BS 6961:1988 download free.British Standard for Open Systems Interconnection:Basic connection-oriented session protocol specification.
O Introduction
BS 6961 is one of a set of International Standards produced to facilitate the interconnection of computer systems. The set of International Standards covers the services and protocols required to achieve such interconnection.
BS 6961 is related to other International Standards in the set as defined by the Reference Model for Open Systems Interconnection (ISO 7498). The Reference Model subdivides the area of standardization for interconnection into a series of layers of specification, each of manageable size. It is most closely related to and lies within the field of application of the Session Service Definition (ISO 8326). It also uses and references the Transport Seniice Definition (ISO 0072), whose provisions it assumes in order to accomplish the aims of the session protocol. The interrelationship of these International Standards is illustrated in figure 1.
BS 6961 specifies a single protocol with a common encoding.
It is intended that the session protocol should be general enough to cater for the total range of session service users without restricting future extensions.
The protocol is structured so that subsets of protocol can be defined.
The pnrnary aim of BS 6961 is to provide a set of rules for communication expressed in terms of the procedures to be carried out by peer session entities at the time of communication. These rules for communication are intended to provide a sound basis for development in order to serve a variety of purposes:
a) as a guide for implementors and designers;
b) for use in the testing and procurement of equipment;
c) as part of an agreement for the admittance of systems into the open systems environment;
d) as a refinement to the understanding of OSI.
As it is expected that the initial users of BS 6961 will be designers and implementors of equipment BS 6961 contains, i notes or in annexes. guidance on the implementation of the procedures defined herein.
It should be noted that, as the number of valid protocol sequences is very large, it is not possible with current technology to verify that an implementation will operate the protocol defined in BS 6961 correctly under afl circumstances. It is possible by means of testing to establish confidence that an implementation correctly operates the protocol in a representative sample of circumstances. It is, however, intended that BS 6961 can be used in circumstances where two implementations fail to communicate in order to determine whether one or both have failed to operate the protocol correctly.
The variations and options available within BS 6961 are essential as they enable a session service to be provided for a wide variety of applications. Thus, a rvnimally conforming implementation will not be suitable for use in all possible circumstances. It is important, therefore, to qualify all references to this International Standard with statements of the options provided or required or with statements of the intended purpose of provision or use.
BS 6961 contains the following annexes:
a) annex A — State tables;
b) annex B Relationship to CCITT Recommendation T.62 encoding;
1 Scope and field of application
This International Standard specifies
a) procedures for a single protocol for the transfer of data and control information from one session entity to a peer session entity;
b) the means of selecting the functional units to be used by the session entities;
c) the structure and encoding of the session protocol data units used for the transfer of data and control information.
The procedures are defined in terms of
a) the interactions between peer session entities through the exchange of session protocol data units;
b) the interactions between a session entity and the session service user in the same system through the exchange of session service primitives;
c) the interactions between a session entity and the transport service provider through the exchange of transport service primitives.
5.4 Functions of the Session Layer
5.4.1 Ov.rvlew of functions
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 en 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.
5.4.2 Connection establishment phase
The purpose of the connection establishment phase is to establish a session connection between two SS-users, and
a) to map session addresses onto transport addresses;
b) to select transport quality of service parameters needed (see 6.1.4);
c) to negotiate session parameters (see 7.1 and 7.2);
d) to transfer session selectors (see 7.1 and 7.2) if required;
e) to distinguish between sessAon connections (see 7.1 and 7.21;
f) to transfer a lirrvted amount of transparent user data (see 7.1 and 7.2).
5.4.3 Data transfer phase
The purpose of the data transfer phase is to transport SSDUs between two SS-users connected by a session connection. This purpose is achieved by means of transmission of SPDUs and by the following functions, each of which may or may not be used, depending on the functional units selected in the session connection establishment phase. These concepts are defined in ISO 86:
a) normal data transfer (see 7.9). which may involve segmenting of SSDUs into SPOUs and reassembly by the destination SPM; and concatenation and separation of certain SPDUs. There are two modes of operation:
1) half -dLplex. when the right to send data is restricted to the owner of the data token;
2) duplex, when there is no restriction on the right to send data.
b) token management (see 7.14 to 7.17), to enable the SSusers to request and transfer tokens which control the exclusive right to exercise certain functions (see table 5);
5.7.5 NegotiatIon of maximum TSDU size
Each SPM proposes a maximum TSDU size that the initiator is permitted to send in the date transfer and connection release phases. The lesser of the two numbers is used. A zero value is interpreted to mean unlimited TSDU size. If either SPM proposes zero, the Initiator may not send segmented SSDUs on the session connection.
Each SPM also proposes a maximum TSDU size that the responder is permitted to send in the data transfer and connection release phases. The lesser of the two numbers is used. A zero value is interpreted to mean unlimited TSDU size. If either SPM proposes zero, the responder may not send segmented SSDUS on the session connection.
5.8 Local variables
BS 6961 uses local variables as a means of darifying the effect of certain actions and clarifying the conditions under which certain actions are valid.
5.8.1 Vact
VactisusedbytheSPMtodetermineifanactivityismprograss when the activity management functional unit has been selected:
Vact = true: an activity is in progress;
Vact = false: no activity is in progress.
5.8.2 Vnextact
Vnextact is used by the SPM when the activity management functional unit has been selected:
Vnextact = true: a MAJOR SYNC POINT SPDU has been sent or received;
Vnextact false: an ACTIVITY END SPDU has been sent or received.
5.8.3 VIA)
V(A) is used by the SPM and is the lowest serial number to which a synchronization point confirmation is expected. No confirmation is expected when V(A) = VIM).
5.8.4 VIMI
V(M) is used by the SPM and is the next serial number to be
used.
5.8.5 VIR)
V(R) is used by the SPM and is the lowest serial number to which resynchronization restart is permitted.
5.8.6 Vsc
Vsc is used by the SPM to determine whether or not the SSuser has the right to send minor synchronization point responses. Vsc has the following values.