Overlay Networks (with a focus on Content Distribution Networks)

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Overlay Networks(with a focus on Content
Distribution Networks)

• Janardhan R. Iyengar
• CISC 856 TCP/IP and Upper Layer Protocols
• 04/23/2002

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What is an Overlay ?
What is the topology of this network? WHICH
network??
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Overlay Networks Overview

• Networks built using an existing network as
  substrate
• Also known as Virtual Networks
• Most popular overlay The Internet Evolved as
  an overlay on the POTS (Plain Old Telephone
  System) network
• Overlays could consist of routing software
  installed at selected sites, connected by
  encapsulation tunnels or direct links

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Overlay Networks Examples

• MBone, 6Bone, ABone
• RON, VNS
• P2P (Napster, FreeNet, Gnutella)
• Content Networks
• Cooperating Caches
• Server Farms
• Content Distribution Networks (CDNs)

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Example Overlays (1) MBone

• Semi-permanent testbed to carry IP multicast
  traffic
• Routing of IP multicast traffic is not commonly
  integrated and deployed in production routers on
  the Internet
• Hence, layered on the Internet to support routing
  of IP multicast packets using tunneling

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Example Overlays (1) MBone
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Example Overlays (2) 6Bone

• 6bone is an IPv6 testbed on the Internet
• Intended to eventually subsume the underlying
  IPv4 network
• IPv4 tunnels used to overlay the 6bone
• ABone is the Active Networks Backbone, for
  experimentation in Active networking. Uses
  tunneling

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Example Overlays (2) 6Bone
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Other known Overlays

• Resilient Overlay Network (RON) Provides fault
  tolerance and faster recovery as compared to
  conventional routing techniques
• Virtual Network Service (VNS) Infrastructure for
  provisioning QoS within Virtual Private Networks
• Peer to Peer Networks Infrastructure for
  distribution and sharing of files (eg Napster,
  Gnutella, Freenet)
• Content Networks
• Server Farms, Caching Proxies, Content
  Distribution Networks (CDNs)
• Today, we will try to focus on CDNs
• What are the motivations for Content Networks?

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Motivations for Content Networks

• More hops between client and Web server gt more
  congestion!
• Same data flowing repeatedly over links between
  clients and Web server

S
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Motivations for Content Networks (contd.)

• Origin server is bottleneck as number of users
  grows
• Flash Crowds (for instance, Sept. 11)
• The Content Distribution Problem Arrange a
  rendezvous between a content source at the origin
  server (www.cnn.com) and a content sink (us, as
  users)

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Example content networks Server Farms

• Simple solution to the content distribution
  problem
• deploy a large group of servers

• Arbitrate client requests to servers using an
  intelligent L4-L7 switch
• Pretty widely used today

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Example content networks Caching Proxies
ISP
Other traffic
Client ren.cis.udel.edu
Intercepters
www.cnn.com
Internet
TCP port 80 traffic
Client merlot.cis.udel.edu
Proxy

• Majorly motivated by ISP business interests
  reduction in bandwidth consumption of ISP from
  the Internet
• Reduced network traffic
• Reduced user perceived latency

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Consider, On September 11, 2001
New Content WTC News!
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Problems with discussed approachesServer farms
and Caching proxies

• Server farms do nothing about problems due to
  network congestion, or to improve latency issues
  due to the network
• Caching proxies serve only their clients, not all
  users on the Internet
• Content providers (say, Web servers) cannot rely
  on existence and correct implementation of
  caching proxies
• Accounting issues with caching proxies.
• For instance, www.cnn.com needs to know the
  number of hits to the webpage for advertisements
  displayed on the webpage

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Again, On September 11, 2001
New Content WTC News!

• Distribution
• Infrastructure

- Surrogate
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Web replication - CDNs

• Overlay network to distribute content from origin
  servers to users
• Avoids large amounts of same data repeatedly
  traversing potentially congested links on the
  Internet
• Reduces Web server load
• Reduces user perceived latency
• Tries to route around congested networks

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CDN vs. Caching Proxies

• Caches are used by ISPs to reduce bandwidth
  consumption, CDNs are used by content providers
  to improve quality of service to end users
• Caches are reactive, CDNs are proactive
• Caching proxies cater to their users (web
  clients) and not to content providers (web
  servers), CDNs cater to the content providers
  (web servers) and clients
• CDNs give control over the content to the content
  providers, caching proxies do not

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CDN Architecture
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CDN Components

• Content Delivery Infrastructure Delivering
  content to clients from surrogates
• Request Routing Infrastructure Steering or
  directing content request from a client to a
  suitable surrogate
• Distribution Infrastructure Moving or
  replicating content from content source (origin
  server, content provider) to surrogates
• Accounting Infrastructure Logging and reporting
  of distribution and delivery activities

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Server Interaction with CDN
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Client Interaction with CDN
Q How did the CDN choose the Delaware surrogate
over the California surrogate ?
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Request Routing Techniques

• Request routing techniques use a set of metrics
  to direct users to best surrogate
• Proprietary, but underlying techniques known
• DNS based request routing
• Content Modification (URL rewriting)
• Anycast based (how common is anycast?)
• URL based request routing
• Transport layer request routing
• Combination of multiple mechanisms

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DNS based Request-Routing

• Common due to the ubiquity of DNS as a directory
  service
• Specialized DNS server inserted in DNS resolution
  process
• DNS server is capable of returning a different
  set of A, NS or CNAME records based on
  policies/metrics

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DNS based Request-Routing
Q How does the Akamai DNS know which surrogate
is closest ?
Akamai DNS
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DNS based Request-Routing
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DNS based Request Routing Caching
Requesting DNS - 76.43.32.4 Surrogate -
145.155.10.15
www.cnn.com A 145.155.10.15 TTL 10s
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DNS based Request Routing Techniques Discussion

• Originator Problem Client may be far removed
  from client DNS
• Client DNS Masking Problem Virtually all DNS
  servers, except for root DNS servers honor
  requests for recursion
• Q Which DNS server resolves pel.cis.udel.edu?
• Q Which DNS server performs the last
  recursion of the DNS request?
• Hidden Load Factor A DNS resolution may result
  in drastically different load on the selected
  surrogate issue in load balancing requests, and
  predicting load on surrogates

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Server Selection Metrics

• Network Proximity (Surrogate to Client)
• Network hops (traceroute)
• Internet mapping services (NetGeo, IDMaps)
• Surrogate Load
• Number of active TCP connections
• HTTP request arrival rate
• Other OS metrics
• Bandwidth Availability

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Value of a CDN

• Scale Aggregate infrastructure size
• Reach Diversity of content locations (diverse
  placement of surrogates)
• Request routing efficiency, delivery techniques

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Content Distribution Internetworking CDI

• Interconnection of content networks
  collaboration between caching proxies and CDNs,
  as well as between individual CDNs
• Greater reach, larger scale, higher capacity,
  increased fault tolerance
• A new area, lots of challenges
• Basic architecture involves gateways between
  various content networks

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CDI Architecture
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Traditional vs. Overlay Content Networks

• Content networks
• Overlay "Content Layer" to enable richer
  services on top of layer 7 protocols (HTTP, RTSP)
• Information processed at layers 4 through 7 of
  the OSI stack
• Units of transported data in content networks are
  images, movies, songs

• Traditional networks
• Information processed at layers 1 through 3 of
  the OSI stack
• Units of transported data are frames and packets

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In Summary

• Overlays is a concept which can be used to
• deploy new services on the Internet
  (Mbone, 6bone, Abone, Peer-to-Peer,
  Content Networks)
• get around problems in the underlying technology
  (Resilient Overlay Networks)
• Further reading - Overlays
• www.savetz.com/mbone/
• www.6bone.net/
• nms.lcs.mit.edu/projects/ron/
• www-2.cs.cmu.edu/hzhang/VNS/
• Further reading - CDNs
• www.ietf.org/internet-drafts/draft-ietf-cdi-model
  -01.txt
• www.ietf.org/internet-drafts/draft-ietf-cdi-known
  -request-routing-00.txt
• Bunch of papers send me mail if you are
  interested
• Questions? Answers? Thoughts?

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Full-Site vs. Partial-Site Content Delivery

• Full-Site delivery is what we have seen so far
  entire webpage is delivered from the CDN
• Partial-Site delivery delivers only embedded
  objects (say, only images on the webpage) from
  the CDN
• Embedded object redirection can be done using DNS
  based request routing or URL rewriting

• Q How many TCP connections are needed to do a
  P-HTTP transfer of a webpage with embedded
  objects using the above 2 techniques?

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CDN with Full-Site Delivery
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CDN with Partial-Site Delivery
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CDN Types (Skeletal)
CDNs
Hosting CDN
Relaying CDN
Full Site Content Delivery
Partial Site Content Delivery
Request Routing Techniques
URL Rewriting
DNS based
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DNS Outsourcing
CDN
CDN DNS (DNS server maintained by CDN company)
Client ISP
Clients
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Client DNS (Local DNS server for client)
1
4
6
2
3
Content Provider
A or CNAME redirection
Customer DNS (DNS containing NS entry for
customer site)
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Tunneling
v6
v6
v6
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Example Overlays (1) MBone

• IP multicast packets are encapsulated for
  transmission through tunnels
• Tunnel endpoints are typically workstation-class
  machines with OS support for IP multicast and
  running the mrouted multicast routing daemon
• DVMRP routing algorithm used in the overlay