OSI/TS Configuration and Management Manual

OSI Transport Services
OSI/TS Configuration and Management Manual—424831-001
5-8
Retention of TPDUs
The OSI/TS Subsystem as Responder
The OSI/TS subsystem initiates passive resynchronization by starting the TWR timer.
TWR determines the length of time OSI/TS waits for a valid TPDU. If TWR times out
before a valid TPDU is received, OSI/TS considers the transport connection released
and informs users by issuing file-system error 140.
If a valid TPDU is received before TWR times out, OSI/TS stops TWR and responds
according to the type of TPDU it received:
•
If it received a disconnect request (DR-TPDU), OSI/TS confirms the request by
issuing a disconnect-confirm TPDU (DC-TPDU) and releases the connection.
•
If it received a CR-TPDU, OSI/TS confirms the request, provided that OSI/TS has
not previously sent either a connection confirmation or a disconnect request.
•
If it did not receive either of the above TPDUs, it should have received either an RJ-
TPDU or ED-TPDU from the remote peer who actively initiated resynchronization.
OSI/TS responds by re-sending any unacknowledged ED-TPDU, returning an RJ-
TPDU that indicates the next DT-TPDU that OSI/TS expects to receive, and re-
sending any unacknowledged DT-TPDUs.
Retention of TPDUs
In classes 1, 3, and 4, the OSI/TS subsystem retains transmitted TPDUs until they are
acknowledged by the correspondent peer. OSI/TS retains all connect-confirm TPDUs
(CC-TPDUs), CR-TPDUs, DR-TPDUs, DT-TPDUs, and ED-TPDUs. This procedure
allows the transport connection to recover data in case of a subnetwork failure.
Retransmission on Timeout
In class 4 only, OSI/TS starts a retransmission timer for each transmitted TPDU. If the
timer expires before the TPDU is acknowledged, OSI/TS retransmits the TPDU and
restarts the timer. After the configured number of retransmissions, OSI/TS releases the
connection and informs the user by issuing file-system error 140. The timer value and
number of retransmissions can be set using SCF.
Resequencing
In class 4 only, OSI/TS delivers data to the user in the order specified by the TPDU-NR
in received DT-TPDUs. Out-of-sequence DT-TPDUs are kept until the sequence is
intact.
Inactivity Control
In class 4 only, OSI/TS detects unsignaled termination of the network connection.
OSI/TS starts an inactivity timer whenever a TPDU is received. If the timer expires
before another TPDU is received, OSI/TS releases the transport connection. The timer
value can be set using SCF.
A transport entity using class 4 over a connection-mode network service does not use
the reassignment and resynchronization procedures used by classes 1 and 3. It reassigns