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.

1. The Great Mobile Slowdown
by Ido Safruti, Cotendo
You see it all the time in New York, San significant cause of slow RTTs. HTTP is data requests behind them slow down
Francisco and other parts of the world. decades old but still dominates data even more. This explains why a mobile
Someone standing on a sidewalk staring traffic. Unfortunately, many of HTTPs page load could lag for 15 seconds, but
at his smart phone. And staring. And characteristics lead to increased when the user reloads the page after
staring. Maybe they are watching a latency. For example, HTTP allows hitting the back button the page loads
video. But more likely they are waiting processing of only one request per far faster on the second try. The slow
for a page to load. And usually they connection, this means that it takes a TCP connection was broken and a faster
look annoyed, which is understandable. minimum of an RTT to complete a single one replaced it. Needless to say, this is
For pages loading over a fast WiFi request, and prevents the browser from not an efficient way to accelerate page
or 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 less
Supposedly fast wireless networks received. On high latency connections, than the underlying protocols for data
average page load times of 8 to 10 like mobile, this results in low utilization delivery. So even when the network is
seconds. In places where high usage of the connection, where most of the not busy and the mobile device obtains
of data services congregate, like the time is spent waiting on one of the a speedy 8Mbps link, loading the same
big cities, those page loads can stretch sides. Browsers circumvent this issue page on mobile device will require far
to 20 seconds more. No, it’s not only by launching as many as 6 separate more time than loading that page on a
about 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 HTTP
Rather, a major cause which is almost on both the end device and the origin problem is called “pipelining,” and in
never discussed for these waits is the server. fact is part of HTTP/1.1. This enables
disparity in the round-trip-times (RTT) the client to send additional requests
experienced by users on wireless versus On wired broadband networks, where on a connection before receiving the
wired 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 better
mobile gateway and back is inevitably and network conditions are reliable due utilization of the link by eliminating
greater 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 pipelining.
at home or at work. That’s despite the and other mobile data networks, where Some servers and network proxies
best efforts of legacy Content Delivery the RTT from a mobile device to the don’t support it, either. Further, HTTP
Networks (CDNs) to speed up delivery mobile gateway alone is typically pipelining cannot handle the sequencing
times and cut latency, both of which will 120ms - 200ms, the dozens of of requests and responses. So for a
boost user engagement. It’s important requests for objects for a page cause page to load properly the responses
to note, too, that very small changes to compounding latency that can easily must be sent in the exact same order
the data footprint of mobile Websites add up to seconds spent waiting on a as the requests. In that case, an object
can result in large reductions in content spinning icon on the screen. Naturally, which is slow to serve will slow the
latency. So let’s examine this question this leads to user abandonment of entire connection, blocking further
at three different levels and discuss content and links. requests.
why mobile content delivery is so much
slower and what can be done to fix that TCP, too, compounds the problem Google is currently championing a
problem. as its efficiency and throughput is newer protocol called SPDY that is
dramatically impacted by variable and fully multiplexed, allows for servers
At the network level, the HTTP inconsistent RTTs on the link. This to initiate a request (“push”), permits
request-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 | info@cotendo.com | www.cotendo.com

2. reduction techniques presently missing images that are optimized for a Web that type of handset. By pushing logic
in HTTP. Google has made SPDY an page or applications on that specific closer and closer to the end user, CDNs
open standard and released their mobile device. Another way to speed will push the envelope on what can
implementation as open-source code. up mobile delivery is to simply speed and cannot be cached and stored in
They have also engineered SPDY into up the delivery of an entire Website. the edge of the network. Further, with
the Chrome browser and will likely This whole site acceleration involves these capabilities CDNs can expand the
include it in a future build of the optimization of all the content – both number of ways they cache. Whereas
Android operating system used by tens static and dynamic - coming off a Web legacy CDNs can cache only by URL and
of millions of mobile devices. However, server. Site Acceleration technologies filename, newer CDNs with advanced
greater client-side adoption of SPDY (by generally compress JavaScript, CSS and edge logic can cache information
handset 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 caching
With regard to the problem of site acceleration shaves off additional will dramatically reduce latency and will
TCP-induced latency, CDN operators are latency and is particularly important for speed up content delivery because far
creating special algorithms to optimize mobile devices. fewer of the dozens of different data
TCP delivery based on specific network requests required to load an average
conditions. 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 far
downward spiral of latency resulting Mobile interactions tend to focus on higher percentage of those requests will
from a slow request continuing to highly personal activities such as be fulfilled by pulling cached data stored
languish 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 based
ways to reduce the time it takes for services). These may include activities While consumers are waiting for their
each TCP request to be processed, such as checking the weather, traffic mobile devices to speed up, carriers
thereby minimizing the impacts of data, or looking at movie times around and content delivery networks will
compounding 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 to
At 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 the
more 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 network
networks. As the link to the device and you get the idea). Users requesting conditions, shrinking file sizes with
may be shaky, reducing the overall this data often experience painfully slow compression, and eliminating unneeded
payload to deliver, reduces the chance loading pages caused by round trips all round trips on HTTP are all needed to
for errors, and ensures faster delivery, the way back to the origin server. reduce latency and improve end-user
within less RTTs. For instance, images experiences on the mobile Internet. The
easily served over wired connections Some of the newer CDN technologies drive to implement and propagate these
may bog down mobile networks when can address the logic level in a number technology improvements is spurred on
network connectivity is weak. Adaptive of innovative ways. First, they can more by the fast growth of the various types
Image Compression, a technology intelligently cache information close of mobile applications and data usage
that 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 these
assessment of the specific user’s mobile this type of logic, cache appropriate newer mobile content acceleration
throughput and network conditions, is detailed weather information for a user technologies. As a result of these and
one way to address this challenge. With in the CDN based on their initial GPS many other newer technologies, we
this technology, a CDN generates on contact point. This weather data can hope that soon a person standing on
the fly different versions of the same be later served to other users located a the street staring at their smart phone
image with varying image sizes and few blocks away, eliminating the need waiting for a page to load is a far rarer
data footprints. As the CDN detects for additional round trips. Alternatively, occurrence.
the type of device and throughput the CDN can quickly detect the type of
conditions for the image request, device and cache assets in the edge
the 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 | info@cotendo.com | www.cotendo.com