3.1.5.1 S20 Protocol MCS Channel

Share v2.0 (S20) is the protocol that is used by Microsoft NetMeeting. It is functionally similar to T.120 but is an earlier legacy protocol.

The S20 protocol MCS channel provides session management for application-sharing between nodes in a share session. The S20 protocol provides the session establishment for application-sharing, and Multipoint Communication Service (MCS) provides the broadcast transport for the S20 protocol.

The S20 protocol functions on the distributed model: one node (the creator node) creates the share session and other nodes join the share session. Each node builds its own share roster and keeps the roster locally. Each roster is built from received S20_RESPOND packets. After a node joins the share session, it can also share its local application with the other nodes in the share session.

The S20 protocol is used on each node in a session share to learn all the names and capabilities of nodes that participate in the share session. Nodes that request application-sharing send control and data packets via the S20 protocol. The S20 protocol sends the received control and data packets to MCS. The S20 protocol also retrieves application-sharing control and data packets from MCS, and forwards application and control data packets from MCS.

For interoperability, the S20 protocol MCS channel is designed to allow rudimentary communication with legacy application-sharing clients. The preferred procedure for establishing a new T.128 application-sharing session is to use the advancements that are available through T.124 Generic Conference Control (GCC) services, rather than the legacy S20 protocol.

The following table lists the S20 protocol session establishment control packets.

Packet Name

Packet Type

Description of Packet Function

S20_CREATE

Control packet

Creates a new application-sharing session.

S20_JOIN

Control packet

Joins an existing share session.

S20_RESPOND

Control packet

Responds to an S20_CREATE, S20_JOIN, or S20_RESPOND message.

S20_DELETE

Control packet

Removes a node from a share session.

S20_LEAVE

Control packet

Used by a node to leave a share session.

S20_END

Control packet

Used by a share creator node to end a share session.

S20_DATA

Data packet

Used by any node as a general transport packet (part of the S20 data packet payload).

S20_COLLISION

Control packet

Used to inform another node that is attempting to create a share that it is already created.