Obsoleted by: 0905
Network Working Group ISO Request for Comments: 892 December 1983 ISO Transport Protocol Specification This document is distributed as an RFC for information only. It does not specify a standard for the ARPA Internet. Note: This document appeared in: ISO/TC97/SC16/WG6. Information Processing Systems - Open Systems Interconnection - Transport Protocol Specification. Computer Communication Review 12, 3-4 (July/October 1982), pp. 24-67. and differs from it only in format. Table of Contents 0. Introduction 1. Scope and Field of Application 2. References Section One - General 3. Definitions 4. Symbols and Abbreviations 5. Overview 5.1 Service provided by the transport layer 5.2 Service assumed from the network layer 5.3 Functions of the transport layer 5.4 Model of the transport layer Section Two - Transport Protocol Specification 6. Protocol Mechanisms 6.1 Assignment to network connection 6.2 Transport protocol data unit (TPDU) transfer 6.3 Data TPDU length and segmenting 6.4 Concatenation and separation 6.5 Connection establishment 6.6 Connection refusal 6.7 Release 6.8 Implicit termination 6.9 Spurious disconnect 6.10 Data TPDU numbering 6.11 Expedited data transfer 6.12 Reassignment 6.13 Reassignment after failure
6.14 Retention until acknowledgement of TPDUs 6.15 Resynchronization 6.16 Multiplexing and demultiplexing 6.17 Explicit flow control 6.18 Checksum 6.19 Frozen references 6.20 Retransmission on timeout 6.21 Resequencing 6.22 Inactivity control 6.23 Treatment of protocol errors 6.24 Splitting and recombining 7. Protocol Classes 7.0 Protocol description of class 0: simple class 7.1 Protocol description of class 1: basic error recovery class 7.2 Protocol description of class 2: multiplexing class 7.3 Protocol description of class 3: error recovery and multiplexing class 7.4 Protocol description of class 4: error detection and recovery class 8. Encoding 8.1 Summary 8.2 Structure 8.3 Connection Request (CR) 8.4 Connection Confirm (CC) 8.5 Disconnect Request (DR) 8.6 Disconnect Confirm (DC) 8.7 Data (DT 8.8 Expedited Data (ED) 8.9 Data Acknowledgement (AK) 8.10 Expedited Data Acknowledgement (EA) 8.11 Reject (RJ) 8.12 TPDU Error (ERR) Section Three - Conformance 9. Conformance 0. Introduction The Transport Protocol Standard is one of a set of International Standards produced to facilitate the interconection of computer systems. The set of standards covers the services and protocols required to achieve such interconnection. The Transport Protocol Standard is positioned with respect to other related standards by the layers defined in the Reference Model for Open Systems Interconnection (ISO 7498). It is most closely related to, and lies within the field of application of the Transport
Service Standard (DP aaaa). It also uses and makes reference to the Network Service Standard (DP bbbb), whose provisions it assumes in order to accomplish the transport protocol's aims. The interrelationship of these standards is depicted in Figure 1. -----------------------------------TRANSPORT SERVICE DEFINITION----- Transport --Reference to aims--------------- Protocol Specification --Reference to assumptions-------- ------------------------------------NETWORK SERVICE DEFINITION------ Figure 1 - Relationship between the transport protocol and adjacent services The standard specifies a common encoding and a number of classes of transport protocol procedures to be used with different network qualities of service. It is intended that the Transport Protocol should be simple but general enough to cater for the total range of Network Service qualities possible, without restricting future extensions. The protocol is structured to give rise to classes of protocol which are designed to minimize possible incompatibilities and implementation costs. The classes are selectable with respect to the Transport and Network Services in providing the required quality of service for the interconnection of two session entities (note that each class provides a different set of functions for enhancement of service qualities). This protocol standard is concerned with optimisation of network tariffs and the following qualities of service: a) different throughput rates; b) different error rates; c) integrity of data requirements; d) reliability requirements. The aim of this standard is primarily to provide a definition for implementors. Since the protocol is complex, the document contains much material which is advisory or descriptive, but mandatory requirements on implementations are clearly identified. 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 this document correctly under all 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 this standard 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 this standard are essential to enable a Transport Service to be provided for a wide variety of applications over a variety of network qualities. Thus, a minimally conforming implementation will not be suitable for use in all possible circumstances. It is important therefore to qualify all references to this standard with statements of the options provided or required or with statements of the intended purpose of provision or use. 1. Scope and Field of Application 1.1 This International Standard Specifies: a) five classes of procedures 1) Class 0. Simple class; 2) Class 1. Basic error recovery class; 3) Class 2. Multiplexing class; 4) Class 3. Error recovery class; 5) Class 4. Error detection and recovery class, for the transfer of data and control information from one transport entity to a peer transport entity; b) the means of negotiating the class of procedures to be used by the transport entities; c) the encoding of the transport protocol data units used for the transfer of data and control information; d) the functional requirements of equipment within a computer system claiming to implement these procedures. 1.2 The procedures are defined in terms of: a) the interactions between peer transport entities through the exchange of transport protocol data units; b) the interactions between a transport entity and the transport service user in the same system through the exchange of transport service primitives; c) the interactions between a transport entity and the network service provider through the exchange of network
service primitives. 1.3 This International Standard is applicable to equipment which supports the Transport Layer of the OSI Reference Model and which wishes to interconnect in an open systems environment. 2. References ISO 7498 Information processing systems - Open systems inter- connection - Basic Reference Model DP aaaa Information processing systems - Open systems inter- connection - Transport service definition (N1169). DP bbbb Information processing systems - Open systems inter- connection - Connection-oriented network service definition (N729) Section One - General 3. Definitions 3.1 Equipment: Hardware or software or a combination of both; it need not be physically distinct within a computer system. 3.2 Transport service user: An abstract representation of the totality of those entities within a single system that make use of the transport service. 3.3 Network service provider: An abstract machine which models the totality of the entities providing the network service, as viewed by a transport entity. Explanatory Notes 1. Definitions 3.1 to 3.3 relate to terms used in clause 1. 2. This standard makes use of the terms, concepts, and definition defined in ISO 7498. 4. Symbols and Abbreviations 4.1 Data Units TPDU Transport protocol data unit TSDU Transport service data unit NSDU Network service data unit 4.2 Types of transport protocol data units
CR TPDU Connection request TPDU CC TPDU Connection confirm TPDU DR TPDU Disconnect request TPDU DC TPDU Disconnect confirm TPDU DT TPDU Data TPDU ED TPDU Expedited data TPDU AK TPDU Data acknowledge TPDU EA TPDU Expedited acknowledge TPDU RJ TPDU Rejected TPDU ERR TPDU Error TPDU 4.3 TPDU fields LI Length indicator (field) CDT Credit (field) TSAP-ID Transport service access point identifier (field) DST-REF Destination reference (field) SCE-REF Source reference (field) EOT End of TSDU mark TPDU-NR DT TPDU number (field) ED-TPDU-NR ED TPDU number (field) YR-TU-NR Sequence number response (field) 4.4 Parameters T (R) Receive sequence number T (S) Send sequence number 4.5 Timer variables T1 Elapse time between retransmissions N The maximum number of retransmissions L Bound for the maximum time between the decision to transmit a TPDU and the receipt of any response relating to it T-WAIT Maximum time for a reassignment to take place before TC failure is assumed I Inactivity timer - Maximum time allowed to elapse between receipt of TPDUs before TC failure is assumed W Window timer - Maximum interval between trans- mission of up to date window information 4.6 Other variables n The number of bits in the sequence number field p The number of bits in the credit field of a CR, CC or AK TPDU
4.7 Miscellaneous TS-user Transport service user TSAP Transport service access point NSAP Network service access point TC Transport connection NC Network Connection 5. Overview of the Transport Protocol 5.1 Service Provided by the Transport Layer The services provided by the protocol described in this document are connection-oriented services. They are defined in document DP aaaa. The Transport Service primitives provided are summarized in Figure 1.
Primitive Parameters ------------------------------------------------------------------------ T-CONNECT Request To Transport Address, From Indication Transport Address, Expedited Data Option, Quality of Service, TS-User data. ------------------------------------------------------------------------ T-CONNECT Response Responding Address, Quality Confirmation of Service, Expedited Data Option, TS-User data. ------------------------------------------------------------------------ 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 Primitives 5.2 Service Assumed from the Network Layer The transport protocol described in this document assumes of the Network Services described in DP bbbb. The Network Service primitives used are summarized in Figure 2.
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 Y ACKNOWLEDGE 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 Primitives 5.3 Functions of the Transport Layer 5.3.1 Connection Oriented Functions 220.127.116.11 Overview of Functions The functions in the transport layer are at least those necessary to bridge the gap between the services available from the network layer and those to be offered to the transport users. The functions in the transport layer are concerned with the enhancement of quality of service, including all aspects of cost optimization. They are described below; the descriptions are grouped into those concerned with the establishment phase, the data transfer
phase, and the release phase. 18.104.22.168.1 Establishment Phase The goal of the establishment phase is to establish a transport connection, i.e., between two transport users. The functions of transport layer during this phase must match the requested class of services with the services provided by the network layer 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. 22.214.171.124.2 Data Transfer Phase The purpose of the data transfer phase is to permit two-way simultaneous transport of TSDUs between the session entities connected by the transport connection. This purpose is achieved by means of two-way simultaneous communication in the Transport protocol and by the following functions. Each of these functions is used or not used in accordance with the result of the selection performed in the establishment 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.
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. 126.96.36.199.3 Release Phase A function to provide a disconnection of the transport connection, regardless of the current activity. 188.8.131.52 Classes and Options
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. 184.108.40.206.2 Options within Classes Options define potential functions which may be used within a class. 220.127.116.11.3 Negotiation Classes and options within classes are negotiated during the connection establishment phase. 18.104.22.168.4 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 terms of quality with respect to error behavior relative to the user requirements. Its main purpose is to provide a basis for the decision regarding which class of transport connection should be used on top of
a 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 the quality of service provided by particular Network connections. 22.214.171.124 Potential Functions The protocol described in this document does not include the following set of functions which have been identified as potential transport 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 connections 5.4 Model of the Transport Layer TSAP TSAP Transport Protocol Transport Protocol Entity Entity NSAP ------- NSAP ------- | (NSAP) | (NSAP) | | | | | |-------------------------|-------- | | ----------------------------------- A Transport Protocol entity within the Transport Layer communicates with a Transport User through a TSAP by means of the
service primitives as defined by the transport service definition DP aaaa. Service primitives will cause or be the result of Transport Protocol Data Unit exchanges between the peer Transport Protocol entities supporting a Transport Connection. These protocol exchanges are effected using the services of the Network Layer as defined by the Network Service Definition DP bbbb through one or more NSAPs. Transport connection endpoints are identified in end systems by an internal, implementation dependent, mechanism so that the Transport User and the Transport Protocol entity can refer to each Transport connection. Section Two - Transport Protocol Specification 6. 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 first transport connection over a network connection, it must also be invoked for any and all other transport connections which use that network connection 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 must be assigned to one (or more if splitting function is being used) network connection(s). Both transport entities involved must become aware of this assignment. A transport connection may be assigned to a suitable existing network connection; one or more new network connections may also be created for the purpose. An existing network connection, which connects the relevant transport entities, is unsuitable for assignment of a transport connection if, for example: o the quality of service needed for the transport connection can not be met by using or enhancing the network
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 service requested is a local matter, though it will normally be related to the requirements of transport connection(s) expected to be assigned to it. A Network Connection with no transport connections will be available after initial establishment or because explicit disconnection of all the transport connections previously assigned to it has taken place. Either Transport entity may as a local matter choose to disconnect the Network Connection or assign other Transport Connections to it. 6.2 Transport Protocol Data Unit (TPDU) Transfer Purpose: To convey transport protocol data unit in user data fields of network service primitives. Network Service Primitives N-DATA N-EXPEDITED DATA Description: This function is inherent. The Transport Protocol Data Units (TPDUs) defined for the protocol are listed in Figure 3. TPDU name Abbreviation Connection Request CR Connection Confirm CC Disconnect Request DR Disconnect Confirm DC Data DT Expedited Data ED Data Acknowledge AK Expedited Acknowledge EA Reject RJ TPDU Error ERR Figure 3. Transport Protocol Data Units
TPDUs are conveyed using the NS-User data parameters of the Network Service primitives, primarily with the N-DATA, but also with N-EXPEDITED primitives. Transport entities shall accept all permissible assignments and may issue any permissible assignments. The permissible assignments of TPDUs to these primitives are shown in Figure 4. Concatenation of TPDUs is also permitted (see section 6.4). Primitive Applicable TPDUs Note N-DATA CR, CC, DR, DT, ED, AK, EA, RJ, DC, ERR N-EXPEDITED ED, EA 1 Notes: 1. This assignment is permissible only when using class 1 and when the network expedited variant has been agreed. Figure 4. Network Service Primitives which can convey TPDUs. 6.3 Data TPDU Length and Segmenting Purpose: Mapping between one TSDU and TPDUs. TPDUs and fields used: DT - End of TSDU (1 bit) Description: The data field of Data TPDUs may contain any number of octets up to an agreed maximum as negotiated at connection time. A transport entity uses an End of TSDU mark as defined below: In each Data TPDU a transport entity may indicate the end of a TSDU. Category 1 Having the End of TSDU mark set to yes. These TPDUs may or may not have the maximum length. Category 2 Having the End of TSDU mark set to no. These TPDUs do not necessarily have the maximum length. A complete Data TPDU sequence is defined as being composed of
either a single category 1 DT TPDU or consecutive category 2 followed by a category 1 DT TPDU. 6.4 Concatenation and Separation Pupose: Conveyance of multiple TPDUs in one NSDU. Description: All TPDUs carry in their TPDU header a length indicator (see Section 8.2.1). Additionally, TPDUs are classified as either Data TPDUs or Control TPDUs. Control TPDUs may or may not contain a data field. For TPDUs containing data the length of the data field is indicated by the length of the NSDU. These provisions permit any number of Control TPDUs that may not contain data to be concatenated with a single control TPDU which may contain data or with a single Data TPDU. The control TPDUs without data must precede the TPDU with data, if any. The number of TPDUs so concatenated is terminated by the end of the NSDU. The concatenated set of TPDUs may be for the same or different transport connections. An implementation shall accept concatenated TPDUs and may concatenate TPDUs before transmission. The transport entity shall not send a concatenated set of TPDUs which exceeds twice the overall maximum TPDU length for all the TCs assigned to the network connection. 6.5 Connection Establishment Purpose: Creation of a new transport connection. Network Service Primitives: N-DATA TPDUs and fields used: CR, CC - source reference (16 bits) - initial credit (if applicable) - calling transport address (optional) - called transport address (optional) - user data (optional) - TPDU size (optional) - sequence number length (optional) - checksum selection (optional) - acknowledgement time (optional) - quality of service (optional) CR - preferred protocol class
- alternative protocol classes (zero or more) - version number (optional) - security (optional) - proposed options CC - destination reference (16 bits) - selected protocol class - selected options Description: This function is inherent: A transport connection is established by means of one transport entity (the initiator) transmitting a Connection Request (CR) TPDU to the other transport entity (the responder), which replies with a Connection Confirm (CC) TPDU. Before sending the CR TPDU, the initiator assigns the transport connection being created to one (or more if the splitting function is being used) network connection(s). It is this set of network connections over which the TPDUs are sent. During this exchange, all information and parameters needed for the transport entities to operate must be exchanged or negotiated. The following information is exchanged: o references. Each transport entity chooses a reference which is 16 bits long and which is arbitrary except for the following restrictions: - it cannot already be in use or "frozen" (see "Frozen References", Section 6.19). - it cannot be zero. Each transport entity is responsible for selecting the Reference which the partner will use. This mechanism is symmetrical and therefore avoids the need to assign a status of master or slave to partners and avoids call collision. This mechanism also provides identification of the transport connection independent of the network connection. The range of References used for transport connections, in a given transport entity, is a local system parameter. o addresses (optional). Indicate the calling and called transport service access points. When either network address unambiguously defines the transport address this information may be omitted. o initial credit. Only relevant for classes which include the Explicit Flow Control Function.
o user data. Not available in class 0. Up to 32 octets in in other classes. The following negotiations take place: o protocol class. The initiator shall propose a preferred class and any number of alternatives. (Except that no alternatives are allowed when class 0 is the preference.) The initiator should assume when it sends the CR TPDU that its preferred class will be agreed to, and commence the functions associated with that class. Note: This means, for example, that when a class which includes resynchronization (see "Resynchronization", Section 6.15) is preferred, resynchronization will occur if a reset is signalled during connection establishment. When the responder has decided which class is to be used, it shall indicate this in the CC TPDU and shall invoke the appropriate functions for the class. The responder may select the preferred class, or any of the alternative classes or may select class 0 if class 1 is proposed or class 2 if class 3 or 4 is proposed. (see Section 9) If the preferred class is not selected, then on receipt of the CC TPDU, the initiator shall adjust its functions accordingly. o TPDU Size. The initiator may propose a maximum size for TPDUs, and the responder may accept this value or respond with any value between the proposed value and 128 in the set of values available (see "Encoding", Section 8). o sequence number length. Either normal or extended is available. When the sequence number is extended, the credit field (if applicable) is also extended. o checksum selection. This defines whether or not TPDUs of the connection are to include a checksum. o version number. This defines the version of the transport protocol standard used for this connection. o security parameter. This parameter and its semantics are user defined. o quality of service parameter. This defines the throughput, delay, priority and residual error rate. o The non-use of explicit flow control in class 2 is negotiated.
o The use of Network Receipt Confirmation and Network expedited is negotiated when class 1 is to be used. The negotiation rules for the options are such that the initiator may propose either to use or not to use the option. The responder may either accept the proposed choice or select the mandatory alternative defined in Section 9. During the establishment phase of the transport connection, the use of the expedited data option field of CR/CC allows both Transport Service user to negotiate the use or non use of the expedited data transport service as described in the transport service definitions. The following table summarizes the negotiation possibilities for the options. Proposition Made Possible by the Initiator Selection by Option the Responder Transport expedited data Yes Yes or No transfer service No No Use of receipt confir- Yes Yes or No mation (class 1 only) No No Use of the network Yes Yes or No expedited variant No No (class 1 only) Non use of checksum Yes Yes or No (class 4 only) No No Non use of explicit Yes Yes or No flow control (class 2 only) No No Use of extended format Yes Yes or No No No In class 2, whenever a transport entity requests or agrees to the Transport Expedited data transfer service or to the use of extended formats, it must also request or agree (respectively) to the use of explicit flow control. 6.6 Connection Refusal Purpose: Refusal of the transport connection. TPDUs and fields used:
DR - reason (1 octet) - user data (maximum of 64 octets) ERR - reject code (1 octet) - rejected TPDU parameter Description: If a transport connection cannot be accepted, the called transport entity shall respond to the CR TPDU with a DR TPDU. The clearing reason shall indicate why the connection was not accepted. The source reference field in the DR TPDU is set to zero to indicate an unassigned reference. If the CR is regarded as an invalid TPDU, the called transport entity will respond by sending an ERR TPDU. On receipt of this TPDU, the calling entity will regard the connection as closed. 6.7 Release Variants: 'implicit' or 'explicit' Purpose: Termination of the transport connection. Network Service Primitives: N-DISCONNECT (implicit variant only) N-DATA TPDUs and fields used: DR - clearing reason (1 octet) - user data (maximum of 64 octets) DC Description: This function is inherent. In the 'implicit' variant, either transport entity disconnects a transport connection by disconnecting the network connection to which it is assigned. Similarly when a transport entity is informed that the network connection has been disconnected by the peer transport entity, this should be considered as a transport disconnect.
In the 'explicit' variant, either transport entity transmits a Disconnect Request (DR) TPDU, and the other responds with a Disconnect Confirm (DC) TPDU. When the DC TPDU is sent or received by a transport entity, that entity should consider the transport connection not to exist (note 1). After the sending of a DR TPDU, other TPDUs received before the DC TPDU are ignored. It is possible that a disconnect collision will occur, when both transport entities send a DR TPDU at about the same time. This results in each transport entity receiving a DR, after sending one. Each transport entity shall consider the received DR TPDU as a confirmation of its DR TPDU, and shall not send or expect to receive a DC TPDU. The DR can convey a limited amount (up to 64 octets) of data. 6.8 Implicit Termination Purpose: Termination of a Transport Connection on the occurrence of a signalled error for which recovery functions are not operative. Network Service Primitives: N-DISCONNECT Indication N-RESET Indication Description: When, on the network connection to which a Transport Connection is assigned, an N-DISCONNECT or N-RESET Indication occurs, both transport entities shall consider that the transport connection no longer exists, and so inform the session entities. Note 1: When a connection has been released, after the exchange of DR and DC, the reference can be re-used immediately (except in Class 4, where the Frozen Reference function is used, see Section 6.19). This is because the releasing transport entity does not know with certainty that the remote transport entity considers use of the reference to be ended. Therefore, the reference should not be re-used for further connections. (In practice, the reference may be re-used after a reasonable period when it is possible to be reasonably certain that the remote transport entity will not continue to use it). 6.9 Spurious Disconnect Purpose: To deal with the arrival of an "unknown" DR TPDU. TPDUs and fields used:
DR, DC - source reference - destination reference Description: A DR TPDU can be received for a transport connection which does not exist. Rather than treating this as an error, a DC TPDU should be send back which reflects the references of the DR TPDU. Note: This only applies when one or more transport connections using a multiplexing class exist over the network connection, or when no transport connections exist. At other times it is a protocol error. 6.10 Data TPDU Numbering Variants: 'normal' or 'extended' Purpose: Numbering of DT TPDUs for use in recovery, flow control, or sequencing functions. TPDUs and Fields Used: DT - TPDU-NR (7 or 31 bits) Description: DT TPDUs transmitted in each direction on a transport connection bear a sequence number 'TPDU-NR'. Its value in the first DT TPDU in each direction after connection establishment will be zero. Thereafter each TPDU had 'TPDU-NR' one greater than the previous. Modulo 2**7 arithmetic is used in the 'normal' variant, and modulo 2**31 in the 'extended' variant. In the sections that follow, the relationships 'greater than' and 'less than' are used in connection with TPDU numbers. In all such uses, the numbers being compared cover a range less than the modulus and in fact lie within a contiguous set of TPDU numbers called a 'window'. The window has a known starting TPDU number and finishing number. The term 'less than' means 'occurring sooner in the window sequence' and the term 'greater than' means 'occurring later in the window sequence'. 6.11 Expedited Data Transfer Variants: 'network expedited' or not Purpose: Provision of the expedited data service
Network Service Primitives: N-DATA N-EXPEDITED DATA TPDUs and Fields Used: ED - ED TPDU-NR (7 or 31 bits) EA - YR-TU-NR (7 or 31 bits) Description: Each expedited TSDU is conveyed as the data field of an Expedited Data (ED) TPDU. Each ED TPDU received must be acknowledged by an Expedited Acknowledge (EA) TPDU. There may only be one ED TPDU unacknowledged at any time for each direction of a transport connection. In the 'network expedited' variant (available in class 1 only), ED and EA TPDUs are conveyed in the data fields of N-EXPEDITED DATA primitives. Otherwise, N-DATA is used. 6.12 Reassignment Purpose: Assignment of a Transport Connection to a different Network Connection. TPDUs and Fields Used: CR - source reference RJ, DR - destination reference Description: When the Network Connection to which a Transport Connection was assigned no longer exists, the Transport Connection can be assigned to another Network Connection. When one transport entity has assigned the Transport Connection, it is important that the other transport entity recognise to which Network Connection it has been assigned. This can only take place when it