Content Delivery Networks

Transcription

1 Content Delivery Networks Hamid R. Rabiee Mostafaalehi, Fatemeh Dabiran, HodaAyatollahi We have used the resources mentioned in the Reference section of this presentation (on page 47) to prepare this lecture. pring 2011

2 What are Content Distribution Networks 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. 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. Data content types often cached in CDNs include web objects, downloadable objects (media files, software, documents), applications, multimedia streams, and database queries. 2

3 Content Distribution Network: idea The content (information) providers are the CDN customers. Content replication (mirror) origin server in North America CDN company installs hundreds of CDN servers throughout Internet. For example: in lower-tier IPs, close to users. CDN distribution node CDN replicates its customers content in CDN servers. When provider updates content, CDN updates servers. CDN server in. America CDN server in Europe CDN server in Asia 3

4 CDN Categories Network Infrastructure: ingle IP Overlay networks Enterprise premise Content types: tatic images and texts Multimedia content: audio and video streams Dynamic HTML and XML pages Customers: Content providers Enterprise 4

5 CDN: Architectural Layout Request Routing(RR) 1 Origin erver 2a 5 4 Distribution Node 2b Mirror erver 6 Client 3 Origin server informs RR of Content Availability..1 Content Pushed to Distribution ystem..2 Client Requests Content, Origin server redirects client to RR. Client sends query to RR..4 RR finds the most suitable Mirror erver..5 5 Mirror server receives client request and provides services..6.3

6 Content Routing Principle (a.k.a. Content Distribution Network) Backbone IP IP Hosting Center IX Backbone IP IP Hosting Center IX IP Backbone IP ite ites routing requests CDN creates a map, indicating distances from leaf IPs and CDN nodes when query arrives at authoritative DN server: server determines IP from which query originates uses map to determine best CDN server 6

7 Content Routing Principle (a.k.a. Content Distribution Network) Hosting Center Hosting Center O Content Origin (CO) here at Origin erver Backbone IP C Backbone IP C Backbone IP C IX IX IP C ite IP IPC ites Content ervers (Cs) distributed throughout the Internet 7

8 Content Routing Principle (a.k.a. Content Distribution Network) Hosting Center Hosting Center O Backbone Backbone Backbone IP C IP C IP C IP C IX IX IP IPC ite ites Content is served from content servers nearer to the client C C 8

9 Two basic types of CDN: cached and pushed Hosting Center Hosting Center O Backbone Backbone Backbone IP C IP C IP C IX IX IP C IP IPC ite ites C C 9

10 Cached CDN Hosting Center Hosting Center O 1. Client requests content. Backbone Backbone Backbone IP C IP C IP C IX IX IP C IP IPC ite ites C C 10

11 Cached CDN Hosting Center Hosting Center O Backbone Backbone Backbone IP C IP C IP C 1. Client requests content. 2. C checks cache, if miss gets content from origin server. IX IX IP C IP IPC ite ites C C 11

12 Cached CDN IP Hosting Center IX IP Hosting Center O 1. Client requests content. 2. C checks cache, if Backbone IP C Backbone IP C Backbone IP C C IX IPC ite ites miss gets content from origin server. 3. C caches content, delivers to client. C C 12

13 Cached CDN IP Hosting Center IX IP Hosting Center O 1. Client requests content. 2. C checks cache, if Backbone IP C Backbone IP C Backbone IP C C IX IPC ite ites miss gets content from origin server. 3. C caches content, delivers to client. 4. Delivers content out of cache on subsequent requests. C C 13

14 Pushed CDN Hosting Center Hosting Center O Backbone Backbone Backbone IP C IP C IP C 1. Origin erver pushes content out to all Cs. IX IX IP C IP IPC ite ites C C 14

15 Pushed CDN Hosting Center Hosting Center O 1. Origin erver pushes content out to all Cs. Backbone IP C Backbone IP C Backbone2. IP C Request served from Cs. IX IX IP C IP IPC ite ites C C 15

16 CDN benefits olve network bandwidth bottleneck Content served closer to client Less latency, better performance Load spread over multiple distributed Cs More robust (to IP failure as well as other failures) Handle flashes better (load spread over IPs) But well-connected, replicated Hosting Centers can do this too 16

17 Content distribution services (CD): business model Benefits for Internet ervice Providers (IPs): customer benefits: provision of high quality content IP benefit: well-defined relationship with only a few CDs CD s are huge bandwidth consumers themselves! Benefits for content-owners owner benefits: focus only on the production of content CD takes care of access-rights, format-conversions, etc Necessary (to be developed) standards resource negotiation/mgt protocol content-adaptation control protocol 17

18 CDN costs and limitations Cached CDNs can t deal with dynamic/personalized content More and more content is dynamic Classic CDNs limited to images Managing content distribution is non-trivial Tension between content lifetimes and cache performance Dynamic cache invalidation Keeping pushed content synchronized and current 18

19 Content delivery service providers We have commercial and non-commercial CDNs. The commercial CDNs are centralized and are client/server based. For example: Advection.NET/ Akamai Technologies/ Amazon CloudFront/ AT&T/ Azion/ Bitgravity/ CDNetworks/ Cotendo The non-commercial CDNs that are created mostly based on P2P architecture to reduce the cost. For example: Globule/ Coral Content Distribution Network/ coblitz/ FreeCast/ MediaBlog/ PeerCast/ PPLive/ PPtream/ Xunlei 19

20 20 Layered architecture of a CDN

21 Layered architecture of a CDN: Basic fabric layer 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. 21

22 Layered architecture of a CDN: Communication and connectivity layer 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 L. There are also some kinds of applications that exist for indexing data to enhance searching contents. 22

23 Layered architecture of a CDN: CDN layer CDN layer: CDN layer consists of sub-layers like CDN services, types of CDN andakindof content that could besaidthat theseare 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. 23

24 Layered architecture of a CDN: End-user layer 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. 24

25 Main Components of building a CDN Content distribution Placing the content to the devices Request routing teer users to a delivery node that is close Content delivery Protocol processing, access control, Qo mechanisms Resource accounting Logging and billing 25

26 Content Distribution Goal: position content objects into delivery devices Different content types use different techniques tatic images and texts: pulled & cached, or pushed Multimedia contents: usually pre-positioned Dynamic pages: requires prior setup 26

27 Distribution Mechanisms HTTP request for pulling Example: standard HTTP reverse proxy FTP of tar files ome equipment vendors use this technique Rate limited tree-form replication Example: Cisco s oda algorithm 27

28 Distribution Mechanisms using Multicast Application-level reliable multicast Example: Inktomi s Fast-Forward Unreliable IP multicast with file-level error correction Example: Digital Fountain, multicast-ftp Unreliable IP multicast Example: RealNetworks 28

29 Content Consistency Mechanisms Expiration times or TTL Renaming in the HTML file Web Cache Invalidation Protocol (WCIP) Nodes receive invalidations when objects change Objects are organized into channels Nodes subscribe to a channel to receive invalidation 29

30 Request Routing Goal: steer the client such that it fetches the content from a close node Methods DN selection HTTP redirection Transparent interception 30

31 DN selection Basic idea: xyz.com s Name erver (N) returns node close to client How to become xyz.com sn? Rewrite URLs (aka Akamizer) Take a subdomaincdn.xyz.com and put all content there Accuracy limited to client s name server Only suitable for IP or overlay networks Not suitable for some enterprise or cable networks 31

32 HTTP Redirection Basic idea: web server tells client to go somewhere else Returns 302 redirect /index.html Mostly used for multimedia objects These objects are usually put together in an index file (.sml or.asx) and clients fetch the index file via HTTP before streaming Accuracy is at individual client level More suitable for enterprise and cable networks 32

33 Transparent Interception Router and switch along the request path can send the request elsewhere Mostly used for distributed data centers front-ended with L7 switches Example: Cisco s C11k WebN 33

34 Algorithms for Request Routing Map-based Create a map of the Internet based on A domains, pick the node with the shortest hop count to client Or, set up coverage zones mapping a node to a collection of subnets Racing-based Let the delivery nodes all race to the client with A-records Winner is selected by client automatically 34

35 Interaction between Content Distribution and Request Routing Don t route request to a node that doesn t have the content! Particularly important for large streaming contents uch content are usually pre-positioned to ensure high-bandwidth playbacks Nodes need to report its content acquisition status to the request router 35

36 Content Delivery Goal: serve content to each client at desired quality of service upported protocols HTTP Microsoft MM Open standard RTP/RTP RealNetworks RTP/RTP Usually part of the larger CDN system 36

37 Qo of Content Delivery erver Qo erver needs to make sure it has enough CPU and disk to service the stream at specified bit rate Network Qo Interoperate with routers via Differvbits Coordination with request router delivery devices should communicate load information to the Request Router 37

38 Resource Accounting Mining the log files Log file aggregation: all device sending log files to a central location Local mining: analyzing the log file at each delivery device Real-time statistics Real-time statistics on throughput/latency based on domain, content type or any HTTP header Example: Cisco C switch billing MIB 38

39 39 Multicasting IP multicasting is efficient but Not deployed Application Level Multicasting (ALM) End systems: Intermediate node Leaf node ALM implementation approaches:

40 CDN for media streaming Works by pushing media to some servers. Each server serves a client that is placed in its specified domain. Each server contains lots of media files, the bandwidth for each file should be limited. ince the cost of maintaining CDN servers is too high, the user should pay a fee to access media files. 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. 40

41 CDN for media streaming (cont.) ome papers propose other methods like stream video with different rates by using techniques like Multiple Description Coding (MDC) or scalable video coding (VC) to provide such a service to users with different capabilities. 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. 41

42 A CDN-P2P for Multimedia treaming P2P: Challenge delivering with constant quality: Churn nature of peers Limited resource CDN: calability problem Costly Maintenance problems CDN-P2P: Could be an efficient approach for delivering video CDN-P2P combines pure P2P architecture with CDN 42

43 CDN-P2P Protocol: OMNI: Overlay Multicast Network Infrastructure [1] Two tire architecture: MNs (Multicast service nodes) end-hosts (or clients) Using tree: Between MNs Under MNs Resilience problem upport Live Video 43

44 CDN-P2P Protocol: A Cost-effective Hybrid P2P-CDN Architecture [2] Consider One CDN server, but it is applicable to all CDN servers Create virtual peer When the seed distribute completely in the network, it is released Using mesh under CDNs CDN servers are NOT participate in other meshes Designed for video-on-demand Different stages of a media data distribution process 44

45 CDN-P2P Protocol: ChinaCahe [3] A commercial approach ervers is deployed in Multiple tree Using mesh under CDNs upport Live video streaming There is no connection between meshes 45

46 Comparison between protocols properties Name calability Load on server tartup Delay interconnection between P2P networks General CDN streaming system Costly High Minimum No peer No General P2P streaming system Yes Low Long One P2P network No Locality awareness OMNI [1] Yes Trade off between load and delay horter No No A cost-effective hybrid architecture [2] Yes Trade off between load and delay horter No No ChinaCache [3] Yes Trade off between load and delay horter between IP's POPs Yes 46

47 Reference [1]. Banerjee, C. Kommareddy, K. Kar, B. Bhattacharjee, and. Khuller, OMNI: an efficient overlay multicast infrastructure for real-time applications, Computer Networks, vol. 50, 2006, pp [2] D. Xu,.. Kulkarni, C. Rosenberg, and H.K. Chai, Analysis of a CDN P2P hybrid architecture for cost-effective streaming media distribution, Multimedia ystems, vol. 11, 2006, pp [3] X. Liu, H. Yin, and C. Lin, A Novel and High-Quality Measurement tudy of Commercial CDN-P2P Live treaming, WRI International Conference on Communications and Mobile Computing, CMC'09., 2009, pp [4] A.M. Pathan and R. Buyya, A taxonomy and survey of content delivery networks, Grid Computing and Distributed ystems (GRID) Laboratory, University of Melbourne, Parkville, Australia, vol. 148, [5] Yasser Aeyyedi, M Thesis, harif University of Technology-Kish Branch, [6] Pei Cao, Technologies for Building Content Delivery Networks, online presentation:, Cisco ystems. 47