Typical NFS clients write in a lazy fashion: they leave dirty pages in the page cache and defer writing to the server until later. This reduces network traffic when applications repeatedly modify the same set of pages. However, this approach can lead to memory pressure, when the number of available pages on the client system is so low that the system must work harder to reclaim dirty pages. System performance is poor under memory pressure. We show examples of this problem and present two mechanisms to solve it: eager write back and eager page laundering. These mechanisms change the client's data management policy from lazy to eager, resulting in higher throughput for sequential writes. In addition, we show that NFS servers suffer from out-of-order file operations, which further reduce performance. We introduce request ordering, a server mechanism to process operations (as much as possible) in the order they were sent by the client, which improves read performance substantially. We have implemented these techniques in the Linux operating system. I/O performance is improved, with the most pronounced improvement visible for sequential access to large files. We see about 33% improvement in the performance of streaming write workloads and more than triple the performance of streaming read workloads. We evaluate several nonsequential workloads and show that these techniques do not degrade performance, and can sometimes improve performance. We also design and evaluate an adversarial workload to show that the eager policies can perform worse in some pathological cases.

Access control in network file systems relies on primitive mechanisms like access control lists and permission bits, which are not enough when operating in a hostile network environment. Network middleboxes, e.g., firewalls, completely ignore file system semantics when defining policies. Therefore, implementing simple context-aware access policies requires modifications to file servers and/or clients, which is impractical. We present FileWall, a network middlebox that allows administrators to define context-aware access policies for file systems using both the network context and the file system context. FileWall interposes on the client-server network path and implements administrator defined policies through message transformation without modifying either clients or servers. In this paper, we present the design and implementation of FileWall for the NFS protocol. Our evaluation demonstrates that FileWall imposes minimal overheads for common file system operations, even under heavy loads.

Background transfers, or transfers that humans do not actively wait on, dominate the Internet today. In today's best-effort Internet, background transfers can interfere with foreground transfers causing long wait times, thereby hurting human productivity. In this paper, we present the design and implementation of a background network, Harp, that addresses this problem. Harp has three significant advantages over recent end-host based background transport protocols; Harp (i) uses multiple paths to exploit path diversity and load imbalance in the Internet to tailor network resource allocation to human needs, (ii) provides better fairness and utilization compared to unipath end-host protocols, and (ill) can be deployed at either end-hosts or enterprise gateways, thereby aligning the incentive for deployment with the goals of network customers. Our evaluation using simulations and a prototype on Planetlab suggests that Harp improves foreground TCP transfer time by a factor of five and background transfer time by a factor of two using just two extra paths per connection.

802.11a, b, and g standards were designed for deployment in cooperative environments, and hence do not include mechanisms to protect from jamming attacks. In this paper, we explore how to protect 802.11 networks from jamming attacks by having the legitimate transmission hop among channels to hide the transmission from the jammer. Using a combination of mathematical analysis and prototype experimentation in an 802.11a environment, we explore how much throughput can be maintained in comparison to the maintainable throughput in a cooperative, jam-free environment. Our experimental and analytical results show that in today's conventional 802.11a networks, we can achieve up to 60% of the original throughput. Our mathematical analysis allows us to extrapolate the throughput that can be maintained when the constraint on the number of orthogonal channels used for both legitimate communication and for jamming is relaxed.

Exponentially growing bandwidth requirements and slowing gains in processor speeds have led to the popularity of multiprocessor architectures. Network stack parallelism is increasingly important to support such architectures. In this paper, we present techniques to improve network stack concurrency using our previous work, TCPServers, a system architecture for offloading network processing within an SMP system. TCPServers dedicates a subset of processors as packet processing engines (PPEs), which handle all asynchronous network events and perform receive processing. We introduce Receive Queues, data structures associated with each socket that store incoming network packets and are accessed exclusively at the PPEs. Using Receive Queues, we modify TCPServers based network stacks to incorporate early packet demultiplexing. We also present an efficient proportional fair scheduling algorithm, which processes incoming packets at the priority of the destination socket. Our experimental evaluation demonstrates that our modifications reduce the scheduling and synchronization overheads and improve the aggregate TCP/IP throughput by up to 75% compared against the default SMP stack. We also show that our system sustains this throughput, even when a large number of short lived connections are present.

We present FRAC, a Framework for role-based access control in network file systems. FRAC is a reference monitor that controls the message flow between file system clients and servers. FRAC supports role hierarchies, user sessions, and static and dynamic separation of duty constraints. It also allows administrators to define dynamic policies based on access history and the environment, e.g., time of day. FRAC introduces a virtual control namespace (VCN) that provides an interface to query and update the state of the access control framework over the standard file system protocol. This namespace eliminates the need for executing specialized user agents either at the client or at the server. Therefore, FRAC does not require any modification to either the file system client or the file server. We have implemented FRAC for the widely deployed NFS protocol using FileWall, a file system proxy previously developed by us. Our experimental evaluation shows that FRAC imposes minimal overheads for the common file system operations.

We propose service continuations (SC), an operating system mechanism that supports seamless dynamic migration of Internet service sessions between cooperating multi-process servers. Service continuations provide a server application with a simple and easy to use abstraction, and a means to migrate the service state along with the serviced connection. SC supports transparent resumption of service to the client of another server, and guaranteed integrity and consistency of communication channels used by server processes. SC is a generic, application independent mechanism that can be used to provide service continuity and availability for today's complex Internet services. We have implemented SC in FreeBSD and used them successfully in three real servers: the Apache Web server, the PostgreSQL transactional database server, and the Icecast streaming server. We present results of an experimental evaluation showing that using SC adds negligible run-time overhead to existing servers and that SC enables efficient dynamic migration of client sessions.