OSI/TS Configuration and Management Manual

OSI Transport Services
OSI/TS Configuration and Management Manual—424831-001
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Data TPDU Numbering
Data TPDU Numbering
OSI/TS assigns a sequence number (TPDU-NR) to all transmitted-data TPDUs
(DT-TPDUs). The sequence begins with 0 and increments by 1, except in class 0, where
TPDU-NR is always 0. After the maximum TPDU-NR is assigned, the counter returns
to 0. The maximum counter number allowed depends on the protocol class and on
whether extended format addressing is used. This function provides extra security
against lost data or packets delivered out of order. DT-TPDU numbering is the basis for
flow control and for recovery from subnetwork resets and disconnects.
Reassigning Transport Connections
Protocol classes 1, 3, and 4 (over connection-mode network service only) attempt to re-
establish a transport connection after a subnetwork connection failure. OSI/TS initiates
reassignment procedures whenever it receives a disconnect indication from the
subnetwork. OSI/TS reassigns all transport connections currently using that subnetwork
connection.
The OSI/TS strategy for reassigning connections after subnetwork-signaled disconnects
depends on the following two conditions:
•
Whether the OSI/TS subsystem was the initiator or the responder during the
connection-establishment phase
•
The current state of the transport connection
The following subsections describe the OSI/TS response to disconnect indications from
the subnetwork.
The OSI/TS Subsystem as Initiator
As initiator of the transport connection, the OSI/TS subsystem actively tries to reassign
the connection to another network connection. However, if OSI/TS has already
requested release of the transport connection with an outstanding DR-TPDU, the
network-disconnect indication is treated as a completion of the release protocol.
If there is no outstanding DR-TPDU when a network-disconnect indication is received,
OSI/TS starts the timer TTR. TTR determines the length of time during which OSI/TS
tries to reassign the transport connection to another subnetwork connection on the same
physical line.
OSI/TS reassigns the transport connection to an existing or new connection on the same
physical line as follows:
•
If OSI/TS owns another network connection between the same NSAPs and the
connection allows multiplexing, OSI/TS reassigns the transport connection to it.
•
If no connection is available, OSI/TS requests one and then waits for a connect
confirmation from the subnetwork. If a confirmation is received before TTR times
out, OSI/TS resynchronizes the data.
If no confirmation is received, OSI/TS repeats the request until the connection is
established or until the timer expires. If TTR times out before a subnetwork connection