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
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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
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
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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
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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.
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)
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
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.
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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. ------------------------------------------------------------------------
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.
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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.
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
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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.
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.
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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.
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
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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.
A Transport Protocol entity within the Transport Layer communicates with a Transport User through a TSAP by means of the
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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.
Purpose: Assignment of transport connections to network connections.
Network Service Primitives:
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
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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.
Purpose: To convey transport protocol data unit in user data fields of network service primitives.
Network Service Primitives
N-DATA N-EXPEDITED DATA
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
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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
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.
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.
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
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- alternative protocol classes (zero or more) - version number (optional) - security (optional) - proposed options CC - destination reference (16 bits) - selected protocol class - selected options
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.
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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.
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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.
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.
DR - clearing reason (1 octet) - user data (maximum of 64 octets)
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.
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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.
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
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.
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).
Purpose: To deal with the arrival of an "unknown" DR TPDU.
TPDUs and fields used:
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DR, DC - source reference - destination reference
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.
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)
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'.
Purpose: Assignment of a Transport Connection to a different Network Connection.
TPDUs and Fields Used:
CR - source reference
RJ, DR - destination reference
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
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has received a TPDU for the Transport Connection on a Network Connection with calling and called network addresses which imply the same transport entities as the old. The TPDU will have been sent as a result of the assigning transport entity commencing resynchronization, and will thus be a RJ, or a retransmitted CR or DR.
The Transport Connection shall be recognised as having been assigned to the Network Connection on which the TPDU was received.
Purpose: To enable and minimize retransmission after possible loss of TPDUs.
Network Service Primitives:
N-DATA N-DATA ACKNOWLEDGE
TPDUs and Fields Used:
CR, CC, DR, DC
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RJ, AK, EA - YR-TU-NR (7 or 31 bits)
DT - TPDU-NR (7 or 31 bits)
ED - ED TPDU-NR (7 or 31 bits)
Copies of the following TPDUs shall be retained upon transmission to permit their later retransmission:
CR, CC, DR, DT, ED.
NOTE: If DR is sent in response to CR there is no need to retain a copy of the DR.
In the 'confirmation of receipt' variant, applicable only in Class 1, transport entities receiving N-DATA Indications which convey DT TPDUs and have the confirmation request field set shall issue a N-DATA Acknowledge Request at the earliest possible opportunity (1).
(1) It is a local matter for each transport entity to decide which N-DATA Requests should have the confirmation request parameter set. This decision will normally be related to the amount of storage available for retained copies of the DT TPDUs. Use of the confirmation request parameter may affect the quality of network service.
After each TPDU is acknowledged, as shown in Figure 5, the copy need not be retained. Copies may also be discarded when the transport connection ceases to exist.
TPDU ACKNOWLEDGED BY
CR receipt of CC, DR, or ERR, TPDU
DR receipt of DC or DR (in case of collision) TPDU
CC receipt of RJ, DT, AK, ED, EA TPDUs (or N-DATA ACKNOWLEDGE Indication.)
DT N-DATA ACKNOWLEDGE Indication when the (Note 1) DT TPDU was sent before or with the oldest N-DATA which had the confirmation request
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DT receipt of Data Acknowledge (AK) or (Note 2) Reject (RJ) TPDU for which 'YR-TU-NR' is greater than 'TPDU-NR' in the DT TPDU.
ED receipt of EA TPDU for which 'YR-TU-NR' is equal to 'ED-TPDU-NR' in the ED TPDU. Notes:
1. Applies to 'confirmation of receipt' variant. 2. Applies to 'AK' variant.
Purpose: To restore the connection to normal after an error.
Network Service Primitives:
TPDUs and Fields Used:
CR, DR, CC, DC
RJ, EA - YR-TU-NR (7 or 31 bits)
DT - TPDU-NR (7 or 31 bits)
ED - ED TPDU-NR (7 or 31 bits)
After the reset of an underlying network connection, the resynchronization procedures below are carried out by both transport entities.
After a network connection failure, the reassignment after failure function is invoked and then the resynchronization function. The sequence of events at the two transport entities is the following:
Events at the transport entity initiating reassignment: (the transport entity immediately commences resynchronization by sending a TPDU)
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o if a CR is retained then retransmit it.
o if a DR is retained then retransmit it.
o otherwise, resynchronize data:
- send RJ TPDU with 'YR-TU-NR' field set to the 'TPDU-NR' of the first unreceived DT TPDU
- when RJ TPDU has been received retransmit any ED TPDUs then DT TPDUs which are unacknowledged
- any ED TPDUs received which are duplicates shall be acknowledged (by EA TPDUs) and discarded.
Events at the other transport entity:
The transport entity shall not send any TPDUs until after receipt of the TPDU which commenced resynchronization. This TPDU therefore serves two purposes, namely indication of re-assignment and commencement of resynchronization.
o if the first received TPDU os a DR, then transmit a DC TPDU.
o if the first received TPDU is a CR and the transport connection is not idle, this means that a CC TPDU is retained: then retransmit it followed by any ED TPDU and then DT TPDUs which are outstanding (that may or may not have been transmitted previously).
NOTE: no TPDUs can be transmitted using network expedited until CC becomes acknowledged, to prevent the network expedited overtaking the CC.
o if the first received TPDU is a RJ, then act as follows:
- if a DR TPDU is retained, then retransmit it
- if a CC TPDU remains unacknowledged, then carry out the data resynchronization procedure described below
- otherwise resynchronize data:
- send RJ TPDU with 'YR-TU-NR' field set to the 'TPDU-NR' of the first unreceived DT TPDU
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- retransmit any ED TPDUs then DT TPDUs which are unacknowledged
- any ED TPDUs received which are duplicates should be acknowledged (by EA TPDUs) and discarded.
NOTE: It is possible for a transport entity using the Class 1 protocol to decide on a local basis to issue an N-RESET Request. The effect of this request at the remote transport entity is to force it to perform the resynchronization mechanism. This possibility may be used to remove congestion within the network connection.
Purpose: To detect corruption of TPDUs by the network service provider.
TPDUs and Fields Used:
All TPDUs - checksum (16 bits - 32 bits)
When a TPDU is to be transmited for a TC which has selected the checksum option, the sending transport entity must generate a checksum for the TPDU and store it in the checksum parameter in the variable part of the TPDU header. The checksum must be generated as follows:
1. Set up the complete TPDU, including the header and user data (if any). The header must include the checksum parameter in its variable part. The value field of the checksum parameter must be set to zero at this point.
2. Initialize two variables to zero. Let these variables be called C0 and C1.
3. For each octet of the TPDU, including the header, variable part of the header and the user data, add the octet value to C0, and then add the value of C0 to C1. Octets should be processed sequentially, starting with the first octet (the Length Indicator) and proceeding through the TPDU. All addition is to be performed modulo 255.
4. Calculate the value field of the checksum parameter as follows. Let the offset into the TPDU of the first octet of the value field be 'n' (where the first octet of the TPDU, the Length Indicator of the header, is considered to be at offset 1). Let the length of the TPDU, i.e. the number of times the above operation was repeated, be 'L'. Let the first octet of the checksum value, i.e., the one at offset 'n' be called 'X', and the second octet, at offset 'n+1', be called 'Y'. Then:
X = (((L - n) * C0) - C1) modulo 255 Y = (((L - n + 1) * (-C0)) + C1) modulo 255
5. Place the computed values of X and Y in the appropriate octets of the TPDU.
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An implementation may use one's complete arithmetic as an alternative to modulo 255 arithmetic. However, if either of the checksum octets X and Y has the value minus zero (i.e., 255) then it must be converted to plus zero (i.e., 0) before being stored.
When a TPDU is received for a TC for which the checksum option has been selected, the TPDU must be verified to ensure that it has been received correctly. This is done by computing the checksum, using the same algorithm by which it was generated. The nature of the checksum algorithm is such that it is not necessary to compare explicitly the stored checksum bytes. The procedure described below may be used to verify that a TPDU has been correctly received.
1. Initialize two variable to zero. Let these variables be called C0 and C1.
2. For each octet in the received TPDU, add the value of the octet to C0 and then add the value of C0 to C1, starting with the first octet and proceeding sequentially through the TPDU. All addition is to be performed modulo 255.
3. When all octets have been sequentially processed, the values of C0 and C1 should be zero. If either or both of them is non-zero, the TPDU has been received incorrectly and the verification has failed. Otherwise, the TPDU has been received correctly and the TPDU should be processed normally.
An implementation may use one's complement arithmetic as an alternative to modulo 255 arithmetic. In this case, if either C0 or C1 has the value minus zero (i.e., 255) it is to be regarded as though it was plus zero (i.e., 0)
If a checksum verification failure occurs, it is not possible to determine the TC that the TPDU relates to, since the Reference field of the TPDU may have been received incorrectly. Therefore, all TCs multiplexed onto the same NC must be treated as though a network signalled error has occurred.
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TPDUs and Fields Used:
ERR - reject cause - TPDU in error (string of octets)
DR - reason code
This function is inherent.
Any received TPDU which is invalid or which cannot be dealt with by any operative function, or which is regarded as a violation of the protocol rules of the class in use (e.g., receipt in a wrong state, window error, sequencing error, TPDU with incorrect format), shall be considered as a protocol error. Such an error shall be signalled to the transport entity responsible by the sending of an TPDU Error (ERR) TPDU or by initiating a release. The ERR TPDU conveys the octets of the offending TPDU up to and including the octet where the error was detected.
In general, no further action is defined for the sender of ERR TPDU, since it is expected that the offender will either correct the error, or close the connection.
Action to be done by the receiver depends on local implementation decision; e.g., freeze the connection, report to management, disconnect.
1. Further action is a local implementation issue. Care should be taken by the transport entity receiving several invalid TPDUs or ERR TPDUs to avoid looping if the error is repeatedly generated.
2. There are two cases in which specific action is defined for the receiver of the ERR TPDU (see Sections 6.6 and 7.0.7).
Purpose: To allow a transport connection to make use of multiple network connections to provide additional resilience against network failure, to increase throughput, or for other reasons.
This function is available only in Class 4.
When this function is being used, a transport connection is assigned (see Section 6.1) to multiple network connections. TPDUs for the
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connection may be sent over any assigned network connection. The resequencing function of Class 4 (see Section 6.21) is used to ensure that TPDUs are processed in the correct sequence.
If the use of Class 4 is not accepted by the remote transport entity following the negotiation rules, only the network connection over which the CR TPDU was sent may be used for this transport connection.
The splitting function should only be used where the supporting network connections provide similar transmit delay.
Protocol Mechanism Variant 0 1 2 3 4
Assignment to Network Conn. * * * * *
TPDU Transfer * * * * *
DT TPDU Length and Segmenting * * * * *
Concatenation and Separation * * * *
Connection Establishment * * * * *
Connection Refusal * * * * *
Release implicit * explicit * * * *
Implicit Termination * *
DT TPDU Numbering normal * m m m extended (1)o o o
Expedited Data Transfer network exp. ao not " m * * * (1)
Reassigment * *
Reassignment after Failure * *
Retention until Acknowledge- Conf. Receipt ao ment of TPDUs AK m * *
Resynchronization * *
Multiplexing and * * * Demultiplexing
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Explicit Flow Control With m * * Without * * o
Checksum (use of) m (non-use of) * * * * o
Frozen References *
Retransmission on Timeout *
Inactivity Control *
Treatment of Protocol Errors * * * * *
Splitting and recombining *
(1) not applicable in class 2 when the non use of explicit flow control is selected.
The details of the implementation of the protocol mechanisms are in certain cases different for different classes. For this reason, the following table is not intended to provide a complete description of the classes, but more to give an overview of how each class works. The exact definition of the protocol is given in the subsequent sections.
* include in the class (always)
m mandatory function (negotiable but always implemented)
o additional function (negotiable but not necessarily implemented)
ao additional function (negotiable but not necessarily implemented). Use of this option depends on the willingness of both transport entities and availability of network service.
The connection establishment function is used with the contraint that only the transport entity which has requested the establishment of the network connection may send the CR TPDU. If the calling transport entity receives a CR TPDU, it shall transfer a TPDU Error (ERR) TPDU to notify the called transport entity of the procedure error.
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For Class 0 the standard maximum transport data TPDU length is 128 octets including the data TPDU header octets.
Other maximum TPDU lengths may be supported in conjunction with the optional transport data TPDU size negotiation function (see Section 8.3 and 8.4). Optional maximum data field lengths shall be chosen from the following list: 256, 512, 1024 and 2048 octets.
TSDUs are transmitted using the segmenting function.
Class 1 provides transport connections with flow control based on the network service provided flow control, error recovery, expedited data transfer, disconnection, and also the ability to support consecutive Transport connections on a network connection.
This class provides the functionality of Class 0 plus the ability to recover after a failure signalled by the Network Service, without involving the user of the Transport Service.
The sending Transport entity keeps a copy of transmitted TPDUs until it receives an acknowledgment which allows copies to be released. After a failure is indicated by the nerwork service (Reset, Disconnect), the resynchronization function is used to determine which TPDUs must be retransmitted.
Resynchronization may also be invoked by a transport entity as a local matter. For that purpose the Resynchronization function is used (see note at the end of Section 6.15).
Acknowledgements are used to release copies of retained TPDUs.
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Two methods of acknowledgment are provided in the Retention until Acknowledgement of TPDUs function:
o use of AK TPDU ("AK" variant) - mandatory
Note: The credit field of the AK TPDU is not used in this class (always Set to zero).
o use of network layer Confirmation of Receipt Service. ('confirmation of receipt' variant) - optional
The variant to be used is negotiated during the Connection Establishment Phase. The default option is the "AK TPDU" variant. Use of Network Layer Receipt Confirmation is allowed only in Class 1, and depends on the availability of the network layer receipt confirmation service, the expected cost reduction, and the agreement of both transport entities to use it.
7.1.4 Connection Establishment Procedures for Class 1
The 'assignment to network connection' and 'connection establishment' mechanisms are used. From the point at which a transport entity issues a CR proposing the use of Class 1 or a CC accepting the use of Class 1 the following mechanisms must be available to deal with signalled errors during connection establishment:
o Reassignment after failure o Retention until Acknowledgement of TPDUs o Resynchronization
If no DT or ED TPDU is to be sent, receipt of a CC should be acknowledged.
Data transfer is accomplished using the 'TPDU transfer' 'Concatenation' and 'DT TPDU Length and Segmenting' mechanisms. 'DT TPDU Numbering' and 'Retention until Acknowledgement of TPDUs' are used in support of error recovery.
After receiving a network reset, the Resynchronization mechanism is invoked. After receiving a network disconnect, the 'Reassignment after Failure' mechanism is invoked after which the 'Resynchronization' mechanism is invoked.
The 'Spurious Disconnect' mechanism is used to deal with receipt of a DR TPDU for an unrecognised Transport
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The Expedited Data Transfer mechanism is used. Two methods are possible to provide the function:
o non network expedited variant
Note: (1) This method is always included in this class.
Note: (2) The EDTPDU-NR of the ED TPDU contains an identification number. This number must be different for successive ED TPDUs. That is, when an ED TPDU has been sent and an EA TPDU for the ED TPDU has been received, the next ED TPDU must have a different value in the EDTPDU-NR field. No other significance is attached to EDTPDU-NR field. It is recommended but not essential, that the values used be consecutive modulo 128.
o network expedited variant
Note: (1) The use of this method is determined through negotiation during transport connection establishment.
The 'explicit' variant of the Release mechanism is used.
Receipt of an error indication by a transport entity, which, prior to this event has sent a DR, causes this transport entity to retransmit DR. Only DC and DR will be accepted and interpreted as the completion of the connection release sequence. The related Reference will become unassigned.
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way to multiplex several transport connections onto a single network connection. This class has been designed to be used in association with type A network connections.
Use of Explicit Flow Control
The objective is to provide flow control to help avoid congestion at end-points and on the network connection. Typical use is when traffic is heavy and continuous, or when there is intensive multiplexing. Use of flow control can optimize response times and resource utilization.
Non Use of Explicit Flow Control (optional)
The objective is to provide a basic transport connection with minimal overheads suitable when independence of transport and network connection lifetime is desirable. The class would typically be used for unsophisticated terminals, and when no multiplexing onto network connections is required. Expedited data is never available.
Class 2 provides transport connections with or without individual flow control - no error detection or error recovery is provided.
If the network resets or clears, the transport connection is terminated without the transport clearing sequence and the transport user is informed.
When explicit flow control is used a credit mechanism is defined allowing the receiver to inform the sender of the exact amount of data he is willing to receive and expedited data transfer is available.
The receiver of data TPDUs holds a count of the sequence number of the next expected TPDU. This count is called the Receive Sequence Number, T(R). The receiver indicated to the sender the number of Data TPDUs he is ready to receive by means of a 'credit' mechanism. Credits are given using the credit field in the AK TPDU. The value of the credit field, in conjunction with the value of T(R) transported by the YR-TU-NR (your TPDU number) field of the AK TPDU, is used by the receiver of the AK TPDU to determine whether and how many Data TPDUs may be accepted by the sender of the AK TPDU. Precise definition of flow control principles appears in Section 188.8.131.52.3.
The non network expedited variant is used. Normal flow is the flow of data subject to the flow control mechanism, expedited flow is the flow of data that the sender may send without explicit agreement of the receiver. This expedited flow has a limited capability and could for example be used to carry session supervisory commands.
The number of expedited data units outstanding at any time is limited to one and the amount of TS-user data is limited (up to 16 octets).
An expedited data may arrive before normal data which was submitted earlier. Normal data submitted after the expedited
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data will arrive after the expedited data.
7.2.4 Connection Establishment Procedures for Class 2
See Section 6.5 for reference assignment. Receipt of any TPDU with a reference that is not assigned to a transport connection other than a Disconnect Request (DR) or Connection Request (CR) will be ignored.
Receipt of a Disconnect Request (DR) for an unassigned Reference will result in a Disconnect Confirm (DC) response.
This phase is achieved by exchange of CR/CC TPDU using the 'connection establishment' function. Since the multiplexing function is in use, then more than one transport connection may be assigned to the same network connection concurrently. The restrictions of Class 0 does not apply to this class and the other higher classes.
184.108.40.206 Sending Data TPDU (No Explicit Flow Control Option)
In this case the data TPDU is built in accordance with the rules stated in Section 6.2 and 6.3 and sent without any additional mechanisms. Thus, the DT TPDU NR field may take any value and no AK TPDU is used.
220.127.116.11 Sending Data TPDU (When Explicit Flow Control is Used)
On each transport connection the transmission of Data TPDUs is controlled separately for each direction and is based on authorization from the receiver.
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This authorization is provided through the use of the TPDUs Credit field. Credit field values are only present in the following TPDUs: CR, CC, AK..
Each Data TPDU transmitted between transport entities, for each direction of transmission in a transport connection, is sequentially numbered.
The sender of Data TPDUs holds a count of the next TPDU to be sent. This count is called the Send Sequence Number T(S). The sender indicates to the receiver the number of the data TPDU he sends by putting the current T(S) value into the TPDU-NR field of the data TPDU.
Sequence numbering is performed modulo 2**n, where n is the number of bits of the sequence number field. The T(S) counter cycles through the entire range 0 to (2**n)-1.
At connection establishment time both Transport entities initialize their T(S) and T(R) counts to zero (i.e. the first Data TPDU to be transmitted between transport entities for a given direction of data transmission after the connection establishment has a TPDU-NR field set to zero).
Receipt of a Data TPDU whose TPDU-NR field is not equal to the expected value T(R), is to be regarded as a protocol error.
Operations described above are summarized as follows:
T(S) = 0 T(R) = 0
Sending of Data TPDU put T(S) into the TPDU-NR field of the Data TPDU to be sent
T(S) = (T(S) + 1) (modulo 2**n)
Receiving of Data TPDU
TPDU-NR field of the received data TPDU which is not equal to T(R) is a protocol error.
T(R) = (T(R) + 1) (modulo 2**n)
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For each transport connection and for each direction of data transmission a 'transmit window' is defined as the (possibly null) ordered set of consecutive data TPDUs authorized to be transmitted in that direction. At any given time, the lowest sequence number of a data TPDU which a transport entity is authorized to transmit is referred to as the 'lowest window edge'. The 'upper window edge' is calculated by adding the credit allocation, given by the value of the Credit (CDT) field contained in a received TPDU, to the lower window edge. Note that a transport entity is authorized to send data TPDUs with sequence numbers up to but not including the upper window edge.
Upper window edge = N (Credit received either in CR or in CC and N < 2**p < 2** (n-1), where P is the number of bits in credit field of CR and CC.
o Sending of a Data TPDU
Send data TPDUs while T(S) is less than the upper window edge. If T(S) equals the upper window edge then wait for additional credit before sending.
o Reception of Data TPDU (with TPDU NR = T(R)
If T(R) is greater than or equal to the upper window edge authorized to the sending transport entity, then the receiving transport entity shall use the Treatment of Protocol Errors function. Otherwise T(R) shall be incremented.
Send AK TPDU with YR-TU-NR = T(R) and Credit equals N. (Where N = number of additional data TPDUs the entity is prepared to receive.)
Receiving Credit in AK.
Lower window edge = YR-TU-NR received.
Upper window edge = Lower window edge + N.
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The value of the upper window edge cannot be decreased in this class. If, at a certain point of time, the upper window edge value is U, the reception of an AK TPDU having YR-TU-NR = M and CDT = N such that:
(U-M) (mod. 2**n) > N
is a protocol error
Provided the previous statements are respected, CDT field may take any value including zero.
The procedure of expedited data transfer allows a transport entity to transmit data to the remote transport entity without following the flow control procedure of the normal data flow. This procedure can only apply in the transfer phase.
The expedited procedure has no effect on the transfer and flow control applying to normal Data TPDUs.
To transmit expedited data, the transport entity sends an expedited data TPDU (ED TPDU). The size of a data field is limited (up to 16 octets). The data field contains a complete ED TSDU. The remote transport will then confirm the receipt of the ED TPDU by transmitting an expedited TPDU acknowledgement (EA TPDU). A transport entity can send another ED TPDU only after having received an EA TPDU for the previously transmitted ED TPDU. In class 2 the ED TPDU NR field of the ED and YR-TU-NR field of the EA TPDU are not defined and may take any value.
The characteristics of Class 3 in addition to those of Class 2 is to mask errors indicated by the network. Selection of this class is usually based upon reliability criteria. Class 3 has been designed to be used in association with type B network connections.
This class provides the functionality of Class 2 (with use of explicit flow control) plus the ability to recover after a failure signalled by the Network Layer without involving the user of the transport service.
The mechanisms used to achieve this functionality also allow the implementation of more flexible flow control.
Class 3 mechanisms include Class 2 (with use of explicit flow control option) mechanisms and the ability to recover after a failure signalled by the network without informing the user of the transport connection.
The sending transport entity keeps a copy of transmitted Data TPDUs and ED TPDUs until it receives a positive aknowledgement which allows copies to be released. It may also receive an RJ command inviting it to retransmit or transmit all Data TPDUs, if any, from the point in the sequence indicated in the RJ command.
This is especially the case, when a failure is indicated by the network. The transport entity sends an RJ command in order to indicate the sequence number of the next expected TPDU.
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Error recovery for ED TPDU is achieved by retransmission (see 18.104.22.168).
22.214.171.124 Relationship between Flow Control and Error Recovery
Acknowledgement is performed by use of the T(R) count. A credit is associated with this acknowledgement which may be equal to or greater than zero. Thus it is possible to acknowledge data without giving the right to send new data.
Credit may be reduced, by the use of the RJ TPDU.
7.3.4 Connection Establishment Procedure for Class 3
The rules for Class 2 (with use of explicit flow control) apply with the addition of the following rules which apply on receipt of an eror indication from the Network layer.
o Reception of an error indication by a transport entity which, prior to this event, has sent a CR and has not yer received a CC, causes the transport entity to retransmit CR.
o Reception of an error indication by a transport entity to wait for reception of CR, RJ or DR TPDU. In this case:
- Reception of CR will cause the transport entity to retransmit CC.
- Reception of RJ will cause the transport entity to transmit an RJ with a YR-TU-NR equal to zero and enter the data phase.
- Reception of a DR will cause termination of the transport connection as for Classes 1 and 2 (see 7.1.4).
TPDU retransmission is a procedure which allows a transport entity to request retransmission of one or several consecutive Data TPDUs from the remote transport entity. A
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transport reject condition is signalled to the remote transport entity by transmission of an RJ TPDU whose YR-TU-NR field indicates the sequence number of the next expected Data TPDU.
On receipt of a RJ TPDU, a Transport entity shall accept credit to the value contained in the credit field and shall re-transmit TPDUs, starting with the one whose number is specified in the YR-TU-NR field of the received RJ TPDU, subject to the new credit.
The transport entity shall not specify a T(R) in the RJ TPDU less than that which has previously been acknowledged. Receipt of an RJ TPDU with a T(R) which has been previously acknowledged will be considered a protocol error.
Additional DT TPDUs pending initial transmission may follow the retransmitted DT TPDU(s) if the window is not closed.
It is possible to decrease the value of the upper window edge down to the sequence number transported by YR-TU-NR field of the RJ TPDU. Receipt of an DT TPDU which would have been inside the window before the reduction is not a protocol error and this TPDU may be discarded.
Note: In such a case the credit equal to zero achieves the effect of a Receive not Ready Condition.
126.96.36.199 Behaviour after an error signalled by the network layer
After receiving an error indication from the Network Service, the transport entity shall tranmit to the remove entity an RJ TPDU with YR-TU-NR field indicating the sequence number of the next expected Data TPDU.
In Class 3, the ED TPDU-NR field of the Expedited Data (ED) TPDU contains an identification number. This number must be different for successive ED TPDUs. That is, when an ED TPDU has been sent and an EA TPDU for the ED TPDU has been received, the next ED TPDU must have a different value in the NR field of the ED TPDU. No other significance is attached to this field. It is recommended, however, that the values used be consecutive modulo 2**n. When a transport entity receives an ED TPDU for a transport connection, it shall respond by transmitting an expedited acknowledgement (EA) TPDU.
It places in the YR-TU-NR field the value contained in
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the NR field of the received ED TPDU. If, and only if, this value is different from the NR field of the previously received ED TPDU, the data contained in the TPDU is to be passed to the session entity.
If an error indication from the Network layer is received before the receipt of the expected Expedited Acknowledgement (EA) TPDU, the transport entity shall retransmit the ED TPDU with the same value in the NR field. By the rule described in the previous paragraph, the session entity does not receive data corresponding to the same expedited TPDU more than once.
The rules for Class 2 apply with the addition of the following rule:
Receipt of an eror indication by a transport entity, which prior to this event has sent a DR, causes this transport entity to retransmit DR. Only DC and DR will be accepted and interpreted as the completion of the connection clearing sequence. The related Reference will become unassigned.
The characteristic of Class 4, in addition to those of Class 3, is the detection of errors which occur as a result of the low grade of service available from the network layer. The kinds of errors to be detected include: TPDU loss, TPDU delivery out of sequence, TPDU duplication. These errors may afect control TPDUs as well as Data TPDUs.
Class 4 has been designed to be usd in association with network connections of type C.
This class provides the functionality of Class 3, plus the ability to detect and recover from lost, duplicated or out of sequence TPDUs without involving the user of the transport service.
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This detection of errors is made by extended use of the sequence numbering of Classes 2 and 3, by a timeout mechanism, and by additional protocol mechanisms.
This class additionally detects and recovers from damaged TPDUs by using a checksum mechamism. The use of the checksum mechanism must be available but its use or its non use is subject to negotiation. Class 4 does not attempt to deal with detection of errors due to the misdelivery of TPDUs.
The network layer is assumed to provide, as an aspect of its grade of service, for a bound on the maximum lifetime of NSDUs in the network. This value is known by the Transport Layer. The maximum time which may elapse between the transmission of an NSDU into the network layer and the receipt of any copy of it is referred to as M.
It is assumed that there is some value of transmit delay in the network, typically much less than M, which will be the maximum delay suffered by all but a small proportion of NSDUs. This value is referred to as E.
Any transport entity is assumed to provide a bound for the maximum time which can elapse between its receipt of a TPDU from the Network Layer and its transmisssion of the Corresponding response. this value is referred to as A/L. The corresponding time given by the remote transport entity is referred to as A/R. The values for these timers may be conventionally established or may be established at connection establishment time.
The local transport entity is assumed to maintain a bound on the time it will wait for an acknowledgement before retransmitting the TPDU. This time is the local retransmission time and is referred to as T1.
T1 = 2*E + X + Ar?
Where X is a value to allow for TPDU processing in the local transport entity.
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The local transport entity is assumed to provide a bound for the maximum time for which it may continue to retransmit a TPDU requiring positive acknowledgment. This value is referred to as R.
The value is clearly related to the time elapsed between retransmission, T1, and the maximum number of retransmissions, N. It is not less than T1*N+X, where X is small quantity to allow for additional internal delays, the granularity of the mechanism used to implement T1 and so on. Because R is a bound, the exact value of X is unimportant as long as it is bounded and the value of a bound is known.
188.8.131.52 Bound on Reference Identifier and Sequence Numbers
Using the above values, a bound L may be established for the maximum time between the decision to transmit a TPDU and the receipt of any response relating to it. The value of L is given by:
L = 2*M+R+Ar
It is necessary to wait for a period L before reusing any reference or sequence number, to avoid confusion in case a TPDU referring to it may be duplicated or delayed.
(Note: In practive, the value of L may be unacceptably large. It may also be only a statistical figure at a certain confidence level. A smaller value may therefore be used where this still allows the required quality of service to be provided).
To protect against unsignalled breaks in the network connection (Half-open connections), each transport entity maintains an inactivity time interval. If the interval passes without receipt of some TPDU, the transport entity will terminate the TC by making use of the release procedure. This interval is referred to as I.
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In class 4, the transport entity associates a response time with TPDUs sent requiring a response. If an appropriate response is not received within time T1, the recovery procedure must be invoked by the sender. This will usually involve the retransmission of the corresponding TPDU.
A TPDU may be transmitted a maximum number of times, This number is referred to a N. The value of N is chosen so that the required quality of service can be provided given the known characteristics of the network connection.
In Class 4 sequence numbering is applied to certain control TPDUs and their acknowledgements, as well as to DT TPDUs. These are ED and its acknowledgement EA.
The length of sequence numbers may be negotiated at connection establishment. Where other than the default length is used, an extended header format is used for sequenced TPDUs containing additional octets of sequence numbers. Extended header format includes a credit field on the appropriate TPDU types allowing extended credit allocation.
7.4.4 Procedures for Connection Establishment Phase
The following features pertain to connection establishment for Class 4:
o In Class 4, a connection is not considered established until the successful completion of a 3-way TPDU exchange. The sender of a CR TPDU must respond to the corresponding CC
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TPDU by immediately sending a DT, ED or AK TPDU.
o As a result of duplication, a CR TPDU may be received specifying a source reference which is already in use with the sending transport entity. If the receiving transport entity is in the data transfer phase, having completed the 3-way TPDU exchange procedure, the receiving transport entity should ignore such a TPDU. Otherwise a CC TPDU should be transmitted.
o As a result of duplication or retransmission, a CC TPDU may be received specifying a paired reference which is already in use. The receiving transport entity should ignore such a CC TPDU.
o A CC TPDU may be received specifying a reference which is in the frozen state. The response to such a TPDU should be a DR TPDU.
When a transport entity transmits a CR TPDU it starts timer T1. If this timer expires before a CC TPDU is received, the CR TPDU is retransmitted and the timer restarted. After transmission of the CR TPDU N times, the connection establishment procedure is abandoned and the failure reported to the transport user. The reference must be placed in the frozen state for a period L (see section 184.108.40.206).
When a transport entity transmits a CC TPDU it starts timer T1. If this timer expires before an AK or DT TPDU is received, the CC TPDU is retransmitted according to the retransmission principles in Section 220.127.116.11
When a CC TPDU is received, the receiving transport entity enters the data transfer phase. It must immediately transmit an AK, ED or DT TPDU to complete the 3-way TPDU exchange. The CC TPDU is subject to the usual rules of the data transfer phase regarding retransmission, see Section 18.104.22.168.
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When an AK, DT or ED TPDU is received the receiving transport entity can enter the data transfer phase. If the entity has data to send it may send DT TPDUs or an ED TPDU. The DT TPDUs are subject to flow control. Otherwise, the transport entity must obey the inactivity principles (see Section 22.214.171.124).
The CR and CC TPDUs may allocate an initial credit value to their respective recipients. This value is limited to 15 by the encoding of the TPDU. Where the extended header format is in use, credit values greater than 15 must be allocated using AK TPDUs.
A transport entity may transmit the value it intends to use for AL at connection establishment, as the 'Acknowledge Time' parameter in the CR or CC TPDU (depending on whether the transport entity is initiating or accepting the transport connection). If this parameter is present in a received CR or CC TPDU, the value of AR should be set accordingly. If this parameter is not present, AR may be assumed to be insignificant in comparison to E the typical maximum transit delay.
When a transport entity has some outstanding DT or ED TPDUs that require acknowledgement, it will check that no T1 interval elapses without the arrival of an AK or EA TPDU that acknowledges one of them. If the timer expires, the first TPDU is retransmitted and the timer is restarted. After N transmissions (N-1 retransmissions) the connection is assumed to have failed and the release phase is entered, and the transport user is informed.
DT TPDUs which fall beyond the current window (due to reduction of the upper window edge) are not retransmitted until advancement of the upper window edge so permits.
Note: This requirement can be met by different means, for example.
1. One timer is associated with each TPDU. If the timer expires, the associated TPDU will be retransmitted, and the timer T1 will be restarted for all subsequent DT TPDUs.
2. One timer is associated with each TC:
if the transport entity transmits a DT TPDU requiring acknowledgement, it starts timer T1,
if the transport entity receives a TPDU that acknowledges one of the TPDUs to be acknowledged, timer T1 is restarted,
if the transport entity receives a TPDU that acknowledges the last TPDU to be acknowledged, timer T1 is stopped.
For the decision whether the retransmission timer T1 is maintained on a per TPDU or on a per TC basis, throughput considerations have to be taken into account.
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A transport entity operating class 4 must acknowledge all TPDUs received requiring acknowledgment. To avoid unnecessary retransmissions and to avoid delays to transmission by the remote transport entity, the delay for acknowledgement should not exceed timer A (see Section 126.96.36.199). L
There are two TPDU types that must be acknowledged: ED and DT. Receipt of an ED TPDU must be acknowledged by an EA TPDU. A DT TPDU is acknowledged with an AK TPDU.
An AK TPDU has the sequence number of the next DT TPDU the receiving transport entity expects to receive. It thus acknowledges receipt of all DT TPDUs with sequence numbers less than the acknowledgement number.
An AK TPDU may be repeated at any time, using the sequence number in the last AK TPDU sent.
Flow control in Class 4 is subject to the same principles as in Classes 2 and 3. The credit mechanism and window principle of those classes still apply, except that in class 4, the upper window edge can be reduced by the receiving transport entity by sending an AK TPDU with a smaller credit.
A receiving transport entity may send an AK TPDU at any time to change the window size. This AK TPDU may acknowledge a new DT TPDU or may repeat a previous acknowledgement.
To ensure the synchronization of flow control information the window timer provokes the frequent exchange of AK TPDUs between transport entities. The window timer maintains a minimum level of TPDU traffic between transport entities cooperating in a transport connection.
In Class 4 the window size can be reduced in any AK TPDU. Due to the possibility of misordering of AK TPDUs and the associated loss of efficiency, the AK TPDU for class 4 includes an additional field called the AK TPDU subsequence parameter.
An AK TPDU should contain the subsequence parameter whenever a duplicate AK TPDU is sent with the same sequence number but with reduced credit. The value of the subsequence parameter is
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set to one for the first time the AK TPDU is resent with reduced credit.
When an AK TPDU is transmitted whose sequence number is increased, the 'sub-sequence' parameter is omitted until credit reduction takes place.
When an AK TPDU is received, it must be processed (i.e., its contents made use of) only if:
o The sequence number is greater than in any previously received AK TPDU, or,
o The sequence number is equal to the highest in any previously received AK TPDU, and the sub-sequence parameter is greater than in any previously received AK TPDU having the same sequence number (where an absent sub-sequence parameter is regarded as having a value of zero), or
o The sequence number and sub-sequence parameter are both equal to the highest in any previously received AK TPDU (where an absent sub-sequence parameter is regarded as having a value of zero), and the credit field is greater than in any previously received AK TPDU having the same sequence and sub-sequence numbers.
When an AK TPDU is transmitted which opens a closed window (i.e. increases credit from zero), it should be retransmitted at an interval of T1. Transmission should occur a maximum of N times, after which the usual inactivity retransmission timer should be reverted to. Retransmission may also cease if the local transport entity becomes sure that the new credit information has been received by the remote transport entity.
If a transport entity receives an AK TPDU containing a 'Flow Control Confirmation' parameter, whose Lower Window Edge and Your-Sub-Sequence fields are equal to its own lower window edge and sub-sequence number, it may note that the credit available at the remote transport entity (relative the Lower Window Edge field) is at least equal to the value conveyed as Your Credit. This enables the transport entity to cease the frequent retransmission of window information, if it thereby knows that the remote window is open.
A transport entity need not retransmit window information (except as described under Inactivity Principles) if it is aware by the receipt of DT TPDUs that the remote transport entity
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has sufficient credit to prevent deadlock. When an AK TPDU is transmitted in response to a DT TPDU, the transport entity may normally assume that the transmitter of the DT TPDU will ensure that the AK TPDU is received, be retransmission of the DT TPDU if necessary. Therefore, it can normally be assumed that the credit conveyed in such an AK TPDU will be available to the remote transport entity, and frequent retransmission is unnecessary.
The assumption that the DT TPDU will be retransmitted may be incorrect if credit reduction has taken place. Therefore, a transport entity may not make this assumption if the sequence number of the DT TPDU is less than or equal to the highest value for which permission to transmit (i.e., credit) has been given and subsequently withdrawn.
Upon receipt of an AK TPDU which increases the upper window edge, a transport entity may transmit an AK TPDU which repeats the information contained in the received TPDU in a 'Flow Control Confirmation' parameter in its variable part an thereby assures the transmitter of the original AK TPDU of its own state. Such an AK TPDU may be tranmmitted:
o Upon receipt of a duplicated AK TPDU (i.e., one which is identical in all fields, including the sub-sequence parameter if present, to the most recently received AK TPDU which was not discarded due to detection of a sequence error), not containing the 'Flow Control Confirmation' parameter.
o Upon receipt of an AK TPDU which increases the upper window edge but does not increase the lower window edge, when the upper window edge was formerly equal to the lower window edge.
The procedure for expedited data is as for Class 3, with the following exceptions.
The ED TPDU has a sequence number which is allocated from a separate sequence space from that of the DT TPDUs. The EA TPDU carries the same sequence number as the corresponding ED TPDU. Only a single ED TPDU may be transmitted and awaiting acknowledgements at any time.
Upon receipt of an unduplicated ED TPDU, a transport entity immediately forwards the data to the transport user. The ED
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TPDU sequence number is recorded in an EA TPDU sent to the other transport entity.
The sender of an ED TPDU shall not send any new DT TPDU with higher T(S) until it receives the EA TPDU. This guarantees the arrival of the ED TPDU before any subsequently sent DT TPDUs.
If the Inactivity Time I passes without receipt of some TPDU, the transport entity will terminate the TC by making use of the release procedure. To present expiration of the remote transport entity's inactivity times when no data is being sent, the local transport entity must send AK TPDUs at suitable intervals in the absence of data, having regard to the probability of TPDU loss. The Window Synchronization Principles (see 188.8.131.52) may ensure that this requirement is met.
Note: It is likely that the release procedure initiated due to inactivity timer expiration will fail, as such expiration indicates probable failure of the supporting NC or of the remote transport entity. This case is described in Section 7.4.6.
The rules for class 3 apply. The DR TPDU is subject to the usual retransmission procedure. After N retransmissions, the transport connection is considered disconnected, the Reference is placed in the frozen state for a period L and retransmission ceases.
The DC TPDU is sent only in response to a DR TPDU, and is not subject to the retransmission procedure.
The DC TPDU when received allows the transport entity to release all resources maintained for the transport connection.
The DR TPDU does not carry a sequence number. Any previously transmitted TPDUs (including DT and ED) which are received after the DR TPDU result in a further DR TPDU but are otherwise ignored. After disconnection, for whatever reason, the Reference is placed in the frozen state for a period L.
When a transport connection is terminated, the corresponding references cannot be immediately reused since TPDUs containing these references may continue to exist in the network for a time up to L. Therefore, these references will be placed in an unassignable, frozen state for this peiod.
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After an event involving loss of transport entity state information, including the status of reference assignments, all references relating to network connections whose transport state information has been lost must be placed in the frozen state for a period L.
If a DC TPDU is received for a local reference which is in the frozen state, or with a remore reference not matching the already recorded one, this DC TPDU shall be ignored.
As defined in the previous sections, all the Transport Protocol Data Units (TPDU) shall contain an integral number of octets. The octets in a TPDU are numbered starting from 1 and increasing in the order of transmission. The bits in an octet are numbered from 1 to 8, where bit 1 is the low-ordered bit.
There are tao types of TPDUs:
o Data TPDUs, used to transfer Transport Service Data Units (TSDUs). The structure of the TSDUs is maintained by means of the TSDU End Mark.
o Control TPDUs, used to control the transport protocol functions, including the optional functions.
This field is contained in the first octet of the TPDUs. The length is indicated by a binary number, with a maximum value of 254 (11111110). The length indicated is the header length, including parameters, but excluding the length indicator field and user data, if any. The value 255 (11111111) is reserved for possible extensions.
The fixed part contains frequently occurring functions including the code of the TPDU. The length and the structure of the fixed part are defined by the TPDU code, defined by bits 5 to 8 of the second octet of the header.
The variable part is used to define parameters relating to optional functions. If the variable part is present, it shall contain one or more parameters. The number of parameters that may be contained in the varialbe part is indicated by the length of
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the variable part which is a LI minus the length of the fixed part.
Since the currently defined minimum fixed part for headers which allow parameters is four octets, and since the length indication field is limited to a maximum of 254, the maximum length of the variable part is 250 octets.
Each parameter contained within the variable part is coded as follows:
o The parameter code field is coded in binary and, without extensions, provides a maximum number of 255 different parameters. However, as noted below, bits 8 and 7 indicates the source of definition, so the practical maximum number of different parameters is less. Parameter code 1111 1111 is reserved for possible extensions of the parameter code.
o The parameter length indication indicates the length, in octets, of the parameter value field. The length is indicated by a binary number, "m" with a theoretical maximum value of 255. The practical maximum value of "m" is lower. For example, in the case of a single parameter contained within the variable part, two octets are required for the Parameter Code and the Parameter Length Indication itself. Thus, the value of "m" is limited to 248. For larger fixed parts of the header and for each succeedimg parameter, the maximum value of "m" decreases.
o The parameter value field contains the value of the parameter identified in the parameter code field.
o No standard parameter codes use bits 8 and 7 with the value 00.
o Implementations shall accept the parameters defined in the variable part in any order. If any parameter is duplicated then the later value will be used.
This field contains data of the TSDU being transmitted. The length of this field is limited to the negotiated TPDU size for this transport connection minus 3 octets in Classes 0 and 1, and minus 5 octets (normal header format) or 8 octets (extended header format) in the other classes. The variable part, if presemt, amy further reduce the size of the user data field.
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Not applicable for Class 0 and Class 2 when the "no explicit flow control" option is selected, and for Class 1 when the network receipt confirmation option is selected.
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Flow Control Confirmation (class 4 only - optionally used)
This parameter contains a copy of the information received in an AK TPDU, to allow the transmitter of the AK TPDU to be certain of the state of the receiving transport entity (See Section 184.108.40.206).
Parameter Code: 100001011
Parameter value field 64 bits, used as follows:
o Lower Window Edge (32 bits) Bit 32 is set to zero, bits 31 to 1 contain the YR-TU-NR value of the received AK TPDU. When normal format is in use, only the least significant seven bits (bits 1 to 7) of this field are significant.
o Your Sub-Sequence (16 bits) Contains the value of the sub-sequence parameter of the received AK TPDU, or zero if this parameter was not present.
o Your Credit (16 bits) Contains the value of the CDT field of the received AK TPDU. When normal format is in use, only the least significant four bits (bits 1 to 4) of this field are significant.