Network-aware content delivery is an attractive approach to mitigate the problems produced by the significant fluctuations of the available bandwidth present in today's Internet. Such network-aware applications require information about the current condition of the network to adapt their resource demands. Such information can be obtained at the network level, the transport protocol level, or directly by the application. This paper compares two kinds of application-level monitoring (at the sender and receiver side) and transport-level monitoring with regard to their ability to provide useful information to network-aware applications. The results indicate that transport-level monitoring can effectively and efficiently satisfy the need for timely and accurate bandwidth information. This observation has direct implications for protocol/OS design. It is desirable to widen the transport layer API to allow the application efficient access to network status information required for adaptation.

The innovative techniques of TCP Vegas have been the subject of much debate in recent years. Several studies have reported that TCP Vegas provides better performance than TCP Reno. However, the question of which of the new techniques are responsible for the impressive performance gains remains unanswered so far. This paper presents a detailed performance evaluation of TCP Vegas. By decomposing TCP Vegas into the various novel mechanisms proposed and assessing the effect of each of these mechanisms on performance, we show that the reported performance gains are achieved primarily by TCP Vegas's new techniques for slow start and congestion recovery. TCP Vegas's innovative congestion avoidance mechanism is shown to have only a minor influence on throughput. Furthermore, we find that the congestion avoidance mechanism exhibits fairness problems even if all competing connections operate with the same round trip time.

Large scale networked image retrieval systems face a number of problems that are not fully satisfied by current systems. On one hand, integrated solutions that store all image data centrally are often limited in terms of scalability and autonomy of data providers. On the other hand, WWW-based search engines proved to be fairly scalable, and data providers retain their autonomy. However, such engines often confront users with links to servers that are not available or to images that no longer exist, i.e., they are unable to keep their meta-database consistent with the repositories' contents. Furthermore, existing solutions often neglect the cost of image delivery. The considerable variations in the effective bandwidth in today's Internet lead to highly unpredictable response times, which are often intolerable from the user's point of view.

Network-aware applications attempt to adjust their resource demands in response to changes in resource availability. E.g., if a server maintains a connection to a client, the server may want to adjust the amount of data sent to the client based on the effective bandwidth realized for the connection. Information about current and future network performance is therefore crucial for an adaptive application. This paper discusses three aspects of the coupling of applications and networks: (1) a network-aware application needs timely information about the status of the network; (2) a simple bandwidth estimation technique performs reasonably well for TCP-Reno connections without timeouts; (3) enhancements proposed to TCP-Reno to reduce the number of timeouts (i.e., SACKs and its variants) increase the bandwidth but also improve the accuracy of bandwidth estimators developed by other researchers. The empirical observations reported in this paper are based on an in-vivo experiment in the Internet. Over a 6-month period, we logged the micro dynamics of random connections between a set of selected hosts. These results are encouraging for the developer of a network-aware application since they provide evidence that a simple widening of the interface between applications and network (protocol) may provide the information that allows an application to successfully adapt to changes in resource availability.

Modern networks provide a QoS (quality of service) model to go beyond best-effort services, but current QoS models are oriented towards low-level network parameters (e.g., bandwidth, latency, jitter). Application developers, on the other hand, are interested in quality models that are meaningful to the end-user and, therefore, struggle to bridge the gap between network and application QoS models. Examples of application quality models are response time, predictability or a budget (for transmission costs). Applications that can deal with changes in the network environment are called network-aware. A network-aware application attempts to adjust its resource demands in response to network performance variations. This paper presents a framework-based approach to the construction of network-aware programs. At the core of the framework is a feedback loop that controls the adjustment of the application to network properties. The framework provides the skeleton to address two fundamental challenges for the construction of network-aware applications: how to find out about dynamic changes in network service quality; and how to map application-centric quality measures (e.g., predictability) to network-centric quality measures (e.g., QoS models that focus on bandwidth or latency). Our preliminary experience with a prototype network-aware image retrieval system demonstrates the feasibility of our approach. The prototype illustrates that there is more to network-awareness than just taking network resources and protocols into account and raises questions that need to be addressed (from a software engineering point of view) to make a general approach to network-aware applications useful.