The Great Mobile Slowdown


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Some of the newer CDN technologies can address the logic level in a number of innovative ways. First, they can more intelligently cache information close to end users with mobile devices, and use the location as a cache-key. So, for example, a weather site can, with this type of logic, cache appropriate detailed weather information for a user in the CDN based on their initial GPS contact point. This weather data can be later served to other users located a few blocks away, eliminating the need for additional round trips.

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The Great Mobile Slowdown

  1. 1. The Great Mobile Slowdown by Ido Safruti, CotendoYou see it all the time in New York, San significant cause of slow RTTs. HTTP is data requests behind them slow downFrancisco and other parts of the world. decades old but still dominates data even more. This explains why a mobileSomeone standing on a sidewalk staring traffic. Unfortunately, many of HTTPs page load could lag for 15 seconds, butat his smart phone. And staring. And characteristics lead to increased when the user reloads the page afterstaring. Maybe they are watching a latency. For example, HTTP allows hitting the back button the page loadsvideo. But more likely they are waiting processing of only one request per far faster on the second try. The slowfor a page to load. And usually they connection, this means that it takes a TCP connection was broken and a fasterlook annoyed, which is understandable. minimum of an RTT to complete a single one replaced it. Needless to say, this isFor pages loading over a fast WiFi request, and prevents the browser from not an efficient way to accelerate pageor home broadband connection the using that connection to send additional load speeds. The upshot of all this?process averages 2 seconds or less. requests until the response is fully Network bandwidth matters far lessSupposedly fast wireless networks received. On high latency connections, than the underlying protocols for dataaverage page load times of 8 to 10 like mobile, this results in low utilization delivery. So even when the network isseconds. In places where high usage of the connection, where most of the not busy and the mobile device obtainsof data services congregate, like the time is spent waiting on one of the a speedy 8Mbps link, loading the samebig cities, those page loads can stretch sides. Browsers circumvent this issue page on mobile device will require farto 20 seconds more. No, it’s not only by launching as many as 6 separate more time than loading that page on aabout the bandwidth or the backhaul, or connections per hostname. But this broadband connection.overloaded towers either. solution is not a panacea as it adds complexity and processing overhead One approach to address the HTTPRather, a major cause which is almost on both the end device and the origin problem is called “pipelining,” and innever discussed for these waits is the server. fact is part of HTTP/1.1. This enablesdisparity in the round-trip-times (RTT) the client to send additional requestsexperienced by users on wireless versus On wired broadband networks, where on a connection before receiving thewired broadband networks. In short, the RTT is typically brief (10ms-50ms) entire response back from the server.the RTT from the mobile device to the and connections are consistently fast Pipelining actually enables bettermobile gateway and back is inevitably and network conditions are reliable due utilization of the link by eliminatinggreater than the last mile RTT from the to fixed nature of these networks, this some unneeded round trips. However,end user back and forth over cable, additional overhead for delivering each this approach has a few problems.fiber or DSL broadband connections object is barely noticeable. But on 3G Most browsers don’t support home or at work. That’s despite the and other mobile data networks, where Some servers and network proxiesbest efforts of legacy Content Delivery the RTT from a mobile device to the don’t support it, either. Further, HTTPNetworks (CDNs) to speed up delivery mobile gateway alone is typically pipelining cannot handle the sequencingtimes and cut latency, both of which will 120ms - 200ms, the dozens of of requests and responses. So for aboost user engagement. It’s important requests for objects for a page cause page to load properly the responsesto note, too, that very small changes to compounding latency that can easily must be sent in the exact same orderthe data footprint of mobile Websites add up to seconds spent waiting on a as the requests. In that case, an objectcan result in large reductions in content spinning icon on the screen. Naturally, which is slow to serve will slow thelatency. So let’s examine this question this leads to user abandonment of entire connection, blocking furtherat three different levels and discuss content and links. requests.why mobile content delivery is so muchslower and what can be done to fix that TCP, too, compounds the problem Google is currently championing aproblem. as its efficiency and throughput is newer protocol called SPDY that is dramatically impacted by variable and fully multiplexed, allows for serversAt the network level, the HTTP inconsistent RTTs on the link. This to initiate a request (“push”), permitsrequest-response protocol is a ensures that those connections and the header compression, and other latency © 2011 Cotendo, Inc. All rights reserved. 866.749.6462 | INTL +44 20 33188363 | |
  2. 2. reduction techniques presently missing images that are optimized for a Web that type of handset. By pushing logicin HTTP. Google has made SPDY an page or applications on that specific closer and closer to the end user, CDNsopen standard and released their mobile device. Another way to speed will push the envelope on what canimplementation as open-source code. up mobile delivery is to simply speed and cannot be cached and stored inThey have also engineered SPDY into up the delivery of an entire Website. the edge of the network. Further, withthe Chrome browser and will likely This whole site acceleration involves these capabilities CDNs can expand theinclude it in a future build of the optimization of all the content – both number of ways they cache. WhereasAndroid operating system used by tens static and dynamic - coming off a Web legacy CDNs can cache only by URL andof millions of mobile devices. However, server. Site Acceleration technologies filename, newer CDNs with advancedgreater client-side adoption of SPDY (by generally compress JavaScript, CSS and edge logic can cache informationhandset makers and mobile software other components of a typical Website based on the device, the location,developers) will take some time. that may be served uncompressed from network conditions, and many other the content origin Web server. Whole key parameters. This advanced cachingWith regard to the problem of site acceleration shaves off additional will dramatically reduce latency and willTCP-induced latency, CDN operators are latency and is particularly important for speed up content delivery because farcreating special algorithms to optimize mobile devices. fewer of the dozens of different dataTCP delivery based on specific network requests required to load an averageconditions. This will make TCP requests At the business logic level, the nature Web page will need to travel all the way“smarter” and less likely to create a of mobile behavior comes into play. back to the origin server. Rather, a fardownward spiral of latency resulting Mobile interactions tend to focus on higher percentage of those requests willfrom a slow request continuing to highly personal activities such as be fulfilled by pulling cached data storedlanguish in the back of the TCP priority communication, use of social networks, in the edge of the network.queue. CDNs are also figuring out new and local services (or location basedways to reduce the time it takes for services). These may include activities While consumers are waiting for theireach TCP request to be processed, such as checking the weather, traffic mobile devices to speed up, carriersthereby minimizing the impacts of data, or looking at movie times around and content delivery networks willcompounding latency. you. Caching this type of information continue to work on a variety of new on a CDN is particularly difficult technologies to speed up content toAt the content level, delivering Web because it is personalized to a user or mobile devices around the world.pages and data to mobile devices is far geographically fragmented into very Bringing content and logic closer to themore complicated than delivering to small increments (Think how many user, more intelligent content caching,PCs or laptops over wired broadband movie theaters there are in a large city optimizing TCP delivery for networknetworks. As the link to the device and you get the idea). Users requesting conditions, shrinking file sizes withmay be shaky, reducing the overall this data often experience painfully slow compression, and eliminating unneededpayload to deliver, reduces the chance loading pages caused by round trips all round trips on HTTP are all needed tofor errors, and ensures faster delivery, the way back to the origin server. reduce latency and improve end-userwithin less RTTs. For instance, images experiences on the mobile Internet. Theeasily served over wired connections Some of the newer CDN technologies drive to implement and propagate thesemay bog down mobile networks when can address the logic level in a number technology improvements is spurred onnetwork connectivity is weak. Adaptive of innovative ways. First, they can more by the fast growth of the various typesImage Compression, a technology intelligently cache information close of mobile applications and data usagethat dynamically compresses image to end users with mobile devices, and patterns (dynamic, personal,content based on the recipient device’s use the location as a cache-key. So, location-based content applications)display capabilities and a real-time for example, a weather site can, with that most benefit from theseassessment of the specific user’s mobile this type of logic, cache appropriate newer mobile content accelerationthroughput and network conditions, is detailed weather information for a user technologies. As a result of these andone way to address this challenge. With in the CDN based on their initial GPS many other newer technologies, wethis technology, a CDN generates on contact point. This weather data can hope that soon a person standing onthe fly different versions of the same be later served to other users located a the street staring at their smart phoneimage with varying image sizes and few blocks away, eliminating the need waiting for a page to load is a far rarerdata footprints. As the CDN detects for additional round trips. Alternatively, occurrence.the type of device and throughput the CDN can quickly detect the type ofconditions for the image request, device and cache assets in the edgethe CDN node will serve compressed of the network that are required for © 2011 Cotendo, Inc. All rights reserved. 866.749.6462 | INTL +44 20 33188363 | |