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ISO-SP: OSI Session Layer Protocol (X.225, ISO 8327)
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-SP: OSI Session Layer Protocol (X.225, X.215, ISO 8327, 8326) 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 |
|
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 |
|
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 (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 |
