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ABSTRACT Insights into PPLive A Measurement Study of a


Insights into PPLive: A Measurement Study of a ? Large-Scale P2P IPTV System
Xiaojun Hei? , Chao Liang? , Jian Liang? , Yong Liu? and Keith W. Ross?
? ? Department of Computer and Information Science Department of Electrical and Computer Engineering Polytechnic University, Brooklyn, NY, USA 11201

ABSTRACT
With over 100,000 simultaneous users (typically), PPLive is the most popular IPTV application today. PPLive uses a peer-to-peer design, in which peers download and redistribute live television content from and to other peers. Although PPLive is paving the way for an important new class of bandwidth intensive applications, little is known about it due to the proprietary nature of its protocol. In this paper we undertake a preliminary measurement study of PPLive, reporting results from passive packet snif?ng of residential and campus peers. We report results for streaming performance, workload characteristics, and overlay properties.

General Terms
Measurement, Performance

Keywords
IPTV, Peer-to-Peer Measurement, PPLive

1.

INTRODUCTION

IPTV is expected to be the next disruptive IP communication technology, potentially reshaping our media and entertainment culture [1]. However, provisioning the IPTV service brings forth signi?cant new challenges [2]. IPTV systems can be broadly classi?ed into two categories: infrastructure-based and peer-to-peer based. In infrastructure-based systems, video servers and application-level multicast nodes are strategically placed in the Internet, and video is streamed from servers to clients via the multicast nodes. The infrastructure-based IPTV systems are expensive to build and dif?cult to maintain. On the other hand, peer-to-peer IPTV systems do not rely on dedicated application-level multicast servers. Instead, each IPTV client is potentially a server, multicasting received content to other IPTV clients. The IPTV clients and the connections between them thus form an overlay network, cooperatively ?This work is supported in part by NSF under grant ITR-0325726 and grant CNS0519998. Any opinions, ?ndings, and conclusions of the authors do not necessarily re?ect the views of NSF.

exchanging video content by leveraging the uploading capacity of the peers. Several P2P IPTV systems have been developed and deployed to date. Among them, CoolStreaming [3] and PPLive [4] have been two of the most popular P2P-based IPTV applications. Zhang et al. [3] reported that more than 4, 000 CoolStreaming users were simultaneously online at some peak time. Using a crawler (to be discussed in a subsequent paper), we observed in our PPLive measurements more than 100, 000 simultaneously online users for a live broadcast of a popular TV program. To the best of our knowledge, PPLive is by far the most popular P2P IPTV application on the Internet today. Given its success and its low-cost P2P architecture, it is our position that the designers of future IPTV systems should understand PPLive’s design, performance, traf?c characteristics and, more broadly, its strengths and its ?aws. Nevertheless, PPLive employs proprietary signaling and video delivery protocols. Details about its performance, streaming workload and overlay characteristics are still largely unknown. In this paper we present results from a preliminary measurement study of PPLive. We have been measuring PPLive with passive packet snif?ng as well as with an active crawler. In this paper, we only present the snif?ng results. Our measurement study focuses on three important aspects of PPLive streaming: streaming performance, workload characteristics, and overlay properties. Quantitative results obtained in our study bring to light important performance and design issues of live streaming over the public Internet.

2. OVERVIEW OF PPLIVE
PPLive is a free P2P-based IPTV application. According to the PPLive web site [4] in January 2006, the PPLive network provides 200+ channels with 400, 000 daily users on average. The bit rates of video programs mainly range from 250 Kbps to 400 Kbps with a few channels as high as 800 Kbps. The PPLive network does not own video content. The video content is mostly feeds from TV channels in Mandarin. The channels are encoded in two video formats: Window Media Video (WMV) or Real Video (RMVB). The encoded video content is divided into chunks and distributed to users through the PPLive P2P network. The PPLive web site [4] provides limited information about its video content distribution mechanism. Nevertheless, various web sites and message boards provide additional information. In this section we describe some of PPLive’s fundamental mechanisms, which we collected from different sources and con?rmed by our own measurement results. The PPLive software implements two major application level protocols: a gossip-based protocol for peer management and channel discovery; and a P2P-based video distribution protocol for high quality video streaming. Figure 1 depicts an overview of the PPLive network. When an end-user starts the PPLive software, it joins the

Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for pro?t or commercial advantage and that copies bear this notice and the full citation on the ?rst page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior speci?c permission and/or a fee. IPTV Workshop, International World Wide Web Conference May 23, 2006, Edinburgh, Scotland, United Kingdom

PPLive network and becomes a PPLive peer node. It then sends out a query message to the PPLive channel server to obtain an updated channel list (step 1). Before a peer actually starts to watch a channel, it does not exchange data with other PPLive peers. After a peer selects one channel to watch, it sends out multiple query messages (step 2) to some root servers to retrieve an online peer list for this channel. Peers are identi?ed by their IP addresses and port numbers on the list. Upon receiving a peer list, the PPLive client sends out probes to peers on the list to ?nd active peers for the channel of interest (step 3). Some active peers may also return their own peer lists, helping the initial peer to ?nd more peers. Peers then share video chunks with each other, as described below.
peer list root server

3. MEASUREMENT SETTING
Our P2P network measurements fall into two categories: passive monitoring and active crawling. In this paper, we describe the results from the passive monitoring platform, which captures PPLive traf?c. We collected multiple PPLive packet traces from four PCs: two PCs were connected to Polytechnic University campus network with 100 Mbps Ethernet access; two PCs were connected to residential networks through cable modem. Most of the PPLive users today have either one of these two types of network connections. The PCs with residential access were located at Manhattan and Brooklyn in New York. Each PC ran Ethereal [5] to capture all inbound and outbound PPLive traf?c. We carefully ?ltered out the cross traf?c of other network activity from the PCs. Table 1 provides an overview of the collected packet traces, which were captured on February 2, 2006. One residential and one campus PC “watched” the channel CCTV3; the other residential and campus PC “watched” the channel CCTV10. Each of these four traces lasted about 2 hours. From the PPLive web site, CCTV3 is a popular channel with a 5-star popularity grade and CCTV10 is a less popular channel with a 3-star popularity grade. We counted an IP address in these traces if there was a TCP connection attempt (TCP SYN packets) between the traced peer and this IP address. We de?ne an IP address to be active if the peer with this IP address exchanges non-zero data with the traced peer.

channel list server

2 1
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Figure 1: Channel and peer discovery The major software component of PPLive is its TV engine. This TV engine is responsible for downloading video chunks from the PPLive network and streaming the downloaded video to a local media player. The streaming process in the PPLive traverses two buffers in local memory: the PPLive TV engine buffer and the media player buffer, as shown in Figure 2. This double buffering mechanism is designed with two goals. One is to pre-cache media content to combat download rate variations from the PPLive network; the other is for ef?cient content distribution between peers.

4. STATISTICS OF PPLIVE SESSIONS
During playback, a PPLive peer normally establishes a large number of sessions to other peers, not only for content exchange but also for signaling. In this section, we present detailed session statistics, such as session duration, packet size and the correlation between them, and traf?c breakdown among sessions.

4.1 Session Duration and Packet Size
A PPLive client utilizes TCP for both signaling and video streaming. TCP signaling sessions normally perform short-duration tasks, including downloading peer lists and probing peers for availability. TCP streaming sessions, on the other hand, have longer durations. Furthermore, TCP streaming packets normally has a large packet size, 1200+ bytes, while small TCP signaling packets are commonly observed. We plot the correlation between TCP session durations and average TCP segment size in Figure 3 for CCTV3Campus. The plots for the other three traces are similar. We clearly observe that long TCP ?ows mainly have large TCP segment sizes. There exists many TCP sessions with short durations and small TCP segment sizes. From Figure 3, we can conclude that signaling sessions typically have short durations and carry mostly small packets; whereas video exchange sessions have long durations and carry many large packets. The presence of signaling and streaming traf?c makes it dif?cult to understand the PPLive working mechanisms. Hence, we separate video and signaling traf?c with the aid of a heuristic: 1. For a given TCP session, we keep track of the cumulative number of large packets (> 1200 bytes) during a session’s lifetime. If the cumulative number of large packets is larger than 10, this session is labelled as a “video session”; otherwise, the session is labelled as a “signaling session”. We ?lter from the traces all signaling sessions. 2. Within a video session, we further ?lter all packets less than 1200 bytes. We also provide of a typical Complementary Cumulative Distribution Function (CCDF) of video session durations in Figure 4.

PPLive TV Engine Internet Queue streaming direcion S

Media Player Media User Interface Queue

Figure 2: PPLive streaming process The cached contents can be uploaded to other peers that are watching the same channel. Speci?cally, the peer client contacts multiple active peers to download media content of the channel. At the same time, this peer client may also upload cached video chunks to multiple peers. Received video chunks are reassembled in order and buffered in the queue of the PPLive TV engine, forming a local streaming ?le in memory. When the streaming ?le length crosses a prede?ned threshold, the PPLive TV engine launches a media player, which downloads video content from the local HTTP streaming server. Most media players, such as windows media player, have built-in video buffering mechanisms. After the buffer of the media player ?lls up to the required level, the actual video playback starts. When PPLive starts, the PPLive TV engine downloads media content from peers aggressively to minimize the playback start-up delay. When the media player receives enough content and starts to play the media, the streaming process gradually stabilizes. The PPLive TV engine streams data to the media player at the media playback rate.

Trace Name CCTV3-Campus CCTV3-Residence CCTV10-Campus CCTV10-Residence

Trace size (Byte) 784,411,647 132,494,879 652,000,813 66,496,909

Duration (Sec) 7676 7257 7285 9216

Table 1: Data sets Playback Rate Total IPs (Kbps) 340 3105 340 1616 312 1008 312 797

Active IPs 2691 1183 910 282

Download (MByte) 360.99 372.53 317.08 385.50

Upload (MByte) 4574.57 352.75 3815.34 7.68

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is that a peer typically receives video from more than one peers at any given time. With this multi-download feature, the aggregate video download rate becomes quite smooth. In conjunction with the buffering mechanisms, which will be discussed in Section 5.4, PPLive is typically able to provide smooth video playback. We also plot analogous curves, in log scale, for video upload in Figure 5(b). Importantly, the top peer video upload session only takes account for about 5% of the total video upload traf?c. Thus this campus node is performing an important multicast function.
500 aggregate-download top-download-session

1 0.1 1 10 100 TCP session duration (sec) 1000 10000

450 400 Video rate (Kbps) 350 300 250 200 150 100 50 0 20 40

Figure 3: TCP session duration vs. TCP average segment size for CCTV3-Campus

Note that the video session duration spreads over a wide range. The median video session is about 20 seconds and about 10% of video sessions last for over 15 minutes or more. Because many sessions are short, a peer may only exchange a few video chunks with its neighbors before the session ends.
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Figure 4: CCDF of video session duration for CCTV3-Campus

(b) Upload Figure 5: Peer download and upload video traf?c breakdown for CCTV3-Campus

4.2 Video Traf?c Breakdown among Sessions
A PPLive peer downloads/uploads video chunks from/to multiple peers. However, due to bandwidth diversity and content availability on peers, download/upload rates of all peering sessions are not evenly distributed. In Figure 5(a), for a campus peer, we compare its aggregate download video rate with the download rate from the greatest contributing peer. This top peer almost contributes about 50% of the total video download traf?c. However, the download rate from this top peer is highly dynamic. This is mostly due to the content availability on the top peer and the inherent rate variation of the TCP session with that peer. One important consequence

5. PPLIVE STREAMING PERFORMANCE 5.1 Start-up Delay
When PPLive ?rst starts, it requires some time to search for peers and then tries to download data from active peers. We record two types of start-up delay: the delay from when one channel is selected until the streaming player pops up; and the delay from when the player pops up until the playback actually starts. The player pop-up

delay is in general 10 ? 15 seconds and the player buffering delay is around 10 ? 15 seconds. Therefore, the total start-up delay is around 20 ? 30 seconds. Nevertheless, we observe that some less popular channels have a total start-up delays of up to 2 minutes. Overall, PPLive exhibits reasonably good start-up user experiences, which is con?rmed by the quick ramp-up of the download traf?c rate shown in Figure 6.

of campus network, this abnormal case requires further investigation.

5.4 Video Buffering
During periods of network congestion and peer churn, the media download rate may not sustain the normal media playback rate, causing the playback buffer to drain. Therefore, the buffer size affects the streaming application’s resilience to network congestion. We estimate the buffer size of both the PPLive TV engine and the media player in the rest of this section. We estimate the media player buffer as follows. We ?rst start to play one streaming channel and wait until the player begins to play. The media playback rate, c, can be read from the media player interface. After a time period, the speed and peer number displayed on the PPLive TV engine become stabilized. We then close the PPLive TV engine at time instance t1 . The media player continues playback the video chunks in its own buffer. Finally, the player reports the end of the program at time instance t2 . We calculate the time interval (t2 ? t1 ) and multiply it with the playback rate c to estimate the buffer size of the media player. Note that after we shut down the PPLive TV engine, the data already stored in the PPLive queue are no longer available for the media player. Therefore, the media player buffer is at least c(t2 ? t1 ). Multiple experiments indicate that this buffer size is at least 5.37 MBytes. We estimate the buffer size of the PPLive TV engine as follows. First, we play one streaming channel to reach a steady streaming state. We physically disconnect the PC from the network. At the same time, we launch an HTTP ?le download software to download the media ?le from the PPLive streaming server. Note that after the network cable is unplugged, the PPLive TV engine still serves as a streaming server. The size of the downloaded video ?le is a rough estimate of the PPLive buffer size. Over multiple experiments for different streaming channels with variable rates, the estimated PPLive buffer size varies from 7.8 MBytes to 17.1 MBytes. It appears that the PPLive adaptively allocates buffer size according to the streaming rate and the buffering time period speci?ed by the media source. Overall, the total buffer size in PPLive streaming 10 ? 30 MBytes. A commodity PC can easily meet this buffer requirement.

5.2 Upload-Download Rates
Figure 6 depicts the upload and download streaming rates for the four traces. Each data point is the average bit rate over 30 second intervals. Note that the download bit rates quickly ramp up to the playback rate. It is also interesting that the upload traf?c rates increase very quickly in the two campus traces; however, the upload rate of CCTV3-Residence increases very slowly (see Figure 6(b)) and there is essentially no upload traf?c from CCTV10-Residence. We observe that in general, the video download rates for both campus peers and residential peers smoothly ?uctuate around their video playback rates. However, there is an obvious download rate decrease for the residential peer in trace CCTV10-Residence around time t = 33 minute. This rate ?uctuation period sustains for around 4 minutes. After this decrease, the PPLive TV engine aggressively downloads from the network and speeds up the download rate for another 3 minutes. Then the download rate becomes steady again. Despite of the PPLive and media player buffering, this download rate ?uctuation may have impacted the quality of video playback. For upload traf?c rates, campus peers and residential peers exhibit distinctly different behaviors. The two campus peers upload signi?cantly more traf?c than the two residential peers. Over the 2-hour period, the two campus peers uploaded about 4.4 GBytes and 3.7 GBytes video traf?c to other peers, respectively. Although not as high as the two campus peers, one of the residential peers contributed traf?c volume comparable to its download traf?c volume. However, the other residual peer only uploaded 4.6 MBytes video chunks to other peers.

5.3 Video Traf?c Redundancy
Due to the distributed nature of PPLive streaming, it is possible that a PPLive peer downloads duplicate media content from multiple peers. The transmission of redundant video chunks wastes network bandwidth; hence, we are interested in the redundancy measurement of the PPLive video traf?c after the streaming player playbacks steadily. To this end, the ?rst 10 minutes of the traces are not used for analysis to minimize the impact of transient behavior of the traces. Excluding TCP/IP headers, we determine the total streaming payload for the download traf?c. Utilizing the video traf?c ?ltering heuristic rule, presented in Section 4, we are able to extract video traf?c. Given the playback interval and the media playback speed, we obtain a rough estimate of the media segment size. We compute the redundant traf?c by the difference between the total received video traf?c and the estimated media segment size. We de?ne the redundancy ratio as the ratio between the redundant traf?c and the estimated media segment size. From Table 2, we observe that the traf?c redundancy in PPLive is limited. This is partially due to the long buffer time period so that PPLive peers have enough time to locate peers in the same streaming channel and exchange content availability information between themselves. The negative redundancy ratio (?3.5%) for CCTV3-Campus indicates that the video download chunks are not suf?cient for smooth video playback. As shown in Figure 6(a), at time 10 < t < 20 minute and 60 < t < 64 minute for CCTV3-Campus, the download rate decreases signi?cantly and the PPLive playback may suffer seriously lacking of video chunks. Given the good connectivity

6. PPLIVE PEERING STATISTICS
Figure 6 plots the number of active video peers. Active video peers are de?ned as those peers which have more than 10 large packet (> 1200 bytes) exchange with the traced peer in its lifetime. There is distinct peer connectivity behavior for campus peers and for residential peers. As expected, a campus peer has many more active video peer neighbors than a residential peer due to its high-bandwidth access network. A campus peer utilizes its highbandwidth connectivity, maintaining a steady number of active TCP connections for video traf?c exchange. It also appears that content popularity has a signi?cant impact on the number of active peer neighbors for the residential peer. In particular, the residential peer with the less popular CCTV10 channel seems to have dif?culty in ?nding enough peers for streaming the media. At time t = 33 minutes, the active video peer number drops to 1. This reduction in video neighbors impacts the download rate of this residential peer signi?cantly, as shown in Figure 6(d). In this experiment, the PPLive client detected this rate reduction quickly and started to search for new peers for additional video download. New peers were quickly found and fresh streaming ?ows were established; hence, the video download rate recovered quickly as a result. During a peer’s lifetime, this peer constantly changes its upload and download neighbors. This is illustrated in Figure 6, in which

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Figure 6: Upload and download video bit rates for the four traces Table 2: Video traf?c redundancy Total download Video download Estimated media segment size (MByte) (MByte) (MByte) 308.3 285.7 296.1 338.4 314.9 276.8 281.0 259.4 257.4 375.5 351.6 321.0

Trace name CCTV3-Campus CCTV3-Residence CCTV10-Campus CCTV10-Residence

Interval (second) 6966.2 6512.6 6600.7 8230.5

Redundancy ratio -3.5% 13.8% 0.76% 9.5%

the number of video peers is sampled every 30 seconds. A changed video peer between two consecutive sampling time points (30 seconds) refers to a peer that either stops to serve as a video peer or becomes a new video peer for the traced peer. Regardless the types of access networks, over 30 seconds period we commonly observe that several video peers are gone and several new video peers start to exchange video chunks with the traced peer. Nevertheless, compared with the total number of video peers, the average number of the changed peers is less than 10% of the total video peers for campus peers. However, the changed peers contribute a large percentage of the total video peers for residential peers. One consequence is that the download video rates of residential peers are likely to ?uctuate more signi?cantly. It would waste network resources to download from another continent if a channel can be downloaded from a source in the same continent. We investigated whether a PPLive peer takes locality into account when it determines which peer to download from. To this end, we employed a simple pre?x matching technique to determine the geographic location of a peer. The ?rst pre?x byte of a

peer’s IP address is selected to estimate the geographic distribution of this peer. For example, 58.a.b.c is regarded as a peer from Asia. Note that there is still a small possibility that two IP addresses with the same pre?x are located in different continents. Table 3 shows the peer geographic distribution of IP addresses for the video sessions from the traces. We observe that a large number of peers are located in Asia and they contribute the majority of the download traf?c for the traced peers as shown in Table 3. On the other hand, the majority of the video traf?c uploaded by our traced peers, located in New York, is to peers in North America. For example, in Table 3(b), this residential peer downloads 81.9% video traf?c from peers in Asia and 17.8% video traf?c from peers in North America; however, it uploads only 5.4% video traf?c to Asia but 64.8% to peers in North America. Nevertheless, Table 3(d) shows that this trend seems not to be valid for trace CCTV10Residence. A closer investigation on this trace reveals that this residential peer uploaded few video chunks to a limited number of peers (see Figure 6(d)). Those video sessions are short-lived and those peers are only transient peers, largely distributed over the

50 45 # of active video peers 40 40 35 30 25 20 15 10 0 20 40 60 Time (min) 80 campus residence 100 120 0 0 20 40 60 Time (min) 80 100 120 # of video peers 30 50 # of video peers # of changed video peers

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40 35 # of active video peers 30 # of video peers 10 25 20 15 10 1 0 20 40 60 Time (min) 80 campus residence 100 120 0 0 20 40 60 Time (min) 80 100 120 5 # of video peers # of changed video peers

(b) CCTV10 Figure 7: Evolution of active video peer connections

(b) CCTV3-Residence Figure 8: Peer departures and arrivals global Internet.

7.

CONCLUSION

We conducted a measurement study on a popular IPTV application, PPLive. Our measurement results show that the PPLive deploys the P2P principles for ef?cient resource discovery and video distribution. Utilizing the best-effort Internet infrastructure, the PPLive streaming maintains satisfactory IPTV performance. This demonstrates that the current Internet infrastructure is capable to provide economic-viable IPTV services while meeting the performance requirements of IPTV. Nevertheless, the emerging IPTV applications exhibit different characteristics from other applications, which may change Internet traf?c pattern signi?cantly. This brings forth new challenges and opportunities for network service providers.

Table 3: Peer geographic distribution of video sessions (a) CCTV3-Campus peer(%) Download(%) Upload(%) Asia 18.6 77.3 1.1 North America 73.0 21.6 83.0 Other Places 8.4 1.1 15.9

(b) CCTV3-Residence Asia North America peer(%) 64.9 28.4 Download(%) 81.9 17.8 Upload(%) 5.4 64.8 (c) CCTV10-Campus peer(%) Download(%) Upload(%) Asia 36.1 94.6 2.6 North America 55.3 4.9 75.8

Other Places 6.7 0.3 29.8 Other Places 8.6 0.4 21.6

8.

REFERENCES

[1] S. Cherry, “The battle for broadband [Internet protocol television],” IEEE Spectrum, vol. 42, no. 1, pp. 24 – 29, Jan. 2005. [2] R. Jain, “I want my IPTV,” IEEE MultiMedia, vol. 12, no. 3, p. 96, 2005. [3] X. Zhang, J. Liu, B. Li, and T.-S. P. Yum, “DONet/CoolStreaming: A data-driven overlay network for peer-to-peer live media streaming,” in Proceedings of INFOCOM, vol. 3, Miami, FL, USA, 13-17 March 2005, pp. 2102 – 2111. [4] “PPLive.” [Online]. Available: http://www.pplive.com [5] “Ethereal.” [Online]. Available: http://www.ethereal.com/

(d) CCTV10-Residence Asia North America peer(%) 60.3 35.6 Download(%) 48.1 50.4 Upload(%) 45.7 24.8

Other Places 4.1 1.5 29.5


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