Globally Distributed Content Delivery

1
Globally Distributed Content Delivery

• John Dilley,Bruce Maggs,
• Jay Parikh,Harald Prokop,Ramesh Sitaraman,Bill
  Weihl
• AkamaiTechnologies
• IEEE INTERNET COMPUTING SEPTEMBER.OCTOBER 2002

2
Outline

• Introduction
• Existing approaches
• Akamai's Network Infrastructure
• Network Service
• Technical Challenges
• Related Work
• Conclusion

3
Introduction

• As web sites become popular
• The flash crowd problem
• Crash a site
• Unusually high response times
• Serious problems while serving Web content from a
  single location
• Site scalability
• Reliability
• Performance
• Devise a system to serve from variable number of
  surrogate origin servers at the network edge
• Many technical challenges
• Direct request
• Handle failure
• Monitor and control the server
• Update software

4
Existing Approaches

• Several approaches to delivering content in a
  scalable and reliable way
• Local clustering
• Improve fault-tolerance and scalability
• Difficult to scale clusters to thousands of
  servers
• Mirroring
• Deploying clusters in a few location
• Mirroring requires synchronizing the site among
  the mirrors
• Multi-homing
• Using multiple ISPs to connect to the internet
• Underlying network protocols dont converge
  quickly

5
Akamais Network Infrastructure

• Allocating more servers to site experiencing high
  load
• The system directs client requests
• Nearest
• Function of network topology and dynamic link
  characteristics
• Available
• Function of load and network bandwidth
• Likely
• Function of which servers carry the content for
  each customer in a data center

6
Akamais Network InfrastructureAutomatic Network
Control

• The direction of requests to content server is
  mapping
• Using dynamic , fault-tolerant DNS system
• Resolve hostname
• service requested
• user location
• network status
• also use DNS for network load-balancing

7
Akamais Network InfrastructureAutomatic Network
Control

• akamai name server resolve host name to IP
  address using the following criteria
• Service requested
• Server health
• Server load
• Network condition
• Client location
• Content requested
• BGP messages
• Network reachability information
• The best routing path among the Internets Ass
• The mapping system
• Using BGP and live network statistics --
  traceroute data
• Network structure and quality measures

8
Akamais Network InfrastructureAutomatic Network
Control
9
DNS Resolution

• Akamai edge servers located using a DNS name.
• Such as a7.g.akamai.net
• Process
• Resolver------------gtroot name server
• a7.g.akamai.net
• lt------------
• the IP of the name servers that handle .net
  domain requests
• Resolver------------gt.net name servers
• lt------------
• the IP of the name servers that handle
  .akamai.net domain requests (the Akamai
  top-level name servers(top-level DNS in F.1))

10
DNS Resolution

• Resolver------------gtAkamai TL DNS server
• lt------------
• return the low-level Akamai name servers
  (low-level DNS in F.1) for .g.akamai.net with a
  TTL of about one hour
• Resolver------------gtAkamai LL DNS server
• lt-----------
• return the IP of servers available to satisfy
  the request with a short TTL(sec to min)

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Akamais Network InfrastructureNetwork Monitoring

• DNS-based load balancing system
• continuously monitors the state of services ,
  their server and networks.
• Each content servers reports its load to a
  monitoring application
• The Monitoring application report to the local
  DNS server
• DNS server determines which IP to return when
  resolving DNS name by some threshold
• Assign some of the servers allocated content to
  additional servers
• The servers IP is no longer available to clients
• The monitoring system also transmits data center
  load to the top-level DNS resolver
• Monitor the entire system end-to-end
• Use agents that simulate end-user behavior

12
Network Services

• Static Content
• Akamais content servers use content type to
  apply
• Lifetime
• Zero to infinite ( never check the objects
  consistency)
• lifetime values for Akamai edge servers can
  differ
• Special feature
• The ability to serve secure content
• Support alternate content

13
Network Services

• Dynamic Content
• Use Edge Side Includes techology
• Similar to server-side include languages
• Add fault-tolerance features
• Can process XML data
• Break a dynamic page into fragments with
  independent cacheability properties
• Must fetch only noncacheable or expired fragments
  from the origin Web site.

14
Network Services

• Streaming Media
• Support live and on-demand in three format
• MS windows media , Real , Quicktime
• Additional challeges
• The content provider typically captures and
  encodes a live stream and sends to entry-point
  server
• Mechanism the will react quickly to a failed
  entry-point server
• Delivery from the entry-point to the edge servers
  must be resilient to failure and packet loss
• Uses information dispersal techniquesmultiple
  redundant paths

15
Technical ChallengesSystem Scalability

• Monitoring and controlling servers while minimize
  monitoring bandwidth
• Monitoring network conditions ,and generate new
  maps
• Minimizing the overhead added to DNS to avoid
  long DNS lookup times
• Dealing with incomplete and out-of-date
  information
• Reacting quickly to changing network conditions
  and changing workloads
• Measuring Internet conditions to attain end-user
  performance
• Customers problems
• Isolating customers to avoid affecting each other
• Data integrity
• Collecting logs

16
Technical ChallengesSystem Reliability

• Akamais monitoring and mapping software ensures
  that server or network failure do not affect end
  users
• DNS system detects failures quickly and hands out
  new IP
• For customers still using cached DNS ansers
• DNS resolution return multiple IP addresses
• A live server assume the failed servers IP
• Prevent when network failures make sites
  unreachable
• Top-level (TL) identify local DNS servers at
  different sites
• Prevent outages
• Avoid single points of failure
• By replicating monitoring and control mechanisms

17
Technical ChallengesSystem Reliability

• Detect and repair software flaw
• New request and response headers appear at any
  time
• Use a test tool that directs a copy of a live
  servers traffic to test version of software

18
Technical Challenges--Software Deployment and
Platform Management

• Software must evolve with new features
• for customers ,improved performance, and better
  operational and monitoring capabilities.
• Cannot upgrade software on the entire network
  atomically
• Unlikely all edge servers (or all networks) will
  be available at the same time
• Inevitably ,miss some servers and have to update
  them at a later time.
• Operating multiple OS platforms and service
• a monitoring platform and tools that run across
  those platforms
• Gave access to servers' service delivery
  parameters--for load balancing

19
Technical Challenges Content Visibility and
Control

• Cache consistency
• Cacheable-object consistency
• Apply a "time to live" (TTL) to objects
• Or use a different URL for each object version
• Unique query string
• Let customers place a version generation number
  in the URL
• Versioned objects often infinite TTLs
• Uncacheable objects' performance
• An edge server between client and origin
• Split the TCP connection
• Edge server can react to packet loss more quickly
  
• Map clients to an efficient edge servers
• Edge server can maintain much longer persistent
  connection with the client

20
Technical Challenges Content Visibility and
Control

• Lifetime Control
• Some case, edge server remove certain objects
  from all servers on demand
• Akamai's edge servers support on-demand purges
  for changed or invalid content
• Authentication and authorization
• Serving protected content ,edge server
• Contain authorization features
• Relay authentication tokens to the origin server
  for authorization
• Edge server must not to evict the protected
  content on a request authorization failure
• Akamai lets content providers authorize every
  user request from their own site

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Technical Challenges Content Visibility and
Control

• Integrity Control
• Server must ensure that
• each client request receives the correct response
  
• Detect when origin servers issue incomplete
  responses
• Content integrity check
• Visibility into access patterns
• Detailed content-access logs
• Aggregate individual server logs
• Real-time delivery rates and client location
  than full log details.
• Billing

22
Related Work in Distributed and Fault-Tolerant
Systems

• Akamai use logically centralized (but distributed
  and replicated) administration and control to map
  request to servers and to manage the entire
• Other systems also use logically centralized
  control
• The Autonet
• Use a centralized algorithm to recompute and
  distribute routing tables when the network
  topology changes
• Web-based Caching
• Update and Management Tools
• Software update and management in large
  distributed computing environments
• Depot
• Reliable system update using modular package and
  internal consistency verification
• Secure data distribution by shared global file
  system

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Conclusion

• Current work
• Running applications at the network's edge
• Advantages
• Capacity on demand
• Cost-effective use of the shared resources
• Respond to users without long distances
• Many challenges
• Visibility into customers' running application
• Applications access the distributed data