The history of P2P networks shows an interesting pattern of innovation on
the Internet. Innovators develop extremely popular applications such as file
sharing, VoIP, and IM. Skype is a good example. The attention they get from
threatened businesses models sparks research in academia that tries to explain
and improve on such innovations. Finally, as an understanding of the innova-
tions develops, standards bodies come into play, and even the threatened busi-
nesses (such as content development studios and telecommunications
providers) start to understand the advantages of P2P networks.
P2P networks are envisaged not only for VoIP but also for other applica-
tions, such as file sharing (the origin of modern P2P systems) and media dis-
tribution in content distribution networks (CDN), sharing of computing
power, application-level multicasting, mobility management, peering between
BGP routers, and last, but not least, as a replacement of the DNS under certain
circumstances.NOTEDavid Bryan has made the remark that “P2P will do to VoIP what VoIP
has done to the PSTN.” For more in-depth information on P2P in general and
P2P SIP specifically, consult David Bryan’s Web site at http://www.p2psip.org.Definitions for P2P Networks
This section examines some P2P fundamentals, including the following:
■■ Overlay networks
■■ Peer-to-Peer networks
■■ Distributed Hash Tables (DHTs)Overlay Networks
An overlay networkis a computer network built on top of another IP computer
network, or on top of the Internet. Figure 20.1 shows an example of an overlay
network.
Note that the overlay network resides at the edge of the IP network and is
completely ignorant of the underlying IP network, as well as any services
residing in the underlying network. The nodes of the overlay network use
only the IP addresses from the underlying network. Both discovery and routing
is done on the application layer at the edge only.No DNS is required for discovery.
Overlay networks have the remarkable characteristic of being self-organiz-
ing, as we will illustrate in the section, “The Chord Protocol,” later in this
chapter.340 Chapter 20