Our previous detouring performance results in Section 8.2.3 were restricted to testing to des- tinations within the PlanetLab testbed. While this gives us the capability to evaluate over hundreds of Internet sites, the diversity of the network is limited by being mostly confined to academic networks. To explore the potential of bandwidth detouring with more realistic des- tination servers on the Internet, we can now demonstrate detouring performance to a set of public Internet web servers located in geographically diverse locations, and two public cloud services.
It would be cost-prohibitive to deploy evaluation servers in a large number of commercial Internet locations. To emulate this evaluation environment, we use mirror sites for the Linux kernel source archive as an example of a widely available and popular source of large file down- loads. Most of the mirrors are hosted at commercial sites, providing a contrast to the academic- network bias of PlanetLab. We select 400 internationally distributed sites at random as targets, and download a single copy of the source archive from each, around 40 MB in size. The mirror sites show a great diversity in network connectivity and host large files that are particularly amenable to detoured downloads. They are not, however, a demonstration of a typical detour- ing use case: by design, mirror sites are distributed so that they can be local to as many clients as possible, and the content that they host is an ideal candidate for CDN or P2P distribution. As well as more traditional private network hosting, many network services are now being de- ployed on cloud-based infrastructures. Such services reduce their operational costs by deploying infrastructure in a limited number of locations. Next, we demonstrate that detouring can also be effective for improving performance to the well-provisioned environments that are typical of cloud providers by using our system to access two cloud storage platforms. Cloud storage services typically hold the private data of users or organisation. Thus, they are inherently not suitable for replication-based distribution approaches, such as those offered by CDNs or P2P, which makes them an ideal target for bandwidth detouring. We provision storage space on Amazon S3 [158], a widely used cloud data storage service that has data centres in 5 inter- national locations: California, Virginia, Ireland, Singapore and Japan. As is typical of cloud services, S3 uses an HTTP API as its primary interface, allowing it to work with our HTTP detouring approach. We host a 40 MB file on each of the sites and make it available through a public HTTP URL.
To demonstrate detouring with a more typical consumer-oriented service, we deploy the same 40 MB file on the Dropbox file store service [33]. Dropbox is a popular online file storage service targeted at individual users and small groups, making content hosted within it not generally suitable for P2P or CDN based distribution systems. It is a commercial service that is deployed solely on Amazon’s cloud infrastructure. We perform detoured downloads from it using its HTTP API, and we observe that connections are directed through a load-balancing system located at the Californian site, making it an example of a globally popular high-bandwidth public service that is only accessible through a single network location.
0.0 0.2 0.4 0.6 0.8 1.0 0.5 1 2 4 8 CDF of paths
Relative bandwidth improvement (logscale) Web Mirrors Dropbox Amazon-S3
Figure 9.2: Detouring improves bandwidth to arbitrary public Internet destinations
We set up the detouring infrastructure as described in Section 9.1, using 50 PlanetLab nodes as the detour sites. Clients are configured to use the best-ranked-5 detouring strategy, meaning that only 5 PlanetLab nodes are used for the purposes of detouring; the rest only participate to take measurements as part of the detour ranking process. We configure a number of detouring clients, also based on PlanetLab. For the public web server experiment, we deploy 10 clients and allocate the 400 target URLs to them in a round-robin fashion. To minimise impact on the third-party infrastructure, each site only serves one download over the course of the experiment. The experiment thus represents the measurement of 400 independent network paths.
For the cloud services evaluation we deploy 78 PlanetLab-based clients. These clients down- load the target file over HTTP from each of the 5 S3 sites, representing 390 independent paths. They also download the target file from Dropbox. For each target, first the file is downloaded directly, then the transfer is repeated through the HTTP detour proxy.
Figure 9.2 shows the distribution of detouring bandwidth improvements relative to the direct path. The most notable observation in all of these results is that the detouring performance is significantly better than that of the intra-PlanetLab detouring evaluated in Section 8.2.3. Detours are successfully discovered for 80% of paths, with median bandwidth improvements ranging from 1.5× for the public webservers and Dropbox targets, to a factor of 2.5× across the S3 targets. Many paths are able to more than double their direct path bandwidth using the detouring system: 28% of the public webserver paths, 35% of Dropbox paths and, most significantly, over 50% of paths to S3.
The improved performance compared to the earlier PlanetLab experiment is likely due to performance limitations of our test webserver, as noted in Section 8.2.3. Where the server itself forms a bottleneck, detouring improvement will be limited. We believe that this also explains why Dropbox achieves a better performance distribution than the public webservers: Amazon’s
network provisioning is likely to be more extensive than most commercial hosting providers, offering more opportunity for improvement before the service becomes the bottleneck.
The 5-site S3 deployment spans three continents, with an equal balance between Asian and North American sites. The majority of our client nodes are based in Europe and the US (such is the distribution of PlanetLab infrastructure). This results in many of the evaluated transfers taking place over extremely long-distance connections between Europe and the US, and Asia. Such paths are particularly amenable to detouring due to the split-TCP effect, which may account for these paths demonstrating larger improvements than the solely US-based Dropbox and the more uniformly distributed public mirror sites.
Detouring for residential broadband
In the previous experiment, we used PlanetLab hosted detouring clients. Such systems will typically exist at academic sites and will have extensively provisioned network connectivity. It is a safe assumption that such connections do not constitute the majority of network connec- tions available to users, who will typically connect from homes or offices with consumer grade infrastructure, such as DSL or DOCSIS based cable Internet, with limited last-mile throughput. We next demonstrate how our detour routing approach performs with clients in such networks. We acquire access to client nodes connected via three different types of residential broadband connection from different local ISPs: a low-speed DSL connection of 3.5 Mbps, a faster 6.5 Mbps DSL connection, and a high-speed DOCSIS-3 cable connection at 50 Mbps. As before, we configure the clients to use the detour nodes of the best-ranked-5 strategy, and direct them each to download from 20 of the public mirror sites.
Table 9.1 summarises detouring performance for the 20 target mirrors over the 3 different connection classes. The slow broadband connection, DSL-low gains little benefit from detouring, locating an effective detour for only 15% of paths. Considering the distribution of bandwidth in Internet paths, as shown in Section 3.4.2, only around 15% of paths would be expected to have a throughput lower than that of the access link, demonstrating that detouring can be effective where bandwidth is not limited by the local network connection.
Similarly, the faster DSL link (DSL-med) shows a slight improvement through detouring, but it remains marginal with successful detouring for only 25% of paths. Again this demonstrates detouring potential being limited by a network bottleneck on the last-mile connection, which is significantly slower than the achievable throughput on most Internet paths.
In contrast, DOCSIS-high illustrates that, with access to a link with a capacity exceeding that of most Internet paths, detouring becomes a feasible approach to improving Internet path performance. We successfully locate detours for 65% of paths, achieving an average (mean) improvement over the direct path of a factor of 3.
Since this experiment encompasses few paths from one specific geographical location, it is appropriate to discuss the specifics of the detour paths that were employed. Using traceroute to compare the geographic locations traversed by the direct and detoured paths, we find that
DSL-low DSL-med DOCSIS-high
Max bandwidth (kbps) 3499 6645 43,045
Detoured paths 15% 25% 65%
Avg gain (kbps) 121 133 4964
Avg relative gain 1.17 1.29 3.21
Table 9.1: Detouring performance behind residential broadband connections
the detour routes chosen by our system could be described as geographically similar to the direct paths. Of the five detour nodes that were available, 52% of detour paths used a node located in Germany, primarily for destinations in Europe, and a further 29% of paths used a detour node in Washington DC, for routes to sites in the Americas and Asia.
Conclusions
By extending our detouring evaluation to include both commercial networks and residential broadband connections, we have demonstrated that the detouring improvements made by our system are not limited to the academic networks represented by PlanetLab, or to well provi- sioned client environments. In contrast, the detouring performance to commercial destinations is generally stronger than to those within PlanetLab, due to the performance limitations that we note to exist on PlanetLab nodes.