Session Layer Protocol: SPDU Exchange, Tokens and Dialogue

Session Layer Protocol and SPDU Exchange

The session protocol establishes the rules for exchanging data and control information between peer entities using a session transport connection. The abstract session protocol machine communicates with the user through a SSAP (Session Service Access Point) via the exchange of session service primitives, and with the supplier via a TSAP (Transport Service Access Point).

The relationship between the exchange of primitives and the procedures of the protocol is as follows:

• If the machine receives a Session Protocol Data Unit (SPDU) delivered by the supplier (transport layer), an indication or confirmation of the user service is generated (for example, an early indication or confirmation).
• If the user sends or receives a request or response primitive, an SPDU is sent from one session entity to another and/or a transport service request is generated.
• An SPDU generated by the session protocol machine can contain parameters whose values depend on the service requested by the user and/or on the information held in the session protocol machine (for example, parameters negotiated at session connection establishment).

Dialogue Management and Token Handling

By default, all connections are full duplex (PDUs in both directions at once). There are hardware and application situations that are half duplex, so dialogue management must control which end can transmit at any time. Dialogue management is achieved using a data token.

When the connection is initiated, half-duplex mode may be negotiated and the initial token holder is identified. Only the entity that holds the token may transmit; the other end must remain silent until it receives the token. The token is passed using the appropriate session primitives. For example:

• The end that passes the token uses S-TOKEN-GIVE.request.
• An entity that does not have the token and wishes to transmit can request it with S-TOKEN-PLEASE.request.
• The token holder may grant or refuse the request.

Specification of the Session Protocol Machine

The session-layer protocol is specified in terms of an abstract session protocol machine (MAS), which is considered part of the session layer. This abstract machine communicates with the user through a SSAP (access point serving the session) by exchanging session service primitives, and with the supplier through a TSAP. The MAS defines the procedures, primitives, and parameter negotiation used to establish and manage session connections.