📄 rfc892.txt
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------------------------------------------------------------------------T-DATA Request TS-User data. Indication------------------------------------------------------------------------T-EXPEDITED Request TS-User data.DATA Indication ------------------------------------------------------------------------T-DISCONNECT Request TS-User data.------------------------------------------------------------------------T-DISCONNECT Indication Disconnect reason, TS-User data.------------------------------------------------------------------------ Figure 1. Transport Service Primitives5.2 Service Assumed from the Network Layer The transport protocol described in this document assumes ofthe Network Services described in DP bbbb. The Network Serviceprimitives used are summarized in Figure 2. ISO Transport Protocol Specification Page 9International Standards Organization Primitive X/Y Parameters X/Y/Z------------------------------------------------------------------------N-CONNECT Request X Called Address, X Indication X Calling Address, X Response X NS-User data, Z Confirmation X QOS. X------------------------------------------------------------------------N-DATA Request X NS-User data, X Indication X Conf. Request Y------------------------------------------------------------------------N-DATA Request YACKNOWLEDGE Indication------------------------------------------------------------------------N-EXPEDITED Request Y DATA Indication NS-User data Y------------------------------------------------------------------------N-RESET Request X Indication X Response X Confirmation X------------------------------------------------------------------------N-DISCONNECT Request X NS-User data Z Indication X------------------------------------------------------------------------ X - The Transport Protocol assumes that this facility is provided in all networks. Y - The Transport Protocol assumes that this facility is provided in some networks and a mechanism is provided to optionally use the facility. Z - The Transport Protocol does not use this parameter. Figure 2. Network Service Primitives5.3 Functions of the Transport Layer5.3.1 Connection Oriented Functions5.3.1.1 Overview of Functions The functions in the transport layer are at least thosenecessary to bridge the gap between the services available from thenetwork layer and those to be offered to the transport users. The functions in the transport layer are concerned with theenhancement of quality of service, including all aspects of costoptimization. They are described below; the descriptions are groupedinto those concerned with the establishment phase, the data transfer ISO Transport Protocol Specification Page 10International Standards Organizationphase, and the release phase.5.3.1.1.1 Establishment Phase The goal of the establishment phase is to establish atransport connection, i.e., between two transport users. Thefunctions of transport layer during this phase must match therequested class of services with the services provided by the networklayer as follows: o Select network service which best matches the requirement of the TS-user taking into account charges for various services. o Decide whether to multiplex multiple transport connection onto a single network connection. o Establish the optimum TPDU size. o Select the functions that will be operational upon entering the data transfer phase. o Map transport addresses onto network addresses. o Provide a means to distinguish between two different transport connections. o Transportation of user's data.5.3.1.1.2 Data Transfer Phase The purpose of the data transfer phase is to permit two-waysimultaneous transport of TSDUs between the session entities connectedby the transport connection. This purpose is achieved by means oftwo-way simultaneous communication in the Transport protocol and bythe following functions. Each of these functions is used or not usedin accordance with the result of the selection performed in theestablishment phase. o Concatenation and Separation A function used to collect several TPDUs into a single NSDU; the destination transport entity separates the TPDUs. o Segmenting and Reassembling The splitting of a single data TSDU into multiple TPDUs which are reassembled into their original format at the destination. ISO Transport Protocol Specification Page 11International Standards Organization o Multiplexing and Demultiplexing A function used to share a single network connection between two or more transport connections. o Splitting and Recombing A function allowing the simultaneous use of two or more network connections to support the same transport connec- tion. o Flow Control A function used to regulate the flow of TPDUs between two transport entities on one transport connection. o Error Detection A function used to detect the loss, corruption, duplication, misordering or misdelivery of TPDUs. o Transport Connection Identification A means to uniquely identify a transport connection between the pair of transport entities supporting the connection during the lifetime of the transport connection. o Error Recovery A function used to recover from detected and signalled errors. o Expedited Data A function used to bypass the flow control of normal data TPDU. Expedited data TPDUs' flow is controlled by separate flow control. o TSDU Delimiting A function used to determine the beginning and ending of a TSDU.5.3.1.1.3 Release Phase A function to provide a disconnection of the transport connection, regardless of the current activity.5.3.1.2 Classes and Options ISO Transport Protocol Specification Page 12International Standards Organization A class defines a set of functions. In this protocol five classes are defined: o Class 0: Simple Class o Class 1: Basic Error Recovery Class o Class 2: Multiplexing Class o Class 3: Error Recovery and Multiplexing Class o Class 4: Error Detection and Recovery Class. Note that with the exception of classes 0 and 1, transport connections of different class may be multiplexed together onto the same network connection.5.3.1.2.2 Options within Classes Options define potential functions which may be used within a class.5.3.1.2.3 Negotiation Classes and options within classes are negotiated during the connection establishment phase.5.3.1.2.4 Choice of the Class of Protocol The choice will be made by the transport entities according to: o the users requirement expressed via T-CONNECT service primitives. In particular, for the choice of the class of protocol, the following rules apply: - if the TS-User requests either transmission of user data during the connection phase, or use of Expedited data transfer, then Class 0 cannot be selected. - if the TS-User requests use of Expedited data transfer, then Class 2 with the non-explicit flow control option cannot be selected. o the quality of the available Network services; o the user required service versus cost ratio acceptable for the transport user. The following is a classification of network services in termsof quality with respect to error behavior relative to the userrequirements. Its main purpose is to provide a basis for the decisionregarding which class of transport connection should be used on top of ISO Transport Protocol Specification Page 13International Standards Organizationa given network connection. Type A: Network connection with acceptable residual error rate (for example not signalled by 'clear' or 'reset') and acceptable rate of signalled failures. Type B: Network connections with acceptable residual error rate (for example not signalled by 'clear' or 'reset') but unacceptable rate of signalled failures. Type C: Network connections with residual error rate not acceptable to the TS-user. It is assumed that each transport entity is aware of thequality of service provided by particular Network connections.5.3.1.3 Potential Functions The protocol described in this document does not include thefollowing set of functions which have been identified as potentialtransport layer functions: o provision for encryption o provision for accounting mechanisms o provision for status exchanges and monitoring of quality of service o provision for blocking o temporary release of network connections5.4 Model of the Transport Layer TSAP TSAP Transport Protocol Transport Protocol Entity Entity NSAP ------- NSAP ------- | (NSAP) | (NSAP) | | | | | |-------------------------|-------- | | ----------------------------------- A Transport Protocol entity within the Transport Layercommunicates with a Transport User through a TSAP by means of the ISO Transport Protocol Specification Page 14International Standards Organizationservice primitives as defined by the transport service definition DPaaaa. Service primitives will cause or be the result of TransportProtocol Data Unit exchanges between the peer Transport Protocolentities supporting a Transport Connection. These protocol exchangesare effected using the services of the Network Layer as defined by theNetwork Service Definition DP bbbb through one or more NSAPs. Transport connection endpoints are identified in end systemsby an internal, implementation dependent, mechanism so that theTransport User and the Transport Protocol entity can refer to eachTransport connection.Section Two - Transport Protocol Specification6. Protocol Mechanisms Several functions are described as 'inherent' or 'pervasive'.Inherent functions must be invoked for every transport connection.Pervasive functions are optional, but if one is invoked for the firsttransport connection over a network connection, it must also be invokedfor any and all other transport connections which use that networkconnection during its lifetime.6.1 Assignment to Network Connection Purpose: Assignment of transport connections to network connections. Network Service Primitives: N-CONNECT N-DISCONNECT Description: This function is inherent. Before a transport connection can be created or used, it mustbe assigned to one (or more if splitting function is being used)network connection(s). Both transport entities involved must becomeaware of this assignment. A transport connection may be assigned to asuitable existing network connection; one or more new networkconnections may also be created for the purpose. An existing network connection, which connects the relevanttransport entities, is unsuitable for assignment of a transportconnection if, for example: o the quality of service needed for the transport connection can not be met by using or enhancing the network ISO Transport Protocol Specification Page 15International Standards Organization connection. o the protocol class preferred or in use for the transport connection is incompatible with the current usage of the network connection as regards the use of pervasive functions (e.g., multiplexing). When a new network connection is created, the quality of⌨️ 快捷键说明
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