BitTorrent, one of the most widespread used P2P application for file-sharing, recently got rid of TCP by introducing an application-level congestion control protocol named uTP. The aim of this new protocol is to efficiently use the available link capacity, while minimizing its interference with the rest of user traffic (e.g., Web, VoIP and gaming) sharing the same access bottleneck. In this paper we perform an experimental study of the impact of uTP on the torrent completion time, the metric that better captures the user experience. We run BitTorrent applications in a flash crowd scenario over a dedicated cluster platform, under both homogeneous and heterogeneous swarm population. Experiments show that an all-uTP swarms have shorter torrent download time with respect to all-TCP swarms. Interestingly, at the same time, we observe that even shorter completion times can be achieved under careful mixtures of TCP and uTP traffic.
The only previous work addressing the impact of uTP on BitTorrent completion time is our own recent work @cite_24 , that however employs ns2 simulations unlike in this work. Interestingly, some of the observations of this study are in agreement with @cite_24 , e.g., showing a larger completion time for increasing buffer occupancy on the data plane. Yet, we point out that @cite_24 does not consider an hardcoded preference for uTP, nor bidirectional uTP connections: hence, an interesting difference with respect to the current work is that @cite_24 forecasted heterogeneous performance for heterogeneous swarms (i.e., larger completion time for TCP peers) that we have shown not to hold on practice.
@cite_24
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BitTorrent, one of the most widespread file sharing P2P applications, recently introduced uTP, an application-level congestion control protocol which aims to efficiently use the available link capacity, while avoiding to interfere with the rest of user traffic (e.g., Web, VoIP and gaming) sharing the same access bottleneck. Research on uTP has so far focused on the investigation of the congestion control behavior on rather simple settings (i.e., single bottleneck, few backlogged flows, etc.), that are fairly far from the P2P settings in which the protocol is deployed. Moreover, prior work typically addressed questions, such as fairness and efficiency, that are natural from a congestion control context perspective, but are not directly related with the performance of the overall P2P system. In this work, we refine the understanding of uTP, by gauging its impact on the primary BitTorrent user-centric metric, namely the torrent download time, by means of packet level simulation. Results of our initial investigations show that: (i) in case uTP clients fully substitute TCP clients, no performance difference arise; (ii) in case of heterogeneous swarms, comprising peers using uTP and TCP congestion control, completion time of uTP peers can possibly benefit of lower uplink queuing delays, as signaling traffic (e.g., chunk requests) are not slowed down by long waits in the ADSL buffers. In this paper, we use a New Radio (NR) simulator, based on ns-3, to assess the impact of 5G NR numerologies on the end-to-end (E2E) latencies in a realistic and complex scenario, including TCP and UDP flows. As expected, we found that TCP goodput increases with the numerology, since a larger numerology allows reducing the round-trip-time. However, although counter-intuitive, simulation results exhibit that the E2E latency of uplink (UL) UDP flows may not be reduced with the numerology. In fact, it depends on two key factors and their relationship: the processing delays (fixed or numerologydependent) and the inter-packet arrival time, which depends on the UDP flow rate and the packet size. We demonstrate how, in some cases, the latency is worsened by an increasing signaling exchange that grows with the numerology. In particular, this is due to a handshake mechanism in UL (scheduling request and UL grant) that is performed each time a data packet encounters empty RLC buffers. For some combination of flow rate, packet size, and processing delays that are not numerology dependent, increasing the numerology may not reduce the E2E delay. Therefore, we conclude that the selection of the numerology in an NR system should be carefully made by taking into account the traffic patterns and the processing delays. A number of analytical models exists that capture various properties of the BitTorrent protocol. However, until now virtually all of these models have been based on the assumption that the peers in the system have homogeneous bandwidths. As this is highly unrealistic in real swarms, these models have very limited applicability. Most of all, these models implicitly ignore BitTorrent's most important property: peer selection based on the highest rate of reciprocity. As a result, these models are not suitable for understanding or predicting the properties of real BitTorrent networks. Furthermore, they are hardly of use in the design of realistic BitTorrent simulators and new P2P protocols. In this paper, we extend existing work by presenting a model of a swarm in BitTorrent where peers have arbitrary upload and download bandwidths. In our model we group peers with (roughly) the same bandwidth in classes, and then analyze the allocation of upload slots from peers in one class to peers in another class. We show that our model accurately predicts the bandwidth clustering phenomenon observed experimentally in other work, and we analyze the resulting data distribution in swarms. We validate our model with experiments using real BitTorrent clients. Our model captures the effects of BitTorrent's well-known ‘tit-for-tat’ mechanism in bandwidth-inhomogeneous swarms and provides an accurate mathematical description of the resulting dynamics. Network latency and packet loss are considered to be an important requirement for realistic evaluation of Peer-to-Peer protocols. Dedicated clusters, such as Grid'5000, do not provide the variety of network latency and packet loss rates that can be found in the Internet. However, compared to the experiments performed on testbeds such as PlanetLab, the experiments performed on dedicated clusters are reproducible, as the computational resources are not shared. In this paper, we perform experiments to study the impact of network latency and packet loss on the time required to download a file using BitTorrent. In our experiments, we observe a less than 15 increase on the time required to download a file when we increase the round-trip time between any two peers, from 0 ms to 400 ms, and the packet loss rate, from 0 to 5 . Our main conclusion is that the underlying network latency and packet loss have a marginal impact on the time required to download a file using BitTorrent. Hence, dedicated clusters such as Grid'5000 can be safely used to perform realistic and reproducible BitTorrent experiments.