BitHoc: BitTorrent for wireless ad hoc networks

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BitHoc: BitTorrent for wireless ad hoc networks

Jointly with:Chadi BarakatJayeoung Choi

Anwar Al HamraThierry Turletti

EPI PLANETE

28/02/2008 MAESTRO/PLANETE INFORMEL SEMINARY

Mohamed Karim Sbaï

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Wireless ad hoc networks

No central administration No base station Nodes are both hosts and routers Need for multi-hop routing approaches The connectivity is ensured thanks to

the collaboration of nodes in forwarding and routing messages of each others

It is the P2P paradigm in the network layer.

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P2P file sharing applications

No central server Users are both clients and servers :

They download content and share some of their upload capacity to serve other users.

The global capacity of the system grows thanks to their collaboration.

This is the P2P paradigm in the overlay.

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P2P file sharing in wireless ad hoc

networks : constraints

Routing overhead Transport protocols performance drops

seriously in wireless multi-hop paths P2P file sharing solutions are topology

unaware : Overlay neighbors are selected independently of their locations.

How will such applications perform in wireless ad hoc networks ? What are the modifications (namely in overlay construction) to be made to ameliorate both download time and sharing ratios ?

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Outline Overview of BitTorrent Framework, scenario and assumptions Performance metrics Impact of the size of pieces Impact of the scope of the

neighborhood BitHoc: Our solution

Choking algorithm Piece selection strategy Some first results Choice of parameters and new

results. Conclusions and future work

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BitTorrent (1/2) a P2P efficient content distribution protocol Peer lookup:

A Client contacts a central rendezvous node named Tracker to get addresses of other clients

Sharing content: A file is cut into pieces (multi-sourcing) Each client shares some of its upload

bandwidth with other clients Torrent : peers cooperating to download

the same content Seed : a peer who finished downloading the

content and still serving other peers leecher : peer who is downloading the file

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BitTorrent (2/2) Choking algorithm: A peer chooses periodically a

set of peers with whom it opens TCP connections to upload pieces.

Only 4 simultaneous connections: 3 best uploaders + 1 a random peer (discovering

new upload capacities )

Enforce cooperation and reciprocity among peers

Piece selection strategy: When a peer receives a piece offer message from a neighbor, it selects the rarest piece in the peer list.

This increases the entropy of pieces in the network and then the chance to replicate pieces after downloading them.

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Framework, scenario and assumptions

Assumptions All nodes are peers No mobility in the network Trackerless BitTorrent : a flooding of

HELLO messages to discover all peers in the network.

Framework We added a module to NS-2

implementing BitTorrent functionalities.

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Framework, scenario and assumptions

Ad hoc scenario A grid topology of N nodes (10

per row) (distance = 40 m between nodes)

No mobility of nodes DSDV proactive routing protocol 802.11 MAC Layer with RTS/CTS-

DATA/ACK Data rate = 1 MB/s, range = 50 m

BitTorrent Scenario 1 file = 10 Mbit 1 original seed Choking period = 40 s Flooding TTL = variable Number of pieces = variable, number of blocks =

variable Block size = 1 KB

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Performance metrics of BitTorrent in ad hoc

networks

Rij : sharing ratio between i and j

Ri: sharing ratio of node i Ri = average on j of Rij ( Uij<>0 or Dij<>0)

Rh: average on nodes i located at h hops from the original seed of Ri

Fi : finish time of peer i. Fh: average on nodes i located at h hops

from the original seed of Fi

),max(

),min(

ijij

ijijij DU

DUR

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Outline Overview of BitTorrent Performance metrics Framework, scenario and assumptions Impact of the size of pieces Impact of the scope of the




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Impact of the size of pieces

Flooding TTL = MAX Small piece size = 100 blocks

Big piece size = 1000 blocks Finish time as a function of number of hops to the seed (Fh):

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Average sharing ratio as a function of number of hops to the seed (Rh):

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Average sharing ratio per couple of nodes for big size of pieces

Average sharing ratio per couple of nodes for small size of pieces

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Impact of the scope of the neighborhood

Flooding TTL = variable piece size = 100 blocks = small Finish time as a function of number of hops to the seed (Fh) for different TTLs:

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Impact of the scope of the neighborhood


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BitHoc: Our solution

Reduced neighborhood + a few connections to far peers good TCP performance in near neighborhood and more entropy of pieces for better sharing.

Peers are classified into 2 tables :NNT : Nearby Neighbors Table (hops <=2)FNT : Far Neighbors Table (hops > 2)

We redefine the choking algorithm and the piece selection strategy

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BitHoc: Choking algorithm

3 best uploaders from NNT and FNT

1 randomly: q times from NNT and 1 time from FNT- uniformly from NNT- probability p to choose a node at h hops in FNT (hm : max hops)

if then else 0

)1()(

mm hh

hhp1 mhh

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BitHoc: Piece selection strategy

Offer from NNT Rarest first in near neighborhood

Offer from FNT Absent piece strategy = accept only pieces that do not exist in near neighborhood

PIECE UPDATE messages need only to be sent to peers in NNT.

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BitHoc: Some first results

Finish time as a function of number of hops to the seed (Fh):

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BitHoc: Some first results


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BitHoc: Static choice of q

We establish an empiric formula for q:q = number of slots near / number of slots farGoal: send a copy of each piece to the end of the network

and wait for it to return to the middle of the network.

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Nhq mhm

αi : average number of pieces sent at i hops in a choking slot.

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Finding optimal q by simulations:

q = 1 for 40 nodes

q = 3 for 80 nodes

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Validation of the empiric formula:

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BitHoc: dynamic choice of q

Topology, network conditions, etc can change each node adapts its q dynamically following its observations.

212 m

m

hhm

d Nh

q

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Comparing static to dynamic choice of q:

average finish time

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Comparing static to dynamic choice of q:

average sharing ratio

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Conclusions and future work

BitHoc gives better finish times and better sharing ratios than other solutions even those limiting the scope of the neighborhood BitHoc finds a good management of neighbor and piece selection strategies

Good performance in near neighborhood Better entropy by adding some few connections to far nodes Only absent pieces are sent to far nodes

Future work: Impact of mobility of nodes Studying the case of sparse overlays Optimizing lookup in wireless ad hoc networks Real implementation and testing of the protocol

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THANK YOU

E-mail: [email protected]

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BitHoc: BitTorrent for wireless ad hoc networks