Technologies for Building Content Delivery Networks Pei Cao Cisco Systems, Inc.

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

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

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

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

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

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

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

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

Overview of Request Arrival Process Client How a request for arrives at : DNS server 1. what is IP addr of Root NS 2. where is name server of xyz.com? xyz.com NS IP: NS record: what is IP of 5. A record: Server switchswitch IP: Router 7. GET /index.html

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

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

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

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

The Boomerang Algorithm Cisco’s research published in WCW’01 –xyz.com’s NS server forwards lookup of 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

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”

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

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

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”

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

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

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!