DiversiFi: Robust Multi-Link Interactive Streaming

ABSTRACT Real-time, interactive streaming for applications such as audio-video conferencing (e.g., Skype) and cloud-based gaming depends critically on the network providing low latency, jitter, and packet loss, much more so than on-demand streaming (e.g., YouTube) does. However, WiFi networks pose a challenge; our analysis of data from a large VoIP provider and from our own measurements shows that the WiFi access link is a significant cause of poor streaming experience. To improve streaming quality over WiFi, we present DiversiFi, which takes advantage of the diversity of WiFi links available in the vicinity, even when the individual links are poor. Leveraging such cross-link spatial and channel diversity outperforms both traditional link selection and the temporal diversity arising from retransmissions on the same link. It also provides significant gains over and above the PHY-layer spatial diversity provided by MIMO. Our experimental evaluation shows that, for a client with two NICs, enabling replication across two WiFi links helps cut down the poor call rate (PCR) for VoIP by 2.24x. Finally, we present the design and implementation of DiversiFi, which enables it to operate with single-NIC clients, and with either minimally modified APs or unmodified APs augmented with a middlebox. Over 61 runs, where the baseline average PCR is 4.9%, DiversiFi running with a single NIC, switching between two links, helps cut the PCR down to 0%, while duplicating wastefully only 0.62% of the packets and impacting competing TCP throughput by only 2.5%. Thus, DiversiFi provides the benefit of multi-link diversity for real-time interactive streaming in a manner that is deployable and imposes little overhead, thereby ensuring coexistence with other applications

The content and access dynamics of a busy Web server: Findings and implications

We study the MSNBC Web site, one of the busiest in the Internet today. We analyze the dynamics of content creation and modification as well as client accesses. Our key findings are (a) files tend to change little upon modification, (b) a small set of files get modified repeatedly, (c) file popularity follows a Zipf-like distribution with an α much larger than reported in previous, proxy-based studies, and (d) there is significant temporal stability in file popularity but not much stability in the domains from which popular content is accessed. We discuss implications of these findings. 1

1 Network Performance of Broadband Hosts: Measurements & Implications

With the rapid growth in the popularity of and the research interest in peer-to-peer (P2P) systems, an interesting question is what the quality of network connectivity between peers in the “real world ” is and what implications this has for applications. In this paper, we describe an effort called PeerMetric to directly measure P2P network performance from the vantage point of broadband-connected residential hosts. Our measurements indicate significant asymmetry in bandwidth, with median downstream and upstream available bandwidths of 900 Kbps and 212 Kbps, respectively. We argue that the availability of last-hop bandwidth is more important than the traditional consideration of locality for overlay multicast over broadband hosts. We also considered the peer selection problem and found that a simple delay-vector based approach is effective for finding proximate peers (in terms of latency). However, P2P latency turns out to be a poor predictor of P2P TCP throughput, which may be the metric of interest for applications such as file sharing. I

The content and access dynamics of a busy Web server: Findings and implications

In this paper, we studythe dynamics of one of the busiest Web sites in the Internet today. Unlike many other efforts that have analyzed client accesses as seen by proxies, we focus on the server end. We analyze the dynamics of both the server content and client accesses made to the server. The former considers the content creation and modification process while the latter considers page popularityand localityin client accesses. Some of our keyresults are: (a) files tend to change little when they are modified, (b) a small set of files tends to get modified repeatedly, (c) file popularity follows a Zipf-like distribution with a parameter α that is much larger than reported in previous, proxy-based studies, and (d) there is significant temporal stabilityin file popularitybut not much stabilityin the domains from which clients access the popular content. We discuss the implications of these findings for techniques such as Web caching (including cache consistencyalgorithms) and prefetching or serverbased “push ” of Web content.