Technologies for Building Content Delivery Networks

1. Technologies for Building Content Delivery Networks Pei Cao Cisco Systems, Inc. cao@cisco.com

2. What are Content Delivery Networks • A centrally managed network of devices that collectively facilitate the delivery of content to end users • Solve network bandwidth bottleneck • Solve server throughput bottleneck

3. CDN Categories • Network Infrastructure: • Single ISP • Overlay networks • Enterprise premise • Content types: • Static images and texts • Multimedia content: audio and video streams • Dynamic HTML and XML pages • Customers: • Content providers • Enterprise

4. Technology Components • Content distribution • Placing the content to the devices • Request routing • Steer users to a delivery node that is close • Content delivery • Protocol processing, access control, QoS mechanisms • Resource accounting • Logging and billing

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

6. Distribution Mechanisms • HTTP request for pulling • Example: standard HTTP reverse proxy • FTP of tar files • Some equipment vendors use this technique • Rate limited tree-form replication • Example: Cisco’s “Soda” algorithm

7. 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

8. 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

9. Request Routing • Goal: steer the client such that it fetches the content from a close node • Methods • DNS selection • HTTP redirection • Transparent interception

10. Overview of Request Arrival Process How a request for www.xyz.com/index.html arrives at 1.2.3.4: Root NS 2. where is name server of xyz.com? DNS server 3. NS record: 1.2.3.1 4. what is IP of www.xyz.com? 1. what is IP addr of www.xyz.com? xyz.com NS 5. A record:1.2.3.4 IP: 1.2.3.1 6. 1.2.3.4 Client s w i t c h 7. GET /index.html Router Server IP: 1.2.3.4

11. DNS selection • Basic idea: xyz.com’s NS returns node close to client • How to become xyz.com’s NS? • Rewrite URLs (aka Akamizer) • Take a subdomain cdn.xyz.com and put all content there • Accuracy limited to client’s name server • Only suitable for ISP or overlay networks • Not suitable for some enterprise or cable networks

12. HTTP Redirection • Basic idea: web server tells client to go somewhere else • Returns “302 redirect … 1.2.4.5/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

13. 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 CSS11k WebNS

14. Algorithms for Request Routing • Map-based • Create a map of the Internet based on AS 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

15. The Boomerang Algorithm • Cisco’s research published in WCW’01 • xyz.com’s NS server forwards lookup of www.xyz.com to all delivery nodes • Delivery nodes all send “A record” response with its own IP address to the client • The one that reaches the client first wins • NS server times the forwarding so that lookup message arrives at all nodes around the same time • Use “simulated annealing” for scalability

16. 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 • Such content are usually pre-positioned to ensure high-bandwidth playbacks • Nodes need to report its content acquisition status to the “request router”

17. Content Delivery • Goal: serve content to each client at desired quality of service • Supported protocols • HTTP • Microsoft MMS • Open standard RTP/RTSP • RealNetworks RTP/RTSP • Usually part of the larger CDN system

18. Content Access Control • Content object attributes • “Publication date” and “Expiration date” • ACL based on user/group/IP • User authentication • HTTP basic • Microsoft NTLM for enterprise environment • other schemes • Media Rights Management

19. QoS of Content Delivery • Server QoS • Server needs to make sure it has enough CPU and disk to service the stream at specified bit rate • Network QoS • Interoperate with routers via DiffServ bits • Coordination with request router • delivery devices should communicate load information to the “Request Router”

20. 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 CSS switch billing MIB

21. Cisco’s CDN Products • Content Distribution Manager (CDM) • Content Router (CR) • Content Engine (CE) • CSS switch

22. Summary • Main components of building a CDN: • Content distribution • Request routing • Content Delivery • Resource accounting • A CDN system requires the four components to work in concert with each other! • Cisco is the only vendor that provide the full solution!