P2P SIP Model
The caller queries the DHT layer (step 1) in the overlay network to determine
the location of the peer node for the called party. The DHT layer routes the
location query to the target peer node (steps 2, 3, and 4), which will answer
and provide its IP address to the SIP UA of the caller (step 5). In step 6, the
DHT layer returns the IP address to the SIP layer of the peer node of the caller.
The SIP UA of the caller can now send a direct INVITEto the called peer node.
This concludes the P2P discovery and call setup.
Note in Figure 20.4b that the SIP layer and the DHT layer can be completely
independent of each other and emerging IETF standards work allows for
future choices of the DHT layer that may not be the Chord protocol, so as to
have a flexible architecture that can keep up with current research in P2P net-
works. The interface between the SP layer and the DHT layer, shown as a dot-
ted line in the peer node of the caller, is an API that may be standardized.
Distributed computing on Planet Lab (http://www.planet-lab.org/)
includes the OpenDHT Layer (http://opendht.org/). The OpenDHT runs
Bamboo and can be used for P2P SIP Communications.Use Cases for P2P SIP
Several descriptions for use cases of P2P communications have been made, the
latest with the intent to develop requirements for a standards-based approach
in the IETF [17]. Such use cases should be considered only examples based on
present knowledge, since you cannot predict innovations.
■■ Public P2P communication service provider—Skype is an excellent example
of a prestandard public P2P communication service. Another example is
Damaka.comas a P2P SIP service provider. Service providers must by
necessity use a central login server that also performs the authentication,
authorization, and accounting (AAA) functions to manage their cus-
tomer base. Note the AAA server has no role whatsoever in the discov-
ery and call setup for communications. Public P2P communication
service providers have the advantage of lowest cost, because of the lack
of any VoIP infrastructure, as will be discussed in the next section.
■■ Open global P2P communications—Anyone on the Internet can enjoy rich
multimedia communications. The only requirement is to have standards-
compliant P2P SIP user devices. The security aspects for global open
P2P communication are, however, not clear at present. Endpoint based
security such as PGP or ZRTP [18] may be a possible approach.
■■ Multimedia consumer devices—As the number of multimedia consumer
devices proliferate in the home, the self-organization of P2P networking
is a necessary ingredient for acceptance by consumers. Current proto-
cols such as UPnP have not taken off, despite the critical need.348 Chapter 20