Through the Wormhole: Low Cost, Fresh Peer Sampling for the Internet
Authors: Roberto Roverso, Jim Dowling, Mark Jelasity
In this paper, we present an alternative approach where the PSS creates enough new connections to ensure the PSS’ quality of service in the face of peer churn, but not too many as to exceed peers’ upper bounds on connection establishment rate. We call our system a wormhole-based PSS (WPSS).
WPSS can tune the number of new stable network connections established per sample to match application-level requirements and constraints. We show that our system can provide the same level of freshness of samples as the state of the art in NAT-aware PSSes, but with a connection establishment rate that is one order of magnitude lower. This, without sacrificing the desirable properties of a PSS for the Internet, such as robustness to churn, NAT-friendliness and local randomness of samples.
The main idea behind WPSS is that we separate the service into two layers. The bottom layer consists of a stable base overlay network that should be NAT-friendly, with private nodes connecting to public nodes, while public nodes connect to one another. On top of this overlay, every node periodically connects to a random public node selected from the base overlay (not necessarily a neighbor in the base overlay). Using the random public node, each node systematically places samples of itself on nodes in the neighborhood of this random public node. We call these links to random public nodes wormholes. That is, a wormhole is a link to a public node that is selected uniformly and independently at random. We do not require hole-punching or relaying to private nodes in this paper, although those techniques can be used as well if there are not enough public nodes. In addition to explaining the WPSS algorithm, our contributions also include a analytical comparison between WPSS and related work, and a thorough evaluation of the protocol in both simulation and our deployed system. The latter experiments include a comparison with the state of the art NAT-resilient PSS, Croupier.
EXPERIMENTAL SETUP: SIMULATION
For simulations, we implemented WPSS on the Kompics platform. Kompics provides a framework for building P2P systems and a discrete event simulator for evaluating those systems under different churn, latency and bandwidth scenarios. All experiments are averaged over 6 runs. Unless stated otherwise, we applied the following settings. The view size (the number of freshest random samples a node remembers) was 50 and we set ∆ = 1 second. For all simulation experiments, we use a scenario of N = 1000 nodes that join following a Poisson distribution with a mean inter-arrival time of 100 milliseconds. In all simulations 20% of the nodes were public and 80% were private, which reflects the distribution observed in the commercial deployments of our P2P application.
EXPERIMENTAL SETUP: DEPLOYMENT
In order to test WPSS in a real environment, we implemented the protocol using , a production-quality framework for building event-based distributed applications. The framework utilizes a UDP-based transport library that implements the same reliability and flow control mechanisms as TCP . We tested WPSS in our test network, where volunteers give us permission to conduct experiments on their machines using a remotely-controlled test agent. The test network contains around 12000 installations. The network included nodes mostly from Sweden (89%) but also some from Europe (6%) and USA (4%). For connectivity, 76% of the nodes were behind NATs or firewalls and 19.2% were public nodes, while the rest (4.8%) could not be determined by our NAT-type identification protocol.
Each experiment is run in wall-clock time and thus it is subject to fluctuations in network conditions and in the number of nodes involved. In order to keep a good level of reproducibility, we selected a subset of 2000 of the more stable nodes, out of an average of 6200 online, which are representative of both our test network and the Internet in Sweden, in general. For each data point, the deployment experiments were repeated a number of times varying from 3 to 10 runs, depending on the variance of the results, and every run lasted 20 minutes. In all deployment experiments, the nodes join at a uniform random point in time within the first 2 minutes from the start of the test. Unless stated otherwise, we set the a view size to 50, and ∆ = 2 seconds.
In this paper, we presented WPSS, a peer sampling service to meet the requirements of commercially deployed P2P systems on the Internet. WPSS executes short random walks over a stable topology and by using shortcuts (wormholes). WPSS provides the same level of sample freshness as other NAT-aware PSSes, but achieves that with a connection establishment rate that is one order of magnitude lower. In addition, the connection establishment rate of WPSS can be tuned from zero per sample to up one per sample, according to the application requirements on freshness and cost. We showed in our deployed live-streaming P2P system that the lowest cost connection creation rate lies between these two extremes. On top top of that, we experimentally demonstrated that our system has the randomness properties required of a peer sampling service, while it is robust to churn and large scale failure scenarios. While we have designed WPSS for the Internet, we believe it is general enough to work over any type of stable base overlay (given a high enough TTL for RWs) and any subset of nodes can act as wormholes. As part of our future work, we will consider applying WPSS to mobile and sensor networks, and overlay networks without NATs or firewalls.