Overview

1Communication Networks

Kolja Eger, Prof. Dr. U. Killat

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From Packet-level to Flow-level Simulations of P2P Networks

Kolja Eger, Ulrich KillatHamburg University of Technology

ITG-Fachgruppentreffen, Aachen

4. Mai 2006



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Overview• P2P Content Distribution

• Packet-level Simulation

• Flow-level Simulation

• Simulation complexity & accuracy

• Conclusion



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P2P Content Distribution• Objective:

Disseminate large files in minimal time to a large number of users

• Swarming principle:

– A file is fragmented into small pieces which can be shared before download of the whole file is completed

– E.g.: BitTorrent protocol

• Our research interests:

– Efficiency: Peer has something of interest for at least one other peer at any point of time

– Fairness: Peers which contribute much should also gain much ⇒ incentive to contribute



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Complexity of P2P Simulation• P2P networks are complex:

– Large and varying peer populations

– Peer behaviour is user-driven

– Peers provide and consume different services, e.g. exchange different pieces of a file with each other

– Services are offered with different quality, e.g. upload bandwidth

– Each peer has only local information about the network

• Only simple cases can be studied analytically, e.g. flash crowd of homogeneous peers

• Simulations must be based on simplifications



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Packet-level Simulations• Assumptions:

– Access line of the peers is the bottleneck in the network

– No packet drops in the core network

• Simplified topology:

– Access link plus overlay link

– Different RTTs between access routers

– No. of links: Z = (NP -1)NP/2 + NP = NP/2 (NP+1)

– Memory increases quadratically with NP

– No. of events is decreased,

because of small no. of hops



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Peer contacts tracker

Connects to other peers

Inform about pieces

Check interest

Peer selection (Unchoke)

Request pieces

Upload

Event 1

Event 2

Download

Have

Check interestTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xTimer xEvent xBitTorrent MessagesP

acke

t-le

vel

Flo

w-l

evel



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TCP Behaviour• In BitTorrent each peer uploads to a number of other peers (default = 5) (called

unchoking)

• Every 10s peers are chosen based on the download rates from them

Exponential increase

RTT RTT RTT RTT RTT

Cup / (No. of uploads) * RTT

Upload Capacity:

1*10 kbit/s to 30*10 kbit/sRTT

1*10ms to 25*10ms

• If uplink of a peer is the bottleneck, TCP reduces to exponential increase at the beginning

TCP throughput /

max. throughput



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Flow-level Simulation• In peer selection algorithm download volume is computed beforehand

• If remote peer needs less, it is redistributed over the remaining connections

• Thus, peer allocates its upload bandwidth max-min fair

Surplus / 3

Surplus / 2

Volume = (Upload Capacity * unchoking interval) /

(No. of uploads)

Demand Demand

Demand



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Simulation Setup• Flash crowd scenario where a single peer holds the complete file at the

beginning

• Time measured until all peers have finished their download

• No peer leaves the network beforehand

• File size: 10MB, piece size: 256KB

• Homogeneous peers with upload capacity of 10KB/s and download capacity 8 times higher (asymmetric access line)

• Packet-level simulation with ns-2

• Flow-level simulation uses timer functionality of ns-2

• Simulation are run on a Pentium 4: 3,2 GHz, 1 GB RAM



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Simulation Time

• approx. 11h for 60 peers with packet-level compared to 2 sec. with flow-level simulation• Calendar queue is used



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Map

List

Calendar

insert: O(log(n))delete: O(log(n))

Heap

insert: O(log(n)+)delete: O(log(n)+)

insert: O(n)delete: O(1)

insert: O(1) to O(n)delete: O(1) to O(n)

Event Scheduler

No. of peers

Sim

ulat

ion

Tim

e [s

]



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Flow-level Simulation

Memory consumption

No. of Peers

MB

10 000 56 MB

20 000 109 MB

40 000 212 MB

80 000 425 MB

120 000 636 MB

170 000 890 MB

170 000 peers in less than 30min.



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Standard-Upload-Kapazität 10.240 B/s = 10 KiB/s = 80 Kib/s

ADSL 6000 76.800 B/s = 75 KiB/s = 600 Kib/s

ADSL 2000 24.576 B/s = 24 KiB/s = 192 Kib/s

ADSL 1000 16.384 B/s = 16 KiB/s = 128 Kib/s

640 B/s = 0,625 KiB/s = 5 Kib/s

Flow-level Simulation (cont.)• Simulation time for a flash crowd of 4000 peers with different upload

capacitiesHigher capacities result in less events for the same download volume



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Simulation Accuracy• Both curves have the same

shape

• But results differ by around 10%

• Reasons:

– Packet headers

– TCP behaviour

– Load for BitTorrent messages



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Conclusion• Packet-level simulation does not scale for P2P networks

• Flow-level simulation is inevitable to study networks of reasonable size

• Results with flow-level simulator are qualitatively comparable but underestimate the true values due to the simplifications made

• Flow-level simulation is a good compromise to study protocol design

• But inadequate to take cross-layer interactions into account, e.g. unchoking is based on TCP throughput which depends on RTT



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Thank you for your attention!

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