The Session Layeradds structure to the data circuit provided by
the Transport Service. It provides an applications oriented
data stream to the session user.
The Session Provider:
Two session entities co-operate using the Session Protocol. In
all twenty-two session services are provided, consisting of fifty
The calling SS-USER is known as the INITIATOR and the called SS-USER
is known as the RESPONDER. The user requesting the service is known as
the requesting user and the corresponding user on the other side is
known as the accepting user.
SS-USERs are addressed at Session Service Access Points (SSAPs). This
is an address composed of two parts:
This address uniquely identifies the SS-USER.
- the Session Selector and
- the Transport address
Session Connection Identifier:
This identifier is created during connection establishment. Its
function is to uniquely name the session connection for that particular
The Connection Identifier is composed of four parts:
- Calling SS-USER Reference (64 octets)
- Called SS-USER Reference (64 octets)
- Common Reference (64 octets)
- Additonal Reference (4 octets)
These are subsets of primitives which are available (the total no.
available is extensive). The Kernel Functional Unit is always
available. There are twelve other functional units available, the use
of which is negotiated during connection establishment.
Various combinations of functional units have proven popular. Upto
1987 three subsets were defined:
Basic Activity Subset(BAS)
Basic Combined Subset (BCS)
Basic Synchronisation Subset (BSS)
The right to invoke some session services is governed by ownership of
Data is exchanged using Session Service Data Units, comprising a
collection of one or more octets. There are three different types of
- NSSDU - normal
- XSSDU - expedited
- TSSDU - typed
Quality of Service (QOS)
Quality of Service is defined as a collection of parameter/value pairs. Most of these are passed directly to
the Transport Service. However there are two that are specific to the Session Service:
Optimised dialogue transfer: The ability to batch several primitives
together for transmission as a single unit. Extended control: The
ability to bypass normal flow control when attempting a
dialogue transfer - boolean
- Extended control - boolean
There are three distinct phases:
State tables indicate in which order these may be invoked.
- Connection Establishment
- Data Transfer
- Connection Release
Provides synchronisation points within the dialogue. Three services provided
- Give Token
- Please Token
- Give Control
- Major Synchronisation Point
- Minor Synchronisation Point
Three services provided.
- Orderly release (all data in transit safely delivered.
- User-initiated abort S_U_ABORT
- Provider-initiated abort (informs two users that connection has been lost) S_P_ABORT