Zhenyu Li

Optimal cache allocation for Content-Centric Networking

Content-Centric Networking (CCN) is a promising framework for evolving the current network architecture, advocating ubiquitous in-network caching to enhance content delivery. Consequently, in CCN, each router has storage space to cache frequently requested content. In this work, we focus on the cache allocation problem: namely, how to distribute the cache capacity across routers under a constrained total storage budget for the network. We formulate this problem as a content placement problem and obtain the exact optimal solution by a two-step method. Through simulations, we use this algorithm to investigate the factors that affect the optimal cache allocation in CCN, such as the network topology and the popularity of content. We find that a highly heterogeneous topology tends to put most of the capacity over a few central nodes. On the other hand, heterogeneous content popularity has the opposite effect, by spreading capacity across far more nodes. Using our findings, we make observations on how network operators could best deploy CCN caches capacity.

Watching videos from everywhere: a study of the PPTV mobile VoD system

In this paper, we examine mobile users behavior and their corresponding video viewing patterns from logs extracted from the servers of a large scale VoD system. We focus on the analysis of the main discrepancies that might exist when users access the VoD system catalog from WiFi or 3G connections. We also study factors that might impact mobile users interests and video popularity. The users behavior exhibits strong daily and weekly patterns, with mobile users interests being surprisingly spread across almost all categories and video lengths, independently of the connection type. However, by examining the activity of users individually, we observed a concentration of interests and peculiar access patterns, which allows to classify the users and thus better predict their behavior. We also find a skewed video popularity distribution and then demonstrate that the popularity of a video can be predicted using its very early popularity level. We further analyzed the sources of video viewing and found that even if search engines are the dominant sources for a majority of videos, they represent less than 10% (resp. 20%) of the sources for the highly popular videos in 3G (resp. WiFi) network. We report that both the type of connections and mobile devices in use have an impact on the viewing time and the source of viewing. Based on our findings, we provide insights and recommendations that can be used to design intelligent mobile VoD systems and help improving personalized services on these platforms.

Efficient and Scalable Consistency Maintenance for Heterogeneous Peer-to-Peer Systems

Consistency maintenance mechanism is necessary for the emerging peer-to-peer applications due to their frequent data updates. Centralized approaches suffer single point of failure, while previous decentralized approaches incur too many duplicate update messages because of locality-ignorant structures. To address this issue, we propose a scalable and efficient consistency maintenance scheme for heterogeneous P2P systems. Our scheme takes the heterogeneity nature into account and forms the replica nodes of a key into a locality-aware hierarchical structure, in which the upper layer is DHT-based and consists of powerful and stable replica nodes, while a replica node at the lower layer attaches to a physically close upper layer node. A d-ary update message propagation tree (UMPT) is dynamically built upon the upper layer for propagating the updated contents. As a result, the tree structure does not need to be maintained all the time, saving a lot of cost. Through theoretical analyses and comprehensive simulations, we examine the efficiency and scalability of this design. The results show that, compared with previous designs, especially locality-ignorant ones, our approach is able to reduce the cost by about 25-67 percent.

Design and Evaluation of the Optimal Cache Allocation for Content-Centric Networking

Content-centric networking (CCN) is a promising framework to rebuild the Internets forwarding substrate around the concept of content. CCN advocates ubiquitous in-network caching to enhance content delivery, and thus each router has storage space to cache frequently requested content. In this work, we focus on the cache allocation problem, namely, how to distribute the cache capacity across routers under a constrained total storage budget for the network. We first formulate this problem as a content placement problem and obtain the optimal solution by a two-step method. We then propose a suboptimal heuristic method based on node centrality, which is more practical in dynamic networks with frequent content publishing. We investigate through simulations the factors that affect the optimal cache allocation, and perhaps more importantly we use a real-life Internet topology and video access logs from a large scale Internet video provider to evaluate the performance of various cache allocation methods. We observe that network topology and content popularity are two important factors that affect where exactly should cache capacity be placed. Further, the heuristic method comes with only a very limited performance penalty compared to the optimal allocation. Finally, using our findings, we provide recommendations for network operators on the best deployment of CCN caches capacity over routers.

CodingCache: multipath-aware CCN cache with network coding

Content Centric Networking (CCN) performance by definition depends on the in-network caching efficiency. We propose CodingCache which utilizes network coding and random forwarding to improve caching efficiency under multipath forwarding. Its advantage is that existing caching strategies can be easily incorporated with it for better performance. We evaluate CodingCache by extensive simulation experiments with the China Telecom network topology and a unique dataset consisting of video access logs from the PPTV system. The results demonstrate that compared with the CCN caching strategy, CodingCache improves the cache hit rate by about 60%.

Churn-Resilient Protocol for Massive Data Dissemination in P2P Networks

Massive data dissemination is often disrupted by frequent join and departure or failure of client nodes in a peer-to-peer (P2P) network. We propose a new churn-resilient protocol (CRP) to assure alternating path and data proximity to accelerate the data dissemination process under network churn. The CRP enables the construction of proximity-aware P2P content delivery systems. We present new data dissemination algorithms using this proximity-aware overlay design. We simulated P2P networks up to 20,000 nodes to validate the claimed advantages. Specifically, we make four technical contributions: 1). The CRP scheme promotes proximity awareness, dynamic load balancing, and resilience to node failures and network anomalies. 2). The proximity-aware overlay network has a 28-50 percent speed gain in massive data dissemination, compared with the use of scope-flooding or epidemic tree schemes in unstructured P2P networks. 3). The CRP-enabled network requires only 1/3 of the control messages used in a large CAM-Chord network. 4) Even with 40 percent of node failures, the CRP network guarantees atomic broadcast of all data items. These results clearly demonstrate the scalability and robustness of CRP networks under churn conditions. The scheme appeals especially to web-scale applications in digital content delivery, network worm containment, and consumer relationship management over hundreds of datacenters in cloud computing services.

PEARL: a programmable virtual router platform

Programmable routers supporting virtualization are a key building block for bridging the gap between new Internet protocols and their deployment in real operational networks. This article presents the design and implementation of PEARL, a programmable virtual router platform with relatively high performance. It offers high flexibility by allowing users to control the configuration of both hardware and software data paths. The platform makes use of fast lookup in hardware and software exceptions in commodity multicore CPUs to achieve highspeed packet processing. Multiple isolated packet streams and virtualization techniques ensure isolation among virtual router instances.

The case for P2P mobile video system over wireless broadband networks: A practical study of challenges for a mobile video provider

Mobile video is becoming extremely popular, and P2P mobile video platforms are being considered for large-scale deployment in this context. However, the design and deployment of realistic P2P video systems have to consider specific characteristics of mobile networks. In this article, we look from the viewpoint of a large-scale commercial P2P mobile video provider system, PPTV, and describe the implementation challenges of a P2P mobile video system over 3G networks. Our analysis is backed by real measurements and experience from PPTV. We extract from these measurements the characteristics of mobile videos and analyze their impact on P2P video systems. We also briefly discuss other practical issues in the design of a mobile P2P system for PPTV.

Locality-Aware Consistency Maintenance for Heterogeneous P2P Systems

Replication and caching have been deployed widely in current P2P systems. In update-allowed P2P systems, a consistency maintenance mechanism is strongly demanded. Several solutions have been proposed to maintain the consistency of P2P systems. However, they either use too much redundant update messages, or ignore the heterogeneity nature of P2P systems. Moreover, they propagate updated contents on a locality-ignorant structure, which could consume unnecessary backbone bandwidth and delay the convergence of consistency maintenance. This paper presents a locality-aware consistency maintenance scheme for heterogeneous P2P systems. Taking the heterogeneity nature, we form the replica nodes into a locality-aware hierarchical structure: the upper layer is DHT-based and a node in the lower layer attaches to a physically close node in the upper layer. An efficient update tree is built dynamically upon the upper layer to propagate the updated contents. Theoretical analyses and simulation results demonstrate the effectiveness of our scheme. Specially, experiment results show that, compared with gossip-based scheme, our approach reduces the cost by about one order of magnitude.

User Behavior Characterization of a Large-scale Mobile Live Streaming System

Streaming live content to mobile terminals has become prevalent. While there are extensive measurement studies of non-mobile live streaming (and in particular P2P live streaming) and video-on-demand (both mobile and non-mobile), user behavior in mobile live streaming systems is yet to be explored. This paper relies on over 4 million access logs collected from the PPTV live streaming system to study the viewing behavior and user activity pattern, with emphasis on the discrepancies that might exist when users access the live streaming system catalog from mobile and non-mobile terminals. We observe high rates of abandoned viewing sessions for mobile users and identify different reasons of that behavior for 3G- and WiFi-based views. We further examine the structure of abandoned sessions due to connection performance issues from the perspectives of time of day and mobile device types. To understand the user pattern, we analyze user activity distribution, user geographical distribution as well as user arrival/departure rates.