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RFC 0892

 
 
 

ISO Transport Protocol specification

Part 3 of 3, p. 53 to 82
Prev RFC Part

 


prevText      Top       Page 53 
        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.

7.4.3.10  Relationship of Times and Intervals

        The following note describes the relationship between 
the time described in Section 7.4.3.1 - 7.4.3.9.

        Note:

             a.   The interrelationship of times for the worst case 
                  is as follows:

                  M:   maximum transit delay of the network (see 
                       7.4.3.1)

                  Ar  maximum acknowledgement time of the remote 
                       transport entity (see 7.4.3.3)

                  R:   maximum local retransmission time (see 
                       7.4.3.5)

                  N:   maximum number of transmission for a single 
                       TPDU (see 7.4.3.9)

                  L:   maximum time for a TPDU to be valid (see 
                       7.4.3.6)

                                             R = T * (N-1)
                                                  1

                            R
                             
                            *
                            M
             L              *

                            A                =2*M  +  A   + R
                             R                         R

                            *
                            M


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                                 t      t

             b.   The interrelationship of times for the average 
                  case is as follows (see 7.4.3.4)

                  E:        average transit delay for the network 
                            (E<<M)

                  X:        TPDU processing time

                  T :       average time from sending a TPDU until 
                   1        the receipt of its acknowledgement (see 
                            7.4.3.4)

                  A :       maximum acknowledgement time of the 
                   R        remote transport entity (see 7.4.3.3)

                         X

                         E

                         A         T  = 2*E + X + A
                          R         1              R

                         E
t                t

7.4.3.11  Sequence Numbering

        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.

7.4.4.1  Connection Request

        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 7.4.3.6).

7.4.4.2  Incomimg Connection Request

        An incoming connection request is processed as for Class 3

7.4.4.3  Connection Confirm

        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 7.4.3.9

7.4.4.4  Incoming Connection Confirm

        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 7.4.5.3.  


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7.4.4.5  Incoming Acknowledgement

        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 7.4.5.8).

7.4.4.6  Unsuccessful Connection

        When a DR TPDU is received in response to a CR TPDU, 
the timer T1 is cancelled and the reference placed in the frozen 
state for a period L (see Section 7.4.6.1).

7.4.4.7  Initial Credit Allocation

        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.

7.4.4.8  Exchange of Acknowledge Time

        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.

7.4.5   Procedure for Data Transfer Phase

7.4.5.1  Sequence Control

        The receiving transport entity is responsible for 
maintaining the proper sequence of DT TPDUs.

        DT TPDUs received out of sequence must not be 
delivered to the TS-user until in-sequence TPDUs have also been 
received.

        AK TPDUs also contain information allowing the 
receiving transport entity to process them in the correct order.

7.4.5.2  Duplicate DT TPDUs

        Duplicate TPDUs can be detected because the T(S) value 
matches that of previously received TPDUs.  T(S) values must not be 


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reused for the period L after their previous use.  Otherwise, a  
new, valid TPDU could be confused with a duplicated TPDU which had 
previously been received and acknowledged.

        Duplicated DT TPDUs must be acknowledged, since the 
duplicated TPDU may be the result of a retransmission resulting from 
the loss of an AK TPDU.

        The data contained in a duplicated DT TPDU should be
ignored.

7.4.5.3  Retransmission Principles

        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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7.4.5.4  Acknowledgement Principles

        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 7.4.3.2).
       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.

7.4.5.5  Flow Control Principles

        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.

7.4.5.6  Window Synchronization Principles

        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.

7.4.5.7  Procedure for Expedited Data

        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.

7.4.5.8  Inactivity Principles

        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 7.4.5.6) 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.

7.4.6   Procedures for Release Phase

        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.

7.4.6.1  Unassigned Frozen References

        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.

7.4.7   Treatment of Invalid TPDUs

        The 'Treatment of Protocol Erorrs' function is used.

7.4.8   Supported Options

        Non use of checksum.

        Use of extended formats.

8.      ENCODING

8.1     Summary

                                    Classes
                                  0  1  2  3  4   Sect.       Code

CR Connection Request             x  x  x  x  x   8.3      1110xxxx

CC Connection Confirm             x  x  x  x  x   8.4      1101xxxx

DR Disconnect Request             x  x  x  x  x   8.5      10000000

DC Disconnect Confirm                x  x  x  x   8.6      11000000

DT Data                           x  x  x  x  x   8.7      11110000

ED Expedited Data                    x  NF x  x   8.8      00010000

AK Data Acknowledgement            NRC  NF x  x   8.9      0110xxxx
    (Note 1)

EA Expedited Data                    x  NF x  x   8.10     00100000
   Acknowledgement

RJ Reject (Note 1)                   x     x      8.11     0101xxxx

ERR TPDU Error                    x  x  x  x  x   8.12     01110000

not available (Note 2)                             -      00000000


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not available (Note 2)                             -      00110000

not available (Note 2)                             -      1001xxxx

not available (Note 2)                             -      1010xxxx

Where xxxx (bits 4-1) is used to signal the CDT

Note 1: In extended header format, the code for AK=0110 0000 and the 
        code for RJ=0101 0000.

Note 2: These codes are already in use in compatible protocols 
        defines by standards organizations other than CCITT/ISO.

NF:     Not available when the non explicit flow control option is 
        selected.

NRC:    Not available when the receipt confirmation option is 
        selected.

8.2     Structure

        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.

Octets  1  2  3  4           n  n+1          p  p+1
        ------------      --------------   --------------   --------   
        LI|  | |  |  ...    |  |   |    .... | |    |   .... |             
        ------------      --------------   --------------   --------
        
         <--- Fixed Part -----><-- Variable Part->
                                   (including checksum   
                                    where applicable)

        <--------------Header-------------------><----Data Field->  

A TPDU is divided into four parts:


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        o  Length Indicator Field (LI)

        o  Fixed Part

        o  Variable Part (may be omitted)

        o  Data Field (may be omitted)

The length Indicator Field, Fixed Part and Variable Part constitute 
the Header of the TPDU.

8.2.1   Length Indicator Field

        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.

8.2.2   Fixed Part

        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.

8.2.2.1  TPDU Code

        This field contains the TPDU code and is contained in 
Octet 2 of the header.  It is used to define the structure of the
remaining header.  This field is a full octet except in the 
following cases:

             1110 xxxx      Connecting Request
             1101 xxxx      Connection Confirm
             0101 xxxx      Reject
             0110 xxxx      Data Acknowledgement

        Where xxxx (bits 4-1) is used to signal the CDT.

        Any other bit pattern may be used to define a TPDU Code.
        Only those codes defined in Section 8.1 are currently valid.

8.2.3   Variable Part

        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:

                      Bits 8   7   6   5   4   3   2   1
        Octets
        n+1                            Parameter Code
        n+2                            Parameter Length 
                                        Indication (e.g."m")
        n+3                            Parameter Value

        n+2+m

        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.

8.2.3.1  Checksum Parameter (Class 4 only)



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        All TPDU types may contain a checksum parameter in 
their variable part.  This parameter must always be present except 
when the non use of checksum option is selected.

             Parameter Code:     1100 0011
             Parameter Length:      2
             Parameter Value:    Result of checksum algorithm.      
                                 This algorithm is specified in 
                                 Section 6.18.

8.2.4   Data Field      This field contains transparent user data.  
Restrictions on its size are noted for each command.

8.3     Connections Request (CR)

8.3.1   Structure

        1     2         3        4     5    6     7     8     p   p+1
        LI  CR  CDT 00000000 00000000 SOURCE-   class  VARIABLE  USER DATA
                                      REFERENCE options   PART

8.3.2   LI

        See Section  8.2.1

8.3.3   Fixed Part (Octets 2 to 7)

             CR:       Connection Request Code:      1110

             CDT:      Initial Credit Allocation (set to 0000 in    
                       Classes 0 and 1 when specified as preferred class).

             SOURCE REFERENCE:        Reference selected by the transport 
                                      entity initiating the CR TPDU to 
                                      identify the requested TC.

             CLASSES:   Bits 8-5 octer 7 defines the preferred Transport 
                        Protocol class to be operated over the requested 
                        TC.  This field may take on one of the following 
                        values.

                                 0000           Class 0
                                 0001           Class 1
                                 0010           Class 2
                                 0011           Class 3
                                 0100           Class 4

        The CR contains the first choice of class in the fixed 
part as above.  Second and subsequent choices are listed in the 
variable part if required.


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        Bits 4-1 of octet 7 are reserved for options to be 
used on the requested transport connection.

        The use of bits 4-1 is as follows:

                  BIT            OPTION

                  4              0    always

                  3              0    always

                  2              =0   use of normal formats
                                 =1   use of extended formats

                  1              =0   use of explicit flow control 
                                      in Class 2

                                 =1   no use of explicit flow 
                                      control in Class 2

        Note:

        1.  It is not valid to request 'use of expedited data 
            transfer' (Additional option parameter) and no use of 
            explicit flow control in Class 2' (bit 1 = 1).

        2.  Bits 4 to 1 are always zero in Class 0 and have 
            no meaning.

8.3.4   Variable Part (Octets 8 to p)

        The following parameters are permitted in the variable part:

        o  Transport Service Access Point Identifier (TSAP-ID)

           Parameter code 11000001 for the identifier of the Calling TSAP.

           11000010 for the identifier of the Called TSAP.

        If a TSPA-ID is given in the request it may be 
returned in the confirmation.

        o  TPDU size

        This parameter defines the proposed maximum TPDU size 
(in octets including the header) to be used over the requested 
transport connection.  The coding of this parameter is:

        Parameter Code 11000000



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        Parameter value field

        00001101  8192 octets (not allowed in Class 0 of 1)

        00001100  4096 octets (not allowed in Class 0 of 1)

        00001011  2048 octets

        00001010  1024 octets

        00001001  512 octets

        00001000  256 octets

        00000111  128 octets

        Default value is 00000111 (128 octets)

        Version Number (not used in Class 0)

        Parameter code 11000100

        Parameter value field 00000001

        Default value 00000001

        Default value 00000001 (not used in Class 0)

       o Security Parameter (not used in Class 0)

         This parameter is user defined.

         Parameter code 11000101

         Parameter value and length field are user defined

       o Checksum (not used in Classes 0 through 3)

             See Section 8.2.3.1

         This parameter must always be present in a CR TPDU  
         requesting Class 4, even if the checksum selection 
         parameter is used to request non-use of the checksum facility.

       o Additional Option Selection (not used in Class 0)

        This parameter defines the selection to be made as to 
        whether or not additional options are to be used.

        Parameter code 11000110


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        Parameter length:  1
        Parameter value field:

        Bits related to options particular to one class are 
        not meaningful and may take any value in the other classes.

             BITS                OPTION

             4    1=   Use of network expedited in Class 1
                  0=   Non use of network expedited in Class 1

             3    1=   Use of receipt confirmation in Class 1
                  0=   Use of explicit AK variant in Class 1

             2    0=   Checksums are to be used in Class 4
                  1=   Checksums are not to be used in Class 4

             1    1=   Use of transport expedited data transfer 
                       service
                  0=   No use of transport expedited data transfer 
                       service

             Default falue is 00000001

        o Alternative protocol class (not used in Class 0)

             Parameter code 11000111

             Parameter length  n

        Parameter value encoded as a sequence of single 
octets.  Each octet is encoded as for octet 7 but with bits 4-1 set 
to zero (i.e., no alternative option selections permitted).

        o  Acknowledge Time

        This parameter conveys the maximum acknowledge time 
        AL to the remote transport entity.  It is an indication only, and 
        is not subject to negotiation (see section 7.4.5.3).

        Parameter Code 10000101

        Parameter Value field: n a binary number (2 octets)

        n is the maximum acknowledge time, expressed in 
        milliseconds.

        o  Throughput      Parameter code:    10001001

                            Length         :    12


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                            1st 3 octets   :    Targer value, 
                                                calling-called user 
                                                direction

                            2nd 3 octets   :    Min. acceptable, 
                                                calling-called  
                                                user direction

                            3rd 3 octets    :   Target value, 
                                                called-calling user 
                                                direction

                            4th 3 octets  :     Min. acceptable, 
                                                called-calling user 
                                                direction

             Values are expressed in octets per second.

        o Residual          Parameter code:     10000110
          error rate
                            Length        :     3

                            1st octet     :     Target value, power 
                                                of 10

                            2nd octet     :     Min. acceptable, 
                                                power of 10

                            3rd octet     :     TSDU size of 
                                                interest, expressed 
                                                as a power of 2

        o  Priority         Parameter code:     10000111

                            Length        :     2

                            Value         :     Integer

        o  Transit          Parameter code:     10001000
             delay

                            Length        :     8

                            1st 2 octets  :     Target value, 
                                                calling-called user 
                                                direction

                            2nd 2 octets  :     Max. acceptable,
                                                calling-called user
                                                direction.


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                            3rd 2 octets  :     Target value, 
                                                called-calling user 
                                                direction.

                            4th 2 octets  :     Max. acceptable, 
                                                called-calling user 
                                                direction

             Values are expressed in milliseconds.

8.3.5   User Data (Octets p+1 to the end)

        No user data are permitted in class 0, and are 
optional in the other classes.  Where permitted, it may not exceed 
32 octets.

8.4     Connection Confirm (CC)

8.4.1   Structure

        1     2     3     4     5     6    7    8    p   p+1

        LI CC  CDT DST-REF SOURCE-REF  class  VARIABLE  USER DATA
          1101                         options  Part             

8.4.2   LT

        See Section 8.2.1.  

8.4.3   Fixed Part (Octets 2 to 7)

             CC                      :          Connection Confirm 
                                                Code:  1101

             CDT                     :          Initial Credit 
                                                Allocation (set to 
                                                0000 in Classes 0 
                                                and 1).

             DST-REFERENCE           :          Reference 
                                                identifying the 
                                                requested transport 
                                                connection at the 
                                                remote transport 
                                                entity.

             SOURCE REFERENCE        :          Reference selected 
                                                by the transport 
                                                entity initiating 
                                                the CC TPDU to 


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                                                identify the 
                                                confirmed TC.

             CLASSES                 :          Defines the selected 
                                                transport protocol class to 
                                                be operated over the accepted 
                                                TC according to the 
                                                negotiation rules specified 
                                                in Section 6.5.

8.4.4   Variable part (Octet 8 to p)

        See Section 8.3.4

8.4.5   User Data (Octets p+1 to the end)

        See Section 8.3.5

8.5     Disconnect Request (DR)

8.5.1   Structure

LI  DR        DST-REF   SOURCE-REF    REASON    VARIABLE  USER DATA 
   10000000                                       PART               

8.5.2   LI

             See Section 8.2.1

8.5.3   Fixed Part (Octets 2 to 7)

        DR                :      Disconnect Request Code:   1000

        DST-REFERENCE     :      Reference identifying the TC at 
                                 the remote transport entity.

        SOURCE REFERENCE  :      Reference identifying the TC at 
                                 the transport entity initiating  
                                 the command.  Value zero when 
                                 reference is unassigned.

        REASON            :      Defines the reason for 
                                 disconnecting the TC.  This field 
                                 shall take one of the following 
                                 values:

                                 The following values can be used 
                                 for class 1 to 4:

                                 128 + 0 - Normal disconnect 


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                                 initiated by session entity.

                                 128 + 1 - Remote transport entity 
                                 congestion at connect request time

                                *128 + 2 - Connection negotiation failed 
                                (i.e. proposed class(es) not supported).

                                 128 + 3 - Duplicated connection detected

                                 128 + 4 - Mismatched references

                                 128 + 5 - Protocol error

                                 128 + 6 - Not used

                                 128 + 7 - Reference overflow

                                 128 + 8 - Connection request refused on this 
                                 network connection

                                 128 + 9 - Not used

                                 128 + 10 - Header or parameter length invalid

             The following values can be used for all classes.

                  0 -  Reason not specified

                  1 -  Congested at TSAP

                  *2   Session entity not attached to TSAP

                  *3   Address unknown

             Note:     Reasons marked with '*' may be reported to 
                       the TS-user as 'persistent', other reasons 
                       as 'transient'.

8.5.4   Variable Part (Octets 8 to 10)

        o    A parameter may be provided to allow additional 
             information related to the clearing of the connection.

             Parameter code:       11100000

             Parameter Value Field:  Additional information.  This 
             field is intended to be used by the transport service 
             provider for internal purposes.



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        o    Checksum (see 8.2.3.1)

8.5.5   User Data (Octets p+1 to the end)

        Not allowed in class 0,

        This field may not exceed 64 octers and is used 
to carry TS-User data.  The successful transfer of this data is not 
guaranteed.

8.6     Disconnect Confirm (DC)

        (Not used in Class 0)

8.6.1   Structure

        1     2         3       4       5      6        7        p
        LI             DST-REFERENCE  SOURCE-REFERENCE  Variable Part
           11000000

8.6.2   LI

        See Section 8.2.1

8.6.3   Fixed Part (Octets 2 to 6)

             DC            :     Disconnect Confirm Code:  1100

             DST-REFERENCE :     See Section 8.3.3

             SOURCE-REFERENCE:   See Section 8.4.3

8.6.4   Variable Part

             Checksum (see 8.2.3.1)

8.7     Data (DT)

8.7.1   Structure

             Normal Format for Class 0 to 1

        1     2          3           4         5

        LI   DT     E    TPDU-NR    User Data
           11110000 0                        
                    T                        

             Normal format for Class 2, 3 and 4



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1        2        3        4          5          6       p     p+1
LI             DST-REFERENCE    E   TPDU-NR   Variable Part  User Data
   11110000                     O
                                T

        Extended Format for optional use in Classes 2,3 and 4

        1       2      3       4     5,6,7,8    9        p p+1

        LI   DT     DST-REFERENCE  E  TPDU-NR  Variable  User Data
           11110000                O
                                   T

8.7.2   LI

        Section 8.2.1

8.7.3   Fixed Part

        (Classes 0 to 1  : -  Octets  2 to 3; classes 2,3,4 
normal format:  Octets 2 to 5; classes 2,3,4 extended format: - 
Octets 2 to 8)

             DT             :    Data Transfer Code:  1111

             DST-REFERENCE  :    See Section 8.4.3

             EOT            :    When set to ONE, indicates that 
                                 the current DT TPDU is the last 
                                 Data Unit of a complete DT TPDU 
                                 sequence (End of TSDU).

             TPDU-NR        :    TPDU Send Sequence Number (Zero in 
                                 Class 0), may take any value in 
                                 Class 2 without explicit flow 
                                 control.

8.7.4   Variable Part

        Checksum (See 8.2.3.1)

8.7.5   User Data Field

        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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8.8     Expedited Data (ED)

        (Not used in Class 2 when "no explicit flow 
         control" option is selected.)

8.8.1   Structure

        Normal Format

     1        2      3        4        EOT 5        6     p        p + 1

     LI       ED    DST-REFERENCE     EDTPDU-NR    Variable Part  User Data
           00010000                 1.

        Extended Format

     1        2        3      4        EOT 5,6,7,8  9    p          p + 1

     LI       ED     DST-REFERENCE       EDTPDU-NR  Variable Part   User Data
           00010000                    1.

8.8.2   LI

        See Section 8.2.1

8.8.3   Fixed Part

        (Octets 2 to 5, normal format: 2 to 8, extended format)

        ED:            Expedited Data command code: 0001

        DST-REFERENCE: Same as Section 8.4.3

        ED TPDU-NR:    Expedited TPDU identification number 
                       (Classes 1, 3, and 4; may take any value in 
                       Class 2).

8.8.4   Variable Part

        Checksum (See 8.2.3.1)

8.8.5   User Data Field

        This field contains an expedited TSDU.  Up to 16 octets.

8.9     Data Acknowledgement (AK)

        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 7.4.5.6).

        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.

8.10    Expedited Data Acknowledgement (EA)

        (Not applicable for Class 0 and Class 2 when the no 
        explicit flow control option is selected).

8.10.1  Structure

        Normal Format

     1       2       3       4         5               6         p

     LI      EA      DST-REFERENCE      . YR-TU-NR     Variable Part
            00100000                  0.

        Extended Format

     1      2         3       4          5,6,7,8            9      p

     LI     EA        DST-REFERENCE       . YR-TU-NR        Variable Part
           00100000                     0.

8.9.1   Structure



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        Normal Format

      1       2          3          4         5            6       p

      LI      AK CDT     DST-REFERENCE         . YR-TU-NR  Variable Part
             0110                            0.

        Extended Format

     1      2         3       4        5,6,7,8       9,10    11    p

     LI     AK        DST-REFERENCE     . YR-TU-NR   CDT     Variable Part
           01100000                   0.

8.9.2   LI

        See Section 8.2.1

8.9.3   Fixed Part

        (Octets 2 to 5, normal format:  2 to 10, extended format)

        AK:            Acknowledgement command code:  0110

        CDT:           Credit Value (set to 0 in class 1)

        DST-REFERENCE: Same as Section 8.4.3

        YR-TU-NR:      Sequence number indicating the next expected 
                       DT TPDU number.

8.9.4   Variable Part

        Checksum (See 8.2.3.1)

        Sub-sequence number (class 4 only - optionally used).

        This parameter is used to ensure that AK TPDUs are 
        processed in the correct sequence.  If it is absent, this is 
        equivalent to transmitting the parameter with a value of zero.

        Parameter Code:  100001010

        Parameter Value: 16-bit sub-sequence number.

8.10.2  LI

        See Section 8.2.1

8.10.3  Fixed Part


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        (Octets 2 to 5, normal format; 2 to 8, extended 
        format)

        EA:            Acknowledgement command code:  0010

        DST-REFERENCE: Same as Section 8.4.3

        YR-TU-NR:      Identification of the ED TPDU being 
                       acknowledged.  May take any value in Class 2.

8.10.4  Variable Part

        Checksum (See 8.2.3.1)

8.11    Reject (RJ)

        (Not used in Classes 0, 2, and 4)

8.11.1  Structure

        Normal Format

       1       2        3       4          EOT 5          6       p

       LI      RJ CDT   DST-REFERENCE       . YR-TU-NR    Variable Part
              0101                        0.

        Extended Format

      1        2       3        4          EOT 5,6,7,8    9,10     11      p
      LI       RJ      DST-REFERENCE        .  YR-TU-NR   CDT      Variable
              0l0l0000                                              Part

8.11.2  LI

        See Section 8.2.1

8.11.3  Fixed Part

        (Octets 2 to 5, normal format; 2 to 10, extended format)

        RJ:            Reject Command Code:  0101

        CDT:           Credit Value (set to 0 in class 1)

        DST-REFERENCE: Same as Section 8.4.3

        YR-TU-NR:      Sequence number indicating the next expected 
                       TPDU from which retransmission should occur.



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8.11.4  Variable Part

        No parameters exclusive to this TPDU type.

8.12    TPDU Error (ERR)

      1         2          3        4        5             6

      LI        ERR        DST-REFERENCE     Reject        Parameters
               01110000                      Cause

8.12.1  LI

        See Section 8.2.1

8.12.2  Fixed Part

        ERR:           TPDU Error Code: 0111

        DST-REFERENCE: Same as Section 8.4.3

        REJECT CAUSE:  
                       00000000  Reason not specified

                       00000001  Invalid parameter code

                       00000010  Invalid TPDU type

                       00000011  Invalid parameter value

8.12.3  Variable Part (Octets 6 to the end)

        Parameter Code:  1100001

        Parameter Value Field:

        Contains the bit pattern of the rejected TPDU up to and 
including the octet which caused the rejection.  This parameter is 
mandatory in Class 0.

        Checksum (See Section 8.2.3.1)

SECTION THREE - CONFORMANCE

9.      CONFORMANCE

        Implementations claiming conformance to this standard shall:

        1.   Implement either Class 0 or Class 2 or both.



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        2.   If other classes are implemented, the following rules 
             shall be observed:

             a)  If Class 3 or Class 4 is implemented then Class 2 
             must be implemented

             b)  If Class 1 is implemented then Class 0 must be 
             implemented.

        3.   The following table defines the requirements for the 
             implementation of the options defined in previous 
             sections:

                                                Class
                                            0     1    2     3    4

        TPDU with Checksum                 no    no   no    no    m
        TPDU without Checksum               m     m    m     m    o

        Expedited Data Transfer            no     m    m     m    m
        No Expedited Data Transfer          m     m    m     m    m

        Flow Control in Class 2            no    no    m    no   no
        No Flow Control in Class 2         no    no    o    no   no

        7 bits format (normal)             m     m    m     m    m
        31 bits format (extended)          no    no    o     o    o

        Use of Receipt Confirmation in     no     o   no    no   no
        Class 1
        No use of Receipt Confirmation in  no     m   no    no   no
        Class 1

        Use of Network Expedited in Class  no     o   no    no   no
        1, if T-EXPEDITED DATA necessary

        No use of Network Expedited in     no     m   no    no   no
        Class 1, if T-EXPEDITED DATA necessary

        o  -  optional:     An implementation may or may not 
                            provide this user-selected option.

        m  -  mandatory:    An implementation must provide for this 
                            option

        no  -               An implementation shall not provide 
                            this option.

        4.   Implementations claiming conformance shall support a 
             TPDU length of 128 octets.  If larger maximum TPDU 


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             sizes can be supported in Classes 1,2,3, or 4, then 
             all permitted TPDU sizes between the maximum and 128 
             octets shall be supported.

        5.   Claims of conformance shall state:

             a)  which class of protocol is supported.

             b)  which additional options indicated by the letter 
             'o' in the above table are supported.