Application Layer – Peer-to-peer - PowerPoint Presentation

Slide1

Application Layer – Peer-to-peer

UIUC CS438: Communication NetworksSummer 2014Fred DouglasSlides: Fred, Kurose&Ross (sometimes edited)

Slide2

A Step Back

Today, everything is megacorps: Google etc.

You

Slide3

A Step Back

What, fundamentally, does the internet do?“Internet”

“End host”

“End host”

mobile network

global ISP

regional ISP

home

network

institutional

network

Slide4

“Pure”

P2P

architecture

no

always-on server

arbitrary end systems directly communicate

peers are intermittently connected and change IP addresses

examples:

F

ile distribution (

BitTorrent

)

Can

be pure

File distribution (

Freenet

)

VoIP (Skype)

Registration + authentication

Many games

Lobby, matchmaking

Slide5

Motivation: File distribution

Question: how much time to distribute file (size F) from one server to

N peers

?

…the traditional way (e.g. from an HTTP server)

…if peers also upload

to each other

u

s

u

N

d

N

server

network (with abundant

bandwidth)

file, size F

u

s

:

server upload capacity

u

i

:

peer i upload capacity

d

i

:

peer i download capacity

u

2

d

2

u

1

d

1

d

i

u

i

Slide6

Client-server vs. P2P: example

Slide7

P2P file distribution: BitTorrent

tracker:

tracks peers

participating in torrent

torrent:

group of peers exchanging chunks of a file

file divided into

fixed size chunks

[list of peers]

Slide8

peer joining torrent:

Starts with no chunks; can only downloadCan pass on any chunk it getsEventually completes the file, andBecomes a “seeder”, orLeaves

During download

P2P file distribution: BitTorrent

while downloading, peer uploads chunks to other peers

peer may change peers with whom it exchanges chunks

churn:

peers may come and go

once peer has entire file, it may (selfishly) leave or (altruistically) remain in torrent

Slide9

BitTorrent: requesting, sending file chunks

requesting chunks:at any given time, different peers have different subsets of file chunksperiodically, Alice asks each peer for list of chunks that they haveAlice requests missing chunks from peers, rarest first

sending chunks: tit-for-tat

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

every 30 secs: randomly select another peer, starts sending chunks

“

optimistically unchoke

”

this peer

newly chosen peer may join top 4

Slide10

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 !

Slide11

Distribute (key, value) pairs over millions of peers

pairs are evenly distributed over peersAny peer can query database with a keydatabase returns value for the keyTo resolve query, small number of messages exchanged among peers

Each peer only knows about a small number of other peers

Robust to peers coming and going (churn)

Distributed Hash Table (DHT)

Slide12

Assign key-value pairs to peers

rule: assign key-value pair to the peer that has the closest ID.convention: closest is the immediate successor

of the key.

e.g., ID space {0,1,2,3,…,63}

suppose 8 peers: 1,12,13,25,32,40,48,60

If key = 51, then assigned to peer 60

If key = 60, then assigned to peer 60

If key = 61, then assigned to peer 1

Slide13

1

12

13

25

32

40

48

60

Silly

Strawman

Circular DHT

each peer

only

aware of

immediate successor and

predecessor.

(Note: circular DHTs aren’t the

only way; e.g.

Kademlia

)

“

overlay network

”

Slide14

1

12

13

25

32

40

48

60

What is the value

associated with key 53 ?

value

O(N)

messages

on avgerage to resolve

query, when there

are

N

peers

Resolving a query

Slide15

Circular DHT with shortcuts (Chord)

each peer keeps track of IP addresses of predecessor, successor, short cuts.reduced from 6 to 3 messages.

possible to design shortcuts with

O(log N)

neighbors,

O(log N)

messages in query

1

12

13

25

32

40

48

60

What is the value for

key 53

value

Slide16

Chord

Me

Finger

Finger

Finger

Finger

Finger

Slide17

Peer churn

example: peer 5 abruptly leaves

1

3

4

5

8

10

12

15

handling peer churn:

peers may come and go (churn)

each peer knows address of its two successors

each peer periodically pings its

two successors to check aliveness

if immediate successor leaves, choose next successor as new immediate successor

Slide18

Peer churn

example: peer 5 abruptly leavespeer 4 detects peer 5’s departure; makes 8 its immediate successor

4 asks 8 who its immediate successor is; makes 8

’

s immediate successor its second successor.

1

3

4

8

10

12

15

handling peer churn:

peers may come and go (churn)

each peer knows address of its two successors

each peer periodically pings its

two successors to check aliveness

if immediate successor leaves, choose next successor as new immediate successor

Slide19

BitTorrent as true P2P

Traditional BitTorrentGet peers from trackerTrackers identified in .torrent file.torrent file hosted on a centralized torrent search engine site

Fully P2P

BitTorrent

Get peers from DHT, and direct exchange

Magnet link provides file hash

Look the hash up in DHT

Magnet links: just text

Search engines, forums, anonymous message boards, …

Slide20

P2P Motivation – Revisited

Client-server dominates the mainstream. Why?PerformanceEconomies of scaleRound trip times(Mass file distribution is a rare exception)So, why peer-to-peer?Avoid single points of failureTechnological…and social

 Robustness, Survivability

OR

 Power to the people

Slide21

Optional

Slide22

Freenet

Pure P2P document storeAlso forums implemented on topGoalsRobustnessNode failuresCensorshipAnonymity

Slide23

Freenet Design

Two components: storage, lookupStorageAll nodes provide ~10+ GB storage to the networkAll content is encrypted: not visible to storerStorer never has any idea what it’s storingLookupKey-based routing, like a DHTLookup path construction: always take the hop closest to the target valueData retrieval and insertion are both “lookups”

Data retrieval

Slide24

Freenet Lookup

First, probe for a pathReceive (or send) data once path is builtNodes relaying the data can cache itData lookup  caching means popular items are replicated, unpopular items forgotten

Slide25

Freenet Topology

Original designStart with connections to some bootstrap nodesRandomly add nodes in lookup paths to neighborsDarknetOnly connect to nodes whose identity the user knows and trustsRelies on social network dynamics to give the necessary fast mixing / “small world” property