content delivery networks - sharif university of...

Content Delivery Networks

Hamid R. RabieeMostafa Salehi, Fatemeh Dabiran, Hoda Ayatollahi

We have used the resources mentioned in the Reference section of this presentation (on page 47) to prepare this lecture.

Spring 2011

What are Content Distribution Networks

² A Content Distribution Network or Content Delivery Network (CDN) is a

system of computers containing copies of data placed at various nodes of a

network.

² When properly designed and implemented, a CDN can improve access to

the data it caches by increasing access bandwidth and redundancy and

reducing access latency.

² Data content types often cached in CDNs include web objects,

downloadable objects (media files, software, documents), applications,

multimedia streams, and database queries.

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² The content (information) providers

are the CDN customers.

Content replication (mirror)

² CDN company installs hundreds of

CDN servers throughout Internet.

² For example: in lower-tier ISPs,

close to users.

² CDN replicates its customers’

content in CDN servers. When

provider updates content, CDN

updates servers.

Content Distribution Network: idea

origin server in North America

CDN distribution node

CDN serverin S. America CDN server

in Europe

CDN serverin Asia

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CDN Categories

²Network Infrastructure:²Single ISP²Overlay networks²Enterprise premise

²Content types: ²Static images and texts²Multimedia content: audio and video streams²Dynamic HTML and XML pages

²Customers:²Content providers²Enterprise

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CDN: Architectural Layout

.1Origin server informs RR of Content Availability..2Content Pushed to Distribution System.

.3Client Requests Content, Origin server redirects client to RR..4Client sends query to RR.

.5RR finds the most suitable Mirror Server..6Mirror server receives client request and provides services.

RequestRequestRouting(RR)Routing(RR)

Distribution Distribution NodeNode

OriginOriginServerServer12a

ClientClient

MirrorMirrorServerServer

2b 3

5

6

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Content Routing Principle(a.k.a. Content Distribution Network)

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ISP

BackboneISP

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BackboneISP

BackboneISP

HostingCenter

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routing requests² CDN creates a

“map”, indicating distances from leaf ISPs and CDN nodes

² when query arrives at authoritative DNS server:² server determines

ISP from which query originates

² uses “map” to determine best CDN server


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Content Origin (CO) hereat Origin Server

Content Servers (CSs) distributed throughout the Internet

OS

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Content is served from content servers nearer to the client

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Two basic types of CDN: cached and pushed

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Cached CDN

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1. Client requests content.

C C

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Cached CDN

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2. CS checks cache, if miss gets content from origin server.

C C

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Cached CDN

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3. CS caches content, delivers to client.

C C

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Cached CDN

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3. CS caches content, delivers to client.

4. Delivers content out of cache on subsequent requests.

C C

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Pushed CDN

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ISP

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1. Origin Server pushes content out to all CSs.

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Pushed CDN

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ISP

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CS CS CS

CSCS

1. Origin Server pushes content out to all CSs.

2. Request served from CSs.

C C

OS

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CDN benefits

² Solve network bandwidth bottleneck

² Content served closer to client

² Less latency, better performance

² Load spread over multiple distributed CSs

²More robust (to ISP failure as well as other failures)

² Handle flashes better (load spread over ISPs)

² But well-connected, replicated Hosting Centers can do this too

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Content distribution services (CDS): business model

² Benefits for Internet Service Providers (ISPs):² customer benefits: provision of high quality content

² ISP benefit: well-defined relationship with only a few CDSs

² CDS’s are huge bandwidth consumers themselves!

² Benefits for content-owners² owner benefits: focus only on the production of content

² CDS takes care of access-rights, format-conversions, etc

² Necessary (to be developed) standards² resource negotiation/mgt protocol

² content-adaptation control protocol

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CDN costs and limitations

² Cached CDNs can’t deal with dynamic/personalized content²More and more content is dynamic² “Classic” CDNs limited to images

²Managing content distribution is non-trivial² Tension between content lifetimes and cache performance² Dynamic cache invalidation² Keeping pushed content synchronized and current

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Content delivery service providers

² We have commercial and non-commercial CDNs.

² The commercial CDNs are centralized and are client/server based. For

example:

² Advection.NET/ Akamai Technologies/ Amazon CloudFront/ AT&T/

Azion/ Bitgravity/ CDNetworks/ Cotendo

² The non-commercial CDNs that are created mostly based on P2P

architecture to reduce the cost. For example:

² Globule/ Coral Content Distribution Network/ coBlitz/ FreeCast/

MediaBlog/ PeerCast/ PPLive/ PPStream/ Xunlei

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Layered architecture of a CDN

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Layered architecture of a CDN: Basic fabric layer

² Basic fabric layer: this layer consists of basic resources for creating CDNs

such as cluster servers that are connected with high bandwidth networks.

The distributed application is run on these cluster servers for indexing and

managing content in a distribution manner.

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Layered architecture of a CDN: Communication and connectivity layer ² Communication and connectivity layer: this layer consists of basic internet

protocols like TCP/UDP and etc as well as CDN specified protocols such as

internet cache protocols (ICP), hypertext caching protocol (HTCP) and cluster

array routing protocols (CARP). Besides, there are some security protocols for

authentication and authorization between cluster servers like the Public Key

Infrastructure (PKI) and SSL. There are also some kinds of applications that

exist for indexing data to enhance searching contents.

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Layered architecture of a CDN: CDN layer

² CDN layer: CDN layer consists of sub-layers like CDN services, types of CDN

and a kind of content that could be said that these are kinds of CDN servers. In

CDN layer redirecting users to the CDN server, load balancing and managing

user communications for sharing resources could be found. In CDNs

multimedia content (i.e. text, audio and video) could be shared for users.

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Layered architecture of a CDN: End-user layer

² End-user layer: in last layer, we have users that send their requested data

from web browsers or multimedia programs to received content from

CDN servers.

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Main Components of building a CDN

² Content distribution

² Placing the content to the devices

² Request routing

² Steer users to a delivery node that is close

² Content delivery

² Protocol processing, access control, QoS mechanisms

² Resource accounting

² Logging and billing

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Content Distribution

² Goal: position content objects into delivery devices

² Different content types use different techniques

² Static images and texts: pulled & cached, or pushed

² Multimedia contents: usually pre-positioned

² Dynamic pages: requires prior setup

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Distribution Mechanisms

² HTTP request for pulling

² Example: standard HTTP reverse proxy

² FTP of tar files

² Some equipment vendors use this technique

² Rate limited tree-form replication

² Example: Cisco’s “Soda” algorithm

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Distribution Mechanisms using Multicast

² Application-level reliable multicast

² Example: Inktomi’s Fast-Forward

² Unreliable IP multicast with file-level error correction

² Example: Digital Fountain, multicast-ftp

² Unreliable IP multicast

² Example: RealNetworks

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Content Consistency Mechanisms

² Expiration times or TTL

² Renaming in the HTML file

² Web Cache Invalidation Protocol (WCIP)

² Nodes receive invalidations when objects change

² Objects are organized into channels

² Nodes subscribe to a channel to receive invalidation

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Request Routing

² Goal: steer the client such that it fetches the content from a close node

² Methods

² DNS selection

² HTTP redirection

² Transparent interception

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DNS selection

² Basic idea: xyz.com’s Name Server (NS) returns node close to client

² How to become xyz.com’s NS?

² Rewrite URLs (aka Akamizer)

² Take a subdomain cdn.xyz.com and put all content there

² Accuracy limited to client’s name server

² Only suitable for ISP or overlay networks

² Not suitable for some enterprise or cable networks

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HTTP Redirection

² Basic idea: web server tells client to go somewhere else

² Returns “302 redirect … 1.2.4.5/index.html…”

² Mostly used for multimedia objects

² These objects are usually put together in an index file (.sml or .asx) and clients fetch the

index file via HTTP before streaming

² Accuracy is at individual client level

² More suitable for enterprise and cable networks

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Transparent Interception

² Router and switch along the request path can send the request elsewhere

² Mostly used for distributed data centers front-ended with L7 switches

² Example: Cisco’s CSS11k WebNS

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Algorithms for Request Routing

² Map-based

² Create a map of the Internet based on AS domains, pick the node with the shortest hop

count to client

² Or, set up coverage zones mapping a node to a collection of subnets

² Racing-based

² Let the delivery nodes all race to the client with A-records

² Winner is selected by client automatically

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Interaction between Content Distribution and Request Routing

² Don’t route request to a node that doesn’t have the content!

² Particularly important for large streaming contents

² Such content are usually pre-positioned to ensure high-bandwidth playbacks

² Nodes need to report its content acquisition status to the “request router”

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Content Delivery

² Goal: serve content to each client at desired quality of service

² Supported protocols

² HTTP

² Microsoft MMS

² Open standard RTP/RTSP

² RealNetworks RTP/RTSP

² Usually part of the larger CDN system

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QoS of Content Delivery

² Server QoS

² Server needs to make sure it has enough CPU and disk to service the stream at specified

bit rate

² Network QoS

² Interoperate with routers via DiffServ bits

² Coordination with request router

² delivery devices should communicate load information to the “Request Router”

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Resource Accounting

² Mining the log files

² Log file aggregation: all device sending log files to a central location

² Local mining: analyzing the log file at each delivery device

² Real-time statistics

² Real-time statistics on throughput/latency based on domain, content type or any HTTP

header

² Example: Cisco CSS switch billing MIB

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Multicasting

² IP multicasting is efficient but Not deployed

² Application Level Multicasting (ALM)

² End systems:

² Intermediate node

² Leaf node

² ALM implementation approaches:

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CDN for media streaming

² Works by pushing media to some servers. Each server serves a client that

is placed in its specified domain.

² Each server contains lots of media files, the bandwidth for each file should

be limited.

² Since the cost of maintaining CDN servers is too high, the user should pay

a fee to access media files.

² One method to control the cost is to place video with different

rates/qualities in the network. Therefore, the users stream the video based

on their available bandwidth.

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CDN for media streaming (cont.)

² Some papers propose other methods like stream video with different rates

by using techniques like Multiple Description Coding (MDC) or scalable

video coding (SVC) to provide such a service to users with different

capabilities.

² In MDC the video is encoded into several descriptions each of which has

the same importance. When video is encoded into M descriptions the

combination of any different description is decodable. The more

description a user can receive, the better quality the video would have.

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A CDN-P2P for Multimedia Streaming

² P2P: Challenge delivering with constant quality:

² Churn nature of peers

² Limited resource

² CDN: Scalability problem

² Costly

² Maintenance problems

CDN-P2P: Could be an efficient approach for delivering video

² CDN-P2P combines pure P2P architecture with CDN

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CDN-P2P Protocol: OMNI: Overlay Multicast Network Infrastructure [1]

² Two tire architecture:² MSNs (Multicast service nodes)

² end-hosts (or clients)

² Using tree:² Between MSNs

² Under MSNs

² Resilience problem

² Support Live Video

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CDN-P2P Protocol: A Cost-effective Hybrid P2P-CDN Architecture [2]

Different stages of a media data distribution process

² Consider One CDN server, but it is applicable to all CDN servers

² Create virtual peer

² When the seed distribute completely in the network, it is released

² Using mesh under CDNs

² CDN servers are NOT participate in other meshes

² Designed for video-on-demand

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CDN-P2P Protocol: ChinaCahe [3]

² A commercial approach

² Servers is deployed in Multiple tree

² Using mesh under CDNs

² Support Live video streaming

² There is no connection between meshes

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Comparison between protocols’ properties

Name ScalabilityLoad on server

Startup Delay

interconnection between

P2P networks

Locality awareness

General CDN streaming system Costly High Minimum No peer No

General P2P streaming system Yes Low Long One P2P network No

OMNI [1] YesTrade off between load and delay

Shorter No No

A cost-effective hybrid architecture [2] YesTrade off between load and delay

Shorter No No

ChinaCache [3] YesTrade off between load and delay

Shorter between ISP's POPs Yes

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Reference

² [1] S. Banerjee, C. Kommareddy, K. Kar, B. Bhattacharjee, and S. Khuller, ―OMNI: an efficient overlay

multicast infrastructure for real-time applications, Computer Networks, vol. 50, 2006, pp. 826–841.

² [2] D. Xu, S.S. Kulkarni, C. Rosenberg, and H.K. Chai, ―Analysis of a CDN–P2P hybrid architecture for

cost-effective streaming media distribution, Multimedia Systems, vol. 11, 2006, pp. 383–399.

² [3] X. Liu, H. Yin, and C. Lin, ―A Novel and High-Quality Measurement Study of Commercial CDN-P2P

Live Streaming, WRI International Conference on Communications and Mobile Computing, 2009.

CMC'09., 2009, pp. 325–329.

² [4] A.M. Pathan and R. Buyya, ―A taxonomy and survey of content delivery networks, Grid Computing

and Distributed Systems (GRIDS) Laboratory, University of Melbourne, Parkville, Australia, vol. 148,

2006.

² [5] Yasser Aeyyedi, MS Thesis, Sharif University of Technology-Kish Branch, 2010.

² [6] Pei Cao, Technologies for Building Content Delivery Networks, online presentation:, Cisco Systems.

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