Content Distribution in Unstructured Peer-to-Peer Networks

Daniel Stutzbach

Committee Members: • Professor Reza Rejaie• Professor Ginnie Lo• Professor Art Farley

Why Peer-to-Peer?

Introduction

Why Study Peer-to-Peer? Peers on the edge band together to

share resources. Peer-to-peer can self-scale. Peer-to-peer applications are becoming

increasingly popular. File Sharing: Kazaa, Gnutella Bandwidth Sharing: BitTorrent Cycle Sharing: SETI@Home UW found that file-sharing uses 3 times as

much bandwidth as the Web.

Introduction

Challenges in Peer-to-Peer Peer-to-Peer is more complicated. Discovering and managing resources is

more difficult because: The resources are distributed. Peers are not under the control of one authority. Peers are unstable. Peers have heterogeneous resources to provide.

It’s harder to measure a deployed system.

Introduction

Research on Peer-to-Peer Characterizing deployed systems:

Provides insight into how P2P systems behave in the real world.

Is needed to develop accurate models for simulation.

Design of new techniques to leverage peer-to-peer resources: Overlay construction Search mechanisms Resource allocation

Introduction

Topics Covered Peer-to-Peer Search

Keyword search for matching filenames Example implementations: Gnutella, Kazaa, eDonkey 2000 Example topologies: ad-hoc mesh, ultrapeers Searching for files to transfer

Peer-to-Peer Transfer Spread parts of files quickly to scale and alleviate flash-crowds Example implementations: eDonkey 2000, BitTorrent, Slurpie Example topologies: ad-hoc mesh, a forest of trees Searching for peers with the right blocks or more bandwidth

Peer-to-Peer Streaming Spread parts of a stream quickly to scale and alleviate flash-crowds Example implementations: CoopNet, SplitStream, PRO Example topologies: ad-hoc mesh, a forest of trees Searching for peers with the right layers, more bandwidth, or lower

delay.

Introduction

Relationship of Topics

Streaming

Similar Functions

Search

Transfer

Components of File-Sharing

Introduction

Overview Background Measurement and Characterization Design:

Peer-to-Peer Search Peer-to-Peer Transfer Peer-to-Peer Streaming

Introduction

Peer-to-Peer Search To the user, all file-sharing

programs look pretty much the same: like a search engine.

However, they operate in different ways.

Background: Peer-to-Peer Search

Gnutella Classic

Background: Peer-to-Peer Search

UltrapeersUltrapeers

Leaves

Background: Peer-to-Peer Search

Peer-to-Peer Transfer

Background: Peer-to-Peer Transfer

• A source has been located.

• Now we need to download the file.

• Split the file into blocks.

• Download blocks from wherever we can.

Peer-to-Peer Streaming

• A source has been located.

• Now we need to view the stream.

• Split the file into encoded sub-streams.

• Listen to as many sub-streams as we can.

• Timing matters.

• But we don’t need all streams.

Background: Peer-to-Peer Streaming

Characterization Measurement Techniques Characterizations

Measurement

Measurement Techniques There are five basic approaches to

measuring peer-to-peer systems: Interception Participation Crawling Probing Centralized

Measurement: Techniques

Interception Pros:

It can monitor many users. It can observe transfers. It can observe throughput.

Cons: It captures a biased cross-section. It misses quiet peers.

Measurement: Techniques

Participation Pros:

It can capture a cross-section of overlay traffic. It can compare different open-source

implementations. Cons:

It assumes the measurement node is “typical”. It tells us nothing about atypical nodes. It’s harder to do with closed-source software.

Measurement: Techniques

Crawling Pros:

It can captures the topology. It provides a global perspective. It can captures the entire peer population.

Cons: The network changes while the crawler

runs. It’s hard to verify the accuracy of the

crawler.

Measurement: Techniques

Probing Pros:

It’s easy to do. It can capture many peer

characteristics. Cons:

Sample population may be biased based on:

Degree Availability Files shared

Measurement: Techniques

Centralized Pros:

It provides global knowledge of some aspects.

It’s easy to do for systems with a central component.

Cons: Most peer-to-peer systems don’t have

a centralized component.

Measurement: Techniques

Characterization Churn File Characteristics Peer Characteristics Query Characteristics Topology Implementation Characteristics

Measurement: Characterization

Churn

Citation Systems Observed

Session Time

[SGG02] Gnutella, Napster

50% <= 60 min.

[CLL02] Gnutella, Napster

31% <= 10 min.

[SW04] Kazaa 50% <= 1 min

[BSV03] Overnet 50% <= 60 min.

[GDS+03] Kazaa 50% <= 2.4 min

Adapted from [RGK04]

Measurement: Characterization: Churn

Churn: Open Issues Existing results are not consistent. The implications of churn on the

topology are not well-understood. The downtime distribution is

unknown. Correlations between uptime,

downtime, and future up and downtimes have not been examined.

Measurement: Characterization: Churn

File Characteristics: Storage The popularity of files stored

follows a Zipf distribution

Measurement: Characterization: File Characteristics

Zipf

From [FHKM04]

Measurement: Characterization: File Characteristics

File Characteristics: Storage The popularity of files stored follows a

Zipf distribution The 10% most popular files make 50% of

all stored bytes. The most popular files are around 4 MB. However, the 3% of files which are videos

make up 21% of stored bytes. Most files are shared by a small fraction

of users. 25%-67% of users share no files at all.

Measurement: Characterization: File Characteristics

File Characteristics: Clustering 30% of files have a correlation of at

least 60% with at least one other file. If two peers have 10 files in common,

there’s an 80% they have at least one more file in common.

Generating a graph by treating users as nodes and assigning edges where there are more than N files in common, results in a small world.

Measurement: Characterization: File Characteristics

File Characteristics: Transfers 90% of files transferred are smaller than

10 MB. Most bytes transferred are part of files

larger than 700 MB. The most popular files are roughly equal

in popularity, while unpopular files follow Zipf.

Measurement: Characterization: File Characteristics

Not-So-Zipf

Taken from [GDS+03]

Measurement: Characterization: File Characteristics

File Characteristics: Transfers 90% of files transferred are smaller than 10 MB. Most bytes transferred are part of files larger

than 700 MB. The most popular files are roughly equal in

popularity, while unpopular files follow Zipf. The most popular 5% of transferred files

account for 50% of all transfers. That’s around 45,000 songs which can be

stored in 175 GB. An inverse cache can result in a savings

between 67%-86%.

Measurement: Characterization: File Characteristics

File Characteristics: Open Issues The shift in popularity of files over time

is not well understood, requiring observations over several months.

It would be interesting to see if correlations between files can be used to predict which files a user will want.

No studies have characterized the swarming download feature included in many modern file-sharing applications.

Measurement: Characterization: File Characteristics

Query Characteristics The most popular queries are of

relatively equal popularity, while less popular queries follow Zipf.

There is little relationship between sharing many files and responding to many queries.

40% of queries are duplicates.

Measurement: Characterization: Query Characteristics

Query Characteristics:Open Issues

The relationship between query, transfer, and file popularities are not well-understood.

Queries are composed of several search terms, which we know little about.

We don’t know how long query results are typically valid for.

Measurement: Characterization: Query Characteristics

Topology: Open Issues The most recent published topology data is

from mid-2001. At the time, Gnutella had around 50,000 peers. Today, it has more than 1 million. The introduction of Ultrapeers has drastically

altered the topology. Those crawls took at an hour or more to

complete, but the median peer lifetime may be just a few minutes.

No topology studies have been done on peer-to-peer networks other than Gnutella.

Measurement: Characterization: Topology Characteristics

Characterization Summary Churn File Characteristics Peer Characteristics Query Characteristics Topology Implementation Characteristics

Measurement: Characterization

Designing Peer-to-Peer Search The Convention Wisdom: Flooding

doesn’t scale. Improvements:

Use ultrapeers Walk instead of flood Index replication Interest-based short-cuts Consider distributed hash tables (DHTs) Overlay-to-Internet topology matching

Design: Search

Walking Directed walks are globally efficient, but

send all the query traffic to certain nodes. Random walks are efficient over an

ultrapeer network, but are slow for less popular results.

K-random walks are efficient and fast. Issues:

How far does it scale? There are subtle issues that make it hard to

implement.

Design: Search: Walking

Index Replication It’s the other side of the coin: bringing the indexing

information closer to the queries. Ultrapeers are a special case of index replication. Most file-sharing systems use some type of proportional

indexing. Napster and DHT-based systems use uniform indexing. However, the optimal index replication system is

square-root indexing. [CS02] Open Issues:

Highly distributed, unstructured indexing schemes may not work well under heavy churn.

Additional indexing doesn’t help for queries that have no matches.

Design: Search: Index Replication

Distributed Hash Tables Using a DHT to index all the files in a

file-sharing network is daunting. Let’s just index the unpopular files,

using a heuristic to decide which files are unpopular.

Issues: How good are the heuristics in practice? DHTs are difficult to incrementally

deploy.

Design: Search: DHT

Designing Peer-to-Peer Search Distributed search has improved

dramatically from the early pure-flooding Gnutella.

We don’t have a good “network health” meter to tell us how well a file-sharing network is performing.

Design: Search

Designing Peer-to-Peer Transfer

No papers on incremental improvements.

Systems: BitTorrent Slurpie Rateless Codes

Design: Transfer

BitTorrent Unique features:

Connect, and the data will come. Reward those uploading the most.

Modeling studies show… BitTorrent handles an initial flash-crowds well. However, it does not do as well if a second a

second flash-crowd. Issues:

We don’t know enough about where BitTorrent’s bottlenecks are, when it works well, and when it doesn’t.

Design: Transfer

Slurpie Unique features:

Estimates the total peer network size Constant load on the root server Chooses a block to download first, then

looks for peers Open Issues:

Just a prototype Limited head-to-head experiments against

BitTorrent

Design: Transfer

Rateless Codes Unique features:

Other systems need to worry about finding the right block.

Using encoding, we can make nearly any block the right block.

Modern rateless codes can generate “practically infinite” codes in O(1) time per block.

Issues: It’s unknown if finding the right block is a

significant problem in existing systems.

Design: Transfer

Peer-to-Peer File Transfers:Open Issues We don’t have a solid

understanding of where the bottlenecks are in existing systems.

We don’t have good metrics for determining where they are.

We don’t have good models for comparing new systems against old ones.

Design: Transfer

Peer-to-Peer Streaming User characteristics are well-understood based on

measurements of client-server streaming. Multiple Descriptor Coding: the breakthrough CoopNet uses multiple trees, organized by a central

server. SplitStream uses multiple trees, organized using a DHT. PRO proposes a decentralized, gossip scheme to build a

loose mesh. Open Issues:

CoopNet and SplitStream are delay-optimized, not bandwidth-optimized.

PRO is still in development. No significant deployment.

Design: Streaming

Conclusion While much has been done in the area of

peer-to-peer content distribution, there are still many open avenues.

I presently have papers under submission regarding: Developing metrics to measure the accuracy of

a new, efficient topology crawler Characterizing the modern Gnutella topoloy Characterizing churn in peer-to-peer networks Demonstrating the effectiveness of peer-to-

peer file transfers to handle flash crowds

Conclusion