Pierpaolo Baccichet

Peer-to-Peer Live Multicast: A Video Perspective

Peer-to-peer multicast is promising for large-scale streaming video distribution over the Internet. Viewers contribute their resources to a peer-to-peer overlay network to act as relays for the media streams, and no dedicated infrastructure is required. As packets are transmitted over long, unreliable multipeer transmission paths, it is particularly challenging to achieve consistently high video quality and low end-to-end delay. In this paper, we focus on error-resilient transport for peer-to-peer video streaming. The algorithms we describe are representative of three broad categories of robust video streaming schemes: forward error correction, multiple descriptions, and prioritized automatic repeat request. We analyze how these techniques can be employed for live peer-to-peer multicast and discuss their relative merits. Our results show that significant gains can be obtained when systems are designed to adapt to the encoding structure of the video streams they are transmitting. They also reveal the importance of avoiding congestion at every peer participating in the multicast to obtain a low-latency system. Finally, we provide insights as to which are the important metrics to compare different peer-to-peer streaming systems.

Low-delay peer-to-peer streaming using scalable video coding

Peer-to-peer (P2P) networks represent a valuable architecture for streaming video over the Internet. In these systems, users contribute their resources to relay the media to others and no dedicated infrastructure is required. In order to ensure a low end-to-end delay, P2P overlay networks are often organized as a set of complementary multicast trees. The source of the stream multiplexes the data on top of these trees and the routing of packets is statically defined. In this scenario, the reliability of the overlay links is critical for the performance of the system since temporary link failure or network congestion can cause a significant disruption of the end-user quality. The novel Scalable Video Coding (SVC) standard enables efficient usage of the network capacity by allowing intermediate high capacity nodes in the overlay network to dynamically extract layers from the scalable bit stream to serve less capable peers. On the other hand, SVC incurs a certain loss in terms of coding efficiency with respect to H.264/AVC single-layer coding. We propose a simple model that allows to evaluate the trade-off of using a scalable codec with respect to single-layer coding, given the distribution of the receivers’ capacities in an error-free network. We also report experimental results obtained by using SVC on top of a real-time implementation of the Stanford Peer-to-Peer Multicast (SPPM) protocol that clearly show the benefits of a prioritization mechanism to react to network congestion.

Frame concealment for H.264/AVC decoders

The H.264/AVC offers a high video coding efficiency and is the first standard that provides an explicit support for the transmission of multimedia content over a packed switched network, specifying a network abstraction layer (NAL). If we consider the transmission of a low resolution video sequence over a wireless LAN network or a UMTS link, we can usually observe that a coded picture typically fits the packet size, therefore a transmission error produces the loss of a whole frame. In this paper, we propose a novel technique to perform the concealment of a whole frame for H.264/A VC video decoders.

Content-Aware P2P Video Streaming with Low Latency

This paper describes the Stanford P2P multicast (SPPM) streaming system that employs an overlay architecture specifically designed for low delay video applications. In order to provide interactivity to the user, this system has to keep the end-to-end delay as small as possible while guaranteeing a high video quality. A set of complimentary multicast trees is maintained to efficiently relay video traffic and a congestion-distortion optimized (CoDiO) scheduler prioritizes more important video packets. Local retransmission is employed to mitigate packet loss. Real-time experiments performed on the Planet-Lab show the effectiveness of the system and the benefits of a content-aware scheduler in case of congestion or node failures.

Performance and Quality-of-Service Analysis of a Live P2P Video Multicast Session on the Internet

We evaluate the performance of a large-scale live P2P video multicast session comprising more than 120, 000 peers on the Internet. Our analysis highlights P2P video multicast characteristics such as high bandwidth requirements, high peer churn, low peer persistence in the P2P multicast system, significant variance in the media stream quality delivered to peers, relatively large channel start times, and flash crowd effects of popular video content. Our analysis also indicates that peers are widely spread across the IP address space, spanning dozens of countries and hundreds of ISPs and Internet ASes. As part of the P2P multicast evaluation several QoS measures such as fraction of stream blocks correctly received, number of consecutive stream blocks lost, and channel startup time across peers. We correlate the observed quality with the underlying network and with peer behavior, suggesting several avenues for optimization and research in P2P video multicast systems.

Peer-to-peer multicast live video streaming with interactive virtual pan/tilt/zoom functionality

Video streaming with virtual pan/tilt/zoom functionality allows the viewer to watch arbitrary regions of a high-spatial-resolution scene. In our proposed system, the user controls his region-of-interest (ROI) interactively during the streaming session. The relevant portion of the scene is rendered on his screen immediately. An additional thumbnail overview aids his navigation. We design a peer-to-peer (P2P) multicast live video streaming system to provide the control of interactive region-of-interest (IROI) to large populations of viewers while exploiting the overlap of ROIs for efficient and scalable delivery. Our P2P overlay is altered on-the-fly in a distributed manner with the changing ROIs of the peers. The main challenges for such a system are posed by the stringent latency constraint, the churn in the ROIs of peers and the limited bandwidth at the server hosting the IROI video session. Experimental results with a network simulator indicate that the delivered quality is close to that of an alternative traditional unicast client-server delivery mechanism yet requiring less uplink capacity at the server.

Robust Low-Delay Video Transmission using H.264/AVC Redundant Slices and Flexible Macroblock Ordering

This paper proposes a scheme for low-delay robust transmission of video signals over packet erasure channels. In applications such as video conferencing, the permissible delay between encoding and playback may be too low to allow retransmission or channel coding approaches which require buffering several video packets. For such a scenario, we present a scheme that provides error robustness using redundant video descriptions applied to pertinent portions of the video signal. In the H.264/AVC specification, this can be efficiently implemented using redundant slices and flexible macroblock ordering (FMO). We describe a model that determines the bit rate of the redundant descriptions such that the expected distortion at the decoder is minimized. Across all the video test sequences used, the average video quality delivered by the proposed scheme is 3.7 dB higher than decoder-based error concealment, and 1.2 dB higher than encoder-based loss-aware rate-distortion optimization.

Stanford Peer-to-Peer Multicast (SPPM) - Overview and recent extensions

We review the Stanford Peer-to-Peer Multicast (SPPM) protocol for live video streaming and report recent extensions. SPPM has been designed for low latency and robust transmission of live media by organizing peers within multiple complementary trees. The recent extensions to live streaming are time-shifted streaming, interactive region-of-interest (IRoI) streaming, and streaming to mobile devices. With time-shifting, users can choose an arbitrary beginning point for watching a stream, whereas IRoI streaming allows users to select an arbitrary region to watch within a high-spatial-resolution scene. We extend the live streaming to mobile devices by addressing challenges due to heterogeneous displays, connection speeds, and decoding capabilities.

Video quality assessment and comparative evaluation of peer-to-peer video streaming systems

We design a test methodology to analyze in detail the video quality received at each peer in a peer-to-peer (P2P) video streaming system. The metrics that we employ at each peer include video PSNR, statistical analysis of frame-freeze events, the amount of time to wait before video playback starts, nature of the data-paths established to serve the peer, protocol overhead and duplicate data received. These metrics are estimated by analyzing the packet reception times at each peer and utilizing information about the original uncompressed video as well as the encoded video. We use this framework to compare the performance of three P2P video streaming systems by deploying them on our controlled traffic-shaped network test-bed. We can emulate the same network conditions and peer behavior for testing different systems and ensure that the experiments are repeatable. These measurements highlight the differences between systems based upon their underlying implementation, overlay architecture, and choice of protocols. This measurement study helps to gauge the performance of currently available P2P video streaming systems and points out desirable performance improvements.

Optimal server bandwidth allocation for streaming multiple streams via P2P multicast

We consider the general scenario where content hosted by the server comprises streams and each peer can subscribe one or more streams. Multiple multicast trees are built to deliver the streams to respective peers while exploiting the overlap of their interests for efficient and scalable delivery. We propose an optimization framework for allocating server bandwidth to minimize distortion across the peer population. We apply the framework to a novel application, peer-to-peer (P2P) multicast live video streaming with virtual pan/tilt/zoom functionality. In this application, each user can watch arbitrary regions of a high-spatial-resolution scene yet the system exploits overlapping interests by building multicast trees. Experimental results indicate that optimal server bandwidth allocation enhances the delivered quality across the peer population.