Peer-to-Peer Sesssion Initiation Protocol (P2PSIP) provides a conceptually simple, standards-based mechanism to distribute functionality, leveraging the power of clusters of devices, rather than a single centralized box. At the heart of P2PSIP is a notion of providing a distributed "fabric" of devices that are able to provide these same services and functionality with little or no centralized infrastructure. P2P systems do not eliminate work that would be done by a centralized device in traditional client-server architecture, but rather distribute that work differently, pushing the functionality to the edge. As a result, there some advantages to a P2P architecture (for example improved scalability and reliability) and some areas where a conventional CS system may be better (for example number of hops involved to route a message).
While it is clear that there are some proposed uses of P2PSIP that are philosophically at odds with the IMS, there are many ways that P2PSIP and IMS can be combined to produce a product that is "greater then the sum of the parts". P2PSIP can help by eliminating some of the complexity aspects of the IMS while simplifying the high-availability and scalability aspects of IMS. This results in reduced OpEx and CapEx while increasing service availability. IMS offers a network architecture that enables network operators to generate revenue from P2PSIP applications. The IMS operator can also use P2PSIP to achieve a near-perfect distribution of resources, as P2P systems use mathematical mechanisms that statistically guarantee that resources are fairly distributed.
In a combined P2PSIP/IMS approach, little (if any) of the capabilities will be pushed entirely to the endpoint. The requirement that mobile IMS devices use as little power as possible means that these devices would be poorly suited to being used as peers, since the additional transmission power needed to serve requests can compromise the life of the battery. P2PSIP does not reinvent basic aspects of SIP, but this also means that certain aspects of a telecommunications system – most notably the charging functions – have not been addressed in P2PSIP. As a result, those deploying even a purely P2PSIP deployment may leverage aspects of the IMS architecture, such as charging, in their systems.