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Communication Protocols

Communications Server 2007

While Session Initiation Protocol (SIP) is still the primary control protocol used by Office Communications Server, the Web Conferencing Server and A/V Conferencing Server and their sub-components also employ other protocols to set up and modify conferences and to set up and break down media streams between different elements in the Office Communications Server network. The following protocols are employed by Office Communications Server:

  • SIP. The industry standard protocol described in IETF RFC 3261 that defines a standard for session setup, termination, and media negotiation between two parties. It is widely used for Voice-over-IP (VoIP) call signaling.
  • HTTPS. The set of rules for exchanging files (text, graphic images, sound, video, and other multimedia files) on the World Wide Web. Relative to the TCP/IP suite of protocols, the basis for information exchange on the Internet, HTTP is an application protocol. HTTPS is the HTTP protocol over SSL/TLS.
  • RTP/RTCP. The industry standard protocol for the transport of real-time data, including audio and video.
  • PSOM. A proprietary protocol for the transport of real-time data, including audio and video. PSOM uses TCP or TLS as the underlying transport.

Table 9 shows the protocols used between conferencing components.

Table 9   Conferencing protocols

  Client Focus Focus Factory Conferencing Server Factory Conferencing Server (MCU)

Client

SIP

SIP

SIP

X

SIP, RTP, PSOM etc.

Focus

SIP

X

X

HTTPS

HTTPS

Focus Factory

SIP

X

X

X

X

Conferencing Server Factory

X

HTTPS

X

X

HTTPS

Conferencing Server (MCU)

SIP, RTP, PSOM etc.

HTTPS

X

HTTPS

X

Figure 10 provides an overview of the protocols and the components that use them to communicate.

Figure 10.   Office Communications Server 2007 protocols
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Interfaces in the diagram identify a specific link, based on the transport and purpose, between two logical elements. The same protocol can be used in different ways over the various interfaces. For example, Scc is used to communicate C3P commands over SIP INFO messages and conference event package notifications over SIP SUBSCRIBE and NOTIFY messages.

C3P stands for Centralized Conferencing Control Protocol and is a new conference manipulation protocol used by the Office Communications Server conferencing servers. C3P is used to modify the conference state. The channels over which C3P can be used in an Office Communications Server deployment are shown in Figure 11.

Figure 11.   C3P channels in Office Communications Server
Bb894519.98b69ed0-5bd0-46b5-86a8-d1c77234a176(en-us,office.12).jpg

C3P has request/pending response/final response semantics similar to SIP. C3P commands include the following:

Conference Level

addConference

deleteConference

modifyConference

getConference

getMCU

modifyConferenceLock

modifyUsersMediaFilters

playRecordedName

User Level

addUser

deleteUser

modifyUser

modifyUserRoles

setUserAccess

Scheduling

getAvailableMcuTypes

getEncryptionKey

getConferences

Endpoint Level

modifyEndpointRole

Endpoint Media Level

addEndpointMedia

deleteEndpointMedia

modifyEndpointMedia

High Availability (HA)/Load Balancing

ping

getConference

PSOM is the media protocol for data collaboration. PSOM will use TLS as the underlying transport. PSOM can be used by conferencing clients to establish media channels with the Web Conferencing Server to negotiate or transfer media.

RTP/RTCP is the standard protocol for the transport of real-time data, including audio and video.

Session Initiation Protocol (SIP) is the industry standard protocol described in IETF RFC 3261 that defines a standard way for session setup, termination, and media negotiation between two parties. It is widely used for Voice over IP (VoIP) call signaling.

Session Description Protocol (SDP) is the industry standard protocol described in IETF RFC 4566 that defines a standard way to convey media details, transport addresses, and other session description metadata to the participants when initiating multimedia teleconferences, Voice over IP calls, streaming video, or other sessions.

This section describes the protocols supported by the various server components and the functionality supported by each of those protocols. Signaling and control protocols are used by clients and servers for session setup and conference management. For each media in a conference or an audio/video call, different media protocols are used.

SIP, as specified in RFC 3261, is used for session setup and termination in Office Communications Server. SIP messages use TCP or TLS as the underlying transport layer for client-to-server communications and TLS with mutual authentication (MTLS) for server-to-server communications. Conferences and call control are established within the context of existing SIP sessions using C3P protocol. C3P commands are sent using SIP INFO messages. A separate SUBSCRIBE/NOTIFY dialog is used to subscribe to conference packages, state change notifications, and the conference participant list.

The Web Conferencing Server uses PSOM as the media protocol for data collaboration. PSOM uses TLS as the underlying transport. As the client for the Web Conferencing Server, Live Meeting functionality also relies on PSOM.

RTP and RTCP are used to provide audio/video functionality. Secure Real-time Transport Protocol (SRTP) and Secure Real-time Transport Control Protocol (SRTCP) are used to provide secure, encrypted audio/video functionality.) RTP/RTCP uses TCP or User Datagram Protocol (UDP) as the underlying transport.

Figure 12 shows the protocol relationships between the client and the server.

Figure 12.   Office Communications Server 2007 client-server protocols and interactions
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