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National Level Conference On Information Processing 2011

CONTENT DISTRIBUTION NETWORKSTrupti V.G [1], Rekha V [2]

1- M.Tech(CE),1st sem, 2-Asst. prof

Dept of Computer science & Engineering

SJB Institute of Technology, Bangalore[1] [email protected] [2][email protected]

Abstract- The paper introduces about Content Delivery Networks. How the traffic is reduced by distributing

the contents across several artificial servers and making the contents highly accessible and available. Success

in Internet applications involves user interactions whose quality is mainly affected by application response

time. Content Delivery Networks (CDNs) have shortly appeared as a distributed solution to serve content

faster than contacting a centralized server. It also specifies the CDN architecture which mainly has a Client,

Server and Surrogate server that form a CDN topology. A set of surrogate servers (distributed around the

world) that cache the origin servers content, Routers and network elements that deliver content requests to

the optimal location and the optimal surrogate server; and An accounting mechanism that provides logs and

information to the origin servers. Under a CDN, the client-server communication is replaced by two

communication flows: one between the client and the surrogate server, and another between the surrogate

server and the origin server. This distinction into two communication flows reduces congestion (particularly

over popular servers) and increases content distribution and availability. This report tries to describe a CDN

from a different point of view, paying much attention on the implementation process of a CDN. More

specifically, CDNs maintain multiple Points of Presence (PoP) with clusters of (the so-called surrogate)

servers that store copies of identical content, such that users requests are satisfied by the most appropriate

site.

Keywords- surrogate servers, Multicast, Computer Networking

I INTRODUCTION

By the growth of World Wide Web the traffic

increased in many of the popular websites i.e. is the

client requests were more which has to be serviced

by the server. These websites have a motivation toprovide better services.

Thus CDNs came into existence and

these have been used to provide better service. A

Content Delivery Network (CDN) is an overlay

network on top of the Internet which pushes

content closer to end users. It is achieved by

strategically placing servers, called surrogates, next

to these users and serving them the desired content.

The surrogates act typically as intelligent and

transparent proxy caches that retrieve content

previously from the origin server before

responding. As the origin server is less accessed,

backbone traffic is reduced and network bandwidthis efficiently used. Besides, load can be balanced

among the servers. CDNs has primarily focused on

techniques for efficiently redirecting user requests

to appropriate surrogates to reduce request latency

and balance load, and placement strategies to place

server replicas in order to achieve better

performance. Many CDN service providers like

Akamai offer some overview whitepapers, they

hide the real implementation as a private secret and

fundamental key of their business success. This

paper tries to provide some implementation hints to

establish a CDN basis. This paper also includes

CDN model architecture with the description of themain building blocks from an implementional point

of view. CDN attempts to reduce network latency

by avoidance of congestion paths.

CDNs maintain multiple Points of

Presence (PoP) with clusters of (the so-called

surrogate) servers that store copies of identical

content, such that users requests are satisfied bythe most appropriate site

II INSITE AND PERSPECTIVENESS OF CDNs

A Content Delivery Network (CDN) is an

overlay network on top of the Internet which

pushes content closer to end users. It is achieved by

strategically placing servers, called surrogates, next

to these users and serving them the desired content.

A. CDN TopologyCDN topology involves:

A set of surrogate servers (distributed around the

world) that cache the origin servers content; Routers and network elements that deliver content

requests to the optimal location and the optimal

surrogate server; and

An accounting mechanism that provides logs and

information to the origin servers.

Under a CDN, the client-server communication is

replaced by two communication flows: one

between the client and the surrogate server, and

another between the surrogate server and theorigin

server and increases content distribution and

availability. To maintain (worldwide) distributed

copies of identical content, the practice for a CDN

is to locate its surrogate servers within strategicdata centers (relying on multiple network

Dept of ISE, AMCEC 18
mailto:[email protected]:[email protected]:[email protected]

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providers), over a globally distributed

infrastructure. This distinction into two

communication flows reduces congestion

(particularly over popular servers)

Some of the content providers are Akamai

Technologies which is the leading one has more

than 20000 servers over 10000 networks. Inktoni, aYahoo company provides services for load

balancing, streaming media.

B. Content Delivery Network World Wide

Organizations offering content to a

geographically distributed and potentially large

audience (such as the Web), are attracted to CDNs

and the trend for them is to sign a contract with a

CDN provider and offer their sites content over

this CDN. CDNs are widely used in the Web

community, but a fundamental problem is that the

costs involved are quite high.

Fig 1. Content Delivery Networks Overview

C.Issues involved in CDNSurrogate Servers Placement: Choosing

the best location for each surrogate server is

important for each CDN infrastructure since the

location of surrogate servers is related to important

issues in the content delivery process. Determining

the best network locations for CDN surrogate

servers (known as the Web server replica

placement problem) is critical for content

outsourcing performance and the overall content

distribution process. CDN topology is built such

that the client-perceived performance is maximized

and the infrastructures cost is minimized

Therefore, effective surrogate server placement

may reduce the number of surrogate servers needed

and the size of content (replicated on them), in an

effort to combine the high quality of services and

low CDN prices. In this context, several placement

algorithms have been proposed (such as Greedy1,

which incrementally places replicas, Hot Spot,

which places replicas near the clients generating

the greatest load, and Tree based replicas). These

algorithms specify the locations of the surrogate

servers in order to achieve improved performance

with low infrastructure cost. Earlier

experimentation has shown that the greedy

placement strategy can yield close to optimalperformance.

Content Selection: The choice of the content that

should be outsourced in order to meet customers

needs is another important issue in the content

selection problem. An obvious choice is to

outsource the entire set of origin servers objects to

other surrogate servers (the so-called entire

replication). The greatest advantage of entire

replication is its simplicity; however, such a

solution is not feasible or practical because

although disk prices are continuously dropping, the

sizes of Web objects increase as well (such asaudio or video on demand). Moreover, the problem

of updating such a huge collection of Web objects

is unmanageable. Therefore, the challenge of the

content selection problem is to find a sophisticated

management strategy for replication of Web

content.

A typical way is to group Web content based on

either correlation or access frequency and then

replicate objects in units of content clusters.

Two types of content clustering have been

proposed:

Users sessions-based: The content of the Web log

files3 is exploited in order to group together a setof users navigation sessions showing similar

characteristics. Clustering users sessions is useful

for discovering both groups of users exhibiting

similar browsing patterns and groups of pages

having related content based on how often URL

references occur together across them.

URL-based: Web content is clustered using the

Web site topology (which is considered as a

directed graph), where Web pages are vertices and

hyperlinks are arcs. The Web pages (URLs) are

clustered by eliminating arcs between dissimilar

pages. The most popular objects from a Web site

are identified, (the so-called hot data), and

replicated in units of clusters where the correlation

distance between every pair of URLs is based on a

certain correlation metric. Furthermore, several

coarse-grain dynamic replication schemes are used.

By using these replication schemes, the

performance of the Web services can be

significantly improved.

D. CDN Pricing

Commercial-oriented Web sites turn to

CDNs to contend with the high traffic problems

while providing high data quality and increased

security for theirs clients in order to increase their


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profit and popularity. CDN providers charge their

customersowners of Web sitesaccording to

their traffic (delivered by their surrogate servers to

the clients).

The most indicative factors affecting the pricing of

CDN services include: Bandwidth cost;

Variation in traffic distribution;

Size of content replicated over surrogate servers;

Number of surrogate servers;

Reliability and stability of the whole system; and

Security issues of outsourcing content delivery.

E. Meet CDN User Preferences:Meeting the user preferences is crucial for CDNs,

adopting a content management task by which the

content is personalized to meet the specific needs

of each individual user. User preferences are learnt

from web usage data by using data miningtechniques. Some indicative objectives of content

personalization over CDNs are

Deliver the appropriate content to the

interested users in a timely, scalable, and

cost-effective manner;

Increase the quality of the published

content by ensuring it is accurate,

authorized, updated, easily searched and

retrieved, as well as personalized

according to various users and audiences

Manage the content throughout its entire

life cycle from creation, acquisition, or

migration to publication and retirement;

and

Meet security requirements since

introducing content personalization on

CDNs will facilitate the security issues

raised such as authentication, signing,

encryption, access control, auditing, and

resource control for ensuring content

security and users privacy.

III. CONTENT DISTRIBUTION

ARCHITECTURE

The process Redirector is mainlycomposed by an algorithm that accepts input The

architecture comprises six basic elements. The

relationships between blocks are as follows:

The origin server delegates its URI namespace to

the request routing system (1), and publishes

content (2) to be distributed to the remote

surrogates (3) by the distribution system. Client

requests content from what he perceives to be the

origin server, but his request is treated by the

request routing system (4) which redirects him to

the optimum surrogate server (5). The surrogate

servers periodically send information to the

accounting system (6), which summarizes it indetail statistics and sends it as feedback to the

origin server(7) and the request routing system

Fig 2. General Architecture of CDN

A. Sequence of action taken during content

transaction

1. The client will connect to a portal, e.g.www.porta1. com, through a web browser.aportal consists of a set of surrogates that

build together a CDN.

2. The request is processed by theauthoritative DNS server, which is

responsible to map the name

www.portal.com into at least one IP

address. This is the best point to introduce

the Request Routing System, and is

mostly used by current CDN companies.

In fact, the DNS server is nothing but an

interface: another process, call it

Redirector, is the one in charge fordetermining the optimal surrogate.

3. parameters and produces a response,

typically a list of IP choice of an

appropriate server depends on client

proximity, server overhead and network

congestion.

4. Server overhead and network congestionimplies some type of continuously

monitoring the system, for example,

through SNMP. This is addressed by

another process, say SNMP Monitor,

responsible for capturing periodical

information of the servers and the

network.

5. The client will retrieve a list of IPaddresses decrementally ordered by

optimal performance estimated by the

Redirectorprocess. Once the client entersthe portal from one of the surrogates, it

has to select a content. This content is

typically in a multimedia format and is

delivered streamlined by a media server.

So we need both a web server and a media

server.

6. Once the desired content is selected by auser, a new resolution phase is needed, as

this selected content supposes a new input


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parameter. It is also important to note that

target web surrogates could be different

from target media surrogates. The

resolution phase takes place at HTTP

level, acting the first contacted surrogate

as interface.

7. In order to distribute the content in a

streamlined multimedia format, some kind

of plug-in is required inside the browser,

such as RealPlayer, QuickTime or, in an

open way, a simple Java applet. This plug-

in connects to the media server in order to

retrieve the content.

B.Look at the Components

Some of the components of the

architecture are given below:

DNS server: The function of our DNS is

to simply map CDN name servers intoCDN identifiers. Once a client request for

a certain website arrives at the DNS

server, it filters it depending on thecontent: if the site is associated to a

certain CDN, then the DNS serverobtainsthe corresponding CDN identifier and

resends the request to the Redirectormodule. Otherwise, the request can be

forwarded to a local DNS server following

the hierarchical DNS operation.

addresses.

The Fig 3. Data Exchange between the modules

Redirector module: The Redirector is a

key process of the whole system, as is the one in

charge of deciding an adequate surrogate for each

client request. There are two different functional

modes, though similar, related with the number of

input parameters that the included algorithm

supports. In the first mode, which takes place at

DNS resolution phase, the Redirector module

retrieves the CDN identifier and a client IP address.

The latter parameter (IP address) is at this stage

unnecessary if only scalability is targeted. The

second mode takes place after the client has

selected the content. This time the surrogate that isserving him has to interact in background with the

Redirectormodule to retrieve an optimal surrogatefor serving this content, which is a key parameter

in the selection strategy.

It is also important to serve content from a nearby

surrogate in order to obtain a low response time;

therefore, client proximity is estimated and taken

into account. If the CDN environment remains

local (iCDN) and the number of surrogates is not

considerable, a simple way of calculating proximity

consists of sending pings from each surrogate to

the client loaded within a time interval, each of

them will serve a client request with the sameprobability if a surrogate is overloaded above an

established limit, it will not be considered in the

algorithm.

The SNMP Monitor captures status informationfrom the surrogates. This information is of two

types: on the one hand, the monitor stores data

about available resources in each portal or

surrogate (memory, CPU utilization and number of

connections); on the other hand, the monitor tracks

information about network status between clients

and portals. Whereas the first type of data is

periodically read, the second type isasynchronously requested from the Redirector

module each time a client issues a request to the

CDN.

The surrogates or portals act as CDNentry points

for the clients and are in charge of serving them the

desired content. The portals store static content

(web pages) and generate dynamic content. Once a

portal receives a client request for a streaming

media content, it firstly interacts with the

Redirectormodule to obtain an optimum surrogate

IP address. After that, the portal generates an applet

that contains a media player and sends it to the

client, including the IP address of the optimum

server. The client then initiates the applet and

reproduces the multimedia content.

The CDNManager is responsible for initializing all

CDNparameter values, as well as managing how

and where to store content according to a certain

policy. That includes cache time control, content

transfers between portals, content inclusion,

content deletion, etc.

C.Database Design

Any system that stores and bases its

behavior on stored data (at least partially) must


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include an effective design of its database structure.

The database design is highly dependent of the

desired content to be published. In the case of our

CDN, there are various important .databases

associated to the different modules of the

architecture.

There is a global content database that includes

three data tables:

table_lessons: it includes some important

information for reference (the title of the lesson, the

correspondent subject, faculty and teacher).

lessons_CDN: it associates a lesson with a portal.copies_lessons: it indicates which surrogate has an

available copy of a certain lesson.

The first two data tables of the contentdatabase are remotely replicated on each surrogate,

so that each surrogate has local knowledge of the

available content in the CDN. The SNMP Monitorhas its own database to store all the information

obtained by the SNMP agents etiher periodically

-CPU usage, used memory and connections or

asynchronously - pings and network hops. Note

that ping mechanisms may suppose a problem if a

client incorporates a firewall that rejects ICMP

messages. The redirection algorithm, as part of the

Redirector Module, also has its own database to

store values of server load and server proximity.

D.Data Exchange between the ComponentsA well performance of a CDN

significantly depends on the correct communicationof each process of the system. This communication

takes place in form of messages, whose exchange is

illustrated in Fig. 3.2.

Two different routes can be distinguished: A DNS

resolution phase: It redirects a client to a portal

using a load balancing algorithm (4 steps), and

A portal resolution phase: When the client has

already entered a portal and is going to select a

streamlined multimedia content from a list of

available ones (7 steps).

If no server is available, an empty list is

sent and an error message is forwarded to the

client. Besides these messages that occur in a

content transaction, there are additional ones

related to management tasks, such as content

transfers, cache control, etc.

IV. CONCLUSIONS

In this paper we described about the

Content Delivery Networks. It tells about how well

the contents are distributed world wide to provide

better accessibility of data for the clients.

CDNs are still in an early stage of

development and their future evolution remains an

open issue. The challenge is to provide a delicate

balance between costs and customers satisfaction.

In this framework, caching-related practices,

content personalization processes, and data mining

techniques seem to offer an effective roadmap for

the further evolution of CDNs.

CDNs deliver all the data to multicast

group, so the clients can join the group anytime

when they need for particular content by sendingsession messages.. This technique saves the

network bandwidth and makes it scalable.

V. FUTURE WORK

The client-server communication flow is

replaced in CDN by two communication flows,

namely one between the origin server and the

surrogate server and the other between the

surrogate server and the client. Thus congestion

control can be done for the communication flow

between client and surrogate server since CDNs

support streaming media contents congestion cantake place which can be reduced further.

REFERENCES

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[3] Floyd, S., Jacobson, V., Liu, C., McCanne, S.

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[4] Pallis, G. and Vakali, A., Insight and

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[5] Molina, B., Palau, C., Esteve, M., Alonso, I.

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[7] Akamai Technologies www.akamai.com.


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