cs3516-10-p2p - worcester polytechnic institute · v assume a centralized index system that maps...

1

CS 3516: Advanced Computer Networks

Prof. Yanhua Li

Welcome to

Time: 9:00am –9:50am M, T, R, and F Location: Fuller 320

Fall 2017 A-term

Some slides are originally from the course materials of the textbook “Computer Networking: A Top Down Approach”, 7th edition, by

Jim Kurose, Keith Ross, Addison-Wesley March 2016. Copyright 1996-2017 J.F Kurose and K.W. Ross, All Rights Reserved.

Application Layer 2-2

Chapter 2: outline

2.6 P2P applications 1. P2P vs Client&Server 2. Unstructured Peer-to-Peer Networks BitTorrent 3. Structured Peer-to-Peer Networks Distributed Hash Table (DHT)


Client-server vs P2P architecture

client/server

peer-to-peer

4

Peer-to-Peer Networks: v  Unstructured Peer-to-Peer Networks

§  Napster §  Gnutella §  BitTorrent


Chapter 2: outline

2.6 P2P applications 1. P2P vs Client&Server 2. Unstructured Peer-to-Peer Networks § Napster § Gnutella §  BitTorrent

6

Peer-to-Peer Networks: How Did it Start?

v  A killer application: Napster §  Free music over the Internet

v  Key idea: share the content, storage and bandwidth of individual (home) users

Internet

7

Challenges

v  Scale: up to hundred of thousands or millions of machines

v  Dynamicity: machines can come and go any time

v  Each user stores a subset of files v  Each user has access (can download) files from all

users in the system

Model

8

Main Challenge

v  Find where a particular file is stored

A B

C

D

E

F

E?

9

Napster: Example

A B

C

D

E

F

m1 m2

m3

m4

m5

m6

m1 A m2 B m3 C m4 D m5 E m6 F

E? m5

E? E

10

Peer-to-Peer Networks: Napster v  Napster history: the rise

§  January 1999: Napster version 1.0 §  May 1999: company founded §  September 1999: first lawsuits §  2000: 80 million users

v  Napster history: the fall §  Mid 2001: out of business due to lawsuits §  Mid 2001: dozens of P2P alternatives that were harder

to touch, though these have gradually been constrained §  2003: growth of pay services like iTunes

v  Napster history: the resurrection §  2003: Napster reconstituted as a pay service §  2006: still lots of file sharing going on

Shawn Fanning,Northeastern freshman

11

Napster Technology: Directory Service v  User installing the software

§  Download the client program §  Register name, password, local directory, etc.

v  Client contacts Napster (via TCP) §  Provides a list of music files it will share §  … and Napster’s central server updates the directory

v  Client searches on a title or performer §  Napster identifies online clients with the file §  … and provides IP addresses

v  Client requests the file from the chosen supplier §  Supplier transmits the file to the client §  Both client and supplier report status to Napster

12

Napster v  Assume a centralized index system that maps files

(songs) to machines that are alive v  How to find a file (song)

§  Query the index system à return a machine that stores the required file

•  Ideally this is the closest/least-loaded machine §  ftp the file

v  Advantages: §  Simplicity, easy to implement sophisticated search

engines on top of the index system v  Disadvantages:

§  Robustness, scalability

13

Napster Technology: Properties v  Server’s directory continually updated

§  Always know what music is currently available v  Peer-to-peer file transfer

§  No load on the server v  As a protocol

§  Login, search, upload, download, and status operations §  No security: cleartext passwords and other vulnerability

v  Bandwidth issues §  Suppliers ranked by apparent bandwidth & response time

14

Napster: Example

A B

C

D

E

F

m1 m2

m3

m4

m5

m6

m1 A m2 B m3 C m4 D m5 E m6 F

E? m5

E? E

15

Napster: Limitations of Central Directory

v  Single point of failure v  Performance bottleneck v  Copyright infringement

v  So, later P2P systems were more distributed

File transfer is decentralized, but locating content is highly centralized

16

Peer-to-Peer Networks: Gnutella

v  Gnutella history §  2000: J. Frankel &

T. Pepper released Gnutella

§  Soon after: many other clients (e.g., Morpheus, Limewire, Bearshare)

§  2001: protocol enhancements, e.g., “ultrapeers”

v  Query flooding §  Join: contact a few nodes to

become neighbors §  Publish: no need! §  Search: ask neighbors, who

ask their neighbors §  Fetch: get file directly from

another node

17

Gnutella v  Distribute file location v  Idea: flood the request v  How to find a file:

§  Send request to all neighbors §  Neighbors recursively multicast the request §  Eventually a machine that has the file receives

the request, and it sends back the answer v  Advantages:

§  Totally decentralized, highly robust v  Disadvantages:

§  Not scalable; the entire network can be swamped with request (to alleviate this problem, each request has a TTL)

18

Gnutella v  Ad-hoc topology v  Queries are flooded for bounded number of hops v  No guarantees on recall

Query: “xyz”

xyz

xyz

19

Gnutella: Query Flooding

v  Fully distributed §  No central server

v  Public domain protocol

v  Many Gnutella clients implementing protocol

Overlay network: graph v  Edge between peer X and

Y if there’s a TCP connection

v  All active peers and edges is overlay net

20

Gnutella: Protocol

Query QueryHit

Query QueryHit

File transfer: HTTP

Scalability: limited scope flooding

•  Query message sent over existing TCP connections

•  Peers forward Query message

•  QueryHit sent over reverse path

21

Gnutella: Pros and Cons

v  Advantages §  Fully decentralized，

v  Disadvantages §  Search scope may be quite large §  Search time may be quite long §  High overhead and nodes come and go often


Chapter 2: outline

2.6 P2P applications 1. P2P vs Client&Server 2. Unstructured Peer-to-Peer Networks BitTorrent

23

BitTorrent: Simultaneous Downloading

v  Divide large file into many pieces §  Replicate different pieces on different peers §  A peer with a complete piece can trade with other

peers §  Peer can (hopefully) assemble the entire file

v  Allows simultaneous downloading §  Retrieving different parts of the file from different

peers at the same time

24

BitTorrent Components v  Seed

§  Peer with entire file §  Fragmented in pieces

v  Leech §  Peer with an incomplete copy of the file

v  Torrent file §  Passive component §  Stores summaries of the pieces to allow peers to verify

their integrity

v  Tracker §  Allows peers to find each other §  Returns a list of random peers

25

BitTorrent: Overall Architecture

Web page with link to .torrent

A

B

C

Peer

[Leech]

Downloader

“US”

Peer

[Seed]

Peer

[Leech]

Tracker Web Server

.torre

nt

26


Web page with link to .torrent

A

B

C

Peer

[Leech]

Downloader

“US”

Peer

[Seed]

Peer

[Leech]

Tracker Web Server

27


Web page with link

to .torrent

A

B

C

Peer

[Leech]

Downloader

“US”

Peer

[Seed]

Peer

[Leech]

Tracker Web Server

28


Web page with link

to .torrent

A

B

C

Peer

[Leech]

Downloader

“US”

Peer

[Seed]

Peer

[Leech]

Tracker

Shake-hand

Web Server

29


Web page with link

to .torrent

A

B

C

Peer

[Leech]

Downloader

“US”

Peer

[Seed]

Peer

[Leech]

Tracker

pieces

Web Server

30


Web page with link

to .torrent

A

B

C

Peer

[Leech]

Downloader

“US”

Peer

[Seed]

Peer

[Leech]

Tracker

pieces

Web Server

31


Web page with link

to .torrent

A

B

C

Peer

[Leech]

Downloader

“US”

Peer

[Seed]

Peer

[Leech]

Tracker

pieces

Web Server

32

Free-Riding Problem in P2P Networks v  Vast majority of users are free-riders

§  Most share no files and answer no queries §  Others limit # of connections or upload speed

v  A few “peers” essentially act as servers §  A few individuals contributing to the public good §  Making them hubs that basically act as a server

v  BitTorrent prevent free riding §  Allow the fastest peers to download from you §  Occasionally let some free loaders download


BitTorrent: requesting, sending file chunks

requesting chunks: v  at any given time, different

peers have different subsets of file chunks

v  periodically, Alice asks each peer for list of chunks that they have

v  Alice requests missing chunks from peers, rarest first

sending chunks: tit-for-tat v  Alice sends chunks to those

four peers currently sending her chunks at highest rate §  other peers are choked by Alice

(do not receive chunks from her) §  re-evaluate top 4 every10 secs

v  every 30 secs: randomly select another peer, starts sending chunks §  “optimistically unchoke” this peer §  newly chosen peer may join top 4


BitTorrent: tit-for-tat (1) Alice “optimistically unchokes” Bob (2) Alice becomes one of Bob’s top-four providers; Bob reciprocates (3) Bob becomes one of Alice’s top-four providers

higher upload rate: find better trading partners, get file faster !


Chapter 2: summary

v  application architectures §  client-server §  P2P

v  application service requirements: §  reliability, throughput,

delay, security v  Internet transport service

model §  connection-oriented,

reliable: TCP §  unreliable, datagrams: UDP

our study of network apps now complete!

v  specific protocols: §  HTTP §  SMTP, POP, IMAP §  DNS §  P2P: BitTorrent


Quiz 5 on Friday P2P networks

Napster Gnutella BitTorrent

Mid-term next Friday (Tentative)

Questions?


cs3516-10-p2p - worcester polytechnic institute · v assume a centralized index system that maps...

Documents