rfc892.txt

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        An existing network connection, which connects the relevant
transport entities, is unsuitable for assignment of a transport
connection if, for example:

        o  the quality of service needed for the transport connection
           can not be met by using or enhancing the network

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           connection.

        o  the protocol class preferred or in use for the transport 
           connection is incompatible with the current usage of the 
           network connection as regards the use of pervasive
           functions (e.g., multiplexing).

        When a new network connection is created, the quality of
service requested is a local matter, though it will normally be
related to the requirements of transport connection(s) expected to be
assigned to it.

        A Network Connection with no transport connections will be
available after initial establishment or because explicit
disconnection of all the transport connections previously assigned to
it has taken place.  Either Transport entity may as a local
matter choose to disconnect the Network Connection or assign other
Transport Connections to it.

6.2     Transport Protocol Data Unit (TPDU) Transfer

        Purpose:  To convey transport protocol data unit in user 
                  data fields of network service primitives.

        Network Service Primitives

        N-DATA
        N-EXPEDITED DATA

        Description:

        This function is inherent.

        The Transport Protocol Data Units (TPDUs) defined for the
protocol are listed in Figure 3.

                TPDU name                Abbreviation

        Connection Request                      CR
        Connection Confirm                      CC
        Disconnect Request                      DR
        Disconnect Confirm                      DC
        Data                                    DT
        Expedited Data                          ED
        Data Acknowledge                        AK
        Expedited Acknowledge                   EA
        Reject                                  RJ
        TPDU Error                              ERR

                Figure 3.  Transport Protocol Data Units

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        TPDUs are conveyed using the NS-User data parameters of the
Network Service primitives, primarily with the N-DATA, but also with
N-EXPEDITED primitives.

        Transport entities shall accept all permissible assignments and
may issue any permissible assignments.  The permissible assignments of
TPDUs to these primitives are shown in Figure 4.  Concatenation of
TPDUs is also permitted (see section 6.4).

Primitive               Applicable TPDUs                        Note

N-DATA                  CR, CC, DR, DT, ED,
                        AK, EA, RJ, DC, ERR

N-EXPEDITED             ED, EA                                  1

Notes:

1.  This assignment is permissible only when using class 1 and when
    the network expedited variant has been agreed.

Figure 4.  Network Service Primitives which can convey TPDUs.

6.3     Data TPDU Length and Segmenting

        Purpose:  Mapping between one TSDU and TPDUs.

        TPDUs and fields used:

        DT
        -  End of TSDU (1 bit)

        Description:

        The data field of Data TPDUs may contain any number of octets
up to an agreed maximum as negotiated at connection time.  

        A transport entity uses an End of TSDU mark as defined below:

        In each Data TPDU a transport entity may indicate the end of a
TSDU.

        Category 1      Having the End of TSDU mark set to yes.  These
                        TPDUs may or may not have the maximum length.

        Category 2      Having the End of TSDU mark set to no.  These
                        TPDUs do not necessarily have the maximum
                        length.

        A complete Data TPDU sequence is defined as being composed of

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either a single category 1 DT TPDU or consecutive category 2 followed
by a category 1 DT TPDU.

6.4     Concatenation and Separation

        Pupose:  Conveyance of multiple TPDUs in one NSDU.

        Description:

        All TPDUs carry in their TPDU header a length indicator (see
Section 8.2.1).  Additionally, TPDUs are classified as either Data
TPDUs or Control TPDUs.  Control TPDUs may or may not contain a data
field.  For TPDUs containing data the length of the data field is
indicated by the length of the NSDU.  These provisions permit any
number of Control TPDUs that may not contain data to be concatenated
with a single control TPDU which may contain data or with a single
Data TPDU.  The control TPDUs without data must precede the TPDU with
data, if any.  The number of TPDUs so concatenated is terminated by
the end of the NSDU.

        The concatenated set of TPDUs may be for the same or different
transport connections.  An implementation shall accept concatenated
TPDUs and may concatenate TPDUs before transmission.  The transport
entity shall not send a concatenated set of TPDUs which exceeds twice
the overall maximum TPDU length for all the TCs assigned to the
network connection.

6.5     Connection Establishment

        Purpose:  Creation of a new transport connection.

        Network Service Primitives:

        N-DATA

        TPDUs and fields used:

        CR, CC
        - source reference (16 bits)
        - initial credit (if applicable)
        - calling transport address (optional)
        - called transport address (optional)
        - user data (optional)
        - TPDU size (optional)
        - sequence number length (optional)
        - checksum selection (optional)
        - acknowledgement time (optional)
        - quality of service (optional)
        CR
        - preferred protocol class

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        - alternative protocol classes (zero or more)
        - version number (optional)
        - security (optional)
        - proposed options
        CC
        - destination reference (16 bits)
        - selected protocol class
        - selected options

        Description:

        This function is inherent:

        A transport connection is established by means of one
transport entity (the initiator) transmitting a Connection Request
(CR) TPDU to the other transport entity (the responder), which replies
with a Connection Confirm (CC) TPDU.  Before sending the CR TPDU, the
initiator assigns the transport connection being created to one (or
more if the splitting function is being used) network connection(s).
It is this set of network connections over which the TPDUs are sent.
During this exchange, all information and parameters needed for the
transport entities to operate must be exchanged or negotiated.

        The following information is exchanged:

        o  references.  Each transport entity chooses a reference which
           is 16 bits long and which is arbitrary except for the following
           restrictions:

           - it cannot already be in use or "frozen" (see "Frozen
             References", Section 6.19).

           - it cannot be zero.

        Each transport entity is responsible for selecting the
Reference which the partner will use.  This mechanism is symmetrical
and therefore avoids the need to assign a status of master or slave to
partners and avoids call collision.  This mechanism also provides
identification of the transport connection independent of the network
connection. The range of References used for transport connections, in
a given transport entity, is a local system parameter.

        o  addresses (optional).  Indicate the calling and called
           transport service access points.  When either network
           address unambiguously defines the transport address this
           information may be omitted.

        o  initial credit.  Only relevant for classes which include
           the Explicit Flow Control Function.


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        o  user data.  Not available in class 0.  Up to 32 octets in
           in other classes.

        The following negotiations take place:

        o  protocol class.  The initiator shall propose a preferred
class and any number of alternatives.  (Except that no alternatives are
allowed when class 0 is the preference.)  The initiator should assume
when it sends the CR TPDU that its preferred class will be agreed to,
and commence the functions associated with that class.

        Note:  This means, for example, that when a class which
includes resynchronization (see "Resynchronization", Section 6.15) is
preferred, resynchronization will occur if a reset is signalled during
connection establishment.

        When the responder has decided which class is to be used, it
shall indicate this in the CC TPDU and shall invoke the appropriate
functions for the class.  The responder may select the preferred
class, or any of the alternative classes or may select class 0 if
class 1 is proposed or class 2 if class 3 or 4 is proposed. (see
Section 9)

        If the preferred class is not selected, then on receipt of the
CC TPDU, the initiator shall adjust its functions accordingly.

        o  TPDU Size.  The initiator may propose a maximum size for
TPDUs, and the responder may accept this value or respond with any
value between the proposed value and 128 in the set of values
available (see "Encoding", Section 8).

        o  sequence number length.  Either normal or extended is
available.  When the sequence number is extended, the credit field (if
applicable) is also extended.

        o  checksum selection.  This defines whether or not TPDUs of
the connection are to include a checksum.

        o  version number.  This defines the version of the transport
protocol standard used for this connection.

        o  security parameter.  This parameter and its semantics are
user defined.

        o  quality of service parameter.  This defines the throughput,
delay, priority and residual error rate.

        o  The non-use of explicit flow control in class 2 is
negotiated.


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        o  The use of Network Receipt Confirmation and Network
expedited is negotiated when class 1 is to be used.

        The negotiation rules for the options are such that the
initiator may propose either to use or not to use the option.  The
responder may either accept the proposed choice or select the
mandatory alternative defined in Section 9.

        During the establishment phase of the transport connection,
the use of the expedited data option field of CR/CC  allows both
Transport Service user to negotiate the use or non use of the
expedited data transport service as described in the transport service
definitions.

        The following table summarizes the negotiation possibilities
for the options.

                                Proposition Made        Possible
                                by the Initiator        Selection by 
        Option                                          the Responder

Transport expedited data             Yes                  Yes or No
transfer service                     No                       No

Use of receipt confir-               Yes                  Yes or No
mation (class 1 only)                No                       No

Use of the network                   Yes                  Yes or No
expedited variant                    No                       No
(class 1 only)

Non use of checksum                  Yes                  Yes or No
(class 4 only)                       No                       No

Non use of explicit                  Yes                  Yes or No
flow control (class 2 only)          No                       No

Use of extended format               Yes                  Yes or No
                                     No                       No

        In class 2, whenever a transport entity requests or agrees to
the Transport Expedited data transfer service or to the use of
extended formats, it must also request or agree (respectively) to the
use of explicit flow control.

6.6     Connection Refusal

        Purpose:        Refusal of the transport connection.

        TPDUs and fields used:

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        DR
        -  reason (1 octet)
        -  user data (maximum of 64 octets)

        ERR
        -  reject code (1 octet)
        -  rejected TPDU parameter

        Description:

        If a transport connection cannot be accepted, the called