which persistent congestion may arise and its consequences, and suggests how we might use REM active queue management to prevent it. We present simulation results that validate our conclusions.

Recent efforts to add new services to the Internet have increased interest in software-based routers that are easy to extend and evolve. This paper describes our experiences using emerging network processors---in particular, the Intel IXP1200---to implement a router. We show it is possible to combine an IXP1200 development board and a PC to build an inexpensive router that forwards minimum-sized packets at a rate of 3.47Mpps. This is nearly an order of magnitude faster than existing pure PC-based routers, and sufficient to support 1.77Gbps of aggregate link bandwidth. At lesser aggregate line speeds, our design also allows the excess resources available on the IXP1200 to be used robustly for extra packet processing. For example, with 8 × 100Mbps links, 240 register operations and 96 bytes of state storage are available for each 64-byte packet. Using a hierarchical architecture we can guarantee line-speed forwarding rates for simple packets with the IXP1200, and still have extra capacity to process exceptional packets with the Pentium. Up to 310Kpps of the traffic can be routed through the Pentium to receive 1510 cycles of extra per-packet processing.

Mobile code makes it easier to maintain, debug, update, and customize a system. Active networks are one of the more interesting applications of mobile code: code is injected into the nodes of a network to customize the network's functionality, such as routing, and to add new features, such as special-purpose congestion control and filtering algorithms. The challenge is to develop a communication-oriented platform for such systems. We refer to mobile code targeted at low-level, communication-oriented systems like active networks as liquid software, the key distinction being that liquid software is focused on the efficient transfer of data, not high-performance computation. To this end, we have designed and implemented Joust, which consists of a complete re-implementation of the Java virtual machine (including both the runtime system and a just-in-time compiler), running on the Scout operating system (a configurable, communication-oriented OS). The result is a configurable, high-performance platform for running liquid software. We present the results of implementing two different applications of liquid software on Joust, including a prototype architecture for active networks. Copyright © 2000 John Wiley & Sons, Ltd.

A TCP forwarder is a network node that establishes and forwards data between a pair of TCP connections. An example of a TCP forwarder is a firewall that places a proxy between a TCP connection to an external host and a TCP connection to an internal host, controlling access to a resource on the internal host. Once the proxy approves the access, it simply forwards data from one connection to the other. We use the term TCP forwarding to describe indirect TCP communication via a proxy in general. This paper characterizes the behavior of TCP forwarding, and illustrates the role TCP forwarding plays in common network services like firewalls and HTTP proxies. We then introduce an optimization technique, called connection splicing, that can be applied to a TCP forwarder, and report the results of a performance study designed to evaluate its impact. Connection splicing improves TCP forwarding performance by a factor of two to four, making it competitive with IP router performance on the same hardware.

This paper reports on a set of experiments that measure the amount of CPU processing needed to decode MPEGcompressed video in software. These experiments were designed to discover indicators that could be used to predict how many cycles are required to decode a given frame. Such predictors can be used to do more accurate CPU scheduling. We found that by considering both frame type and size, it is possible to construct a linear model of MPEG decoding with R 2 values of 0.97 and higher. Moreover, this model can be used to predict decoding times at both the frame and packet level that are almost always accurate to within 25% of the actual decode times. This is a surprising result given the large variability in MPEG decoding times, and suggests that it is feasible to design systems that make quality of service guarantees for MPEG-encoded video. 1 Introduction The ability of modern processors to decode MPEG compressed video in realtime has led to users wanting to run a mix of application...

Scout is a communication oriented operating system that can be specialized for different information appliances. It uses paths as an explicit first class object to describe the flow of information through the system. Escort is the security architecture for Scout. It uses the explicit knowledge provided by a path abstraction to secure information flow in a flexible manner.

No abstract available.

This paper describes several techniques designed to improve protocol latency, and reports on their effectiveness when measured on a modern RISC machine employing the DEC Alpha processor. We found that the memory system---which has long been known to dominate network throughput---is also a key factor in protocol latency. As a result, improving instruction cache effectiveness can greatly reduce protocol processing overheads. An important metric in this context is the memory cycles per instructions (mCPI), which is the average number of cycles that an instruction stalls waiting for a memory access to complete. The techniques presented in this paper reduce the mCPI by a factor of 1.35 to 5.8. In analyzing the effectiveness of the techniques, we also present a detailed study of the protocol processing behavior of two protocol stacks---TCP/IP and RPC---on a modern RISC processor.

Simulation is a critical tool in developing, testing, and evaluating network protocols and architectures. This paper describes x-Sim, a network simulator based on the x-kernel, that is able to fully simulate the topologies and traffic patterns of large scale networks. It also illustrates the capabilities and usefulness of the simulator with case studies. Finally, based on our experiences using x-Sim, we identify a set of principles (guidelines) for network simulation, and present concrete examples that quantify the value of these principles, along with the cost of ignoring them.

This article presents a software-only solution to the synchronization problem for uniprocessors. The idea is to execute atomic sequences without any hardware protection, and in the rare case of pre-emption, to roll the sequence forward to the end, thereby preserving atomicity. One of the proposed implementations protects atomic sequences without any memory-accesses. This is significant as it enables execution at CPU-speeds, rather than memory-speeds. The benefit of this method increases with the frequency at which atomic sequences are executed. It therefore encourages the building of systems with fine-grained synchronization. This has the additional advantage of reducing average latency. Experiments demonstrate that this technique has the potential to outperform even the best hardware mechanisms. The main contribution of this article is to discuss operating-system related issues of rollforward and to demonstrate its practicality, both in terms of flexibility and performance.

Scout is new communication-centric operating system. The principle scout abstraction (the path) is an attempt to capture all of the operating system infrastructure necessary to insure that a given network connection can achieve high and predictable performance in the face of other connections and other system loads.

Vegas is an implementation of TCP that achieves between 37 and 71% better throughput on the Internet, with one-fifth to one-half the losses, as compared to the implementation of TCP in the Reno distribution of BSD Unix. This paper motivates and describes the three key techniques employed by Vegas, and presents the results of a comprehensive experimental performance study, using both simulations and measurements on the Internet, of the Vegas and Reno implementations of TCP.>

Vegas is a new implementation of TCP that achieves between 40 and 70% better throughput, with one-fifth to one-half the losses, as compared to the implementation of TCP in the Reno distribution of BSD Unix. This paper motivates and describes the three key techniques employed by Vegas, and presents the results of a comprehensive experimental performance study—using both simulations and measurements on the Internet—of the Vegas and Reno implementations of TCP.

This paper describes our experiences, from a software perspective, with the OSIRIS network adaptor. It first identifies the problems we encountered while programming OSIRIS and optimizing network performance, and outlines how we either addressed them in the software, or had to modify the hardware. It then describes the opportunities provided by OSIRIS that we were able to exploit in the host operating system (OS); opportunities that suggested techniques for making the OS more effective in delivering network data to application programs. The most novel of these techniques, called application device channels, gives application programs running in user space direct access to the adaptor. The paper concludes with the lessons drawn from this work, which we believe will benefit the designers of future network adaptors.

Consul is a system designed to support the use of fault-tolerance techniques in the construction of fault-tolerant distributed systems structured according to the state machine approach. The way in which modularity has been used in the design and implementation of Consul is described. The approach makes it easy to configure a system customized to the needs of a specific application and facilitates the development of the individual components that make up Consul.>

We have designed and implemented a new operating system facility for I/O buffer management and data transferacross protection domain boundaries on shared memory machines. This facility, called fast buffers (fbufs), combines virtual page remapping with shared virtual memory, and exploits locality in I/O traffic to achieve high throughput without compromising protection, security, or modularity. goal is to help deliver the high bandwidth afforded by emerging high-speed networks to user-level processes, both in monolithic and microkernel-based operating systems.This paper outlines the requirements for a cross-domain transfer facility, describes the design of the fbuf mechanism that meets these requirements, and experimentally quantifies the impact of fbufs on network performance.

Abstract The use of modularity in the design and implementation of complex software simplifies the development process, as well as facilitating the construction of customized configurations. This paper describes our experience using modularity in Consul, a communication substrate used for constructing fault‐tolerant distributed programs. First, Consul is presented as a case study of how modularity is feasible in both the design and the implementation of such systems. Secondly, general lessons about modularity in fault‐tolerant systems based on our experience with Consul are given. Issues that are addressed include deciding how the system is divided into various modules, dealing with problems that result when protocols are combined, and ensuring that the underlying object infrastructure provides adequate support. The key observation is that the modularization process is most affected by dependencies between modules, both direct dependencies caused by one module explicitly using another's operation and indirect dependencies where one module is affected by another without direct interaction. Although our observations are based on designing and implementing Consul, the lessons are applicable to any fault‐tolerant distributed system.

Morpheus is a special-purpose programming language that facilitates the efficient implementation of communication protocols. Protocols are divided into three categories, called shapes, so that they can inherit code and data structures based on their category. The programmer implements a particular protocol by refining the inherited structure. Morpheus optimization techniques reduce per-layer overhead on time-critical operations to a few assembler instructions even though the protocol stack is not determined until run time. This supports divide-and-conquer simplification of the programming task by minimizing the penalty for decomposing complex protocols into combinations of simpler protocols.>

Integrating protocol data manipulations is a strategy for increasing the throughput of network protocols. The idea is to combine a series of protocol layers into a pipeline so as to access message data more efficiently. This paper introduces a widely-applicable technique for integrating protocols. This technique not only improves performance, but also preserves the modularity of protocol layers by automatically integrating independently expressed protocols. The paper also describes a prototype integration tool, and studies the performance limits and scalability of protocol integration.>

This paper defines a simple model, called a preference hierarchy, that provides a framework for using the information available to a naming system to compute the object(s) identified by a given name. The preference hierarchy therefore serves as a formal tool for designing and reasoning about naming systems.

The Jade file system, which provides a uniform way to name and access files in an Internet environment, is introduced. Jade is a logical system that integrates a heterogeneous collection of existing file systems in which underlying file systems support different file access protocols. Because of autonomy, Jade is designed under the restriction that the underlying file systems may not be modified. In order to avoid the complexity of maintaining an Internet-wide, global name space, Jade permits each user to define a private name space. Jade's name space supports two features: it allows multiple file systems to be mounted under one directory, and it permits one logical name space to mount other logical name spaces. A prototype of Jade has been implemented to examine and validate its design. The prototype consists of interfaces to the Unix File System, the Sun Network File System, and the File Transfer Protocol. An overview of Jade's design is reported, and the authors' experiences in designing and implementing a large scale file system are reviewed.>

It is argued that a modular operating system architecture should provide support for modularity independent of protection domains. Given such support, modules and interfaces can be designed according to sound software engineering principles, without concern for cross-domain invocation costs. The partitioning of modules into domains and across machines becomes a matter of configuration, rather than design. Current micro-kernel-based architectures do not sufficiently address this issue since their communication mechanisms are designed for the nonlocal, i.e., cross-domain, case. An architecture that provides location-transparent binding and access of modules optimized for the local case, thereby decoupling the orthogonal concepts of modularity and protection, is proposed.>

Morpheus is special-purpose programming language that facilitates the efficient implementation of communication protocols. Protocols are divided into three categories, called shapes, so that they can inherit code and data structures based on their category; the programmer implements a particular protocol by refining the inherited structure. Morpheus optimization techniques reduce per-layer overhead on time-critical operations to a few assembler instructions even though the protocol stack is not determined until runtime. This supports divide-and-conquer simplification of the programming task by minimizing the penalty for decomposing complex protocols into combinations of simpler protocols.

The transfer of digital images between data archives and scientific workstations is likely to consume a significant amount of network bandwidth in the very near future. This paper examines the image transfer problem from an end-to-end perspective, that is, it describes a complete image transfer protocol that takes into account both the nature of digital imagery and the properties of the underlying network. Specifically, it describes a simple algorithm for encoding images into network packets in such a way that the receiver can recover from dropped packets without requiring the sender to retransmit them. Avoiding retransmissions has the advantages of improving response time and eliminating the need to buffer data at the sender.

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 ISO 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 architecture, illustrate the advantages of using the architecture, and demonstrate that the architecture results in efficient network software.

A description is given of an operating system kernel, called the x-Kernel, that provides an explicit architecture for constructing and composing network protocols. The authors' experience implementing and evaluation several protocols in the x-Kernel shows that this architecture is general enough to accommodate a wide range of protocols, yet efficient enough to perform competitively with less-structured operating systems. Experimental results demonstrating the architecture's generality and efficiency are provided. The explicit structure provided by the x-Kernel has the following advantages. First, the architecture simplifies the process of implementing protocols in the kernel, making it easier to build and test novel protocols. Second, the uniformity of the interface between protocols avoids the significant cost of changing abstractions and makes protocol performance predictable. Third, it is possible to write efficient protocols by tuning the underlying architecture rather than heavily optimizing protocols themselves.>

Abstract Univers is a generic attribute‐based name server upon which a variety of high‐level naming services can be built. This paper defines Univers' underlying attribute‐based naming model. It also describes several aspects of its implementation and demonstrates how various naming services—including a global white‐pages service, a local yellow‐pages service and a conventional name‐to‐address mapper—can be built on top of Univers.

This paper reports our experiences implementing remote procedure call (RPC) protocols in the x-kernel. This exercise is interesting because the RPC protocols exploit two novel design techniques: virtual protocols and layered protocols. These techniques are made possible because the x-kernel provides an object-oriented infrastructure that supports three significant features: a uniform interface to all protocols, a late binding between protocol layers, and a small overhead for invoking any given protocol layer. For each design technique, the paper motivates the technique with a concrete example, describes how it is applied to the implementation of RPC protocols, and presents the results of experiments designed to evaluate the technique.

Psync is an IPC protocol that explicitly preserves the partial order of messages exchanged among a set of processes. A description is given of how Psync can be used to implement replicated objects in the presence of network and host failures. Unlike conventional algorithms that depend on an underlying mechanism that totally orders messages for implementing replicated objects, the authors' approach exploits the partial order provided by Psync to achieve additional concurrency.>

When processes in a network communicate, the messages they exchange define a partial ordering of externally visible events. While the significance of this partial order in distributed computing is well understood, it has not been made an explicit part of the communication substrate upon which distributed programs are implemented. This paper describes a new interprocess communication mechanism, called Psync , that explicitly encodes this partial ordering with each message. The paper shows how Psync can be efficiently implemented on an unreliable communications network, and it demonstrates how conversations serve as an elegant foundation for ordering messages exchanged in a distributed computation and for recovering from processor failures.

The x-kernel is a configurable operating system kernel designed to support experimentation in interprocess communication and distributed programming. The x-kernel's underlying architecture provides a rich set of abstractions that are used to construct and compose communication protocols. The architecture is interesting because the abstractions are both general enough to accommodate a wide range of protocols and efficient enough to provide a useful testbed in which protocol performance can be accurately measured.

Abstract A conversation‐based message system groups related messages into conversations and orders messages within conversations according to the context in which they were submitted. This paper describes the implementation of a comprehensive conversation‐based message system, called Dragonmail, that extends an earlier prototype in two ways. First, it integrates conversations with conventional memo‐based mail in a way that preserves the semantics of the conversation abstraction. Secondly, it provides for the distribution of software components across multiple hosts to support remote interfaces.

Profile is adescriptivenaming service used to identify users and organizations. This paper presents a structural overview of Profile's three major components: a confederation ofattribute-basedname servers, a name space abstraction that unifies the name servers, and a user interface that integrates the name space with existing naming systems. Each name server is an independent authority that allows clients to describe users and organizations with a multiplicity of attributes; the name space abstraction is a client program that implements a discipline for searching a sequence of name servers; and the interface provides a tool with which users build customized commands. Experience with an implementation in the DARPA/NSF Internet demonstrates that Profile is a feasible and effective mechanism for naming users and organizations in a large internet.

A new message communication paradigm based on conversations that provides an alternative to memo - and conference -based mail is described. A conversation-based message system groups messages into conversations, and orders messages within a conversation according to the context in which they were written. The message context relation leads to an efficient implementation of conversations in a distributed environment and supports a natural ordering of messages when viewed by the user. Experience with a prototype demonstrates the workability of conversation-based mail and suggests that conversations provide a powerful tool for message communication.

Although the principle goal of the DARPA Domain Name project is to provide a distributed name resolution mechanism for host names in the DARPA Internet, it is envisioned that the domain naming scheme will also support names for mailboxes in a global mail system that extends to mail networks beyond the DARPA Internet. In addition to overviewing the fundamental ideas behind the domain name scheme and outlining a proposal for using domain names as an umbrella naming system for computer mail, we discuss the administrative and technical problems involved in using domain names in the global mail system.