6.2.1. TokensTokens provide a mechanism for limiting the use of certain session services to one ofthe two session users at a time. Four tokens are provided:· Data Token. This is used for half-duplex connections. The service userpossessing the token has the exclusive right to issue S-DATA requests. Dataexchanges using S-EXPEDITED-DATA and S-TYPED-DATA requests arenot affected by this token. This token is irrelevant to and unavailable in full-duplex connections.· Release Token. This is used for connections which have successfullynegotiated the use of the Negotiated Release functional unit. The service userpossessing 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 successfullynegotiated the use of the Minor Synchronize functional unit. The service userpossessing the token has the exclusive right to issue S-SYNC-MINOR requests.This token is irrelevant and unavailable when the Symmetric Synchronizefunctional unit is being used instead.· Sync-Major/Activity Token. This is used for connections which havesuccessfully negotiated the use of the Major Synchronize or the ActivityManagement functional unit. The service user possessing the token has theexclusive right to issue S-SYNC-MAJOR and S-ACTIVITY requests.Token distribution is managed by three service primitives. S-TOKEN-PLEASEis used by a service user to request the possession of one or more tokens from theother user. S-TOKEN-GIVE is used by the possessor of the token(s) to forwardthem to the other user. Finally, S-CONTROL-GIVE enables a service user toforward all its tokens to the other user.