instances, more than one TSAP and/or more than one NSAP may be
associated with a particular transport entity.
26
SECTION TWO. TRANSPORT PROTOCOL SPECIFICATION
6 ELEMENTS OF PROCEDURE
This clause contains elements of procedure which are used in the
specification of protocol classes in clauses 7 to 12. These
elements are not meaningful on their own.
The procedures define the transfer of TPDUs whose structure and
coding is specified in clause 13. Transport entities shall
accept and respond to any TPDU received in a valid NSDU and may
issue TPDUs initiating specific elements of procedure specified
in this clause.
NOTE - Where network service primitives and TPDUs and parameters
used are not significant for a particular element of procedure,
they have not been included in the specification.
6.1 Assignment to network connection
6.1.1 Purpose
The procedure is used in all classes to assign transport
connections to network connections.
6.1.2 Network service primitives
The procedure makes use of the following network service
primitives:
a) N-CONNECT;
b) N-DISCONNECT.
27
6.1.3 Procedure
Each transport connection shall be assigned to a network
connection. The initiator may assign the transport connection to
an existing network connection of which it is the owner or to a
new network connection (see Note 1) which it creates for this
purpose.
The initiator shall not assign or reassign the transport
connection to an existing network connection if the protocol
class(es) proposed or the class in use for the transport
connection are incompatible with the current usage of the network
connection with respect to multiplexing (see Note 2).
During the resynchronization (see 6.14) and reassignment after
failure (see 6.12) procedures, a transport entity may reassign a
transport connection to another network connection joining the
same NSAPs, provided that it is the owner of the network
connection and that the transport connection is assigned to only
one network connection at any given time.
During the splitting procedure (see 6.23), a transport entity may
assign a transport connection to any additional network
connection joining the same NSAPs, provided that it is the owner
of the network connection and that multiplexing is possible on
the network connection.
The responder becomes aware of the assignment when it receives
a) a CR TPDU during the connection establishment procedure
(see 6.5); or
b) an RJ TPDU or a retransmitted CR or DR TPDU during the
resynchronization (see 6.14) and reassignment after
failure (see 6.12) procedures; or
c) any TPDU when splitting (see 6.23) is used.
28
NOTES
1. When a new network connection is created, the quality of
service requested is a local matter, although it will
normally be related to the requirements of transport
connection(s) expected to be assigned to it.
2. An existing network connection may also not be suitable
if, for example, the quality of service requested for the
transport connection cannot be attained by using or
enhancing the network connection.
3. A network connection with no transport connection(s)
assigned to it, may be available after initial
establishment, or because all of the transport connections
previously assigned to it have been released. It is
recommended that only the owner of such a network
connection should release it. Furthermore, it is
recommended that it not be released immediately after the
transmission of the final TPDU of a transport connection -
either a DR TPDU in response to CR TPDU or a DC TPDU in
response to DR TPDU. An appropriate delay will allow the
TPDU concerned to reach the other transport entity
allowing the freeing of any resources associated with the
transport connection concerned.
4. After the failure of a network connection, transport
connections which were previously multiplexed together may
be assigned to different network connections, and vice
versa.
6.2 Transport protocol data unit (TPDU) transfer
6.2.1 Purpose
The TPDU transfer procedure is used in all classes to convey
transport protocol data units in user data fields of network
service primitives.
29
6.2.2 Network Service Primitives
The procedure uses the following network service primitives:
a) N-DATA;
b) N-EXPEDITED DATA
6.2.3 Procedure
The transport protocol data units (TPDUs) defined for the
protocol are listed in 4.2.
When the network expedited variant has been selected for class 1,
the transport entities shall transmit and receive ED and EA TPDUs
as NS-user data parameters of N-EXPEDITED DATA primitives.
In all other cases, transport entities shall transmit and receive
TPDUs as NS-user data parameters of N-DATA primitives.
When a TPDU is put into an NS-user data parameter, the
significance of the bits within an octet and the order of octets
within a TPDU shall be as defined in 13.2.
NOTE - TPDUs may be concatenated (see 6.4).
6.3 Segmenting and reassembling
6.3.1 Purpose
The segmenting and reassembling procedure is used in all classes
to map TSDUs onto TPDUs.
30
6.3.2 TPDUs and parameter used
The procedure makes use of the following TPDU and parameter:
DT TPDUs;
- End of TSDU.
6.3.3 Procedure
A transport entity shall map a TSDU on to an ordered sequence of
one or more DT TPDUs. This sequence shall not be interrupted by
other DT TPDUs on the same transport connection.
All DT TPDUs except the last DT TPDU in a sequence greater than
one shall have a length of data greater than zero.
NOTES
1. The EOT parameter of a DT TPDU indicates whether or not
there are subsequent DT TPDUs in the sequence.
2. There is no requirement that the DT TPDUs shall be of the
maximum length selected during connection establishment.
6.4 Concatenation and separation
6.4.1 Purpose
The procedure for concatenation and separation is used in classes
1, 2, 3 and 4 to convey multiple TPDUs in one NSDU.
31
6.4.2 Procedure
A transport entity may concatenate TPDUs from the same or
different transport connections.
The set of concatenated TPDUs may contain:
a) any number of TPDUs from the following list: AK, EA, RJ,
ER, DC TPDUs, provided that these TPDUs come from
different transport connections;
b) no more than one TPDU from the following list: CR, DR,
CC, DT, ED TPDUs; if this TPDU is present, it shall be
placed last in the set of concatenated TPDUs.
NOTES
1. The TPDUs within a concatenated set may be distinguished
by means of the length indicator parameter.
2. The end of a TPDU containing data is indicated by the
termination of the NSDU.
3. The number of concatenated TPDUs referred to in 6.4.2.a is
bounded by the maximum number of transport connections
which are multiplexed together except during assignment or
reassignment.
6.5 Connection establishment
6.5.1 Purpose
The procedure for connection establishment is used in all classes
to create a new transport connection.
32
6.5.2 Network service primitives
The procedure uses the following network service primitive:
N-DATA
6.5.3 TPDUs and parameters used
The procedure uses the following TPDUs and parameters:
a) CR TPDU;
- CDT;
- DST-REF (set to zero);
- SRC-REF
- CLASS and OPTIONS (i.e. preferred class, use of extended
format, non-use of explicit flow control in class 2);
- calling TSAP-ID;
- called TSAP-ID;
- TPDU size (proposed);
- version number;
- security parameter;
- checksum;
- additional option selection (i.e. use of network
expedited in class 1, use of receipt confirmation in
class 1, non-use of checksum in class 4, use of
transport expedited data transfer service);
- alternative protocol class(es);
- acknowledge time;
- throughput (proposed);
- residual error rate (proposed);
- priority (proposed);
- transit delay (proposed);
- reassignment time;
- user data.
b) CC TPDU;
- CDT;
- DST-REF;
33
- SRC-REF;
- CLASS and OPTIONS (selected);
- calling TSAP-ID;
- called TSAP-ID;
- TPDU size (selected);
- security parameter;
- checksum;
- additional option selection (selected);
- acknowledge time;
- throughput (selected);
- residual error rate (selected);
- priority (selected);
- transit delay (selected);
- user data.
NOTE - The transport service defines transit delay as
requiring a previously stated average TSDU size as a basis
for any specification. This protocol, as specified in
13.3.4(n), uses a value of 128 octets. Conversion to and
from specifications based upon some other value is a local
matter.
6.5.4 Procedure
A transport connection is established by means of one transport
entity (the initiator) transmitting a CR TPDU to the other
transport entity (the responder), which replies with a CC TPDU.
Before sending the CR TPDU, the initiator assigns the transport
connection being created to one (or more if the splitting
procedure is being use) 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 shall be exchanged or negotiated.
NOTE - Except in class 4, it is recommended that the
initiator starts an optional timer TS1 at the time the CR
TPDU is sent. This timer should be stopped when the
connection is considered as accepted or refused or
unsuccessful. If the timer expires, the initiator should
34
reset or disconnect the network connection and, in classes 1
and 3 freeze the reference (see 6.18). For all other
transport connection(s) multiplexed on the same network
connection the procedures for reset or disconnect as
appropriate should be followed.
After receiving the CC TPDU for a class which includes the
procedure for retention until acknowledgement of TPDUs the
initiator shall acknowledge the CC TPDU as defined in table 5
(see 6.13).
When the network expedited variant of the expedited data transfer
(see 6.11) has been agreed (possible in class 1 only), the
responder shall not send an ED TPDU before the CC TPDU is
acknowledged.
The following information is exchanged:
a) references. Each transport entity chooses a reference
which is to be used by the peer entity is 16 bits long and
which is arbitrary except for the following restrictions:
1) it shall not already be in use or frozen (see 6.18),
2) it shall not be zero.
This mechanism is symmetrical and 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
matter.
b) 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.
c) initial credit. Only relevant for classes which include
the explicit flow control function.
d) user data. Not available if Class 0 is the preferred
class (see note). Up to 32 octets in other classes.
35
NOTE - If class 0 is a valid response according to table
3, inclusion of user data in the CR TPDU may cause the
responding entity to refuse the connection (e.g. if it
only supports class 0).
e) acknowledgement time. Only in class 4.
f) checksum parameter. Only in class 4.
g) security parameter. This parameter and its semantics are
user defined.
The following negotiations take place:
h) protocol class. The initiator shall propose a preferred
class and may propose any number of alternative class
which permit a valid response as defined in table 3. The
initiator should assume when it sends the CR TPDU that its
preferred class will be agreed to, and commence the
procedures associated with that class, except that if
class 0 or class 1 is an alternative class, multiplexing
shall not commence until a CC TPDU selecting the use of
classes 2, 3 or 4 has been received.
NOTE - This means, for example, that when the preferred
class includes resynchronization (see 6.14) the
resynchronization will occur if a reset is signalled
during connection establishment.
The responder shall select one class defined in table 3 as a
valid response corresponding to the preferred class and to the
class(es), if any, contained in the alternative class parameter
of the CR TPDU. It shall indicate the selected class in the CC
TPDU and shall follow the procedures for the selected class.
If the preferred class is not selected, then on receipt of the CC
TPDU the initiator shall adjust its operation according the
procedures of the selected class.
36
+------------------------------------------------------------+
| Pre- | Alternative class |
|ferred |----------------------------------------------------|
|class | 0 | 1 | 2 | 3 | 4 | none |
|-------|--------|--------|--------|--------|--------|-------|
| 0 |not |not |not |not |not |class |
| |valid |valid |valid |valid |valid | 0 |
|-------|--------|--------|--------|--------|--------|-------|
| 1 |class |class |not |not |not |class |
| |1 or 0 |1 or 0 |valid |valid |valid |1 or 0 |
|-------|--------|--------|--------|--------|--------|-------|
| 2 |class |not |class |not |not |class |
| |2 or 0 |valid |2 |valid |valid | 2 |
|-------|--------|--------|--------|--------|--------|-------|
| 3 |class |class 3,|class |class |not |class |
| |3,2 or 0|2,1 or 0|3 or 2 |3 or 2 |valid |3 or 2 |
|-------|--------|--------|--------|--------|--------|-------|
| 4 |class |class 4,|class |class |class |class |
| |4,2 or 0|2,1 or 0|4 or 2 |4,3 or 2|4 or 2 |4 or 2 |
+------------------------------------------------------------+
Table 3.
Valid responses corresponding to the preferred class and any
alternative class proposed in the CR TPDU
NOTES:
1. The valid responses indicated in table 3 result from both
explicit negotiation, whereby each of the classes proposed
is a valid response, and implicit negotiation whereby:
a) if class 3 or 4 is proposed then class 2 is a valid
response;
b) if class 1 is proposed then class 0 is a valid
response.
37
2. Negotiation from class 2 to class 1 and from any class to
an higher-numbered class is not valid.
3. Redundant combinations are not a protocol error.
j) TPDU size. The initiator may propose a maximum size for
TPDUs, and the responder may accept this value or respond
with any value between 128 and the proposed value in the
set of values available (see 13.3.4.b).
NOTE - The length of the CR TPDU does not exceed 128
octets (see 13.3).
k) normal or extended format. Either normal or extended is
available. When extended is used this applies to CDT,
TPDU-NR, ED-TPDU-NR, YR-TU-NR and YR-EDTU-NR parameters.
m) checksum selection. This defines whether or not TPDUs of
the connection are to include a checksum.
n) quality of service parameters. This defines the
throughput, transit delay, priority and residual error
rate.
p) the non-use of explicit flow control in class 2.
q) the use of network receipt confirmation and network
expedited when class 1 is to be used.
r) use of expedited data transfer service. This allows both
TS-users to negotiate the use or non-use of the expedited
data transport service as defined in the transport service
(ISO 8072).
The following information is sent only in the CR TPDU:
s) version number. This defines the version of the transport
protocol standard used for this connection.
t) reassignment time parameter. This indicates the time for
which the initiator will persist in following the
reassignment after failure procedure.
38
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 an
alternative choice as defined in table 4.
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 shall also request or agree (respectively)
to the use of explicit flow control.
+-------------------------------------------------------------+
| Option | Proposal Made | Valid Selection |
| | by the Initiator | by the Responder |
|-----------------------|------------------|------------------|
|Transport expedited | Yes | Yes or No |
|data transfer service | No | No |
|(Classes 1,2,3,4 only) | | |
|-----------------------|------------------|------------------|
|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 | No | No |
|(Class 2 only) | | |
|-----------------------|------------------|------------------|
|Use of extended format | Yes | Yes or No |
|(Classes 2,3,4 only) | No | No |
+-------------------------------------------------------------+
Table 4. Negotiation of options during connection establishment
39
NOTE - Table 4 defines the procedures for negotiation of options.
This negotiation has been designed such that if the initiator
proposes the mandatory implementation option specified in clause
14, the responder has to accept use of this option over the
transport connection except for the use of the transport
expedited data transfer service which may be rejected by the TS-
user. If the initiator proposes a non-mandatory implementation
option, the responder is entitled to select use of the mandatory
implementation option for use over the transport connection.
6.6 Connection refusal
6.6.1 Purpose
The connection refusal procedure is used in all classes when a
transport entity refuses a transport connection in response to a
CR TPDU.
6.6.2 TPDUs and parameters used
The procedure makes use of the following TPDUs and parameters:
a) DR TPDU;
- SRC-REF;
- reason;
- user data.
b) ER TPDU;
- reject code;
- rejected TPDU parameter.
40
6.6.3 Procedure
If a transport connection cannot be accepted, the responder shall
respond to the CR TPDU with a DR TPDU. The reason shall indicate
why the connection was not accepted. The source reference field
in the DR TPDU shall be set to zero to indicate an unassigned
reference.
If a DR TPDU is received the initiator shall regard the
connection as released.
The responder shall respond to an invalid CR TPDU by sending an
ER or DR TPDU. If an ER TPDU is received in response to a CR
TPDU, the initiator shall regard the connection as released.
NOTES
1. When the invalid CR TPDU can be identified as having class 0
as the preferred class, it is recommended to respond with an
ER TPDU. For all other invalid CR TPDUs either an ER TPDU or
DR TPDU may be sent.
2. If the optimal supervisory timer TS1 has been set for this
connection then the entity should stop the timer on receipt
of the DR or ER TPDU.
6.7 Normal release
6.7.1 Purpose
The release procedure is used by a transport entity in order to
terminate a transport connection. The implicit variant is used
only in class 0. The explicit variant is used in classes 1,2,3
and 4.
41
NOTES
1. When the implicit variant is used (i.e. in class 0), the
lifetime of the transport connection is directly correlated
with the lifetime of the network connection.
2. The use of the explicit variant of the release procedure
enables the transport connection to be released independently
of the underlying network connection.
6.7.2 Network service primitives
The procedure makes use of the following network service
primitives:
a) N-DISCONNECT (implicit variant only),
b) N-DATA
6.7.3 TPDUs and parameters used
The procedure makes use of the following TPDUs and parameters:
a) DR TPDU;
- clearing reason;
- user data;
- SRC-REF;
- DST-REF.
b) DC TPDU.
42
6.7.4 Procedure for implicit variant
In the implicit variant either transport entity disconnects a
transport connection by disconnecting the network connection to
which it is assigned. When a transport entity receives an N-
DISCONNECT this should be considered as the release of the
transport connection.
6.7.5 Procedure for explicit variant
When the release of a transport connection is to be initiated a
transport entity
a) if it has previously sent or received a CC TPDU (see note
1), shall send a DR TPDU. It shall ignore all
subsequently received TPDUs other than a DR or DC TPDU.
On receipt of a DR or DC TPDU it shall consider the
transport connection released;
b) in other cases it shall:
1) For classes other than class 4 wait for the
acknowledgement of the outstanding CR TPDU; if it
receives a CC TPDU, it shall follow the procedures in
6.7.5.a.
2) For class 4 either send a DR TPDU with a zero value in
the DST-REF field or follow the procedure in
6.7.5.b.1.
A transport entity that receives a DR TPDU shall
c) if it has previously sent a DR TPDU for the same transport
connection, consider the transport connection released;
d) if it has previously sent a CR TPDU that has not been
acknowledged by a CC TPDU, consider the connection refused
(see 6.6).
43
e) in other cases, send a DC TPDU and consider the transport
connection released.
NOTES
1) This requirement ensures that the transport entity is
aware of the remote reference for the transport
connection.
2) When the transport connection is considered as released
the local reference is either available for re-use or is
frozen (see 6.18).
3) After the release of a transport connection the network
connection can be released or retained to enable its re-
use for the assignment of other transport connections (see
6.1.).
4) Except in class 4, it is recommended that, if a transport
entity does not receive acknowledgement of a DR TPDU
within time TS2, it should either reset or disconnect the
network connection, and freeze the reference when
appropriate (see 6.18). For all other transport
connection(s) multiplexed on this network connection the
procedures for reset or disconnect as appropriate should
be followed.
5) When a transport entity is waiting for a CC TPDU before
sending a DR TPDU and the network connection is reset or
released, it should consider the transport connection
released and, in classes other than classes 0 and 2,
freeze the reference (see 6.18).
6.8 Error Release
44
6.8.1 Purpose
This procedure is used only in classes 0 and 2 to release a
transport connection on the receipt of an N-DISCONNECT or N-RESET
indication.
6.8.2 Network service primitives
The procedure makes use of the following service primitives:
a) N-DISCONNECT indication;
b) N-RESET indication.
6.8.3 Procedure
When, on the network connection to which a transport connection
is assigned, an N-DISCONNECT or N-RESET indication is received,
both transport entities shall consider that the transport
connection is released and so inform the TS-users.
NOTE - In other classes, since error recovery is used, the
receipt of an N-RESET indication or N-DISCONNECT indication will
result in the invocation of the error recovery procedure.
6.9 Association of TPDUs with transport connections
6.9.1 Purpose
This procedure is used in all classes to interpret a received
NSDU as TPDU(s) and, if possible, to associate each such TPDU
with a transport connection.
45
6.9.2 Network service primitives
This procedure makes use of the following network service
primitives:
a) N-DATA indication;
b) N-EXPEDITED DATA indication.
6.9.3 TPDUs and parameters uses
This procedure makes use of the following TPDUs and parameters:
a) any TPDU except CR TPDU, DT TPDU in classes 0 or 1 and AK
TPDU in class 1;
- DST-REF
b) CR, CC, DR and DC TPDUs;
- SCR-REF.
c) DT TPDU in classes 0 or 1 and AK TPDU in class 1.
6.9.4 Procedures
6.9.4.1 Identification of TPDUs
If the received NSDU or Expedited NSDU cannot be decoded (i.e.
does not contain one or more correct TPDUs) or is corrupted (i.e.
contains a TPDU with a wrong checksum) then the transport entity
shall:
46
a) if the network connection on which the error is detected
has a class 0 or class 1 transport connection assigned to
it, then treat as a protocol error (see 6.22) for that
transport connection;
b) otherwise
1) if the NSDU can be decoded but contains corrupted
TPDUs, ignore the TPDUs (class 4 only) and optionally
apply 6.9.4.b.2.
2) if the NSDU cannot be decoded issue an N-RESET or N-
DISCONNECT request for the network connection and for
all the transport connections assigned to this network
connection (if any), apply the procedures defined for
handling of network signalled reset or disconnect.
If the NSDU can be decoded and is not corrupted, the
transport entity shall:
c) if the network connection on which the NSDU was received
has a class 0 transport connection assigned to it, then
consider the NSDU as forming TPDU and associate the TPDU
with the transport connection (see 6.9.4.2).
d) otherwise, invoke the separation procedures and for each
of the individual TPDUs in the order in which they appear
in the NSDU apply the procedure defined in 6.9.4.2.
6.9.4.2 Association of individual TPDUs
If the received TPDU is a CR TPDU then, if it is a duplicate, as
recognized by using the NSAPs of the network connection, and the
SRC-REF parameter, then it is associated with the transport
connection created by the original value of the CR TPDU;
otherwise it is processed as requesting the creation of a new
transport connection.
If the received TPDU is a DT TPDU and the network connection has
a class 0 or 1 transport connection assigned to it, or an AK TPDU
47
where a class 1 transport connection is assigned, then the TPDU
is associated with the transport connection.
Otherwise, the DST-REF parameter of the TPDU is used to identify
the transport connection. The following cases are distinguished:
a) if the DST-REF is not allocated to a transport connection,
the transport entity shall respond on the same network
connection with a DR TPDU if the TPDU is a CC TPDU, with a
DC TPDU if the TPDU is a DR TPDU and shall ignore the TPDU
if neither a DR TPDU nor CC TPDU. No association with a
transport connection is made.
b) if the DST-REF is allocated to a connection, but the TPDU
is received on a network connection to which the
connection has not been assigned then there are three
cases:
1) if the transport connection is of class 4 and if the
TPDU is received on a network connection with the same
pair of NSAPs as that of the CR TPDU then the TPDU is
considered as performing assignment,
2) if the transport connection is not assigned to any
network connection (waiting for reassignment after
failure) and if the TPDU is received on a network
connection with the same pair of NSAPs as that of the
CR TPDU then the association with that transport
connection is made.
3) Otherwise, the TPDU is considered as having a DST-REF
not allocated to a transport connection (case a).
c) If the TPDU is a DC TPDU then it is associated with the
transport connection to which the DST-REF is allocated,
unless the SRC-REF is not the expected one, in which case
the DC TPDU is ignored.
d) If the TPDU is a DR TPDU then there are three cases:
1) if the SRC-REF is not as expected then a DC TPDU with
DST-REF equal to the SRC-REF of the received DR TPDU
is sent back and no association is made;
48
2) if a CR TPDU is unacknowledged then the DR TPDU is
associated with the transport connection, regardless
of the value of its SRC-REF parameter;
3) otherwise, the DR TPDU is associated with the
transport connection identified by the DST-REF
parameter.
e) if the TPDU is a CC TPDU whose DST-REF parameter
identifies an open connection (one for which a CC TPDU has
been previously received), and the SRC-REF in the CC TPDU
does not match the remote reference, then a DR TPDU is
sent back with DST-REF equal to the SRC-REF of the
received CC TPDU and no association is made.
f) if none of the above cases apply then the TPDU is
associated with the transport connection identified by the
DST-REF parameter.
6.10 Data TPDU numbering
6.10.1 Purpose
Data TPDU numbering is used in classes 1, 2 (except when the
non-use of explicit flow control option is selected), 3 and 4.
Its purpose is to enable the use of recovery, flow control and
re-sequencing functions.
6.10.2 TPDUs and parameters used
The procedure makes use of the following TPDU and parameter:
DT TPDU;
- TPDU-NR.
49
6.10.3 Procedure
A Transport entity shall allocate the sequence number zero to the
TPDU-NR of the first DT TPDU which it transmits for a transport
connection. For subsequent DT TPDUs sent on the same transport
connection, the transport entity shall allocate a sequence number
one greater than the previous one.
When a DT TPDU is retransmitted, the TPDU-NR parameter shall have
the same value as in the first transmission of that DT TPDU.
Modulo 2**7 arithmetic shall be used when normal formats have
been selected and modulo 2**31 arithmetic shall be used when
extended formats have been selected. In this International
Standard the relationships 'greater than' and 'less than' apply
to a set of contiguous TPDU numbers whose range is less than the
modulus and whose starting and finishing numbers are known. 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
6.11.1 Purpose
Expedited data transfer procedures are selected during connection
establishment. The network normal data variant may be used in
classes 1, 2, 3 and 4. The network expedited variant is only
used in class 1.
6.11.2 Network service primitives
The procedure makes use of the following network service
primitives:
a) N-DATA;
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b) N-EXPEDITED DATA.
6.11.3 TPDUs and parameter used
The procedure makes use of the following TPDUs and parameters:
a) ED TPDU;
- ED TPDU-NR.
b) EA TPDU;
- YR-EDTU-NR.
6.11.4 Procedures
The TS-user data parameter of each T-EXPEDITED DATA request shall
be conveyed as the data field of an Expedited Data (ED) TPDU.
Each ED TPDU received shall be acknowledged by an Expedited
Acknowledge (EA) TPDU.
No more than one ED TPDU shall remain unacknowledged at any time
for each direction of a transport connection.
An ED TPDU with a zero length data field is a protocol error.
51
NOTES
1. The network normal data variant is used, except when the
network expedited variant (available in Class 1 only), has
been agreed, in which case ED and EA TPDUs are conveyed in
the data fields of N-EXPEDITED DATA primitives (see
6.2.3).
2. No TPDUs can be transmitted using network expedited until
the CC TPDU becomes acknowledged, to prevent the network
expedited from overtaking the CC TPDU.
6.12 Reassignment after failure
6.12.1 Purpose
The reassignment after failure procedure is used in Classes 1 and
3 to commence recovery from an NS-provider signalled disconnect.
6.12.2 Network service primitives
The procedure uses the following network service primitive:
N-DISCONNECT indication
6.12.3 Procedure
When an N-DISCONNECT indication is received from the network
connection to which a transport connection is assigned, the
initiator shall apply one of the following alternatives:
a) if the TTR timer has not already run out and no DR TPDU is
retained then:
52
1) assign the transport connection to a different network
connection (see 6.1) and start its TTR timer if not
already started.
2) while waiting for the completion of assignment if:
- an N-DISCONNECT indication is received, repeat the
procedure from 6.12.3.a,
- the TTR timer expires, begin procedure 6.12.3.b.
3) when reassignment is completed, begin
resynchronization (see 6.14) and:
- if a valid TPDU is received as the result of the
resynchronization, stop the TTR timer, or
- if TTR runs out, wait for the next event, or
- if an N-DISCONNECT indication is received, then
begin either procedure 6.12.3.a or 6.12.3.b
depending on the TTR timer.
NOTE - After the TTR timer expires and while waiting for
the next event, it is recommended that the initiator
starts the TWR timer. If the TWR timer expires before the
next event the initiator should begin the procedure in
6.12.3.b.
b) if the TTR timer has run out, consider the transport
connection as released and freeze the reference (see
6.18).
c) if a DR TPDU is retained and the TTR timer has not run
out, then follow the actions in either 6.12.3.a or
6.12.3.b.
The responder shall start its TWR timer if not already started.
The arrival of the first TPDU related to the transport connection
(because of resynchronization by the initiator) completes the
reassignment after failure procedure. The TWR timer is stopped
and the responder shall continue with resynchronization (see
6.14). If reassignment does not take place within this time, the
53
transport connection is considered released and the reference is
frozen (see 6.18).
6.12.4 Timers
The reassignment after failure procedure uses two timers:
a) TTR, the time to try reassignment/resynchronization timer;
b) TWR, the time to wait for reassignment/resynchronization
timer.
The TTR timer is used by the initiator. Its value shall not
exceed two minutes minus the sum of the maximum disconnect
propagation delay and the transit delay of the network
connections (see note 1). The value for the TTR timer may be
indicated in the CR TPDU.
The TWR timer is used by the responder. If the reassignment time
parameter is present in the CR TPDU, the TWR timer value shall be
greater than the sum of the TTR timer plus the maximum disconnect
propagation delay plus the transit delay of the network
connections.
If the reassignment time parameter is not present in the CR TPDU,
a default value of 2 minutes shall be used for the TWR timer.
NOTES
1. Provided that the required quality of service is met, TTR may
be set to zero (i.e. no assignment). This may be done, for
example, if the rate of NS-provider generated disconnects is
very low.
2. Inclusion of the reassignment time parameter in the CR TPDU
allows the responder to use a TWR value of less than 2
minutes.
3. If the optional TS1 and TS2 timers are used, it is
recommended:
54
a) to stop TS1 or TS2 if running when TTR or TWR is
started;
b) to restart TS1 or TS2 if necessary when the
corresponding TPDU (CR TPDU or DR TPDU respectively is
repeated);
c) to select for TS1 and TS2 values greater than TTR.
55
6.13 Retention until acknowledgement of TPDUs
6.13.1 Purpose
The retention until acknowledgement of TPDUs procedure is used in
classes 1, 3 and 4 to enable and minimize retransmission after
possible loss of TPDUs.
The confirmation of receipt variant is used only in Class 1 when
it has been agreed during connection establishment (see note).
The AK variant is used in classes 3 and 4 and also in Class 1
when the confirmation of receipt variant has not been agreed
during connection establishment.
NOTE - Use of confirmation of receipt variant depends on the
availability of the network layer receipt confirmation service
and the expected cost reduction.
6.13.2 Network service primitives
The procedure uses the following network service primitives:
a) N-DATA;
b) N-DATA ACKNOWLEDGE.
6.13.3 TPDUs and parameters used
The procedure uses the following TPDUs and parameters:
a) CR, CC, DR and DC TPDUs;
b) RJ and AK TPDUs;
- YR-TU-NR.
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c) DT TPDU;
- TPDU-NR.
d) ED TPDU;
- ED-TPDU-NR.
e) EA TPDU;
- YR-EDTU-NR.
6.13.4 Procedures
Copies of the following TPDUs shall be retained upon transmission
to permit their later retransmission:
CR, CC, DR, DT and ED TPDUs
except that if a DR is sent in response to a CR TPDU there is no
need to retain a copy of the DR TPDU.
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
an N-DATA ACKNOWLEDGE request (see notes 1 and 2).
After each TPDU is acknowledged, as shown in table 5, the copy
need not be retained. Copies may also be discarded when the
transport connection is released.
57
NOTES
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.
2. Use of the confirmation request parameter may affect the
quality of network service.
58
+-------------------------------------------------------------+
|RETAINED| | |
| TPDU | VARIANT | RETAINED UNTIL ACKNOWLEDGED BY |
|--------|--------------|-------------------------------------|
| CR | both |CC, DR or ER TPDU. |
| | | |
| DR | both |DC or DR (in case of collision) TPDU.|
| | | |
| CC | confirmation |N-DATA Acknowledge indication, RJ, |
| | of receipt |DT, EA or ED TPDU. |
| | variant | |
| | | |
| CC | AK variant |RJ, DT, AK, ED or EA TPDU. |
| | | |
| DT | confirmation |N-DATA ACKNOWLEDGE indication cor- |
| | of receipt |responding to an N-DATA request which|
| | variant |conveyed, or came after, the DT TPDU.|
| | | |
| DT | AK variant |AK or RJ TPDU for which the YR-TU-NR |
| | |is greater than TPDU-NR in the DT |
| | |TPDU. |
| | | |
| ED | both |EA TPDU for which the YR-EDTU-NR is |
| | |equal to the ED-TPDU-NR in the |
| | |ED TPDU. |
+-------------------------------------------------------------+
Table 5. Acknowledgement of TPDUs
59
6.14 Resynchronization
6.14.1 Purpose
The resynchronization procedures are used in Classes 1 and 3 to
restore the transport connection to normal after a reset or
during reassignment after failure according to 6.12.
6.14.2 Network service primitives
The procedure makes use of the following network service
primitive:
N-RESET indication.
6.14.3 TPDUs and parameters used
The procedure uses the following TPDUs and parameters:
a) CR, DR, CC and DC TPDUs
b) RJ TPDUs;
- YR-TU-NR.
c) DT TPDU;
- TPDU-NR
d) ED TPDU;
- ED TPDU-NR.
e) EA TPDU;
- YR-EDTU-NR.
60
6.14.4 Procedure
A transport entity which is notified of the occurence of an N-
RESET or which is performing 'reassignment after failure'
according to 6.12 shall carry out the active resynchronization
procedure (see 6.14.4.1) unless any of the following hold:
a) the transport entity is the responder (see note). In this
case the passive resynchronization procedure is carried
out (see 6.14.4.2).
b) the transport entity has elected not to reassign (see
6.12.3.c). In this case no resynchronization takes place.
6.14.4.1 Active resynchronization procedures
The Transport entity shall carry out one of the following
actions:
a) if the TTR timer has been previously started and has run
out (i.e. no valid TPDU has been received), the transport
connection is considered as released and the reference is
frozen (see 6.18).
b) otherwise, the TTR timer shall be started (unless it is
already running) and the first applicable of the following
actions shall be taken:
1) if a CR TPDU is unacknowledged, then the transport
entity shall retransmit it;
2) if a DR TPDU is unacknowledged, then the transport
entity shall retransmit it;
3) otherwise, the transport entity shall carry out the
data resynchronization procedures (6.14.4.3).
The TTR timer is stopped when a valid TPDU is received.
61
6.14.4.2 Passive resynchronization procedures
The transport entity shall not send any TPDUs until a TPDU has
been received. The transport entity shall start its TWR timer if
it was not already started (due to a previous N-DISCONNECT or N-
RESET indication). If the timer runs out prior to the receipt of
a valid TPDU which commence resynchronization (i.e. CR or DR or
RJ TPDU) the transport connection is considered as released and
the reference is released (see 6.18).
When a valid TPDU is received the transport entity shall stop its
TWR timer and carry out the appropriate one of the following
actions, depending on the TPDU:
a) if it is a DR TPDU, then the transport entity shall send a
DC TPDU;
b) if it is a repeated CR TPDU (see note 1) then the
transport entity shall carry out the appropriate action
from the following:
1) if a CC TPDU has already been sent, and acknowledged:
treat as a protocol error;
2) if a DR TPDU is unacknowledged (whether or not a CC
TPDU is unacknowledged): retransmit the DR TPDU, but
setting the source reference to zero;
3) if the T-CONNECT response has not yet been received
from the user: take no action;
4) otherwise; retransmit the CC TPDU followed by an
unacknowledged ED TPDU (see note 2) and any DT TPDU;
NOTES
1. A repeated CR TPDU can be identified by being on a
network connection with the appropriate network
addresses and having a correct source reference.
62
2. The transport entity should not use network expedited
until the CC TPDU is acknowledged (see 6.5). This
rule prevents the network expedited from overtaking
the CC TPDU.
c) if it is an RJ or ED TPDU then one of the following
actions shall be taken:
1) if a DR TPDU is unacknowledged, then the transport
entity shall retransmit it;
2) otherwise, the transport entity shall carry out the
data resynchronization procedures (6.14.4.3).
3) If a CC TPDU was unacknowledge, the RJ or ED TPDU
should then be considered as acknowledging the CC
TPDU. If a CC TPDU was never sent, the RJ TPDU should
then be considered as a protocol error.
6.14.4.3 Data Resynchronization Procedures
The transport entity shall carry out the following actions in the
following order:
a) (re)transmit any ED TPDU which is unacknowledged,
b) transmit an RJ TPDU with YR-TU-NR field set to the TPDU-NR
of the next expected DT TPDU;
63
c) wait for the next TPDU from the other transport entity,
unless an RJ or DR TPDU has already been received; if a DR
TPDU is received the transport entity shall send a DC,
freeze the reference, inform the TS-user of the
disconnection and take no further action (i.e. it shall
not follow the procedures in 6.14.4.3.d). If an RJ TPDU
is received, the procedure of 6.14.4.3.d shall be
followed. If an ED TPDU is received the procedures as
described in 6.11 shall be followed. If it is a
duplicated ED-TPDU the transport entity shall acknowledge
it, with an EA TPDU, discard the duplicated ED TPDU and
wait again for the next TPDU.
d) (re)transmit any DT TPDUs which are unacknowledged,
subject to any applicable flow control procedures (see
note);
NOTE - The RJ TPDU may have reduced the credit.
6.15 Multiplexing and demultiplexing
6.15.1 Purpose
The multiplexing and demultiplexing procedures are used in
Classes 2, 3 and 4 to allow several transport connections to
share a network connection at the same time.
6.15.2 TPDUs and parameters used
The procedure makes use of the following TPDUs and parameters:
CC, DR, DC, DT, AK, ED, EA, RJ and ER TPDUs
- DST-REF
64
6.15.3 Procedure
The transport entities shall be able to send and receive on the
same network connection TPDUs belonging to different transport
connections.
NOTES
1. When performing demultiplexing the transport connection to
which the TPDUs apply is determined by the procedures
defined in 6.9.
2. Multiplexing allows the concatenation of TPDUs belonging
to different transport connections to be transferred in
the same N-DATA primitive (see 6.4).
6.16 Explicit Flow Control
6.16.1 Purpose
The explicit flow control procedure is used in Classes 2, 3 and 4
to regulate the flow of DT TPDUs independently of the flow
control in the other layers.
6.16.2 TPDUs and parameters used
The procedure makes use of the following TPDUs and parameters:
a) CR, CC, AK and RJ TPDUs
- CDT.
b) DT TPDU
- TPDU-NR.
65
c) AK TPDU
- YR-TU-NR;
- subsequence number;
- flow control confirmation.
d) RJ TPDU
- YR-TU-NR.
6.16.3 Procedure
The procedures differ in different classes. They are defined in
the clauses specifying the separate classes.
6.17 Checksum
6.17.1 Purpose
The checksum procedure is used to detect corruption of TPDUs by
the NS-provider.
NOTE - Although a checksum algorithm has to be adapted to the
type of errors expected on the network connection, at present
only one algorithm is defined.
6.17.2 TPDUs and parameters used
The procedure uses the following TPDUs and parameters:
All TPDUs
- checksum
66
6.17.3 Procedure
The checksum is used only in Class 4. It is always used for the
CR TPDU, and is used for all other TPDUs except if the non-use of
the procedure was agreed during connection establishment.
The sending transport entity shall transmit TPDUs with the
checksum parameter set such that the following formulas are
satisfied:
SUM(from i=1 to i=L) OF a[i] EQUALS <zero> (module 255)
SUM(from i=1 to i=L) OF i*a[i] EQUALS <zero> (module 255)
where
i = number (i.e. position) of an octet within the TPDU
(see 13.2);
a[i] = value of octet in position 1;
L = length of TPDU in octets.
A transport entity which receives a TPDU for a transport
connection for which the use of checksum has been agreed and
which does not satisfy the above formulas shall discard the TPDU
(see also note 2).
NOTES
1. An efficient algorithm for determining the checksum
parameters is given in annex B.
2. If the checksum is incorrect, it is not possible to know
with certainty to which transport connection the TPDU is
related; further action may be taken for all the transport
connections assigned to the network connection (see 6.9).
3. The checksum proposed is easy to calculate and so will not
impose a heavy burden on implementations. However, it
will not detect insertion or loss of leading or trailing
zeros and will not detect some octets misordering.
67
6.18 Frozen references
6.18.1 Purpose
This procedure is used in order to prevent re-use of a reference