Effect of Public-First Delivery

To examine the efficiency of the public-first delivery policy, we implement the FIFO policy for comparison, in which, the data requests will be processed based on the time. Using the public-first policy, a peer will process the data requests from private peers only after requests from public peers are fulfilled.

To evaluate the file download time for a P2P file sharing system, we configure the

simulations as follows. There are 101 peers in the simulation, including a source peer and 100 peers to download a file from the source peer. All peers stay on throughout the simulations. The file is divided into 200 segments. The source peer is capable of serving 10 segments each round. The other peers are capable of serving 3 segments and requesting 6 segments each round. Every peer is connected with every other peer.

Before a peer requests a segment, the peer solicits the segment availabilities from other peers and determines the segment to request next based on the above mentioned rarest first policy. When a peer is requesting a segment, it will search by the rarest first order and ask its neighbors sequentially. This makes the peer may not fetching the rarest segment when it holds all the rarest ones. However, this is close to how the real peer-to-peer transmission systems work.

The performance metric in this simulation is the elapsed time to complete the file transmission. We vary the percentage of public peers in the network and see how the proposed policy scales to the amount of public peers in the network. Fig. 4-1. is the simulation result.

Before a peer requests a segment, the peer solicits the segment availabilities from other peers and determines the segment to request next based on the above mentioned rarest first policy. Fig. 4-1 shows the performance of P2P file transmission using public-first policy vs. not using the public-first policy. Since public-first delivery policy has better utilization on upload bandwidth. We can see the public-first policy reduces Fig. 4-1. Performance plot of peer-to-peer file sharing system simulation for different

elapsed rounds by about 20%. The benefit of public-first policy is significant even when the percentage of public peers is low. The performance improvement comes from the public-first makes data segments distributed effectively.

To evaluate the continuity of the video stream, we extend the setting to capture operations of a P2P multimedia stream broadcasting system. Unlike P2P file transmission systems, peers in the multimedia streaming system will not hold entire downloaded file. Peer will, usually, maintain a buffer that stores partial streaming data.

With streaming buffer, peer will not store entire file that may take hundreds of MB.

A larger amount of buffer benefits the quality of video playback. We have configured in this set of simulations that every peer holds a 60 segments buffer space.

Note that the video is divided into segments and each segment contains one second of video. When peer begins playing, it waits 20 rounds of initial buffering. The simulations run for 1000 rounds. Other settings are like those in the file sharing simulation. Segment continuity is considered the main performance metric in the simulation. Segment continuity, which is called continuity index in [6], indicates the percentage of timely received data segments. A higher score means a better video streaming quality. Figure 4-2 has shown the simulation result of a peer-to-peer streaming system. We may see that the public-first delivery does not benefit in terms of video segment continuity index. In Figure 4-3, the average buffer utilization shows the

public-first delivery benefits buffer utilization. However, the average continuity index doesn’t improve much. More simulations were conducted for explanation of this phenomenon.

Fig. 4-2. Performance plot of peer-to-peer streaming system simulation for different

percentage of public peers

It might be puzzling that the fact of using public-first delivery or not does not benefit peer-to-peer streaming systems performance. The reason comes from how the peer-to-peer system performs scheduling. In this simulation, when a peer is scheduling a data source, it will search its neighbors they hold the segments it lacks in a rarest first fashion. That makes the transmission happens when the requested peer has the ability of uploading a segment and holds the segment. In the previous simulation, the buffer size is 60 segments, which is large enough for a requesting peer finds a segment that it lacks in. That makes the performance bottleneck is only the available upload bandwidth, which remains unchanged in both scenarios.

Fig. 4-3. Average buffer utilization of peer-to-peer streaming system simulation for

different percentage of public peers in the network

To illustrate this, we may think of how the peer-to-peer networking transmission works. The transmission in peer-to-peer networking systems fails when the data provider does not has the upload bandwidth or the requested data. Using public-first delivery make public peers has better chance of getting data. However, the performance is limited by available upload bandwidth in the system.

The author has conducted simulations that are illustrative to how the public-first delivery affects the peer-to-peer streaming system performance. The reason of having identical continuity index comes from how the scheduling works. The peer will request a segment which is short of. The requesting peer can always find a deficient segment that is hold by another public peer when the buffer size is large enough. That is, when the buffer size is small, the video segment cannot be distributed effectively. When the public peer cannot receive video before the segment is shifted out of the buffer, it obviously cannot distribute the segment. Public-first, by definition, sends video segment to public peers first. That makes public peers able to contribute their bandwidth as soon as possible.

Hence, using public-first would not suffer from the performance degradation brought by smaller buffer size. Figure 4-4 has shown the effect of varying the buffer size.

In this buffer size varying simulation, the percentage of the public peers is 20%.

Previous simulation result shows that when the buffer size is large (60 segments), the Fig. 4-4. The effect of varying buffer size

Fig. 4-5. Performance plot of peer-to-peer streaming system simulation for different

start-up time of private peers in the network.

continuity index is around 0.68. Reducing buffer size results in continuity index decreasing. We may see the continuity index drops dramatically when the buffer size shrinks. The reason comes from that it takes time for public peers to get the data and forward to others. Since the public-first delivery sends the data to the public peers first, the public peers may contribute their bandwidth as soon as possible. This makes performance difference in terms of continuity index.

The author cannot stop wondering if there are other scenarios that the public-first benefits peer-to-peer streaming systems. In the previous peer-to-peer file sharing system simulation, public-first outperforms 20% in terms of file transmission time. That may indicates if the sliding window is not moving, public-first delivery has better transmission efficiency. Inspired by this, the author has conducted simulation that is changing the start-up time of private peers.

In Figure 4-5, the start-up time is changed to see if the public-first benefits. In this simulation, the start-up time for public peers is fixed to 5 rounds and the start-up time for private peers varies. The buffer size for each peer is 60 segments. The percentage of public peers is 20 percent. The goal for this simulation is to see if delay the start-up time of private peers benefits.

In this simulation, the performance of using public-first delivery is better by around 3-5% in terms of continuity index. The reason for this phenomenon comes from

the public-first delivery has better delivery efficiency while the sliding window is not moving. Due to the start-up delay is relatively small to the entire streaming playback time, the improvement is relatively small.