OSI/TS Configuration and Management Manual

Managing a Transport Connection
OSI/TS Configuration and Management Manual—424831-001
6-13
Connection Release
When receiving DT-TPDUs, OSI/TS uses AK-TPDUs to advance the remote transmit
window. OSI/TS sends an AK-TPDU only if a READ call is outstanding. If OSI/TS
cannot forward a received TPDU to the user because a READ call is not outstanding, no
AK-TPDU is sent. Receipt of an AK-TPDU that sets the lower window edge to more
than one greater than the TPDU-NR of the last TPDU is not considered an error, unless
it acknowledges DT-TPDUs that have never been sent.
Connection Release
During normal operation of a transport connection, a transport entity initiates connection
release at the request of a transport user. The Transport Layer can provide host-to-host
confirmation of connection release to ensure a coordinated shutdown of the transport
connection.
On connection-mode subnetworks, if correspondent transport entities do not provide
error-recovery services, they release the transport connection upon indication of a reset
or disconnect by the communications subnetwork. In this case, each connection
endpoint initiates the error-release procedure independently and does not provide host-
to-host confirmation.
Correspondent transport entities can also use the connection-release protocol at any time
during the connection-establishment phase to refuse or abandon the connection-
establishment procedure.
OSI/TS provides different connection-release procedures, according to the protocol
class, as described in the following subsections.
Connection Release in Class 0
Protocol class 0 uses subnetwork procedures to manage connection release. There is no
host-to-host confirmation of connection release. When a transport user issues a
T-DISCONNECT service primitive, OSI/TS issues a disconnect request to the
communications subnetwork and immediately returns a T-DISCONNECT indication to
the user.
If OSI/TS receives an indication that the subnetwork connection has been reset or
disconnected, the OSI/TS subsystem completes all pending file-system requests by
issuing error 140.
Connection Release in Classes 1 Through 4
Class-1 through class-4 protocols use an explicit procedure that provides host-to-host
confirmation of connection release. When a transport user requests release of a
transport connection, OSI/TS responds as follows:
1. OSI/TS issues a DR-TPDU to the correspondent peer and starts the disconnect timer
DISCONNECTTIMEOUT. OSI/TS then ignores any incoming TPDU on the
connection unless it is a DC-TPDU or a DR-TPDU.
2. If a DC-TPDU or DR-TPDU is received, OSI/TS considers the connection released
and completes all pending file-system requests by issuing error 140.