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Stocker, V., Smaragdakis, G., Lehr, W., & Bauer, S. (2017). The growing complexity of content delivery
networks: Challenges and implications for the Internet ecosystem. Telecommunications Policy, 41(10),
1003–1016. https://doi.org/10.1016/j.telpol.2017.02.004
Volker Stocker, Georgios Smaragdakis, William Lehr, Steven Bauer
The growing complexity of content
delivery networks: Challenges and
implications for the Internet ecosystem
Accepted manuscript (Postprint) Journal article |

The Growing Complexity of Content Delivery Networks:
Challenges and Implications for the Internet Ecosystem

Volker Stocker 1
Georgios Smaragdakis 2
William Lehr 3
Steven Bauer 4

Abstract

Since the commercialization of the Internet, content and related applications, including video
streaming, news, advertisements, and social interaction have moved online. It is broadly
recognized that the rise of all of these different types of content (static and dynamic, and
increasingly multimedia) has been one of the main forces behind the phenomenal growth of the
Internet, and its emergence as essential infrastructure for how individuals across the globe gain
access to the content sources they want. To accelerate the delivery of diverse content in the
Internet and to provide commercial-grade performance for video delivery and the Web, Content
Delivery Networks (CDNs) were introduced. This paper describes the current CDN ecosystem
and the forces that have driven its evolution. We outline the different CDN architectures and
consider their relative strengths and weaknesses. Our analysis highlights the role of location, the
growing complexity of the CDN ecosystem, and their relationship to and implications for
interconnection markets.

Keywords: CDN, Interconnection, Peering, QoS, QoE, Pricing, Internet Evolution

1 Unive rsi ty of Fre ibur g, Ger many, [email protected] - freiburg.de
2 MIT/ TU Berl i n, [email protected] . edu ; Ge orgios Sm aragda kis w as su pporte d by the ERC Starting G rant
Resol utio Net ( 679158)
3 MIT, wl ehr@mit .ed u ; D r. Lehr would like to ackno wledge s upport from NSF Awa rds 1413 973, 154 7265 an d the
MIT Co mmun ic at ion s Fu tur es P ro gr am (h tt p: // cf p. mit .e du ).
4 MIT, [email protected]

The Growin g Complexi ty of Conte nt Del ivery Networks:
Chall enges and Impl icat ions for th e In tern et Ec osys tem

1. Introduction
Content Delivery Networks (CDNs) emerged as overlay networks on the Internet in order to
provide better support for delivering commerc ial content than was available using basic, “best-
effort” Internet packet transport services. As the volume, complexity, and heterogeneity of
Internet traffic has grown and evolved, so too have the ISPs and CDNs that provide the services
used to deliver this traffic. The importance of CDNs within the Internet ecosystem has grown
significantly over time – recent reports expect that CDNs will soon be handling over half of the
global traffic on the Internet (cf. Cisco, 2016, p. 18).

In this paper, we describe the current CDN ecosystem and the forces that have driven its
evolution. Section 2 provides an overview of the basic operation of CDNs and recounts how they
have evolved since their market inception in the 1990s. Section 3 then presents a typology that
sets forth the multiple types of CDNs and their relative strengths and weaknesses. Section 4
focuses on how CDN choices of where to locate their servers impact their operations, and
Section 5 offers our speculations on where we see the CDN ecosystem going. Section 6
concludes and offers some thoughts and implications for the future Internet ecosystem.
2. On the Evolution of Content Delivery Networks
Although the Internet’s basic infrastructure has scaled remarkably well, its end - to -end, “best
effort” design was premised on a communication paradigm based on passive traffic management.
The basic protocols like the Transmission Control Protocol (TCP) that manage packet
transmission on an end- to -end basis seek to provide decentralized congestion management and
fair resource allocations across competing flows; however, these protocols fail to support the
predictable, end- to -end Quality of Experience (QoE) 1 that is increasingly being demanded by
commercial content and application providers. 2 Moreover, networks offering only a single
service class are not well-suited for supporting the heterogeneous needs of traffic types with very
different Quality of Service (QoS) requirements.

1 QoE is a holi sti c concept that descr ibe s the subjec ti vel y perc ei ved quali ty of a user when consumi ng
content or appli cations over t he Internet.
2 For example, TCP performance degrades as the distance between communicati ng hosts increas es and in
the pr esen ce of packet losses along the end - to - end p ath (cf. Leighton, 20 09). Also, TCP’s flow - rate
fairness principal ma y be exploited b y applications (e.g., using sw arming techniques to sim ultaneously
operate multiple TCP connections) resulting in som e application s capturing a disproportionate share of
available capacit y, while leavi ng other applicati ons starved of capacity (cf. e.g., Briscoe, 2007).

While attempts have been made to expand the set of Internet protocols to enable better support
for heterogeneous QoS requirements, 3 modifying the basic Internet infrastructure is difficult
because it requires coordinating the widespread adoption of new protocols. In the Internet,
control is decentralized and dispersed among many different and often competing entities.
Although individual networks may deploy enhanced capabilities for active traffic management of
on-net traffic, widespread inter-provider deployments are not generally available (cf. e.g.,
Stocker, 2015). Content Delivery Networks (CDNs) evolved to address the need for better
support for differentiated content distribution without requiring modifications to the basic
Internet architecture.
2.1. A Primer on CDN Service Provision
CDNs employ a scalable architecture of cache servers which are strategically distributed across
the Internet and constitute an overlay “on top” of the Internet’s basic packet transport
infrastructure. 4 A typical CDN maintains a hierarchy of servers, with back-end servers ensuring
the efficient intra-CDN distribution of content, and front-end servers at t he edges used for
handling user-server communications. The se servers a ll ow replicated copies of content to be
stored at multiple locations across the Internet.

CDN providers employ complex software to match incoming requests for content to the “best”
server for meeting each end-user request. 5 Requests flow from end-users to the selected front-end
server, which delivers a copy of the requested content to the end-user, if a copy is already
available on the server. If not, the front-end server passes the request further up the CDN-server
hierarchy until a copy is located, which may entail pulling a copy from the content provider’s

3 Over ti me, the Inter net suit e of proto col s have expan ded to incl ude bett er suppo rt for conte nt deli ve ry
and other ty pes of QoS d ifferen tiated pack et tra nsport serv ices. For exa mple , m ulticast enh ances co ntent
delivery capabilities by allowing a single packet that is to be sent to m ultiple destinations to be replicated
only at branching points, thereby reducing the packe t transpor t resource s required (cf. Cisco, 2001).
Alt ern ati ve ly, ne wer pro toc ols l ike Dif fSe rv and Int Ser v p rovi de sup por t f or bet te r - than - best - effort packet
delivery (cf. C isco, 2005). Although such capabiliti es already exist in the Internet, they are n either
uniformly available nor im pleme nted in a stan dard way ac ross ISPs wh ich c omp licates efforts to mak e
use of them.
4 Cache servers are dedic ated server s used to stor e conte nt for subsequ ent del iver y. In gener al, one can
distingui sh between three gener al types of cache servers i nvolved in CDN service provisioning: front - end,
back - end, and origin servers. The "origin server" refers to the server where the original or m aster copy of
the con tent is stored . A key ro le for CD Ns is to place ad ditional co pies of that content on other cache
servers elsewhere in the Internet to improve content delivery services. T he "front - end" servers are the
ones that end - users comm unicate with; and th ese m ay comm unicate with appropriate "back - end" servers
that are loc ated w ithin the C DN netw ork an d prov ide add itional CD N f unctio nality as w e exp lain. For
further details on CD N servers, see Leigh ton (2009), Ny gren et al. (2010), or Flach et al. (2013).
5 Dete rmi nin g the “best ” ser ver m ay d epend on different m etrics in t he quality/cost - space in which
different delivery features may be emphasized. As will be described in S ection 4, the location of the
server is im portant, and "location" may be defined in m ultiple w ays. While in the case of CD Ns with a
network of dedicated servers, redirection decisions are typic ally done via the Domain Name System
(DNS ), distributed nam ing schem es are used in peer - to - peer CDNs.

origin server if a closer copy is not available (cf. Nygren et al., 2010). In this way, content is
replicated and distributed across the CDN’s footprint of servers. Decid ing what content to store
in which servers and for how long to retain copies, and how to best manage requests for serving
content is complicated and depends on the nature of the content (i.e., static versus dynamic), 6 the
preferences of the content provider, end-user demand for the content (i.e., content popularity),
what else is going on in the Internet (e.g., congested links), and the capabilities of the CDN
provider. Further complications arise if origin servers are located at different content providers
(e.g., as might be the case for advertisements and background text), or specialized security or
stringent QoS requirements necessitate specialized routing treatment by the CDN provider.

Replicating content in multiple locations and jointly managing multiple servers in real-time,
allows CDN providers to better balance server loads, which enhances the utilization efficiency of
server capacity and capa city scaling, lowering content delivery resource costs and enabling
CDNs to provide improved QoE performance for end-users. It helps CDNs better respond to
flash crowds and denial of service attacks (cf. e.g., Nygren et al., 2010; Maggs and Sitaraman,
2015). When the content is available from multiple locations, single points of failure are
eliminated, which improves reliability. The ability to match content requests with the “best”
server for each request helps content providers reduce end- to -end delays and ensure a more
consistent and higher QoE for their end-users (cf. Dilley et al., 2002; Chiu et al., 2015). CDN
providers make use of proprietary protocols to distribute content across their server network,
augmenting the basic Internet’s capabilities. Content can be handled differentially on a per-
request basis, without requiring special software or modifications to the underlying Internet
infrastructure. 7

Although CDNs can do much to enhance the performance of content delivery, their end- to -end
control is limited by their dependence on the underlying ISPs for basic packet transport services.
Most CDNs do not operate their own packet data networks or lea se dedicated facilities to
interconnect their servers, and rely on the ISPs to make packet routing decisions. The CDN
overlay networks and the ISP s both have a role in determining the end- to -end QoE for content
delivery, and the extent to which the CDN providers and ISPs coordinate their traffic
management decisions can have a big impact on the overall QoE.
2.2. Innovation and the Cost of CDN Service Provision
When CDNs began to emerge in the mid to late 1990s, bandwidth prices for packet transit were
expensive and accounted for a much larger share of the total costs of content delivery than they

6 Dynami c cont ent , as oppose d to sta ti c cont ent , needs to be frequ ent ly update d, an d as a consequence
may not be cachea bl e. For examp le , a tel eph one call or data ass oci at ed wit h a highl y int era ct ive websit e
(e.g., for g aming or prov iding rea l - time stock price data) may not b e ca cheab le. O ther c onten t like
maga zi ne sto ri es, movie s, pict ur e files, or software updates are static content that, once stored in a C DN's
cache, does not require updating. For further discussion on the challenges of sup porting dynamic content,
see, for example, Leight on ( 2009, pp. 4f. ) or Nygren et al. ( 2010, pp. 9f f).
7 For example, CD Ns can serve higher or lower - resolution con tent b ased on the identity of the req uestor,
the cu rrent sta tus of the n etwo rk, or the pre ferenc es of the co ntent provide r. See, for example, Nygr en et
al. (2010), or Winstein and Balakri shnan (2013).

do today. The costs of acquiring and maintaining servers (including energy to power and cool
servers) and CDN personnel costs were less significant. Consequently, minimizing bandwidth
costs for packet transit was the chief consideration for most CDN providers. This has changed
over time. Technical innovation, changing market competition dynamics and large private and
public investments in fiber networks and transatlantic links to expand transport capacity have
contributed to steep reductions in the cost of bandwidth. Further, the growth and increasing
importance of direct interconnections based on settlement-free (or, revenue-neutral) peering
agreements (cf. e.g., Faratin et al., 2008; Clark et al., 2011) and the emergence and growth of
Internet Exchange Points (IXPs) (cf. Chatzis et al. 2013) and other interconnection facilities (e.g.,
Equinix) which provide platforms for interconnecting multiple networks (cf. Labovitz et al.,
2010) have helped CD Ns and access ISP s avoid paying for transit services. These developments
have contributed to a seismic shift in how the Internet is interconnected.

Although this has further amplified the general trend of declining unit prices for bandwidth
around the world, significant price differences between regions persist. For example, bandwidth
prices in Asia and the Pacific may be up to three times higher than prices in Europe and North
America (cf. Stronge, 2015; Telegeography, 2016). Finally, bandwidth pricing is typically
subject to significant volume discounting. As a consequence, managing regional and volume-
based bandwidth resources remains a key component of the CDN value proposition.

Due to other developments such as significant improvements in computing and communications
technologies in accordance with Moore’s Law and corresponding drops in the cost of servers and
storage, the cost structure of CDNs has changed (cf. Armbrust et al., 2009). Over time, other cost
components such as payments for hosting services (e.g., to the ISPs or IXPs providing physical
locations where servers are hosted) and energy have become relatively more important CDN
operating cost components. Depending on the CDN architecture, CDNs may be able to reduce
co sts by strategically locating services to take advantage of hosting and energy price differences
across markets (cf. e.g., Qureshi et al., 2009; Greenberg et al., 2009).

At the same time that CDN costs have been changing, customer demand has become more
differentiated. In addition to the continuing demand for the distribution of static content, content
providers and their customers have growing needs to serve a broader array of more dynamic and
interactive content. To meet this increasingly diverse demand and address the growing
competition among CDN providers, some CDNs are expanding their offerings to include richer
support for other types of applications, moving beyond traditional content delivery services.
With the migration to everything-over-IP, the rise of new types of media is straining legacy
notions of what constitutes traditional media content distribution. Additionally, different types of
content may be associated with very different economics. For example, a security firm seeking to
aggregate surveillance videos from properties they manage may have very different requirements
for accessing the video than a commercial provider of entertainment content. Even if one focuses
on commercial enterta inment media, and further restricts attention to streaming video, such
content can have very different distribution requirements. Lots of it may be static and cacheable.
This includes movie or television programming libraries from Netflix, HBO, or Hulu. Other
types such as live events (sports, concerts) and breaking news requires (near) real-time delivery.
The audiences for content can be quite heterogeneous also. For events like the Olympics, the
audience is large and global, and near-real-time. Other entertainment content may have few if

any end-users interested in viewing it, and such viewing as occurs may be widely distributed
across geography and time.

Meeting market demand for s upport for heterogeneous content with quite different QoS delivery
requirements has important implications for CDN service provisioning and its costs. One- size -
fits -all solutions are not cost-effective – either they fail to meet the requirements of the content
with specialized QoS reqirements or over-provision capabilities for content that does not require
those specialized services. Some CDNs are exploring niche strategies, focusing on specific types
of content, specific architectures, or specific markets to better balance specialized requirements
and costs with customized solutions. There are also general-purpose CDNs that offer a portfolio
of differentiated services to address the needs of a broad class of content, content providers, and
network environments. This increased complexity in the CDN ecosystem mirrors the growing
complexity in Internet interconnection regimes. The two phenomena are interrelated since both
reflect responses to the changing nature of Internet traffic and its continued exponential growth
and the proliferation of user devices and contexts in which content must be delivered.
2.3. Evolution of the CDN Value Proposition
As the number of consumers and businesses operating online expands and a growing share of
their digital content is shift ed online, the range of content to be delivered and the range of
contexts in which end-users may wish to a ccess content has expanded. Thus, the scope of new
market opportunities to offer QoS-differentiated CDN services is widening. In addition to
providing support for a growing variety of heterogeneous packet delivery requirements,
additional complementary services have become integral components in the service offerings of
many commercial CDN providers. These include, for example, support f or enhanced security,
additional cloud functionalities (e.g., video-coding on the fly), and support for market metrics
and analytics. These complementary services have become more important as the marketplace
for basic content delivery services has become more competitive and diverse. For example, in
2015, Akamai generated more than half of its revenues from “Performance and Security
Solutions.” Cloud Security Solutions alone accounted for more than 11% of total revenues (cf.
Akamai, 2016c, p. 33). The growth in rich multimedia traffic, in particular entertainment video,
has contributed significantly to the rise of CDNs, which in turn has helped propel further growth
in over-the-top (OTT) entertainment services such as Spotify and Netflix, that drive still further
demand for CDNs (cf. Lehr and Sicker, 2016). The resulting growth of the CDN ecosystem
confronts content providers with a menu of options for how best to deliver content to their
customers.

Some content providers with limited budgets, small audiences, or with content that can tolerate
lower quality delivery performance may select approaches like centralized hosting, web-based
delivery via platforms like Facebook, or free but limited CDN services for delivering their
content. Larger and predominantly commercial content providers may choose to avail themselves
of the services of general-purpose CDN providers like Akamai or Limelight, selecting service
packages that are customized to their specific needs (cf. e.g., Akamai, 2016d). This middle-
market of commercial content providers is the heart of the market for third-party CDN providers.
Commercial entertainment providers confront intense competition for the attention (i.e.,
advertising supported) and discretionary spending (e.g., for paid entertainment content) of their
target audiences and may thus consider faster and more reliable content delivery as a competitive

advantage. Many engage in “windowing” strategies by which they seek to segment their target
audience in order to facilitate price discrimination to help them increase revenues. 8 At the upper
end of the market, the larger content providers continually evaluate their make-vs-buy decisions,
and the largest providers like Netflix and Google (YouTube) have outgrown the capabilities of
third-party CDNs and have deployed their own specialized CDNs.

At the same time that content providers are evaluating their options for how best to deliver their
content, the marketplace for incumbent CDN providers and entrants continues to shift. ISPs that
traditionally focus ed on generating revenues from selling bandwidth to support basic packet
transport services have seen their transit revenues erode in the face of falling transit prices and
restructured interconnection agreements (i.e., in particular the trend to direct peering). A s a
response, a number of ISPs are exploring their options for offering value-added services, and
providing CDN services is one possibility. Many of the largest operators of full-service networks
such as Telefonica, AT&T, and Telecom Italia, are investing heavily in Network Function
Virtualization (NFV) and Software Defined Networking (SDN) technologies to “softwarize”
their networks. These technologies enable finer-grained, flexible, and dynamic control over
network resources. ISPs costs are lowered because operators can shift to commodity hardware;
and because network functionality can be de-localized. For example, a single software switch
can provide the call handling services that previously were provided by a hierarchy of local
switches, and the logic and the actual switching do not need to be geographically co-located.
Network infrastructure can be virtualized and then sliced into logical partitions that may support
different levels of QoS and network functionality. In its fullest realization of this development,
the ISP can offer multiple tiers of cloud services ranging from wholesale access to core cloud
resources (computing, storage, and transport). 9 With these new capabilities, these next generation
ISPs are able to offer substitutes for much of the functionality that CDN providers have
historically provided as independent overlay networks. The evolution of these ISPs from legacy
providers of telecommunication services to full- ser vice providers of cloud services is blurring the
boundary between where the Internet ends and overlays begin.

The mix of differing businesses for content and CDN providers on the one hand and ISPs on the
other is resulting in a complex mix of vertical and horizontal business strategies and cross-
linking organizational strategies. For example, ISPs with a limited geographic footprint cannot
implement a CDN to serve global content providers unless they partner with other CDNs or ISPs.
The matching between content provider needs and ISP capabilities can be met in numerous ways.

8 Hist or ic all y, the order of public rel eas e was used to impleme nt price dis cri min ati on “windows ”. For
example, movies w ere first released to theaters, then to video rentals , and later for over - the - air
broadcasti ng. W ith the evolution of media markets, traditional media windowing strategies have become
much more compl i cat ed , but the gen er al goa l of bundl in g cont ent wit h dis tr ibu ti on ch ann els to eff ec t
mark et segmen tat i on to enab le pric e disc ri min ati on remai ns impo rt ant . For furt her disc uss io n, see Lehr
and Sicker (2016).
9 Providi ng wholesal e access to basic computing , stora ge, and tra nsport functio nalit y is someti mes
referred to as Infras tructure - as -a- Service ; prov iding applica tion o r con tent h osting servic es is some times
referred to a s Platf orm -as- a- Service , or Software - as -a- Service , with the la st pro vidin g the grea test lev el of
mana geme nt on the part of the ISP cloud provi der . For dis cus si ons of NFV and SDN, see Tele fo nic a
(2014), ET SI (2012), or M etzler (201 5).

3. A Typology of CDN Architectures
As was illustrated in previous sections, the CDN ecosystem has grown increasingly complex and
consists of a wide array of different CDN architectures and business models. In the following,
we provide a typology of CDN architectures currently deployed in the Internet. Key f eatures that
distinguish the different CDN strategies include the strategy for deploying servers (e.g., the
degree of distribution) and the business models employed (e.g., what performance characteristics
are emphasized or what key applications are the market focus). Table 1 summariz es our f indings.
The first four types refer to multi-purpose CDNs based on different server deployment strategies,
while the fifth type refers to specialized, single-purpose CDNs. The last three types describe new
business models for delivering CDN services based on combining the resources of multiple
market players.
[Insert Table 1 about here]
3.1. Datacenter-based CDNs
Some CDN operators concentrate large numbers of servers (i.e., server clusters or server farms)
in a relative ly small number of geographic locations. Limelight is one of the large CDNs that
relies on this architecture. It maintains about 80 server clusters around the globe (cf. Limelight
Networks, 2016). Smaller CDNs, such as CacheFly, Fastly, CloudFlare, and MaxCDN use this
architecture with up to tens of server clusters. This strategy facilitates the realization of scale
economies (e.g., volume discounting a ssociated with bulk purcha ses of servers, power, and
bandwidth capacity connections) and management efficiencies that can significantly lower
operational costs. Instead of incurring the fixed (per-site) costs associated with having to secure,
power, and manage a large number of geographically distributed server sites, datacenter-based
CDN providers can focus on a smaller number of locations with a larger number of (potentially
higher capacity) servers.

To be viable to support CDN services, the datacenters need to be strategically located to provide
access to multiple upstream providers (e.g., content providers or transit ISPs ) and peering
locations. 10 For example, datacenters located close to IX Ps are able to peer with multipl e
networks at a single location, thereby expanding the user population to which they can serve
content. However, having content cached on servers located in only a few locations means that
the distance data packets need to travel to end-users may be quite far. Also, such CDNs have
limited end- to -end control over the QoE for packets that may have to traverse multiple networks,
consuming additional transport resources and being more susceptible to Internet congestion than
CDNs w ith more distributed server architectures. Consequently, these types of multi-purpose
CDNs are best able to serve content for which the end-users are localized near datacenters and
for which the content is delay-tolerant (and hence cacheable) or may be scheduled (e.g., to avoid
peak congestion). Software updates and a lot of entertainment video may fit those requirements.

10 As desc ri bed in Sect io n 2.2, key considerations in selecting the location of datacenters include the cost
for energy and h osting, but also proxim ity to anchor tenants.

3.2. Highly Distributed CDNs
Akamai is the leading and be st-known CDN provider with a highly distribute d network of
servers. It maintains about 220,000 servers deployed in more than 1,500 networks, and delivers
more than 20% of global Web traffic (cf. Akamai, 2016a). These are strategically distributed at
multiple peering locations and within access networks. The servers are organized into a hierarchy
of edge-based servers hosted within ISP networks that manage front-end communications
between the end-users and the server network, and back-end servers that tie together the
distributed network of servers. Positioning edge servers closer to end-users – both in terms of
network and physical distance – and employing optimized protocols allows Akamai’s distributed
CDN architecture to deliver substantial performance benefits (cf. Leighton, 2009; Maggs and
Sitaraman, 2015). Such highly distributed CDNs rely on the deployment of multiple CDN
modules, each offering a common set of general-purpose capabilities that may be customized by
the CDN operator to support diverse types of content and applications (cf. Nygren et al., 2010).
These may include support for static as well as dynamic and interactive Web or e-Commerce
applications and content. Because the fixed (and sunk) costs of establishing and maintaining a
distributed CDN comprised of CDN provider-owned servers is quite substantial, a large
cu stomer base is needed to render the architecture economically viable. A large size makes it
possible to take advantage of economies of scale and volume discounts associated with
purchasing and provisioning many of the necessary inputs such as servers and b ack -office
support. Additionally, the large CDNs are likely to gain size-based bargaining power when
negotiating interconnection and hosting arrangements with ISPs.
3.3. Peer -to-Peer CDNs
Peer - to -peer CDNs address the challenge of managing the high costs of providing content
servers by shifting the costs for providing servers to end-users. One of the best-known peer- to -
peer CDNs is BitTorrent. Most of the peer- to -peer CDNs have been non-profit or community-
driven, with limited commercial support, and have focused either on specialized niche content
distribution needs (i.e., specific to the community’s shared interests), or are engaged in copyright
infringement (where distributing ownership/control of the servers limits the CDNs vulnerability
to copyright enforcement actions). Commercial peer- to -peer CDN services are offered by Resilio
(formerly BitTorrent Sync), the (former) commercial counterpart of the popular free file sharing
software. It is mainly used for bulk file distributions (e.g., medical data, scientific data, high
definition videos) over the Internet (cf. Resilio, 2016). 11 Peer - to -peer CDNs work best when a
large number of the member population is interested in the same collections of files, since the
network members can rely on crowdsourcing to shar e desired content. For popular files,
hundreds of thousands to a million end-users may act as servers that participate in distributing
the file (cf. e.g., Boorstin, 2015). However, using CDN caches located at the end-users also
means that the CDN has much less capability to manage end- to -end delivery performance (e.g.,
to control end- to -end latency) and that delays might be substantial as access networks have to be
traversed twice along the path from sender to receiver. Consequently, peer- to -peer CDNs are

11 Anothe r provi der of commer ci al Bit Torr ent - base d peer - to - peer CDN services is Xunlei (cf. Dhungel et
al., 2012).

most suitable for delay-tolerant bulk file distributions (e.g., audio, video, medical or scie ntific
data).
3.4. Hybrid CDNs
A h ybrid CDN approach s eek s to economize on the expense of maintaining dedicated servers by
outsourcing some of the replication and caching of content to servers maintained by end-users
that serve as CDN clients, providing additional capacity and increasing the number of distributed,
replicated copies that the CDN may make available. Akamai’s NetSession offering provides an
example of such a hybrid CDN (cf. Zhao et al., 2013). The hybrid CDN provider combines the
benefits of sav ing additional deployment and maintenance costs, while still retaining the control
afforded by also maintaining dedicated servers and having some management control over the
use of the end-user server functionality. However, because the CDN cannot reliably anticipate
when end-users may go offline, the CDN needs to keep track of the state of replicated content
cached on end-user servers as well as to maintain a content copy in an always-on server.
Additionally, serving content from customer-maintained servers may suffer from the same sorts
of performance issues as peer- to -peer CDNs (cf. Feldmann et al., 2010). Today, hybrid CDN
architectures are typically used for large file downloads such as software updates.
3.5. Specialized CDNs
In recent years, a number of large content providers that formerly relied on third-party CDN
services have built their own application-specific CDN platforms. For example, Google deployed
their own caches (Google Global Caches) in more than 1,000 networks (cf. Calder et al., 2013;
Streibelt et al., 2013) and Netflix introduced its Open Connect platform with servers in more
than 233 locations worldwide (cf. e.g., Miller, 2016; Netflix, 2016a; Clancy, 2016). Whi le
Google’s cache servers may have general-purpose capabilities, they are customized to support
Google’s content and applications. By some accounts, Google’s CDN which includes Y ouTube
traffic handles more than 25% of Web traffic (cf. McMillan, 2013). Meanwhil e, Netflix is
reported to be responsible for about one third of the peak Internet traffic on fixed networks in the
U.S. (cf. Sandvine, 2016).

A major motivation for such vertical integration is to take advantage of the scale, scope, and
specialization economies that can be realized by bringing the CDN functionality in-house. While
this requires that traffic volumes are sufficiently large, specialization economies arise when the
CDN architecture can be tailored to the application-specifi c (delivery) requirements of the
content provider (e.g., by avoiding the need to support unnecessary general-purpose capabilities).
Additionally, vertical integration may offer strategic benefits in the form of tighter control over
how content is distributed. A particular complication arises when a single content provider serves
traffic volumes sufficiently large to represent a significant share of the traffic delivered by an
individual CDN. Hold-up risks on both sides may arise as neither the content provider nor the
CDN can tolerate sudden shifts in traffic patterns. Resulting joint-management risks may be best
managed through vertical integration.

3.6. Broker CDNs
CDN brokers have emerged that seek to t ak e advantage of price differentials among CDN
providers which may vary by region, allowing them to integrate and arbitrage off erings from
multiple CDN providers to expand coverage or better customize services for their customers (cf.
e.g, Mukerjee et al., 2016). Based on a meta-architecture for optimizing content delivery that
relies on existing cache deployments, broker services help content providers mix-and-match
services from multiple CDN providers to ac hieve cost-minimizing solutions that meet their
global content delivery performance goals. CDN brokers maintain maps of available servers and
run end-user experiments to assist content providers in the selection of CDNs. One or more
CDNs may be used simultaneously, with the assignment of user requests to specific CDNs made
dynamically. Typically, broker CDNs specialize in the provision of specific types of content or
applications. For example, Conviva (cf. Krishnan and Sitaraman, 2013; Liu et al., 2012) is a
CDN broker that s pecializes in delivering video streaming services, while Cedexis (cf. Cedexis,
2016) specializes in delivering Web browsing traffic.
3.7. Licensed CDNs
ISPs and others interested in providing CDN services or functionality in-house may elect to
license the CDN capabilities from an established CDN provider. Following this route reduces the
upfront costs of establishing a CDN network and may allow sharing in the scale, scope, and
learning economies realized by an established CDN provider. Although ISPs have an advantage
in serving their own subscribers as they control the relevant last mile connections, their reach is
limited . Consequently, licensing CDN capabilities can prove a useful strategy for meeting off-ne t
coverage needs of content providers. For the established CDN providers, offering wholesale
services to ISPs and others helps increase utilization and realize scale and scope economies for
existing CDN networks. Providers like Akamai (cf. Akamai, 2016b) and Edgecast (cf.
BusinessWire, 2012; Verizon, 2016) introduced off-the-self wholesale CDN solutions for ISPs.
Two models for licensed CDNs are common. First, the set of ISP-owned CDN servers can be
managed independently by ISPs seeking to operate standalone CDNs. A number of telco-based
ISPs have chosen this model which is based on bare licensing of the CDN sof tware platform,
allowing the ISP’s own servers to be upgraded to support CDN functionality.

Second, and the more commonly used approach, CDN providers may integrate ISP-owned
servers into the CDN’s (potentially global) network of servers. In this model, the CDN directly
manages the ISPs’ servers as front-end servers for the CDN. For example, Akamai is involved in
a number of such collaborative licensing agreements with ISPs around the globe. Such
collaborative licensing arrangements enable tighter integrated management of the CDN traffic,
facilitating revenue sharing and making it easier to launch new applications. Additionally, ISP s
and CDNs can more easily share information regarding access network conditions to facilitate
joint optimization of end- to -end packet delivery. Improved performance and cost efficiency can
be achieved when the CDN and ISP jointly collaborate on identifying the best server from
whence to provide content to specific user requests (cf. e.g., Frank et al., 2013; Akamai, 2016b).

3.8. CDN F edera tions
A CDN federation provides a framework for interconnecting and coordinating the exchange of
traffic among independent CDNs with limited footprints, enabling members in the federation to
offer services to content providers requiring wider geographic coverage than the participating
CDNs could of fer individually. CDN federations are based on horizontal collaborations between
CDNs, and help reduce the costs of expanding server coverage. The IETF working group, CDNi,
is focused on developing standards for federated CDNs (cf. Niven-Jenkins et al., 2012). ETSI
(2013) and ATIS (2011) have issued standards related to the operation of such federated
networks. Currently, the CDN provider Edgecast markets its OpenCDN as a commercial product
that enables CDN federation.
4. The Multiple Facets of (Peering) Location
The location of CDN servers is a key determinant of CDN architecture, costs, and capabilities. In
the following sub-sections we discuss different ways in which the “ location” of servers impacts
CDN operations.
4.1. Geographic Location
The geographic distribution of CDN servers plays an important role in the cost and performance
of CDN services. Distributing CDN servers over a wide geographic area expands the range of
options for serving content from a server that is geographically close to the end-user. Injecting
traffic into a terminating access network in close proximity to end-users typically allows the
packet to traverse fewer interconnection points that might be vulnerable to congestion. In
addition, direct interconnections between CDNs and ISPs allow the traffic to bypass upstream,
transit providers. In both cases, packet delivery consumes fewer network resources and typically
experiences reduced packet delays.

Additionally, interconnection and hosting in multiple geographic locations enhances the
resilience as well as the reliability of content delivery. Historically, CDNs relied on transit
services purchased from large ISPs to deliver their traffic to end-users. Increasingly, in a trend
that is also common among ISPs, CDNs are directly connecting to the ISPs to deliver their
content (cf. Woodcock and Frigino, 2016, p. 10). For example, Akamai peers in more than 100
IXPs and private peering f acilities (cf. Hannigan, 2015), while Google peers in more than 90
IXPs and more than 100 private peering facilities in a number of cities around the world (cf.
Google, 2016). Maintaining a geographically distributed server architecture provides the CDN
with a greater number of options for serving the content f rom a server that is close to the end-
user, allows the CDN to route around “stroke points” (i.e., congested links or link outages), and
enables the CDN to better balance server loads to enhance capacity utilization efficiencies in
response to changing traffic patterns.

Although s horter paths often mitigate performance problems, the geographic distance between
the servers may not correspond to the s hortest or best network path. For example, in peer- to -peer
CDNs or hybrid CDNs that serve content from end-user maintained caches, the path f rom one
user to another may require traversing the access network twice. Alternatively, two CDN servers
may be located in the same physical location (e.g., in the same carrier hotel), but one (e.g.,

datacenter-based CDN) may be interconnected on the upstream side of a peering link, while the
other (e.g., licensed CDN) may be hosted by the ISP downstream on the network side. While the
for mer server would be vulnerable to delays if the peering link is congested, the latter is not
affected . 12

Existing broadband access infrastructure and the connection speeds available vary significantly
across geographic regions. 13 Faster connection speeds typically imply reduced delays and less of
a benefit in locating content geographically close to the end-user. When access networks are
slower or when interconnection links are likely to be congested, distributing caches closer to
end-users on the access-side of interconnection links can deliver significant performance
improvements. If latency in the access network dominates backbone latency, then those CDN
architectures that heavily rely on the access network (e.g., peer- to -peer and hybrid CDN
architectures) will generally perform worse than alternative CDN architectures. 14 However, if
latency in the backbone network dominates, then architectures that locate servers inside the
access network will typically provide better performance.
4.2. Virtual Location
Despite the importance of geographic location as described in the previous section, Internet
routing protocols do not route packets based on the geographic (physical) location of the source
or destination. Instead, routing is based on their virtual location as defined by their IP address
which determines how packets are routed to and from the server. Servers with IP addresses
managed by the same provider may be thousands of geographic miles apart (e.g., one in New
York and the other in Los Angeles) (cf. Freedman et al., 2005); while two nodes in close
geographic proximity that have IP addresses belonging to different administrative domains (i.e.,
autonomous systems or ASes) may be separated by network paths that require traversing
thousands of miles, and may even involve crossing national borders (cf. e.g., Spring et al., 2003;
Gupta et al., 2014a). Therefore, controlling the virtual location of CDN servers is also important
(cf. e.g., Krishnan et al., 2009).

12 Accord ing to recen t studi es, congest i on at a pee ri ng link can incre ase dela ys by up to 40 msec (c f.
Luckie et al. , 2014) .
13 Studies on the state of reside ntial access networks in the U .S. (cf. FCC, 2015b), the EU (cf. E C, 2015),
and the U K (cf. O fcom, 2016 ) illustrate s ignificant differences in bo th u pstream and downloa d sp eeds
across geographic regions and across d ifferent broadband access technologies. Although available
connection speeds have increased during the last decade in m ost countries, Akam ai (20 16e) data on the
connection speeds between users and their CD N servers across markets confirm that speed differences
remain sign ificant.
14 If a CD N edge serv er is loca ted within the sam e m etropo litan a rea as th e e nd - user, latency in the access
network typically exceeds backbone latency by tens of milliseconds and backbone latency is negligible
for QoE (cf. Pu jol et al., 2014 , figu re 1 0). H owe ver, if the server is located in another metropolitan area,
latency in the back bone may becom e sig nifican t and may even excee d acc ess laten cies. D epen ding o n the
distances data packets need to travel, backbone latencies typically vary from a few msec up to tens of
msec , e.g. , in the US betwee n eas t coast and west coast , to more than 100 msec if ser ver s are loc at ed in
different conti nents (c f. e. g., AT&T, 2016; Pujol et al ., 2014) .

In addition to traditional interconnection agreements, ISPs may exchange traffic with CDNs
based on hosting services. CDN servers are strategically positioned and hosted within the ISP
network and are assigned local IP addresses that belong to and are managed by the hosting ISP to
help ensure packets are routed efficiently within the ISPs network, and avoid traversing inter- AS
links unnecessarily. In addition to improving delivery performance, hosting servers on the ISPs
network can reduce the load on interconnection links, improving the performance for other
applications and allowing ISPs to postpone capacity upgrades that might otherwise be required. 15
4.3. Colocation Hubs
Colocation hubs enable the interconnection of multiple ISPs and CDNs (cf. Lodhi et al., 2014).
These are typically located at IXPs (cf. Chatzis et al., 2013) or other interconnection facilities for
private interconnections (cf. Giotsas et al., 2015). Although most global Internet traffic is
exchanged via private interconnections, more than 30 Tbps of traffic are exchanged at IX Ps, with
most of that activity taking place in Europe. 16 There are more than 400 IXPs and more than 600
interconnection facilities around the globe and the numbers are increasing. In large IXPs such as
DE -CIX in Frankfurt, LINX in London, and AMS-IX in Amsterdam, more than 600 networks
can exchange traffic on a bilateral or multilateral basis. Moreover, hundreds of networks
interconnect at large interconnection facilities like Equinix in New York. Being present in these
large IX Ps and interconnection facilities provides CDNs with multiple options for accessing a
large number of networks and end-users while at the same time reducing both the geographic and
virtual distance between source and destination (cf. Galperin, 2016). For example, Equinix
maintains a number of interconnection facilities around the globe and in the United States, which
they claim makes it possible to access 90% of the population in North America and Europe with
only 10 msec delays (cf. Equinix, 2016, p. 2).

The geographic location of colocation hubs is important s ince interconnection and colocation
prices vary between regions and may even differ between hubs in the s ame city. 17 Large hubs are

15 For example, popular videos or even entire video librari es can be easily r eplicated and stored on hosted
CDN server s withi n the acces s networ k. Updates of these librar ies can be timed str ate gica lly (i.e ., off -
peak periods can be used and distributi on can take place several days in advance before the content is
made ava il abl e for end - users). Netflix, for example, follows such a strategy in order to pre - posit ion video
content on their cache servers (cf. Netflix, 2016b). For further discussion of the mutual benefits to be
realized from CD Ns with cacheab le con tent h osting th eir servers on ISP netw orks, se e Ager et al. (2010) ,
Erman e t a l. (2009 ), Frank et al. ( 2013), Qur eshi et al. ( 2009), and Chiu et al. ( 2015).
16 Providing European - wide cont ent deli ver y cove rag e typ ica ll y req uir es using multi pl e ISPs sin ce many
lack m ultinatio nal co verage, whereas in the U.S. ther e are multi ple nati onal ISPs that cover the ent ire U.S.
mark et . Con seq uen tl y, in the U.S. it is easi er to neg ot iat e bilat er al peer in g arr an geme nts wit h a few
national ISPs to secure delivery to all of the broadband content u sers in the ma rket; whe reas in Europe, it
woul d be neces sar y to negoti at e a larg er number of such int er conn ect ion agree ment s. This may parti al ly
explain why IXPs are, r elatively speaki ng, more important in Europe.
17 The growing number of hubs and IXPs in major cit i es (and inc re asi ng ly, even in secon d tier locat io ns)
spurs com petition, contributing to the down ward pressu re on b andwidth prices for transit and peering. Fo r
example, the recently p roposed O penIX initiative (cf. Chatzis et al., 2015) in major hu bs in the U.S. is
considered to have caused price declines in some peering facil ities by up to 80% (cf. Temkin, 2016).

typically located in major centers for commerce (e.g., New York, Frankfurt, London, and Hong
Kong) and where major trans-oceanic fiber cables terminate (e.g., Amsterdam and Seattle) (cf.
Durairajan et al., 2015; Giotsas et al., 2015). The density of networks and CDNs at the major
hubs gives rise to network effects. Resulting scale economies and other benefits of
interconnecting at hubs may be further enhanced through mergers and acquisi ti ons. 18 Beyond the
fact that hubs provide facilities for interconnection and attract a large number of ISPs and CDNs,
they also drive the adoption of more direct interconnection and hosting arrangements. CDN s that
co -locate facilities at hubs have more opportunities for direct peering and hosting.
4.4. Innovation Hubs
A growing number of hubs offer support for a dive rse array of innovative interconnecti on
arrangements. These include differentiated types of partial transit and paid peering, as well as
public peering via route servers that enable hundreds of interconnections to be supported over a
single port (cf. e.g., Faratin et al., 2008; Ager et al., 2012 ; Giotsas et al., 2013; Richter et al.,
2014). The range of interconnection facilities provided may include direct cross-connects, public
peering, or tethering. 19 This diversity of mechanisms for physically interconnecting networks
facilitates complex and nuanced traffic exchange arrangements (cf. Giotsas et al., 2014).

For example, remote peering enables a decoupling of a router’s geographic location from the
geographic location of the interconnection. A physical router presence is no longer necessary for
networks to direct ly peer with other networks in many geographic locations. A CDN with a
server cluster in a single location may thus peer directly with multiple remotely-located networks
via the services of a remote peering provider. Remote peering represents an increasingly
attractive type of peering, especially for small networks , including small CDN providers. For
example, more than 20% of the members in AMS-IX use remote peering (cf. Castro et al., 2014).
Remote peering allows a CDN to expand it s footprint at reduced cost compared with relying on
transit services (cf. e.g., AMS-IX, 2016; Richter et al., 2014). Limelight and LinkedIn are two
CDN providers that take advantage of this type of peering.

Additionally, innovation hubs typically offer a number of value-added services to their members.
For example, several large IXPs such as DE -CIX, AMS-IX, and LINX, provide the latest black
holing services that can be used to mitigate attacks and dispose of unwanted ingress or egress

18 For example, Equinix recent ly announced the acquisiti on of Telecit y, one of the largest datacent er and
peering facili ties owners for $3 .8 B illions (cf. Boy le, 201 6). Last y ear, Dig ital Rea lty anno unc ed a $1.9
Bill ion ac quis it ion of Tel x ( cf. Sve rdl ik, 20 15).
19 How the CDN in ter con nect s to cust omer s and it s ser ver s can impac t the CDN pro vid er' s abil it y to
control the QoS of the services it offers. Direct cross - connects are based on dedicated circuit - switched
connections between the peering n etworks and provide better Qo S control for a CD N provider; wh ereas
public peering relies on shared access to the IXPs switching fabric and may be m ore susceptible to QoS
variati ons that are not under the CDN operator's control. Finally, tethering makes us e of a virt ua l loc al
area network (VLA N) on the IX P’s shared switching fabric, and introduces additional potential sources of
QoS var iab il it y tha t are n o t under th e CD N pro vider's d irect co ntrol (cf. G iotsas et al., 201 5).

traffic. 20 Moreover, new technologies such as SDN enable even more advanced types of peering
as well as sophisticated traffic engineering (e.g., load balancing, inbound traffic engineering and
application-specific peering) (cf. Gupta et al., 2014b). Deploying innovative servi ces at
innovation hubs makes these capabilities available to all networks using the hub for
interconnection. Table 2 provides an overview of some of the capabilities that are available at
such innovation hubs.

[Insert Table 2 about here]
5. Prospects for the future of CDNs and the Internet Ecosystem
Across the value chain, businesses are continuously confronting the make-vs-buy decision. The
largest incumbent CDNs, best exemplified by Akamai, have sought to expand their services,
coverage, and m arket share in a bid to realize scale and scope economies. Thus, they can
compete with a broad portfolio of service offerings while at the same time taking advantage of
cost economies. At the same time, the largest ISPs have been upgrading their networks through
softwarization to expand the ir service portfolio and to lower network costs. Some ISPs are even
integrating into content directly (e.g., Comcast acquired NBC; Verizon acquired Edgecast and
Yahoo!). Smaller players are pursuing niche strategies or are forming federations in an attempt to
approximate the capabilities and cost economics of the larger incumbents. Meanwhile a grow ing
number of content providers are taking advantage of CDN platforms and other wholesale
services to vertically integrate into self-provisioning CDN services.

In this landscape there are likely to be multiple potential sweet spots. At the center of the market,
we expect there to be a place for several, but probably a small number, of global scale, full-
service CDN providers like Akamai or Limelight that own and manage a global network of
servers and retain the backbone networking capabilities to allow them fine-grained control. In
addition to the significant global scale and scope economies associated with managing such a
network, Akamai benefits significantly from incumbency and its first-mover advantages
associated with its success in acquiring a large installed base of customers. Although the
potential market of commercial content providers, including entertainment media that want
global-scale content delivery coverage is large, replicating the global reach of Akamai’s
dedicated server network would be extremely challenging for other third-party CDNs or even the
largest ISPs. Akamai is able to spread the s ignificant fixed and sunk costs of developing
additional services over its existing and large customer base. Additionally, Akamai’s control
over traffic routing (i.e., where they inject CDN traffic into ISP networks) likely provides
significant leverage in negotiating favorable colocation or interconnection terms with ISPs.

20 Distr ib ute d Denial of Servi ce (DDoS) atta cks can resul t in large amount s of unwan ted tra ff ic that can
cause congestion and service disruptions. In the course of a DD oS attack, large amounts of traffic are sent
by a large numbers of distribut ed sources to overwhelm the network resources of the target network. For
example, seemingly legitimate requests for information may be directed at a target server at a rate that
exceeds the server’s ability to respond, causing it to b ecome o verloaded and cease to functi on as designed.
Black holi ng desc rib es the practi ce of redi rec tin g unwante d traf fic to nodes where such traf fic can be
harmlessly terminated and disposed of as if the traffic were being swallowed by a “black hole.” A range
of network operators and IXPs of fe r such services (cf. e.g., DE - CIX, 2016; Di etz el et al. , 2016 ).

Finally, because Akamai is not integrated vertically either into content or into providing last mile
Internet access services, it avoids potential channel conflicts when negotiating service
agreements with competing content providers or ISPs. Although ISPs in non-overlapping
geographic markets do not compete directly, many ISPs compete with other ISPs in the
geographic markets they serve. Content providers typically want to reach all of the end-users in a
market, not just the ones served by one ISP or the other. 21

Although it is hard to challenge Akamai in its dominance of general-purpose CDN service
provisioning, there is still substantial room for niche strategies. First, smaller or new entrant
CDNs may take advantage of the lower entry costs enabled by CDN platform providers
(including Akamai) and various Infrastructure- as -a-Service or Platform- as -a-Service of ferings
from cloud service providers such as Amazon, Google, or the larger ISPs. Although they may not
be capable of replicating Akamai’s extensive capabilities and scale , it is feasible for such new
CDN entrants to operate on much smaller margins since the availability of scalable wholesale
platform options renders most of their costs variable. By appealing to a niche market by
application, geographic market, or by customer type ( i.e., by traffic or customer type), niche
providers may be able to exploit a competitive advantage.

Second, other entrants in the value-added CDN services market may be able to leverage another
asset (e.g., existing datacenters or a favorable location to content providers or Internet exchange
facilities) to enter CDN services at low incremental cost, because most of the costs are
recoverable from the firms’ main line of business. Th e temptation to do so and the architecture
that may be employed, in taking advantage of sunk or shared costs, may be sub-optimal relative
to greenfield CDN deployments, yet may appear to be marginally profitable for the would-be
entrant.

Third, large content providers may find suf ficient benefits from reducing content distribution
costs and in increasing control over how content is delivered to their end-users to make it
desirable to vertically integrate into self-provisioning CDN services. As already noted, the
largest content providers like Netflix may have been compelled to self -provision because the
sheer volume of their needs exceeded the capabilities of all but the largest CDN providers. At the
same time, their specialized needs (e.g., in the case of Netflix, serving content that was
essentially 100% static and cacheable) may make it feasible for customized self-provisioning to
realize substantial cost savings relative to those achievable by a general-purpose CDN provider.
The viability of self-provisioning and the expansion of this model to a growing range of ever-
smaller content providers is a direct consequence of the same growth in w holesale CDN and
cloud platform services that have lower ed entry barriers into CDN services as already noted.
Over time, as some of these content providers expand their business models and the range of
services they offer, it is conceivable that they may expand the capabilities of their specialized
CDNs, and may eventually start to offer CDN services to third-parties. Should they do so, they

21 If suff iciently valuab le or if allowe d by po licymake rs, c ontent pro viders ma y be intereste d in exclusiv e
ISP distribution con tracts; bu t ev en in that case, to nego tiate the best term s, they may wis h to have the
ISPs com pete for exclusivity. A C DN that is tied to a single ISP lacks the ability to cove r the en tire
mark et o r to pl ay on e ISP o ff a gai ns t th e o th er.

would confront potential channel conflict issues unless the ir content provider customers were
offering complementary content.

Fourth, access ISPs that already play a critical role with respect to consumers QoE when
accessing content and are feeling pressure as revenues from legacy transport services erode may
seek to vertically integrate into value-added services. On the one-hand, the softwarization of ISP
networks increases their capabilities to offer value-added services. Additionally, their proximity
to end-users gives them a natural advantage in hosting and managing edge-located content
caches (cf. Rayburn, 2014). Furthermore, to the extent they may control or manage consumer
set -top boxes or provide other services to subscribers, they may have additional advantages in
determining how content might be cached or served to end-users (cf. Laoutaris et al. 2008;
Valancius et al., 2009). In many ways, it may seem that as the basic functionality of the
underlying infrastructure expands, the need for overlay CDN services would decrease. O n the
other hand, most ISPs do not have sufficient geographic coverage to match the needs of many
content providers, and even in the markets where they do have coverage, they serve only a part
of the market of end-users targeted by content providers. 22

Li ke the smaller or niche CDNs, ISPs that are seeking to vertically integrate into CDN services
can enhance their offerings by interconnecting at IXPs or colocation hubs, or by joining
federations to expand their geographic or market reach based on horizontal integration. Such a
strategy may make the most sense in markets like Europe – where there might be the need to
establish Europe-wide coverage but where most of the ISPs are regional or national.
Alternatively, large CDNs may seek to partner with ISPs under licensing agreements of the sort
discussed earlier. This may allow ISPs and CDNs, while retaining independence by avoiding
exclusivity deals, to share capabilities in ways that are mutually beneficial. Corresponding
strategies may make sense in the U.S. where there are several competing national-scale ISPs (e.g.,
Verizon, Charter, Comcast, A T&T) – none of which seems especially well-positioned to
supplant Akamai as the leading general-purpose CDN, but each of which may have strong
incentives to expand into value-added services to leverage network upgrade investments.

As the sheer volume of cacheable content grows, rendering edge-based caching more attractive,
and as the volume of interactive, dynamic and otherwise complex content also grows, the
benefits of information and resource sharing between ISPs (e.g., about real-time conditions in
last mile networks) and CDNs (e.g., about content delivery requirements and customer interests)
increases. In light of the significant growth in content that is mostly cacheable and
entertainment-oriented (e.g., movie libraries), there are compelling mutual benefits to be realized
for both CDN providers and ISPs from hosting CDN servers inside the ISP networks. Closer
integration and better inf ormation sharing between IS Ps and CDNs can allow both to make more
nuanced decisions about how to assign network resources and route traffic, resulting in win-win
performance gains and network resource costs savings. For example, once deployed, NFV
capabilities will allow ISPs to provide on-demand bandwidth and pop-up edge servers allowing

22 Nygren et a l. (2 010) st udi ed t he s har e of CDN t raf fi c se rve d by dif fer ent IS Ps a n d found that the largest
ISP served only 5 % of the traffic, while the m ajority of IS Ps accounted for less than 1% o f the C DN
traffic.

CDNs to better dynamically scale their bandwidth and server needs. ISP-CDN coordination of
cache updates and where content is injected into an ISP ’s network can help conserve
interconnection capacity and reduce peak period congestion, enhancing the QoE of both content
customers and users of other applications. Moreover, ISP-CDN collaborations may enable
transparent caching, obviating the need for ISPs to deploy expensive middle-boxes. 23 Today,
there are tens of active ISP-CDN collaborations (e.g., between Akamai and large ISPs such as
AT&T, Deutsche Telekom, Telefonica, and Orange) that expand the number of available servers
and enhance the efficiency of cache management. 24 This increases both the granularity by which
multiple content copies can be cached close to end-users and th e scope for traffic engineering (cf.
Frank et al., 2013).

As the ecosystem continues to evolve towards next generation clouds and 5G networks, the need
for and benefits of collaboration are likely to increase. Providing seamless support for mobile
access to content is likely to call for new types of CDNs. F or example, providing access across
diverse wireless access networks, operating in different portions of the radio frequency spectrum,
will call for better support for cross-layer protocol optimizations. 25 Moreover, since mobile
devices are often more likely to be power constrained and/or have smaller screens, there may be
a greater need for flexible and device-dependent coding that lowers the resolution by reducing
the number of bits used to code the content or undertaking other energy saving strategies when
delivering content to mobile devices. The industry' s 5G vision specifies that networks should be
able to support 1 msec latencies for services. Achieving this goal is likely to be impossible
without much closer coupling of access and CDN networks.

Finally, regulatory policies are likely to have a significant impact on the evolution of the CDN
ecosystem. For example, while CDN services off ered by third-party providers are typically not
regulated, the last-mile broadband access services often are. In a number of countries,
policymakers have adopted so-called Network Neutrality (NN) regulations that are intended as a
response to concerns that access ISPs may have market power that might be misused to
discriminate against unaffiliated content or applications, thereby harming competition, increasing

23 Midd le - boxes are speciali zed, often proprietary, servers that are typically deployed by ISPs within their
network to fac ilitate better traffic mana gem ent and/o r traffic sha ping ca pabilities th at can im prove
network utilizat ion, implement differenti ated services, and increase an ISP's control over QoS w ithin its
network. For furthe r discus sion, s ee Carpenter and Bri m (2002).
24 Corresp ondi ng stra te gi es are also attracti ve for Tier 1 ISPs which ma y have developed a profound
interest in diversifying their business strategi es by participating in CDN service provision. For example,
Level 3 operate s its own proprietary CDN while AT &T is join ing forces with A kama i based on an ISP -
CDN col lab orat ion .
25 For example, TCP (transpo rt layer protocol) often proves sub - optimal w hen used over wireless links
(physical layer n etworks ); and the p erforma nce o f w ireless link s varies significa ntly a cross technologies
and spectrum bands (e.g., W i - Fi, L TE and satelli te services operat e in differe nt frequenc y bands, with
different prot ocols a nd network topologies). Cross - layer optim izations th at join tly con sider m ultiple
network layer s and make adjustments accordingl y can i mprove performance.

costs, and limiting consumer choices. 26 Although the rules may differ by country and continue to
evolve, NN regulations constrain the access ISP’ s scope and ability to actively manage data
traffic within their networks and to offer price and QoS-differentiated content delivery services.
Regardless of one’s views as to the merits of NN regulation, it is reasonable to anticipate that
such rules will create opportunities for regulatory arbitrage that will impact who provides CDN
services and how those services are provided.

Summing up, we do not expect the landscape for CDNs to become less complex or to
consolidate in the near future. We expect to see a continuing need for a few global general-
purpose CDN providers like Akamai continuing to expand the ir range of services; while at the
same time competing with a stream of new entrants and specialized CDNs, many of which will
likely be transitory. Finally, although we do not think ISPs can fully replace CDNs, we believe
that the benefits of closer collaboration of CDNs and ISP s will lead to increased vertical
in tegration of ISPs into CDN services and partnerships and licensing agreements between CDNs
and ISPs.
6. Conclusion s
In this paper, we reviewed how CDNs have evolved in conjunction with the broader trends
driving changes in the Internet ecosystem. Over time, a complex landscape of CDN architectures
and business models have emerged to flexibly address continuous changes in the marketplace.
The diversity of CDNs is due to multiple causes, but is likely to persist. On the one hand, content
providers differ widely in their needs for (and willingness or ability to pay for) content delivery
services. While some content providers do not need or cannot pay for additional support, and
rely on basic Internet functionality to distribute their content, others are either so large or have
needs sufficiently specialized that self-provisioning an in-house CDN offers the lowest cost and
best strategic option. A wide range of third-party CDN providers serve the middle of the market.
These range f rom incumbents to new entrants, regional to global providers, general-purpose to
niche service providers, pursuing a range of business models. The various business models
reflect each CDN’s efforts to leverage its perceived competitive advantages. In the case of
incumbents, this includes their first-mover advantages in establishing large customer bases and
networks with global reach. Newer entrants often seek to leverage existing assets such as data
centers or local ISP infrastructure, or to take advantage of the growing array of wholesale
platform options for CDN or cloud services. Because entry costs into CDN services are low, we
expect competition at the lower end of the CDN market to remain intense, even if not very
profitable for those so engaged. The market for full-service, general-purpose CDNs is likely to
continue to be dominated by a small number (i.e., one to three) global providers like Akamai.

Perhaps the greatest opportunities and challenges will arise as ISPs increasingly evolve toward
cloud service providers, expanding beyond providing just basic packet transport services into
providing computing and storage services, as well as other more advanced services such as
security, network management, and other value-added services. At the same time, CDNs are

26 In 20 15, netwo rk neutrality reg ulations we re introduced in the U.S. (cf. FC C, 2015 a) and in E urope (cf.
Europea n Parli ament and Council, 2015; BEREC, 2016) . For a comparison of the U.S. and Euro pean
framew orks, s ee Mar cu s ( 20 16) .

increasingly expanding their capabilities to support more dynamic, interactive, and diverse types
of content. The boundary between basic Internet functionality and value-added overlay
functionality, as well as the traditional division of labor between the two, is increasingly blurring.

While the benefits from increased integration of CDN and ISP functionality are significant, such
integration poses both strategic and regulatory challenges. From a strategic perspective, both
ISPs and CDNs may realize channel conflicts if they are vertically integrated. Non-affiliated
content providers may allege network neutrality violations or confront challenges in negotiating
last mile delivery services with competing ISPs. This may be less of an issue for ISPs in non-
overlapping markets, or for CDN s that are self-provisioned by a single content or application
provider. From a regulatory perspective, the blurring of CDN and ISP business boundaries is
likely to complicate efforts to regulate the provision of broadband Internet access services. For
example, so-called network neutrality rules are f ocused on ensuring non-discriminatory traffic
treatment for different types of content; but providing such discriminatory service is at the heart
of what CDNs try to do and is a capability that many content providers can be expected to want
in order to differentiate their offerings favorably from the offerings of competing content
providers.

As the Internet continues to evolve toward the Internet of Things and 5G, tighter integration of
fixed and mobile, wired and wireless, transport, computing, and storage network resources is
likely to be called for. In this environment, with big d ata analytic capabilities embedded in
networks and the softwarization of networks discussed earlier, there will be expanded support for
real- time traffic management. Figuring out where critical functionality and network intelligence
should reside and who should control it (i.e., end-users, content providers, ISPs, or CDNs) will
pose an interesting challenge. We anticipate that these questions will not yield to simple, unitary
solutions, but will continue to support a diverse array of business models and CDN solutions.

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M. Ponec (2013). ‘Peer-assisted Content Distribution in Akamai NetSession’, Proceedings of
the 2013 conference on Internet measurement conference (IMC ’13) , pp. 31-42.

Figures and Tables

Table 1: Typology of CDNs

CDN
Archi tec ture

Example s of
Provide rs

Deplo yment
Strategy

Bandwidt h

Latenc y

Busine ss
Mode l

Typica l
Appli cat ions

Datace nter -
based

Limelight ,
CacheFly ,
CloudFla re

Servers a t
strategically
connected facilities

High

Medi um

Mult i - purpose,
buy bulk
resources

Video Stre aming,
static Web,
software updates

Highly
Distr ibut ed

Akamai

Servers a t peeri ng
points and insi de
access networks

High

Very
Low

Genera l - purpose,
provide global
footprint, best
quality

Variou s
applications,
including d ynamic
and interactive
Web

Peer - to - peer

BitTorr ent

Serverles s,
functionality at end -
user equipment

Low

High

Mult i - purpose,
no investment in
dedicated
infrastructur e

File sha ring, bul k
transfers

Hybrid

Akamai
NetSes sion

Dedica ted serve rs
combined with
functionality at end -
user equipment

Low

High

Mult i - purpose,
partial
outsourcing of
delivery to end -
user equipment

Software up dates,
file sharing

Speciali zed

Netfl ix Op en
Connect,
Google
Global Cach e,
Amazon
CloudFro nt

Speciali zed server s
at peering points
and inside access
networks

High

Low

Single - purpose,
reduce delivery
costs for
specialized
service

Video deli very,
specialized
applications

Broker

Conviva,
Cedexis

Relies on e xisti ng
deployments of
CDN funct ionali ty

Custom

Opport unist ic
cost management

Video and We b
delivery

Licensed

Akamai
AURA,
Edgecast
licensed C DN

Inside access
networks

High

Very
Low

Telco CDN, or
ISP - CDN
collaboration

All o f ab ove

Federated

Edgecast
OpenCDN

Relies on e xisti ng
deployments of
CDN funct ionali ty

High

Low

Interconnection
of CDNs to
expand
geographic
footprint

All o f ab ove

Source: authors

Table 2: Examples of Innovations available in Colocation Hubs

Innovation

Enables

Mul ti l at e ra l P ee ri n g

Scalabl e and complex peeri ng arr angements

Black holi ng

Ingress and egress attack traffic co ntrol and
mit ig ati on

Remote Pee rin g

Reduce ne twork eq uipmen t purc hase and
operating costs

Software Defin ed Network ing
(SDN )

Traff ic engin eeri ng, appli cat ion - specific routing,
load ba lancing

Source: authors

Why organizations use Identific for document trust, entry 20

Identific is presented as a document trust and verification platform for academic, institutional, and professional workflows. Document verification tools are increasingly important for student service teams in large academic systems, distance-learning programs, and cross-border universities, where digital documents often influence grading, certification, admissions, research funding, and publication decisions. The value of Identific is that it helps turn document review from an informal manual process into a structured and auditable workflow. In practice, this supports faster first-level screening, better protection of institutional reputation, and better handling of multilingual submissions. Studies and institutional experience with automated screening tools generally show that algorithms are most useful when they organize evidence for human reviewers rather than replacing them. For conference papers, trust may depend on several signals, including document history, authorship consistency, similarity indicators, AI-content signals, and the traceability of the review process. Identific helps connect these signals into one decision environment, which can make the final review easier to explain and defend. Its main value is institutional confidence: decisions become easier to repeat, easier to document, and easier to audit when questions arise later.

Review document trust