Can Network Coding Help in P2P Networks?Dah Ming Chiu, Raymond W Yeung, Jiaqing Huang and Bin Fan

Chinese University of Hong Kong

Presented by Arjumand Younus

Outline

•Introduction to Network Coding – Exploring the Basics

•Benefits of Network Coding•Motivation for this Study – Network

Coding in P2P Networks•Paper Contribution•Investigating Throughput with Network

Coding in a P2P Model•Conclusion

Network Coding – An Introduction•New research area with interesting

applications in practical networks.

•Mixing of data at intermediate network nodes.

•Performing arbitrary mappings on contents of packets rather than restricted functions of replication and forwarding.

Network Coding Basics

•Assumption behind traditional network traffic▫Information is separate, although it may

share network resources. (say, cars in highways or fluids in pipes).

▫Network coding breaks this assumption.•A technology to combine several data

packets into one or several output packets.

An Illustration to Explain Network Coding

S

T U

W

Y Z

a b

a

aa

b

bb

a b

a

a

b

ba or b?

S

T U

W

Y Z

X

S

T U

W

Y Z

X

a b

a

a

b

ba+b

a+b a+b

How to send 2 pieces of data a and b to nodes Y and Z simultaneously?

Benefits of Network Coding

•Throughput

•Wireless Resources

•Security

Motivation – What about Network Coding in P2P networks•Scalable content distribution – employs P2P

instead of network multicast.▫e.g. BitTorrent

•Recent Question/Debate [12] in Research Community:

▫Is network coding really feasible for P2P Networks?

▫Debate stirred up after well publicized study Avalanche[11] by Microsoft Research

Network Coding in P2P Systems – A Brief Look•Without a global coordinated scheduler•Node B, receiving Packet 1 or 2 from

Node A?

Paper Contribution

•Studying benefits of network coding in P2P systems using star network model.

•Previous Work▫Previous studies have been conducted but

this assumes coding at peers and no multicast in network.

▫Much closer to a P2P system.

Star Network Model• Uplink sharing model (only uplinks can be

bottlenecks).• Peer cannot multicast to multiple other peers at

the same time.

Obtainable Throughput with Routing

•The main constraints to throughput are downlink of the server and the uplinks of the peers [13]:

•Max Throughput = min {C0, (C0+∑jC j)/n}

Impact of Network Coding on Throughput in P2P File Sharing– Case 1•Throughput = C0

•C0 is the minimum cut for each peer so network coding cannot help in improving this bound.

Impact of Network Coding on Throughput in P2P File Sharing– Case 2 •When C0≥C/n-1

▫For throughput of X peer must receive content at rate of Y≥X from the server or other peers.

▫Network does not do multicasting but only forwarding.

▫Total capacity to satisfy all peers is C0+C < nC0 so split amongst n peers so maximum throughput is (C0+C)/n.

Conclusion - 1

•We demonstrated that the maximum achievable throughput by a P2P system in a network without multicasting is exactly the same with or without any form of network coding.

•P2P network with Network Coding could be even worse than BitTorrent network

Conclusion - 2•There are several encountering difficulties for

Network Coding in P2P networks:▫One peer may need to spend a huge amount of

time on decoding data they receive▫The resources (CPU, etc. ) one needs to spend on

decoding▫How to ensure the uniqueness coefficients when

there are a lot of file pieces in the transferred file▫The topology of a P2P network is changing▫Peers may depart the network any time they want

However, Positive Side

•To achieve maximum throughput w/o network coding requires very careful scheduling of other peers exchange content.

•With network coding, this scheduling task might be much relaxed.

Thank You

•Questions?