PROLOG

An introduction to the paper by Shen, Luo, Zimmermann,and Vasilakos

Peer-to-Peer Media Streaming:Insights and New DevelopmentsBY JIM ESCH

The amount of Internet traffic being driven by digital

multimedia is burgeoning. Video content was responsible

for over one-third of all consumer network traffic in 2009,

a number that is forecasted to go as high as 57% by 2014,

posing challenges for service providers seeking high

quality-of-service delivery for demanding users. To meet

that demand, peer-to-peer (P2P) media streaming is in-creasingly popular, because it is so eminently scalable:

participating clients downloading content also make

available their upload capacity to the system.

Early P2P focused on tree-based architectures: a media

stream is pushed from a root server to leaf nodes. Peers will

arrive and leave the network independently by the thou-

sands or millions, creating an effect

known as peer churn. Tree-based ar-chitectures do not respond so well to

churn, so a variety of mesh-based

pull architectures have been devel-

oped that address this issue, wherein

each peer node keeps a local con-

nectivity list of partner peers. Peri-

odically, availability information is

exchanged with the partners, andmissing data are pulled from them.

Recently, commercial P2P sys-

tems have favored the mesh-based

schemes, and the focus has now

turned to other issues: explaining

and predicting P2P behavior; moni-

toring effects, defects, and inefficiencies; traffic localiza-

tion techniques; hybrid solutions that maximize strengthsand minimize weaknesses of particular schemes; net-

worked and layered coding to improve throughput; adapt-

ing to wireless mobile networks; and adapting schemes to

newer multiview video and 3-D mesh objects. This paper

offers a survey of the latest developments in these areas.

As a prelude to this discussion, it is helpful to under-

stand the concept behind content delivery networks

(CDNs). A CDN is an improvement on the traditional

client–server model by expanding the definition of a

server. In a CDN, a video source server pushes content to

those content delivery servers that are closer to the clients

that need the content. An adequately dimensioned and

strategically located set of servers will provide quality

service, as can be found for instance on networks likeYouTube. CDNs do have some drawbacks, most notably,

their inability to utilize client upload bandwidth, which

places excessive loads on the infrastructure. Hybrid sys-

tems that combine the quality of CDNs with P2P features

have long been desired. Such hybrid systems can result in a

finer balance between scalability and streaming quality, as

well as smaller latency and reduced

cross-ISP traffic. To reduce thetraffic load on the server side, P2P

media streaming architectures (the

aforementioned tree-based and

mesh-based pull systems) have been

designed. In a tree-based push sys-

tem, every client can become a

server to other clients. Mesh-based

systems require peers to share infor-mation about their media holdings,

so peers can pull needed chunks

from one another.

We can classify media streaming

systems into two general categories:

live streaming and video on demand

(VoD). Most studies to date have focused on live stream-

ing. Many techniques can be applied to both categories;however, differences exist between them. Live streaming

requires synchronization with a broadcast server; VoD

does not, which means VoD has difficulty finding peers

with the content it needs. Live streaming generates its

contents in real time, whereas VoD contents are most

often prepared ahead of time, making VoD more versatile

with respect to available peer resources. VoD requires

more user interaction controls like pause and random seek,which makes it challenging to find quickly the required

stream data when the peer seeks a new position.

To meet the increasingdemands on the Internetbecause of growing videoand other new types ofmedia, peer-to-peer (P2P)media streaming isincreasingly popular,because it is so eminentlyscalable.

Digital Object Identifier: 10.1109/JPROC.2011.2170752

Vol. 99, No. 12, December 2011 | Proceedings of the IEEE 20870018-9219/$26.00 �2011 IEEE

Theoretical analysis of P2P media streaming has re-sulted in two types of modeling techniques: stochastic

modeling and combinatorial modeling. Stochastic models

help solve the problem of understanding system through-

put and how it is affected by churn and system settings.

Combinatorial models help to construct dissemination

trees for media streaming.

Many new technological developments in P2P stream-

ing are worthy of notice. They are summarized below.Overlay Network Monitoring and Diagnosing. P2P

streaming is cost efficient and popular, resulting in large

user communities. Large-scale deployments need to moni-

tor behavior and performance. Real system measurement

and optimization is key. We need to know what are the

best measurable indicators that would permit inferences

about network-wide quality. One solution is the use of

buffer maps to advertise chunk availability. Also, systemsneed to deal with peer heterogeneity (varying upload

bandwidths and session lengths). P2P systems need to

identify superior peers in the system, those with higher

bandwidth and longer session lengths. Finally, there is a

need to improve the efficiency of bandwidth provisioning

on the server side. In short, how can server bandwidth be

allocated among concurrent channels to maximize stream-

ing quality? One solution is a bandwidth provisioningalgorithm called RATION, described in this paper.

Traffic Locality. Because P2P streaming is by definition

distributed, traffic is random and disparate. Long-distance

traffic in particular adds stress to networks. Cross-ISP

traffic is especially troublesome. ISPs have resorted to

throttling cross-ISP links or caching proxies. Analysis of

the problem has turned up some likely causes: current P2P

systems use a peer-selection method that ignores the net-work underlay information, which means that connection

topologies are a random affair. Studies concerned with this

issue on live streaming systems have found that most

chunks requested by a peer are downloaded from peers in

the same ISP, and partners from the same ISP respond

faster to the requesting peer. An ISP-friendly chunk sched-

uling strategy can proactively promote local degrees of

connectivity. It is based on the idea that downloads fromdistant peers are permitted only when the video data buffer

is close to empty. In addition to optimized chunk sched-

uling, the peer selection mechanism can be modified; it

adapts the locality degree according to feedback on

streaming quality.

Hybrid P2P Infrastructures. Tree-based push and mesh-

based pull architectures have their advantages and disad-

vantages for P2P streaming. Some have proposed hybridapproaches that bring together push and pull techniques.

These include a pull–push hybrid approach, two-tier hybrid

approach, and a hybrid-approach with delay optimal push.

Coding Enhanced P2P Streaming. The demands of P2P

media streaming require new coding mechanisms that will

collect sufficient chunks before playback without incurring

redundant transmissions, while also adapting media qual-

ity to available bandwidth. Network coding and layeredcoding techniques can help. Described in this paper are the

following approaches: random push with random network

coding; pull scheduling for layered streaming; and layered

streaming with network coding.

Streaming across heterogeneous networks. The mobile

video market has expanded alongside the expansion of

third-generation (3G) wireless networks. Mobile data traf-

fic is likely to double every year through 2014. That beingsaid, P2P architectures have been slow to the mobile mar-

ket, in part because of lagging network characteristics and

limitations in mobile handsets. Aggregate bandwidth is

limited; video streaming is extremely bandwidth intensive

and requires large amounts of energy consumption; the

mobility of the technology complicates the process of dis-

tinguishing peers; and data usage fees can be prohibitive.

The paper surveys some studies that address these problems,including P2P-friendly infrastructure upgrades; collaborative

video streaming among mobiles; and energy-aware collabo-

rative streaming.

Media streaming beyond 2-D video. New applications

like multiview video and networked virtual environments

(NVEs) are pushing bandwidth demand ever higher. Three

studies that aim at extending the reach of P2P into this

area are covered: multiview video streaming; 3-D meshobject streaming; and 3-D content streaming in NVE.

There are some obstacles to the widespread adoption of

P2P streaming. We do not yet know for certain whether

traffic locality actually will make improvements for con-

tent providers and end users; quantitative studies will be

needed. Also, we do not know the precise tradeoffs be-

tween the many system parameters involved in hybrid

overlay infrastructures. And while CDN-P2P approachesoffer excellent performance, they are quite costly to

deploy. Cloud services may provide a solution here, though

many details and optimizations need to be worked out.

Last, mobile media content delivery will remain challeng-

ing for the foreseeable future; in addition to improved

coding schemes, overlay networks will need to be jointly

built with underlying support mechanisms. h

Prolog to the paper by Shen, Luo, Zimmermann, and Vasilakos

2088 Proceedings of the IEEE | Vol. 99, No. 12, December 2011