Routing Overview

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Peer-to-Peer Computing CS587x Lecture
Department of Computer Science Iowa State
University
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What to Cover

• Review on some P2P applications
• Napster
• Gnutella
• Freenet
• Discussion and summary

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Resource Sharing

• Questions to answer in order to design a
  resource-sharing network
• How to add new nodes to the network
• How can one node know about others
• How can a node find and retrieve data
• How to manage the shared data

users
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Client/Server Architecture

• Create a server to store the information that
  these nodes want to share
• The server is the only data source
• Clients request data from server
• Example mp3.com
• A client registers to mp3.com and uploads its
  music files to the server
• The songs are then stored and indexed on a server
  that is part of the web site
• Other uses can connect to the web site and
  downloads the songs they are interested in
• Limitation of C/S model
• Scalability is hard to achieve
• Presents a single point of failure
• Requires administration
• Unused resource at the network edge

Central Server

Client-1
Client-n
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Peer-to-Peer Models

• Napster
• Gnutella
• Freenet

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Napster

• Each node registers to napster.com and provides a
  list of its song titles
• The napster server knows the music titles and
  their sites
• The songs themselves are still stored locally
• For a node to download a song,
• the node contacts the server
• The server returns a list of nodes that have the
  song
• The requesting node selects one of the nodes in
  the list to download the file directly from the
  node

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Highlights of Napster

• Main innovation a client downloads a music
  directly from another client, i.e., P2P
  communication
• After a client downloads a music, it can serves
  other clients
• Napster server itself does not have any music
  files
• It acts as a directory or broker
• Advantages
• Each consumer contributes its resource (disk and
  bandwidth) and content to the community
• Contents are more reliable because the same file
  is stored in many nodes, which are geographically
  distributed
• Administration and service cost are minimal
• Drawback
• Napster is a hybrid P2P system since a central
  server is required to coordinate file sharing
• The central server presents a single point of
  failure

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Gnutella

• Creating a Gnutella network
• A node joins the network with a PING to announce
  self
• IP address, port, number/size of shared files
• Receivers forward the Ping to their neighbors
• Receivers back-propagate a PONG to announce self
• Each Pong includes senders IP address,
  number/size of shared files
• Maintaining a Gnutella network
• PING neighbors periodically
• PING Well-known root nodes if starting from
  scratch

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Search Protocol

• For node A to request a file (any kind), it
• creates a query (A, S, N, T), where S is search
  string, N unique request ID, T Time-to-Live
• checks local system, if not found
• Sends (A, S, N, T) to all Gnutella neighbors
• B receives a query (A, S, N, T)
• If B has already received query N or T 0, drops
  the query
• Otherwise, B looks up S locally and sends (N,
  Result) to A if anything found
• Any kind of look up (could simply grep, or
  construct some sql cmd)
• If not found locally,
• B sends (B, S, N, T-1) to all of its Gnutella
  neighbors
• B records the fact that A has made the request N
• When B receives a response of the form (N,
  Result) from one of its neighbors, it forwards
  the response to A

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(No Transcript)
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Gnutella Messages

• PING
• request the transitive closure of connected nodes
  to identify them, essentially asking the question
  "Are you there?
• PONG
• response by a node upon receiving a PING the
  responding node provides its IP address and
  number of sharable files it contains. This gives
  the answer that "Yes, I am here.
• QUERY
• request to locate a set of files matching some
  filter criteria. These are messages stating, "I
  am looking for x".
• HITS
• response to a query giving a list of files
  matching the filter criteria and the IP address
  of the provider, can be many in number.
• GET/PUSH
• request a file provider to contact the requester.
  This provides a simple mechanism trying to get
  through firewalls

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Partial Map of a Gnutella Network
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Highlights of Gnutella

• Pure P2P
• Unlike Napster, Fully decentralized, no single
  point of failure
• Limitations
• Scalability if you send out a request with a TTL
  of 7, and each site contacts six other sites, up
  to 6162636465 6667 messages could be
  exchanged
• Not anonymous since result contains the URL
  string, the source provider can be tracked this
  is addressed in Freenet

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Freenet

• Freenet is a pure P2P system mainly designed to
  support
• distributed information storage and retrieval
• anonymity for producers, consumers and holders of
  information
• adaptive respond to usage patterns
• Freenet differentiates from Gnutella mainly in
• Retrieving data
• Storing data
• Managing data

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Architecture

• Each file is identified by a binary key
• The key is generated using some hash function
• Every file is stored, retrieved, and maintained
  with its file key
• Each node maintains a local data store and a
  routing table
• data store maintains a set of files
• routing table keeps information about neighboring
  nodes and the keys that they are thought to hold
• A sequence of (file key, node address)
• Used for file retrieval

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Retrieving data

• A user first obtains or calculates a key
• The user sends a search request message (keyTTL)
  to local node
• When a node receives a request, it checks its own
  data storage
• If the specified data is found, returns it
• Otherwise, the node looks up its routing table
  and forwards the request to the node that has the
  nearest key
• why do this - the similarity of two keys actually
  has nothing to do with that of their
  corresponding files?
• If this request is successful, the node that has
  the target data
• returns the data through the search path,
• caches the file in its own data store, and
• creates a new entry in its routing table

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Example
Cache file in datastore Create new entry in
routing table
1. Calculate binary file key 2. Check routing
table for node with nearest key
Cache file in datastore Create new entry in
routing table
FOUND
NOT FOUND
NOT FOUND
A
B
1. Check datastore for file
2. Check routing table for node with nearest key
to requested one
3. Try the node with second nearest key
FAILURE
C
D
E
File request (key, hops to live)
Cache file in datastore Create new entry in
routing table
Data reply actual data source
Failure message
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Effect of Retrieving Mechanism

• Anonymity
• Uncontrolled replication allows one to deny
  responsibility of having the file
• Quality of routing improved over time
• Nodes specialize in locating sets of similar keys
• Files with similar keys are stored in clustering
  (why?)
• Files are key-clustering instead of
  subject-clustering
• Transparent replication of popular data
• Improved data availability
• Replication degree depends on data popularity
• Increasing connectivity
• The graph becomes more and more connected

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Effect of Retrieving Mechanism

• Major difference from Gnutella searching
• Breadth-first search vs. Depth-first search
• Replication over the retrieval path
• Limitation
• Searching for a document that does not exist?

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Storing data

• Calculate binary file key and send insert message
  like request (key, hops to live)
• When a node receives an insert proposal, it first
  checks its own data store
• If the key already exists, the users need to try
  again using different key
• Otherwise, the node looks up the nearest key in
  its routing table and forwards the insert to the
  corresponding node
• If key collision occurs at the adjacent node, the
  node notifies the inserted to try another key
• If TTL expires without a key collision, an all
  clear result will be backwarded to the original
  inserter

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Storing data

• Effects of insert mechanism
• New files are placed on nodes possessing files
  with similar keys
• Limitation
• How long it takes to insert a file?
• How about version management?
• Two different files could have the same key and
  both may exist in network
• Different users must have different name space
• The same user must use different file description
  (e.g., keywords) for different file
• Security is a concern

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Managing data

• File replacement is done using LRU
• Data items sorted in decreasing order by time of
  most recent request/insert
• Outdated documents fade away naturally as routing
  table entry will remain for a time
• File lifetime
• The time period of keep a file is unknown
• You cannot delete a file from a Freenet a file
  will not disappear unless it is not accessed for
  a while
• No guarantee that a document you submit today
  will exist tomorrow

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Highlights of Freenet

• Pure P2P - similar to Gnutella,
• Provides anonymity
• Neither data producer and retriever can be
  identified
• Searching/Storing/Managing are all different
• for anonymity and performance purpose

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P2P Advantages

• Efficient use of resources
• Client/Server architecture cannot take advantage
  of the unused bandwidth, storage, processing
  power at the edge of network
• Scalability
• Each user contributes its resource to the entire
  community, instead of just a burden
• Reliability
• Replicas
• Geographic distribution
• No single point of failure
• Ease of Administration
• Nodes self organize
• No need to deploy servers to satisfy demand
• Built-in fault tolerance, replication, and load
  balancing

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P2P Computing Summary

• P2P computing is the sharing of computer
  resources by direct exchange between systems
• Such resource includes information, processing
  cycles, storage, etc.
• A P2P network has the following characteristics
• Each node behaves as client, server, and router
• Nodes are autonomous (no administrative
  authority)
• Network is dynamic nodes enter and leave the
  network frequently
• Nodes collaborate directly with each other (not
  through well-known servers)
• Nodes have widely varying capabilities