Session Layer Role Session Services

www.pragsoft.com Chapter 6: The Session Layer 79 look at various protocol-related concepts which primarily deal with the data transfer phase.

6.2.1. Tokens

Tokens provide a mechanism for limiting the use of certain session services to one of the two session users at a time. Four tokens are provided: • Data Token . This is used for half-duplex connections. The service user possessing the token has the exclusive right to issue S-DATA requests. Data exchanges using S-EXPEDITED-DATA and S-TYPED-DATA requests are not affected by this token. This token is irrelevant to and unavailable in full- duplex connections. • Release Token . This is used for connections which have successfully negotiated the use of the Negotiated Release functional unit. The service user possessing the token has the exclusive right to issue an S-RELEASE request. Disconnections using S-U-ABORT requests are not affected by this token. • Sync-Minor Token . This is used for connections which have successfully negotiated the use of the Minor Synchronize functional unit. The service user possessing the token has the exclusive right to issue S-SYNC-MINOR requests. This token is irrelevant and unavailable when the Symmetric Synchronize functional unit is being used instead. • Sync-MajorActivity Token . This is used for connections which have successfully negotiated the use of the Major Synchronize or the Activity Management functional unit. The service user possessing the token has the exclusive right to issue S-SYNC-MAJOR and S-ACTIVITY requests. Token distribution is managed by three service primitives. S-TOKEN-PLEASE is used by a service user to request the possession of one or more tokens from the other user. S-TOKEN-GIVE is used by the possessor of the tokens to forward them to the other user. Finally, S-CONTROL-GIVE enables a service user to forward all its tokens to the other user.

6.2.2. Activities and Dialogue Units

An activity represents a convenient way of referring to a set of related tasks as a single entity. It has a clear beginning and a clear end, both of which are marked by major synchronization points. An activity consists of one or more atomic tasks, called dialogue units. Like an activity, the beginning and end of each dialogue unit is marked by a major synchronization point. Dialogue units and therefore activities can be interrupted and resumed later.