Identity and Presence

As discussed in Chapter 3, "P2P Application Types," users of instant messaging (IM) systems must be uniquely identified. How a user is identified is fundamental to the operation of the system.

Identity has also proven fundamental to discovery and P2P systems in general. Our simple ping map example was unable to satisfy the critical requirements of P2P networks. It had no way to resolve the dynamic and transient nature of peer participation.

Peers and resources need to be uniquely identifiable. This identity must not be limited to current session or current IP address identification. It must persist to enable contextual information and historical interactions to be stored and subsequently restored. In effect, it is required to accumulate the knowledge necessary to support sophisticated P2P networks. Presence information tied to identity can be used to ensure that peer maps are consistent and represent the current state of the network. Knowing when a peer is online is required for building efficient, distributed, and user-centric systems.