Mobile Network Performance and Content Delivery - Part 2

Most Content Delivery Networks (CDNs) attempt to pick the closest replcia server to your location. We see large inefficiencies in replica selection for cellular networks, with remote Public DNS resolvers retrieving better replicas a significant fraction of time.

Led by John Rula

This series looks at the current state of network performance and content distribution on cellular networks. Download Namehelp Mobile for Android in the Play Store to find which DNS give you the best performance on your phone. In Part 1 we looked at the impact DNS performance and compared your default DNS service on your mobile device to several public options. In this part we look at how Content Delivery Networks (CDNs) attempt to optimize content delivery on mobile networks.

Content delivery networks (CDNs) are the unseen engine which drive the Web. Companies like Akamai and Limelight operate a global network of replica caches, with the intention of delivering content from the closest (and thus fastest) server to its users. For instance, when you go to the New York Time's website, your browser resolves to its IP address by its locally configured DNS, typically provided by your internet service provider. This DNS tthen asks the authoritative DNS server (in this case run by its CDN Akamai), which trys to choose the closest server. Since the CDN can only see the origin of the last request, it has to approximate your location based on the location of your DNS server.

When selecting replica servers, CDNs try to choose the server with the lowest latency, typically the closest geogaphically, to its intended destination. Latency is important factor for web performance since the typical content request include several round trip times before the user receives data, including a DNS resolution, a TCP connection and HTTP header exchange. The multiple round trips mean that any latency improvements to the content server are compounded throughout the transfer.

Conventional widsom in replica selection is that the closer you are to your DNS server, the closer your content servers will be. It is very hard to improve upon the DNS server of your internet service provider when it comes to replica server selection. For example, our previous investigation into the impact of Public DNS on replica server selection (paper here )found that replicas selected using public DNS resolves had over twice the latency when compared to those chosen by your ISP's DNS.

Replica selection in cellular networks seems to contradict these traditional beliefs. In Part 1 of this series, we saw that public DNS resolvers like GoogleDNS and OpenDNS are typcaily farther away from clients resulting in longer DNS resolution than the ISP configured resolver. However, when we look at the latency to the replica servers returned by each DNS, we see that public DNS resolvers returned servers which had equal or lower latency 74% of the time in the case of OpenDNS! Figure 1 below shows the percentage of time the replicas returned by public DNS servies had equal or improved latencies for all hostnames we tested. It is also interesting to note that both public DNS resolvers returned servers with a 20% latency improvement around 20% of the time, and a 40% improvement 10% of the time.

% of Replica Selections
(larger is better)
Percentage Improvement
Figure 1. Percentage of times each public DNS service returned equal or better replica servers to devices.

When we look at replica selection on a per domain level, we see that it varies depending on the hostname chosen, and on the CDN they employ. The greatest example of disparity we saw was with both and, where OpenDNS performed as well or better 95% of the time. Figure 2 shows the percentage of time replicas returned had latencies which had equal or greater performance than the Local DNS options. The figure displays the wide disparity between public DNS services as well as across different hostnames. It seems clear that the current system of replica selection for cellular networks is far from optimal.

Fraction of Replica Selections (%)
(larger is better)
Figure 2. Percentage of times each public DNS service returned equal or better replica servers to devices for each hostname.

We have released a tool called Namehelp Mobile which measures the performance of your ISP provided DNS service against several public DNS options. In addition, it also measures the role that your DNS service has with Content Delivery Networks (CDNs) in mobile networks, and how your ISP provided DNS service can actually deliver worse performance in many cases.