CS219 BitTorrent Talk

8/18/2019 CS219 BitTorrent Talk

1/24

The BitTorrentcontent distribution system

Nikitas Liogkas

CS219 – P2P SystemsUniversity o Ca!iornia" Los #nge!es


2/24

$otivation

!ash cro%d &aka s!ashdot' eect many c!ients" e% servers

Prob!em( servers cannot hand!e !oad

So!ution( s%arming c!ients do%n!oad )ieces o the i!e

rom each other has been )roven to have good sca!ing and

)erormance )ro)erties


3/24

Presentation out!ine

*oining the system

+ncoding , metadata i!e

Tracker )rotoco!

Peer %ire )rotoco!

Piece se!ection

Peer se!ection

C!ient im)!ementations

-esources


4/24

ne% !eecher

*oining a torrent

Peers divided into( seeds( have the entire i!e

leechers( sti!! do%n!oading

data

re.uest

)eer !ist

metadata i!e

/oin

1

2 0

seed,!eecher

%ebsite

tracker

1 obtain the metadata file &out o band'2 contact the tracker 0 obtain a peer list &contains seeds 3 !eechers' contact )eers rom that !ist or data


5/24

4

+5changing data

6 have!eecher #

7

8eriy pieces using hashes7 o%n!oad sub:)ieces &b!ocks' in parallel

7 Advertise received )ieces to the entire )eer !ist

7 interested ( need )ieces that a given )eer has

seed

!eecher B

!eecher C


6/24

Bencoding

encoding ormat o a!! e5changed messages

our ty)es

byte strings integers

!ists

dictionaries &ma))ing keys to va!ues'

e5am)!es (s)am re)resents the string ;s)am<

i1=e re)resents the integer 1=


7/24

$etadata i!e structure

contains inormation necessary to contact the

tracker and describes the i!es in the torrent

announce U-L o tracker i!e name

i!e !ength

)iece !ength &ty)ica!!y 2>?@B'

SA#:1 hashes o )ieces or veriication

and creation date" comment" creator"


8/24

Tracker )rotoco!

communicates %ith c!ients via ATTP,ATTPS c!ient +T re.uest

inoDhash( uni.ue!y identiies the i!e

)eerDid( uni.ue!y identiies the c!ient c!ient 6P and )ort &ty)ica!!y ?EE1:?EE9' num%ant( ho% many )eers to return &deau!ts to >=' stats( bytes u)!oaded" do%n!oaded" !et

tracker +T res)onse interva!( ho% oten to contact the tracker !ist o )eers" containing )eer id" 6P and )ort stats( com)!ete" incom)!ete

tracker:!ess modeF based on the @adem!ia AT


9/24

Presentation out!ine

*oining the system

+ncoding , metadata i!e

Tracker )rotoco!

Peer %ire )rotoco!

Piece se!ection

Peer se!ection


-esources


10/24

Peer %ire )rotoco!

im)!emented on to) o TCP

messages handshake &maybe %ith bitie!d' kee):a!ive choke , unchoke interested , not interested

have &advertisement o a ne%!y:ac.uired )iece' re.uest , )iece cance! &on!y used in ;endgame mode


11/24

Piece se!ection

%hen do%n!oading starts( choose at random get com)!ete )ieces as .uick!y as )ossib!e obtain something to trade

ater %e have )ieces( &!oca!' rarest irst achieves the astest re)!ication o rare )ieces obtain something o va!ue to trade get uni.ue )ieces rom the seed

endgame mode deense against the ;!ast:b!ock )rob!em< send re.uests or missing sub:)ieces to a!!

)eers in our )eer !ist

send cance! messages u)on recei)t o a sub:)iece


12/24

Last:b!ock )rob!em

at the end o the do%n!oad" a )eer may have

troub!e inding the missing )ieces

based on anecdota! evidence

other )ro)osa!s net%ork coding Gkantsidis et al " 6nocomH=>I

)reer to u)!oad to )eers %ith simi!ar i!ecom)!etenessF unair or the )eers %ith most

o the )ieces GTian et al." 6nocomH=?I


13/24

Last:b!ock )rob!em – a mythJ

is it a )rob!em ater a!!J

igure rom GLegout et al., 6N-6#:T-:2==?I" %ith )ermission


14/24

Peer se!ection : unchoking

!eecher #

seed

!eecher B

!eecher C

K ca!cu!ate data:receiving rates

K u)!oad to &unchoke' the astestK rate ca!cu!ation is )erormed )eriodica!!y&a round occurs ty)ica!!y every 1= seconds'

K constant number o unchoking s!ots

K attem)t to achieve Pareto efficiency


15/24

)timistic unchoking

)eriodica!!y se!ect a )eer at random

and u)!oad to it ty)ica!!y )erormed every 0 rounds &0= seconds'

mu!ti:)ur)ose mechanism a!!o% bootstra))ing o ne% c!ients

continuous!y !ook or the astest )artners

kee) the net%ork connectedF every )eer has a

non:Mero chance o interacting %ith any other )eer


16/24

Seed unchoking

o!d a!gorithm unchoke the astest do%n!oaders

)rob!em( astest )eers may mono)o!iMe seeds

ne% a!gorithm )eriodica!!y sort a!! !eechers according to

their !ast unchoke time

)reer the most recent!y unchoked !eechersF

on a tie" )reer the astest

&)resumab!y' achieves e.ua! s)read o seed band%idth


17/24

ne% !istre.uest

)eer !ist

o%n!oading on!y rom seeds

!eecher #

seed

!eecher B

!eecher C

tracker

7 -e)eated!y .uery the tracker or )eer !ists7 istinguish the seeds" and receive data rom them

7 8io!ates airness mode!F may be harmu! to honest )eers


18/24

+va!uation in )rivate torrents

Limit band%idth o !eechers 1 to ?" no !imit on seed $odest airness vio!ation &22 better rate'

%hen se!ish )eer is ast

-obustness does not suer( most honest s!o%er by O1> min

median

ma5

2>i!e

>i!e

22

o%n!oad rates or a!! )eers


19/24


20/24

-ate: vs vo!ume:based se!ection

Pro)onents o rate:based decision metrics(

GCohen" P2P+CNH=0I and

G6N-6# T-H2==?I Pro)onents o vo!ume:based metrics(

GBharambe et al." $S-:T-:2==>I"

Gkantsidis et al " 6nocomH=>I"

G*un et al." P2P+CNH=>I" and

eonkey i!e:sharing system

No c!ear %inner yet4


21/24


main!ine( %ritten in PythonF right no%" the on!yone em)!oying the ne% seed unchoking a!gorithm

#Mureus( the most )o)u!ar" %ritten in *avaF

im)!ements a s)ecia! )rotoco! bet%een c!ients&eg )eers can e5change )eer !ists'

other )o)u!ar c!ients( #BC" BitComet" BitLord"BitTornado" QTorrent" )era bro%ser

various non:standard e5tensions reta!iation mode( detect com)romised,ma!icious )eers anti:snubbing( ignore a )eer %ho ignores us su)er seeding( mas.ueraded seed


22/24

-esources R1

Basic BitTorrent mechanisms

GCohen" P2P+CNH=0I

BitTorrent s)eciication ikihtt)(,,%ikitheoryorg,BitTorrentS)eciication

$easurement studies

G6Ma! et al., P#$H=I"GPou%e!se et al " e!t T- 2== and 6PTPSH=>I"

Guo et al., 6$CH=>I" and

GLegout et al." 6N-6#:T-:2==?I


23/24

-esources R2

Theoretica! ana!ysis and mode!ing

Giu et al." S6C$$H=I" and

GTian et al." 6nocomH=?I Simu!ations

GBharambe et al." $S-:T-:2==>I

6ncentives and e5)!oiting themGShneidman et al." P6NSH=I"

G*un et al." P2P+CNH=>I" and

GLiogkas et al." 6PTPSH=?I


24/24

Conc!usion and ood or thought

BitTorrent is ast and robust

et" many )arameters are arbitrari!y set number o unchoking s!ots round duration siMe o )ieces,sub:)ieces

hat can %e !earn rom BitTorrent orthe design o uture P2P content

distribution )rotoco!sJ

CS219 BitTorrent Talk

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