The Unified Communications vision that is embodied in Office Communications Server 2007 R2 is built on Session Initiation Protocol (SIP), an industry-standard, application-layer signaling protocol that is used for starting, controlling, and ending communication sessions in an IP-based network. SIP is formally described in the Internet Engineering Task Force (IETF) reference specification Request for Comments (RFC) 3261. By using SIP, one user can explicitly invite another to join a conversation or multimedia session. A SIP session begins when the second user accepts a SIP INVITE request.

In Office Communications Server 2007 R2, SIP is used for instant messaging (IM), conferencing, presence subscriptions, video, and Voice over IP (VoIP), providing a common user experience across all these communication modes. Signaling for phone calls coming from the public switched telephone network (PSTN) is converted to SIP by the media (PSTN) gateway.

Although SIP sessions can include the sharing of real-time media, SIP itself does not handle the actual media data, such as audio, video, and application sharing. In practical terms, this separation means that SIP and various media protocols can evolve independently.

Another key protocol is Transport Layer Security (TLS), which enhances security and data integrity for communications over IP networks. By default, Office Communications Server 2007 R2 is configured to use TLS for client-to-server connections. Additionally, Office Communications Server uses mutual TLS (MTLS) for server-to-server connections.

Other key protocols that are used in Office Communications Server 2007 R2 include the following:

To download a complete list of Office Communications Server protocols, see .