Unlike typical client-server web application or Jabber presence system, a SIP user agent is both client and server. Which means, a SIP user agent can send as well as receive a SIP request. In theory, a SIP proxy is not a required component in a SIP system. Ideally, the caller does not need to know whether a call to "sip:firstname.lastname@example.org" will be received by a SIP user agent directly or by an intermediate SIP proxy server first. In practice, people often refer to an intermediate SIP proxy (and registration) server as the SIP server, and such SIP servers are norm rather than exceptions.
In theory, once the end-to-end SIP session is established between the caller and callee user agent, the media can traverse directly between the two user agents on the IP network. This is what makes SIP systems support peer-to-peer (or end-to-end) media path. In practice, presence of network address translators and firewalls prevent direct IP connectivity between the two user agents for media path. This requires workarounds such as interactive connectivity establishment or network relay services such as media-proxy.
A bigger problem in practice is that due to business reasons, the VoIP provider does not want the media path to be end-to-end, so that it can have control over the "service" for accounting, billing, advertisement or other reasons.
In short, SIP is a tool that enables peer-to-peer (or end-to-end) media as well as signaling path. Protocols are like tools, which vendors use to build applications and systems, similar to how a construction worker uses various tools to build a house. In the current Internet, many vendors have used SIP to build closed walled gardens of managed services, unlike what SIP was initially envisioned for.
On the other hand, P2P-SIP is a realization of this problem to explicitly define SIP-based communication without using managed servers. Instead of central SIP servers managed by your provider, which can impose constraints to break end-to-end media and communication services, P2P-SIP aims at decentralizing the registration and proxy functions so that the signaling path is peer-to-peer.
The main benefits of P2P-SIP are as follows:
- Organizations and services providers can save cost of server maintenance. In particular, there is no new training or position required specifically for a VoIP IT staff. There is no need to host dedicated servers in data centers with 99% availability and pay for energy and bandwidth.
- The VoIP industry can move away from traditional service provider oriented business to a more open end-to-end user application. Essentially, it becomes more inline with your other Internet services such as web and email: how web browser don't distinguish between servers or domains, and how you can send email using any mail client and any provider to anybody else.
- Finally, the most important reason is that the cost saving eventually propagates to end-user, who can enjoy free VoIP service as long as they are paying for their IP network access. Peer-to-peer infrastructure enables highly scalable communication system such as Skype at a very low cost. A small vendor's VoIP is able to scale to millions of users only if it can save cost of server maintenance and bandwidth.
The important aspects of P2P-SIP are:
- It does not depend on a VoIP service provider for signaling and media path. Hence there is not much money for managed services in P2P-SIP.
- It can use end-user devices on public Internet to relay media path for end-users behind restricted networks. Hence it requires incentive for public end-users to help restricted end-users.
As you can see, benefits of P2P-SIP are for end-users, at the cost of service provider businesses. Most of existing VoIP effort is driven by large corporations and carriers who do not have any interest in making the service open to end-users and lose control over them.