Mary Baker

Mitigating routing misbehavior in mobile ad hoc networks

This paper describes two techniques that improve throughput in an ad hoc network in the presence of nodes that agree to forward packets but fail to do so. To mitigate this problem, we propose categorizing nodes based upon their dynamically measured behavior. We use a watchdog that identifies misbehaving nodes and a pathrater that helps routing protocols avoid these nodes. Through simulation we evaluate watchdog and pathrater using packet throughput, percentage of overhead (routing) transmissions, and the accuracy of misbehaving node detection. When used together in a network with moderate mobility, the two techniques increase throughput by 17% in the presence of 40% misbehaving nodes, while increasing the percentage of overhead transmissions from the standard routing protocols 9% to 17%. During extreme mobility, watchdog and pathrater can increase network throughput by 27%, while increasing the overhead transmissions from the standard routing protocols 12% to 24%.

Measuring link bandwidths using a deterministic model of packet delay

We describe a deterministic model of packet delay and use it to derive both the packet pair [2] property of FIFO-queueing networks and a new technique packet tailgating) for actively measuring link bandwidths. Compared to previously known techniques, packet tailgating usually consumes less network bandwidth, does not rely on consistent behavior of routers handling ICMP packets, and does not rely on timely delivery of acknowledgments.

Analysis of a local-area wireless network

To understand better how users take advantage of wireless networks, we examine a twelve-week trace of a building-wide local-area wireless network. We analyze the network for overall user behavior (when and how intensively people use the network and how much they move around), overall network traffic and load characteristics (observed throughput and symmetry of incoming and outgoing traffic), and traffic characteristics from a user point of view (observed mix of applications and number of hosts connected to by users).nAmongst other results, we find that users are divided into distinct location-based sub-communities, each with its own movement, activity, and usage characteristics. Most users exploit the network for web-surfing, session-oriented activities and chat-oriented activities. The high number of chat-oriented activities shows that many users take advantage of the mobile network for synchronous communication with others. In addition to these user-specific results, we find that peak throughput is usually caused by a single user and application. Also, while incoming traffic dominates outgoing traffic overall, the opposite tends to be true during periods of peak throughput, implying that significant asymmetry in network capacity could be undesirable for our users.nWhile these results are only valid for this local-area wireless network and user community, we believe that similar environments may exhibit similar behavior and trends. We hope that our observations will contribute to a growing understanding of mobile user behavior.

Analysis of a metropolitan-area wireless network

We analyze a seven-week trace of the Metricom metropolitan-area packet radio wireless network to find how users take advantage of a mobile environment. Such understanding is critical for planning future large-scale mobile network infrastructures. Amongst other results, we find that users typically use the radios during the day and evening. Of the users who move around during the trace (over half), we find that the more locations a user visits on a daily basis, the closer together, on average, those locations are. While these results are only known to be valid for this particular network, we hope future analysis of other networks will add to a growing understanding of mobile network behavior.

Peer-to-Peer Caching Schemes to Address Flash Crowds

Flash crowds can cripple a web sites performance. Since they are infrequent and unpredictable, these floods do not justify the cost of traditional commercial solutions. We describe Backslash, a collaborative web mirroring system run by a collective of web sites that wish to protect themselves from flash crowds. Backslash is built on a distributed hash table overlay and uses the structure of the overlay to cache aggressively a resource that experiences an uncharacteristically high request load. By redirecting requests for that resource uniformly to the created caches, Backslash helps alleviate the effects of flash crowds. We explore cache diffusion techniques for use in such a system and find that probabilistic forwarding improves load distribution albeit not dramatically.

The mobile people architecture

People are the outsiders in the current communications revolution. Computer hosts, pagers, and telephones are the addressable entities throughout the Internet and telephony systems. Human beings, however, still need application-specific tricks to be identified, like email addresses, telephone numbers, and ICQ IDs. The key challenge today is to find people and communicate with them personally, as opposed to communicating merely with their possibly inaccessible machines---cell phones that are turned off or PCs on faraway desktops.We introduce the Mobile People Architecture which aims to put the person, rather than the devices that the person uses, at the endpoints of a communication session. We describe a prototype that performs person-level routing; the prototype allows people to receive communication regardless of the network, device, or application they use, while maintaining their privacy.

Using IEEE 802.11e MAC for QoS over wireless

We study the behavior of the new MAC protocols for QoS in the proposed IEEE 802.11e draft standard and analyze them for their ability to fulfill their goals of better QoS and higher channel efficiency. We study the response of these mechanisms to various choices in available protocol parameters. We show that HCF reduces channel contention and allows better channel utilization. However, both the proposed MAC coordination functions, EDCF and HCF, are highly sensitive to protocol parameters. We believe that the effectiveness of these functions also depends on the scheduling algorithms. The effects of the various policy choices need to be understood and validated before the draft becomes a standard.

Flexible network support for mobility

Fueled by the large number of powerful tight-weight portable computers, the expanding availability of wirel= networks, and the popularity of the Internet, there is an increasing de mand to connect portable computers to the Internet at any time and in any place. However, the dynamic nature of such connectivity requires more flexible network support than has typically been atiable for stationary workstations. ThM paper introducw the following two me~anisms, in the context of Mobile 1P [24], to ensure a mobile host’s convenient and efficient communication with other hosts in a &anging environment. One me&anism supports multiple packet defivery methods (such as regular 1P or Mobile 1P) and adaptively selects the most appropriate one to use Wcordlng to the char=teristim of eti trfic flow. The other medanism enables a mobile host to make use of multiple wtive network interfww simultaneously and to control the selection of the most dwirable network interfaces for both outgoing and incoming pdets for different traffic flows. We demonstrate the usefdnm of these two network layer me&anisms and describe how they are implemented.

User-friendly access control for public network ports

We are facing a growing user demand for ubiquitous Internet access. As result, network ports and wireless LANs are becoming common in public spaces inside buildings such as lounges, conference rooms and lecture halls. This introduces the problem of protecting networks accessible through these public ports from unauthorized use. In this paper, we study the problem of access control through public network ports. We view this problem as a special case of the more general problem of access control for a service on a network. We present an access control model on which we base our solution. This model has three components: authentication, authorization, and access verification. We describe the design and implementation of a system that allows secure network access through public network ports and wireless LANs. The design requires no special hardware or custom client software, resulting in minimal deployment cost and maintenance overhead. The system has a user-friendly, Web-based interface, offers good security, and scales to a campus-sized community.

The Roma personal metadata service

People now have available to them a diversity of digital storage facilities, including laptops, cell phone address books, handheld devices, desktop computers and web-based storage services. Unfortunately, as the number of personal data repositories increases, so does the management problem of ensuring that the most up-to-date version of any document in a users personal file space is available to him on the storage facility he is currently using. We introduce the Roma personal metadata service to make it easier to locate current versions of personal files and ensure their availability across different repositories. This centralized service stores information about each of a users files, such as name, location, timestamp and keywords, on behalf of mobility-aware applications. Separating out these metadata from the data respositories makes it practical to keep the metadata store on a highly available, portable device. In this paper we describe the design requirements, architecture and current prototype implementation of Roma.