Content Delivery Networks Terena 2000 ftp://ftpeng.cisco.com/sgai/t2000cdn.pdf
Silvano Gai Cisco Systems, USA Politecnico di Torino, IT
[email protected] Terena 2000
© 2000, Cisco Systems, Inc.
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Agenda
• • • • • •
Terena2000CDN
What are Content Delivery Networks? DNS based routing Server Load Balancing Content Routers Ethical questions Conclusion
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At the beginning were Web Caches S1
S2
IP Network
S3
IP Network
Web Cache A Web Cache is a device that stores a local copy of more recently required HTTP objects and reacts as proxy server to clients’ requests
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Motivations for Content Delivery Networks • Web site performance improvement • Static Web pages or nonlive streaming media • Server farms are far from users Better RTT better user’s experience
• Bandwidth • Cost saving • Less probability to incur in a bottleneck
• Availability • Load balancing • Traffic peaks crashes sites Terena2000CDN
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Content Delivery Networks (CDNs)
• Distributed Web Hosting • Video-On-Demand • MPEG on LAN • Low/Mid-rate streaming on WAN
• Scalable Live Streaming • Dynamic Content • Conditional-Access Content • advertisements
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Example of CDN
Content Updates
Live Streams
Server
1K to 10M Client Requests
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An Overlay Network over Internet
SIGHTPATH
SIGHTPATH
SIGHTPATH
SIGHTPATH
SIGHTPATH
SIGHTPATH
SIGHTPATH
SIGHTPATH
SIGHTPATH
SIGHTPATH
SIGHTPATH
SIGHTPATH
SIGHTPATH
SIGHTPATH
SIGHTPATH
SIGHTPATH
SIGHTPATH
SIGHTPATH
SIGHTPATH
• A CDN is an overlaid network of Caches, a.k.a. Content Servers, a.k.a. Delivery Nodes, a.k.a. Replicas
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Agenda
• • • • • •
Terena2000CDN
What are Content Delivery Networks? DNS based routing Server Load Balancing Content Routers Ethical questions Conclusion
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The idea: a new DNS Server Architecture DNS queries for www.terena2000.com
Standard DNS interface Routing Engine {DNS-names, IP-SA} IP-DA
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DNS-based CDNs
• Host Names are used to redirect the traffic to the best replica • the replica selections happens when the name is translated to an IP address
• DNS servers become “Content Routers” • they measure as many metric as possible (RTT, Server Load, Layer 3 metrics, response time, etc.) to compute a replica routing table {{DNS-names, IP-SA} IP-DA} • Metric measurement is not easy • Layer 3 metrics alone are not particularly meaningful Terena2000CDN
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Traditional Browsing
Access Provider
Backbone Provider
Hosting Provider
MSPG
DNS
XYZ
UUNET
AT&T
MCI
ABOV
EXDS DNS
Content Provider Terena2000CDN
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CNN 11
DNS-based CDN Browsing
Access Provider
MSPG
DNS
XYZ
Replica
Backbone Provider
Hosting Provider
Replica
UUNET
AT&T
Replica
MCI
GBLX
EXDS DNS
Content Provider Terena2000CDN
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CNN
CDN DNS 12
Example “Akamaized” content
Akamaization
HTML delivered by CNN Entire page delivered by CNN
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DNS-based CDNs Limitations • There are limitations • The granularity of redirection is an host name, not a URL • Content of large web sites cannot be split into multiple caches • It is difficult to use the same host name for static and dynamic content
• The Akamai approach: • Akamaized URLs: http://a836.g.akamaitech.net/7/836/123/e358f5db0045e/ www.terena2000.com/logo.gif Terena2000CDN
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How Akamai works (1)
• It creates new domain names for each client content provider • E.g., a128.g.akamai.net • The CDNs DNS servers are authoritative for the new domains CDN’s
• The content provider modifies its content so that embedded URLs reference the new domains • http://cnn.com/a.gif --> http://a128.g.akamai.net/7/23/cnn.com/a.gif • Apache module or IIS ISAPI filter Terena2000CDN
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How Akamai works (2)
• Using multiple domain names for each client allows the CDN to further subdivide the content into groups • DNS sees only the requested domain name, but it can route requests for different domains independently • www.repubblica.it may point to mainly Italian caches • www.elpais.es may point to mainly Spanish caches
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Extension to DNS-based CDNs
• How to implement more granular DNS-based CDNs (e.g. how to look for the complete URL)? • HTTP/RTSP Redirect
• Redirection can be obtained in two ways • every server in the farm is capable to redirect • An SLB (Server Load Balancer) is capable to redirect
• Effective only in a Local Area
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The Next Step: URL-based CDNs
• URLs are used to redirect the traffic to the best Content Server • URL routing requires TCP termination • TCP termination is complex and expensive • TCP termination introduces delay
• There will be only one TCP termination point • Close to the client? • Close to the server?
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Agenda
• • • • • •
Terena2000CDN
What are Content Delivery Networks? DNS based routing Server Load Balancing Content Routers Ethical questions Conclusion
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Server Farms • A reality today • Clients see a unique Virtual Server (IP address) • Traffic destined to the Virtual Server is load balanced among different Real Server
R Data Server
Application Server
IP Network
Client (Browser)
Web Server
Server Farm Terena2000CDN
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Server Load Balancing
Real Server
S1
Real Server
S2
Real Server
S3
SLB
IP Network Client (Browser)
Virtual Server
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Server Load Balancing • Content-unaware (layer 4 switching) • TCP connections are not terminated by the SLB All packets belonging to a given TCP connection must be terminated always on the same server
• Examples Load distribution based on source IP address Load distribution based on hash (IP src/dst, TCP sport/dport), …
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• Content-aware (layer 7 switching) • TCP connections with both clients and servers are terminated • To support SSL (https) the SLB requires the server keys
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TCP Proxy Client
SYN
Content Aware Router (Layer 7 Switch)
Server
SYN/ACK ACK GET URL SYN SYN/ACK
ACK GET URL Data Terena2000CDN
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Data 23
Limitations with SLB • Some applications require that TCP connections from the same client are redirected to the same server (Sticky Connections): • Shopping Cart • Searches • Forms • Economic Transactions
• Stickiness may be addressed/complicated by: • source IP address • cookies • SSL ID Terena2000CDN
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Agenda
• • • • • •
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What are Content Delivery Networks? DNS based routing Server Load Balancing Content Routers Ethical questions Conclusion
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URL routing
• Can we build a router that routes on URLs? • YES, but: • statefull (we must terminate TCP) • complex packet parsing (we need the URL) • anycast router (a URL is associated to multiple replicas) • Do we have URL routing tables? • Do we have URL routing protocols? • Do we have metrics? How do we compute them?
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IP vs. Content Routing IP routing H1
R1
R2
R3
H2
Content routing
or CR2 H1
CR1
H2
CR3 Terena2000CDN
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Or even more complex
H1
H1
CR1
CR2
CR2
CR4
CR1
CR5
CR3
H2
H2
CR3
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Content Delivery Control Protocols • Content Routers in series cannot all terminate the TCP session: • we don’t want to reinvent X.25
• URL must be • extracted by the first Content Router • propagate by a Content Delivery Control Protocol
• Some protocols have been proposed: • HUP • Christmas Tree • ICAP
• Still in a very preliminary phase: • if successful, they can be integrated in the hosts. Terena2000CDN
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Agenda
• • • • • •
Terena2000CDN
What are Content Delivery Networks? DNS based routing Server Load Balancing Content Routers Ethical questions Conclusion
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The Ethical question
• Is it ethical to deploy Content Routers in the Internet? • They hijack the packets • They spoof the addresses • They break the end-to-end model of IP
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Where, is the question
Access Provider
Here No, or may be
Backbone Provider
NOT HERE !!! Hosting Provider
Content Provider Terena2000CDN
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Here YES 32
Agenda
• • • • • •
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What are Content Delivery Networks? DNS based routing Server Load Balancing Content Routers Ethical questions Conclusion
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Sometime CDNs are very good!
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Sometime are not so good!
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Content Peering Content Provider CDN1
CDN2
Control POP Routing POP Delivery POP
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Conclusions • Content Delivery Networks (CDNs) DNS-based will be widely deployed • CDNs are not only for web traffic, but also for multimedia streaming • Replicas will have slightly different content (e.g. local advertisement) • Content Peering is still an unsolved problem
• Server Farms and Server Load Balancing will be widely deployed • Intrusive content routing poses: • ethical questions • scalability concerns Terena2000CDN
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The End
Thank You Terena2000CDN
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