scalability

11

Scalability

“You can’t scale better than by utilising someone elses computer.”

Paul James

1

P2P has led to a recent renewal of interest in decentralized systems and a

number of scalable applications being deployed on the Internet. Although the

underlying Internet itself is the largest decentralized computer system in the

world, most systems have employed a completely centralized topology in the

In this chapter, we look at the ways in which peers are organized within ad

hoc, pervasive, multi-hop networks by providing an overview of two common

scalable approaches often referred to broadly as structured and unstructured

P2P networks. Structured networks adopt a somewhat hierarchical approach

by creating a structured overlay across the network and dividing content across

the distributed network by using hash functions. An unstructured approach,

however, adapts dynamically by adding caching centres (super peers) across

the network in an ad hoc fashion. Both approaches have been shown to scale

to large numbers of participants and both have somewhat equal popularity at

the time of writing. Although the specifics of the various approaches within

198 11 Scalability

11.1 Performance in P2P Networks

So, how do we organize P2P networks in such a way as to get optimum

performance? Well, performance in P2P networks is non-deterministic. A P2P

network is often constructed in an ad hoc fashion and due to its nature and

transient availability of its cooperating peers, it is therefore impossible to

stabilise the network for optimal performance. However, this does not mean

that we should give up; it simply means we have to devise complicated robust

algorithms that make the best of this situation. Therefore, performance in P2P

networks cannot be measured precisely; it rather involves taking empirical

measurements such as:

• Does the network scale as we add more peers? Such networks can rapidly

expand from a few hundred peers to several thousand or even millions.

The answers to these questions have a direct impact on the success and

usability of a system. Remember that P2P networks operate in unreliable en-

vironments where peers are continuously connecting and disconnecting. Re-

sources may suddenly become unavailable for a variety of reasons: e.g., users

may disconnect from the network because they do not want to participate any

more. Further, there are more random failures, e.g., cable and DSL failures,

power outages, hackers and attacks, as personal machines are much more vul-

nerable than dedicated servers. In such an environment, therefore, there need

1. Communication: An obvious fundamental necessity of any P2P network.

Most users connect through dial-up, cable or DSL and so their connection

speed is the main bottleneck. However, this problem is amplified by the

highly parallel, multi-hop nature of a P2P network where it often takes

a number of hops for a packet to reach its destination. Therefore other

users’ connection speeds also create a bottleneck; i.e., if one peer on your

multi-hop request is running a 56K modem, then this slows things down

en route. Network traffic minimization and load balancing therefore are

important considerations to overall performance.

11.2 Unstructured P2P 199

2. Searching: There is no central server in P2P networks so many more

peers are involved in the search process, which typically occurs over sev-

eral parallel network hops. Each hop of such a search adds to the total

bandwidth load and therefore the time to set up a connection becomes a

critical factor. Combine this with the inherently unreliable nature of the

individual peers and delivery time starts to grow alarmingly; e.g., if a peer

dicated that around 70 percent of Gnutella users share no files at all [65].

Too many free riders will degrade the performance of the network for oth-

ers and therefore care must be taken to factor in the correct proportion so

that measures can be taken to optimize the current topology, not assume

a static and perhaps unrealistic one.

11.2 Unstructured P2P

Unstructured P2P networks are dynamically formed in an ad hoc fashion

through the use of super peer nodes. Such a topology is often referred to as a

centralized-decentralized topology because it combines a centralized structure

of connectivity from edge peers to super-peers and then employs a decentral-

tralized system, unstructured protocols, such as Gnutella, propagate arbitrary

advertisements and queries across the overlay. Retrieval of a certain resource,

or even its existence, is not guaranteed. They employ the use of super-peer

caches for lesser peers, such as caching services, file locations, endpoints or

advertisements, and can build their own overlay amongst themselves. This ap-

proach leads to the dynamic development of layers, reducing network flooding

of messages which can happen in topologically flat systems. This layered ap-

proach can be extended through the formation of groups that form their own

neighbourhoods, in systems such as Jxta, described in Chapter 15.

However, the theory of unstructured networks is not new. To illustrate