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. 6.13 Reassignment After Failure Purpose: Recovery from network provider initiated disconnect. Network Service Primitives: N-DISCONNECT Indication Description: When a N-DISCONNECT Indication arrives for the network connection to which a transport connection is assigned, the transport connection must be reassigned by its initiator (see "Reassignment") If the reassignment has not successfully occurred within a time of T-wait seconds, then the transport connection must be considered as non-existent by both transport entities.1 1. The CR TPDU does not have a destination reference; nevertheless it can be distinguished from a new connection attempt by having the same source reference. NOTE: The value of T-wait has to be agreed by the communicating transport entities. 6.14 Retention Until Acknowledgement of TPDUs Variants: 'confirmation of receipt' or 'AK' 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
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) Description: 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
field set. 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. Figure 5. Acknowledgement of TPDUs 6.15 Resynchronization Purpose: To restore the connection to normal after an error. Network Service Primitives: N-RESET Indication 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) Description: 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)
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
- 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. 6.16 Multiplexing and Demultiplexing Purpose: Concurrent sharing of a network connection by several transport connections. TPDUs and Fields Used: CC, DR, DC, DT, AK, ED, EA, RJ, ERR - destination reference Description: This function is pervasive. When this function is in operation, more than one transport connection can be simultaneously assigned to the same network connection. Every TPDU (including DT TPDUs) must carry the destination reference, to identify the transport connection to which it refers. 6.17 Explicit Flow Control Purpose: Regulation of flow of DT TPDUs independently of the flow control in the other layers. TPDUs and Fields Used: CR, CC, AK, RJ - CDT (4 or 16 bits) DT - TPDU-NR (7 or 31 bits) AK, RJ - YR-TU-NR (7 or 31 bits) - subsequence number (optional) - flow control confirmation (optional)
Description: The mechanism depends on the class. Thus the description can be found in the section describing the class. 6.18 Checksum Purpose: To detect corruption of TPDUs by the network service provider. TPDUs and Fields Used: All TPDUs - checksum (16 bits - 32 bits) Description: 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.
NOTE 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. NOTE 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. 6.19 Frozen References Purpose: To prevent re-use of a reference while TPDUs associated with the old use of the reference may still exist. Description: When a transport entity determines that a particular connection has terminated, the reference shall be placed in a frozen state
during which time it will not be reused. The circumstances under which this is done, and the period of time for which the reference remains frozen depends on the class. 6.20 Retransmission on Timeout Purpose: To cope with unsignalled loss of TPDUs by the network service provider. TPDUs and Fields Used: CR, CC, DR, DT, ED, AK Description: The description is given in the section related to class 4. 6.21 Resequencing Purpose: To cope with misordering of TPDUs by the network service provider. TPDUs and Field Used: DT - TPDU NR ED - ED TPDU NR Description: The description is given in the section related to class 4. 6.22 Inactivity Control Purpose: To cope with unsignalled termination of a network connection. TPDUs and Fields Used: AK Description: The description is given in the section related to class 4. 6.23 Treatment of Protocol Errors Purpose: To deal with invalid TPDUs.
TPDUs and Fields Used: ERR - reject cause - TPDU in error (string of octets) DR - reason code Description: 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. NOTES: 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). 6.24 Splitting and Recombining 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. Description: 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
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
Explicit Flow Control With m * * Without * * o Checksum (use of) m (non-use of) * * * * o Frozen References * Retransmission on Timeout * Resequencing * 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. 7. PROTOCOL CLASSES 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. KEY * 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. na not applicable. 7.0 PROTOCOL DESCRIPTION OF CLASS 0: SIMPLE CLASS 7.0.1 Characteristics of Class 0 The characteristic of this class is that it provides the simplest type of transport connection and fully compatible
with the CCITT recommendations S.70 for Teletex terminals. The class is designed for use in association with network connections of type A (see 220.127.116.11.4.). 7.0.2 Functions of Class 0 This class is designed to have minimum functionality. It provides only the functions needed for connection establishment with negotiation, data transfer with segmenting and protocol error reporting. Class 0 provides transport connections with flow control based on the network service provided flow control, and disconnection based on the network service disconnection. 7.0.3 Protocol Mechanisms of Class 0 18.104.22.168 Connection Establishment Phase Connection shall be made in accordance with the general rules (Assignment of Network Connection, Connection Establishment and Connection Refusal) with the following restrictions: o No exchange of user data is allowed. o Only TSAP-ID and TPDU size parameters are allowed. 22.214.171.124 Data Transfer Phase o Segmenting (DT TDPU length and Segmenting) o Detection and indication of procedural errors. 126.96.36.199 Release Phase There is no explicit transport connection release procedure for this class. The lifetime of the transport connection is directly correlated to the lifetime of the network connection. 7.0.4 Connection Establishment for Class 0 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.
7.0.5 Data Transfer Procedures 188.8.131.52 General The data transfer procedures described in the following subsections apply only when the transport layer is in the data transfer phase, that is after completion of Transport Connection establishment. 184.108.40.206 Transport Data TPDU maximum length 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. 7.0.6 Release Procedure The "implicit" variant of the release function is used. 7.0.7 Treatment of invalid TPDUs The "treatment of protocol errors' function is used. 7.0.8 Behaviour after an error signalled by the network service. The implicit termination function is used and the high layer is informed about this disconnection. 7.0.9 Supported Options None 7.1 PROTOCOL DESCRIPTION OF CLASS 1: BASIC ERROR RECOVERY CLASS 7.1.1 Characteristics of Class 1 The characteristic of this class is that it provides a basic transport connection with minimal overheads. The main purpose of the class if to recover from network signalled errors (network disconnect or reset). Selection of this class is usually based on
reliability criteria. Class 1 has been designed to be used in association with type B network connections. 7.1.2 Functions of Class 1 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. 7.1.3 Protocol Mechanisms of Class 1 Class 1 protocol mechanisms include Class 0 protocol mechanisms plus the following: 220.127.116.11 User Data in the Connection Phase Class 1 provides the possibility of conveying data in the connection request and confirm commands. 18.104.22.168 Numbering of Data TPDU Each Data TPDU transmitted between transport entities for each direction of transmission in a transport connection is sequentially numbered. 22.214.171.124 Release The "explicit" variant of the release function is used. 126.96.36.199 Error Recovery 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). 188.8.131.52 Acknowledgement Acknowledgements are used to release copies of retained TPDUs.
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. 7.1.5 Data Transfer Phase 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. 184.108.40.206 Behaviour after an error 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
Connection. 220.127.116.11 Procedure for Expedited Data Transfer 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. 7.1.6 Release Procedures 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. 7.1.7 Treatment of Unknown TPDUs The 'Treatment of Protocol Errors' mechanism is used. 7.1.8 Supported Options Use of network receipt confirmation. Use of network expedited. 7.2 PROTOCOL DESCRIPTION OF CLASS 2: MULTIPLEXING CLASS 7.2.1 Characteristics of Class 2 The characteristic of this class is to provide a
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. 7.2.2 Functions of Class 2 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. 7.2.3 Protocol Mechanisms of Class 2 18.104.22.168 Connection Establishment Phase The connection establishment function shall be used. 22.214.171.124.1 User Data in the Connection Phase Class 2 provides the possibility to convey data in the connection request and confirm commands. 126.96.36.199 Connection Identification In Class 2 each TPDU conveys a Destination Reference.
This uniquely identifies the transport connection within the receiving transport entity and thus allows multiplexing. 188.8.131.52 Release Phase The release of a transport connection results either from the use of the 'explicit' variant of the release function or from the Implicit Termination function. 184.108.40.206 Protocol Mechanisms when Explicit Flow Control is used. The following mechanisms are provided: 220.127.116.11.1 Numbering of Data TPDU Each Data TPDU transmitted between transport entities for each direction of transmission in a transport connection is sequentially numbered. Each Data TPDU contains a Send Sequence Number T(S). 18.104.22.168.2 Flow Control Principles 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 22.214.171.124.3. 126.96.36.199.3 Expedited Flow 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
data will arrive after the expedited data. 7.2.4 Connection Establishment Procedures for Class 2 188.8.131.52 References 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. 184.108.40.206 Connection Eastablishment 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. 7.2.5 Data Transfer Procedures for Class 2 The data transfer procedures described in the following section apply independently to each transport connection existing between two transport entities. 220.127.116.11 TPDU Maximum Length and Segmenting The general rules defined in Section 6.3 apply. 18.104.22.168 Concatenation The general rules defined in Section 6.4 apply. 22.214.171.124 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. 126.96.36.199 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.
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.. 188.8.131.52.1 Numbering of Data TPDUs 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: o initalization 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)
184.108.40.206.2 Window Definition 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. 220.127.116.11.3 Flow Control Flow control is performed as follows: o initialization time Lower window edge = 0 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. Sending Credit 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.
18.104.22.168.4 Reducing the Upper Window Edge 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. 22.214.171.124.5 Procedure for Expedited Data Transfer 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. 7.2.6 Release Procedures for Class 2 The data phase ends after a transport entity has sent or received a Disconnect Request (DR). The transport entity will ignore any incoming TPDU except DC or DR. If the network resets or clears the network connection, all transport connections are terminated without the transport clearing sequence. The References become frozen. For Class 2 the explicit variant of the 'release' mechanism is used, enabling transport connections to be cleared independently of the underlying network connection. 7.2.7 Treatment of Invalid TPDUs
The 'Treatment of Protocol Error' mechanism in Section 6.23 is used. 7.2.8 Behaviour after an Error signalled by the Network Layer. The implicit termination mechanism is used. 7.2.9 Supported Options Non use of explicit flow control. Extended formats. 7.3 PROTOCOL DESCRIPTION OF CLASS 3: ERROR RECOVERY AND MULTIPLEXING CLASS 7.3.1 Characteristics of Class 3 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. 7.3.2 Functions of Class 3 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. 7.3.3 Protocol Mechanisms of Class 3 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. 126.96.36.199 Error Recovery Principles 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.
Error recovery for ED TPDU is achieved by retransmission (see 188.8.131.52). 184.108.40.206 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). 7.3.5 Data Transfer Procedures for Class 3 220.127.116.11 Acknowledgement The 'AK' variant of the Retention until Acknowledgement of TPDUs function is used. 18.104.22.168 Retransmission Procedure 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
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. 22.214.171.124 Reducing the upper window edge 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. 188.8.131.52 Procedure for Expedited Data Transfer 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
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. 7.3.6 Release Procedures for Class 3 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. 7.3.7 Treatment of Invalid TPDUs The 'Treatment of Protocol Errors' mechanism is used. 7.3.8 Supported Options Extended formats. 7.4 PROTOCOL DESCRIPTION OF CLASS 4: ERROR DETECTION AND RECOVERY CLASS 7.4.1 Characteristics of Class 4 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. 7.4.2 Functions of Class 4 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.
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. 7.4.3 Protocol Mechanisms of Class 4 184.108.40.206 Network Service Data Unit Lifetime 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. 220.127.116.11 Average Transit Delay 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. 18.104.22.168 Remote Acknowledge Time Assumptions 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. 22.214.171.124 Local Retransmission 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.
126.96.36.199 Persistence Time 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). 184.108.40.206 Inactivity Time 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. 220.127.116.11 Window Time A transport entity maintains a time to ensure that there is a maximum interval between transmission of up-to-date window information. This interval is referred to as the window time, W. 18.104.22.168 Class 4 Error Recovery Principles