Click is a new software architecture for building flexible and configurable routers. A Click router is assembled from packet processing modules called elements. Individual elements implement simple router functions like packet classification, queueing, scheduling, and interfacing with network devices. A router configuration is a directed graph with elements at the vertices; packets flow along the edges of the graph. Configurations are written in a declarative language that supports user-defined abstractions. This language is both readable by humans and easily manipulated by tools. We present language tools that optimize router configurations and ensure they satisfy simple invariants. Due to Click’s architecture and language, Click router configurations are modular and easy to extend. A standards-compliant Click IP router has sixteen elements on its forwarding path. We present extensions to this router that support dropping policies, fairness among flows, quality-of-service, and

This paper describes a new operating system kernel, called the x-kernel, that provides an explicit architecture for constructing and composing network protocols. Our experience implementing and evaluating several protocols in the x-kernel shows that this architecture is both general enough to accommodate a wide range of protocols, yet efficient enough to perform competitively with less structured operating systems. 1 Introduction Network software is at the heart of any distributed system. It manages the communication hardware that connects the processors in the system and it defines abstractions through which processes running on those processors exchange messages. Network software is extremely complex: it must hide the details of the underlying hardware, recover from transmission failures, ensure that messages are delivered to the application processes in the appropriate order, and manage the encoding and decoding of data. To help manage this complexity, network software is divi...

innovative system offering application developers an extensively flexible group communication model is described. The emergence of process-group environments for distributed computing represents a promising step toward robustness for mission-critical distributed applications. Process groups have a “natural’ ’ correspondence with data or services that have been replicated for availability or as part of a coherent cache. They can be used to support highly available security domains, and group mechanisms fit well with an emerging generation of intelligent network and collaborative work applications.

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...

Filing services have experienced a number of innovations in recent years, but many of these promising Ideas have faded to enter into broad use. One reason is that current filing environ-ments present several barriers to new development. For example, file systems today typically stand alone instead of building on the work of others, and support of new filing services often requires changes that invahdate existing work. Stackable file-system design addresses these issues m several ways. Complex fihng services are constructed from layer “building blocks, ” each of which may be provided by independent parties. There are no syntactic constraints to layer order, and layers can occupy different address spaces, allowing very flexible layer configuration Independent layer evolution and development are supported by an extensible interface bounding each layer This paper discusses stackable layering in detad and presents design techniques it enables We describe an implementation providing these facilities that exhibits very high performance. By lowering barriers to new filing design, stackable layering offers the potential of broad third-party file-system development not feasible today.

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.

Plan 9 is a distributed computing environment. It is assembled from separate machines acting as CPU servers, file servers, and terminals. The pieces are connected by a single file-oriented protocol and local name space operations. By building the system from distinct, specialised components rather than from similar general-purpose components, Plan 9 achieves levels of efficiency, security, simplicity, and reliability seldom realised in other distributed systems. This paper discusses the building blocks, interconnections, and conventions of Plan 9. Introduction. Plan 9 is a general-purpose, multi-user, portable distributed system implemented on a variety of computers and networks. It lacks a number of features often found in other distributed systems, including (i) A uniform distributed name space, (ii) Process migration,

As we approach nation-wide integration of computer systems, it is clear that file replication will play a key role, both to improve data availability in the face of failures, and to improve performance by locating data near where it will be used. We expect that future file systems will have an extensible, modular structure in which features such as replication can be "slipped in" as a transparent layer in a stackable layered architecture. We introduce the Ficus replicated file system for NFS and show how it is layered on top of existing file systems. The Ficus file system differs from previous file replication services in that it permits update during network partition if any copy of a file is accessible. File and directory updates are automatically propagated to accessible replicas. Conflicting updates to directories are detected and automatically repaired; conflicting updates to ordinary files are detected and reported to the owner. The frequency of communications outages rendering i...

The ADAPTIVE Communication Environment (ACE) is an object-oriented toolkit that implements strategic and tactical design patterns to simplify the development of concurrent, event-driven communication software. ACE provides a rich set of reusable C++ wrappers, class categories, and frameworks that perform common communication software tasks across a range of operating system platforms. The communication software tasks provided by ACE include event demultiplexing, event handler dispatching, connection establishment, interprocess communication, shared memory management, message routing, dynamic (re)configuration of network services, multi-threading, and concurrency control. ACE is targeted for developers of high-performance concurrent network applications and services. The primary goal of ACE is to simplify the development of concurrent OO communication software that utilizes interprocess communication, event demultiplexing, explicit dynamic linking, and concurrency. In addition, ACE auto...