rfc892.txt

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transport entity shall respond to the CR TPDU with a DR TPDU.  The
clearing reason shall indicate why the connection was not accepted.
The source reference field in the DR TPDU is set to zero to indicate
an unassigned reference.

        If the CR is regarded as an invalid TPDU, the called transport
entity will respond by sending an ERR TPDU.  On receipt of this TPDU,
the calling entity will regard the connection as closed.

6.7     Release

        Variants:  'implicit' or 'explicit'

        Purpose:  Termination of the transport connection.

        Network Service Primitives:

        N-DISCONNECT (implicit variant only)
        N-DATA

        TPDUs and fields used:

        DR
        - clearing reason (1 octet)
        - user data (maximum of 64 octets)

        DC

        Description:

        This function is inherent.

        In the 'implicit' variant, either transport entity disconnects
a transport connection by disconnecting the network connection to
which it is assigned.  Similarly when a transport entity is informed
that the network connection has been disconnected by the peer
transport entity, this should be considered as a transport
disconnect.


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        In the 'explicit' variant, either transport entity transmits a
Disconnect Request (DR) TPDU, and the other responds with a Disconnect
Confirm (DC) TPDU.  When the DC TPDU is sent or received by a
transport entity, that entity should consider the transport connection
not to exist (note 1).  After the sending of a DR TPDU, other TPDUs
received before the DC TPDU are ignored.  It is possible that a
disconnect collision will occur, when both transport entities send a
DR TPDU at about the same time.  This results in each transport entity
receiving a DR, after sending one.  Each transport entity shall
consider the received DR TPDU as a confirmation of its DR TPDU, and
shall not send or expect to receive a DC TPDU.

        The DR can convey a limited amount (up to 64 octets) of data.

6.8     Implicit Termination

        Purpose:  Termination of a Transport Connection on the
occurrence of a signalled error for which recovery functions are not
operative.

        Network Service Primitives:

        N-DISCONNECT Indication
        N-RESET Indication

        Description:

        When, on the network connection to which a Transport
Connection is assigned, an N-DISCONNECT or N-RESET Indication occurs,
both transport entities shall consider that the transport connection
no longer exists, and so inform the session entities.

Note 1:

        When a connection has been released, after the exchange of DR
and DC, the reference can be re-used immediately (except in Class 4,
where the Frozen Reference function is used, see Section 6.19).  This
is because the releasing transport entity does not know with certainty
that the remote transport entity considers use of the reference to be
ended.  Therefore, the reference should not be re-used for further
connections.  (In practice, the reference may be re-used after a
reasonable period when it is possible to be reasonably certain that
the remote transport entity will not continue to use it).

6.9     Spurious Disconnect

        Purpose:  To deal with the arrival of an "unknown" DR TPDU.

        TPDUs and fields used:


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        DR, DC
        - source reference
        - destination reference

        Description:

        A DR TPDU can be received for a transport connection which
does not exist.  Rather than treating this as an error, a DC TPDU
should be send back which reflects the references of the DR TPDU.

Note:
        This only applies when one or more transport connections using
a multiplexing class exist over the network connection, or when no
transport connections exist.  At other times it is a protocol error.

6.10    Data TPDU Numbering

        Variants:  'normal' or 'extended'

        Purpose:   Numbering of DT TPDUs for use in recovery, 
                   flow control, or sequencing functions.

        TPDUs and Fields Used:

        DT
        - TPDU-NR (7 or 31 bits)

        Description:

        DT TPDUs transmitted in each direction on a transport 
connection bear a sequence number 'TPDU-NR'.  Its value in the first
DT TPDU in each direction after connection establishment will be zero.
Thereafter each TPDU had 'TPDU-NR' one greater than the previous.  
Modulo 2**7 arithmetic is used in the 'normal' variant, and modulo 2**31
in the 'extended' variant.

        In the sections that follow, the relationships 'greater than'
and 'less than' are used in connection with TPDU numbers.  In all 
such uses, the numbers being compared cover a range less than the 
modulus and in fact lie within a contiguous set of TPDU numbers called
a 'window'.  The window has a known starting TPDU number and finishing 
number.  The term 'less than' means 'occurring sooner in the window
sequence' and the term 'greater than' means 'occurring later in the
window sequence'.

6.11    Expedited Data Transfer

        Variants:  'network expedited' or not

        Purpose:  Provision of the expedited data service

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        Network Service Primitives:

        N-DATA
        N-EXPEDITED DATA

        TPDUs and Fields Used:

        ED
        - ED TPDU-NR (7 or 31 bits)

        EA
        - YR-TU-NR (7 or 31 bits)

        Description:

        Each expedited TSDU is conveyed as the data field of an Expedited
Data (ED) TPDU.  

        Each ED TPDU received must be acknowledged by an Expedited
Acknowledge (EA) TPDU.  

        There may only be one ED TPDU unacknowledged at any time for each
direction of a transport connection.  

        In the 'network expedited' variant (available in class 1 only),
ED and EA TPDUs are conveyed in the data fields of N-EXPEDITED DATA
primitives.  Otherwise, N-DATA is used.  

6.12    Reassignment

        Purpose:  Assignment of a Transport Connection to a different
Network Connection.

        TPDUs and Fields Used:

        CR
        - source reference

        RJ, DR
        - destination reference

        Description:

        When the Network Connection to which a Transport Connection was
assigned no longer exists, the Transport Connection can be assigned to
another Network Connection.  

        When one transport entity has assigned the Transport Connection,
it is important that the other transport entity recognise to which 
Network Connection it has been assigned.  This can only take place when it

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has received a TPDU for the Transport Connection on a Network Connection
with calling and called network addresses which imply 
the same transport entities as the old.  The TPDU will have been sent 
as a result of the assigning transport entity commencing resynchronization,
and will thus be a RJ, or a retransmitted CR or DR.  

        The Transport Connection shall be recognised as having been
assigned to the Network Connection on which the TPDU was received.  

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

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        RJ, AK, EA
        - YR-TU-NR (7 or 31 bits)

        DT
        - TPDU-NR (7 or 31 bits)

        ED
        - ED TPDU-NR (7 or 31 bits)

        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

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                        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)