The OSI Session Layer Protocol (ISO-SP) provides the session management, e.g. opening and closing of sessions. In case of a connection loss it tries to recover the connection. If a connection is not used for a longer period, the session layer may close it down and re-open it for next use. This happens transparently to the higher layers. The Session layer provides synchronization points in the stream of exchanged packets.
The Session Protocol Machine (SPM), an abstract machine that carries out the procedures specified in the session layer protocol, communicates with the session service user (SS-user) through an session-service-access-point (SSAP) by means of the service primitives. Service primitives will cause or be the result of session protocol data unit exchanges between the peer SPMs using a transport connection. These protocol exchanges are effected using the services of the transport layer.
Session connection endpoints are identified in end systems by an internal, implementation dependent, mechanism so that the SS-user and the SPM can refer to each session connection.
The functions in the Session Layer are those necessary to bridge the gap between the services available from the Transport Layer and those offered to the SS-users.
The functions in the Session Layer are concerned with dialogue management, data flow synchronization, and data flow resynchronization.
These functions are described below; the descriptions are grouped into those concerned with the connection establishment phase, the data transfer phase, and the release phase.
Protocol Structure
ISO Session Protocol (ISO-SP) Messages:
| Functional unit | SPDU code | SPDU name |
| Kernel | CN OA CDO AC RF FN DN AB AA DT PR |
CONNECT OVERFLOW ACCEPT CONNECT DATA OVERFLOW ACCEPT REFUSE FINISH DISCONNECT ABORT ABORT ACCEPT DATA TRANSFER PREPARE |
| Negotiated release | NF GT PT |
NOT FINISHED GIVE TOKENS PLEASE TOKENS |
| Half-duplex | GT PT |
GIVE TOKENS PLEASE TOKENS |
| Duplex | Â | No additional associated SPDUs |
| Expedited data | EX | EXPEDITED DATA |
| Typed data | TD | TYPED DATA |
| Capability data exchange | CD CDA |
CAPABILITY DATA CAPABILITY DATA ACK |
| Minor synchronize | MIP MIA GT PT |
MINOR SYNC POINT MINOR SYNC ACK GIVE TOKENS PLEASE TOKENS |
| Symmetric synchronize | MIP MIA |
MINOR SYNC POINT MINOR SYNC ACK |
| Data separation | Â | No additional associated SPDUs |
| Major synchronize | MAP MAA PR GT PT |
MAJOR SYNC POINT MAJOR SYNC ACK PREPARE GIVE TOKENS PLEASE TOKENS |
| Resynchronize | RS RA PR |
RESYNCHRONIZE RESYNCHRONIZE ACK PREPARE |
| Exceptions | ER ED |
EXCEPTION REPORT EXCEPTION DATA |
| Activity management | AS AR AI AIA AD ADA AE AEA PR GT PT GTC GTA |
ACTIVITY START ACTIVITY RESUME ACTIVITY INTERRUPT ACTIVITY INTERRUPT ACK ACTIVITY DISCARD ACTIVITY DISCARD ACK ACTIVITY END ACTIVITY END ACK PREPARE GIVE TOKENS PLEASE TOKENS GIVE TOKENS CONFIRM GIVE TOKENS ACK |
Related protocols: CLNP, CONP, TP0, TP1, TP2, TP3, TP4, ISO Presentation Protocol, ACSE, ROSE
Sponsor Source: The ISO-SP (OSI Session Layer protocol) is defined in ISO (http://www.iso.org) documents 8326 and 8327 and ITU (www.itu.org) documents X.215, X.225 and X.235.
Reference:
http://www.doc.ua.pt/arch/itu/rec/product/X.htm:
X.215: Information technology - Open Systems Interconnection - Session service definition
X.225: Information technology - Open Systems Interconnection - Connection-oriented Session protocol: Protocol specification
X.235: Information technology - Open Systems Interconnection - Connectionless Session protocol: Protocol specification