cs 414 - spring 2012 cs 414 multimedia systems design lecture 37 introduction to p2p (part 1)...

CS 414 - Spring 2012

CS 414 – Multimedia Systems Design

Lecture 37 – Introduction to P2P (Part 1)

Klara Nahrstedt

Administrative MP3 deadline Saturday April 28, 5pm

Demonstrations of MP3, April 30, 5-7pm Groups should sign up as follows:

5-6pm – 3rd floor 3401 SC for groups that work with laptops and need wireless connectivity for MP3

6-7pm - 216 SC basement for groups who work with PCs and need wired connectivity for MP3

Top four groups will be decided Monday, April 30 at 7pm (via email, also posted on the newsgroup/classwebsite) - these groups will compete in front of the judges on Tuesday, May 1


Administrative Competition of final four groups on

Tuesday 5-7pm in 3401 SC/ 216 SC ByteMobile Inc. company – judging competition (and

TA/Instructor) The top four groups should prepare 3-4 power-point

slides to present Intro Slide – name of your system and your names (1 slide) Surveillance System Design – overall architecture (1 slide) Features of Your System - interface (1 slide) Features of Your System – other features (1 slide)

Administrative Homework 2 is posted

Deadline May 2, Wednesday midnight 11:59pm Peer Evaluations – due Friday, May 4, midnight

Peer Evaluation Form and Explanation - available on the class website

Submit your Peer Evaluation to [email protected] Note: if you do not submit your peer evaluations, you get

0 for self-evaluation and 100% for your group mates. ¼ Unit projects – due Friday, May 4 midnight (if

you need more time, arrange deadline with instructor)


Why P2P?

People like to get togetherShare thingsShare information

How can I find others to share? How can I find what they are sharing?


Why P2P?

WWW to share?Not everyone can set up and maintain a

websiteMight not have a good enough search engine

rank Sharing on a social network?

Only possible if you created the contentPeople have to find you, instead of the

contentCS 414 - Spring 2012

Napster

Created in 1999 to share music


Napster’s servers• Store file directory

(Filename/location)

Clients/peers• Store files

Centralized Peer Management: Each Peer registers withthe Napster server and Napster server(s) keep Peer List

Centralized File Management:Searching for other peer file is central, i.e., Peers go to Napster servers which keep file list of files and which peer keeps what files

Napster- Search Operation


1. Query (keyword)

2. Servers search using ternary tree

3. Response

4. Ping candidates5. Download

Napster’s drawbacks

Asymmetric operation – servers vs. client peersCentralized Peer Management and File

ManagementScalability and congestion issuesSingle point of failure

Napster responsible for abetting users’ copyright violation


Contemporary P2P systems Symmetric operation – all nodes (peers)

have the same functionalitySystem naturally scales with new peers

Distributed Peer Management Each peer when it comes online, advertises its own presence Each peer keeps a peer list to whom it is connected

Distributed File Management Each peer keeps file list of files it keeps. If a peer wants a file, it queries/searches for a file.


Contemporary P2P systems

Basic operations Insert fileSearch fileDelete fileMaintain an overlay network


physical network

Overlay P2P network

Contemporary P2P Systems (Classification) Usually classified depending on how peers

connect to each-other how the overlay network is created and maintained

Unstructured – no particular connection pattern (e.g., randomly connected) Gnutella Fast Track

Skype BitTorrent, etc.


Contemporary P2P Systems (Classification) Structured – defines a distributed data

structure (e.g., distributed hash table)ChordPastryCANEtc.


Gnutella – Example of Unstructured P2P


Servents (peers)

Peer pointer

Peers store:• Their files• Peer pointers (peer management)

Gnutella, searching for files


Query message: <id, QUERY, ttl, hops, payload length, min speed, keywords>Query hit message: <id, QUERY HIT, ttl, hops, payload length, num hits, port, ip, speed, (fileindex, filename, filesize), servent id>

“jazz”??

1. Flood query ( )2. Ignore repeated

messages3. Answer if local

match 4. Query hit sent

using reverse path ( )

5. Establish connection and fetch file ( )

“jazz”

Gnutella, maintaining overlay (peer management)


VX

A

Ping: <id, PING, ttl, hops, payload length (zero)>Pong: <id, PONG, ttl, hops, payload length, port, ip, num. files, num. KBs>

Neighbor list:•“A”

“V”

“X”

1. Periodically flood ping ( )

2. Pong sent using reverse path( )

3. Update neighbor list with received pongs

• Why periodically?

Gnutella, maintaining the overlay (peer management)


VX

Peers can leave or fail at any time – P2P systems can have high churn rate!.

Neighbor list:•“A”•“V”•“X”

Gnutella: some issues

Ping/Pong constitutes 50% of traffic Flooding causes excessive traffic Repeated searches with same keywords Large number of freeloaders (70% of

users in 2000)


DHTs (Distributed Hash Tables) – Example of Structured P2P Hash table allows these operations on

object identified by key: InsertLookup Delete

Distributed Hash Table – same but in a distributed setting (object could be files)


DHT performance comparisonMemory Lookup latency lookup overhead

Napster O(1) at client; O(N) at server

O(1) O(1)

Gnutella O(N) O(N) O(N)Chord (DHT) O(log(N)) O(log(N)) O(log(N))


Conclusion

P2P technology – very viable technology for multimedia distribution


cs 414 - spring 2012 cs 414 multimedia systems design lecture 37 introduction to p2p (part 1)...

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