Unit VI Overlays
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Unit VI Overlays






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  • Tunneling is a technique which can be used for transporting new — not yet universally supported — protocols or services, such as IPv6 or multicasting, over the Internet. The unsupported packets are "wrapped" in standard IP packets and transported across the Internet between routers that do support the protocol
  • Hosting CDNs or Relaying CDNs that use URL rewriting do not have the Client DNS masking problem since the domain of the CDN (a1055.g.akamai.com) is within the CDN’s control. The DNS server for the CDN can refuse recursion. Typically, a domain name such as akamai.worldnet.att.com will not be chosen.
  • Tunneling is a technique which can be used for transporting new — not yet universally supported — protocols or services, such as IPv6 or multicasting, over the Internet. The unsupported packets are "wrapped" in standard IP packets and transported across the Internet between routers that do support the protocol
  • Tunneling is a technique which can be used for transporting new — not yet universally supported — protocols or services, such as IPv6 or multicasting, over the Internet. The unsupported packets are "wrapped" in standard IP packets and transported across the Internet between routers that do support the protocol

Unit VI Overlays Unit VI Overlays Presentation Transcript

  • Overlay Networks (with a focus on Content Distribution Networks)
  • What is an Overlay ? What is the topology of this network? WHICH network?? Figure borrowed from www.isi.edu/xbone
  • 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
  • Overlay Networks: Examples
    • MBone, 6Bone, ABone
    • RON, VNS
    • P2P (Napster, FreeNet, Gnutella)
    • Content Networks
      • Cooperating Caches
      • Server Farms
      • Content Distribution Networks (CDNs)
  • 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
    Mbone node Mbone node Mbone node Internet router Internet router Internet router Internet router
  • 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
  • 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 ?
  • Motivations for Content Networks
    • More hops between client and Web server => more congestion!
    • Same data flowing repeatedly over links between clients and Web server
    S C1 C4 C2 C3 - IP router
  • 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)
  • Example content networks: Server Farms
    • Arbitrate client requests to servers using an “intelligent” L4-L7 switch
    • Pretty widely used today
    • Simple solution to the content distribution problem:
    • deploy a large group of servers
    L4-L7 Switch Request from grad.umd.edu Request from ren.cis.udel.edu Request from ren.cis.udel.edu Request from grad.umd.edu www.cnn.com (Copy 1) www.cnn.com (Copy 3) www.cnn.com (Copy 2)
  • Example content networks: C aching Proxies Client ren.cis.udel.edu Client merlot.cis.udel.edu Intercepters Proxy www.cnn.com Internet TCP port 80 traffic Other traffic ISP
    • Majorly motivated by ISP business interests – reduction in bandwidth consumption of ISP from the Internet
    • Reduced network traffic
    • Reduced user perceived latency
  • Consider, On September 11, 2001 New Content WTC News! Web Server www.cnn.com User merlot.cis.udel.edu 1000,000 other hosts 1000,000 other hosts old content request request - Caching Proxy ISP
    • Congestion /
    • Bottleneck
  • Problems with discussed approaches: Server 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
  • Again, On September 11, 2001 New Content WTC News! - Surrogate
    • Distribution
    • Infrastructure
    Web Server www.cnn.com User merlot.cis.udel.edu request new content 1000,000 other users 1000,000 other users FL IL DE NY MA MI CA WA
  • 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
  • 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
  • CDN Architecture Surrogate Surrogate Request Routing Infrastructure Distribution and Accounting Infrastructure CDN Origin Server Client Client
  • 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
  • Server Interaction with CDN Distribution Infrastructure 1
    • Origin server pushes new content to CDN
    • OR
    • CDN pulls content from origin server
    Accounting Infrastructure 2 2. Origin server requests logs and other accounting info from CDN OR CDN provides logs and other accounting info to origin server CDN Origin Server www.cnn.com
  • Client Interaction with CDN Q: How did the CDN choose the Delaware surrogate over the California surrogate ? Request Routing Infrastructure 1 1. Hi! I need www.cnn.com/sept11 2
    • Go to surrogate
      • delaware.cnn.akamai.com
    3 3. Hi! I need content /sept11 Client Surrogate (DE) Surrogate (CA) CDN california.cnn.akamai.com delaware.cnn.akamai.com
  • 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
  • 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
  • DNS based Request-Routing Akamai DNS Q: How does the Akamai DNS know which surrogate is closest ? DNS query: www.cnn.com DNS response: A Session local DNS server (louie.udel.edu) DNS query: www.cnn.com DNS response: A www.cnn.com Surrogate Surrogate Akamai CDN merlot.cis.udel.edu delaware.cnn.akamai.com california.cnn.akamai.com
  • DNS based Request-Routing DNS query DNS response Session Akamai DNS www.cnn.com Surrogate Surrogate Akamai CDN merlot.cis.udel.edu local DNS server ( louie.udel.edu) DNS query DNS response Measure to Client DNS Measure to Client DNS Measurement results Measurement results Measurements Measurements
  • DNS based Request Routing: Caching Requesting DNS - Surrogate - www.cnn.com A TTL = 10s www.cnn.com Client DNS Surrogate Surrogate Akamai DNS Akamai CDN Client Requesting DNS - Available Bandwidth = 10 kbps RTT = 10 ms Requesting DNS - Available Bandwidth = 5 kbps RTT = 100 ms
  • 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
  • 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
  • Value of a CDN
    • Scale: Aggregate infrastructure size
    • Reach: Diversity of content locations (diverse placement of surrogates)
    • Request routing efficiency, delivery techniques
  • 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
  • CDI: Architecture CDN1 CDN2 CDN3 CN4 For instance,cache network of some ISPx - Content Peering Gateway
  • Traditional vs. Overlay Content Networks
    • Traditional networks
    • Information processed at layers 1 through 3 of the OSI stack
    • Units of transported data are frames and packets
    • 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
  • 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?
  • 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?
  • CDN with Full-Site Delivery Surrogate Server CDN Origin Server Client GET index.html GET image1.gif, image2.gif index.html, image1.gif, image2.gif index.html embedded image1.gif image2.gif
  • CDN with Partial-Site Delivery Origin Server Surrogate Server CDN Client GET index.html GET image1.gif, image2.gif image1.gif, image2.gif index.html embedded image1.gif image2.gif
  • CDN Types (Skeletal) CDNs Hosting CDN Relaying CDN Partial Site Content Delivery Full Site Content Delivery URL Rewriting DNS based Request Routing Techniques
  • DNS Outsourcing Customer DNS (DNS containing NS entry for customer site) Content Provider CDN DNS (DNS server maintained by CDN company) CDN Client DNS (Local DNS server for client) Client ISP Clients 1 6 2 3 5 4 A or CNAME redirection
  • Tunneling v6 v6 v6 v4 v4 v4 v4 v4 IP proto = 6 (TCP) IP proto = 6 (TCP) IP proto = 41 (IPv6) v6 header v4 header IPv6-SDU IPv6-SDU v6 header
  • 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