CDA 6506 Network Architecture and ClientServer Computing

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CDA 6506 Network Architecture and Client/Server
Computing
Peer-to-Peer Computing and Content
Distribution Networks by Zornitza Genova
Prodanoff
09/13/04
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Lecture Outline

• Client/Server Model
• Peer-to-Peer Model
• Content Distribution Networks

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Client/Server Communication

• The client (process)
• Runs on a desktop
• Makes requests for services
• Executes a portion of the application code
• The server (process)
• Executes a set of functionally related services
• Never initiates a message exchange with a client
• Implements a queue to hold client requests as
  they arrive
• Request-reply protocol client sends requests and
  server replies
• Queue of client requests at the server
• Server throughput is the number of requests
  served per unit time

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Client/Server Communication (continued)

• Server Types
• File servers
• Database servers
• Application servers
• Groupware servers
• Object servers
• Web servers
• Software servers

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C/S Example Hyper Text Transfer Protocol (HTTP)

• Hypertext Markup Language (HTML)
• Integrate images and other multimedia objects, as
  well as software applications
• Application layer protocol
• Runs on top of TCP/IP

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C/S Example Hyper Text Transfer Protocol (HTTP)

• HTTP 1.0 vs. HTTP 1.1
• HTTP 1.0
• Allows for embedded documents in the HTML file
• The client browser opens a separate connection
  with the server for each requested file html,
  image, etc.
• Client does not need to wait for connection
  termination before another connection request is
  sent
• Images are downloaded transparent to user HTTP
  response, containing the URL for the image is
  parsed before a new connection request is made

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C/S Example Hyper Text Transfer Protocol (HTTP)

• HTTP 1.1
• Allows for embedded documents in the HTML file
• Persistent connections The client browser
  opens a (single) connection with the server for
  the html file. Then, embedded documents are
  requested within this same TCP connection
• Pipelined requests multiple requests are sent
  without waiting for response (ACK)
• Pipelined HTTP 1.1 outperforms parallel
  connection based HTTP 1.0

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Peer-to-Peer Model

• Problems with C/S model
• Server seen as a central node
• Scalability
• Reliability
• Peer-to-Peer model a collaborative and
  distributed approach to computing
• A study conducted at a major company with 10,000
  workstations
• 75 of CPU cycles in the workstations or network
  nodes such as routers are idle
• 50 of disk space is free

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Peer-to-Peer Model (continued)

• 10 billion MHz 10,000 TB not utilized at the
  edges of the Internet (see openP2P.com)
• The size of the networks and the
  complexity/requirements from the protocols
  steadily increase
• Bandwidth consumption attributed to popular file
  sharing applications reaches 60 of the total
  Internet traffic
• Must be able to locate the resources efficiently

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Peer-to-Peer Model (continued)

• Peer-to-Peer model sharing of resources
  available at the edges of the Internet
• In a P2P system, distributed computing nodes of
  equal roles or capabilities exchange information
  directly with each other
• Resources could be content, storage, CPU-cycles,
  bandwidth
• P2P paradigm has many plausible characteristics
• Scalability
• No centralized authority,
• Robustness
• Cooperation, sharing
• Anonymity

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Peer-to-Peer Model (continued)

• Routing Indexes
• No-indexing (Gnutella)
• Centralized index (Napster)
• Distributed index (FreeNet)

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Peer-to-Peer Model (continued)

• Advantages of Peer-to-Peer model
• Performance gain with much less cost
• Self-organization
• Load balancing
• Adaptation
• Fault-tolerance
• Mobile devices

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Peer-to-Peer Model (continued)

• Open problems
• Locating and addressing
• Caching of passing through objects
• How to identify an Object
• Network usage and participation

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Peer-to-Peer Model (continued)

• Queries propagate across nodes, each node does
  some processing and uses resources
• Bandwidth The average over a set of
  representative queries of the aggregate BW
  consumed (in bytes) over each edge on behalf of
  the query
• Processing The average over a set of
  representative queries of the aggregate
  processing power consumed at each node on behalf
  of the query

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Peer-to-Peer Model (continued)

• Gnutella BFS technique is used with depth limit
  of D, where D TTL of the message. At all levels
  lt D query is processed by each node and results
  are sent to source and at level D query is
  dropped.
• Freenet uses DFS with depth limit D. Each node
  forwards the query to a single neighbor and waits
  for a definite response from the neighbor before
  forwarding the query to another neighbor (if the
  query was not satisfied), or forwarding the
  results back to the query source (if query was
  satisfied).

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Content Distribution Networks

• Next generation Internet - Content Distribution
  Networks
• A CDN is an overlay network on the Internet
• A CDN co-locates content throughout the world
• CDNs are of a great commercial and research
  interest
• 15 million in NSF funding for Web services
  research
• Akamai is one major CDN provider

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Content Distribution Networks
Global content distribution in a CDN
http//214.29.2.15/page
http//www.some.com/page
http//334.249.2.8/page
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Content Distribution Networks

• HTTP redirection in a CDN
• (1) HTTP request and redirect
• (2) HTTP re-request and response

Reverse cache
Origin site
Proxy cache
Clients
Distributed server
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Content Distribution Networks

• Caching
• reduces both Internet traffic and server load
• implemented
• local to the client
• as a caching hierarchy in the Internet
• at the server site
• content information best shared as compressed
  digests
• update interval - seconds to minutes in length

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Content Distribution Networks

• Selection of the best server with
• satisfactory response time to the user and
• minimized network traffic.
• (Selection criteria server load, server
  contents, and network path characteristics)

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Content Distribution Networks

• Scaling from local to distributed cluster
• internet connections constrained by bandwidth and
  delay
• state information shared with less frequency
• state information smaller in size
• herd effect
• chose least-loaded server
• unbalanced and oscillating loads.

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Content Distribution Networks
Architecture of a new URL router
One armed URL router
HTTP requests and redirects
Network links
Layer 3 switch
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